Gillian Whitenett

Stable single-mode operation of a narrow-linewidth, linearly polarized, erbium-fiber ring laser using a saturable absorber

This paper describes the design and operation of a stable narrow-linewidth linearly polarized fiber ring laser using a polarization-maintaining (PM) erbium-doped fiber as a saturable absorber. The effect of the PM fiber on suppressing mode hopping is experimentally demonstrated and optimum conditions for single-mode operation are identified. Laser output power is /spl sim/ 4.7 mW at 1535 nm for a pump power of 94 mW, the polarization extinction ratio is 24.8 dB, the SNR is larger than 45 dB, the relative intensity noise is below -104 dB/Hz at frequencies above 150 kHz, and the linewidth is less than 1.5 kHz. Potential applications of the fiber laser for interferometric or spectroscopic fiber sensors are briefly discussed.

Investigation of a tuneable mode-locked fiber laser for application to multipoint gas spectroscopy

This paper reports on an initial investigation into the operation of a mode-locked fiber laser system for application in gas spectroscopy as a multipoint multigas sensor. Wavelength selection is performed by use of multiple chirped gratings and fine tuning by using the dispersion properties of the chirped gratings. A tuning rate of /spl sim/0.014 nm per kHz change in mode-lock frequency (at the third harmonic) has been demonstrated, which is suitable for scanning across gas absorption lines. Key issues that have an important bearing on the tuning are discussed, including gain flattening and polarization drift. This paper investigates a method of multiplexing the sensor cells with the mode-locked system. Preliminary results for a two cavity system are presented to verify the principles of the technique.

Optical fibre instrumentation for environmental monitoring applications

We report our research on the development of optical fibre trace gas sensors for environmental applications. We describe the operation of a 64-point fibre-optic methane sensor, which has been installed on a landfill site in Glasgow, UK, where methane is used for power generation as part of the current trend for renewable energy programmes. Although the environmental conditions are harsh, the sensor has performed satisfactorily, detecting methane in the range of ~50 ppm to 100% methane. Another area of our current research is the application of erbium-doped fibre lasers and amplifiers in gas spectroscopy. One system under investigation consists of an all-fibre cavity ring-down loop employing a fibre amplifier for the compensation of loop loss. We have been able to obtain ring-down times as long as 0.2 ms, corresponding to ~1100 pulses in the loop, producing an effective increase in a gas cell length from 5 cm to 55 m. The mode-locked operation of fibre lasers is also under investigation and, using dispersion effects, we demonstrate fine tuning of the wavelength which is important for absorption line scanning, with a typical tuning rate of ~0.014 nm kHz−1 at the third harmonic, closely matching the theoretical predictions. Techniques for extending fibre laser systems to form multi-point, multi-species gas sensors are explored.

Design of a tunable L-band multi-wavelength laser system for application to gas spectroscopy

We present multi-wavelength operation of single and double pass fibre laser systems by use of a micro-optic cell that doubles as both a Fabry?Perot cavity and a gas cell. The lasers utilize a silicon wafer to provide multiple lasing wavelengths with spacing between each of the peaks of 1 nm in the L-band wavelength region. The comb spectrum produced by the laser can be tuned to match the L-band absorption lines of a gas by the addition of a programmable variable attenuator into the laser. One gas of interest is hydrogen sulphide (H2S); it has absorption lines over the region 1570 nm?1580 nm with spacing of around 1 nm. The multiple lasing peaks could be coarsely tuned with large attenuation (5?9 dB) close to the absorption lines and then finely tuned by variation of 0.05?0.1 dB. Finally, we were able to equalize the power of three of the lasing peaks in both the lasers by adding polarization controllers, allowing us to reduce the amplitude variation to 0.6 dB for the single pass laser.

