Staff Publications (Engineering & Physical Sciences)
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Item Compact low repetition rate optical parametric oscillators(Heriot-Watt University, 2024-08) Allan, Ewan James Duncan; McCracken, Doctor RichardOptical parametric oscillators (OPO) offer a potential route to a cost effective low repetition rate ultrafast infrared laser sources. The current favoured technology is optical parametric amplifiers (OPA), which require high powered pump laser sources to generate sufficient pulse energies, but these are large and expensive systems. For an OPO to achieve the same level of performance it would require overcoming some demanding engineering challenges. The cavity length of the OPO typically must match that of its pump laser, meaning to achieve sub-50 MHz repetition rates the cavity length of the OPO needs to be >6 m and for 1 MHz the OPO cavity would need to be 300 m long. This thesis explores OPO cavity designs that target the construction of a low repetition rate OPO within a compact footprint. The primary method investigated utilises an intracavity Herriott cell to store a large portion of the cavity length within a compact footprint. A Herriott cell is a type of multipass cell which is made up of two opposing spherical mirrors usually with a hole machined in one or both mirrors to allow a beam to enter and exit the cell. Once the beam enters the cell it is reflected multiple times, forming an elliptical pattern on the mirrors, with the number of reflections being determined by the separation distance of the mirrors. Incorporating a Herriott cell into a OPO cavity presents challenges for optimising the stable resonating mode, maintaining a Boyd-Kleinman focusing ratio near 1, and achieving the cavity length required for synchronous pumping. This was demonstrated in a synchronously pumped 49.16 MHz Herriott cell OPO producing femtosecond pulses from 1440 nm to 1530 nm with average signal powers up to 312.6 mW when pumped with 1.8W from the Yb:fiber pump laser, and extended to 12.29 MHz in a 12.2-m cavity. The next method demonstrated was a harmonically pumped idler resonant OPO, in which the OPO cavity mirrors are coated so that the longer wavelength idler now resonates in the cavity, and the signal leaves the cavity immediately after the nonlinear crystal. The cavity is made compact by constraining the cavity length to be a harmonic of the pump cavity length. This increases the repetition rate of the resonant idler, but the signal remains at the repetition rate of the pump laser. A 294.96 MHz idler resonant cavity was demonstrated producing femtosecond signal pulses with average powers up to 88 mW when pumped with 1.7 W of pump power. This is reduces the cavity to just 1/6th the size of a synchronously pumped OPO. Finally, fiber feedback OPOs have been demonstrated as a method to achieve a compact low repetition rate OPO, however the additional dispersion from the fiber can limit operation to the picosecond range. To address this modelling work was done investigating cascaded fiber systems where the fibers have complimentary dispersion coefficients, minimising the pulse broadening. A combination of SMF-28 and UHNA7 was used to show that at key wavelengths of 1550 nm, 1700 nm and 2090 nm a 50 fs pulse propagating thorough 1 m of this cascaded system sees minimal broadening with the shortest pulse seen for 1700 nm at just 59.7 fs.Item Fibre interferometry for differential measurements(Engineering and Physical Sciences, 2015-10) Smith, Martin Dignan; Maier, Doctor Robert; MacPherson, Doctor WilliamThis thesis investigates the use of interferometry as an interrogation technology for the measurement of differential length at two widely separate locations. Differential length measurements are essential and can have many applications in industrial processes, therefore accurate measurements can be a critical. Such differential length measurements can be applied to aspects of differential pressure. Using an all optical fibre approach, the research utilises the effects of light interference for both low coherent and high coherent light sources for the determination of a differential length between individual sensing cavities separated by up to 10’s of meters. The construction of the differential length interrogation system makes use of two Fabry-Perot cavities arranged in a tandem configuration, as a means of determining the differential length between them. Such an arrangement provides a common path through which an optical broadband light source at a central wavelength of 1550 nm can propagate. As a consequence of this configuration, differential lengths are made simply using one single measurement, removing the need to determine each individual length. An additional benefit of this common optical path prevents environmental factors such as temperature and air pressures from affecting the measurement length in question. Using a scanning reference Michelson interferometer to induce an optical path change, low coherence interference effects are present when the optical path length of the differential Fabry-Perot cavities is