This PDF file contains the front matter associated with SPIE Proceedings Volume 8966, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

Average power scaling in a thin disk geometry supports more than <10 kW from Yb-doped solid-state and <100 W from vertical emitting semiconductor lasers. Both lasers can be passively mode-locked with SESAMs pushing the performance frontier into a regime previously assumed to be impossible. A Yb-YAG thin disk laser generates femtosecond pulses with more than 80 μJ pulse energy without any external pulse amplification. With semiconductor thin disk lasers (also referred to as VECSELs and MIXSELs) we can obtain <1W average power with both femtosecond and picosecond pulses and a pulse repetition rates ranging between 100 MHz to 100 GHz.

This tutorial gives an overview of the microscopic approach developed to describe equilibrium and nonequilibrium effects in optically excited semiconductor systems with an emphasis to the application for VECSEL modelling. It is outlined how nonequilibrium quantum theory is used to derive dynamic equations for the relevant physical quantities, i.e. the optically induced polarization and the dynamical carrier occupation probabilities. Due to the Coulombic many-body interactions, polarization and populations couple to expectation values of higher-order quantum correlations. With the help of a systematic correlation expansion and truncation approach, we arrive at a closed set of equations. Formally these can be combined with Maxwell’s equations for the classical light field, yielding the Maxwell-semiconductor Bloch equations (MSBE). However, instead of the more traditional approach where losses and dissipative processes are treated phenomenologically and/or through coupling to external reservoirs, we derive fully microscopic equations for the carrier-carrier and carrier-phonon scattering as well as the effective polarization dephasing. Due to their general nature, the resulting equations are fully valid under most experimentally relevant conditions. The theory is applied to model the high-intensity light field in the VECSEL cavity coupled to the dynamics of the optical polarization and the nonequilibrium carrier distributions in the quantum-well gain medium.

Quasi-stable self-mode-locking of an InGaAs vertical external-cavity surface-emitting laser (VECSEL) emitting around 1020 nm has been observed, resulting in 500 fs pulses at a repetition rate of 1 GHz. The mechanism is attributed to negative ultrafast Kerr lensing in the semiconductor gain structure. Our calculations show that a mode narrowing on the order of 0.5% can be obtained at the concave cavity end-mirror or at the gain medium. This is consistent with experimental observations, as mode-locking can be achieved by placing a (hard) aperture before the concave cavity end mirror inside the VECSEL cavity, or by the soft aperture created by changing the pump spot size in relation to the lasing mode on the gain chip. The pulse train generated by the VECSEL has been analyzed by a fast InGaAs photo diode and oscilloscope, RF spectrum analyzer, and second harmonic intensity autocorrelation. The effect of dispersion on pulse width has been studied, hinting at soliton-like pulse formation.

Mode-locking an optically pumped semiconductor disk laser has been demonstrated using low-loss saturable absorption containing a mixture of single-walled carbon nanotubes in PMM polymer. The modulator was fabricated by a simple spin-coating technique on fused silica substrate and was operating in transmission. Stable passive fundamental modelocking was obtained at a repetition rate of 613 MHz with a pulse length of 1.23 ps. The mode-locked semiconductor disk laser in a compact geometry delivered a maximum average output power of 136 mW at 1074 nm.

In the past decade, passively modelocked optically pumped vertical external cavity surface emitting lasers (OPVECSELs), sometimes referred to as semiconductor disk lasers (OP-SDLs), impressively demonstrated the potential for generating femtosecond pulses at multi-Watt average output powers with gigahertz repetition rates. Passive modelocking with a semiconductor saturable absorber mirror (SESAM) is well established and offers many advantages such as a flexible design of the parameters and low non-saturable losses. Recently, graphene has emerged as an attractive wavelength-independent alternative saturable absorber for passive modelocking in various lasers such as fiber or solid-state bulk lasers because of its unique optical properties. Here, we present and discuss the modelocked VECSELs using graphene saturable absorbers. The broadband absorption due to the linear dispersion of the Dirac electrons in graphene makes this absorber interesting for wavelength tunable ultrafast VECSELs. Such widely tunable modelocked sources are in particularly interesting for bio-medical imaging applications. We present a straightforward approach to design the optical properties of single layer graphene saturable absorber mirrors (GSAMs) suitable for passive modelocking of VECSELs. We demonstrate sub-500 fs pulses from a GSAM modelocked VECSEL. The potential for broadband wavelength tuning is confirmed by covering 46 nm in modelocked operation using three different VECSEL chips and up to 21 nm tuning in pulsed operation is achieved with one single gain chip. A linear and nonlinear optical characterization of different GSAMs with different absorption properties is discussed and can be compared to SESAMs.

