In-line polarizer used in all-0°-splice resonator fiber-optic gyro Huilan Liu,1,2 Wei Wang,1,2,* Junjie Wang,1,2 Lishuang Feng,1,2 and Yinzhou Zhi1,2 1
Science and Technology on Inertial Laboratory, Beihang University, 100191 Beijing, China
2
Key Laboratory of Precision Opto-mechatronics Technology, Ministry of Education, Beihang University, 100191 Beijing, China *Corresponding author:
[email protected] Received 19 August 2013; revised 10 October 2013; accepted 15 October 2013; posted 16 October 2013 (Doc. ID 196023); published 8 November 2013
A method to suppress the polarization-fluctuation-induced drift in a resonator fiber-optic gyro is proposed in this paper. By inserting one in-line polarizer whose polarization extinction ratio is 30 dB into a polarization-maintaining fiber resonator with 0° polarization-axis splices, the unwanted resonance is introduced to high loss and therefore the ratio of the resonance height of the desired eigen-states of polarization (ESOP) to the unwanted ESOP is 74 dB theoretically; thus the polarization-fluctuationinduced drift is adequately suppressed. The new scheme has excellent operability and high temperature stability simultaneously. Compared to the resonator with twin 90° polarization-axis rotated splices, this scheme does not need precise length difference control. This work is of great importance in the research on resonator integrated optic gyros. © 2013 Optical Society of America OCIS codes: (060.2370) Fiber optics sensors; (060.2800) Gyroscopes. http://dx.doi.org/10.1364/AO.52.007821
1. Introduction
Resonator fiber-optic gyros (R-FOGs) employing a fiber ring resonator obtain the rotation rate through measuring the resonant frequency difference between the clockwise (CW) and the counter clockwise (CCW) propagating lightwaves based on the Sagnac effect. R-FOGs have the potential to satisfy the Inertial Navigation System (INS) requirement (10 − 7 rad∕s) with a short fiber coil (5–10 m) [1]. However, noises from various effects limit the practical performance of R-FOGs, such as polarization-fluctuation-induced drift, the Kerr effect, and the Shupe effect [2,3,4]. The polarization-fluctuationinduced drift is one of the most important noises. In a polarization-maintaining fiber (PMF) resonator, two eigen-states of polarizations (ESOPs) are excited [4]. The phase difference between the desired ESOP 1559-128X/13/327821-05$15.00/0 © 2013 Optical Society of America
and the unwanted ESOP drifts rapidly as the temperature changes, which causes large detection errors [5]. To suppress the polarization-fluctuation-induced noise sufficiently, a resonator employing a single 90° polarization-axis rotated splice was proposed [6]. The two ESOPs excited in this method are separated from each other by half of the free spectral range (FSR). This scheme cannot effectively suppress the interference-type error. A PMF resonator with twin 90° polarization-axis rotated splices was also proposed to excite a single ESOP by setting the length difference of the fiber segments between two 90° polarization-axis rotated splicing points (Δl) to a half of the beat-length of the fiber (B/2) [7]. Unfortunately, keeping Δl at B/2 has difficult operability because the short beat length (1–5 mm) varies with the environmental disturbance. And this scheme cannot operate over a wide temperature range. Besides, a PMF resonator inserting two in-line polarizers with twin 90° polarization-axis 10 November 2013 / Vol. 52, No. 32 / APPLIED OPTICS
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rotated splices was reported [8]. The polarizers introduce high loss for the unwanted ESOP and the resonator excites only one resonance. However, the number of polarizers is two in this attempt. This gives rise to high loss for the resonator which decreases the finesse. In this paper, a new scheme to suppress the polarization-fluctuation-induced drift is proposed, which adopts a PMF resonator with only one in-line polarizer employing 0° polarization-axis splices. The principle of this new scheme has been explained by numerical simulation, and the effectiveness in polarization-fluctuation reduction has been demonstrated experimentally. Furthermore, the research of this new scheme has guiding significance on the resonator integrated optic gyro (R-IOG) based on a single-polarization waveguide, which is regarded as the miniaturization direction of the optic gyro [9,10,11], whose resonator has difficulty in adopting 90° polarization-axis rotation currently. 2. Principle
