High precision Raman lasers
Solid-state Raman lasers have emerged as powerful technology for wavelength shifting of conventional lasers. After recent advances in diamond growth technology, there is interest in diamond Raman lasers (DRLs) owing to their supreme thermal performance. While steady progress has been made in high-power Raman lasers including access to a wider range of wavelengths, single longitudinal mode (SLM) operation of Raman laser is a less-studied topic of investigation. Also, to date little effort has been made in the field of Raman cavity-enhanced pumping. In this thesis, we investigate cavity enhanced pumping of cascaded Raman lasers, both theoretically and experimentally.
A theoretical model was derived to describe the behavior of the pump resonant Raman lasers. The model can describe the output performance of such lasers for arbitrary cascading of the Stokes process. The impact of the configuration parameters of the cavity (e.g., beam radius, mirror reflectivities, crystal length etc.) on laser performance are analyzed. The equations for optimum mirror designs are derived and studied.
We design and build several different pump resonant Raman lasers to verify the model and generate commercially-useful laser designs. A diamond Raman laser pumped at 851 nm achieved up to 364 mW of second Stokes output at 1101.3 nm, with stable SLM output up to 140 mW. The slope efficiency was 33%. Under 1064 nm pumping, we generated 1548 mW of second Stokes lasing at 1485 nm using 17.05 W of pump power. For the further verification of our model, a vanadate-based pump resonant Raman laser was built. This laser reached 417.5 mW of second Stokes lasing with a slope efficiency 10% using 1064 nm pumping. The thermal effects in the vanadate laser confirmed the benefits of using diamond even at these power levels.
Finally, we investigated a different Raman laser design, with the pump not resonated but instead just double passed. Such lasers tend not to run on a single longitudinal mode. By inserting a frequency-doubling crystal and adjusting the doubled power, we could use this to stabilize the infrared output power to achieve SLM operation. We get stable first Stokes operation at 4.3 W with the aid of 1250 mW of frequency doubled conversion. We analyse the stabilizing mechanism of axial mode suppression due to the doubling process.
The presented experimental and theoretical work is a big step towards bringing our understanding of pump-resonant Raman lasers up to the level of other Raman designs. The model derived in this thesis provides a good reference to predict the laser performance, and to aid people to design lasers for optimum output. The methods to achieve SLM operation show the potential of narrow linewidth Raman lasers, and close the capability gap between Raman lasers and other wavelength shifting technologies.
Versatile ultrafast Raman laser sources for biophotonics
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