Ultra-low-noise frequency synthesis, comparison and dissemination using femtosecond optical frequency combs
Abstract
This thesis presents research into ultra-low noise photonic microwave synthesis
and the development of a novel, frequency comb-based, fibre optic time transfer technique.
The focus in the first area is on reducing the noise introduced in the optical-to-
electrical conversion process using balanced optical-microwave phase detectors.
Two mainly free-space and two mainly fibre-based devices were built
and their performance was characterised. The phase noise of the optical-to-electrical conversion of the free-space device was -119 dBc Hz⁻¹ at 1 Hz and
-143 dBc Hz⁻¹ at 20 kHz from an 8 GHz carrier which is the best performance
reported for a free-space balanced-optical microwave phase detector.
The improved fibre-based set-ups demonstrated a state-of-the-art amplitude-to-phase noise suppression of 60 dB and a phase noise of the optical-to-electrical conversion of -131 dBc Hz⁻¹ at 1 Hz and 148 dBc Hz⁻¹ at 20 kHz from an 8 GHz carrier.
The novel time transfer technique developed in the second part superimposes
timing information onto the optical pulse train of an ITU-channel-filtered
frequency comb using an intensity modulation scheme. Time transfer over
a 50 km long, delay-stabilised fibre spool produced a state-of-the-art time
deviation of 300 fs and an accuracy of approximately 0.01 ns which is close
to the best performance achieved using amplitude modulated cw lasers. Using
this technique on a 159 km long installed fibre link between NPL and
Reading, the same time deviation was achieved and an accuracy of approximately
0.08 ns was obtained, limited by uncertainty of the time interval
counter. Using the same fibre link, microwave frequency transfer of the
ITU-channel-filtered comb was demonstrated with a fractional frequency instability
of 2x10⁻¹⁷ at 5000 s which is approximately at the same level as
the best previously reported results which were obtained with a 30 nm wide
optical frequency comb.
Type
Thesis, DEng Doctor of Engineering
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