Channeled substrate nonplanar laser analysis--Part II: Lasers with tapered active regions

Light versus current (LversusI) characteristics are calculated for double-heterostructure diode lasers whose active regions decrease in thickness laterally from a maximum on axis. This variation produces lateral real refractive index waveguiding which in turn stabilizes the spatial mode such that the modal field becomes anastigmatic and theLversusIplot becomes linear. In addition to determining threshold current and differential quantum efficiency, we compute the TE00mode patterns, active region charge density distribution, and the power levelP*_{1}at which spatial hole burning causes the TE01mode to begin lasing. The maximum power density at the facet for that power levelP*_{1}is also obtained. All these characteristics are presented as functions of the various device parameters including carrier spontaneous recombination time, diffusion length, optical gain, unpumped band-to-band absorption, internal losses, antiguidance index, wavelength, cladding Al content, active region dimensions, current spreading resistance, facet reflectivity, laser length, and stripe width. Utilizing this information, a design is developed for a laser with low threshold current (40-50 mA) and high differential quantum efficiency (50-65 percent) that operates stable single lowest order (TE00) spatial mode to powers well in excess of 50 mW.     

Carrier density dependence of refractive index in AlGaAs semiconductor lasers

Carrier density dependence of the refractive index in the active layers of semiconductor lasers is evaluated from the wavelength shift with increases in current by taking into account effects of the active layer temperature rise and lateral carrier and optical field distributions on the wavelength shift. The derived refractive index change due to carrier density increase is-4 times 10^{-27}m³, which is in good agreement with the theoretical value.     

Performances of a frequency offset locked He-Xe laser system at 3.51 µm

A highly stabilized frequency offset locked He-Xe laser system was constructed for high resolution laser spectroscopy of H2CO [5_{1,5}(upsilon = 0) rightarrow 6_{0,6}(upsilon_{5}= 1)] at 3.51μm. It is composed of three He-Xe lasers. The first laser is H2CO-stabilized and is used as a frequency reference in the system. The second laser is frequency offset locked to the first laser by using the beat frequency between these lasers, and is used as a local oscillator. The third laser is frequency offset locked to the second laser, and is used to observe the H2CO spectrum by slowly varying the beat frequency between these lasers. The frequency stability of the first laser, measured against a similarly stabilized and synchronously modulated laser, was1.0times10^{-14}attau = 100s, where τ represents the integration time. The frequency traceability of the second laser to the first laser was expressed as8.0times10^{-13} cdot tau^{-1}for 10 msleq tau leq 100s. It was found that this value of the traceability was independent of the frequency modulation of the first and second lasers. The frequency traceability of the third laser to the second laser was nearly equal to that of the second laser described previously. The variable range of the frequency of the third laser was 19 MHz. In this range, the frequency traceability of the third laser to the second laser was independent of the beat frequency between these two lasers. From these results, it was concluded that this system can be used for the observation of the H2CO spectrum.     

Precision Threshold Current Measurement for Semiconductor Lasers Based on Relaxation Oscillation Frequency

The soft turn-on of semiconductor lasers leads to uncertainty in defining and measuring the laser threshold injection current, ${I}_{rm th}$. Previously, practical calculation algorithms have been developed to achieve high-accuracy measurement of a clearly defined and reproducible quantity which is called ${I}_{rm th}$. We demonstrate a new and higher accuracy measurement of ${I}_{rm th}$ using the dependency of the relaxation oscillation frequency on injection current, as compared to the existing standardized approaches. Further, if it is accepted that relaxation oscillations do not occur below laser threshold, this may be regarded as a more fundamentally based definition and measurement method to determine the laser threshold injection current in a semiconductor laser. The method may also be applicable to other types of lasers.      

