Vertical External Cavity Surface Emitting Lasers (VECSELs) XIV

Ultrafast semiconductor lasers offer a more compact alternative to diode-pumped ion-doped solid-state lasers. Operating in the gigahertz pulse repetition rate regime, these lasers achieve emission wavelength customization through bandgap engineering and can reach repetition rates up to 100 GHz without Q-switching instabilities. Semiconductor modelocked lasers can be divided into edge emitters and top emitters with their own strengths and weaknesses. Optically pumped top emitters, also known as semiconductor disk lasers (SDLs) or vertical external-cavity surface-emitting lasers (VECSELs), combine the advantages of a semiconductor gain material with the benefits of conventional diode-pumped solid-state lasers with low-noise operation and power scaling of thin disk lasers. A higher level or integration can be achieved with the modelocked-integrated external cavity surface emitting laser (MIXSEL). This keynote will review advancements in output power, pulse duration, wavelength range from near-infrared to short-wave infrared (SWIR), and dual-comb modelocking with proof-of-principle spectroscopy demonstration since the first demonstration in 2000.

We present a broadband laser system utilizing a passively mode-locked Vertical-External-Cavity Surface-Emitting-Laser (VECSEL) for high-speed, multi-contrast nonlinear imaging. The system generates a 300 nm wide supercontinuum via a Photonic Crystal Fiber (PCF) and a Multiphoton Intrapulse Interference Phase Scan (MIIPS) scheme for advanced pulse shaping enabling precise spectral window selection and compression of pulses to their transform-limited durations. The 1.5 GHz repetition rate of the system coincides with the typical fluorescence lifetimes of many biological samples and commonly used fluorophores. The spectral range spans 900 to 1200 nm, covering critical wavelengths for deep tissue imaging and offers a high versatility across various biomedical applications The ability to control our pulse’s amplitude, wavelength, and phase allows for a tailored excitation and enhanced signal-to-noise ratio while the intrinsically high repetition rate mitigates some risks of photodamage. The system’s capability to drive a nonlinear response from biological samples is also demonstrated.

Passively modelocked, optically pumped semiconductor disk lasers (VECSELs or MIXSELs) offer wavelength versatility, wafer scalability, high beam quality, and significant average output power. Typically, V-shaped cavities are used for SESAM-modelocked VECSELs, whereas Modelocked Integrated eXternal-cavity Surface Emitting Laser (MIXSELs) employ a straight cavity, integrating the saturable absorber into the VECSEL chip. We demonstrate a dual-comb MIXSEL in the short-wave infrared (SWIR) regime via spatial multiplexing, using InGaSb quantum well gain and saturable absorber layers. This dual-comb MIXSEL generates distinct microwave comb lines without active stabilization, with a tunable repetition rate difference allowing alias-free sampling and rapid acquisition. The phase of heterodyne beat interferograms can be tracked for coherent averaging. This advancement supports dual-comb spectroscopy in the 2-µm regime. Proof-of-principle spectroscopy measurements of CO2 highlight the dual-comb MIXSEL's potential for accurate, rapid acquisition spectroscopy, demonstrating its capability for ambient pressure gas sensing in the SWIR range.

Ultrafast lasers in the short-wave infrared region (SWIR) have attracted significant attention due to sensing and spectroscopic applications. GaSb-based materials are utilized for semiconductor saturable absorber mirrors (SESAMs) and optically pumped semiconductor lasers operating between 2.0-2.8 µm. This study demonstrates how the recovery time of SWIR SESAMs can be controlled by altering the strain in the quantum well (QW) absorber and choosing different barrier materials. At 2 μm, SESAMs with ternary InGaSb or GaAsSb, and quaternary InGaAsSb QWs embedded in GaSb were grown with varying levels of lattice mismatch, changing the strain from compressive (–1.71 %) to tensile (+0.77 %). Both compressive and tensile strain result in short recovery times. Lattice-matched QWs exhibit the maximum of 340 ps. Also, increasing the aluminum content in AlGaAsSb barriers increases recovery time through defect filtering. Both observations were validated at 2.3 μm. Moreover, quaternary QWs impact nonlinear reflectivity due to changes in valence band energy offsets, reducing hole confinement. The study advances the understanding of GaSb-based materials for both gain and absorber applications.

