Fibre Lasers and Photonic Components
Low-power optical fibre ring lasers have a number of possible applications as coherent sources for optical fibre sensors based on interferometry (for example, for acoustic and magnetic field sensing) and in optical fibre spectroscopy (for gas and chemical sensing). However to realise their full potential, there is a need to ensure stable, single-mode operation as well as the ability to tune the laser over the full gain-bandwidth available from erbium-doped fibre (~1500–1620nm) or from other fibre dopants. Current research involves the investigation of a variety of techniques for stabilisation and tuning, including the use of secondary optical cavities and saturable absorbers and on the development of scanning methods for interrogation of gas absorption lines. The research work on fibre laser systems also involves the use of intra-cavity techniques where a sensor element is directly placed within the fibre loop cavity in order to improve sensitivity (for example, when detecting the weak near-IR absorption lines of gases such as carbon dioxide or hydrogen sulphide). The two methods currently under investigation are “active-cavity ring-down spectroscopy” and “intra-cavity laser absorption spectroscopy” (ICLAS). For this work, a number of theoretical models of laser operation, both to predict the steady-state operation and to model the dynamic response of fibre lasers when the pump power is subjected to step or pulsed inputs, have been developed. In other areas, mode-locked operation of fibre lasers, with dispersion tuning, has been investigated.
High power fibre laser systems offer high efficiency and high output power without the need for active cooling. As such they are replacing bulk systems for many applications including surgery, industrial marking and materials processing/machining. To achieve high output power, both fibre lasers and semi-conductor lasers in combination with high power erbium, ytterbium and Raman amplifiers are being investigated. One of the key applications being addressed is gas sensing from airborne and /or land based vehicles e.g. searching for gas leaks in long distance pipelines using helicopter based systems.
Single Mode, Tunable Operation of Fibre Lasers For Sensor Applications
Design and Construction of a Multi-Wavelength Fibre Laser in the C- or L-Band for Optical Sensing
Micro-structured Optical Fibre (MOF): A UK - India (UKIERI) Collaboration
Optical MEMS and Microsystems
The MEMS/microsystem activity has concentrated on applying foundry manufacturing technology for the realisation of single and multi-layer silicon MEMS. Expertise lies in all stages from modelling and design through to experimental realisation of MEMS devices. Complex, multi-layer silicon micromechanical systems with applications in the optoelectronics and radio frequency domains have been designed and fabricated. State-of-the-art design packages, such as COVENTOR, MEMSPRO and ANSYS, which are relevant to simulation, design and modelling of microstructures and microsystems are used. Optoelectronic MEMS devices, including adjustable micromirrors, tunable filters, variable optical attenuators, variable slits and optical fibre positioning devices, have been investigated. In the RF domain, we have been involved in designing low-loss microwave waveguides and tunable RF components on silicon substrates. Many of our MEMS devices have been fabricated using POLYMUMPs (a polysilicon surface micromachining process), SOIMUMPs (a silicon-on-insulator surface micromachining process) and METALMUMPs (a metal/silicon surface micromachining process). All of these are commercial foundry processes. The advantage of working with a commercial foundry is in the seamless transfer to manufacturing from initial proof-of-concept research.
Non-silicon MEMS work is also undertaken by use of an excimer laser based projection micromachinng system which allows microscale patterning of polymer and other organic films, ceramic surfaces and thin layers of metal. Novel laser micromachined substrates have been fabricated for investigating the growth of cartilage cells in tissue engineering research.
Alignment and Fixing of Micro-Optical Components using MEMS
Design and Characterisation of Dual Shutter for Optical Chopper Application
Electrically Settable Vernier Latching Mechanism for Micro-Optical Components using MEMS
Electro-Thermal-Mechanical Modelling of MEMS Actuators
Force Characterisation of Scratch Drive Actuator
Hybrid Optical MEMS Tunable Filter
Microactuated Spherical Retroreflector for Free Space Communication
Modelling of Scratch Drive Actuator
Modification of the Classical Micro-Electro-Thermal Actuator
Tissue Engineering Microdevice for Cartilage Cell Monitoring
Photonic Sensors and Systems
Optical techniques for gas sensing are growing in popularity on the strength of their intrinsic safety, electromagnetic immunity and ease of maintenance. In particular, interest in tunable diode laser spectroscopy (TDLS) is growing dramatically as practical applications benefit from its calibration stability, its large dynamic range and its compatibility with optical fibre sensing. The following key areas of TDLS gas sensing are being addressed:
- Simultaneous measurement of gas concentration and pressure from TDLS signals for industrial process control e.g. for fuel cell applications and autoclave sterilisation processes. Concentration is obtained from line strength measurements and pressure is obtained from the linewidth.
- Translation of near infra-red (IR) TDLS techniques to the mid IR using amplified near IR diode lasers and waveguide parametric difference frequency generators. This will enable the powerful combination of addressing the stronger absorption lines of the mid IR with the powerful signal recovery techniques of TDLS to achieve enhanced sensitivity.
Innovative material testing using photonic systems is the second major theme of sensor research. Here, very high frequency (hundreds of MHz and beyond) ultrasound generated using laser sources offers unparalleled spatial and temporal bandwidth with precise control of optical energy densities. We have detected ultrasound using innovative interferometry within these frequency bands so that complete micro miniature systems can be realised. The basic technique has been demonstrated at medium scale (millimetres to centimetres) and recovery of physical parameters such as Young’s modulus, shear modulus and Poisson ratio is relatively straightforward. These non-contact techniques point towards both measurements on nano and microstructures and non-contact damage assessment and monitoring systems for critical areas in structures such as aircraft and ground based transportation systems.
Calibration of Pressure Effects in Water Vapour Sensing using the 1370nm Absorption Line
Damasens: Damage Detection in Carbon Fibre Automotive Structures
Design and Construction of a Fibre Laser System with Application to Gas Sensing
Optical Correlation Spectroscopy
Distributed Fibre Optic Fluid Sensors
Intra-Cavity Laser Absorption Spectroscopy With Fibre Lasers
Optical Fibre Vibration Detection
Optical Techniques for Examining Mechanical Materials
Prospect of Low Power Diode Laser to Generate Lamb Wave in a Plate-like Structure
Tunable L-Band Multi-Wavelength Fibre Laser with Application to Gas Sensing
Difference Frequency Generation of Light in the 3micron Range