This PDF file contains the front matter associated with SPIE Proceedings Volume 8606, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

The design of optically pumped semiconductor disk lasers is discussed with emphasis on the optimization for high power conversion e_ciencies. Main topics are the compensation of strain in the epitaxial layer sequence, the realization of a low-absorption Bragg reector which has a high reectivity for pump and laser wavelength and a low thermal resistance, and the e_ect of a surface coating reducing optical losses inside the semiconductor disk. As an alternative concept, quantum-well pumping may be more e_cient because of the reduced quantum defect. E_cient intra-cavity second-harmonic generation can be obtain in folded cavity setups.

We report high power operation of a vertical external-cavity surface-emitting laser (VECSEL) operating around 1180 nm. The gain chip of the VECSEL comprises 10 strain-compensated GaInAs/GaAs quantum wells in a top-emitting configuration. A maximum output power of 23 W was achieved with a mount temperature of about 0 ‡C, and 20.5 W with the mount temperature of about 12 °C. By introducing a birefringent filter inside the laser cavity we demonstrate a tuning range of 67 nm. The gain chip was also used to construct a VECSEL for single-frequency operation. In this configuration, a maximum output power of about 11 W was recorded.

The broad gain-bandwidth and the high output powers make vertical-external-cavity surface-emitting lasers (VECSELs) promising candidates as femtosecond laser sources. Besides an effective design of the gain structure, the major challenges for high power VECSELs are the thermal management of the chips as well as the homogeneity of the epitaxial growth. In this work, we present results of passively mode-locking of our highly efficient VECSELs and demonstrate femtosecond operation with record output powers and pulse energies. At a repetition rate of 1.7 GHz, we achieved nearly transform-limited pulses with 682 fs duration and an average output power exceeding 5 W.

Our femtosecond VECSELs have generated 1.05 W average output power. Numerical simulations have been successfully used to gain a better understanding, but initially have not predicted the average output power correctly. Only after we directly determined the correct gain parameters we got very good agreement. Numerical simulations show that weak gain saturation is beneficial for high-power operation. With a high-precision reflectivity measurement setup we measured the nonlinear change in reflectivity of the optically-pumped (OP) VECSEL gain chip as function of the incident pulse fluence, pump intensity, and heat-sink temperature. We also determined the small signal gain and the gain bandwidth.

Significant progress has been made over the last year towards generating frequency combs using VECSELs. Here, I will discuss recent progress made generating < 4kW peak power femtosecond pulse VECSELs, where we have achieved 3.3 W average power with 400 fs pulse duration at 1.7 GHz repetition rate. This has been achieved by exploiting the rapid power scaling progress made in the field of CW VECSELs [1]. The gain structure used here is grown and processed by the University of Marburg, and the window layer is etched for anti-resonance to increase the gain bandwidth and reduce the dispersion [2]. We have used this to generate supercontinuum, achieving 45 % throughput in a 2.2 micron core photonic crystal fiber when the VECSEL produced 1 W average output power. A continuum with a width of 175 nm is generated. At higher average powers heating of the fiber tip reduces coupling efficiency which limits the supercontinuum bandwidth and we will discuss measures to avoid this. Finally, I will outline approaches to further reduce the pulse length, whilst maintaining the average power, to a point where generating coherent octave spanning supercontinuum, suitable for F-2F stabilization should become a reality.

We present a mode-locked VECSEL emitting 400-fs pulses at a 3 GHz repetition rate at 1040 nm, amplified by a cascaded ytterbium doped fiber amplifier system to an average power of 40 W. The 3-ps duration amplified pulses are recompressed to their original 400-fs duration using a high-throughput transmission grating compressor. The recompressed pulses are used to generate supercontinuum with two different photonic crystal fibers (PCFs); an all-normal dispersion PCF and a PCF with a zero-dispersion wavelength at 1040 nm, creating spectra with 20 dB bandwidths of 200 nm with 3.9 W average power and 280 nm with 2.5 W average power respectively.

Laser cooling of solids to 148 K has been demonstrated in a Yb:YLF crystal using intracavity absorption enhancement in an InGaAs MQW VECSEL at 1020 nm. This is the lowest temperature achieved in the intracavity geometry to date and presents a significant advancement towards an all-solid-state compact cryocooler.

