Gufran S. Khan

Subaperture stitching for measurement of freeform wavefront

A method based on subaperture stitching for measurement of a freeform wavefront is proposed and applied to wavefronts calculated from the slope data acquired using a scanning Shack Hartmann sensor (SHS). The entire wavefront is divided into a number of subapertures with overlapping zones. Each subaperture is measured using the SHS, which is scanned over the entire wavefront. The slope values and thus the phase values of separately measured subapertures cannot be connected directly due to various misalignment errors during the scanning process. The errors lying in the vertical plane, i.e., piston, tilt, and power, are minimized by fitting them in the overlapping zone. The radial and rotational misalignment errors are minimized during registration in the global frame by using active numerical alignment before the stitching process. A mathematical model for a stitching algorithm is developed. Simulation studies are presented based on the mathematical model. The proposed mathematical model is experimentally verified on freeform surfaces of a cubic phase profile.

Experimental investigation on butane diffusion flames under the influence of magnetic field by using digital speckle pattern interferometry

In this paper, the effect of magnetic fields on the temperature and temperature profile of a diffusion flame obtained from a butane torch burner are investigated experimentally by using digital speckle pattern interferometry (DSPI). Experiments were conducted on a diffusion flame generated by a butane torch burner in the absence of a magnetic field and in the presence of uniform and nonuniform magnetic fields. A single DSPI fringe pattern was used to extract phase by using a Riesz transform and monogenic signal. Temperature inside the flame was determined experimentally both in the absence and in the presence of magnetic fields. Experimental results reveal that the maximum temperature of the flame is increased under the influence of an upward-decreasing magnetic gradient and decreased under an upward-increasing magnetic gradient while a negligible effect on temperature in a uniform magnetic field was observed.

Experimental investigations on characterization of freeform wavefront using Shack–Hartmann sensor

The metrology of freeform wavefront can be performed by the use of a noninterferometric method, such as a Shack–Hartmann sensor (SHS). Detailed experimental investigations employing an SHS as metrology head are presented. The scheme is of nonnull nature where small subapertures are measured using an SHS and stitched to give the full wavefront. For the assessment of complex misalignment errors during the spiral scanning, a library of residual slope errors has been created, which makes the alignment process fast converging for minimizing the scanning errors. A detailed analysis of the effects of slope and positioning error on reproducibility is presented. It is validated by null test where a null diffractive optical element has been used in a Mach–Zehnder configuration for compensating the freeform shape. A freeform optics is measured by both measurement schemes, and the results are in good agreement. Further, the nonnull-based scanning subaperture stitching scheme is also validated by performing measurements on an aspheric surface and compared with the measurements from the interferometric method (Zygo Verifire).

Development of metrology for freeform optics in reflection mode

The increased range of manufacturable freeform surfaces offered by the new fabrication techniques is giving opportunities to incorporate them in the optical systems. However, the success of these fabrication techniques depends on the capabilities of metrology procedures and a feedback mechanism to CNC machines for optimizing the manufacturing process. Therefore, a precise and in-situ metrology technique for freeform optics is in demand. Though all the techniques available for aspheres have been extended for the freeform surfaces by the researchers, but none of the techniques has yet been incorporated into the manufacturing machine for in-situ measurement. The most obvious reason is the complexity involved in the optical setups to be integrated in the manufacturing platforms. The Shack-Hartmann sensor offers the potential to be incorporated into the machine environment due to its vibration insensitivity, compactness and 3D shape measurement capability from slope data. In the present work, a measurement scheme is reported in which a scanning Shack-Hartmann Sensor has been employed and used as a metrology tool for measurement of freeform surface in reflection mode. Simulation studies are conducted for analyzing the stitching accuracy in presence of various misalignment errors. The proposed scheme is experimentally verified on a freeform surface of cubic phase profile.

Measurement of elastic and thermal properties of composite materials using digital speckle pattern interferometry

In the present work, application of digital speckle pattern interferometry (DSPI) was applied for the measurement of mechanical/elastic and thermal properties of fibre reinforced plastics (FRP). Digital speckle pattern interferometric technique was used to characterize the material constants (Poisson’s ratio and Young’s modulus) of the composite material. Poisson ratio based on plate bending and Young’s modulus based on plate vibration of material are measured by using DSPI. In addition to this, the coefficient of thermal expansion of composite material is also measured. To study the thermal strain analysis, a single DSPI fringe pattern is used to extract the phase information by using Riesz transform and the monogenic signal. The phase extraction from a single DSPI fringe pattern by using Riesz transform does not require a phase-shifting system or spatial carrier. The elastic and thermal parameters obtained from DSPI are in close agreement with the theoretical predictions available in literature.

Study the effect of magnetic field on gaseous flames using digital speckle pattern interferometry

An experimental investigation on the behavior of gaseous flames in the presence of magnetic field by using digital speckle pattern interferometry is presented. Premixed, partially premixed and diffusion flames generated by butane torch burner were exposed to the magnetic field of 0.35 Tesla. Phase has been extracted from a single DSPI fringe pattern by the application of Riesz transform and the monogenic signal and from which refractive index and the temperature were calculated. Experimental results show that the temperature and the width of the flames are increased under the influence of magnetic gradient.

Ultra-precision Diamond Turning Process

Higher quality and reliability of products is the main demand of manufacturing industries. Ultra-precision machining is the efficient technique to produce highly precise surface with complex shapes and micro-features. Surface quality in ultra-precision machining is only assured by strictly following the optimized process conditions. This chapter covers the various stages of the ultra-precision diamond turning process especially considering the practical aspects. This chapter provides the understanding of, importance and effects of each stage of ultra-precision machining including metrology.

Phase contrast imaging of red blood cells using digital holographic interferometric microscope

In this paper, digital holographic interferometric microscope (DHIM) in conjunction with Fresnel reconstruction method is demonstrated for phase contrast imaging of red blood cells (RBCs). The advantage of using the DHIM is that the distortions due to aberrations in the optical system are avoided by the interferometric comparison of reconstructed phase with and without the object.

Design and evaluation of laser spot illuminator for a laser projection engine design

An easily implementable illumination optical element enabling collection and redirection of multiple precollimated laser beams to be used as a laser spot illuminator (LSI) is proposed. Unlike the lenses, this element is free from any aberration and focusing/defocusing effects which may arise due to curved surfaces. The nonfocusing nature of beams reduces risk of the thermal instability at the target spot if compared with focused beam, especially for high brightness applications where multiple laser beams are used. Further, the illuminating spot is more uniform and pupil filling is higher which can help in the reduction of speckle contrast for imaging applications. In this research, we present two designs of LSI named biprism and sleek design, suitable for conventional grinding-polishing and molding methods of fabrication, respectively. It is shown that the proposed spot illuminator is suitable for conversion of lamp/LED-based projection engine design into a laser projector engine without any modification in its relay and imaging optics. Display performance parameters such as efficiency and uniformity across the modulator plane are reported and found acceptable for the projection engine designs using light tunnels as well as lenslet arrays for homogenization. Further, a prototype of a biprism laser illuminator for 2×4 laser arrays has been fabricated and evaluated to demonstrate its manufacturability.

Investigations on sub-aperture stitching approach for testing freeform optics

In this paper, an experimental investigation for the measurement of freeform surfaces using a sub-aperture scanning and stitching approach is presented. The Shack Hartmann Sensor (SHS) and a Modified Talbot Wavefront Sensor are used. The experimental results are compared with those obtained by simulations.