Conversion between Optical ASK and Optical FSK Using Nonlinear
Dynamics of Semiconductor Lasers
Sheng-Kwang Hwang,1,2 Sze-Chun Chan,3 Shiuan-Li Lin,1 Cheng-Hao Chu,1 and Yu-Han Hung1
1
Department of Photonics, National Cheng Kung University, Tainan, Taiwan
2
Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan, Taiwan
3
Department of Electronic Engineering, City University of Hong Kong, Hong Kong, China
Due to the continuous demand in transmission capacity and distance, many advanced optical modulation formats have been proposed to substitute for conventionally adopted amplitude-shift keying (ASK), including frequency-shift keying (FSK) and phase-shift keying (PSK). Therefore, different optical modulation formats would be adopted for different optical communication networks in the future depending on their scales and applications. Accordingly, optical modulation format conversion becomes a key functionality bridging different optical networks. In fact, many conversion schemes based on different device nonlinearities have been proposed over the past years, which however focus mainly on conversion from ASK to PSK. In this study, we propose to use the period-one (P1) nonlinear dynamics of semiconductor lasers for optical modulation format conversion between ASK and FSK. When a semiconductor laser is subject to an incoming optical carrier, equivalently an external optical injection, it can enter, under proper injection conditions, into P1 dynamics through Hopf bifurcation due to the radical modification in field-carrier coupling of the injected laser resulting from the dynamical competition between injection-imposed laser oscillation and injection-shifted cavity resonance. Equally-separated spectral components appear, of which intensity and frequency depend strongly on the injection level and frequency. This suggests that a dynamical amplitude or frequency variation of the incoming optical signal, such as ASK or FSK, respectively, would lead to corresponding dynamical variation in amplitude and frequency of each spectral component. Therefore, by properly selecting the optical frequency of the output optical carrier and by minimizing the residual ASK and FSK modulation, both ASK-to-FSK and FSK-to-ASK conversions can be achieved, where bit-error ratio down to 10-12 is achieved with a slight power penalty. Only a typical semiconductor laser is necessary as the key conversion unit. In addition, different output modulation depths and different frequency shifts of the optical carrier can also be achieved. The latter allows a simultaneous frequency conversion of the optical carrier if required. These characteristics may increase the flexibility and re-configurability of the proposed system.
