Showing papers in "IEEE Journal of Quantum Electronics in 1994"

Space-time duality and the theory of temporal imaging

Abstract: There exists a beautiful duality between the equations that describe the paraxial diffraction of beams confined in space and the dispersion of narrow-band pulses in dielectrics. We will see that this duality leads naturally to the conclusion that a quadratic phase modulation in time is the analog of a thin lens in space. Therefore, by a suitable combination of dispersion and quadratic phase modulation (now a "time lens"), we can synthesize the time-domain analog of an imaging system. Such a temporal-imaging system can magnify time waveforms in the same manner as conventional spatial-imaging systems magnify scenes. We analyze this space-time duality and derive expressions for the focal length and f-number of a time lens. In addition, the principles of temporal imaging are developed and we derive time-domain analogs for the imaging condition, magnification ratio, and impulse response of a temporal-imaging system. >

High-performance uncooled 1.3-/spl mu/m Al/sub x/Ga/sub y/In/sub 1-x-y/As/InP strained-layer quantum-well lasers for subscriber loop applications

Abstract: Design considerations for fabricating highly efficient uncooled semiconductor lasers are discussed. The parameters investigated include the temperature characteristics of threshold current, quantum efficiency, and modulation speed. To prevent carrier overflow under high-temperature operation, the electron confinement energy is increased by using the Al/sub x/Ga/sub y/In/sub 1-x-y/As/InP material system instead of the conventional Ga/sub x/In/sub 1-x/As/sub y/P/sub 1-y//InP material system. To reduce the transparency current and the carrier-density-dependent loss due to the intervalence-band absorption, strained-layer quantum wells are chosen as the active layer. Experimentally, 1.3-/spl mu/m compressive-strained five-quantum-well lasers and tensile-strained three-quantum-well lasers were fabricated using a 3-/spl mu/m wide ridge-waveguide laser structure. For both types of lasers, the intrinsic material parameters are found to be similar in magnitude and in temperature dependence if they are normalized to each well. Specifically, the compressive-strained five-quantum-well lasers show excellent extrinsic temperature characteristics, such as small drop of 0.3 dB in differential quantum efficiency when the heat sink temperature changes from 25 to 100/spl deg/C, and a large small-signal modulation bandwidth of 8.6 GHz at 85/spl deg/C. The maximum 3 dB modulation bandwidth was measured to be 19.6 GHz for compressive-strained lasers and 17 GHz for tensile-strained-lasers by an optical modulation technique. The strong carrier confinement also results in a small k-factor (0.25 ns) which indicates the potential for high-speed modulation up to 35 GHz. In spite of the aluminum-containing active layer, no catastrophic optical damage was observed at room temperature up to 218 mW for compressive-strained five-quantum-well lasers and 103 mW for tensile-strained three-quantum-well lasers. For operating the compressive-strained five-quantum-well lasers at 85/spl deg/C with more than 5 mW output power, a mean-time-to-failure (MTTF) of 9.4 years is projected from a preliminary life test. These lasers are highly attractive for uncooled, potentially low-cost applications in the subscriber loop. >

Wave mixing in semiconductor laser amplifiers due to carrier heating and spectral-hole burning

Abstract: Theoretical results on wave mixing in traveling-wave semiconductor laser amplifiers are presented. Starting from rather general density-matrix equations, including the effects of carrier-density pulsations, carrier heating, and spectral-hole burning, we derive equations capable of treating the range from nearly degenerate to highly nondegenerate four-wave mixing. The equations are solved numerically, and comparison to published experimental results is made. The theoretical results explain different experimental results, which have been taken to support either the role of spectral-hole burning or that of carrier heating in mediating wave mixing under highly nondegenerate conditions. >

Thermal beam distortions in end-pumped Nd:YAG, Nd:GSGG, and Nd:YLF rods

Abstract: The thermally induced beam distortions in end-pumped Nd:YAG, Nd:GSGG, and Nd:YLF rods were analyzed and the influence of edge- and face-cooling was investigated. The distributions of temperature, stress, and strain in the crystals were calculated by finite element analysis. Based on these data, the space-resolved changes of the refractive index were determined considering thermal dispersion, surface deformation, and strain-induced birefringence. The resulting optical path difference for one round-trip in the end-pumped rods was integrated numerically. For each rod, the induced thermal lens was determined over the extent of the pump spot radius. The calculations of the optical path difference were experimentally confirmed by investigations using a modified Twyman-Green interferometer with a polarized HeNe probe beam at 633 nm under lasing and nonlasing conditions. >

