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We report on the pulse compression of an 18.5 MHz repetition rate pulse train from 230 fs to sub-40 fs by nonlinear spectral broadening in a multi-pass cell and subsequent chirp removal. The compressed pulse energy is 4.5~\mu \textJ , which corresponds to 84 W of average power, with a compression efficiency of 88%. This recently introduced compression scheme is suitable for a large pulse energy range and for high average power. In this paper, we show that it can achieve three times shorter pulses than previously demonstrated. [ABSTRACT FROM AUTHOR]

A system scheme is presented, which allows non-reciprocal wave transmission or directional amplification of electromagnetic signals, using a boxed four-node method. Edges represent strong hopping interactions and diagonals stand for weak parametric interactions. Using careful optimization of values for design parameters, we are able to obtain non-reciprocity in excess of 12 and 130 dB for intrinsic and extrinsic configurations at identical input/output frequencies. For the directional amplification, an isolation as high as 40 dB is demonstrated with the forward/backward gains of ±20 dB. Cascading two such systems potentially can offer high isolations at high gains. [ABSTRACT FROM AUTHOR]

We report on a quenching behavior of the 3.4~\mu \textm dual-wavelength pumped fiber laser that can be accounted by the adverse contribution from a new excited state absorption transition in Er3+:ZrF4 glasses. The transition occurs between levels 4F9/2 and 4F7/2 and partially overlaps with the 1976 nm pump wavelength. We show that this upconversion process causes strong quenching of the lasing power at 3.4~\mu \textm under insufficient 974 nm pumping. Intracavity fluorescence at 550 nm from levels (2H11/2, 4S3/2) is also monitored as a witness of this excited state absorption mechanism. Through numerical modeling, we estimate the cross section value of the process and show that it is essential to reproduce both power and fluorescence curves. This paper also emphasizes the critical role played by the pump wavelengths. [ABSTRACT FROM AUTHOR]

The performance of mid-infrared Er3+-doped fiber lasers has dramatically improved in the last few years. In this paper, we present a numerical model that provides valuable insight into the dynamics of a dual-wavelength pumped fiber laser that can operate on the 3.5- and 2.8- \mu \textm bands. This model is a much needed tool for optimizing and understanding the performance of these laser systems. Comparisons between simulation and experimental results for three different systems are presented. [ABSTRACT FROM PUBLISHER]

We recently demonstrated orders of magnitude enhancement of two-photon absorption (2PA) in direct gap semiconductors due to intermediate state resonance enhancement for photons of very different energies. It can be expected that further enhancement of nondegenerate 2PA will be observed in quantum wells (QWs), since the intraband matrix elements do not vanish near the band center, as they do in the bulk, and the density of states in QWs is larger near the band edge. Here, we present a perturbation-theory-based theoretical description of nondegenerate 2PA in semiconductor QWs, where both the frequency and the polarization of two incident waves can vary independently. Analytical expressions for all possible permutations of frequencies and polarizations have been obtained, and the results are compared with the degenerate 2PA in QWs along with the degenerate 2PA and the nondegenerate 2PA in bulk semiconductors. We show that using QWs in place of bulk semiconductors with both the beams in the transverse magnetic-polarized mode leads to an additional order of magnitude increase in the nondegenerate 2PA. Explicit calculations for GaAs QWs are presented. [ABSTRACT FROM AUTHOR]

A quantum dot (QD) middle-far-infrared (MFIR) detection and imaging approach based on all-optical-readout (AORO) is proposed. Using the intersubband (ISB) optical transition to change the interband (IB) absorption, it converts the ISB MFIR signal into the IB near-infrared signal. It might overcome the problems in the electrical readout infrared detection and imaging, such as dark current, low-temperature operation, low resolution, and vulnerability to the high radiation intensity. Based on the In0.5Ga0.5As/GaAs lens-shaped QD structure, our calculation shows the potential of this AORO approach for the MFIR detection and high-resolution imaging, the potential for room-temperature operation, and the ability for frequency selectivity and spectrum scanning. [ABSTRACT FROM PUBLISHER]

