Xingshu Wang

Tunable multiwavelength mode-locked Tm/Ho-doped fiber laser based on a nonlinear amplified loop mirror.

We propose and demonstrate a tunable multiwavelength mode-locked Tm/Ho-doped fiber laser based on a nonlinear amplified loop mirror (NALM). Without using polarization-maintaining fiber, only passive fibers with low birefringence were inserted into the NALM to help overcome mode competition and realize mode-locking. The spacing between adjacent channels was measured to be ∼6  nm. By adjusting the polarization controllers (PCs) to an appropriate position, self-started mode-locking was achieved, which further overcame the mode competition in the fiber laser. A multiwavelength mode-locked fiber laser with at least three available channels were tunable in the widest range of 30 nm (from 1935 to 1965 nm) with a 3 dB channel bandwidth of ∼1.6  nm. This multiwavelength mode-locked fiber laser is quite stable with the maximum peak fluctuation within 0.47 dB in long-term observations.

Attitude-correlated frames approach for a star sensor to improve attitude accuracy under highly dynamic conditions

The attitude accuracy of a star sensor decreases rapidly when star images become motion-blurred under dynamic conditions. Existing techniques concentrate on a single frame of star images to solve this problem and improvements are obtained to a certain extent. An attitude-correlated frames (ACF) approach, which concentrates on the features of the attitude transforms of the adjacent star image frames, is proposed to improve upon the existing techniques. The attitude transforms between different star image frames are measured by the strap-down gyro unit precisely. With the ACF method, a much larger star image frame is obtained through the combination of adjacent frames. As a result, the degradation of attitude accuracy caused by motion-blurring are compensated for. The improvement of the attitude accuracy is approximately proportional to the square root of the number of correlated star image frames. Simulations and experimental results indicate that the ACF approach is effective in removing random noises and improving the attitude determination accuracy of the star sensor under highly dynamic conditions.

An Improved Method for Dynamic Measurement of Deflections of the Vertical Based on the Maintenance of Attitude Reference

A new method for dynamic measurement of deflections of the vertical (DOV) is proposed in this paper. The integration of an inertial navigation system (INS) and global navigation satellite system (GNSS) is constructed to measure the bodys attitude with respect to the astronomical coordinates. Simultaneously, the attitude with respect to the geodetic coordinates is initially measured by a star sensor under quasi-static condition and then maintained by the laser gyroscope unit (LGU), which is composed of three gyroscopes in the INS, when the vehicle travels along survey lines. Deflections of the vertical are calculated by using the difference between the attitudes with respect to the geodetic coordinates and astronomical coordinates. Moreover, an algorithm for removing the trend error of the vertical deflections is developed with the aid of Earth Gravitational Model 2008 (EGM2008). In comparison with traditional methods, the new method required less accurate GNSS, because the dynamic acceleration calculation is avoided. The errors of inertial sensors are well resolved in the INS/GNSS integration, which is implemented by a Rauch–Tung–Striebel (RTS) smoother. In addition, a single-axis indexed INS is adopted to improve the observability of the system errors and to restrain the inertial sensor errors. The proposed method is validated by Monte Carlo simulations. The results show that deflections of the vertical can achieve a precision of better than 1″ for a single survey line. The proposed method can be applied to a gravimetry system based on a ground vehicle or ship with a speed lower than 25 m/s.

Region-confined restoration method for motion-blurred star image of the star sensor under dynamic conditions.

Under dynamic conditions, the centroiding accuracy of the motion-blurred star image decreases and the number of identified stars reduces, which leads to the degradation of the attitude accuracy of the star sensor. To improve the attitude accuracy, a region-confined restoration method, which concentrates on the noise removal and signal to noise ratio (SNR) improvement of the motion-blurred star images, is proposed for the star sensor under dynamic conditions. A multi-seed-region growing technique with the kinematic recursive model for star image motion is given to find the star image regions and to remove the noise. Subsequently, a restoration strategy is employed in the extracted regions, taking the time consumption and SNR improvement into consideration simultaneously. Simulation results indicate that the region-confined restoration method is effective in removing noise and improving the centroiding accuracy. The identification rate and the average number of identified stars in the experiments verify the advantages of the region-confined restoration method.

High-precision rolling angle measurement for a three-dimensional collimator.

We propose a precise rolling angle measurement for a collimator to extend its application in 3D angular deformation measurement, with performance significantly superior to that of the traditional 2D technique. The rolling angle measurement is realized by taking full advantage of the point array image, which is projected in terms of the collimated beam. The measurement error is estimated according to the proposed algorithm. The characteristics of the point array are analyzed to optimize the point array for precise measurement, including the point distribution, the point array resolution, and the point array area. Both simulations and experiments demonstrate that subarcsecond precision rolling angle measurement is achieved by our method, which is superior to those attained by other proposed targets.

