What’s new in our Optical Modeling and Design Software?

Being able to vary the parameters of an optical system is a key part of the analysis of any setup, in order to better understand how the system will behave in the face of anything from manufacturing errors to potential misalignment of the components. Designing a system that shows robustness when confronted with these unavoidable deviations from the idealized intended design can be just as important as, if not even more than, finding an initial design that perfectly fulfills all the specifications.

With this in mind, the fast physical optics modeling and design software VirtualLab Fusion offers its Parameter Run document: a tool that allows the user to flexibly configure the variation of all system parameters and analyze the corresponding results. In today's newsletter, we would like to show how this tool can improve the optical engineer's workflow with two examples. In the first example, we examine the properties of a collimating lens and automatically export the detector results to a specified file path (for additional post-processing, for instance). In the second example, we use the fully customizable programmable mode of the Parameter Run to realize different random distributions for the variations of the parameters of interest, in order to perform a tolerance analysis of a sawtooth grating.

Due to their ability to split a single laser beam into multiple beams in combination with well-defined power ratios, diffractive beam splitters are widely used for applications such as laser material processing and optical metrology. But because of the small feature sizes required for non-paraxial or even high-NA splitting or diffraction angles, the design and optimization of this type of device can be challenging. VirtualLab Fusion provides optical engineers with several tools to assist them in this task.

To illustrate the general workflow, we showcase two examples: In the first example, we employ the Iterative Fourier Transform Algorithm (IFTA) alongside a structure design based on the Thin Element Approximation (TEA) to generate a series of initial designs for a beam splitter, which are then rigorously analyzed and further optimized rigorously with the Fourier Modal Method/Rigorous Coupled Wave Analysis (FMM/RCWA). In order to define a suitable and efficient merit function for that last optimization step, the Programmable Grating Analyzer is applied. The second example covers this part of the process in more detail.

Mirau interferometry is a well-known technique that allows the measurement of surfaces with an accuracy of up to one hundredth of the wavelength used. To fully investigate and design such a system, a non-sequential simulation approach is helpful because it automatically incorporates the interference effects that arise from the internal reflections.

Therefore, this week we not only present such a device, but also elaborate on the measurement principle by investigating the interference effects of differently shaped etalons.

In spectroscopy of gases, in order to obtain a sensitive enough measurement of the absorption, it is often required to have long optical path lengths. Multiple-pass cells, where the gas-filled volume is encased between mirrors, are a way of fulfilling this requirement while at the same time controlling beam divergence on the way and preempting the need for extremely large devices. The Herriott cell is one example of this kind of system, characterized by the use of two spherical mirrors with a single off-axis hole drilled into one of them to allow for the entry and exit of the beam. The curvature of the mirrors redirects the beam and controls its divergence.

In today’s newsletter we want to demonstrate the simulation of one such Herriott cell. We have used the Parameter Coupling to link several system parameters together, in order to ensure the correct configuration of the setup while allowing the user to investigate the effect of varying, for instance, the distance between mirrors.

Conical refraction is a well-known phenomenon caused by optical anisotropy. It occurs when a convergent beam propagates through a biaxial crystal along one of its optic axes: the transmitted field evolves into a cone that is highly dependent on the polarization state of the input beam. Several applications have been developed based on this phenomenon; using it as the basis for polarization metrology is one of the most interesting.

With the fast physical optics modeling and design software VirtualLab Fusion, this effect and its applications can be fully investigated. Take a look at the examples below, where we first demonstrate the basic principles of conical refraction with a circularly polarized input beam, and then analyze the design of a polarimeter with two biaxial crystals in separated arms.

In diffractive optics, when a periodic structure is illuminated by collimated light, it is possible to observe the image of the periodic structure forming at periodic distances behind the object. This is the well-known Talbot effect (with the so-called Talbot distance describing the periodic intervals) which has seen regular application in e.g. lithography.

With the Field Tracing technology of the fast physical optics modeling and design software VirtualLab Fusion, this effect and its applications can be fully investigated. See the examples below, where we demonstrate the basic principles of the Talbot effect with linear and crossed patterns, and have a closer look at a particular lithography application to produce nanostructures.

While providing handy tools to enable the user to obtain fast and accurate results for a desired optical task is the main purpose of any optical simulation software, the value of a versatile post processing capabilities should not be underestimated. The customization of the appearance of the resulting data enables to either fit specific requirements for a publication in a journal or kind of reports, but moreover to emphasize and highlight interesting and crucial aspects of the results.

In the selected Use Cases, different options for the customization of detector results and the appearance of VirtualLab Fusion in general are presented.

"Littrow configuration” is the name given to those optical systems containing a reflective grating in which the orientation of the grating is such that the working order (most often the first diffraction order) travels back along the direction of the incident beam. This can be useful for various different applications, for example, in the context of laser resonators, where the grating can act as one of the mirrors of the resonator, or in monochromators and spectrometers.

In this week’s newsletter, we present two examples related to gratings in Littrow configuration. In the first, we demonstrate how to employ the Parameter Coupling tool in VirtualLab Fusion to ensure that the Littrow condition is fulfilled, by automatically adjusting the orientation of the grating, and the orientation and position of the detectors according to the wavelength and grating period. In the second example, we go over the optimization of a grating intended for use under Littrow configuration, where the goal is to design the grating structure so as to minimize its polarization effects.