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High Precision Photonic Crystal Fiber Optic Gyroscope for Space Application Xiaobin Xu,* Ningfang Song, Zhihao Zhang, Wei Cai and Fuyu Gao Department of Instrument Science and Opto-electronic Engineering, Beihang University, Xueyuan Road 37#, Beijing 100191, P. R. China *
[email protected]
Abstract: A low-loss (~1.7dB/km) PM-PCF has been designed and fabricated, based on which a PCFOG having a bias stability of ~0.002°/h is developed. The PCFOG has a flight test in “Tianzhou-1” Cargo Spaceship. OCIS codes: (060.4005) Microstructured fibers; (060.5295) Photonic crystal fibers; (060.2800) Gyroscopes
1. Introduction Photonic crystal fiber (PCF) is a class of optical fiber based on two-dimension photonic crystal, and strongly attracts people’s interest owing to its special characteristics such as endless single mode, easy to design and so on. Since the first PCF was born in 1996 [1], PCF has experienced great development whether in theory or in fabrication. For single-mode PCF, the loss is close to the intrinsic loss of silica glass [2]; for polarization-maintaining PCF, the loss is smaller than 2dB/km; for other PCFs for special applications, such as large mode, high nonlinearity, high power and so on, they are also very mature [3]. Optic characteristics of PCF are primarily decided by periodical air-hole structure that hardly depends on environment, e.g. the birefringence’s dependence on temperature for PCF is about two orders of magnitude smaller than conventional PANDA fiber [4]. Besides, PCF is made of pure silica, so it has lower sensitivity to radiation as compared to conventional PANDA fiber which core is made of Ge-dopped silica. Those special properties make PCF very suitable for the application in space, but it has never been really applied in space. In this paper, we, to the best of our knowledge, first report the PCFOG for real application in space. 2. Photonic crystal fiber PCF is the key component of the PCFOG, and generally a FOG requires that the fiber should have low loss, high and stable birefringence to guarantee performance, and as thin as possible to reduce volume of the FOG coil. According to these requirements, we have analyzed the dependence of the confinement loss on number of layers of the cladding, and choose four-layer cladding to confine the light. Four layers are fewer than conventional five layers and increase some confinement loss, but reduce to a large degree the stacking and fabrication complexity, moreover, the diameter of the cladding and fiber is cut down. Based on the four-layer cladding structure, the model is established with the finite element method, as illustrated in Fig.1. We simulate and optimize the structure parameters (small hole diameter d, big hole diameter D and ptich Λ), and finally d/Λ and D is determined as ~0.56 and ~5.7, respectively.
Fig. 1 Model for the photonic crystal fiber.
Based on the design structure, the popular “stack-and-draw” technique is used to fabricate the PCF. In fact, single-mode PCF has achieved a loss of ~0.18dB/km many years ago [2], but polarization-maintaining PCF is still relatively larger. We have spent a few years on the optimization of fabrication process in order to reduce the loss, and recently we have realized a polarization-maintaining PCF prototype having a loss of smaller than 1.5dB/km. For the PCF used here, it was fabricated at least one year ago. Its cross section is shown in Fig.2(a), which has a coating diameter of ~135μm and a cladding diameter of ~100μm; some cones drawn from preforms are shown in Fig.2(b); the transmission spectrum is given in Fig.2(c), indicating a very narrow water peak and a loss of
Su1A.3.pdf
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~1.7dB/km; the index difference between the slow and fast birefringent axis is ~ 6×10-4. What is more, mode is also very important for the FOG application, multimodes are harmful for the FOG performance, especially when the coupling exists among these modes. Therefore, we test the distribution of the modes within the PCF using optical coherence domain polarimeter, as illustrated in Fig.2(d). It is obvious that there exist high-order modes, which can be explained by the fact that the air filling ratio in the cladding is slightly enlarged to reduce loss, and large air filling ratio must cause high-order modes. However, the amplitude of high-order modes decreases as the fiber gets longer, e.g. it decreases from ~ -20dB to ~ -30dB when the length increases from ~0.6m to ~3m, respectively. Therefore, those high-order modes can attenuate to an ignorable level for a fiber coil as long as thousands of meters.
