Acknowledgements

gas phase materials within the hollow core. This provides an excellent means for strong light/molecular interaction inside the fiber core over long distance.

Critical issues related to distributed, high sensitivity, fast response sensors are mainly governed by the time required for gas to diffuse into the holes and open ends of the sensor fiber. One can introduce a number of transverse holes by using micro-structuring techniques. Various techniques for transverse micro-structuring have been reported. Hensley et al. [45] have reported gas cell fabrication using femtosecond micro-machining. Tightly focused laser pulses are used to produce micro-meter diameter radial channels in hollow core photonic crystal fiber, and through these micro-channels, the core of the fiber is filled with test gas. There are some reports about fabrication of microchannel by using 193 nm ArF laser. Viola et al. [43] have reported micro-hole drilling in hollow core mid-IR transmitting

Distributed gas sensors rely on depositing specific gas sensitive coatings as cladding in multi-mode fibers or gratings and OTDR method. Recently, many designs and configurations using PCFs have been reported which targeted better sensitivity and distributed detection capability. The advantage of waveguide-based sensors over open path multiple reflection cells included low gas volume, high photon to molecular interaction, and reduction of fringe effects. Whitenett et al. [47] reported the operation of a 64 point fiber optic methane sensor installed on a landfill site in Glasgow, UK. Though the environmental conditions are harsh, the sensor has performed satisfactorily, detecting methane in the range of 50 ppm to 100% methane. Viola et al. [42] have reported development of a distributed nerve gas sensor based on mid-IR spectroscopy. This has been possible due to availability of tunable quantum cascade lasers in the 9–10 μm range, suitable hollow fibers for

India is poised to use nuclear power in a big way. The safety of these power stations will depend on monitoring the radiation levels near plant and at waste disposal sites. In such environments, conventional sensors have certain limitations. Fiber-based sensors are being developed all over the world and are expected to make significant contribution to safe operation of nuclear fuel cycle. New technologies of laser micro- and nano-processing, mid-IR transmitting fibers, and hollow fibers have opened development of new structures and devices like fast response PCF-based gas sensors, high temperature Bragg gratings, distributed nerve gas sensors, and distributed nuclear radiation sensors for home land security. On the other hand, availability of all silica nuclear resistant fibers and nano-fibers and possibility of grating writing in such fibers using focused ion beam source and femto-sec lasers have attracted their use for different parameter monitorings like structural health monitoring in nuclear reactors, tokamaks, and storage facilities.

6.6.3 Enabling technologies

Applications of Optical Fibers for Sensing

fibers by means of high power CO2 laser shots.

this band, and low noise cooled detectors.

7. Conclusion

74

6.6.4 Recent developments and existing fiber-based systems

The authors are thankful for the help received from Shri Jai Kishore, Smt. Smita Chaubey, and Shri Sanjai Kumar of Fiber Sensors Lab., RRCAT, Indore, in conducting various experiments.
