Volume 49 Issue 12
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Li Xiang, Wang Hong, Qiao Zhongliang, Zhang Yu, Niu Zhichuan, Tong Cunzhu, Liu Chongyang. Repetition frequency variation of a 2 μm GaSb-based passively mode-locked laser (Invited)[J]. Infrared and Laser Engineering, 2020, 49(12): 20201054. doi: 10.3788/IRLA20201054
Citation: Li Xiang, Wang Hong, Qiao Zhongliang, Zhang Yu, Niu Zhichuan, Tong Cunzhu, Liu Chongyang. Repetition frequency variation of a 2 μm GaSb-based passively mode-locked laser (Invited)[J]. Infrared and Laser Engineering, 2020, 49(12): 20201054. doi: 10.3788/IRLA20201054

Repetition frequency variation of a 2 μm GaSb-based passively mode-locked laser (Invited

doi: 10.3788/IRLA20201054
  • Received Date: 2020-10-15
  • Rev Recd Date: 2020-11-20
  • Available Online: 2021-01-14
  • Publish Date: 2020-12-25
  • Multi-gigahertz optical pulse trains generated from mode-locked semiconductor lasers are promising for a number of applications in many areas. For most of these applications, a fixed and stable pulse repetition frequency is necessary. Since the repetition frequency of such lasers is primarily determined by the effective refractive index of the laser waveguide and the laser cavity length, uncertainties during device fabrication as well as cleaving process may bring deviations to the repetition frequency. To gain better knowledge of how working conditions of such lasers effect their repetition frequency and thus compensate the above-mentioned deviations, a novel 2 µm InGaSb/AlGaAsSb single quantum well (SQW) mode-locked laser (MLL) was presented in this work. It has a two-section configuration (gain section and saturable absorber section separated by an electrical isolation region) and stable mode locking was achieved in this laser under a variety of bias conditions up to 60 ℃. Repetition frequency variations of this mode-locked laser with bias condition (gain section current Ig, absorber section voltage Va) and working temperature (T) were systematically recorded, and the mechanisms behind these variations were analyzed. It is believed that this work enables us to have a better understanding of passively mode-locked semiconductor lasers and is of interest to better meet the application-required frequencies.
  • [1] Rafailov E U, Cataluna M A, Sibbett W. Mode-locked quantum-dot lasers [J]. Nature photonics, 2007, 1(7): 395-401. doi:  10.1038/nphoton.2007.120
    [2] Thompson M G, Rae A R, Xia M, et al. InGaAs quantum-dot mode-locked laser diodes [J]. IEEE Journal of Selected Topics in Quantum Electronics, 2009, 15(3): 661-672. doi:  10.1109/JSTQE.2008.2012265
    [3] Kuntz M, Fiol G, Laemmlin M, et al. High-speed mode-locked quantum-dot lasers and optical amplifiers [J]. Proceedings of the IEEE, 2007, 95(9): 1767-1778. doi:  10.1109/JPROC.2007.900949
    [4] Merghem K, Teissier R, Aubin G, et al. Passive mode locking of a GaSb-based quantum well diode laser emitting at 2.1 μm [J]. Applied Physics Letters, 2015, 107(11): 111109. doi:  10.1063/1.4931364
    [5] Holc K, Weig T, Pletschen W, et al. Picosecond pulse generation in monolithic GaN-based multi-section laser diodes[C]//Gallium Nitride Materials and Devices VIII. International Society for Optics and Photonics, 2013, 8625: 862515.
    [6] Kemal J N, Marin-Palomo P, Panapakkam V, et al. Coherent WDM transmission using quantum-dash mode-locked laser diodes as multi-wavelength source and local oscillator [J]. Optics express, 2019, 27(22): 31164-31175. doi:  10.1364/OE.27.031164
    [7] Sadeev T, Arsenijević D, Franke D, et al. 1.55 μm mode-locked quantum-dot lasers with 300 MHz frequency tuning range [J]. Applied Physics Letters, 2015, 106(3): 031114. doi:  10.1063/1.4906451
    [8] Li X, Wang H, Qiao Z, et al. High temperature characteristics of a 2 μm InGaSb/AlGaAsSb passively mode-locked quantum well laser [J]. Applied Physics Letters, 2019, 114(22): 221104. doi:  10.1063/1.5096447
    [9] Huang X, Stintz A, Li H, et al. Passive mode-locking in 1.3 μm two-section InAs quantum dot lasers [J]. Applied Physics Letters, 2001, 78(19): 2825-2827. doi:  10.1063/1.1371244
    [10] Sulmoni L, Lamy J M, Dorsaz J, et al. Static and dynamic properties of multi-section InGaN-based laser diodes [J]. Journal of Applied Physics, 2012, 112(10): 103112. doi:  10.1063/1.4768163
    [11] Thompson M G, Rae A, Sellin R L, et al. Subpicosecond high-power mode locking using flared waveguide monolithic quantum-dot lasers [J]. Applied Physics Letters, 2006, 88(13): 133119. doi:  10.1063/1.2186110
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Repetition frequency variation of a 2 μm GaSb-based passively mode-locked laser (Invited

