Integrated planar cavities for external cavity diode lasers

Abstract

External cavity diode lasers (ECDL) have demonstrated single mode operation, narrower linewidths, wider tuneability and lower noise operation when compared to fully monolithic diode lasers. An ECDL is a laser system that integrates a diode laser into a larger cavity permitting greater control over the laser properties. The external cavities are usually based on diffractions gratings and moving parts. This thesis is focused on making more stable, compact and versatile platforms based on integrated optical gratings. The fabrication and characterisation of three ECDL geometries is covered, all hosting direct UV-written planar Bragg gratings.

A planar glass-on-silicon chip with a UV-written Bragg grating and waveguide was coupled with a gain-chip to form a single mode 1651 nm laser suitable for methane gas sensing. The relative intensity noise (RIN) was measured to be <150 dB at 100 MHz. The laser was mixed with a replica to generate a beat note where the Lorentz linewidth was measured to be 220 kHz. Frequency offset locking to a replicated laser was demonstrated using a commercial phase locked loop chip. Using the laser, the R4 methane gas line at 1650.96 nm was measured down to a concentration of 1250 ppm at room temperature and atmospheric pressure.

A new ECDL for acetylene, based on a new optical platform consisting of a fibre-planar composite, coined integrated optical fibre (IOF), hosting a Bragg grating inscribed at 1532.83 nm. The RIN was measured and demonstrated improved noise characteristics compared to commercial ECDL. The linewidth was measured to be <14 kHz similar to commercial system. Finally, this system scanned the P13 acetylene line at 20 Torr at 20 °C, confirming the IOF 1532.83 nm operation.

In a third experiment, an ECDL was constructed at a wavelength of 1560.48 nm suitable for frequency doubling to 780 nm Rubidium spectroscopy. This system was adhered onto a silicon substrate and packaged in a small aluminium enclosure, demonstrating improved stability with improved low frequency RIN compared to the previous systems.

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