An extremely fast and high-power laser diode driver module

A new extremely fast and high-power laser diode driver module is introduced, which is made of a fast high-power MOSFET and based on discharging capacitor. The main factors are analysed in theory, which determine the main performance of this kind of laser diode driver module. The whole performance of the laser diode driver module is simulated with SPICE module in detail, and the testing results of the produced laser diode driver module are described. The main methods to change the output peak power and current pulse width of the laser diode driver module are presented. The output peak power of the laser diode driver module is very high, which can reach 50 W. The output current pulse width of the laser diode driver module is very short, which is less than 8 ns. The laser diode driver module can directly drive many kinds of pulse laser diode in the market, and be used as the high performance transmitter driver module of time-of-flight ladar and laser range finder.

Extraction methed of the motion blurred star image for the star sensor under high dynamic conditions

Star sensor is beyond dispute the most accuracy absolute attitude determination sensor and it is widely used in deep space exploring and earth observing. Star image spots captured in the image sensor of the star sensor will elongate when it is under high dynamic conditions especially the rotation motion resulting in the blurred star images. The motion blurred star images with low SNR lead to the increase of difficulty in the extraction of the star areas. Normal processing procedures as the estimation of background threshold, image binarization and connected components analysis are not profitable anymore. A extraction method of the motion blurred star image based on template-correlation matching of the star sensor under high dynamic conditions is proposed in this paper. Parameters of the motion degradation function is precisely measured with the strap down laser gyro unit. So the template is generated in real time according to different dynamic conditions through the degradation model of the motion blurred image. Correlation matching criterion like minimum correlation difference and maximum correlation product are alternatives. Simulation and experimental results show that this method readily identifies and extracts the star spot area and removes the noises outside its region simultaneously. It is effective in extracting faint blurred star images unless they are totally submerged by the noise.

Hull Structure Monitoring Using Inertial Measurement Units

Hull structure monitoring is needed to provide healthy suggestions for a ship. Inertial Measurement Units (IMUs) are available for this application. By installing IMUs on different points of the ship hull, the angular velocity and acceleration of each point can be measured. Once the hull flexure changes, the movement parameters of those points will be different, and the hull flexure can be estimated through matching these parameters by a Kalman filter. We deduced proper measurement equations of the Kalman filter, which is essential for the flexure estimation, and designed real data and simulation data experiments to verify the equation. Results showed that the hull structure can be effectively monitored by the proposed method.

A new integrated Gaussian-Markov process model for precision shipboard transfer alignment

In shipboard transfer alignment (TA), Kalman filter used to estimate the misalignment angle requires accurate ship dynamic flexure model. Traditionally, the ship dynamic flexure is modelled as a second-order Gaussian-Markov process according to experience. However, this model has not been validated in real applications, when the current model is differ with this used in Kalman filter design will result in a large measurement error. To solve this problem, an integrated Gaussian-Markov process model is proposed in this contribution which is based on the hydroelastic analysis and statistic from previous measured data. Specifically, theoretical analysis shows ship dynamic flexure is the response of elastic ship hull to sea wave loads, while the sea wave spectrum is of double-peaked in frequency filed for the swell and wind sea waves occurring simultaneously, and therefore, the dynamic flexure is also with double-peaked spectrum distribution. Furthermore, the frequency analysis based on our previous measured ship dynamic flexure data also demonstrates the power spectrum density (PSD) of actual dynamic flexure angle is of double-peaked distribution, which can be modelled more accurately by combined using two independent second-order Gaussian-Markov process models. Experimental results show that Kalman filter utilizing the proposed integrated Gaussian-Markov process model provides more accurate measurement of the misalignment angle compared with using traditional second-order Gaussian-Markov process model in shipboard TA.

A new time discrimination circuit for the 3D imaging Lidar

In order to enhance the time discrimination precision in the 3D imaging lidar, we propose a new time discrimination circuit, which improves both the delayer and the attenuator in the previous CFD (Constant Fraction Discriminator) circuit. The proposed circuit mainly includes a delayer, a low-pass filter, and a comparator. The delayer is implemented with a series of inductors and capacitors, which has some advantages: low signal distortion, small volume, easy adjustment, etc. The low-pass filter attenuates the signal amplitude and broadens the signal width, as well as reduces the noise by decreasing the equivalent noise bandwidth, and increases the signal slope at the discrimination time. Therefore, the time discrimination error is reduced significantly. This paper introduces the proposed circuit in detail, carries out a theoretical analysis for the noise and time discrimination error in the proposed circuit and compares them with the previous CFD circuit. The comparison results show that the proposed circuit can reduce the time discrimination error by about 50% under the same noise level. In addition, some experiments have been carried out to test the performances of the circuit. The experiments show that the time delay of the circuit is about 14ns, the time discrimination error is less than 150 ps when the voltage SNR ranges from 18.2 to 81.8, and the time discrimination error is less than 100 ps when the signal amplitude ranges from 0.2 V to 1.86 V. The tested time discrimination error is well in accordance with the theoretical calculation.