Figure 5. Timing diagrams of the sample-and-hold-subtractcircuit
1
1.1 1.2 1.3 1.4 1.5 1.6
1.7 1.8 1.9
2
Refractiveindex (NJ Figure 3. Plots of the fraction of coupled light (F) passing from one fiberto another against the refractive index (NJ of the medium with ylr = 5 (upper curve) and 10 (lower curve).Values of other parameters are given in the text.
where r is the radius of the fiber core. The first term on the right-hand side of eq 2 represents the effective transmittance after reflection a t the two cordmedium interfaces. The second term gives the ratio of the cross-sectional area of the fiber core to that of the light cone at the receiving end of the output fiber. Thus, it gives a measure of the portion of the light loss that is due to the divergence of the beam in the sensing medium. Since any alteration in the physicalor the chemical properties of the medium in SV can cause a variation in the refractive index N and, consequently, the factor F, it is worthwhile to study the relationship between these two parameters. ~ i g u r e - gives 3 plots of the fraction of coupled light F against the refractive index of the medium with ylr = 5 (upper curve) and 10 (lower m e ) . In the calculations, we assume that the transmittance, the numerical aperture, and the core refractive index of the fiber had the values 1.000, 0.470, and 1.492, respectively. I t can he seen that the fraction of coupled light between two fibers vanes fairly linearly with the refractive index of the medium. ~heref&e,by measuring the parameter F, one can deduce the chanee in refractive index of the meDesign and Fabrication of Electronic Circuitry for FOS
Figure 4 shows a block diagram of the FOS proposed in this work. A signal generator (SG), which consists of an
with (a) output of the I-kHz signal generator (SG), (b)output of the photodiode (PD), (c) triggerlng pulses on SHI, (d)triggerlng pulses
on SH2, and (e)outputs of SH1 and SH2.
MC1455 chip (Motorola), is used to produce a train of 1-kHz square waves. These electric pulses are used to drive the LED (HLMP-4101. Hewlett Packard with peak power a t 650 nm) on and off..They also directly trigg& the sample-and-hold chip SH1 (IC LF398. National) and. th&igh an inverting &cuit, an identicalehip SH2. In thi$ manner, SH1 and SH2 are enabled alternatively. The sample-and-hold-subtract circuit (9) thus devised allows the sensor to work under ambient light and other photo disturbances. Light generated from the LED passes through the input fiber and is modulated by themedium in the SV. Then the photon signals are collected by the output fiber. They are directed to the photodiode (BPW21, RS Components) and converted to voltage signals, which are then inputed to SH1 and SH2. Timing
Figure 5 shows the schematic timing diagram for the sample-and-hold-subtract circuit. The train of light signals thus produced by the LED according to the input of the SG (Fie. 5a) passes tbroueh the input fiber. the sensine volume, and the outputfiber-finally &aching thephotodiode. When the LED is turned on, the reading obtained from the PD is 5 V, with 1V due to ambient light (Fig. 5b). The sample-and-hold-subtract circuit devised in this work can contribute a net 4 V that oridnates onlv from the modulated light as described below. When the LED is turned on, SH1 is enabled (Figs.4 and 5c) and maintains an output of 5 V,as shown in Figure 5e. SH2 is triggered on when the LED is off (Fig. 5d), and thus outputs only 1V due to the ambient light (Figs. 4 and 5e). The output signals of SH1 and SH2 are then fed to a differential amplifier (Fig. 4) to give the net photon signal with the ambient noise eliminated. The resulting signals are then amplified and either displayed directly from a multimeter or stored in a computer &ugh a computer interface. The Electronic Circuit
Figure 4. Blockdiagram of various electronic components ofthe fiber optic sensor. The symbols are explained in the text.
Figure 6 gives a schematic layout of the electronic circuit as designed in this work. I t is self-explanatorv. All integrated circuit components are readily available on the market as mentioned in the previous discussion. The PD chosen has a sensitive area of 2 x 2 mm2 and effectively collects most of the coupled light. The LED used has high brightness. I t has a light intensity of 1000 mcd and peaks at 650 nm a t a driving current of 20 mA. It also has a tapered lens a t its front to concenVolume 70 Number 4 April 1993
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For the best performance, the fiber ends should be smooth and flat. This can be accomplished by polishing them with progressively finer lapping sheets. Aplastic coupler can be used to hold the fiber against a lapping sheet, which should be placed on a hard planar surface such as glass. For the grinding, add a small puddle of water. Then start, for example, with a 60-pm-grit lapping sheet. Then use the 9-wm and l-pm sheets. Finally use a 0.3-pm-grit sheet for fme polishing. A figure "8" motion should be used in these Drocesses. In eeneral. 30 or 40 strokes are sufficien
