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We propose and experimentally demonstrate a demodulation technique for real-time monitoring the change process of liquid concentration based on a tilted fiber Bragg grating (TFBG) and differential amplification detection. Continuous variations of the TFBG transmission power are measured by using two photodiodes (or a precision reference voltage generator instead of the referenced photodiode) along with two integrated operational amplifier circuits (IOAC). The voltage-concentration curves are obtained, by recording the output voltages of the two IOACs at different time. The experimental results show that such a fast demodulation technique is capable of measuring the effective glycerol solution concentration over the range of about 89%~69%.
©2012 Optical Society of America
1. Introduction
Real-time monitoring of the liquid concentration or refractive index is of great importance in many widely dispersed applications, especially in the fields such as chemical, biomedical, environmental science, and so on. As a new kind of passive sensing elements, fiber grating has drawn a considerable attention in the sensor field and capable of being employed in flammable, explosive, and corrosive chemical environment. However, in ordinary fiber Bragg grating (FBG), the strong coupling is permitted only between the forward and backward propagating core modes, for which the light is confined near the fiber axis and isolated from the surroundings by a relatively thick cladding [1]. As a result, in the absence of any special alteration in the fiber cladding, such as due to etching or polishing, the FBG is insensitive to liquid refractive index or concentration [2–5]. Long-period grating (LPG) is vulnerable to the external environment, and can be used as sensors of the concentration or refractive index [6–8], but the strong cross-sensitivity nature of LPGs prevents their use in most applications, except for a few ones where high packaging cost is insignificant. Tilted FBG (TFBG) with special structure and properties [1, 9], allow the detection of environmental parameters near the fiber surface [10–18], provided the protective jacket of the fiber has been removed. The measurement of these parameters is made by employing a suitable demodulation technique. At present, the TFBG demodulation approaches are mainly based on directly recording or detecting the changes of the transmission power or spectrum by using an optical spectrum analyzer (OSA) or a photo-detector, and then obtaining the corresponding variations of the parameters, including the resonant wavelength or bandwidth of the cladding mode [10–12], the area enclosed by the upper and lower envelope curves [13–15], and so on. It is difficult to realize fast measurement of the dynamic change of liquid concentration or refractive index using an OSA. And because the background light signal in the TFBG transmission spectrum is very strong, the measurement sensitivity and range will be subjected to certain restrictions by the direct use of a photo-detector [16], or some special data processing is needed [17].
In this paper, we propose a novel scheme for real-time monitoring the liquid concentration based on detecting the change of the TFBG transmission power. Taking a 4° TFBG to monitor the concentration change process at the interface between water and glycerol as an example, the feasibility of this scheme is experimentally demonstrated. In the detection system, one photodiode (PD) and another referenced PD (or using a precision reference voltage generator (PRVG) to replace) are employed to detect small variation of the TFBG transmission power in a strong background. Moreover, the ability to measure the instantaneous change of refractive index is also demonstrated by using this scheme. Therefore, the proposed scheme without complicated data processing is expected to be applied in the fast, remote, and real-time measurement of the liquid concentration, refractive index, volatilization, and other parameters.
2. Sensing principle and measuring system
TFBG belongs to the short-period gratings family, but their index modulation pattern is slanted with respect to the fiber axis. The tilt of the grating planes enhances the coupling of light from the forward-propagating core mode to the backward-propagating cladding mode, while simultaneously reducing the backward coupling of the core mode. As a result, several cladding mode resonances appear at discrete wavelengths shorter than the Bragg resonance in the core mode. Since these modes are guided by the interface between the cladding and the surrounding medium, the refractive index change in the medium adjacent to the fiber produces obvious spectral variation in the TFBG spectrum, and hence the transmission power will also be changed.
