This PDF file contains the front matter associated with SPIE Proceedings Volume 10087, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

Keys to high-power operation of disk lasers are a thin active layer, a small Stokes shift and an efficient cooling, best realized with a limited number of QWs which are pumped close to the laser wavelength and which are in close contact with one or two diamond heat sinks. To get sufficient pump absorption many passes of the pump radiation are needed. This can be realized either by taking advantage of intrinsic resonances (designed for the pump radiation) or by an external multi-pass optics (known from Yb disk lasers) or a combination of both. The various options will be discussed and some results for AlGaInP disk lasers will be presented.

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. The Vertical External Cavity Surface Emitting Laser (VECSEL) technology [1] has been identified for years as a good candidate to reach high power, high coherence and broad tunability while covering a wide emission wavelength range exploiting III-V semiconductor technologies. Offering such performances in the Near- and Middle-IR range, GaAs- and Sb-based VECSEL technologies seem to be a well suited path to meet the required specifications of demanding applications. Built up in this field, our expertise allows the realization of compact and low power consumption marketable products, with performances that do not exist on the market today in the 0.8-1.1 μm and 2-2.5 μm spectral range.

Here we demonstrate highly coherent broadly tunable single frequency laser micro-chip, intracavity element free, based on a patented VECSEL technology, integrated into a compact module with driving electronics. VECSEL devices emitting in the Near and Middle-IR developed in the frame of this work [2] exhibit exciting features compared to diode-pumped solid-state lasers and DFB diode lasers; they combine high power (>100mW) high temporal coherence together with a low divergence diffraction limited TEM00 beam. They exhibit a class-A dynamics with a Relative Intensity Noise as low as -140dB/Hz and at shot noise level reached above 200MHz RF frequency (up to 160GHz), a free running narrow linewidth at sub MHz level (fundamental limit at Hz level) with high spectral purity (SMSR >55dB), a linear polarization (>50dB suppression ratio), and broadband continuous tunability greater than 400GHz (< 30V piezo voltage, 6kHz cut off frequency) with total tunability up to 3THz. Those performances can all be reached thanks to the high finesse cavity of VECSEL technology, associated to ideal homogeneous QW gain behaviour [3]. In addition, the compact design without any movable intracavity elements offers a robust single frequency regime with a long term wavelength stability better than few GHz/h (ambient thermal drift limited).

Those devices surpass the state of the art commercial technologies thanks to a combination of power-coherence-wavelength tunability performances and integration.

The (AlGaIn)(AsSb) material system has been shown to be ideally suited to realize VECSELs for the 2-3 μm wavelength range. In this report we will present results on increasing the output power of the SDL chips with special emphasis on the 2.8 μm emission wavelength by means of low quantum defect pumping. Further on we have investigated concepts for a VECSEL-pumped Q-switched Ho:YAG laser in order to convert the high cw-power of the VECSEL into pulses with a high peak power. Up to 3.3 mJ of pulse energy were achieved with a compact setup (corresponding to a peak power of 30 kW at 110 ns pulse length) combined with stable pulsing behavior.

A systematic study of microscopic many-body dynamics is used to analyze a strategy for how to generate ultrashort mode locked pulses in the vertical external-cavity surface-emitting lasers with a saturable absorber mirror. The field propagation is simulated using Maxwell’s equations and is coupled to the polarization from the quantum wells using the semiconductor Bloch equations. Simulations on the level of second Born-Markov are used to fit coefficients for microscopic higher order correlation effects such as dephasing of the polarization, carrier-carrier scattering and carrier relaxation. We numerically examine recent published experimental results on mode locked pulses, as well as the self phase modulation in the gain chip and SESAM.

