Valiyaparambil Chacko Pretheesh Kumar

Bio: Valiyaparambil Chacko Pretheesh Kumar is an academic researcher from Indian Institute of Technology Madras. The author has contributed to research in topics: Tilt (optics) & Adaptive optics. The author has an hindex of 2, co-authored 2 publications receiving 9 citations.

Increasing the sensitivity for tilt measurement using a cyclic interferometer with multiple reflections

Abstract: Measurement of tilt plays an important role in metrological applications and consequently, several methods have been proposed in the recent past. Classical interferometric methods can measure angles with high accuracy but are easily susceptible to external turbulences. We propose to use a cyclic interferometer to measure tilt in which the sensitivity to tilt measurement is double when compared with that of the classical Michelson interferometer. Since the counter propagating beams travel identical paths, the interferometer is insensitive to external vibrations and turbulence and thus can be used under harsh environmental conditions. The novelty in the technique lies in creating multiple reflections in the tilt mirror to enhance the measurement accuracy by the way of increasing the sensitivity. This paper presents the basics of the interferometer and experimental results to quantify the increase in sensitivity. By increasing the number of reflections, it is shown that sensitivity can be further improved to measure tilt angles below 5 μ rad .

Tolerance analysis of misalignment in an optical system using Shack–Hartmann wavefront sensor: experimental study

Abstract: The wavefront aberrations induced by misalignments due to decentration and tilt of an optical component in an optical measurement system are presented. A Shack–Hartmann wavefront sensor is used to measure various aberrations caused due to the shifting of the axis and tilt of a lens in the path of an optical wavefront. One of the lenses in an optical system is decentered in the transverse direction and is tilted by using a rotational stage. For each step, wavefront data have been taken and data were analyzed up to the fourth order consisting of 14 Zernike terms along with peak-to-valley and root mean square values. Theoretical simulations using ray tracing have been carried out and compared with experimental values. The results are presented along with the discussion on tolerance limits for both decentration and tilt.

Standards for reporting the optical aberrations of eyes

Abstract: In response to a perceived need in the vision community, an OSA taskforce was formed at the 1999 topical meeting on vision science and its applications (VSIA-99) and charged with developing consensus recommendations on definitions, conventions, and standards for reporting of optical aberrations of human eyes. Progress reports were presented at the 1999 OSA annual meeting and at VSIA-2000 by the chairs of three taskforce subcommittees on (1) reference axes, (2) describing functions, and (3) model eyes.

Predictor-corrector framework for the sequential assembly of optical systems based on wavefront sensing.

Abstract: Alignment of optical components is crucial for the assembly of optical systems to ensure their full functionality. In this paper we present a novel predictor-corrector framework for the sequential assembly of serial optical systems. Therein, we use a hybrid optical simulation model that comprises virtual and identified component positions. The hybrid model is constantly adapted throughout the assembly process with the help of nonlinear identification techniques and wavefront measurements. This enables prediction of the future wavefront at the detector plane and therefore allows for taking corrective measures accordingly during the assembly process if a user-defined tolerance on the wavefront error is violated. We present a novel notation for the so-called hybrid model and outline the work flow of the presented predictor-corrector framework. A beam expander is assembled as demonstrator for experimental verification of the framework. The optical setup consists of a laser, two bi-convex spherical lenses each mounted to a five degree-of-freedom stage to misalign and correct components, and a Shack-Hartmann sensor for wavefront measurements.

Nano scale tilt measurement using a polarizing phase shifting cyclic interferometer

Abstract: 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.

Effect of beam quality on tilt measurement using cyclic interferometer

Abstract: 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.

Improving Angular Accuracy of a Scanning Mirror Based on Error Modeling and Correction.

Abstract: Scanning mirrors appear to be key components in optoelectronic systems for line-of-sight (LOS) stabilization. For improving the angular accuracy of a scanning mirror based on the eddy current displacement sensor measurement, an angular error-correction method is proposed and demonstrated. A mathematic angular error model with physical parameters was developed, and the cross-validation method was employed to determine the reasonable order of the Maclaurin series used in the error model, which increased the exactitude and robustness of the correction method. The error parameters were identified by accurately fitting the calibrated angular errors with the error model, which showed excellent error prediction performance. Based on the angular calculation model corrected by the error model, the closed-loop control system was established to obtain accurate deflection angles. Experimental results show that within the deflection angle of ±1.5 deg, the angular accuracy was improved from 0.28 deg to less than 1.1 arcsec, and the standard deviation for six measurements was less than 1.2 arcsec, which indicates that the angle correction method was effective in improving the linearity of the eddy current sensors and reducing the influence of manufacturing and installation errors.