This PDF file contains the front matter associated with SPIE Proceedings Volume 8242, including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

Mode-locked vertical-extended-cavity-surface emitting lasers (ML-VECSEL) are promising candidates for the generation of stable short pulses at multi-GHz rate. However, the poor thermal behavior of quaternary InP-based semiconductor compounds often limits the performance of ML-VECSELs operating at 1.55 μm. In this work, we report on a specific approach using downward heat sinking to optimize the heat dissipation out of the active region. VECSEL chips with a low thermal resistance are fabricated using a hybrid metal-metamorphic GaAs/AlAs mirror and bonding to a highly thermally conductive host substrate. We show that superior performance can be obtained with a CVD diamond substrate, while electroplated copper host substrate can afford a flexible and low cost alternate approach for moderate (~100 mW) output power. The VECSEL chip assembled with a 1.55μm fast InGaAs(Sb)N/GaAsN semiconductor saturable absorber mirror (SESAM) produces nearly Fourier transform-limited mode-locked pulses at ~ 2 GHz repetition frequency, and the RF linewidth of the free running laser is measured to be less than 1000 Hz. When the resonance and group delay dispersion of the SESAM microcavity are tuned by selective etching of specific top phase layers, the modelocked pulse width is reduced from several picoseconds to less than 1 ps.

We review the development of the first GaSb-based passively mode-locked VECSEL generating sub-picosecond pulses at 2 μm wavelength range. The general goal of this development was to leverage the unique features of the mode-locked VECSELs (i.e. high-average power, sub-ps operation, high repetition rate, low-noise properties) to the 2-3 μm wavelengths. Such lasers could have a significant impact on the development of practical ultrafast systems required for frequency-combs, time-resolved molecular spectroscopy, THz generation, or as seeders for optical amplifiers and mid-IR supercontinuum sources. By using semiconductor gain mirrors and saturable absorber mirrors incorporating InGaSb/GaSb quantum wells, we have been able to demonstrate a VECSEL producing near transform-limited 384 fs pulses at a wavelength of 1950 nm. Important part of this development has been focused on understanding the ultrafast absorption recovery dynamics of the SESAM. An interesting observation is that the absorption recovery time of asgrown InGaSb SESAMs is within ps range and is not much affected by a change of the growth parameters.

Femtosecond mode-locked semiconductor disk lasers (SDLs) have the potential to replace rather complex, expensive laser systems and to establish ultrashort-pulse applications outside of scientific laboratories. We report about almost Fourier-limited pulses with a duration close to 100 fs obtained in the single-pulse regime. The SDL cavity consisted of only three elements, an InGaAs/AlGaAs gain chip, a fast semiconductor saturable absorber mirror and an output coupler. The pulse in our mode-locked SDL was shaped mainly by the spectro-temporal behavior of saturable absorption and gain and the associated self-phase modulation. The group delay dispersion is small. Pulses as short as 107 fs were generated with a spectral width of 10.2 nm (FWHM), centered at 1030 nm. This results in a time-bandwidth product of ≈0.31, which is close to the transform-limit. The output power amounted to 3 mW at a pulse repetition rate of 5 GHz, corresponding to fundamental mode-locking. Harmonically mode-locked SDLs are capable of operating at much higher pulse repetition rates, which was also investigated. A maximum pulse repetition rate of 92 GHz was achieved while preserving the pulse duration shorter than 200 fs.

Optically-pumped semiconductor (OPS) lasers are power-scalable, wavelength-flexible, infrared brightness converters. Adding intra-cavity frequency doubling turns them into efficient, low noise, high power visible laser sources. We report on a laser combining an InGaAs gain medium with an LBO nonlinear crystal to produce more than 20 Watt CW in single transverse mode at 532 nm. Efficient cooling of the single gain chip using advanced mounting techniques is the key to making the laser reliable at high CW powers. A rugged and compact package withstands significant environmental excursions. The laser's low noise makes it suitable for demanding Ti:Sapphire pumping applications.

The latest achievements of quantum dot based semiconductor disk lasers are reviewed. Several lasers operating at 1040 nm - 1260 nm were studied. All the structures were grown with molecular beam epitaxy on GaAs substrates. The number of quantum dot layers was varied and the gain was provided either by the ground or the excited state transition of the quantum dots. Frequency doubling of the lasers was demonstrated and the dual-gain laser geometry was found to be practical solution for intracavity frequency conversion. Intracavity heat spreader and thinned device heat management approaches are studied and compared.