We report on recent developments in the characterisation of non-linear lensing in semiconductor disk laser gain samples. We find that there is a significant nonlinear lens present and the magnitude and sign of this depend on the conditions under which it is being observed. Under experimental conditions which are, to date, the closest to intra-cavity conditions, with 350 fs pulses at the same wavelength a mode-locked SDL using that gain chip would operate at we find that the lens is always negative and its magnitude is almost independent of pump intensity. We also report on the experimental observation of different mode-locking regimes in SDLs including dual wavelength mode-locking and pulse molecule formation and compare these experimentally observed modes of operation with predictions from microscopic modelling previously reported in by Kilen et. al. [1]

[1] I. Kilen et. al. Optica, 1 (4) 192-197 (2014)

Ultrafast optically pumped semiconductor disk lasers (SDLs) provide an enabling combination of gigahertz pulse repetition rates, short pulse durations, high peak power and good beam quality. However, the successful demonstration of shorter pulse durations with pulses as short as 100 fs has come at the expense of lower optical-to-optical pump efficiency for ultrafast SDLs based on active InGaAs quantum wells (QWs). The optical pump efficiency, which also depends on the pulse repetition rate, decreases with the pulse duration to values typically below 1% in the sub-300-fs regime. For a better understanding of this trade-off between shorter pulse durations and optical efficiency we present an empirical model based on three time constants: the carrier lifetime in the conduction band, the time required for a pulse to empty the carrier reservoir of a QW and the time needed to refill the QW through the continuously pumped GaAs barriers. With the time constants used as fitting parameters we obtain a reasonably good agreement for the measured efficiency of all previously published MIXSEL results. Furthermore we have investigated the SDLs gain dynamics and our measurements have confirmed that shorter pulses significantly reduce the gain saturation fluence. A simplified description of the QWs as a 3-level system could accurately reproduce the gain saturation curves and let us identify spectral hole burning as the main cause of the reduced gain and efficiency. This will be a challenge for further improvements of the ultrafast performance of ultrafast SDLs based on InGaAs QWs.

Self-heating of Optically Pumped Semiconductor (OPS) chip has been identified as the major limiting factor of power scaling in OPS-based lasers in continuous wave (cw) mode. In this work, characterization of OPS lasers in short pulse (100 ns) and low duty cycle (1%) regime, where self-heating is negligible, as a function of the heat sink temperature is presented. This data, combined with a rigorous thermal model, allows us to predict OPS chip performance in new cooling configurations for power scaling.

We report on the highly coherent modeless broadband continuous wave operation of a semiconductor vertical-external cavity-surface-emitting laser. Its design is based on a frequency-shifted-feedback scheme provided by an acousto-optic frequency shifter inserted into the cavity. Chirping the optical wave inside the cavity leads to the elimination of the traditional spectral resonances occurring in usual laser cavities, leading to a wideband (>330GHz here) continuous spectrum. The light is of high coherence concerning the transverse mode and the polarisation state, as well as in the time domain (time coherence ≈ 19μs, like a 1MHz DFB) in spite of its wideband emission.

We present a novel Vertical External Cavity Surface Emitting Laser (VECSEL) cavity design which makes use of multiple interactions with the gain region under different angles of incidence in a single round trip. This design allows for optimization of the net, round-trip Group Delay Dispersion (GDD) by shifting the GDD of the gain via cavity fold angle while still maintaining the high gain of resonant structures. The effectiveness of this scheme is demonstrated with femtosecond-regime pulses from a resonant structure and record pulse energies for the VECSEL gain medium. In addition, we show that the interference pattern of the intracavity mode within the active region, resulting from the double-angle multifold, is advantageous for operating the laser in CW on multiple wavelengths simultaneously. Power, noise, and mode competition characterization is presented.

We demonstrate a surface-emitting laser, based on III-V semiconductor technology with an integrated metasurface, generating vortex-like coherent state in the Laguerre-Gauss basis.²⁴ We use a first order phase perturbation to introduce a weak orbital anisotropy, based on a dielectric metasurface and non-linear laser dynamics, allowing selecting vortex handedness. Moreover, similarly to linear Doppler Shift, light carrying orbital angular momentum L, scattered by a rotating object at angular velocity, experiences a rotational Doppler shift L. We show that this fundamental light matter interaction can be detected exploiting self-mixing in a vortex laser under Doppler-shifted optical feedback, with quantum noise-limited light detection.²⁵ This will allow us to combine a velocity sensor with optical tweezers for micro-manipulation applications, with high performances, simplicity and compactness. Such high performance laser opens the path to widespread new photonic applications.

