Microwave Photonic Filter with High Q Based on Optical Frequency Comb
Highly nonlinear fibers are used to broaden the spectrum of input pulses with self phase modulation, and then a waveshaper compensates the time delay distortion brought by higher order dispersion.
A result of the highest Q value ever got for microwave photonic filters(MPFs) based on optical frequency comb(OFC) is reported in Chinese Optics Letters Volume 12 Issue 8, 2014 (/col/abstract.cfm?uri=col-12-8-080601). The high Q tunable MPF can be applied in many fields including high speed communication, high performance radar, radio astronomy, etc.
Microwave photonic filter technology converts the microwave signal into optical signal, then processes it with the optical subsystem in optical domain and recovers the signal with the photodetector. Compared with the conventional electrical filter, it features flexibility and reconfigurability. MPF based on transversal delay taps with finite impulse response is also stable and prone to being tuned, which has a wide range of potential applications. The difficulty of this kind of MPF is to obtain larger Q value.
The research group, led by Prof. Xiaoping Zheng, from Tsinghua University, China, took advantage of the highly nonlinear effect of optical fibers to realize the self-phase modulation spectrum broadening of the narrow pulses from a mode locking laser and obtained a broadband OFC with 356 comb lines, which were used as the optical source for MPFs. They then used a liquid crystal spatial light modulator to construct a cubic phase modulation function and to eliminate the effect caused by broadband optical source. Finally they implemented a MPF with a high Q value of 296. Besides, the filter was also tunable.
Currently, when large scale optical phase process is applied, optical power decays in the liquid crystal, and therefore the insertion loss of the filter is relatively large. The following research plan will be focused on decreasing the insertion loss of MPFs.