In this work, highly sensitive measurements of strain and temperature have been demonstrated using a fiber Bragg grating (FBG) sensor with significantly enhance sensitivity by all-optical signal processing. The sensitivity enhancement is achieved by degenerated Four Wave Mixing (FWM) for frequency chirp magnification (FCM), which can be used for magnifying the wavelength drift of the FBG sensor induced by strain and temperature change. Highly sensitive measurements of static strain and temperature have been experimentally demonstrated with strain sensitivity of 5.36 pm/με and temperature sensitivity of 54.09 pm/°C. The sensitivity has been enhanced by a factor of five based on a 4-order FWM in a highly nonlinear fiber (HNLF).
© 2013 OSA
In recently years, fiber optical sensing has been attracting a lot of interests for applications in strain and temperature measurements [1, 2]. Fiber Bragg grating (FBG) has been widely used in fiber optical sensing with advantages including compact size, all-fiber structure, high sensitivity, simple fabrication and low cost [3–5]. The strain and temperature sensitivities of an FBG arises from the refractive index change and grating period variation. Usually, the strain sensitivity is about 1 μm/με (με=microstrain) and the temperature sensitivity is about 13 pm/℃ for a standard FBG sensor around 1550 nm wavelength [6–8]. Such a high sensitivity enables us to measure strain and temperature with comparably high resolution and precision. However, the measured resolution of strain and temperature is also limited by the resolution of the interrogator .
Consider an optical spectrum analyzer (OSA) as an interrogator with 0.02 nm spectrum resolution, the measured strain and temperature resolution would be limited to 20 με and 1.5 °C for FBG sensors with 1-pm/με strain sensitivity and 13-pm/°C temperature sensitivity. The wavelength drifts induced by smaller train beneath 20 με and lower temperature change less than 1.5 °C will be invisible to the OSA due to the limited resolution.. For instance, the strain induced by the Earth’s crustal deformation is at the scale of 10−9ε [9–11]. Thus, the required wavelength resolution is about 1 fm which makes the interrogation very difficult (impossible for an OSA).
In order to obtain a high resolution strain and temperature sensing, one not only should improve the interrogator’s resolution , but also need to increase the sensitivity of the FBG sensor. The intrinsic sensitivity is decided by the material and structure of the FBG. Thus, it usually needs pre-processing of the sensor head to improve the sensitivity [13–17]. For example, by tapering the FBG or fabricating FBG over nano-fiber, one can obtain an improved strain sensitivity of 2.5 pm/με . By using polymer based FBG, temperature sensitivity of 360 pm/°C has been achieved [15, 16]. However, pre-processing of the FBG sensor head is limited by the effectiveness. The pre-processing usually increases the instability and complexity, which weakens the key-advantage of the FBG sensor. Thus, it is more preferable to use post-processing techniques to enhance the sensitivity, which is the main objective of this study.
In this work, we demonstrate a post-processing method for enhancing the sensitivity of an FBG sensor in the strain and temperature measurements. The sensitivity enhancement is achieved by frequency chirp magnification (FCM) based on degenerated four wave mixing (FWM) in a highly nonlinear fiber (HNLF). The strain and temperature change induced wavelength drift of the FBG sensor has been magnified in a high order FWM process. Highly sensitive measurement of static strain and temperature has been experimentally demonstrated with 5.36-pm/με strain sensitivity and 54.09 pm/°C temperature sensitivity, which has been magnified by a factor of five.
In a degenerated FWM process, the generated first-order idler light has an electrical field Ei described by19, 20]. This phenomenon can be utilized for applications like low-voltage phase modulation, optical phase erasure, frequency sweeping bandwidth enhancement and sensing [19, 21–24]. In a high order FWM process, the frequency for the kth order idler would be: ωi-k = (k + 1)(ωp + δω) - kωs. Then, one can significantly magnify the frequency chirp along with the increase of the idler order. Consider the wavelength drift as one kind of frequency chirp, one can obtain magnified wavelength drift as shown in Fig. 1(a). Therefore, with a wavelength drift of δλ0 at the pump wavelength, one can obtain a magnified wavelength drift of δλ2 at ilder-2 as shown in Fig. 1(b), which has been magnified by a factor of three. In a high order FWM process, the wavelength drift δλn at ilder-n would be (n + 1)δλ0 .