doi: 10.3788/IRLA20201054
  • 1. Temasek Laboratories, Nanyang Technological University, Singapore 637553, Singapore
  • 2. School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore
  • 3. School of Physics and Electronic Engineering, Hainan Normal University, Haikou 571158, China
  • 4. State Key Lab for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
  • 5. State Key Lab of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, China

Abstract: Multi-gigahertz optical pulse trains generated from mode-locked semiconductor lasers are promising for a number of applications in many areas. For most of these applications, a fixed and stable pulse repetition frequency is necessary. Since the repetition frequency of such lasers is primarily determined by the effective refractive index of the laser waveguide and the laser cavity length, uncertainties during device fabrication as well as cleaving process may bring deviations to the repetition frequency. To gain better knowledge of how working conditions of such lasers effect their repetition frequency and thus compensate the above-mentioned deviations, a novel 2 µm InGaSb/AlGaAsSb single quantum well (SQW) mode-locked laser (MLL) was presented in this work. It has a two-section configuration (gain section and saturable absorber section separated by an electrical isolation region) and stable mode locking was achieved in this laser under a variety of bias conditions up to 60 ℃. Repetition frequency variations of this mode-locked laser with bias condition (gain section current Ig, absorber section voltage Va) and working temperature (T) were systematically recorded, and the mechanisms behind these variations were analyzed. It is believed that this work enables us to have a better understanding of passively mode-locked semiconductor lasers and is of interest to better meet the application-required frequencies.

    • 高重复频率的超短光脉冲序列在光通信/存储,传感,激光雷达,时钟信号和种子光产生等领域都有着广泛的应用[1-6]。半导体锁模激光器由于具备灵活的波长调节、体积小巧、高效率,能够方便地电泵浦,以及能够跟硅光子进行集成等优点,成为了一种用于产生这种光脉冲序列的理想光源,在近年来引起了人们的广泛关注。除此之外,半导体锁模激光器典型的两段式/多段式布局大大简化了它的结构,并且使人们能够得到一体化的单片器件。

      在几乎所有上述提到的应用中,一个固定而准确的重复频率是必须的,有时还需要能够在小范围内进行调节。已知锁模激光器的重复频率f由激光器波导的有效折射率neff和腔长L通过公式f=c/2neffL进行确定,所以首要的且能够简单控制f的因素便是解理时形成的腔长L,然而半导体激光器在解理时普遍存在着一定的不确定性,也就是说腔长并不能绝对精确地进行控制,这就需要找出方法来对重复频率进行微调。已知锁模激光器的偏置条件(增益区电流Ig,饱和吸收体区电压Va)会对重复频率产生影响,比如Sadeev等人已经报导由于偏置条件的变化使重复频率产生了多达300 MHz的变化[7],这就能够很大程度上弥补解理带来的不确定性。然而,还有另一个能够显著影响重复频率的因素,那就是工作温度(T),其对半导体锁模激光器重复频率的影响,尤其是对长波长GaSb基锁模激光器的影响则鲜有提及,然而高温环境在上述的很多应用中都是不可避免的。综上,研究半导体重复频率随各个因素的变化是很有必要的,这能够让人们更加清楚地认识锁模激光器的特性,从而更好地使其达到各种应用需要的重复频率。

