This PDF file contains the front matter associated with SPIE Proceedings Volume 10901, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

The progress in developing practical vertical external cavity surface emitting lasers (VECSELs) continues to open new application opportunities in spectroscopy, sensing, life science, and more recently quantum technology. One of the most important features of VECSEL platform is its wavelength versatility, addressing specific application needs. While there has been a tremendous progress in wavelength extension and power scalability of VECSELs [1], the performance level is rather diverse and to large extent reflects the maturity and efforts put behind developing the gain mirror technology. From this perspective, this tutorial presentation covers the challenges in advancing the VECSEL technology with a particular focus on engineering the semiconductor gain mirrors. Starting from general design guidelines for thermal management, pumping, and band-gap engineering, we discuss the key features underpinning the developments of the major classes of gain mirror material systems (i.e. based on GaN, GaAs, InP, and GaSb). The progress in the performance level of VECSELs is discussed in connection with the challenges and opportunities associated with the gain mirror technology. References 1. M. Guina et al., “Optically pumped VECSELs: review of technology and progress”, Journal of Physics D: Applied Physics Vol. 50, no. 38, 383001 (2017).

Optically-pumped vertical external cavity surface emitting lasers (VECSELs) based on flip-chip gain mirrors emitting at the 1.55-μm wavelength range are reported. The gain mirrors employ wafer-fused InAlGaAs/InP quantum well heterostructures and GaAs/AlAs distributed Bragg reflectors, which were incorporated in a linear and a V-cavity configurations. A maximum output power of 3.65 W was achieved for a heatsink temperature of 11°C and employing a 2.2% output coupler. The laser exhibited circular beam profiles for the full emission power range. The demonstration represents more than 10-fold increase of the output power compared to state-of-the-art flip-chip VECSELs previously demonstrated at the 1.55-μm wavelength range, and opens a new perspective for developing practical VECSEL-based laser system for applications such as LIDAR, spectroscopy, communications and distributed sensing.

The bandwidth of an optically pumped semiconductor laser (OPS) is determined by the bandwidth of the material gain, the bandwidth of the longitudinal confinement factor (LCF), and the bandwidth of the Distributed Bragg Reflector (DBR). For a typical OPS structure at 1064nm, the bandwidth of the DBR is the largest among them. In this work, we demonstrate a tunable OPS structure with broadened material gain and LCF, so that the bandwidth of the OPS is close to the bandwidth of the DBR. The laser outputs more than 2W, tunable in a wavelength range of 1035 – 1100nm.

Subwavelength grating structures have been studied in the last two decades for a wide range of integrated optoelectronic applications that include narrow-band filters, lasers, couplers and broadband high reflectors. In particular, using high-contrast refractive indices, broadband reflectivities exceeding 99% have been demonstrated in such grating structures. In this study, we investigate an active mirror structure by embedding a gain medium. We call these devices gain-embedded meta-mirrors (GEMMs). Our optical wave propagation analysis uses a RCWA software to identify optimum GEMM structures that provide highest active reflectivity. An RCWA design procedure that can be interpreted in terms of elementary grating diffraction theory is presented. The procedure optimizes high reflectivity as well as manufacturing tolerance. A key advantage of DBR-free membrane VECSELs has been their potential for efficient heat removal. GEMM devices equally offer this advantage with the added benefit that the optical modes are only evanescent in the thermal substrate (diamond, SiC or sapphire). We will show that this property will allow us to perform more efficient heat-sinking of the thermal substrate. We will also present our preliminary experimental results on fabricating a GEMM device using e-beam lithography and AlGaAs /GaAs/AlGaAs double heterostructures as the gain medium.