Figure 1 depicts the proposed scheme for real-time monitoring the change of liquid concentration. A broadband light signal from an ASE source is launched into a chirped FBG (CFBG) via a fiber optic circulator. The reflected signal after passing through the circulator, splits at 1 × 2 fiber coupler and reaches the sensing TFBG and PD2. The TFBG transmitted signal is further divided by using another 1 × 2 coupler; one is forwarded to OSA for monitoring the transmission spectrum and serves as a calibration signal while the other is received at PD1. The PD1 and PD2 are the fiber-coupled InGaAs p-i-n photodiodes. The output current signals of PDs are converted into the voltage signals and after differential amplification by using an integrated operational amplifier circuit (IOAC)-1 received at channel-1 of a voltmeter (or oscilloscope). As the voltage signal from PD2 does not change by neglecting the power variations of the light source, a PRVG is used instead of PD2 at IOAC-2 and the resultant differential amplified signal is obtained at channel-2 of voltmeter and acts as a reference. In subsequent experiment, a 4° TFBG as sensing unit is placed in water-glycerol diffusion zone which produces a continuous concentration (or refractive index) change.
Fig. 1 Schematic diagram of real-time monitoring system of liquid concentration (The dashed region contains the sample cell of liquid phase diffusion). PD: Photodiode; PRVG: Precision reference voltage generator; IOAC: Integrated operational amplifier circuit; C: Channel; IMG: Index matching gel.
In this system, the bandwidth of ASE source is very large with the range of about 1520~1610nm, so a CFBG as a broad band-pass filter with the range of 1525~1555nm is used to eliminate the redundant bandwidth of the source, which primarily reduces the strong background light introduced by the light source. The ASE source, CFBG reflection and TFBG transmission spectra are shown in Fig. 2 . Each dip in the TFBG transmission spectrum corresponds to light that has been removed from the single-mode core and coupled to one or several backward propagating cladding modes.
Fig. 2 Transmission spectrum (blue line) of a 4° TFBG after filtering through CFBG. (Red line: reflection spectrum of CFBG; black line: spectrum of ASE source).
Assuming the power spectral density of ASE source is G(λ), the CFBG reflectivity and TFBG transmissivity are RCFBG(λ) and TTFBG(λ), respectively, then by neglecting the insertion loss of the fiber, the power signals detected by PD1 and PD2 can be written as
where, λmin and λmax are taken as about 1525 and 1555nm, respectively, according to the effective spectral window of the CFBG. When the refractive index around the TFBG varies, the coupling intensity and hence, the dips in the transmission spectrum will also change, thus causing the function TTFBG(λ) to alter while the functions G(λ) and RCFBG(λ) will remain same. Therefore, from Eqs. (1a) and (1b), the power P2 is almost constant, but P1 depends on the function TTFBG(λ) related to the liquid refractive index or concentration around the TFBG.3. Experimental results and discussions
In the experiment, pure water and 99% glycerol solution are tardily injected into the sample cell in turn with a fine plastic tube attached to an injector at the bottom. Two small holes are drilled in the adjacent vertical walls of the sample cell for the fiber’s passing through. A 4° TFBG placed at the water-glycerol interface is used to sense the concentration change. Figure 3 (Media 1) directly shows the evolution of the TFBG transmission spectra with the time. It can be seen that the coupling intensity of the cladding mode first decreases and then increases with the diffusion time. This behavior is observed, because the refractive index of 99% glycerol is slightly higher than that of the fiber cladding.
For observing the response of the coupling intensity with time more significantly and evidently, the normalized areas enclosed by the upper and lower envelope curves (marked as ξup and ξlow) of the cladding modes are calculated at different time, as shown in Fig. 4 . The normalized area A can be defined as follows [13]
where, λ1, λ2 are the limits of the spectral window of interest (taken as 1528.00 nm and 1548.96 nm, respectively, in the following experiments), and, are the reference upper and lower envelope curves when TFBG is surrounded by air. The relationship between the normalized area A (represents the envelope area of TFBG cladding mode) and the power signal P1 (which is the integral result of the transmission spectrum) can be determined by a calibration measurement. And the variation of the glycerol concentration associated to the normalized area changes through the calibration relationship obtained by the same TFBG, as referred in our earlier publication [15], is also shown in Fig. 4.Fig. 4 Variation of the normalized area and glycerol concentration with the diffusion time. The inset shows the TFBG transmission spectrum with marked the upper and lower envelope curves of the cladding mode at T = 90min.
