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ABSTRACT: A 3-D numerical model is developed for a continuous wave, intra-cavity frequency doubled Nd:YVO<sub>4</sub>/MgO:PPLN green laser. A fast Fourier transform beam propagation method is employed to solve the 3-D coupled-wave equations in the laser cavity. Both analytical solution and experimental results are made to validate the 3-D numerical model. By employing the developed numerical model, key material and device parameters that affect the device performance, such as nonlinear crystal length and loss, pump beam spot size and cavity length are investigated.
IEEE Journal of Quantum Electronics 11/2011; · 1.88 Impact Factor
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ABSTRACT: High surface area, sol-gel derived macroporous silica films doped with gold nanoparticles (AuNP) are used as a platform for high-density affinity-based immobilization of functional structure-switching DNA aptamer molecules onto Michelson interferometer long-period grating (LPG) fiber sensors, allowing for label-free detection of small molecular weight analytes such as adenosine triphosphate (ATP). The high surface area afforded by the sol-gel derived material allowed high loading of DNA aptamers, while the inclusion of gold nanoparticles within the silica film provided a high refractive index (RI) overlay, which is required to enhance the sensitivity of the LPG sensor according to our numerical simulations. By using a structure-switching aptamer construct that could release an oligonucleotide upon binding of ATP, the effective change in RI was both enhanced and inverted (i.e., binding of ATP caused a net reduction in molecular weight and refractive index), resulting in a system that prevented signals originating from nonspecific binding. This is the first report on the coupling of aptamers to LPG fiber sensors and the first use of high RI AuNP/silica films as supports to immobilize biomolecules onto the LPG sensor surface. The dual functionality of such films to both improve binding density and LPG sensor cladding refractive index results in a substantial enhancement in the sensitivity of such sensors for small molecule detection.
Analytical Chemistry 09/2011; 83(20):7984-91. · 5.86 Impact Factor
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Microwave and Optical Technology Letters 07/2011; · 0.62 Impact Factor
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ABSTRACT: General time-domain traveling wave rate equations are employed for the simulation of the intra-cavity second harmonic generation (IC-SHG) of semiconductor lasers by a periodically poled nonlinear crystal waveguide. A 1060 nm high power, single-mode, ridge waveguide semiconductor laser is used in the simulation. The SHG crystal is a MgO-doped periodically poled lithium niobate with a single-mode ridge waveguide. Comparisons are made between the computed and experimental results of single-pass SHG for the purposes of model validation. The design of an IC-SHG green laser is further compared to the single-pass SHG laser in terms of the SHG output power, conversion efficiency, temperature tolerance, and high speed modulation capability. It is theoretically found that by using the IC-SHG configuration, SHG power and efficiency can be improved with a relatively short nonlinear crystal.
IEEE Journal of Quantum Electronics 05/2011; · 1.88 Impact Factor
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ABSTRACT: A polymeric-nanoparticle composite film is used, for the first time, to increase cladding mode evanescent field and, therefore, the sensitivity of an all-in-fiber Michelson interferometer for refractive index (RI) sensing. Utilization of the layer-by-layer polyelectrolyte deposition and metal nanoparticle synthesis within the polymeric composite film allows us to form precisely a very high RI thin overlay required for cladding mode reorganization. As a result, the developed refractomer demonstrates significant sensitivity enhancement over ambient RI change. Potential applications of the refractometer as a biosensor are outlined and discussed.
IEEE Journal of Selected Topics in Quantum Electronics 07/2010; · 3.78 Impact Factor
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ABSTRACT: In this paper, modulation crosstalk from 1310 nm upstream optical signals to 1490 nm downstream optical signals in an distributed feedback laser diode and monitor photodiode (DFB-LD and MPD) monolithically integrated optical transceiver has been studied numerically through the time-domain traveling wave (TDTW) model. Interaction between the 1310/1490 nm wavelengths throughout the cavity originated from both cross gain modulation and cross phase modulation are the physical source of the crosstalk in the device, which were analyzed numerically in detail in the paper. A new structure is proposed to reduce the modulation crosstalk resulted from the interaction. Simulation shows that the modulation crosstalk can be drastically reduced by applying an adjustable current on the phase tuning section inserted between the DFB-LD and MPD, which verifies our design. Polarization and temperature dependence on the modulation crosstalk are also discussed.