The mid-infrared (MIR) region above 3 microns is of great interest for spectroscopic applications. Because it is difficult to produce mode-locked laser sources that emit natively in this region, difference frequency generation (DFG) is a popular method to produce mid-IR output using more traditional laser oscillators. Previous examples include fiber based DFG sources and OPOs, which are typically limited to repetition rates on the order of tens to hundreds of MHz. VECSELs allow access to higher repetition rates, while the use of highly nonlinear waveguides enables the requisite spectral broadening despite the lower pulse energy. In this work we present a VECSEL-based frequency comb that uses DFG to produce output in the 3-4 micron range. This system is based on a mode-locked VECSEL emitting at a 1030 nm wavelength with a 1.6 GHz repetition rate. A Yb fiber amplification system is used to increase the power to over 10W and compress the pulses to sub-90 fs. Coherent spectral broadening out to 1560 nm is achieved with a nonlinear waveguide. By combining the 1030 nm and 1560 nm beams in a PPLN DFG crystal, 290 mW of mid IR output between 3.0 and 3.5 microns is produced. Since the DFG light is produced by two wavelengths from the same oscillator, the carrier envelope offset frequency is cancelled, producing an offset free comb requiring stabilization of only a single degree of freedom. We characterize this VECSEL based frequency comb and discuss the advantages it provides for spectroscopic applications.

The VECSEL architecture is shown to be an effective approach for building THz quantum-cascade lasers with scalable watt-level output power in a high quality beam pattern. The enabling component is a “metasurface” made up of sub-wavelength arrays of antenna-coupled sub-cavities loaded with quantum-cascade active material. By using a sub-cavity antenna based upon a third-order resonance (rather than a first order resonance), metasurfaces with higher fill factors are demonstrated which are suitable for large output powers. Watt-level pulsed output powers have been demonstrated in a single mode, with tunability achieved by intra-cryostat tuning of the cavity length.

Laser technology is finding applications in areas such as high resolution spectroscopy, radar-lidar, velocimetry, or atomic clock where highly coherent tunable high power light sources are required. Offering such performances in the Near- and Middle-IR range, GaAs- and Sb-based Vertical External Cavity Surface Emitting Laser (VeCSEL) technologies [1] seem to be a well suited path to meet the required specifications of demanding applications. Here, we report on the realization of industry ready packaged low noise single frequency VeCSEL devices emitting in the 0.8-1.1 µm and 2-2.5 µm spectral range with high performances thanks to a combination of power-coherence-wavelength tunability and compactness. A fundamental study of the non-linear multimode laser dynamics was carried out to avoid dynamic phase-amplitude instability. We demonstrate both experimentally and theoretically the existence of a deterministic dynamics of the laser field, with either a regular multimode non-stationary regime, or a route to robust single frequency operation. Integration of flat photonics technology allows the realization of devices emitting new coherent light states (VORTEX or dual frequency lasers) for applications to optical tweezers or THz emission, for instance.

With ultrathin gain media, traditional vertical-external-cavity surface-emitting lasers (VECSELs) in theory allow for ideal power scaling with mode area given the one-dimensional heat flow from the active material and into the underlying heatsinking structure. In experiments, however, the integrated semiconductor distributed Bragg reflector (DBR) and its large thermal resistance hamper that scalability. DBR-free semiconductor disk lasers (SDLs), using gain membranes without DBRs and taking advantage of direct bonding, allow for heat dissipation from both sides of the gain medium. Our previous numerical thermal analysis has shown potential advantages in thermal management for this dual-heatspreader configuration, or membrane external-cavity surface-emitting laser (MECSEL) structure, over traditional VECSELs. In this paper, we present both theoretical and experimental performance comparisons between DBR-free SDLs in both single- and dual-heatspreader configurations. Under similar cavity and pumping conditions, the dual-heatspreader configuration has a comparable slope efficiency but experiences thermal roll-over at twice the incident pump power when compared to the single-heatspreader configuration. After optimization of the output coupling efficiency, a maximum output power of 16 W near 1040 nm is collected with the dual-SiC-heatspreader configuration at a coolant temperature of 10 ͦC. With the availability of wafer-scale SiC heatspreaders, we show the potential for mass production of SDLs employing a dual-heatspreader configuration.

We present a new method to simulate the formation of transverse modes in VECSELs. An expression for the gain as a function of carrier density and temperature is derived from a simulation of the structure reflectivity, while the field propagation in the cavity is computed with the Huygens-Fresnel integral. A rate equation model is employed to calculate the field and gain dynamics over numerous round-trips. The optimal mode size for single mode operation for a given pump shape is calculated and compared to experimental results. The effect of pump geometry, thermal lensing and structure design will be discussed.

