The paper presents an analysis of the efficiency of using bimorph deformable mirrors and mirrors on piezoactuators for the problem of focusing scattered laser radiation. In this paper, we study the efficiency of a piezoactuator deformable mirror with 61 piezopackets and an aperture of 60 mm, as well as a bimorph mirror with 48 electrodes and an aperture of 50 mm. It is shown that such mirrors can be used to optimize the focal spot formed by scattered radiation in the far-field.
Wavefront correction efficiency by bimorph deformable mirror could be estimated through the modal functions’ reconstruction. Zernike polynomials at this point could be considered as an effective and easy-to-use tool for this purpose. The 37-channel bimorph deformable mirror was able to reproduce wavefront aberrations up to 7th order with reasonable amplitude and residual RMS of less than 0.05 μ.
A fast adaptive optical system, operating at frequencies up to 2000 Hz (frames per second), was used to analyze turbulence created in the laboratory by using a fan heater. The turbulent distortion bandwidth was approximately 100 Hz. The expansion of the wavefront in terms of Zernike polynomials was used when processing the raw data. Then the statistical analysis was performed separately for each polynomial. As a result, the degree of predominance of definite aberrations in the wavefront of laser radiation was obtained. Taylor's hypothesis is confirmed: low-order aberrations are slower than high-order ones. The dependence of the correction quality on the number of corrected Zernike polynomials is also shown.
KEYWORDS: Field programmable gate arrays, Mirrors, Photodiodes, Data communications, Zernike polynomials, Wavefronts, Laser systems engineering, Turbulence, Data conversion, Wavefront sensors
We present an adaptive optical system to stabilize the position of a laser beam passed through the turbulent atmosphere. The system uses two tip-tilt mirrors and is controlled by an FPGA to increase the bandwidth. An internal FPGA structure is presented. FPGA reads the error signal from the sensors formed by quadrant photodiodes and calculates the voltages to be applied to the piezo-driven tip-tilt mirrors by the control units.
A bimorph deformable mirror with a diameter of 320 mm, including 127 control electrodes, has been developed and investigated. The flatness of the initial surface of the mirror RMS = 0.16 μm was achieved due to mechanical adjustment in the system of fixing the mirror substrate in the frame. An adaptive system with deformable mirrors and a ShackHartmann-type wavefront sensor was installed in a 4.2 PW Ti: Sa laser. Correction of the wavefront made it possible to obtain a record radiation intensity in the focusing plane of 1.1x1023 W/cm2, while the Strehl ratio was 0.84.
The results of experiments carried out on the setup of a fast adaptive optical system are presented. A fan heater was used as a source of wavefront aberrations, the air flow of which was directed perpendicular to the laser beam. The processing of the experimental data consisted in determining the spectral characteristics of the disturbing effect from the dynamics of oscillations of the coordinates of the focal point of the lens array. To ensure sufficient resolution in the frequency domain, a sample of the original data was recorded for 10 s, which provided a resolution along the frequency axis of 1/10 Hz. The graphs of the spectral energy for the full set of wavefront aberrations calculated from the fluctuations of the lens array focal spot are shown. For a more detailed consideration, the wavefront aberrations were expanded in terms of Zernike polynomials, after which a spectral analysis of each aberration was carried out. It is shown that more than 90% of all turbulence energy is concentrated in lower-order aberrations, which makes it possible to use a bimorph mirror as a wavefront corrector, which reproduces well the lower-order aberrations.
Deformable mirrors in adaptive system for high power lasers should have high damage threshold and thermally stabilized surface. The cooling is required for average beam power above 100W. In this paper, the problem of the thermo-stabilizing of the deformable mirrors is studied. Bimorph deformable mirrors (BDM) and stack actuator deformable mirrors (SADM) are considered. For BDMs the active and passive cooling designs have been implemented. In active design, the waffletype cooling system for circulation of the cooling liquid was made inside of the thin substrate. In passive design, the periphery surface is thermally contacted with the condenser which temperature is constant. Such BDMs not damaged at CW power density 20kW/cm2 when the beam fills full surface of the mirror and can be used in powerful solid state ceramic YAG lasers. SADM is most suitable to correct for small-scale and high-speed aberrations. We developed SADM where reflecting substrate is cooled through the actuator bodies. Individual actuators are housed in metal holders and can be replaced if they fail due to electrical breakdown. SADM has the diameter 120 mm and included 121 actuators. The deformation stroke of the actuator was 7μ. The first resonant frequency was 18.5 kHz, which allowed operating in adaptive system in kHz range.
