Resumen de High-power, continuous-wave optical parametric oscillators from visible to near-infrared
Goutam Kumar Samanta
This thesis presents the development of a new class of high-power, continuous-wave (cw) optical parametric oscillators (OPOs) with extended tunability from visible to near-infrared (near-IR). While lasers have been in use for nearly 50 years, it is still difficult to develop laser systems that can cover many regions of the optical spectrum, from ultraviolet (UV) and visible to the near and mid-infrared wavelength range, with potential applications in the fields such as spectroscopy, remote sensing, trace gas detection, and many more. Development of cw OPOs in singly-resonant oscillator (SRO) configurations, the focus of this thesis, is challenging due to the high threshold pump power (several watts). In addition, with visible pumping, photorefractive effect and thermal lensing effects become important issues to overcome. Therefore, the realization of practical cw SROs requires optimal cavity design, suitable nonlinear materials, and high-power laser with high spectral and spatial quality. High-power, single-frequency, cw SROs based on 30-mm-long MgO-doped, stoichiometrically grown, periodically-poled LiTaO3 (MgO:sPPLT) have been developed. The oscillators were pumped in the green by a frequency-doubled cw diode-pumped Nd:YVO4 laser at 532 nm. With a single grating period of 7.97 µm, continuous signal and idler coverage over 848-1430 nm is obtained by temperature tuning between 52 oC and 248 oC. In a linear cavity configuration and double-pass pumping, an oscillation threshold of 2.88 W has been obtained, and single-pass idler powers in excess of 1.51 W have been generated over 1104-1430 nm for 6 W of pump power at an extraction efficiency of 25.2% and photon conversion efficiency of 56.7%. For single-frequency performance of the cw SRO across 848-1430 nm, we have used a compact ring cavity configuration along with a frequency selecting element (etalon). Using the same MgO:sPPLT crystal, the SRO oscillation threshold of 2.84 W has been obtained, and single-pass idler powers in excess of 1.59 W have been generated over 1104-1430 nm with a maximum SRO extraction efficiency of 25.2% and pump depletions as much as 67%. The single-frequency idler output has a linewidth of ~7 MHz. Under free-running conditions and in the absence of thermal isolation, the idler power exhibits a peak-to-peak stability of 16% over 5 hours. Although the cw SRO can provide optical radiation across 848-1430 nm, the high output power was only available over 1104-1430 nm, due to the high reflectivity of the cavity mirrors for SRO operation. Using finite output coupling of the resonant wave, we have extended the available practical output power across the entire tuning range. The cw out-coupled SRO (OC-SRO) can deliver total power of up to 3.6 W at 40% extraction efficiency with a linewidth of 3 MHz across 848-1430 nm. The signal power shows a peak-to-peak power stability <10.7% over 40 minutes in a TEM00 spatial mode with M2 <1.52. Without any active stabilization, the resonant signal exhibits a natural long-term frequency stability <75 MHz over 15 minutes and short-term frequency stability <10 MHz over 10 seconds, demonstrating the potential of the system for spectroscopic applications. Using internal second-harmonic-generation of the resonant near-infrared signal radiation of the MgO:sPPLT cw SRO in a 5-mm-long BiB3O6 crystal, we have generated 1.27 W of cw, single-frequency blue power over a tunable range of 425-489 nm with a linewidth of 8.5 MHz and a Gaussian spatial beam profile. The blue source is frequency-stable to better than 280 MHz, limited by the resolution of the wavemeter. We have also developed cw green sources, in a simple single-pass experimental configuration by frequency-doubling a fiber laser using MgO:sPPLT and periodically-poled KTiOPO4 (PPKTP) crystals, generating as much as 9.6 W of green radiation in TEM00 spatial beam profile (M2 <1.33) with a single-pass efficiency of 32.7% in MgO:sPPLT. This green source has also successfully been used to pump cw SROs and will be used to pump Ti:sapphire lasers. Using this frequency-doubled green source, we have demonstrated cw OC-SRO providing a stable single-frequency output power up to 2 W across the tuning range of 855-1408 nm, with peak-to-peak power stability <11.7%, frequency-stability <10 MHz over 10 seconds, in TEM00 (M2 <1.26) spatial beam profile.
