Degenerate correlated photon pairs (DCPPs) have been widely used in quantum information science, especially in the areas of quantum computation, quantum state control and precision measurement, which are typically generated in aχ(2) nonlinear crystal through the spontaneous parametric down-conversion. However, such a source is not compatible with optical fiber as large coupling losses occur when the pairs are launched into it, which restricts its direct application to quantum information processing system. More recently, DCPP generation from spontaneous four-wave mixing inχ(3) optical fiber has aroused strong interest, due to its advantages of compatibility with existing fiber networks and free of alignment. The process of generating DCPP in fiber can be described as follows: two pump photons at different frequenciesωp1 andωp2 scatter through theχ(3) nonlinearity to create a pair of identical photons at the mean frequencyωc, such thatωp1+ωp2=2ωc. Because the collinear tensor componentχ(3)xxxx in a Kerr nonlinear medium is 3 times as large as the tensor componentχ(3)xyxy, the co-polarized four-wave mixing is preferred, which means the two pump photons and new-born twin photons are both co-polarized. Therefore, it is very challenging to deterministically separate the fiber-based DCPP, since the twin photons share the same properties in all degrees of freedom: frequency, polarization and spatial. Sagnac fiber loop (SFL), composed of a piece of nonlinear fiber and 50/50 coupler, is presented as the splitter for DCPP based on the reversed Hong-Ou-Mandel quantum interference of counter-propagating DCPPs. The SFL can be configured as a total reflector, total transmitter or equally transmissive and reflective state, which sets the differential phases of counter-propagating DCPPs meeting at 50/50 coupler to beπ, 0 and−π, respectively. In order to satisfy the differential phase requirement for completely splitting the DCPP, the SFL is always set to be equally transmissive and reflective state, however, the polarization-mode matching of counter-propagating DCPPs is not easily achieved due to the disturbance of fiber birefringence. According to the Jones matrix derivation of DCPP propagating in the SFL, the polarization mode of counter-propagating DCPPs when interference at 50/50 coupler is automatically matched, if the SFL is set as a total reflector or total transmitter. In experimental scheme, utilizing the SFL as a total reflector, the 1.1 nm bandwidth and 1544.53 nm central wavelength DCPPs are generated by two pulsed light beams pumping the 300 m dispersion-shifted fiber in the SFL. Using the two pieces of single mode fiber connecting the 300 m dispersion-shifted fiber and 50/50 coupler, whose length difference is fixed at 3.3 m, the differential phase of counter-propagating DCPPs highly dependent on the dispersion properties of single mode fiber is managed at 2π for fully distributing DCPPs into two spatial modes. The SFL is also cooled by liquid nitrogen to suppress the spontaneous Raman scattering photons, which degrades the fidelity of DCPP source. The measured ratio of coincidence to accidental-coincidence of DCPPs from one port is approximately 1.8 : 1, which indicates that the coincidence counts mainly originate from accidental coincidence counts and extra coincidence counts from photon bunching and there are not any DCPPs outputting from one port. Meanwhile, the ratio of best measured coincidence to accidental-coincidence of DCPPs from two ports reaches 47 : 1, when the average power of two pumps is fixed at 0.026 mW. The experimental results demonstrate that the high purity and fully spatial separation DCPPs are successfully prepared in optical fibers, which is a very useful tool for realizing various quantum information systems. How the spatial state of outputting DCPPs depends on the length difference between single-mode fiber and detuning wavelength is also discussed in detail.%光纤中自发四波混频过程产生的频率简并关联光子对是实现量子信息处理和高精密测量的重要资源。 Sagnac光纤环是制备简并关联光子对的典型装置，利用环中的对向传播光子对在50/50分束器的量子干涉，实现两个孪生简并关联光子的空间分离。本文利用两束不同波长的脉冲光抽运由300 m色散位移光纤和50/50分束器组成的Sagnac光纤环，通过环中单模光纤色散引入的相位差控制光子对的对向传播相位差，获取了空间模式分离的窄带简并关联光子对。
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