      文中,笔者系统地记录了一种工作在2 μm波段的InGaSb/AlGaAsSb单量子阱锁模激光器在稳定锁模状态下重复频率随偏置条件和工作温度的变化,并且对产生这些变化的原因进行了分析。

    • 此次实验用于加工锁模激光器的外延片是使用分子束外延的方法在n型掺杂的GaSb衬底上生长得到的。外延结构包括了一个10 nm宽的In0.2Ga0.8Sb单量子阱,量子阱两侧是270 nm厚的与GaSb晶格匹配的Al0.25GaAsSb。完整的外延结构以及器件加工方法可以参考笔者团队之前的工作[8]。文中用于测试的锁模激光器条宽约为5 μm,这使它能够实现单横模工作。增益区和饱和吸收体区的长度分别为1.89 mm和0.23 mm,两者之间被一个宽度为10 μm的隔离沟隔开,隔离电阻大约为1.1 kΩ。激光器的腔面没有镀膜。当工作在稳定的锁模状态时,增益区加正向偏压(Ig),同时饱和吸收体区一般需要加反向偏压(Va)。锁模激光器的工作温度由一个电制冷器进行控制。

      图1给出了当饱和吸收体区开路时(Va=0 V)激光器在不同工作温度下出射光功率(从增益区腔面测试得到)随注入电流变化的曲线(L-I)。此激光器可以在连续模式(cw)下至少工作到60 ℃。可以观察到在阈值处光功率出现了一个突然的增加,这一现象在包含饱和吸收体的器件中普遍存在[9-10],它主要归因于当激光器激射时饱和吸收体中载流子的迅速增加,而电场在Va=0时并不足以将载流子从量子阱中迅速扫出,这一过程减弱了饱和吸收体的吸收,从而导致了光功率的跳变。

      Figure 1.  L-I curves of the tested laser at different temperatures when Va=0 V

      被测试的激光器在至少60 °C下都在很大范围的偏置条件达到了稳定的锁模状态。图2给出了20 ℃时的一个典型的射频(RF)谱线(Ig=180 mA, Va=−2 V)。谱线由一个高速光探测器(EOT ET-5000F)后接一个50 GHz的RF频谱分析仪(Agilent N9030A)测量得到。从图中可以看出,位于13.34 GHz处的锁模激光器的基础重复频率具有高达55 dB的信噪比。这一基础重复频率对应于光子在2.94 mm长的激光谐振腔中走一个来回所需要的时间。位于26.68 GHz处的二次谐波也可以在图中清晰的看到。图2的插图(上)还给出了放大的基频RF信号,信号的半高宽约为60 kHz,表明了激光器非常有效的锁模机制并且具有较低的相位噪声。图2的另一个插图(下)则是在示波器上(DSO 93004L)观察到的光脉冲序列,两个相邻脉冲间的时间间隔约为75 ps,对应于13.34 GHz的基础重复频率。

      Figure 2.  A typical RF spectrum at 20 °C under the bias condition of Ig=180 mA, Va=−2 V. The upper inset shows the RF signal at the fundamental repetition rate, while the lower inset shows the pulse train

      激光器在与图2相同偏压条件(Ig=180 mA, Va=−2 V)下的光谱在图3中给出。它可以完美地被高斯曲线拟合,这说明超短光脉冲的包络也应为高斯型。光谱的中心波长位于约1958 nm处,半高宽约为7.4 nm,至少有超过40个纵模被锁为了同相位。如果假设啁啾现象并不明显,此时高斯型脉冲的时间-带宽乘积约为0.441,则激光器理论上能达到的最短的脉冲宽度大约为760 fs。

      Figure 3.  Optical spectrum under the same bias condition as that in Fig. 2 (Ig=180 mA, Va=−2 V)

