This PDF file contains the front matter associated with SPIE Proceedings Volume 10515, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

The influence of microscopic non-equilibrium dynamics on vertical external-cavity surface-emitting lasers (VECSELs) is investigated through a systematic numerical study of single- and dual-wavelength operation. In single-wavelength operation the microscopic dynamics can be adiabatically eliminated, however in dual-wavelength operation the microscopic dynamics varies with the spectral location of the modes. The optically active quantum wells (QWs) are modeled microscopically using the Semiconductor Bloch equations while the CW laser field is simulated using Maxwell’s equations. Higher order correlation terms, such as carrier scattering and polarization dephasing, are treated on the level of second Born-Markov or as effective rates. Results are presented on the modeling, stability, and non-equilibrium effects in dual-wavelength operation.

We investigate the influence of the external cavity geometry on a vertical-external-cavity surface-emitting laser (VECSEL). In particular, we consider a V-shaped device geometry with the gain chip in between the out-coupling facet and saturable absorber mirror, such that mode-locked pulses travel through the gain chip twice per roundtrip. We analyze the dynamic changes in the laser output in dependence of the gain chip position within the external cavity using a numeric modeling approach based on the optical Bloch equations. We show that the cavity geometry can be engineered to favor certain dynamic regimes of laser operation, e.g., tuning the pulse repetition rate by higher harmonic mode-locking. In between the stable operation conditions, regions of complex dynamics and chaos are found.

We consider a VCSEL micropillar laser with an internal absorber section, which has been shown to show excitable behaviour before its threshold. The addition of optical feedback offers the possibility to generate a pulse train of low jitter with a repetition time that is controlled by the length of the feedback loop. We perform a bifurcation analysis of the governing Yamada equations for the intensity, gain and absorption and with delayed feedback. This reveals an increasing variety of new dynamics and a considerable degree of multistability when the delay time is increased. More concretely, we present a bifurcation/stability diagram in the plane of feedback delay and feedback strength --- the two parameters of the external feedback loop. The organising feature is a winding curve of Hopf bifurcations, which develops an increasing amount of self-intersections as the delay of the feedback is increased. Along certain parts of the Hopf curve stable periodic solutions are created, which are physically self-pulsations with an increasing number of pulses in the external cavity. Several of these self-pulsations may coexist stably, and their basins of attractions are intermingled in a complicated way. As a result, the micropillar laser is very sensitive to small perturbations and noise while in such a multistable configuration. We also find multifrequency dynamics, where the amplitude of the self-pulsations is strongly modulated.

Recent experiments with an excitable VCSEL micropillar laser with delayed optical feedback demonstrated that the system is able to sustain trains of optical pulses. The laser has two layers of gain and one layer of absorption in the VCSEL cavity, and it is an excitable single longitudinal and transverse mode laser. With optical feedback, a past pulse can trigger a new pulse, creating a pulse train with repetition rate given by the delay time. It is possible to trigger and retime pulses by appropriate external perturbations, in the form of appropriately timed short optical pulses. In particular, several pulse trains can be triggered independently by optical perturbations, and sustained simultaneously in the external cavity, with different timing in between pulses. Such dynamics are also called localised structures, and are investigated here theoretically. It has been verified experimentally and theoretically that the phase of the electric field is not relevant. The Yamada model – a well-established system of ordinary differential equations for intensity, gain and absorption – is thus a suitable model. As we show, the Yamada model with delayed intensity feedback describes the pulsing micropillar laser system in good agreement with the experiment. A bifurcation analysis of this model shows that several pulsing periodic solution with different repetition rates coexist and are stable. Although coexisting pulse trains can seem independent on the timescale of the experiment, we show that they correspond here to extremely long transient dynamics toward one of the stable periodic solutions, with equidistant pulses.

DBR-free membrane VECSELs, also called MECSELs are a recent development in the field of VECSELs. They offer an alternative set of design parameter controls compared to traditional VECSELs. Here we will report on recent developments of 1 micron membrane VECSELs for CW and mode-locked operation, including achieving >10 W CW output power using a membrane VECSEL mounted on a silicon carbide intra-cavity heat spreader.

