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Hi. I have a question. Do substances(for example Fe or Benzene ) in trace amounts (for example micrograms per liter) cause light refraction? and if they do, is this refraction large enough to be detected? and also if they do, is this refraction unique for each substance?
I also need to know if we have a solution with different substances, can refraction help us determine what the substances are? can it measure the concentration?
Thanks for your help
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It is also on ResearchGate. Author Shangli Pu. Measurement of refractive index of magnetic fluid by Retro - reflection on fiber optics end face.
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I want to understand how I may use a Cyclic Shearing Interfermeter to measure Spatial coherence.My objective is to measure the Coherence function between the interfering beams.Is it possible to use a CCD to measure the intensity of the fringes ?If not,then how to measure the intensity of the brigh fringes of the fringe pattern.
Thank you.
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the fringes captured the CCD can be imported into the matlab for further image processing. The number of pixels in both dark and bright fringes determines the intensity of the fringes using matlab of ImageJ software.
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I like to do project in laser measurement system. My objective is to eliminate or reduce the cosine error, abbe error in Laser measurement system. I have Rhenishaw XL80 system.
Please tell the procedure to eliminate that particular error. And help me to complete my project.
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Dear Joshua,
Let me try to help you.
# In order to eliminate or reduce the cosine error, in my opinion, you should have to tilt your laser head so that the misalignment angle (theta) is sufficiently small. You can do that by trial and error.
# In order to eliminate or reduce the abbe error, in my opinion, you should have to reduce or remove abbe offset (d). In my understanding, there are any 2 (two) methods, i.e. Zero abbe offset configuration and Abbe error compensation. Please see on the attachment file. The disadvantage of Zero Abbe offset configuration is the system size becomes large. For Abbe error compensation, I did not have experience with it. I am still learning about Abbe error compensation. Generally, you need a double pass interferometer. I suggest you read the publication from Dr. @Jong-Ahn Kim. I remember that he published a paper in 2012 about angular compensation to remove abbe error. He is an expert in this field.
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Lately, I have received a manuscript containing IR-spectra of a single crystal for optic applications. Authors have shown two IR-spectra (due to copyright statement the figure is schematically attached to the question) at which both materials have 5N purity synthesized through Czochralski method. The blue is the control sample while the orange is their synthesized sample. As it is obvious, the orange curve is deviated from 1000 to 2400 cm-1 and it is not as smooth as blue one. They realized that the difference is due to presence of impurities in their sample, while I think it is not.
Does anyone can help me in this regard? What factor leads to such deviation of not being smooth (from 1000 to 4400 cm-1)? any references related to factors leading to deviation of IR-spectrum of single crystal samples?
Waiting for your respectful answers
Regards
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Read the article
I.A. Kaplunov, V.E. Rogalin Optical properties and application of germanium in photonics. // Photonics 2019. Vol. 13. No. 1. P. 88-106. Doi: 10.22184 / FRos.2019.13.1.88.106
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Dear colleagues,
I am investigating the dependence of the number of diffraction rings on the concentration in third order non-linear organic dyes (due to nonlinear refraction and nonlinear absorption). Prof. Pramodini [1] claims that the number of diffraction rings depends linearly on the concentration. However, Prof. Hussain A Badran [2] assume that the number of rings increases exponentially with respect to the concentration. Our experimental curves on aniline blue and Acid blue 29 showed a linear relationship. However, for Oil Red O, experimental curve is not the straight line and the exponential curve. So, is this relationship linear or exponential?
Thank you and hoping for your insightful response.
1.S. Pramodini, P. Poornesh, Effect of conjugation length on nonlinear optical parameters of anthraquinone dyes investigated using He –Ne laser operating in CW mode, Optics & Laser Technology
2. Badran, Hussain A.; Ali Hassan, Qusay Mohammed; Imran, Abdulameer, A Quantitative Study of the Laser-Induced Ring Pattern and optical limiting From 4-Chloro-3-methoxynitrobenzene solution, Basrah Journal of Agricultural Sciences . 2015, Vol. 41 Issue 2, p51-57. 7p.
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what is the effect of the increases of the number of rings
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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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Introducing myself, i am Senior Lecturer/Researcher, Deputy Director (Corporate Planning Division) and Editor-in-chief (EDUCATUM Journal of Science, Mathematics and Technology) in Sultan Idris Education University, Malaysia.
Currently, i am working on optical glass, nanoparticles, glass-ceramics and graphene/reduced-graphene oxide to develop new materials for novel fiber optics and laser glass.
I am looking for international collaborations to extend the research on glass materials. The activity of the collaborations are as follows:
1. Exchange knowledge
2. Mutual research collaboration
3. Invited speaker or keynote speaker in international conference on optical glass
4. Research grant opportunity
5. Research article collaboration
6. Student exchange
7. Visiting professor
8. Memorandum of Agreement (MOA) or Memorandum of Understanding (MOU)
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I am Dr. shoaib from Pakistan working on glass and single crystal materials. our group is working on preparation and characterizations, e.g. physical, optical, luminescence, radiation, shielding and defect study.
please let me know how we can work together?
our lab head: Gul Rooh (gulrooh@gmail.com)
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I have a question on how peak fluence is calculated. For a Gaussian circular beam, the peak fluence is the total pulse energy divided by πω2/2, where ω is the Gaussian beam radius. Now my question is about a Gaussian elliptical beam with a 10um x 500um beam spot size, is there a way to calculate the peak fluence? Thanks for the inputs!
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You need to find an expression for your elliptical beam. I am assuming it is a simple product of a Gaussian beam in the x-direction and the y-direction. You want to have this normalized so that you know that the total integral of your beam profile is equal to 1. Once you have this normalized you know that the power distribution is the total power times this normalized distribution.
In brief, I think you want the normalization constant. In your case this appears to be \pi*w_x*w_y
Note: there is a factor 2 in your equation compared to mine. Will you be careful with the definition of 'beam radius'? Is this at half intensity? 1/e intensity? 1/e for the field? etc? etc?
