A novel fiber Bragg grating aided fiber loop ringdown (FLRD) sensor array and the wavelength-time multiplexing based interrogation technique for the FLRD sensors array are proposed. The interrogation frequency of the system is formulated and the interrelationships among the parameters of the system are analyzed. To validate the performance of the proposed system, a five elements array is experimentally demonstrated, and the system shows the capability of real time monitoring every FLRD element with interrogation frequency of 125.5 Hz.
© 2015 Optical Society of America
Optical fiber loop ringdown (FLRD) sensor has not only the intrinsic advantages of the traditional cavity ringdown sensor, such as, the capability of detecting tiny loss, and immunization to the influence of source power fluctuation, but also the advantage of that the miniaturization enhances the flexibility of the system. Therefore, in recent two decades, the FLRD sensor has been successfully applied on detecting various parameters [1–5]. As an advantage of the fiber sensors, many sensors can be multiplexed into a network with shared demodulation equipment, and the concept of the FLRD based sensor network has also been proposed . However, up to now, the experimental demonstration of the multiplexing technology for FLRD sensors array is limited. The only reported FLRD sensor network is based on time division multiplexing (TDM) technique . In this TDM system, the power of the pulsed laser is divided into many FLRD sensors, and the intensity of the output signal decreases with increasing number of sensors. Consequently, to detect the weak ringdown signals, a photo-detector (PD) with extremely high sensitivity is required. In addition, for the TDM technology, the strict requirement on the time sequence of the ringdown signal makes the optical paths length between any adjacent elements and the length of a ringdown loop must be precisely controlled.
In the paper, a novel interrogation technique based on wavelength-time multiplexing technology for FLRD sensor array is proposed and demonstrated. By introducing an uniform fiber Bragg grating (FBG) to form the FBG-FLRD hybrid sensor element, each FLRD sensor can be addressed by a unique wavelength determined by a FBG in real-time. Light output from the wavelength-swept fiber laser (WSFL) is modulated by an acousto-optic intensity modulator (AOM) into pulsed light and then injected into the serial FBG-FLRD sensors array. Based on the time sequence of light reflected by FBGs, each FBG-FLRD sensor element can be addressed.
2. Configuration and theory
The schematic diagram of the proposed system is shown in Fig. 1. It contains a WSFL, an AOM, several FBG-FLRD sensor elements, and digital signal decoupling unit.
In this paper, a WSFL is used to illuminate the FBG-FLRD sensors array which has a similar architecture in , and a tunable filter is introduced into the resonant cavity to repeatedly sweep wavelength of lasing. Because the Fabry–Pérot filter restricts the maximum input power, the output power of the WSFL is limited. An additional EDFA is adopted to boost the power of the WSFL. The wavelength-time relation of the WSFL is determined by the driven signal of the Fabry–Pérot filter, which is generated by signal generator 1(SG1). The AOM is employed to generate the pulsed light with the wavelength-time relation from WSFL. And then the pulsed light is used to illuminate the FLRD array.
Every FLRD sensor element consists of a fiber loop, a FBG and an isolator. As shown in Fig. 1, the ports of the fiber loop are designated as numbers of 1, 1', 2, 2' 3, 3′, 4, and 4'. The pulses generated by AOM are injected into FLRD sensor via port 1. The port 2 is linked to other cascaded FBG-FLRD elements via the corresponding FBGi. The pulsed light is selected out by FBGi from the whole set of input pulse strings, and feed into fiber loop via port 2 and port 1'. One fiber loop is composed of two identical 2 × 2 fiber couplers with the split ratio of 1:99. Two couplers are spliced together to form a fiber loop. In a way that the transmission loss from port 4' to port 3′ is 1%, and the loss from port 2' to port1' is also 1% Ultimately, most of the pulse power remains in the fiber loop via each round trip. The ringdown signal is coupled out from the port 3. The port 4 is idle. Due to the loss of the loop, an exponential decay of light intensity will be incurred. Obviously, the pulsed light reflected by FBGi will be not only coupled into ith fiber loop, but also other fiber loops (i.e., 1st element … (i-1)th element). To avoid the unwanted interference among FLRD elements, an optical isolator is used between two adjacent FLRD units to prevent unwanted feedback.