The voltage readings from channel-1 and −2 are taken after every 5min interval by using a digital multimeter. At first, the two voltages are adjusted to about 3V, for the readings to remain significant within the system saturation limits of ± 4.5V throughout the measurement process. In actual measurement, the time interval can be finalized by the rate at which the solution concentration changes. Finally, the two voltage response curves with the diffusion time corresponding to channel-1 and −2 are obtained, as shown in Fig. 5(a) . It can be seen that both the curves are in good agreement and are just the opposite to the response of the normalized area of the cladding mode. It can also be observed that the turning point in the curve between the normalized area and the time of Fig. 4 will disappear because of a little resonance happening and the approximate symmetry variation of upper and lower envelope curves at the beginning stage of the diffusion. As the diffusion process continues, the increase of the coupling intensity of the cladding mode, and then the integral area (P1) of the spectrum increase gradually with the decrease of the refractive index, causing the decrease of the two voltages because of IOAC-1 and −2 with reverse amplification. Moreover, the difference between the voltages of channel-1 and −2 are due to the dissimilar amplifications of IOAC-1 and −2, adjusted during the measurement process to observe the feasibility of the technique.
Fig. 5 Response curves of the voltages from channel-1 (C-1) and channel-2 (C-2) with the diffusion time (a) and the glycerol concentration (b).
During the water-glycerol diffusing process, the concentration changes continuously with the time. Therefore, by using the calibration relationship between the concentration and the normalized area, the voltage response curves with the concentration can be obtained, as shown in Fig. 5(b). It is evident that the voltages of channel-1 and −2 reduce with the decrease of the concentration in the range from about 89% to 69%. However, within the concentration range of 99%-89%, for similar reasons as in Fig. 5(a), the two voltages doesn’t change significantly with the glycerol concentration. From Fig. 5(b), the curves show an exponential behavior in the range of 89%-69%, and can be expressed as
where the R-square values of the exponential fitting curves are 0.9957 and 0.9978, respectively. Since the detected signals of the two channels are identical in nature, we can real-time monitor the concentration process using either of them. Moreover, according to the maximum fluctuation of 10mV and the range of the voltage signals, the measurement errors of the two channels are about 1.7% and 1.4%F.S., respectively.By using a 4° TFBG system to monitor the concentration change in our experiment, it should be noted that the effective measuring range of about 89%-69% is relatively narrow, but it is dependent on the initial TFBG transmission spectrum, and is expected to be enhanced by increasing the tilt angle of TFBG resulting in stronger cladding mode resonances.
In order to demonstrate the fast measurement aspect of the proposed scheme, the TFBG initially surrounded by air is instantaneously immersed in 99% glycerol solution lying on a glass plate. The voltage from channel-2 will suddenly rise, which is obtained by using an oscilloscope (Tektronix DPO 3054). The voltage from channel-1 has similar change. The voltage curve after smoothing and its gradient (or first-order derivative) can be shown in Fig. 6 . After calculation, the voltage change of this process requires only about 0.11s, and the maximum change rate of the voltage can be up to 98.54V/s. The results show that this process is very fast, as it is difficult to detect such a change by use of an OSA. Therefore, due to the limitations involved in the conventional spectral measurement scheme incorporating OSA, e.g. slow scanning rate, bulky and impractical for field measurement, the proposed scheme is a better candidate for the widespread applications.
Fig. 6 Voltage waveform and its gradient when the TFBG is immersed in the glycerol. The inset shows the original waveform from an oscilloscope.
4. Conclusions
A TFBG demodulation technique for measuring the liquid concentration process has been presented. The continuous change of the concentration due to the water-glycerol diffusion and refractive index transients when the TFBG immersed in the glycerol, are real-time monitored by measuring the power variation of the TFBG transmission spectrum using two methods. Compared with spectral measurement method, this scheme can achieve fast, cost-effective, real-time measurement of continuous changes in the liquid concentration or refractive index, reduce follow-up complex data processing, and directly obtain the measurand by a further simplified calculation according to the actual requirement.
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