The measurement range of sensors based on optical resonators is often limited by the cavity linewidth. The usual Pound–Drever–Hall current feedback can extend the sensor’s measurement range to the tuning range of the laser. However, this method is still insufficient for sensors with high dynamic range requirements. We present an optical frequency shift detection technique that combines temperature and current tuning, which expands the dynamic range of sensors based on optical resonators. When the optical frequency shift in the optical resonators is beyond the current feedback measurement range, temperature feedback is used to alter the wavelength of the laser. As a result, the resonance curve of the resonators is adjusted to fit the current tuning measurement range. The method has high accuracy for current feedback and a large measurement range for temperature feedback. Experimental results show that the method has a range of measurement of 8.14 GHz, 14 times higher than the traditional current feedback method, with a measurement accuracy of 38 kHz and a dynamic range of 106 dB.
The resonant micro optical gyro (RMOG) is considered to be a unique type of optical gyroscope with great application prospects because of its high precision and miniaturization. However, high precision RMOG systems are generally required to have the narrow full width at half maximum (FWHM) of the resonance spectrum. The dynamic range of the open-loop detection method based on FWHM demodulation output is correspondingly narrowed. Therefore, we propose a triple closed-loop control system based on optoelectronic hybrid feedback in this work. First, the sawtooth wave feedback loop is used to the track the angular velocity, which reduces the influence of the nonlinear error and improves the dynamic range of the system. Second, the laser frequency locking loop (LFLL) achieves frequency locking by locking the laser’s output frequency at the static resonance frequency of the waveguide ring resonance. Third, the light intensity feedback loop is introduced to reduce the optical Kerr noise by minimizing the light intensity difference between the clockwise (CW) and counterclockwise (CCW) directions. Experimental results show that, when using this method, the dynamic range of the gyroscope system is increased from ±686°/s to ±5365°/s compared with the open-loop output detection scheme, which is eight times higher. The light intensity difference between the CW and CCW channels decreases from 7.91 to 0.615 mV. The backscattering noise width under dual phase modulation technology decreases from 9.2 to 0.76 mV. The nonreciprocal noises, mainly optical Kerr noise and backscattering noise, is reduced by about one order of magnitude. The long-term bias stability measurement of the RMOG system is 0.926 °/h.
Backscatter noise is a key limiting factor in the performance of resonant fiber optic gyroscopes. To enhance the accuracy of resonant fiber optic gyroscopes, a gyroscope system based on sideband locking technology is proposed. In this scheme, two tunable semiconductor lasers independently provide the clockwise and counterclockwise optical paths. An optical phase-locked loop is employed to achieve independent subharmonic phase locking of the two output lights, with their optical frequency difference equaling one free spectral range (FSR). By applying sinusoidal modulation at different frequencies to generate sidebands, the sideband locking technique is used to lock the first-order sidebands of the two lights to adjacent resonant frequencies. Through the filtering effect of the fiber ring resonator, the carrier intensity of the transmitted light and other sidebands are effectively suppressed. With this signal processing technique, the influence of two types of backscatter noise is completely eliminated, and the additional suppression of laser frequency noise is achieved. Experimental results show that, based on the Allan deviation, the bias instability of the gyroscope output is 1.42°/h. This method significantly improves the detection accuracy of the resonant fiber optic gyroscope under the same parameter precision or device performance requirements.
In the development of miniaturized optical gyroscopes, the resonant micro-optical gyroscope (RMOG) is the most representative sensor of new optical gyroscopes. However, in the manufacturing process of optical elements, the influence of backscattering and optical Kerr noise limits the performance of RMOG. A method of suppressing optical Kerr noise generated by the front end by compensating the optical power difference while suppressing the backscattering noise is introduced. In this method, we study the optimal strategy of double-phase modulation frequency to obtain the maximum spectral gap of clockwise and counterclockwise light, so as to suppress the error related to backscattered light. At the same time, the influence of Kerr noise on the optical path is reduced through the high-precision feedback loop of the intensity modulator. Based on Allan variance, the long-term deviation stability of RMOG reaches 0.98 deg/h.
The whispering gallery mode resonator based on a fluoride crystalline material offers the advantage of an ultrahigh quality factor, however, its fast tuning remains a challenge. We propose a tuning method for a CaF2 crystalline resonator using external pressure loading. Through our theoretical calculations and simulations, we discovered that the frequency shift induced by deformation during the pressure tuning of the crystalline resonator is greater than that induced by changes in refractive index. Both shifts were of the same order of magnitude, indicating that both factors must be considered. The experimental results show tuning rates of 1114.6 and 888.2 MHz/MPa in the transverse electric and transverse magnetic modes, respectively. The tuning system can respond to a signal frequency of up to 1.2 MHz.