Due to its unique zero-bandgap structure, linear disperion of electrons and compatibility with various optoelec- tronic platforms, graphene has become one of the principal materials of interest with strong development for many nonlinear optical devices. Functionalized graphene-composites exhibit excellent optical limiting properties while single layer graphene speci cally, has shown great promise as a saturable absorber in mode locking ber and solid state lasers from the visible to infrared regime. However, more recently work has been done to in- tegrate graphene in a vertical-external-cavity-surface-emitting-lasers (VECSELs). Currently VECSELs employ semiconductor-based saturable absorbers which have a narrow tuning range and require complex fabrication procedures. By developing a graphene-based saturable absorber, one can take advantage of its zero bandgap structure and therefore its frequency-independent absorption as well as its thermal and mechanical stability to passively modelocking lasers over a wide frequency range { potentially through the terahertz regime. Here we report on recent developments in graphene mode-locking of VECSELs, speci cally for high power operation. Further, work done in studying the nonlinear optical properties of graphene pertaining to the development of sat- urable absorbers as well as optical limiters will be presented. Finally preliminary fabrication and characterization work conducted to integrate the graphene-based materials in a VECSEL will be presented.

The modelocked integrated external-cavity surface emitting laser (MIXSEL) combines the active region and basic layout of vertical-external-cavity surface-emitting lasers (VECSELs) with the saturable absorber of a semiconductor saturable absorber mirror (SESAM) in a single semiconductor layer stack. This concept allows stable and self-starting passive modelocking in a simple straight cavity. Record-high average output power has been demonstrated previously with a MIXSEL based on a quantum dot saturable absorber, but with a minimum pulse duration of only 17 ps up to now. Here we present a femtosecond MIXSEL emitting 620-fs pulses in 101 mW of average output power at 4.8 GHz pulse repetition rate. A novel single quantum well saturable absorber, whose parameters are discussed in detail, enabled the strong reduction in pulse duration by over an order of magnitude.

We demonstrate the highest free running single frequency power from a single chip VECSEL reported to date, with more than 15W in continuous operation at room temperature. The GaAs-based structure presents an emission wavelength of 1020nm and a tuning range <15nm, with a continuous tunability of 9GHz. The TEM₀₀ output beam exhibits very low transverse phase fluctuations across the entire mode, leading to a beam quality M² <1.2. To identify and reduce the different sources of noise, the relative intensity noise and frequency noise spectral density are investigated and the intensity and the frequency of the laser were independently stabilized. The laser frequency is controlled and tuned varying the cavity length using a high bandwidth piezoelectric element while intensity fluctuations are reduced by varying the pump intensity. Intrinsic and stabilized frequency and intensity noise are compared.

We present timing jitter measurements of a free-running and actively stabilized modelocked integrated external-cavity surface-emitting laser (MIXSEL) generating ps-pulses around 2-GHz repetition rate and over 600 mW of average output power with <0.15% rms amplitude modulation (AM) noise. The free running rms timing-jitter was 129 fs [100 Hz to 10 MHz] and 70 fs [300 Hz to 10 MHz], which is the lowest timing jitter of a free-running passively modelocked semiconductor laser to date. Actively stabilized to an electronic reference source using a piezo-actuator, an rms timing jitter of 31 fs was obtained, representing the lowest value ever measured from a passively modelocked semiconductor disk laser between 100 Hz and 100 MHz.

