在基于四波混频（FWM）效应实现频率变换的全光纤光学参量振荡器（OPO）中，信号光通常只能在较短的距离内获得有效的参量增益。根据光时域反射（OTDR）技术的基本原理，通过测量信号光的后向散射光，获得了信号光在光子晶体光纤（PCF）中的增长曲线，进一步计算得到了PCF中的参量增益分布；将PCF的起始端到参量增益急剧下降处的距离定义为有效作用长度，并且通过水浴实验验证了测量结果。测量发现，在34.5 m的PCF中，有效作用长度仅有10 m，信号光仅在PCF的前10 m获得了有效增益，近似以指数形式增长，在后24.5 m的PCF中，信号光与抽运光已不再满足相位匹配条件，信号光不再增长。这种测量全光纤OPO参量增益分布的方法有助于优化全光纤OPO的结构,提高全光纤OPO的转化效率。

In the all-fiber optical parametric oscillator (OPO) based on four wave mixing (FWM), the signal laser gets effective gain only in a short distance. The increasing trend of signal laser in photonic crystal fiber (PCF) is measured by optical time domain reflecting (OTDR) technology, and the backscatter signal light is measured. The parametric gain distribution in PCF is calculated, and the distance between the start of the PCF and the sharp drop in the parametric gain is defined as the effective interaction length. Moreover, the results are verified by PCF heating experiment. The results show that there is only 10 m of the effective interaction length in the 34.5 m PCF. The signal laser keeps growing exponentially only in the first 10 m PCF, and cannot increase any more in the rest 24.5 m PCF for the signal and the pump light is no longer meet the phase matching conditions. An effective method for measuring the parametric gain distribution is proposed, and the result is useful for optimizing the configuration and improving the efficiency of all-fiber OPO.