To ensure successful addressing of the FBG-FLRD sensors array, we need to calculate and elaborately design the timing relationship. The timing relationships of the system is shown in Fig. 2.
Assuming the 3dB bandwidth of all FBGs are identical, represented as ΔλFBG-3dB, then the time slot Δt that the lasing wavelength of the WSFL sweeps through ΔλFBG-3dB of FBG can be expressed as Δt = ΔλFBG-3dB /(fWSFL × Δλspan). The fWSFL is the sweeping frequency of the WSFL and the Δλspan is the wavelength span of the WSFL. The pluses number in time slot of FBGi is
Assuming the round trip time of each fiber loop is τtrip, and the pulse width of AOM is denoted by τAOM. τAOM = η/fAOM , where η is the duty cycle of AOM pulsed signal. From Fig. 2, there are twokinds of crosstalks in this proposed system. One crosstalk is the overlap of adjacent round trip signal. When τAOM > τtrip, this overlap occurs as shown crosstalk1 in Fig. 2.
To avoid this crosstalk, τtrip should be longer than τAOM. The other crosstalk is the overlap of two ringdown signal sequences of adjacent two FBG-FLRD sensor elements or itself. When τi > 1/fAOM, τi is the ringdown time of the ringdown signal sequence, and this overlap occurs as shown crosstalk 2 in Fig. 2. To avoid this crosstalk, 1/fAOM should be longer than τi. In summary, in order to satisfy the interrogation conditions, all the signals of the proposed interrogation system should meet
The FLRD sensor elements will be addressed periodically with the frequency of the driving signal of the WSFL, which is saw toothed wave in our experiment. Similar to , in order to make sure that the ringdown signal of ith element can be separated from convolution of multiple sensor units, a parallel switches array controlled by the CPLD is employed . Based on the wavelength-time relation of the WSFL, the switch Si is “on” level and other switches are “off” level at the ringdown time slot ti-ti + τi, and the ringdown signal of the ith sensor will exactly pass through the switch Si. So the ringdown signal from different elements can be decoupled into the parallel channels. And thus the real-time interrogation for the FBG-FLRD sensor array can be realized. A diagram presentation of the signal decoupling processing is shown in the Fig. 3. In order to control time sequence of the system, tThe trigger signal of the SG1 and SG2 are synchronously triggered by the CPLD.
3. Experimental results
The proposed system with five FBG-FLRD elements (m = 5) is exemplified. As shown in Fig. 1, the five FBGs are connected to form a one-dimensional array. In the experiment, the length L of each fiber loop is 120 m, and accordingly τtrip is about 600 ns. The absorption coefficient of SM-28 fiber α is 0.20 dB/km, the total absorption loss of fiber is 0.024 dB. The insertion loss of each fiber coupler is about 0.09 dB. Each fiber fusion loss is about 0.03 dB. And thus the total light loss of each round trip is about 0.264 dB. When fiber loop is free, the initial ringdown time τi0 of the FLRD is given by ,
In the experiment, ΔλFBG-3dB of all FBGs is 0.18 nm, the sweeping range of the WSFL is from 1539.5 nm to 1555.7 nm, namely Δλspan is 16.2 nm. For nΔt = 1, according to Eq. (1), fWSFL should be 125.5 Hz. The sweeping signal for driving F-P filter is generated by a signal generator (Stanford research systems, DS345). In summary, all timing parameters for the system are shown in the Table 1.
