Huilian Ma

Reduction of polarization-fluctuation induced drift in resonator fiber optic gyro by a resonator integrating in-line polarizers

A method to decrease the polarization-fluctuation induced drift in a resonator fiber optic gyro (R-FOG) is demonstrated by inserting two in-line polarizers in a polarization-maintaining fiber resonator with twin 90° polarization-axis rotated splices. The in-line polarizers attenuate the unwanted resonance by introducing high loss for the unwanted eigenstates of polarization in the resonator. Compared to the resonator without in-line polarizers, the polarization-fluctuation induced drift is reduced by 6×10(3) times. The desired resonance in the resonator can keep excellent stability in a wide temperature range; thus the temperature-dependent polarization-fluctuation drift in the R-FOG is sufficiently suppressed. A typical bias stability of 4.7°/h over 6500 s with an integration time of 10 s has been carried out. To the best of our knowledge, the long-term bias stability and high temperature stability are the best ever demonstrated in an R-FOG.

Open-loop operation experiments in a resonator fiber-optic gyro using the phase modulation spectroscopy technique.

A detection system in the resonator fiber-optic gyro is set up by the phase modulation (PM) spectroscopy technique. The slope of the demodulated curve near the resonant point is found to affect the ultimate sensitivity of the gyro. To maximize the demodulated signal slope, the modulation frequency and index are optimized by the expansion of the Bessel function and optical field overlapping method. Using different PM frequencies for the light waves, the open-loop gyro output signal is observed. The modulation frequency in this PM technique is limited only by the cutoff frequency of the LiNbO3 phase modulators, which can reach several gigahertz. This detection technique and system can be applied to the resonator micro-optic gyro with a less than 10 cm long integrated optical ring.

Resonator fiber optic gyro employing a semiconductor laser

Resonator fiber optic gyro (RFOG) based on the Sagnac effect has the potential to achieve the inertial navigation system requirement with a short sensing coil. Semiconductor laser is one of the key elements for integration and miniaturization of the RFOG. In this paper, an RFOG employing a semiconductor laser is demonstrated. The model of the laser frequency noise induced error in the RFOG is described. To attenuate the laser frequency noise induced error, active frequency stabilization is applied. An online laser frequency noise observation is built, as a powerful optimum criterion for the loop parameters. Moreover, the laser frequency noise observation method is developed as a new measurement tool. With a fast digital proportional integrator based on a single field programmable gate array applied in the active stabilization loop, the laser frequency noise is reduced to 0.021 Hz (1σ). It is equivalent to a rotation rate of 0.07°/h, and close to the shot noise limit for the RFOG. As a result, a bias stability of open-loop gyro output is 9.5°/h (1σ) for the integration time 10 s in an hour observed in the RFOG. To the best of our knowledge, this result is the best long-term stability using the miniature semiconductor laser.

Improving thermal stability of a resonator fiber optic gyro employing a polarizing resonator

To improve the thermal stability of a resonator fiber optic gyro (R-FOG), a transmission-type polarizing resonator by inserting two in-line polarizers in a polarization-maintaining fiber resonator with twin 90° polarization-axis rotated splices is proposed and experimentally demonstrated. The in-line polarizers attenuate the unwanted resonance by introducing high loss for the unwanted eigenstates of polarization in the resonator. The desired resonance in the resonator can keep excellent stability in a wide temperature range, thus the temperature-related polarization error in the R-FOG is dramatically suppressed. Both our numerical simulation and experimental verification are carried out, which for the first time to our best knowledge demonstrate that the open-loop output of the R-FOG is insensitive to environmental temperature variations. A bias stability below 2°/h in the temperature range of 36.2°C to 33°C is successfully demonstrated.

Low-Noise Low-Delay Digital Signal Processor for Resonant Micro Optic Gyro

A low-noise low-delay digital signal processor is constructed on a single field-programmable gate array. An equivalent input noise as low as 3.752 nV/√Hz is demonstrated for the digital signal processor, which can detect an equivalent Sagnac effect of 0.003°/s in a resonant micro optic gyro (RMOG) with a 2.5-cm diameter ring resonator. With the processing time reduced from hundreds of seconds to 1.1 μs , this processor significantly increases the loop gain of the feedback loop and reduces the reciprocal noise in the RMOG. Owing to the fast speed of this processor, the lock-in frequency accuracy is reduced to 0.78 Hz (1σ), which is equivalent to a rotation rate of 0.004°/s. Relationship between this digitalized RMOG output signal and angular rate is obtained from ±0.25°/s to ±400°/s. The standard deviation of the residuals between RMOG output results and linear fit curve is 0.0236°/s.