In this paper, we present a novel actuation technique of diffractive optics based on magnetic interaction forces, permitting a fine resolution tuning as well as a tuning in discrete steps. This actuation combines advantages of "analog" and "digital" tuning in conventional diffractive optics. The application of a magnetic field perpendicularly to the beams of a freestanding grating coated with magnetic material generates intermagnet forces causing these beams to deform slightly or to clump together. A theoretical analysis of the forces has been carried out, followed by actuation simulations in both linear and nonlinear regimes for different types of gratings, showing examples of exponential and parabolic chirp profiles in addition to beam clumping. In experiment, the diffraction patterns of distinct fabricated gratings have shown both a continuous shift and smear as well as a discrete change in the diffracted angles proving the concept of analog and digital tuning. [ABSTRACT FROM AUTHOR]

We present an analytical model for the optical emission produced by sources located in a thin-film weak-microcavity formation and study the effects of the ensemble spectral and spatial distribution on the device emission properties. However derived for a general stratified media configuration, the formulation results are highly applicable for the study of nanometric organic light-emitting devices. Rigorously developed into closedform analytical expressions using the device's thin-film weakmicrocavity characteristics, they enable clear observation of the underlying physical processes that determine the emission properties of the device, as well as the impact of the exciton ensemble spectral and spatial distributions on these properties. For the sake of simplicity and clarity, we focus on a 2-D canonical configuration excited by impulsive (line) sources. Our results show that the spectral distribution of the ensemble diminishes interference effects originated in the weak microcavity formed between the substrate/air and cathode/active layer interfaces, while the spatial distribution can only impact the slow-varying component of the emission pattern, which is the consequence of the source-image interference near the highly reflecting cathode. For a typical device, the quasi-Lambertian emission pattern reported experimentally is reproduced. It should be pointed out that the incorporation of both rigorous electromagnetic analysis and the source spectral and spatial broadening effects is addressed in our report, to the best of our knowledge, for the first time. This results in a precise model capable of repeating and interpreting experimental and simulated data. [ABSTRACT FROM AUTHOR]

A record of 680-mW single-frequency red emission at 671.1. nm is demonstrated using a Nd:YVO4 ring laser intracavity frequency-doubled by a type-I critically phase-matched BiB3O6 crystal. Single-frequency tuning over Δλred ∼ 1.25 nm can be achieved with a thin etalon with reflectivity of 40%. At 1342.5 nm and with a T = 2% transmission output coupler, the laser provides an optimal 1.55 W of single-frequency power. [ABSTRACT FROM AUTHOR]

A new and efficient method for waveguide field calculation is proposed. In the proposed Fourier cosine beam propagation method, both dispersion and waveguide effects for each propagation interval are treated in a matrix form. Such a calculation method converts the conventional point-by-point analysis skill into a matrix-based one. By using the matrix method, the computing time required for calculating the field evolution is much less than that required by using the commonly used finite-difference beam propagation method. Two y-branches with, respectively, a rib-type and a channel-type cross sections are tested. The numerical results demonstrate the efficiency of the proposed method. [ABSTRACT FROM AUTHOR]

We present a detailed analysis and comparison of dielectric waveguides made of CdTe, GaP, GaAs and InP for modal phase matched optical difference frequency generation (DFG) in the terahertz domain. From the form of the DFG equations, we derived the definition of a very general figure of merit (FOM). In turn, this FOM enabled us to compare different configurations, by taking into account linear and nonlinear susceptibility dispersion, terahertz absorption, and a rigorous evaluation of the waveguide modes properties. The most efficient waveguides found with this procedure are predicted to approach the quantum efficiency limit with input optical power in the order of kWs. [ABSTRACT FROM AUTHOR]

This paper presents the lineshape analysis of the beat signal between the optical carrier and the shifted and delayed sidebands produced by sinusoidal amplitude modulation. It is shown that the beat signal has a typical lineshape with a very narrow δ-peak superposed on a quasi-Lorentzian profile. Theoretical explanation for the appearance of this peak has been given based on optical spectral structure constructed by a large number of optical wave trains. It is predicted that the -peak is originated from the beat between the wave trains in the carrier and those in the delayed sidebands when their average coherence length is longer than the delay line. Experiments carried out using different delay lines clearly show that the -peak is always located at the modulation frequency and decreases with the increasing delay line. Our analysis explicitly indicates that the linewidth is related to the observation time. It is also suggested that the disappearance of the δ-peak can be used as the criterion of coherence elimination. [ABSTRACT FROM AUTHOR]