A PMF resonator with 0° polarization-axis splices will support two orthogonal ESOPs even if the input lightwave is linearly polarized along one of the polarization axes of the fiber because of the misalignment of polarization-axis rotated splicing and the polarization crosstalk at the coupler [12]. While the unwanted ESOP can be sufficiently suppressed if an in-line polarizer whose orientation corresponds to the input lightwave is inserted into the resonator, and so is the polarization-fluctuation-induced drift. The resonator structure of this new scheme is shown in Fig. 1. We assume that the input lightwave is linearly polarized along the x axis of the PMF. The resonator adopts two fused PMF couplers, C1 and C2, with coupling ratios of 97∕3 whose excess losses are both 0.2 dB. The diameter of the resonator is 0.15 m. All the pigtails of the couplers, L1 and L4, L2 and L3, have the length of 2.1 m. The four fiber pigtails are connected with 0° polarization-axis splices. The x-axis-oriented in-line polarizer P with a total length of 2.4 m is spliced between the pigtails L1 and L2. The insertion loss and the polarization
Fig. 1. Basic configuration of resonator with in-line polarizer. C, PM fiber coupler; P, in-line polarizer; F, PM fiber; L, PM fiber with length of Li, i 1, 2, 3, 4; and FRR, fiber ring resonator. 7822
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extinction ratio of the polarizer are 0.3 and 30 dB, respectively. The fiber F with the same length of the in-line polarizer is spliced between the pigtails L3 and L4. Compared to the resonator with twin 90° polarization-axis rotated splices that need a precisely adjusted length difference between (L1 L4) and (L2 L3) through the feedback control loop [13], this resonator does not need precise control on the length difference because there are no polarization-axis rotated splices inside the resonator. Although it will lead to variation of the phase difference between the desired ESOP and the unwanted ESOP theoretically, the latter is so weak due to the in-line polarizer that its maximum attribution to the output is below the shot noise limit, according to the numerical simulation. It means that the new scheme has a wide tolerance for the R-FOG’s fabrication. Figure 2 depicts the calculated resonant curves of the resonator with the in-line polarizer P applied or not. In the resonator of 0° polarization-axis splices without a polarizer, as shown in Fig. 2(a), the unwanted ESOP (small peak) in the resonant curve drifts across the desired ESOP (large one) rapidly as temperature changes, which causes large polarization-fluctuation-induced noise and deteriorates the performance of R-FOG seriously. The polarization
Fig. 2. Simulation of the resonant curves (a) under the condition of the resonator without in-line polarizer and (b) under the condition of the resonator with in-line polarizer.
simulation; therefore the polarization-fluctuationinduced drift is largely reduced. 3. Experiment and Result
Fig. 3. Resonant curves of the resonator with an in-line polarizer when the phase separation between the two ESOPs is 0°.
angle between the incident light and the x axis of the PM fiber is set at 20° to make the unwanted ESOP easy to observe. We define the resonance height as the difference of the corresponding peak value and the minimum value of the resonant curve and the polarization suppression ratio (PSR) as the ratio of the resonance height of the desired ESOP and the unwanted ESOP. Assuming that the polarization crosstalk at the coupler is 20 dB, which can be replaced by an incident polarization angle of 5.71° equivalently, then the PSR is about 16.5 dB. When the x-axis-oriented polarizer is inserted into the resonator, the lightwave propagating along the y axis of the fiber will attenuate greatly. Thus the unwanted ESOP is almost suppressed and only the desired ESOP is excited, as shown in Fig. 2(b). When the phase separation between the two ESOPs is 0° in the resonator without in-line polarizer, the two ESOPs’ resonances coincide and have equal intensities, which causes the largest polarization noise [8]. By contrast, the resonant curve of the resonator with in-line polarizer when the phase separation between the two ESOPs is 0° is shown in Fig. 3, in which the black line and the blue line stand for the desired ESOP (ESOP1) and the unwanted ESOP (ESOP2), respectively. Assuming that the polarization extinction ratio of the polarizer is 30 dB, the PSR is 74 dB approximately by