We report on the development of a high-efficiency frequency doubled vertical-external-cavity surface-emitting laser with an output power of 20 W and emission spectrum centered at 588 nm. The MBE-grown gain chip incorporated 10 GaInAs quantum wells and emitted in the 1180 nm range. The frequency conversion was achieved using a lithium triborate nonlinear crystal in an intra-cavity configuration. In addition to the nonlinear crystal, the V-shaped cavity also included a birefringent filter and an etalon for linewidth narrowing and wavelength tuning. The maximum optical-to-optical conversion efficiency obtained was ~28 % for 16 W of output power and the VECSEL had a tuning bandwidth of ~26 nm ranging from about 576 to 602 nm. We were also able to generate yellow pulses down to 570 ns duration by directly modulating the VECSEL’s pump laser.

We demonstrate a continuous wave, single frequency terahertz (THz) source based on parametric difference frequency generation within a nonlinear crystal located in an optical enhancement cavity. Two single-frequency VECSELs with emission wavelengths spaced by 6.8 nm are phase locked to the external cavity and are used as pump sources for the nonlinear down conversion. The emitting THz radiation is centered at 1.9 THz and has a linewidth of less than 100 kHz. The output power of the source exceeds 100 μW. We show that the THz source can be used as local oscillator to drive a receiver used in astronomy applications.

Optically pumped wafer fused 1310 nm VECSELs have the advantage of high output power and wavelength agility. Gain mirrors in these lasers are formed by direct bonding of InAlGaAs/InP active cavities to Al(Ga)As/GaAs DBRs. We present for the first time Watt-level 1310 nm wafer-fused VCSELs based on gain mirrors with heat dissipation in the “flip-chip” configuration. Even though output power levels in this approach is lower than with intra-cavity diamond heat-spreaders, the “flip-chip configuration demonstrates higher quality optical emission and is preferable for industrial applications in optical amplifiers, intra-cavity doubled lasers, etc.

VECSELs are characterized by an outstanding brightness of 100kW/mm²/sr and a small spectral width. Electrical pumping and the potential to combine many emitters in arrays allow for highly integrated and easy to manufacture laser sources which can be scaled towards high power. This almost ideal value proposition is affected by the penalty in efficiency which reduces the output power from VCSELs towards multimode VECSELs and finally single mode VECSELs. The root causes for this lower efficiency are optical losses in the extended cavity, a mismatch of pump and mode profile and losses related to the oxide aperture which is used for current confinement. The reduction of losses requires a careful design of spatial doping distributions in the epitaxially grown layers as these losses have to be balanced against the requirement of low electrical resistance across the many hetero-interfaces in the DBR mirrors. The mismatch of pump and mode profile and the aperture related losses are addressed by an improved current injection enabled by a tailored electrical contact. In this paper optimized structures will be presented which enable a significant increase of efficiency and output power towards more than 150mW in a single mode and more than 300mW in multimode operation. The optical concept of the extended cavity can use a plane mirror in the simplest case thus facilitating the power scaling in arrays with many individual VECSEL apertures combined on a single chip.

A numerical investigation of the current injection into the active region of electrically-pumped vertical-external-cavity surface-emitting lasers (E-VECSELs) is presented. To achieve high power of emission, a broad aperture is necessary, but such geometry favors multimode operation as the result of undesired current crowding. To reduce this effect, we propose a novel approach of selectively etched tunnel junctions in the form of coaxial rings. The paper presents the optimization of this novel geometry as an efficient approach for increasing the single mode output power of such laser.

Optically pumped vertical external cavity surface emitting lasers (OP-VECSELs) evolved to high-power laser sources offering excellent beam-quality, wavelength flexibility and low-noise properties in a compact and simple cavity. Passively modelocked with a semiconductor saturable absorber mirror (SESAM), VECSELs demonstrated fs-pulses with multi-Watt average output powers at gigahertz repetition rates. Electrical pumping (EP) is an obvious step to make these semiconductor lasers even more compact and suitable for chip integration, potentially enabling access to applications such as data communication or optical clocking. With SESAMmodelocked EP-VECSELs, 57-ps pulses with an average output power of 40 mW and 9.5-ps pulses with 7.6 mW have been obtained. However, due to the intrinsic trade-off between electrical and optical properties in the design of EPVECSELs, short pulses at high average output power are difficult to achieve. This challenge was previously addressed in our theoretical guidelines for power scaling and modelocking optimization and later experimentally verified. Here, we report on the successful implementation of an improved design and fabrication scheme for EP-VECSELs, grown and fabricated at ETH Zurich. These lasers enabled a further decrease in pulse duration to 7.3 ps while increasing the average output power to 13.1 mW at 1.46-GHz repetition rate. The shortest pulse duration measured was 6.3 ps with an average power of 6.2 mW. In addition to the modelocking experiments, we present a thorough cw-characterization of our EP-VECSELs of different sizes and in different cavity configurations, pointing out the inevitable trade-off between high-power multi-mode and low-power single-mode operation limiting the modelocking performance.

