大幅度增加弛豫振荡频率来实现毫米级外腔半导体激光器的外腔机制转换

混沌外腔半导体激光器输出明显存在弛豫振荡特征,弛豫振荡频率小于外腔振荡频率时,外腔半导体激光器输出态是短腔机制;反之,外腔半导体激光器输出态是长腔机制.首先对比分析了弛豫振荡频率为5.6 GHz,腔长对频谱有效带宽的影响.然后同时调节注入电流和载流子寿命来大幅度地增加弛豫振荡频率.最后在弛豫振荡频率为40 GHz、腔长为毫米级(4—20 mm)时,实现由短腔机制到长腔机制的转换,进而分析了外腔反馈率和外腔长对外腔半导体激光器频谱带宽的影响.分析结果表明:短腔机制下,输出混沌态不稳定,0.1 mm的偏差就会导致混沌态与非混沌态之间的转化;长腔机制下,输出混沌态稳定,输出混沌区域较大,证明长腔机制下更有益于获得宽带连续的混沌区域.在弛豫振荡频率为40 GHz、外腔长度为毫米级时,实现了外腔半导体激光器的长腔机制,从而增大了高带宽混沌的参数空间.

Conversion of external cavity mechanism of millimeter-level external cavity semiconductor laser by significantly increasing relaxation oscillation frequency

Optical chaos has conducted in-depth investigation and attracted widespread attention in recent years,owing to its important applications in chaos-based secure communication,fast physical random bit generation,chaotic laser radar, lidar,chaotic optical time domain reflectometer,distance measurement,and optical fiber sensor.The key to these applications is a compact and broadband chaotic light source,because the integrated circuits have an advantage over those setups composed of discrete components in some unique virtues such as smaller size,lower cost,better stability, and better reproducibility via mass production.In order to combine the advantages of the chaotic application and integrated circuits,the integrated chaotic external-cavity semiconductor laser has aroused great interest.Note that, the integrated chaotic external-cavity semiconductor laser can work in both short-and long-cavity mechanisms,which depends on the laser relaxation oscillation frequency.The output of chaotic external cavity semiconductor laser has obvious relaxation oscillation characteristic.When the relaxation oscillation frequency is less than the external-cavity oscillation frequency,the external-cavity semiconductor laser works in short-cavity mechanism.Otherwise,it works in long-cavity mechanism.In this paper,we comparatively analyze the effects of fine-tuning cavity length on the effective bandwidth of the integrated external-cavity semiconductor laser under both short-and long-cavity mechanisms.
First,we comparatively analyze the effects of fine-tuning cavity length and external-cavity feedback rate on the effective bandwidth of the integrated external-cavity semiconductor laser when relaxation oscillation frequency is 5.6 GHz. At the same time,the injection current and carrier lifetime are adjusted to observably increase the relaxation oscillation frequency.Finally,we comparatively analyze the effects of fine-tuning cavity length and external-cavity feedback rate on the effective bandwidth of the integrated external-cavity semiconductor laser when relaxation oscillation frequency is 40 GHz.Results show that for short-cavity mechanism,the chaotic output is not stable:0.1-mm deviation will lead to the conversion from chaotic state into non-chaotic state.By contrast,for the long-cavity mechanism,the chaotic output is more stable and has a larger chaotic area.It proves that the long-cavity mechanism is more feasible and conducive to the continuous achievement of a broadband chaotic laser and broadband continuous chaotic region.According to this feature,we realize the transition from short to long cavity regime by adjusting the injection current and carrier lifetime to substantially increase the relaxation oscillation frequency at the same time.We realize the transition from short to long cavity regime in a cavity length range from 2 mm to 10 mm,and then analyze the influences of the external cavity rate and external cavity length on the spectrum bandwidth of the external cavity semiconductor laser.The results show that under the long cavity mechanism,it is more conducive to the achievement of a broadband continuous chaotic region in a cavity lengt range from 4 mm to 20 mm.Considering the refractive index of integrated material,the external-cavity length for long-cavity mechanism can be shortened to a range from 1 mm to 2 mm.This length fully conforms to the butterfly packaging size.