The signal-to-noise ratio (SNR) of heterodyne detection is seriously reduced by the spatial phase distortion, so the compensation method of the phase distortion is of great significance for improving the performance of the heterodyne system. By replacing the single detector with an array detector in the system, the previous compensation method based on sequence shift and optimization algorithm has a certain effect. However, the method also has various shortcomings, such as long processing time, poor search stability, and possible false alarm, which severely limits its practical application. To solve the above-mentioned problems, the autocorrelation operation is used to achieve the equiphase superposition of the signals output by the elements of the array detector, thereby realizing the phase compensation. In comparison to the previous method, our method does not need an optimization algorithm, which avoids the long iterative operations and considerably improves the processing efficiency. Besides, this method avoids the false alarm caused by the optimization algorithm. The numerical calculation indicates that in case of severe phase distortion, the proposed method can increase the SNR by dozens of dB compared with the single-point detector system, thus proving the effectiveness of this method. The study results may provide a feasible and effective phase compensation method for improving and promoting the laser heterodyne detection performance.
By constructing an experimental system, the heterodyne detection of the photon counting system is realized. The effects of photon counting rate, sampling rate, background light noise and beam mismatching angle on the single-photon heterodyne detection performance based on power spectrum averaging are studied experimentally. The results show that the power spectrum average results have a saturated signal-to-noise ratio (SNR) at a certain count rate, and the saturation SNR increases with the increase of the count rate. As the sampling rate increases, a higher saturation SNR can be obtained. The local oscillator and signal light mismatch angles have the same effect on single photon heterodyne detection compared to linear detection. The research results in this paper have a promoting significance for the practical application of single-photon heterodyne detection technology.
Adaptive optics (AO) schemes are often applied to the inertial confinement fusion (ICF) system, such as SG-Ⅲ Prototype which has been set in China. This laser systm mainly includes a pulsed seed laser source, a multi-pass laser amplifier with the configuration of beam rotate-90° and expansion. When AO system is employed in this sytem, the beam bounces twice on the deformable mirror (DM) which works as the cavity mirror (CM) of the multi-pass laser amplifier, moreover, after the first bounce on the DM, the beam rotate 90° and expansion with a ratio. Therefore, the relationship between the DM’s correction stroke and the aberrations within the laser sytem must be known before applying a adaptive correction. This paper demonstrates that any output wave-front aberrations within the DM’s correction stroke range can be well corrected, as well as illuminates that the expansion ratio of beam and the types of output wave-front aberrations both affect the correction stroke range of DM. Furthermore, through building a theoretical calculating model and some simulation. the relationship between the DM’s surface stroke needed and different aberrations within the laser sytem is ascertained clearly. Results show that this configuration is proper for compensting most low order aberrtions besides some special ones. As a result, it will provides a useful guidance for those rotate-90°laser systems adopting adaptive optics technique.
Walsh functions have been modified and utilized as binary-aberration-mode basis which are especially suitable for representing discrete wavefronts. However, when wavefront sensing techniques based on binary-aberration-mode detection trying to reconstruct common wavefronts with continuous forms, the Modified Walsh functions are incompetent. The limited space resolution of Modified Walsh functions will leave substantial residual wavefronts. In order to sidestep the space-resolution problem of binary-aberration modes, it’s necessary to transform the Modified-Walsh-function expansion coefficients of wavefront to Zernike-polynomial coefficients and use Zernike polynomials to represent the wavefront to be reconstructed. For this reason, a transformation method for wavefront expansion coefficients of the two aberration modes is proposed. The principle of the transformation is the linear of wavefront expansion and the method of least squares. The numerical simulation demonstrates that the coefficient transformation with the transformation matrix is reliable and accurate.
Based on a 979-actuator adaptive optics system, this paper analyzes the different performance of iterative wavefront
reconstruction algorithms. Under the condition of dynamic wavefront errors, the iteration number, storage space and the
number of multiplication of steepest descent method, conjugate gradient method and G-S iterative method are studied.
The steepest descent method and conjugate gradient method need smaller storage space. Both the G-S iterative method
and the conjugate gradient method converge faster than steepest descent method, while the latter takes the least number
of multiplication. Finally, the optimal iterative algorithm is selected considering storage space, iteration numbers and the
number of multiplication.
Thin observation module by bounded optics (TOMBO) is an optical system substituting a micro lens-let array with smaller apertures for a conventional large full aperture. This array allows us to capture multiple low resolution sub-images of the same scene and use them to reconstruct a high resolution image. While lost resolutions can be recovered, there has been very little work on experimentally evaluating restored resolution performance in the TOMBO system. Our work focuses on resolution comparisons among a 4×4 lens-let TOMBO and Nikon lenses in the same f number condition. Experimental results present the equivalent focal length of the experimental TOMBO system.