As for sensors based on FBG, the measurement of strain and temperature can be realized by interrogating the resonant wavelength drift. Thus, the strain or temperature sensitivity can be interpreted by the wavelength drift after the loading of an unit strain (1 με) or an unit temperature change (1 °C).
By taking the wavelength drift of the FBG sensor as the frequency chirp for the pump in a degenerated FWM process, the sensitivity of the FBG sensor can thus be magnified due to the FCM as shown in Fig. 1(b). The enhancement by a factor of (n + 1) can be obtained for idler-n in a single-stage high-order degenerated FWM process. The enhancement of the sensitivity is merely realized by the post processing of the signal, which makes this method an effective solution that can be extended to a wide range of applications.
3. Experimental results and discussions
Shown in Fig. 2 is the experimental setup which consists of the fiber laser system based on FBG and the frequency chirp magnifier system based on degenerated FWM. In the fiber laser system, the output port of the EDFA1 is connected to a circulator which directs the light to the FBG. The reflected light from FBG is redirected to a PC by the CIR and then launched into the OC which has a splitting ratio of 30:70. The 70% port of the OC is connected to an ISO and the ISO is connected with the input port of the EDFA1. Thus, we can construct the fiber laser with the output lasing wavelength located at the FBG resonant wavelength. In the frequency chirp magnifier system, the FBG laser with output power of 3.5 dBm and the TL with output power of 10.1 dBm are combined and launched into EDFA2. The linewidth for the FBG laser and the TL are respectively 400 MHz and 100 KHz. The amplified pump from FBG laser and signal from TL are then directed into 500-m HNLF after a BPF. Then, the intense interaction between the pump and signal in the HNLF generates the idler wavelengths due to the FWM effect which arises from the third order nonlinearity in the optical fiber. The HNLF used in this work has a nonlinear coefficient of about 11 W−1km−1 and its zero-dispersion-wavelength is 1550 nm.
The fiber laser in Fig. 2 has a lasing wavelength located at 1550.32 nm which corresponds to the FBG’s initial resonant wavelength at room temperature. By stretching the FBG, certain amount of strain can be loaded onto the FBG. Therefore, the resonant wavelength of the FBG will be shifted. Consequently, the lasing wavelength of the fiber laser will drift along with the strain loading. In the experiment, we fix one end of the FBG pigtail and fasten the other end of the FBG pigtail using the holder of a splicer (Fitel S178). Thus, by controlling the splicer in manual mode, we can drive the motor to move the holder and then stretch the FBG. Therefore, certain strain can be loaded onto the FBG. The total length of the FBG plus the fiber pigtail is 1130 mm.
Shown in Fig. 3 are the optical spectra after the FCM with the loading of 50-μm stretching. The degenerated FWM in the HNLF generates high order idler components. Four idlers have been generated consider the fiber laser at 1550.32 nm as the pump. The EDFA2 has an output power of 480-mW. After the degenerated FWM process in FCM, the wavelength drift induced by the 50-μm stretching over 1130-mm length has been significantly magnified. Shown in the zoom-in spectra in Fig. 3(b), the wavelength drift of 0.045 nm for the pump (from 1550.316 nm to 1550.361 nm) has been magnified to 0.242 nm for idler-4 (from 1557.830 nm to 1558.072 nm).
By stretching 1130-mm fiber with 50-μm length increment, a strain of 44.25 με is obtained. With such a strain, the wavelength drifts are measured to be 0.045, 0.076, 0.114, 0.174 and 0.238 nm respectively for pump, idler-1, idler-2, ider-3 and idler-4. Thus, the corresponding strain sensitivities would be 1.02, 1.72, 2.58, 3.93, and 5.38 pm/με. The optical spectra in Fig. 4 show the wavelength drift magnification with different stretching length to load different strain to the FBG. The largest stretching of 300 μm which results in a strain of 256.49 με are demonstrated with wavelength drift of 0.272, 0.546, 0.836, 1.12 and 1.376 nm respectively for pump, idler-1, idler-2, ider-3 and idler-4. The magnification of the strain sensitivity is shown in Fig. 5, in which, the curves have different slopes indicating magnified sensitivity. The slope of idler-4’s curve in Fig. 5 indicates a strain sensitivity of 5.36 pm/με, corresponds to an initial strain sensitivity of 1.07 pm/με for the FBG.