In this paper electro-optic cavity dumping of a 2 μm semiconductor disk laser is reported. Using this approach, pulsed mode operation with 3 ns pulse length, 30 W of peak power and pulse repetition frequencies between 87 kHz and 1 MHz has been achieved. For cavity dumping, a birefringent polarizer prism was inserted into the V-shaped external cavity formed by high-reflectivity mirrors, in order to establish a linear polarization of the intra-cavity field. The polarization of the intra-cavity field could be rotated by 90° when applying a voltage to a Pockels cell placed inside the cavity close to the end mirror. Photons with rotated polarization undergo then total internal reflection inside the polarizing prism and are coupled out of the cavity sideways. Furthermore, a model based on standard laser rate equations has been developed, which excellently reproduces the measured pulse shape and temporal evolution of the intracavity power. Based on this model, we have studied the pulse length and shape as well as peak power and pulse energy as a function of cavity length in order to explore routes for further increasing peak output power and pulse energy.

Lasers with emission wavelength around 2 _m have been traditionally based on InGaSb quantum wells grown on GaSb. An alternative is to use self assembled InAs Quantum Dashes grown on InP by the Stranski-Krastanov growth mode. More speci_cally, InAs quantum dashes embedded in strained GaInAs quantum wells, grown in InAlGaAs waveguides lattice matched to InP substrates have been successfully used as active medium in edge emitting lasers with wavelengths in the range from 1.45 _m to 2.1 _m. Advantages of this material system compared to the GaSb based system include easier lattice matching; i.e. only one group V element is involved. Many optoelectronic properties of the InAs/InP quantum dash material system are similar to those of InAs quantum dots grown on GaAs substrates. The latter material system has been very successfully used for VECSELs in the wavelength region around 1 _m, leading to the highest power VECSEL at this wavelength, mode locking, wide range tunability as well as intra cavity SHG to generate red light. A challenge in the material system based on InP substrates is to fabricate a DBR. A lattice-matched DBR can consist of GaAsSb/AlAsSb. Alternatively one can grow a metamorphic DBR based on either GaAs/AlAs or GaSb/AlSb. The latter requires the DBR to be grown after the active region. The resultant VECSEL is then a bottom emitter, where the substrate has to be removed. This can be achieved by introducing an etch stop layer between substrate and active region. Lastly, the DBR can be grown separately and subsequently wafer bonded to the active region. This paper will discuss details of these technologies and present results.

The antimonide based vertical external cavity surface emitting lasers (VECSELs) operating in the 1.8 to 2.8 Tm wavelength range are typically based on InGaAsSb/AlGaAsSb quantum wells on AlAsSb/GaSb distributed Bragg reflectors (DBRs) grown lattice-matched on GaSb substrates. The ability to grow such antimonide VECSEL structures on GaAs substrates can take advantage of the superior AlAs based etch-stop layers and mature DBR technology based on GaAs substrates. The growth of such III-Sb VECSELs on GaAs substrates is non-trivial due to the 7.78% lattice mismatch between the antimonide based active region and the GaAs/AlGaAs DBR. The challenge is therefore to reduce the threading dislocation density in the active region without a very thick metamorphic buffer and this is achieved by inducing 90 ° interfacial mist dislocation arrays between the GaSb and GaAs layers. In this presentation we make use of cross section transmission electron microscopy to analyze a variety of approaches to designing and growing III-Sb VECSELs on GaAs substrates to achieve a low threading dislocation density. We shall demonstrate the failure mechanisms in such growths and we analyze the extent to which the threading dislocations are able to permeate a thick active region. Finally, we present growth strategies and supporting results showing low-defect density III-Sb VECSEL active regions on GaAs.

Recent developments of wafer-fused long-wavelength VECSELs resulted in reaching record high CW output power of 6.6 W at 1300 nm and a coherence length longer than 5 km in fiber and 1 Watt of output power in single frequency regime at 1550 nm. First wafer-fused electrically pumped VECSELs emitting at 1470 nm demonstrate maximum CW output power of 6.5 mW which represents more than 10-times improvement compared with previously published results.