Vertical-external-cavity surface-emitting lasers (VECSELs), also called semiconductor disk lasers (SDLs), have developed strongly during the last two decades. Additionally, the range of available wavelengths has been drastically extended during this time, especially when second harmonic generation is taken into account. Nevertheless, these systems run into limits when the refractive indices of the materials used for the necessary distributed Bragg reflectors (DBRs) approach too much. This leads to a much higher number of necessary layer pairs, which increases the structure thickness and makes growth of such DBRs at least extremely difficult. Another limit occurs when the band gap of the gain material used in the VECSEL approaches too close to the band-gap in the used DBR materials. Absorption losses in the DBR are the consequence. Additionally, the performance of VECSELs in general suffers from heat incorporation into the active region caused by the excess energy of the pump photons together with the low thermal conductivity of the substrate and the included DBR. The recently shown membrane external-cavity surface-emitting laser (MECSEL) concept opens the potential to overcome all the above named challenges as only an isolated active region membrane, sandwiched between intra-cavity heat spreaders is used as gain material. Furthermore, active region membranes in the GaInP/AlGaInP material system aiming on the yellow and red-orange spectral region where direct laser emission has not been realized yet, grown on high-index substrates, open the possibility to deliver sufficient gain realizing a MECSEL.

Optically pumped semiconductor (OPS) vertical external-cavity surface-emitting lasers (VECSELs) are an important category of power scalable lasers with a wide range of applications in bio photonics, medicine technologies and for example spectroscopy. The possibility of band-gap engineering, a laser emission in the fundamental Gaussian mode and the technical simplicity leads to ongoing growth of the area of applications for these tunable laser sources. The open laser resonator allows inserting frequency selective and converting intra-cavity elements as well as absorptive elements to create mode locking. In addition, intra cavity gas cells allow absorption spectroscopy. Aiming on spectroscopic applications for rubidium one important absorption line is located at 780 nm. Nowadays, laser emission in this spectral range has not been shown by VECSELs, neither in direct nor in frequency doubled emission although the available III-V semiconductor materials would provide such a band-gap. A very low charge carrier confinement may be the main challenge here. We present several strategies to create gain structures based on the AlGaAs- and the AlGaAs/AlGaInP material system. The expected high thermal sensitivity can be counteracted by realizing this VECSEL structure also as a membrane external-cavity surface-emitting laser (MECSEL) to improve the heat transfer out of the active region. Investigations comparing barrier pumping with in-well pumping are also possible. A MECSEL would be in both cases beneficial here as not absorbed pump light is just transmitted instead of being absorbed in the DBR creating unnecessary heat.

Semiconductor disk lasers, also called vertical external-cavity surface-emitting lasers (VECSELs) have advantageous properties such as high output power, wavelength flexibility due to bandgap engineering and near-diffraction limited beam quality. The possibility to insert intra-cavity elements – filters, frequency doubling crystals or semiconductor saturable absorber mirrors (SESAMs) – enables wavelength tuning, second harmonic generation or mode locking with ultra-short pulses. A major challenge for these laser sources is the removal of heat which is introduced by optical pumping. The thermal management can be improved by placing only the active region directly between two heat spreaders. This membrane external-cavity surface-emitting laser (MECSEL) allows emission in an even larger wavelength range, since the growth is not restricted by a distributed Bragg reflector. We present the fabrication, processing and characterization of VECSELs and MECSELs using different material systems for laser emission at various wavelengths in the visible and in the infrared spectral range. Our semiconductor structures are grown by metal-organic vapor-phase epitaxy and contain quantum wells or quantum dots in the active regions. We discuss our latest results including the membrane laser concept with investigations of strain effects on the photoluminescence and the laser emission, different pumping schemes and ultra-short pulse generation.

We report on the simulation of cavity-dumped semiconductor disk lasers utilizing an intracavity Pockels cell. This technique is used to generate high peak power pulses with pulse lengths of nominally one cavity round-trip. These results are compared to experiments demonstrated using InGaAs quantum-well gain region operating at approximately 1 μm to generate micro-Joule level nanosecond pulses.

Processing of information with optical spikes could present an alternative path with a reduced energy consumption. It could also be well suited in the framework of novel brain-inspired computation paradigms. We investigate the spiking and pulse train dynamics in a micropillar laser with integrated saturable absorber. The optically-pumped microcavity laser is based on a specifically optimized design. The solitary laser can emit sub-nanosecond Q-switched pulses above laser threshold. Below threshold, the laser is in the so-called excitable regime, a generic all-or-none kind of response also found in biological neurons. We demonstrate several neuromimetic properties of the micropillar laser including the relative and absolute refractory periods and the temporal summation. The latter gives rise to sensitive and fast coincidence detectors of optical signals. In the configuration with delayed optical feedback, the system is shown experimentally and theoretically to sustain controllable trains of dissipative temporal solitons controlled by adequate optical perturbations. We show that the pulse train can be started or resynchronized (retiming) with a single perturbation and that the system can store a large variety of temporal pulse patterns. We discuss the role of pump noise that may terminate a pulse train. We demonstrate a strong asymmetry in the effect of noise on the switch on and off processes, as well as a peculiar role played by noise timing. Besides its interest as a compact source of controllable pulses, this system can be arranged if needed in arrays leading to interesting prospects for artificial optical neural networks.

