Xuhui Yu

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.

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.

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.

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.

Full investigation of the resonant frequency servo loop for resonator fiber-optic gyro

Resonator fiber-optic gyro (RFOG) is a high-accuracy inertial rotation sensor based on the Sagnac effect. A high-accuracy resonant frequency servo loop is indispensable for a high-performance RFOG. It is composed of a frequency discriminator, a loop filter, and a laser actuator. Influences of the loop parameters are fully developed. Optimized loop parameters are obtained by considering the noise reduction and wide dynamic performance of the RFOG. As a result, with the integration time of 10 s, the accuracy of the resonant frequency loop is increased to 0.02 Hz (1σ). It is equivalent to a rotation rate of 0.067°/h, which is close to the shot noise limit for the RFOG, while a minimum rotation of ±0.05°/s has been carried out simultaneously. These are the best results reported to date, to the best of our knowledge, for an RFOG using the miniature semiconductor laser that benefits from the optimization of the resonant frequency servo-loop parameters.

Sensitive birefringent temperature sensor based on a waveguide ring resonator

A sensitive birefringent thermometer based on a SiO2 waveguide ring resonator is demonstrated in this paper. It can be used to fabricate a terahertz thermal detector. The temperature sensitivity is enhanced by the resonances of two polarization modes in the waveguide ring resonator. A high degree of common rejection exists for external influence. A linear temperature range from 6°C to 40°C has been detected with resolution of 0.025°C.

Reduction of the effect of the overflow reset in resonant frequency servo loop for resonator fiber optic gyro

Digital proportional-integral (PI) controller is always adopted in the resonant frequency servo loop in a highperformance resonator fiber optic gyro (R-FOG). Large reset pulse at the output of the R-FOG occurs on the overflow reset of the digital PI controller, which limits the lock-in frequency accuracy and system response. To reduce the effect of the overflow reset in the digital PI, an auto-controlled reset technique is proposed and experimentally demonstrated. As a result, the time back to the lock-in state falls from 8 s to 5 ms. With the integration time of 1 s, the equivalent accuracy of the resonant frequency servo loop is improved to 0.18 deg/h.

Double closed-loop resonator fiber optic gyro with improved digital serrodyne modulation

A double closed-loop resonator fiber optic gyro (RFOG) with an improved digital serrodyne scheme is experimentally demonstrated. To overcome the effect of the imperfect serrodyne modulation, an improved frequency shifting module is designed and constructed on a single FPGA. The frequency resolution is improved to 0.01Hz, which is equivalent to a rotation rate of 0.04°/h. A bias stability of 2°/h is achieved in a double closed-loop RFOG with a 14.25-m long fiber ring resonator. This is the best result reported to date, to the best of our knowledge, for closed-loop RFOGs. Moreover, good linearity and large dynamic range are also experimentally demonstrated thanks to the closed-loop operation.

Digital resonator fiber optic gyro based on a miniature laser source

A resonator fiber optic gyro (RFOG) based on a fiber-coupled semiconductor DFB-LD with an FPGA-based digital processor is set up. A bias stability of 23deg/h over one hour is successfully demonstrated. This is the best result in long time stability reported to date, to the best of our knowledge, for the RFOG based on a miniaturized laser source.