IEEE Journal of Quantum Electronics 04/2010; · 1.88 Impact Factor
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ABSTRACT: A sensitivity-enhanced long-period grating in-fiber Michelson interferometer with diameter-reduced cladding was proposed for the first time, and was also demonstrated as a label-free biosensor based on refractive index measurement. The cladding structure dependence of the sensitivity for antibody-antigen immuno-sensing was studied experimentally. The proposed biosensor has shown an 8.5-fold sensitivity enhancement as compared to sensors without cladding reduction.
IEEE Journal of Selected Topics in Quantum Electronics 12/2007; · 3.78 Impact Factor
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ABSTRACT: Ultrafast all-optical optical time-division multiplexing demultiplexing based on the cascaded second-harmonic generation and difference-frequency generation wavelength conversion in quasi-phase-matched periodically poled lithium niobate waveguides are studied theoretically. For a typical 160- to 10-Gb/s demultiplexing process, conversion efficiency, pulse shape, time delay, phase variation, and crosstalk are investigated in two different arrangements of the 160-Gb/s signals, and both the wavelength conversion and pulse reshaping can be realized
IEEE Photonics Technology Letters 05/2007; · 2.19 Impact Factor
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ABSTRACT: External perturbation such as transverse force can increase the birefringence of most fiber Bragg gratings (FBGs). As a result, the interrogation accuracy of an FBG-based sensor may be significantly deteriorated due to an uncertain wavelength shift caused by the variation of polarization state of the input light. In the experiments, the reflection spectra of the FBG-based strain sensors made of the spun and conventional single-mode fibers were compared under different transverse forces when the polarized and unpolarized light was injected, respectively. It is shown that the spun FBG sensors have lower polarization dependence on, and consequently stronger resistance to, transverse force than the conventional FBG sensors
IEEE Photonics Technology Letters 05/2007; · 2.19 Impact Factor
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ABSTRACT: A comprehensive review of the optical transceivers for fiber-to-the-premises applications is presented, with emphasis on system requirements and enabling technologies. Recent progress in photonic integration to realize low-cost, high-performance, and compact optical transceivers for the optical network terminal is reviewed
Journal of Lightwave Technology 02/2007; 25(1):11-27. · 2.78 Impact Factor
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ABSTRACT: The cladding-modified long-period-grating (LPG) refractive-index sensors with a high-refractive-index overlay are optimized based on systematical studies on the structure-modified LPG. Dependence of the cladding-mode effective indexes on the parameters of the structure-modified LPG, such as the overlay refractive index (RI), the overlay thickness, the cladding-layer radius, and the ambient RI, as well as the order of cladding mode, are investigated in detail. An optimization procedure based on reducing the cladding radius, keeping the grating period as an adaptive parameter, and employing the HE<sub>13 </sub> mode notch-wavelength shift is proposed for the LPG RI sensors. By using the proposed optimization method, it is shown that an ambient RI sensitivity as large as 5980 nm/RI can be achieved, representing a threefold sensitivity enhancement, as compared to the best result obtained from the reported structure, in which the high HE<sub>17</sub> mode resonate notch wavelength was employed in sensing
Journal of Lightwave Technology 02/2007; 25(1):372-380. · 2.78 Impact Factor
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ABSTRACT: Second-harmonic generation (SHG) of 40-GHz picosecond optical pulses with different pulsewidths, pulse energies, and central wavelengths in a MgO-doped periodically poled lithium niobate (PPLN) waveguide is studied experimentally and theoretically. In the experiments, the picosecond pulses are generated from a 40-GHz mode-locked fiber laser and two tunable filters, with which the lasing wavelength can be tuned from 1530 to 1570 nm, and the pulsewidth can be tuned from 2 to 7 ps. The second-harmonic (SH) pulses are generated when the picosecond pump pulses pass through the MgO-doped PPLN waveguide. Dependences of SHG on pump pulsewidth, average pump power, and pump central wavelength are then investigated systematically. Meanwhile, dynamic behaviors of both pump and SH pulses in propagation are simulated numerically. Based on the temporal and spectral characteristics of SHG, a quantitative and systematic analysis on SHG efficiencies in terms of both pulse energy and spectral peak is presented. The simulation results are in good agreement with the measured data.
