Vertical-external-cavity Surface-emitting Semiconductor Laser Research Articles

Monolithic VECSEL for stable kHz linewidth.

Vertical-external-cavity surface-emitting semiconductor lasers (VECSELs) are of increasing interest for applications requiring ultra-coherence and/or low noise at novel wavelengths; performance that is currently achieved via high-Q, air-spaced resonators to achieve long intra-cavity photon lifetimes (for the so-called class-A low noise regime), power scaling and high beam quality. Here, we report on the development of a compact, electronically tunable, monolithic-cavity, class-A VECSEL (monolithic VECSEL) for ultra-narrow free-running linewidths. A multi-quantum-well, resonant periodic gain structure with integrated distributed Bragg reflector (DBR) was optically-bonded to an air-gap-free laser resonator created inside a right-angle fused-silica prism to suppress the influence of environmental noise on the external laser oscillation, thus achieving high stability. Mode-hop-free wavelength tuning is performed via the stabilized temperature; or electronically, and with low latency, via a shear piezo-electric transducer mounted on the top of the prism. The free-running linewidth, estimated via the frequency power spectral density (PSD), is sub-kHz over ms timescales and <1.9 kHz for time sampling as long as 1s, demonstrating at least two orders-of-magnitude improvement in noise performance compared to previously reported single frequency VECSELs. The stable, total internal reflection resonator concept is akin to the prevalent monolithic non-planar ring oscillator (NPRO), however the monolithic VECSEL has several important advantages: tailored emission wavelength (via semiconductor bandgap engineering), no relaxation oscillations, no applied magnetic field, and low requirements on the pump beam quality. This approach is power-scalable in principle and could be applied to VECSELs at any of the wavelengths from the visible to the mid-infrared at which they are already available, to create a range of robust, ultra-coherent laser systems with reduced bulkiness and complexity. This is of particular interest for remote metrology and the translation of quantum technologies, such as optical clocks, from research laboratories into real world applications.

Phase-Incoherent Photonic Molecules in V-Shaped Mode-Locked Vertical-External-Cavity Surface-Emitting Semiconductor Lasers

Passively mode-locked vertical external-cavity surface-emitting semiconductor lasers (VECSELs) composed of a gain chip and a semiconductor saturable absorber have been drawing much attention due to their excellent performance figures. In this work we investigate how localized structures and incoherent, non-locally bound, pulse molecules emerge in a long cavity VECSELs using a V-shaped cavity geometry. We show that these states are bistable with the laser off state and that they are individually addressable. Using a model based upon delay differential equations, we demonstrate that pulse clusters result from the cavity geometry and from the non-local coupling with the gain medium; this leads to locally independent, yet globally bound phase, incoherent photonic molecules. Using a multiple time-scale analysis, we derive an amplitude equation for the field that allows us to predict analytically the distance between the elements of a cluster.

Material research on the InGaAs-emitting-layer VECSEL grown on GaAs substrate

Wafers of InGaAs-emitting-layer vertical external cavity surface emitting semiconductor laser (VECSEL) gain chip and separate active region were grown on semi-insulator GaAs substrates by low pressure metal–organic vapor phase epitaxy (MOVPE). Photoluminescence (PL) wavelength of the active region could be adjusted linearly about 1nm for increasing 1sccm H2 flow rate through TMIn under AsH3 flow rates of 150sccm. The complicated surface-emitted PL signal of the VECSEL gain chip was strongly modulated by interferences within the multilayer and was interpreted by the aberrance of the quantum wells emission with a profile filtered by a micro-cavity resonance in the longitudinal confinement factor. Material tests of the VECSEL wafer showed the reflectivity of the DBR mirrors was in good agreement with the active region photoluminescence, and the wafer was obtained with high crystal quality.

