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Abstract
We propose and demonstrate a nondestructive and contamination-free method for azimuth angle orientation of the photonic crystal fiber (PCF) and its application for fiber side-polishing. This technique is based on the interference pattern analysis of the light forward-scattered from the PCF that is side illuminated by a laser beam. The scattering pattern is analyzed by introducing a characteristic value which is the sum over the intensities of the upper-half or lower-half regions of the scattering pattern. The characteristic value correlates closely with the azimuth angle of PCF, which enables characterizing the azimuth angle through scattering pattern analysis. Three kinds of PCFs are studied for the determination of their azimuth angles, and an angular accuracy better than 0.5° is obtained. This method is subsequently applied to orient the angular azimuth in side-polishing of the PCF, and the accuracy of polishing angle is about 0.5°. This technique is nondestructive, contamination-free, easy to implement, and able to serve for in-line angular orientation during the fabrication of PCF-based optical devices and manipulation of the PCF.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Since the emergence, the photonic crystal fibers (PCFs) [1–3] have been widely used, due to their unique guiding properties, for realizing various optical devices such as fiber lasers [4], interferometers [5], refractive index sensors [6], temperature sensors [7], filters [8] and super-continuum sources [9]. The unique guiding properties of PCFs benefit from the inner microstructures of the PCFs. The functionality of the PCF-based optical devices depends on the fiber internal microstructures. For example, during the fabrication of Bragg gratings and long-period gratings in PCFs, the transverse-coupling efficiency between the lightwave and the fiber core depends on the orientation of the PCF [10-11]. A high grating growth rate requires accurate determination of the azimuth angle of PCF. In addition, the azimuth angle must be considered for side-polished PCF since the symmetry of the PCF is broken after side polishing. It has been reported that the mode field distribution varies with the polishing angles especially when the residual radius of the side-polished PCF is smaller than 0.5 μm [12]. The PCF couplers were typically implemented by the fused biconical taper method [13], but the air holes were easily deformed under high temperature. In contrast, the side-polishing technique is ideal for fabrication of PCF couplers [14–16]. The polishing angle is a key factor for determining the coupling ratio of the PCF coupler, because the evanescent field coupling between the cores of the fibers relies on the polishing orientation [17]. Therefore, it is important to precisely determine and control the azimuth angle of PCF during the fabrication of the optical device. In addition, accurate determination of the azimuth angle of PCF is an essential requirement for manipulating the PCF, such as splicing, connecting, and coupling.
Several techniques have been proposed to retrieve the PCF inner structure. For example, the structure of the PCF was monitored by examining the cleaved ends of the PCF using optical and scanning electron microscope (SEM). However, this method is destructive and polishing the cross-section of the fiber is required before examining the fiber ends. The refractive index profile of PCF was tomographically reconstructed by imaging the fiber at various fiber orientations [18], while data-processing of the images is complicated and time consuming. Digital holographic microtomography was also applied to image the three-dimensional refractive index profile of PM PANDA fiber [19] and measure the geometric parameters of the single-mode fiber [20]. Side-scattering technique by perpendicularly illuminating the fiber with a laser beam, on the other hand, is a nondestructive technique for retrieving the microstructure in PCF [21–23]. The scattering patterns carry information regarding the fiber inner microstructure, which were applied to derive the information such as the pitch and azimuth angle of the PCF by measuring the Doppler shift of the scattered light [21]. The bandgaps in the transverse PCF were observed in the measured transmission spectra [24]. In addition, the crystallographic symmetry axes of the PCF were determined by measuring the transverse intensity patterns of the light scattered from the PCF [25]. However, precise alignment of the launching and probing SMFs is required in this method. In addition, it requires a proper power of the incident light and the angle accuracy is limited to 3°. Since the PCF has more complicate microstructure and larger index contrast between the air holes and the silica glass compared with the SMF, its inner microstructure can result in multiple scattering which changes as the fiber is rotated under laser illumination, making the analysis of the scattering patterns complicated. An effective solution to overcome multiple scattering is to fill the hollow channels of PCF with index-matching fluid, which was previously used to reduce the scattering from air-glass interfaces in Bragg grating writing [26]. A precise measurement of the twist profile of structural rocking filters in PCF was realized by employing side-scattering [22], where the hollow channels were filled with a fluid of index close to that of silica so as to avoid multiple scattering. However, the PCF is contaminated by the index-matching fluid. More importantly, this approach is not applicable to shorter wavelengths, and the heating effects of the fluids can damage the fiber as a result of higher-order photon effects during grating writing [27]. Side images obtained through illuminating the fiber with an incoherent light were used to determine the azimuth angle of PCF [23]. In this method, the multiple scattering was reduced by using the incoherent light, and a characteristic value defined as the maximum intensity of the image was introduced to characterize the azimuth angle. However, an objective lens was required, and a CCD camera was fixed for recording the side images, which needs precise optical alignment and limits its in-line applications such as manipulating the PCF.