Additive-pulse modelocking in fiber lasers

Abstract: A self-starting additive-pulse modelocked (APM) all-fiber ring cavity laser operating in the negative group velocity regime is analyzed. The advantages of using a ring cavity with an isolator are discussed. The "figure eight" and the Sagnac loop reflector fiber laser are investigated in a novel way, and one form of their operation is related to the master equation of APM. We point out that the ring laser has an advantage with respect to self-starting. >

Experimental study of sideband generation in femtosecond fiber lasers

Abstract: We present a thorough experimental investigation of the phenomenon of soliton resonance sideband generation in femtosecond fiber lasers. The dependence of the sideband wavelengths on the dispersion and pulse length is confirmed. Third order dispersion is found to play a significant role in determining the sideband spectrum. We show that the minimum pulse length obtained in a fiber laser is determined by the cavity dispersion and relate this to loss into the sidebands. We show how the sideband spectrum can be used as a diagnostic of the fiber parameters and of the formation of ultrashort pulses in the laser. >

Ultrabroadband femtosecond lasers

Abstract: The exploitation of soliton-like pulse shaping mechanisms and the optimization of phase dispersion in broadband self-modelocked solid state laser oscillators have led to unprecedented advances in ultrafast laser technology. This paper reviews the basic physical mechanisms governing pulse formation and addresses the requirements for optimum performance of these novel ultrashort pulse laser sources. These considerations together with the demonstration of /spl ap/10 fs pulse generation from a Ti:sapphire laser suggest that a family of solid-state laser oscillators with comparable performance can be developed. >

Analytical model for rare-earth-doped fiber amplifiers and lasers

Abstract: An analytical model for two-, three-, and four-level system rare-earth-doped fiber amplifiers and lasers is presented. The theory is applicable to dopants such as erbium, neodymium, thulium; praseodymium, and ytterbium. Fiber-amplifier gain is expressed in terms of attenuation coefficients, intrinsic saturation powers, and cross-saturation powers at the pump and signal wavelengths. These parameters can be directly determined from one- and two-beam fiber-transmission measurements. System-independent formulas are given for the slopes and thresholds of ring and linear fiber lasers. Good agreement between theory and experiment has been shown for erbium-doped fiber amplifiers and lasers and thulium-doped fiber lasers. Because of the finite-pump-level lifetime, three- and four-level models predict a flattening of the fiber laser slope at higher pumping powers when the fiber is shorter than the optimum length. Approximate system-independent solutions are also given for fiber amplifiers with excited-state absorption at either the pump or signal wavelengths. A novel technique, requiring only one tunable light source, is proposed for finding the best pump wavelength when pump ESA is present. The two-level analytical model recently developed for erbium-doped fibers is a special case of this theory. >

Band-structure engineering in strained semiconductor lasers

Abstract: The influence of both compressive and tensile strain on semiconductor lasers and optical amplifiers is reevaluated in the light of recent experimental and theoretical work. Strain reduces the three-dimensional symmetry of the lattice and helps match the wave functions of the holes to the one-dimensional symmetry of the laser beam. It can also decrease the density of states at the valence band maximum and so reduce the carrier density required to reach threshold. These two effects appear to adequately explain the TE and TM gain in compressive and tensile structures, including polarization-independent amplifiers, the behavior of visible lasers and the improved frequency characteristics of InGaAs/GaAs lasers. In 1.5 /spl mu/m InGaAsP/InP lasers phonon-assisted Auger recombination appears to remain the dominant current path and can explain why the temperature sensitivity parameter to remains >

Dynamic analysis of radiation and side-mode suppression in a second-order DFB laser using time-domain large-signal traveling wave model

Abstract: In this paper, we have developed a relatively simple algorithm to calculate the large-signal dynamic response of DFB lasers by solving the time-dependent coupled wave equations directly in the time domain. The spontaneous emission noise, longitudinal variations of carrier (hole burning) and photon densities as well as that of the refractive index are taken into consideration. To demonstrate the power of this straightforward algorithm, the model shows how the side-mode suppression ratio in devices with high /spl kappa/L and a /spl lambda4: phase shift is significantly affected by the radiation in the second-order DFB laser. The time-dependent radiation pattern in grating-coupled surface-emitting lasers is also calculated for the first time. >

