Surface-emitting semiconductor lasers can make use of external cavities and optical pumping techniques to achieve a combination of high continuous-wave output power?, ? and near-diffraction-limited low divergence circular beam quality that is not matched by any other type of waveguide semiconductor source (Fig. ??,??). The ready access to the laser mode that the external cavity provides has been exploited for applications such as intra-cavity frequency doubling,? passively mode-locked ultrashort pulse,? tunable single-frequency ultra-low noise operation in the 0.8 to 2.3 µm spectral range,?,?,?,?,? low noise dual frequency operation for metrology, remote sensing, and communications applications,? gyrolaser operation for avionic/space,? high sensitivity laser absorption spectroscopy for gas analysis.? The growing current interest in Vertical-External-Cavity Surface-Emitting Lasers (VECSELs) is at first sight paradoxical, since these lasers incorporate precisely the feature that conventional semiconductor lasers most prize the lack of; a bulky external cavity that requires alignment. They are frequently, moreover, optically pumped, exploiting the diode beam coupling technologies that have been developed for diode-pumped solid state lasers (DPSSL). Electrical pumping over large area is also possible to reduce chip volume, with reduced power performances and a more complex technology. On further investigation, however, it becomes apparent that this family of devices offers a distinctive combination of properties not easily matched by any other type of laser source. Since the area of the spatial mode on the surface of the gain structure can be large, the power that can be extracted in a fundamental spatial mode before the optical damage threshold is reached greatly exceeds that which can be obtained from a conventional diode laser where the waveguide section size as to be on the order of few λ (Fig. ??). This high-finesse mm-long air-filled stable vertical cavity design boosts the spatial (low wavefront phase noise) and temporal (low frequency and intensity noise) light coherence due to its long photon lifetime tp (Fig. ??), generating continuously-tunable sub-MHz linewidth single-frequency laser with large diameter TEM00 mode ??, exibiting a linear polarization state.?,?,?,? The emitted laser radiation exhibits low Relative Intensity Noise or fluctuations, as the laser is in a class-A regime with a short cutoff frequency < 50 Mhz (Fig ??), above which the intensity fluctuations reache the shot noise level. It tends to a class-B regime with moderate relaxation oscillations if the cavity length is decreased.? This contrasts with conventional monolithic semiconductor lasers which exhibit large amplitude/noise resonance above the GHz frequency level (Fig. ??,??). Single longitudinal mode tunable DFB lasers (Fig. ??) and high-power solid-state lasers rely on strong intracavity filtering. A compact powerfull laser design can be thus achieved using a short cavity Quantum-Well VECSEL without any intracavity filter,?,?,? as it exhibits an ideal homogeneous gain behavior - as with QW’s -, where amplified spontaneous emission (Fig. ??) and non-linear mode interactions - like Spatial hole Burning - are negligible? - in constrast to monolithic semiconductor lasers.
We will present an overview of the laser results we achieved with such GaAs-based and Sb-based laser geometry, emitting in the 0.9-1.1µm and 2-2.7µm windows respectively, together with a full physical study of the emitted coherent wave. We will discuss the targeted photonics applications. The Mid-IR window is especially well adapted for gas analysis applications (CH4, NH3, CO, HF and H2O, CO2…). The design principles can be extended to any wavelength.
HIGH POWER VECSEL DEVICE DESIGN AND TECHNOLOGY
1/2-VCSEL structure design and fabrication : the gain mirror - membrane on SiC for thermal management
The 1/2-VCSEL structures are composed of a high reflectivity Bragg mirror, typically 6 quantum-wells and a SiN antireflection coating on top?,?,? (Fig. ??). The GaAs-based structuresare grown by MOCVD? and the Sb-based ones by MBE,? and fully characterized (Fig ??) for emission at 1µm and 2 2-2 7µm For higher power operation requiring a low thermal impedance, the structures are grown in reverse order, with an etch stop layer added, bonded on SiC by solid-liquid inter-diffusion process, and the substrate removed by selective chemical etching:?,? the semiconductor membrane is then bonded on a low thermal impedance carrier substrate. A 300nm gold layer is evaporated on the bragg, before bonding, to enhanced the reflectivity. This design allows reducing the bragg layer number, therfore the thermal device impedance by a factor 3 to 10 depending on the III-V technology (Fig. ??).?,? The resulting Sb-based 1/2-VCSEL membrane is shown in scanning electron microscopy cross section on Fig. ??. We measured a device thermal resistance in rather good agreement with the 2D symmetric simulation (FEMLAB) [Fig. ??].
Device design: short plano-concave cavity principle pumping at large angle
The devices are formed by a 1/2-VCSEL, a small 0.3-15 mm air gap to stabilize single transverse and longitudinal mode operation in a plano-concave type stable cavity. The cavity is closed by a commercial dielectric mirror on glass or CaF2 (99% of reflectivity) having either a concave surface (2-10 mm radius of curvature) or a plane surface (taking advantage of pump induced thermal lens). A Piezoelectric Transducer (PZT) is used to tune the cavity length over 5 µm, thus the laser frequency over several cavity free-spectral-range (FSR). The main advantage of VECSEL for broad continuous frequency tuning, in contrary to monolithic semiconductor devices, is that the ultra-thin gain chip is not coupled with the optical cavity: the gain wavelenght and the cavity frequency comb postion can be tuned independently, with the pump power/chip temperature and with the cavity length (PZT voltage) respectively. The external mirror is held by an ultra-stable mirror mount (New Focus 9882). High power 8W and 200mW singlemode 800nm commercial diode lasers are used as pump sources, lunched at large incidence angle on the chip (> 45 °).?,? All the components are glued in a compact lab prototype (sealed box) to reduce mechanical and thermal fluctuations (Fig. ??). In the plano-plano type configuration? at high power, we calculated that our thermal lens-based cavity behaves like a single transverse mode optical resonators,? where higer transverse modes can not oscillate. To be in a single frequency light state, the laser has to operate on a single transverse and longitudinal mode, and light polarization state (linear along  here).? To stabilize only one longitudinal mode in the gain bandwidth, we took advantage of the ideal homogeneous gain behavior of QW VECSEL,? where non-linear mode interactions are weak. Then we chose a short cavity length L < 10 mm, without the need of any intracavity filter, to prevent from multimode operation, or mode hopping, due to technical perturbations, specially while using a multimode noisy pump.?,?,? We chose a cavity length short enough so that the characteristic time (≪ 1 ms here) to stabilize a single longitudinal mode? is shorter than the characteristic time of strong technical fluctuations (mechanical, thermal…), already potentially large in the kHz frequency range (see Fig. ??,??).