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Using laser-induced breakdown spectroscopy for quantitative analysis, the researcher aims to use linear regression in finding the relationship between signal intensity and the concentration of the substance. So far, laser-induced breakdown spectroscopy is a young but promising method (shown to be effective and quick in qualitative analyses) but on the aspect of quantitative analyses, it is still somewhat semi-quantitative (that is if it's interpreted from the view of a beginner using the device) such that the relationship in between the signal intensity and concentration is found rather than directly looking for a more direct unit such as grams, milligrams, or parts per million. 
Using signal intensity as a variable is somewhat difficult especially when an undergraduate interested (but not highly skilled) in the use of Laser induced breakdown spectroscopy tries to interpret data. This is because the signal intensity can be disrupted by many other factors (such as the disruption of the atmosphere of the sample towards the light path as well as the need for uniformity of the sample compositions). For heavy metal analyses (such as cadmium, lead, and chromium), what can be a good calibration curve in order to prevent false results as well as underestimation/overestimation of data? :) 
This is for our study on mycoremediation of tropical white-rot fungi on heavy metal contaminated soil sites, and we're very interested and set in the use of LIBS since unlike other spectroscopic methods, it does not need to have sample preparations and other tedious solubilizations of samples; which are methods that can contaminate our samples. :) 
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In light of above discussions, It is clear that only two methods are possible for getting concentration of unknown samples, First is based on calibration curve and other on is calibration free technique. For getting a good calibration curve with minimum error, one should plot a calibration curve of required element in the similar matrix samples in similar experimental condition.
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Hi there,
Do you know how to measure longitudinal and transverse modes in lasers? I need to know the cutting edge and experimental techniques related to this subject.
Bests,
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Dear Sourav,
Thank you for your answer. Could you please explain more about S2 measurement technique ?
Thanks,
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Ruby (Al2O3 doped with Cr3+ ions), when exposed to ionizing radiation (for e.g. X-rays), emits luminescence. What is the mechanism that causes this 'Radio-luminescence? Does Cr3+ accept electrons or give up?
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@Raju: It may happen that we use such conversion for understanding but possibly no such experimental evidences are available in many cases. Even if conversion is there, it may be well below detection limit and negligible. See sizes of different states, they may find difficult to adjust in lattice etc.In CaSO4:Dy we assume Reduction as per Nambi Model, but experimental evidence is not there. There is no change in colour evan after huge irradiation for CaSO4:Dy, if reduction holds, colour should change. You may contact Dr. B. C. Bhatt, who is also on ResearchGate and has done pioneering work in these fields. I am also sharing the question with him.
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Which simulation software can simulate a Diode side-piumped solid state laser Nd:YAG with one or more gain media or amplifier with multipass configuration. Any suggestions for how to use MATLAB simulink to perform this configurations?
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You can MathCad, if you have a full formula and coefficients
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I'm trying match the phase between different links(all the links has same devices with same parameters). As i know i have to place an exact same length of the fiber between all the links to perfectly match the phase between all the links. I'm unable perform exact calculations to match the phase to over wide bandwidth(around 10 MHz to 3000 MHz). I''m able to match at lower frequencies by cutting the fiber length approximately. Once i over but even by few mm length im going out of phase with the link. I need to have 6 degree phase difference between each link. Can some one give exact calculations to match the phase over wide band.?
Thanks
Surya
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In addition to the previous comments, bearing in mind the length of the fiber links and the environmental factors, for fine tuning you can use commercially available devices.
See in the link:
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The question is upon the manipulation of micro particles (of size perhaps ranging from several microns to hundred of microns). We may desire to trap or capture a micro particle (dielectric, opaque or organic materials) and control its three-dimensional location in air/ or manipulate the particle to move along a certain 3D trajectory in relatively high speed (perhaps about 0.1m/s to 1m/s) in ambient air.
Is there any possible/ reliable approaches?
I was wondering if using optical vortices (laser beams of orbital angular momentum OAM in the manner of optical tweezers) is feasible? It seems that currently, they were limited in the manipulations of nanoparticles in a 2D plane and could not support 3D spatial moving big particle of high speed in the air?
I was wondering if some other potential morphologies such as photophoretic trapping with optical bottle beams, thermophoresis or other method can work better?
Thanks.
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Dear Nils-Frederik Hasselmann,
Thank you so much for your answer. I think this paper that you provided helps a lot!
Best,
Yuan
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I'm looking for a polymer (or even a type of tape) that can be used with high energy lasers in photoacoustic experiments.
The material must be able to repeatedly be exposed to 500 mJ/cm^2 from a nanosecond (5 nanoseconds) or picosecond laser pulse (8 picoseconds) without burning AND it must have favorable acoustic properties (an acoustic impedance close-ish to water at 1.5 MRayls) AND it must be optically transparent in the range from 400-700 nm.
Is there anything even remotely close to being able to handle these types of energies/powers in a polymer form or would it all have to be glass?
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I think Cyclic Olefin Copolymer (COC) is good option
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Dear all,
I have an optical setup for cleaning up the laser diode beam, which includes coupling the laser light into a polarization-maintaining single mode FC/APC fiber.
I have a technical question on coupling the laser light into an FC/APC connector. Should I introduce any tilt on the fiber holder for that? Or should the angled end be tilted with 8 degrees from the optical axis by simply pluging the fiber into the holder?
I understand that with tilting I will lose the advantages of reducing the back reflections into the source. However, the proper adjustment of such fiber is not clear to me.
Thank you in advance for your help!
Marta
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I would recommend buying a collimator matching to your prefered beam diameter:
and a kinematic tilt mount like
maybe also an x/y-shift stage. Precise angle adjustment is much more sensitive and important than sideways shift.
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Is there any limit for generating narrow beam? How do I generate a narrow beam of 5 micron beam size?