In this paper, we report a nondestructive and contamination-free technique for determining the azimuth angles of PCF from analysis of the scattering patterns. The scattering patterns can be recorded with flexible locations of the CCD camera, which can ease the alignment problem. The irregular spikes in the scattering pattern caused by multiple scattering are reduced through introducing a characteristic value which is the sum over the intensities of the upper-half or lower-half region of the scattering pattern. The correlation between the characteristic value and the azimuth angle is used to determine the azimuth angle of PCF. We show how this technique can be used to determine azimuth angles of PCFs including the endless single mode (ESM), large-mode area (LMA), and hybrid lattice microstructured (HLM) fibers. In addition, this technique is applied for the angular orientation during the side-polishing of the ESM fiber along particular directions, and the accuracy of polishing angular is about 0.5°. To the best of our knowledge, this is the first demonstration of utilizing forward-scattering technique for in-line angular orientation during the PCF side-polishing process.
2. Method and results
Figures 1(a)-1(c) depict the cross sections of the investigated PCFs, and Figs. 1(d)-1(f) show their corresponding SEM images, respectively. The X-axis is set to be along the horizontal direction and Y-axis is along the vertical direction. The ГK and ГM directions are defined in Figs. 1(a)-1(c). The azimuth angle θ is defined as the angle between the X-axis and the ГK direction of the PCF. Figure 2 is the experimental setup of the side-scattering technique. The light source is a semiconductor laser with a wavelength of 650 nm, which illuminates the fiber under test on a direction perpendicular to the longitudinal axis. The fiber is fixed along the Z-axis by a fiber chuck and mounted on a motorized rotation stage, enabling rotation along the fiber axis step by step. The light is scattered by the inner structures of the fiber, and the scattering pattern is captured using an imaging screen. The distance between the fiber and the imaging screen is about 1 m. A CCD camera is employed for recording the images of the scattering patterns. In order to obtain the information on the actual azimuth angle of the PCF, another CCD camera is used to record the cross section image of PCF under rotation. The scattering patterns and cross section images of PCF are recorded simultaneously, which provides the correlation between the scattering pattern and the azimuth angle of PCF.
Fig. 1 Definition of the azimuth angles of the (a) ESM, (b) LMA, and (c) HLM fibers, and SEM images of the (d) ESM, (e) LMA, and (f) HLM fibers.
Fig. 2 (a) Schematic of the experimental setup for nondestructive determination of azimuth angle of the PCF. (b) Two-dimensional schematic of setup.
Before starting rotation of the fiber, the direction of the laser beam is adjusted carefully so as to illuminate the side of the PCF, and the cameras are well tuned to obtain clear images. The entire PCF is illuminated by the laser beam without any aperture or focusing lens. The rotation of the PCF along its axis is controlled by the motor, starting from a position with an initial azimuth angle of 0° (the ГK direction along the X-axis). The fiber is rotated in an anti-clockwise manner with a step of 0.5°. The scattering pattern and the cross section images are recorded simultaneously for each step. The measurement stops after a 360° rotation cycle is completed. We repeat this measurement for the ESM, LMA, and HLM fibers.