equal to the optical path length difference in the Michelson interferometer. Using a separate DFB laser light source to illuminate the reference interferometer high coherence interference fringes, present when the optical path length of one interferometer arm is changing due to a piezo fibre stretcher, can be analysed to provide an accurate length determination. Taking into consideration the noise within the system the interrogation technique has a length measurement resolution of 27.43 nm. Demonstrations show that a differential length of 82.539 μm could be measured with an uncertainty of 41.00 nm. Through the characterisation of a deformable silicon diaphragm, it would be possible to construct a sensing system capable of measuring a differential pressure of 1 Pa in 100 kPa. This however would require a 9.13 mm thick diaphragm, with a radius of 0.35 m. Such a diaphragm would be out of the question and so further investigation into reducing the length measurement resolution would need to be carried out.Item Development of tunable and miniature microwave filters for modern wireless communications(Heriot-Watt University, 2014-05) Ni, Jia; Hong, Jia-ShengDue to the increasing demand for new wireless services and applications, the high level of integration and the coexistence of multi-standard (MS) or multi-band operations into a single device are becoming defining trends in designing microwave filters. This has driven considerable technological advances in reconfigurable/tunable and miniaturized filters. More specifically, reconfigurable/tunable filters that tune to different frequency bands instead of classical filter banks have great potential to significantly reduce the system size and complexity; while reducing the filter size becomes essential to achieve the highest degree of integration density in compact and portable wireless devices. In the light of this scenario, the objective of this dissertation is to develop the new design technologies, concepts and filtering configurations for tunable microstrip filters and compact passive microwave filters. To this aim, this dissertation is divided into two main parts. The first part (Part I) focuses on the designs of novel varactor-tuned microstrip filters with advanced performances. In this aspect, new topologies for realizing tunable lowpass and highpass filters are firstly developed. State-of-the-art performances, including wide tuning range, high selectivity with multiple transmission zeros, low insertion loss and compact size for all the tuning states are obtained in both of these filters. Secondly, two novel classes of tunable bandpass filters are presented. One of them is designed based on varactor-loaded parallel-coupled microstrip lines (PCML) and short-circuited stubs, which allows the lower passband edge together with two transmission zeros located around the lower passband skirt to be reconfigured separately. While the other tunable bandpass filter is constructed by the combination of tunable bandpass and lowpass filters, featuring both centre frequency and bandwidth tunabilities, as well as high selectivity with abundant transmission zeros. Furthermore, a new concept of tunable lossy filter is demonstrated, which attempts to achieve an equivalent high-Q tunable performance by using low-Q resonators. This concept makes the presented tunable combline filter interesting for some frequency-agile applications in which the low in-band loss variation and high selectivity are much desired while the absolute insertion loss can be a tradeoff. The second part (Part II) is devoted to the design of miniaturized passive microwave filters with improved characteristics. For this, the concept of artificial right-handed and left-handed transmission lines are applied to the signal interference filtering topology, which results in a compact circuit size and good out-of-band performance. In particular, for a further size reduction, such filter is implemented in the forms of multilayered structure by using liquid crystal polymer (LCP) technology. Additionally, another two types of miniaturized bandpass filters using stepped impedance resonators are demonstrated, which are implemented based on different fabrication processes (i.e. LCP bonded multilayer PCB technology and a standard planar PCB technology). Among their main features, the compact size, wide passband, broad stopband with multiple transmission zeros and circuit simplicity are highlighted. For all the proposed design techniques and filtering structures, exhaustive theoretical analyses are done, and design equations and guide rules are provided. Furthermore, all the proposed schemes and/or ideas have been experimentally validated through the design, implementation and measurement of different filters. The fabrication processes of multilayer technology utilized: liquid crystal polymer (LCP) technology and liquid crystal polymer (LCP) bonded multilayer printed circuit board (PCB) technology, are also demonstrated for reference. All of the results achieved in this dissertation make the proposed filters very attractive for their use in modern wireless communication systems.