VECSELs have low intensity and frequency noise at both low and high Fourier frequencies due to their unique combination of fast gain, long intracavity photon lifetime, and low spontaneous emission factor. In their typical air-spaced resonator format, however, they are susceptible to environmental noise such that narrow integrated linewidth over reasonable measurement times (~1 s) is usually achieved via active frequency-stabilisation to an external stable reference, thus increasing the bulk and complexity of the laser system. We have recently demonstrated class-A, single frequency, monolithic-cavity VECSELs for the first time via resonators based on total internal reflection in a silica prism. This new laser platform has enabled record stability of the free-running VECSEL, maintaining sub-kHz integrated linewidth for up to 40 s averaging time, without active frequency stabilisation. Here we review recent work with the monolithic VECSELs, including management of frequency drift.

We present an intra-cavity frequency doubled membrane external cavity surface emitting laser (MECSEL) at 556nm with >20GHz mode-hop free tuning and >3 W of single transverse mode output power with 20kHz line width. The laser is operated with closed loop dither optimization to prevent mode-hop events during operation and tuning of the center wavelength. The optically pumped MECSEL chip was analyzed with a Shack-Hartmann-Sensor to characterize pump-induced higher order spherical aberrations. These measured aberrations are compared to finite-element model results for the chip. We show that temperature-dependent amplified spontaneous and resonating emission must be considered in the analysis to achieve sufficient agreement with the experimental results. The dynamic response of the temperature profile in the chip upon modulation of the pump power was derived from the model and compared to the emission phase response of the laser.

Quantum squeezed states of light require laser systems exhibiting superior low intensity noise and quantum phase noise. Within this framework, optical parametric oscillators (OPOs) have demonstrated significant promise due to their ability to produce highly-correlated non-classical output photons. In a recent publication, we have illustrated the effectiveness of vertical external-cavity surface-emitting lasers (VECSELs) as pump sources for OPOs by achieving single-frequency operation with high stability. In this investigation, we present the first OPO device pumped by a frequency-locked VECSEL system (AlGaInP-based) and conduct a thorough evaluation of the noise characteristics of the OPO VECSEL, resulting in narrow integrated (1 s) linewidths (and intensity noise reductions) for the VECSEL fundamental (691 nm), signal (1.25 µm), and idler (1.55 µm) of 54.3 kHz (31.1 dB), 72.6 kHz (33.5 dB), and 25.3 kHz (18.3 dB), respectively. Further noise reduction will be investigated, via locking the OPO cavity, targeting reduced overall system noise compared to OPOs reported in the literature.

Laser threshold magnetometry (LTM) promises a path towards significant reductions in optical power and volume for high sensitivity diamond nitrogen-vacancy (NV) magnetometers. LTM utilizes the difference in laser threshold due to intra-cavity diamond absorption while driving the RF spin resonance. To gain the maximum benefit from this technique, the diamond containing vertical external cavity surface emitting laser (VECSEL) is held as close to the lasing threshold as possible. Since laser cavities near threshold tend to experience higher noise, a combination of power and frequency stabilization have been implemented to reduce intensity noise while operating 0.1% above threshold. We have experimentally shown contrasts above 34% and photon shot noise limits of 26 pT/Hz^1/2 and will demonstrate the possibility of further improvement through optimized cavity design.

Nitrogen-vacancy (NV) centers in diamond can be used as a highly sensitive sensor for magnetic fields. One concept to reach the fT/sqrt(Hz) regime is laser threshold magnetometry (LTM). In this talk, we present a self-sustainable, magnetic field-dependent cw NV laser system. To achieve this, we place the NV-diamond within the external cavity of a red VECSEL. When the NV-diamond is pumped, we observe a decrease of the pump power at laser threshold of the VECSEL while the cavity output increases by up to 10%. We demonstrate a strong magnetic-field dependence of the laser threshold as a function of the VECSEL pump power. Our results demonstrate a practical concept for self-sustainable LTM devices based on NV-gain.