Investigation of the Dynamic Response of Erbium Fiber Lasers With Potential Application for Sensors

We derive a theoretical description of the transient response for erbium fiber ring lasers, which includes the effects of amplified spontaneous emission and the large number of longitudinal modes that are present in large cavities. Based on these modified rate equations, the full transient period is simulated and compared with experimental measurements performed at 1560 nm, showing reasonable agreement with theory. Approximate analytical relations are also derived for the key characteristics of the laser response, including steady-state and transient parameters such as build-up time, frequency, and the decay constant of relaxation oscillations. The results are useful in the measurement of fiber laser parameters and in the design of novel fiber laser sensors, such as intracavity laser absorption sensors based on spectral narrowing during the transient period of power build-up in a laser cavity.

Phase noise characteristics of a diode-pumped Nd:YAG laser in an unbalanced fiber-optic interferometer

The measurement sensitivity of interferometric fiber sensors can be limited by the laser phase noise. We investigate the phase noise characteristics of a diode-pumped Nd:YAG laser in an unbalanced fiber-optic interferometer. Measurements were made of the phase-induced intensity noise of an interferometer with varying optical path differences. The experimental results of the relation between the noise and the optical path difference are given, and the application of the results in the design of a fiber-optic accelerometer is discussed.

Design of fiber laser and sensor systems for gas spectroscopy in the near-IR

Because of the ready availability of fibre optic components from the communications industry, fibre optic systems operating in the near-IR are well suited for remote, multi-point monitoring of hazardous and environmentallyimportant gases. However a number of challenges have to be met in order exploit the potential commercial opportunities and applications for such sensors. Here we review our research on gas sensors based on fibre laser systems and absorption spectroscopy. Fibre lasers are of particular interest for sensors since on-going developments have extended their wavelength range of operation over ~1480-1620nm, encompassing the near-IR absorption lines of numerous gases. We discuss several configurations for fibre laser systems which offer the prospect of either enhanced performance or the possibility of multiplexing a number of sensor cells. However, because gas absorption lines in the near-IR spectral region are relatively weak, high sensitivity techniques are required for a number of species and we discuss methods for path-length enhancement through ring-down and intra-cavity absorption spectroscopy. Effective interrogation methods are required to attain the benefits of the various forms of cavity enhanced spectroscopy in fibre optic systems and several techniques are under investigation to realise this potential.

A mode-locked fibre laser system for multi-point intra-cavity gas spectroscopy

We have described a new way of realising a multi-point, multi-gas sensor system based on a mode-locked fibre laser with potential application for trace gas detection in environmental and safety monitoring. Although based specifically on erbium-fibre lasers, the principles could be extended to other laser types, for example, Tm/sup 3+/ fiber lasers in silica glass or the use of semiconductor optical amplifiers which are commercially available for the 1300-1600 nm region, extending the range of gas sensors possible. Preliminary experiments have been conducted to test the principles and research is currently in progress to demonstrate a fully multiplexed system.

Remote gas analysis using fibre optic links and near infrared absorption

This paper describes the potential application of optical fibre addressed systems in remote gas spectroscopy. The paper will first describe the basic principles of the spectroscopic measurements and the reasons why such measurements find applications complementing more established mid infrared and electrochemically-based systems. We shall describe some practical field trials and the results obtained there from. Finally we shall discuss the potential offered by new approaches in laser design and system architectures to enhance the range of addressable species and the sensitivity which these remote detection systems may achieve.

Optical fibre sensors and networks for environmental monitoring

Describes the authors work on the development of fibre sensors and networks for monitoring trace gases such as methane, acetylene, carbon dioxide, carbon monoxide, hydrogen sulphide and for detection of spills of gasoline, diesel and organic solvents, all of which are important in environmental and safety management. As an example, a 45‐point fibre optic sensor network has been installed on a landfill site to assess the distribution of methane generation across the site. System operation is based on near‐IR absorption and is currently being extended to monitor other gases such as carbon dioxide and hydrogen sulphide. Concurrently, research is being conducted on fibre lasers for the realisation of multi‐point, multi‐gas monitoring systems. Based on other principles (periodic micro‐bending loss effects), detection of hydrocarbon fuel spills has been demonstrated at multiple locations along the length of a specially designed fibre optic cable using standard optical time domain reflectometry (OTDR) measurements.