Ultra-narrow angle-tunable Fabry–Perot bandpass interference filter for use as tuning element in infrared lasers

• A mid-infrared bandpass filter with extremely narrow bandwidth suitable for tuning an external-cavity QCL has been developed.
• The transmission bandwidth is 0.14% of the central wavelength of approx. 4 μm and the filter tunes with tilt angle.
• The low-index material of choice is yttrium fluoride, one of the most useful materials for high-Q infrared filters.
• The deposition of YF3 in thick layers is challenging, so the recipes for filters in the 3–12 μm range are provided.
• Filters with single-cavity and coupled-cavity Fabry–Perot designs are compared.

We have developed a bandpass infrared interference filter with sufficiently narrow bandwidth to be potentially suitable for tuning a self-stabilizing external-cavity quantum-cascade laser (ECQCL) in single-mode operation and describe the process parameters for fabrication of such filters with central wavelengths in the 3–12 μm range. The filter has a passband width of 6 nm or 0.14% with peak transmission of 55% and a central wavelength of approximately 4.0 μm. It can be tuned through over 4% by tilting with respect to the incident beam and offers orders of magnitude larger angular dispersion than diffraction gratings. We compare filters with single-cavity and coupled-cavity Fabry–Perot designs.