Vertical-External-Cavity Surface-Emitting Lasers (VECSEL) have emerged as an attractive single-frequency laser platform for quantum technology applications utilizing trapped ions, neutral atoms, and cold molecules. VECSELs exhibit a unique combination of features including high-power single-frequency operation, excellent beam, and the ability to cover a broad wavelength range from the ultraviolet to visible and to infrared. Here recent developments of compact and modular VECSELs for the industrial scale-up of quantum applications are presented, together with most recent data for wavelength versatility and linewidth narrowing to sub-Hz.
VECSELs have already been successfully used for quantum experiments based on ion and atom systems, where the main desired features are narrow linewidth, low noise, and stability. Here we are exposing the broad tuning capability of the VEXLUM VALO single-frequency laser platform for the resonant excitation of recently developed GaSb-based semiconductor quantum dots, which can provide single-photon emission in the 3rd telecom widow. The VECSEL allows direct excitation of a single quantum dot and elimination of complex charge relaxation paths in the surrounding semiconductor matrix, thus improving the charge stability of the quantum dot.
Vertical-external-cavity surface-emitting lasers (VECSELs) have emerged as an attractive single-frequency laser platform for quantum computing and -sensing applications including optical clocks. VECSELs exhibit a unique combination of features including high-power single-frequency operation, excellent beam, and the ability to cover a broad wavelength range from the ultraviolet to visible and to infrared. Here a compact commercial VECSEL platform with sub-Hz linewidth for optical lattice clocks is presented including recent data for Sr and Yb systems.
Progress of commercial single-frequency VECSELs for quantum technology applications is reviewed. Availability of practical laser systems with specific wavelength matching an atomic transition is instrumental for the quantum technology research experiments and is becoming increasingly important for the upscaling of commercial quantum systems. To this end, we present a versatile commercial single-frequency VECSEL platform operating in the ultraviolet, visible and NIR -spectral ranges. The suitability of the laser systems for a wide variety of quantum information processing tasks, including spectroscopy, photoionization, and laser cooling is demonstrated.
We review the progress of commercial single-frequency VECSELs for quantum technology applications. VECSELs are ideal laser sources for applications requiring low-noise watt-level single-frequency light with excellent beam quality. In particular, we present features of lasers addressing the need for spectroscopy, laser cooling and trapping for selected atoms and molecules, such as Be, Cd, Sr and Yb. Power scaling abilities in single-frequency operation are also discussed.
Over the last 30 years, Sodium Guidestar Lasers (SGLs) have proved to be an important element of adaptive-optics (AO) image correction techniques for astronomical observatories. In recent years, the astronomy community has employed Raman shifted fiber lasers to meet the need. However, emerging applications would greatly benefit by a reduction in the cost per Watt of on-sky power and the Size Weight and Power (SWAP) required by the laser. Small (meter-class) observatories seek to incorporate AO systems to meet space situational awareness and free space laser communication applications. Simultaneously, large (10 meter class) observatories require larger numbers of lasers on-sky to implement multi-conjugate AO systems, Further, techniques such as re-pumping and frequency-chirping are being developed to increase returns from the sky for a given laser power. The next generation of SGLs (Sodium Guidestar Lasers) must be suited for such modes of operation while reducing cost and SWAP. Optically pumped semiconductor lasers (OPSLs), also referred to as Vertical External Cavity Surface Emitting Lasers (VECSELs), represent a technology pathway to realizing SGLs with high performance, compact size, high reliability, and low acquisition and maintenance costs. In pursuit of the next generation of SGL, we demonstrate 8W of single-frequency power at 589 nm based on in intracavity frequency doubling of 1178 nm fundamental wavelength VECSEL. Our work investigates the key challenges of the laser design; especially frequency selection, tuning, and locking the laser to sodium resonance, laser power, and gain-mirror lifetime.
A MECSEL emitting around 825nm is reported. With a tuning range from 807nm to 840 nm, the MECSEL extends the coverage of high beam quality semiconductor based lasers in the short 8XXnm region and opens new perspectives for scanning ground-based water-vapor differential absorption lidar. 1.4W maximum output power has been achieved at room temperature operation and at 12.5W absorbed power using a 532 nm emitting pump laser. The beam quality has been investigated by M2 measurements at different pump power. The effect from a growing pump mode and thermal lensing has been observed as the beam divergence angle decreases and the beam waist radius enlargens with increasing pump power.