    • 接下来具体来看激光器重复频率的变化。首先,Va被设定在−1 V保持不变,这是一个适中的反向偏压。图4(a)给出了20、40、60 ℃时重复频率随Ig的变化趋势。从图中可以看出,在各个温度下,重复频率都随着Ig线性降低,不同温度下降低的速度有所不同,分别是−0.60、−0.29 、 −0.15 MHz/mA。随Ig的这个下降趋势已经在两段式锁模激光器中被广泛观察到[4, 7]。造成它的原因主要是注入电流增大引起的温度上升。具体来说,包括热膨胀导致的腔长变长,禁带收缩造成的有效折射率增大等等。

      Figure 4.  (a) Repetition frequency as a function of Ig ; (b) Corresponding spectra change at three different temperature ( Va was fixed at −1 V)

      另一方面,从图4(a)中可以看出:重复频率随着工作温度(T)的升高整体出现了明显的上升。笔者认为造成这一现象的主要原因是温度升高时激光器波长的红移,进而降低了有效折射率,图4(b)很明显地展示了这个波长变化。当然,温度上升时,上一段提到的使得重复频率下降的效应也都存在。所以当工作温度升高时,两个对重复频率施加相反作用的过程同时存在,共同作用于重复频率。图5对这个过程进行了总结。在图4(a)展示的Ig范围内,整体来说波长改变造成的折射率变化是占优势的,除了在150 mA左右时,两个过程相互抵消。

      Figure 5.  Frequency variation mechanism caused by operation temperature rise. RI: effective refractive index, L: cavity length

      下一步,将Ig设定在了190 mA保持不变,这个电流远高于20、40、60 ℃时激光器激射的阈值。如图6所示,在每个温度下重复频率随Va的变化趋势保持一致:在比较小的反偏时上升,然后保持平稳,最后缓慢下降。其他的课题组也报导了这个上升的过程[7],而另一些课题组则观察到了下降的过程[3, 11]。在笔者看来,这整个过程仍然是由两个对立的过程共同作用形成的。首先,当反偏增大时,饱和吸收体会引入更大的损耗,增益区的有源区中载流子的浓度要增大来弥补这些损耗,从而由于等离子体色散效应,增益区的有效折射率会下降从而使得重复频率增大。另一方面,更大的反向偏压使得饱和吸收体区有了更显著的禁带收缩,从而增大了饱和吸收体区的有效折射率。这两个过程总结在了图7中,它们共同作用造成了图6(a)中重复频率的变化趋势,图7中,RI为有效折射率。

      Figure 6.  (a) Repetition frequency as a function of Va (Ig was fixed at 190 mA); (b) Corresponding spectra change

      Figure 7.  Frequency variation mechanism caused by Va changing to a more negative value

      跟之前同样的原因,图6(a)中工作温度升高时波长的红移使得重复频率整体变得更高。当激光器工作在20和40 °C时,温度升高导致的两个相反的作用在Va=−1.6 V时互相抵消,使被测激光器在两个不同温度下呈现出了非常接近的重复频率。

      除此之外图4图6还有值得注意的一点,40 ℃时的光谱要比20 ℃时有了显著的展宽。以图4(b)中两个温度下两个形状对称的光谱为例,半高宽从20 ℃时的2.7 nm变为40 ℃时的7.4 nm,几乎扩大了3倍。图6(b)在40 ℃下更大的反偏电压时甚至进一步展宽到了12.4 nm,甚至比很多基于量子点的半导体锁模激光器的光谱都要宽[2, 7, 9]。40 ℃时更宽的光谱表明此时有更多的纵模被锁为了同相位,原因可能是由于40 ℃时增益带宽变宽造成的。更宽的光谱不仅为得到更窄的光脉冲提供了可能性,同时当把锁模激光器当做多波长光源时,也有更多的波段可以选择。

      综上,文中展示了一种2 μm InGaSb/AlGaAsSb单量子阱锁模激光器。激光器可以在高达60 ℃实现稳定的锁模工作模式。笔者系统地记录了其重复频率随三个主要因素(IgVaT)的变化规律。并且对产生这些变化的原因进行了分析。这些工作能够让人们对如何改变两段式锁模激光器重复频率有一个更加清楚的认识,从而指导人们对激光器进行调节以使其达到实际应用所需的频率。

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