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Highly sensitive bend sensor based on Mach–Zehnder interferometer using photonic crystal fiber
Optics Communications 284 (2011) 2849–2853
Contents lists available at ScienceDirect
Optics Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o p t c o m
Highly sensitive bend sensor based on Mach–Zehnder interferometer using photonic crystal fiber Ming Deng a,⁎, Chang-Ping Tang a, Tao Zhu a, Yun-Jiang Rao b,a a
Key Laboratory of Optoelectronic Technology and Systems (Education Ministry of China), Chongqing University, Chongqing 400044, China Key Lab of Broadband Optical Fiber Transmission & Communication Networks Technologies (Education Ministry of China), University of Electronic Science and Technology of China, Chengdu, Sichuan 610054, China
b
a r t i c l e
i n f o
Article history: Received 22 October 2010 Received in revised form 22 February 2011 Accepted 22 February 2011 Available online 9 March 2011 Keywords: Fiber-optic sensors Mach–Zehnder interferometer Photonic crystal fiber Bend measurement
a b s t r a c t Optical fiber bend sensor with photonic crystal fiber (PCF) based Mach–Zehnder interferometer (MZI) is demonstrated experimentally. The results show that the PCF-based MZI is sensitive to bending with a sensitivity of 3.046 nm/m−1 and is independent on temperature with a sensitivity of 0.0019 nm/°C, making it the best candidate for temperature insensitive bend sensors. To that end, another kind of bend sensor with higher sensitivity of 5.129 nm/m−1 is proposed, which is constructed by combining an LPFG and an MZI with zero offset at the second splice mentioned above. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Optical fiber sensors offer unique advantages, such as immunity to electromagnetic interference, ruggedness even in corrosive and other harsh environments, fast response, high sensitivity, good stability and repeatability [1]. Therefore, they have been widely adopted and applied in different areas for the measurement of many physical parameters including strain, temperature, displacement, and pressure [2–6]. Special attention has been paid to the development of optical fiber bend sensors which have many applications in the fields of composite material structures, robot arms and artificial limbs. For instance, a number of optical fiber bend sensors based on long-period fiber grating (LPFG) have been proposed. The typical mechanisms used for LPFG bend sensor are based on a bend-induced wavelength shift, or on the splitting of some attenuation band [7,8]. However, the broad transmission resonance features of LPFG sensors cause difficulty in reading the exact wavelength of the loss dip, limiting the measurement accuracy. To overcome the problem mentioned above, a hybrid structure consisting of an LPFG/MMF and a tilted fiber Bragg grating (TFBG) was proposed [9,10]. In this configuration, an LPFG/MMF is located upstream from the TFBG and re-couples one of the cladding modes excited by the TFBG, so the curvature can be determined by monitoring the reflected powers of
⁎ Corresponding author. Tel.: +86 023 65111973; fax: +86 023 65111973. E-mail address: [email protected] (M. Deng). 0030-4018/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2011.02.061
the Bragg mode and of the re-coupled cladding mode. Recently, with the development of optical fiber technology, a macrobend sensor was fabricated by fusion-splicing a section of hollow-core fiber between two single mode fibers, and the measured macrobend loss was different in terms of the radius of curvature [11,12]. However, the light intensity modulation bend sensors are easily disturbed by the light source, showing poor stability. Alternatively, another kind of optical fiber bend sensor based on Sagnac loop with low-birefringence photonic crystal fiber has been demonstrated [13]. The low-birefringence photonic crystal fiber of ~400 mm long is inserted into a Sagnac loop and a section of it about 155 mm is used as a sensing element. However, the bending sensitivity of the sensing element is relatively low. On the other hand, an all-fiber bend sensor based on Mach–Zehnder interferometer (MZI) formed by two identical LPFGs in a photonic crystal fiber was also demonstrated [14]. Such a device has a higher bending sensitivity of 3 nm/m− 1 ranging from 0 to 1 m− 1, however, its fabrication process is complicated and precise. In this paper, we demonstrate an alternative fiber-optic bend sensor based on MZI, which is fabricated by simply splicing a section of photonic crystal fiber (PCF) in between two single mode fibers (SMFs) with a commercial available fusion splicer by using a manual program. At the first splice point, the cladding air holes of PCF are fully collapsed, resulting in that the cladding modes propagating in the PCF can be effectively excited. And then, at the second splice point, a part of cladding modes is coupled back into the core of a leadout SMF. As a result, modal interferences occur in the lead-out SMF due to the coupling between the core mode and the cladding modes.
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M. Deng et al. / Optics Communications 284 (2011) 2849–2853
We have fabricated PCF-based MZIs with regular and high-contrast fringe pattern by introducing small intentional lateral offsets at the second splice point. Therefore, such a device has the advantages of simple and compact structure, small size, and easy fabrication. Moreover, the proposed MZI is temperature insensitive due to its ultra-low thermal characteristics, making it the best candidate for the temperature insensitive bend sensors. To that end, we propose another kind of Mach–Zehnder interferometer using a hybrid structure combined with an LPFG and an MZI with zero offset at the second splice above mentioned. An LPFG located upstream couples a part of the core mode to a cladding mode, and at the collapsing region, the cladding mode is coupled back into the core. It is also demonstrated that such a structure can be used as a bending sensor, and exhibits the capability of detecting very small bending with higher sensitivity.
