VECSEL-based frequency comb in the MIR

The mid-infrared (MIR) region above 3 microns is of great interest for spectroscopic applications. Because it is difficult to produce mode-locked laser sources that emit natively in this region, difference frequency generation (DFG) is a popular method to produce mid-IR output using more traditional laser oscillators. Previous examples include fiber based DFG sources and OPOs, which are typically limited to repetition rates on the order of tens to hundreds of MHz. VECSELs allow access to higher repetition rates, while the use of highly nonlinear waveguides enables the requisite spectral broadening despite the lower pulse energy. In this work we present a VECSEL-based frequency comb that uses DFG to produce output in the 3-4 micron range. This system is based on a mode-locked VECSEL emitting at a 1030 nm wavelength with a 1.6 GHz repetition rate. A Yb fiber amplification system is used to increase the power to over 10W and compress the pulses to sub-90 fs. Coherent spectral broadening out to 1560 nm is achieved with a nonlinear waveguide. By combining the 1030 nm and 1560 nm beams in a PPLN DFG crystal, 290 mW of mid IR output between 3.0 and 3.5 microns is produced. Since the DFG light is produced by two wavelengths from the same oscillator, the carrier envelope offset frequency is cancelled, producing an offset free comb requiring stabilization of only a single degree of freedom. We characterize this VECSEL based frequency comb and discuss the advantages it provides for spectroscopic applications.