Progress in long-wavelength strained-layer InGaAs(P) quantum-well semiconductor lasers and amplifiers

Abstract: The progress in long-wavelength compressively and tensile-strained InGaAs(P) quantum-well semiconductor lasers and amplifiers is reviewed. By the application of grown-in strain, the device performance is considerably improved such that conventional bulk and unstrained quantum-well active-layer devices are outperformed, while a high reliability is maintained. >

Coupled quantum well semiconductors with giant electric field tunable nonlinear optical properties in the infrared

Abstract: Coupled quantum wells present unique opportunities for engineering new semiconductors with large optical nonlinearities associated with intersubband transitions in the infrared. In this paper we report an in depth study of these properties in the AlInAs/GaInAs heterostructure material system grown by molecular beam epitaxy. We show that by judicious control of the tunnel coupling between wells and of the thickness of the latter one can design the wavefunctions and the energy levels in such a may that these new structures behave as quasi-molecules with extremely large dipole matrix elements and strongly field tunable nonlinear optical properties. Structures with giant nonlinear susceptibilities /spl chi//sup (2)/(2/spl omega/) and /spl chi//sup (3)/(3/spl omega/) (compared to the bulk constituents of the quantum wells) have been designed and demonstrated. They exhibit large linear Stark shifts of the intersubband transitions which have been used to efficiently tune the nonlinear susceptibilities. The second order nonlinear susceptibility |/spl chi//sup (2)/(2/spl omega/)| exhibits a peak as a function of the electric field corresponding to the energy levels being made equally spaced via the Stark effect. In a three-coupled-well structure triply resonant third harmonic generation has been observed. This process is associated with four equally spaced bound states. The corresponding |/spl chi//sub (3/spl omega/)//sup (3)/| (10/sup -14/ (m/V)/sup 2/ at 300 K and 4/spl times/10/sup -14/ (m/V)/sup 2/ at 30 K) is the highest measured third order noniinear susceptibility in any material. The equivalent of multiphoton ionization of a molecule has also been investigated in this structure. Electrons are photoexcited to a continuum resonance above the barrier via a three-photon transition enhanced by intermediate energy levels. The effect of this resonance on /spl chi//sup (3)/(3/spl omega/) as the electric field is varied is also investigated. Finally, appropriate figures of merit are discussed. >

Optimization of the carrier-induced effective index change in InGaAsP waveguides-application to tunable Bragg filters

Abstract: A model for the refractive index of InGaAsP and its dependence on the carrier density is presented. Full transport calculations for electrons and holes in heterostructures, including temperature effects, are used to show that the maximum injected carrier density is limited by electron heterojunction leakage for low-cladding doping and by recombination in the tuning layer for high-cladding doping. With the effective index method, we can then compute the maximum propagation constant change as a function of the waveguide geometry. Thermal effects are important only in the case of poor heatsinking. Using these results, we can estimate a maximum tuning range of more than 15 nm for optimized Bragg reflection gratings. >

Four-wave mixing and optical modulation in a semiconductor laser

Abstract: There is a direct connection between nearly degenerate four-wave mixing in a semiconductor laser and optical modulation in the laser field. It can be understood using a model of an unlocked, optically injected laser, which emphasizes the effect of the laser resonator on the optical interactions. This model correctly describes the observed spectral characteristics and their dependence on the intrinsic parameters of the semiconductor laser. This is used to develop a simple and accurate technique using a single experimental setup for the parasitic-free characterization of the intrinsic laser parameters, including the relaxation resonance frequency, the total relaxation rate, the nonlinear relaxation rate, and the linewidth enhancement factor. Other parameters, such as the spontaneous carrier lifetime, the photon lifetime, the differential and nonlinear gain parameters, and the K factor, are determined from the power dependencies of these parameters. This technique requires only two CW lasers closely matched in wavelength and is applicable to semiconductor lasers of any wavelength and any dynamic bandwidth. >

