Vertical External Cavity Surface Emitting Lasers (VECSELs) XIII

The development of VECSELs and many of their unique features are governed by the ability to customize the compound semiconductor heterostructures providing the gain. The progress in developing VECSELs is discussed in connection with the pefforts made to develop gain structures based on GaN, GaAs, InP, and GaSb compound semiconductor systems.

Over twenty years ago Coherent introduced the first products based on the then novel optically pumped semiconductor laser technology (OPSL). Since then OPS lasers have found application in a variety of fields and today Coherent offers a full product line based on the technology. In this presentation we’ll cover the history of the development, from laboratory demonstrations to commercial success. We’ll describe some of the technical hurdles that we overcame along the way and discuss advantages and disadvantages of the technology in the marketplace as well as other practical considerations.

Vertical external cavity surface emitting laser (VECSEL) and membrane external surface emitting laser (MECSEL) have demonstrated their superior performance due to the flexibility in cavity design as well as power scalability. The epitaxy of different materials and band gap engineering allows the coverage of a wide spread of emission wavelength. We present in this contribution strategies for different gain regions for devices from the red to the near-infrared spectral range, fabricated by metal-organic vapor-phase epitaxy (MOVPE) and show the optical properties of the different gain chips and the laser performances of produced VECSEL and MECSEL.

Ultra-narrow-linewidth visible lasers are required for a range of next generation quantum technologies, with particularly challenging performance parameters for those based on neutral cold atoms. VECSELs offer a wavelength-engineerable, brightness-scalable, intrinsically-low-noise solution with potential for lower size, weight, power and cost (SWaP-C) than many current systems used at visible wavelengths. Here we provide an overview of our work developing high power and ultra-narrow-linewidth VECSELs for application in strontium optical lattice clocks, which require multiple stable laser systems at blue and red wavelengths. We will include: laser design; implementation of active stabilisation of the pump and resonator; characterisation of intensity, frequency, and phase noise; and determination of linewidth. We will also present an electronically-tunable monolithic architecture for sub-kHz free-running linewidth and discuss competing and emerging technologies.

Vertical-external-cavity surface-emitting lasers (VECSEL) have emerged as an attractive single-frequency laser platform for quantum technology applications utilizing trapped ions, neutral atoms, and cold molecules. VECSELs exhibit a unique combination of features including high-power single-frequency operation, excellent beam, and the ability to cover a broad wavelength range from the ultraviolet to visible and to infrared. Here recent developments of compact and modular VECSELs for the industrial scale-up of quantum applications are presented, together with most recent data for wavelength versatility and linewidth narrowing to sub-Hz.

Quantum information processing based on trapped ion technology is one of the leading platforms, heavily relying on a set of single-frequency lasers in its core operations. Narrow linewidth lasers perform atom photoionization, cooling, state-preparation and read-out. In this work we demonstrate in-house designed and fabricated optically pumped semiconductor laser gain mirror comprised of InGaAs quantum wells and GaAs/AlAs distributed Bragg reflector. We demonstrate in-house designed and fabricated single-frequency laser operating at 493 nm for Ba+ cooling. Inherent power scaling potential, efficient intracavity frequency conversion, coupled with sub-MHz linewidth and wide gain tuneability make VECSELs advantageous semiconductor laser platform for various quantum technology applications.

Recent progress will be reviewed on high-power, single-frequency GaSb-based VECSEL, covering the 2.0 – 2.4 µm wavelength range. Up to 2.8 W of continuous-wave, single-frequency output power was reached at 18°C heatsink temperature and 2.23 µm emission wavelength. We will discuss the tuning capabilities of different setups and means for absolute wavelength stabilization.

SESAMs (Semiconductor Saturable Absorber Mirrors) are crucial in ultrafast laser systems and have been extensively used in the near-infrared region. Extending them to the short-wave infrared (SWIR) regime is essential for many sensing and spectroscopic applications. This investigation examines how wavelength, strain, and barrier material affect SWIR SESAM performance. At 2 μm, SESAMs with InGaSb quantum wells and GaSb barriers demonstrate fast recovery times (<30 ps) due to defect states in confined quantum wells. However, using AlAsSb barriers improves rollover parameters but slows down recovery (> 500 ps). Strain-compensated InGaAsSb quantum well SESAMs consistently show slow recovery times, which can be partially explained by delocalized hole states. In contrast strain-compensated SESAMs with AlAsSb barriers have localized hole states with good quantum well confinement, yet still exhibit slow recovery, suggesting an unidentified mechanism related to the AlAsSb barrier. We will give an overview for controlling SESAM characteristics with different quantum well designs and MBE growth parameters over a wavelength range of 2 to 2.4 µm.

Passively modelocked, optically pumped semiconductor disk lasers, commonly referred to as VECSELs or MIXSELs, offer a unique combination of wavelength versatility, wafer scalability, high beam quality, and substantial average output power. While V-shaped cavities are typically used for SESAM-modelocked VECSELs, MIXSELs utilize a simplified straight cavity, integrating the saturable absorber into the VECSEL chip. Here, we demonstrate a dual-comb modelocked MIXSEL in the short-wave infrared (SWIR) regime, employing InGaSb quantum well gain and saturable absorber layers. The free-running dual-comb MIXSEL generates distinct microwave comb lines based on a few interferograms, eliminating the need for stabilization. Two distinct repetition rates enable sampling without aliasing while maintaining rapid acquisition times. Moreover, the phase of the heterodyne beat interferograms can be tracked, allowing for the application of coherent averaging algorithms. This breakthrough lays the foundation for dual-comb spectroscopy in the 2-µm regime providing direct access to CO2 spectroscopy.

