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Hi ,
I want to monitor supercontinuum on OSA and for that purpose first I couple supercontinuum into SMF (PM-980) and other end to OSA. I am wondering since the loss is wavelength dependent and at shorter wavelengths we have high attenuation constant (~6dB/km), I could see red light in the generated supercontinuum but OSA does not show any IR wavelength and SC starts from 800nm to 1600nm. Is it because of high attenuation on those wavelengths or something else is going on. What type of fiber one should use in such situation.
Thanks
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By the way, here is a trick to optimize coupling using the OSA. You can scan the wavelengths for only a single wavelength, for example from 900 nm to 900 nm. This allow to see in real time the variation of the power at this wavelength (while moving the 3-axes stage) instead of waiting the full scan and it is easier to visualize precisely an increase or decrease. I hope you understand what I mean!
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Hi we want to see the laser pulses(femtosecond pharos, 1030nm, 100KHz, 180 fs) with oscilloscope, what should be the settings we need to play with to see the signal from the photo diode detecting the laser pulses. we try to change the vertical and horizontal divisions(50 mV and 2 ns) this is what we got we got.
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Hi Melika,
Make sure your PD has higher bandwidth than your laser repetition rate.
secondly, also make sure PD didn't run out of battery. After making sure all of these points, you can auto-reset the Oscope setting and give you the expected signal.
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or we can use light with lower frequencies in the range of MHz?
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Dear Mahdi Vazvani,
What is the rise time of a photodetector?
The rise time is equivalent to the response speed of the device. It is defined as the time required for the photodetector output signal to change from 10% to 90% of the peak level. A good rule of thumb is to choose a photodiode with a rise time that is one fifth of the laser pulse width to be measured.
How can we measure the laser pulse duration less than pS?
Pulse durations down to roughly 10 ps can be measured with the fastest available photodiodes in combination with fast sampling oscilloscopes. For the measurement of shorter pulse durations, streak cameras can be used.
How is laser pulse measured?
The pulse duration, which is typically in the nanosecond regime, can be directly measured with a fast photodetector. The pulse energy may be measured directly (e.g. with a pyroelectric detector). In some cases with repetitive operation, it is calculated from the average power (from a power meter) and repetition rate.
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Hi there,
I hope you are doing well.
Do you know, which camera is used recently for the GRENOUILLE technique in order to measure the pulse length of ultra-short pulses?
what features the camera should provide? Should it be fast or regular camera works as well?
This article names a camera but I am not sure it is used recently in experiments or it is out of date!
Attached is a document in this regard.
I appreciate your time and any feedback.
Best regards,
Aydin
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Dear Hassan Nasser I really appreciate your response.
Thank you!!
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I have shadowgraphs of plasma filament formed in air by focusing a femtosecond laser and I am trying to solve for refractive index change in the region using the shadowgraph. The contrast value at each pixel is a source term for 2D-Poisson equation.
I can apply the Dirichlet boundary condition at the top and bottom of the image. But the left and right corners contain sections modulated by the filament.
What type of boundary conditions should I use in this case?
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You have three possibilities:
- Dirichlet b.c. - specifies the value of the function at the boundary: if the boundary is sufficiently extended from the source/filament you can use Dirichlet b.c. assuming no signal there (e.g. n=1).
- Neumann b.c. (specifies derivative of the function, so at boundary you would like to have no change in the processed value, so d(n)=0;
- Cauchy b.c. (specifies both: value e.g. n=1 and its derivative (d(n)=0).
It depends on the geometry of your imaging system, the field of view, known information (maybe you have specific values at the boundary?). The simplest case is for D.b.c. (1st point), if you can assume an environmental index of refraction equal to 1 for a sufficiently large distance from the source. Hope this helps.
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Is there any nonlinear process through which a 0.8 micron femtosecond laser pulse can be used to efficiently produce radiations in the range of 1 - 1.5 microns.
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It depends on the application and conversion efficiency you are expecting. While OPOs are mostly at nJ pulses with high pump to signal/Idler conversion efficiency. But if you have mJ pulse, you can go with Parametric generation like in most OPAs. In Supercontinuum based on PCF, you will not have much control over the output bandwidth and tuning cannot be independent and arbitrary.
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I want to generate second harmonic using the long-wavelength (1600nm) part of the supercontinuum as pump. How to decide on choosing the right focal length lens for SHG. I am using 10mm-long MgO-PPLN. The average power at the output of supercontinuum is ~100mw and ~40mw for SHG (after long-pass dichroic mirror). Since the temporal profile of the signal after SC is not known (specifically for the 1600nm part ), is it suitable to treat that signal as continuous-wave for SHG pump. For CW pump, the literature suggests that the ratio of crystal length to the confocal parameter (2*Rayleigh range) should be 2.84 for optimal SHG. From where we may calculate the right focal length of the lens to focus light into the SHG crystal.
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you welcome Abbas N..
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I would like to know your recommendations for books/online courses (MIT-OCW/Youtube/Udemy etc.) available on Photoacoustic Signals (Basic/Advanced/any level). I would prefer literature with more emphasis on their mathematics/physics.
I eventually want to observe the effects of these signals post Absorption spectroscopy (obtained from a nanosecond/femtosecond laser pulse source).
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Usually the terahertz source used for terahertz communication is a continuous terahertz wave generated by differential frequency, then can terahertz pulses generated by femtosecond laser be used for terahertz communication?
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In principle yes however there is serious problem: how to achieve good sensitivity? To extract information, one needs to perform down-conversion or envelope demodulation / pulse position detection. For Thz continuous wave one can do down-conversion by some kind of mixer however of Thz pulses we mostly left with option of square (envelope) detector and it requires much stronger signal. Some limited distance can obtained by providing rather high power of transmission.
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I want to generate second harmonic of 1600nm component from the octave-spanning supercontinuum to beat it with 800nm component and detect CEO signal. What are important specs on deciding an optimal SHG crystal.
Also, What should be the crystal acceptance bandwidth or how to estimate it.
Thanks
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I would recommend using a periodically poled crystal here, e.g., PPLN or MgO:PPLN. You can probably use a standard crystal for your 1600 nm wavelength. A few nanometers conversion bandwidth are sufficient for f-2f interferometry. In the end, it only matters how many 800 nm photons you generate. A longer PPLN crystal has a narrower acceptance bandwidth but a higher conversion efficiency. In my opinion, the crystal length is therefore not so important. We always used 1 cm long crystals in our Ti:sapphire based experiments.
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I wanna calculate spatial profile of laser-induced heating with a temporal evolution.
System is like a 100-nm thick film of gold (Au) deposited on a 7.4-um thick yttrium iron garnet (YIG) on a gallium gadolinium garnet (GGG) substrate. This system is heating up by absorbing of 100-fs laser pulse, e.g. 525-nm wavelength.
Looks like it is possible to use Comsol Multiphysics or Lumerical for this task but I haven't found any real solutions or applications of performing this calculation in such softwares.
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May this help:
Modeling of femtosecond laser damage threshold on the two-layer metal films
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Hello everyone,
Normally, I use the autocollimation technique in the case of a cw laser which implies the use of an additional objective lens (OL) right away the laser to wider the beam. However, in the case of a pulsed fs laser, I would be concerned about the group delay dispersion (GDD) of the OL. Obviously one would suggest to use a GDD-minimized OL for this aim but maybe there is another way to collimate the beam without having to buy an additional OL. Beam divergence of the laser is < 1 mrad.
Would be thankful for sharing your expertise (if any).
