Yanxiong Niu

Wavelength Switchable High-Power Diode-Side-Pumped Rod Tm: YAG Laser around 2µm

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.

High-power diode-side-pumped rod Tm:YAG laser at 2.07 μm

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.

False star detection and isolation during star tracking based on improved chi-square tests

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.

On-orbit calibration for star sensors without priori information

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.

Accurate and Autonomous Star Acquisition Method for Star Sensor under Complex Conditions

Star sensor is a preferred attitude measurement device for its extremely high accuracy. Star acquisition is the essential and critical procedure, which is aiming at acquiring accurate star areas. However, degenerated acquisition results under complex conditions become one of the major restrictions for modern star sensor. In this paper, an accurate and autonomous star acquisition method is proposed. Mathematical morphology and variable thresholding are combined for accurate star extraction; motion PSF is estimated in frequency domain and nonlinear filter is adopted for star restoration. Accurate star acquisition can be achieved based on only one star image. Simulations and laboratory experiments are conducted for verification. Several existing methods are also reproduced for comparison. Acquisition results demonstrate that the proposed method is effective and an excellent performance can be achieved autonomously under complex conditions, along with more detected stars and improved acquisition accuracy.

Measurement of phase retardation of optical multilayer films based on laser feedback system.

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.

Frequency difference stabilization in dual-frequency laser by stress-induced birefringence closed-loop control.

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.

Angular velocity estimation based on star vector with improved current statistical model Kalman filter.

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.

Numerical analysis of the thermal and mechanical effects of laser windows of a high-power all-solid-state 2-μm laser system

Abstract. The output window of a high-power laser system is vulnerable to damage, and this is the main limiting factor on the power scaling and structure integrity of the laser system. In endeavoring to obtain higher output powers from the laser system, the impact of the thermal and mechanical effects and the damage mechanism of the output window must be considered. In order to study these issues, a thermal model of the laser window is established based on the heat transfer and thermoelastic theories, and the expressions for the transient thermal and mechanical stress distributions of the output window are deduced in terms of the integral-transform method. Taking the infrared quartz window material as an example, the temperature and mechanical field distributions of a high-power all-solid-state 2-μm laser system window are simulated, and the laser-induced damage mechanism is deeply analyzed. The calculation results show that the laser window-induced damage is mainly caused by melting damage when the temperature exceeds the melting point of the material. The presented theoretical analysis and numerical simulation results are significant for the design and optimization of high-power laser windows.

KTP crystal thickness distribution measurements based on laser feedback interferometry.

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.