Introduction to Adaptive Optics System
The adaptive optics system mainly consists of three basic components: wavefront sensor, wavefront corrector, and wavefront controller. The active component in adaptive optics systems is the wavefront corrector, which corrects wavefront distortion by changing the optical path length at each point on the cross-section of the beam. Generally, the change in optical path length can be achieved by moving the position of the reflective mirror or changing the refractive index of the transmission medium. The most widely used in adaptive optics systems is the wavefront corrector based on the position movement of the reflective mirror surface (usually referred to as a deformable mirror), which has excellent characteristics such as fast response speed, large deformation displacement, working spectral bandwidth, high optical utilization, and multiple implementation methods.
Adaptive optics systems can measure and compensate for wavefront distortions caused by various interferences in real time, enabling the optical system to automatically adapt to changes in external conditions and maintain optimal working conditions. Based on these advantages, adaptive optics has been widely used in fields such as astronomical observation and laser transmission, and has gained great recognition. At the beginning of this century, with the increasing interest in adaptive optics in other fields, its application scope began to expand, including human retina imaging systems, laser communication systems, and so on.
Application of Adaptive Optics System
Most practical adaptive systems are used in the field of astronomical imaging, but with the advancement of adaptive optics technology, mainly due to the diversity of device implementation methods, the application areas of adaptive optics have also been greatly expanded.
1. Adaptive optics system for imaging observation
All large aperture telescopes are now using adaptive optics systems to improve the imaging quality of the system. These systems have various application targets and utilize various technologies such as deformable mirrors and wavefront sensors. In 2003, the ALTAIR adaptive optics system equipped with Gemini North used a 177 unit deformable mirror (DM) and a separate tilt mirror (TTM), and employed a Hartmann Shack wavefront sensor to detect wavefront errors in the visible light band. The system operated at a frequency of 1kHz and achieved a resolution of 0.1 arcseconds in the K-band. The adaptive system equipped on the KECK II telescope with a 10 meter aperture uses a 349 element deformable mirror combined with a Hartmann Shack wavefront sensor, enabling the telescope to achieve resolutions of 0.022 arcseconds and 0.04 arcseconds in the 0.85um and 1.65um bands, respectively. At the summit of Mauna Kea, the Canada France Hawaii 3.6-meter telescope is equipped with an adaptive system called "Hokupa'a". The special feature of this system is that it uses a 36 element dual piezoelectric deformable mirror and a 36 element curvature sensor, greatly reducing the cost of the adaptive optics system. In early practical observations, it was found that the use of adaptive systems increased the peak intensity of imaging by 30 times. This data is in the observation band of 0.936um, and the corrected Strell ratio reaches 0.3. The 3.67-meter Advanced ElectroOptical System Telescope (AEOS) at the US Air Force Base on Maui uses 941 unit deformable mirrors, mainly for space target recognition, and its system scale is extremely large.
2. Adaptive optics system for laser devices
The use of adaptive optics technology for beam purification of laser beams is an important means to improve the quality of laser output beams, which can generally be divided into intracavity adaptive optics technology and extracavity adaptive optics technology. Intra cavity adaptive optics technology is to place a wavefront corrector inside a laser resonant cavity to correct the static and dynamic aberrations of the cavity, maintain the correct resonance conditions of the laser resonant cavity, improve the intensity and phase distribution of the laser, and increase the output power; Out of cavity adaptive optics technology is to place the wavefront corrector outside the laser resonant cavity and use the principle of wavefront compensation to improve the phase distribution of the laser output beam, in order to achieve the goal of increasing the far-field energy concentration. The technology of intracavity adaptive optical correction is relatively more complex, as the process of generating laser intracavity modes is already complex and requires numerical simulation for iterative analysis. As early as the 1980s, there were a series of theoretical analyses and experimental results for calibrating unstable cavity CO2 lasers. However, the experimental results showed that it was difficult to achieve good calibration results and often only a small amount of artificially introduced errors could be corrected. After the 1990s, Russian researchers conducted calibration work on Nd: YAG laser, and Cherezova et al. summarized their research results in their paper. They successfully doubled the divergence angle of the multimode beam. They also found that certain deformable mirror patterns can produce square or triangular pattern structures. Kudryashov and Samarkin used water-cooled piezoelectric deformable mirrors to perform intracavity calibration on high-energy CO2 lasers. Research has shown that by changing the focal length of the deformable mirror, the resonant cavity parameters can be adjusted to modulate the output intensity distribution. In contrast, external adaptive optics systems are more well-known, with typical representatives being inertial confinement fusion (ICF) and laser weapon systems. The existing major inertial confinement fusion systems in the world, such as the National Ignition Facility (NIF) in the United States, the Megajoule Laser Facility (LMJ) in France, the GEKKO facility in Japan, and the Shin Kong facility in China, all use adaptive optics technology to improve and control the quality of laser beams. In addition, the US military further applied previous research results to the field of strategic and tactical laser weapons. In 2001, the vehicle mounted solid tactical laser weapon system successfully intercepted ballistic missiles at the Baisha Range, and the more ambitious Air Born Laser (ABL) program made adaptive optics technology one of its core technologies. Although the system ultimately failed to achieve its expected strategic goals and was terminated in 2011, its mid-term performance demonstration has become an excellent advertisement for adaptive optics technology.
