ABSTRACT
Backscattering-induced noise is a dominant noise in a resonant optic gyroscope (ROG). This paper proposes a method to suppress the carrier and backscattering-induced noise with a sideband locking technique. The resonant cavity can be taken as a band-pass filter, and the carrier frequency component can be located at the stop-band while one sideband is locked to the cavity resonance. Then, the carrier will be suppressed by the cavity itself, which will reduce the interference with carrier backscattering. For the adoption of different modulation frequencies in clockwise (CW) and counter-clockwise (CCW) directions, the first-order sidebands of CW and CCW have a frequency offset to each other. Therefore, the first-order sideband backscattering can be eliminated when the sideband is locked to the cavity resonance. Also, both high and low modulation frequencies are applied to the phase modulator, which will further suppress the carrier, and the demodulation of low frequency will reduce the sensitivity to phase fluctuation noise in the system. The method has low requirements for parameter accuracy or device performance.
ABSTRACT
Polarization-induced noise is a dominant noise that seriously hinders the progress of resonator optic gyroscopes. Many countermeasures have been developed but showed insufficient performance. In this paper, we propose a quadrature demodulation technique (QDT) that adopts references of both sine and cosine to demodulate the signal. Theoretical analyses of the polarization effect and QDT are shown, and experimental results are listed and compared. Experimental results are consistent with theoretical analyses. QDT shows great performance in suppressing environment- and polarization-induced phase fluctuation of signal. We also demodulate the intensity-dependent coefficient of 2Ωt term, which is demonstrated effective for compensating polarization-induced intensity noise together with QDT. The scheme shows significant progress in improving long-term stability.
ABSTRACT
In this Letter, a microbottle-resonator-based strain sensor with individual mode distribution and recognizable resonance spectrum was proposed and demonstrated. A cleaned-up spectrum was achieved by inscribing horizontal microgroove scars close to the bottle center. The inscribing parameters of these grooves were designed according to the field distribution of the modes, and the obtained spectrum showed excellent consistency with theoretical analysis. The shift in the resonance peak with increasing stretching force was investigated, and the corresponding strain sensitivities were 0.085 pm/µÏµ for transverse electric polarization and 0.136 pm/µÏµ for transverse magnetic polarization, which could be further increased by using materials with smaller elastic moduli.
ABSTRACT
High-precision ground calibration is essential to ensure the performance of star sensors. However, the complex distortion and multi-error coupling have brought great difficulties to traditional calibration methods, especially for large field of view (FOV) star sensors. Although increasing the complexity of models is an effective way to improve the calibration accuracy, it significantly increases the demand for calibration data. In order to achieve high-precision calibration of star sensors with large FOV, a novel laboratory calibration method based on a regularization neural network is proposed. A multi-layer structure neural network is designed to represent the mapping of the star vector and the corresponding star point coordinate directly. To ensure the generalization performance of the network, regularization strategies are incorporated into the net structure and the training algorithm. Simulation and experiment results demonstrate that the proposed method can achieve high precision with less calibration data and without any other priori information. Compared with traditional methods, the calibration error of the star sensor decreased by about 30%. The proposed method can satisfy the precision requirement for large FOV star sensors.
ABSTRACT
A novel sensing peak identification method for high accuracy refractive index (RI) sensing is proposed. The implementation takes the intensity of interference maximum as the characteristic to distinguish interference peaks, tracking the sensing peak continually during a RI changes, with high measurement accuracy and simple computation. To verify the effect of the method, the extrinsic Fabryâ»Perot interferometer (EFPI) sensor has been fabricated using the large lateral offset splicing technique. In the RI range from 1.346 to 1.388, the measurement range of the EFPI with the proposed method reaches at least 6 times larger than that of EFPI with the wavelength tracking method and the largest measurement error is -4.47 × 10-4. The EFPI refractive index (RI) sensor identified the sensing peak is believed to play an important role in RI, concentration and density sensing, etc., for superior performance.
ABSTRACT
Microchip lasers are applied as the light sources on various occasions with the end-pumping scheme. However, the vibration, the temperature drift, or the mechanical deformation of the pumping light in laser diodes in the end-pumping scheme will lead to instability in the microchip laser output, which causes errors and malfunctioning in the optic systems. In this paper, the side-pumping scheme is applied for improving the disturbance-resisting ability of the microchip laser. The transverse mode and the frequency purity of the laser output are tested. To ensure unicity in the frequency of the laser output, numerical simulations based on Fresnel-Kirchhoff diffraction theory are conducted on the parameters of the microchip laser cavity. Film-etching technique is applied to restrain the area of the film and form the microcavity mirrors. The laser output with microcavity mirrors is ensured to be in single frequency and with good beam quality, which is significant in the applications of microchip lasers as the light sources in optical systems.
