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Principles Of Optics Electromagnetic Theory Of Propagation Interference And Diffraction Of Light

Optical materials capable of advanced functionality in the infrared will enable optical designs that can offer lightweight or small footprint solutions in both planar and bulk optical systems. The University of Central Florida’s Glass Processing and Characterization Laboratory, together with our collaborators, have been evaluating compositional design and processing protocols for both bulk and film strategies employing multicomponent chalcogenide glasses (ChGs). These materials can be processed with broad compositional flexibility that allows tailoring of their transmission window, physical and optical properties, which allows them to be engineered for compatibility with other homogeneous amorphous or crystalline optical components. We review progress in forming ChG-based gradient refractive index (GRIN) materials from diverse processing methodologies, including solution-derived ChG layers, poled ChGs with gradient compositional and surface reactivity behavior, nanocomposite bulk ChGs and glass ceramics, and metalens structures realized through multiphoton lithography. We discussed current design and metrology tools that lend critical information to material design efforts to realize next-generation IR GRIN media for bulk or film applications.

https://www.spiedigitallibrary.org/journals/optical-engineering/volume-59/issue-11/112602/Advances-in-infrared-gradient-refractive-index-GRIN-materials--a/10.1117/1.OE.59.11.112602.pdf

Advances in infrared gradient refractive index (GRIN) materials: a review

The paraxial focal length is still the most important parameter in the design of a lens. As presented at the SPIE Optics + Photonics 2016, the measured focal length is a function of the aperture. The paraxial focal length can be found when the aperture approaches zero. In this work, we investigate the dependency of the Zernike polynomials on the aperture size with respect to 3D space. By this, conventional wavefront measurement systems that apply Zernike polynomial fitting (e.g. Shack-Hartmann-Sensor) can be used to determine the paraxial focal length, too. Since the Zernike polynomials are orthogonal over a unit circle, the aperture used in the measurement has to be normalized. By shrinking the aperture and keeping up with the normalization, the Zernike coefficients change. The relation between these changes and the paraxial focal length are investigated. The dependency of the focal length on the aperture size is derived analytically and evaluated by simulation and measurement of a strong focusing lens. The measurements are performed using experimental ray tracing and a Shack-Hartmann-Sensor. Using experimental ray tracing for the measurements, the aperture can be chosen easily. Regarding the measurements with the Shack-Hartmann- Sensor, the aperture size is fixed. Thus, the Zernike polynomials have to be adapted to use different aperture sizes by the proposed method. By doing this, the paraxial focal length can be determined from the measurements in both cases.

Determination of the paraxial focal length using Zernike polynomials over different apertures

Optical nanoscale positioning measurement with a feature-based method

In this contribution, we demonstrate the first referenceless measurement of a THz wavefront by means of shear-interferometry. The technique makes use of a transmissive Ronchi phase grating to generate the shear. We fabricated the grating by mechanical machining of high-density polyethylene. At the camera plane, the +1 and -1 diffraction orders are coherently superimposed, generating an interferogram. We can adjust the shear by selecting the period of the grating and the focal length of the imaging system. We can also alter the direction of the shear by rotating the grating. A gradient-based iterative algorithm is used to reconstruct the wavefront from a set of shear interferograms. The results presented in this study demonstrate the first step towards wavefield sensing in the terahertz band without using a reference wave.

Terahertz referenceless wavefront sensing by means of computational shear-interferometry.

In the design of a lens the most important parameter is the paraxial focal length. Though, most focal length measurement
methods are not measuring the paraxial focal length, but a focal length influenced by aberrations. We have developed a
method to determine the paraxial focal length of strong focusing lenses using experimental ray tracing in a 2D cross
section measurement. The method developed by us gives a much higher accuracy in measuring the paraxial focal length
than the compared methods according to the German Institute for Standardization and Neal et al.. It shows an accuracy
of up to 0.15% in measurement.