We compared single-side pumping (SSP) and double-side pumping (DSP) of a semiconductor membrane external-cavity surface-emitting laser (MECSEL). The MECSEL's active region was based on a 4×3 AlGaAs quantum well (QW) structure. This structure was embedded between two silicon carbide (SiC) wafer pieces that were used as transparent intra-cavity (IC) heat spreaders creating a symmetrical cooling environment. The MECSEL structure targeted emission at 780nm and was operated at 20°C heat sink temperature. Via DSP the differential efficiency was improved from 31.9% to 34.4 %. The laser threshold was reduced from 0.79 W to 0.69 W of absorbed pump power while the maximum output power was increased from 3.13 W to 3.22 W. The DSP configuration enabled these improvements by a reduced thermal resistance of the gain element by 9 %. The MECSEL operated at a fundamental Gaussian TEM₀₀ mode profile and the beam quality was measured to be M² <1.09. We further demonstrate a maximum tuning range from 767 nm to 811 nm. A similar active region with about half the thickness (2×3 AlGaAs QWs) was investigated using the DSP configuration and first results are presented here. 500-μm-thick sapphire IC heat spreaders were used instead of SiC. The output power exceeded 0.5W and the emission was spectrally located around 770 nm.

Non-equilibrium multi-wavelength operation of vertical external-cavity surface-emitting lasers (VECSELs) is investigated numerically using a coupled system of Maxwell semiconductor Bloch equations. The propagation of the electromagnetic field is modeled using Maxwell’s equations, and the semiconductor Bloch equations simulate the optically active quantum wells. Microscopic many-body carrier-carrier and carrier-phonon scattering are treated at the level of second Born-Markov approximation, polarization dephasing with a characteristic rate, and carrier screening with the static Lindhard formula. At first, an initialization scheme is constructed to study multi-wavelength operation in a time-resolved VECSEL. Intracavity dual-wavelength THz stabilization is examined using longitudinal modes and an intracavity etalon. In the latter, anti-correlated noise is observed for THz generation and investigated.

Depending on the operation conditions of mode-locked semiconductor lasers different dynamical regimes exist and are well understood. However, if the amplifying and the absorbing material are embedded in a V-shaped external cavity, new multi-pulse (pulse cluster) solutions emerge. We theoretically model such a device by a system of multi-delay differential equations and apply numeric integration as well as path-continuation methods to understand the underlying bifurcation scenarios. Our investigations indicate that by repositioning the gain-chip, the laser can be operated in different stable regions e.g. fundamental, higher harmonic or multi-pulse mode-locking. Furthermore, our bifurcation analysis shows how the multi-pulse solutions evoke from the fundamental mode-locking branch and that different operation regimes can be favored by introducing an asymmetry to the cavity.

Ultra-short pulse generation with saturable-absorber-free vertical-external-cavity surface-emitting-lasers (VECSELs) has raised significant interest in recent years due to the promises it holds for further peak-power scaling and cost-efficiency as well as for the design of more flexible, compact and simpler cavities. Although demonstrated for various devices, the self-mode-locking phenomenon in VECSELs still lacks a consistent explanation. Here, nonlinear lensing in a VECSEL gain chip as a possible mode-locking mechanism, directly measured via Z-scans at laser-relevant wavelengths, and the role of the microcavity resonance on the strength and dispersion of the Kerr nonlinearity are discussed. Furthermore, the impact on self-mode-locking is considered.

Dual frequency comb generation is a field which has seen considerable interest in recent years, with notable implementations such as dual wavelength operation of a Mode-locked Integrated External-cavity Surface Emit- ting Laser (MIXSEL), CW pumping of orthogonal polarisation states in a microring resonator, and optical phase-locking of discrete frequency combs. Dual frequency operation of CW Vertical External Cavity Surface Emitting Lasers (VECSEL) has been demonstrated in a particularly well controlled way using sub-wavelength metallic masks fabricated onto the surface of the laser gain structure. We present a variation of this technique in which patterned loss masks are machined onto a VECSEL cavity mirror using a Digital Micromirror Device (DMD)-enabled femtosecond-laser ablation system, where the DMD is used as an intensity spatial light mod- ulator. Interaction of the loss mask with the laser mode area results in the VECSEL oscillating preferentially on the spatial modes that observe the least loss within the aperture, and modulation of pump power enables control of the oscillating mode frequency separation. We describe the characteristics of the masks and the properties of the laser operation as progress towards eventual pulsed emission. Our technique has the advan- tages of discrete gain and Semiconductor Saturable Absorber Mirror (SESAM) structures, very fast fabrication times and the ability to fabricate multiple apertures on a single mirror.