ABSTRACT
The star sensor is a precise attitude measurement device for a spacecraft. Star tracking is the main and key working mode for a star sensor. However, during star tracking, false stars become an inevitable interference for star sensor applications, which may result in declined measurement accuracy. A false star detection and isolation algorithm in star tracking based on improved chi-square tests is proposed in this paper. Two estimations are established based on a Kalman filter and a priori information, respectively. The false star detection is operated through adopting the global state chi-square test in a Kalman filter. The false star isolation is achieved using a local state chi-square test. Semi-physical experiments under different trajectories with various false stars are designed for verification. Experiment results show that various false stars can be detected and isolated from navigation stars during star tracking, and the attitude measurement accuracy is hardly influenced by false stars. The proposed algorithm is proved to have an excellent performance in terms of speed, stability, and robustness.
ABSTRACT
The star sensor is a prerequisite navigation device for a spacecraft. The on-orbit calibration is an essential guarantee for its operation performance. However, traditional calibration methods rely on ground information and are invalid without priori information. The uncertain on-orbit parameters will eventually influence the performance of guidance navigation and control system. In this paper, a novel calibration method without priori information for on-orbit star sensors is proposed. Firstly, the simplified back propagation neural network is designed for focal length and main point estimation along with system property evaluation, called coarse calibration. Then the unscented Kalman filter is adopted for the precise calibration of all parameters, including focal length, main point and distortion. The proposed method benefits from self-initialization and no attitude or preinstalled sensor parameter is required. Precise star sensor parameter estimation can be achieved without priori information, which is a significant improvement for on-orbit devices. Simulations and experiments results demonstrate that the calibration is easy for operation with high accuracy and robustness. The proposed method can satisfy the stringent requirement for most star sensors.
ABSTRACT
Avalanche photodiode is widely used in laser rangefinder due to high gain characteristics, but introduces highly additive noise during the time of current's multiplication that makes laser rangefinder's SNR meet bottleneck. This paper proposes a method of designing a high SNR's graphene photodetector based on microcavity. The graphene's unique optoelectronic properties make it an ideal platform for a variety of photonic applications, such as fast lasers, optical modulators, transparent electrodes, and ultrafast photodetectors. It has been recognized internationally to have dominant advantages in photodetectors due to its high carrier mobility, gapless spectrum, and frequency-independent absorption coefficient. With the wavelength of 1.06 µm, the mechanism of light waves' transmission in the cavity and the graphehne's absorption are studied by using optical transmission matrix method and scattering matrix method; the light absorption model of the graphene photodetector based on microcavity is established. Device's final quantum efficiency reaches 91.2%, respectively reaches 0.778 A·W(-1), its full width at half maximum (FWHM) reaches 6 nm; the influence between graphene's position in the microcavity and device's absorption shows that device's absorption's peak value changes periodically with graphene's position under resonant condition, and the variety of length of microcavity does not have any influence on the peak value, but changes the graphene's position when absorption reaches peak value, on the condition that the length of microcavity is n times of half of wavelength, the number of device's absorption peak value is 2n with the variety of graphene's position, and all the peak values are symmetrical with respect to the center of microcavity, the final graphene's position is 0.402 8 mm away from the top mirror of microcavity, and the absorption reaches 94%, Compared with single layer graphene, the absorption rate increases 16 dB; By solving SNR equation of the graphene photodetector based on microcavity and SNR equation of the avalanche photodiode, eventually finds that the SNR of the graphene photodetector based on microcavity is 90.3, which raises 10 dB compared with the avalanche photodiode's. Theoretical analysis shows the graphene photodetector based on microcavity has high absorption rate, high quantum efficiency, and high SNR. In this paper, the research achievements provide a theoretical reference to update and design higher SNR photodetector used in laser rangefinder's receiving system.
ABSTRACT
Angular velocity information is a requisite for a spacecraft guidance, navigation, and control system. In this paper, an approach for angular velocity estimation based merely on star vector measurement with an improved current statistical model Kalman filter is proposed. High-precision angular velocity estimation can be achieved under dynamic conditions. The amount of calculation is also reduced compared to a Kalman filter. Different trajectories are simulated to test this approach, and experiments with real starry sky observation are implemented for further confirmation. The estimation accuracy is proved to be better than 10-4 rad/s under various conditions. Both the simulation and the experiment demonstrate that the described approach is effective and shows an excellent performance under both static and dynamic conditions.
ABSTRACT
The frequency difference of dual-frequency lasers is increasingly becoming an area of focus in research. The stabilization of beat frequency is of significance in fields such as synthetic wavelength and shows great potential in precise measurement. In this paper, a novel device based on stress-induced birefringence closed-loop control is proposed. Experiments are carried out on a dual-frequency He-Ne Zeeman-birefringence laser with the output mirror sealed in the opposite direction. The results show that the device is capable of controlling the frequency difference variation in 1.3%, in a convenient and highly cost-effective way, and it can increase the quantity of frequency difference, which is crucial to the application of precise measurement through dual-frequency lasers.