The results of experiments conducted on a laboratory setup of a fast adaptive optical system based on the use of FPGA as the main control element and a bimorph mirror as a wavefront corrector are presented. The adaptive system bandwidth ranged from a dozen Hertz to 2,000 Hertz. For independent control of the quality of correction the intensity distribution in the far field was recorded. It is shown that for a good correction of the wavefront the system bandwidth should be an order of magnitude higher than the upper boundary of the spectrum of wavefront distortions caused by turbulence. A comparison of the model and experimental data is also presented.
The article discusses the use of stacked-actuator adaptive mirrors to improve the focusing of laser radiation. The criterion of focusing efficiency is the fraction of the energy of the laser radiation passing through the pinhole located in the focal plane of the focusing lens.
The results of experiments carried out on the installation of a fast adaptive optical system with a bandwidth of 1500 Hz are presented. Graphs of the spectral power and normalized energy of Shack-Hartmann wavefront sensor focal spot fluctuations are presented. Theoretical calculations and experimental data are compared. It is shown that in order to achieve a satisfactory correction of distortions caused by turbulence, the system operation frequency should be an order of magnitude higher than the maximum frequency of turbulent distortions.
One of the main problems in tasks of laser beam propagation though Earth’s atmosphere is decrease the efficiency of the optic-electronic systems operation due to atmospheric turbulence influence that leads to laser beam’s wavefront distortions. Use of fast adaptive optical system are suggested to solve this problem. It allows to compensate the wavefront distortions, which upper bound of the spectrum is up to 150 Hz, in real time. Owing to the fact that adaptive optical system is discrete (it’s defined by digital camera included in the system), the sampling rate shall be at least 1500 Hz (frames per second).
The results of numerical simulations and experiments on the correction of turbulent distortions of a laser beam are presented. The experiments were carried out using an adaptive optical system with a bandwidth of 2000 Hz. It was shown that for effective correction the bandwidth of the adaptive optical system should be an order of magnitude larger than the bandwidth of turbulent distortions.
An adaptive optical system that implements a phase conjugation algorithm designed to compensate for the effect of atmospheric turbulence the propagating laser beam is presented. The system allows compensating for the influence of atmospheric disturbances up to 200 Hz (in terms of sine). To achieve the compensation effect system operates at a frequency of 2000 Hz (in terms of fps - frames per second). Such high performance can be achieved only when using FPGA as the master control element of the system. The results of correction of disturbances obtained by using a heat fan, simulating the turbulence to frequencies of 200 Hz, are presented.
Bimorph deformable mirror with 63 electrodes on 20 mm aperture is discussed. Methods of dividing all round electrode into sectors with a square of 2-4 mm2 are described. Results of flat-top beam formation by means of 50 mm bimorph deformable mirror with 48 electrodes and 20 mm miniature bimorph mirror with 27 electrodes are presented.
Laser beam shaping technology nowadays requires as small diameter of the adaptive optics as possible. In our lab we usually control for laser radiation by means of bimorph deformable mirrors with a typical size of more than 50 mm. To fit the most of industrial and scientific applications the aperture of the corrector should be reduced because the use of extra optics instead makes the whole optical scheme more complicated and introduces extra distortions. But in a bid to reduce the size of the mirror we should care of the response of the mirror electrodes which obviously should not decrease drastically. Here we present 20 mm bimorph mirror with high density of electrodes which is manufactured using laser engraving technology to divide the electrode on the piezoceramic disc into a large number of the controlled sectors. The ability of laser beam formation by means of this mirror is discussed, the results are compared with the ones obtained using 50 mm bimorph deformable mirror.
