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Photonic Crystal Fiber Strain Sensor Based on Modified Mach–Zehnder Interferometer Volume 4, Number 1, February 2012 L. M. Hu C. C. Chan X. Y. Dong Y. P. Wang P. Zu W. C. Wong W. W. Qian T. Li

DOI: 10.1109/JPHOT.2011.2180708 1943-0655/$26.00 ©2011 IEEE

IEEE Photonics Journal

PCF Strain Sensor Based on Modified MZI

Photonic Crystal Fiber Strain Sensor Based on Modified Mach–Zehnder Interferometer L. M. Hu, 1;2 C. C. Chan, 2 X. Y. Dong, 1 Y. P. Wang, 1 P. Zu, 2 W. C. Wong, 2 W. W. Qian, 1;2 and T. Li 1;2 1

Institute of Optoelectronic Technology, China Jiliang University, Hangzhou 310018, China 2 Division of Bioengineering, School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637457 DOI: 10.1109/JPHOT.2011.2180708 1943-0655/$26.00 Ó2011 IEEE

Manuscript received November 3, 2011; revised December 10, 2011; accepted December 13, 2011. Date of publication December 20, 2011; date of current version January 3, 2012. This work was supported by the National Basic Research Program of China (973 Program) under Grant 2010CB327804, the National Natural Science Foundation of China under Grant 60807021, and the Zhejiang Provincial Natural Science Foundation of China under Grant R1080087. Corresponding author: C. C. Chan and X. Y. Dong (e-mail: [email protected]; [email protected]).

Abstract: In this paper, a novel strain sensor is proposed and demonstrated by employing a modified photonic crystal fiber (PCF)-based Mach–Zehnder interferometer, in which a collapsed region is introduced at the middle point of the PCF to improve the extinction ratio. Experimental results show that this proposed structure has a high sensitivity of 11.22 dB/m" over a range of 1.28 m" and high-temperature stability. Index Terms: Photonic crystal fiber (PCF), strain sensor, interferometer, temperatureinsensitive, intensity modulated.

1. Introduction Photonic crystal fiber (PCF) is a new class of optical fiber which has a series of air holes running along the length of the fiber. Due to its pure silica and air holes structure, PCF-based sensors have extremely low thermal dependence, and a compensation process is avoid. They have attracted lots of research interest in recent years in the measurement of strain [1]–[6], refractive index [7], pressure [8], temperature [9], and so on. The sensor setups are usually based on structures of fiber loop mirror [1]–[3], [9], Fabry–Pe´rot interferometer [4], [5], Mach–Zehnder interferometer (MZI) [6], [10]–[17], etc. PCF-MZI sensors can be formed by sandwiching a short length of PCF between two standard single-mode fibers (SMFs). The two beams can be introduced and combined by fusion splicing points or long period gratings [6], [10]–[14]. They are used for the measurement of strain [10]–[12], curvature [13], refractive index [14], and so on. Recently, several types of modified PCF-MZIs have been proposed based on polarization maintaining PCF [15], hollow core PCF [16], etc. A sensor array with several PCF-MZIs for multiple point measurement was also demonstrated [17]. In this paper, a simple modified PCF-MZI strain sensor is proposed by introducing a collapsed region to the middle of a normal PCF-MZI. As a result, the extinction ratio of the transmission spectrum of the PCF-MZI is significantly enhanced, which is good for increasing the measurement accuracy. The sensor design and principle, experimental results and discussions, and conclusion are described in the following sections.

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Fig. 1. Design of the modified PCF-MZI. Inset: (a) Image of the PCF’s end face. (b) Image of the splicing point including CR1 .

2. Sensor Design and Principle The proposed modified PCF-MZI strain sensor consists of a short section of index-guiding PCF (LMA-10, NKT Photonics) which was sandwiched between two SMFs longitudinally, as illustrated in Fig. 1. It has three collapsed regions, located respectively at the two endpoints and the middle of the PCF. The length of PCF is about 9.2 cm, and its end face is shown in inset (a) of Fig. 1. Fabrication of the modified PCF-MZI was carried out easily by using a conventional fusion splicer (Fujikura FSM-40S), together with a mechanical fiber cleaver. The collapsed regions were introduced in the arc fusion splicing process and their lengths could be adjusted by changing the intensity and duration of the arc discharge, as well as the offset between the central axis of the arc discharge and the joint of SMF and PCF [18]. In the experiment, a normal PCF-MZI was fabricated at first by fusion splicing the PCF to two SMFs with two collapsed regions, CR1 and CR2 , at its both ends, but the extinction ratio of its transmission spectrum was relatively low. In order to improve the performance, one more collapsed region, i.e., CR3 , was introduced to the middle point of the PCF, by using the same fabrication process as described above. The lengths of CR1 and CR2 are nearly the same, i.e., about 135 m. Inset (b) of Fig. 1 shows the picture of one of the splice points. CR3 is relatively long, i.e., 291 m, because the fusion splicing was happened for PCF only, which is different from the fabrication of CR1 and CR2 , where both PCF and SMF existed. As is known, in the collapsed region the PCF is not an SMF any more because there is no corecladding structure. A part of the optical beam coming from the core of the lead-in SMF will be coupled into cladding modes in CR1 . Then the two beams propagating along the core and cladding of the PCF will combine and interfere in the collapsed regions of CR3 and CR2 , successively. Therefore, the modified PCF-MZI is actually a combined one with two cascaded MZIs (MZI1 and MZI2 ). The interference happens twice so that the transmission spectrum measured at the lead-out SMF has a higher extinction ratio than that of the normal PCF-MZI. Assuming that the number of the excited cladding modes in MZI1 is n, then the total intensity of the transmission from MZI1 can be described as T1 ¼ E02 þ

n X i¼1

Ei2 þ 2

n n 1 X n X X 2ðni nj ÞL 2ðn0 ni ÞL E0 Ei cos Ei Ej cos þ2 i¼1 i¼1 j¼iþ1

(1)

where E0 , Ei , and Ej are the magnitudes of electric field of the core mode and the ith- and jth-order cladding mode of the PCF in MZI1 , respectively. n0 , ni , and nj are the effective indices of the core mode, the ith- and jth-order cladding mode of the PCF in MZI1 , respectively. L is the physical lengths of the MZI1 and MZI2 , and is the operating wavelength of the optical source.


