Hollow-core photonic crystal fibres have excited interest as potential ultra-low loss telecommunications fibres because light propagates mainly in air instead of solid glass. We propose that the ultimate limit to the attenuation of such fibres is determined by surface roughness due to frozenin capillary waves. This is confirmed by measurements of the surface roughness in a HC-PCF, the angular distribution of the power scattered out of the core, and the wavelength dependence of the minimum loss of fibres drawn to different scales.

Figure 4 shows the normalized output power against the tilt angle with FBG (squares). The normalized output power shows a monotonic decrement against the tilt angle, as expected. For comparison, the normalized output power against the tilt angle without FBG was also measured, and the results are given in Figure 4 (circles). From the results, we can see that the FBG can effectively enhance the tilt angle sensitivity of the sensor.The Bragg wavelength shift of the FBG with temperature was directly recorded by monitoring the wavelength shift using an optical spectrum analyzer. The resolution of the optical spectrum analyzer is limited (0.01 nm). The temperature response of the senor is shown in Figure 5. A sensitivity of about 0.0121 nm/°C was obtained. The temperature sensitivity of the sensor can be enhanced by packaging with a material with higher temperature sensitivity. CONCLUSIONA sensor based on a fiber taper and an FBG for simultaneous measurement of the tilt angle and temperature has been presented. The sensor is operated in the reflection mode. The magnitude of the tilt angle can be obtained by monitoring the tilt-induced loss of the light that passes the taper and is reflected by the FBG. The fiber grating can not only enhance the tilt angle sensitivity but also measure the surrounding temperature by the wavelength shift. The sensor is simple and compact. SURFACE-MOUNT LOOP ANTENNA FOR AMPS/GSM/DCS/PCS

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