Schematic of the in-band-pumped Tm:KYW CW laser inside the dry-air-purged enclosure. LD – 1720 nm fiber-coupled laser diode; L1, L2 – AR-coated lenses imaging the fiber output into the crystal; Tm:KYW – 3-mm gain crystal with a dichroic coating (DC) that is anti-reflective (AR) for the pump and highly-reflective (HR) for the laser deposited on the facet towards the pump; CM – concave mirror (ROC −100 mm); PM – plane mirror; OC – plane output coupler; BRF – 2-mm-thick quartz birefringent filter used only for wavelength-tuning experiments (dashed box).