The fiber F-P filter in WSFL has a 3 dB bandwidth of 40 pm (Micron Optics, Inc. FFP-TF), and the maximum input power of the F-P filter is 20 mW. The output power of the boost stage is 80 mW. Theoretically, the maximum tuning frequency is not only limited by the maximum tuning frequency of F-P filter, but also by the building up time of the fiber laser for stable laser output. As discussed in , the highest frequency of WSFL could reach several thousands of hertz. In fact, the operating frequency of WSFL is also limited by other parameters as illustrated in Eq. (1) and Eq. (2). In this experiment, the driving signal of F-P filter is 125.5 Hz saw-toothed wave, and the peak-to-peak value and DC bias of F-P filter are 3.00 V and 5.65 V respectively.
In this experiment, the central wavelengths of the five FBGs are 1543.7 nm, 1545.8 nm, 1547.9 nm, 1550.1 nm, and 1551.9 nm, respectively. In addition, the reflectivity of all FBGs is about 85%. The ringdown signals and the driving signal of the WSFL and the waveform of the AOM driving signal are recorded with an oscilloscope (Tektronix TDS3034B) and shown in Fig. 4.
To ensure that the complete ringdown signal of ith element is decoupled to the corresponding channel, the window time for keeping “on” level of the switch Si should match ringdown time and the reflected time of light reflected by FBGi. According to the wavelength-time relationship of the WSFL and the wavelength of FBGs, The reflected times of five FBGs are 2.066 ms, 3.099 ms, 4.132 ms, 5.214 ms, and 6.099 ms, respectively. The time slot in FBG bandwidth of 0.18 nm is 88.5 µs, and ringdown time is about 9.5 µs. To ensure that only one perfect ringdown signal of ith element passes through the switch Si, the “on” level time of switch Si should be more than is 88.5 µs. In this experiment, we make the “on” level time of the switches S1, S2, S3, S4, S5 are 1.95-2.15 ms, 3.00-3.20 ms, 4.05-4.25 ms, 5.15-5.35 ms and 6.0-6.2 ms. The coupled signals and decoupling results are shown in Fig. 5. We can see the ringdown signal of each sensor can be successfully decoupled into the parallel channels, and the real-time interrogation of the FBG-FLRD sensor array can be realized.
To examine real-time interrogation technique for FBG-FLRDs sensor array, a 40 × 80 mm2 microbend force head, as shown in the left illustration of Fig. 1, with a saw tooth pitch of 2 mm is used to apply external force. Two 80 mm fiber sections of fiber loop (2nd or 5th sensors) are parallelly placed inside the microbend force head. When the external force, F, is applied on the microbend force head, it will excite the higher order transmission modes of the fiber, which causes additional loss in the fiber loop. If the force-induced loss is much smaller than the intrinsic loop loss, the change of ringdown time can be described by :
Here, the τi0 and τi are the ringdown time of the ith FLRD element without and with the force F, the β is the force-induced loss coefficient, whose unit is g−1∙cm−1, and li is the length of the fiber on which the force is applied. Figure 6 shows the ringdown signal of 2nd FBG-FLRD sensor under different external forces. When the force F gradually increases, the ringdown time gradually decrease, which is consistent with theoretical expectations.
The change of ringdown time versus the force applied on the 2nd sensor and the 5th sensor is plotted in Fig. 7. It shows that the term (1/τi –1/τi0) is linearly proportional to force, and the slopes of 2nd and 5th sensor are almost equal with force threshold of 4N. It is in good agreement with that predicted in Eq. (5) and Ref .
In this paper, we proposed and experimentally demonstrated a wavelength-time multiplexing based real-time interrogation technique for the FBG-FLRD hybrid sensor system. The FBG-FLRD hybrid sensor was built by introducing FBG to the conventional FLRD sensor as an identifier. A five elements wavelength-time multiplexing system based on FBG-FLRD sensor array was demonstrated firstly. The timing parameters of the system were theoretically analyzed. The proposed system provides the possibility of real-time, multi-parameter, and multi-position measurement using fiber ringdown sensor array. By multiplexing technology, the demodulation cost of FLRD sensors array was greatly reduced in the proposed system.
This research was supported by the National Nature Science Foundation of China under Grant 11274099, National “863” project under Grand 2011AA010300, and the High Level Innovation Teams of Heilongjiang University under Grant HD-028.
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