Stable Locking Dynamics of Semiconductor Lasers for Optical Single-Sideband Modulation and Transmission over Long Optical Fibers
Kun-Lin Hsieh1*, Sheng-Kwang Hwang1,2
1Department of Photonics, National Cheng Kung University, Tainan, Taiwan
2Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan, Taiwan
* presenting author:Kun-Lin Hsieh,
In these two decades, Radio-over-Fiber technology has attracted much research interest for future cellular networks, especially in the fields of 4G or even 5G. Such the scheme optically distributes microwaves over fibers and radiates data at remote base stations through photodetection. In this way, the total cost of each base station can be considerably reduced owing to the absence of expensive high-frequency local oscillators. The straightforward way to superimpose microwaves upon optical carriers is to directly or externally modulate laser diodes. Due to the natural inherence, double-sideband modulation (DSB) signals are generally produced and however give rise to chromatic dispersion-induced microwave power fading effect (MPFE) over optical fibers. Many people therefore paid many efforts to propose various schemes to generate single-sideband modulation (SSB) signals, which can mitigate severe MPFE. In this study, we propose a scheme to achieve DSB-to-SSB conversion using stable locking dynamics of semiconductor lasers. For a laser diode subject to a continuous-wave (CW) optical injection, its cavity resonance will be red-shifted due to the anti-guidance effect when the injected signal regenerates. A pair of asymmetric oscillation sidebands is therefore generated. By taking advantage of such the intensity-asymmetry, a SSB output can be produced after injecting a DSB signal. For instance, a 30-GHz SSB signal with 14-dB sideband rejection ratio (SRR) can be achieved, which improves the microwave power variation from more than 90 dB to less than 4 dB over fibers. Besides, the oscillation frequency of stable locking dynamics is adjustable to meet the operating microwave frequency by properly controlling the injection conditions, e.g., the injection strength and the detuning frequency. It can be widely tunable from less than 15 GHz to more than 60 GHz. Based on the previous research, it is found that the SRR of SSB outputs depends on that of stable locking dynamics under CW injection, determined by the injection conditions. It enables a desired SRR with various operating microwave frequencies as well as a specific operating microwave frequency with different SRRs. However, the inherent noise of the injected laser diodes will simultaneously impose on the lower modulation sidebands of DSB inputs, giving rise to increase in phase noise of the beat microwave signals. It depends on not only the resulted SRR but also the operating microwave frequency. But thanks for the enhancement of lower modulation sidebands, the modulation index of DSB signals is relatively improved, thus amplifying the microwave power. It effectively compensates the deterioration of phase noise, and eventually benefits the improvement of detection sensitivity at base stations. Moreover, such the conversion system is also operable although the operating microwave frequency is unequal to the relaxation resonance frequency. This feature ensure the operation of stability under fluctuations in operating conditions due to ambiance variation.

Keywords: Radio-over-Fiber, Semiconductor Laser, Stable Locking Dynamics, Microwave Power Fading Effect, Single-Sideband Modulation