Figures 3(a)-3(c) show scattering patterns and the corresponding cross section images of the ESM fiber for 0°, 30°, 60°, respectively. The scattering patterns are interference patterns of forward-scattered light [28]. It is clearly shown that different azimuth angles of PCF result in scattering patterns with different information. Note that the upper and lower-half regions of the scattering patterns are asymmetric with respect to the middle bright spot. In addition, the scattering patterns shift slightly in the horizontal and vertical directions and rotate slightly. This is caused by slight fluctuations of the output power of laser and changes of the fiber position during our experiment. Multiple scattering in the fiber results in irregular spikes in the scattering pattern. Therefore, it is impossible to retrieve the azimuth angle directly from the scattering pattern. In order to obtain the correlation between the scattering pattern and the azimuth angle, we introduce a characteristic value that is the sum over local intensities of the scattering pattern. The characteristic value CV is defined as
where p(i,j) is intensity of the scattering pattern in row i and column j, M and N are numbers of pixels in vertical and horizontal directions, respectively. The characteristic values for various regions in the scattering pattern are analyzed for different azimuth angles. It is found that the characteristic value for the entire pattern is not correlated with the azimuth angle. Using the sums over intensities of the upper and lower-half regions of the scattering pattern, the irregular spikes are reduced, and much cleaner patters are obtained. Figure 4(a) shows the upper and lower-half regions of the scattering pattern of the ESM fiber with an azimuth angle of 60°. The characteristic values of the upper and lower-half regions for angles from 0° to 360° are shown in Figs. 4(b) and 4(c), respectively. It is clearly seen that there are strong peaks at angles around multiples of 60° which correspond to the hexagonal structure of the ESM fiber. Six strong peaks are observed within a complete rotation cycle of 360°. The angles corresponding to peaks in Fig. 4(b) are slightly larger than multiples of 60°, while the angles of the peaks in Fig. 4(c) are slightly smaller than multiples of 60°.Fig. 3 Scattering patterns and the corresponding cross section images of the ESM fiber at azimuth angles of (a) 0°, (b) 30°, (c) 60°.
Fig. 4 Scattering pattern of the ESM fiber. (a) Scattering pattern for an azimuth angle of 60°, showing the upper and lower-half regions. Characteristic value as a function of azimuth angle for the (b) upper-half region and (c) lower-half region of the scattering pattern.
The intensity as a function of azimuth angle in the range from 55° to 65° for the upper and lower-half regions are shown in Figs. 5(a) and 5(b), respectively. The intensities reach their peaks at 62.73° and 57.17° for the upper and lower-half regions, respectively. The average of 62.73° and 57.17° gives 59.95°. The azimuth angle 60° is the ГК direction of the ESM fiber. Therefore, the average of angles corresponding to the intensity peaks of the upper and lower-half regions can be used to identify the ГК direction of ESM fiber. The measured azimuth angles are compared with the integer multiples of 60°, and the deviations are plotted in Fig. 5(c). As shown in Fig. 5(c), the deviations of measured azimuth angles with respect to the ГК directions are smaller than 0.16°. Therefore, the accuracy of determination the orientation is 0.16° for the ESM fiber. The deviations of azimuth angles are smaller than 3° for both the upper and lower-half regions. The investigation on the ESM fiber confirms that analysis of the sum over intensities in the upper and lower-half regions of the scattering pattern can be used to determine the azimuth angle.
Fig. 5 Scattering pattern analysis of the ESM fiber. Intensity versus azimuth angle for (a) the upper-half region and (b) the lower-half region of the scattering pattern. (c) The measured azimuth angle deviations with respect to the ГК directions.