Stabilization of single frequency operation of coupled-cavity lasers

We present a theoretical analysis of cleaved coupled-cavity lasers. Mode selectivity arises from two mechanisms. The first is diffraction loss in the gap between the cleaved sections. The second is based on one section acting as a resonant reflector. Our analysis includes the change of refractive index with carrier density which shifts the cavity resonances and causes mode switching. Above threshold the gains g1and g2of the two cavities are not pinned, but are related for each mode in the form of a curve in the (g_{1}, g_{2}) plane. The separation of mode curves along a 45° line determines the ratio of mode intensities. Single mode operation above threshold is described by zones in the (C_{1}, C_{2}) plane where C1and C2are the currents driving the two cavities. Cases of stable, unstable, and bistable operation are discussed.     

Tunnel generation and locking stimulated Raman radiation components

Stimulated Raman radiation is considered in a ring optical resonator. It is supposed that the resonator is partly filled with a condensed medium at the full length of the medium close to several tunnel lengths. A Raman-active medium and an optical waveguide are assumed to occupy a part of the condensed medium. The feasibility is shown of tunnel generation and synchronization of a great number of Stokes components of stimulated Raman radiation. Here, femtosecond light pulses are shown to be formed in a spacing adjacent to the Raman-active medium, the pulses being single over the periodT = 2pi/omega_{R}(ωRis the Raman frequency shift). Predicted and considered also are tunnel generation and locking of new light modes in an optical parametric oscillator. The effects are due to a correction for the dispersion of the full effective refractive index of the medium inside the resonator. Numerical calculations of the correction for a glass thin-film optical waveguide, quartz, and the Raman-active media CaCO3, C6H6, CS2inside the resonator are represented. Here, five or six Stokes components are shown to be generated and locked for CaCO3and C6H6, eight Stokes components for CS2. A similar numerical calculation is represented for the active medium LiNbO3in the corresponding optical parameteric oscillator. Here, new modes covering a frequency bandwidth of 5000 cm^-1are shown to be feasible. Noted and discussed is also a higher-order correction for the effective refractive index dispersion making it feasible to broaden the frequency bandwidth for tunnel generation and locking.     

The photoionization cross sections of the Rg*2[3Σ+u] excimer states for Ne*2, Ar*2, and Kr*2

Based on the recently determined Rydberg series of the³Sigma+_{u}excimer states of Ne*2, Ar*2and Kr*2, the photoionization cross sections of these molecules are calculated using a single-channel quantum defect method. These cross sections are found to differ considerably from those of the asymptotic metastable atomic Rg*(ns^{3}P_{2}) states, but are in good agreement with recently reported experiments at isolated wavelengths. The implications of these results for VUV and XUV lasers are discussed.     

Effective refractive index and first-order-mode cutoff conditions in InGaAsP/InP DH laser structures (λ = 1.2-1.6 µm)

The virtual wavelength-invariance of the ratio between bandgap wavelength and the square root of the dielectric-constant step in InGaAsP/InP DH structures [i.e., (lambda_{g}/sqrt{Deltaepsilon}) = 0.95 pm 0.03for 1.2 μm< lambda_{g} < 1.6 mum] and an analytical approximation for the transverse propagation constantb, allow the derivation of an accurate, closed-form expression for the effective refractive index Neffof InGaAsP/InP planar DH lasers emitting in the1.2-1.6 mum range. Then, Neffis only a function of two readily measurable parameters: emission wavelength and active-layer thickness. Furthermore, the mode cutoff conditions for various lateral waveguides: buried-rectangular, buried-crescent, and ridge-guide, become wavelength-independent analytical expressions. First-order-mode cutoff conditions for these lateral waveguides are derived, plotted and compared to experimental data from mode-stabilized 1.3 and 1.55 μm DH lasers.     

Femtosecond time domain measurements of group velocity dispersionin diode lasers at 1.5 μm

The authors have measured the group velocity dispersion of bulk V-groove semiconductor lasers and multiple quantum well lasers operating at a wavelength near 1.5 μm. The data yield group velocity dispersions in the range from -0.63 to -0.95 μm^-1 and indicate that material dispersion is the dominant factor in the diodes. Cross-correlation traces of transmitted femtosecond pulses confirm the measured values of dispersion     

Lateral analysis of quasi-index-guided injection lasers: Transition from gain to index guiding