Fig.2 Cross section and optical properties of the PCF. (a) Cross section; (b) Cones drawn from preforms; (c) Loss spectrum; (d) High-order modes distribution for PCFs having different lengths.
3. Photonic crystal fiber optic gyroscope The schematic diagram of photonic crystal fiber optic gyroscope (PCFOG) is shown in Fig.3 [5]. A high-power (>15mW) amplified spontaneous emisstion (ASE) source is used to provide 1550-nm light. A special integrated optic chip (IOC) having an extinction ratio of larger than 70dB is used to polarize the light and suppress the polarization nonreciprocity error. A ~2000-m fiber coil is wound with the fabricated PCF based on quadrupolar winding technique.
Fig.3 Schematic diagram of the photonic crystal fiber optic gyroscope
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Asia Communications and Photonics Conference (ACP) © OSA 2017
Fusion splicing is applied to connect the PCF coil to the IOC pigtails in order to guarantee reliability that is extremely important for the space application. Backreflection always exists at fusion splicing points, which causes large nonreciprocity error and noise when the backreflection is strong, so fusion splicing process has to be optimized to avoid collapse of the air holes, and PCF mode field is optimized to match that of the IOC pigtails. Finally, the backreflection in the PCFOG is ~60dB/mm, as shown in Fig.4, which corresponds to an error of ~10-4°/h [6].
Fig.4 Test results of backreflection at fusion splicing points between PCF coil and IOC pigtails.
A PCFOG engineering prototype (Fig.5(a)) is developed based on the technologies mentioned above, and the static performance is given in Fig.5(b), which shows a bias stability of ~0.002°/h. What is more, the PCFOG has stood the rigorous tests performed according to aerospace standard, including thermal cycles, vibration, shock, radiation and so on. Finally, the PCFOG has a flight test in “Tianzhou-1” Cargo Spaceship that was launched in April 2017 and it has been working normally with stable precision. This experiment has verified the feasibility and advantage of the PCFOG’s application in space for the first time.
Fig.5 (a) PCFOG prototype and (b) static performance at room temperature.
4. Conclusions A PM-PCF having a four-layer cladding and a coating diameter of ~135μm has been designed and fabricated. The PM-PCF has a loss of ~1.7dB/km, an index difference of ~6×10-4 between slow and fast birefringent axis. A fiber optic gyroscope engineering prototype based on PM-PCF coil is developed, exhibiting a bias stability of ~0.002°/h. The PCFOG has a flight test in “Tianzhou-1” Cargo Spaceship that was launched in April 2017, and the experiment has verified the feasibility and advantage of the PCFOG’s application in space. 5. References [1] [2] [3] [4] [5] [6]
J. C. Knight, T. A. Birks, P. St. J. Russell and D. M. Atkin, “All-silica single-mode optical fiber with photonic crystal cladding,” Opt. lett. 21, 1547-1549 (1996). K. Tajima, “Low loss PCF by reduction of hole surface imperfection,” Optical communication-post deadline papers. VDE, 2007 pp.1-2. http://www.nktphotonics.com/lasers-fibers/en/ P. Ma, N. Song, J. Jin, J. S. X. Xu, “Birefringence sensitivity to temperature of polarization maintaining photonic crystal fibers,” Opt. laser technol. 44, 1829-1833 (2012). H. C. Lefèvre, The Fiber-optic Gyroscope, 2nd ed. (Artech House: Boston, United States, 2014). X. Xu, N. Song, Z. Zhang, J. Jin, “Backward secondary-wave coherence errors in photonic bandgp fiber optic gyroscope,” Sensors 16, 851 (2016).