Polarization-induced noise in resonator fiber optic gyro

An optical fiber ring resonator (OFRR) is the key rotation-sensing element in the resonator fiber optic gyro (R-FOG). In comparing between different OFRR types, a simulation model that can apply to all cases is set up. Both the polarization crosstalk and polarization-dependent loss in the coupler are fully investigated for the first time to our knowledge. Three different splicing schemes, including a single 0°, a single 90°, and twin 90° polarization axis rotated spices, are compared. Two general configurations of the OFRR are considered. One is a reflector OFRR, the other is a transmitter OFRR. This leads to six different OFRR types. The output stability of the R-FOG with six OFRR types is fully investigated theoretically and experimentally. Additional Kerr noise due to the polarization fluctuation is discovered. The OFRR with twin 90° polarization axis rotated splices is of lower additional Kerr noise and hence has better temperature stability. As the coupler is polarization dependent, we notice that in a reflector OFRR, the straight-through component of the output lightwave, which can be isolated by a transmitter configuration, would produce large polarization fluctuation-induced noise. The experimental results show that the bias stability of the transmitter OFRR is 8 times improved over that of the reflector OFRR, which is in accord with the theoretical analysis. By the analysis and experiments above, it is reasonable to make a conclusion that an R-FOG based on a transmitter OFRR with twin 90° polarization axis rotated splices is of better temperature stability and smaller additional Kerr effect noise.

Reducing polarization-fluctuation induced drift in resonant fiber optic gyro by using single-polarization fiber

A novel hybrid single-polarization (SP) fiber ring resonator is demonstrated by using a polarization-maintaining coupler formed by splicing a section of SP fiber into the resonator. The SP fiber selectively eliminates the unwanted resonance by introducing high loss for the unwanted eigenstates of polarization in the resonator. The calculated result shows that this hybrid SP resonator is a good candidate for a tactical-grade performance gyro with a high environmental temperature stability. The experiment shows that the desired resonance in the resonator can keep an excellent stability in a wide temperature range, thus the temperature-dependent polarization-fluctuation drift in the resonant fiber optic gyro is sufficiently suppressed. As a result, a random walk coefficient of 0.08°/√h and a typical bias stability below 0.3°/h for an integration time of 300 s have been carried out.

Resonant micro-optic gyro using a short and high-finesse fiber ring resonator

A novel hybrid integrated scheme is proposed for a high-performance resonant micro-optic gyro (RMOG), which requires a low-loss micro-ring resonator for mass production. A new record for the RMOG is established experimentally with a short fiber ring resonator and an integrated signal detecting and processing circuit. The finesse of the short fiber ring resonator with a length of 60 cm and a diameter of 4.77 cm is as high as 202, and the theoretical sensitivity of the RMOG is better than 0.3°/h assuming the average optical intensity at the photodetector is 1 mW. The 60 cm long spliceless micro-ring resonator is experimentally proved to be sufficient for a tactical-grade RMOG. An angle random walk coefficient of 0.64°/√h and a typical bias stability below 9.6°/h for the integration time of 50 s are successfully demonstrated using an innovative open-loop approach for an operation time of 1600 s.

Reduction of optical Kerr-effect induced error in a resonant micro-optic gyro by light-intensity feedback technique

As a type of main optical error source in the resonant micro-optic gyro (RMOG), the optical Kerr-effect brings a nonzero bias to the output of the RMOG. The light-intensity fluctuations are interpreted as the cause. To eliminate the drifts due to the optical Kerr-effect, the intensities of the clockwise (CW) and counterclockwise (CCW) lightwaves circulating in the resonator should be equal at all times. Through theoretical analysis and experimental investigation, a linear relationship between the second-harmonic demodulated signal and the light-intensity input to the resonator is demonstrated for the sinusoidally phase modulated RMOG. Both our numerical simulation and experimental verification are carried out, which, for the first time to the best of our knowledge, demonstrate that the second-harmonic demodulated signal can be used as a feedback error signal to reduce both the input-intensity mismatch between the CW and CCW lightwaves and their intensity fluctuations. By applying the light-intensity feedback loop to the closed-loop RMOG, the light-intensity fluctuations are reduced to 2.7×10⁻⁵, down from 5.86%. As a result, the optical-Kerr effect induced error is effectively reduced.

Closed-loop resonant fiber optic gyro with an improved digital serrodyne modulation

To widen the linear dynamic range and improve the linearity, a closed-loop resonant fiber optic gyro (RFOG) is proposed and experimentally demonstrated. To overcome the effect of the imperfect serrodyne modulation, an improved frequency shifting module is designed and constructed on a LiNbO3 phase modulator. Its frequency resolution is improved to 0.01 Hz which is equivalent to a rotation rate of 0.04°/h for an RFOG with a 12-cm diameter fiber ring resonator. With the frequency shifter applied in the RFOG, a closed-loop detection is demonstrated, whose bias stability is around 2 °/h, close to that of the open-loop output. Moreover, good linearity and wide dynamic range are also experimentally demonstrated thanks to the closed-loop operation. The measured result shows that the open-loop linear detection range of ± 215°/s is improved to ± 1076°/s. It is improved by a factor of 5. The open-loop scale factor nonlinearity of 1.2% is decreased to 0.02% (200 ppm), which is improved by a factor of 60. These are the best results reported to date, to the best of our knowledge, for closed-loop RFOGs.