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Hi Michael,
Thanks for the suggestion. As I understand, an off-axis parabolic silver-coated mirror could be the option (as such kind of mirrors introduce negligible dispersions of any orders).
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Who have the phd thesis on the spectrum of terahertz radiation from fs laser induced air plasma? If you have one, can you tell me the title of the thesis? Thank you for your kindly help!
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Not a thesis but a recent paper, which you might be interested in:
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I understand at high pulse energies you can machine visible patterns on quartz. How do we tune the energy/other parameters to form patterns which are only visible after etching with HF or KOH?
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You have to control the following:
1. Spot diameter of the beam, which is controlled by the focal length and input light diameter.
2. The energy of the LASER. By controlling the input energy you can actually make a pattern in the quartz that is smaller than the diffraction limit.
3. Beam shape (profile) of your input LASER beam; typically a Gaussian beam is used.
4. Ellipticity of the beam.
5. The LASER beam pulse width; a shorter pulse beam will have higher fluence. In general it is the peak fluence that causes index of refraction difference in the crystal from the high photon flux. At a molecular level the substrate cannot handle the thermal in flux of photons, so energy still has to be conserved, therefore the electric dipoles (quantum level) absorb the energy and cause the dipoles to be displaced. This displacement of the dipoles in the short pulse focus beam area causes an index of refraction change from the rest of the substrate.
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how to do f-2f interferometry from an octave spanning supercontinnum with frequency spacing 120MHz. Which would be the most suitable optical filter in this range for filtering f and 2f components from supercontinnum, to detect carrier envelope offset frequency of a mode-locked laser.
Thanks in advance!
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The optical filter has to be matched to the phase-matching bandwidth of your SHG crystal. For Ti:sapphire based combs, one typically uses some off-the-shelf PPLN crystals, e.g., for 532 nm wavelength. You can find matching interference filters from Thorlabs or Edmund Optics. You can also use a simple diffraction grating instead. Details on a suitable setup are here: https://www.nature.com/articles/nphoton.2010.91
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We are a research group working in the field of laser processing. We want to build our own femtosecond (or picosecond laser) for micromachining (fine marking for example). Do your know any low cost solution ? (publications, reports, do-it-yourself instructions, simple low cost source, etc...). Thanks for your advice and contact me if you having complete discussion : mflury@unistra.fr
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Still an interesting topic in 2021. Ultrafast lasers are still expensive! The smallest available laser which can be used for material processing (means 3-5 W of average power, 15-100ps pulses, 30-60uJ pulse energy) still costs 25kEUR-45kEUR . :/
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The principles of wavefront reconstruction based on a geometric-optical reflection of reconstructing light from the surfaces with constant phase differences between the object and reference waves can also be used for a temporal reconstruction of the object ultrashort pulse [1]. This can be illustrated by the following simple example. Let the object beam consists of two δ-pulses delayed with respect to each other by τ. We suppose that the object and reference beams propagate in opposite directions forward to each other and also that δ-pulse is used as the reference one. In that case the interference fringe structure will consists of two parallel planes separated by a distance τc/2 where c is the velocity of light. If we use the δ-pulse for reconstruction it will be reflected sequentially from one plane and then from the other. The time delay between two reflected pulses will be equal to τ. So, the object pulse temporal structure was reconstructed by simple geometric-optical reflection. The question is: How this mechanism of the object pulse temporal reconstruction relates to the known methods of time-resolved holography ([2, 3] and other)? 1.https://www.researchgate.net/publication/238033164_Ultrashort_light_pulse_scattering_by_3D_interference_fringe_structure
2. Rebane, A., & Feinberg, J. (1991). Time-resolved holography. Nature, 351(6325), 378-380.
3. Mazurenko, Y. T. (1990). Holography of wave packets. Applied Physics B, 50(2), 101-114.
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This question is discussed in my recent paper:
Article Geometrooptical Mechanism of Wave-Front Reconstruction
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Working on a femtosecond laser which is mode locked, we can not change the pulse energy and wavelength so we have to change its fluence
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Hi! The simplest thing to do is to use a variable attenuator. You can build your own variable attenuator using a half-wave plate and a polarizer. Then you can vary the power without even touching the laser
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hi,we want to build an Ultrafast Transient Absorption Spectrometer system, how to choose a spectrometer?
We already have femtosecond lasers system (coherent) ,pump probe system(home built), white light white-light supercontinuum(use CaF2 or Al2O3),
We don't know how to choose a suitable spectrometer, what products can be recommended?
We would appreciate it if you could help.
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It depends a bit on whether you want to measure a spectrum per single laser pulse, or integrate the spectra of several consecutive pulses into one average spectrum.
In general, an Echelle grating in combination with a 2D detector array (like a camera chip) is recommendable for fast pulses. If you want to integrate several pulses, then an integrating camera chip like a CCD is best. If you, however need individual pulse spectra, then a CMOS camera chip is best.
Success!
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There is a femtosecond pump beam with a central wavelength of 800 nm, pulse duration of 150fs and repetition rate of 1kHz used to pump the OPO/OPA system with Ti: Sapphire (to get white light) and BBO crystals to get tunable infrared radiation in the spectral range of 1200-1600nm. Interested to know how much the initial 150fs pulse duration could change and how to calculate it. There is no current option to measure it, unfortunately. Thank you.
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I think is a rather challenging task with a not so accurate outcome. First of all you should know all the components of your OPA (which I believe is what you are using). Normally you generate white light (supercontinuum generation), then you have a first opa interaction with fundamental in a noncollinear arrangement and then usually a second opa interaction with some more fundamental light. For all of this, you have normally a saphire plate, two nonlinear crystals, some ND filters and a lot of lenses and mirrors. You should know all the properties of all of this elements. For propagation in linear media you can have an estimation by fourier transform the pulse at the entrance and calculating E(x_0+Dx,omega)=E(x_0,omega)exp(i*k(omega)*Dx), and inverse fourier transform the results (where k=n(omega)omega/c). Basically k(omega)*Dx is a spectral phase accumulated by each monochromatic
wave during propagation. This spectral phase is normally approximated by a Taylor expansion around the pulse carrier frequency w for which the first terms are normally called group delay (GD), group delay dispersion(GDD) and third order dispersion (TOD), which you can normally find for each material. For all nonlinear interactions is a little harder, because you need to know how the phase mismatch depends on omega. On a first order approximation, this is proportional to the group velocity mismatch, GVM=(1/v_g1-1/v_g2). From here you can calculate bandwidth and using the time-bandwidth product estimate a lower bound to the pulse width. I think is easier to buy/build and autocorrelator, for example an interferometric one. I hope this helps, and best luck.
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Consider the scenario,
input pulse width and spectrum = 700fs/ 2.23nm (after propagating thru 20cm SMF)
Output pulse width and spectrum become = 1.183 ps/ 38.357nm
How to select a compressor (Grating pair or the one more suitable) to compress output pulse with of 1.183ps to its fourier limit.
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There are several ways to compress the pulses. Common are Prism compressor, Grating compressor and Chirped mirrors.
It depends also on the pulse energy. For example if your pulse energy is in the range on mJ, you might not want to use Prism compressor as it is not suitable for higher energies due to nonlinear effects in the prism. Grating compressor avoids the nonlinearity problem, but it lacks throughput efficiency. In this case using Chirped mirrors is the best option as it gives high throughput and you can also control the amount of GDD suitable number of reflections reflections on the mirror.
This article might be of help.
Cheers,
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I am aware of many femtosecond laser based approaches to excite phonon modes e.g. in crystals and minerals. Quantum Cascade Lasers (QCLs) seem like an interesting alternative, especially since they can be tuned by frequency and can be operated continuously. Has someone tried to use QCLs to excite phonon modes? Or is there a catch?
