Development of Photonic Crystal Fiber Based Gas/ Chemical Sensors

The development of highly-sensitive and miniaturized sensors that capable of real-time analytes detection is highly desirable. Nowadays, toxic or colorless gas detection, air pollution monitoring, harmful chemical, pressure, strain, humidity, and temperature sensors based on photonic crystal fiber (PCF) are increasing rapidly due to its compact structure, fast response and efficient light controlling capabilities. The propagating light through the PCF can be controlled by varying the structural parameters and core-cladding materials, as a result, evanescent field can be enhanced significantly which is the main component of the PCF based gas/chemical sensors. The aim of this chapter is to (1) describe the principle operation of PCF based gas/ chemical sensors, (2) discuss the important PCF properties for optical sensors, (3) extensively discuss the different types of microstructured optical fiber based gas/ chemical sensors, (4) study the effects of different core-cladding shapes, and fiber background materials on sensing performance, and (5) highlight the main challenges of PCF based gas/ chemical sensors and possible solutions

Stable single-mode operation of a narrow-linewidth, linear polarization erbium fibre 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 ∼ 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

Optical fibre sensors based on the dynamic response of fibre lasers

We propose a optical fibre sensor system based on performing measurements during the transient period of operation of fibre lasers which may have potential for realising several types of sensor. Detailed experimental and theoretical study of the dynamics of a erbium fibre ring laser have been carried out, with simulation of the full transient period in order to characterise its operation. The results reveal how certain features of the dynamic response may be analytically related to the cavity parameters but also indicate the important role of spontaneous emission in determining the exact details of the response due to the involvement of a large number of cavity modes during the relaxation period. The principles may be applied for sensing by use of an intra-cavity sensor element which modifies the cavity properties in response to the measurand and by analysis of the cavity output in both time and spectral domains

Investigation of a tuneable mode-locked fibre laser for application to multi-point 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

Investigation of the dynamic response of erbium fibre 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

Tuneable L band multi-wavelength fibre laser using silicon wafers

We discuss a continuous-wave (CW) multi-wavelength fibre laser operating in the L-band, which has applications to gas sensing. Multi-wavelength operation of between two to eight lasing peaks with a spacing of around 0.9nm between each peak was demonstrated by placing silicon wafers in a micro-optic cell in order to produce a Fabry-Perot effect. Tuning over a region of 1565 to 1595nm was done by adjustment of the attenuation within the laser cavity. For our system we are interested in wavelength tuning for the detection of gases such as Hydrogen Sulphide (H2S) and Carbon Dioxide (CO2)

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

This paper looks at a mode-locked fibre laser system for multi-point intra-cavity gas spectroscop

Design of fibre 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 environmentally-important 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 similar to1480-1620nm, 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

Optical fibre sensors for environmental monitoring of trace gases

The work described here concerns the development of fibre sensors and networks for monitoring trace gases such as methane, acetylene, carbon dioxide, carbon monoxide and hydrogen sulphide, all of which are important in environmental or safety monitoring. A 45-point fibre sensor network using a single DFB laser source has been installed on a landfill site to assess the distribution of methane generation across the site, with detection levels from <100ppm to 100% methane. The system is currently being extended for carbon dioxide and hydrogen sulphide monitoring. Concurrently, fibre lasers sources are under investigation to provide a single source for several gases using techniques such as mode-locked operation for interrogation of multi-point systems and ring-down spectroscopy for high sensitivity measurements