Generation and characterisation of ultrashort diode laser pulses
Abstract
This thesis is concerned with the development of a compact diode laser source
of picosecond optical pulses having enhanced average powers. This is realised
by the application of a large amplitude sinusoidal modulation to a single-contact,
single-mode, narrow stripe, InGaAs/GaAs ridge-waveguide diode laser. The
operational characteristics of the device when in continuous wave and gain-
switched regimes are presented.
In the gain-switched regime, a minimum pulse duration of 30ps is
demonstrated, at average and peak powers up to ≈150mW and ≈1.8mW
respectively. A sonogram technique is employed to determine the sign and
magnitude of the frequency chirp in the optical pulses. On the basis of this
information aperiodic gratings are designed and fabricated in germanosilicate
optical fibres and lithium niobate crystals to realise temporal pulse compression
and efficient second harmonic generation respectively.
The effect of self-injection optical feedback is described, along with the
corresponding realisation in the reduction in the spectral bandwidth of the
optical pulses from ≈11nm to 0.05nm. When the optical feedback is provided
by a standard diffraction grating, a tuning range of 70nm is demonstrated. The
addition of a second grating results in two independently tunable outputs, with
an adjustable spectral separation of up to 53nm.
Bragg gratings are fabricated in the cores of photosensitive germanosilicate
optical fibres. It is demonstrated that when such a fibre is used in an external
cavity configuration, both temporal and spectral compression of the optical
pulses is observed.
Direct frequency conversion of the diode laser output by using quasi-phase
matched crystals of lithium niobate and KTP is demonstrated. High efficiencies
are obtained with a KTP crystal containing a waveguide structure and a Bragg
grating section to provide optical feedback to the diode laser. By this approach
impressively high average second harmonic powers of up to 7.3mW in the blue
spectral region are achieved for this frequency-doubled picosecond diode laser.
Type
Thesis, PhD Doctor of Philosophy
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