Vertical External Cavity Surface Emitting Laser Research Articles

Switchable double wavelength generating vertical external cavity surface-emitting laser

Switchable, double wavelength generation is demonstrated from a single vertical external cavity surface-emitting laser chip. Power of ~0.5 W for two wavelengths λ≈967 nm and 1,018 nm i.e. within the spectral distance of 51 nm were registered. In the semiconductor heterostructure a single set of nominally identical quantum wells was enclosed in a single, two-mode resonant microcavity. The wavelength switching was induced by the change of the pump power. The increase or decrease of the pump power changes the active region temperature and thus tunes spectrally the gain spectrum to the one of two modes.

Nonequilibrium and thermal effects in mode-locked VECSELs

Ultrafast femtosecond timescale dynamics in Vertical External Cavity Surface Emitting Lasers (VECSELs) have recently been employed to achieve record average power and duration mode-locked pulses by employing different types of saturable absorbers and Kerr Lens elements. Microscopic many-body dynamics are expected to dominate when attempting to push pulse durations below 100 fs. We present a preliminary microscopic simulation of ultrafast mode-locking in order to expose the role of hot carrier distributions in establishing ultrafast mode-locking.

Pulse repetition rate scaling from 5 to 100 GHz with a high-power semiconductor disk laser

The high-power semiconductor laser studied here is a modelocked integrated external-cavity surface emitting laser (MIXSEL), which combines the gain 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. The MIXSEL concept allows for stable and self-starting fundamental passive modelocking in a simple straight cavity and the average power scaling is based on the semiconductor disk laser concept. Previously record-high average output power from an optically pumped MIXSEL was demonstrated, however the long pulse duration of 17 ps prevented higher pulse repetition rates and many interesting applications such as supercontinuum generation and broadband frequency comb generation. With a novel MIXSEL structure, the first femtosecond operation was then demonstrated just recently. Here we show that such a MIXSEL can also support pulse repetition rate scaling from ≈5 GHz to >100 GHz with excellent beam quality and high average output power, by mechanically changing the cavity length of the linear straight cavity and the output coupler. Up to a pulse repetition rate of 15 GHz we obtained average output power >1 W and pulse durations <4 ps. Furthermore we have been able to demonstrate the highest pulse repetition rate from any fundamentally modelocked semiconductor disk laser with 101.2 GHz at an average output power of 127 mW and a pulse duration of 570 fs.

Photon-pair generation in photonic crystal fibre with a 1.5 GHz modelocked VECSEL

Four-wave mixing (FWM) in optical fibre is a leading technique for generating high-quality photon pairs. We report the generation of photon pairs by spontaneous FWM in photonic crystal fibre pumped by a 1.5GHz repetition-rate vertical-external-cavity surface-emitting laser (VECSEL). The photon pairs exhibit high count rates and a coincidence-to-accidental ratio of over 80. The VECSEL׳s high repetition-rate, high average power, tunability, and small footprint make this an attractive source for quantum key distribution and photonic quantum-state engineering.

Spectroscopy of 14NH3 and 15NH3 in the 2.3 μm spectral range with a new VECSEL laser source

A spectroscopic study of ammonia molecule in the 2.3μm water atmospheric window is presented. Compared to previous experimental works a narrow, broadly tunable, continuous laser source was applied in order to obtain high resolution spectra in this range. It was the first application of the newly developed Vertical External Cavity Surface Emitting Laser (VECSEL). The analysis of the absorption spectra of two different isotopologue admixtures allowed to identify 336 14NH3 and 244 15NH3 lines in the studied region from 4275 to 4340cm−1. These data were compared to previous line list based on experimental data – the HITRAN database and the work of Urban et al., as well as to the theoretical predictions by Yurchenko et al. and Huang et al. Together for 235 14NH3 and 183 15NH3 lines a complete assignment was proposed. From those 63 14NH3 and 114 15NH3 transitions were newly assigned. The complete peaklist of transitions containing ro-vibrational state identifications is presented in Supplementary materials.