2. Sensor fabrication and operation principle The proposed MZI is formed by splicing a section of index-guiding photonic crystal fiber (PCF) in between two single-mode fibers (SMF-28e), as shown in Fig. 1. The PCF (ESM-12-01, Crystal Fiber Inc.) has a pure solid-silica core and a micro-structured cladding formed by a concentric ring of air holes in silica, as shown in Fig. 2. The diameters of air holes and the entire holey region are ~ 3.68 μm and ~ 60 μm, respectively. The core and outer diameters of the PCF are 12 μm and 125 μm, respectively. The mode field diameter of the PCF is ~ 6.4 μm, which is smaller than that of the SMF of ~ 10.4 μm. By utilizing the cladding modes of the PCF, we can make interference between the core mode and cladding modes in the lead-out SMF. So it is expected that, at the first splice point, a part of core mode beam coming from the lead-in SMF is coupled to a single or several cladding modes of PCF, and at the second splice point, the cladding modes of PCF are coupled back into the core of lead-out SMF. As a result, the cladding mode beam makes interference with the beam that has propagated only as the core mode, leading to smooth and regular interference fringe with higher contrast. In the splicing process, a high-accuracy optical spectrum analyzer (OSA, Si720, Micron Optics, USA) was used to in situ monitor the interference spectrum of the sensor with a wavelength resolution and precision of 0.25 pm and 1 pm, respectively. As a section of unjacketed PCF is spliced to a SMF-28e lead-in fiber, the suitable splicing parameters (e.g. the arc fusion current of 80 mA, pre-fuse time of 165 ms, and the arc duration of 800 ms) were set to ensure the air holes of PCF to completely collapse at a limited region, leading to a glass-air waveguide structure and hence the fundamental mode field coming from the lead-in fiber will broaden due to the diffraction in the region of collapsed holes. For this reason, a part of fundamental mode of the SMF-28e lead-in fiber can be coupled to several cladding modes of the PCF and propagated along the sensing segment [15–17]. At the second splice point, a small intentional lateral offset is introduced to limit the number of excitable cladding modes of PCF recoupled into the lead-out fiber. Moreover, a weaker fusion power and a shorter fusion time (i.e. fusion current of 55 mA, pre-fuse time of 150 ms, and the arc duration of 700 ms) were adopted to ensure the
Collapsing
SMF
Offset
PCF
SMF
Fig. 1. The schematic diagram of the PCF-based MZI.
Fig. 2. The structure of PCF.
selective cladding modes of PCF to enter into the SMF-28e lead-out fiber steadily. We observed the modes by using nanoscan near-field profiler systems (Photon Inc.) for the splicing point and lead out fiber, and the experimental results show that a few cladding modes occur, but only one dominates. As a result, the dominant cladding mode makes interference with the core mode in the SMF-28e lead-out fiber, making the interference fringe more smooth and uniform. A collection of samples with different interference lengths (e.g. ~ 2.1 cm, ~ 3.2 cm, and ~ 5.3 cm) was fabricated, and the interference fringes are shown in Fig. 3(a). From this figure, we can see that the interference spectra are more uniform and the fringe contrast is in the 7–20 dB range, which are better than that of all-PCF interferometers [18]. In addition, it can be seen that the longer length PCF based interferometer has narrow FSR. In order to determine the number and power distribution of the interference modes, the wavelength spectra in Fig. 3(a) are Fourier transformed to obtain the spatial frequency of the interference fringes, as shown in Fig. 3(b). It can be seen that there is only one dominant spatial frequency corresponding to an interference fringe, verifying that the interference effect did not occur among many strong modes, in accordance with the aforementioned analysis. Fig. 4 illustrates a hybrid structure combined with an LPFG and a PCF-based MZI with zero offset at the second splice point above mentioned, which is similar with that in Ref. [19]. An LPFG located upstream couples a part of the input core mode beam to multiple forward-propagating cladding modes of SMF, and then at the collapsing region, a certain amount of the light propagating in the cladding mode of SMF is coupled back into the core of PCF. After the collapsing region, two optical waves with a differential optical path delay propagate in the core of PCF, resulting in an interference pattern. The distance between the LPFG and the collapsing region corresponds to the physical length of the interferometer, which is ~220 mm in the experiment. The LPFG with a period of 640 μm and 38.4 mm length was arc-induced in the SMF-28e fiber by using the high frequency CO2 laser pulses exposure method [20]. The obtained central wavelength for this condition is 1540 nm. The length of the PCF is ~ 5.3 cm. The red line in Fig. 5 shows the interference pattern of the hybrid structure. It can be seen that such a device has a little narrower resonance wavelength, compared with that of the aforementioned PCF-based MZI. Moreover, the fringe contrast of the device is ~7 dB, which is about three times of the MZI based on LPFGs [21]. So it is anticipated that such a structure can be used as a sensor with higher resolution.
