Laser inertial reference device for submicron line-of-sight stabilisation

Abstract

FIELD: optoelectronics; precision laser technology.SUBSTANCE: invention can be used for submicron stabilisation of the line of sight of optoelectronic means of space systems. A laser inertial reference device for submicron stabilisation contains a base, an inertial unit, a three-axis fibre-optic gyroscope and a three-axis microelectromechanical accelerometer, an optical unit, a semiconductor laser fixed on a platform, a collimator, two two-axis movable mirrors, the reflective surfaces of which are located parallel at a distance of 50 mm, at an angle of 70° to the direction of the optical axis, a receiving system consisting of two beam splitters, two position-sensitive photodetectors (PSPD) of quadrant photodiodes, as well as an objective in front of one of the PSPDs.EFFECT: reducing weight and dimensions, increasing accuracy and speed due to the use of a system of two two-axis movable mirrors and two position-sensitive photodetectors.3 cl, 2 dwg

Description

[0001] Field of technology to which the invention relates

[0002] The invention relates to the field of optoelectronics and precision laser technology.

[0003] Technology Level

[0004] An analogue of the proposed invention is an optical inertial reference unit (OIRU) ["Device for automatic stabilization of the spatial position of a light beam", patent SU 1364878 A1, IPC: G01C 15 / 00 (2006.01), published 01 / 07 / 1988, Bulletin No. 1]. The device comprises a radiation source, a deflector in the form of a mirror fixed on a bimorphic piezoelectric plate, a beam splitter, an objective and a position-sensitive photodetector (PSPD) in the form of a quadrant photodiode. The device operates as follows: part of the light flux emitted by the source and passing the deflector is directed by the beam splitter in the direction of the telescopic system, the objective and the PSPD. Thus, a closed photoelectric tracking system is formed. The remaining part of the light flux is measuring and can be used as a reference line.When the laser beam's position fluctuates, the center of the light spot on the surface of the PF shifts, and the measured angular shift is compensated for by the deflector. Thus, automatically maintaining the center of the light spot at the center of the PF, located in the focal plane of the objective, ensures that the direction of the beam incident on the objective is independent of fluctuations in the angular position of the beam at the output of the radiation source and is determined only by the spatial stability of the principal point of the objective, the photoelectric center of the PF, and the focal length of the objective.

[0005] The analog has the following disadvantages: 1) the impossibility of linear beam shift, since maintaining the position of the reference beam by changing the shape of the bimorph mirror and controlling the position on the quadrant diode when passing the lens does not allow tracking and linearly shifting the beam, 2) the difficulty in controlling the bimorph mirror, which consists in the need to control and monitor ten or more piezoelectric drives, 3) the limitations of its response speed (modern models no more than 500 Hz, with a natural oscillation frequency of 50 Hz), which is not sufficient for submicron stabilization in real time of the sighting axis during vibration of the spacecraft, the frequency of which reaches from 20 to 300 Hz, as, for example, presented in the article (Weida XING et al. "Micro-vibration suppression and compensation techniques for in-orbit satellite: A review", Vol. 37(9), 2024, 19 p.).

[0006] Also known is the Optical Axis Jitter Measurement Method for Space Optical Camera and Device [patent CN 105045030 A, IPC: G03B 43 / 00 (2006.01), published 11 / 11 / 2015]. The device comprises a platform, two non-parallel radiation sources mounted on the platform and four gyroscopes, as well as two additional FPs in the form of square photodiodes, which are installed in the focal plane of the space telescope matrix. The device operates as follows: two non-parallel laser beams of different intensity and shape for their subsequent differentiation on detectors are directed into the optical system of the space telescope; passing through its elements, they hit the two FPs. Based on measurements taken from a gyroscope combined with an astrosensor, which shows the displacement of two laser beams, the beam displacement on the PCF due to vibration is determined and compared with the actual data on the detectors; they become different only in the case of a shift in the optical components of the telescope.This allows us to determine how much the image is shifted at the moment of vibration, and the method allows us to determine not only the linear shift on the matrix surface, but also the angular shift around the normal to the matrix surface.

[0007] The analog has the following disadvantages: 1) the impossibility of forming a true reference laser beam in the inertial frame of reference, since the beam does not move, it remains stationary only in the telescope coordinate system, which makes it limited in application, for example, for other optoelectronic systems, 2) the need to increase calculations and build new models of beam path with an increase in the number of optical elements in the telescope system, 3) the bulkiness of the system and high requirements for mutual calibration of all components of the device.