In double-heterostructure stripe-geometry semiconductor lasers an effective lateral index stepDelta n_{L}over the stripe region can be induced through evanescent-field coupling. Such a quasi-index-guided device exhibits a transition from the gain-guided to the index-guided regime whenDelta n_{L}is progressively increased. Using parameters appropriate to a 1.3-μm InGaAsP laser, the transition is shown to occur aroundDelta n_{L} sim 5 times 10^{-3}. The exact value ofDelta n_{L}depends on the extent of carrier-induced antiguiding. In the transition region the threshold current decreases rapidly, the lateral mode contracts, and the far field changes from a twin-lobe to a single-lobe pattern. Our analysis suggests that a quasi-index-guided device operates most efficiently for values ofDelta n_{L}at which the index-guided regime is just approached. With a further increase ofDelta n_{L}, the mismatch between the gain and mode profiles leads to lower differential quantum efficiencies. Among other structures, the analysis is applicable to a ridge waveguide laser. For a 1.3-μm laser the optimumDelta n_{L}can be obtained using 0.2-μm-thick cladding layers for a 0.2-μm thick active layer.     

Minimal group refractive index dispersion and gain evolution in ultra-broad-band quantum cascade lasers

The group refractive index dispersion in ultra-broad-band quantum cascade (QC) lasers has been determined using Fabry-Perot spectra obtained by operating the lasers in continuous wave mode below threshold. In the wavelength range of 5-8 /spl mu/m, the global change of the group refractive index is as small as +8.2 /spl times/ 10/sup -3/ /spl mu/m/sup -1/. Using the method of Hakki and Paoli (1975), the subthreshold gain of the lasers has furthermore been measured as a function of wavelength and current. At the wavelength of best performance, 7.4 /spl mu/m, a modal gain coefficient of 16 cm/spl middot/kA/sup -1/ at threshold and a waveguide loss of 18 cm/sup -1/ have been estimated. The gain evolution confirms an earlier assumption that cross-absorption restricted laser action to above 6 /spl mu/m wavelength.     

Determination of Refractive Indices From the Mode Profiles of UV-Written Channel Waveguides in ${hbox {LiNbO}}_{3}$-Crystals for Optimization of Writing Conditions

We report on a method for the simultaneous determination of refractive index profiles and mode indices from the measured near-field intensity profiles of optical waveguides. This method has been applied to UV-written single-mode optical waveguides in ${hbox {LiNbO}}_{3}$ for the optimization of the writing conditions. The results for the waveguides written with light of the wavelengths 275, 300.3, 302, and 305 nm for different writing powers and scan speeds reveal that for optimum writing conditions a maximum possible refractive index change of $sim$0.0026 can be achieved at a value of 632.8 nm transmitting wavelength. The computation process used in the presented technique may also become useful to extract absolute refractive index values of any slowly varying graded index waveguide.      

AlInP–AlGaInP Quantum-Well Lasers Grown by Molecular Beam Epitaxy

We have examined a possibility to use an Al $_{x}$In $_{1 - {x}}$P layer as an active region of a 650-nm semiconductor laser. Encouraging results have been obtained with compressively strained oxide-stripe AlInP–AlGaInP quantum-well lasers, which operated in continuous-wave mode at room temperature, producing an optical power of 460 and 320 mW per uncoated facet at 10 $^{circ}$C and 20 $^{circ}$C, respectively. In pulsed mode, a power level of 780 mW/facet was achieved at 2-A drive current at 5 $^{circ}$C. The results indicate that wide-bandgap AlInP affords an opportunity to develop lasers for the wavelengths $600leq lambda leq 650$ nm, which is difficult to achieve by any semiconductor heterostructure.      

Lateral waveguiding in stripe-geometry double-heterostructure lasers below the lasing threshold

Structure in the far-field pattern of current-confined stripe-geometry double-heterostructure lasers in the angle rangepm 20degis observed at current levels of less thanfrac{1}{4}of threshold value. The behavior is explained by leaky-mode guiding due to a gain maximum under the stripe and to a negative refractive index step on the order of -0.01.     