M. Deng et al. / Optics Communications 284 (2011) 2849–2853
2851
-10
Intensity(dBm)
-15
-20
~5.3cm
-25 ~3.2cm ~2.1cm ~3.2cm ~5.3cm
-30
-35 1520
a
~2.1cm
1530
1540
1550
1560
1570
Wavelength (nm) 8
Fig. 5. Interference spectra of the hybrid bending sensor.
x 104
7 ~2.1cm
~2.1cm ~3.2cm ~5.3cm
~3.2cm
6
between the core mode and cladding mode and includes a term due to the different propagation constants of the core mode and cladding modes propagating toward the re-coupled region. When external bend is applied to the sensors, the phase difference will change, which can be illustrated by the wavelength shifts in the optical spectrum analyzer. If the phase difference change is less than 2π, the phase ambiguity issue can be avoided. In this case, the characteristic spectral positions such as the interference peak, the center point of the interferogam or the interference valley can be monitored to measure the bending.
Intensity
5 4 ~5.3cm 3
3. Experimental results and discussion
2
b
1 0 0
0.1
0.2
0.3
0.4
0.5
0.6
Spatial frequncy(#/nm) Fig. 3. The interference patterns of the PCF-based MZI with different interference lengths.
For both aforementioned structures, the core and cladding paths constitute the arms of the MZI, whose transmission spectrum can be mathematically described by: I = Icore + Icladding + 2
Φ=
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Icore Icladding;i cosðΦÞ
The bend characteristic of the implemented PCF-based MZI was evaluated using the experimental setup presented in Fig. 6. A tested PCF-based MZI with interference length of ~320 mm was set in the middle of a section of fiber, and then two ends of the fiber were pasted on two thin metal sheets inserted in the slots of two disks, respectively. The thin metal sheets can turn the fiber on its axis when the two disks rotate synchronously. On the other hand, the bend-induced axis strain in the MZI can be eliminated by means of the thin metal sheets moving slightly along the axis of the disks. A metal beam was placed on the fiber, and two 5-g masses were attached to two ends of the fiber in order to ensure the fiber adhere to the bent metal beam. The fiber was bent at the center of the metal beam with a precise micrometer driver, the resulting curvature (C) of the sensor is defined as [7]:
ð1Þ C=
core cladding;i L 2π neff −neff
ð2Þ
λ
cladding, i where Icore and Icladding,i, ncore are the intensities and the eff and neff effective refractive indices of the core mode and the ith cladding mode, respectively. L is the physical length of the interferometer, and λ is the input wavelength in vacuum. Φ is the phase difference
LPFG
Collapsing
SMF
PCF
SMF
Fig. 4. The hybrid structure by cascading an LPFG and a PCF-based MZI with zero offset.
1 2d = 2 R d + l2
ð3Þ
where R is the curvature radius, d is the bending displacement at the center of the PCF-based MZI sensing section and l is the half-distance between the edges of the two supports, the initial length of which is ~580 mm. In the experiment, a high-accuracy optical spectrum analyzer (OSA, Si720, Micron Optics, USA) was used to measure the MZI transmission spectra as the curvature increased or decreased, as shown in Fig. 7. The initial main resonance valley was located near 1540 nm, and it shifted to a longer wavelength with the increment of bending curvature. It is noted that the reduction of the resonant spectral bandwidth and increment of the re-coupling strength were simultaneously observed for increasing bending curvature, which is contrary with that in Ref. [14]. This may be mainly because the coupling coefficients are placed in groups (with small Δβ distance), corresponding to different eigenvalues of modes. The plot of the
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M. Deng et al. / Optics Communications 284 (2011) 2849–2853
dial
P
steel beam
MZI
Si720 OSA
d 2l
metal sheet
5-g mass
Fig. 6. Schematic diagram of the experimental setup for curvature measurement.
Fig. 7. Wavelength shifts of a PCF-based MZI against the applied bending.