We review recent results concerning the development of dilute nitride based semiconductor disk lasers. We have demonstrated over 7.4 W of output power at the second harmonic wavelength (around 590 nm) using a β-BBO crystal. Over 10 W has been demonstrated at ~1.2 μm, and multi-watt output power has been achieved at 589 nm with narrow linewidth (δν < 20 MHz).

Strategies for power scaling VECSELs, including improving thermal management, increasing the quantum well gain/micro-cavity detuning that increases the threshold but increases roll-over temperature, and double-passing the excess pump via reflection from a metalized reflector at the back of a transparent distributed Bragg reflector (DBR) were studied. The influence of the heat spreader thickness and the pump profile on the temperature rise inside the active region was investigated using commercial finite element analysis software. Improvement was observed in optical efficiency of the VECSEL devices with a transparent DBR by double passing the pump light. Higher dissipated power at maximum output power was found in devices with larger spectral detuning between the quantum well gain and the micro-cavity detuning.

In this paper we report on the wavelength tuning of a VECSEL by changing the cavity geometry. The development and demonstration of a tunable high power single frequency Vertical External Cavity Surface Emitting Lasers (VECSEL) operating at various wavelengths from the UV to the IR region of the spectrum have been reported in many papers. However, it is important to understand that in many instances a precise lasing wavelength is required for proper operation. For example, VECSELs have been designed to specifically interact with the sodium spectral lines. If the VECSEL growth is not adequate, it may not be possible to reach the desired wavelength in a traditional cavity where the intracavity mode interacts with the VECSEL chip at normal incidence. Here we notice that if a fold angle is introduced at the VECSEL chip, a spectral blue shift occurs, and extended tunability may be possible. Therefore, by altering the cavity geometry it may be possible to further optimize a VECSEL design to obtain maximum output power at a desired wavelength.

The (AlGaIn)(AsSb) semiconductor materials system has been shown to be ideally suited to realize optically pumped Vertical External Cavity Surface Emitting Lasers (VECSELs) for the 2-3 μm wavelength range. For a 2 μm emitting single-chip VECSEL with a ternary GaInSb quantum well active region, employing a linear planar-concave resonator geometry, a maximum continuous wave (cw) output power of 4.2 W at 20°C heatsink temperature (6 W at 0°C) has been achieved. Standard fiber-coupled 980 nm diode lasers have been used for optical barrier pumping. Employing a W-shaped resonator with two optically pumped gain chips acting as planar folding mirrors, a maximum cw output power of 5.5 W has been achieved at a heatsink temperature of 20°C, increasing to 10.4 W when reducing the heatsink temperature to -10°C. The resonator versatility of a VECSEL also allows the insertion of additional optical elements into the optical cavity for wavelength selection and linewidth control. Employing a V-shaped folded cavity with the gain chip acting as the planar end mirror, single-mode operation with a linewidth <100 kHz at 1 W optical output power has been achieved at a lasing wavelength of 2.05 μm.

Here we report on the development and demonstration of a tunable high power single frequency Vertical External Cavity Surface Emitting Laser (VECSEL) operating at 589nm. A highly strained InGaAs/GaAs VECSEL designed to operate at ~ 1178nm is used in conjunction with an intracavity Birefringent Filter (BF) and low finesse Fabry-Perot (FP) etalon to achieve the single frequency operation at the fundamental wavelength. An internal non-linear optical element is then used to obtain the single frequency output at the desired wavelength of 589nm. Our results show outputs in excess of 4W at 589nm with a FWHM linewidth of the fundamental frequency to be less than 10MHz. We demonstrate the measurement of the sodium D1 and D2 lines by passing the output through a reference cell.

We present in this work the study of a short vertical external cavity semiconductor laser in single longitudinal operation at 852 nm without intracavity elements. Two different configurations were studied, a plane-plane configuration, stabilized by the thermal lens and a plane-concave configuration. The influence of the output coupler transmission and the thermal lens has been studied. In the plane concave configuration we have demonstrated more than 100mW in stable single frequency operation using a very compact cavity emitting around 852 nm.

In this paper we report on the development of narrow-linewidth vertical-external-cavity surface-emitting laser (VECSEL) at a wavelength of >2 μm. Starting from a laboratory setup, we designed a highly stable VECSEL module machined from a solid block of aluminum. For linewidth precise measurements, heterodyne beatnote measurements were employed. For this firstgeneration module a linewidth of 9 kHz was achieved when actively stabilizing the laser wavelength, whereas without stabilization the linewidth amounted to 45 kHz at an output power of 100 mW, both data referring to a 100-μs sampling time. To further increase the output power, a second-generation module was fabricated, for which the on-chip mode diameter was increased. This allowed operation at a larger pump-spot diameter and still maintaining TEM00 operation, while increasing the maximum pump power and hence the output power. This module yielded an output power above 1 W in single-mode operation at a linewidth of 60 kHz (100 μs sampling time) without active wavelength stabilization. Modehop-free single-mode operation could be maintained for more than 18 hours. This new multiple-Watt, narrow-linewidth VECSEL module is apt for plane-to-ground communications without the necessity of amplifiers.

