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Volume Holographic Gratings (VHGs) are well-known for their high spectral and angular sensitivity combined with adjustable diffraction efficiencies. Hence these gratings are an appropriate tool in optical systems that require accurate spectral and angular filtering. Due to their characteristic three-dimensional and smooth modulation of the refractive index, the modeling can pose numerical challenges, e.g. determining a fine-enough discretization to respect the continuous nature of the refractive-index profile. This can make a simulation of such structures quite demanding computationally, especially when the volume grating is used as just one component in a full, more complex system.

The fast physical optics modeling and design software VirtualLab Fusion enables a fast and accurate simulation of even complex optical systems containing Volume Holographic Gratings by combining specialized solvers for the individual components, such as the Fourier Modal Method in the case of VHGs. Check out the use cases below for an example of an angular-filtering volume grating used in conjunction with a diffractive beam splitter to improve the initial design, as well as an overall introduction on how to configure a volume grating in VirtualLab Fusion.

For many applications in the field of high-power laser scanning systems it is important to ensure that off-axis focal spots lie on the focal plane, and not, as happens with regular spherical lenses, along a curved surface. F-theta lenses have been developed with this requirement in mind and are designed to focus incoming collimated beams into a focal spot whose lateral displacement ideally depends linearly on the scan angle.

The fast physical optics and design software VirtualLab Fusion provides several tools that allow the optical engineer to examine the performance of a particular f-theta design. These include the Distortion Analyzer, which calculates the deviation between the actual and desired spot positions, and a scanning source that allows for the simultaneous configuration of a set of field-of-view modes with different incident directions, for a more convenient investigation of the system. Furthermore, the powerful Field Tracing engine gives the user the possibility to investigate the behavior of the focal spots (point spread function) with physical optics, which is capable of uncovering additional effects not taken into account with a pure ray tracer.

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.