Design of a single-mode tapered waveguide for low-loss chip-to-fiber coupling

Abstract: The novel waveguide structures described in this paper have nonlinearly tapered shapes that result in low radiation losses despite their relatively short lengths. The core at the waveguide endface connected with the fiber has a very small cross section and an expanded mode field with a non-Gaussian shape. The taper structures are analyzed by using an improved step-transition method. This method is a based on the theory of enclosing a waveguide within electrical walls and that can therefore treat the radiation modes in a tapered waveguide as discrete mode spectra. Analyzing the relationships between the lengths and shapes of the tapers and the radiation loss due to the tapers show that appropriately tapered semiconductor waveguides operating at an optical wavelength of 1.55 /spl mu/m and having a taper length of less than 0.7 mm can have a radiation loss of only 0.1 dB and a coupling loss with a conventional single-mode fiber of less than 0.5 dB. >

Chaos and locking in a semiconductor laser due to external injection

Abstract: We have analyzed the behavior of a semiconductor laser subjected to increasing external injection. Numerical simulations show the well-known nonlinear modulation and locking regimes, followed by an intermediate chaotic region that precedes definitive locking to the external source at significantly higher injection levels. >

Laser, optical, and thermomechanical properties of Yb-doped fluorapatite

Abstract: The laser performance of Yb-doped fluorapatite (Ca/sub 5/(PO/sub 4/)/sub 3/F or FAP), is assessed by employing a Ti:sapphire laser operating at 905 nm as the pump source. We have measured slope efficiencies to be as high as 79%; the residual decrement from the quantum defect-limited efficiency of 87% is accounted for by the presence of passive loss at the 1043-nm laser wavelength. The important spectral properties of Yb:FAP were evaluated, including the absorption and emission cross sections, excited-state lifetime, and ground-state energy-level splitting. The emission and absorption cross sections of Yb/sup +3/ in FAP are found to be substantially larger than those of other Yb-doped media. The thermal, physical, and optical properties of the FAP host are reported as well. >

Temperature-tuned 90/spl deg/ phase-matching properties of LiB/sub 3/O/sub 5/

Abstract: A single LiB/sub 3/O/sub 5/ crystal cut normal to x(=a) and z(=b) can be used for the 90/spl deg/ phase-matched SHG at 0.475-0.875 /spl mu/m by heating the crystal from /spl sim/20 to 320/spl deg/C. The improved Sellmeier's equations and new formula of the temperature variation of the refractive indexes are presented. >

Traveling wave analysis of semiconductor lasers: modulation responses, mode stability and quantum mechanical treatment of noise spectra

Abstract: We present a traveling wave analysis of a general class of semiconductor lasers, which includes multisection DFB/DBR lasers and gain-coupled DFB lasers. The analysis leads to new semianalytic expressions for the small-signal IM and FM modulation responses, the intensity and FM noise spectra, and the linewidth. The expressions are given in terms of solutions to four coupled linear homogeneous differential equations and can easily be evaluated numerically. We also derive a stability parameter /spl sigma/, for which /spl sigma/ >

Optical-feedback-induced chaos and its control in multimode semiconductor lasers

Abstract: The effects of optical feedback in multilongitudinal mode semiconductor lasers are studied through computer simulations. Two separate regimes are found based on the length of the external cavity. For long external cavities (external-cavity mode spacing larger than the relaxation-oscillation frequency), the laser follows a quasi-periodic route to chaos as feedback is increased. For short external cavities, the laser can undergo both quasi-periodic and period doubling routes to chaos. When the laser output becomes chaotic, the relative-intensity noise is greatly increased (by more than 20 dB) from its solitary-laser value. Considerable attention is paid to the effects of optical feedback on the longitudinal-mode spectrum. The stabilization of the mode spectrum and the reduction of the feedback-induced noise through current modulation are studied and compared with experimental results. Current modulation eliminates feedback-induced chaos when the modulation frequency and depth are suitably optimized. This technique of chaos control has applications in optical data recording. >

Band lineup and in-plane effective mass of InGaAsP or InGaAlAs on InP strained-layer quantum well

