The design, fabrication and characterisation of a silicon nitride optical phased array (OPA) for waveguide integration with flip-chip bonded rubidium (Rb) atomic MEMS vapour cells are demonstrated. A one-dimensional OPA with a symmetric splitter tree design at 780 nm wavelength is fabricated by electron-beam lithography and plasma dry-etching. The waveguides were cladded with silicon dioxide before bonding to an Rb MEMS vapour cell. The characterisation was performed by end-fire coupling a 780.24 nm laser to the waveguide by a lens fibre. A saturated absorption spectroscopy measurement revealed the ~ 6 MHz hyperfine transition absorption dips that can be utilised for laser frequency stabilisation for atomic cooling.
GaAs/AlGaAs Distributed Feedback Semiconductor (DFB) lasers with laterally-coupled gratings are demonstrated at 778.1 nm wavelength with output powers up to 48 mW, over 35 dB side-mode suppression ratios, tuning ranges of 0.8 nm, and vertical beam divergences of 20.5. An asymmetrical mode expander and aluminum-free active layers have been used in the material epilayer to reduce the linewidth to 3.67 kHz and relative intensity noise (RIN) of –140 dBc/Hz while maintaining high output powers. The fabricated lasers demonstrate high-resolution spectroscopy of the hyperfine levels of the 87Rb two-photon transitions and are suitable for integration into miniaturized cold atom systems.
KEYWORDS: Laser applications, Cladding, Semiconductor lasers, Chemical species, Active optics, Wave propagation, Reactive ion etching, New and emerging technologies, Mirrors, Atomic clocks
A GaAs/AlGaAs distributed feedback semiconductor (DFB) laser with a laterally-coupled grating is demonstrated at a wavelength of 780.24 nm with an output power up to 60 mW. A mode expander and aluminum-free active layers have been used in the material epilayer to reduce the linewidth to 612 kHz while maintaining high output power. The fabricated laser demonstrates over 40 dB side-mode suppression ratio with tuning range > 0.3 nm, which is suitable for atom cooling experiments with the D2 87Rb atomic transition and provides substantial potential for the laser to be integrated into miniaturized cold atom systems.
The growing interest in quantum technology applications such as laser cooling and quantum sensing has generated a large demand for narrow linewidth and high-power laser sources in the visible and near-infrared wavelength emission range. Semiconductor lasers are ideal candidates for developing these sources as they combine low cost and low-power consumption with small size and unparalleled potential for integration.
This work presents experimental results on two complementary design strategies that can be effectively used to reduce the laser linewidth in a Distributed FeedBack (DFB) semiconductor laser: i) the optimisation of the epilayer structure and ii) the apodisation of the grating geometry. The design of the epilayer stack was optimised to vertically shift the optical mode towards the n-doped region so as to reduce the interaction with the more lossy p-doped region and therefore decrease the internal losses of the waveguide. Such design also reduces the modal overlap with the quantum well (QW) gain region. As predicted by the Schawlow-Townes relationship, both of these factors translate into a reduction of the laser linewidth. The DFB lasers were fabricated with a sidewall grating geometry that simplifies the fabrication process and allows to engineer the feedback profile. In this work, the gratings were apodised to alleviate the spatial hole burning, which substantially worsen the laser linewidth at high power levels. The optimisation of the epilayer and grating designs allowed to fabricate robust devices with measured linewidths as small as a few hundred kHz and power output of several tens of mW.
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