resonant valley wavelength of the PCF-based MZI versus bending curvature is shown in Fig. 8. The bending curvature ranges from 0 to 1.4 m− 1, the calculated bending sensitivity is 3.046 nm/m−1 without hysteresis observed, which is about ten times of Sagnac loop based bending sensor (typically –0.33 nm/m− 1) and comparable with that of the LPFG pairs-based MZI [13,14]. As the resolution of the OSA was 0.25 pm, the bending resolution for the PCF-based MZI was 0.00008 m− 1. On the other hand, the fabrication process of such a
device is simpler than that of the aforementioned two kind bending sensors. The temperature response of the PCF-based MZI with interference length of ~ 350 mm is shown in Fig. 9. It can be seen that the central wavelength slightly shifted to a longer wavelength when the temperature is raised from 30 °C ~ 100 °C. The sensitivity of the wavelength to temperature is 0.0019 nm/°C. So, such a device is temperature insensitive due to the small difference between the cladding and core temperature coefficients of the PCF [6], which makes it the best candidate for the temperature insensitive bend sensors. In the same way, the bending characteristic of the aforementioned hybrid structure was also investigated. For such a device, the interferometric phase change caused by curvature may depend on its any parts, i.e., the PCF section itself, the single mode fiber length between the LPFG and the PCF, and, eventually, the LPFG. To investigate this issue, the curvature action was applied to the LPFG, but no wavelength change in the device behavior was observed. Then, the LPFG and the single mode fiber length remained fixed and the curvature was induced only in the PCF section. Again, no wavelength change was noticed, however the coupling coefficients between the core mode and cladding mode should show some dependence on curvature, an indication that there is an increment of the resonant spectral bandwidth in the process. Finally, the LPFG and the PCF were kept straight and curvature was applied to the single mode fiber between them. In this situation a curvature-induced interferometric phase variation appeared. These tests indicate that the single mode fiber between the LPFG and the PCF is the crucial element to turn this structure a curvature-sensing head. To detail this behavior and to get
1542 1531 1530.75 y = 3.046x + 1537.3 R2 = 0.9965
1540
1530.5
Wavelength(nm)
Wavelength (nm)
1541
1539 1538
Increased Decreased
1530.25 1530 1529.75 1529.5
1537 1529.25 1536
0
0.2
0.4
0.6
0.8
1
1.2
1.4
Curvature (1/m) Fig. 8. Relationship between the applied bending and the wavelength shifts of the PCFbased MZI.
1529 30
40
50
60
70
80
90
100
Temperature(°C) Fig. 9. Temperature response of the PCF-based MZI with interference length of ~ 350 mm.
M. Deng et al. / Optics Communications 284 (2011) 2849–2853
1540.8
Wavelength (nm)
is used for measuring physical parameters. Experimental results show that the resonant wavelengths of the PCF-based MZI is sensitive to external bending with sensitivity of 3.046 nm/m− 1 for the bending curvature ranging from 0 to 1.4 m− 1. Moreover, such a device is insensitive to temperature, making it the best candidate for the temperature insensitive bending sensors. To that end, this paper also demonstrated another kind of Mach–Zehnder interferometer using a hybrid structure combined with an LPFG and an MZI with zero offset at the second splice above mentioned. It has been found that such a structure can also be used as a bending sensor, and exhibits the capability of detecting very small bending of 0.00004 m− 1. It is anticipated that this sensing structure will find applications in robot arms and artificial limbs where measurement of small curvatures with high sensitivity is often required.
y = 5.1293x + 1538.7 R2 = 0.9971
1540.4 1540 1539.6 1539.2
Increased Decreased
1538.8 1538.4
0
0.05
0.1
0.15
0.2
0.25
0.3
2853
0.35
Curvature(1/m) Fig. 10. Relationship between the applied bending and the wavelength shifts of the hybrid structure.
insight on the particular structure, the single mode fiber between the LPFG and PCF was bent at the center of the metal beam with a precise micrometer driver and the experimental results are illustrated in Fig. 5. Similarly, the initial main resonance valley near 1539 nm shifted to a longer wavelength by the increment of bending curvature, and the relationship between the applied bending and the wavelength shifts is shown in Fig. 10. It can be seen that the bending sensitivity is 5.129 nm/m−1 for the bending curvature ranging from 0 to 0.4 m− 1. Hence, the bending resolution for such a device was 0.00004 m− 1, which is higher than that of PCF-based MZI mentioned above. 4. Conclusion This paper reported an in-line photonic crystal fiber based Mach– Zehnder interferometer, which is fabricated by splicing a section of photonic crystal fiber in between two single mode fibers with a commercial available fusion splicer by using a manual program. The fabrication process only involves splicing and cleaving. So this MZI has the advantages of simple and compact structure, small size, and easy fabrication. The fabricated devices have regular and high-contrast fringe patterns, making it easier for the next signal processing when it
