Influence of mirror reflectivity on laser performance ofvery-low-threshold vertical-cavity surface-emitting lasers

The influeuce of mirror reflectivity on laser performance of InGaAs-GaAs vertical-cavity surface-emitting lasers fabricated by selective oxidation is investigated by the stepwise change of the number of pairs in top mirror stack after device fabrication. Devices with 18-pair stacks in the top mirror, which is the optimized number of pairs in this structure, show an output power over 1.9 mW and a slope efficiency of 55% while maintaining a low threshold current of 212 μA. The analysis of the threshold current and differential efficiency related to mirror reflectivity shows an internal quantum efficiency of 95%, an internal round-trip loss of 0.072, and a transparency current density of 71 A/cm²     

Optical gain in GaAs/GaAlAs graded-index separate-confinementsingle-quantum-well heterostructures

Optical model gain in both the TE and TM polarizations of graded-index separate-confinement single-quantum-well heterostructure lasers measured at various levels of injection current on samples with different quantum-well widths is discussed. Lasers with wide quantum wells (⩾120 Å) have emission and gain spectra which exhibit two peaks, caused by the n=1 and n=2 subband transitions. With ordinary cavity parameters, the saturation gain of the n=1 subband transitions is lower than the cavity loss of the laser, and the lasers always lase at the n=2 transitions. Reducing the quantum-well width increases the saturation gain of the n=1 transitions enough to allow lasing from them, even in cases of higher cavity loss. Further, for a fixed cavity loss, reduction of the quantum-well width decreases the threshold current density for n =1 lasing transitions, while that for n=2 lasing increases. The superlinear increase of the material gain with the decrease of the well width reduces the minimum cavity length for n =1 subband lasing. Narrower quantum wells with higher mirror reflectivity allow shorter cavity lengths while retaining n=1 lasing, resulting in low threshold current     

S/N and Error Rate Performance in AlGaAs Semiconductor Laser Preamplifier and Linear Repeater Systems

The applications of AlGaAs semiconductor laser preamplifier and linear repeaters in single mode optical fiber transmission systems were studied through the baseband signal-to-noise ratio and bit error rate performance measurement. Experiments were carried out with the Fabry-Perot cavity laser amplifiers whose characteristics are improved by reducing the input mirror reflectivity to 6 percent. The use of a preamplifier improves the minimum detectable power by 7.4 dB over the Si-APD direct detection level when the received signal is amplified by 30 dB before photodetection. The use of two linear repeaters increases the regenerative repeater gain by 37 dB. These experimental results are in good agreement with theoretical predictions based on the photon statistic master equation analysis.     

S/N and error rate performance in AlGaAs semiconductor laser preamplifier and linear repeater systems

The applications of AlGaAs semiconductor laser preamplifier and linear repeaters in single mode optical fiber transmission systems were studied through the baseband signal-to-noise ratio and bit error rate performance measurement. Experiments were carried out with the Fabry-Perot cavity laser amplifiers whose characteristics are improved by reducing the input mirror reflectivity to 6 percent. The use of a preamplifier improves the minimum detectable power by 7.4 dB over the Si-APD direct detection level when the received signal is amplified by 30 dB before photodetection. The use of two linear repeaters increases the regenerative repeater gain by 37 dB. These experimental results are in good agreement with theoretical predictions based on the photon statistic master equation analysis.     

一种可降低Littman-Metcalf光栅外腔半导体激光器衍射损耗的结构设计

基于原有Littman-Metcalf型光栅外腔半导体激光器的工作原理,设计了一种可以降低衍射损耗的外腔结构。在Littman-Metcalf结构的基础上增加一个反射镜,将闪耀光栅二次衍射产生的零级衍射光反馈回半导体激光器本征腔。推导了新结构模型外腔损耗的表达式,通过等效腔的概念对两种结构激光器的外腔损耗、阈值电流、输出线宽以及输出功率进行了仿真分析。结果表明：将二次衍射产生的零级光反馈回有源区可有效降低Littman-Metcalf结构激光器的外腔损耗,提高了系统的耦合效率,从而降低阈值电流,提高了激光器的输出功率。同时,由于提高了外腔反射效率,该外腔结构进一步压窄激光器的输出线宽。对影响低损耗Littman-Metcalf外腔激光器输出线宽以及输出功率的因素(端面反射率、内外腔长、闪耀光栅衍射效率以及反射镜反射率等)也进行了仿真分析,为后期激光器制作提高了理论指导。     

Effects of well number, cavity length, and facet reflectivity onthe reduction of threshold current of GaAs/AlGaAs multiquantum welllasers