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Fig. 2. Transmission spectra of the normal and the modified PCF-MZIs.

Although the total physical length of the PCF is not changed after introducing CR3 , the free spectral range of the interference pattern is doubled because the cavity number becomes from one to two and the cavity length is halved. After the second MZI ðMZI2 Þ of the modified PCF-MZI, the transmission can be easily expressed as the following [7]: T2 ¼ kT12

(2)

where k is the factor that describes the insertion loss of the transmission light at CR3 . The extinction ratio of the cascaded MZIs should, in theory, be twice of that of the MZI1 or MZI2 [7]. When an axial strain is applied on the total length of the PCF, the physical length of each cavity will change, and the effective refractive index for each mode of the PCF will change also due to the photoelastic effect [12]. As a consequence, the phase difference of each MZI is changed by the applied strain and wavelength shift of the interference pattern will be observed. By measuring this wavelength shift, the applied strain can be detected and evaluated.

3. Experimental Results and Discussions In order to apply strain to the modified PCF-MZI, one of its both ends, i.e., the lead-in and the leadout SMFs, was fixed while the other is stretched with a translation stage. A broad band light source (AS3223-BA2) with an output wavelength range from 1520 nm to 1600 nm and an optical spectrum analyzer (YOKOGAWA, AQ6370) with the highest wavelength resolution of 0.02 nm were employed to measure the interference transmission spectrum. A tunable laser source and an optical power meter (Newport, 1935T-C) were employed to carry out intensity-modulated measurement. Before the introduction of CR3 , the corresponding spectra and output intensities of the normal PCF-MZI including only CR1 and CR2 were recorded under different strain levels for comparison purpose. Afterwards, the CR3 was introduced and the measurement was carried out again. Fig. 2 shows the transmission spectra of the modified PCF-MZI against strain in comparison with that of the PCF-MZI without the third collapsed region. The applied strain was changed from 0 to 1.28 m". With increase of the strain, both interference patterns shifted to the shorter wavelength direction. It can be seen that extinction ratio of the modified PCF-MZI, i.e., 27.88 dB, is much larger than that of the normal PCF-MZI, i.e., 4.05 dB. It agrees with the theoretical prediction in general. The much better, 7 times, improvement in extinction ratio may be caused by various splitting ratios of light in the three collapsed PCF regions. The relatively large additional insertion loss, i.e., 5 dB, introduced by CR3 may be related to the relatively large length of the collapsed PCF region of CR3 [18]. Wavelength shifts of two transmission dips of the normal and the modified PCF-MZIs around 1595 nm were recorded against applied strain, as shown in Fig. 3. Both wavelength responses are linear and have R 2 values of over 0.99. The wavelength changing rates are 1.89 and 1.98 nm/m"


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Fig. 3. Measured wavelength shift versus applied strain for the normal and the modified PCF-MZIs.

Fig. 4. Optical intensity versus applied strain for the normal and the modified PCF-MZIs.

for the normal and the modified PCF-MZIs, respectively. It indicates that the third collapsed region of the PCF nearly does not affect the wavelength shift of the PCF-MZI. As wavelength measurement needs expensive equipment such as optical spectrum analyzer, intensity measurement is a better choice from this point of view. In the experiment, the output wavelength of the tunable laser source was set to 1537.79 nm and 1534.92 nm successively to measure the strain responses of the normal and the modified PCF-MZIs. These two wavelengths are the deepest transmission dips of the two PCF-MZIs under zero strain. The measured relationships between optical signal intensity and applied strain are shown in Fig. 4. The achieved sensitivity are 1.87 and 11.22 dB/m" for the normal and the modified PCF-MZIs, respectively. Therefore, the sensitivity of intensity measurement is also enhanced significantly for the modified PCF-MZI. Corresponding to the resolution of 0.01 dBm of the optical power meter, a high strain resolution of 0.89 " can be achieved, while it is 5.35 " for the normal PCF-MZI. The thermal stability of the proposed strain sensor was tested by placing the modified PCF-MZI on a hot plate (Corning PC-4200) with a gap of less than 0.5 mm and changing the temperature between 20 C and 80 C. The responses of wavelength and intensity were both recorded. The results are shown in Fig. 5. There were 0.089 nm red shift in wavelength and 0.558 dBm variation in intensity observed for the total range, i.e., 60 C, of temperature change. The thermal stabilities, i.e., 0.0015 nm= C and 0.009 dBm= C, are quite good. The error caused by temperature change is only 0:83 "= C for intensity measurement of strain. Considering the large measurement range of 1.28 m" of the proposed sensor, the small thermal response can be totally neglected in common applications.

4. Conclusion A temperature insensitive strain sensor has been proposed and experimentally demonstrated by employing a modified PCF-MZI, in which a collapsed region has been introduced to the middle point


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Fig. 5. Temperature response of the modified PCF-MZI strain sensor.

of the PCF to improve the extinction ratio. Strain sensitivity of 11.22 dB/m" with resolution of 0.89 " has been achieved within a large range of 1.28 m". Attributed to its simple configuration, easy fabrication, and cost-efficient intensity-modulated measurement, the proposed PCF-MZI strain sensor has good potential in various application areas.

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