Journal of Lightwave Technology 11/2006; 24(10):3698-3708. · 2.78 Impact Factor
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ABSTRACT: Deposition of a high refractive index (RI) overlay on long-period grating (LPG) was recently studied to enhance the sensitivity of LPG sensor to the surrounding ambient RI changes and offset the sensitive operational range of LPG RI sensor to low ambient RI. The enhancement mechanism relies on cladding mode reorganization due to the existence of the overlay. In this paper, the reorganization of the cladding mode in a cladding radius reduced LPG coated with high RI overlay is studied with full vector transfer matrix method. The dependence of the cladding modes effective indices on the fiber parameters and ambient RI is systematically investigated. Based on the simulation result, an LPG refractive index sensor optimization procedure is proposed, in which the grating period is set as an adaptive parameter and the cladding radius of the LPG is reduced. It is found that by using the proposed optimization method, the notch band corresponding to low order cladding mode resonance has higher sensitivity to the ambient refractive index. It is shown that an ambient refractive index sensitivity as large as 4012nm/RI can be achieved, representing a 3-fold sensitivity enhancement as compared to the best result obtained from the reported structure in which the high order cladding mode resonate notch wavelength was employed in sensing.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
09/2006;
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ABSTRACT: What is believed to be a novel long-period grating (LPG) refractive index sensor with a modified cladding structure is proposed and studied. In the proposed structure, the cladding of the fiber has a two-layer structure, i.e., a cladding layer of low refractive index with a reduced radius and an overlay of high refractive index. The sensitivity of the structure-modified LPG sensor to the ambient refractive index change as a function of the cladding layer and overlay parameters is investigated by way of modeling. It is found that an increase of the ambient refractive index causes a field redistribution of the cladding mode into the overlay when the parameters of the overlay are properly selected. It is shown that by reducing the radius of the cladding layer, the operational range of the LPG refractive index sensor can be as large as 0.195 (from 1.244 to 1.440) with a minimum sensitivity of 660 nm/refractive index, which represents a 31% increase of operational range in comparison with the operational range obtained from the reported structure. The design guidelines for achieving this large operation range and high sensitivity are explained by investigating the dependence of the cladding modes on the radius of the cladding layer.
Applied Optics 09/2006; 45(24):6142-7. · 1.41 Impact Factor
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ABSTRACT: The wavelength conversion of picosecond optical pulses based on the cascaded second-harmonic generation-difference-frequency generation process in a MgO-doped periodically poled lithium niobate waveguide is studied both experimentally and theoretically. In the experiments, the picosecond pulses are generated from a 40 GHz mode-locked fiber laser and two tunable filters, with which the lasing wavelength can be tuned from 1530 to 1570 nm, and the pulse width can be tuned from 2 to 7 ps. New-frequency pulses, i.e., converted pulses, are generated when the picosecond pulse train and a cw wave interact in the waveguide. The conversion characteristics are systematically investigated when the pulsed and cw waves are alternatively taken as the pump at the quasi-phase-matching wavelength of the device. In particular, the conversion dependences on input pulse width, average power, and pump wavelength are examined quantitatively. Based on the temporal and spectral characteristics of wavelength conversion, a comprehensive analysis on conversion efficiency is presented. The simulation results are in good agreement with the measured data.