The most technologically mature optically pumped semiconductor lasers (OPSL) are based on InGaAs quantum wells (QW) for emission in the 900-1200 nm range. The low wavelength boundary is set by both the bandgap of InGaAs and the most common pump wavelength of 808 nm. To extend the wavelength coverage into 700 – 900 nm, a different QW system and a different pump wavelength are needed. In this work, we present the progress and result in the development of AlGaAs-based OPSL.

Here we present the gain and SESAM structure design strategy employed for the demonstration of ultrashort pulses and we present a comprehensive study outlining the influence of the cavity geometry on the pulse duration and peak power achievable with a state of the art VECSEL and SESAM structure. We will discuss the physical mechanisms limiting the output power with near 100fs pulses and we will compare experimental results obtained with different cavity geometries, including a V-shaped cavity, a multi-fold cavity, and a ring cavity in a colliding pulse modelocking scheme. The experimental results are supported by numerical simulations.

We present the self-referenced stabilization of the carrier-envelope offset (CEO) frequency of a semiconductor disk laser. The laser is a SESAM-modelocked VECSEL emitting at a wavelength of 1034 nm with a repetition frequency of 1.8 GHz. The 270-fs pulses are amplified to 3 W and compressed to 120 fs for the generation of a coherent octavespanning supercontinuum spectrum. A quasi-common-path f-to-2f interferometer enables the detection of the CEO beat with a signal-to-noise ratio of ~30 dB sufficient for its frequency stabilization. The CEO frequency is phase-locked to an external reference with a feedback signal applied to the pump current.

We present preliminary results showing the potential of VECSEL technology for the generation of high power coherent supercontinuum. Among these results, we demonstrate a stable output power of 16 W with a pulse duration of 71 fs and a repetition rate of 1.7 GHz from a VECSEL oscillator and Ytterbium fiber amplifier. This system was used to generate a coherent supercontinuum averaging 3 W of power using a highly nonlinear photonic crystal fiber. In addition, we discuss the possible methods for the detection and stabilization of the carrier offset frequency. The beatnote between a VECSEL seeded supercontinuum and an external CW laser reveals a relatively stable signal, well above the detection noise. A discussion about system design considerations for noise reduction and increased offset frequency stability is also included.

The THz time domain spectrometer (THz-TDS) has revolutionized the adoption of THz science in fields such as medicine, material characterization, pharmaceutical research and biology among others. Traditionally a THz-TDS was based on a titanium sapphire laser, while most of the commercially sold spectrometers today adopt fiber lasers. Vertical External Cavity Surface emitting lasers or VECSELs have potential to be the future laser of choice for the implementation of THz spectrometers, as they are small, low-cost, low noise and high repetition rate. Here I will outline the progress in our laboratory and the general community concerning VECSEL-THz technology and I will account the problems that have to be solved for the VECSEL-THz technology to succeed.

We investigated cw intra-cavity third-harmonic generation (THG), where both the second- and third-harmonic NLO processes are type-I. The concept and results from a prototype with output of 28mW at 307nm are presented here.

This paper will present developments in narrow-linewidth semiconductor-disk-laser systems using novel frequencystabilisation schemes for reduced sensitivity to mechanical vibrations, a critical requirement for mobile applications. Narrow-linewidth single-frequency lasers are required for a range of applications including metrology and highresolution spectroscopy. Stabilisation of the laser was achieved using a monolithic fibre-optic ring resonator with free spectral range of 181 MHz and finesse of 52 to act as passive reference cavity for the laser. Such a cavity can operate over a broad wavelength range and is immune to a wide band of vibrational frequency noise due to its monolithic implementation. The frequency noise of the locked system has been measured and compared to typical Fabry-Perotlocked lasers using vibration equipment to simulate harsh environments, and analysed here. Locked linewidths of < 40 kHz have been achieved. These developments offer a portable, narrow-linewidth laser system for harsh environments that can be flexibly designed for a range of applications.

This paper presents the latest efforts in the development of commercial optically-pumped semiconductor disk lasers (SDLs) at M Squared Lasers. Two types of SDLs are currently being developed: an ultrafast system and a continuous wave single frequency system under the names of Dragonfly and Infinite, respectively. Both offer a compact, low-cost, easy-to-use and maintenance-free tool for a range of growing markets including nonlinear microscopy and quantum technology. To facilitate consumer uptake of the SDL technology, the performance specifications aim to closely match the currently employed systems.