The optimum design for reducing the threshold current of GaAs/AlGaAs multiquantum well lasers is determined experimentally. The lowest threshold current density is realized by using single and multiquantum wells at long and short cavity lengths, respectively. The threshold current has a minimum at the optimum cavity length: the minimum threshold current is smaller for a larger number of quantum wells, and the optimum cavity length is inversely proportional to the number of wells. Experiments are compared to the theory developed by P.W.A. McIlroy, et al. (ibid., vol.21, no.12, p.1958-63, 1985) and limiting performances of quantum well lasers with various numbers of wells are presented. The reduction of the threshold current by high reflectivity coatings is also demonstrated, and a threshold current as low as 1.86 mA at 15°C is reported     

Minimum bit rate of DPSK transmission for semiconductor laser with a long external cavity and strong linewidth reduction

A numerical model for a laser diode, weakly coupled to an external cavity, is presented. Using this model, the actual frequency noise spectrum is considered rather than a white frequency noise for estimating the minimum bit rate in a differential-phase-shift-keying (DPSK)-system. Despite possible linewidth reductions by up to 200 with long external cavities, the minimum bit rate (BER = 10^-9) remains nearly unchanged. However, a longer laser cavity (approx600 mum) connected to an external cavity of moderate length ( ≈10 cm) yields a significant reduction of the minimum bit rate (BER = 10^-9) for DPSK-systems at low feedback levels. If more stringent criteria are applied (e.g., receiver penalty < 1 dB) the minimum bit rate remains high also for longer laser cavities.     

Butt joint integrated extended cavity InAs/ InP (100) quantum dot laser emitting around 1.55 μm

The first butt joint integrated extended cavity InAs/InP (100) quantum dot (QD) Fabry-Perot laser emitting around 1.55 mum is demonstrated. Continuous wave lasing at room temperature on the QD ground state transition is achieved. The threshold current is comparable to that of all-active QD lasers. The Butt joint reflectivity for straight waveguides is below -40 dB.     

基于光纤光栅谐振腔的双包层光纤激光器

理论分析了线型腔双包层光纤激光器的输出特性,包括光纤长度、光纤损耗及后腔镜反射率对激光输出功率和阈值泵浦功率的影响.设计了基于光纤光栅谐振腔的双包层光纤激光器,采用锥度光纤实现了泵浦模块与双包层光纤之间的低损耗连接,实现了全光纤化的掺Yb3 双包层光纤激光器,其阈值泵浦功率为300 mW,在泵浦入纤功率为17 W时达到了10.5 W的最大激光输出功率,斜率效率为62%.     

High-reflectivity cavity mirrors for high-performance (GaAl) As diode lasers

Effects of the high-reflectivity mirrors on the performance of (GaAl)As diode lasers are investigated by using a laser diode model. An optimum value for the mirror reflectivity is obtained by using a figure of merit which reflects low threshold and high slope efficiency. An optimum reflectivity valueR = 0.75is obtained for a laser with a cavity lengthl= 300 mum and an internal loss coefficientgamma_{i} = 10cm^-1. A higher internal photon flux density relative to the output photon flux density is shown to occur with the high-reflectivity mirror. High-reflectivity mirrors are shown to reduce optical feedback effect from external devices and the lowered threshold value to reduce the thermal effects on the modulation characteristics and the degradation.     

Analysis and application of theoretical gain curves to the design of multi-quantum-well lasers

Gain/current curves for a single quantum well are calculated. The optimum well number, cavity length, threshold current, and current density of multi-quantum-well (MQW) lasers are derived in terms of this gain curve. The limiting performance of MQW lasers is found to be better than that of graded refractive index (GRIN) lasers, assuming comparable efficiencies and spontaneous emission linewidths. The optimum threshold current for an MQW laser with a 7 μm cavity and 90 percent facet reflectivity issim50 muA/μm.     

连续激光光腔衰荡法精确测量高反射率

提出了基于宽谱半导体激光器连续激光光腔衰荡法测量高反射率的方法,给出了方波调制连续激光光腔衰荡法理论。采用锁相技术记录光腔输出信号一次谐波的振幅和相位,由振幅和相位随调制频率的变化曲线分别拟合衰荡时间。给出了不同腔长下两种拟合方式分别得到的衰荡时间,最终确定腔镜反射率为99.70%,误差小于0.01%。由频域拟合方式得到反射率结果,使可靠性得到提高。与脉冲激光光腔衰荡法相比,采用连续半导体激光器作光源,大大降低了成本,且由于光束质量高,更有利于提高测量精度。     

Parallel pump-shared linear cavity laser array using 980-nm pumpreflectors or N pieces of gain fibers as self-equalizers

By using fiber gratings as pump reflectors with various reflectivities or N pieces of gain fibers with various lengths externally to linear cavity as self-equalizers, a parallel-type, pump-shared linear cavity laser array (LCLA) is proposed and numerically studied. The average output power is increased and the maximum power variation among channels is reduced from 7.2 to less than 0.1 dB when the reflectivity of each pump reflector or the length of each gain fiber is adjusted appropriately     

