Learn about Beam Deflection Technology Based on Space Optical Communication in 3 Minutes

Beam deflection technology is a key component of free-space laser communications, and its performance determines whether free-space laser communications can meet fast and stable communication needs. Beam deflection technology can be divided into two categories: mechanical beam deflection technology and non-mechanical beam deflection technology. Among them, mechanical beam deflection technologies include scanning galvanometers, fast control mirrors, and micro-electromechanical system deformable mirrors; non-mechanical beam deflection technologies include acousto-optic deflection technology, deflection technology based on liquid crystal materials, and electro-optical deflection technology.

Let's take a look at the characteristics of various beam deflection technologies and their application prospects in the field of space optical communications.

1.Scanning galvanometer

The most mature mechanical beam deflection device is a scanning galvanometer, which is essentially a light reflector with a step response time of milliseconds/sub-milliseconds and a pointing accuracy of microradians, as shown in Figure 1.

Figure 1 Schematic diagram of scanning galvanometer

The galvanometer scanning system has a simple structure, small size, high scanning accuracy, fast speed, and relatively low cost. However, it has problems such as limited working range, pincushion distortion, and galvanometer wear. This device has reached excellent performance standards in terms of deflection angle. For example, the XG210 series scanning galvanometer launched by the American THORLABS company has a deflection angle of up to ±20°. Currently, researchers at home and abroad are working on increasing the scanning speed and using methods such as femtosecond laser pulses and multi-dimensional galvanometer structures to improve its performance.

However, for two-dimensional galvanometers and higher-dimensional galvanometer scanning technologies, the system structure is more complex, and orientation errors will occur in practical applications, and good correction methods are required to correct the errors. In the future, we can consider using variable structure control technology and thick and thin two-level composite axis control technology to assist in suppressing residual errors. They can be applied in satellite constellations with good space environments and short working cycles to achieve high-precision tracking and scanning with maximum efficiency. In addition, the power of lasers in laser communications is generally very high, so choosing galvanometer mirror materials with higher reflectivity to reduce surface damage is also a problem that needs to be solved in the future.

2.Fast Steering Mirrors

There are two structures for Fast Steering Mirrors, FSM (as shown in Figure 2): one is the X-Y axis frame structure, also called the shaft system structure; the other is the flexible axis structure, which is the main development direction of FSM at present.

Figure 2 (a) X-Y axis frame structure diagram of the Fast Steering Mirrors; (b) Flexible axis structure diagram of the Fast Steering Mirrors

The fast control mirror has the advantages of high positioning accuracy, high angular resolution, fast response speed, and compact size. It is widely used in a variety of optomechanical systems, and the flexible support structure also reduces mechanical friction, but in practical applications it requires Combined with the large inertia frame structure, it will lead to a certain optical axis error.

At present, on the one hand, domestic research in this field mainly focuses on the structural simulation and system control of fast reflectors, and progress in the development of new reflectors is slow. This is also related to the need for continuous iterative verification and high research and development costs. Therefore, developing a joint simulation system so that physical verification can be simulated by adjusting certain parameters in the system, thereby greatly shortening the development cycle, finding high-performance fast mirror parameters faster, and improving optimization efficiency is something that needs to be explored in the future.

On the other hand, thermal disturbances and fundamental vibrations existing in the space environment will cause optical axis distortion and jitter when pointing high-precision beams. Currently, the existing method is to use a beam composed of a Michelson interferometer and a fast control mirror. Pointing alignment system to compensate for the problem of optical axis error. However, this method has low accuracy in handling dynamic measurement errors. Enhancing the accuracy of dynamic measurement errors to compensate for errors in real time is a problem that should be solved in the future.

3.MEMS deformable mirror

Micro-Electro-Mechanical System-Deformable Mirror (MEMS-DM) has various types such as electrothermal drive, piezoelectric drive, electrostatic drive and electromagnetic drive. In view of the fact that electrostatic drive has simple structure, It has the advantages of fast response speed and the ability to work under high-frequency signals, so it is mostly driven by electrostatic force, and is mostly implemented in the form of flat capacitors. Its structure is shown in Figure 3.

Figure 3 MEMS deformable mirror drive structure diagram

Microelectromechanical system deformable mirrors have the advantages of high unit density, short response time, low power consumption, low cost, and good integrated circuit compatibility, and are more widely used in the imaging field; however, they also have slow scanning speed and low light energy utilization. , problems such as more stray light. In recent years, researchers have begun to develop more unit actuators for deformable mirrors in order to increase the wavefront stroke and obtain a higher frame rate; at the same time, deformable mirrors with more actuators will lead to greater mechanical stress, so choosing Lighter, lower-hardness base materials are the way forward.

