A simple optical signal demodulation technique is demonstrated for multi-point interrogation of fiber Bragg grating sensors. The wavelength-division responses of polarization-maintaining fiber Sagnac loop filters are flexibly controlled to interrogate the simultaneous optical intensity changes from the independent wavelength shifts of multiple fiber Bragg grating sensors. Experimental performance is also demonstrated to show the simultaneous detection of the separated FBG sensing signals by individual photo detectors.
©2006 Optical Society of America
Various interrogation techniques in optical sensor networks have been thoroughly investigated to obtain sensing information, such as the variation of temperature, strain, and bending, from the wavelength shifted optical signal coming from the fiber Bragg grating (FBG) sensor heads . For the cost-effective interrogation technique, linearly wavelength-dependent devices based on various optical mechanisms, such as Fabry-Perot etalon filter , wavelength division multiplexing (WDM) coupler , long-period fiber gratings , and Sagnac loop filter , have been intensively developed, but these techniques still require more improvements in stability , flexibility , and multi-point sensibility .
In recent years, as a prospective linearly wavelength-dependent interrogator, the polarization-maintaining fiber (PMF) Sagnac loop filter has been widely studied because of its high polarization independence and stability, compared to other interferometeric demodulators, such as Mach-Zehnder or Michelson interferometer . Recently, one of the authors has proposed the tuning method of effective length of PMF by adjusting the polarization controllers (PC) between multiple PMF segments in the Sagnac loop . The temperature insensitive Sagnac loop interferometer is also suggested by using the lower temperature-sensitivity of photonic crystal fiber (PCF) PMF, which are made of a single glass material . Although these solutions provide a simple passive demodulation system, the available sensing point has been limited to only one FBG sensing head in the conventional interrogation system of one Sagnac loop filter because it is hard to distinguish multiple optical signals coming from two or more FBG sensing heads.
In this research, we demonstrate a practical interrogation system using multiple PMF Sagnac loop filters with the high flexibility of both spectral periodicity and wavelength position. The proposed scheme has a lot of advantages like the increased number of sensing heads, low cost and simple all-optical fiber configuration. We show experimental results of sensing measurement for two different wavelengths with two FBG-based sensing heads by using the proposed multi-point interrogation system based on cascaded flexible PMF Sagnac loop filters.
2. Flexible wavelength-division Sagnac loop filters
The scheme for the proposed interrogation configuration for multiple sensor heads is shown in Fig.1. The proposed scheme is composed of three Sagnac loop filters with the effective Lyot PMF length of L A, L B and L C, respectively, one isolator and one circulator, as shown in the dash line of Fig. 1. Each Sagnac loop filter consists of a polarization insensitive 50:50 fiber coupler, a Lyot PMF section with an effective length of L eff and a birefringence of Δn, and polarization controllers (PCs).
The Lyot section of the PMF has multiple PMF segments with different lengths, which are connected with PC. The PC on the inside of PMF segments can be used to adjust the relative orientation of adjacent segments for the tuning of the effective length, L eff. The transmission and reflection spectra of PMF Sagnac interferometer can be easily derived using Jones matrixes  and can be written as
where λ is the operation wavelength and ϕ is the phase retardation of polarized modes. Due to the conservation of energy, the wavelength-division multiplexing between transmission and reflection ports can be performed in the flexible PMF Sagnac loop filter, which can be useful for re-configurable inter-band or inter-channel router .
Since the length of the PMF is effectively adjusted by changing the relative polarization states between multiple PMF segments in each Sagnac loop, the spectral period is modified by changing the Δn·L eff product value of PMF . By changing the fast axis angles of two PMF segments with respect to each other, we can control the whole effective length of the Lyot filter sections discretely . Since the spectral peak-peak period, Δλpeak-peak, is inversely proportional to the effective fiber length of the Lyot PMF sections, L eff, that is,
it allows the tuning of the spectral periodicity by adjusting the effective length, L eff. For example, with the relative angle change of 0° between two axes of PMF segments, L 1 and L2, we can get the effective length, L eff, of L1+L2. In the case of orthogonal axis angle between L1 and L2, we obtained the effective length, L eff, to L1–L2.