FREE RUNNING TUNABLE SINGLE FREQUENCY OPERATION AT HIGH OUTPUT POWER
Single longitudinal and transverse Mode operation at the quantum and diffraction limit
These low noise sources operate in cw at RT, with up to 2.1 W of output power (pump limited; with thermal management)? at 1µm (Fig. ??), and > 7mW at 2.3µm and 150 µW at 2.7µm without thermal management (Fig. ??) in a single transverse and longitudinal mode (Fig. ??). The short stable external optical cavity enforces circular TEM00 diffraction limited beam (RMS phase fluctuation < λ/100 measured) (Fig. ??) with a very low divergence, and single frequency broadly tunable operation (SMSR=60dB) without any intracavity spectral filter (Fig. ??). The far field phase map was recorded with a wavefront sensor. The second moment measurement and far field phase map shows that the beam is close to diffraction limit with a quasi-Gaussian shape, in spite of a poor quality elliptical multimode pump diode beam, in good agreement with the simulation. VeCSELs are linearly polarized due to QW gain dichrosm along the  crystal axis (> 30 dB orthogonal polarization exctinction ratio), which breaks the circular symetry. ?,?,? A wide mode-hope-free tuning range > 500 GHz is obtained by moving the external mirror of a short cavity with a PZT as shown in Fig. ?? at 2.3µm for absorption spectroscopy application. ?
Intensity and frequency noise in free running operation
This high-Q VECSEL design leads to low RIN and frequency noise operation despite the use of multimode high power pump diode. These free running lasers were studied in terms of Relative Intensity Noise and linewidth showing a highly coherent wave. Thanks to the high-Q external cavity approach, the dynamics is in the relaxation oscillation-free regime with a laser cutoff frequency of 41 MHz for a 7 mm long cavity here (Fig. ??). The dynamics is thus in the relaxation-oscillation-free regime, in contrary to conventional monolithic semiconductor lasers, as the photon lifetime tp > τe the carrier lifetime ??, with a laser cutoff frequency below 100 MHz. The laser is in a class-A regime (Fig. ??) and tends to a class-B regime if the cavity length is decreased.? The emitted optical power is ultra stable - much more than conventional solid-state lasers - exhibiting relative RMS fluctuations lower than 0.1% (Fig. ??) in the frequency range 10Hz-40MHz even at 2.1 W of output power, and reaches the shot noise limit above 41MHz.? Between laser cutoff frequency and the cavity Free-Spectral-Range value (f=40MHz-20 GHz), the laser intensity noise is at the shot-noise fundamental limit, leading to an ultra low noise source in this frequency range. We performed frequency noise measurements with a Fabry-Perot interferometer. Below 1 kHz the frequency noise is limited by thermal/mechanical contributions and 1/f technical noise. The frequency noise spectral density comes close to the Hz level fundamental quantum limit at high frequencies. We experimentally deduced a Gaussian-like shape linewidth of 37 kHz over 1 ms even at 2.1 W of output power,? limited by pump induced thermal fluctuation (Fig. ??). This linewidth value is similar to what would be measured using a standard heterodyne technique, and is 2-3 order of magnitude lower than the limit of conventional semiconductor integrated lasers above 5 MHz. Longer cavity design (cm) would allow to reach a linewidth below the kHz level. By frequency locking to a high finesse fabry-perot or to an atomic transition, this source would allow to reach a linewidth close to the Hz level still at high power.
We demonstrated a compact highly coherent and efficient high power QW semiconductor laser in the near and middle-IR range (1-2.7µm range) at the multiwatt level. The single frequency laser exhibits low intensity and frequency noise (sub-40 kHz linewidth) and is broadly continuously tunable. It is based on a short plano-concave type cavity VECSEL, where a VECSEL membrane is bonded on SiC for thermal management. We obtained diffraction limit quasi-Gaussian beam even pumping with a low beam quality beam diode laser. We showed that our thermal lens-based cavity behaves like a single transverse mode optical resonators. We showed that pump properties define the cavity design and laser coherence. The laser RIN and linewidth can be further reduced using noiseless excitation, like low noise pump or electrical pumping.
This design principle and the laser properties obtained can be extended to any wavelength by using suitable semiconductor materials, and is scalable to higher output powers using larger active diameters. This low noise compact device shows higher quality than standard commercial laser diodes or solid-state lasers, with noise levels several order of magnitude lower together with output powers several order of magnitude higher.
These highly coherent high power semiconductor sources are now being used in various photonic applications: high sensitivity spectroscopy instruments,? tunable dual frequency operation device for metrology, remote sensing, and communications,? gyrolaser operation for avionic/space,? atomic clock for atomic frequency standards as well as atomic inertial sensors.?
This work was supported by the French MICPHIR, MIREV, 2POLEVF and NATIF ANR programs, the French GLACE DGA PREI program, and the GONG European Space Agency program.
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