Chirped pulse waveguide amplifier

Cr:ZnS and Cr:ZnSe lasers – truly named by the inventor a “Ti:sapphire of the infrared” - have underpinned advances in ultrafast lasers, in many respects outperforming Ti:sapphire. Recently, even single optical cycle pulses with outstanding pulse-to-pulse stability in amplitude and phase, as well as multi-octave frequency combs covering the whole near- and mid-IR wavelength range between 1 and 18 microns became a reality in bulk format. The current trend in photonics goes towards integration and the first steps have been recently made towards integration of a first Ti:sapphire waveguide amplifier on insulator. Cr:ZnS laser is perfectly compatible with silicon photonics though and as such presents a particular interest as a laser in waveguide format. We introduce the first 34-mm long truly waveguide large cross-section (up to 50 μm) broadband ultrafast Cr:ZnS crystalline amplifier with remarkably high spatial output beam quality and M² factor of 1.13x1.25 (in vertical and horizontal planes, respectively) which is close to fundamental Gaussian mode spatial quality. The depressed-cladding buried waveguide is produced by an ultrafast laser writing procedure in a single pulse modification mode with a nonlinear defect formation mechanism, which allows a high degree of flexibility in fabrication when the geometry, size, and even refractive index can be modified along the waveguide. A designed numerical model that includes both pump and pulse propagation allows calculation and optimization of the waveguide design. In a chirped pulse amplifier (CPA) arrangement with a volume Bragg grating-based stretcher/compressor the amplifier provides a high gain of 5.5 dB/cm and 2.35 W of average output power, while the amplified pulse compression down to 204 fs assuming Sech² temporal shape is demonstrated. Remarkably, we demonstrate that a truly waveguide geometry with a non-centrosymmetric active medium like Cr:ZnS generates multiple harmonics even at moderate pulse energies, making an integrated CEP-stabilization feasible. Such frequency comb sources, combined with the CEP-controlled coherent spectra from visible to the mid-IR in an integrated Si-compatible format will further revolutionize fundamental science and will open new horizons in quantum technologies, environmental sensing, optogenetics and bio-medical applications.