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You may get a 5 micrometer beam but at the expense of a large divergence (the beam diameter increases as a function of the propagation distance. To understand this you need to look into the ubject of gaussian beams. It is quite srightforward and very useful if you work with lasers. A perfectly collimated beam really does not exist. Rather than that what you can obtain is a laser that focuses into a waist, at which it is collimated and then it diverges again. The angle at which the beam spreads from the waist is called the divergence angle. The narrower the waist, the larger the divergence angle. Divergence_angle=2*lambda/(Pi*w0). w0 is the beam radius at the waist. This formula is approximate and is valid far from the waist where the divergence is linear with propagation distance.
I recommend you grab any book on optics that has a chapter on gaussian optics to read this subject. It is very interesting.
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I want to simulate the interaction of laser with composite materials. i need to know the  thermo-physical and optical properties and their changes with temperature.
In fact, I'd like an overview of the characteristics of composites and import them in heat transfer model.(The required parameters are :Absorption coefficient , thermal conductivity,Reflectance coefficientوDensity , specific heat capacity)
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you have to specify type of the composite (fibers type and matrix type).
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I am trying to lock laser at half transmission point of scanning fabry perot etalon (SA200 ) using electrical feedback to laser driver current source.How can I achieve it.  my experiment consists of a DFB-LD , input to circulator port 1, at port 2 etalon is connected and observing laser reflected spectrum.the transmitted signal from etalon is photodetected and feedback to laser driver current source.
Thanks in advance.
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Dear sir @ David A. Ackerman , @ Patrick Joseph Bisson , I am very much thankful for your helpful and invaluable comments. 
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I'm trying to couple an argon laser into an FC terminated single-mode fiber, and then collimate the output using a 1", 30-mm EFL achromatic aspheric lens. However, I keep seeing a diffraction pattern, like an Airy disc or a Bessel beam when the output is projected on a wall some distance away. Here is what I've tried:
1. Coiling the fiber to eliminate cladding modes.
2. Cleaning both ends of the fiber.
3. Swapping out for another SMF.
4. Swapping input/output ends of the fiber.
In all cases, the pattern persists. Any suggestions?
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How long is you fiber? Remember that higher modes and cladding modes may propagate, with loss, along a short length of fiber (up to 2-3m, perhaps more). Are you sure that you're launching a single mode only (spot size and converging radiance angle)?
A long, long time ago, I was tasked with couple an visible argon laser into a multi-mode 50/125 fiber with considerable power. To ensure proper input alignment, I created an microscope/imager with very good rejection for 500nm and shorter wavelengths. This allowed me to inspect the fluorescence produced at the fiber tip to precisely center the spot. If your application lends itself to inspecting irradiance profiles at the fiber tips, it may give more insight. But be very careful regarding eye-safety and keep your laser safety officer involved.
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I want to make optical measurements (fluorescence) of a liquid sample (fluorophore dissolved in 2-methyltetrahydrofuran) inside of a cryostat. The problem is that when I make my sample (in a cuvette) and introduce it into the cryostat once I put it under vacuum the seal on the cuvette does not always withstand the high vacuum of the cryostat.  As a result I will sometimes evaporate my solvent before I freeze it.
I would also like to make measurements above the freezing point of the solvent but at temperature that need me to keep the vacuum running this also leads to solvent evaporation. This changes the concentration of the sample possibly adding additional error to the measurement. 
I am currently using a 1 or 2 mm pathlenght cuvette with a Teflon plug that I hole down with Teflon tape but this is not working that well.
Are there any special sample holders for this kind of measurement?
if yes, what are they and where can I get them?
If not, is there anybody with experience using cryostats that can offer me some advice on how to make my measurements?      
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Would something like the cuvette holder sold by Janis work for you? It has a spring that holds down the lid.
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We know that if we use a single cycle/half cycle pulse to generate high harmonics, we may generate single attosecond pulse giving us a broad spectrum as indicated by the continuum. However, we only get discrete frequency combs by using multicycle pulses. Is it that there occurs some kind of destructive interference between various generated harmonics?
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"Is it that there occurs some kind of destructive interference between various generated harmonics?"
Yes, that is correct. In a centro-symmetric medium harmonics emitted at different half cycles of the driving laser field interfere destructively in even orders an constructively in the odd orders. However, in a non-centrosymmetric medium the amplitude from different half cycles is not equal and even harmonic emission is observed (see https://doi.org/10.1103/PhysRevLett.109.113901).
Another way to think about it: anything that is periodic in time must also be periodic in frequency. When harmonics are emitted with equal amplitude every T_L/2 (where the laser period is T_L) then the corresponding periodicity in the spectral domain is 2/T_L (i.e. twice the fundamental frequency). 
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In a Raman set up we need very collinear pulsed nanosecond beams. A colleague with experience recommending using two cylindrical lenses (one for each beam before a combining optic), which we can put into the beam path to focus the light to a line. If these lines are collinear they should appear dim due to the destructive interference (they will not be overlapped in time nor do they need to be). Does anyone have experience with this or recommendations for creating very collinear beams? Is this as simple as my colleague made it sound? What sort of focal lengths do you use? Logistical concerns here?
Our use: We plan to use a combining optic to transmit one pulsed ns beam and reflect another ns beam off this combining optic. We will subsequently align our two beams through two sets of irises (this is pretty typical I believe). The irises may not get sufficiently collinear light or may take many iterations and I would like to know other ways people address this problem.
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If your intention is to make the two beams parallel to each other, and they are close enough to put thru a single spherical lens, the best method is to overlap the beams at the focal plane of the lens. The focal plane of a lens is where the angle of a beam entering the lens is totally transformed into position on that plane (Fourier transform of angle to position). Use a lens with a long enough focal length so that spherical aberration is not a factor. Depending on the intensity of the beams you could use first surface reflection off a quality uncoated optical flat (preferably wedged) to reduce the intensity to the point where you can examine the focused spots. Make sure that you observation is at the focal of the lens for the laser wavelength. This will be the position of the smallest spot size only if the beams are well collimated. There is also a slight difference between the focal plane and the position of the smallest spot from a collimated beam, but this should be negligible if you are using a high f# configuration.  Focal length / diameter of 20 will do this, as well as keep you away from spherical aberrations.
