Research On Theory And Applications Of The Novel Ultra Narrow Band Optical Filter

Ultra narrow band optical filter (filtering bandwidth is not larger than MHz level) has great value in the areas of high precision optical signal processing,single-longitudinal-mode narrow-linewidth optical source and so on.Narrowing filtering bandwidth is one development tendency of optical filter.But fabricating this kind of filter faces many challenges because of the ultra-high frequency of lightwave and limited manufacturing technology at present. These challenges focus on 3 points: the first one is ultra-narrowing the filtering bandwidth, the second one is tuning the center frequency broadly,the third one is stabilizing the center frequency for a long time. This dissertation researches based on these 3 challenges and get some innovative achievements as below:1. Aiming at the challenges of ultra-narrowing filtering bandwidth and broadband tunability, this dissertation proposes the equivalent optical filter based on delay match. This new filter combines narrowband microwave filter with photonic technology and realizes equivalent ultra narrow band optical filtering. The bandwidth of this equivalent optical filter equals to that of microwave filter we use, and the center frequency of this equivalent optical filter mainly depands on the local oscillator (LO) we use. Further, the dissertation uses this optical filter into the research of single-longitudinal-mode narrow-linewidth optical source and proposes two methods. The first method is single longitudinal mode extraction from a femtosecond optical frequency comb (fs-OFC). A single longitudinal mode can be extracted from 100 MHz fs-OFC successfully, whose 3-dB Lorentz linewidth (1.3 kHz) is only 1/35 of that of LO we use. The second method is combing this optical filter with optical resonator. The dissertation proposes a novel "photonic & electronic hybrid cavity" which is different from the traditional opto-electronic oscillator (OEO). This new structure can suppress the phase noise introduced by LO obviously (the highest suppression ratio is more than 65 dB) and achieve single-longitudinal-mode narrow-linewidth laser.2. Aiming at the challenge of ultra-narrowing filtering bandwidth, this dissertation designs and builds a fiber ring filter with 1.2 MHz filtering bandwidth. Then this dissertation introduces fiber grating and waveshaper to realize single transmission peak selecting of fiber ring filter. Based on coherent detection and combination of these filters, this dissertation realizes the microwave photonics filter (MPF) with variable filtering bandwidth,broadband tenability in 40 GHz and single transmission peak simultaneously.3. Aiming at the challenge of filtering stability, this dissertation uses PID control to improve the stability of optical filter and realize 3 technologies:real-time compensation of optical phase, real-time wavelength tracking of two lasers and accurately locking the laser wavelength to a resonance peak of optical filter. The experiment results show that real-time compensation of optical phase technology can work both in linear and nonlinear region, and improving the stability of out:put light obviously; real-time wavelength tracking technology can lock two laser wavelengths for a long time and the accuracy of locking is less than 1MHz; laser wavelength can be locked with a resonance peak of optical filter whose 3dB bandwidth is only 500kHz and the structure is much more simple than the typical PDH system.