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I am trying to couple light from Laser with M^2=1, wavelength = 1030nm and beam diameter = 1.3mm. Please suggest a lens for efficient coupling into PCF (say 2um core). As far as I know, I have two options, either to use Aspheric lens or Microscope objective, however, I am not really sure which one would be a best choice. As I understood (Please advice if right or wrong)
For efficient coupling, these points should be considered.
1) N.A of lens should be equal or close to that of fibre (PCF)
2) Lens focus diameter should be equal to core size of the fibre (PCF)
3) Input beam should be parallel to Optical axis (Collimated)
Please advice.
Thanks
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Abbas N. regarding your 3 points:
1) There is no need for the lens NA to equal that of the fibre. Usually, coupling will be more efficient if the lens NA exceeds that of the fibre, provided the lens aperture (entrance pupil) is sufficiently large to avoid vignetting the input beam.
2) Coupling is optimised when the beam is focussed to a spot whose field profile matches the field distribution of the fibre mode. Typically, both launch spot and mode profile are approximately Gaussian, and the matching criterion is for the launch spot diameter to equal the mode field diameter.
3) Launching the collimated input beam parallel to the optical axis of the lens which in turn is coincident with the optical axis of the fibre will minimise mode mismatch due to tilt errors and lens aberrations.
More specifically, you specify a beam quality factor M2 = 1, so the beam is close to an ideal Gaussian beam. When focussed by a lens, the radius of the beam waist at 1/e2 intensity, w0, is related to the wavelength, λ, and the divergence half-angle, θ, also at 1/e2 intensity by: θ = M2 λ / π w0
Your collimated input beam width is W=0.65 mm. If the focal length of the lens is f, the radius of the beam at the lens principal plane is W = θ f = f λ / π w0
provided the lens NA > sin θ. https://www.newport.com/n/gaussian-beam-optics
The fibre mode field diameter will be comparable to the fibre core diameter, but need not be identical. The relationship depends on wavelength, and the extent to which the evanescent field extends into the cladding.
If we assume that the beam spot radius is equal to the fibre radius, w0 = a = 1 μm, then we require a lens focal length f = π W w0 / λ = 1.98 mm.
A larger mode field diameter will require a proportionately larger focal length. A 2.8 μm mode field diameter will require a focal length of approximately 2.8 mm. The exact value is not too critical. If the mode field radius is r0, the coupling efficiency (assuming Gaussian profile for both fibre mode and excitation) is:
η = 4 w02 r02 / (w02 + r02)2
A 40% mismatch between launch spot diameter and mode field diameter degrades the theoretical coupling efficiency by only 11% (to 89%).
Regarding the choice between microscope objective and aspheric lens, both are capable of diffraction-limited performance for a monochromatic beam aligned with the optical axis. The microscope objective will be more complex, because it must deliver high resolution imaging over a relatively wide field of view, with illumination comprising a broad range of wavelengths.
The exact focal length of microscope objectives is not always stated. Magnification depends on the distance between objective and eyepiece. Tube lengths of order 160 mm are common, so a crude estimate for high power objectives is focal length f = 160 mm / magnification.
The 60x objective suggested by Suchita Yadav. would have a focal length around 2.7 mm, and would be a reasonable choice, especially if the fibre MFD is larger than 2 μm, but an 80x objective may be better. A 40x objective with 4 mm focal length is probably be too long.
If the microscope objective is optimised for visible wavelengths, there may be some additional losses due to surface reflections at infra-red wavelengths.
Note that a weak supplementary lens can be used to modify the diameter of the beam as it enters the focussing lens, and adjust the spot size for optimum coupling.
Hope this helps.
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Dear professors and colleagues,
I am going to to study effect of the two photons absorption in Safranin O. Safranin O is organic material, so I think that the power needed to activate this effect doesn't need to be too high. However, the two photons absorption is a third-order nonlinear optical effect, so it is usually implemented with high-power pulsed lasers. I cannot afford to buy high-power pulsed lasers. So, can I stimulate effect of two photons absorption in safranin O by continuous wave laser (808 nm or 1064 nm)? And how much is the required power of laser? I hope the colleagues who have experience in this experiment share me useful information.
I look forward to hearing from you. Thanks in advance.
Yours sincerely.
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Dear Nguyen, I beg You pardon, please read with attention my above given advice.. or at least tell us for what task You are going to apply TPA excitation then it would be possible to figure out correctly what laser beam intensity W/mm2 You was needed...
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Do there exist any compounds, fluorophores, dyes, quantum dots, phosphors etc that can emit or fluoresce non-linearly without needing ultra-high intensity illumination (such as pulsed femtosecond laser)? That is, with a UV led for example and with this non-linear emission being dominant?
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Dear Sammy,
Prof. A. Tikhonov already nicely summarised the most important issues concerning up-conversion. As far as phosphors are concerned, there are a couple of materials that can show rather efficient NIR to VIS up-conversion at moderate irradiation levels. These are: NaGdF4:Er,Yb; Gd2O2S:Er,Yb; BaY2F8:Er,Yb and YOCl:ErYb.
All of these materials make us of the mechanism of Sensitised Energy Transfer Up-conversion (S-ETU), while Yb3+ is the sensitiser and Er3+ the green or red emitter. Important is that the host material has solely low phonon energies, otherwise the process is easily quenched by electron-phonon coupling. Good luck!
Thomas
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When analyzing geoloical samples (element or isotope composition) with ICP-MS which type of laser ablation system would provide better result - nanosecond laser (193//213 nm) or femtosecond one?
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Hi Dany
We have never found a "finer" aerosol formed after LA in silicates. In fact Reto observed even more particles > 20 nm with 795 nm and 265 nm fs LA in silicate and zircon (Glaus et al. Spectrochimica Acta Part B 65 (2010) 812–822).
For such insulators, there is apparently sufficiently low heat dissipation on the ns time scale to avoid a deep melt and the corresponding splashed drops.
Best regards
Bodo
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I am using a femtosecond laser (Ti-sapphire) which has pulse width 200 fs. Now, I want to change the pulse width from 200 fs to ps range using glass rods for our microscopy application. But I do not have any idea how to check the pulse width?
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The most important thing is that the oscilloscope bandwidth must be greater than 1/T where T is the pulsewidth. So you need a scope with about 10 GHz bandwidth. You need also that photo detector capacitance Cd to be very small to make with the transimpedance amplifier resistance Rf a time consatnt Cd Rf that must be smaller than the pulse width of the optical narrow pulse. So, Also about i ns time constant may be suitable to great extent. The more safe condition not to shape the pulse is a rise time about T/10.
To see the effect of the Cd and Rf on the optical pulses please see the paper in the link:Conference Paper A comprehensive study of an optical transceiver
Best wishes
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As part of a project proposal that I am working on as an intern at the Canadian Light Source, we are looking at the effects on the production and transport of an electron bunch produced by photoemission of chirping a pulse from a femtosecond laser from approximately 35 fs to approximately 10 ps.
In order to design an appropriate system to produce this chirp and understand what the chirped pulse profile will look like so that I can model the resulting electron bunch, I have been trying to find some open source software that would allow me to model the pulse chirper and its effect on the pulse profile without success. Does anyone know of a program that would be suited to this task?
