| Article ID | Journal | Published Year | Pages | File Type |
|---|---|---|---|---|
| 1784249 | Infrared Physics & Technology | 2014 | 4 Pages |
•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.
