Ultra-coherent 780 nm VECSEL for rubidium-based quantum applications

The development of stable ultra-coherent lasers is crucial for further advancement of the so-called quantum technologies (QT), including cold atom interferometry-based gyroscopes and accelerometers for position, navigation and timing applications [1]. Such laser performance has direct impact on the accuracy and efficiency of QT systems, also affecting their deployability for field trials. In this context, vertical-external-cavity surface-emitting-laser (VECSEL) technology is being investigated as a candidate for the next generation of quantum devices given advantageous characteristics such as low-noise performance with sub-Hz intrinsic linewidth, broad wavelength coverage and high brightness [2]. In this work, a novel 780 nm VECSEL (gain chip fabricated by Vexlum Ltd) is developed and characterized for low noise single frequency operation, the first step towards the development of more sophisticated laser systems for rubidium-based QT. Multimode output powers exceeding 1 W and 0.5 W (OC = 1%) are achieved when barrier-pumped at 638 and 532 nm, respectively, with the expected efficiency reduction at the lower pump wavelengths caused by a higher absorption coefficient and larger quantum defect. In the latter case, the VECSEL thermal resistance was characterized [3] to be 1.7(4) K/W for a pump spot diameter of 182 μm. For single frequency operation, a table-top two-mirror cavity with an intracavity birefringent filter (BRF) was built, with the sample temperature stabilized (precision ~ mK) and the output coupler mounted on a piezo-electric transducer for frequency stabilization purposes. The resulting laser presented a 10 nm tuning range (774 – 784 nm), with 0.45 pm wavelength stability and power stability of 1.03 %. The frequency noise and linewidth performance were characterized when the VECSEL was stabilized to a reference cavity (Finesse = 400; FSR = 1 GHz) via the Pound-Drever-Hall technique [4], resulting in low frequency noise with a linewidth of 6.0(1) kHz for an averaging time of 4 s. Further optimization of the current table-top system and noise performance characterization will be performed, including laser cavity engineering and packaging, to improve further the 780 nm VECSEL performance which will be used as a first step to produce more complex laser systems for integration with Rb-based cold atom interferometers.

References
[1] B. Battelier, et al., Proc. SPIE 9900, SPIE Photonics Europe - Quantum Optics, 990004 (2016).
[2] M. Kuznetsov et al, IEEE J. Sel. Top. Quantum Electron 5, 561-573 (1999)
[3] B. Heinen et al., IEEE J. Quantum Electron. 48, 934-940 (2012)
[4] R. W. Drever et al., Appl. Phys. B 31, 97-105 (1983)