Electrically pumped vertical external cavity surface emitting laser is passively mode-locked at record-low repetition rate of 216 MHz demonstrating potential peak power scalability. A quantum dot saturable absorber is used to achieve stable operation.

Coherent population trapping (CPT) is an interesting technique for the development of compact atomic frequency references. We describe an innovating laser source for the production of the two cross-polarized coherent laser fields which are necessary in CPT-based atomic clocks. It relies on the dual-frequency and dual-polarization operation of an optically-pumped vertical external-cavity semiconductor laser. This particular laser emission is induced by intracavity birefringent components which produce a controllable phase anisotropy within the laser cavity and force emission on two cross-polarized longitudinal modes. The laser emission is tuned at the Cs D₂ line (λ = 852.14 nm), and the frequency difference Δν between the two laser modes is tunable in the microwave range. The laser line wavelength is stabilized onto an atomic hyperfine transition, and concurrently the frequency difference is locked to an ultra-low noise RF oscillator at 9.2 GHz. The high spectral purity of the optically-carried microwave signal resulting from the beatnote of the two cross-polarized laser lines is assessed through its narrow spectral linewidth (<30 Hz) as well as its low phase noise (≤ -100 dBrad²/Hz). The performance of this laser source is already adequate for the interrogation of atoms in a CPT atomic clock, and should result in an estimated relative stability of 3.10^-13τ^-1/2 – one order of magnitude better than commercial atomic clocks.

We investigate, both experimentally and theoretically, the spectral behavior of the intensity noises as well as the phase noise of the radio frequency (RF) beatnote generated by optical mixing of two orthogonally polarized modes of a dual-frequency VECSEL. To be more speci c, we measure the relative intensity noises (RINs) and the correlation between the intensity noises of the two laser modes for di erent nonlinear coupling strengths between them within frequencies 10 kHz to 50 MHz. Moreover for these frequencies, we explore the spectral behavior of the phase noise of the RF beatnote generated by optical mixing of two laser modes and the dependence of this RF phase noise spectrum on the strength of non-linear coupling between the laser modes. The theoretical model considers pump intensity uctuations as the only source of noise within the considered frequency range. The pump uctuations, entering into the two spatially separated laser modes on the active medium, are measured to be white noises of identical amplitudes, partially correlated, and in phase. To model the RF phase noise, we take into account two di erent physical mechanisms: (i) the coupling of intensity noise with phase noise due to large Henry factor of the semiconductor gain medium and (ii) the thermal uctuations of the refractive index of the semiconductor active medium induced by pump intensity uctuations. For all the results, theory shows very good agreement with the experiment.

We systematically study the single- and multi-mode emission of vertical-external-cavity surface-emitting lasers (VECSELs) using streak camera measurements and interferometric measurement techniques. In all experiments, the VECSEL chip is based on (GaIn)As multi-quantum wells as active medium designed for laser emission around 1010 nm. The emission is analyzed in dependence of the pump power, employing two resonator designs as well as different output couplers. We monitor the evolution of emission bandwidth and show that in our setups a stable two-color lasing –with both lasing intensities sharing the same gain region on the chip– is related to a sufficiently high number of longitudinal modes participating in the laser emission.

The MIXSEL combines the gain of a VECSEL with the saturable absorber of a SESAM in one semiconductor structure to achieve fundamental modelocking in a simple straight cavity. We present a high-power MIXSEL with sub-10-ps pulse durations that can be scaled easily in repetition rate from a few GHz to <100 GHz. At 5.1 GHz repetition rate an average output power of 1.05 W in 2.4-ps-pulses was achieved. By scaling the repetition rate to 10 GHz (3.9-ps-pulses at 1.29 W), then to 20.7 GHz (2.35-ps-pulses at 607 mW) and most recently to even more than 100 GHz makes this high-power MIXSEL an attractive source suitable for applications such as optical clocking or optical sampling.