As shifting linearly the periodic spectrum by tuning the phase retardation, ϕ, using the PC on the outside of PMF segment, the wavelength slope between peak and deep can be shifted to optimal wavelength position, and this can be used for the optical signal from FBG sensing head to convert the wavelength shift to intensity change. Precise and rapid tuning of polarization can be performed by an electro-optic polarization controller based on lithium-niobate waveguide .
3. Experimental results and discussion
For the two FBG-based sensing heads, it is required to prepare at least three Sagnac loop filters, which have the tunability of spectral period and wavelength position. As described in the previous section, both optimal spectral period and optimal wavelength position can be flexibly selected by adjusting PCs in the Sagnac loop. The function of the channel routing filter A in Fig. 1 is to separate to the overall demodulation ranges of other two steep slope filters like B and C. The center peak region of transmission spectrum, TA, is adjusted around the wavelength shift region of FBG1 and, simultaneously, the high reflection spectrum of RA is inversely routed to the wavelength channel of FBG2 because the periodicity of Sagnac loop filter, Δλpeak-peak, can be adjusted to the half of center wavelength separation between FBG 1 and 2. It is also easy to adjust the transmission slope steepness, TB and TC, of two Sagnac loop filters to the suitable demodulation range and sensitivity for the wavelength shift region of FBG1 and FBG 2, respectively.
For the experimental demonstration, we select two FBG sensors with the center wavelength of 1534.20 nm and 1537.09 nm. Figure 2 shows the reflection spectrum of two FBGs measured at the port 3 of circulator 1 before entering the interrogator in the dash lines of Fig. 1. From this spectrum, we can easily find that it is impossible to discriminate the independent wavelength-shift effects of two FBG peaks by a single photodetector (PD) and a single Sagnac loop filter as monitoring the overall reflected intensity variation . However, with the aid of a channel routing filters A and a steep slope filters B and C, it makes possible to measure wavelength variation of multiple resonant peaks separately.
Figure 3(a) shows the measured transmission (T) or reflection (R) spectra of TA, RA, TB and TC, respectively, and the combined transmission spectra of TA × TB and RA × TC are also shown in Fig. 3(b). For the reliable performance of interrogation of TA × TB, it is necessary for the filter transmission of FBG1 region to obtain much higher intensity spectrum than any transmission in FBG 2 region. We can easily find that the filter spectrum of TA × TB has a linearly wavelength-dependent response on FBG 1 and negligible transmission response on FBG 2. The wavelength positions of peak and deep of TB can be easily adjusted to correspond to those of TA, and, similarly, the spectral position of TC is match with those of RA. It is required to have broader periodicity of channel routing filter A than the wavelength shift region of each FBG sensors to route two FBG responses separately. To measure the applied temperature within 50 °C, we assume the variation region of center wavelength will be within a dynamic range of +0.5 nm when the temperature sensitivity of FBG sensor is 0.01 nm/°C.
Similarly, the combined spectrum of RA × TC is transmitting for the wavelength region of FBG 2, but not for that of FBG 1. It is noted that an isolator is necessary between filter A and B to block the influence of RB into the clear spectrum of RA. In the experiment with the filters of two pieces of PMF (Δn of 0.00038, L1 of 2 m and L2 of 1 m), the effective PMF length of channel routing filter A is tuned to 1 m (L1–L2) from Eq. (3) due to the FBG center wavelength difference of 2.89 nm. In steep slope filters B and C, the effective PMF lengths are adjusted to 3 m (L1+L2), which is three times longer than that of filter A, because the period of the steep slope filters B and C, is necessary to be odd integer times smaller than the period of the channel routing filter A.
Figure 4 shows the experimental result of the interrogated sensor signal spectra with external variation on both FBG’s. The spectra are measured using a broadband source and an optical spectrum analyzer at the position of PD1 and PD2, respectively. In Fig. 4(a), optimally filtered spectrum is measured at the position of PD1, which demonstrated the conversion of the wavelength shift of FBG1 to intensity change linearly. The wavelength shift of FBG 2 is not monitored in the 1537.09~1537.5 nm region. When we add an extra circulator in the position of the isolator of Fig. 1, the power ratio between transmitted and reflected signal of steep slope filter B can be additionally compared for the purpose of self-referencing the power fluctuation in the broadband light source . Therefore, PD1 can easily and simply monitor the intensity response of FBG 1 only and, similarly, the sole response of FBG 2 is detected at PD2 by the help of a channel routing filter A and a steep slope filter C (RA × TC), as shown in Fig. 4(b). In the experiment, a large insertion loss over 10 dB is monitored due to many FC/PC connections, which can be further reduced by arc-splicing method.
