Optimal modified lateral shearing interferometer with axial range extension by using a dual optical plate.

We present a method to extend the axial range of digital holographic microscopy based on the optimal modified lateral shearing interferometer (MLSI). The proposed system can extend the axial range by using a dual optical plate. The interference pattern with two spatial wavelengths is generated by the plate with different thicknesses. These spatial wavelengths transfer a dual spatial frequency into the Fourier plane by using FFT. Two phases are extracted by a dual spatial frequency and combined to create a synthetic wavelength, which is applied to measure the micrometer-scale object without phase unwrapping. Also, the noise-reducing algorithm is used to reduce phase noise caused by the amplified noise of the synthetic wavelength. The experimental result confirms the feasibility of the optimal MLSI by using a dual optical plate.

Optimal modified lateral shearing interferometer for submicro-defects measurement of transparent objects.

Transparent optical components, which are required for high-precision and high-performance, are made by a plastic injection molding method. This can cause inner defects to appear because of the difference in cooling rate and pressure. However, submicron inner defects, caused by the refractive index error, are extremely difficult to determine. Thus, digital holographic microscopy based on an optimal modified lateral shearing interferometer is proposed. This is to measure submicron defects in transparent objects, such as a defective microlens. In addition, the optimal lateral shearing distance is proposed to efficiently remove duplicate images. Optical experiments using the suggested lateral shearing interferometer are presented, and the experimental results with a test microlens confirm the feasibility of the proposed method.

Digital holographic microscopy based on a modified lateral shearing interferometer for three-dimensional visual inspection of nanoscale defects on transparent objects.

A new type of digital holographic microscopy based on a modified lateral shearing interferometer (LSI) is proposed for the detection of micrometer- or nanometer-scale defects on transparent target objects. The LSI is an attractive interferometric test technique because of its simple configuration, but it suffers from the so-called 'duplicate image' problem, which originates from the interference of two sheared object beams. In order to overcome this problem, a modified LSI system, which employs a new concept of subdivided two-beam interference (STBI), is proposed. In this proposed method, an object beam passing through a target object is controlled and divided into two areas with and without object information, which are called half-object and half-reference beams, respectively. Then, these two half-beams make an interference pattern just like most two-beam interferometers. Successful experiments with a test glass panel for mobile displays confirm the feasibility of the proposed method and suggest the possibility of its practical application to the visual inspection of micrometer- or nanometer-scale defects on transparent objects.