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For laser propagation codes like MALAPROP, you make use extensively of FFT's (fast Fourier Transforms) and their cousins in cylindrical coordinates. See original articles by Fox and Li demonstrating numerically the open resonator modes in the 60's, and for example Laser Program Annual Report: Volume 1 January 1, 1979 Lawrence Livermore National Laboratory (for MALAPROP, page 2-147). The latter is a free e-book download once you sell your soul to Google and sign into your Google acct. Then you propagate individually each Fourier component (each perfect sinusoid), modelling the cgrating compression by taking slightly different lengths of propagation for each frequency. Sorry I can't give more details, it;s been 20-30 yrs since I've looked into these things.
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Recently, I ran into an interesting (now, it's rather annoying) problem while mode-locking Coherent (Mira-HP) femtosecond laser. When the laser beam path is blocked by a metal plate as shown in the attached figure, we observe a stable mode-locked spectrum; however, it no longer stays mode-locked once the metal plate is removed.
I would greatly appreciate if experts would shed some light in this regard.
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An expensive solution could be to insert an optical isolator, but keep in mind dispersion of the crystal in this case.
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The femtosecond laser ionized air can form a filament of a length of centimeter, and broadband THz wave can be radiated from the filament. The plasma density at both ends of the filament is different, and the phase of the radiated terahertz will be different.
So, which factor can change the phase of the THz waveform? What is the physical mechanism behind the change of the phase of the THz waveform?
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The time delay of the pump.
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Hello
I am searching to find a code for describing and simulating interaction electron beam with femtosecond laser in vacuum.
notice : I don't have ions or plasma in my simulation. I have just electrons.
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I wanted to install Smilei(4.1 and 4.0) on Ubuntu but I could not.
I had this error in terminal
==============
Compiling src/picsar_interface/interface.cpp Linking smilei lto1: fatal error: bytecode stream generated with LTO version 6.0 instead of the expected 4.1 compilation terminated. lto-wrapper: fatal error: /usr/bin/g++ returned 1 exit status compilation terminated. /usr/bin/ld: error: lto-wrapper failed collect2: error: ld returned 1 exit status makefile:181: recipe for target 'smilei' failed make: *** [smilei] Error 1 ============
can you help me?
 I tried to remove this error by  this command:
make clean
make
but I could not solve this problem.
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I am trying to build a pump-probe system and I need to find the "time zero" between a pump pulse (@800 nm, ~50fs generated by Ti:Sapphire fs amplifier) and a probe pulse (@530 nm, ~50 fs, generated by an OPA). The delay line I built can create a maximum time travel difference of ~4ns. I have measured approximately the length of the optical path of the two beams (I set the delay line in approximately the middle point of the travel line) and set up the system so that the two beams meet after approximately the same travel distance. My plan is the following: I will set the OPA @ 800nm and I will put a 50:50 beam splitter in the meeting point of the two beams and finally collimate after that the two beams. Then I will monitor with a fiber spectrometer their spectrum and by scanning the delay I am expecting to see the spectral interference of the two pulses. Finally I will set the OPA at the desired wavelength and replace the beamsplitter with a dielectric mirror and realign and begin the experiment. I hope it will work but I am wondering if there is an easier and more efficient way?
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The easiest is to get SFG in nonlinear crystal. Since you have OPO, you can get any wavelength. If you have a nonlinear crystal (BBO) for SHG of 800 nm, you may generate from OPO wavelength closer to 800 nm, e.g. 750 nm, and use the same crystal for SFG. Check polarizations.
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"Beyond this point, with shorter focal length lenses, the diameter of the self-focal zone can no longer increase because the geometrical focal diameter becomes smaller. That is to say, external focusing becomes dominant and there is practically no distinction between geometrical focusing and self-focusing. Therefore, the diameter of the laser beam is limited by the strong external focusing of the lens, as observed in insets of Fig. 3 where the plasma column diameter decreases for shorter focal length lens."
what is the difference between external focusing , geometrical focusing and self-focusing. I am confused with this three concepts. Who can tell me the difference between this three physics concpts?
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the external focusing is using mirror while self focusing me be done with non linear material even liquids
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A two-color femtosecond laser is transmitted in the air and a positive group velocity dispersion occurs. Negative group velocity dispersion is introduced when a-BBO crystal is added to the optical path. Can someone explain to me what is a positive group velocity dispersion and negative group velocity dispersion in simple sentences?
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Hello, As Pr. Rüdiger Mitdank said, the group velocity is the First derivative of the frequency w ( w= 2pi*f) to the wave vector k ( v = d w/ dk ).
We deal hear with the dispersion of the group velocity which means its variation with k.
A positive group velocity dispersion means, that w increases with k, a negative means, that w decreases with k. This is the forward and direct explanation of negative/positive group velocity dispersion.
all the best
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The femtosecond laser can form a plasma in the air, and broadband terahertz radiation is formed in the plasma due to the oscillation of positive and negative charges. We know that there are two main frequencies involved. One is the oscillation frequency of the plasma and the other is the peak frequency of the terahertz spectrum. The usual study considers the peak frequency of terahertz to be the intrinsic oscillation frequency of the plasma. Is the oscillation frequency of the plasma and the peak frequency of the terahertz really equal? What are the main factors determining the peak frequency of the terahertz spectrum?
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The femtosecond laser ionizes the air to form a plasma from which terahertz waves can be radiated. The current mainstream models describing the mechanisms generated by terahertz waves are ionized photocurrent models and four-wave mixing models. I have always had a confusion: we know that terahertz waves belong to electromagnetic waves, and electromagnetic wave radiation can be explained by the energy level transition theory. Why cannot one use the energy level transition theory to explain the generation mechanism of terahertz waves? Or does the theory of energy level transitions not apply to the generation of terahertz waves at all?
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It indeed looks like a confusion. Energy levels characterize some material system (electronic levels of Atoms; Electronic rotational and vibrational levels of molecules; electronic bands or phonons in solids and many other material systems). Electromagnetic waves exist with no need of material. The thing is, that electromagnetic wave might interact with material. The interaction is particularly strong if the frequency of electromagnetic wave fits the characteristic frequency of the material system (some of its degrees of freedom). THz frequency range might fit typical frequencies of vibratons of rather heavy molecules, also it might meet some rotational spectra. Such studies have a lot of sense, and there is a whole branch of THz spectroscopy. The THz generation mechanism as you described might, in principal be described in this quantum language, if we will look at the electronic energy levels of nitrogen molecules close to dissociation. The problem, nevertheless, is that close to dissociation, the electronic energy levels become so close, that they merge, so we effectively get a continuum of energy states. Basically it means that electron can have any energy within an allowed energy band (range), so, in such a situation, it becomes much more intuitive to use classical physics of electrons moving in electromagnetic field, rather than keep quantum-mechanical formalism.
After all: any type of theory is just a toolbox of mathematical tricks to explain a behavior of physical objects. Classical physics in this case is simply easier to use, and in most cases gives similar results to a more complicated quantum picture.
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I need information for reflective(not refracting) type microscope objective for working with femtosecond laser pulse system.I find maximum maximum 40x reflective objective, but i needed more magnification. Can any body suggest me higher magnification reflective type objective which is dispersion free in visible range.
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Dear Bhuvan,
They provide broadband reflective objectives so I think they cover from the visible to MIR.
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Which is suitable microscope objective with mangification 100x or less for time dependent studies and Imaging in Femtosecond laser system?
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Zeiss and a number of other companies stock microscope objectives up to 100x magnification.
However something to bear in mind is that traditional microscope objectives contain a lot of glass so your femtosecond beam will undergo a lot of dispersion, this can be corrected in two ways you can either pre-chirp your femtosecond pulse beforehand with a compressor . Or you can build your own custom microscope using thin lenses designed for femtosecond lasers (i am not aware of any microscope objectives that are specifically designed for this purpose).