ABSTRACT
The phase property of optical films is becoming a new research focus in optoelectronics. The retardation caused by reflection on multilayer optical films can be utilized to make phase retarders and modulate the polarization state of optical systems. In this paper, a novel method based on laser feedback is presented to measure the retardance. The laser feedback system can realize fast and stable measurement with high accuracy. Two samples of K9 glass covered with different multilayer optical films are measured at different angles of incidence. The results show that the retardance is sensitive to the incident angle and can provide guidance for the usage of reflective optical films.
ABSTRACT
Multilayer graphene, with wide absorption spectrum and unique photoelectric properties, is an ideal material to make the next generation of photoelectric detector. Taking graphene interband tunneling theory as the foundation, a photoelectric detector model with the structure of multilayer graphene nanoribbons was proposed. Nanoribbons which contacted with source and drain electrode at the end were sandwiched between the semiconductor substrate and the top and back gate. Using this model, a photoelectric conversion mechanism of multilayer graphene nanoribbon detector was established. It discussed the working principle of the detector at different top gate voltage, studied the relationship between the source-drain current and the incident light energy, researched the influence of the bias voltage, the length of depletion and the values of band gap on the dark current, and analyzed the change of detector responsibility and detectivity with the incident light energy under the different parameters. The results show that, the responsibility of detector increases with the layers of nanoribbons, and are affected by the band gap, the length of depletion and the bias voltage. The maximum responsibility up to 10(3) A·W(-1); By limiting on the top gate voltage, the band gap and other variables can control the dark current of system and increase the detectivity, the detectivity up to a maximum value of 10(9) cm Hz(1/2)·W(-1). The structure of multilayer graphene nanoribbons can enhance the absorption of the incident light, improve the sensitivity of the detector and the detection capability of weak light, and realize the detection from THz to far infrared wavelength of incident light. The detection performance is far better than that of many quantum structures and narrow-band semiconductor structure of photoelectric detector.
ABSTRACT
KTiOPO(4) (KTP) crystal is a widely used nonlinear optical crystal, and it can meet high requirements for parallelism of crystal surfaces and the length in the light propagation direction in its application fields. In this paper, we present a method for measuring the thickness distribution of cuboid KTP crystals based on laser feedback interferometry (LFI). The accuracy of the measurements for the relative thickness distribution was 8.8 nm, and that of the absolute thickness can be improved by increasing the accuracy of the refractive index. This method is applicable to measurements of all light transmissive birefringent materials, and the results provide detailed instructions for crystal processing and polishing.
ABSTRACT
We present a theoretical model study of a quasi-three-level laser with particular attention given to the Tm:YAG laser. The oscillating conditions of this laser were theoretically analyzed from the point of the pump threshold while taking into account reabsorption loss. The laser oscillation at 2.02 µm with large stimulated emission sections was suppressed by selecting the appropriate coating for the cavity mirrors, then an efficient laser-diode side-pumped continuous-wave Tm:YAG crystal laser operating at 2.07 µm was realized. Experiments with the Tm:YAG laser confirmed the accuracy of the model, and the model was able to accurately predict that the high Stark sub-level within the H36 ground state manifold has a low laser threshold and long laser wavelength, which was achieved by decreasing the transmission of the output coupler.
Subject(s)
Computer-Aided Design , Lasers, Solid-State , Lasers , Models, Theoretical , Refractometry/instrumentation , Computer Simulation , Equipment Design , Equipment Failure Analysis , Light , Scattering, RadiationABSTRACT
We report a high-power diode-laser (LD) side-pumped rod Tm:YAG laser of around 2 µm. The laser was water-cooled at 8°C and yielded a maximum output power of 267 W at 2.07 µm, which is the highest output power for an all solid-state cw 2.07 µm rod Tm:YAG laser reported as far as we know. The corresponding optical-optical conversion efficiency was 20.7%, and the slope efficiency was about 29.8%, respectively.
Subject(s)
Lasers, Solid-State , Thulium , Equipment Design , Equipment Failure AnalysisABSTRACT
We report a high-power diode-side-pumped rod Tm:YAG laser operated at either 2.07 or 2.02 µm depending on the transmission of pumped output coupler. The laser yields 115W of continuous-wave output power at 2.07 µm with 5% output coupling, which is the highest output power for all solid-state 2.07 µm cw rod Tm:YAG laser reported so far. With an output coupler of 10% transmission, the center wavelength of the laser is switched to 2.02 µm with an output power of 77.1 W. This is the first observation of high-power wavelength switchable diode-side-pumped rod Tm:YAG laser around 2 µm.