We further characterize the LMA and HLM fibers by using this scattering pattern analysis. The scattering pattern of LMA fiber for an azimuth angle of 60° is depicted in Fig. 6(a). Similar to the ESM fiber, the intensity profile of the LMA fiber has peaks around integer multiples of 60°, as shown in Figs. 6(b) and 6(c). This is expected because the LMA fiber has rotational symmetry with a period of 60°, as depicted in Fig. 1(b). The angles corresponding to peaks in Fig. 6(b) are slightly larger than angles of the ГК directions, while the angles corresponding to peaks in Fig. 6(c) are slightly smaller than angles of the ГК directions. Figures 7(a) and 7(b) show the dependence of the intensity on the azimuth angle in the range from 55° to 65° for the upper and lower-half regions, respectively. The peak intensities locate at 63.60° and 56.53° for the upper and lower-half regions of the scattering pattern, respectively. The average of 63.60° and 56.53° is 60.065°. Figure 7(c) shows the azimuth angle deviations with respect to the ГК direction. The accuracy of determination the orientation is 0.46° for the LMA fiber. The deviations of azimuth angles are smaller than 4° for both the upper and lower-half regions.
Fig. 6 Scattering pattern analysis of the LMA fiber. (a) The scattering pattern for an azimuth angle of 60°, where the upper and lower-half regions are indicated. (b) Characteristic value versus azimuth angle for the (b) upper-half region and (c) lower-half region.
Fig. 7 Scattering pattern analysis of the LMA fiber. Intensity as a function of azimuth angle for (a) the upper-half region and (b) the lower-half region of the scattering pattern. (c) The measured azimuth angle deviations with respect to the ГК directions.
The scattering pattern of the HLM fiber at an azimuth angle of 180° is shown in Fig. 8(a). The characteristic value of the HLM fiber versus the azimuth angle for the upper and lower-half regions is displayed in Fig. 8(b) and 8(c), respectively. The angles corresponding to the peaks of Fig. 8(b) are 183.60° and 363.51°. In Fig. 8(c), the angles of the peaks are 176.10° and 356.34°. The variation of intensity with azimuth angle in the range from 174° to 186° for the upper and lower-half regions are shown in Figs. 9(a) and 9(b), respectively. The angles near 180° for the maximum intensities give an average value of 179.85°. The azimuth angle deviations with respect to the ГК direction are shown in Fig. 9(c). The accuracy of determination the orientation is 0.15° for the HLM fiber. For both the upper and lower-half regions, the azimuth angle deviations are smaller than 4°. In terms of the repeatability, we have tested the proposed method with three different kinds of PCFs (two samples for each kind), and all obtained similar measurement accuracy (i.e., <0.5°), which indicates a reasonable good repeatability.
Fig. 8 Scattering pattern of the HLM fiber. (a) The scattering pattern for an azimuth angle of 180°, which is divided into the upper and lower-half regions. Characteristic value as a function of azimuth angle for (b) the upper-half region and (c) the lower-half region.
Fig. 9 Scattering pattern analysis of the HLM fiber. Intensity versus azimuth angle for (a) the upper-half region and (b) the lower-half region of the scattering pattern. (c) The measured azimuth angle deviations with respect to the ГК directions.