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I am conducting research to investigate the safeguards technology applications of cavity ring down spectroscopy with a femtosecond laser. I am inquiring to see if any past CRDS experiments have been conducted with femtosecond lasers.
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Femto second laser has been used for cavity ring down spectroscopy. Following paper may be useful
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Hi there,
I hope every thing goes well.
As we know there are some techniques in generating ultra short pulses such as active mode lock (Acousto-optic / Electro-optic modulator) and passive mode lock (saturable absorbtion or kerr lens effect) and also hybrid mode lock (which contain both active and passive methodes).
And we know that kerr lens effect is a phenomena that works in ti:sapphire lasers.
Question here is why by focusing the pulse in the medium, it causes mode locking and what is the the exact role of adjustable slit in the cavity ?
Please explain more detail about this.
Bests,
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Dear Raul,
Thank you for your response. I got the point. I will find that talks and will study them.
Best regards,
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Hi there,
I hope everything goes well.
I want to be familiar with molecular dynamics and Coulomb Explosion Imaging (CEI).
Could you please explain what COLTRIMS, VMI and TOFMS do? and in which applications they are used? what are differences?
Do we choose them related to our applications? Do they depend on light sources (X-Ray, Femtosecond laser pulses, Ion impact / electron impact collision) or other things?
What are advantages and disadvantages of them?
and at the end, are there such other apparatuses like them to image the molecules?
Bests,
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Dear Amit Kumar,
Thank you for sharing these articles and books. They sounds good. I will study them in upcoming days. Actually I am in my first steps and if possible give me a summery and short data about them.
Bests,
Aydin
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Hi there,
I hope every thing goes well.
Today I face with a question that I want share with you.
What is the exact difference between Electron impact ionization , Highly charged ion ionization, X-ray ionization and femtosecond pulsed laser ionization?
and which method do this ionization process? for instance, Two/ multi photon absorption, distortion of potential well, tunneling, single photon absorption and so on.
Please let me know.
Bests,
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Thank you all
I appreciate your answers. As you said the main cause of ionization in molecules by electron impact and ion impact ionizations are referred to the transferring kinetic Energy of ions and electrons ( ~~100-200 eV) in to the molecule system and eject some electrons from the system.
In X-Ray ionization the more energetic photons (wavelength less that 100 nm ) eject electrons from the system.
in 800 nm femtosecond pulse that the photon energy is below the ionization energy we have two different path as describe below:
If the photon energy is lower than the ionization energy of the system, however the light intensity is high enough, then the system can be ionized by absorbing multiple photons. Multi-photon ionization of atoms and molecules depends on the intensity and the wavelength of the incoming light. It can be classified with the Keldysh parameter
if Keldysh parameter is more greater than 1 (gamma>>1) the process refereed to multi-photon ionization and if Keldysh parameter is more smaller than 1 (gamma<<1) the interaction belongs to tunneling ionization.
Within the tunneling regime, if the intensity of the incoming light continues to increase to a level that the Coulomb barrier of the system is suppressed significantly, and the electrons start to spill into the continuum without tunneling out of the Coulomb barrier, the ionization process is often called over the barrier ionization
Bests,
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In a chirped pulse amplifier (CPA) one uses a short femtosecond laser pulse and then amplifies that to the pulse energy scales of terawatts and pentawatts. In doing so a stretcher and a compressor has to be used along with the pulse amplifier. Initial pulse is stretched then amplified and then compressed again. Such ultra short laser pulses have broad spectral bandwidth, a property which is used in it's controlled stretching/re-compression.
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CPA utilizes dispersion for both stretching and compressing the pulse. Dispersion elements can be either prisms or diffraction gratings.
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Would appreciate for sharing the trustworthy references for measured nonlinear refractive index for fused silica (quartz) for femtosecond laser pulses preferably in the region of 800nm wavelenght with around 120fs pulse duration. After checking the literature been found that the quartz value for n2 differs for several times (excluding quite strange values) but somehow is of the order of 10(-16) cm2/W. Thank you.
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Dear Vincent, thank you very much for your answer. Appreciate much. The mentioned values are in a good match with the values I have found in the literature. Would be nice to get the pdf of Prof. Agrawal on nonlinear fiber optics.
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corneal rings considered one of the new surgical procedures for the treatment of keratoconus, femtosecond laser allow precise location of the tunnel deep in stroma and also precise heigt that prevent dislocation of the rings during surgery
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ICRS are only mechanical devices that support the diseased corneal tissues by the induced corneal flattening effect thus improving dramatically the refractive and visual outcomes. However, ICRS have no role in halting KC progression, so that ICRS are only valid when CXL is valid. In other words, every keratoconic cornea not valid for CXL, then ICRS are of no use
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1. How to detect change in absorbance (delta A) spectra in transient absorption spectroscopy using CCD, mechanical chopper?
2. How to measure I(unpumped) and I(pumped) both at different frequencies using chopper synchronized with detector?
3. We tried triggering CCD with chopper frequency and external triggering of chopper to fs laser (1K rep rate). But did not get success in detecting absorbance change.
4. How exactly this process of synchronization works?
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Actually, you need to synchronize the ccd triggered by your femtosecond laser. The chopper also synchronizes with the laser at a certain phase.
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This is feasible - sse the following publication:
A. Schiffrin et al, "Optical-field-induced current in dielectrics", Nature, v. 493, p. 70 (2013).
I am looking for a collaborator to do experiments. I have a femtosecond laser to excite the photocurrents. I have no expertise in ultra high-speed electronics and detection of the current.
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It requires integrated photonics design. With a suitable Lock in amplifier and allied electronics, it can be executed. However, the internal noise figure has to be effectively controlled so that there is accurate retrieval of the signal.
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in the current context, the automotive industries are forced to reduce carbon dioxide emissions as part of the European H2020 program.
To do this the reduction of friction is a primary lever. In fact, automotive manufacturers are trying to find solutions such as downsized engines for example to lighten the weight of vehicles but they compensate for their relief by a higher need for power
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I think you can
good luck
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The non-holographic mechanism of achromatic wavefront reconstruction is based on a geometric-optical reflection of reconstructing radiation from surfaces with constant phase differences between the object and reference waves used to record the interference fringe structure in the medium bulk [1]. This mechanism was realized by femtosecond recording of the interference fringe structure in very thick medium [2]. However, it seems that some experimentally easier ways of realization are possible. Maybe some other waves instead of light can be used, etc. Can anybody suggest a new method of realization of the non-holographic mechanism of achromatic wavefront reconstruction?
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My previous comment can be considered as a program of future experimental research in this direction for anybody who is interest in it. Now I can suggest the question for theoretical study. This is a problem: what types of fields could be reconstructed by geometric-optical mechanism? It is clear that this is not arbitrary field. We tried to make a first step in this direction at the end of [1]. However, this is just the first step only.
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PLD technique relies on laser with high peak power to ablate various targets. Mostly nanosecond pulsed lasers (UV) are integrated commercially. Why not they use picosecond or femtosecond laser which could deliver high peak power and less thermal effect?
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Nanosecond laser can ablate almost all the targets and one can also get a decent deposition rate of materials using ns laser. These are the two main parameters required for a PLD setup.
So you don't need a ps or fs laser to ablate targets in PLD technique. So using ps or fs lasers in commercially available PLD system will increase the overall cost of the system. Until and unless you have some specially designed experiments which requires these ultrafast lasers in PLD system other then that I think ns laser can do the job.