Acknowledgment This work is supported by the Project no. CDJZR10120002 supported by the Fundamental Research Funds for the Central Universities. References [1] B. Culshaw, G.W. Day, A.D. Kersey, Y. Ohtsuka, J. Lightwave Technol. 13 (1995) 1187. [2] H. Guo, F. Pang, X. Zeng, N. Chen, Z. Chen, T. Wang, Appl. Opt. 47 (2008) 3530. [3] E. Cibula, D. Donlagic, Opt. Express 15 (2007) 8719. [4] Y. Zhu, A. Wang, IEEE Photonics Technol. Lett. 17 (2005) 447. [5] Y.J. Rao, M. Deng, D.W. Duan, T. Zhu, Sens. Actuators A. 148 (2008) 33. [6] C.L. Zhao, L. Xiao, J. Ju, M.S. Demokan, W. Jin, J. Lightwave Technol. 26 (2008) 220. [7] H.J. Patrick, C.C. Chang, S.T. Vohra, Electron. Lett 34 (1998) 1773. [8] Y. Liu, L. Zhang, J.A.R. Williams, I. Bennion, IEEE Photonics Technol. Lett. 12 (2002) 531. [9] Y.X. Jin, C.C. Chan, X.Y. Dong, Y.F. Zhang, Opt. Commun. 283 (2009) 2690. [10] L.Y. Shao, A. Laronche, M. Seietana, P. Mikulic, W.J. Bock, J. Albert, Opt. Commun. 283 (2010) 2690. [11] Y. Jeong, S. Baek, B. Lee, S. Choi, K. Oh, Opt. Eng. 41 (2002) 1815. [12] W. Jin, H.F. Xuan, H.L. Ho, Meas. Sci. Technol. 21 (2010) 094014. [13] H.P. Gong, C.C. Chan, P. Zhu, L.H. Chen, X.Y. Dong, Opt. Commun. 282 (2010) 3905. [14] W. Shin, Y.L. Lee, B.A. Yu, Y.C. Noh, T.J. Ahn, Opt. Commun. 283 (2010) 2097. [15] B. Bourliaguet, C. Pare, F. Emond, A. Croteau, A. Proulx, R. Vallee, Opt. Express 11 (2003) 3412. [16] J.H. Chong, M.K. Rao, Opt. Express 11 (2003) 1365. [17] L.M. Xiao, M.S. Demokan, W. Jin, Y.P. Wang, C.L. Zhao, J. Lightwave Technol. 25 (2007) 3563. [18] H.Y. Choi, M.J. Kim, B.H. Lee, Opt. Express 15 (2007) 5711. [19] H.Y. Choi, K.S. Park, B. Lee, Opt. Lett. 33 (2008) 812. [20] Y.J. Rao, Y.P. Wang, Z.L. Ran, T. Zhu, J. Lightwave Technol. 21 (2003) 1320. [21] J.H. Lim, H.S. Jang, K.S. Lee, J.C. Kim, B.H. Lee, Opt. Lett. 29 (2004) 346.
Contents lists available at ScienceDirect
Optics Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o p t c o m
Highly sensitive bend sensor based on Mach–Zehnder interferometer using photonic crystal fiber Ming Deng a,⁎, Chang-Ping Tang a, Tao Zhu a, Yun-Jiang Rao b,a a
Key Laboratory of Optoelectronic Technology and Systems (Education Ministry of China), Chongqing University, Chongqing 400044, China Key Lab of Broadband Optical Fiber Transmission & Communication Networks Technologies (Education Ministry of China), University of Electronic Science and Technology of China, Chengdu, Sichuan 610054, China
b
a r t i c l e
i n f o
Article history: Received 22 October 2010 Received in revised form 22 February 2011 Accepted 22 February 2011 Available online 9 March 2011 Keywords: Fiber-optic sensors Mach–Zehnder interferometer Photonic crystal fiber Bend measurement
a b s t r a c t Optical fiber bend sensor with photonic crystal fiber (PCF) based Mach–Zehnder interferometer (MZI) is demonstrated experimentally. The results show that the PCF-based MZI is sensitive to bending with a sensitivity of 3.046 nm/m−1 and is independent on temperature with a sensitivity of 0.0019 nm/°C, making it the best candidate for temperature insensitive bend sensors. To that end, another kind of bend sensor with higher sensitivity of 5.129 nm/m−1 is proposed, which is constructed by combining an LPFG and an MZI with zero offset at the second splice mentioned above. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Optical fiber sensors offer unique advantages, such as immunity to electromagnetic interference, ruggedness even in corrosive and other harsh environments, fast response, high sensitivity, good stability and repeatability [1]. Therefore, they have been widely adopted and applied in different areas for the measurement of many physical parameters including strain, temperature, displacement, and pressure [2–6]. Special attention has been paid to the development of optical fiber bend sensors which have many applications in the fields of composite material structures, robot arms and artificial limbs. For instance, a number of optical fiber bend sensors based on long-period fiber grating (LPFG) have been proposed. The typical mechanisms used for LPFG bend sensor are based on a bend-induced wavelength shift, or on the splitting of some attenuation band [7,8]. However, the broad transmission resonance features of LPFG sensors cause difficulty in reading the exact wavelength of the loss dip, limiting the measurement accuracy. To overcome the problem mentioned above, a hybrid structure consisting of an LPFG/MMF and a tilted fiber Bragg grating (TFBG) was proposed [9,10]. In this configuration, an LPFG/MMF is located upstream from the TFBG and re-couples one of the cladding modes excited by the TFBG, so the curvature can be determined by monitoring the reflected powers of
⁎ Corresponding author. Tel.: +86 023 65111973; fax: +86 023 65111973. E-mail address: [email protected] (M. Deng). 0030-4018/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2011.02.061
the Bragg mode and of the re-coupled cladding mode. Recently, with the development of optical fiber technology, a macrobend sensor was fabricated by fusion-splicing a section of hollow-core fiber between two single mode fibers, and the measured macrobend loss was different in terms of the radius of curvature [11,12]. However, the light intensity modulation bend sensors are easily disturbed by the light source, showing poor stability. Alternatively, another kind of optical fiber bend sensor based on Sagnac loop with low-birefringence photonic crystal fiber has been demonstrated [13]. The low-birefringence photonic crystal fiber of ~400 mm long is inserted into a Sagnac loop and a section of it about 155 mm is used as a sensing element. However, the bending sensitivity of the sensing element is relatively low. On the other hand, an all-fiber bend sensor based on Mach–Zehnder interferometer (MZI) formed by two identical LPFGs in a photonic crystal fiber was also demonstrated [14]. Such a device has a higher bending sensitivity of 3 nm/m− 1 ranging from 0 to 1 m− 1, however, its fabrication process is complicated and precise. In this paper, we demonstrate an alternative fiber-optic bend sensor based on MZI, which is fabricated by simply splicing a section of photonic crystal fiber (PCF) in between two single mode fibers (SMFs) with a commercial available fusion splicer by using a manual program. At the first splice point, the cladding air holes of PCF are fully collapsed, resulting in that the cladding modes propagating in the PCF can be effectively excited. And then, at the second splice point, a part of cladding modes is coupled back into the core of a leadout SMF. As a result, modal interferences occur in the lead-out SMF due to the coupling between the core mode and the cladding modes.
