NOVEL COMPACT NARROW-LINEWIDTH MID-INFRARED LASERS FOR SENSING APPLICATIONS

The mid-infrared (2-14 μm) spectral region contains the strong absorption lines of many important molecular species, which make this region crucial for several well-know applications such as spectroscopy, chemical and biochemical sensing, security, and industrial monitoring. To fully exploit this region through absorption spectroscopic techniques, compact and low-cost narrow-linewidth (NLW) mid-infrared (MIR) laser sources are of primary importance. This thesis is focused on three novel compact NLW MIR lasers: demonstration and characterization of a new glass-based spherical microlaser, investigation of the performance of a novel fiber laser, and the design of a monolithic laser on a silicon chip. Starting with fabrication of spherical microcavities based on MIR transparent materials, I showed the feasibility of achieving quality factors of more than 10 million in whispering- gallery mode (WGM) microresonators made of different types of fluoride glasses. Next using Erbium doped ZBLAN glass spherical microresonators, I demonstrated a new ultra- low threshold NLW MIR microlaser. In particular, all aspects of this room temperature continuous-wave (CW) microlaser with a wavelength of 2.71 μm are carefully characterized and studied and the origin of the measured mode structure and polarization is described using a simple analysis. To amplify the output power of this laser, I designed and fabricated a MIR fiber amplifier with a record gain of about 30 dB at 2.71 μm that facilitated the characterization process and boosted the MIR power level to usable level while preserving the laser linewidth. To demonstrate the application of MIR microresonators and microlasers, I studied intracavity absorption spectroscopy based on active and passive high quality WGM MIR microlasers and microresonators. I also estimated the sensitivity and detection limit of gas sensors based on these devices. The outcome of my analysis shows that ppm level sensitivity should be achievable using both active and passive microresonators. Next, I modeled the performance of two newly proposed configurations for NLW MIR generation based on stimulated Raman scattering. First, I studied a new family of Raman fiber lasers that are capable of generating any NLW MIR line in the 2.5-9.5 μm spectral region. I demonstrated the feasibility of this MIR laser family, calculated the threshold conditions, identified the condition for its single-mode operation, and laid the foundation for the first experimental demonstration of such lasers. Finally, I explored the performance of silicon-based on-chip Raman lasers and the parameters that have prevented expanding their wavelength to MIR range. Using the outcomes of this study, I proposed and then analyzed a new architecture for on-chip silicon Raman lasers capable of generating single NLW lines around 3.2 μm with sub-mW threshold pump power

Development of multi-element fibres for applications in space-division multiplexing

This thesis presents a novel multi-element fibre (MEF) technology for implementing space-division multiplexing (SDM) in optical fibres. MEF comprises multiple fibre-elements that are drawn and coated together using a common polymer coating. In MEF, the fibre-elements are compatible with current technology i. e. the fibre-elements can be directly fusion spliced to standard single mode pigtail fibre. Thus, a smooth upgrade from WDM based systems to SDM system is possible. In this work, MEF technology has been implemented for both, passive SDM fibres and SDM amplifiers.Erbium-doped Core-pump MEF amplifiers have been demonstrated exhibiting similar gain and noise figure performance to conventional Er-doped fibre amplifier while maintaining ultralow crosstalk levels. In addition, an Erbium/Ytterbium-doped cladding-pumped MEF amplifier has been developed, and a novel technique to achieve a broadband gain has been demonstrated which could cover wavelength region of 1536nm-1615nm using a single multimode pump. Furthermore, MEF technology has been combined with mode-division multiplexing to show that higher spatial multiplicity could be achieved by implementing the MEF with other SDM technologies.In passive MEFs, the fabricated fibres have been characterised for their loss and transmission properties, showing low loss and error-free transmission. Also, the MEFs are proof-tested showing high strength. The compatibility of MEF fibres have been tested in a concatenated SDM system demonstrating their flexibility in the telecom network