Double-diamond high-contrast-gratings vertical external cavity surface emitting laser

A new design of vertical external cavity surface emitting laser (VECSEL) with diamond-based high contrast gratings is proposed. The self-consistent model of laser operation has been calibrated based on experimental results and used to optimize the new proposed device and to perform comparative thermal and optical analysis of conventional and double-diamond high-contrast-grating VECSELs. The proposed design considerably reduces the dimensions and complexity of the device and provides up to 80% increase of the maximum emitted power as compared with the conventional design.

Control of light polarization using optically spin-injected vertical external cavity surface emitting lasers

We fabricated and characterized an optically pumped (100)-oriented InGaAs/GaAsP multiple quantum well Vertical External Cavity Surface Emitting Laser (VECSEL). The structure is designed to allow the integration of a Metal-Tunnel-Junction ferromagnetic spin-injector for future electrical injection. We report here the control at room temperature of the electromagnetic field polarization using optical spin injection in the active medium of the VECSEL. The switching between two highly circular polarization states had been demonstrated using an M-shaped extended cavity in multi-modes lasing. This result witnesses an efficient spin-injection in the active medium of the LASER.

Exploring ultrafast negative Kerr effect for mode-locking vertical external-cavity surface-emitting lasers

We present analytical considerations of "self-mode-locked" operation in a typical vertical external-cavity surface-emitting laser (VECSEL) cavity geometry by means of Kerr lens action in the semiconductor gain chip. We predict Kerr-lens mode-locked operation for both soft- and hard-apertures placed at the optimal intra-cavity positions. These predictions are experimentally verified in a Kerr-lens mode-locked VECSEL capable of producing pulse durations of below 500 fs at 1 GHz repetition rate.

Phase Noise of the Radio Frequency (RF) Beatnote Generated by a Dual-Frequency VECSEL

We analyze, both theoretically and experimentally, the phase noise of the radio frequency (RF) beatnote generated by optical mixing of two orthogonally polarized modes in an optically pumped dual-frequency Vertical External Cavity Surface Emitting Laser (VECSEL). The characteristics of the RF phase noise within the frequency range of 10 kHz - 50 MHz are investigated for three different nonlinear coupling strengths between the two lasing modes. In the theoretical model, we consider two different physical mechanisms responsible for the RF phase noise. In the low frequency domain (typically below 500 kHz), the dominant contribution to the RF phase noise is shown to come from the thermal fluctuations of the semicondutor active medium induced by pump intensity fluctuations. However, in the higher frequency domain (typically above 500 kHz), the main source of RF phase noise is shown to be the pump intensity fluctuations which are transfered to the intensity noises of the two lasing modes and then to the phase noise via the large Henry factor of the semiconductor gain medium. For this latter mechanism, the nonlinear coupling strength between the two lasing modes is shown to play an important role in the value of the RF phase noise. All experimental results are shown to be in good agreement with theory.

Lead-chalcogenide mid-infrared vertical external cavity surface emitting lasers with improved threshold: Theory and experiment

Mid-infrared Vertical External Cavity Surface Emitting Lasers (VECSEL) based on narrow gap lead-chalcogenide (IV-VI) semiconductors exhibit strongly reduced threshold powers if the active layers are structured laterally for improved optical confinement. This is predicted by 3-d optical calculations; they show that lateral optical confinement is needed to counteract the anti-guiding features of IV-VIs due to their negative temperature dependence of the refractive index. An experimental proof is performed with PbSe quantum well based VECSEL grown on a Si-substrate by molecular beam epitaxy and emitting around 3.3 μm. With proper mesa-etching, the threshold intensity is about 8-times reduced.

