MID-IR generation based on difference frequency generation (DFG) and femtosecond fiber laser systems

Difference frequency generation-based laser systems provide great flexibility for applications in infrared microscopy and spectroscopy. In contrast to light generation in an active laser medium, the DFG process is not limited to specific wavelength ranges [1, 2]. Thus, DFG systems can provide access to a large spectral domain starting in the near infrared and extending to THz radiation. Furthermore, DFG systems can operate at low repetition rates of a few kHz as well as at high repetition rates of several hundreds of MHz, or even GHz. Depending on the pump laser system and nonlinear crystals used, DFG systems can provide output power levels of hundreds of mW and narrow spectral bandwidths of a few wavenumbers as well as a large spectral bandwidth of hundreds of wavenumbers [1, 2]. Additionally, the DFG process inherently generates carrier envelope offset frequency free radiation (fceo=0) providing easy access to vibrational spectroscopy with extremely high precision.
Here, we present multiple system designs for DFG-based offset-free MID-IR laser systems in the 3 15 µm range. Our system designs utilize two-color femtosecond fiber laser systems based on Menlo Systems’ highly reliable and low noise figure 9TM laser technology. Starting with either an Yb-based or Er-based fiber laser system, we use frequency shifting in nonlinear fibers to generate a second wavelength output between 1 µm and 2.2 µm, which is overlapped with the fundamental wavelength for DFG generation in different nonlinear crystals. Menlo Systems’ DFG system designs can cover various repetition rates, output power levels, and spectral bandwidths for MID-IR radiation (see Figure 1). We demonstrate MID-IR generation between 3 µm and 15 µm with output power levels of up to 200 mW and spectral bandwidths between 40 cm-1 and 400 cm-1 with repetition rates between 100 MHz and 250 MHz.
The presented DFG systems are versatile tools for infrared microscopy and spectroscopy. Furthermore, Menlo Systems’ offset-free DFG systems allow to increase precision and simplify detection without requiring an FTIR spectrometer when used in a dual-comb configuration [3].