Determination of the paraxial focal length of strong focusing lenses using Zernike polynomials in simulation and measurement

Ray tracing is mostly known from simulation. However, Hausler et al. also proposed an experimental implementation of ray tracing. We have taken the proposal to real life and covered a lot of applications with it.

Experimental Ray Tracing – from simulation to reality

New methods enabling the production of custom-tailored Gradient Index (GRIN) optical components brings the next challenge to the lens manufacturers. Simultaneously, for testing these optics, metrology has to evolve to accommodate new optics. In this paper, we describe how Experimental Ray Tracing (ERT) can be used to test GRIN optics produced using additive manufacturing. To evaluate this technique, we compare the results to those obtained using Phase Shifting Diffraction Interferometry (PSDI). The common way of lens manufacturers to verify their products is the measurement of the surface, e.g. using surface profilers or reflective interferometry. Determination of optical performance solely from surface topography includes the assumption of a completely homogeneous structure inside the lens. Since GRIN lenses introduce material inhomogeneity on purpose, these measurement techniques exceed their limits, as surface measurement techniques cannot see the material structure inside the lens. To overcome this problem, we propose the measurement of GRIN lenses using ERT. This reference free measurement technique measures the device under test in transmission. A narrow laser beam is introduced into the device under test (DUT) at a known position. By measuring the direction of the beam behind the DUT, its optical function at this position can be determined. Evaluating these local measurements to an optical powermap over the full aperture, details of the inside structure of the DUT can be seen. The results of the proposed measurement technique show good agreement with the results from measurements using PSDI. However, differences can be seen between the two techniques. Therefore, the results of both measurement techniques are evaluated and compared and the advantages and disadvantages of both techniques are presented.

Characterization of gradient index optical components using experimental ray tracing

The precision of measurements as well as the need for precise measurements are increasing more and more. Thus, the
importance of a good calibration of a setup is increasing, too. In the world of topography measurement a huge variety of
techniques are available. Some of these techniques are using known shadow patterns reflected by the device under test
(DUT). The reflected patterns are recorded using a camera with imaging optics. From the changes of the patterns, the
topography can be resolved. Other measurement techniques are using a tactile sensing head, which is in contact with the
surface to determine its topography. However, these techniques need a reference surface to calibrate movements. If this
reference surface presents deviations from its expected form, errors are introduced.
 
We have developed a calibration method for reflective surface measurements based on experimental ray tracing (ERT)
without the need of a reference surface. In our measurement setup, a narrow laser beam introduced in the measurement
under a certain angle is reflected by the device under test. After the reflection the position and the direction of the ray in
terms of the coordinate system of the camera is detected. Thus, no errors are introduced by using an additional imaging
optic. To calibrate position and direction of the incident ray in respect to the coordinate system of the camera, the
reflected rays from the measurement are used only. From these rays, the incident ray is determined by detecting the line,
all reflected rays are intersecting with. This leads to two major advantages. First, there is no calibration run needed, since
the measurement data can be used directly for the calibration. Second, for the calibration no well-known reference
surface is needed. However, some regulations have to be considered for a stable process of this calibration method. In
terms of peak-to-valley values of the sag of the surface as well as of the change of the surface slope, the surface has to
show values deviating from zero. If a surface like this is measured, a separate measurement run can be performed using
another surface fulfilling these requirements. Since the DUT is scanned by moving the DUT itself, the position and the
direction of the incident ray is not changed from one measurement to another and can be reused.
We describe the newly introduced calibration method for the incident ray in detail and present the necessary boundary
conditions. The calibration has been tested using simulations and has been implemented in a measurement setup. Within
this measurement setup, the expected performance resulting from the simulations has been examined.

Tobias Binkele

Papers

Determination of the paraxial focal length using Zernike polynomials over different apertures

Determination of the paraxial focal length of strong focusing lenses using Zernike polynomials in simulation and measurement

Experimental Ray Tracing – from simulation to reality

Characterization of gradient index optical components using experimental ray tracing

Calibration of the incident beam in a reflective topography measurement from an unknown surface