Parametric wavelength exchange (PWE) has been demonstrated as a versatile device in providing different functionalities. In this paper, we will concentrate, numerically and experimentally, on one of these functionalities, namely, all-optical time demultiplexing of 160-Gb/s return-to-zero (RZ) signals based on a pulsed-pump PWE in a 400 m highly nonlinear dispersion-shifted fiber. Experimental results show power penalties ⩽ 2.7 dB at bit-error rate of 10-9 for all demultiplexed 10-Gb/s RZ signals. We also derive theoretical expressions for the conversion/residual efficiencies and investigate the impact of pump pulse width and phase mismatch on these efficiencies. Furthermore, the impacts of pulsed-pump wavelength and power level on the characteristics of the switching window are investigated numerically. As a result, the demultiplexer can be easily upgraded to an add-drop multiplexer because of the complete exchange nature of PWE, which is justified by the surviving channels' waveform performance. [ABSTRACT FROM AUTHOR]

In this paper, we show that patterned metallic surfaces can support localized resonant modes if a suitable defect in the periodicity of the structure is introduced. For those modes, optical losses due to radiative leakage perpendicular to the metallic plane can be significantly reduced by properly tuning the geometry of the defect-mode surface cavity. A design strategy based on the investigation of the electromagnetic field in the reciprocal space is suggested. [ABSTRACT FROM AUTHOR]

The concept of soliton dispersion management pertaining to the effect of varying dispersion with external harmonic oscillator potential for chirped solitons have been studied in detail with emphasis on the various aspects of dispersion management of solitons. The interaction scenarios pertaining to in-phase- and off-phase injection with equal and unequal amplitudes are dealt elaborately. Suppression of interaction forces between the dispersion managed solitons, optical soliton pulse amplification and robustness of bound soliton states to collisions, are some of the main results of this article. [ABSTRACT FROM AUTHOR]

A numerical model for multiwavelength actively mode-locked erbium-doped fiber lasers (AML-EDFL) is presented. Using the model, the influences of four-wave mixing (FWM) and phase modulation effects (i.e., self-phase modulation (SPM) and cross-phase modulation (XPM)) can be studied, respectively. We use it to investigate AML-EDFLs incorporating a highly nonlinear fiber (HNLF) and explore the mechanisms for gain competition suppression (GCS). The net dispersion of the cavity is found critical for GCS. In the small dispersion regime, FWM dominates GCS, but SPM and XPM deteriorate the output pulses. In the large anomalous dispersion regime, SPM and XPM, however, enhance GCS and produce high-quality pulses. Degree of GCS is also quantitatively evaluated. The simulation results are at last confirmed by experiments. [ABSTRACT FROM AUTHOR]

Using a wedged and antireflection-coated diamond heatspreader, a continuously tunable semiconductor disk laser with intracavity second-harmonic generation (SHG) is demonstrated. Output powers of > 600 mW tunable over 10 nm around 530 nm are obtained. Finite-element modeling shows that the use of a diamond heatspreader for thermal management—in contrast to substrate thinning approaches—permits power scaling across the 670-2300-nm range of these lasers. Using a green laser as an exemplar, this paper details the issues involved in translating this spectral coverage to the ultraviolet and visible via SHG. Polarization and wavelength selection are discussed and the adopted approaches presented. Almost 1 W of second-harmonic light at 530 nm is demonstrated, with an efficiency of 11% with respect to the incident pump power. [ABSTRACT FROM AUTHOR]

We investigated the dispersion and modal properties of a photonic crystal fiber (PCF) with larger air holes (LAH). The theoretical and numerical results show that large anomalous group-velocity dispersion at short wavelength down to around 700 ∼ 800 nm can be achieved in a LAH-PCF with small pitch. Furthermore, the high-order modes usually are excited in a LAH-PCF when the optical field is launched from a traditional single-mode fiber, and all the excited modes will interfere when they propagate along the PCF. Finally, the properties of the excited modal spectrum and the multimode interference are determined by the normalized pitch and the normalized hole size of the PCF. All of these will provide references for the smoothing applications of the LAH-PCF. [ABSTRACT FROM AUTHOR]