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I may obtain a titanium:sapphire oscillator. I need to build an amplifier that is cheap. I have no prior experience in femtosecond physics, though I deal with optics and in the past dealt with nanosecond sources.
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It is all doable, you do not need a specialist. I would recommend a regen amplifier based on the 100 femtosecond pulse length and not explicitly looking for high energy. The major cost is the pump laser to the Ti:sapphire crystal in your amplifier. The other expensive components is the Faraday rotator and Q switch. I assume that you will need the pulse stretcher and pulse compressor will require gratings also even though sometimes this is not part of the amplifier. The cost is dependent on the price for the parts. My experience is price is always negotiable. Could look for a cheap old amplifier and refurbish to be cost effective.
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We would like a 2P optogenetics rig for as cheap as possible. Ti:Sapph are nice tunable lasers but not really cheap. Ideally we would want a femtosecond pulsed laser about 920nm, but lasers are a lot cheaper around 1040nm.
It looks like that the excitability of ChR2 is low but not zero at 1040nm. I am not sure if this means 1040nm is suitable for ChR2, or maybe it would require power levels that exceed safe levels for the animal. Has anyone tried this?
I know we could use 1040nm with a red-shifted opsin, but we would prefer not to.
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I am aware my comment may not be extremely useful as I am not familiar with the details of lasers used in 2P optogenetics, and not too familiar with the prices of Ti:Sa lasers.
However, I still want to ask: have you considered using a 1040nm laser to pump an optical parametric amplifier? Perhaps such a setup would still be within the budget? 920nm is a wavelength easily accessible for an OPA pumped with the SH of 1040nm lasers.
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I'm studying brass drilling with a femtosecond laser. The holes I get are narrow (10-20 microns) and with a depth greater than 50 microns. The confocal microscope I have available is not able to detect the bottom of the hole. Any Suggestions?
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Hi Jose,
two suggestion to try and one comment:
(1) Use a good (!) optical microscope at large magnification and with a calibrated (!) wheel for adjusting the imaging position. Check brightfield imaging in reflection or transmission mode. By adjusting the imaging position you may be able to "focus through" the depth of your hole (always seeing a different ring shaped part in a "sharp" way) and take a note of the wheel position at the deepest position where you can see the crater bottom. The difference of the postions of your wheel for properly imaging (i) the sample surface and (ii) the crater bottom then represents your crater depth.
(2) You may check whitelight interference microscopy for directly characterizing the topography z(x,y) of your craters:
However, the large depth-to-width ratio may reduce the amount of light  (back reflected from the crater bottom) available for the measurement. You have to try..
(3) For many AFMs, the depth of 50 microns may be too large..
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I am using DPSS continuous wave laser to study Nonlinear optical properties say it Z-scan technique. What else I can research using these sources, I just want to try something new with the available facility. Any answers would be appreciable. Thank you for your ideas in advance..
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Dear Shivaraj R Maidur,
You also investigate optical limiting with CW laser.
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I used femtosecond laser to do ablation in producing silver nanoparticle. In order to create the good coloid, i have to make the beam focused, how can i get the new focused beam in medium? How to calculate the new focused beam size, so that the focused beam touch the silver plate
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Michael's response is more than adequate for the focus location of an incoherent ray bundle. The answer for the coherent field propagation is a bit more complicated than that. The optical field that is converging to the focus in air has a spherical wavefront. For a Gaussian beam, the minimum spot location is not at the center of curvature of the spherical wave. It's actual location depends on both the radius of curvature and the size of the distribution at the location that spherical wave is considered.
The dielectric interface between the media of propagation (considered here to be flat with no curvature) will affect the radius of curvature of the wavefront (this change is consistent with Michael's treatment). Since the radius curvature change, the location of the spot will also change given the same considerations as the location in air before the interface.
Al Siegman gives these considerations a formal treatment for an ideal Gaussian beam distribution that is monochromatic and coherent (originally based on papers from Kogelnik and Li publish by Bell Labs in the 1960s). The problem you have is that these treatments don't pertain well to your problem.
First, the assumption of a Gaussian spatial distribution may not be valid. You did not describe the laser, but if it's based on fiber lasers and fiber gain segments, then the assumption is probably adequate for your purposes.
Second, the field is not monochromatic. The effective bandwidth (wavelength range) of the optical field will increase as the pulse width decreases. Since the dielectric containing your sample is dispersive, the index for each wavelength will be slightly different and hence will the change in the radius of curvature for that field component will change differently. That means the minimum spot location will shift as well. This shift may not be significant but should be considered as it may increase the focus spot size and reduce the field amplitude accordingly.
Third, any absorption in the dielectric medium may produce heat which in turns leads to thermal lensing effects that are often stochastic for media that are gaseous or liquid phase. Random thermal lensing effects give rise to spurious focal power that may not only shift the focus but also move it laterally due to a linear phase component in the random focal power.
Lastly, a femto second laser has a very high peak electric field amplitude. This may introduce nonlinear effects in the material. This is beyond my direct area of expertise but I felt I should mention this for your consideration as well.
If you need to calculate the focus shift with some level of confidence, then the problem is a simple or complicated as you feel it should be to arrive at your level of confidence (for a thesis for example). If you need to maximize nan-particle generation, then an empirical study is probably your best choice. I would start with the numbers you get by assuming a Gaussian distribution and Siegman's ABCD math. You can probably find these equations on the internet somewhere. Be aware that 2 approaches to the ABCD matrix values are in the literature. I prefer the use of matrices that always have a unity determinant because the wavelength that is used to get spot size, radius or curvature, waist size and location are always based on the wavelength in a vacuum. The other approach gives the determinant as the index of the final material, then it is very important to use the wavelength in that material to calculate the results accurately.
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I have prepared nanoparticles using laser ablation method. I have used 2nd harmonic beam, and prepared four different samples in de-ionized water  by changing laser energy (60, 120, 180 and 240 mJ) . I kept the time constant i.e 40 min.
I have observed that the PL and Raman peaks increases when the laser energy was 120mJ then peaks intensity decreases at 180mJ laser energy and then again increases at 240 mJ.  What is the reason behind it?
I have read that the intensity increases due to the increase in particle size
However in my case the particle size increases with the increase of laser energy.
The peaks intensities are increasing randomly
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Hello
I agree with Mr. Yuri Mirgorod. So I think you must consider the marked side of your substrate to repeat your experiment again and again with the same substrate and different powers. In this experiment temperature has a key role. You must control the temperature of laser in Raman spectroscopy because the more the temperature, the more the intensity. Then, control key is in your hand and you can obtain very good spectra.
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I know it must be less than 1 ps, but anyone have a more precise knowledge about it?
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@Pengji: Yes.
Allow me to elaborate a bit more. There are roughly two major factors regarding the time scales:
1. The (reduced) mass of the molecule. Everything goes slower with heavier molecules, and from classical mechanics you should get a scaling factor of sqrt(mu) here.
2. The forces acting on the molecule during the dissociation.
Under the crude assumption that the repulsive and the binding forces are similar in magnitude, you get my rule of thumb above. However, these assumptions are only good enough if you only need an approximate number without spending days on it.
If you _do_ care about the difference between, say, 50 fs and 200 fs, you have to do more theory; in particular, you need the potential for the dissociating state. Also, you need to define what it means for the molecule to be dissociated. When the atoms are 5 Angstroem distant? 10? Obviously, this changes your numbers somewhat, and it depends on your specific use case.
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Soliton Formation and Superluminality Effect due to Nonlinear Absorption of Femtosecond Laser Pulse Energy.
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Yes!