In tasks related to free-space communications, a significant role has a turbulent atmosphere which influences lead to a decrease in the efficiency of systems. Since the characteristic turbulence spectrum rarely exceeds 100 Hz for typical paths, it is proposed to use a discrete adaptive optical system with a frequency of 1500 frames per second to reduce the influence of the atmosphere. The structure of the system based on the use of FPGA as a computing device as well as the main results associated with the correction of both static and dynamic components of aberrations are presented.
The article discusses the use of stacked-actuator adaptive mirrors to improve the focusing of laser radiation. The criterion of focusing efficiency is the fraction of the energy of the laser radiation passing through the pinhole located in the focal plane of the focusing lens.
As well known, the quality of radiation of laser beam is reduced during propagation along optical trace, because of various reasons (for example, atmospheric turbulence, scattering, thermal fluctuations etc.). We propose small-size deformable mirrors with high spatial resolution of control elements, that will allow to compensate for wavefront aberrations in wide range. Developed wavefront corrector could be used in different scientific areas: free-space communications, destruction of space debris, etc.
In this work, we investigate the efficiency of the use of the bimorph deformable mirror to focusing laser beam in the pinhole. Pinholes of different diameters are used as an instrument for focusing verification. Different algorithms are discussed and analyzed for the investigation of the process of the beam focusing. It is shown that tip-tilt correction is an essential condition for increasing the focusing efficiency.
The most important part of any adaptive optical system is a deformable mirror. One of the most widely used type of such mirrors are the bimorph ones. In fact, there is no problem to manufacture a wide aperture bimorph wavefront correctors that perfectly can compensate low-order laser aberrations. But if one needs a tiny deformable mirror to correct for high order aberrations with reasonable amplitude, he usually will use stacked actuator mirror or a MEMS one. In this presentation we suggest the new design and technology of production of a small size bimorph mirrors to be used to correct for atmospheric phase fluctuations. Our mirror has the diameter of 30 mm and 37 control electrodes (mirror with 20 mm and 63 control electrodes is being developed). The resonance frequency of 13.2 kHz is due to its small diameter. At the same time, large number of electrodes allows to reproduce high order aberrations. To manufacture this device two modern technologies are used: ultrasonic welding and laser engraving technology.
High-power laser systems are widely used for technological purposes or laser fusion experiments, but also to deliver the energy to some remote places here on the Earth as well as in space. Probably the only instrument to improve the phase structure of the laser beam is to use methods and elements of adaptive optics. The heart of any adaptive optical system is of course the deformable mirror. That is why the research and development of such optical elements continues for many years already. Here we propose a new stacked actuator mirror to be used to correct for not just the turbulent phase fluctuations, but also it should work with high CW power laser beams. Therefore, our mirror is a water-cooled one. Another important feature of this device is that each actuator could be easily substituted in case of failure. The mirror is 120 mm in diameter and has 121 control elements. Main characteristics of a mirror are presented in this work.
This article discusses the use of bimorph adaptive mirrors to improve the focusing of laser radiation. The criterion of
focusing efficiency is the fraction of the energy of the laser radiation passing through the pinhole located in the focal
plane of the focusing lens.
We present our latest research results on intensity distribution transformation from Gaussian to a flattop and doughnut. The theoretical calculations and experimental results of the efficiency of different types of deformable mirrors are given. During the experiments the wavefront was measured with Shack-Hartmann sensor and then modified with bimorph deformable mirror to reach the desired intensity distribution in the far-field. Then the bimorph mirror was substituted with the stacked-actuators deformable mirror to confirm the simulations.
Thermal effects in the active laser medium influence on the quality of the laser beam in high-power laser systems. Temperature fluctuations lead to refractive index modulation in the medium, thus the intensity of the radiation is significantly reduced. Historically, to solve this task an adaptive optics technique is used. It allows to compensate for the wavefront aberrations. Stacked-actuator deformable mirror is a traditional technology that is used in order to improve the quality of the incident wavefront. This type of wavefront correctors has one significant shortcoming – the impossibility of the replacement of broken actuators. We developed a stacked-actuator deformable mirror with aperture of 120 mm and 121 control actuators to correct for the high-power laser radiation. Actuator arrangement was hexagonal. In our design, the broken piezoactuators could be easily replaced.