Output power of VECSELs had been scaled by lateral scaling to tens of watts and beyond. Longitudinal scaling, employing multiple VECSEL devices in a single resonator, has the potential to scale up the power as well. However, some of the devices need to be placed at a fold of the resonator and inherently suffers from the spectral instability. The standing wave pattern created at a fold of a standing wave cavity exhibits that of 4-wave interference, and the resulting pattern shows high contrast modulation in the plane of the quantum well. The phase of that modulation depends on the phase relationship between the forward and backward beams, which differs for different longitudinal modes. This results in a situation similar to the special hole burning effect in solid-state lasers in which case the standing wave pattern is in the longitudinal direction. Because of the resonant periodic gain structure, VECSELs do not suffer from spatial hole burning if the device is placed at the end of the standing-wave cavity and single-frequency operation can be obtained relatively simply. This no longer holds when the VECSEL device is placed at the fold of a standing wave cavity. Twisted-mode configuration addresses this and allows narrow-linewidth or single-frequency operation of multidevice VECSELs. By having forward and backward modes in oppositely rotating circular polarization, the standing wave pattern does not show modulation in the planes of quantum wells, recovering the advantage of resonant periodic gain.
We compared single-side pumping (SSP) and double-side pumping (DSP) of a semiconductor membrane external-cavity surface-emitting laser (MECSEL). The MECSEL's active region was based on a 4×3 AlGaAs quantum well (QW) structure. This structure was embedded between two silicon carbide (SiC) wafer pieces that were used as transparent intra-cavity (IC) heat spreaders creating a symmetrical cooling environment. The MECSEL structure targeted emission at 780nm and was operated at 20°C heat sink temperature. Via DSP the differential efficiency was improved from 31.9% to 34.4 %. The laser threshold was reduced from 0.79 W to 0.69 W of absorbed pump power while the maximum output power was increased from 3.13 W to 3.22 W. The DSP configuration enabled these improvements by a reduced thermal resistance of the gain element by 9 %. The MECSEL operated at a fundamental Gaussian TEM00 mode profile and the beam quality was measured to be M2 <1.09. We further demonstrate a maximum tuning range from 767 nm to 811 nm. A similar active region with about half the thickness (2×3 AlGaAs QWs) was investigated using the DSP configuration and first results are presented here. 500-μm-thick sapphire IC heat spreaders were used instead of SiC. The output power exceeded 0.5W and the emission was spectrally located around 770 nm.
Optically pumped semiconductor (OPS) vertical external-cavity surface-emitting lasers (VECSELs) are an important category of power scalable lasers with a wide range of applications in bio photonics, medicine technologies and for example spectroscopy. The possibility of band-gap engineering, a laser emission in the fundamental Gaussian mode and the technical simplicity leads to ongoing growth of the area of applications for these tunable laser sources. The open laser resonator allows inserting frequency selective and converting intra-cavity elements as well as absorptive elements to create mode locking. In addition, intra cavity gas cells allow absorption spectroscopy. Aiming on spectroscopic applications for rubidium one important absorption line is located at 780 nm. Nowadays, laser emission in this spectral range has not been shown by VECSELs, neither in direct nor in frequency doubled emission although the available III-V semiconductor materials would provide such a band-gap. A very low charge carrier confinement may be the main challenge here.
We present several strategies to create gain structures based on the AlGaAs- and the AlGaAs/AlGaInP material system. The expected high thermal sensitivity can be counteracted by realizing this VECSEL structure also as a membrane external-cavity surface-emitting laser (MECSEL) to improve the heat transfer out of the active region. Investigations comparing barrier pumping with in-well pumping are also possible. A MECSEL would be in both cases beneficial here as not absorbed pump light is just transmitted instead of being absorbed in the DBR creating unnecessary heat.
Vertical-external-cavity surface-emitting lasers (VECSELs), also called semiconductor disk lasers (SDLs), have developed strongly during the last two decades. Additionally, the range of available wavelengths has been drastically extended during this time, especially when second harmonic generation is taken into account. Nevertheless, these systems run into limits when the refractive indices of the materials used for the necessary distributed Bragg reflectors (DBRs) approach too much. This leads to a much higher number of necessary layer pairs, which increases the structure thickness and makes growth of such DBRs at least extremely difficult. Another limit occurs when the band gap of the gain material used in the VECSEL approaches too close to the band-gap in the used DBR materials. Absorption losses in the DBR are the consequence. Additionally, the performance of VECSELs in general suffers from heat incorporation into the active region caused by the excess energy of the pump photons together with the low thermal conductivity of the substrate and the included DBR.