In recent years, M Squared Lasers have successfully commercialized a range of mode-locked vertical external cavity surface emitting lasers (VECSELs) operating between 920-1050nm and producing picosecond-range pulses with average powers above 1W at pulse repetition frequencies (PRF) of ~200MHz. These laser products offer a low-cost, easy-to-use and maintenance-free tool for the growing market of nonlinear microscopy. However, in order to present a credible alternative to ultrafast Ti-sapphire lasers, pulse durations below 200fs are required.

In the last year, efforts have been directed to reduce the pulse duration of the Dragonfly laser system to below 200fs with a target average power above 1W at a PRF of 200MHz. This paper will describe and discuss the latest efforts undertaken to approach these targets in a laser system operating at 990nm. The relatively low PRF operation of Dragonfly lasers represents a challenging requirement for mode-locked VECSELs due to the very short upper state carrier lifetime, on the order of a few nanoseconds, which can lead to double pulsing behavior in longer cavities as the time between consecutive pulses is increased.

Most notably, the design of the Dragonfly VECSEL cavity was considerably modified and the laser system extended with a nonlinear pulse stretcher and an additional compression stage. The improved Dragonfly laser system achieved pulse duration as short as 130fs with an average power of 0.85W.

We present a passive and robust mode-locking scheme for a Vertical External Cavity Surface Emitting Laser (VECSEL).We placed the semiconductor gain medium and the semiconductor saturable absorber mirror (SESAM) strategically in a ring cavity to provide a stable colliding pulse operation. With this cavity geometry, the two counter propagating pulses synchronize on the SESAM to saturate the absorber together. This minimizes the energy lost and creates a transient carrier grating due to the interference of the two beams. The interaction of the two counter-propagating pulses in the SESAM is shown to extend the range of the modelocking regime and to enable higher output power when compared to the conventional VECSEL cavity geometry. In this configuration, we demonstrate a pulse duration of 195fs with an average power of 225mW per output beam at a repetition rate of 2.2GHz, giving a peak power of 460W per beam. The remarkable robustness of the modelocking regime is discussed and a rigorous pulse characterization is presented.

Ultrafast vertical external-cavity surface-emitting lasers (VECSELs) are versatile laser sources and feature high-power operation. To date the best modelocking results have been achieved with a semiconductor saturable absorber mirror (SESAM). Ultrafast optically pumped semiconductor disk lasers (SDLs) are compact, cost-efficient and provide excellent beam quality at gigahertz pulse repetition rates for applications such as for example multi-photon imaging, ultrafast communication and in particular self-referenced gigahertz frequency combs. The highest peak power obtained with an ultrafast VECSEL is 4.35 kW in 400-fs pulses and the shortest pulses until now are 107 fs at 3 mW average output power. Here we present a SESAM-modelocked VECSEL with pulses as short as 96 fs and 100 mW average output power. These are to the best of our knowledge the shortest pulses achieved by a fundamentally modelocked SDL and result in a very high peak power of 0.56 kW at a pulse repetition rate of 1.63 GHz. The short pulse duration was achieved by introducing a small amount of positive group delay dispersion with a single path through an external 2-mm thick ZnSe window plate that compensated the initially negatively chirped 107-fs output pulses.

Currently the power is limited by the transition from fundamental modelocking to multi-pulse operation, which reduces the pulse peak power and introduces additional noise. Therefore, we present a study of the multi-pulse behavior of the high-power 100-fs SDL resulting from the complex modelocking mechanism. This study also provides an insight into special issues of pulse characterization that may suggest stable fundamental modelocking even if this is not the case.