We present a highly efficient optical field concentrator that is capable of confining optical field in nanometer-thin low-index media with very high optical confinement factor. The structure is made of multiple-layered low-index nanolayers embedded in high-index silicon waveguides. By creating multiple high-index-contrast interfaces, the normal field in the low-index nanolayer regions is significantly enhanced. It subsequently results in a very high optical confinement and power density in these regions. With the help of numerical simulation tools, the guiding and propagating characteristics of the new structure are studied and presented. The optimal structures have demonstrated confinement factors and normalized power densities in the range of 30%–60% and 20–160 μm-2 for the 5–20-nm thin low-index multiple nanolayers. [ABSTRACT FROM AUTHOR]

Integrated nonlinear-optic (NLO) devices for generation of twin-photon beams are a key device for quantum information techniques. This paper presents a quantum theory analysis on the effect of optical losses in twin-photon generation. The analysis uses a multi-mode-pair description, and is based upon unitary transformations of amplitude operators and state vectors for describing the NLO and loss phenomena in infinitesimal sections in the interaction range. Full description of the quantum states of the twin-photon beams affected by the losses is obtained. Mathematical expressions useful for prediction of the performances degradation are deduced. [ABSTRACT FROM AUTHOR]

An optimized diode-laser side-pumped grazing-incidence Nd:YVO4 amplifier was used to increase the power of a 50-mW 150-MHz continuous-wave (CW)-pumped mode-locked oscillator up to 6.1 W in single pass, with 22% optical-to-optical efficiency, or up to 8.4 W in double pass, with 30% efficiency. Both beam quality (M² < 1.4 from TEM00 seed pulses) and pulse duration (7.5 ps from 6.9 ps) were preserved. Single- or double-pass small-signal gain greater than 40 dB was achieved. These experimental results have been corroborated by a numerical model analysis of the amplifier. [ABSTRACT FROM AUTHOR]

An alternative segmented-contact method for accurate measurement of the optical gain and absorption of quantum-dot and quantum-dash active materials with small optical gain is reported. The usual error from unguided spontaneous emission is reduced by subtracting signals acquired from three independently controlled sections as opposed to just two found in the conventional technique. The quantum-dot gain spectra are measured to a precision of less than 0.2 cm-1 at nominal gain values below 2 cm-1, and gain spectrum of quantum-dash sample is calculated with an error less than 0.3 cm-1 at a gain less than 1 cm-1. These accuracies are checked with a self-calibrating method. The internal optical mode loss measurement is also described. [ABSTRACT FROM AUTHOR]

We demonstrate optical processing based on spectral line-by-line pulse shaping of a frequency comb generated by phase modulating a CW laser and show various applications including CW-to-pulse conversion, width/wavelength tunable return-to-zero pulse generation, pulse-to-CW conversion, wavelength conversion, and microwave photonics. The correlations between optical spectra, time-domain waveforms, and RF spectra are quantitatively studied. [ABSTRACT FROM AUTHOR]

A two modes rate equation model for vertical-cavity surface-emitting lasers (VCSELs) subject to optical feedback from an extremely short external cavity (ESEC) is presented. By making use of it we develop a map of bistability to investigate the parametric dependence of polarization properties of VCSELs in such configuration, finding out a periodic dependence of the polarization switching (PS) currents on the ESEC length. By increasing the external mirror reflectivity we can make this periodic dependence stronger and strongly asymmetric providing the possibility to prevent PS for any injection current achieving polarization stabilization in VCSELs. Further numerical simulations with isotropic and non isotropic feedback show how parameters as the compression coefficients, the frequency splitting between the linearly polarized (LP) modes, the differential gain and the top mirror reflection coefficient, affect the map of bistability. [ABSTRACT FROM AUTHOR]

Coherent beams at soft X-ray (SXR) wavelengths can be generated using extreme nonlinear optics by focusing an intense laser into a gas. In this paper, we discuss phase-matching and quasi-phase-matching techniques that use gas-filled modulated waveguides to enhance the frequency conversion process. This leads to the generation of SXR beams that are both spatially and temporally coherent. [ABSTRACT FROM AUTHOR]