Femtosecond pulses from a modelocked integrated external-cavity surface emitting laser (MIXSEL)

Novel surface-emitting optically pumped semiconductor lasers have demonstrated >1 W modelocked and >100 W continuous wave (cw) average output power. The modelocked integrated external-cavity surface emitting laser (MIXSEL) combines the gain of vertical-external-cavity surface-emitting lasers (VECSELs) with the saturable absorber of a semiconductor saturable absorber mirror (SESAM) in one single semiconductor structure. This unique concept allows for stable and self-starting passive modelocking in a simple straight cavity. With quantum-dot based absorbers, record-high average output power was demonstrated previously, however the pulse duration was limited to 17 ps so far. Here, we present the first femtosecond MIXSEL emitting pulses with a duration as short as 620 fs at 4.8 GHz repetition rate and 101 mW average output power. The novel MIXSEL structure relies on a single low temperature grown quantum-well saturable absorber with a low saturation fluence and fast recovery dynamics. A detailed characterization of the key modelocking parameters of the absorber and the challenges for absorber integration into the MIXSEL structure are discussed.

Growth and testing of vertical external cavity surface emitting lasers (VECSELs) for intracavity cooling of Yb:YLF

Optically-pumped vertical external cavity surface emitting lasers (VECSELs) have unique characteristics that make them attractive for use in intracavity optical cooling of rare earth doped crystals. We present the development of high power VECSELs at 1020nm for cooling ytterbium-doped yttrium lithium fluoride (Yb:YLF). The VECSEL structures use AlAs/GaAs distributed Bragg reflectors and InGaAs/GaAsP resonant periodic gain epitaxially grown by metal-organic vapor phase epitaxy. To achieve the necessary output power, we investigated thinning the substrate to improve the thermal characteristics. We demonstrated a VECSEL structure that was grown inverted, bonded to the heat sink, and the substrate removed by chemical etching. The inverted structure allows us to demonstrate 15W output with 27% slope efficiency. Wavelength tuning of 30nm around 1020nm was achieved by inserting a birefringent quartz window into the cavity. The window also narrows the VECSEL emission, going from a FWHM of 5nm to below 0.5nm at a pump power of 40W.

Intracavity Nonlinear Frequency Down-Conversion in a Continuous-Wave Operation Regime of a Dual-Wavelength Vertical-External-Cavity Surface-Emitting Laser

Steady-state operational characteristics of a dual-wavelength vertical-external-cavity surface-emitting laser (VECSEL) with a nonlinear quasi-phase-matched crystal placed in the laser cavity have been simulated. We have demonstrated the dramatic effect of three-wave nonlinear optical interaction on both the intensity of fundamental optical fields and the carrier concentration in the quantum wells of the laser. Potential capability of the dual-wavelength VECSEL for efficient generation of mid- or far-infrared radiation by means of the intracavity nonlinear frequency conversion has been shown.

Ultrafast Electrically Pumped VECSELs

We present an improved design for electrically pumped vertical external-cavity surface-emitting lasers (EP-VECSELs) optimized for passive modelocking with a semiconductor saturable absorber mirror (SESAM). In continuous-wave (cw) multimode operation, up to 170 mW of output power are demonstrated at a wavelength of 967 nm. Overcoming our prior limitation, we demonstrate fundamental transverse mode operation with 26 mW of average output power. Passively modelocking one of the fabricated EP-VECSELs with a quantum-well SESAM, we have achieved 6.3-ps pulses with an average output power of 6.2 mW at 1.46-GHz repetition rate, the shortest pulse duration from this type of laser to date. With slightly longer pulses of 7.3 ps, an average output power of 13 mW is obtained. Tradeoffs for further output power scaling and reduced pulse duration are discussed.

