661 research outputs found
Controlling nonlinear optics with dispersion in photonic crystal fibres
Nonlinear optics enables the manipulation of the spectral and temporal features of light.
We used the tailorable guidance properties of photonic crystal fibres to control and
enhance nonlinear processeswith the aim of improving nonlinearity based optical sources.
We utilised modern, high power, Ytterbium fibre lasers to pump either single photonic
crystal fibres or a cascade of fibres with differing properties. Further extension of our
control was realised with specifically tapered photonic crystal fibres which allowed for a
continuous change in the fibre characteristics along their length.
The majority of our work was concerned with supercontinuum generation. For continuous
wave pumping we developed a statistical model of the distribution of soliton
energies arising from modulational instability and used it to understand the optimum
dispersion for efficient continuum expansion. A two-fold increase in spectral width was
demonstrated, along with studies of the noise properties and pump bandwidth dependence
of the continuum. For picosecond pumping we found that the supercontinuum
bandwidth was limited by the four wave mixing phase-matching available in a single
fibre. A technique to overcome this by using a cascade of fibres with different dispersion
profiles was developed. Further improvement was achieved by using novel tapered PCFs
to continuously extend the phase-matching. Analysis of this case showed that a key role
was played by soliton trapping of dispersive waves and that our tapers strongly enhanced
this effect. We demonstrated supercontinua spanning 0.34-2.4 ¹mwith an unprecedented
spectral power; up to 5 mW/nm.
The use of long, dispersion decreasing photonic crystal fibres enabled us to demonstrate
adiabatic soliton compression at 1.06 ¹m. From a survey of fibre structures we found
that working around the second zero dispersion wavelength was optimal as this allows
for decreasing dispersion without decreasing the nonlinearity. We achieved compression
ratios of over 15
New materials, regimes and applications of fibre laser technology
Nonlinear optics enables the manipulation of spectral and temporal characteristics of optical pulses interacting with a dielectric medium. Optical fibres, as a uniquely practical medium, provide an environment for effectively exploiting the nonlinear effects. This has facilitated the rapid growing interest in this field focused on the investigation of fibrebased
sources incorporated with various novel saturable absorber devices for ultrashort pulse generation.
This thesis reports a series of experiments exploring the ongoing research in the field of nonlinear optics, including the development of ultrafast mode-locked fibre sources and their applications in supercontinumm generation and third order parametric interactions
in new carbon materials.
Firstly, the integration of carbon-based materials with rare-earth doped media allows the demonstration of ultrafast mode-locked laser sources operating at wavelengths across the near-infrared region in a compact, low cost and environmentally robust scheme. Power scaling of such sources can be achieved by operating in the all-normal dispersion
regime making use of a glass-substrate saturable absorber device that exhibits a higher damage threshold.
Supercontinuum generation has been used as an effective method for spectral broadening. Pumping with a conceptually simple and reliable fibre-based system, a continuum covering from 2 to 3 μm is generated in a highly nonlinear GeO2 fibre. This experiment demonstrates a robust and long-term stable source of radiation in an important band, coincident with a portion of the atmospheric transmission window.
Finally, the demonstration of a simple and compact nano-material based dual-wavelength system shows the performance of such devices as a simultaneous saturable absorber and passive synchroniser. An experimental study of coherent frequency mixing at large frequency
shifts in a graphene sample, pumped by a two-colour fibre-integrated source, proves the strong nonlinear response of this new carbon material.Open Acces
Pulsed Tm-fiber Laser For Mid-ir Generation
The thulium fiber laser has gained interest due to its long emission wavelength, large bandwidth (~1.8 – 2.1 µm), high efficiencies (~60 %), and high output power levels both in cw as well as pulsed regimes. Applications like remote sensing, machining, medical tissue ablation, and mid-infrared generation benefit from high peak power thulium laser sources. Pulsed thulium fiber laser systems are advancing rapidly towards higher peak power levels and are becoming the preferred sources for these applications. This dissertation work describes the development of novel nanosecond pulsed thulium fiber laser systems with record high peak power levels targeting mid-infrared generation. The peak power scaling in thulium fiber lasers requires new fiber designs with larger mode field area (MFA) than commercially available step index large mode area (SI-LMA) fibers. Two different prototypes of thulium doped photonic crystal fibers (PCF) were investigated for high peak power generation. The first prototype is a flexible-PCF with MFA twice as large as SILMA fiber and the second prototype is a PCF-rod with six times larger MFA. A robust single stage master oscillator power amplifier (MOPA) source based on flexible-PCF was developed. This source provided narrow linewidth, tunable wavelength, variable pulse duration, high peak power, and high energy nanosecond pulses. The PCF-rod was implemented as a second stage power amplifier. This system generated a record level of ~1 MW peak power output with 6.4 ns pulse-duration at 1 kHz repetition rate. This thulium doped PCF based MOPA system is a state of the art laser source providing high quality nanosecond pulses. iv The single stage MOPA system was successfully implemented to pump a zinc germanium phosphide (ZGP) crystal in an optical parametric oscillator (OPO) cavity to generate 3 - 5 µm wavelengths. The MOPA source was also used to demonstrate backside machining in silicon wafer. The PCF based laser system demonstrated an order of magnitude increase in the peak power achievable in nanosecond thulium doped fiber laser systems, and further scaling appears possible. The increase in peak power will enable additional capabilities for mid-infrared generation and associated applications
