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Photonic crystal fiber interferometer composed of a long period fiber grating and one point collapsing of air holes Hae Young Choi, Kwan Seob Park, and Byeong Ha Lee* Department of Information and Communications, Gwangju Institute of Science and Technology (GIST), 1 Oryong-dong, Buk-gu, Gwangju 500-712, South Korea *Corresponding author:
[email protected] Received January 4, 2008; revised March 5, 2008; accepted March 6, 2008; posted March 12, 2008 (Doc. ID 91238); published April 10, 2008 We present an all-fiber interferometer fabricated with a single piece of an endless single-mode photonic crystal fiber (PCF) by an electric arc discharge. By forming a long period grating (LPG) at a point and collapsing the air holes at another point along the PCF, the simple but effective interferometer could be implemented. The LPG made a strong wavelength selective mode coupling between the core and cladding modes in the interesting wavelength range, while the air-hole collapse induced wavelength independent mode couplings. By cascading them, we could implement the all-fiber interferometer. As a potential application of the proposed all PCF interferometer, strain sensing is experimentally demonstrated. © 2008 Optical Society of America OCIS codes: 060.2370, 050.2770, 060.5295.
A photonic crystal fiber (PCF), which is generally composed of a single material with an array of air holes running along its length, has been widely studied over the last decade. Since the structure of the air holes is adjustable, unique characteristics, such as a single-mode operation over a wide wavelength range and a large mode area have been obtained [1,2]. These properties make the PCF attractive not only in optical fiber communications but also in fiber sensor applications. The long period fiber grating (LPG) imprinted in the PCF is one of the good sensing elements owing to its strong wavelength dependency. Several implementing methods have been reported [3–5] and applications, such as high temperature, bending, and strain sensors, have also been proposed. To enhance the resolution of the LPG-based sensors, the interferometer technique based on an LPG pair is usually utilized. However, it is not simple to form two identical LPGs along a PCF unlike with conventional single-mode fibers because one cannot form the LPGs by using the UV exposing method. In general, the PCF composed of pure silica has no photosensitivity. Recently, the possibility of fabricating an all-PCF interferometer based on the LPGs induced by periodic mechanical pressing was demonstrated [6]. However, the pressing method is not suitable or practical for sensing applications owing to its complicated structure and bulkiness. In this Letter, we present the PCF interferometer composed of a single LPG and a short region of airhole collapse made along a single piece of pure silica PCF. The LPG is imprinted with the electric arc discharge of a fusion splicer by using the point-by-point technique [7]. The PCF-LPG induces strong mode coupling from the fundamental core to the cladding modes of the PCF. To get the mode coupling back from the cladding to the core modes, the air holes of the PCF at a short section along its length are collapsed. By simply cascading these two coupling elements, the LPG and the collapsing section, a very 0146-9592/08/080812-3/$15.00
sensitive Mach–Zehnder interferometer can be effectively implemented. By adjusting the separation between coupling elements the spectral properties of the interferometer, specifically the interference fringe spacing, can be controlled. The spectral property of an implemented all-PCF interferometer is investigated and its strain-sensitivity is experimentally presented. An LPG was formed in a PCF (Crystal Fibre Co., LMA10) by inducing a series of periodic local deformations with a commercial fusion splicer (FITEL Co., S183PM). A piece of unjacketed PCF was fixed onto a motor-driven translation stage and loaded by a small weight to apply a constant tension along the fiber under the deformation process. The PCF was heated by the electric arc discharge while moving the stage in steps of the grating period. This process was repeated several times until a required resonant peak was obtained. The transmission spectrum of the fabricated PCF-LPG was measured by using a broadband LED and an optical spectrum analyzer (OSA). Figure 1 shows the transmission spectra of the fabricated PCF-LPGs having a grating period of 490 m and 15–17 grating elements. The resonant wavelength of the PCF-LPG formed with the 17 grating elements was located at ⬃1.26 m and its peak coupling efficiency was ⬃25 dB. The PCF had four layers of hexagonally structured air holes around a solid core of an 11 m diameter as shown in Fig. 2(a), which was designed to have the endless single mode property. When the PCF was locally heated by strong electric arc discharges, the air holes in the cladding region were collapsed. Figures 2(b) and 2(c) show the side and cross-sectional views of the collapsed region, respectively, which reveal that the air holes in the heated region were fully collapsed. At the collapsed region, the PCF no longer becomes a single-mode waveguide, since the fiber has no cladding part that can confine the core mode. Thus, the beam that has originally been in the core of © 2008 Optical Society of America
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Fig. 3. (Color online) Schematic of the all-PCF interferometer composed of an LPG and air-hole collapsing in a single piece of PCF.