Abstract: We describe the band lineups of InGaAlAs on (001) InP as well as InGaAsP on (001) InP system with strain effects, based on the Harrison model. We show that the compressive strain does not affect the band position so much, and tensile strain raises the band position in the InGaAsP system. It is also shown that both compressive and tensile strains raise the band positions in the InGaAlAs system. The conduction and valence band positions of InGaAs, InGaAsP, and InGaAlAs relative to InP valence band are given in approximate formulas as a function of the strain. We calculate the energy versus in-plane wave vector relationship of the InGaAsP/InGaAs(P) InGaAlAs/InGa(Al)As on InP strained quantum-well systems. We obtain the in-plane effective mass of the strained quantum-well system by fitting the dispersion relationship to a parabolic curve. The in-plane effective masses of several kinds of strained quantum-well systems are listed. >

Strained Ga/sub x/In/sub 1-x/P/(AlGa)/sub 0.5/In/sub 0.5/P heterostructures and quantum-well laser diodes

Abstract: The properties of (AlGa)/sub 0.5/In/sub 0.5/P, strained Ga/sub x/In/sub 1-x/P/(AlGa)/sub 0.5/In/sub 0.5/P heterostructures, and single quantum well (QW) laser diodes with Al/sub 0.5/In/sub 0.5/P cladding layers, prepared by low pressure organometallic vapor phase epitaxy, are described. The influence of biaxial strain upon the relative positions of the valence band edges are examined by analyzing the polarized spontaneous emission. Laser diodes with wavelength 620 >

Temporal aberrations due to misalignments of a stretcher-compressor system and compensation

Abstract: Sensitivity of alignment is one of the major issues in the stretcher-compressor devices used for chirped pulse amplification. In this paper we discuss the temporal aberrations induced by the different kinds of misalignments of the gratings and we emphasize the importance of spatially transverse effects. We discuss simple criteria for minimizing these faults. Finally, we introduce the basic principles of self-compensation and present such a set-up based on symmetry considerations. >

High-frequency oscillations and self-mode locking in short external-cavity laser diodes

Abstract: An effect of compound-cavity mode competition for a laser diode with a short external cavity is investigated analytically and numerically on the basis of a round-trip approximation for emitted photons in the external cavity. Due to the noncoincidence of the compound-cavity modes with possible frequencies of stationary lasing, such competition may cause self-locking of two compound-cavity modes with deep high-frequency oscillations of the laser output for a short (typically /spl les/5 mm) external cavity. Simple analytical expressions for critical feedback parameters and the frequencies of self-oscillations are given, as well as a generalized diagram of different dynamic regimes of a short external-cavity laser, which shows these regimes of high-frequency self-oscillations as well as coherence collapse instability. The results may be considered a guideline for the design of high-speed laser diodes with an integrated passive cavity. In particular, such a device may be a very efficient pulse source in the microwave (>20 GHz) region. >

Theoretical modeling of optical amplification in Er-doped Ti:LiNbO/sub 3/ waveguides

Abstract: An accurate theoretical analysis is presented describing optical amplification in Er-diffused Ti:LiNbO/sub 3/ channel waveguides. It follows as far as possible the theory already developed for Er-doped fibers. As optical pumping around /spl lambda//sub p//spl ap/1.48 /spl mu/m is considered, a quasi-two-level model for the Er/sup 3+/ ions is used with wavelength-dependent cross sections. The optical gain in the 1.53 /spl mu/m >

Degradation behavior of 0.98-/spl mu/m strained quantum well InGaAs/AlGaAs lasers under high-power operation

Abstract: The degradation behavior of 0.98-/spl mu/m strained quantum well (QW) InGaAs/AlGaAs lasers with facet coating films is systematically clarified at high-power operations of more than 100 mW/facet. The degradation is mainly caused by the instability of the interface between the laser material (facet) and the antireflecting (AR) coating film. This phenomenon is associated with a high rate of facet oxidation and generation of instantaneous catastrophic optical damage (COD) at a relatively low optical output power in lasers without facet coating films. Throughout the clarification of those phenomena, the main reliability problem in 0.98-/spl mu/m strained QW lasers under high output power operation is clarified. >

Measurement of the anisotropy of two-photon absorption coefficients in zincblende semiconductors