As it is well-known, the laser radiation, propagated through different mediums, is affected by wavefront distortions and thus the quality of the radiation is significantly decreased. To compensate for the wavefront aberrations adaptive optics means are used. We developed miniature bimorph mirror with 37 electrodes. To manufacture this type of mirrors two technologies were used. Those are laser engraving technology for drawing electrode grid on the piezoceramic disc, and ultrasonic welding technology to connect wires to the control electrodes. Main parameters of such a deformable mirror were investigated and presented in this paper.
During propagation of the high-power laser radiation through atmosphere the quality of the beam is significantly reduced due to atmosphere turbulence. For compensation of the wavefront distortions and improving quality of radiation, as a rule, adaptive optics approaches are used. The design and main characteristics of 121-element water-cooled stacked-actuator deformable mirror for this kind of applications is discussed in this paper.
Fast adaptive optical system can be used, for example, for correction of laser beam passed through a strong turbulent atmosphere. The frequency that such a system should operate with to achieve an acceptable level of wavefront correction is about 1 - 1.5 kHz. There are two most popular methods to develop this system: by using a standard PC computer and by using FPGA. This paper presents the advantages and disadvantages of each of these approaches. The results obtained with the use of these systems are presented. Recommendation for achieving higher performance are given.
The latest results on intensity distribution transformation from Gaussian to a flattop and doughnut are presented in the paper. The wavefront was modified with bimorph deformable mirror to reach the desired intensity distribution in the farfield. LC phase modulator was also considered as an alternative device for laser beam shaping. The theoretical calculations and experimental results of the efficiency of different types of wavefront correctors are given.
The process of remapping the intensity profile of a laser beam is presented. Bimorph deformable mirror was used to change the beam phase; the control signals for the mirror were calculated in accordance with both phase analysis and far-field intensity distribution measurements.
The transformation of an intensity distribution from Gaussian to a flattop, doughnut, etc. still is a very interesting and important task. And the necessary result could be obtained with the use of adaptive optics that changes the phase of the beam and modifies the shape of the focal spot in the far-field zone. In this paper, we present the flattop and doughnut beam formation result with the use of a bimorph and stacked-actuator deformable mirrors as well as LC phase modulator. The experimental results are also given.
In this paper we present recent results of formation of different beam intensity distribution by means of bimorph deformable mirrors. We discuss the results of such formation as well as the problems that one faces on this way. A new method for beam structure modification is suggested based on the use of Shack-Hartmann wavefront sensor with the combination of standard M2 meter.
This paper presents the design of the closed loop adaptive system to measure and correct for the aberrations of CO2 laser radiation. We considered two wavefront sensors - one sensor is based on commercially available IR camera while the second one – on the so-called thing film sensors. Also we present the design of two bimorph deformable mirrors to be used under high power laser radiation. We discuss both positive and negative attributes of these devices and the possibility to use them in the real laser high-power systems.
This paper presents the two types of Hartmann wavefront sensors to measure the aberrations of CO2 laser radiation. One
sensor is baser on commercially available IR camera while the second one - on so-called thing film sensors. We discuss
both positive and negative attributes of these sensors and the possibility to use them in the real laser systems.
This paper discusses the novel adaptive optical closed loop system with water-cooled bimorph mirror as a wavefront
corrector to compensate for the aberrations of high-power CW laser beam. Shack-Hartmann wavefront sensor is used as
an element for feedback control. Comparison of phase conjugation and modified hill-climbing technique is shown.
Shack-Hartman wavefront sensors are widely used in scientific investigations of wavefronts and also, as a
component of the closed-loop adaptive optical system, intended to correct for laser beam aberrations. This paper
presents successful application of such type sensors in the laser systems investigations to obtain high quality laser
radiation. Results of investigations of wavefronts of modern high power solid-state lasers are given in this paper.
Here we investigate closed-loop adaptive optical system to compensate for laser beam aberrations. A bimorph mirror is
used as a wavefront corrector and Shack-Hartmann wavefront sensor is an element for feedback control. Comparison of
phase conjugation and multi-dither technique is shown.
