Abstract:
This thesis describes two external cavity laser diode designs. The first utilises a Bragg
grating fabricated in highly biréfringent optical fibre and offers, through the use of a
waveplate, the ability to switch between modes that are separated in both polarisation
and wavelength, due to the differing refractive index in either eigenmode. The laser
offers three stabilised states of operation, single mode operation for either axis of the
fibre, or a third state in which both modes lase simultaneously with a wavelength
separation of 0.3 nm. The application of transverse strain on the fibre Bragg grating
was also demonstrated as a method of tuning the wavelength separation between these
modes.
The second external cavity laser design utilises two spatially and spectrally
separate Bragg gratings fabricated in mono-mode fibre. This allows two longitudinal
modes corresponding to the Bragg wavelengths to oscillate simultaneously. The
application of longitudinal strain allowed either fibre Bragg grating to be tuned, thus
generating a stabilised tuneable beat frequency, which was demonstrated between
130 GHz - 2.28 THz. A three fibre Bragg grating laser is also presented which
allowed three modes to oscillate simultaneously.
Both the laser based on a Bragg grating fabricated in highly biréfringent fibre and
the laser based on multiple Bragg gratings fabricated in single mode fibre
demonstrated reduced injection current threshold and mode hop free operation over
the full injection current range.
A practical application for the two fibre Bragg grating lasers is presented where the
properties o f independently tuneable dual wavelength operation are used to
interrogate a miniature fibre Fabry-Perot sensor. The wavelength separation o f the
two oscillating modes can be tuned to reach a point of quadrature for the sensor
cavity. A sensing system demonstrating this technique is presented which detected
vibrations up to 500 Hz in cavities of 16-34 pm.
