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The idea of this technique is to measure the reflected optical power due to Fresnel reflection from the fibre–fluid interface. A change in the reflected optical power measures the change in the refractive index of the fluid. However, to increase measurement resolution, the reflected power is normalized by a similar reflected power from a reference fibre–air interface, with both reflected signals derived from the same optical source, and detected by a common detector. The normalization procedure corrects for fluctuations that are common to the signal and reference arms. The idea mentioned above was implemented by the experimental setup shown in figure 1. A train of optical pulses, derived from a diode laser driven by a pulse generator, is launched into a standard telecommunications graded singlemode optical fibre (SMF-28 fibre). The optical pulse is divided into two paths by a 2×2 single-mode fused fibre couplerwith a nominal 50%:50% splitting ratio. The end of the signal fibre is immersed in the test fluid while the end of the reference fibre rests free in air. Returned pulses due to Fresnel reflections from both fibre ends are detected by a dc-coupled detector. The pulses are subsequently amplified and processed by a computer. The experiment is conducted in the pulse mode so that the reflected power from the signal and reference fibres can be temporally separated. Thus, the signal path consists of only a few metres of fibre while the reference path is a spool of long fibre. The length of the long fibre used is arbitrary as long as the returned signal and reference pulses can be temporally separated. The long fibre length is not a problem because the optical wavguide effect of the fibre remains intact independent of the fibre length, unless the fibre is excessively bent during the course of the experiment causing radiation loss. A pair of time-separated optical pulses reflected from the fibre–fluid and fibre–air interface is shown in figure 2. It is important to note that both pulses are derived from a common optical pulse but delayed in time by 2L/c, where L is the length difference between the two optical paths and c is the speed of light in the fibre. This assures that any pulse amplitude fluctuation will not limit the measurement precision. The first pulsewhich represents the overlay of two reflected pulses from the fibre–fluid interface with two different NaCl concentrations of 0 and 0.03 g cm−3 (3% salt) shows the amplitude dependence on solute concentration. The second pulse is the reference pulse. It is noted that, unlike the second pulse, the first pulse does not have a flat top. This is due to the additional contribution originating from backscattered Rayleigh light from the reference pulse travelling in the long fibre. However, the Rayleigh backscattered light does not affect the differential index measurement because its magnitude is proportional to the laser power which is corrected by this normalization technique. The amplitude of each pulse is made almost equal by adjusting the splice loss while splicing the reference arm to the fibre coupler when the tip of the signal arm is immersed in pure water. This gives the reference value for the amplitude ratio. Any deviation from this ratio is indicative of an index change. However, any NaCl concentration can be taken as the reference ratio. The technique gives a refractive index measurement resolution of about 2 × 10−5 and salt concentration resolution of about 2 × 10−4 g cm−3 limited by temperature and polarization effects. This ratio meter should be appropriate for various measurements, in particular for measurements of the spatially and temporally dependent concentration field in an environment such as a water tank. From: Benjamin Tay Chia Meng O-SING Issue 3 Page 10 of 12 AUGUST 26, 2004 Disclaimer: O-SING is published by the Student Chapter OSA(NTU), School of Electrical and Electronic Engineering, Nanyang Technological University. Block S1, Nanyang Avenue, Singapore 639798. Opinions expressed herein do not necessarily reflect the views of the University. Copyright c 2003 Student Chapter OSA(NTU), All rights reserved. Material in this publication may not be reproduced without written permission.
The first overseas conference for any PhD student is always a scary prospect, presenting your work in front of all the top researchers in your field while being a greenhorn is not that appealing for a student’s holiday trip. But I suppose it is a part of any graduate student’s life. This time, my time has come, I was summoned to the far ends of the world….USA. The air travel was a great drag as I took around 20 hours from Singapore to Denver on two transits and three airports at Japan, Los Angeles and finally Denver. Once I reached Denver, it was a direct trip to my hotel room to relax myself. I checked into the WARWICK Hotel situated on Grant Street in Denver Downtown taking the SKYShuttle from the airport at a cost of USD$18 per trip. My BEDS ARE WAITING FOR ME A VERY LARGE BATHROOM After night’s sleep, it was TREASURE HUNT time where I have to search for the convention centre in the whole of Denver Downtown. Luckily I got there in the nick of time. Capital Hill, Denver 16th Street, California St. A old USA building before reaching the convention centre THE DENVER’s Convention Centre O-SING Issue 3 Page 11 of 12 AUGUST 26, 2004 Disclaimer: O-SING is published by the Student Chapter OSA(NTU), School of Electrical and Electronic Engineering, Nanyang Technological University. Block S1, Nanyang Avenue, Singapore 639798. Opinions expressed herein do not necessarily reflect the views of the University. Copyright c 2003 Student Chapter OSA(NTU), All rights reserved. Material in this publication may not be reproduced without written permission.
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