Current dependence of pulsations in semiconductor lasers due to photon induced modulation of optical loss

Pulsations in semiconductor lasers are frequently observed to occur within a range of injection currents above threshold. A model is proposed to account for the existence of the current range and attributes it to a critical optical power level at which photon induced modulation of optical loss takes place, such as due to the effect of self-focusing or lateral mode deformation. This value is estimated to be0.36 times 10^{15}photons- cm^-3for double heterostructure oxide-striped lasers.     

Diode-Pumped Passively Mode-Locked Nd : LuVO$_{4}$ Laser With a Semiconductor Saturable Absorber Mirror

A high output power diode-pumped passively mode-locked all-solid-state ${hbox{Nd}:hbox{LuVO}_{4}}$ laser is demonstrated with a semiconductor saturable absorber mirror. Both the $Q$-switched and continuous-wave mode-locked operation are experimentally realized. The mode-locked pulses have a pulsewidth of 7.9 ps and a repetition rate of 105 MHz. At the incident pump power of 22 W, a maximum average output power of 7.6 W is obtained giving an optical conversion efficiency of 34.5%.      

Wavelength Tunable Laser Using an Interleaved Rear Reflector

A novel integrated reflector concept for use in widely tunable semiconductor lasers is introduced. Two concatenated Fabry–PÉrot cavities with low partial reflections and different cavity lengths are interleaved to realize a wavelength-selective reflective element. Differential current injection in the interleaved cavities is used to tune the reflection peak. The reflector is integrated in an all-active multisection device. The laser tunes across 37 wavelength channels on a 100-GHz grid over a wavelength span of 30 nm. The linewidth of each mode is ${≪}$ 7 MHz while the power deviation is ${≪}$6 dB.      

RMS width of pulses in nonlinear dispersive fibers

An approximate analytical expression for the root means square (RMS) width of pulses propagating in nonlinear, dispersive fibers is derived. This result is useful for predicting how far a pulse can travel before it suffers significant distortion due to the combined influence of the nonlinearity of the refractive index and the dispersive properties of the fiber. This theory applies to pulses operating near the zero-dispersion wavelength where dispersion alone has a negligible influence, but where the combined influence of nonlinear self-phase modulation and dispersion can produce a significant effect     

An efficient numerical approach, for determining the dispersioncharacteristics of dual-mode elliptical-core optical fibers

Based on vectorial formulations which combine the surface integral equation method and the finite-element method, a novel numerical approach is proposed for calculating the dispersion coefficients of dual-mode elliptical-core fibers with arbitrary refractive index profiles. By differentiating the original formulations involving the propagation constant β and the guided mode fields H_x and H_y once and twice with respect to the normalized frequency V, the new formulations for {dβ/dV, dH_x/dV, dH_y/dV} and for {d²β/dV², d² H_x/dV², d²H_y/dV² } are obtained respectively. Once {β, H_x, H_y } is solved through the eigenvalue procedure which dominates the computing time, only a few matrix manipulations are required to obtain {dβ/dV, dH_x/dV, dH_y/dV} and {d²β/dV², d²H_x/dV² , d²H_y/dV²}. Some numerical examples are examined to see the influence of different refractive index distributions with dips on the dispersions of the four nondegenerate LP ₁₁ modes for elliptical-core fibers     

注入锁定半导体激光器全光波长转换技术

波长转换器是光通信网络中的一个重要器件。而除半导体光放大器（SOA）外，半导体激光器也是进行波长变换的一种很好选择。基于半导体激光器的注入锁定波长变换技术具有转换带宽较大、啁啾小、消光比特性好、结构简单、成本低廉等诸多优点。将探测光与信号光同步注入法布里-珀罗（F-P）半导体激光器，可以通过信号光功率的变化控制激光器锁模与失锁，导致腔内纵模变化，探测光随之被共振放大或减弱，从而将信息由信号光转换到探测光频率上。从静态实验入手，对半导体激光器的注入锁定现象及光信号控制法布里-珀罗纵模移动等问题分别进行了研究。分析了动态转换激光器工作点的选取问题，在动态实验中实现了较宽范围的正相与反相波长转换，转换速率达到了10Gb／s。