Using a novel design approach, high-performance large-port-count switches based on cascaded multimode interference (MMI) couplers are shown to be feasible in low-index-contrast materials. This approach combines the transfer matrix method, optimization of the MM! dimensions, and mode propagation analysis (MPA) for calculation of phase shifts. 1 × 4,1 × 8,1 × 16, and 1 × 32 switches are designed, with insertion losses of 1 dB and crosstalk as low as -31.7 db. Also, it is shown that this approach can balance the losses for different switch states. [ABSTRACT FROM AUTHOR]

We propose a new multiple quasi-phase-matched wavelength converter based on the continuous phase modulation of a X(2) grating for use in variable wavelength conversion. A numerical study shows that the proposed device exhibits a high conversion efficiency, flexible design, and robust fabrication tolerance. A waveguide device fabricated by annealed proton exchange agrees well with the numerical design. Fine-tuning the device enabled us to demonstrate variable wavelength conversion between signals on the standard optical frequency grid. Using the device, we also demonstrated fast (<100 ps) wavelength switching of 4-channel 40-Gb/s signals. The obtained results clearly show that the proposed multiple quasi-phase-matched devices will be useful when constructing future flexible photonic networks. [ABSTRACT FROM AUTHOR]

Serrodyne frequency translation of continuous optical signals by ±1.28 GHz is reported, significantly exceeding the highest previously published serrodyne shifts. To achieve these shifts, an ultrawide-band high-power electrical sawtooth created by spectral modulation of dispersed optical pulses was used. Its amplification involved a novel predistortion technique to compensate for the gain and phase ripple in the amplifier bandwidth. [ABSTRACT FROM AUTHOR]

The Bloch mode propagation through the chirped periodical structure is defined by its local dispersion relation. In a slowly varying structure its time delay is the integral of the local inverse group velocity along the propagation direction. The integration can be strongly simplified for linearly chirped structures if the assumption is made that the local dispersion relation is just a scaled and shifted version of the dispersion relation at the input. This assumption leads to exact solutions for the structures with locally uniaxial deformation and provides a good approximation for arbitrary structures with. small chirps. The approach is demonstrated for high index contrast chirped Bragg mirrors and complicated photonic crystal waveguide structures, including coupled waveguides and a slow group velocity waveguide. [ABSTRACT FROM AUTHOR]

Thermooptic coefficients (dnE/dT and dnO/dT) of AgGaS2 have been measured at temperatures of ∼308 and 97 K, using temperature-induced shift in the frequency of the interference fringes in the Fourier transform infrared (FTIR) transmittance spectrum of a 2-mm-thick etalon. Values of dnE/dT and dnO/dT are reported for the spectral regime from 12.4 μm to 1.06 μm; the subscripts E and O refer to extraordinary and ordinary rays, respectively. Our values of dnE/dT and dnO/dT at 308 K are significantly lower (∼1/3) than those reported previously. [ABSTRACT FROM AUTHOR]

It is experimentally shown that both two- and three-photon absorption in a high-molar-concentration chromophore system can be more efficiently utilized to accomplish optical power limiting and stabilization at laser wavelengths of 1.064 and ∼1.3 μm, respectively. The nonlinear absorbing medium is a novel liquid dye system, trans-4-[4-(dihexylamino)styryl]-N-(2-{2-[2-(2-hydroxy-ethoxy)-ethoxy]-ethoxy} -ethyl)-pyridinium p-to- sylate (abbreviated as ASEPT), consisting of chromophore molecules capable of multiphoton absorption in the near IR range. The nonlinear transmission property and output/input characteristics have been studied based on this liquid dye system, using nanoseconds 1.064-μm laser pulses for two-photon excitation and ∼1.3-μm subpicoseconds laser pulses for three-photon excitation. A fairly good optical stabilization capability of this new material has been demonstrated at both laser wavelengths. The relative intensity fluctuation of the laser pulses can be remarkably reduced by simply passing through this multiphoton absorbing medium. [ABSTRACT FROM AUTHOR]

A new analysis leading to an exact and efficient algorithm is presented for calculating directly and without numerical differentiation the mode dispersion characteristics of cylindrical dielectric waveguides of arbitrary refractive-index profile. The new algorithm is based on the equivalent transmission-line (T-L) technique. From Maxwell's equations, we derive an equivalent T-L circuit for a cylindrical dielectric waveguide. Based on the TL-circuit model we derive exact analytic formulas for a recursive algorithm which allows direct calculation of mode delay and dispersion. We demonstrate this technique by calculating the fundamental mode dispersion for step, triangular, and linear chirp optical fiber refractive index profiles. The accuracy of the numerical results is also examined. The proposed algorithm computes dispersion directly from the propagation constant without the need for curve fitting and subsequent successive numerical differentiation. It is exact, rapidly convergent, and it results in savings for both storage memory and computing time. [ABSTRACT FROM AUTHOR]

