Stable Locking Dynamics of Semiconductor Lasers for Photonic Microwave Amplification
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,
The radio-over-fiber technology has attracted considerable attention in the last two decades since it enables the distribution of microwaves over long distances through optical fibers. The simplest scheme to superimpose microwaves on optical carriers is direct or external modulation of semiconductor lasers. However, due to their low modulation efficiency, especially at high frequencies, low optical modulation depth is typically generated. This leads to low microwave power after photodetection at a remote base station and therefore limits the distance of microwave radiation. The most straightforward approach to enhance the microwave power is to increase the optical power received by the photodetector, which however may induce unnecessary fiber nonlinearity and may also result in severe detector damage. To overcome these problems, a variety of different approaches based on, for example, optical filtering, fiber Bragg grating, and Brillouin scattering have been proposed for microwave power amplification under a fixed optical power received by the photodetector through improving the modulation depth of a microwave-modulated optical signal. These approaches rely on considerable power suppression of the optical carrier and therefore require optical amplifiers to compensate for the optical power loss. In this study, the stable locking dynamics of semiconductor lasers are applied to achieve modulation depth enhancement through sideband enhancement, instead of carrier suppression, without significant power dissipation. Under proper continuous-wave optical injection, while the optical oscillation of a semiconductor laser is stably frequency-locked to the injection, the lower sideband of its relaxation resonance becomes resonantly enhanced due to the red-shift of the cavity resonance. By taking advantage of such a resonant enhancement characteristic, the modulation depth of a microwave-modulated optical signal can be considerably enhanced by sending it into the laser under the same operating conditions. As a result, the power of the photodetected microwave is amplified by more than 17 dB under the same received optical power. Accordingly, the sensitivity of the phtodetection is significantly improved by more than 8 dB. The phase quality, such as linewidth and phase noise, of the microwave is preserved after amplification.

* S.K. Hwang's work is supported by the Ministry of Science and Technology of Taiwan under Contract MOST103-2112-M-006-013-MY3.

Keywords: Semiconductor Laser, Nonlinear Dynamics, Stable Locking Dynamics, Radio-over-Fiber, Photonic Microwave Amplification