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I am on my way to setup a measurement to estimate thermal lensing in my setup with double pass of a probe beam through the crystal. The only problem is how to interpret the data I will receive. Most of the papers give end-result with little insight on the method. So I would appreciate if someone could recommend me a useful paper/thesis/book which could help to understand it more deeply.
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Dear Arvydas,
I suggest for you a links and attached files in topics.
-shack-hartmann wavefront sensors: Topics by Science.gov
-lens shack-hartmann wavefront: Topics by Science.gov
-shack hartmann sensor: Topics by WorldWideScience.org
-shack-hartmann wavefront sensor: Topics by WorldWideScience.org
Best regards
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thank you for answer
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Dear Dr Razi
I agree with Dr Raid
Regards
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I would like to know the maximum rate of pairs of photons  that can be generated by SPDC (and detected) with a continuous wave laser (405 nm). The crystal used is a 1 cm long 10 um period PPKT collinear type II crystal. Any reference where I can find this information will be strongly apreciated.
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Optics Express 24, 2941-2953 (2016)_Polarization-entangled photon-pair source obtained via type-II non-collinear SPDC process with PPKTP crystal
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Hi!
A colleague of mine was doing some laser diffraction analysis for a waste water. We observed some strange effect which we couldn't explain: an analysis with and without ultrasound showed similar results (it said, that there are too large particles, although there are NO visible ones; the range was 0-350 micron). This cannot be true. We cleaned and checked the device (with a standard) and remeasured the sample which we had before: the one already treated with US. The result seems ok. A repition of an original sample with longer US lead to "too large particles" again. 
Now my question is:
Are there any molecules/chemical componentes which influence the analysis and pretend to be large particles? Acids, colour, solvents, ....
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Hello Johanna  I still have no idea what the level of concentration of particles is in you sample.  Can you post data on this site (if you're using a Malvern) or send to me?  alan.rawle@malvern.com 
How do you measure the background for your sample?  Normally one has the dispersant liquid in the accessory and then adds drops of suspension, paste or emulsion to the accessory until the appropriate optical concentration for measurement is reached.  If you're trying to measure the background first in DI water, then emptying, then adding your sample then you'll have a major problem in at least 3 cases - dissolved salt(s) altering the RI and thus the alignment of the system, possible thermal effects if the temperatures are different and thermal equilibration isn't achieved, the inability to remove dissolved air (easy if DI water is the background as sonication can be used here.
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Setup (refer attachment): laser, sample, detection of fluporescence at 180  degrees from excitation, cutoff filter, band pass filter, detector, TCSPC hardware board, software control for detector and laser. 
I carry out these experiments to find out the lifetimes of Erythrosin, Coumarin 540 and L-Tryptophane.
At my early test, I found out that with or without the laser excitation, my TCSPC system gives the same spectrum (refer attachment). It must be something wrong.
By going through the literature, I found out that these kind of experiments are done at magic angle conditions and without the pileup effect. And we also need to measure the IRF (Instrument Response Function) of the systems.
Do I need to insert a polarizers between sample holder and PMT? How important it is? How to find the right one for my system?
Please refer the attachment. Many thanks!
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Thank you so much Jayita Patwari,
Yes, You are totally right. I should set up the detector at 90 degree from the excitation.
 I was aslo suggested to re set up the SPCM  by using the oscilloscope.
I would like to let you all know the result here after I finished.
Thank you so much.
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In your opinion what would be the optimum optical scanner for elliptical scanning trajectories (general elliptical, i.e. from line to circular in different sizes).
Which current solutions would you suggest for compact and fast response scanners (1KHz) other than e.g. classical salvo scanners?
Thank you in advance.
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Excellent, thank you very much
I know that glavos do well, but are not as compact as I would Need
AO deflectors, I have only used them as attenuators (AOM) or pulse pickers, but never as true steering System.
I am a bit concerned of the Degradation of optical Quality.
I was originally thinking of compact Risley prisms Scanners, but cannot find any manufacturer for those.
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We have a MCP PMT which detects in the wavelength range starting from 950 nm to 1700 nm. I have to use 510 nm picosecond pulsed laser to excite the samples I have which usually have emissions around 1200-1300 nm. How should I take the instrument response function with the 510 nm excitation source? No scattering is detected by the PMT as it is sensitive to the NIR region and I am exciting in the visible region.
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Often in these cases you are forced to use a dye that can absorb at your excitation wavelength and emits around your spectral region of interest. Use a quencher like KI to make the dye have a very short lifetime so its emission is like an IRF.
Also see the attached paper for the general procedure
I would suggest maybe the laser dyes Styryl 20 (LC 9940) or Styryl 14 (LC 9450). They are able to absorb around ~530 nm and emit around 930-980 nm. You can check out their specs here:
These dyes may be inappropriate if they are not emitting the IRF in the same spectral region you are planning to do measurements though.
Edit: Those dyes I suggested may not be so good. I re-read your comment and see you are looking at the 1200-1300nm region.
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I am looking for a exact relation to determine the laser beam radius at focus.
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Thank you Shivaraj R Maidur.
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I need to know the refractive index of polymers CR39 at wavelength of 293 nm
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Rana,  This site may give answer to your question.  Currently, they plot in visible wavelength region but not in deep UV region.
Regards, Shigeo.
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Have to make a research about it,some help plz
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The topology or surface structure of a metal could be modified through either laser ablation or laser deposition.
This will change properties such as wetting, which would have an impact on adhesion.
Reading into both of these methods might be useful, and I imagine there has been other work in this field as laser based processes are not new in manufacturing, in fact a quick google search came up with the following paper on exactly that topic:
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Frequency comb generation
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In one of the experimentation I observed that the linewidth as well the FSR both are higher than that of required for a comb. So still the question persists whether we can overcome this..
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Hello ,
Researcher's saying that "It is really wrong to assume that the cladding index is 1 (ncl=1). The properties of PCFs are exactly different from step-index fibers because of the frequency dependent cladding index".