Growth and Optimization of 2-μm InGaSb/AlGaSb Quantum-Well-Based VECSELs on GaAs/AlGaAs DBRs

We report the growth of optically pumped vertical-external-cavity surface-emitting lasers (VECSELs) based on InGaSb/AlGaSb quantum wells grown on GaAs/AlGaAs distributed Bragg reflectors (DBRs). The 7.78% lattice mismatch between GaSb and GaAs is accommodated by an array of 90° misfit dislocations at the interface. This results in spontaneous relaxation of the GaSb epilayer and also significantly reduces the threading dislocation density. The VECSELs are operated in both pulsed (with 340-W peak output power) and continuous wave mode (with 0.12-W peak output power). We investigate the effects of the GaSb/GaAs interface by comparing the lattice mismatched III-Sb VECSEL grown on GaAs/AlGaAs DBRs to a lattice matched III-Sb VECSEL grown on GaSb/AlAsSb DBRs. The lattice matched VECSEL outperforms the lattice mismatched VECSEL in terms of threshold pump density, efficiency, and maximum continuous-wave output power. This can be attributed to the presence of threading dislocations throughout the active region of the mismatched VECSEL, which is confirmed by cross-sectional transmission electron microscopy. The optical properties of the III-Sb active regions are characterized by time-resolved photoluminescence, which can be used to optimize the IMF interface.

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.

Mode-locked VECSEL emitting 5 ps pulses at 675 nm

A picosecond GaInP/AlGaInP/GaAs vertical external-cavity surface-emitting laser (VECSEL) at 675 nm is reported. The laser is mode-locked with a GaInP/AlGaInP/GaAs saturable absorber mirror and emitted ~5.1 ps pulses at a 973 MHz repetition rate and an average power of 45 mW. To our knowledge, this is the first demonstration of a passively mode-locked VECSEL emitting fundamental laser radiation at the visible part of the spectrum.

Measurement of Gain and Device Performance of a 1050-nm Vertical External Cavity Surface Emitting Laser

In this paper, we measure and analyze the pump power and wavelength dependent gain of an optically pumped infrared 1050-nm vertical external-cavity surface-emitting laser vertical external-cavity surface-emitting laser (VECSEL) developed for an application as a frequency doubled green laser in mobile projectors. Increasing the reflectivity of the chip coating from 3% to 20% for 1050-nm is found to result in a maximal gain of 7.9%, more than twice as high as that of the 3% coating, and in a spectral narrowing of the wavelength-dependent gain as a consequence of the enhanced coupling between the optical field and the quantum well gain medium. From our gain measurements, the wavelength dependent laser threshold can directly be obtained and compared to the VECSEL device characteristics. A very good agreement between the data extracted from the gain measurements and the measured laser threshold in VECSELs with different output mirrors is found.

Sub-60-fs Timing Jitter of a SESAM Modelocked VECSEL

We present noise measurements of a pulse train emitted from an actively stabilized semiconductor-saturable-absorber-mirror (SESAM) modelocked vertical external cavity surface emitting laser (VECSEL). The laser generated 6-ps pulses with 2-GHz pulse-repetition rate and 40-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 detected with a highly linear photodiode and measured with a signal source analyzer. The resulting RMS timing jitter integrated over an offset frequency range from 1 Hz to 100 MHz gives a value of below 60 fs, lower than previous modelocked VECSELs and comparable with the noise performance of ion-doped solid-state lasers. The RMS amplitude noise was below 0.4% (1 Hz to 40 MHz) and not influenced by the timing phase stabilization.

Intensity noise correlations in a two-frequency VECSEL

We present an experimental and theoretical study of the intensity noise correlation between the two orthogonally polarized modes in a dual frequency Vertical External Cavity Surface Emitting Laser (VECSEL). The dependence of the noise correlation spectra on the non-linear coupling between the two orthogonally polarized modes is put into evidence. Our results show that for small coupling the noise correlation amplitude and phase spectra remain nearly flat (around -6 dB and 0° respectively) within the frequency range of our interest (from 100 kHz to 100 MHz). But for higher values of the coupling constant the low frequency behaviors (below 1-2 MHz) of the correlation amplitude and phase spectra are drastically changed, whereas above this cut-off frequency (1-2 MHz) the correlation spectra are almost independent of coupling strength. The theoretical model is based on the assumptions that the only source of noise in the frequency range of our interest for the two modes are pump noises, which are white noises of equal amplitude but partially correlated.