[0008] Also known is the Optical Inertial Reference Unit for kilohertz bandwidth submicroradian optical pointing and jitter control (OIRU) [patent US 7227111 B2, IPC: G01J 1 / 20 (2006.01), G01C 21 / 02 (2006.01), published 05.06.2007], adopted as a prototype. The OIRU comprises a base, a laser, a movable piezoelectric platform including four piezoelectric drives, four angular velocity sensors, and a linear displacement sensor. The device operates as follows. Fluctuations in the position of the piezoelectric platform and laser are measured using an angular velocity sensor and compensated by piezoelectric drives, which rotate the platform around two axes, forming a closed movable system. The generated laser beam becomes relatively stationary (with an accuracy limited by the accuracy of the platform movement and the measurements of the angular velocity sensor), this beam can be used as a reference beam, independent of the angular jitter of the device.

[0009] The prototype has the following drawbacks: 1) the inability to change the linear beam displacement caused by linear external disturbances; 2) specific requirements for the placement of piezoelectric actuators and sensors: all elements must be arranged with fourfold symmetry around an axis perpendicular to the stable platform and parallel to the center of the optical beam. When the device's center of mass is located below the laser, additional linear beam displacement occurs when the platform's angular position changes.

[0010] The measurement of the beam position accuracy and its jitter in the prototype is described in the article (Dan-Eckelkamp-Baker. Magnetohydrodynamic Inertial Reference System, In Acquisition, Tracking and Pointing XIV, Vol. 4025, 2000, pp. 99-110.) and is carried out using simulation modeling of the control system in the Matlab Simulink software package or using full-scale experiments, since the derivation of analytical dependencies from systems of differential equations of motion of piezoelectric drives is impossible.

[0011] The accuracy and performance characteristics of the current devices and prototype are described in articles ("Inertial stabilization technology in an optical-electric tracking system," "Design of a space telescope for vibration control," "Model analysis and resonance suppression of a wide-bandwidth inertial reference system," "Mechanical design and determination of bandwidth for a two-axis inertial reference unit," "Image Motion Measurement and Image Restoration System Based on an Inertial Reference Laser") and on the websites of Bluehalo and L3Harris Technologies. The angular positioning accuracy reaches at least 4 μrad / mrad, angular jitter is at least 400 nrad, and the response time reaches 2500 Hz, allowing the device to operate in real time with a latency of 0.4 ms.

[0012] Disclosure of the essence of the invention

[0013] The objective of the invention is to generate a reference laser beam, namely, to stabilize the laser beam under conditions of linear and angular external microvibrations. This objective is achieved by a closed-loop photoelectric tracking system consisting of a base, an inertial unit, and an optical unit. This system is characterized by two biaxial movable mirrors that provide linear and angular beam displacement, creating four degrees of freedom of beam position, using measurements from a fiber-optic gyroscope and a microelectromechanical accelerometer and beam position tracking by a receiving system consisting of two beam splitters, two position-sensitive filters, and a lens in front of one of the position-sensitive filters. Moreover, the base, inertial unit, and optical unit can be positioned asymmetrically. The technical result of the invention is a reduction in weight and dimensions, and an increase in accuracy and response time due to the use of a system of two biaxial movable mirrors (PZ) and two position-sensitive photodetectors.

[0014] The essence of the invention is explained by drawings (Fig. 1-3).

[0015] Brief description of drawings

[0016] Fig. 1 shows the basic diagram of the laser inertial reference unit (LIRU) and introduces the following designations: 1 - base; 2 - fiber-optic gyroscope; 3 - microelectromechanical accelerometer; 4 - semiconductor laser; 5 - collimator; 6, 7 - two-axis movable mirrors; 8, 9 - beam splitters; 10, 11 - position-sensitive photodetectors; 12 - lens.

[0017] Fig. 2 shows the structural diagram of the LIRU control and the following designations are introduced: 13 - inertial unit; 14, 19, 24, 27 - elements taking into account the parameters of the optical block; 15-18 - PID controllers; 20-23 - piezoelectric drives; 25, 26 - PCF.