My question is how to calculate the exact value of cladding refractive index "nclad " when most of the part in cladding is air.
the waveguide is attached
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REFRACTIVE NEAR FIELD METHOD CAN HELP TO DETERMINE.
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A spatial filter is a device to 'clean up' a laser beam with an irregular intensity profile, and create a smooth Gaussian profile at the output.
It is usually said (e.g. here) that you need a microscope objective and a pinhole for this. The microscope objective creates the Fourier transform of the laser beam at its output. The pinhole acts as a low-pass filter in the Fourier plane of the lens, to remove unwanted high spatial frequency components of the beam.
Why is the microscope objective necessary, instead of any other lens with diffraction-limited performance (e.g. aspheric lenses)? What makes microscope objectives more suitable?
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There are many cases where you would not want to use a microscope objective for spatial filtering a laser beam, actually. Microscope objectives can damage from high energy pulsed lasers, and their input apertures are small. For high energy pulsed lasers, I have used a plano-convex lens with a long focal length (1 meter) to prevent air breakdown at focus, and a diamond pinhole instead of a typical pinhole, because the laser would burn through the standard pinholes meant for a HeNe. It just depends on what kind of laser you are using, and the intensity/fluence.
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I am looking for an analytical expression describing the time evolution of the spatially averaged density (in practice, the average concentration in particles/m3) of a system of particles hopping with a given frequency on a d=3 lattice where some traps are randomly placed. The idea is that when a particle meets a trap it disappears from the system, so that the average density decreases to zero with some decay law. I expect to find something like a stretched exponential decay and I would like to relate the parameters of this decay curve with the input data of my system (e.g. intial particle density, traps concentration etc.) The lattice can be ordered (say, cubic) or disordered (like in glassy media or polymers) and the traps can have a finite size. As a further complication, the effect of traps saturation can be also taken into account. 
Thanks!
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A lot depends on whether your traps are mobile (that is, if they too diffuse) or are fixed, but placed at random. In the former case, decay is eventually exponential in the number of distinct sites visited, S(t). S(t) itself depends on the dimension
S(t)=sqrt(t)     d=1
S(t)=t/ln(t)       d=2
S(t)= const. t for all higher dimensions.
If, on the other hand, the traps are immobile, then you have a stretched exponential
exp[-t^(d/(d+2))]
where d is the dimension. This arises from the fact that you might start in a region where there are exceptionally few traps, and survival is dominated by these events.
However, even in the simplest case of a point particle diffusing on a discrete lattice, there are no analytic expressions to my knowledge.
The literature is rather huge: the following somewhat random choice may get you started:
Havlin, S., Weiss, G. H., Kiefer, J. E., & Dishon, M. (1984). Exact enumeration of random walks with traps. Journal of Physics A: Mathematical and General, 17(6), L347.
Grassberger, P., & Procaccia, I. (1982). The long time properties of diffusion in a medium with static traps. The Journal of Chemical Physics, 77(12), 6281-6284.
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I am trying to get decent power out of a fiber I am coupling to my laser (dynamic range: 100mW) but am having a hard time.  The fiber is 400um multimode, 0.39 NA and is connected to an SMA adapter threaded to a mount whose angle you can adjust.  The fiber post is mounted on a translational stage.  I do not have any convex lenses between the fiber and lens at the moment because my spot size is already very, very small.  But the power that comes out of my fiber when the laser is set to ~60mW is around 1uW... Any tips?
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You may check if you can use a diverging lens in front of the laser output. That is a lens with convex-concave surfaces. The concave side should face the laser output. 
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Is it possible to evaluate a B-integral of amplified laser pulse only from experiment data ?
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If you are interested in 4-wave mixing processes transforming post- to prepulses via B-Integral you might take a look at Didenko et al. 'Contrast degradation in a chirped-pulse amplifier due to generation of prepulses by postpulses,' Optics Express, 2008.
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Hello. I have the following  situation.
Laser beam: D=5 mm diameter 1/e2, lambda=1064 nm, CW, TEM00.
Fiber: NA 0.13-0.15, mode field diameter w=5.3-6.7 um
In a technical note from Newport, I found that I should be using a lens with focus f=D*Pi*w/(4*lambda).
I found an aspheric lens with f=20 mm, which is close to what I calculated. I aligned it in front of the laser, with flat side facing the laser. I go zero power despite my attempts to align everything, and the beam diameter in focus was about 500 um.
At the same time, according to the Newport tech note, it should match the mode field diameter!
What lens or objective would you recommend to use?
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Hi Elena,
  • I did couple 18-20% laser light into SMF twice last month. The focal length calculation is right. The crucial part is alignment. I use Nanomax 3 axis stage and associated components such as lens mount and fiber holder from Thorlabs for alignment. Here is the link.
  • If your laser beam is in free space, use two mirrors before 3 axis stage to control the beam height and beam angle. I have shared the photo of the assembly with you. Make sure that beam is parallel to the top of the axis stage using a pinhole.
  • Reversing the side of the lens helps sometimes.
  • Make sure that your SMF is cleaved properly. Re-cleaving the fiber improves the performance a lot.
Good luck and have lot of patience.
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When optic pulse propagates in a fiber it always experience small environment disturbance of it polarization state. Is any pulse stable polarization that hardly can be destroyed ? If such a state exist, pulse polarization have to develope to that from any other state and be stable.
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In Conventional Single Mode Fibers, due to change of refractive index along the fiber in x-y direction, where the two state of polarized light propagate, the two polarized modes experience different phase velocity. If the z is the propagation direction, say, at z=L, the two polarized modes, under polarization mode dispersion (PMD), would lag each other, resulting in spread in time of the propagating pulse with a particular input polarization. 
To avoid this effect, polarization-maintaining fiber (PMF) could be a remedy, of course for a short distance applications, such as sensors.  Polarizing optical fibers, such as D-shape fiber also can be used to maintain polarized pulses. 
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I see in literature some mixtures between Co2, Ne2, He2 1:1:3 ratio, and 1:1:8... Plus 5% of Xenon, exist a optimum gas for a slab waveguide resonator? Or its depends on the architecture of each one? 