4.Acoustic and light deflection technology

Acousto-optic deflection technology converts high-frequency electrical signals into ultrasonic waves and transmits them to the working medium through a transducer to form a grating, which uses light wave diffraction to deflect the beam, as shown in Figure 4. The acousto-optic diffraction effect is divided into Ramanes diffraction and Bragg diffraction according to the length of the acousto-optic area. Since Ramanes diffraction has low light utilization efficiency and Bragg diffraction has high diffraction efficiency, Bragg diffraction is generally used.

Figure 4 Principle diagram of acoustic and light deflection

Acousto-optic deflection devices have the advantages of small size, light weight, low driving power, and high diffraction efficiency. At the same time, acousto-optic deflection technology also has real-time parallel processing capabilities, large time bandwidth, easy compatibility with computers, and automatic control. However, there are also the following shortcomings: most of the diffracted light is first-order diffracted light, which results in the acousto-optical deflection device having obvious shortcomings in the large-angle deflection range, low deflection accuracy, difficulty in achieving fine control of the beam, and low resolution. , a "chirp effect" will appear under high-speed scanning.

By using methods such as ultrasonic tracking and single crystal multi-frequency, the effective bandwidth can be increased to solve the problem of low resolution. For the "chirp effect", a cylindrical lens can be added after the deflector to eliminate its influence. At present, there are many studies on the frequency of incident acoustic waves, and different methods of experimental improvement have been carried out to improve the diffraction efficiency and frequency response performance of the acousto-optic deflector under the incidence of ultrasonic waves, but the performance of increasing the deflection angle has rarely been analyzed.

In the future, controllable acoustic wave vector technology can be considered to change the incident direction of the acoustic wave to expand its deflection scanning angle. Other indicators of the deflection performance of acousto-optic deflectors, including bandwidth performance, antistatic ability, and thermal stability, are also current research hotspots.

5.LCD deflection technology

Beam deflection technologies based on liquid crystal materials mainly include: liquid crystal phased arrays, liquid crystal microlens arrays, and liquid crystal polarizing gratings.

Liquid Crystal Optical Phased Array (LCOPA) technology refers to applying voltage to liquid crystal molecules through electrodes. Since liquid crystal molecules have an electronically controlled birefringence effect, the applied voltage controls the degree of deflection of liquid crystal molecules in different states, thereby affecting the beam wave. It plays the role of phase modulation in front to realize beam scanning, as shown in Figure 5.

Figure 5 Principle diagram of liquid crystal phased array deflection

LCOPA has the advantages of high power and low voltage driving, and can achieve high-precision beam deflection with dexterity and no mechanical inertia. However, it has shortcomings such as long response time and short operating spectrum width. In addition, the small deflection angle also limits the application range of LCOPA, which requires an angle amplification device to achieve a larger deflection angle. However, due to factors such as the effective aperture and walk-away angle of the angle amplification device, it is currently difficult for the angle amplification device to achieve higher angle magnification. At the same time, the liquid crystal phased array will have multiple diffraction orders during operation, and coupled with the influence of nonlinear correlation effects, the deflection efficiency of LCOPA will be reduced.

Liquid Crystal Micro-lens Array (LCMLA) consists of 3 lens arrays, as shown in Figure 6. Compared with LCOPA, LCMLA has a larger deflection angle and is not affected by the optical return zone, so the deflection efficiency is higher; affected by the change time of the LC molecular arrangement in the liquid crystal material, the optical path difference required by LCMLA is longer than that of LCOPA. Small, the thickness can be made smaller, so LCMLA has a smaller response time than LCOPA. However, to achieve continuous beam deflection scanning, LCMLA needs to be used in combination with some fine-angle deflection devices, which increases the complexity of application implementation. Moreover, LCMLA is composed of a multi-layer lens array, and the system stability is worse than LCOPA. LCMLA achieves beam deflection by changing the main large diffraction order of the emitted light. The spatial coherence of the microlens array affects its resolution, which requires a very small error in the size of the microlens, which is a major problem that needs to be solved. 

Figure 6 Schematic diagram of liquid crystal microlens array

The principle of Liquid Crystal Polarization Grating (LCPG) is that the incident light passes through the polarizer to form left-handed light and right-handed light, and then passes through the LCPG to deflect the light beam in two different directions. The deflection light path is shown in Figure 7. LCPG is not affected by the electric field edge effect and has high resolution, programmable control, lightness and flexibility. LCPG only needs to generate the optical path difference of the equivalent half-wave plate, and the required thickness of the liquid crystal layer is thinner, thus making its response time shorter. It is fast and does not have the impact of optical return caused by phase resetting. In addition, it can also achieve wide spectrum operation. However, it is difficult for a single LCPG to achieve the index requirements of multiple angles and a large field of view at the same time, and multi-layer LCPG has high requirements on the preparation process and system stability.

