Novel Cavities in Ultrafast Vertical External Cavity Surface Emitting Lasers for High Power Harmonic Generation

Abstract

Vertical external cavity surface emitting lasers (VECSELs) have seen tremendous growth and development over the the past twenty years since its initial demonstration. A key feature that has driven its development is the direct access to the external laser cavity. When combined with various cavity elements, these continuous wave semiconductor lasers can provide high output powers, tunability, high-quality diffraction-limited beams, and nonlinear frequency conversion to different spectral regions that are difficult, or impossible, to reach with more traditional laser technology. Passively mode locked, or ultrafast, VECSELs are able to combine many of the same features in ultrashort pulses with high output powers and fast repetition frequencies. The vast majority of work that has been published on ultrafast VECSELs has been at its fundamental wavelength. This is in stark contrast to its continuous wave counterpart, which has seen nonlinear frequency conversion utilized to target spectral regions from the UV to mid-IR with excellent results. It is the primary objective of this dissertation to explore the combination of intracavity nonlinear frequency conversion with ultrafast VECSELs to achieve high peak power, ultrashort pulses in the visible and UV spectral regions. This work is concluded with a new wafer and cavity design for an ultrafast VECSEL for telecommunications applications and, finally, initial results are presented for an ultrafast VECSEL generating higher order Hermite-Gaussian modes. This dissertation begins with an introduction to continuous wave VECSELs, including their origin and applications, semiconductor gain chip design, microfabrication process, and general operating principles. This is followed by an in-depth look at passive mode locking of VECSELs, the fundamentals and cavity design criteria that lead to stable mode locking, and the characterization of ultrashort pulses. The methods of resonator cavity design and pulse simulation are detailed, along with an introduction to second harmonic generation with ultrashort pulses and the dominant criteria for pulse broadening effects. These introductory sections are followed by the demonstration of a record-setting ultrafast VECSEL utilizing intracavity second harmonic generation for high peak power, sub-picosecond pulse duration in the green spectral region. This was accomplished by careful optimization of the resonator cavity to satisfy key lasing parameters simultaneously. This work was then expanded upon with an overlapped cavity design that fully encloses the high peak power of the second harmonic within its own high Q resonator cavity for all-intracavity fourth harmonic generation into the UV spectral region. This work is followed by a discussion and analysis of the gain chip design process and parameters for a new multi-quantum well wafer emitting in the 1550~nm spectral region that will be used in passively mode locked VECSELs. Finally, this dissertation concludes with the initial results for an ultrafast VECSEL generating higher order Hermite-Gaussian (HG) modes for conversion into Laguerre-Gaussian (LG) modes, with a brief discussion about future work to further develop this concept and combine it with other VECSEL technology for the next generation of laser systems.