The modelocked integrated external-cavity surface-emitting laser (MIXSEL) integration allows modelocking in a simple straight linear cavity. The MIXSEL integrates the saturable absorber of a semiconductor saturable absorber mirror (SESAM) and the gain of a vertical external-cavity surface-emitting laser (VECSEL) in a single chip. With birefringent crystals, the MIXSEL cavity can be extended to provide a low noise, compact and cost-efficient dual frequency comb source for multiheterodyne applications. The crystals enable the generation of two orthogonally polarized frequency combs with tunable line spacing from the same cavity. For many applications, a broadband light source with excellent beam quality is desired. While the latter is resulting from the external cavity, the semiconductor layer design of the MIXSEL chip defines the spectral properties of the optical frequency combs. We present the techniques that were necessary to build the first broadband femtosecond dual-comb MIXSEL. Optimization of growth techniques and thermal properties enable the next generation of MIXSEL chips. With careful management of the roundtrip group delay dispersion, the new MIXSEL is capable of generating pulses as short as 139 fs with an average output power of 30 mW in single comb operation. In dual comb operation sub-400 fs with more than 3 nm of optical bandwidth are achieved. Two intracavity birefringent crystals counteract aliasing effects occurring due the broad optical bandwidth of the chip. This laser was optimized for dual-comb spectroscopy of acetylene but has also been used for LIDAR measurements without adjustments of the laser cavity.

The generation of optically carried radio frequency (RF) is an important issue in the area of microwave photonics (MWP), which has gained tremendous developments and has a bright future [1]. Dual-frequency vertical-external-cavity surface-emitting laser (DF-VECSEL) [2], sustaining two cross-polarized laser in a same optical cavity, is an attractive solution to generate such an optically carried RF signal. With class-A dynamics, DF-VECSEL can exhibit a low intensity noise and be free of relaxation oscillation (RO), due to the fact that the lifetime of the photons inside the external cavity can be longer than the lifetime of the semiconductor gain media carriers. Additionally, DF-VECSEL is also interesting for atomic clocks. Indeed, coherent population trapping (CPT) atomic clock [3] is promising to realize miniaturized atomic clocks, but requires a trade-off between performance and size. Recently, a report gives out a solution using two pulses and a double lambda atomic system to obtain high contrast Ramsey fringes [4]. It is expected that the trade-off can be improved by the combination of DF-VESCEL and the double lambda atomic system configuration of CPT atomic clocks [5]. One key parameter of the DF-VECSEL for such applications is its noise. We report in this paper our efforts to understand the origin of the amplitude and phase noises of the laser and to reduce these noises by optimizing the laser cavity design and the pumping architecture. 1. J. Capmany, José and D. Novak. Nature Photonics 1, 319 (2007). 2. G. Baili, L. Morvan, M. Alouini, D. Dolfi, F. Bretenaker, I. Sagnes, and A. Garnache. Opt Lett. 34, 3421 (2009). 3. J. Vanier, Appl. Phys. B 81, 421 (2005). 4. T. Zanon et al., Phys. Rev. Lett. 94, 193002 (2005). 5. P. Dumont et al. J. Lightwave Technol. 32, 3817 (2014.