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Dear Cassio Alvarenga your, at first sight, net technical issue raises the main deep questions of the nature (or God).
We will give some, most known examples on this, the subject touched by you, from various areas.
1. All devil sits in trifles.
2. Fly in the ointment.
3. Small additional additives very strongly change taste of tea. The ingenious English physicist Michael Faraday paid attention to it and invented alloyed and stainless steels by entering of small additives in iron of other metals, for example, of molybdenum.
4. The washing capability of anion detergents raises in case of small additives cationic surfactant that contradicts logic. As mixed among themselves in an identical proportion cationic and anion surfactant is compensated each other and this mix doesn't wash at all, etc.
5. Thus, it is possible to draw a conclusion that efficiency of the active environment depends on a right choice of the main gases of the gas laser and additional small additives. It is a lot of work, to touch and mix together different gases in big and small proportions.
Sincerely,
Alexander, PHD "Laser Physics", 1992.
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Does anybody have experience with v-groove  array coupling with  photodiodes on the chip? I need to couple the laser light from V-grooves to the 30um photodiode. I already tried once using the optical bench to couple  the PD and Vgrooves. But the efficiency seems very small (<1%). It seems the reflection from the mirror not matched with the PD size or may be the reflection is not uniform. Does anyone have idea about the on chip coupling? I would be also grateful if you kindly describe the gaussian mode matching. 
Thank you. 
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Dear Naim,
If you are using standard V-grooves, for coupling, you may try graded index (GRIN) optics. As you know, graded-in and graded-out structure of this type optics will help you to collect the light and then focus it on PD more efficiently.
Good Luck
Farmarz
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I would appreciate very much, if somebody can give me a connection, where we can get/order standard sample for testing performance of Laser particle size analyser.
Thank you for your help in advance.
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Dear Mirka,
I will also add to Alan's nice comprehensive description that you can, if more convenient, order certified reference materials (CRM's) through Whitehouse Scientific via Sigma-Aldrich.  Just in case that offers some convenience to your local situation.   Also see Sigma-Aldrich's BCR-067.
I have used the Malvern QAS materials and found them helpful in several instances.
Much depends on what size range you plan on operating in as to which CRM's you chose.  So know that prior to speaking with the vendors about possible CRM's.
Best regards,
Mike
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I am looking for high power intracavity optics (lens and polarizer). 
Wavelength: 755 nm
Pulse energy density: 1.6 J/cm2
Pulse width: 500 ps
Repetition rate: 100 MHz (during pulse build-up for ~ 2 microseconds)
Since the laser system is q-switched, modelocked and cavity dumped, the damage threshold requirement seems to be significantly higher than general stock optics. I have already damaged stock lenses, and some custom-made brewster thin film polarizer before the laser can reach the target energy.
The beam size has been expanded as much as possible inside the cavity with telescope configuration. Only option seem to be to look for a extremely sturdy optical coating.
Does anybody know any custom coating services that specializes in high damage threshold optics or intracavity optics?
(I've heard that hafnium coating is required for high LIDT but do not know about this in detail...)
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Hi Chanju,
Laser Pulse damage is always a difficult topic, as polishing, coating, cleaning and operational temperature play a role.
To my knowledge ATF is a supplier not only for high reflectivity but also for high power coatings:
However we in Germany usually order the coatings from Layertec and LaserOptic:
I hope presenting this variety the keeps me from the accusation of being biased.
However I have a very different question: What is the q-switching repetition-rate and the average power? We as disk laser developer normally use lenses just for the pump light. Intra cavity the only transmittive elements are the active medium, wave plates and Pockels cell. Are you sure, the lens was not affected by the average power of nonlinear absorption? Just an idea,
 I hope I could help.
All the best,
Karsten
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If I have a 1000 um optical fiber, is there any near field effect from the tip of the fiber if the fiber tip is not tapered? For example, in near field scanning optical microscopy, the tapered tip allows for a near field effect to interact with a surface for imaging, but is there a field that is measurable even with an untapered tip?
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Dear Benjamin,
Certainly, near field exists at the tip of an untapered fiber, as well. Please refer to Petermann Formula (Type II).
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I ablated stainless steel target in Hexane and water by UV laser. When I tested the absorbance of the suspension  of nanoparticles in Hexane, I observed positive and negative absorbance, The positive absorbance was in UV, while the negative absorbance was in visible, The negative absorbance was not observed in case of water. Please note that both of pure liquids have zero absorbance in UV-V. Can anyone explain the reason?
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Assuming your instrument is calibrated properly: If the instrument also detects scattered light from the sample (depends on the geometry), then you should also think of fluorescence as the reason behind increased signals.
Also, take into account that laser ablation in hexane may cause reaction products in the liquid, with a different spectroscopic characteristic than before laser ablation.
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How does the factor 2*pulse energy/cross section area comes for the gaussian beam and for top hat it it not present? Is this because of the energy distribution within the beam? Does anyone know the proof for this formula or recommend any book where this is mentioned ? 
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Dear Justin,   Integrating the Gaussian intensity function over full beam area (implying infinity) reveals finite integral of the same Gaussian intensity function up to the radius of its beam waist w fills less than 50% of the volume of cylinder I0 pi w^2.   Please see attached my calculation.   Regards, Shigeo.
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An Eye safe laser of Erbium 1535 micro meter wavelength, in pulse mode with peak power of 1 MW.
Under what class of laser it will fall...3b or 4?
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But, I think, the classification of laser is not your job. If you buy a laser, the manufacture have to classify the laser! Because it depends on some parameters: beam diameter, repetition rate, peak intensity, avg. power, pulse duration, etc.
And second, if you have a laser in the lab, before you use it, you have to contact with special "laser safety person" at your department, who is a certificated safety person.
Best regards,
Zsuzsanna
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How to calculate the beam waist of  Gaussian beam from its intensity in a crystal? Is there any direct relationship between the beam waist and intensity of a gaussian beam?  
