Mohamed Bichra

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.

Chromatic information coding in optical systems for hyperspectral imaging and chromatic confocal sensing

Dispersion causes the focal lengths of refractive and diffractive optical elements to vary with wavelength. In our contribution we show how it can be used for chromatic encoding and decoding of optical signals. We specifically discuss how these concepts can be applied for the implementation of systems with applications in the growing fields of hyperspectral imaging and chromatic distance coding. Refractive systems as well as hybrid combinations of diffractive and refractive elements are used to create specific chromatic aberrations of the sensors. Our design approach enables the tailoring of the sensor properties to the measurement problem and assists designers in finding optimized solutions for industrial applications. The focus of our research is on parallelized imaging systems that cover extended objects. In comparison to point sensors, such systems promise reduced image acquisition times and an increased overall performance. Concepts for three-dimensional profilometry with chromatic confocal sensor systems as well as spectrally resolved imaging of object scenes are discussed.

Hybrid hyperchromats for chromatic confocal sensor systems

Abstract The combination of diffractive and refractive elements in hybrid optical systems allows for precise control of the longitudinal chromatic aberration. We provide comprehensive design strategies for hybrid hyperchromatic lenses that maximise the longitudinal chromatic aberrations. These lenses are mainly used in chromatic confocal sensor systems for efficient non-contact profilometry as well as for measurements of distances and wall thicknesses of transparent materials. Our design approach enables the tailoring of the sensor properties to the specific measurement problem and assists designers in finding optimised solutions for industrial applications. We, for example, demonstrate a hybrid system that significantly exceeds the longitudinal chromatic aberration of purely diffractive elements.

Parallelized chromatic confocal sensor systems

In this paper we present chromatic confocal distance sensors for the parallelized evaluation at several lateral positions. The multi-point measurements are performed using either one- or two-dimensional detector arrays. The first sensor combines the concepts of confocal matrix sensing and snapshot hyperspectral imaging to image a two-dimensional array of laterally separated points with one single shot. In contrast to chromatic confocal matrix sensors which use an RGB detector our system works independently from the spectral reflectivity of the surface under test and requires no object-specific calibration. Our discussion of this sensor principle is supported by experimental results. The second sensor is a multipoint line sensor aimed at high speed applications with frame rates of several thousand frames per second. To reach this evaluation speed a one-dimensional detector is employed. We use spectral multiplexing to transfer the information from different measurement points through a single fiber and evaluate the spectral distribution with a conventional spectrometer. The working principle of the second sensor type is demonstrated for the example of a three-point sensor.

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.

Freeform characterization based on nanostructured diffraction gratings

The in-line characterization of freeform optical elements during the production cycle is challenging. Recently, we presented a compact sensor setup for the characterization of the wavefront generated by freeform optical elements in transmission. The sensor is based on a common-path interferometer consisting of diffractive components and Fourier filtering being adapted to the subsequent numerical post processing. Additionally, it offers several degrees of freedom for enlarging the measurement range of the wavefront gradients. In this contribution, we propose an advanced sensor setup for the measurement of wavefronts generated by freeform elements in reflection. The main advantage is the uni-axial illumination of the test object and the measuring system without the need for conventional beamsplitters. Due to this uni-axial arrangement, the main challenge is to avoid the effect of stray light and back reflections on the measurement signal-to-noise ratio. This is achieved by implementing a highly absorbing amplitude grating based on nanostructured silicon. We demonstrate the experimentally realized measurement system and compare its performance to a commercial Shack-Hartmann sensor.

Wavefront sensing by numerical evaluation of diffracted wavefields

A novel wavefront sensor principle based on diffraction theory and Fourier analysis with a modified angular spectrum propagator has been developed. We observe the propagation of a wavefront behind a two-dimensional cross grating and derive a universal method to extract the phase gradient directly from a captured intensity image. To this end the intensity distribution is analyzed in the spectral domain, and the processing is simplified by an appropriate decomposition of the propagator kernel. This method works for arbitrary distances behind the grating. Our new formulation is verified through simulations. The wavefront generated by a freeform surface is measured by the new method and compared with measurements from a commercial Shack–Hartmann wavefront sensor.

Tunable multisegment Si x N y /AlN piezo lenses for wavefront correction

A combined layer structure of aluminum nitride (AlN) and silicon nitride (SixNy) enables the fabrication of multisegment piezo-actuated micro lenses with a precise control of the lens surface. It is demonstrated that these micro lenses offer free aspheric deformation of the lens surface and can operate at high repetition rates along with reproducible and precise tunability. The AlN/SixNy micro lenses are highly advantageous to be used as wave front filter or for fast focus correction.

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.