Abstract
Actively tunable optical filters based on chalcogenide phase-change materials (PCMs) are an emerging technology with applications across chemical spectroscopy and thermal imaging. The refractive index of an embedded PCM thin film is modulated through an amorphous-to-crystalline phase transition induced through thermal stimulus. Performance metrics include transmittance, passband center wavelength (CWL), and bandwidth; ideally monitored during operation (in situ) or after a set number of tuning cycles to validate real-time operation. Measuring these aforementioned metrics in real-time is challenging. Fourier-transform infrared (IR) spectroscopy provides the gold-standard for performance characterization yet is expensive and inflexible—incorporating the PCM tuning mechanism is not straightforward, hence in situ electro-optical measurements are challenging. In this work, we implement an open-source MATLAB®-controlled real-time performance characterization system consisting of an inexpensive linear variable filter (LVF) and mid-wave IR camera, capable of switching the PCM-based filters while simultaneously recording in situ filter performance metrics and spectral filtering profile. These metrics are calculated through pixel intensity measurements and displayed on a custom-developed graphical user interface in real-time. The CWL is determined through spatial position of intensity maxima along the LVF’s longitudinal axis. Furthermore, plans are detailed for a future experimental system that further reduces cost, is compact, and utilizes a near-IR camera.
Highlights
Optical bandpass filters are critical components utilized in a plethora of systems and applications, from fluorescence microscopy to remote sensing.[1,2,3] These filters are designed to transmit only a certain band of wavelengths and block all others
We present an approach to real-time phase-change materials (PCMs)-based tunable filter characterization
Using the mid-wave infrared (MWIR) camera, the linear variable filter (LVF) was imaged with seven different bandpass filters behind it
Summary
Optical bandpass filters are critical components utilized in a plethora of systems and applications, from fluorescence microscopy to remote sensing.[1,2,3] These filters are designed to transmit only a certain band of wavelengths (passband) and block all others. Conventional optical bandpass filters are passive—offering discrete static passbands, arising from the interference of alternating index dielectric thin films.[4,5] There is a growing number of imaging applications requiring precise spectral filtering across a range of wavelengths (tunability), with motorized filter wheels typically utilized.[6,7] filter wheels are bulky, have limited spectral coverage, and offer slow switching speeds. The dielectric materials in the LVF enabled a wavelength bandwidth of Δλ 1⁄4 2.5 μm and a reference wavelength of λmin 1⁄4 2.5 μm. If the incoming light to the LVF first passes through collimating optics, the following relationship results:[19] f
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