Development of ultrafast chips operating at speeds exceeding 100 GHz is of paramount importance for increasing the transmission capacity of fiber-based networks, directly impacting 5G wireless networks, internet, local area networks, metropolitan area networks, and long-haul backbones, thus bringing closer to the concept of networked society. Although complex modulation formats combined with digital signal post-processing are usually preferred to reach ultra-high modulation bandwidth, the long latency introduced by electronic processing results in a severe communication bottleneck.
To this end, direct-detection systems implemented with directly modulated semiconductor lasers constitute a simple and compact solution for low-cost fiber optic communications. In these systems, the electro-optic 3-dB modulation bandwidth (in GHz) of a directly modulated laser is the most important figure-of-merit that determines the maximum data rate (in Gbps) achievable. To keep increasing the data rate, the enhancement of the electro-optic 3-dB modulation bandwidth without causing other impairments is highly desired. Such improvements are usually obtained either from the development of novel in-plane semiconductor materials or from nonlinear architectures like the optical injection-locked gain lever laser, which is the solution proposed by J. M. Sarraute, et al. in this research. The work is published in Photonics Research, Volume 5, No. 4, 2017 (J. M. Sarraute, et al., Effects of gain nonlinearities in an optically injected gain lever semiconductor laser).
As described in the figure, the gain lever laser is composed of a short modulation section and a long gain section continuous wave biased. In order to maximize the gain-lever effect, the gain section is biased at high gain and the modulation section at low gain level as seen in the corresponding diagram. In such a case, a strong radio frequency optical gain will result, leading to an increase of the electro-optic 3-dB bandwidth. To further enhance the modulation dynamics, the gain lever laser is coupled to an external master laser. In particular, when the strength of the injected light and the frequency detuning between the two lasers fall within a certain range, an injection-locking regime takes place, hence pushing forward the electro-optic 3-dB bandwidth close to 100 GHz. In this work, the researchers also reveal that unlike any directly modulated semiconductor lasers, the compression factor originating from gain nonlinearities does not affect the dynamical performance. For instance, calculations unveil that considering a practical injection strength, a high gain lever effect and a relatively large compression value allows us to maintain a modulation bandwidth of about 85 GHz that is four times larger than that for free-running laser operating without gain lever.
Overall, these results give realistic guidelines for the development of novel types of broadband optical sources for direct modulation of light and operating at high-speeds. Further work will focus on investigating the potential of using quantum dot lasers from which large compression factors are usually observed.
要实现低成本、高效率的光纤通信，必须设计一个简单而紧凑的能够实现超高带宽调制的方案。由直接调制的半导体激光器构建的直接检测系统提供了这样一种解决思路。在这些系统中，直接调制激光器的电光3 dB调制带宽（GHz量级）是最为重要的品质因子，其决定了可实现的最大数据率（Gbps量级）。因而为了保持数据率的进一步提高，在对其他方面不造成负面影响的情况下增大电光3 dB调制带宽是十分必要的。这一改进通常是通过开发新型平面半导体材料或者通过利用诸如光注入锁定增益杠杆激光之类的非线性结构来实现的。后者正是J. M. Sarraute等人在本项研究中提出的解决思路。这一工作发表在Photonics Research 2017年第5卷第4期上(J. M. Sarraute, et al., Effects of gain nonlinearities in an optically injected gain lever semiconductor laser)。
如图所示，增益杠杆激光由一个短的调制部分和一个长的由连续波偏置的增益部分构成。为了使得增益杠杆效应最大化，增益部分偏置于高增益水平，而调制部分偏置于低增益水平。在这种情况下，系统会产生强的射频光学增益，从而导致电光3 dB调制带宽的增加。为了进一步增强调制动力学，增益杠杆激光器与外部主激光器耦合在一起。特别是，当注入光的强度和两个激光器的频率失谐处于特定范围时，注入锁定机制就会发生，从而将电光3 dB调制带宽推进至接近100 GHz。在该工作中，研究人员发现，与直接调制半导体激光器不同，由非线性增益所致的压缩因子并不影响动力学表现。例如，计算表明，当考虑通常的注入强度时，高增益杠杆效应和相对大的压缩值可以使调制带宽达到约85 GHz，这比没有增益杠杆的自由运转激光器高四倍。