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M. Deng et al. / Optics Communications 284 (2011) 2849–2853
We have fabricated PCF-based MZIs with regular and high-contrast fringe pattern by introducing small intentional lateral offsets at the second splice point. Therefore, such a device has the advantages of simple and compact structure, small size, and easy fabrication. Moreover, the proposed MZI is temperature insensitive due to its ultra-low thermal characteristics, making it the best candidate for the temperature insensitive bend sensors. To that end, we propose another kind of Mach–Zehnder interferometer using a hybrid structure combined with an LPFG and an MZI with zero offset at the second splice above mentioned. An LPFG located upstream couples a part of the core mode to a cladding mode, and at the collapsing region, the cladding mode is coupled back into the core. It is also demonstrated that such a structure can be used as a bending sensor, and exhibits the capability of detecting very small bending with higher sensitivity.
2. Sensor fabrication and operation principle The proposed MZI is formed by splicing a section of index-guiding photonic crystal fiber (PCF) in between two single-mode fibers (SMF-28e), as shown in Fig. 1. The PCF (ESM-12-01, Crystal Fiber Inc.) has a pure solid-silica core and a micro-structured cladding formed by a concentric ring of air holes in silica, as shown in Fig. 2. The diameters of air holes and the entire holey region are ~ 3.68 μm and ~ 60 μm, respectively. The core and outer diameters of the PCF are 12 μm and 125 μm, respectively. The mode field diameter of the PCF is ~ 6.4 μm, which is smaller than that of the SMF of ~ 10.4 μm. By utilizing the cladding modes of the PCF, we can make interference between the core mode and cladding modes in the lead-out SMF. So it is expected that, at the first splice point, a part of core mode beam coming from the lead-in SMF is coupled to a single or several cladding modes of PCF, and at the second splice point, the cladding modes of PCF are coupled back into the core of lead-out SMF. As a result, the cladding mode beam makes interference with the beam that has propagated only as the core mode, leading to smooth and regular interference fringe with higher contrast. In the splicing process, a high-accuracy optical spectrum analyzer (OSA, Si720, Micron Optics, USA) was used to in situ monitor the interference spectrum of the sensor with a wavelength resolution and precision of 0.25 pm and 1 pm, respectively. As a section of unjacketed PCF is spliced to a SMF-28e lead-in fiber, the suitable splicing parameters (e.g. the arc fusion current of 80 mA, pre-fuse time of 165 ms, and the arc duration of 800 ms) were set to ensure the air holes of PCF to completely collapse at a limited region, leading to a glass-air waveguide structure and hence the fundamental mode field coming from the lead-in fiber will broaden due to the diffraction in the region of collapsed holes. For this reason, a part of fundamental mode of the SMF-28e lead-in fiber can be coupled to several cladding modes of the PCF and propagated along the sensing segment [15–17]. At the second splice point, a small intentional lateral offset is introduced to limit the number of excitable cladding modes of PCF recoupled into the lead-out fiber. Moreover, a weaker fusion power and a shorter fusion time (i.e. fusion current of 55 mA, pre-fuse time of 150 ms, and the arc duration of 700 ms) were adopted to ensure the
Collapsing
SMF
Offset
PCF
SMF
Fig. 1. The schematic diagram of the PCF-based MZI.
Fig. 2. The structure of PCF.
