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Optical circulators [Ramaswami et al., 2009; Hecht, 2005; Mynbaev & Scheiner, 2000] are important nonreciprocal devices that can direct a light from one port to another in only one direction. They are essential components in the construction of fundamental network modules, such as optical add–drop multiplexers, dispersion-compensation, optical amplifiers, and timedomain reflectometry. Different kinds of design of optical circulator have been proposed [Iwamura et al., 1979; Shirasaki et al., 1981; Yokohama et al., 1986; Koga, 1994; Wang, 1998]. According to the operation principles, optical circulators can be divided into three types, traditional, waveguide, and holographic. The traditional optical circulators mainly apply spatial walk-off polarizers (SWPs) [Nicholls, 2001], Faraday rotators (FRs), and half-wave plates (Hs) to implement its function. The waveguide optical circulators utilize a waveguide Mach–Zehnder interferometer to implement the function of SWPs. The holographic optical circulators apply holographic optical elements to replace traditional SWPs. Accordingly, the spatial walk-off polarizer is a key component in the design of optical circulator that significantly influences the performances and cost of a device. Traditional spatial walk-off polarizers are essentially birefringent crystals that can split an optical beam into two orthogonally polarized beams. However, birefringent crystals suffer from challenges of highly optical qualities, crystal manufacturing, and hard optical fabrications. The highly optical qualities mean high transparency and optical uniformity for a wide spectrum range, high birefringence, and enough hardness. The main crystal growth thechnologies are Czochralski method and Verneuil process. The hard fabrications include xray orientation, slicing, polishing, coating, cleaning, testing, packaging, and related processes. Therefore, the cost is hard to down. In addition, limited by the finite birefringence, the beam splitting distance is small. Therefore, the device length is hard to shorted. Compare to the crystal-type SWPs, polarization-selective substrate-mode volume holograms (PSVHs) [Huang, 1994] have a large splitting angle and several superior advantages. A PSVH are phase volume holograms stacked on a glass or plastic substrate and signals transmit in the substrate by total internal reflection. With this planar structure, PSVHs have advantages of easy fabrication, low cost, high efficiency, compactness, easy coupling, and easily to combine with other elements. Due to these merits, PSVHs had been widely applied

Polarization-Selective Substrate-Mode Volume Holograms and Its Application to Optical Circulators