Advances in small lasers

M.T.H was supported by an Australian Research council Future Fellowship research grant for this work. M.C.G. is grateful to the Scottish Funding Council (via SUPA) for financial support.Small lasers have dimensions or modes sizes close to or smaller than the wavelength of emitted light. In recent years there has been significant progress towards reducing the size and improving the characteristics of these devices. This work has been led primarily by the innovative use of new materials and cavity designs. This Review summarizes some of the latest developments, particularly in metallic and plasmonic lasers, improvements in small dielectric lasers, and the emerging area of small bio-compatible or bio-derived lasers. We examine the different approaches employed to reduce size and how they result in significant differences in the final device, particularly between metal- and dielectric-cavity lasers. We also present potential applications for the various forms of small lasers, and indicate where further developments are required.PostprintPeer reviewe

Ultrashort, High Power, And Ultralow Noise Mode-locked Optical Pulse Generation Using Quantum-dot Semiconductor Lasers

This dissertation explores various aspects and potential of optical pulse generation based on active, passive, and hybrid mode-locked quantum dot semiconductor lasers with target applications such as optical interconnect and high speed signal processing. Design guidelines are developed for the single mode operation with suppressed reflection from waveguide discontinuities. The device fabrication procedure is explained, followed by characteristics of FP laser, SOA, and monolithic two-section devices. Short pulse generation from an external cavity mode-locked QD two-section diode laser is studied. High quality, sub-picosecond (960 fs), high peak power (1.2 W) pulse trains are obtained. The sign and magnitude of pulse chirp were measured for the first time. The role of the self-phase modulation and the linewidth enhancement factor in QD mode-locked lasers is addressed. The noise performance of two-section mode-locked lasers and a SOA-based ring laser was investigated. Significant reduction of the timing jitter under hybrid mode-locked operation was achieved owing to more than one order of magnitude reduction of the linewidth in QD gain media. Ultralow phase noise performance (integrated timing jitter of a few fs at a 10 GHz repetition rate) was demonstrated from an actively mode-locked unidirectional ring laser. These results show that quantum dot mode-locked lasers are strong competitors to conventional semiconductor lasers in noise performance. Finally we demonstrated an opto-electronic oscillator (OEO) and coupled opto-electronic oscillators (COEO) which have the potential for both high purity microwave and low noise optical pulse generation. The phase noise of the COEO is measured by the photonic delay line frequency discriminator method. Based on this study we discuss the prospects of the COEO as a low noise optical pulse source

Rare-earth elements doped novel photonics sources

This thesis presents the work carried out on the development of novel photonic sources based in rare-earth doped ions. It discusses in detail the properties of rare earth ions and its applications. The three major components of this work, namely, rare-earth doped solid state hosts, rare-earth doped speciality fibres, and rare-earth doped waveguide lasers have been presented in different chapters. The host glasses for the rare-earth doped gain mediums have been prepared by the traditional melt-quenching technique and spectroscopic studies have been carried out on them. Experiments to realise multi-wavelength lasers operating in the visible range have been carried out on the samarium doped phosphate glasses, owing to samarium‟s unique multiple emission peaks at 561 nm, 596 nm, and 643 nm with violet-blue excitation. Due to the relatively low emission cross section value of trivalent samarium ions (3.911 X 10-22 cm2 at 596 nm), it requires a much higher pump power. Due to the lack of high pump power diodes in the violet wavelength range, laser action could not be demonstrated. Further spectroscopic investigations on the samarium doped glasses and crystals revealed that the presence of excited state absorption could be a factor as well which discourages the realisation of laser emission in the sample. Rare-earth doped multicore optical fibres have been designed and fabricated for the realisation of active multiplexer elements and multi-wavelength lasers. Optical fibres with six cores and two cores respectively have been fabricated. Each of the six cores of the fibre were doped with erbium with the aim to develop active multiplexer elements which could incorporate multiplexing and amplification together. The cores showed considerable gains, with the maximum gain of around 30 dB – 40 dB in the wavelength range of 1500 nm – 1600 nm. The cores of the two core fibre were doped with ytterbium and erbium/ytterbium with the aim to demonstrate simultaneous laser action at 1 μm and 1.5 μm. The fibre, upon cladding pumping at 976 nm, demonstrated simultaneous laser emissions at 1061 nm and 1536 nm from the ytterbium and erbium/ytterbium doped cores, respectively. The laser action was observed with Fresnel reflection from the parallel cleaved facets of the fibre. The slope efficiency of the emission for both the cores were ~1%, which is quite low, considering the Fresnel reflection lasing. CW modelocked waveguide laser has been demonstrated in ytterbium doped bismuthate glasses. The waveguides were inscribed by the ultrafast laser inscription technique. The waveguide laser operated at the repetition rate of around 1.94 GHz with the pulse duration of about 1.1 ps at the wavelength of 1029 nm