In modelocked electrically pumped VECSELs (EP-VECSELs) the gain saturation strongly influences the pulse formation. Here we present a detailed gain characterization of EP-VECSELs as published the first time in [1]. The spectral gain-distribution and the gain saturation behavior of two devices with different field-enhancement in the quantum-well gain layers are investigated. Comparing spectral bandwidth, small-signal gain and saturation fluence of the three devices, we chose the most suitable for modelocking experiments. Using a low-saturation fluence SESAM we have generated 9.5-ps-pulses with an average output-power of 7.6 mW at 1.4 GHz repetition-rate, which have been the shortest pulses from an EP-VECSEL to date [1].

We present a model and results of simulations and experiments investigating the L-I characteristics of electrically pumped (EP-) VECSELs in the single- and multi-mode regime. In our model we use a mode expansion ansatz to treat the electromagnetic field inside the VECSEL cavity. The eigenmodes of the passive cavity are computed using the bidirectional beam propagation method (BDBPM) to solve the Helmholtz equation. The BDBPM allows us to account for the complex refractive index distribution within the semiconductor heterostucture, composed of approximately thousand interfaces along the optical axis in addition to lateral refractive index variations in oxide-confined devices as well as the macroscopic external cavity. We simulate the time evolution of the modal powers of several transverse modes and the spatial distribution of the inversion carriers in the quantum well plane. Therefore we solve an differential equation system composed of multimode rate equations and the carrier diffusion equation. With this ansatz we are able to identify cavity geometries suitable for single-mode operation assuming typical current profiles that are taken from photoluminescence measurements of the devices under investigation. Furthermore, we identify effects limiting the single-mode efficiency, such as poor gain and mode matching, reabsorption in unpumped regions of the quantum wells or enhanced carrier losses due to strong spatial hole burning. Critical parameters of the equations, such as optical losses, injection effciency, carrier recombination constants and gain parameters are obtained from experiments, microscopic models and literature. The simulation results are compared to experimental results from EP-VECSELs from Philips Technologie GmbH U-L-M Photonics.

Due to the architectural advantage of an external cavity architecture that enables the integration of additional elements into the cavity (e.g. for mode control, frequency conversion, wavelength tuning or passive mode-locking) VECSELs are a rapidly developing laser technology. Nevertheless they often have to compete with direct (edge) emitting laser diodes which can have significant cost advantages thanks to their rather simple structure and production processes. One way to compensate the economical disadvantages of VECSELs is to optimize each component in terms of quality and costs and to apply more efficient (batch) production processes. In this context, the paper presents recent process developments for the automated assembly of VECSELs using a new type of desktop assembly station with an ultra-precise micromanipulator. The core concept is to create a dedicated process development environment from which implemented processes can be transferred fluently to production equipment. By now two types of processes have been put into operation on the desktop assembly station: 1.) passive alignment of the pump optics implementing a camera-based alignment process, where the pump spot geometry and position on the semiconductor chip is analyzed and evaluated; 2.) active alignment of the end mirror based on output power measurements and optimization algorithms. In addition to the core concept and corresponding hardware and software developments, detailed results of both processes are presented explaining measurement setups as well as alignment strategies and results.

We report the effects of cryogenic temperatures on the performance of two CW VECSELs. Firstly we make use of a liquid nitrogen cooled copper cold finger cryostat to house an entire VECSEL device within the vacuum space and report a 3.3 fold reduction in lasing threshold and a 3.6 times increase in the slope efficiency when the gain chip is held at 133 K compared to 293 K. Secondly we show that a VECSEL utilising an unprocessed gain chip and pumped with constant pump-spot size, exhibits thermal rollover at twice the incident pump power when held at 20 K and compared to operation at 293 K. This enhanced performance will motivate the development of processed gain chips that are robust against thermal contraction.

This paper was inadvertently published and has been withdrawn by the publisher.

NonLinear Microscopy techniques, such as Two-Photon Excited Fluorescence and Second Harmonic Generation provide advantages over conventional Confocal Laser Scanning Microscopy. A key element in a NonLinear Microscope is an ultrafast laser which produces short pulses with the high intensities needed for exciting nonlinear processes. Semiconductor Disk Lasers potentially offer an alternative to expensive Ti:Sapphire lasers. The reported 200MHz operation of a modelocked Semiconductor Disk laser is to our knowledge the lowest repetition rate as yet demonstrated.

