26th Australian Institute of Physics Congress and the Australian and New Zealand Conference on Optics and Photonics

Brillouin amplifiers for optical frequency comb line selection

Not scheduled

20m

Belinda Hutchinson Building (The University of Sydney )

Contributed Oral ANZOS | Photonics and Optics (ANZCOP)

Speaker

Ryan Russell (The University of Sydney)

Description

Optical frequency combs (OFCs) bridge the optical to the microwave (electronic) domain and enable frequency counting in optical atomic clocks (OACs) and low noise microwave synthesis, amongst other applications. Using the OFC directly for spectroscopy is challenging due to the low power per comb line (typically nanowatts to microwatts) and the narrow line spacing of fibre-based OFCs; this results in low signal-to-noise ratio and multi-line excitation which deteriorates atomic clock readout, contributing to timing instability. Hence, methods to provide narrowband selection and amplification of individual comb lines is highly sought after, yet elusive.
Here, we show that Stimulated Brillouin scattering (SBS) is an ideal nonlinear amplifier with gain bandwidth (~10–40 MHz) narrower than the line spacing of most OFCs. Narrowband amplification of comb lines addresses both above constraints (low power, close comb line-spacing) and converts the OFC into effective continuous-wave lasers. We systematically compare two vastly different SBS amplifiers: a 2.25 km telecommunications fibre and a compact 20cm long chalcogenide integrated photonic waveguide on a chip. The chip offers record high gain >60 dB in a 0.8 cm² footprint. The SBS amplifiers are low-noise, particularly in clock-relevant frequency bands which includes a ‘quiet point’ in the frequency noise between ~10-100 kHz. Consequently, we utilise the SBS amplifiers to demonstrate direct comb spectroscopy of the two-colour 5S1/2 → 5D5/2 transition in rubidium-87, relevant to OACs.
Our demonstration opens a pathway to more compact optical clocks based on direct-comb spectroscopy with reduced size, weight, power and cost (SWaP-C). Furthermore, the concept is applicable to OFC-based photonic signal processing applications e.g. low-noise microwave signal generation, frequency transfer over long-haul fibre links, and carrier recovery in coherent telecommunications. Importantly, centimetre-scale chip-integrated Brillouin amplifiers can now deliver performance previously achievable only in kilometre-scale fibres.