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

High power mode locked lasers monolithically integrated with semiconductor optical amplifiers

This thesis is concerned with the design, fabrication and characterisation of high power semiconductor mode locked lasers (SMLLs), operating at ~ 1.5 μm. The devices are based on a novel epitaxial structure with three quantum wells (QW) in the active region. The novel epitaxial structure was based on a commercially available five-QW AlGaInAs/InP epitaxial structure, with the number of QWs reduced to increase the gain saturation energy and a farfield reduction layer (FRL), and a spacer layer were inserted in the n-cladding layer. SMLLs based on both the five-QW and three-QW material, were fabricated and comparatively investigated. The devices based on the three-QW material exhibited an increased average output power, as well as reduced RF linewidth and pulse widths. The average output power obtained in the mode locked operation from a 40 GHz MLL, based on this three-QW material was limited to 28 mW. Single mode ridge waveguide semiconductor optical amplifiers (SOAs) were monolithically integrated with the SMLLs, which increased the average output power to 130 mW. The devices performance was investigated at both the saturable absorber (SA) and SOA facets. Transform limited pulses with a minimum duration of 3.3 ps and a peak power of > 1 W were obtained at the SOA facet. To take advantage of the higher saturation output power of the tapered SOAs, SMLLs were monolithically integrated with 2° and 6° tapered SOAs, respectively. The devices integrated with 2° tapered SOAs were mounted on Aluminium Nitride (AlN) sub-mounts using Gold-Tin (AuSn) soldering for better heat sinking. These devices resulted in an average output power of 200 mW, with corresponding peak power > 1.2 W. The SMLLs integrated with 6° tapered SOAs, mounted on conventional brass sub-mounts resulted in a lower output power (105 mW), compared to the devices integrated with ridge waveguide and 2° tapered SOAs, respectively

Investigation of in-situ parameter control in novel semiconductor optical amplifiers

Fibre optic networks form the backbone of modern communications systems. As demand for ever increasing bandwidth continues to grow, technologies that enable the expansion of optical networks will be the key to future development. The semiconductor optical amplifier (SOA) is a technology that may be crucial in future optical networks, as a low cost in-line amplifier or as a functional element. As fibre networks extend closer to the end user, economical ways of improving the reach of these networks are important. SOAs are small, relatively inexpensive and can be readily integrated in photonic circuits. Problems persist with the development of SOAs, however, in the form of a relatively high noise figure and low saturation output power, which limits their use in many circumstances. The aim of this thesis is to outline a concept for control of these parameters such that the SOA can achieve the performance required. The concept relies on the control of the carrier density distribution in the SOA. The basic characteristics of the SOA and how they are affected by changes in the carrier density are studied. The performance of the SOA in linear and high power transmission of CW and pulsed signals is determined. Finally, the wavelength conversion characteristics of the SOA are outlined. The role of the carrier density control in shaping all of these characteristics will be explained

Harnessing optical micro-combs for microwave photonics

In the past decade, optical frequency combs generated by high-Q micro-resonators, or micro-combs, which feature compact device footprints, high energy efficiency, and high-repetition-rates in broad optical bandwidths, have led to a revolution in a wide range of fields including metrology, mode-locked lasers, telecommunications, RF photonics, spectroscopy, sensing, and quantum optics. Among these, an application that has attracted great interest is the use of micro-combs for RF photonics, where they offer enhanced functionalities as well as reduced size and power consumption over other approaches. This article reviews the recent advances in this emerging field. We provide an overview of the main achievements that have been obtained to date, and highlight the strong potential of micro-combs for RF photonics applications. We also discuss some of the open challenges and limitations that need to be met for practical applications.Comment: 32 Pages, 13 Figures, 172 Reference