selective cladding modes of PCF to enter into the SMF-28e lead-out fiber steadily. We observed the modes by using nanoscan near-field profiler systems (Photon Inc.) for the splicing point and lead out fiber, and the experimental results show that a few cladding modes occur, but only one dominates. As a result, the dominant cladding mode makes interference with the core mode in the SMF-28e lead-out fiber, making the interference fringe more smooth and uniform. A collection of samples with different interference lengths (e.g. ~ 2.1 cm, ~ 3.2 cm, and ~ 5.3 cm) was fabricated, and the interference fringes are shown in Fig. 3(a). From this figure, we can see that the interference spectra are more uniform and the fringe contrast is in the 7–20 dB range, which are better than that of all-PCF interferometers [18]. In addition, it can be seen that the longer length PCF based interferometer has narrow FSR. In order to determine the number and power distribution of the interference modes, the wavelength spectra in Fig. 3(a) are Fourier transformed to obtain the spatial frequency of the interference fringes, as shown in Fig. 3(b). It can be seen that there is only one dominant spatial frequency corresponding to an interference fringe, verifying that the interference effect did not occur among many strong modes, in accordance with the aforementioned analysis. Fig. 4 illustrates a hybrid structure combined with an LPFG and a PCF-based MZI with zero offset at the second splice point above mentioned, which is similar with that in Ref. [19]. An LPFG located upstream couples a part of the input core mode beam to multiple forward-propagating cladding modes of SMF, and then at the collapsing region, a certain amount of the light propagating in the cladding mode of SMF is coupled back into the core of PCF. After the collapsing region, two optical waves with a differential optical path delay propagate in the core of PCF, resulting in an interference pattern. The distance between the LPFG and the collapsing region corresponds to the physical length of the interferometer, which is ~220 mm in the experiment. The LPFG with a period of 640 μm and 38.4 mm length was arc-induced in the SMF-28e fiber by using the high frequency CO2 laser pulses exposure method [20]. The obtained central wavelength for this condition is 1540 nm. The length of the PCF is ~ 5.3 cm. The red line in Fig. 5 shows the interference pattern of the hybrid structure. It can be seen that such a device has a little narrower resonance wavelength, compared with that of the aforementioned PCF-based MZI. Moreover, the fringe contrast of the device is ~7 dB, which is about three times of the MZI based on LPFGs [21]. So it is anticipated that such a structure can be used as a sensor with higher resolution.
M. Deng et al. / Optics Communications 284 (2011) 2849–2853
2851
-10
Intensity(dBm)
-15
-20
~5.3cm
-25 ~3.2cm ~2.1cm ~3.2cm ~5.3cm
-30
-35 1520
a
~2.1cm
1530
1540
1550
1560
1570
Wavelength (nm) 8
Fig. 5. Interference spectra of the hybrid bending sensor.
x 104
7 ~2.1cm
~2.1cm ~3.2cm ~5.3cm
~3.2cm
6
between the core mode and cladding mode and includes a term due to the different propagation constants of the core mode and cladding modes propagating toward the re-coupled region. When external bend is applied to the sensors, the phase difference will change, which can be illustrated by the wavelength shifts in the optical spectrum analyzer. If the phase difference change is less than 2π, the phase ambiguity issue can be avoided. In this case, the characteristic spectral positions such as the interference peak, the center point of the interferogam or the interference valley can be monitored to measure the bending.
Intensity
5 4 ~5.3cm 3
3. Experimental results and discussion
2
b
1 0 0
0.1
0.2
0.3
0.4
0.5
0.6
Spatial frequncy(#/nm) Fig. 3. The interference patterns of the PCF-based MZI with different interference lengths.
For both aforementioned structures, the core and cladding paths constitute the arms of the MZI, whose transmission spectrum can be mathematically described by: I = Icore + Icladding + 2
Φ=
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Icore Icladding;i cosðΦÞ
The bend characteristic of the implemented PCF-based MZI was evaluated using the experimental setup presented in Fig. 6. A tested PCF-based MZI with interference length of ~320 mm was set in the middle of a section of fiber, and then two ends of the fiber were pasted on two thin metal sheets inserted in the slots of two disks, respectively. The thin metal sheets can turn the fiber on its axis when the two disks rotate synchronously. On the other hand, the bend-induced axis strain in the MZI can be eliminated by means of the thin metal sheets moving slightly along the axis of the disks. A metal beam was placed on the fiber, and two 5-g masses were attached to two ends of the fiber in order to ensure the fiber adhere to the bent metal beam. The fiber was bent at the center of the metal beam with a precise micrometer driver, the resulting curvature (C) of the sensor is defined as [7]:
ð1Þ C=
core cladding;i L 2π neff −neff
ð2Þ
λ
cladding, i where Icore and Icladding,i, ncore are the intensities and the eff and neff effective refractive indices of the core mode and the ith cladding mode, respectively. L is the physical length of the interferometer, and λ is the input wavelength in vacuum. Φ is the phase difference
LPFG
Collapsing
SMF
PCF
SMF
Fig. 4. The hybrid structure by cascading an LPFG and a PCF-based MZI with zero offset.
1 2d = 2 R d + l2
ð3Þ
where R is the curvature radius, d is the bending displacement at the center of the PCF-based MZI sensing section and l is the half-distance between the edges of the two supports, the initial length of which is ~580 mm. In the experiment, a high-accuracy optical spectrum analyzer (OSA, Si720, Micron Optics, USA) was used to measure the MZI transmission spectra as the curvature increased or decreased, as shown in Fig. 7. The initial main resonance valley was located near 1540 nm, and it shifted to a longer wavelength with the increment of bending curvature. It is noted that the reduction of the resonant spectral bandwidth and increment of the re-coupling strength were simultaneously observed for increasing bending curvature, which is contrary with that in Ref. [14]. This may be mainly because the coupling coefficients are placed in groups (with small Δβ distance), corresponding to different eigenvalues of modes. The plot of the
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M. Deng et al. / Optics Communications 284 (2011) 2849–2853
dial
P
steel beam
MZI
Si720 OSA
d 2l
metal sheet
5-g mass
Fig. 6. Schematic diagram of the experimental setup for curvature measurement.