We report the growth and fabrication of bound-to- bound In0.53 Ga0.47As-InP quantum-well infrared photodetectors using metal-organic vapor phase epitaxy. These detectors have a peak detection wavelength of 8.5 μm. The peak responsivities are extremely large with Rpk = 6.9 A/W at bias voltage Vb = 3.4 V and temperature T = 10 K. These large responsivities arise from large detector gain that was found to be gn = 82 at Vb = 3.8 V from dark current noise measurements at T = 77 K and gp = 18.4 at Vb = 3.4 V from photoresponse data at T = 10 K. The background-limited temperature with F/1.2 optics is TBLIP = 65 K for 0 < Vb ⩽ 3.4 V. The highest value of peak detectivity is DΛ* = 5.4 × 109 cm√ Hz/W at Vb = 2.9 V and T = 77 K, while for T < TEMP = 65 K, the back- ground-limited detectivity, which exhibits negligible bias dependence, is DBLIP* = 3 × 1010 cm√Hz/W. [ABSTRACT FROM AUTHOR]

We present the dispersive characteristics of an electronically tunable optical filter, with application toward delaying high speed digital and analog signals. Square waves can he dynamically delayed and reshaped through a change in injection cur- rent to the filter. Since the filter is of an all-pass nature, its usable bandwidth is limited primarily by the phase response (dispersion). We show that for 2-GHz square waves, dispersion is not a limiting factor and the filter can be operated over the full range of delays. At 5 GHz, the pulses can still be delayed, however the range of operation is dispersion limited. Additionally, at 5 GHz this dispersion can be harnessed for pulse reshaping applications. The filter is studied both experimentally and theoretically, demonstrating good agreement. [ABSTRACT FROM AUTHOR]

A high-repetition-rate ytterbium fiber laser, harmonically mode-locked using a phase modulator, is investigated experimentally, numerically, and analytically. Experimental results agree well with numerical simulations using the measured parameter values. By employing a few approximations, our model is cast in terms of a Ginzberg-Landau equation. This equation has known analytic solutions that agree well with the results of the full model in the appropriate limit. Pulse stability is also investigated numerically with an emphasis on the role of third-order dispersion. [ABSTRACT FROM AUTHOR]

A novel ultracompact 2 × 2 wavelength division multiplexer (WDM) for 1.55-μm operation based on highly dispersive two-mode interference (TMI) was designed, theoretically modeled, and verified using a finite-difference-time-domain (FDTD) method. A two-moded waveguide assisted with a dispersive tooth- shaped grating provided a mode-dependent reflection band of central wavelength at 155 μm. The wavelengths of 1538 and 1572 nm that were at the band edges and had the lowest reflection losses and relatively high dispersion were selected for wavelength multiplexing. The result showed that the wavelengths were separated by grating dispersion in a coupler length of 75 μm which was much shorter than the required length of 1.1 mm in a regular TMI multiplexer of no grating. Insertion loss of about 1.7 dB and channel contrast of about 12 dB were achieved. [ABSTRACT FROM AUTHOR]

We have studied terahertz emission from interdigitated finger photomixers coupled to planar antenna structures. Using both pulsed and continuous-wave excitation, polarization measurements reveal that the antenna design dominates the properties of the radiated output at frequencies below 0.6 THz, while the efficiency at higher frequencies is additionally dependent on the design of the photomixer fingers. we have produced terahertz maps of the device, characterizing the photomixer by measuring the generated power as a function of the excitation position. Together, these measurements have allowed us to understand better the distinct roles of the photomixer and antenna in emission at different frequencies and, hence, independently optimize these components, [ABSTRACT FROM AUTHOR]