Figure 7 Schematic diagram of liquid crystal polarization grating

The traditional LCOPA is light and flexible and can achieve fine deflection within a small angle range. The system complexity is relatively simple and the preparation process is relatively mature. However, it is affected by the optical return zone caused by phase resetting, and there are obvious deficiencies in deflection efficiency, response time and other indicators. , still needs continuous improvement and development. LCMLA and LCPG are not affected by the optical return zone and have greatly improved the deflection efficiency. However, they both need to be equipped with fine-angle deflection devices to achieve quasi-continuous deflection scanning of the beam, and both use multi-stages to achieve the maximum deflection angle. The series structure will lead to a system that is too long and has relatively poor stability. Compared with LCOPA and LCMLA, LCPG not only has the characteristics of large deflection angle and high deflection efficiency, but also has the unique advantage of wide spectrum operation, but it can only achieve beam deflection scanning with a large angular interval. At present, liquid crystal deflection technology is the most widely studied in non-mechanical deflection, but there are significant limitations in achieving large angles and high efficiency under non-polarized light conditions. To solve this problem, the device architecture and material type can be considered; when using liquid crystal polarizing grating devices, it is difficult to achieve continuous angle deflection at large angle deflections. These are problems that need to be solved in the future.

6.Electro-optical deflection technology

Electro-optical deflection technology is realized by utilizing the deflection generated by the refractive index gradient perpendicular to the direction of beam propagation, as shown in Figure 8. Compared with other technologies, beam deflectors based on electro-optic crystals have the advantages of arbitrary deflection angle, small size, fast response speed, and high sensitivity, but they have the problem of low resolution.

Figure 8 Principle diagram of electro-optical deflection

In recent years, electro-optical materials with secondary electro-optical effects have been reported at home and abroad, such as lithium niobate, barium titanate, etc. Compared with crystals with linear electro-optical effects, they are superior in performance such as response speed and deflection voltage. Among them, KTN crystals The most representative.

KTN crystal is the currently known crystal with the largest secondary electro-optical effect. It has outstanding characteristics such as large dielectric constant, low dielectric loss, obvious ferroelectricity and excellent nonlinear optical properties. It has a very wide range of applications in the field of beam deflection. prospect. At present, foreign companies such as Japan's NTT Company and the University of Pennsylvania in the United States, as well as domestic Harbin Institute of Technology, Nankai University, and Shandong Academy of Sciences, have done a lot of research on the deflection characteristics of KTN crystals.

NTT Company and the University of Pennsylvania mainly studied KTN crystal beam deflection technology based on space charge injection; Shandong Academy of Sciences mainly studied the beam deflection technology induced by the composition gradient of KTN crystal; Harbin Institute of Technology and others mainly studied the electrodes of KTN crystal beam deflectors. Engineering issues such as structure and operating temperature were studied.

The following problems currently exist: it is difficult to achieve high optical uniformity in crystal growth and meet the needs of practical applications; applications near the Curie temperature require precise temperature control methods; there are questions about the space charge injection mechanism and polarity at the Curie temperature. Scientific issues such as the nano-region and the control mechanism of beam deflection require further research.

In order to more intuitively display the advantages and disadvantages of each deflection technology, a comparative analysis was conducted, as shown in Table 1.

Table 1 Comparison of beam deflection technologies

Summary

Commonly used mechanical micro-electromechanical deformable mirrors, fast reflection mirrors and scanning galvanometers change the direction of the emitted optical axis through mechanical means. Their accuracy can reach microradians and the deflection angle can reach dozens of radians. They have wide application prospects in medicine and other fields. . However, there are problems such as complex structure, bulky size, and high energy consumption. Due to the large size of adaptive optical systems, MEMS deformable mirrors at home and abroad are mainly used in the imaging field. In the field of beam deflection, it is difficult to meet the needs of small-scale spaceborne environments. to meet the high requirements of chemicalization and lightweight.

Acousto-optic deflection equipment has a large working bandwidth, but it is difficult to meet the deflection accuracy of microradians, and it has high requirements on the wavelength, angle and energy of the incident light and consumes large energy losses.

Methods such as liquid crystal phased arrays and microlens arrays have low power consumption and low driving voltage, but they have slow response speed, discontinuous angular deflection, large deflection angles but low deflection efficiency at large angles, making it difficult to meet the task requirements of large-bandwidth transmission.

Compared with other technologies, beam deflectors based on electro-optic crystals have the advantages of arbitrary deflection angle, small size, fast response speed, and high sensitivity. They are considered to be the most suitable for realizing one of the leading directions of high-speed light deflection technology. Among various types of electro-optical materials, electro-optical deflectors based on KTN crystals have the advantages of large-angle deflection, fast response speed, high deflection efficiency, high deflection accuracy, wide-bandwidth operation, etc., and have greater potential in applications in fields such as space optical communications, becoming a Research hotspots around the world. On the one hand, the subsequent work must analyze and study the growth characteristics and conditions of KTN crystals to grow high-quality crystals with uniform composition and regular shape; on the other hand, we must gradually study the microscopic deflection mechanism of KTN crystals, which is very important. practical significance.