Broadly tunable CW green-to-red laser source based on frequency doubling of a quantum-dot external cavity diode laser in a PPKTP waveguide

Here we present a compact tunable all-room-temperature frequency-doubling scheme, using a periodically poled potassium titanyl phosphate (PPKTP) waveguide and a QD-ECDL. A broad wavelength tunability of the second harmonic generated light (SHG) in the spectral region between 567.7 and 629.1 nm was achieved, with maximum conversion efficiencies in range of 0.34%-7.9%. The maximum output power for the SHG light was 4.11 mW at 591.5 nm, achieved for 52 mW of launched pump power at 1183 nm, resulting in a conversion efficiency of 7.9%.

Optimization of DFB fiber laser performance based on a superimposed cavities structure

A simple technique based on superimposed cavities structure for direct real-time assessment of a DFB fiber laser mode condition during operation is demonstrated and used to provide a cavity mode conditioning feedback mechanism to optimize output performance. Significant improvements to the output performance and robustness are achieved over the entire pump power range without ambient isolation.

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.

Raman fibre laser based amplification in coherent transmission systems

The thesis presents a detailed study of different Raman fibre laser (RFL) based amplification techniques and their applications in long-haul/unrepeatered coherent transmission systems. RFL based amplifications techniques were characterised from different aspects, including signal/noise power distributions, relative intensity noise (RIN), mode structures of induced Raman fibre lasers, and so on. It was found for the first time that RFL based amplification techniques could be divided into three categories in terms of the fibre laser regime, which were Fabry-Perot fibre laser with two FBGs, weak Fabry-Perot fibre laser with one FBG and very low reflection near the input, and random distributed feedback (DFB) fibre laser with one FBG. It was also found that lowering the reflection near the input could mitigate the RIN of the signal significantly, thanks to the reduced efficiency of the Stokes shift from the FW-propagated pump. In order to evaluate the transmission performance, different RFL based amplifiers were evaluated and optimised in long-haul coherent transmission systems. The results showed that Fabry-Perot fibre laser based amplifier with two FBGs gave >4.15 dB Q factor penalty using symmetrical bidirectional pumping, as the RIN of the signal was increased significantly. However, random distributed feedback fibre laser based amplifier with one FBG could mitigate the RIN of the signal, which enabled the use of bidirectional second order pumping and consequently give the best transmission performance up to 7915 km. Furthermore, using random DFB fibre laser based amplifier was proved to be effective to combat the nonlinear impairment, and the maximum reach was enhanced by >28% in mid-link single/dual band optical phase conjugator (OPC) transmission systems. In addition, unrepeatered transmission over >350 km fibre length using RFL based amplification technique were presented experimentally using DP-QPSK and DP-16QAM transmitter.