Quasi-lossless data transmission with ultra-long Raman fibre laser based amplification
The project consists of an experimental and numerical modelling study of the applications of ultra-long Raman fibre laser (URFL) based amplification techniques for high-speed multi-wavelength optical communications systems. The research is focused in telecommunications C-band 40 Gb/s transmission data rates with direct and coherent detection. The optical transmission performance of URFL based systems in terms of optical noise, gain bandwidth and gain flatness for different system configurations is evaluated. Systems with different overall span lengths, transmission fibre types and data modulation formats are investigated. Performance is compared with conventional Erbium doped fibre amplifier based system to evaluate system configurations where URFL based amplification provide performance or commercial advantages
Wavelength extension in speciality fibres
Since the invention of the laser and its first application, there has been an almost continuous
stream of new applications - many of which require specific laser sources. These
applications often require a laser source with a specific power, pulse duration, energy
and wavelength. In some cases these demands are easily met, whilst in others they have
proven rather more difficult to achieve. Traditionally, wavelength versatility has been
limited to the regions for which rare earth or gas gain media are available. These lasers
themselves can be used to generate other wavelengths through the nonlinear processes of
second and third harmonic generation, as well as sum frequency generation. Despite all
of this, there still exists a significant section of the visible and infrared spectrum for which
no convenient sources exist. This thesis is concerned with the development of sources in
these regions along with broadband sources covering significant portions of the spectrum
by themselves.
These new wavelengths are generated in a variety of speciality fibres using either
nonlinear processes or new gain media doped into standard silica fibres. Three types
of speciality fibre are used: low concentration bismuth doped fibre which provides gain
in the 1.0-1.4 μm region; photonic crystal fibres; and very high (75%) concentration
germanium fibres to generate a laser source at 2.1 μm based upon stimulated Raman
scattering. Photonic crystal fibres provide high nonlinearities and controllable dispersion
which enables the generation of broadband supercontinuum sources based upon the
interaction of many nonlinear effects. Each source will be described in depth, with
particular attention given to the underlying physics that gives rise to the source. Previous
and current limitations will be examined and an outlook of the future development of
such sources will be discussed
Advances in Optical Amplifiers
Optical amplifiers play a central role in all categories of fibre communications systems and networks. By compensating for the losses exerted by the transmission medium and the components through which the signals pass, they reduce the need for expensive and slow optical-electrical-optical conversion. The photonic gain media, which are normally based on glass- or semiconductor-based waveguides, can amplify many high speed wavelength division multiplexed channels simultaneously. Recent research has also concentrated on wavelength conversion, switching, demultiplexing in the time domain and other enhanced functions. Advances in Optical Amplifiers presents up to date results on amplifier performance, along with explanations of their relevance, from leading researchers in the field. Its chapters cover amplifiers based on rare earth doped fibres and waveguides, stimulated Raman scattering, nonlinear parametric processes and semiconductor media. Wavelength conversion and other enhanced signal processing functions are also considered in depth. This book is targeted at research, development and design engineers from teams in manufacturing industry, academia and telecommunications service operators
Photonic devices for integrated optical applications
work presented in this thesis encompasses an investigation into the use of
ultrafast laser inscription in the fabrication of glass based photonic devices for
integrated optical applications. Waveguide fabrication and characterisation
experiments were carried out in three categories of glass substrate.
Firstly, waveguides were inscribed in an erbium doped glass with the aim of
fabricating optical amplifiers and lasers operating in the 1.5 μm spectral region.
Low loss waveguides were fabricated in substrates with different dopant
concentrations. Fibre to fibre net gain was achieved from one substrate
composition, however it was found that ion clustering limited the amount of
achievable gain. Laser action was demonstrated by constructing an optical fibre
based cavity around the erbium doped waveguide amplifier.
Waveguides were also inscribed in bismuth doped glass with the aim of
fabricating optical amplifiers and lasers operating in the 1.3 μm spectral region.
Low loss waveguides were fabricated, however the initial composition was
incapable of providing gain. A proven substrate material was employed,
demonstrating ultra-broadband gain spanning more than 250 nm. High losses
prevented the achievement of net gain, however the broad potential of the
substrate material was highlighted.
Finally, waveguides were inscribed in a Chalcogenide glass. Strong refractive
index contrasts were observed, with a wide range of waveguiding structures
produced. Supercontinuum experiments were carried out in order to confirm the
nonlinear behaviour of the waveguides. A spectrally smooth supercontinuum
spanning 600 nm was generated, providing a potentially useful source for optical
coherence tomography
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