Thermal Management for High-Power Single-Frequency Tunable Diode-Pumped VECSEL Emitting in the Near- and Mid-IR

We demonstrate a technological process for reducing thermal impedance of 1/2 vertical external-cavity surface-emitting semiconductor lasers (VECSEL) chips on both GaSb- and GaAs-based III-V materials for 1 and 2.3 μm emission, respectively. The structure is grown in reverse order and gold electroplated, and then the substrate is removed by wet etching. Gold allows us both to reduce the number of Bragg mirror pairs and to increase the material thermal conductivity leading to a strong reduction of thermal impedance. We then characterized the VECSEL laser emission.

Repetition-frequency-tunable mode-locked surface emitting semiconductor laser between 278 and 787 GHz

We report a repetition frequency tunable, passively mode-locked vertical-external-cavity surface-emitting semiconductor laser (VECSEL) with continuous repetition frequency tuning between 2.78 and 7.87 GHz using mechanical tuning of the laser cavity length. The laser emits near-transform-limited, sub-500-fs pulses over almost an octave tuning range between 2.78 and 5 GHz. At repetition rates above 6 GHz the pulse duration increases to ~2.5 ps. Over the entire tuning range the laser emits an average output power of 40 ± 5 mW in a fundamental transverse mode. The change in pulse duration highlights a change in the dominant modelocking mechanism which forms the pulses. At high repetition frequencies the pulse duration is set by the saturable absorber recovery time. At low repetition frequencies the fluence and peak intensity on the SESAM increases to a point where the fast pulse shaping mechanisms of the optical Stark effect and carrier thermalization dominate the pulse shortening.

Sub-MHz-Linewidth 200-mW Actively Stabilized 2.3-μm Semiconductor Disk Laser

We report on the realization of an actively stabilized single-frequency vertical-external-cavity surface-emitting semiconductor laser (VECSEL) at a wavelength of 2.3 μ m. The laser was locked to a high-finesse Fabry-Pérot interferometer with a free-spectral range of 1 GHz. From the error signal of the control feedback loop a laser linewidth of 390 kHz could be derived (1-s sampling time). Reducing the sampling time to 100 μs, a much narrower linewidth of 55 kHz was obtained, indicating the dominant effect of low-frequency technical noise. The output power exceeded 200 mW. By rotating the intracavity birefringent filter, a wide tuning range of 62 nm could be achieved. Changing the resonator length with the help of a piezoelectric transducer mounted to a cavity mirror, a modehop-free fine tuning range of 5.5 GHz was achieved.

Simultaneous dual-wavelength generation from vertical external-cavity surface-emitting semiconductor lasers

We demonstrate dual-wavelength emission from optically pumped vertical-external-cavity surface-emitting lasers (VECSELs) with two different gain mirror designs. The first gain mirror design (S1) generated an optical output power of 0.39 W at λS = 984 nm and 0.30 W at λL = 1042 nm. A VECSEL using the second gain mirror design (S2) generated a maximum optical output power of 0.75 W at λS = 966 nm and 1.38 W at λL = 1047 nm. VESCELs using either gain mirror design exhibited self-pulsating behaviour in the high output power regime. In the VECSEL using the S1 gain mirror design, selfpulsation started beyond the λS output power of 140 mW and λL output power of 115 mW. The corresponding output power limits in the VECSEL using the S2 gain mirror were 300 mW for both wavelengths. Keywords: vertical external-cavity surface-emitting lasers, resonant periodic gain structure, semiconductor lasers

7 W high-efficiency continuous-wave green light generation by intracavity frequency doubling of an end-pumped vertical external-cavity surface emitting semiconductor laser

The authors report the demonstration of highly efficient continuous-wave green light at 535nm generation by intracavity frequency doubling of an end-pumped vertical external-cavity surface emitting semiconductor laser (VECSEL). The beta-barium borate (β-BaB2O4) crystals, cut for critical type I phase matching at room temperature, are used for second harmonic generation of the laser. In combination with an end-pumped VECSEL and efficient intracavity frequency doubling with a folded three-mirror cavity, more than 7W green light output power is obtained at pump power of 26W. The optical conversion efficiency of 27% is achieved from pump power to second harmonic power.