Fibre-based devices for next generation photonics communication systems

The future broadband information network will undoubtedly integrate the mobility and flexibility of wireless access systems with the huge bandwidth capacity of photonics solutions to enable a communication system capable of handling the anticipated demand for interactive services. Towards wide coverage and low cost implementations of such broadband wireless photonics communication networks, various aspects of the enabling technologies are continuingly generating intense research interest. Among the core technologies, the optical generation and distribution of radio frequency signals over fibres, and the fibre optic signal processing of optical and radio frequency signals, have been the subjects for study in this thesis. Based on the intrinsic properties of single-mode optical fibres, and in conjunction with the concepts of optical fibre delay line filters and fibre Bragg gratings, a number of novel fibre-based devices, potentially suitable for applications in the future wireless photonics communication systems, have been realised. Special single-mode fibres, namely, the high birefringence (Hi-Bi) fibre and the Er/Yb doped fibre have been employed so as to exploit their merits to achieve practical and cost-effective all-fibre architectures. A number of fibre-based complex signal processors for optical and radio frequencies using novel Hi-Bi fibre delay line filter architectures have been illustrated. In particular, operations such as multichannel flattop bandpass filtering, simultaneous complementary outputs and bidirectional nonreciprocal wavelength interleaving, have been demonstrated. The proposed configurations featured greatly reduced environmental sensitivity typical of coherent fibre delay line filter schemes, reconfigurable transfer functions, negligible chromatic dispersions, and ease of implementation, not easily achievable based on other techniques. A number of unique fibre grating devices for signal filtering and fibre laser applications have been realised. The concept of the superimposed fibre Bragg gratings has been extended to non-uniform grating structures and into Hi-Bi fibres to achieve highly useful grating devices such as overwritten phase-shifted fibre grating structure and widely/narrowly spaced polarization-discriminating filters that are not limited by the intrinsic fibre properties. In terms of the-fibre-based optical millimetre wave transmitters, unique approaches based on fibre laser configurations have been proposed and demonstrated. The ability of the dual-mode distributed feedback (DFB) fibre lasers to generate high spectral purity, narrow linewidth heterodyne signals without complex feedback mechanisms has been illustrated. A novel co-located dual DFB fibre laser configuration, based on the proposed superimposed phase-shifted fibre grating structure, has been further realised with highly desired operation characteristics without the need for costly high frequency synthesizers and complex feedback controls. Lastly, a novel cavity mode condition monitoring and optimisation scheme for short length, linear-cavity fibre lasers has been proposed and achieved. Based on the concept and simplicity of the superimposed fibre laser cavities structure, in conjunction with feedback controls, enhanced output performances from the fibre lasers have been achieved. The importance of such cavity mode assessment and feedback control for optimised fibre laser output performance has been illustrated.

Ultrashort pulse generation from diode laser devices

This thesis presents a detailed, experiment-based study of generation of ultrashort optical pulses from diode lasers. Simple and cost-effective techniques were used to generate high power, high quality optical short pulses at various wavelength windows. The major achievements presented in the thesis is summarised as follows. High power pulses generation is one of the major topics discussed in the thesis. Although gain switching is the simplest way for ultrashort pulse generation, it proves to be quite effective to deliver high energy pulses on condition that the pumping pulses with extremely fast rising time and high enough amplitude are applied on specially designed pulse generators. In the experiment on a grating-coupled surface emitting laser (GCSEL), peak power as high as 1W was achieved even when its spectral bandwidth was controlled within 0.2nm. Another experiment shows violet picosecond pulses with peak power as high as 7W was achieved when the intensive electrical pulses were applied on optimised DC bias to pump on InGaN violet diode laser. The physical mechanism of this phenomenon, as we considered, may attributed to the self-organised quantum dots structure in the laser. Control of pulse quality, including spectral quality and temporal profile, is an important issue for high power pulse generation. The ways to control pulse quality described in the thesis are also based on simple and effective techniques. For instance, GCSEL used in our experiment has a specially designed air-grating structure for out-coupling of optical signals; hence, a tiny flat aluminium mirror was placed closed to the grating section and resulted in a wavelength tuning range over 100nm and the best side band suppression ratio of 40dB. Self-seeding, as an effective technique for spectral control of pulsed lasers, was demonstrated for the first time in a violet diode laser. In addition, control of temporal profile of the pulse is demonstrated in an overdriven DFB laser. Wavelength tuneable fibre Bragg gratings were used to tailor the huge energy tail of the high power pulse. The whole system was compact and robust. The ultimate purpose of our study is to design a new family of compact ultrafast diode lasers. Some practical ideas of laser design based on gain-switched and Q-switched devices are also provided in the end.

Narrow-band generation and mode correlations in a short Raman fibre laser

We design a Raman fibre laser with a short cavity providing narrow-band generation. The laser is based on a commercial single-mode fibre (980-HP) span of 12 m length. The laser generates up to 11 W of intracavity power. Even at high generation power, the laser spectrum is narrow (less than 200 pm) - several times narrower than for conventional Raman fibre lasers based on longer fibres. The intensity dynamics reveals indications of mode correlations. © 2014 Astro Ltd.