Fig. 7. Wavelength shifts of a PCF-based MZI against the applied bending.
resonant valley wavelength of the PCF-based MZI versus bending curvature is shown in Fig. 8. The bending curvature ranges from 0 to 1.4 m− 1, the calculated bending sensitivity is 3.046 nm/m−1 without hysteresis observed, which is about ten times of Sagnac loop based bending sensor (typically –0.33 nm/m− 1) and comparable with that of the LPFG pairs-based MZI [13,14]. As the resolution of the OSA was 0.25 pm, the bending resolution for the PCF-based MZI was 0.00008 m− 1. On the other hand, the fabrication process of such a
device is simpler than that of the aforementioned two kind bending sensors. The temperature response of the PCF-based MZI with interference length of ~ 350 mm is shown in Fig. 9. It can be seen that the central wavelength slightly shifted to a longer wavelength when the temperature is raised from 30 °C ~ 100 °C. The sensitivity of the wavelength to temperature is 0.0019 nm/°C. So, such a device is temperature insensitive due to the small difference between the cladding and core temperature coefficients of the PCF [6], which makes it the best candidate for the temperature insensitive bend sensors. In the same way, the bending characteristic of the aforementioned hybrid structure was also investigated. For such a device, the interferometric phase change caused by curvature may depend on its any parts, i.e., the PCF section itself, the single mode fiber length between the LPFG and the PCF, and, eventually, the LPFG. To investigate this issue, the curvature action was applied to the LPFG, but no wavelength change in the device behavior was observed. Then, the LPFG and the single mode fiber length remained fixed and the curvature was induced only in the PCF section. Again, no wavelength change was noticed, however the coupling coefficients between the core mode and cladding mode should show some dependence on curvature, an indication that there is an increment of the resonant spectral bandwidth in the process. Finally, the LPFG and the PCF were kept straight and curvature was applied to the single mode fiber between them. In this situation a curvature-induced interferometric phase variation appeared. These tests indicate that the single mode fiber between the LPFG and the PCF is the crucial element to turn this structure a curvature-sensing head. To detail this behavior and to get
1542 1531 1530.75 y = 3.046x + 1537.3 R2 = 0.9965
1540
1530.5
Wavelength(nm)
Wavelength (nm)
1541
1539 1538
Increased Decreased
1530.25 1530 1529.75 1529.5
1537 1529.25 1536
0
0.2
0.4
0.6
0.8
1
1.2
1.4
Curvature (1/m) Fig. 8. Relationship between the applied bending and the wavelength shifts of the PCFbased MZI.
1529 30
40
50
60
70
80
90
100
Temperature(°C) Fig. 9. Temperature response of the PCF-based MZI with interference length of ~ 350 mm.
M. Deng et al. / Optics Communications 284 (2011) 2849–2853
1540.8
Wavelength (nm)
is used for measuring physical parameters. Experimental results show that the resonant wavelengths of the PCF-based MZI is sensitive to external bending with sensitivity of 3.046 nm/m− 1 for the bending curvature ranging from 0 to 1.4 m− 1. Moreover, such a device is insensitive to temperature, making it the best candidate for the temperature insensitive bending sensors. To that end, this paper also demonstrated another kind of Mach–Zehnder interferometer using a hybrid structure combined with an LPFG and an MZI with zero offset at the second splice above mentioned. It has been found that such a structure can also be used as a bending sensor, and exhibits the capability of detecting very small bending of 0.00004 m− 1. It is anticipated that this sensing structure will find applications in robot arms and artificial limbs where measurement of small curvatures with high sensitivity is often required.
y = 5.1293x + 1538.7 R2 = 0.9971
1540.4 1540 1539.6 1539.2
Increased Decreased
1538.8 1538.4
0
0.05
0.1
0.15
0.2
0.25
0.3
2853
0.35
Curvature(1/m) Fig. 10. Relationship between the applied bending and the wavelength shifts of the hybrid structure.
insight on the particular structure, the single mode fiber between the LPFG and PCF was bent at the center of the metal beam with a precise micrometer driver and the experimental results are illustrated in Fig. 5. Similarly, the initial main resonance valley near 1539 nm shifted to a longer wavelength by the increment of bending curvature, and the relationship between the applied bending and the wavelength shifts is shown in Fig. 10. It can be seen that the bending sensitivity is 5.129 nm/m−1 for the bending curvature ranging from 0 to 0.4 m− 1. Hence, the bending resolution for such a device was 0.00004 m− 1, which is higher than that of PCF-based MZI mentioned above. 4. Conclusion This paper reported an in-line photonic crystal fiber based Mach– Zehnder interferometer, which is fabricated by splicing a section of photonic crystal fiber in between two single mode fibers with a commercial available fusion splicer by using a manual program. The fabrication process only involves splicing and cleaving. So this MZI has the advantages of simple and compact structure, small size, and easy fabrication. The fabricated devices have regular and high-contrast fringe patterns, making it easier for the next signal processing when it
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