The recently shown membrane external-cavity surface-emitting laser (MECSEL) concept opens the potential to overcome all the above named challenges as only an isolated active region membrane, sandwiched between intra-cavity heat spreaders is used as gain material. Furthermore, active region membranes in the GaInP/AlGaInP material system aiming on the yellow and red-orange spectral region where direct laser emission has not been realized yet, grown on high-index substrates, open the possibility to deliver sufficient gain realizing a MECSEL.
We report the development of an intracavity-frequency-doubled vertical external-cavity surface-emitting laser (VECSEL) emitting at 571 nm for photoionization of magnesium. The laser employs a V-cavity geometry with a gain chip at the end of one cavity arm and a lithium triborate (LBO) crystal for second harmonic generation. The gain chip has a bottom-emitting design with ten GaInAs quantum wells of 7 nm thickness, which are strain compensated by GaAsP. The system is capable of producing up to 2.4 ± 0.1 W (total power in two separate output beams) in the visible. The free-running relative intensity noise was measured to be below −55 dBc/Hz over all frequencies from 1 Hz to 1 MHz. With acoustic isolation and temperature regulation of the laser breadboard, the mode-hop free operation time is typically over 5 hrs. To improve the long-term frequency stability, the laser can be locked to a Doppler-free transition of molecular iodine. To estimate the short-term linewidth, the laser was tuned to the resonance of a reference cavity. From analysis of the on-resonance Hänsch-Couillaud error signal we infer a linewidth of 50 ± 10 kHz. Light at 285 nm is generated with an external build-up cavity containing a β-barium borate (BBO) crystal. The UV light is used for loading 25Mg+ ions in a surface-electrode RF Paul trap. These results demonstrate the applicability and versatility of high-power, single-frequency VECSELs with intracavity harmonic generation for applications in atomic and molecular physics.
We report intracavity Raman conversion of a long-wavelength InGaAs-QW VECSEL to ~1320 nm, the longest wavelength yet achieved by a VECSEL-pumped Raman laser. The setup consisted of a VECSEL capable of emitting >17W at 1180nm and tunable from 1141-1203nm and a 30-mm-long KGd(WO4)2 (KGW) Raman crystal in a coupled-cavity Raman resonator. The Raman cavity was separated from the VECSEL resonator by a tilted dichroic mirror, which steers the Raman beam to an output coupler external to the VECSEL. The spectral emission of the VECSEL, and consequently of the Raman laser, was set by a 4-mm-thick quartz birefringent filter in the VECSEL cavity. The KGW Raman laser was capable of emitting 2.5W at 1315 nm, with M2~2.7 and >4% diode-to-Stokes conversion efficiency. The Raman laser emission was tunable from 1295-1340 nm, limited by the free spectral range of the birefringent filter. Spectral broadening of the fundamental emission was observed during Raman conversion. At the maximum Raman laser output power, the total linewidth of the VECSEL spectrum was ~0:7nm FWHM. As a consequence, the Raman laser emission was also relatively broad (~0.9nm FWHM). Narrow (<0.2nm FWHM) Raman emission was obtained by inserting an additional 100 µm etalon within the VECSEL cavity. With this configuration the fundamental intracavity power clamped at its value at the Raman threshold, suggesting an enhanced effective Raman gain, but the maximum output power of the Raman laser was 1.8 W.
We report on the development of a high-power vertical-external-cavity surface-emitting laser (VECSEL) emitting around 1180 nm. The laser emitted 50 W of output power when the mount of the gain chip was cooled to -15°C. The output power was measured using a 97% reflective cavity end-mirror. The VECSEL was arranged to form an I-shaped cavity with a length of ~100 mm; the gain chip and a curved dielectric mirror (RoC=150) acting as cavity end mirrors. The gain chip was grown by molecular beam epitaxy (MBE) and incorporated 10 GaInAs/GaAs quantum wells. For efficient heat extraction, the chip was capillary bonded to a diamond heat spreader which was attached to a TEC-cooled copper mount. The maximum optical-to-optical conversion efficiency of 28% was achieved for 42 W of output power and -15°C mount temperature.
We report a high-power VECSEL emitting <8W around 615 nm. The gain chip of the laser was grown by plasmaassisted molecular beam epitaxy and it comprised 10 GaInNAs quantum wells. The VECSEL cavity had a V-shaped geometry and a 10-mm-long non-critically phase-matched LBO crystal for second harmonic generation. The cavity incorporated also an etalon and a birefringent filter for controlling the output wavelength. With the aid of the secondharmonic output and the infrared light leaking out from the laser cavity, the single-pass conversion efficiency of the crystal was estimated to have a value of 0.75%.
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