9.1-W High-Efficient Continuous-Wave End-Pumped Vertical-External-Cavity Surface-Emitting Semiconductor Laser

We report the first demonstration of an end-pumped vertical-external-cavity surface-emitting semiconductor laser (VECSEL) with more than 9.1-W continuous-wave output power at a wavelength of 1079 nm. In order to enable the VECSEL to operate in an end-pumping scheme, a liquid capillary bonding method is used to bond the VECSEL chip to the surface of an optically transparent heat spreader. Moreover, a modified distributed Bragg reflector is applied to the VECSEL chip structure to maximize the pump efficiency. These will result in an end-pumped VECSEL that is characterized by high output power with good beam quality without additional antireflection coatings of the samples. In combination with an end-pumped scheme and desirable output beam, the end-pumped VECSEL will prove useful in a variety of applications requiring compact module and efficient source with high output power

50-GHz Passively Mode-Locked Surface-Emitting Semiconductor Laser With 100-mW Average Output Power

We have developed a passively mode-locked optically-pumped vertical-external-cavity surface-emitting semiconductor laser (VECSEL) which delivers up to 100 mW of average output power at a repetition rate of 50 GHz in nearly transform-limited 3.3-ps pulses at a wavelength around 960 nm. The high-repetition-rate passive mode locking was achieved with a low-saturation-fluence semiconductor saturable absorber mirror (SESAM) incorporating a single layer of quantum-dots. The output power within a nearly diffraction-limited beam was maximized using a gain structure with a low thermal impedance soldered to a diamond heat spreader. In addition, we systematically optimized the laser resonator to accommodate for the strong thermal lens caused by the optical pumping. We measured the thermal lens dioptric power and present a numerical model which is in good agreement with the measurements and is useful for optimizing resonator designs. The experimental setup is very versatile and its design and construction are discussed in detail

Mode-locking dynamics in electrically driven vertical-external-cavity surface-emitting lasers

We develop a novel description of electrically driven vertical-external-cavity surface-emitting semiconductor lasers (VECSELs) mode-locked by saturable absorber mirrors. Our approach is based on an analytical solution of the bidirectional traveling-wave equations for fundamental transverse mode operation. The resulting time-domain equations describe the evolution of the electric fields and carrier-densities at the quantum-well layers of the emitter and absorber structures which are coupled through delayed boundary conditions. For the design considered, we obtain stable mode-locked pulses of few tens of picoseconds at 15-GHz repetition rate in agreement with recently reported experimental results.

10 GHz passively mode-locked external-cavity semiconductor laser with 1.4W average output power

We present a 10GHz passively mode-locked vertical external-cavity surface-emitting semiconductor laser (VECSEL) with 1.4W average output power in 6.1ps pulses. The output features a very good pulse quality with a time–bandwidth product of 0.42 in a nearly diffraction-limited beam. This demonstrates that passively mode-locked VECSELs are suitable for generating high powers in high-repetition-rate pulse trains.

Sensitive instrumental developments in high-resolution laser and Fourier transform spectroscopies

Two recent instrumental developments involving laser and Fourier transform spectroscopies, and their applications are discussed. The first one is of metrological relevance, and provides a stable frequency reference in the near-infrared region thanks to diode laser instrumentation. The second one combines intracavity laser absorption spectroscopy (ICLAS) and time-resolved Fourier transform spectroscopy (TRFTS). Spectra around 1 μm, exhibiting kilometric absorption path length, are obtained from the high-resolution time-resolved step-scan interferometer of the Laboratoire de Photophysique Moléculaire (LPPM), with an intracavity laser absorption set-up based on a vertical-external-cavity surface-emitting semiconductor laser (VECSEL).

Actively stabilized single-frequency vertical-external-cavity AlGaAs laser

We report on actively stabilized single-frequency operation of a vertical-external-cavity surface-emitting semiconductor laser (VECSEL). The VECSEL was locked to a 300-MHz reference cavity allowing a relative frequency measurement that indicated a laser linewidth of 3 kHz. Coarse tuning over an 8.5-nm range was achieved, with fine tuning over 250 MHz. The laser produced up to 42 mW of output power in single-frequency operation.