The temperature sensitivity of FBG can also be magnified based on the proposed method. Usually, 1-°C temperature variation can result in a wavelength drift of only about 0.013 nm. Such a small amount of wavelength drift is barely visible for an OSA with a high resolution of 0.02 nm. However, based on such a frequency chirp magnifier, one can magnify the wavelength drift of the FBG sensor by post-processing the signal.
Shown in Fig. 6 are the wavelength drifts for the pump and idler-4 with the temperature decreasing from 43.24 °C to 42.07 °C. Such a 1.17-°C temperature change would result in an estimated wavelength drift of about 0.015 nm, assuming a standard temperature sensitivity of 13pm/°C for a common FBG. The OSA using in this work has a resolution of 0.02 nm, which can barely resolve the wavelength drift for the pump (Fig. 6, lower curves). Therefore, the measured wavelength drift of 0.011 nm would be unreliable due to the limited resolution.
However, with the FCM in the HNLF, the wavelength drift can be significantly enlarged and thus make it possible for precise identification with clearly resolved spectra as shown in Fig. 6 (upper curves). The wavelength drift is 0.061 nm for idler-4, which has been magnified by a factor of five, indicating a temperature sensitivity of 52.14 pm/°C.
We measured the wavelength drift spectra at several temperatures (70.2, 66.2, 56.8, 56.1, 48.1 and 44.5°C) as shown in Fig. 7. The wavelength drift from 70.2 to 44.5°C is measured to be 0.27, 0.55, 0.81, 1.12 and 1.39 nm respectively for pump, idler-1, idler-2, ider-3 and idler-4. Therefore, the corresponding temperature sensitivities are 10.51, 21.40, 31.52, 43.58, and 54.09 pm/°C. Shown in Fig. 8 are temperature sensing curves for the pump and idlers. The temperature sensitivity, which refers to the slope of the curve, has been magnified by a factor of five at idler-4. The shadowed area indicating the overlapped region between idler-3 and idler-4 due to the magnified wavelength drift.
The measurements of the sensitivity are based on the spectrum analysis which is obtained from the 0.02-nm resolution OSA. Thus, the results are limited by the accuracy for small signals (small value of strain or temperature change) due to the limited resolution. Particularly, the FBG laser has a wavelength drift of about 5 pm which also affects the measurement precision. Theoretically, the magnification of the sensitivity should be linear along with the increase of the idler order. However, one can find that the magnification is quite nonlinear for low-order idlers with small wavelength drifts. For example, one can get the calculated strain sensitivities of 1.02 and 1.72 pm/με for pump and idler-1 with 44.25 με, which did not show a linear magnification of the sensitivity, indicating an unreliable value of measured wavelength drift. However, by magnifying the wavelength drift with a factor of five at idler-4, the measured strain sensitivity is 5.38 pm/με (corresponds to an initial strain sensitivity of 1.08 pm/με,) which is much more reliable in good agreement with the initial strain sensitivity of 1.07 pm/με, measured at 256.49 με. Therefore, the results indicate the improvement of the measurement reliability for small signals in strain and temperature sensing, which is in fact one of the major benefits of the enhanced sensitivity.
The FWM in this work is realized in a 500-m HNLF in a single-stage operation. Further improvement can be achieved by using longer HNLF with higher nonlinearity and pump power with multiple stage operation to generate more idlers for obtaining a higher sensitivity. On the other hand, by improving the sensitivity, one can therefore measure very small values of strain and temperature change using interrogators with standard resolution. Thus, ultra-high resolution strain and temperature sensing can be obtained based on this method with further improvement by using phase-shift FBG with narrower bandwidth.
The proposed FCM method is a post-processing technology, which does not require sensor head processing. Therefore, it can be applied to a wide range of fiber sensing applications.
As a conclusion of this work, we demonstrated a method for enhancing the sensitivity of FBG sensors. The enhancement is achieved by degenerated FWM which is one kind of all-optical signal processing technologies. Highly sensitive strain and temperature measurements with 5.36-pm/με strain sensitivity and 54.09-pm/°C temperature sensitivity have been experimentally realized, which has been magnified by a factor of five. The proposed method is of important significance to high resolution FBG sensing applications. Since it is a post-processing technique which does not require pre-processing of the sensor head, we believe it has great potential to be applied to a wide range of fiber sensing applications.
This work was supported by the National Science Foundation of China under Grant 61275097, 61327812, 61307107, and Shanghai STCSM Science Foundation under Grant 13ZR1456200.
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