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I want to build a noncolinear geometry pump probe femtosecond setup.I want to know how to determine the time zero ?in a colinear geometry,this is easy. thank you for your help
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I use the pinhole of 5 micro meter. 
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Hello there,
I've been working on fs laser processing on the surface of transparent materials, i.e. BK7, sodalime glass slide, polymers such as PMMA. I am trying to optimize laser process (given that pulse duration is 400fs and wavelength is 1030nm) so that after laser ablation (trenching, grooving, free form cavities) the surface roughness remains as small as possible (ideally still transparent).
I can achieve some 500nm Ra after laser machining myself. Anyone here doing better or knows how to do it better than that?
Cheers
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You can try a laser polish after ablation for glasses with longer pulses. There is some literature about this. The pulse duration depends on how much topology you would like to be flattened. Probably ns for flattening shallow topography, longer for higher. Be aware of strain induced by this.
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What are the ways to applying chirp effect on the frequency of electric field in a femtosecond laser pulse?
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If you think about it from the Fourier point of view, femtosecond laser pulse generation requires two things: a broad frequency bandwidth and a reasonably-flat phase relationship between those frequencies. Because the treatment of the phase is so fundamental to the production of ultrafast laser pulses, researchers have literally come up with hundreds of ways to purposely chirp/unchirp/modify/etc. femtosecond laser pulses. (This, of course, says nothing of all the unintentional ways to change the phase of a laser pulse!)
For example: a typical Ti:sapphire-based mode-locked oscillator might use chirped mirrors or intracavity prism pairs to compensate for the phase chirp due to the material dispersion of the gain medium. In some cases, the dispersion of bulk glass or an optical fiber might be used to stretch or compress a pulse before or after amplification; in other cases, perhaps prism, grating, or grism pairs might be used, or even acousto-/electro-optic modulators (e.g. an acousto-optic programmable dispersive filter). Pulse shapers/phase modulators might be employed to correct higher-order material phase accrual or to construct novel temporal profiles; the active elements in such setups could be liquid crystal-based spatial-light modulators or deformable/micro-machined mirrors at the Fourier plane of a zero-dispersion stretcher, for example, or other devices also based on the acousto- or electro-optic effect. Even out of these few methods for chirping laser pulses mentioned above, there are countless types and variations of each.
As suggested previously, this topic is broad and basic enough that you should find some general information in any basic laser textbook. (For a simple introduction to the mathematics of chirp, for example, see the link below.) For any additional information, it might be better to be more specific with your question.
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I have already asked this question physics stack exchange but I thought I'd post it here as well.
I am trying to solve the dual-hyperbolic two temperature model (TTM) using a modified smoothed particle hydrodynamics (SPH) scheme known as CSPM (corrective smoothed particle method), as demonstrated in these two papers.
1. Numerical study of ultrashort laser pulse interactions with metal films, J. K. Chen, J. E. Beraun, Numerical Heat Transfer, Part A: Applications, Vol. 40, Iss. 1, 2001
2. Chen, J. K., Beraun, J. E. and Carney, T. C. (1999), A corrective smoothed particle method for boundary value problems in heat conduction. Int. J. Numer. Meth. Engng., 46: 231–252.
This essentially models the ablation of a thin metal film by a femtosecond laser pulse, T represents temperature (electron and lattice) and q is the heat flux.
I coded the CSPM scheme successfully and tested it through some simple examples (including a simple 1 temperature model in paper [2]). However, for the TTM, the results are close to what I need (i.e. qualitative shape of curves), but they're off numerically. I think it might be the boundary conditions and/or the way I'm assigning particle mass and density.
The boundary conditions I need to use are:
$q_{E, L}(z = 0, t) = q_{E, L}(z = L, t) = 0$ where L is the film length.
My questions are as follows:
1. How do I correctly pick the boundary particle positions? I have tried:
- A: Boundary particles with their centers on the boundary (z = 0, z = L). (BP2 in [2])
- B: Boundary particles which do not lie on the boundary (z = dz, z = L-dz). (BP1 in [1]).
For both cases, the particles are equally spaced, with h = dz ([2] recommends this value of $h$). Boundary particle B gives me better results (peak temp. is around 750 compared to expected 650 in once calculation, for example). Particle (A) has worse results (peak 850 vs 650). The boundary particle has half the mass of the internal particles, otherwise, the zigzaging bhavior in the picture below prevails. Curves have the correct shape but they're not smooth. Most SPH papers deal with hydrodynamics and I haven't found an in-depth discussion of how to position my boundary particles, especially that CSPM is slightly different from SPH. I am just enforcing my $q$ boundaries to 0 and not updating them as you would in as mesh-based method.
2. The bigger issue, how do I define mass and density in this case? Other SPH applications seem to have a physical meaning for both these quantities (and the particles themselves), and density seems to be involved in the PDEs themselves in most cases. However, in my case I have no idea how to define these concepts since the particles are supposedly fictitious and are completely static. Maybe treat the particles as metal atoms and use those values?
Currently I'm using:
$m = dz\cdot1\times10^9$ for internal particles, and half that for boundary particles. My $dz$ is in nm, if I use such a small value for mass it just doesn't work.
For density I tried two formulations:
- Constant rho = 1. Gives best results.
- Using the equation $\rho_i = \sum_j^N W_{ij}m_j$. Results are zig-zagging as shown above.
Note that in all cases $\sum_j W_{ij} m_j/\rho_j$ is not equal to 1 for the first few particles near the boundary, but is equal to 1 for internal particles. Is this expected?
I apologize for the long question, I am just completely stuck and I have run out of ideas.
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you may mail me on eoomole@jabu.edu.ng
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I am doing research on tin and tin-oxide nanoparticle using laser ablation method and used de-ionized water, acetone, ethanol and methanol as liquid media. I have varied the energy from 60 to 240 mJ for de-ionized water however for other liquid varied it from 30 to 60mJ. The oxidation rate increases with the decreases in energy when liquid was de-ionized water whereas for other liquid its opposite occurs. Why these liquid behaving differently so?
In two different research paper i have studied that that there is a threshold of laser flunece after which laser interact more with the liquid which causes an increase in oxidation rate.
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Dear Tanzeeha Jafari,
In my opinion, you have two main points related to this subject. First, your energy distribution profile is nearly Gaussian, so the energy distribution is not homogeneous in the area affected by the laser radiation. Second, you need to understand better the properties of your liquids. Certainly, the energy required to break the bonds is different depending on the liquid. The work function changes. I think that at least these two factors influence the variation on oxidation rate.
I hope you can find it out.
Best regards,
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I know as least for nanosecond, or even picosecond, laser in which there is enough time dumping out energy that will be absorbed by Si. However, as for femtosecond laser, the times seems to be too short for Si to absorb making electrons jumping into higher energy band level. I have seen some papers in which femtosecond laser is used to treat Si material surface. I fail to understand why it works and what is the detailed explanation towards scenario. 
Thanks in advance
Chenjin
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Dear Chenjin, the femtosecond lasers enables non linear interactions with the materials on which interact, like the multiphoton absorption, since the electron recombination from the conduction band to the valence band or the thermal relaxing from higher levels of the conduction band to the bottom of the latter are phenomena that happen in picoseconds. Regarding the absorption, instead, under certain conditions well described in the previous answer to your question, there are no problems using femtosecond pulses.
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what is mean by broad band and why the short laser pulse (ps or fs laser) means broad band laser ?
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There are a couple ways to look at this: one is that the time of a pulse is related to the energy by way of a Fourier transform. A narrow peak in time in represented by a wide band in energy (or equivalently, frequency). Also, you might simply realize that if you isolate a pulse quite tightly in time, the uncertainty principle requires that the energy of that pulse be rather broad. Energy (frequency) and time are complementary.
