Automated real-time spectral characterization of phase-change tunable optical filters using a linear variable filter and infrared camera

TL;DR

This paper presents a cost-effective, real-time spectral characterization system for phase-change tunable optical filters using a linear variable filter and infrared camera, enabling in situ performance monitoring.

Contribution

The authors develop an open-source MATLAB system that allows real-time, in situ measurement of tunable filter metrics with a simple, inexpensive setup.

Findings

Successfully measures transmittance, CWL, and bandwidth in real-time

Enables in situ spectral performance monitoring during tuning cycles

Provides a low-cost alternative to FTIR for filter characterization

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 spectroscopy (FTIR) 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 infrared 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-infrared camera.