Optically pumped VECSEL (vertical external cavity surface emitting lasers) based on IV-VI semiconductors grown on Si cover the entire wavelength range between 3.0 and 10 μm. Thanks to their simple structure and large wavelength coverage they are an interesting alternative laser technology to access the mid-infrared wavelength region. The active layers consist either of homogeneous "bulk" layers, double heterostructures or quantum well structures of the PbSe, PbTe or PbS material system. Maximum operation temperatures of 325 K are achieved with output powers above 200 mW_p. Further, continuously tunable VECSEL are presented, emitting between 3.2 and 5.4 μm. The single emission mode is continuously tunable over 50-100 nm around the center wavelength, yielding an output power > 10 mW_p. The axial symmetric emission beam has a half divergence angle of < 3.3°.

The microscopic theory for the nonequilibrium optical properties of VECSELs is summarized. Detailed experiments of VECSELs under two-color operation conditons are performed utilizing streak camera measurements of the laser output. A statistical analysis reveals the stability range of two-color emission and shows that this operation mode is possible even in the presence of relatively large losses.

The paper reports on M Squared Lasers' recent work in the field of narrow-linewidth, continuous wave (CW) optical parametric oscillators (OPO) that utilise the semiconductor disk laser (SDL) format for intra- and extracavity pumping schemes. For the experimental investigation, InGaAs quantum well gain structures were employed to produce compact SDLs which were subsequently used to pump OPOs based on periodically poled lithium niobate (PPLN). Using the intracavity architecture, a compact, CW, narrow-linewidth OPO was developed, capable of delivering up to 250 mW of narrow-linewidth idler output power (at ~3.3 μm). The SDL maintained stable single longitudinal mode operation and the spectral bandwidth of the OPO output was measured to be <0.14nm. A full discussion of the experimental set-up and results will be presented, together with a review of the potential issues and limitations in pursuing the development of intracavity pumped, single frequency OPOs. In addition to the intracavity work, extracavity approaches will also be discussed and their performance compared to those produced from the intracavity device. For the purpose of pumping singly resonant extracavity PPLN OPOs, high power (>15W), single frequency SDLs were developed using single- and multichip configurations. The extracavity OPO pumped by these sources delivered up to 950 mW of idler output power, with a minimum threshold of 2.4 W and tuning range from 2.8 μm to 3.6 μm. Further details of the high power, single frequency SDLs as well as the extracavity OPO will be discussed, with specific focus on the advantages and disadvantages of multi-chip and single-chip arrangements for single frequency SDLs.

We present the design, technology and performances of a tunable Sb-based diode-pumped type-I quantum-well vertical-external-cavity surface-emitting lasers emitting at 2.7μm. The half VCSEL structure was grown by MBE with quantum-well growth temperature of 440 °C. The sample was thermally annealed to optimize the QW gain design. We report on room-temperature continuous-wave laser with 0.17mW output power and low threshold incident pump intensity of 0.7kW/cm2 while pumping at 830nm with a commercial diode laser. The external cavity provides a circular TEM00 beam with a low divergence of 3.6°. The short mm-long optical cavity laser exhibits tunable single frequency operation, with a side mode suppression ratio 23 dB, a linewidth 4GHz and a linear light polarization. Thermal and optical properties are studied.

The wide range of applications in biophotonics, television or projectors, spectroscopy and lithography made the vertical external cavity surface-emitting lasers an important category of power scalable lasers. The possibility of bandgap engineering, inserting frequency selective and converting elements into the external laser cavity and laser emission in the fundamental Gaussian mode leads to ongoing growth of the area of applications for tunable laser sources. We present an intra cavity frequency-doubled VECSEL with emission wavelength around 330 nm and a maximum tuning range of more than 7nm with output powers exeeding 100mW. Frequency doubling is realized with an anti-reflection coated beta barium borate crystal, while a birefringent filter, placed inside the laser cavity under Brewster's angle, is used for frequency tuning. The fundamental laser, pumped by a 532nm Nd:YAG laser under an angle of 50° normal to the surface, is realized by a multi quantum well structure consisting of 20 compressively strained GaInP quantum wells in an AlxGa_1-xInP separate confinement heterostructure and it emits around 660 nm. The VECSEL-chip with its n-λ cavity is completed by a 55 λ/4 pairs Al_0.50Ga_0.50As/AlAs distributed Bragg reflector. Next to the optical properties of the device, we show results of different arrangements of the quantum wells, namely five times four and ten times two packages.

