Yuntao Qiu

Beam quality improvement by joint compensation of amplitude and phase

The M² factor could be decomposed as amplitude term and phase term. A method to improve the beam quality of laser beams is proposed. In our method, the amplitude and phase of a laser beam are both compensated in order to improve the beam quality completely. In experiment, a laser amplifier is set to modulate the amplitude to Gaussian type, and a deformable mirror is used to compensate the phase aberration. The laser beam is well compensated by our method; the beam quality factor of the laser is improved from 1.7 to 1.1.

Weighted Fried reconstructor and spatial-frequency response optimization of Shack–Hartmann wavefront sensing

Two modified Fried wavefront reconstructors are proposed, both based on an enhanced geometry that combines and balances those of Fried and Hudgin with an additional weight. The optimal weights for both of them are derived with the analytical frequency response functions, which can provide near-unity spatial frequency response over broad bandwidth to the Shack-Hartmann wavefront sensing system. Comparisons between the proposed reconstructors and the classical ones are presented in the frequency domain, and simulations have further confirmed the frequency characteristic and the reconstruction performance of the new reconstructors. It is expected that the optimized reconstructors can help improve the performance of adaptive optics systems for high-power laser wavefront control and other relevant optical systems that require high-accuracy wavefront sensing.

Structure and closed-loop control of a novel compact deformable mirror for wavefront correction in a high power laser system

A deformable mirror using a new preload structure is presented and characterized in a high power laser system. The structure is analyzed by the finite-element method and the character parameters of the deformable mirror are selected based on simulated results. The device reveals a high fidelity in static flatness aberration experiments. The closed-loop control method is introduced and analyzed. The results from closed-loop control, including a residual aberration P–V value of only 0.040 μm and an RMS value of 0.010 μm, show practically perfect aberration correction. The experiment results prove the fidelity of the deformable mirror with the preload structure and the validity and accuracy of closed-loop control.

Research on the particular temperature-induced surface shape of a National Ignition Facility deformable mirror

We investigate the changes in the shape of a deformable mirror used at the National Ignition Facility caused by differences in temperature between the working environment and the mounting temperature of the mirror. In general, the temperature-induced profile change of the mirror is dominated by a few low-order aberrations, which mainly result in defocus. However, after these low-order distortions are corrected, there remain special, higher-order, surface distortions caused by the particular arrangement, construction, and mounting of the mirror actuators. This work analyzes these special aberrations, and their dependence on the particular actuator design, using the finite element method. Experiments are carried out to verify the computational results, and finally, design considerations to help minimize the temperature-induced high-order aberrations are suggested.

Method to evaluate beam quality of Gaussian beams with aberrations.

Zernike polynomials are commonly used to describe the aberration of light beams, and the beam quality of Gaussian beams with aberrations can be deduced when the order of Zernike polynomials is limited. In this paper, Hermite polynomials are utilized to reconstruct the aberrations of Gaussian beams. The beam quality factor is directly related to the coefficient and terms of Hermite polynomials and has no limit on its index. We analyzed the beam quality of a Gaussian beam with a quartic aberration and other former 11th Zernike aberrations by Hermite polynomial expansion. The result corresponds with the published research work.

The beam quality of a truncated Gaussian beam with aberrations

We present a method for the calculation of the beam quality factor of an aberrated laser beam. The closed-form equations are proved, and it is shown that the beam quality factor of an aberrated Gaussian beam depends directly on the Hermite coefficient of the wavefront, as well as the truncation ratio. Further numerical calculation proves that the proposed model leads to results matching those in published work.

Flexible high-resolution wavefront sensor using a double-frequency two-dimensional acousto-optic modulator

Abstract. A versatile wavefront sensor is described, using a two-dimensional acousto-optic modulator (AOM) driven with double radio frequencies in both the orthogonal directions. Inserted in the spectral plane in a 4-f system, the AOM acts like a moving double-frequency crossed grating. The proposed wavefront sensor inherits the advantages of a double-frequency grating lateral shearing interferometer in the aspects of high-resolution slope estimations and structure compactness. The sensor naturally performs as a heterodyne detection system, thanks to the inherent property of traveling waves in acousto-optic modulators. Variable shear can be acquired conveniently by electronically adjusting the driving frequencies, with the optimal performance achieved under different circumstances involving parameters such as accuracy, dynamic range, and signal-to-noise ratio. Beyond the proposed concept, theory analysis is presented in detail, and simulations are carried out to demonstrate the principle and validation of the proposed wavefront sensor.