Etalons, when used in transmission mode, have a series of transmittance peaks with periodic frequency spacing and are typically used in telecom for wavelength locking. Etalons are characterized by their Free Spectral Range (FSR) – the frequency spacing between peaks – and their Finesse – which essentially governs the shape of the transmission peaks.

Etalon filters typically consist of a single spacer layer with two reflector stacks on either side.

For FSR values below 1000 GHz, the spacer is usually fabricated from a substrate material (rather than grown in thin film deposition). To achieve etalon quality polishing the parallelism of the substrate needs to be better than 1-5 arc-seconds.

Different substrate materials are chosen depending on the application – fused silica is used when the etalon FSR and peak variation with temperature is to be minimized while Si is used when a high FSR temperature coefficient is required for thermal tuning.

Iridian fabricates custom high quality fused silica and Si etalons at competitive prices with FSR values in the range of 12.5 GHz to 500 GHz, Here are some typical fused silica  and single cavity etalon products manufactured by Iridian.

https://www.iridian.ca/product-category/telecom-filters/etalon/