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Please explain the difference between the spot size and the crater diameter and how to find them? the ablation threshold range of steels machined by femtosecond laser?
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Most popular method being D-square method (https://www.osapublishing.org/ol/abstract.cfm?URI=ol-7-5-196). You plot the square of the crater diameter against laser power/pulse fluence/fluence(if you know w0). This method allows you deduce ablation threshold, w0 and incubation coefficient of any given material.
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After reading through Swamp Optics' website (http://www.swampoptics.com) I got convinced that the best way to measure and characterize ultrashort laser pulses (ps/fs long) is using their products, and specifically GRENOUILLE.
I'll be happy to hear more on this subject, preferably from someone who has worked in this field or even with this device and can share some insights about its pros and cons.
Thanks,
Assaf
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So this really depends on what you want. If your just want the basic information an auto-correlator will do the job, you can build your own, just need a crystal some optics and a translation stage. (simple! :-)) you can do this for an order of magnitude less money than paying Swamp Optics $10k. However, the auto-correlator will only give you symmetric information, i.e. you won't know if that feature is a prepulse or a postpulse. But, for that matter you can build your own FROG for the same $1000, but then you need software to analyze the spectral info, which might be great for a student to do, but isn't really thesis work. In any case, for more detailed info you need a cross-correlator. (Or a FROG or SPIDER). This much more involved, and is commercially available. Del Mar Photoics makes a good cross-correlator, their web site SUCKS, but their products are good. But if you need spectral and phase info, or worry about pulse stability buy the FROG. Del Mar makes one too, have no idea how it compares to the GRENOUILLE. Hope this helps.
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I study the picosecond self-mode-locked laser recently, i want to know how can i determine the dispersion regime of mode-locked operation  with spectral shape and the time‐bandwidth product?
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From general point of view a gain band of any laser media is dislocated in  region of the negative dispersion region...
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Currently we are using a standard telekom phase modulator for the repetition rate control of a femtosecond laser. When upscaling the intracavity power the laser doesnt operate properly. Are there commercial compact fiber coupled MgO doped LN phase modulators at 1550 nm available ? The Lithium Niobate could show whether a waveguide or bulk design. Nevertheless the bulk PPLN should be confined within a micro-optic package hence it needs to be very compact. Cheers, Sebastian
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I have a femtosecond laser (800nm, 1 kHz, 50 fs, 4 mJ) and want to focus with off axis parabolic mirror to a spot size of less than 10 micron. I can focus but difficult to get good quality of focus. generally I obtain bright tiny yellow (if seen on pink paper) spot at the center after focusing however the periphery has also either the hot spots or some distribution of high intensity component. I mean the beam is not fully concentrated at the center.  What is remedy you can suggest to me to achieve my goal. what is the proper shape of focused spot? Thanks in advance and appreciation for any kind of suggestions and comments.
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Do you really believe what you can see by eye on a paper about focusing a 4mJ pulse in a 10 micron spot? This can be very misleading because neither the response of the paper nor your eye is linear. You may think the part outside the main peak is very intense, but it's probably not. Try to measure the spot with a linear device like a CCD.
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What is the cause/origin of non linearity or non linear behavior in materials like dyes,2-d materials etc and what are the unique properties that material should possess in order to show non linear behavior,when it is irradiated by a Laser beam?   
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Dear Aamir! If You wont to look nonlinear interaction of light with some  substances ask your friends who has Q-switch Nd3+- laser to  make with You following experiments (or one of them):
1) Take a short focusing lens and  direct to it laser beam. Without lens beam pass free and You see nothing. With lens You will see creation of bright white point-like discharge that result due to many photon absorption and air ionization. It is eye observed impressive experiment from nonlinear optics.
2) Second  eye observed experiment  You can do if You take small sized crystal powder of dye- Brilliant green and locate it between 2 glass  plates so to direct on it laser beam. When unseen Nd-laser beam 1064nm illuminate spot on dyes the spot will emit green light 532nm in all directions because small dye  crystals are  oriented randomly and every one generate  second harmonic of  incident light. But You must be quite careful against accidental reflected beam of very intensive light from Nd-laser in your eyes. I send  You my  old work with similar  experiments to find efficient substances  for  second harmonic generation. If You make less intensity of Your laser no one  of described results You will observe.
It would be enough for You at the start. And I wish You any success.
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Suppose there are two Raman peaks and both of them show a shift with pressure. But one of them after certain pressure start to come back to original Raman peak position i.e. without pressure position. What does this mean? Please comment on this
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thanks Dustin; its a nice article and enlighten many things. But still i am not able to find what i am looking for.
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Suppose there are two Raman peaks and both of them show a shift with pressure. one of them after certain pressure start to come back to its original Raman peak position i.e. without pressure position. Please comment on this.
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In another approach, it might be mode softening of the particular mode with pressure. If this is the case how one can explain it. Please comment on this.
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There are two major techniques to measure ps - fs pulses of the mode-locked lasers:
I) Steak camera
II) Optical autocorrelation
According to the space-time transformation in second-order autocorrelator, what is the function of the nonlinear crystal ( KDP or BBO) leading to the interferometric autocorrelation to display the amplitude-coherence behavior of laser sources?
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The NL crystal is used to generate the SHG signal. For temporally separated pulses the SHG signal is constant in intensity, irrespective of the magnitude of the pulse delay. In case the pulses start to overlap in time and space on the crystal and the interferometer is stable, the SHG signal is oscillatorily modulated with respect to the pulse delay due to the interference of the two pulses. The generated signal is instantaneous, as such the temporal duration of your signal reflects the pulse duration best. If the SHG signal is separately detected behind the crystal, the result is the second order autocorellation function which you can easily use to calculate the pulse duration, phase modulation and spectrum.
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Looking back on about 30y of femtosecond optics and spectroscopy, I would like to start a discussion on which achievements active researchers today consider as, e.g. most valuable, as a break-through, most visionary etc. To be specific, I'm not looking for top-ranked papers but a more general idea/concept/invention/technique/method that has in your opinion revolutionalized the field, has paved the way for succeeding studies and/or has been picked up by other branches / has inspired researchers in other fields such as biology, chemistry or even industry.
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I'd like to add three of my favorites:
1) ultrafast spectroscopy and coherent control of single molecules. This goes hand in hand with the already mentioned pulse-shaping techniques, ultrafast nanoscopy and in-depth characterization of ultrafast pulses. I'm still amazed by the fact that it is possible to detect and manipulate the optical response of single particles and to compare the result to theoretical predictions. 
2) 4D ultrafast electron microscopy. It extends the concept of ultrashort pulse generation to electrons and opens up completely new insights into structure and dynamics of matter under various physical and chemical conditions, for instance phase transitions, particles in optical near fields or spatially-corellated systems.
3) although already fairly old and not very surprising, starting out from the photon-echo experiments, I still consider the development of multi-dimensional optical spectroscopy as ground-breaking. Clear visualization of chemical bond transformation, energy transfer between molecules and relation to chemical structure is in my opinion a very important development that still holds great potential for instance in medical research and energy related science.
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I am working on a project------temperature effects on supercontinuum generation using femtosecond laser in water. when the temperature of water changes ,many optical properties change. The critical power Pcr (3.77λ2/8πn0n2) also changes, I already know the liner refractive index of water n0(different temperature) ,but don't know the nonliner part n2 .So I can't analyse the critical power quantitative with the change of water's temperature.
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