We present the performance of a compact, non expensive, easy to use ultrafast semiconductor disk laser (1W average power, 1.5 ps, 500MHz) for multiphoton imaging. The laser's operating wavelengths of 970 nm makes it ideal for nonlinear excitation of GFP as it has a two-photon action cross section peak at this wavelength. This property relaxes the required peak powers for TPEF imaging. We show the suitability of this laser for in-vivo imaging with GFP and other dyes; at different penetration depths; time-lapse studies and SHG imaging. The laser performance is evaluated in commercial microscopes and in comparison with Ti:sapphire lasers.

Quantum well femtosecond pulse VECSELs are ideally suited to generate pulse trains at GHz repetition frequencies. Here I will report on recent progress in developing laser systems which exploit the unique features of the VECSEL, such as 'class A' dynamics and the short upper level lifetime of the gain. Continuous tunable repetition rate operation through cavity length variation, covering the range 2.8 to 7.9 GHz is described. Pulses as short as 290 fs were achieved with femtosecond pulse operation between 2.8 and 5 GHz. To increase the repetition rate further, into the hundreds of GHz range, harmonic mode locking is investigated. We demonstrated 400 fs pulses at 175 GHz repetition frequency with a combined average output power of 300 mW.

We investigate experimentally and theoretically the influence of non-radiative carrier losses on the performance of VECSELs under pulsed and CW pumping conditions. These losses are detrimental to the VECSEL performance not only because they reduce the pump-power to output-power conversion efficiency and lead to increased thresholds, but also because they are strong sources of heat. This heating reduces the achievable output power and eventually leads to shut-off due to thermal roll-over. We investigate the two main sources of non-radiative losses, defect recombination and Auger losses in InGaAs-based VECSELs for the 1010nm-1040nm range as well as for InGaSb-based devices for operation around 2μm. While defect related losses are found to be rather insignificant in InGaAs-based devices, they can be severe enough to prevent CW operation for the InGaSb-based structures. Auger losses are shown to be very significant for both wavelengths regimes and it is discussed how structural modifications can suppress them. For pulsed operation record output powers are demonstrated and the influence of the pulse duration and shape is studied.

We present an overview of recent advances in generation of non-diffracting (Bessel) beams from surface-emitting lasers, such as electrically and optically pumped VECSELs, and discuss their applications in optical trapping/tweezing and manipulation of micromachines. Our experiments on VECSEL-generated watt power level Bessel beams with central lobe diameters of a few to tens micrometers suggest that the semiconductor surface-emitting lasers are the best candidates for replacement of gas and solid-state counterparts for power-demanding applications in optical manipulation.

Vertical external cavity surface emitting lasers (VECSELs) have proven themselves to be a suitable semiconductor answer to many solid-state lasers. Their simplicity makes them a very versatile platform for accessing wavelengths from the UV through the THz through direct and frequency-converted emission. This wavelength flexibility, combined with an optical cavity accommodating additional tuning or nonlinear elements, make the VECSEL a uniquely suited solution to a variety of applications. We will present recent AFRL progress in VECSELs and potential applications for these lasers.

We present gain characterization measurements for different VECSEL structures with quantum well (QW) and quantum dot (QD) layers in the active region. We use a high-precision reflectivity measurement setup to determine the change in reflectivity of the pumped gain chip with varying pulse fluences. In this way the gain saturation behavior and the smallsignal gain for several structures on different heat-spreaders were measured. The characterization was performed with femtosecond and picosecond pulses for varying pump powers and heat-sink temperatures. We measured a small-signal gain of up to 5% and saturation fluences in the range of 30-80 μJ/cm² for both QW and QD VECSELs. With an additional measurement setup we determined the gain spectra of two gain chips using a tunable cw probe beam. We measured gain bandwidths FWHM of 26 nm (QD-structure) and 30 nm (QW-structure).