Experimental interrogation for two FBG sensing heads has been demonstrated by using three flexible PMF Sagnac loop filters; one is for channel routing and the other two are for slope response. Higher number of FBG sensing head can be interrogated by using similar configuration with increased number of Sagnac filters. As increasing the number of FBG sensing heads by one, we need one more Sagnac routing filter to demultiplex for the additional wavelength channel and one more Sagnac slope filter is also required to convert the routed signal to PD response. For the linear relation between PD output and the center wavelength change of the FBG sensing head, it is needed to select a linear response wavelength region of this system and calibrate the converting relation quantitatively .
We have demonstrated a simple interrogation system to demultiplex optical signals from two FBG sensors and detect individual shifts in the center wavelength of those FBGs. The FBG signals are first sent to a Sagnac loop filter which transmits the portion of the signal containing the channel of FBG and reflects the portion of the signal containing the channel of the other FBG. Those two signals are then sent to fine comb filters tuned so that their associated PDs can detect changes in the center wavelength of each FBG. Using cascaded flexible wavelength-division Sagnac loop filters, experimental tuning of both wavelength position and spectral period is demonstrated to separate the response channels of multiple FBG’s and to change the steepness of linear wavelength-dependent slope, respectively. Multi-point sensible, spectral flexible, tunable and inexpensive FBG sensing system can be achieved with the proposed optical signal demodulation technique.
This work was supported by 2006 PNU-IGB Joint Research Center Grant of Pusan National University.
References and links
1. B. Lee, “Review of the present status of optical fiber sensors,” Opt. Fiber Technol. 9, 57–79 (2003). [CrossRef]
2. A. D. Kersey, T. A. Berkoff, and W. W. Morey, “Multiplexed fiber Bragg grating strain-sensor system with a fiber Fabry-Perot wavelength filter,” Opt. Lett. ,18, 33–39 (1993). [CrossRef]
3. M. A. Davis and A. D. Kersey, “All-fiber Bragg grating strain sensor demodulation technique using a wavelength division coupler,” Electron. Lett. 30, 75–77 (1994). [CrossRef]
4. R. W. Fallon, L. Zhang, L. A. Everall, J. A. R. Williams, and I. Bennion, “All-fibre optical sensing system: Bragg grating sensor interrogated by a long-period grating,” Meas.. Sci. and Technol. 9, 1969–1973 (1998). [CrossRef]
5. S. Chung, J. Kim, B. A. Yu, and B. Lee, “A fiber Bragg grating sensor demodulation technique using a polarization maintainging fiber loop mirror,” IEEE Photon. Tech. Lett. 13, 1343–1345 (2001). [CrossRef]
6. Y. G. Han, S. B. Lee, C. S. Kim, J. U. Kang, U. C. Paek, and Y. Chung, “Simultaneously measurement of temperature and strain using dual long-period fiber gratings with controlled temperature and strain sensitivities,” Opt. Express 11, 476–481 (2003). [CrossRef] [PubMed]
8. C. S. Kim and J. U. Kang, “Multi-wavelength switching of Raman fiber ring laser incorporating composite polarization-maintaining fiber Lyot-Sagnac filter,” Appl. Opt. 43, 3151–3157 (2004). [CrossRef] [PubMed]
9. D. H. Kim and J. U. Kang, “Sagnac loop interferometer based on polarization maintaining photonic crystal fiber with reduced temperature sensitivity,” Opt. Express 12, 4490–4495 (2004). [CrossRef] [PubMed]
10. X. Yang, C.-L. Zhao, Q. Peng, X. Zhou, and C. Lu, “FBG sensor interrogation with high temperature insensitivity by using a HiBi-PCF Sagnac loop filter,” Opt. Commun. , 250, 63–68 (2005). [CrossRef]
11. G. Chen and J. U. Kang, “Frequency discriminator based on ring-assisted fiber Sagnac filter,” IEEE Photon. Tech. Lett. 17, 109–111 (2005). J. U. Kang “Multi-wavelength switching of Raman fiber ring laser incorporating composite polarization-maintaining fiber Lyot-Sagnac filter,” Appl. Opt. 43, 3151–3157 (2004). [CrossRef]