In DTS technique, a pulse laser is coupled to an optical fiber through a directional coupler. The light is backscattered throughout the fiber due to changes in density and composition as well as due to molecular and bulk vibrations. The backscattered light consists of Rayleigh component, a Brillouin component and a Raman component. Thermally influenced molecular vibrations cause the Raman backscattered component to change and therefore it is sensitive to temperature. The anti-stokes component is strongly dependent upon temperature while, stokes component is very weakly dependent on temperature. Therefore, the ratio of anti-stokes to stokes signal provides an absolute value of temperature irrespective of laser power, launch conditions, fiber geometry etc. Combining the temperature measurement technique of Raman backscatter with distance measurement through time-of-flight of light, the DTS provides temperature measurements along the length of fiber.

The commercially available DART systems with sub-meter resolution are very expensive and utilize complex signal processing techniques such as time-correlated single photon counting and/or use expensive high power pulsed lasers. It is in this context significant that we achieved sub-meter DART system with in-expensive diode laser and relatively straightforward signal processing technique. A portable Raman sensor system has been installed in one of the DAE units and it is expected that the unit will help improve the safety aspects of operation of the units.

We have reported the highest sensitivity of 0.38 nm / 0 C in photosensitive fiber by optimizing various grating writing parameters. Efforts are on to develop device grade sensors using long period gratings.

Fabrication of photonics crystal fibre, like in conventional fibre fabrication, starts with a fibre preform. Conventional preforms are formed by using either modified chemical vapour deposition (MCVD) or vapour axial depostion (VAD) process. Microstructured preforms are formed by stacking a number of capillary silica tubes and rods to form the desired air/silica structure, fusing the stack into a perform, and then pulling the perform to fibre at a temperature sufficiently low (~ 1950 0 C) to avoid hole collapsing. When the desired preform has been constructed, it is drawn to a fibre in a conventional high-temperature drawing tower and hair-thin photonic crystal fibres are readily produced in kilometer lengths. This way of creating the preform allows a high level of design flexibility as both the core size and shape as well as the index profile throughout the cladding region can be controlled. Through careful process control, the air holes retain their arrangement all through the drawing process and even fibres with very complex designs and high air filling fraction can be produced. Finally, the fibres are coated to provide a protective standard jacket that prevents micro bending and also allows robust handling of the fibres.

Photonic crystal fibres have been fabricated of various hole sizes from 25 μm to 2.5 μm. In a typical PCF fibre, we could fabricate hole of 4 μm in a 125 μm fibre with the fibre length of few tens of meters. In most of the cases, the PCF fibres showed endlessly single mode behavior from 544 nm to 1.55 μm. The fibres with large air-holes showed high sensitivity to pressure. Since, making of micron size air-holes uniform along lengths is a significant technological challange, our efforts were directed to develop this part. The figure depicts the microscope image of cross section of such type of small hole PCF fibre developed in our laboratory.

Figure: Photonic crstal fibre drawn at RRCAT. Diameter of fibre is 125 μm, with air holes of 4 μm running across the fibre