Generation of CW mid-infrared radiation in the mW power range and tuneable over 400 nm

Miniaturization of mid-infrared (MIR) spectroscopy sources has progressed significantly during the past two decades, but a solution able to provide full integration, high optical power and wide tuneability in the so-called atmospheric window (2.5 - 5 µm) is still missing. In this context, we investigated a broadband frequency-tuneable source relying on difference frequency generation (DFG) in a periodically poled lithium niobate (PPLN) ridge waveguide. By employing tuneable lasers for the pump and signal wavelengths emitting at around 1 µm and 1.55 µm, respectively, we were able to fully cover the ≈ 3 - 3.5 µm spectrum, thus translating the technological maturity of data communication photonic sources to the MIR wavelength band. Moreover, the use of a relatively large cross-section for the here-proposed PPLN ridge waveguide compared to commonly employed thin-film lithium niobate (TFLN) waveguides has allowed us to achieve low propagation and coupling losses together with high damage threshold, thereby allowing us to reach mW-level power in the MIR wavelength band.