Towards dual frequency comb VECSELs with DMD laser-ablated spatial mode masks

Dual frequency comb generation is a field which has seen considerable interest in recent years, with notable implementations such as dual wavelength operation of a Mode-locked Integrated External-cavity Surface Emitting Laser (MIXSEL), CW pumping of orthogonal polarisation states in a microring resonator, and optical phase-locking of discrete frequency combs. Dual frequency operation of CW Vertical External Cavity Surface Emitting Lasers (VECSEL) has been demonstrated in a particularly well controlled way using sub-wavelength metallic masks fabricated onto the surface of the laser gain structure. We present a variation of this technique in which patterned loss masks are machined onto a VECSEL cavity mirror using a Digital Micromirror Device (DMD)-enabled femtosecond-laser ablation system, where the DMD is used as an intensity spatial light modulator. Interaction of the loss mask with the laser mode area results in the VECSEL oscillating preferentially on the spatial modes that observe the least loss within the aperture, and modulation of pump power enables control of the oscillating mode frequency separation. We describe the characteristics of the masks and the properties of the laser operation as progress towards eventual pulsed emission. Our technique has the advantages of discrete gain and Semiconductor Saturable Absorber Mirror (SESAM) structures, very fast fabrication times and the ability to fabricate multiple apertures on a single mirror.