Fig. 1. (Color online) Transmission spectra of the PCFLPGs having a grating period of 490 m and 15–17 grating elements. The resonant peak grows with the number of elements. The maximum coupling was ⬃25 dB for the 17 grating elements.
a PCF can be partly directed into the cladding region and vice versa, which enables the mode coupling between the fundamental core and cladding modes of the PCF. Moreover, one important property of the airhole collapsing method is that the mode coupling does not depend on the wavelength, unlike the LPG case. Figure 3 shows the schematic of the proposed allPCF interferometer. A part of the core mode beam is coupled to a cladding mode by the PCF-LPG. This mode coupling is wavelength selective owing to the resonant property of the LPG. After the LPG, two optical waves separately propagate through the core and cladding regions of the PCF. Finally, the beam in the cladding mode is partly recoupled to the core mode at the collapsing region of the PCF. This recoupling is not wavelength selective. Since the recoupled core mode makes interference with the original core mode that has not been coupled at both coupling elements, a simple all-fiber Mach–Zehnder interferometer could be formed. The distance from the center of the LPG to the center of the collapsing region corresponds to the physical length of the interferometer, and the optical path length difference of the interferometer becomes proportional to the difference in the propagation constants of the involved modes as the LPG pair case [8]. Figure 4 shows the experimentally measured transmission spectra of a single LPG (dashed curve)
and the implemented interferometer (solid curve). We can see the well-developed interference fringes within the stop band of a typical LPG spectrum. The spectral spacing between adjacent interference fringes was measured as 9.1 nm, which is known as inversely proportional to the difference between the effective group indices of the involved modes [8]. In the experiment, the length of the interferometer was 80 mm, the period of the grating was 480 m, the number of grating elements was 10, and the length of the air-hole collapsing region was 200 m. The transmission loss due to air-hole collapsing was measured as ⬃3 dB. To investigate the usefulness of the proposed allPCF interferometer as a strain sensor, an all-PCF interferometer having a physical length of 80 mm was utilized and the variation of its transmission spectrum according to longitudinal strain was monitored. Figure 5(a) shows the transmission spectra measured without strain and with a 2200 strain. The straininduced variation of the interference peak centered on 1.314 m was followed and is plotted in Fig. 5(b). The interference peak was linearly decreased or blueshifted with a sensitivity of −1.8 pm/ . Interestingly, the blueshift of the interference fringe is opposite that of the strain sensor based on a conventional single-mode fiber [9]. In the interferometer of a physical length L, the phase difference between the two beams (one has passed as the core mode and the other as a cladding mode) is given by 2共nco − ncl兲L / , where is the wavelength and nco and ncl are the effective indices of the core and cladding modes, respectively. When the involved modes are nondispersive, the differential effective index 共nco − ncl兲 becomes constant. In that case, the strain causes only the increment of the interferometer length L; thus, the resonant wavelength increases with the strain (redshift) to keep the same phase. However, when the modes of the PCF are very dispersive, the differential effective index can no longer be treated as constant. Considering only dis-
Fig. 2. (Color online) Cross-sectional view of the PCF (a) before and (c) after arc discharge and (b) its side view. The air holes in the cladding region of the PCF were fully collapsed with the arc discharge.
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Fig. 4. (Color online) Transmission spectra of an LPG having a 480 m period (dashed curve) and the proposed interferometer (solid curve). Interference fringes are well developed.
persions, or neglecting the elasto-optic effect, we can say that, from the blueshift of the resonant peak, effective index of the core mode varies more slowly with the wavelength than that of the cladding mode
of the PCF. A similar behavior was reported with the LPGs inscribed in a PCF with various grating periodicities [3,10]. The most common modal interferometer based on optical fibers is composed of a pair of 3 dB LPGs, which needs two identical LPGs to get the best performance [8,11]. However, the proposed interferometer requires only one LPG but has the performance of the LPG pair. The operating wavelength is solely determined by the resonant property of the single LPG since the mode coupling made at the air-hole collapsing region is not wavelength dependent. Therefore, we can have much freedom in choosing the operating wavelength; in other words, the effort for matching the resonant wavelengths of two LPGs is not necessary. Moreover, the simple structure of the proposed interferometer allows easy fabrication and versatile applications. In summary, we have proposed and demonstrated the all-fiber interferometer that was implemented in a single piece of pure silica PCF by an LPG and airhole collapsing. The LPG made a mode coupling between the core and cladding modes of the PCF at a narrow wavelength range, while the air-hole collapsing induced wavelength independent mode coupling. The differential effective index of the PCF and the separation between the LPG and the collapsing region mainly governed the interference property of the interferometer. It was measured that the interference fringes were linearly shifted toward the shorter wavelength direction with the strain applied along the device. This type of modal interferometer is very simple to fabricate and thus expected to be very powerful not only as a strain sensor but as a high temperature sensor. Because the PCF and thus the interferometer are composed of only fused silica, we can overcome the core material diffusion problem of conventional single-mode fibers at high temperatures. This work was supported by the BK21 project. References
Fig. 5. (Color online) (a) Transmission spectra of the interferometer measured at 0 and 2200 strain. (b) Strain response of the interference peak centered at 1.314 m. The interference fringe was blueshifted with the strain having a sensitivity of −1.8 pm/ .
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