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Optical Shop Testing

A new type of wave front shearing interferometer is described in which two images of the wave front under test which are of different sizes are made to interfere. When the centres of the two images coincide, this results effectively in shear along the radial direction. It is shown that the interpretation of the resulting interferogram is comparatively easy. In addition, the instrument is extremely convenient to handle, since all the adjustments can be made with a white light source.

Radial shearing interferometer

Speckle correlation based optical levers (SC-OptLev) possess attractive characteristics suitable for sensing small changes in the angular orientations of surfaces. In this study, we propose and demonstrate a spatial multiplexing technique for improving the dynamic range of SC-OptLev. When the surface is in its initial position, a synthetic speckle intensity pattern, larger than the area of the image sensor is created by transversely shifting the image sensor and recording different sections of a larger speckle pattern. Then, the acquired images are stitched together by a computer program into one relatively large synthetic speckle pattern. Following the calibration stage, the synthetic speckle intensity pattern is used to sense changes in the surface’s angular orientation. The surface is monitored in real-time by recording part of the speckle pattern which lies within the sensor area. Next, the recorded speckle pattern is cross-correlated with the synthetic speckle pattern in the computer. The resulting shift of the correlation peak indicates the angular orientations of the reflective surface under test. This spatial-multiplexing technique enables sensing changes in the angular orientation of the surface beyond the limit imposed by the physical size of the image sensor.

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Spatial Multiplexing Technique for Improving Dynamic Range of Speckle Correlation based Optical Lever.

A torsion balance has been built for the measurement of the Newtonian gravitational constant that takes advantage of the particular properties of a wide thin torsion strip under heavy load. The test mass is in the form of an assembly of four 0.47 kg masses suspended from a Cu-Se torsion strip 1.25 mm wide and 30 micrometers thick and 80 mm long. The source mass is an assembly of four 10 kg masses that can be turned to different positions with respect to the test masses. Preliminary tests of the system show that the relatively large gravitational signal produces a gravitational signal that, in air at atmospheric pressure, should easily be measurable to 0.1%. As we expect from our previous experience with torsion strips, an excellent zero stability is observed. A measurement of G with an uncertainty of 1% will be made soon to test all the systems and it is hoped to reach an uncertainty of 0.1% within a reasonably short time.

A novel torsion balance for the measurement of the Newtonian gravitational constant

Precise measurement of extremely small tilt angles is of immense importance in various scientific and technological applications. Interferometry has always been a tool of great importance in such applications. Most of the conventional interferometric techniques use a Michelson configuration and the problem with this interferometer is that it is extremely sensitive to environmental turbulances and vibrations. In our privious works, we had introduced a cyclic interferometer for the measurement of tilt angles which showed excellent stability against environmental turbulances and vibrations as well as twice the sensitivity. Also, with the introduction of multiple reflections, sensitivity as low as 5 micro radian had been achieved by us. To improve the sensitivity further, we had employed phase shifting techniques. The cyclic configuration being a same path interferometer, we used a polarizing phase shifting technique. For acieving this, we developed a new scheme of polarizing phase shifting techique which is rather simpler compared to those reported in the literature. With this we could precisely measure angles as low as 2 nano radians. However, in these measurements we found that the precise alignment of the quarter wave plate plays an important role in the visibility of the fringes which affects the accuracy of measurement. In this work, we numerically investigate the effect of the misalignment of the quarter wave plate on the visibility of the fringes and consequently on the accurcy of the measurement.

Effect of Visibility of the Fringes on the Tilt Measurement Using a Cyclic Interferometer and Polarization Phase Shifting

High-accuracy tilt or roll angle measurement is required for a variety of engineering and scientific applications. A cyclic interferometer with multiple reflections has been developed to measure small tilt angles. To accomplish this task, a novel and simple method, phase shift by polarization, was developed. The results of these studies show that the multiple reflection cyclic interferometers can be used to measure object tilts on the order of 0.2 nano-radians. We develop the theory for polarization phase step and show that accurate measurements can be made with the cyclic interferometer.

Nanoscale tilt measurement using a cyclic interferometer with polarization phase stepping and multiple reflections.

This paper presents an improved method of high precision tilt measurement using a phase shifting cyclic interferometer. Tilt measurement with a cyclic interferometer is a highly stable and reliable experimental technique and tilts as low as 5 μrad has been reported using the same. Here we employ Polarizing Phase Shifting Interferometry (PPSI) as well as multiple reflections for improving the sensitivity. Using a combination of these two techniques tilts as low as 500 nrad has been measured.

Nano scale tilt measurement using a polarizing phase shifting cyclic interferometer

Accurate measurement of angles is extremely important in various metrological applications. Interferometry has always
been an excellent technique for accurate measurements. Several methods have been proposed for accurate tilt measurement
using interferometric techniques. Almost all of them use the Michelson configuration which is extremely sensitive to
environmental vibrations and turbulences. We know that a cyclic interferometer is extremely stable. Even though it is not
sensitive to displacement changes, it is twice sensitive to tilt compared to that of a Michelson interferometer. We have
enhanced the sensitivity to measure tilt using multiple reflections in a cyclic interferometer. Since the input beam is
collimated, we have studied the effect of aberration of the input beam on the accuracy of tilt measurement. Experimental
results on this study are presented in this paper.

Effect of beam quality on tilt measurement using cyclic interferometer

Abstract. We present a method for increasing the dynamic range of a Shack–Hartmann wavefront sensor with a hexagonal lenslet array. Increasing the dynamic range is equivalent to calculating the spot displacement correctly, when it is bigger than the subaperture radius. Our method does this by sorting and indexing the spots according to centered hexagonal number. This allows tracking of the wandering of each spot separately. Further, we also show that the system can work in a reference-free mode if we are not interested in the tip and tilt terms. The results of the work are presented along with the details of the algorithm and theory.

V. C. Pretheesh Kumar

Papers

Nanoscale tilt measurement using a cyclic interferometer with polarization phase stepping and multiple reflections.

Nano scale tilt measurement using a polarizing phase shifting cyclic interferometer

Effect of beam quality on tilt measurement using cyclic interferometer

Effect of Visibility of the Fringes on the Tilt Measurement Using a Cyclic Interferometer and Polarization Phase Shifting

Shack–Hartmann wavefront sensor with enhanced dynamic range and reference-free operation