Joint Research on Gain-Coupled DFB Semiconductor Laser

1. 增益耦合系数的测量：增益耦合系数是增益耦合（GC）分布反馈（DFB）激光器中最重要的器件参数，但目前尚无测量方法。本文与荷兰根特大学合作，利用最小二乘法拟合理论亚阈值谱，对DFB激光器的折射率和增益耦合系数等参数进行了提取。因此，首次可以测量和提取出复耦合DFB激光器的指数耦合系数56.4cm_<-1>和增益耦合系数22.3 3cm_<-1>。此外，该方法允许确定面反射率和相位。此外，在不同偏置水平下测量亚阈值谱，可以提取线宽增强因子。InP衬底上层状InGaAs/InGaAIAs量子阱的无序性：上述量子阱在加州大学通过分子…More r束外延生长。然后用等离子体辅助化学气相沉积（p-CVD）技术在样品上覆盖SiO_2和/或SiN薄膜，然后制成图案。然后，对整个样品进行快速热退火（RTA），使量子阱无序化。从这些样品中取光致发光（PL），以便通过PL波长的位移来评估无序程度。结果证实SiO_2作为无序启动子，而SiN作为保护帽抑制无序。发现无论是否沉积帽膜，在应变量子阱中存在的应力都会导致RTA加热过程的无序。最后，通过显微PL测量评价了选择性无序的面内空间分辨率。结果表明，通过这样的处理，可以将微加工精度降低到2微米。分布式前向和后向耦合波长可调谐激光器的分析与制造：在波分复用（WDM）光通信系统中，半导体激光器需要具有宽范围的波长可调谐性。然而，在GC - DFB激光器中，振荡波长可调至今仍不可能实现。在这里，我们与英国根特大学合作，提出并研究了“分布式正向和反向耦合（DFBC）”结构，该结构在GC DFB激光器中带来了宽范围的波长可调谐性。该结构在有源层上方有一个采样光栅，在有源层下方有一个用于移动折射率的共向无源波导。采样光栅提供了激光振荡所需的后向反馈，而主动式和无源波导之间的共向前向耦合提供了波长滤波功能。共向无源波导折射率的微小变化会导致滤波器选择的波长发生很大变化，从而使大范围调谐成为可能。利用f矩阵分析，我们模拟了1.55 μ m纯增益耦合DFBC激光器的波长调谐特性，发现同向无源波导中0.03的折射率变化就会导致激光波长位移达到100nm。接下来，我们设计了0.8mum波长区域的DFBC激光器，并进行了初步的制作实验。在第一步MOCVD（有机-金属气相外延）之后，采用新开发的双光阻涂层方法形成采样光栅。然后，进行第二步MOCVD和多个光刻步骤，完成DFBC结构。我们在制备的样品中验证了电二极管的特性和自发发射，从而建立了DFBC激光器制备的基础。GC - DFB激光器与外部EA调制器的集成：我们与清华大学合作，制造了一个集成电吸收（EA）调制器的GC - DFB激光器。通过使DFB波长相对于增益曲线峰值向较长侧失谐，我们可以在激光器和调制器部分使用相同的量子阱。因此，整个结构仅由两个MOCVD生长步骤制成。由于GC - DFB的面反射不灵敏，使得激光器单模振荡，且调制器具有足够的消光比。GC - DFB激光器模拟调制失真分析：与英国根特大学合作，对影响模拟应用的二阶和三阶谐波失真进行了分析。因此，GC - DFB激光器由于更小的纵向空间孔燃烧，通常比传统DFB激光器具有更小的畸变。在某些情况下，改进幅度可达10分贝。少

1. Measurement of gain coupling coefficient : Although the gain-coupling coefficient is the most imporant device parameter in the gain-coupled (GC) distributed-feedback (DFB) laser, there has been no measurement method for it. Here we studied, with the University of Gent, extraction of parameters including index and gain coupling coeffcients from DFB lasers by minimum-square-fitting theoretical subthreshold spectrum with the experimental one. As a result, it has become possible for the first time to measure and extract, for example, an index-coupling coefficient of 56.4cm_<-1> and a gain-coupling coefficient of 22.3cm_<-1> out of a complex-coupled DFB laser. Also, this method permits to determine the facet reflectivity and phase. Moreover, the linewidth enhancement factor can be extracted if the subthreshold spectrum is measured at different bias levels.2. Disordering of stratined InGaAs/InGaAIAs quantum wells on InP substrates : The above-mentioned quantum wells were grown by molecula … More r beam epitaxy at the University of California. The samples were next capped with SiO_2 and/or SiN films by plasma-assisted chemical vapor deposition (p-CVD) that were made into patterns later. Then, rapid thermal annealing (RTA) was applied to the whole sample to disorder the quantum wells. Photoluminescence (PL) from such samples was taken so that the degree of disordering could be evaluated by the PL wavelength shifts. Consequently, it was verified that the SiO_2 worked as a disordering promoter whereas the SiN worked as a protective cap which suppressed the disordering. The stress existing in the strained quantum wells was found to cause the disordering the RTA heating process regardless of whether the cap film was deposited or not. Finally, the in-plane spatial resolution of the selective disordering was evaluated through microscopic PL measurement. As a result, it turned out that microfabrication down to 2mum was possible by such processing.3. Analysis and fabrication of distributed forward-and backward-coupling wavelength-tunable laser : In wavelengthdivision multiplexed (WDM) optical communication systems, wide-range wavelength tunability is required in semiconductor lasers. However, in the GC DFB laser, tunable oscillation wavelength has not been possible so far. Here we proposed and studied, in collaboration with University of Gent, "distributed forward-and backward-coupling (DFBC) " structure that brings about wide-range wavelength tunability in the GC DFB laser. This structure has a sampled grating above the active layr, and a codirectional passive waveguide for shifting refractive index below the active layr. The sampled grating provides backward feedback necessary for laser oscillation, whereas the codirectional forward coupling between the active and the passive waveguides provides wavelength-filtering function. A small change in the index of the codirectional passive waveguide results in a large change in the wavelength selected by the filter, and thereby a wide-range tuning is made possible. Making use of the F-matrix analysis, we simulated wavelength tuning characteristics of a 1.55mum purely-gain-coupled DFBC laser, and found that index change as small as 0.03 in the codirectional passive waveguide could give rise to lasing wavelength shift as large as 100nm. Next, we designed a DFBC laser in 0.8mum wavelength region, and did preliminary fabrication experiment. After the first step MOCVD (organo-metallic vapor phase epitaxy), the sampled grating was formed by a newly-developed method of dual photoresist coating. Then, the second step MOCVD as well as a number of photolithography steps were carried out to complete the DFBC structure. We verified electrical diode characteristics and spontaneous emission in the fabricated sample, and thereby the fundamentals of the DFBC laser fabrication were established.4. Monolithic integration of a GC DFB laser and an external EA modulator : In collaboration with Tsinghua University, we fabricated a GC DFB laser integrated with an electro-absorption (EA) modulator. By detuning the DFB wavelength toward the longer side with respect to the gain profile peak, we could use identical quantum wells in both the laser and the modulator sections. As a result, whole structure was made by only two MOCVD growth steps. Due to the facet reflection insensitivity of the GC DFB,the laser oscillated in a single mode, and the modulator had sufficient extinction ratio.5. Analysis of analog modulation distortion in GC DFB lasers : The second-and third-order harmonic distortions which affect analog applications very much were analyzed in collaboration with University of Gent. Consequently, GC DFB lasers were found to generally possess a little smaller distortion than conventional DFB lasers due to smaller longitudinal spatial hole burning. The improvement could be as large as 10 dB in some cases. Less