Tenghao Li

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.

A thermo-field bimetal deformable mirror for wavefront correction in high power lasers

A novel thermo-field bimetal deformable mirror (TBDM) is proposed, which is mainly composed of a face plate, a base plate and some thermal elements, such as thermoelectric coolers (TECs). The TBDM utilizes the continuous thermal field to bend the face plate, via the TECs heating or cooling. A prototype is described, and the first results are presented. The simplicity and ultra-low cost make the TBDM attractive in correcting aberrations in high power lasers.

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.

High-precision system identification method for a deformable mirror in wavefront control

Based on a mathematic model, the relation between the accuracy of the influence matrix and the performance of the wavefront correction is established. Based on the least squares method, a two-step system identification is proposed to improve the accuracy of the influence matrix, where the measurement noise can be suppressed and the nonlinearity of the deformable mirror can be compensated. The validity of the two-step system identification method is tested in the experiment, where improvements in wavefront correction precision as well as closed-loop control efficiency were observed.

Wavefront sensing for a nonuniform intensity laser beam by Shack–Hartmann sensor with modified Fourier domain centroiding

Abstract. A fast and simple solution is proposed to wavefront sensing for laser beams with spatial smoothly varying intensity, where Shack–Hartmann wavefront sensor and the modified Fourier domain centroiding algorithm are applied. Detailed analysis has been conducted to Fourier domain centroiding, simplifying the origin double-frame method to work with merely single frame, and revealing its interesting property of robustness to nonuniform intensity perturbation. The key parameter is also analyzed to optimize the algorithm’s performance. Numerical simulations demonstrate that the proposed method outperforms the center of gravity algorithm, with an average phase error reduction up to approximately one order in root mean square. Application of the proposed method in joint amplitude–phase compensation system is also verified by simulation with practical models and parameters. The modified Fourier domain centroiding is promising to enhance real-time actual wavefront measurement for adaptive optics in high-power lasers, such as joint amplitude–phase compensation.

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.

Wavefront sensing based on phase contrast theory and coherent optical processing

A novel wavefront sensing method based on phase contrast theory and coherent optical processing is proposed. The wavefront gradient field in the object plane is modulated into intensity distribution in a gang of patterns, making high-density detection available. By applying the method, we have also designed a wavefront sensor. It consists of a classical coherent optical processing system, a CCD detector array, two pieces of orthogonal composite sinusoidal gratings, and a mechanical structure that can perform real-time linear positioning. The simulation results prove and demonstrate the validity of the method and the sensor in high-precision measurement of the wavefront gradient field.

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.

Beam phase-distortion correction in a high power laser based on the stochastic parallel gradient descent technique

The feasibility of the closed-loop adaptive optic system based on laser far-field information was experimentally demonstrated, which can significantly increase the working efficiency of the multi-beamline high power solid laser facility. A stochastic parallel gradient descent algorithm with mean-radius cost function was applied in the adaptive optics that was installed in one chosen beamline in the SG-III laser facility. The far-field diameter from a reference laser was corrected from about 275 to 100??m, very close to the ideal limit of 75??m. Multiple-wavefront correction can be performed with this far-field detection based adaptive optics system, and the experimental result validates the practicability of the proposed system in the multi-beamline high power solid laser facilities.