We propose a novel wavefront sensing technique based on binary-aberration-mode filtering and detection. Rather than
Zernike polynomials, the orthogonal binary two-dimensional Walsh functions are transferred to circular mode-fieldfitted
Walsh functions and used as binary aberration modes to expand the wavefront. A Digital Micromirror Device
(DMD) is employed as an intensity spatial light modulator (SLM). It generates each of the intensity modulation patterns
prescribed by the mode-field-fitted Walsh functions to modulate the intensity of the incident beam before it is focused to
impinge on a single-mode optical fiber. The single-mode optical fiber, as a spatial mode filter, supports only fundamental
binary aberration mode. A detector collects the amount of the intensity after each modulation. By building the
relationship with the intensity, the binary-aberration-mode coefficients can be calculated. This technique turns the
complex two-dimensional wavefront sensing into simple intensity detection. Therefore, many limitations, such as low
response frequency and weak far-infrared detection capability of most photosensor arrays can be easily eliminated just
by adopting a photosensor such as a photodiode. Thus, this technique is especially suitable for weak and far-infrared
light detection. The numerical simulation demonstrates that the wavefront reconstruction with the binary aberration
modes is reliable and the technique can easily meet the demands of high speed atmospheric measurements and has a
promising application in atmospheric fields.
A low cost adaptive optical system for improving solid-state laser beam quality has been set up. This
system consists of a deformable mirror, a high voltage amplifier, a set of solid-state laser system, a
CCD camera, a control software and corresponding optimization algorithm. This adaptive optical
system doesn't employ a wave-front sensor to detect the phase information, but optimize the light
intensity within a fixed aperture in the focal plane by a 19-element piezoelectricity deformable mirror.
In order to find the optimum surface profile of the deformable mirror, which is applied to correct the
phase aberrations in a solid-state laser system, a global genetic algorithm is introduced. The far-field
light intensity signal, which is measured by a CCD camera, is used as a fitness function of the genetic
algorithm. In this paper, the performance and efficiency of this wave-front sensor-less adaptive optical
system based on the genetic algorithm are presented. Both the simulation results and the experimental
results are given and discussed.
In an inertial confinement fusion (ICF) system, wave-front aberrations existed in laser beam will enlarge the focal spot
size and decrease power density at the target. Fortunately, an adaptive optical system (AO) could be employed in ICF
system to correct the beam aberrations. As a powerful wave-front detector, Hartmann-Shack (H-S) sensor is often
utilized as a wave-front sensor in AO. However, H-S sensor can not detect the aberrations after the sampling location. A
new method is presented to measure the aberrations of entire ICF beam path in this paper. Based on the AO, a CCD is
installed in the target chamber to detect the focal spot distribution. The deformable mirror's (DM) is yielded to different
surface shapes; the extra different aberrations are modulated and added to ICF beam path, and then create their
corresponding focal spots. The extra aberrations and the corresponding focal spots intensity could be recorded
simultaneously by H-S sensor and CCD respectively. An amendatory phase-retrieval algorithm which is introduced can
reconstruct the aberrations of entire ICF beam path from the pairs of extra aberrations and their corresponding focal spots
intensity. The numerical simulation show that the AO can correct the aberrations of entire beam path of ICF successfully
based on this method.
A conventional adaptive optical system (AOS) often measures the wavefront slope or curvature straightly by a wavefront sensor. However, another alternative approach allows the design of an AOS without an independent wavefront sensor. This technique detect the image quality affected by phase aberration in laser wavefront rather than measuring the phase aberration itself, and then the image quality is taken as a sharpness metric. When wavefront phase aberration is corrected, the sharpness metric reaches its maximum value. In this paper, a wavefront sensorless adaptive optical system (AOS) has been set up. This system mainly consists of a 19-element piezoelectricity deformable mirror (DM), a high voltage amplifier, a set of 650nm laser, a CCD camera and an industrial computer. The CCD camera is used to measure the light intensity within an aperture of the focus plane, and then this intensity is regarded as the sharpness metric to optimize. A Modified Hill Climbing Algorithm (MHC) and a Genetic Algorithm (GA) are used to control the DM to correct the phase aberrations in this system. Experimental results show that both of these two algorithms can be used successfully in this indirect wavefront measurement AOS. However, the GA can obtain better performance than the MHC. After phase aberrations are corrected, the βfactor are reduced from 5.5 to 1.5 and 1.9, from 30 to 1.2 and 1.4 respectively.
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