[0018] Implementation of the invention

[0019] The device consists of a base 1, an inertial unit 13 containing a three-axis fiber-optic gyroscope 2 and a three-axis microelectromechanical accelerometer 3, an optical block including a semiconductor laser 4, a collimator 5, two biaxial reflectors 6 and 7, a receiving system of two beam splitters 8 and 9, two PCRs 10 and 11, and an objective 12 in front of one of the PCRs. The reflective surfaces of the reflectors are located parallel at a distance of 50 mm, at an angle of 70° to the direction of the incident and reflected beams (the direction of the optical axis). All elements of the system 2-12 are rigidly fixed to the base 1 and are located asymmetrically relative to the center of mass of the base 1. Rigid fixing of the movable mirrors 6 and 7 means fixing their housing (base). In this case, the formed beam is independent of the linear, longitudinal optical axis and angular microvibrations of the device around the optical axis.The device operates as follows: the laser beam enters collimator 2, reaches the required beam diameter, then falls on a system of two movable mirrors 6 and 7, after which it is divided by the first beam splitter 8. Part of the light flux enters the second beam splitter 9, which splits the beam into two PCRs 10 and 11. This creates a closed photoelectric tracking system. The remaining portion of the light flux exiting the first beam splitter 8 is used for measurement and can be used as a reference marker beam. Changes in the position of the device due to external microvibration are recorded by a three-axis fiber-optic gyroscope 2 and a three-axis microelectromechanical accelerometer 3, the angular and linear displacement of the beam in inertial space is compensated by a system of two two-axis movable mirrors 6 and 7, and the actual displacement of the beam is controlled by a receiving system of two beam splitters 8 and 9, PCR 10 and 11, and lens 12.Thus, the automatic displacement of the center of the light spot by the one calculated in the plane of the PF leads to the fact that the error in the rotation of the mirrors is minimized, and the direction of the beam leaving the device after the first beam splitter 8 turns out to be inertially fixed (with an accuracy limited by the measurement accuracy of the inertial unit and the PF).

[0020] To evaluate the accuracy and performance characteristics of the proposed invention, a control system for the device was developed and simulated in the Matlab Simulink package. The signal flow and control order are determined by the control block diagram shown in Fig. 2. The device has four degrees of freedom for beam control, two linear and two angular (Px, Py, θx, θy). Therefore, the input data of the block diagram are two linear vibrations and two angular velocities. The control objects are piezoelectric drives 20-23, which shift the mirror planes around two axes. In this model, the specified movements of the piezodrives dx1(t), dy1(t), dx2(t), dy2(t) are converted into real movements (dielectric and mechanical losses are taken into account) dx1r(t), dy1r(t), dx2r(t), dy2r(t).Transducers 14, 19, 24, and 27 are used to convert one physical quantity into another more convenient for use in the control process. In the developed model, they represent a set of equations linking the displacements of the photodetectors with the vibration parameters required by the displacements of the piezoelectric actuators. PID controllers 15-18 are used to ensure the specified dynamic properties of the closed-loop system. They ensure high operating accuracy in steady-state conditions and dampen strong oscillatory processes. In this model, the controllers generate control signals u1(t), u2(t), u3(t), u4(t), tending to zero errors εX1(t), εY1(t), εX2(t), εY2(t) between the detected X1d(t), Y1d(t), X2d(t), Y1d(t) and the specified beam positions X1(t), Y1(t), X2(t), Y1(t) on the photodetector. Inertial unit 13 serves to convert ideal vibration data into data measured with delay and error by the gyroscope and accelerometer.The 25 and 26 frequency converters serve to form a feedback loop and determine the detected positions of the laser beam on the photodetectors X1d(t), Y1d(t), X2d(t), Y1d(t).

[0021] The angular and linear positioning accuracy of the laser beam reaches at least 3 μrad / mrad and μm / mm, respectively, the angular and linear beam jitter reaches at least 350 nrad and 320 nm, respectively, the response speed reaches 3800 Hz, which allows the device to operate in real time with a delay of 0.26 ms.

Claims

1. A laser inertial reference device for submicron stabilization, consisting of a base, an inertial unit, characterized in that it contains a three-axis fiber-optic gyroscope and a three-axis microelectromechanical accelerometer, an optical unit, and has a semiconductor laser fixed on the platform, a collimator, two two-axis movable mirrors (MZ), the reflective surfaces of which are located in parallel at a distance of 50 mm, at an angle of 70° to the direction of the optical axis, a receiving system consisting of two beam splitters, two position-sensitive photodetectors (PSPD) of quadrant photodiodes, as well as a lens in front of one of the PSP.

2. The device according to paragraph 1, characterized in that the combination of the base, inertial and optical units does not require symmetrical placement relative to the center of mass of the base.

3. A device according to any one of paragraphs 1, 2, characterized in that the position of the beam can be shifted along four degrees of freedom, which allows the beam to be shifted both angularly and linearly, forming a reference laser beam that is stationary in the inertial coordinate system.

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