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you can use a power meter and a knife edge.
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What is the magnitude of  the relaxation oscillation frequency of a semiconductor laser?
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Thank you Mr. Daniel and Mr. David for your advice.
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I'm looking for penetration depth of GaAs. I was using 532nm laser. Is it simply 1/a? (a:absoprtion coefficient) Help me, plz. T.T
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As I know, depth is related to focal spot size and  wavelength of incident wave in vacume.
depth=k.w^2(w=spot size)
k=(2*3.14)/(landa/n) (landa:wavelength)(n:refractive index)
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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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In the context of a Joint spectral amplitude of an SPDC effect is it possible to model a cw-laser with a dirac delta function?
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It is not fully clear from the question which properties of the laser you are interested to model with the delta function. The most obvious candidate would be the spectral power density. As mr. Tikhonov pointed out, the linewidths of proper single-longitudinal-mode CW lasers can be quite narrow, and, depending on the process you're modeling, it is indeed likely that a Dirac function may be a good approximation of the narrow Gaussian or Lorentzian spectral line-shape.
I suppose you would have to compare the linewidth of your laser to other spectral widths in your problem to see if it's much narrower.
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I am trying to figure out the lifetime of a LASER expressed in hours when given the number of shots fired.
I was thinking that 1x pulse is equal to one shot.
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Dindo, is quite impossible to answer with the little info you have. I would just add that it is important to know your  PRF  for the future and there is an easy way to do that using a photodiode and an oscilloscope. I am not sure what is your goal. People log the number of shots for maintenance purpose ( for example, in order to know when to change the flashlamps or filters). However, many components that require frequent maintenance can also be inspected visually in case you don't know the no. of shots/lifetime. For example, If the lamp is old you will see a degradation of the cathode (a bunch of powder accumulated on the inside of the glass) and it needs to be changed. I recommend starting to log all of this info in a laser notebook from now on (daily power output, no. of shots/duration of use, last day when you changed components etc). Normally, each laser should have a notebook associated with it!
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Hi everyone,
I want to introduce in the scan head of the microscope two laser lines (different lasers). One is already aligned with the system and I need the other one to follow the same path. The problem is the mirror in between that blocks one of the lasers.
That is why I am trying to find a solution and something that reflects what comes front the front and refracts what comes from behind, could work.
This is the only thing I can think of. Any other ideas are more than welcome.
Many thanks,
Yurema
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you can try using a mixing multimode fiber, and align all the lasers through that.
it will decrease the brightness a little bit.
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I'm particulary intersted to learn what is the the maximal average power and/or peak power achieved using second-harmonic generation that result in 100-500 ns pulses, especially  @ 515/532 nm.
Thanks,
Assaf
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I suspect that the answer to both is probably at the National Ignition Facility or somewhere similar. They use a two-stage amplifier to generate 351 nm from 1053 nm, with energies in the 100s to kJ to ~1 MJ per pulse. Pulse duration is 10 ns, so of order 1013-14 W peak power in the 527 nm beam.
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I have a micro projector using a 860nm laser diode.
There is a focal point in the optical path very close to an optical surface.
We would like to use polycarbonate molded optics with AR coatings.
Is there any data supporting this type of testing? I have only been able to find data for nanosecond pulse 1064 lasers.
I'm looking for intensity data for surface damage, bulk damage, and coating damage in W/cm^2.
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Polycarbonate has an exceptionally low intrinsic absorption compared to the AR coating materials. This will probably be no issue. The AR coating will be the first to fail. Assuming the optics will be clean (no dust) and have a surface roughness as expected for the laser optics, the damage threshold will depend on the absorption of the AR coating. You should specify to the coating supplier that only materials with low absorption at 806nm should be used. In most cases, the supplier will do that even without special request, since 808 is a standard for pumping Nd: solid state lasers, with pretty high power in many cases, although most of such optics is made of a fused silica glass. It is impossible to provide a "generic" damage threshold number, but just for order of magnitude, for a "high power laser quality" coating on fused silica  I would suggest an estimation of 1 to 10 kW/cm^2 CW. I am not sure if those numbers will hold for plastic.
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By Transparent I mean we could see what is happening inside from outside
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It is possible to see into a semiconductor laser if you are willing to use a camera that is sensitive to the laser wavelength.  In this case, a metal contact on the substrate side with a gap parallel to the laser cavity would afford a clear view of the cavity.  The semiconductor material is transparent to the laser wavelength because it must be of higher index to produce transverse confinement.  Looking through the gap in the metal contact gives a very interesting view -- but the light seen is limited to spontaneous emission and a small fraction of scattered stimulated emission.  Nevertheless, you can see the effects of cavity dynamics should a higher order mode take over at some operating current.  Perhaps this is not what you were hoping for -- but such a simple system might be easier to produce than a one-off gas laser with a transparent enclosure.  'All you need' is access to a fab line that makes semiconductor lasers, say at a university, and and a friend who can run a wafer with windows.
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all
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I I have little idea oflaser power
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I installed a simple system consists of 10Gbps NRZ signal and cw pump laser mixed together across pump coupler co-propagating to a SMF. at the end of the fiber, the pump coupler counter-propagation connected. the Raman pump laser connected to this pump coupler counter-propagation. Raman pumping should enhance the conversion efficiency even with lower pump power.
but I didn't see any effect of the Raman pump on the signal.
the wavelengths were, 1557, 1553, and 1455 nm for the signal, pump, and Raman pump respectively. I think the problem in the pump coupler counter-propagation,
 Please could any one can help me to solve this problem? 
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Thank you Silver Sauceda, 
Thank you S. Selvendran 
thank you for your responses
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when we consider a fabry perot resonator
the number of longitudinal modes which can be excited = half width / separation of longitudinal modes
why are we taking half width shouldn't we consider the full width of the gain profile
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Dear Noel! Similar evaluation provides just approximate description. Practically lasing bandwidth depends  from other parameters: cavity, nature of gain band broadening, lasing operation regime (standing or running way, transient or steady state, spatial broadening of gain and mode interaction). The most  bright example  is when laser cavity has so short length  that never mind what is type laser media we have one mode  oscillation. If cavity length is increased to allow two longitudinal modes we will observe them if both fits approximately to half width of gain band. At last  when  intermode gap is small to compare  with gain width lasing is getting many-mode type. All the same total lasing width never be equal to half of gain width due to gain saturation: it stay lesser.
