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.

具有高Q值的光频梳微波光子滤波器

图片说明：利用高非线性光纤对输入光脉冲进行自相位调制频谱展宽，然后利用光谱整形器补偿高阶色散带来的延时畸变。

一项来自清华大学郑小平教授课题组的，关于光频梳微波光子滤波器取得目前最高Q值的研究成果发表在Chinese Optics Letters 2014年第8期上（/col/abstract.cfm?uri=col-12-8-080601 ）。这种高Q值可调谐微波光子滤波器可用于高速通信、高性能雷达、射电天文等多个领域。

微波光子滤波首先将微波信号转换为光信号，然后对光信号进行适当处理，最后用光电探测器将光信号转换成电信号。相比传统电学滤波器，它具有结构灵活、重构性强等优点。基于有限冲激响应的横向延时抽头微波光子滤波器同时还有稳定性好、容易调谐等特点，应用前景看好。该类滤波器的一个难点在于如何获得高Q值。

该课题组利用光纤高非线性效应对锁模激光器输出的窄脉冲进行自相位调制频谱展宽，得到了谱线条数为356的宽带光频梳，并以此作为微波光子滤波器的光源。利用液晶空间光调制器构造立方相位函数，消除高阶色散对宽谱光源引入带来的负面影响，最终实验获得了Q值高达296的具有调谐能力的微波光子滤波器。

目前由于液晶在进行光域相位处理时存在信号光功率的损失，因此滤波器的插损较大。如何进一步降低滤波器插损是研究人员下一步工作的重点。