In this paper, we have proposed an all-silicon and compact transverse electric (TE)-pass polarizer with large bandwidth and high polarization extinction ratio (PER), where the polarizer uses a semi-arc waveguide to cause polarization-dependent bending loss (PDBL). The combination of the subwavelength grating (SWG) and double adiabatic tapers serve as an equivalent cladding to get strengthened PDBL. The parameters of SWG and double adiabatic tapers are analyzed in detail to improve the performance of the device. In the proposed TE-pass polarizer, the injected fundamental TE mode (i.e., TE0) gets protection and the injected fundamental transverse magnetic mode (i.e., TM0) lets out. The footprint is only about 6 μm × 10 μm, which is substantially reduced compared with the previous counterparts. From the results, its bandwidth is as high as 228 nm (from 1.482 to 1.710 μm) with PER > 39 dB and low excess loss (EL < 1 dB), and it has an ultra-high PER of 42.5 dB and a low EL of 0.1 dB at 1.55 μm. In addition, large fabrication tolerances of 20 nm ( ± 10 nm) to the key structural parameters are analyzed.
In this study, a polarization-maintaining fiber optical gyro (PMFOG) with orthogonal-polarization states is constructed, and the non-reciprocity error induced by varying temperatures has been studied. Compared with the traditional optical gyro system based on single-polarization, theory and experiments have shown that the non-reciprocity error induced by varying temperatures is more evident in the PMFOG with orthogonal-polarization states. It is approximately five times larger than that of the traditional PMFOG. To decrease the non-reciprocity error induced by varying temperatures, we proposed an innovative structure in this study, which enhances the symmetry of the light path without the addition of redundant optical devices. The clockwise and counterclockwise light beams experience the same change of refractive index along the fast and slow axes of the fiber, and the non-reciprocity error induced by varying temperatures can be reduced to zero when the whole gyro is heated up nearly uniformly. This research, which overcomes a design limitation in PMFOGs with orthogonal-polarization states, is significant for the miniaturization and optimization of the environmental adaptability of fiber optic gyroscopes.
A self-injection locking method for resonant micro-optical gyroscope (RMOG) is proposed. The resonator is used as the sensitive element of the gyroscope and the device of light compression locking. The fiber and waveguide ring resonator (WRR) are designed and fabricated, consecutively. Both of the resonators can narrow the linewidth by three orders of magnitude, and the phase noise and the relative intensity noise can be reduced to −120 and −150 dBc / Hz, respectively. By evaluating the locking effect, WRR is more suitable for optical locking of a resonant micro-optical gyroscope (RMOG) light source, and its theoretical locking accuracy can reach 6.02 deg/s.
The magnetic-thermal coupling effect, which can result in a serious non-reciprocal error, can’t be ignored in high-performance depolarized interferometric fiber optic gyroscope (De-IFOG). In this paper, we research on the magnetic-thermal coupling effect under varying temperature field in De-IFOG theoretically and experimentally. The mechanism of the coupling effect is thoroughly investigated and the related theoretical calculation model is established. The essential differences between the errors caused by varying temperature field and magnetic-varying temperature field are analyzed respectively. Simulations and experiments are consistent with the theoretical model. The experimental results show that, when the temperature varies from -30℃ to 20℃ at the speed of 14℃/h, the peak to peak value of error is up to 35 °/h with a constant magnetic field of 10 Gauss. The results can be used to enhance environmental adaptability of devices such as De-IFOGs, which are in great demands for aerospace applications.
We demonstrate a method based on transverse second harmonic generation (TSHG) for in-situ modal inspection of single nonlinear micro/nanowaveguide. CdS nanobelts (NBs) with width of several micrometers and length of a few hundred micrometers are synthesized by a thermal evaporation process. Transverse second harmonic (TSH) signal is observed as pumping light coupled inside a single CdS NB from both ends simultaneously. As a result of multimode interference, periodic interference patterns are observed along the whole NB. The influence of multimode interaction on TSHG effect inside single nonlinear micro/nanowaveguide is studied both theoretically and experimentally. Using fast Fourier transform (FFT), the TSH patterns are analyzed. The results indicate several particular modes inside the NB, which is in great well with calculated results. The method can be in principle operated in varieties of nonlinear micro/nanowavegudies including ZnO, CdS, CdTe, GaAs and LiNbO3.
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