Anhu Li

Laser coarse–fine coupling scanning method by steering double prisms

Recent developments in optical communication systems have presented an emerging need for scanning and tracking dynamic targets with high accuracy. Unfortunately, conventional scanners have difficulty supplying either sufficient vision information or high scanning resolution because of the fixed optical parameters and optomechanical structure. This paper introduces a novel cascaded double-prism scanner that combines the two scanning modes of rotating and titling motions into a nested device. The analysis results show that both the vertical field angle range and the horizontal field range of the coarse scanning are no less than ±10°, with the scanning accuracy superior to 50 μrad, while the vertical field angle range and the horizontal field range of the fine scanning are, respectively, no less than 2500 μrad and 1200 μrad, with the scanning accuracy superior to 1 μrad. The scanning method not only meets the requirements of collaboratively steering the beam deviation in a large range with high accuracy but also can obtain a variety of scanning modes and trajectories, which has important engineering significance for the solution of steering a fine beam to track and position a dynamical target.

Inverse solutions for tilting orthogonal double prisms

An analytical reverse solution and actual examples are given to show how to direct a laser beam from a pair of orthogonal prisms to given targets in free space. Considering the influences of double-prism structural parameters, a lookup table method to seek the numerical reverse solution of each prisms tilting angle is also proposed for steering the double-prism orientation to track a target position located in the near field. Some case studies, as well as a specified elliptical target trajectory scanned by the cam-based driving double prisms, exhibit the significant application values of the theoretical derivation. The analytic reverse and numerical solutions can be generalized to investigate the synthesis of scanning patterns and the controlling strategy of double-prism tilting motion, the potentials of which can be explored to perform the orientation and position tracking functions in applications of precision engineering fields.

Investigation of beam steering performances in rotation Risley-prism scanner.

Rotation Risley-prism scanner appears to be the most promising solution to high-accuracy beam scanning and target tracking. In the paper, some important issues crucial to the function implementation are thoroughly investigated. First the forming law of scan blind zone relative to double-prism structural parameters is explored by a quantitative analysis method. Then the nonlinear relationship between the rotation speeds of double prisms and the change rate of beam deviation angle is presented, and the beam scan singularity is indicated as an essential factor that confines the beam scan region. Finally, the high-accuracy radial scan theory is verified to illustrate the important application owing to the high reduction ratio from the rotation angles of double prisms to the deviation angles of the emergent beam. The research not only reveals the inner mechanisms of the Risley-prism beam scanning in principle, but also provide a foundation for the nonlinear control of various beam scan modes.

Design of an optical testbed for in-lab testing and validation for the intersatellite lasercom terminals

For on-ground performances testing and verification for intersatellite laser communication systems, a compact and multi-function optical testbed and a 2-D sub-micro-radian beamsteerer were designed. The testbed consists mainly of an optical simulator for long-distance propagation of laser beams, two CCD detectors, an autocollimation system, an interferometer, and a noise light source, for the uses is to measure far-field beam characteristics, transmitting powers, the wavefront errors and to evaluate the communication performances of any lasercom terminals with a aperture less than 280 mm in link level. The range of beam propagation distance of the optical simulator is from 2.8km to 3,5000km. An angular deviation yielded by the beamsteerer is to simulate the pointing error caused by all error sources. And a light source is to provide the noise-like illumination of changeable levels. Combined the testbed with the beamsteerer, it can measure the pointing errors and the tracking errors of the lasercom terminals, and estimates dynamic communication performances in the presence of random angular jitter. So the testbed has the potential to become all-purpose and tests the optical, communication and track performances for intersatellite lasercom terminals (ISLTs).

Beam distortion of rotation double prisms with an arbitrary incident angle.

The distortion of beam shape in rotation Risley prisms is discussed in this paper. Using the ray-tracing method based on vector refraction theorem, a rigorous theoretical model of beam distortion with an arbitrary incident angle is established to explore the influencing factors. For a specified double-prism pair, the emergent beam is squeezed in one direction while stretched in the mutual perpendicular direction, the distortion of which is determined by the relative rotation angle. Moreover, the distortion of beam shape is greatly influenced by the wedge angles and the refractive indices of the prisms, as well as different double-prism configurations, while uncorrelated to the prism thickness and the distance between two prisms. This paper demonstrates the regular change of the beam shape with multiparameter variations in rotation double prisms, which can be applied to the design of rotation double-prism systems.