A low-noise, single-mode laser system with a few watts of output power is proposed for the optical excitation of quantum sensors based on NV-diamonds. The laser system consists of an 808 nm pump-source, a MECSEL at 1064 nm and an intracavity crystal for frequency doubling to 532 nm. The development of this pump-laser is designed to improve the performance of NV centre-based magnetometry and will contribute to reach the photon shot noise limit of the diamond sensor.

VECSELs have rapidly gained a position as an attractive laser solution for optical atomic clocks and quantum computers utilizing trapped ions and cold atoms. Half of VECSELs’ popularity is thanks to the efficient intracavity doubling, providing high-power wavelength coverage through all visible colors without the need for amplifiers or doubling cavities. The second half is thanks to the unique surface-emitting semiconductor laser geometry with external cavity, filtering out any amplified spontaneous emission and high-frequency noise. We report a commercial VECSEL-system (VXL) designed for industrial system integration and present recent developments in wavelength coverage (e.g. for Ba 1762 nm and Rb 780 nm), power scaling (e.g. for Rb Rydberg transitions), long-term stability, noise suppression, and linewidth narrowing (sub-Hz for optical clock transitions).

Power scaling of ultra-coherent laser systems, without impacting complexity and bulkiness, is highly desirable for quantum technologies based on cold atoms such as atom interferometers and atomic lattice-enabled computers and simulators, which benefit a range of civilian and scientific endeavors. For current systems at relevant wavelengths limited solutions are available, most of which depend on external amplifiers, complex frequency stabilization setups and beam correction optics to achieve performance requirements. In contrast, VECSELs are well-known for their high beam quality and low-noise properties, but their capacity for compact power scaling - especially important at challenging wavelengths such as 689 nm, which is used for cold Strontium-based quantum devices – makes them attractive as high-power single-frequency sources. Here we demonstrate a power-scaled AlGaInP VECSEL producing 2 W (multimode) direct output power around 689 nm and (at lower pump powers) 750 mW single-frequency at the Sr 1S0-3P1 cooling transition frequency (434.8282 THz) with intracavity filtering.

In this contribution we will report on the realization of single frequency, high-power VECSEL modules with wavelengths of 2064.4 nm and 2128.0 nm for two specific QFC processes. The modules show a coarse tuning range of >45 nm and a mode-hop-free fine-tuning range of > 2.5 GHz. The output power of both modules is > 1.8 W in single-frequency operation. For absolute wavelength stabilization, a locking scheme using an optical frequency comb was implemented with a 10-9 accuracy level RF-reference. In that way, an absolute wavelength stability of 150 kHz could be achieved (2 fm). Intensity noise measurements on the 2062.4 nm VECSEL demonstrated a low frequency relative intensity noise (RIN) floor around -110 dBc/Hz. RIN and frequency noise measurements will be shown.

Optically pumped semiconductor disk lasers (i.e. VECSELs or MIXSELs) combine the wavelength versatility and wafer scalability of semiconductors, with high beam quality and good average output power. Both cw and passively modelocked semiconductor disk lasers have been demonstrated in GaAs- and GaSb-based material systems. We have introduced the first GaSb-based membrane external cavity surface emitting laser (MECSEL) at 2.08 µm and report here also the performance parameters for different MECSEL designs. The first GaSb-based MECSEL, bonded to a SiC heatspreader, generated 1.5 W cw output power with an optical-to-optical efficiency of 5.85 %. A double-pass pump configuration increases efficiency to 8.9 % and cw output power to 2.5 W. An in-well pumped MECSEL generated 1.06 W using single-pass pumping. We further investigate how absorption, efficiency, and threshold power changes based on the locations of the QWs with regards to the standing wave pattern of the lasing or pump electric field.