The experimental setup of R-FOG with a resonator employing 0° polarization-axis splices in which one in-line polarizer is integrated is shown in Fig. 4. Lightwave from a 1550 nm fiber laser (FL) (linewidth ≤1 kHz) is equally divided and injected into the resonator in the CW and CCW direction. The LiNbO3 integrated optical phase modulators PM1 and PM2 are driven by triangular waves. The triangular waves are generated from the signal process unit to be with the same amplitudes and frequencies but opposite phases to reduce the Rayleigh backscatteringinduced noise [14]. The CW and the CCW lightwaves from the resonator are detected by the photodetectors PD1 and PD2, respectively. Two isolators, ISO1 and ISO2, are placed in front of PD1 and PD2 to suppress the backreflection from the photodetectors. The output of PD1 is used to lock the central frequency of the light source to the resonant frequency of CCW pathway through the signal process unit. Then the rotation rate of the gyro can be obtained by demodulating the CW lightwave. The resonant curve is first measured at different incident polarization angles (0°, 30°, 60°, 90°) by scanning the central frequency of the input lightwave by applying triangular waveform voltage to the piezo transducer (PZT) of the light source, as shown in Fig. 5. Only one resonance is obviously observed. Therefore, this new scheme largely reduces the requirement for the alignment of incident polarization angle. The resonance height decreases as the incident polarization angle varies from 0° to 90°, which is determined by the projection of input lightwave electric field onto the desired ESOP. Furthermore, the measured resonant curve at different temperatures is also obtained, as shown in Fig. 6. Similarly, only one resonance is observed either at the room temperature or after heated. The unwanted resonance is greatly suppressed. It is verified that the resonant curve has high temperature stability. Then the demodulation curve is obtained to estimate the effectiveness of suppression experimentally, as depicted in Fig. 7. Only the desired ESOP is observed in the demodulation curve with
Fig. 4. Experimental setup of R-FOG with a resonator integrating one in-line polarizer. FL, fiber laser; PM, phase modulator; C, coupler; ISO, isolator; and PD, photodetector. 10 November 2013 / Vol. 52, No. 32 / APPLIED OPTICS
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Fig. 7. Demodulation curve of R-FOG.
Fig. 5. Measured resonant curve at different incident polarization angles.
peak-to-peak value of 3189, and the corresponding demodulated digital output of the unwanted ESOP is less than 1, so the PSR is over 35 dB. The difference between the simulation and experimental
Fig. 8. Rotation test result of the R-FOG prototype.
Fig. 6. Measured resonant curve at different temperature (a) under the condition that the resonator is in room temperature and (b) under the condition that the resonator is heated. 7824
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Fig. 9. Static test result of the R-FOG prototype.
results is mainly attributed to the open-loop dynamic range of the test system. The rotation measurement result of the R-FOG is shown in Fig. 8. The rotation rate is 3°∕s. Simultaneously, the R-FOG bias stability is tested based on the setup shown in Fig. 4. The result is stated in Fig. 9, giving the bias stability of 23°∕h over 600 s with an integration time of 10 s. If an all-0°-splice PMF ring resonator without in-line polarizer is used in the R-FOG system, the frequency-lock servo loop cannot work regularly as the temperature-induced polarization fluctuation is too large. Therefore, it is shown that the new scheme is an effective measure in reducing the polarization-fluctuation induced drift.
4. Conclusion
In summary, we propose a new scheme of R-FOG adopting a PMF resonator with one in-line polarizer employing 0° polarization-axis splices. And we have theoretically and experimentally demonstrated the effectiveness in suppressing the polarizationfluctuation-induced drift in the R-FOG. The ratio of the resonance height of the desired ESOP and the unwanted ESOP is 74 dB theoretically based on the new scheme, which is verified that the unwanted ESOP is greatly suppressed due to the in-line polarizer in the resonator. Compared with the resonator with twin 90° polarization-axis rotated splices, this scheme does not need precise length difference control. The system has excellent operability and high environmental temperature stability. A bias stability of 23°∕h has been successfully carried out. Furthermore, this work is of great importance in the research of R-IOG based on single-polarization waveguide. The authors would like to acknowledge financial support from the National Natural Science Foundation of China (No. 61171004) and the Institute of Opto-Electronic Technology of Beihang University.
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