Abstract:
The ability to identify the range of an interferometric signal is very useful in interferometry,
allowing the suppression of parasitic signal components or permitting
several signal sources to be multiplexed. Two novel range-resolved optical interferometric
signal processing techniques, employing very different working principles,
are theoretically described and experimentally demonstrated in this thesis. The first
technique is based on code-division multiplexing (CDM), which is combined with
single-sideband signal processing, resulting in a technique that, unlike prior work,
only uses a single, regular electro-optic phase modulator to perform both range-based
signal identification and interferometric phase evaluation. The second approach
uses sinusoidal optical frequency modulation (SFM), induced by injection current
modulation of a diode laser, to introduce range-dependent carriers to determine phase
signals in interferometers of non-zero optical path difference. Here, a key innovation
is the application of a smooth window function, which, when used together with
a time-variant demodulation approach, allows optical path lengths of constituent
interferometers to be continuously and independently variable, subject to a minimum
separation, greatly increasing the practicality of the approach.
Both techniques are applied to fibre segment interferometry, where fibre segments
that act as long-gauge length interferometric sensors are formed between pairs of
partial in-fibre reflectors. Using a regular single-mode laser diode, six fibre segments
of length 12.5 cm are multiplexed with a quadrature bandwidth of 43 kHz and a phase
noise floor of 0.19 mrad
·
Hz
-0.5
using the SFM technique. In contrast, the 16.5 m
spatial resolution achieved with the CDM technique points towards its applicability
in medium-to-long range sensing. The SFM technique also allows high linearity,
with cyclic errors as low as 1 mrad demonstrated, and with modelling indicating
further room for improvement. Additionally, in an industrial measurement, the SFM
technique is applied to single-beam, multi-surface vibrometry, allowing simultaneous
differential measurements between two vibrating surfaces.