You are welcome
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My work is related to theoretical modeling of a phenomenon in semiconductor lasers and I want to submit it in a reputed journal. Most journals in optics and photonics require experimental work. Can anyone suggest some jounrnal which deals with theoretical modeling?
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Optical and Quantum Electronics
Wireless Personal Communication
Optik
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I am using continuous laser for my experiment and photo detectors to detect the laser light, from the detectors I am getting photon counts only, Now I just want to convert photon counts into Intensity in terms of W/m2.
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Well, you want W/m². but you seem to have only a number of photons.
Obviously you need two things:
- the surface of the spot, to obtain an intensity in xxx/m²
- to convert photon into W, you need to know the amount of photon which arrive per unit of time. Then each one of your photon has an energy, in eV, or in J. With the time normalization, J becomes W.
Is that clear ?
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When two counter-traveling modes produce a standing wave pattern in a semiconductor ring laser, population inversion grating is formed which couples one mode to another, leading in unidirectional operation. How is then bidirectional operation still possible? 
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Dear Arpit,
It is likely to have coupling between two directions, due to non-linear effects in the SOA. But most of them have same effects on both directions. 
If you want to suppress one, then you need to build up an effect which is only strongly effective for one direction.
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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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A plane wave (laser beam) with an optical power density 0.5 W/cm2 propagate in vacuum (n = 1). That beam travels into a new medium, optically denser than vacuum (n = 2). Is the optical power density still the same? (time-averaged Poynting vector) or the optical power density is multiply by refractive index of new medium?
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Intensity depends on the refractive index of the medium, look it up in Saleh-Teich Photonics or any similar book. Attached the correct definition (intensity is the modulus of the time-averaged Poynting vector, E_0 the amplitude of the light-electric field) and the equation including the refractive index.
Cheers,
Martin
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I'm looking for the least expensive picosecond laser with the following minimum specifications. Ideally, my upper end in cost would be ~$10,000 so anything around that or less would be nice.
Wavelength: 532 nm or 1064 nm (or both)
Pulse duration: <300 pS (shorter is better)
Energy per pulse: 1-5 mJ (for a 6 mm beam, or less energy with a smaller beam to have the same effective radiant exposure)
Repetition rate: Even 1 Hz would be okay, but ideally 10 Hz or more
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He need pulse energy ~ 1 mJ. You can not get this level with cheap fiber laser.
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What should be the easiest way to measure laser beam waist at focus and How?
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The D86 is also called encircled energy (but is in fact encircled intensity that is measured and energy is computed), this is generally the way that all short-pulse laser systems measure the intensity on target. This is the technique used in my post above to calculated the spot size from a CCD as you need some point of reference to know where to stop your contour, and this number can be essentially arbitrary because the spot may deviate significantly from Gaussian. Also, this number can be manipulated to make your laser look better than it is, i.e. taking the 50% contour instead of the 86% contour can yield a much higher average intensity, thus as a best practice one should not quote a number, but rather show the encircled energy curve. See attached Fig 2, encircled energy at best focus, and attached image of the focal spot through focus.
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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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Is laser spot size (beam diameter at the waist) depends on the laser intensity?
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Firstly I fully agree with Prof. Muhammad Aslam Baig that intensity is measured as power per unit area.
Moreover, "... spot size is directly proportional to the laser wavelength and lens focal length but inversely proportional to the diameter of the laser beam  falling on the focusing lens.".  
I want to comment on the bold-faced part of the text. 
If we have a laser beam of Gaussian diameter, say, 0.5mm and wavelength of 632.8nm (He-Ne laser) and want to focus it to the minimum spot size (and hence maximum  focus intensity). Then first we should expand it  to a larger beam diameter (and hence lower the intensity), say, 20mm by a magnification of X40 and then use a suitable focusing lens to achieve the min. spot size (and max. intensity). Now larger diameter of the incoming beam  means lesser intensity , and that the spot size can be minimized by maximizing the beam diameter, hence, therefore the spot size may depend upon the incoming beam diameter.
Further, if incoming beam parameters are fixed like its diameter, wavelength. Also the focal length and diameter of the lens is also fixed. Then we might ask if there is any dependence of spot size upon  upon laser beam intensity? The answer lies in the Kerr effect. If the laser is intense enough to produce Kerr non-linearity at the focus, then the beam can be 'self-focused' and the spot size might be further reduced. In the absence of Kerr non-linearity, the beam optics remains independent of laser intensity. Therefore, the spot size will not depend upon intensity.
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Any book or paper explaining key differences in the construction, working and dynamics of semiconductor lasers and gas lasers.
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Laser Physics by Milloni
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What material can be used to filter based on the intensity of laser beam
Not wavelength
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If you want to filter high intensity or to reduce the fluctuations in the intensity, you can use suitable optical limiters (reverse saturable absorbers).
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Hi all,
I am focusing a Gaussian beam using a High NA objective (NA=1.3). The laser is linearly polarized. I focus the laser at an oil/water interface and image the back reflected light. The image is attached. The profile is not near what you expect, i.e. asymmetric Gaussian. If I lower the NA of the objective the profile is Gaussian again which is expected. I can trap fine. At this point am not sure what is causing this. I appreciate any help.
Regards
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Hi,
I have been trying to revive an old Continuum Minilite series laser.  The main problem it has is that after a short period of running the measured laser power drops significantly (approx 160 mW to begin with, dropping to ~50 mW after 20 minutes of running).
  • My testing so far has suggested that the pump is operating as it should do according to specification and that the cooling system is fine. We do not think that heating of the cavity is the problem.
  • The flashlamp has als