We will discuss fabrication of twin core photonic crystal fiber (TC-PCF) using the stack-and-draw method and its application for in-line Mach-Zehnder interferometers. The small difference in the effective indexes of the two core modes leads to interference fringes and the birefringence of the twin cores results in polarization-dependent fringe spacing. The strain sensitivity was negative and wavelength-dependent. A novel intensity-based bend sensor is also demonstrated with bend-induced spatial fringe shift. High air filling fraction of fabricated TC-PCF cladding provides immunity to bend-induced intensity fluctuation.
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Various optical fiber sensors have been recently proposed for smart structure, structural health monitoring and security systems. Mach-Zehnder interferometric sensors with two independent optical paths, in which one of the paths acts as the sensing part, provide high sensitivity . For example, special multi-core fiber can be used for in-line Mach-Zehnder interferometer and four-core optical fiber has been used in Mach-Zehnder interferometer system for two-dimensional optical profilometry . An integrated Mach-Zehnder interferometer that utilizes a twin core fiber with electrodes was implemented for electro-optic switching .
Photonic crystal fibers (PCFs) are specialty optical fibers that are composed of microscopic air holes in the cladding that run parallel to the propagation direction with an air hole diameter d and period Λ . PCFs have drawn much interest because of their unique optical properties such as an endlessly single-mode operation, photonic bandgap effect, unique dispersion tailoring, and controllable optical nonlinearity [5–7]. The unique optical properties of PCFs are governed by strongly wavelength-dependent cladding effective index which provide large degrees of freedom in designing the optical properties . Numerous PCF sensors have been demonstrated, such as gas phase sensing [9,10] and PCF-based long-period fiber grating (LPFG) sensors for strain and temperature measurements . PCF-based LPFG interferometer with an arc discharging method was recently demonstrated .
In this work, we propose an in-line Mach-Zehnder interferometric sensor with twin core photonic crystal fiber (TC-PCF) for strain measurement. By splicing a TC-PCF between standard single-mode fibers (SMFs), all fiber in-line Mach-Zehnder interferometer can be configured. We also propose an intensity-based in-line vectorial bend sensor where the fringe shift is caused by bend-induced optical path difference between two identical silica cores. Our fabricated TC-PCF was found insensitive to bend loss. This novel property can be utilized for intensity-based smart bend sensing systems.
2. Fiber fabrication and dimensions
Our fabrication procedure of the TC-PCF is based on the well-known stack-and-draw method. The fiber was fabricated by stacking thin circular capillaries into the triangular lattice and drawing the preform into intermediary canes which are later drawn to the fiber of microscopic structure. Firstly, a number of end-sealed silica capillaries are stacked in a hexagonal close-packed structure. Then the two-core structure is formed by replacing four capillaries near the center with solid silica rods. This stack was over-jacketed and fed into high temperature furnace at the temperature of ~1820°C and then drawn to intermediary canes. During the cane drawing process, interstitial air holes (IAHs) are eliminated by pressurizing the entire cladding region. This cane was inserted inside sleeve tube and drawn to fiber while evacuating the sleeve tube. Figure 1 shows the scanning electron micrograph of the cross-section of a fabricated sample fiber. The microstructure has two cores. The larger (shorter) axis of the core is about 8.4 (2.9) μm. The diameter of 7-cells of microstructure which includes the twin cores is around 11 μm. It is similar to the core size of conventional SMFs. During the fiber drawing process, it is possible to modify the cladding air filling fraction and twin core spacing by controlling the gas pressure. The final fiber diameter was around 125 μm.
2. Experimental scheme of the strain and temperature measurement
Figure 2 shows the experimental setup of the proposed strain measurement using the TC-PCF-based in-line Mach-Zehnder interferometer. The two cores of the TC-PCF are not exactly identical and the slight difference in the effective indexes of the two core modes results in difference between the optical path lengths of the interferometer arms. In order to construct an in-line interferometer, the TC-PCF was fusion spliced between two SMFs (Samsung SMF). Light from a broadband source that passes through a polarizer is coupled into two cores at the splice point and then re-coupled beams make interference at the other end. Since the cores of the TC-PCF have birefringence, the interference fringes depend on the polarization state of the input beam. The transmission spectra were monitored by an optical spectrum analyzer (OSA). Figure 3 shows interference fringes for two orthogonal input polarization states with 87 cm length of TC-PCF. It is seen that the interference fringes have different peak spacing depending on the polarization state.