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.

We present the MEXL, a novel type of vertically emitting semiconductor laser for biomedical applications. The core component of this laser system is the gain crystal based on a thin semiconductor membrane, which enables the fabrication of extremely compact laser modules. We discuss the current status of our MEXL developments and demonstrate multi-watt output powers in the visible spectral range.

Using quantum dots (QDs) as active region for a semiconductor laser can be preferable for various laser properties. However, the internal strain and the fabrication of an active region membrane can alter the structure properties and, consequently, the laser emission properties as well. In this contribution, we present the emission characteristics of a MECSEL based on InP QDs and emitting in the red spectral range. We discuss the influence of different membrane fabrication methods on the laser emission and the composition of the gain region.

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 on red emitting MECSELs will be discussed and an overview of future perspectives will be given. Full characterization of a new structure design containing 40 quantum wells will be presented.

This research delves into state-of-the-art enhancements in laser design, focusing on achieving high power and superior beam quality. Innovations being explored include customizing intracavity elements, external beam modifications, and advanced pump beam shaping methods. A significant area of interest is compact laser resonators and the role of a 4F lens system and silicon spatial light modulator. Mainly, the study delves deeply into examining mode selection within a Membrane External-Cavity Surface Emitting Laser (MECSEL). Experimental results have shown encouraging advances in reducing beam divergence and optimizing spatial modes. This research potentially sets a new course for digital lasers, which could revolutionize material processing and light-matter interactions.

Mode-locked gain coupled VECSEL cavities exhibit novel lasing properties with dual cavities supporting independently running mode-locked fs pulse trains. Using an extended Maxwell – Semiconductor Bloch microscopic model, we show that initial simulations support experimentally observed independent mode-locking at separated wavelengths with little evidence for gain competition. Our extended SBE model expands the microscopic carrier populations and polarizations in a nonparaxial grating basis to capture large angle incident beams on the gain chip. Preliminary simulations support uncoupled mode-locking showing pulses simultaneously incident on the gain chip and we identify a novel independent mechanism for the wavelength selectivity,

There is significant interest in developing high-power lasers with excellent beam quality and tunable wavelength in the short-wave infrared (SWIR) to mid-infrared range. Type-II quantum well (QW) VECSELs have been demonstrated in the GaAs material system. However, their true potential lies in suppressing Auger recombination at wavelengths beyond 2.3 µm in the GaSb material system where type-I QWs face increasing challenges. Therefore, our research focuses on investigating type-II QW configurations to extend the emission wavelength of VECSELs. Here, we explore VECSEL operation at 2.3 µm using w-like AlSb/InAs/AlGaSb/InAs/AlSb QWs, which offer longer operation wavelength by adjusting their thickness. We aim to compare these novel type-II QW VECSELs with conventional type-I InGaAsSb QWs. Careful optimization of QW number, pump absorption, and overall design is crucial due to reduced wavefunction overlap in the type-II configuration. Precise control of the growth is also essential to achieve accurate bandgap engineering and smooth interfaces for efficient radiative recombination.

VECSELs have already been successfully used for quantum experiments based on ion and atom systems, where the main desired features are narrow linewidth, low noise, and stability. Here we are exposing the broad tuning capability of the VEXLUM VALO single-frequency laser platform for the resonant excitation of recently developed GaSb-based semiconductor quantum dots, which can provide single-photon emission in the 3rd telecom widow. The VECSEL allows direct excitation of a single quantum dot and elimination of complex charge relaxation paths in the surrounding semiconductor matrix, thus improving the charge stability of the quantum dot.

We study coherent laser arrays operating in epitaxially grown semiconductor membrane quantum wells. The samples are deposited by transfer on substrates of oxidised silicon and we record the real and reciprocal space of the laser emission. The Laser arrays are in a lateral emission geometry and are waveguides lasers where the end mirrors are the end-facets of the cleaved membranes which usually form cavities in the order of 100 microns. We are able to create waveguide laser arrays with modal widths of approximately 5-10 microns separated by 10-20 microns. We use real and reciprocal space imaging to examine the emission characteristics of the lasing cavity. Remarkably, we discover that the mutual coherence is preserved whether the cavity operates on a single longitudinal mode or multiple modes. We will show how their emission and coherence can be controlled using a digital micromirror device to control the position and shape of the pump illumination studying threshold, coherence and frequency.

Optically pumped semiconductor disk lasers – known as vertical-external-cavity surface-emitting lasers (VECSELs) – are promising devices for ultrashort pulse formation. A SESAM-free approach labeled “self-mode-locking” has risen attention for many years, relying solely on a chip-related nonlinear optical property which can establish stable ultrashort pulsed operation.

Nitrogen-vacancy (NV) centers have considerable promise for use as magnetometers, however miniaturization is limited by inefficient collection of the photoluminescence used to read out magnetic fields. Laser threshold magnetometry (LTM) enables efficient signal collection, providing a path towards compact high sensitivity magnetometry. We demonstrate an infrared LTM using a diamond plate doped with NV centers integrated into a VECSEL tuned to the spin dependent absorption line of the NV centers. Magnetic field sensing was achieved by monitoring the VECSEL output power. Furthermore, the contrast and the projected sensitivity limit are shown to improve when operating close to the lasing threshold.