Upcoming applications like indoor navigation, 3D object recognition or autonomously driving vehicles have increased the demand for high-resolution 3D LIDAR systems. The time of flight (TOF) technique for distance measurement in combination with a scanning system requires a beam source with low divergence and high peak power in pulse operation. The detailed application and its operating distance define the output peak power. Furthermore, the needed distance and lateral resolution leads to a maximum pulse length and beam divergence of the light source. Parameters like wavelength, pulse shapes and repetition rates can be derived from the interaction between source, receiver and environmental conditions of the LIDAR system. For a scanning 3D-LIDAR system for consumer or autonomous applications with a working distance of 1-200m, a diffraction-limited beam with pulses in the nanosecond regime is needed. The external cavity of VECSELs allows decreasing the divergence and simultaneously increasing the active area and as a result, the maximum output power. By using electrically pumped VECSELs in a modulated operation, one can achieve the demanded pulse conditions. Low duty-cycle operation allows using significant higher peak current pulses and generating much higher optical peak pulses by pushing the thermal rollover point to higher currents. The external cavity as well as the combination between electronic driver and VECSEL design shows a significant influence on pulse shape and limits the overall performance. This paper will show a first prototype of electrically pumped VECSEL for LIDAR applications and different measurements and investigations of electrically pumped VECSELs in short pulse operations by electrical modulation.

Optical frequency combs (OFCs) have been revolutionizing numerous fields in metrology and spectroscopy. So far, self-referenced OFCs have been mainly based on modelocked fiber or solid-state lasers at wavelengths imposed by the respective gain materials. VECSELs have a large flexibility in their emission wavelength offered by bandgap engineering, making them ideally suited for applications in spectroscopy and sensing. In addition, VECSELs can easily operate at GHz repetition rates, thus enabling a high power per comb mode and the ease of access to individual optical lines. Multi-GHz OFCs are also advantageous for low-noise RF generation by optical-to-microwave frequency division, because it enables operation at lower noise levels in the photo-detection of the comb pulse train. In this presentation, we will first review the initial work on the detection, noise characterization and stabilization of the carrier envelope offset (CEO) frequency of VECSELs. Then we discuss future application areas, such as their use in low noise microwave generation. In the traditional approach of locking the OFC to an ultrastable reference laser, the achieved phase noise of the generated microwave directly depends on the properties of the optical lock. This is a challenge for VECSEL combs, which currently exhibit higher noise than state-of-the-art ultrafast fiber or bulk lasers. A new RF-generation scheme appears promising for this task, in which a free-running or RF-locked OFC acts as a transfer oscillator. The method does not require any optical lock of the OFC and circumvents the need for high bandwidth actuators.

An ultrafast semiconductor disk laser (SDL) and a single frequency continuous SDL, respectively named Dragonfly and Infinite are currently under development at M Squared lasers with the aim of offering an alternative to Titanium-sapphire based systems. Such SDLs-based systems have the potential to be easy-to-use, low-cost and maintenance free tools for markets including nonlinear microscopy and quantum technologies. Introducing the SDLs technology into the nonlinear microscopy market requires reproducing the performances of currently employed systems: pulses with 1-W average power, duration below 200fs and a typical repetition rate of 80MHz. Such a low repetition rate is particularly challenging for SDLs which are limited by their short carrier lifetime and preferably operate at GHz repetition rates. With Dragonfly, a repetition rate of 200 MHz has been found as a good compromise to balance the mode-locking instabilities while reducing the repetition rate. By adding external pulse compression and spectral broadening stages, pulses as short as 130fs and an average power of 0.85W have been achieved. Developing low-footprint, single-frequency narrow-linewidth CW SDLs could enable quantum technologies to move from the lab to successful commercialisation. In this context, we have been developing a variant of the Infinite system suitable for Sr atom cooling. About 1W and a sub-MHz linewidth at 461nm are required. As direct emission is not an option, an SDL emitting at 922nm followed by an M Squared Lasers SolTiS ECD-X doubler is currently under development. The SDL oscillator delivered > 1W at 922nm with an RMS frequency noise < 150kHz.

In this contribution, we investigated the design of an AlGaAs/AlGaInP electrically-pumped VECSEL structure emitting at a wavelength of 665 nm. With the finite element method (FEM) of an electro-thermal numerical model, we analyzed the current density distribution in the active region of different laser structures by changing the structure geometry, doping concentration of current spreading layer, and bottom contact size. A complete flip-chip processing is proposed according to the optimized designed structure. The measured results of the electroluminescence (EL) profile indicate that the diameter of the emission area with quasi-Gaussian distribution can be up to 100 μm, which is in good agreement with the numerical simulation.