Then numerical simulations are also performed for analyzing the fiber. In numerical simulations, we use the ESM fiber, as depicted in Fig. 1(a). The forward-scattering from an ESM fiber side illuminated by a laser beam at 650 nm is simulated based on the ray tracing method using the commercial software COMSOL Multiphysics. The parameters of the ESM fiber used in the numerical simulations are the same as those used in experiments. The diameters of the air-holes and cladding are 2.28 and 125 μm, respectively. The pitch is 5.70 μm, and the refractive index of the cladding is 1.45. The intensity distributions of the scattering pattern are expected to repeat every 60° when the ESM fiber is rotated, since the fiber has a six fold rotational symmetry in the air-hole lattice. Figures 10(a) and 10(b) show the intensity in the upper and lower-half regions of the scattering pattern, respectively, which are obtained by summing over scattered fields in the upper and lower-half regions, respectively. The angles of 0°-360° are analyzed for comparing the numerical results with experimental ones. The intensity changes with azimuth angle periodically, and the peaks locate at angles near the integer multiples of 60° which corresponds to the symmetric axis of the ESM fiber. The peaks near 60° locate at 64.75° and 55.09° for the upper and lower-half regions, respectively. The average of the two angles is 59.92°. Figure 10(c) shows the simulated results on the azimuth angle deviations with respect to the ГК directions. The azimuth angle deviations are smaller than 0.08°, and for both the upper and lower-half regions the deviations are smaller than 5.20°. The simulation results reveal analysis of the scattered fields in the upper and lower-half regions of the scattering pattern can be used to characterize the azimuth angle of PCF. The results are in good agreement with the experimental results.
Fig. 10 Simulation results of the ESM fiber for scattered fields in the (a) upper-half region and (b) lower-half region. (c) The azimuth angle deviations with respect to the ГК directions.
3. Method application in side-polishing of PCF
The side-polished fiber (SPF), which is fabricated by removing a portion of the fiber cladding, provides an ideal platform for evanescent filed coupling. The SPF has been widely used in sensors [29,30], optical fiber amplifiers [31], optical polarizers [32], all-optical modulates [33], and filters [34]. For side-polishing of the PCF with particular azimuth angle, in-line angular orientation of the PCF is required. As an application of the forward-scattering technique, the ESM fiber is side-polished with particular directions with the assistance of the method we proposed here. In our experiment, the experimental setup shown in Fig. 2 is firstly added to the side-polishing setup. Then the azimuth angle of the fiber is determined by scattering pattern analysis. After that, the ESM fiber is rotated to the desired position. Finally, the fiber is polished with fixed axes. We polished two ESM fiber samples with polished surfaces perpendicular to the ГM and ГК directions, respectively, and their cross section images are shown in Figs. 11(a) and 11(b), respectively. The angular deviations between the polished surface (solid line) and the desired surface (dashed line) are both approaching 0.5°.
Fig. 11 Cross-section view of the side-polished ESM fibers with polishing directions perpendicular to (a) the ГM and (b) ГК directions.
4. Conclusions
In conclusion, a nondestructive and contamination-free method for determination of the azimuth angle of PCF as well as its practical application for fiber side-polishing have been demonstrated. A characteristic value is introduced for analyzing the scattering pattern. The correlation between the characteristic value and the azimuth angle is applied to characterize the azimuth angles of three kinds of PCFs with an angular accuracy of better than 0.5°. By applying this technique, the ESM fiber is side-polished along particular directions, the polishing angular accuracy is about 0.5° for polishing directions that are perpendicular to the ГM and ГК directions. The results suggest that this method has general applicability for other designs of fiber with different inner microstructures in a nondestructive and contamination-free manner, and it is an effective solution for precise determining and controlling the azimuth angle in fabrication of PCF-based optical devices and in-line manipulation of the PCFs.
Funding
National Natural Science Foundation of China (NSFC) (61705087, 61575084, 61361166006, 61401176, 61405075, 61475066, 61505069); Natural Science Foundation of Guangdong Province (2015A030313320, S2013050014606, 2014A030313377, 2014A030310205, 2015A030306046, 2016A030311019, 2016A030313079, 2016A030310098); Science and Technology Projects of Guangdong Province (2017A010101013, 2012A032300016, 2014B010120002, 2014B010117002, 2015A020213006, 2015B010125007, 2016B010111003, 2016A010101017); Science & Technology Project of Guangzhou (201707010500, 201506010046, 201607010134, 201605030002, 201610010026, 201605030002, 20160404005).
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