We will review recent advances in Membrane-External-Cavity Surface-Emitting-Laser (MECSEL) technology, including beam quality, tunability and wafer-scale bonding to SiC heat spreaders. Using a hybrid MECSEL with a DBR bonded to one of the heat spreaders and in-well pumping to reduce the quantum defect, we demonstrate high-power operation at 1178 nm. Using a birefringent filter and etalon, and intra-cavity frequency doubling, we achieve single-longitudinal-mode operation at 589 nm with more than 10 W of power and a linewidth below 8 MHz. Saturated absorption spectroscopy in a sodium cell is used to lock the laser to the D2a transition.
Membrane-external-cavity surface-emitting lasers (MECSELs) were originally demonstrated by Prof. Mansoor Sheik-Bahae’s group at the University of New Mexico (UNM) in 2015. MECSELs consist of an epitaxial active region combined with at least one transparent heatspreader, typically SiC or diamond, employing external cavity mirrors for feedback. The standalone gain element allows for significant flexibility in emission wavelength and is amenable to enhanced power scaling via optimized thermal management. In an extremely fruitful collaboration with UNM beginning in 2017, we progressed from serially-produced chip-scale prototypes, to 4” wafer-scale manufacturing of double-bonded (SiC/epi/SiC) devices capable of single-mode frequency-doubled output powers in excess of 10 W. Leveraging this unique architecture, we see a bright future ahead for multi-Watt-output optically-pumped semiconductor laser systems emitting throughout the visible and infrared spectral regions.
We demonstrate an in-well pumped high-power hybrid MECSEL (H-MECSEL) formed by sandwiching the MQW semiconductor gain membrane between two SiC heat spreaders, one of which is mirrored with a DBR. We obtain 28 W CW output power around 1178 nm with a slope efficiency of 38% using multipass pumping at 1070 nm. Employing intracavity spectral filtering and frequency doubling, we demonstrate single-mode operation with 8 W of output power at 589 nm and a linewidth of ~4 MHz. We demonstrate preliminary (low-power) wavelength stabilization to the Na D2a transition. Work is underway for full power stabilization towards an on-sky demonstration.
We outline the development of a high-power-handling deformable mirror device, based on a modified Thorlabs DMH40, employing a low-loss substrate-transferred crystalline coating as the reflective element. In standard products, this system features a metal coated (Ag or Al) 18 mm diameter × 150 μm thick BK10 glass substrate mounted to a 40-segment piezoelectric actuator, enabling Zernike compensation up to 4th order, with a peak-to-valley stroke up to ±17.6 μm. In the modified variant described here, the metal coating is replaced with a high-reflectivity (~99.998%) and low-stress (compressive, ~130 MPa) monocrystalline GaAs/AlGaAs Bragg stack transferred to the thin glass substrate via direct bonding. While maintaining similar physical performance, this custom system exhibits a substantial enhancement in power handling, with laser-induced damage tests (performed by Spica Technologies, Inc.) yielding a continuous-wave damage threshold of 75 MW/cm2 at 1070 nm with a 1/e2 spot diameter of 32.8 μm.
Pushing the boundaries of cavity-enhanced spectroscopy experiments in the mid-IR is strongly tied to the availability of suitable mid-IR frequency combs and mirror coatings with well-characterized properties.
Recently, substrate-transferred crystalline coatings (e.g. epitaxial GaAs/AlGaAs multilayers bonded on silicon substrates) have emerged as a groundbreaking new concept for the fabrication of high-performance thin-film interference coatings in the mid-IR, circumventing limitations of established material systems and physical vapor deposition technology.
In this presentation, I will talk about state of the art, mid-IR frequency combs and present a detailed characterization of substrate-transferred crystalline mirrors centered at a wavelength of 4.55µm
Substrate-transferred crystalline coatings have emerged as a groundbreaking new concept in optical interference coatings. Building upon our initial demonstration of this technology in 2013, we have recently realized significant improvements in the optical performance of these novel single-crystal GaAs/AlGaAs multilayers. In the near-infrared, for center wavelengths spanning 1064 to 1560 nm, we have reduced the excess optical losses (scatter + absorption) to less than 5 ppm, with the direct measurement of sub-ppm optical absorption in these films, enabling the realization of a cavity finesse exceeding 600,000 at the telecom-relevant wavelength range near 1550 nm. In this presentation we outline preliminary measurements of the laser-induced damage threshold (LIDT) of these novel semiconductor-based interference coatings. For pulsed excitation (ns pulse durations at 1064 nm), the narrow bandgap of the constituent mirror materials limits the LIDT to 3-5 J/cm2. Under these conditions, laser damage is driven by two-photon absorption (TPA) in the semiconductor multilayer, primarily the high-refractive-index GaAs films. Note that improved performance may be realized for illumination wavelengths >1740 nm, where TPA is eliminated. For continuous-wave (CW) illumination, the high thermal conductivity (~30 Wm-1K-1) and low intrinsic absorption yield the potential for excellent performance. Here we present preliminary CW damage measurements for a 10-ppm transmission quarter-wave GaAs/AlGaAs Bragg mirror transferred to super-polished fused silica, with only a 1.4 K temperature rise for an intensity of ~1.5 MW/cm2. Further efforts will continue to push the limits of the structure with the aim of determining the maximum CW intensity that such mirrors can tolerate.
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