An extended Dragonfly system is being developed targeting the nonlinear microscopy market, which typically requires 1-W average power pulse trains with pulse durations below 200 fs. The pulse repetition frequency (PRF) of the commonly used laser systems, typically Titanium-sapphire lasers, is 80 MHz. This property is particularly challenging for mode-locked SDLs which tend to operate at GHz repetition rates, due to their short upper state carrier lifetime. Dragonfly has found a compromise at 200 MHz to balance mode-locking instabilities with a low PRF. In the ongoing development of Dragonfly, additional pulse compression and nonlinear spectral broadening stages are used to obtain pulse durations as short as 130 fs with an average power of 0.85 W, approaching the required performance.

A variant of the Infinite system was adapted to provide a laser source suitable for the first stage of Sr atom cooling at 461 nm. Such a source requires average powers of approximately 1 W with a sub-MHz linewidth. As direct emission in the blue is not a viable approach at this stage, an SDL emitting at 922 nm followed by an M Squared Lasers SolTiS ECD-X doubler is currently under development. The SDL oscillator delivered >1 W of single frequency (RMS frequency noise <150kHz) light at 922 nm.

Mode-locked VECSELs using SESAMs are a relatively less complex and cost-effective alternative to state-of-the-art ultrafast lasers based on solid-state or fiber lasers. VECSELs have seen considerable progress in device performance in terms of pulse width and peak power in the recent years. However, it appears that the combination of high power and short pulses can cause some irreversible damage to the SESAM. The degradation mechanism, which can lead to a reduction of the VECSEL output power over time, is not fully understood and deserves to be investigated and alleviated in order to achieve stable mode-locking over long periods of time. It is particularly important for VECSEL systems meant to be commercialized, needing long term operation with a long product lifetime.

Here, we investigate the performance and robustness of a SESAM-modelocked VECSEL system under intense pulse intensity excitation. The effect of the degradation on the VECSEL performance is investigated using the SESAM in a VECSEL cavity supporting ultrashort pulses, while the degradation mechanism was investigated by exciting the SESAMs with an external femtosecond laser source. The decay of the photoluminescence (PL) and reflectivity under high excitation was monitored and the damaged samples were further analyzed using a thorough Transmission Electron Microscopy (TEM) analysis. It is found that the major contribution to the degradation is the field intensity and that the compositional damage is confined to the DBR region of the SESAM.

Tantalum pentoxide (Ta₂O₅) is a promising material for mass-producible, multi-functional, integrated photonics circuits on silicon, exhibiting robust electrical, mechanical and thermal properties, as well as good CMOS compatibility. In addition, Ta₂O₅ has been reported to demonstrate a non-linear response comparable to that of chalcogenide glass, in the region of 3-6 times larger than that of materials such as silica (SiO₂) or silicon nitride (Si₃N₄). In contrast to Si-based dielectrics, it will accept trivalent ytterbium and erbium dopant ions, opening the possibility of on-chip amplification. The high refractive index of Ta₂O₅ is consistent with small guided mode cross-section area, and allows the construction of micro-ring resonators. Propagation losses as low as 0.2 dB=cm have been reported. In this paper we describe the design of a planar Ta₂O₅ waveguides optimised for the generation of coherent continuum with near infrared pulse trains at kW peak powers. The Pulse Repetition Frequency (PRF) of the VECSEL can be tuned to a sub-harmonic of the planar micro-ring and the optical pump power applied to the VECSEL can be adjusted so that mode-matching of the VECSEL pulse train with the micro-ring resonator can be achieved. We shall describe the fabrication of Ta₂O₅ guiding structures, and the characterisation of their nonlinear and other optical properties. Characterisation with conventional lasers will be used to assess the degree of coherent spectral broadening likely to be achievable using these devices when driven by mode-locked VECSELs operating near the current state-of- art for pulse energy and duration.

A concept and numerical study of a continuous-wave (CW) nitride-based vertical-external-cavity surface-emitting laser (VECSEL) with an InGaN/GaN active region is presented. The structure is designed to generate radiation around 450 nm. An array of nitride-based continuous-wave laser diodes is proposed to pump directly the quantum wells in the active region. We expect that it enables CW operation of the presented laser, in contrast to the GaN-based VECSELs demonstrated so far. Moreover, employing in-well pumping instead of barrier pumping reduces pump-laser quantum defect, which contributes to better thermal properties of the device. An external efficiency as high as 26% can be theoretically achieved by using a special multi-pass pump setup.