Membrane-external-cavity surface-emitting lasers (MECSELs) consist of an epitaxial active region directly bonded to at least one transparent heatspreader with external cavity mirrors for feedback. These devices enable significant flexibility in emission wavelength and are amenable to enhanced power scaling via optimized thermal management. In this presentation, we describe novel production techniques based on low-temperature direct bonding with the realization of: i) wafer-scale production of high-performance hybrid MECSELs incorporating a back reflector on one face of a dual-SiC-heatspreader (SiC/epi/SiC) stack and ii) high-yield chip-scale double-bonded diamond devices (diamond/epi/diamond) with the potential for significantly enhanced power scaling in optically-pumped semiconductor disk lasers.

We demonstrate frequency-doubled in-well pumped high-power membrane external-cavity surface-emitting lasers. The gain element is fabricated using a wafer-scale process, where a multi-quantum-well semiconductor gain membrane is placed between two silicon carbide heat spreaders. One of these heat spreaders includes a distributed Bragg reflector (DBR) directly attached and is mounted similarly to traditional solid-state thin-disk lasers. To compare thermal performance, we examined two samples: one with a semiconductor DBR and the other with a dielectric DBR. We achieved multi-watt single-mode operation of the second harmonic (~589 nm) by implementing intracavity spectral filtering with an etalon, a birefringent filter, and active frequency stabilization.

We demonstrate the first microchip semiconductor membrane external-cavity surface-emitting laser (MECSEL) consisting of an InGaAsP gain membrane sandwiched between two HR coated silicon carbide heat-spreaders. Similar to VCSELs, the resonator stabilizes itself by the formation of a thermal lens. The microchip MECSEL has an output power of more than 3.3W at a pump power of 8.1W. Further, the beam quality factor M2 was measured and the device operates in a transverse single mode with up to 1W output power. Additionally, effects of thermal lensing were investigated and will be discussed.

THz quantum-cascade vertical-external-cavity surface-emitting-lasers (VECSELs) are desirable as potential frequency comb sources, however they do not naturally lase in a multimode state due to the lack of spatial hole burning within the amplifying metasurface. RF modulation of the injected current of a quantum-cascade VECSEL is observed to induce multimoding when the injected signal is close to at the cavity round trip frequency. Furthermore, we observe injection locking and multimoding when the RF signal is at an integer multiple or submultiple of the cavity round-trip frequency, i.e. harmonic and sub-harmonic injection locking (respectively).

Mode-Locking VECSEL have shown interesting application potentials and are subject to further refinement and exploration. Recently, their achievable operation regimes have motivated the examination of a number of different properties, such as nonlinear coefficients, dispersion and so forth, to improve their performance. In this direction, machine-learning studies indicate to benefit prospective VECSEL and MEXL applications. This talk summarizes a SWIFTS characterization of an frequency-modulated comb VECSEL and ongoing efforts towards a MEXL-applied machine-learning technique.

Key elements for advanced all-optical photonic systems are microsources of coherent light and light amplifiers that can be readily integrated and offer tunability. In this study, we investigate a photonic device capable of flexibly and tunably pumping and potentially amplifying optical signals. Specifically, we focus on coherent laser arrays operating within epitaxially grown semiconductor membrane quantum wells. These laser arrays are designed in a lateral emission geometry, functioning as waveguide lasers with end mirrors formed by the cleaved membranes. We analyze the emission characteristics of the laser arrays and we find that adjacent lasers in the array are coherent and coherence is a function of pump power. Additionally, we demonstrate how emission and coherence can be controlled using a digital micromirror device. When we adjust the position and shape of the pump illumination, we manage to switch on and off coherence and select the phase of the adjacent cavities.

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 2.4 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 and MECSELs. Here, we explore w-like AlSb/InAs/AlGaSb/InAs/AlSb type-II QWs operating at 2.4 µm, 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 mitigate reduced wavefunction overlap in the type-II configuration. Precise control of the growth is essential to achieve accurate bandgap engineering and smooth interfaces for efficient radiative recombination.