Lasers operating in the transmission window of tissue at wavelengths between 700 and 800 nm are needed in numerous medical and biomedical applications, including photodynamic therapy and fluorescence microscopy. However, the performance of diode lasers in this spectral range is limited by the lack of appropriate compound semiconductors. Here, we review our recent research on 750 nm VECSELs. Two approaches to reaching the 750 nm wavelength will be discussed. The first approach relies on intra-cavity frequency doubling a wafer-fused 1500 nm VECSEL. The VECSEL gain chip comprises a GaAs-based DBR and an InP-based gain section, which allows for optical pumping with low-cost commercial diodes at 980 nm. With this scheme we have achieved watt-level output powers and tuning of the laser wavelength over a 40 nm band at around 750 nm. The second approach is direct emission at 750 nm using the AlGaAs/GaAs material system. In this approach visible wavelengths are required for optical pumping. However, the consequent higher costs compared to pumping at 980 nm are mitigated by the more compact laser setup and prospects of doubling the frequency to the ultraviolet range.

Semiconductor disk lasers with all their advantages¹ became an important stand-alone class of solid-state lasers during the last years. However, these systems suffer from heat incorporation into the active region caused by the excess energy of the pump photons. To overcome this limitation we realized the semiconductor membrane external-cavity surface-emitting laser as a diamond heat spreader sandwiched active region design. A detailed process description towards the MECSEL² approach is given as well as fundamental performance values. Furthermore, parasitic lateral lasing effects are discovered and investigated. Nevertheless, the MECSEL approach indicates enormous potential to revolutionize the semiconductor based disk lasers regarding available output powers at room temperature and material combinations.

Semiconductor disk lasers (SDLs) are attractive for applications requiring good beam quality, wavelength versatility, and high output powers. Typical SDLs utilize the active mirror geometry, where a semiconductor DBR is integrated with the active region by growth or post-growth bonding. This imposes restrictions for the SDL design, like material system choice, thermal management, and effective gain bandwidth. In DBR-free geometry, these restrictions can be alleviated. An integrated gain model predicts DBR-free geometry with twice the gain bandwidth of typical SDLs, which has been experimentally verified with active regions near 1 μm and 1.15 μm. The lift-off and bonding technique enables the integration of semiconductor active regions with arbitrary high quality substrates, allowing novel monolithic geometries. Bonding an active region onto a straight side of a commercial fused silica right angle prism, and attaching a high reflectivity mirror onto the hypotenuse side, with quasi CW pumping at 780 nm, lasing operation was achieved at 1037 nm with 0.2 mW average power at 1.6 mW average pump power. Laser dynamics show that thermal lens generation in the active region bottlenecks the laser efficiency. Investigations on total internal reflection based monolithic ring cavities are ongoing. These geometries would allow the intracavity integration of 2D materials or other passive absorbers, which could be relevant for stable mode locking. Unlike typical monolithic microchip SDLs, with the evanescent wave coupling technique, these monolithic geometries allow variable coupling efficiency.

Since the invention of VECSELs, vast spectral coverage has been demonstrated with emission wavelengths in the range from the UV to almost the MIR. Accordingly, a great variety of different quantum well and quantum dot gain designs have been applied so far to achieve such versatility. A novel gain design for GaAs based VECSELs emitting at wavelengths >1.2 μm employs type-II quantum wells, which exhibit spatially indirect charge-carrier recombination. The first VECSEL based on such a design has been demonstrated very recently. Our device consists of ten (GaIn)As/Ga(AsSb)/(GaIn)As heterostructures arranged as a resonant periodic gain. We summarize the development of this pioneering structure and discuss the fundamental laser characteristics, such as carrier densities, gain temperatures and slope efficiency. Remarkable output powers up to 4 W are demonstrated in multi-transverse mode operation at 1.2 μm. Also, the performance in TEM00 operation is investigated, with an M² < 1.13. One major difference to conventional type-I gain structures is a characteristic blue shift of the material gain. Due to the importance of the detuning in quantum well based surface-emitters, the blue shift has to be considered as a critical designing parameter. Hence, we carry out a detuning study in order to determine an optimal detuning. As an important part of the optimization, the experimental results are compared with fully microscopic simulations.

