Sub 15 ps Self Mode-Locked Nd:YVO₄ Laser through Intra-Cavity Sum-Frequency Mixing

Abstract

Ultrafast lasers have proven to be a great asset in many fields such as biological imaging [1], high-precision machining [2] and nonlinear spectroscopy [3]. This has resulted in a great interest in the further development of passive mode-locked sources. The most common passive mode-locking techniques today rely on semiconductor saturable absorbers (SESAM) [4] or other artificial saturable absorbers, such as Kerr lensing [5]. While these methods are well established, they still have issues with the durability of the SESAM or the need to reach sufficient intensities for reliable operation of the Kerr lens mechanism.In this work we present a new mechanism for passively mode-locking solid-state lasers using intra-cavity sumfrequency mixing (SFM). The underlying idea is to have two laser cavities with a shared section in which a nonlinear crystal is placed. The nonlinear medium phase-matches the SFM between the two operating wavelengths. By matching the roundtrip time of the two cavities, the same temporal part of the light in the two cavities will always interact. This forces one of the lasers to form a dark pulse and the other a bright pulse. Advantages of this approach is that it works for high repetition rates, can be used for any wavelengths in the transparency window for the nonlinear material and is easy to setup. Moreover, the phase-mismatched frequency doubling in the same crystal might infer cascaded F (2) :F (2) Kerr mechanism which could lead to spectral broadening and solitary mode-locking regime.The setup is shown in Fig. 1, where two Nd:YVO4 lasers resonate in a folded y-cavity, one operating at 1064 nm and the other at 1342 nm. In the shared section of the cavity a periodically poled nonlinear RKTP crystal is placed which is quasi-phase matched (QPM) for SFM between the two lasing wavelengths.