A small-diameter polarization-maintaining photonic crystal fiber (PM-PCF) for mini coils of spaceborne miniature fiber-optic gyroscopes is proposed in this paper. To ensure the strength of the small-diameter PM-PCF, a four-ring air holes structure is adopted. Using the full vector finite element method, dependence studies of modal field distribution, birefringence, and confinement loss on several key structure parameters are numerically investigated. The optimized parameter region is obtained. An optimized PM-PCF is fabricated, which can achieve similar to or even better optical properties than that of commercial PM-PCFs. The coating and cladding diameters of the optimized PM-PCF are 135 μm and 100 μm, respectively. Meanwhile, the optimized small-diameter PM-PCF shows a proof test level of 0.5%. The attenuation of the PM-PCF at 1550 nm is $\sim 2\mathrm{dB}/\mathrm{km}$. Typical volume of a mini coil wound with 300 m optimized PM-PCF is $5.9{\mathrm{cm}}^3$, which is decreased by $\sim 60\%$ compared with a commercial PM-PCF coil of the same length. The bias stability of this coil is comparable with that of a conventional PMF coil of comparable length. Thus, the optimized small-diameter PM-PCF is suitable for mini coils of spaceborne miniature fiber-optic gyroscopes.

We propose a method based on the white-light interference technique for measuring the Verdet constant of a polarization-maintaining air-core photonic bandgap fiber (PM-PBF). The experimental results show that the Verdet constant of the PM-PBF is ~3.3 mrad/T/m for the broadband light with a spectral width of ~38 nm and a mean wavelength of ~1550 nm, which is ~124 times less than that of a conventional stress-induced birefringent fibers called PANDA fibers (~0.41 rad/T/m for the same broad-spectrum light). The results indicate that the nonreciprocal error induced by the Faraday effect in a fiber optic gyroscope (FOG) made of the PM-PBF is theoretically ~25 times less than that of a conventional FOG made of the PANDA fiber when other conditions, such as the fiber twist, fiber coil area, and so on, are the same.

We investigated the steady state gamma-ray radiation response of pure-silica-core photonic crystal fibers (PSC-PCFs) under an accumulated dose of 500 Gy and a dose rate of 2.38 Gy/min. The radiation-induced attenuation (RIA) spectra in the near-infrared region from 800 nm to 1700 nm were obtained. We find that the RIA at 1550 nm is related with hydroxyl (OH⁻) absorption defects in addition to the identified self-trapped hole (STH) defects. Moreover, it is proposed and demonstrated that reduced OH⁻ absorption defects can decrease the RIA at 1550 nm. The RIA at 1550 nm has effectively declined from 27.7 dB/km to 3.0 dB/km through fabrication improvement. Preliminary explanations based on the unique fabrication processes were given to interpret the RIA characteristics of PSC-PCFs. The results show that the PSC-PCFs, which offer great advantages over conventional fibers, are promising and applicable to fiber sensors in harsh environments.

A theoretical model of the thermal phase noise in a square-wave modulated solid core photonic crystal fiber-optic gyroscope has been established, and then verified by measurements. The results demonstrate a good agreement between theory and experiment. The contribution of the thermal phase noise to the random walk coefficient of the gyroscope is derived. A fiber coil with 2.8 km length is used in the experimental solid core photonic crystal fiber-optic gyroscope, showing a random walk coefficient of 9.25 × 10⁻⁵ deg/√h.

The transverse magneto-optic error is a significant nonreciprocal error of fiber optic gyroscopes, particularly for the miniature fiber optic gyroscope. In this paper, the transverse magneto-optic error of a solid-core photonic-crystal fiber optic gyroscope is theoretically analyzed and experimentally measured, as well as compared with that of the conventional fiber optic gyroscopes. The results show that the transverse magneto-optic error per turn of the miniature photonic-crystal fiber optic gyroscope is ~ 3.5 times smaller than that of the conventional fiber optic gyroscopes. A thinner $\mu $ -metal shield can be used in a miniature photonic-crystal fiber optic gyroscope, resulting in less cost and weight. Meanwhile, the experimental and the calculated results agree well with each other, which illustrates that the transverse magneto-optic error derives mainly from the bend-induced asymmetry distribution of the refractive index in fiber. Considering the remarkable properties of the solid-core photonic-crystal fibers, they are the good candidates for the future miniature fiber optic gyroscopes.