In this paper, a Ge-doping square solid-core photonic bandgap (PBG) fiber is proposed and the effective index of defect mode is investigated. Simulations show that, at the starting wave- length of the defect mode, neffd (the effective index of defect mode) changes rapidly due to the PBG effect, which induces large negative dispersion in fiber. This mechanism is different from the mode coupling case in dual-core concentric fibers, and the dispersion characteristic is more robust to structural deviations. [ABSTRACT FROM AUTHOR]

without adaptive optics high-power lasers suffer either from poor beam quality or limited power ranges. A laser can be forced to operate on its stability limit by self-adaptive overcompensation resulting in constant and good beam quality over a large power range. With this technique, the brightness of a Nd:YAG laser was increased by almost an order of magnitude despite highly aber-rated thermal lens in the laser rod. [ABSTRACT FROM AUTHOR]

A new terahertz (THz) photoconductive photomixer/antenna device Is presented. A continuous-wave THz signal is generated in a dc-biased photoconductive film by employing Optical heterodyne scheme, and at the same time, the size of the film on the grounded dielectric substrate is designed to have an efficient broadside radiation. Incorporating the photoconductive film as the photomixing media and the radiation element not only eliminates source to antenna coupling problem but also makes the proposed device attractive for THz array source configurations. Analytical expressions for the photocurrent and the radiation power are derived under high dc bias condition, and the effects of the device configuration on its performance Is studied. Two possible photomixer configurations, namely longitudinal and transversal, are introduced and their photomixing efficiencies and radiation power are compared. The typical nW output power is achievable by mW laser pump power for frequencies up to 10 THz. [ABSTRACT FROM AUTHOR]

Second-harmonic generation (SHG) of ultrashort pulse in both phase- and group-velocity mismatch structures is studied theoretically in temporal and frequency domains taking account of the effects of group-velocity dispersion (GVD) and high-order phase mismatch. Under the plane-wave and pump undepletion approximation, a set of simple analytical expressions is derived in frequency domain for Gaussian pulse. Furthermore, a formula describing the SH pulse is obtained under pump depletion condition. The obtained expressions are valid in large range of interaction length and have features such as explicit form, high accuracy, and less computing time. Meanwhile, the influences of intensity, pulse duration, chirp coefficient and shape of pump pulses, as well as the linear loss of material, on the pulsewidth preservation length are investigated by numerical method. In the case of both phase- and group-velocity mismatch structures, the conversion efficiency larger than 45 % could be obtained without pulse distortion for weak input ultrashort pulse. [ABSTRACT FROM AUTHOR]

Two-dimensional (2-D) photonic crystal (PC) directional couplers (DCs) that have a triangular lattice pattern of air holes in a planar dielectric slab are theoretically and experimentally analyzed. Unlike the 2-D PC DC structure with a dielectric rod in air, which is frequently used in theoretical studies, more practical PC DCs tend to be multimode in nature and exhibit a large group velocity dispersion, thus creating decoupling points in the dispersion relation without any additional modifications to the structure. The multimode nature and large dispersion lead to interference which degrades the coupling properties. By inserting three rows of air holes between neighboring line-defect waveguides in order to separate them, we have successfully reduced the multimode region and obtained a single-mode region. In this case, the large dispersion allows, the creation of a PC DC with wavelength selectivity and a coupling length as short as 30a, ∼ 10 μm for a = 345 nm, where a is the lattice constant. The transmission spectra obtained experimentally showed good agreement with the theory whereas their transmission ranges were restricted to those of bent waveguides. These results are encouraging for practical application to optical communications. [ABSTRACT FROM AUTHOR]

Mode loss or gain can have a significant effect on the operation of waveguide transmission gratings as optical filters. Taking for example a long-period grating (LPG), we present a computer simulation that accounts for these effects. We show that cladding loss or gain result in qualitatively different LPG spectral behavior that can be used as a tuning mechanism in guiding structures where cladding mode properties can be controlled. One possible control mechanism is to have pump-driven gain in a doped cladding, which can perform gradual loss compensation and eventually gain excitement. Based on simulation, results, we propose new approaches to tuneable amplifying filter designs in special fibers, as well as grating assisted co-directional couplers for integrated optics platforms. [ABSTRACT FROM AUTHOR]