This work presents several optical experiments to investigate the phenomenon of self-mode locking (SML) in optically pumped semiconductor lasers (OPSLs). First of all, we systematically explore the influence of high-order transverse modes on the SML in an OPSL with a linear cavity. Experimental results reveal that the occurrence of SML can be assisted by the existence of the first high-order transverse mode, and the laser is operated in a well-behaved SML state with the existence of the TEM0,0 mode and the first high-order transverse mode. While more high-order transverse modes are excited, it is found that the pulse train is modulated by more beating frequencies of transverse modes. The temporal behavior becomes the random dynamics when too many high-order transverse modes are excited. We observe that the temporal trace exhibits an intermittent mode-locked state in the absence of high-order transverse modes. In addition to typical mode-locked pulses, we originally observe an intriguing phenomenon of SML in an OPSL related to the formation of bright-dark pulse pairs. We experimentally demonstrated that under the influence of the tiny reflection feedback, the phase locking between lasing longitudinal modes can be assisted to form bright-dark pulse pairs in the scale of round-trip time. A theoretical model based on the multiple reflections in a phase-locked multi-longitudinal-mode laser is developed to confirm the formation of bright-dark pulse pairs.

Since 2000, semiconductor saturable absorber mirrors (SESAMs) have been used to realize mode locking of vertical external-cavity surface-emitting lasers (VECSELs), achieving femtosecond pulse durations, GHZ repetition rates and several Watts of average output power. Despite these excellent results, SESAMs which have to be carefully adjusted to the gain structure can be a limiting factor for the development of a cost-effective pulsed laser system. In recent years, a new concept of VECSEL mode locking, the self-mode locking technique, has been demonstrated. While the mechanism behind this kind of mode locking is not yet fully explained, most publications focus on the effect of Kerr lensing.

We present first experiments on SESAM-free mode locking of red-emitting AlGaInP-VECSELs with different cavity geometries based on the assumption of Kerr lensing in the active region. Our semiconductor samples are grown by metal-organic vapor-phase epitaxy with an active region containing GaInP quantum wells embedded in AlGaInP barriers and cladding layers. In order to exploit the effect of Kerr lensing, a slit is placed inside the cavity acting as a hard aperture. When the beam width is confined, pulsed operation is observed by oscilloscope and autocorrelation measurements. Ongoing research is focusing on a detailed characterization of the pulsed laser to improve one's understanding of the obtained SESAM-free mode-locked operation.

Mode-locked Vertical External-Cavity Surface-Emitting Lasers (ML-VECSELs) have seen advances in pulse energy and peak power thanks to improved power handling techniques and structure designs. The significant increase in gain and intra-cavity power, coupled with the VECSEL's accessible external-cavity, has made the addition of intra-cavity elements for frequency conversion possible even for lossy conversion mechanisms. In this paper, we report a gold-patterned Semiconductor Saturable Absorbing Mirror (SESAM) that functions both as a slow saturable absorber in a ML-VECSEL and as an intracavity strip line Photo-Conductive Antenna (PCA) for THz emission. Here we describe the design of the strip emitter, THz-Time Domain Spectroscopy (TDS) performed with a ML-Yb fibre laser and the mode-locked characterisation of a ML-VECSEL built with the patterned SESAM.

Publisher’s Note: This conference presentation, originally published on 21 April 2017, was withdrawn per author request.

Two different approaches to developing new laser sources operating in the mid-infrared range based on vertical-external cavity surface-emitting lasers (VECSELs) are studied with the aid of numerical modelling. The first one consists in enhancing a maximal emission wavelength of currently available GaSb-based structures beyond 3 μm. The second approach consists in using dual-wavelength VECSEL (DW-VECSEL), emitting two coaxial laser beams of different wavelengths, to generate radiation from the 3-5 μm spectral range with the aid of difference frequency generation.

In AlGaInP based VECSELs, a low thermal conductivity of the substrate with included distributed Bragg reflector leads to a strong temperature-dependent performance due to the limited charge-carrier confinement. For efficient heat removal, a good bonding between VECSEL-chip and intra-cavity heat spreader is indispensable. Here, a new designed sample holding device which allows improved bonding is presented. With this device, the laser performance of a barrier-pumped AlGaInP VECSEL emitting at 665 nm could be improved tremendously which resulted in an output power of more than 1W at a heatsink temperature of 10°C. We present a full characterization of the laser system including a comparison between standard and the new device.