We report a 1-μm mode-locked VECSEL using a 4.5λ/2 antiresonant microcavity gain structure. The pulses were generated using two different semiconductor saturable absorber mirrors (SESAMs). The first was grown for operation at 1000 nm and the SESAM was heated to 85 °C and the gain cooled to -23 °C to wavelength match the gain and absorber. The second SESAM was designed for 1030 nm and mode-locked operation was achieved with both gain and SESAM at -12 °C. The first approach generated 205-fs pulses with an average power of 2 mW, the second 260-fs pulses with an average power of 13 mW.

This paper was inadvertently published and has been withdrawn by the publisher.

This work consists in an in-depth analysis of the VeCSELs beam coherence thanks to the physical study of the noise properties for both the frequency and the intensity of the optical beam.

We demonstrate the utility of optically pumped semiconductor lasers (OPSLs) in the eld of precision atomic spectroscopy. We have constructed an OPSL for the purpose of laser-cooling and trapping neutral Hg atoms. The OPSL lases at 1015 nm and is frequency quadrupled to provide the trapping light for the ground state cooling transition. We report up to 1.5 W of stable, single-frequency output power with a linewidth of < 70 kHz with active feedback. From the OPSL we generate deep-UV light at 253.7 nm used to form a neutral Hg magneto-optical trap (MOT). We present details of the MOT. We also report initial results for spectroscopy of the 6¹S₀ - 6³P₀ clock transition in the Hg¹⁹⁹ isotope.

We describe the dual-frequency operation of an optically-pumped vertical external cavity semiconductor laser (VECSEL) stabilized onto an Cs atomic transition. It is based on the simultaneous emission of two cross-polarized adjacent longitudinal modes inside the same laser cavity, which provides a strong correlation between the two laser lines. The frequency difference, in the GHz range, is fixed by the intracavity phase anisotropy, and precisely tuned with an electro-optic modulator (EOM). For this work, we additionally take benefit of the class-A dynamical behaviour of VECSEL which results in a shot-noise limited relative-intensity-noise on a wide spectral range.

The GaAs/AlGaAs active structure is pumped with a 1W-fiber coupled laser diode at 670 nm. The laser cavity has been carefully designed for improved thermal and mechanical stability, and compactness. It consists in a 15-mm concave output coupler, a glass Fabry-Perot etalon, a YVO₄ birefringent plate and a MgO:SLT EOM. The output power at each frequency reaches 20 mW. The frequency difference is phase-locked to a microwave reference source through the EOM voltage with a MHz bandwidth, resulting in a high-purity optically-carried microwave signal. Simultaneously, one laser line is locked on a Cs atomic hyperfine transition at 852 nm through a low-bandwidth servo-loop on the cavity length. The performance of our laser source is thus fully compatible with the excitation of Cs atoms in coherent population trapping atomic clocks.

An efficiency improvement of 87% is demonstrated in cooling of concentrated heat loads when using thermoelectric coolers (TECs) constructed with thermally conductive printed circuit boards (TCPCBs) as compared to traditional ceramic-based TECs. Laser diodes and infrared detectors must be actively cooled but are smaller than typical TECs. As a result, heat spreading must occur between the optical component and the semiconductor pellets near the edge of the TEC. Typically, TECs based on aluminum nitride circuit boards are chosen and in some cases an AlN plate is added between the optical component and the TEC. To address this, TECs have been developed that replace the ceramic circuit boards with laminated TCPCBs containing a thick copper backing. The copper backing improves heat spreading within the TEC. A study was conducted to quantify differences in coefficient of performance (COP, heat pumped divided by electrical power consumed) when cooling concentrated heat loads. A heat source 3 mm wide was cooled by TECs ~12 mm wide, comparing ceramic-based and TCPCB-based TECs of otherwise identical design. With a fixed hot side temperature and heat load, each TEC was powered to achieve a desired temperature at the heat source. Ceramic-based and TCPCB-based TECs exhibited COPs of 0.235 and 0.440 respectively, an 87% improvement. Further improvements are achievable: adding a thick copper plate between the heat source and the TEC resulted in a COP of ~0.59 for both TEC types.