Vertical External Cavity Surface Emitting Lasers (VECSELs) XII

Membrane external-cavity surface-emitting lasers (MECSELs) have experienced a rapid progress in recent years. Based on the membrane geometry of MECSELs, an intrinsically excellent beam quality is one important benefit of this new vertically emitting laser kind. The latest developments will be discussed and an overview of future perspectives will be given. The most important recent progress, like continuous wave broadband tuning and anti-resonant gain membrane design play a major role.

We present continuous wave bi-frequency operation in an optically pumped membrane external-cavity surface-emitting laser (MECSEL). A laser ablation system utilising a digital micromirror device is used to define areas of intra-cavity loss by removing Bragg layers from the surface of the cavity mirror in a crosshair pattern with an undamaged central area. Our MECSEL simultaneously operates on two Hermite-Gaussian spatial modes, the fundamental and a higher order mode, by aligning the laser cavity to be centred on a masked area. We demonstrate bi-frequency operation with a wavelength separation on the order of 5 nm around 1005 nm.

We present coherent laser arrays in a silicon photonics compatible waveguide geometry in optically pumped semiconductor membrane quantum well lasers (MQWLs) on oxidised silicon and silicon carbide substrates. Real and reciprocal space imaging is used to investigate the emission of the laser arrays and mutual coherence is seen to be maintained while operating on single and multiple longitudinal modes in each cavity. Further, we investigate writing laser cavity arrays through micro-structuring of the MQWL and also through the utilisation of a spatial light modulator (SLM) to define areas of gain in the MQWL by shaping the pump beam.

It took thirty years from the early demonstration of optical pumping of semiconductor lasers in the 1960’s to the first modern Vertical-External-Cavity Surface-Emitting Laser, VECSEL, in the 1990’s. It took another thirty years to today’s flourishing of these lasers, with their multitude of operating characteristics and applications. This talk reviews this history, with the emphasis on our work in a small startup company Micracor in the 1990’s, that brought these lasers from scientific curiosity to the powerful tool of science and technology today.

The key limiting factor for output power scaling of VECSELs is the thermal resistance of the structure owing to the reflector thickness and the heat conductivity of the semiconductor materials. We have successfully fabricated a flip-chip processed VECSEL emitting at 2 µm using a GaSb/AlAs0.08Sb0.92 hybrid Bragg reflector with 10.5 mirror pairs and a 100–nm copper layer. The flip-chip processed VECSEL reaches a record high continuous wave average output power of 3 W. The device thickness is reduced by 2.5 µm (36%) compared to the standard 19.5 layer semiconductor-only Bragg reflector design.

A mode-locked VECSEL is reported using a novel hybrid SESAM consisting of a semiconductor absorber region bonded to a curved dielectric partial reflector. The hybrid SESAM is realized by direct bonding of the saturable absorber to a commercially available ultrafast output coupler, nominally 99.4% reflectivity and GDD of < 20 fs2 with a radius of curvature of 10 cm. In a linear cavity where the curved output coupler is the hybrid SESAM, a pulse-width of 410 femtoseconds is achieved at a repetition rate of 4.2 GHz for a VECSEL operating at a wavelength of 1030 nm.

Progress of commercial single-frequency VECSELs for quantum technology applications is reviewed. Availability of practical laser systems with specific wavelength matching an atomic transition is instrumental for the quantum technology research experiments and is becoming increasingly important for the upscaling of commercial quantum systems. To this end, we present a versatile commercial single-frequency VECSEL platform operating in the ultraviolet, visible and NIR -spectral ranges. The suitability of the laser systems for a wide variety of quantum information processing tasks, including spectroscopy, photoionization, and laser cooling is demonstrated.

Terahertz (THz) quantum-cascade VECSELs are strong candidates for frequency-agile local oscillators for next generation heterodyne instruments for astrophysical observations. In this work, a THz QC-VECSEL with a tuning range of 2.48 THz to 2.95 THz (17% fractional) is demonstrated. Additionally, the effects of the output coupler are studied since the frequency dependent reflectance of the output coupler causes variation in the laser properties with tuning. To suppress Fabry-Perot oscillations, a silicon output coupler with an etch-based anti-reflective coating is demonstrated.

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 report on the development of 2.1 µm GaSb-based VECSEL, specially designed to meet the requirements for quantum-frequency-converter pumping. Different approach for the heat-management of the VECSEL-chips were tested and will be compared in regard to the needed specifications. Long-term stability and noise measurements as well as means for wavelength stabilization will be presented.

We build and model coupled GHz rep rate mode-locked VECSEL cavities sharing a common gain medium. The goal is to understand both experimentally and theoretically, gain competition between pulse trains in each cavity while varying relative rep rates and explore applications.

The open cavity concept of a VECSEL is often used to incorporate additional intracavity elements such as: filters, nonlinear crystals, and SESAMs. This leads to expanded functionality of devices to include: larger spectral coverage, single frequency operation, short pulse generation, etc. Here we discuss the design of coupled cavity VECSELs where multiple laser cavities can be “linked” in succession to allow for even more advanced output characteristics.

We demonstrate a THz quantum-cascade vertical-external-cavity surface-emitting laser (QC-VECSEL) based on a disordered amplifying metasurface. One-dimensional disorder is introduced into the metasurface by pseudo-randomly varying the width of uniformly spaced ridge antennas. A mid-sized QC-VECSEL was characterized as a function of its external cavity length. In general, short cavities exhibited more modes: as many as seven were observed. Typical beam patterns were overall circular, albeit with several hot spots. We hypothesize that extending the disorder to two dimensions and increasing the metasurface size should increase the mode number by at least a factor of 10.

We report on a SESAM modelocked GDD balanced VECSEL embedding the active region in quaternary Al15Ga85AsSb. The GDD is flattened by a multilayer semiconductor dielectric top-coating allowing for stable femtosecond operation with a standard SESAM and high-quality YAG Brewster windows. We avoid tradeoffs that would limit the output power. This GDD balanced VECSEL is the next step towards higher level of integration: pump-DBR implementation, demonstration of 1:1 modelocking, and absorber integration will yield a 2-µm MIXSEL, where gain medium and absorber are grown in one monolithic structure.

We demonstrate a MECSEL with broad emission characteristics at 660nm. MECSELs represent a cost-effective light source with high brightness and near-diffraction limited beam quality. Their potential application as a sweepable light source for optical coherence tomography is being investigated. The absence of a DBR gives MECSELs an enhanced freedom of design, such as the incorporation of two or more different quantum wells in the same active region. The incorporation of two transparent heat spreaders gives MECSEL devices enhanced heat removal characteristics when compared to VECSELs. Our latest results show 18 nm tuning range at 670nm, and a linewidth of 0.17nm.