Single-frequency mid-infrared source based on unidirectional ring-laser exploiting differential parametric gain from tunable, single-frequency pump source

Unidirectional operation of a solid-state ring laser is achieved through differential parametric gain, using a single pump source for both the solid-state laser and the nonlinear difference-frequency mixing process. The pump source is a singlefrequency tapered diode laser, tunable over a wide range from 780 to 815 nm, delivering up to 1.6 W of power. This diode laser pumps a 10 mm long Nd:YVO₄ crystal, ensuring high absorption across a wide pump wavelength span. Differencefrequency generation between the 1064 nm solid-state laser and the tunable diode laser, in a 20 mm long periodically poled lithium niobate crystal, produces additional signal photons at 1064 nm in the co-propagating direction, enforcing unidirectional ring laser operation. In this regime, the output power in the co-propagating direction approximately doubles and single-frequency operation is confirmed by Fabry–Perot interferometer measurements. The generated idler wave is tunable from 3250 to 3430 nm by adjusting the diode wavelength and temperature of the nonlinear crystal, with up to 5 mW of single-frequency output power at 3290 nm. At a fixed pump wavelength, the laser operates unidirectionally over more than 10°C temperature variation of the nonlinear crystal. This approach paves the way for simple, cost-effective implementations of tunable single-frequency sources in the mid-infrared spectral range.