We present passive mode locking of a vertical external-cavity surface-emitting laser (VECSEL) in the red spectral range. The gain structure includes 20 compressively strained GaInP quantum wells (QWs), which are arranged in a resonant periodic gain design containing five packages of four quantum wells each. We use tensile strained AlGaInP barriers and cladding layers to compensate the strain introduced by the quantum wells. The semiconductor saturable absorber mirror (SESAM) includes two of the same quantum wells as used in the gain structure, positioned close to the surface. The semiconductor structure is grown by MOVPE in a near-resonant design and coated with a fused silica layer for an overall anti-resonant design. For tight focussing of the laser mode onto the absorber, we use a v-shaped cavity with an overall length of 179mm. Autocorrelation measurements show a FWHM pulse duration below 250 fs with side pulses arising due to the diamond heatspreader bonded onto the gain chip. The laser spectrum consists of a soliton-like part at 664.5 nm and a “continuum” which is also found in autocorrelation measurements perfomed in a Hanbury-Brown and Twiss type setup. An FFT based frequency analysis of the emitted pulse train shows a repetition rate of 836MHz. The SESAM charge carrier dynamics were investigated by pump-probe measurements. We observe a tri-exponential decay with a dominant fast decay time in the range of the pulse duration.

Femtosecond pulse mode-locked VECSELs have become a significant focus of research in the VECSEL community, with recent progress being made in several directions including power scaling. Power scaling advances in femtosecond VECSELs have included increasing the average power to over 5W [1], producing 3.3W average power with 400 fs pulses [2]. Here I report our recent work reducing the pulse duration of Watt-level VECSELs to 355 fs, and also developing approaches to reach sub-250-fs pulse durations using coherent broadening in photonic crystal fiber in the normal dispersion regime and a grating pulse compressor.

This paper presents recent advances of 2-μm GaSb-based vertical external cavity surface emitting laser (VECSEL) with special emphasis on quantum deficit reduction and miniaturization. Operating the VECSEL in a 5-cm long cavity, we could demonstrate an increase in maximum cw output power from 4.2 W to 7.2 W at room temperature when barrier pumping a 2.0-μm emitting VECSEL at a pump wavelength of 1.5 μm instead of 980 nm. Furthermore, miniaturized VECSELs were realized by depositing a high-reflectivity (~97 %) coating on top of a 375-μm thick SiC heat spreader, which acts as output coupler of the micro cavity (μC) formed. This planar cavity is rendered stable by thermal lensing induced by the absorption of pump light. At the same time, thermal lensing influences the beam quality. We will report a detailed study of the influence of the thermal lens on the stability and beam diameter of the μC-VECSEL by using two different VECSEL structures optimized for 980 nm and 1.5 μm barrier pumping, respectively. Using different pump photon energies results in different amounts of heat generated at a given pump photon flux, and thus thermal lenses with different focal lengths. Using the low-quantum deficit pumping scheme we could achieve a factor-7 increase in output power in TEM₀₀ emission from the μC-VECSEL compared to the 980 nm pumped device, as well as a maximum output power of 2.2 W. This 2-μm μC-VECSEL exhibits 110-nm tunable single-frequency emission at a 7-MHz linewidth at an output power of up to 90 mW. The linewidth of the μC-VECSEL is comparable to that of VCSELs, which typically emit output powers in the milli-Watt range.

Since years, the VeCSEL concept is pointed out as a technology of choice for beyond-state-of-the-art laser light sources. The targeted coherent state in CW is typically the common gaussian TEM₀₀, single frequency, linearly polarized lightstate. In this work, we take advantage of the VeCSEL technology for the generation of other kinds of coherent states, thanks to the insertion of intracavity functions, such as low-loss intensity and phase filters integrated on a semiconductor chip. This technological development permitted to demonstrate very pure high-order Laguerre-Gauss mode, both degenerate and non-degenerate(vortex)modes, preserving the coherence properties of usual TEM₀₀ VeCSELs. This technology paves the way for the generation of other coherences (Bessel beams) or new functionnalities (wavelength filtering, etc.). We also explore new time domain coherence : owing to a high gain semiconductor chip design and the insertion of intracavity AOM, we demonstrated the first Frequecy-Shifted-Feedback VeCSEL, with a broadband coherence state as wide as 300 GHz.