Efficient yellow-green light generation at 561 nm by frequency-doubling of a QD-FBG laser diode in a PPLN waveguide

A compact high-power yellow-green continuous wave (CW) laser source based on second-harmonic generation (SHG) in a 5% MgO doped periodically poled congruent lithium niobate (PPLN) waveguide crystal pumped by a quantum-dot fiber Bragg grating (QD-FBG) laser diode is demonstrated. A frequency-doubled power of 90.11 mW at the wavelength of 560.68 nm with a conversion efficiency of 52.4% is reported. To the best of our knowledge, this represents the highest output power and conversion efficiency achieved to date in this spectral region from a diode-pumped PPLN waveguide crystal, which could prove extremely valuable for the deployment of such a source in a wide range of biomedical applications.

The essentials of teamworking:international perspectives

The team, rather than the individual, is increasingly seen as the building block of organizations and a key source of competitive advantage. Despite this, not enough is understood about how to build successful teams in modern organizations. The Essentials of Teamworking broadens this understanding by offering a selection of key chapters on teamwork from the International Handbook of Organizational Teamwork and Cooperative Working. This concise paperback edition reveals the complexity of teamwork and offers empirically based guidance on how teamwork can be effectively developed in modern organizations. Bringing together leading international scholars, The Essentials of Teamworking offers challenging perspectives on teamwork that will inform future research and practice. It is an invaluable resource for professionals, researchers and students alike.

A refractometer based on a micro-slot in a fiber Bragg grating formed by chemically assisted femtosecond laser processing

A liquid core waveguide as a refractometer is proposed. Microtunnels were created in standard optical fiber using tightly focused femtoscond laser inscription and chemical etching. A 1.2(h)x125(d) x500(l) µm micro-slot engraved along a fiber Bragg grating (FBG) was used to construct liquid core waveguide by filling the slot with index matching oils. The device was used to measure refractive index and sensitivity up to 10- 6/pm was obtained.

Directional bend sensor based on an asymmetric modification of the fiber cladding by femtosecond laser

A femtosecond laser was used to modify a part of the cladding of a standard LPG bend sensor. The device produced wavelength shifts depending upon the direction of bend, thus making a shape sensor.

Vector bending sensors based on fibre Bragg gratings inscribed by infrared femtosecond laser

A novel, direction-sensitive bending sensor based on an asymmetric fiber Bragg grating (FBG) inscribed by an infrared femtosecond laser was demonstrated. The technique is based on tight transverse confinement of the femto-inscribed structures and can be directly applied in conventional, untreated singlemode fibers. The FBG structure was inscribed by an amplified, titanium sapphire laser system. The grating cross-section was elongated along the direction of the laser beam with the transverse dimensions of approximately 1 by 2 μm. It was suggested that the sensitivity of the device can be improved by inscribing smaller spatial features and by implementing more complex grating designs aimed at maximizing the effect of strain.

Vector bending sensors based on fiber bragg gratings inscribed by an infrared femtosecond laser

A direction-sensitive bend sensor in standard single-mode fiber is demonstrated for the first time based on an axially-offset fiber Bragg grating, directly written by an infrared femtosecond laser.

A new method for microwave generation and data transmission using DFB laser based on fiber Bragg gratings

A novel architecture for microwave/millimeter-wave signal generation and data modulation using a fiber-grating-based distributed feedback laser has been proposed in this letter. For demonstration, a 155.52-Mb/s data stream on a 16.9-GHz subcarrier has been transmitted and recovered successfully. It has been proved that this technology would be of benefit to future microwave data transmission systems.

Single- and dual-wavelength switchable erbium-doped fiber ring laser based on intracavity polarization selective tilted fiber gratings

We propose and demonstrate a single- and dual-wavelength switchable erbium-doped fiber laser (EDFL) by utilizing intracavity polarization selective filters based on tilted fiber gratings (TFGs). In the cavity, one 45° TFG functions as an in-fiber polarizer and the other 77° TFG is used as a fiber polarization dependent loss (PDL) filter. The combined polarization effect from these two TFGs enables the laser to switch between the single- and the dual-wavelength operation with a single-polarization state at room temperature. The laser output at each wavelength shows an optical signal-to-noise ratio (OSNR) of >60 dB, a side mode suppression ratio (SMSR) of >50 dB, and a polarization extinction ratio of ~35 dB. The proposed EDFL can give stable output under laboratory conditions.