3. Adaptive optics system for optical communication
Atmospheric optical communication refers to a communication system that uses lasers as information carriers and the atmosphere as transmission channels for information transmission, including communication between satellites and ground stations, as well as between ground stations. Atmospheric optical communication combines the advantages of optical communication and wireless communication, allowing for high-capacity, high-speed transmission of data, voice, images, and other information without the need for any wired channels. So it has great potential for application and development in satellite communication, local broadband access, and military communication fields. The huge demand for applications has directly promoted the development of atmospheric optical communication technology, but the impact of atmospheric turbulence on communication quality has also caused difficulties for researchers. Since the 1990s, many researchers have attempted to use adaptive optics technology to reduce the impact of atmospheric turbulence on communication quality, and have achieved some important research results. The use of phase compensation principle for transmission correction in satellite ground links has shown good results. However, the transmission of horizontal links has not yet achieved ideal results due to strong atmospheric scintillation and other reasons, and further research is needed.
In the field of optical communication, the tremendous development of fiber optic technology has made it necessary for optical switches to replace electronic switches. Adaptive optics technology
The technique can improve the coupling efficiency of optical fibers. The experimental application of single-mode fiber switches using deformable mirror technology can eliminate aberrations and improve coupling efficiency. The maximum frequency of the switch can reach 1KHZ, and the coupling efficiency has increased from 9% to 46%. The phase modulation technology of deformable mirrors can also be used for experiments in optical information encoding, holographic recording systems, and laser free space communication technology. Adaptive optics technology will become one of the supporting technologies for optical communication.
Adaptive technology is becoming increasingly mature in optical network applications. Adaptation goes further than automatic switching and is the development direction of the next generation optical network. Compared to ASON, adaptive optical networks have better adaptability and self-organization capabilities, and can adaptively access various services. Adaptively adjust node transmission parameters and optimize network performance based on business requirements and actual network conditions. It can be said that adaptive optical networks can achieve automatic management and optimization of the optical transport layer on the basis of ASON automatic connection management. Adaptive optical network technology is of great significance for the future development of the field of optical communication.
4. Retinal adaptive optics imaging system
The eyes are the "window of information" through which humans perceive the world, with approximately 80% to 90% of external information entering the world of human consciousness through visual channels. Therefore, the visual analysis of the human eye, especially high-resolution imaging of the retina region, has always been a research focus in the field of biomedical science abroad. Experiments have shown that if diffraction limited imaging can be achieved even with a pupil diameter of 7mm, the photoreceptor cells on the retina can be easily seen using instruments. However, due to the imperfect structure of the cornea and lens, the human eye produces wavefront errors in the passing light, and their size and form vary from time to time, making it impossible to solve them by applying fixed corrections. This makes it impossible for general ophthalmic imaging systems to reach the diffraction limit, and thus cannot achieve high-resolution ophthalmic imaging. Adaptive optics can precisely solve such problems. Through fundus retinal images, various human diseases and pathological information can be discovered, such as cardiovascular and endocrine disorders, normal individuals and age-related macular degeneration, central serous chorioretinopathy, etc; However, in addition to defocus and astigmatism, human eye aberrations also include higher-order aberrations, which reduce imaging resolution. Traditional ophthalmic measurement techniques cannot overcome these higher-order aberrations, while adaptive optics technology used in human retinal imaging systems can obtain clearer retinal images. Junzhong Liang et al. from the Center for Visual Sciences at the University of Rochester in the United States used a 217 sub aperture Hartmann Shack wavefront sensor combined with a 37 unit deformable mirror to achieve adaptive optics retinal imaging with a lateral spatial resolution of 2um, which has enabled the differentiation of visual cells. Afterwards, scientists combined optical coherence tomography (OCT) and confocal scanning laser ophthalmoscopy (CSLO) with adaptive optics, enabling both longitudinal and transverse resolutions to reach the cellular level, making three-dimensional cell resolved retinal imaging possible. These technologies have become new tools for human visual science research. In recent years, systems have been developing towards high resolution, miniaturization, affordability, safety, and stability, and a large number of research results have been reported.
On some specialized optical instruments, such as the long path laser interferometer LIGO for measuring cosmic gravity waves and the multiphoton confocal scanning microscope, adaptive optics technology can be applied to correct the static or dynamic aberrations introduced by laser pump amplification of the instrument, thereby improving stability and ensuring detection sensitivity.
In short, due to the widespread application of optical instruments in military, industrial, medical, communication, testing and other fields, adaptive optics technology plays a unique role in improving the performance, anti-interference, stability and other aspects of instruments. With the continuous development, improvement and maturity of system integration and unit technology, and the continuous reduction of costs, this scientific and technological field will undoubtedly have broader development space in various military and civilian industries, and create social and economic benefits.