Polarization of the output of InGaAsP semiconductor diode lasers

Measurements of the degree of polarization (ρ) of the output of 1.3-μm InGaAsP semiconductor diode lasers as a function of current are discussed. It is found that ρ measured for well below threshold (ρ_b) is a better indicator of mechanical strain in the active region than ρ measured near or above threshold. ρ_b exhibits little dependence on drive level, mode reflectivity, threshold, or saturation of the gain. A possible correlation between the magnitude of ρ_b and device operating characteristics was found. More work is required to confirm this possibility. Data which provide experimental evidence for the magnitude of the difference between the reflectivities of the TE and TM modes are presented     

宽谱连续波复合衰荡光腔技术测量高反射率

提出了一种连续波复合衰荡光腔(CRD)技术,可同时确定腔镜和待测镜高反射率,无需测量系统响应,提高了测量精度。以方波调制的连续激光耦合进衰荡腔,采用锁相方式探测衰荡信号一次谐波的振幅和相位。在数据处理中消除了系统响应的影响,由对应调制频率时直腔和折叠腔衰荡信号一次谐波的振幅比值和相位差值进行频域拟合,同时得到腔镜和待测平面镜的反射率。理论上推导了频域拟合函数。对100 cm共焦腔和110 cm稳定腔分别进行了直腔和折叠腔实验,最终确定腔镜反射率99.784%,待测平面镜反射率99.668%,测量精度达到10-5量级,与测量并消除系统响应后得到的反射率结果一致。     

Very low threshold current density 1.29 μm GaInNAs triple quantum well lasers grown by MBE

Very low threshold Ga_0.62In_0.38N_0.007As_0.993/GaN_0.011As_0.989/GaAs triple quantum well (QW) lasers emitting at 1.29 mum are demonstrated. The laser structure was grown by molecular beam epitaxy after extensive optimisations of growth and in situ annealing conditions. As- cleaved broad area lasers with a cavity length of 1 mm showed a record low threshold current density of 400 A/cm² (~130 A/cm² / QW), a high differential efficiency of 0.32 W/A/facet and a characteristic temperature of 94 K in the temperature range 10 to 110degC.     

Low-threshold 833-nm GaAsP-AlGaAs tensile-strained quantum-well laser diodes

We report the demonstration of TM polarized laser emission at 833 nm from GaAsP-AlGaAs quantum well laser structures grown on GaAs substrates by low-pressure organometallic vapor phase epitaxy for the first time. This is the longest wavelength near infrared TM polarized laser ever reported. Broad area laser diodes containing single GaAs/sub 0.95/P/sub 0.05/ quantum well lased at a low threshold current density of 280 A/cm/sup 2/ for a cavity length of 1 mm and with a high differential quantum efficiency of 32%/facet for cavity length less than 500 /spl mu/m.     

Design studies for distributed Bragg reflectors for short-cavityedge-emitting lasers

In conventional edge-emitting lasers, mirror reflectivity is ~0.3-0.4 so that very short cavities have an unacceptable loss. To decrease the mirror losses, one can conceive of an external distributed Bragg reflector (DBR) structure produced by etching. We examine the design considerations for such mirrors. Using a finite-difference time-domain (FDTD) scheme, we numerically simulate the propagation of a Gaussian wave packet in a laser cavity with an external DBR structure on one end. We find that despite the divergence of light at the semiconductor-air interface, high reflectivities (>90%) can be obtained using only a few mirrors, provided a low refractive index material (such as air) is one of the components of the DBR period. In our modeling, we include fluctuations based on factors such as lithographical error, etch error, and off-design wavelength lasing. Our results indicate that the external DBR is highly tolerant of such factors. In addition, simulations done using material with higher refractive index than air in the DBR structure indicate that it is possible for insulators to be deposited in the DBR structure, yet allow high reflectivity     

Continuous-wave cavity ring-down technique for accurate measurement of high reflectivity

A continuous-wave cavity ring-down (CWCRD) technique employing a broadband diode laser is developed for high reflectivity measurement. The theory of square-wave modulated CW-CRD is presented. The spectrum of the broadband CW diode laser covers numerous free spectral ranges (FSRs) so that sufficient laser power is coupled into the cavity. Both amplitude and phase-shift of the first harmonic of the CRD signal,measured at an appropriate frequency range, are detected by a lock-in method and fitted to obtain the ring-down time and reflectivity. The measurements are repeated with five different cavity lengths and all the fitted reflectivities are in excellent agreement, indicating a high reliability of the CW-CRD technique. The reflectivity of the cavity mirror is determined statistically to be 99.70%,with an uncertainty of 0.01%.     

量子阱激光器的阈值电流与腔长关系

一般激光器的阈值电流会随着腔长增加而增大,但是最近发现单量子阱激光器的阈值电流在很大的腔长范围内保持恒定。造成这样现象的原因是由于激光器有源区不掺杂和采用了线形渐变折射率导引机制的量子阱激光器结构,这样可以有效的减少载流子泄漏和非辐射复合,因此激光器内损耗很小,阈值增益主要由腔面损耗决定,致使阈值电流在很大的腔长范围内保持恒定。