Applied Optics 08/2006; 45(21):5391-403. · 1.41 Impact Factor
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ABSTRACT: The photorefractive effect (PRE) in quasi-phase-matched LiNbO3 waveguides is studied systematically by employing the pump-probe technique. It is found that the PRE is gradually eased with an increase of the device operation temperature. With increasing light irradiation power, the PRE becomes severe, the buildup (relaxation) time constant decreases (increases), while the second harmonic generation (SHG) efficiency is enhanced for the devices used in studies. The PRE under a shorter wavelength (980 nm) light irradiation is much stronger than that at a longer wavelength one (1550 nm), and the PRE for the latter is mainly caused by the SHG of the pump.
Applied Physics Letters 07/2006; 89(4):043510-043510-3. · 3.84 Impact Factor
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ABSTRACT: Characteristics of multimode fiber Bragg gratings (MMFBGs) are systematically studied. Dependence of the reflection spectra of MMFBG on excitation conditions, refractive-index change profile, and grating length is investigated experimentally. It is observed that the transmission spectra of the MMFBGs are more sensitive to the polarization state of input light than that of single-mode FBGs (SMFBGs). The influences of the fabrication conditions of MMFBGs (i.e., UV-exposure time) and the rotation of MMF on the output spectra of the MMFBG-based external-cavity semiconductor lasers are then investigated. It is found that the side-mode-suppression ratio and the spatial wavelength-locking region at a specific Bragg wavelength of an MMFBG are dependent on the refractive-index change profile of the grating.
Journal of Lightwave Technology 05/2006; 24(4):1903- 1912. · 2.78 Impact Factor
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ABSTRACT: The numerical modeling of actively Q-switched fiber lasers is systematically presented. On the basis of typical Q-switched ytterbium-doped double-clad fiber lasers under forward and backward pump, the dynamic characteristics of pulse energy, pulse width, population inversion, and stored energy at tens-of-kilohertz repetition rates are studied by using the traveling-wave method. The laser performance is further investigated for different fiber core diameters, doping rates, cavity lengths, fiber losses, signal and pump wavelengths, reflectivities of output coupler, and switching speed of an acousto-optic modulator; the laser optimization is also quantitatively discussed. Some simulation results are also compared with previous experimental results.
Applied Optics 04/2006; 45(9):2058-71. · 1.41 Impact Factor
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ABSTRACT: The output spectra of external-cavity semiconductor lasers (ECSLs) formed by using different antireflection (AR) coated laser diodes (LDs) with multimode fiber Bragg gratings (MMFBGs), are studied systematically. It is found that the side mode suppression ratio (SMSR) of the output spectra of ECSL is dependent on the relative position of the Bragg wavelength and the intracavity modes of the LD, and this effect is more apparent when the reflectivity of AR coating of LD is increased. Numerical simulations are provided to explain the experimental observations. Furthermore, the requirements of wavelength locking of ECSLs with MMFBGs are found to be different from those with single-mode fiber Bragg gratings (SMFBGs). The conditions of wavelength locking of the MMFBG-based ECSLs in terms of gain margin between the material gain peak and the gain corresponding to the selected Bragg wavelength, reflectivity of AR coating of LD and reflectivities of the Bragg wavelengths of MMFBG are comprehensively investigated, and compared with the experimental results.
IEEE Journal of Quantum Electronics 01/2006; · 1.88 Impact Factor
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ABSTRACT: A novel method for measuring modal power distribution (MPD) in multimode fibers is proposed using built-in tilted fiber Bragg gratings (TFBGs). For each degenerated guided mode group, a portion of the guided power can be coupled to the corresponding radiation mode through a TFBG imprinted into the same fiber, and then measured separately with a spatial filter. The power coupling coefficients between the guided and radiation modes are obtained by solving a set of linearly independent equations acquired under a number of launching conditions. With these coefficients obtained, the MPD under any excitation condition can then be obtained by simply measuring the powers of radiation modes. The proposed method is successfully demonstrated with a few-mode fiber.
IEEE Photonics Technology Letters 11/2005; · 2.19 Impact Factor
