Superresolution technology based on a heterodyne detection system.

Diffractive superresolution elements (DSEs) placed on a pupil plane can generate a diffractive main lobe whose width is smaller than that of an Airy disk, allowing for the realization of superresolution technology based on pupil filtering. However, the energy of the main lobe decreases dramatically with the decreasing of main lobe width, namely, the implementation of this superresolution technology is at the cost of effective signal power. This restricts greatly the development of this technology. In order to solve this problem, this study suggests the use of a heterodyne detection system (HDS) with this technology. The resolution characteristics of the HDS are analyzed through theoretical deduction. According to research results, HDS has the same longitudinal resolution and twice as high transverse resolution as a direct detection system (DDS). More significantly, the theoretical analyses show that HDS can increase detection sensitivity significantly compared with DDS. Hence, the proposed method makes it possible to detect extremely faint signals using this superresolution technology. In addition, because HDS lowers the requirements on main lobe energy due to its high sensitivity, the design of DSE can achieve a smaller width of main lobe, which can further improve the resolution of the superresolution technology based on pupil filtering.

Analysis of false alarm in heterodyne coherence accumulation based on sequence shifting and the genetic algorithm.

As a new method, coherent accumulation based on sequence shifting and the genetic algorithm (GA) has been proposed to detect a weak heterodyne signal. The excellent performance of the new method was proved by an experiment in previous studies. In this paper, the phenomenon of false alarm caused by the method is revealed through numerical simulation. It is found that, when only noise exists or signal-to-noise ratio (SNR) is low, the false alarm will occur with a high probability. Based on the statistical property of the false alarm, a solution approach using multiple GA modules is proposed to avoid the false alarm. According to the approach, only when all search results of the modules are the same, can one determine that there is an actual heterodyne signal in the detection; otherwise, the inputs of the modules will be judged as noise. Thus, the false alarm can be eliminated completely even if the SNR is quite low. The studies of this paper promote the practical application of the new coherent accumulation method.

Experimental study of coherent accumulation based on sequence shifting and a genetic algorithm.

Because there is a certain phase difference between the two adjacent samples of an analog-to-digital converter for a heterodyne signal, we propose to eliminate the random phases in the coherent accumulation by shifting the sampling sequences with different steps. A genetic algorithm is used to determine the shifting steps of the sequences. An experiment has been conducted to verify the effectiveness of our approach. The results prove that the approach can greatly increase the strength of the heterodyne signal compared with the accumulation with random phases. Our approach is easier to implement in comparison with existing technologies.

Shack-Hartmann wavefront sensing based on binary-aberration-mode filtering.

Spot centroid detection is required by Shack-Hartmann wavefront sensing since the technique was first proposed. For a Shack-Hartmann wavefront sensor, the standard structure is to place a camera behind a lenslet array to record the image of spots. We proposed a new Shack-Hartmann wavefront sensing technique without using spot centroid detection. Based on the principle of binary-aberration-mode filtering, for each subaperture, only one light-detecting unit is used to measure the local wavefront slopes. It is possible to adopt single detectors in Shack-Hartmann wavefront sensor. Thereby, the method is able to gain noise benefits from using singe detectors behind each subaperture when used for sensing rapid varying wavefront in weak light. Moreover, due to non-discrete pixel imaging, this method is a potential solution for high measurement precision with fewer detecting units. Our simulations demonstrate the validity of the theoretical model. In addition, the results also indicate the advantage in measurement accuracy.

Analysis of heterodyne detection in the frequency domain.

As opposed to previous works, our research on heterodyne detection is performed in the frequency domain. Based on the planar wave, it is pointed out that the current amplitude of a heterodyne signal is proportional to the spectrum value of the quantum efficiency function of a detector, and the frequency corresponding to the value is decided by the beam's incident angle. The spectrum of the quantum efficiency function has a low-pass characteristic, and its cut-off frequency is determined by the diameter of the photosensitive surface. Consequently, the strength of the heterodyne signal current depends on the position of a specific frequency in the passband. Our study method is extended to the arbitrary wave and proven to be still effective. According to our analyses, it is concluded that, in addition to the energy decline of interference field, the main cause of performance deterioration resulting from various influential factors can be generally explained as a spectrum mismatch between the interference field and the quantum efficiency function. Based on the Gaussian wave mode, the fact is illustrated by making theoretical deductions, in which the effects of the misaligned angle and beams' spot position offset are treated as examples. Our work provides the possibility of introducing frequency-domain theories into further studies on heterodyne detection.

Wavefront sensing by means of binary intensity modulation.

We propose a kind of wavefront sensing technique by means of binary intensity modulation. A digital micromirror device operates as a binary intensity modulator and a pinhole works as a binary-aberration-mode filter. Through modulating intensity distribution of incident light, light emitting from the pinhole is capable of containing information on binary aberration coefficients. With the amount of light acquired by a single detector, the coefficients of binary aberration modes for reconstructing incident wavefront can be calculated. Differing from the conventional wavefront sensing technique, this method turns the complex two-dimensional wavefront sensing into simple total-light-intensity detection. The simulation experiment has validated the feasibility of the theoretical model.

Way of detecting entire beam path aberrations of laser systems based on a phase-retrieval method.

A method is presented to detect entire beam path aberrations of many laser systems, especially the SG-III inertial confinement fusion (ICF) laser system, which is under construction in China. Instead of a Hartman-Shack (HS) sensor, a CCD will be introduced into the target chamber of the ICF laser system. The HS sensor and the CCD could record the near-field wavefront distributions and the intensity signals of their corresponding focal spots, respectively. An amendatory phase-retrieval method is introduced to reconstruct the aberrations of the entire beam path from near-field wavefront distributions and their corresponding focal spots intensity. Principle experimental results indicate that the aberrations of the entire beam path of a laser system can be precisely detected based on this method.

Intracavity transverse modes controlled by a genetic algorithm based on Zernike mode coefficients.

A new adaptive optics (AO) system for controlling the mode profile of a diode-laser-pumped Nd:YAG solid laser has been set up in our laboratory. A 19-element piezoelectric deformable mirror (DM), which is used as the rear mirror of the solid-state laser, is controlled by a genetic algorithm (GA). To improve the system convergence rate, the GA optimizes the first 10 orders of Zernike mode coefficients rather than optimize 19 voltages on the DM. The transform matrix between the 19 voltages and the first 10 orders of Zernike mode coefficients is deduced. Comparative numerical results show that the convergence speed and the correction performance of the AO system based on optimizing Zernike mode coefficients is far better than that of optimizing voltages. Moreover, experimental results showed that this AO system could change TEM(10), TEM(11), and TEM(20) transverse modes into a TEM(00) mode successfully.