We study the various nonlinear effects produced during the detection of high-intensity signals in semiconductor photodiodes, Experimental results on few commercial detectors are compared with theoretical analysis. This analysis is based on the tensor coefficients from second- and third-order optical nonlinearities. Both contributions are measured using collinear orthogonally polarized beams in a pump-probe setup. The photoelectric signal is obtained for several orientations of the crystal optic axis. [ABSTRACT FROM AUTHOR]

We present a detailed theoretical study of a vertical- cavity surface-emitting laser (VCSEL) with two optically coupled, active cavities. The study is based on a rate-equation model written for carriers and photons under steady-state conditions. The model allows one to determine all the relevant parameters-carrier densities, gains, and output powers-starting from two input parameters: the injection currents in each cavity. The system of equations is solved for different operating regimes of the device and the results provided by the model are shown to be in very good qualitative and quantitative agreement with the experimental data. [ABSTRACT FROM AUTHOR]

A tunable highpower cladding-pumped neodymium- doped aluminosilicate fiber laser is demonstrated. The maximum power reached was 2.4 W with a slope efficiency of 41 % and a threshold pump power of 1.68 W, both with respect to launched pump power, when cladding pumped by two 808-nm diode pump sources at both fiber ends. The dependence of the tuning range on the fiber length is investigated. The tuning range changed from 922 to 942 nm for a 25-m-long fiber to 908-938 nm with a 14-m-long fiber, because of reabsorption effects. The output llnewidth was 0.26 nm in a diffraction-limited beam. Operation on the challenging 0.9-μm three-level transition in neodymium-doped double-dad fiber laser was facilitated by a W-type core refractive index profile. This filtered out the unwanted and competing strong transition at 1.06 μm while guidance of 0.9 μm remained intact. [ABSTRACT FROM AUTHOR]

A time domain algorithm for fiber Bragg grating simulation is formulated. It provides a time-domain equivalent to the well-known spectral-domain transfer matrix algorithm for fiber Bragg grating simulation, and allows much more accurate impulse response to be simulated directly from a known fiber Bragg grating. Two different time-domain grating reconstruction algorithms are formulated, which are much more accurate and efficient than their spectral-domain equivalents due to their intrinsic infinite spectral representation and truly one-dimensional nature. Error studies are also conducted for the time-domain grating reconstruction algorithms. The high accuracy and efficiency of the new time-domain algorithms are well demonstrated with several simulation, reconstruction, and design examples. [ABSTRACT FROM AUTHOR]

A continuous-wave optical parametric oscillator (OPO) was phase locked to an optical frequency comb in the 830-nm region. The optical frequency of the OPO was controlled by changing the cavity length of the pump laser. The residual phase noise under phase locking was 220 mradrms. and the energy concentration to the carrier was 95 %. Furthermore, the optical frequency fluctuations of a free-running OPO were measured by using an optical frequency comb that was phase locked to an atomic clock. The measured fluctuations were around 10 MHz in an hour. [ABSTRACT FROM AUTHOR]

Radiation scattered from diffraction ratings on the surface of waveguides is analyzed using the volume current method. This technique allows the interpretation of the grating elements as scattering centers of the underlying waveguide mode, and radiating sources for the diffracted fields. The framework allows separation of the effects o! the grating array global periodicity and the effects of the specific shape of the individual grating elements. A straightforward analogy between the effects of the grating element shape and the behavior of phased-antenna array systems allows a clear and intuitive understanding of the effects of blazed gratings on the directionality of grating-coupled radiation. The analysis is applied to some specific geometries. [ABSTRACT FROM AUTHOR]

We develop a vector theory of cross-phase modulation (XPM) capable of describing nonlinear coupling between two pulses of different wavelengths and arbitrary states of polarization. We focus for simplicity on the pump-probe configuration and use it to investigate the temporal and spectral polarization effects occurring inside an optical fiber. Using the Stokes-vector formalism we show that the probe polarization changes in general through XPM-induced nonlinear polarization rotation. In the absence of dispersion-induced probe broadening, such nonlinear changes in the probe polarization do not affect the temporal shape of the probe pulse but produce a multipeak spectrum whose different spectral peaks have different states of polarization. When dispersive effects are included, even the shape of the probe pulse becomes polarization dependent, and different parts of the pulse develop different states of polarization. Such nonlinear polarization effects lead to novel phenomena such as polarization-dependent compression and splitting of the probe pulse. [ABSTRACT FROM AUTHOR]