Demanding applications such as LIDAR, velocimetry, gas analysis or atomic clock rely on a highly coherent laser. Offering high coherence at high power and flexible wavelength, the GaAs- and Sb-based VECSEL technologies seem to be a well suited path to fulfill the required specifications of demanding applications. Till now, technical and physical knowledge of high power high coherence single frequency compact diode-pumped VECSELs have been developed at IES [1], with low intensity and frequency noise, but this promising technology is still at laboratory stage. The expertise built up in this field allows considering the realization of user-friendly marketable products, with performances that do not exist on the market today at 1 μm and 2.3 μm. Our goal is to develop a single frequency diode-pumped VECSEL, intracavity element free, achieving the desired performances, and to integrate this component into a compact module. The VECSEL prototypes developed in the frame of this work exhibit exciting features compared to diode-pumped solidstate lasers; they combine high power high coherence in a single TEM00 mode emission, free running narrow linewidth with high SMSR, a linear polarization, broadband continuous tunability, and compact design without any movable intracavity elements. All these specifications can be reached thanks to the high finesse cavity of VECSEL technology, associated to ideal homogeneous laser QW gain behavior.

Electrically-injected vertical external cavity surface emitting laser (VECSEL) arrays are an attractive source for lowcost, high-brightness applications. Optical pumping can be used to investigate the emission properties of such devices without undergoing complex device fabrication. The design of such arrays is based on a single VECSEL chip, a 2D lens array, and a flat output coupling dichroic mirror. In this work, we report on the demonstration of an optically pumped, coherently-coupled VECSEL array. The array achieves a maximum total output power of >60 mW and lasing spectrum indicates single-mode operation. Near-field characterization reveals 37 individual lasing elements in a hexagonal array. Far-field measurements show an interference pattern which is consistent with inphase coherent coupling, with >60% of the total output power present in the on-axis central lobe. The physical origin of coherent coupling is attributed to diffractive coupling. The simplicity of the optical cavity design suggests scalability to much larger arrays, making the result of particular interest to the development of low-cost, highbrightness diode sources.

We report mode-locking of an optically pumped VECSEL using a graphene-based saturable absorber mirror (GSAM). Self-starting and stable modelocked operation is demonstrated with 473 fs pulses at 1.5 GHz repetition rate and 949 nm center wavelength. Wavelength tuning is achieved over a 46 nm bandwidth. We discuss the mirror design, the fabrication of the GSAMs, and give an outlook on further optimization of the design, including dielectric top coatings to protect the graphene and to increase the flexibility in the design.

We present an experimental study on beam combining techniques with multiple vertical external cavity surface emitting lasers (VECSELs) using volume Bragg gratings (VBGs). The specially designed holographic gratings introduce frequency specific feedback for the near infrared wavelength VECSELs to achieve both spectral linewidth narrowing and beam combination effects. For coherent addition, we obtained >3W output power with 8% slope efficiency in a coherent power scaling cavity scheme. In the multiplexed VBGs (MVBGs) wavelength beam combining compound cavity scheme, we measured >6W combined output with nearly 100% combining efficiency. Both beam combining/power scaling schemes produced spectrally narrowed and near diffraction limited outputs.

We report a novel single-frequency, injection-locked vertical external-cavity surface-emitting laser (VECSEL) for the first time to the best of our knowledge. The wavelength of the injection locked output can be tuned with the master laser. A lower power single-frequency VECSEL served as the master oscillator that provides wavelength tunability with frequency selective elements such as a birefringent filter, and/or an etalon. The master laser is mode-matched to the slave VECSEL ring resonator. By varying the injecting power and wavelength of the master VECSEL oscillator, we investigated the locking ability of the laser. With 200 mW of the injection power, we generated above 4 W of stable output in single frequency. With the ability to provide narrow linewidth, good beam quality, and stable output with sufficient power at specific wavelengths, this kind of laser sources can be useful in many laser applications, such as precision spectroscopy.