3. Strain response of twin core photonic crystal fiber based interferometer
The strain sensitivity of the proposed interferometer was measured with a 60 cm section of stripped TC-PCF using the input polarization state that produced the interference fringes of smaller peak spacing (Fig. 3(b)). The strain sensitivity was measured while applying tensile strain to the TC-PCF at room temperature (25°C) with a fixed input polarization. Figure 4 shows the shift of the transmission spectrum with the change of the applied strain. It was observed that the interference fringes shifted to the shorter wavelength almost linearly as the applied strain increased. The measured data for the wavelength shift of three peaks as a function of the applied strain are summarized in Fig. 5 . In the three spectral ranges near 1450 nm, 1500 nm and 1550 nm, linear fitting of the measured data up to 1800 με yielded the estimated strain sensitivities of −1.8pm/με, −2.0pm/ με and −2.18pm/με, respectively.
If the difference between the effective indexes of the two core modes were constant, the strain-induced peak shift would be red-shifted since application of the strain would increase only the lengths of the interferometer arms . The actual observation of the blue shift implies that the group birefringence sensitivity between the two core modes is negative in the TC-PCF.
4. Bending loss of twin core photonic crystal fiber
The typical d/Λ parameter for our fabricated TC-PCF was larger than 0.83. This high air-filling fraction in microstructured cladding provides strong immunity to bending due to the large index contrast between the silica core and the cladding. To confirm this, we measured the macro-bending loss of TC-PCF at the wavelength of 633 nm using He-Ne laser. The schematic of the bending loss measurement setup is shown in Fig. 6(a) . A number of bend turn was applied to a 1-meter long TC-PCF using a mandrel with a diameter of 6 mm. Figure 6(b) shows the macro-bending loss of TC-PCF with different number of bend turns. The inset summarizes the measured bending loss together with that of an SMF included for comparison.
5. Application of the twin core photonic crystal fiber as a vectorial bend sensor
The experimental setup for bend-induced interferometric fringe shift measurement is shown in Fig. 7 , which employs a fiber-coupled He-Ne laser and TC-PCF spliced to the SMF. The He-Ne laser beam was illuminated on SMF end and both cores of TC-PCF were excited. The launched light beams experience two independent optical paths to the cleaved end-face of the TC-PCF.
The two diffracted beams from the twin cores interfere with each other and form fringe pattern along the direction parallel to the axis of the twin cores. The TC-PCF shows very different fringe shift behaviors along the two orthogonal bend directions. Maximum fringe shift occurs when the fiber is bent in the direction of the axis of the twin cores and virtually no fringe shift occurs when the fiber is bent in the direction orthogonal to the axis.
We counted the number of projected fringe shifts with different bend angles by applying directional bending to the TC-PCF. Figure 8 shows measured fringe shift for the two orthogonal directions of bending (see Fig. 7) for the full range of directional bend angles together with the projected interferometric fringe pattern in the inset. Since the fabricated TC-PCF also has strong immunity to bend induced loss, fringe shifts can be easily counted without noisy intensity fluctuation. Our sample TC-PCF also showed less than 0.02dB bend loss at 633 nm with the same experimental configuration in Fig. 7 over the full range of the applied bend angle.
In conclusion, a twin core photonic crystal fiber (TC-PCF) has been fabricated with the stack-and-draw method. We investigated strain response of the TC-PCF-based in-line Mach-Zehnder interferometer and we observed that the peak wavelength shifted to shorter wavelength when the applied strain increased. This blue shift with application of strain implies that the group birefringence sensitivity between the two core modes is negative in the TC-PCF. The strain sensitivity was also shown to have wavelength dependence.
It is expected that TC-PCF-based in-line sensor will be promising especially for high temperature sensors since our fabricated TC-PCF is composed of single material that is free from the thermal diffusion problems. We also proposed and experimentally demonstrated intensity-based in-line Mach-Zehnder interferometric bend sensor using the TC-PCF which has strong bend loss immunity. The proposed bend sensor is simple and economical since it does not require optical spectrum analyzer (OSA). Our proposed in-line sensor also provides architectural design flexibility because photonic crystal fiber technology allows large degree of freedom in designing the optical characteristics. Further investigation regarding different designs of TC-PCF is currently under consideration.
This work was supported by the Photonics2020 research project through a grant provided by the Gwangju Institute of Science & Technology and by BK-21 Information Technology Project, Ministry of Education and Human Resources Development, Republic of Korea.
References and links
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