Zhigang Han

Lensless reflective point diffraction interferometer.

A lensless reflective point diffraction interferometer (LRPDI) is proposed for dynamic wavefront measurement. The point diffraction interferometer is integrated on a small substrate with properly designed thin film, which is used for generating the interferogram with high carrier frequency at a CCD target. By lensless imaging, the complex amplitude at the CCD target can be propagated to the conjugated plane of the exit pupil of an incident wavefront, which not only avoids the edge diffraction in the interferogram, but also eliminates systematic error. The accuracy of LRPDI is demonstrated by simulation and experiment, and a precision better than 1/150 wavelength is achieved. The new design with lensless imaging processing is suitable for dynamic wavefront measurement.

High power cladding light stripper using segmented corrosion method: theoretical and experimental studies

We present the segmented corrosion method that uses hydrofluoric acid to etch the fiber of a fiber laser for removing high-power cladding light to improve stripping uniformity and power handling capability. For theoretical guidelines, we propose a simulation model of etched-fiber stripping to evaluate the relationship between the etched-fiber parameters and cladding light attenuation and to analyze the stripping uniformity achieved with segmented corrosion. A two-segment etched fiber is fabricated with cladding light attenuation of 19.8 dB and power handling capability up to 670 W. We find that the cladding light is stripped uniformly and the temperature distribution is uniform without the formation of hot spots.

Absolute flatness testing of skip-flat interferometry by matrix analysis in polar coordinates

A new method utilizing matrix analysis in polar coordinates has been presented for absolute testing of skip-flat interferometry. The retrieval of the absolute profile mainly includes three steps: (1) transform the wavefront maps of the two cavity measurements into data in polar coordinates; (2) retrieve the profile of the reflective flat in polar coordinates by matrix analysis; and (3) transform the profile of the reflective flat back into data in Cartesian coordinates and retrieve the profile of the sample. Simulation of synthetic surface data has been provided, showing the capability of the approach to achieve an accuracy of the order of 0.01 nm RMS. The absolute profile can be retrieved by a set of closed mathematical formulas without polynomial fitting of wavefront maps or the iterative evaluation of an error function, making the new method more efficient for absolute testing.

Suppressing the influence of charge-coupled device vertical blooming on the measurement of laser beam quality factor (M^2) of a near-infrared laser

In this paper, a new method, which is based on reconstructing the original intensity distribution of a laser with images captured by a charge-coupled device (CCD) in two orthogonal directions, is proposed for suppressing the influence of CCD vertical blooming on the measurement of the laser beam quality factor (M2). A simplified theoretical model for the distribution of CCD blooming is also proposed. With the proposed method and model, the influence of CCD vertical blooming on the measurement of M2 is simulated. The experimental results demonstrate that the new method can be an effective means to measure the M2 of a near-infrared laser with a silicon CCD camera. The proposed method can be applied to a beam quality analyzer in order to suppress the influence of blooming on the measurement of M2.

Defect detection based on a lensless reflective point diffraction interferometer

We propose a defect detection system to identify phase defects on optics based on a lensless reflective point diffraction interferometer (LRPDI). The optics under test are illuminated by a collimated beam to produce a signal wavefront carrying the defect information, and then the signal wavefront is recorded in a high carrier interferogram using the LRPDI. By lensless imaging, amplitude and phase defects, as well as the accurate phase of a phase defect, can be identified. The simulation and experiment demonstrate the success of the proposed system in detecting phase defects, and its high-accuracy and high-resolution dynamic detection abilities are verified.

Phase extraction from fringe pattern via light propagation

A phase demodulation method via light propagation is proposed, where one or two fringe patterns are viewed as the superposition of complex amplitudes, and then the phase is reconstructed by separating the light field via light propagation. Simulation and experimental results indicated that the proposed method can extract the phase from a single shot effectively, thereby realizing dynamic phase retrieval. In addition, the accuracy of phase reconstruction can be improved by adding another fringe pattern with an unknown phase shift. The carrier requirement is relatively low, and, thus, the proposed method can be applied to the measurements with an environment disturbance, an inaccurate phase shift, and the requirement of a high speed capture.

Determination of the laser beam quality factor (M 2 ) by stitching quadriwave lateral shearing interferograms with different exposures

A complete complex amplitude reconstruction method for the determination of the laser beam quality factor M2 based on the multiple exposure of a quadriwave lateral shearing interferometer (QWLSI) is presented. The theoretical analysis and simulation of the influence of the information in the small signal area on the calculation of the M2 factor is provided. The experimental results demonstrate that the new method can be an accurate means to measure the M2 factor. The proposed method can avoid the influence of phase inaccuracy in the small signal area of the interferogram, during the measurement of the M2 factor.

Experimental research of improved subaperture stitching method able to eliminate high-order defocus error

Subaperture stitching is often adopted when measuring large size optic with interferometry method. To reduce the error of defocus in the process of stitching, an improved subaperture stitching method which can eliminate the high-order defocus error is presented based on the experimental study of defocused wavefront. Through detecting the wavefront difference with variable defocus, the actual defocus wavefront is proposed to replace the traditional paraboloid defocus basement so as to realize eliminating high-order defocus. A reflector sample is tested, the roughness parameter Ra is 1.252nm and 0.403nm respectively using the paraboloid and the actual decocus wavefront as the basement. The amendatory defocus basis is taken into the subaperture stitching process, for the two subaperture wavefront, different area of the actual defocus wavefront is used. A SiC super-smooth surface is measured with the improved subaperture stitching method, the Ra is 0.236nm after eliminating high-order defocus. Results show that it is a useful way to erase high-order defocus error without changing the low frequency information of test wavefront.

Collimation testing of the wavefront of interferometer by scanning cube corner retro-reflector method

The wavefront collimation of the optical interferometric systems must be controlled strictly. A scanning cube corner retro-reflector is introduced to divide the wavefront of the interferometer into a series of sub-wavefronts. The tilt angles of all the sub-wavefronts are obtained to reconstruct the tested wavefront thus the collimation test of the wavefront can be realized. The wavefront in the horizontal direction of a 100mm aperture interferometer is tested and reconstructed. The PV and RMS value of the collimated wavefront are 0.629λ and 0.130λ. The effect of system error on the experimental results is analyzed. The experimental results show that this method is of low cost and high accuracy, and is particularly applicable to the collimation test of a large aperture wavefront.