We report a continuous wave 1 μm laser based on InAs Stranski-Krastanov quantum dots (SK-QD) which is optically pumped on a wetting layer absorption band at 915 nm. The slope efficiency of this laser relative to absorbed pump power was measured to be 56% with wetting layer pumping, 1.75 times larger than when pumped with 830 nm light absorbed into the barriers between the SK-QD layers. Compared to barrier pumping, wetting layer pumping benefits from a smaller quantum defect, with less heat deposited in the active region, at the expense of weaker pump absorption in the thin (~1 nm) wetting layer. When a 50 μm thick intracavity diamond heatspreader was contacted to the optically pumped gain structure, a 10-fold increase in output power, up to 2.25W, was obtained in the barrier pumped case. A much smaller 2-fold increase in power, to a maximum of 0.35 W, was seen for the wetting layer pumped case. The diamond heatspreader is more effective in removing heat from the active region, where it is deposited by barrier pumping, than from the substrate, which absorbs residual pump radiation in the barrier pumping case. A gain sample with a doubly periodic DBR to back reflect pump radiation, will allow the full potential of wetting layer pumping to be realised, both by increasing pump absorption due to the double pass through the active region, and by localising heat generation in the active region.

We present timing jitter measurements of an actively stabilized SESAM modelocked VECSEL generating 4-ps pulses with 2-GHz repetition rate and 20-mW average output power. The repetition rate was phase-locked to a reference source using a piezo actuator. The timing phase noise power spectral density of the laser output was measured with an Agilent E5052B Signal Source Analyzer. The resulting rms timing jitter integrated over an offset frequency range from 1 Hz to 1 MHz gives a timing jitter of below 80 fs, several times lower than previous modelocked VECSELs and comparable to the noise performance of ion-doped solid-state-lasers.

We present high average power femtosecond VECSELs based on both quantum dot (QD) and quantum well (QW) gain with extremely low dispersion. 1.05 W in 784-fs pulses could be achieved from a QD-VECSEL modelocked by a QDSESAM with fast recovery dynamics. A similar QW-gain structure modelocked by the same SESAM enabled stable 480-fs with an average output power of 300 mW at a repetition rate of 7 GHz. Furthermore, we investigated repetition rate scaling by changing the cavity length. We demonstrated fundamentally modelocked pulses over a tuning range from 6.5 GHz to 11.3 GHz. Without any realignment of the cavity over the whole tuning range, the pulse duration remained nearly constant around 625 fs (±3.5%) while the output power was 169 mW (±6%). The center wavelength changed only about ±0.2 nm around 963.8 nm. A tunable repetition rate can be of interest for various metrology application such as optical sampling by laser cavity tuning.

We analyze the effect of strain compensation on the crystalline quality of InGaAs/GaAs quantum well gain mirrors that are designed for emission above 1100 nm. The gain mirrors used in this study were grown by molecular beam epitaxy and they utilize GaAsP strain compensation. The effect of strain compensation has been assessed by measuring the curvature of the wafers and by mapping photoluminescence to identify non-emissive dark areas. We present that about 70 % strain compensation is sufficient to prevent dark line defect generation for gain mirrors designed for up to 1170 nm operation. Rapid thermal annealing studies revealed that the strain compensation is efficient in preventing appearance of dark lines even for samples that have been annealed at temperatures as high as 700 °C for a considerable time. Finally, we demonstrate multi-watt operation at 1115-1190 nm using 70-90 % strain compensated gain mirrors.

We report a mode-locked Vertical-External-Cavity Surface-Emitting Laser (VECSEL) that exhibits 13.7 nm of tuning around a centre wavelength of 1042 nm. The wavelength tuning is achieved by incorporating an uncoated, 25 μm thick, fused silica etalon into the cavity of the laser at Brewster's angle. The etalon is then tilted with respect to the cavity axis. The etalon has a calculated free spectral range of 14 nm at normal incidence. The repetition rate of the laser is measured to be 1.88 GHz. The pulse duration, averaged over the tuning range, is 1.9 ps corresponding to a mean time bandwidth product of 0.46. For a sech² pulse this is 1.46 times larger than the transform limit. The average power of the laser does not fall below 2.6 mW and, over the tuning range, averages 3.5 mW. With appropriate amplification, such a laser would be highly suited to the generation of heralded single photons in photonic crystal fibre.

VECSELs are excellent high power semiconductor lasers with diffraction-limited circular output beam and outstanding modelocking performance. The output power can be scaled up by simply increasing the mode area on the gain region. Electrical pumping requires doped layers and also requires changes in the epitaxial design. Crucial for high power operation is a low electrical resistance, because electrical power heats the device. The p-doped mirror gives the largest contribution to the electrical resistance. There are certain possibilities to reduce the resistance while keeping the optical losses as low as possible. Among these techniques are graded interfaces and improved doping schemes.