Optical Path Difference Data Research Articles

OPD analysis and prediction in aero-optics based on dictionary learning

When aircraft flying at a high speed, the density and reflective index of atmosphere around it become uneven. Thus images or videos observed from the observation window on the aircraft are usually blur or quivering, which is called the aero-optical effect. To recover the images from poor quality, it is necessary to learn about the wavefront distortion of the light, described as optical path difference (OPD). Among the existing methods, the method of computational fluid dynamics (CFD) simulation followed by ray tracing is very time consuming, and the method of real-time OPD measurement with OPD sensor has a certain lag for OPD with high frequency. In this paper, a reconstructible dimension reduction method based on dictionary learning is employed to map the high-dimensional OPD data into a low-dimensional space, and the OPD data are calculated when rays travel across the supersonic shear layer. All the parameters of training and test datasets remain the same except the convective Mach numbers (Mc number). According to the dimension reduction results of training sets, we find that OPD is obviously periodic and its distribution characteristics have a strong correlation with Mc number. By fitting the OPD data in the low-dimensional space, every point on the fitting curve can be reconstructed to the original high-dimensional space, which works as prediction. Compared with the truthful data, the average similarity coefficient of the prediction for the test datasets is up to 83%, which means that the prediction result is credible.

Measurement of aspheric surfaces using annular subaperture stitching interferometry based on an automatic positioning method: theory and application

A simple and effective automatic positioning method (APM) is proposed for the application of annular subaperture stitching interferometry in the stage of precision polishing. In the testing process, a series of optical path difference (OPD) data of subaperture are obtained since the interferometer is shifted relative to the tested aspheric surface. These OPD data are analyzed by the APM to get the key stitching parameters (e.g., aspheric departure) without a precision motion system. The basic principles of the APM are described. The performance of the method is simulated in some pertinent cases. Finally, we study the applicability of the proposed method to subaperture stitching tests of a hyperbolic mirror. The stitching results agree with the full-aperture test results. It demonstrates the validity and practicability of the proposed algorithm.

Measurement of aspheric surfaces using an improved annular subaperture stitching interferometry (IASSI)

An improved annular subaperture stitching interferometry (IASSI) is proposed for testing aspheric surfaces in the stage of precision polishing. It includes a reasonable stitching model and an automatic positioning operation. In the testing process, a series of optical path difference (OPD) data of annular subapertures is obtained as the interferometer is gradually shifted relative to the tested aspheric surface. Then these OPD data can be analyzed by the automatic positioning operation to get the key stitching parameters, and can be stitched together based on a reasonable mathematical model. To verify its validity, we study the applicability of the method to subaperture stitching tests of two conic aspheric surfaces. The stitching results agreed with the full-aperture test results.

Optical path difference measurement techniques for SLMs

A simple measurement method to obtain 2-D spatial point-by-point optical path difference (OPD) data for spatial light modulators (SLMs) is described. A simple imaging system is used that allows measurements of all SLM parameters on the same laboratory system. Simple digital processing of the output image for a uniform applied input voltage provides OPD data. Simple methods to confirm amplitude-mostly SLM operation are also provided. Results are presented for Kopin and CRL liquid crystal SLMs.

Phase error model for simple Fourier transform lenses

The effects of amplitude nonuniformities and phase errors on the accuracy of the optical Fourier transform are considered for the case of a simple lens. A quadratic phase model is derived, analyzed, and compared to experimental and point-by-point optical path difference data. The goal is to determine the lens specifications needed to produce an optical Fourier transform of given accuracy rather than the design of the lens itself.