Physical basis and corresponding instruments for PAT performance testing of inter-satellite laser communication terminals

The propagation of laser beam in inter-satellite laser communications belongs to the far-field diffraction, but in the optical test and verification of pointing, acquisition and tracking (PAT) function on the laboratory the beam from a terminal propagates within the near-field. In this paper, in terms of the Fresnel diffraction theory the inherent difference is found that in the far-field diffraction the optical tracking position error is resulted from both the mutual movement between two laser communication terminals and the tilting of the receiver terminal, but the tilting of the transmitter has no effect on the error position; and that in the near-field diffraction the position error is caused by the tilting of the transmitter or the receiver, but the mutual movement has no effect. It is furthermore found that the use of a beam scanner in the test in the near-field can simulate exactly the mutual movement of satellites in the far-field, and the trajectory formula for the beam scanning is deduced that is the same as the mutual angular trajectory from one satellite to another. Therefore a practical PAT test bed of a double-focus laser collimator, a beam scanner and a fine beam steering device is developed by us to test and verify the PAT function of inter-satellite laser communication terminals. The optical aperture is about φ440mm for this use. And a test bed for concurrent test and verification of both PAT function and communication performance is also demonstrated. The test bed consists of a conventional laser collimator, an optical scanner and a far-field beam transmission simulator, which is a combination of a Fourier-transform lens an a followed multiple-stage imaging amplifier. The details of configurations are given. It is clear that these test beds can be also used to test and verify the functions of laser radar, passive optical tracker, and so on.

Comparison of refractive rotating dual-prism scanner used in near and far field

A pair of wedge prisms can be used to scan a laser beam over a specified angular range with a high resolution. The structural parameters of dual prisms directly influence the optical path and generate a tracking trajectory difference. For a remote target, the influences can be ignored, but for a near field target, the opposite is the case. The paper makes a comparison of rotating dual-prism scanner used in near and far field, as well as a forward solution and a reverse one of a laser beam through the rotating dual-prism system. The conclusion is valuable for a rotating dual-prism scanner to perform the optical tracking and targeting direction.

Dynamic characteristics analysis of a large-aperture rotating prism with adjustable radial support

Support elements as key components in performing the opto-mechanical function have been an important topic for optical system development. Focusing on a rotation prism with a large aperture and asymmetric loading, a radial multi-segment support is developed to solve the dynamic mounting issue. In order to explore the actual surface deformations over the full rotation, a novel dynamic analysis method to extract the transient load spectrum is established to access the surface deformations, including dynamic load extraction to connect varying loads with corresponding rotation positions, typical position analysis to obtain maximum deformation values, and vibration analysis. The results show that a maximum peak-to-valley value on the plane side reaches 103.16 nm when the prism rotates to 159.84°, and that of the wedge side is 74.38 nm when the prism rotates to 213.84°, both of which are less than λ/4 (λ=632.8 nm). However, when excited by the external loads with response frequency, the surface deformations become more serious. Because the dynamic characteristics obtained can reflect the actual usage situation, the proposed method is preferable for system development.

Submicroradian accuracy scanning system with a double-wedge rotating around the orthogonal axes

A scanning system of a high accuracy double-wedge is presented, which allows us to perform very small angle deviation of a passing beam in a simple way. The first wedges principal section is perpendicular to the horizontal axis, and the second is to the vertical axis correspondingly. They respectively rotate around the horizontal axis and the vertical axis as they work. So different small rotation angles of two wedges determine the corresponding orientation and position of the passing beam, and then high accuracy and very small-angle beam deviation can be performed. According to the design result: when the wedge angle is 5°, the refraction light beam will change about 1μrad if the wedge is rotated 1arcmin; the scanning range of light beam in the horizontal direction and the vertical direction can be not less than 600μrad, and the scanning precision of the device can be superior to 0.2 micro μrad

Forward and inverse solutions for three-element Risley prism beam scanners

Scan blind zone and control singularity are two adverse issues for the beam scanning performance in double-prism Risley systems. In this paper, a theoretical model which introduces a third prism is developed. The critical condition for a fully eliminated scan blind zone is determined through a geometric derivation, providing several useful formulae for three-Risley-prism system design. Moreover, inverse solutions for a three-prism system are established, based on the damped least-squares iterative refinement by a forward ray tracing method. It is shown that the efficiency of this iterative calculation of the inverse solutions can be greatly enhanced by a numerical differentiation method. In order to overcome the control singularity problem, the motion law of any one prism in a three-prism system needs to be conditioned, resulting in continuous and steady motion profiles for the other two prisms.