Abstract: The imaginary parts of all of the independent two-photon-resonant susceptibility tensor elements in GaAs and CdTe are determined by using a two-beam coupling technique to measure the anisotropy of the two-photon absorption coefficient p as a function of crystal orientation and probe polarization. Anisotropy parameters of -0.76 and -0.46 are measured for GaAs and CdTe, respectively, at a wavelength of 950 nm. These correspond to a 45% variation in /spl beta/ for GaAs, between 19 and 30 cm/GW, for radiation polarized along the [001] and the [111] crystallographic axes, respectively, and a 25% variation between 14 and 18 cm/GW for CdTe. By invoking intrinsic and zincblende symmetry, we present macroscopic expressions that accurately account for the dependence of single-beam two-photon absorption on the orientation of the crystal with respect to the polarization of the light and also expressions that describe the two-photon absorption of a probe when it is polarized either perpendicular or parallel to the pump in degenerate-four-wave-mixing experiments. Finally, we discuss the microscopic origins of this anisotropy of two-photon absorption in terms of simple k/spl middot/p models of the band structure, and we find the anisotropy to be caused predominantly by the mixing of the valence band with a higher conduction hand. This simple theory produces magnitudes consistent with experimental results and predicts that the anisotropy scales linearly with the ratio of the lower bandgap to the higher bandgap: E/sub g//E'/sub g/. >

A self-consistent model for the discharge kinetics in a high-repetition-rate copper-vapor laser

Abstract: A self-consistent computer model has been developed to simulate the discharge kinetics and lasing characteristics of a copper-vapor laser (CVL) for typical operating conditions. Using a detailed rate-equation analysis, the model calculates the spatio-temporal evolution of the population densities of 11 atomic and ionic copper levels, four neon levels, and includes 70 collisional and radiative processes, in addition to radial particle transport. The long-term evolution of the plasma is taken into account by integrating the set of coupled rate equations describing the discharge and electrical circuit through multiple excitation-afterglow cycles. A time-dependent two-electron group model, based on a bi-Maxwellian electron energy distribution function, has been used to evaluate the energy partitioning between the copper vapor and the neon-buffer gas. The behavior of the plasma in the cooler end regions of the discharge tube near the electrodes, where the plasma kinetics are dominated by the buffer gas, has also been modeled. Results from the model have been compared to experimental data for a narrow-bore (/spl phi/=1.8 cm) CVL operating under optimum conditions. Close agreement is obtained between the results from the model and experimental data when comparing electrical I-V characteristics of the discharge tube and circuit, and spatio-temporal evolution of the population densities of the laser levels and other excited Cu I and Ne I states, and lasing characteristics. During the period of lasing action, the populations of the laser levels are perturbed by 10-20 percent due to stimulated emission. >

Noncritical quasi-phase-matched second harmonic generation in an annealed proton-exchanged LiNbO/sub 3/ waveguide

Abstract: We report the demonstration of dimensional noncritical phase matching, a phase-matched interaction length exceeding 10 mm, and an internal conversion efficiency of 204%/W for second harmonic generation of 976 nm radiation in a periodically poled, annealed proton-exchanged LiNbO/sub 3/ waveguide. Using models for the linear and nonlinear optical properties of annealed proton-exchanged LiNbO/sub 3/ waveguides and the observed ferroelectric domain grating, the phase-matching wavelength was predicted to within several nm and the conversion efficiency to within /spl ap/20% of the measured values. Optimization of waveguide second harmonic generation devices is discussed. >

Spectroscopic properties and efficient diode-pumped laser operation of neodymium-doped lanthanum scandium borate

Abstract: We report on the spectroscopic properties and laser performance of Czochralski-grown LaSc/sub 3/(BO/sub 3/)/sub 4/ (LSB) crystals with high neodymium concentrations up to 2.5/spl middot/10/sup 21/ cm/sup -3/. The low-concentration quenching of the upper laser level of neodymium and the polarization dependence of the spectra indicate that LaSc/sub 3/(BO/sub 3/)/sub 4/ crystalizes in a huntite-type structure like Nd/sup 3+/:YAl/sub 3/(BO/sub 3/)/sub 4/ (NYAB). With diode laser pumping at 808 nm, a multimode Nd/sup 3+/:LaSc/sub 3/(BO/sub 3/)/sub 4/ (NLSB) laser at 1063 nm is demonstrated. Optical slope efficiency was 64% with respect to absorbed pump power. The possibility of second-harmonic generation in the laser crystal is discussed. >