Some peculiarities of the use of adaptive optical elements and the whole system to correct for the aberrations of high power
single pulse lasers are discussed in this paper. The examples of the use of adaptive system to correct for the aberrations of
some lasers are presented. As a corrector we used bimorph multi electrode deformable mirror while as a sensor - Shack-
Hartmann wavefront sensor.
We discuss the new type of the closed loop adaptive optical system with bimorph corrector and M2 meter. The study of some multi-dither algorithms to be used in the system is presented. Multi-dither approach is applied to 40-TW TiS fs laser in JAERI, Japan, to improve the focusability of laser beam. We demonstrate that it is possible to obtain 75% of input power in first diffraction maximum with use both phase conjugate and multi-dither adaptive optical systems.
We present an adaptive optical closed loop system to obtain a good focused beam. A bimorph mirror is used as a wavefront corrector and CCD camera at the focal plane of the lens is a sensor. Such adaptive system can correct for the low-order wavefront aberrations without any sophisticated wavefront sensors.
This paper presents adaptive optical closed loop system with bimorph mirror as a wavefront corrector and Shack-Hartmann wavefront sensor to compensate for the aberrations of the high power lasers. Adaptive system can correct for the low-order aberrations in the real-time -- the frequency of corrected aberrations is less than 25 (30) Hz. The amplitude of such aberrations -- about 7 microns. These parameters are mostly determined by utilized Shack-Hartmann wavefront sensor. Number of corrected aberrations -- up to 30th Zernike polynomial (excluding tip-tilt). We are presenting the results of the use of our adaptive system in several high-power laser systems such as ATLAS, LULI, JAERI and Beijing Institute of Physics.
This paper discusses the novel adaptive optical closed loop system with bimorph mirror as a wavefront corrector and Shack-Hartmann wavefront sensor to compensate for the aberrations of the laser beam occurred during the distribution of the beam from laser to processed material. Adaptive system can correct for the low-order aberrations in the real-time, the frequency of corrected aberrations is less then 25 (30) Hz. The amplitude of such aberrations, about 7 microns. These parameters are mostly determined by utilized Shack-Hartmann wavefront sensor. Number of corrected aberrations - up to 15th Zernike
polynomial (excluding tip-tilt).
We describe an adaptive optical closed loop system with bimorph mirror as a wavefront corrector and CCD camera at the focal plane of the lens as a sensor to obtain a good focal spot. Adaptive system can correct for the low-order aberrations with the frequency of corrected aberrations about 5 Hz. These parameters are mostly determined by the deformable mirror properties and multi-dither algorithm.
This paper discusses the novel adaptive optical closed loop system with bimorph mirror as a wavefront corrector and Shack-Hartmann wavefront sensor to compensate for the aberrations of the laser beam occurred during the distribution of the beam from laser to processed material. Adaptive system can correct for the low-order aberrations in the real-time - the frequency of corrected aberrations is less then 25 (30) Hz. The amplitude of such aberrations - about 7 microns. These parameters are mostly determined by utilized Shack-Hartmann wavefront sensor. Number of corrected aberrations - up to 15th Zernike polynomial (excluding tip-tilt).
This paper discusses the novel adaptive optical closed loop system with bimorph mirror as a wavefront corrector and Shack-Hartmann wavefront sensor to compensate for the aberrations of the laser beam occurred during the distribution of the beam from laser to processed material. Adaptive system can correct for the low-order aberrations in the real-time - the frequency of corrected aberrations is less than 25 (30) Hz. The amplitude of such aberrations - about 7 microns. These parameters are mostly determined by utilized Shack-Hartmann wavefront sensor. Number of corrected aberrations - up to 15th Zernike polynomial (excluding tip-tilt).
This paper discusses the novel adaptive optical closed ioop system with bimorph mirror as a wavefront corrector and Shack- Hartmann wavefront sensor to compensate for the aberrations of the laser beam occurred during the distribution of the beam from laser to processed material. Adaptive system can correct for the low-order aberrations in the real-time —the frequency of corrected aberrations is less then 25 (30) Hz. The amplitude of such aberrations — about 7 microns. These parameters are mostly determined by utilized Shack-Hartmann wavefront sensor. Number of corrected aberrations —up to 15th Zernike polynomial (excluding tip-tilt).
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