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Abstract
We develop a local birefringence determination method of measuring the distribution of external force-induced birefringence in spun high-birefringence (HiBi) fiber (spun HiBi fiber) using polarimetric optical frequency domain reflectometry (P-OFDR). By constructing the similarity between the measured Mueller matrices and fiber under test (FUT) matrices using two input states of polarization, the total phase retardance caused by the local birefringence of FUT can be determined from the trace of the measured matrices. We measure the local birefringence of spun HiBi fibers from two different manufacturers and telecom SMF (G652.D) caused by bending, twist, and transverse stress using our presented P-OFDR system. From the experimental results, we find that bending- and twist-induced birefringences of spun HiBi fiber are much lower than those of standard SMF. More remarkably, the coating package influences the transverse stress induced birefringence of spun HiBi fibers significantly. These experimental results verify that our presented method is beneficial to evaluating and improving spun HiBi fibers’ quality.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
6 beiyangkl@tju.edu.cn
7lytus@126.com
1. Introduction
The current sensor is a significant device in the field of current measurement for the electric power system. Optical fiber current sensor (OFCS) has many prominent advantages over traditional current sensors of light weight, small size, immunity against electromagnetic interference, excellent insulation performance, corrosion resistance, intrinsic safety, high sensitivity and broad dynamic range [1,2]. A large majority of OFCSs are interferometric OFCS, which are based on a polarization-rotated reflection interferometer. The current generates a magnetic field and induces a nonreciprocal phase delay between the right and left circular polarization states after propagating a distance in a sensing fiber coil under the magnetic field’s effects.
In an OFCS with high performance, its sensitivity should remain stable, even though OFCS is used in varied environmental conditions, such as temperature changing, external force variation, vibration etc. The sensitivity of OFCS is related to the circular and residual linear birefringences of the sensing fiber coil, which need to be stable [3]. However, the standard telecom single-mode optical fiber (SMF) can’t achieve this goal, because the circular and linear intrinsic birefringences of standard SMF are very low and they are significantly sensitive to external perturbations induced birefringence. Since external perturbations induced birefringence is varied with external environmental conditions, the sensitivity of OFCS using standard SMF will be unstable. Using spun fiber as a sensing fiber in OFCS became a solution to this issue. One type of spun fiber is a low linear and high circular intrinsic birefringence fiber (spun LoBi fiber), which is produced by spinning a standard SMF during the drawing process in the molten state [4–6]. Unfortunately, spun LoBi fiber still suffer from external perturbations induced linear birefringence such as bending, which will still make the sensitivity of OFCS using spun LoBi fiber to be unstable. Another type of spun fiber is a spun high birefringence fiber (spun HiBi fiber), which is produced by a high linear birefringence fiber (e.g. a bow-tie fiber or a PANDA fiber) preforms during the drawing process in the molten state [7–9]. The effect is that the fiber has elliptically birefringence, namely, high circular and linear intrinsic birefringences, which impart a high resistance to external perturbations induced birefringences. The spun HiBi fiber became an ideal choice for the sensing fiber in OFCS [3].
Although the spun HiBi fiber has a high resistance to external perturbations induced birefringence, we still need to control external perturbations induced birefringence of spun HiBi fiber. The first thing is to measure perturbation induced birefringences in spun HiBi fiber, which is useful to screen out sub-quality spun HiBi fiber for OFCS manufacturers, more importantly, to improve production technique for spun HiBi fiber manufacturers. Xu et al. measure temperature variation coefficients of the circular and linear birefringence in a spun HiBi fiber using a polarization measurement system based on binary polarization rotators [10]. The current investigations on assessing capability of resistance to external force perturbations are aimed to the performance of OFCS. Whereas, the investigations of external force induced birefringence measurements in spun HiBi fiber are very limited. Polarimetric optical frequency domain reflectometry (P-OFDR) is a useful tool to achieve distributed birefringence measurements along the fiber under test (FUT). P-OFDR has a capability of analyzing bending [11,12], twist [13] and transverse stress [14] induced birefringence or polarization mode dispersion (PMD) [15] along standard SMF as fiber under test (FUT). More importantly, P-OFDR can locate the external force induced birefringence and avoid the influence from the birefringence of connectors and pigtail fiber compared with non-distributed polarization measurement system. Galtarossa, Palmieri et al. also use P-OFDR to analyze the characterizations of spun low birefringence fiber for telecom applications [16] such as spin profile [17,18], bending induced birefringence [19]. The previous P-OFDR systems presented in [15,20] require FUT to be stable to avoid polarization fluctuation during the test, which is not suitable for practical applications [14]. Wei et al. present a P-OFDR system to solve this issue, in which the local birefringence along FUT was measured by analyzing the states of polarization (SOP) evolution between two neighboring locations [14]. However, this system only use one input SOP, so the distributed birefringence curves measured were influenced by the degree between the input SOPs and the axis of birefringence [21]. Feng et al. present a distributed polarization analysis (PA) OFDR system using binary polarization rotators and achieve bending-induced birefringence measurements with high accuracy [22], which forms a commercial product (OFDR-1000, General Photonics Inc.) [23]. However, PA-OFDR need at least 16 times measurements for a full Muller matrix of a local fiber segment, which is complex and time-consuming. Any polarization fluctuations during 16 times measurements will cause an error for birefringence measurements.
In this paper, we develop a method of distributedly measuring external force induced birefringence in spun HiBi fiber using P-OFDR. We present a local birefringence determination method in Mueller matrix using two input SOPs, namely two independent measurements. By constructing the similarity between the measured matrices and FUT matrices, the total phase retardance caused by the local birefringence of FUT can be determined from the trace of the measured matrices. We measure the local birefringence of spun HiBi fibers from two different manufacturers and telecom SMF (G652.D) caused by bending, twist and transverse stress using our presented P-OFDR system. From the experimental results, we find that bending and twist induced birefringence of spun HiBi fiber are much lower than those of standard SMF. Whereas, more remarkably, the transverse stress induced birefringences of FUT with and without coating are dramatically different. With coating, the transverse stress induced birefringence of spun HiBi fiber is much higher than that of the standard SMF. Without coating, the transverse stress induced birefringence of spun HiBi fiber is slightly lower than that of the standard SMF. The coating package influences the transverse stress induced birefringence of spun HiBi fibers significantly, which is needed to be noticed for spun HiBi fiber and OFCS manufacturers. In addition, the bending and transverse stress induced birefringence of spun HiBi fibers from different manufacturers are also different. These experimental results above verify that our presented method is beneficial to evaluate and improve the quality of spun HiBi fibers.
2. Principle
2.1 P-OFDR system configuration
We will firstly describe the system configuration and the principle of our P-OFDR for measuring the space-resolved polarization properties along FUT. Figure 1 shows our P-OFDR system configuration. The starting wavelength, tuning speed and tuning range of the tunable laser source (TLS, Keysight 81607A) are 1520 nm, 2.5 × 103 GHz/s (20 nm/s) and 5 × 103 GHz (40 nm), respectively. A 1: 99 polarization maintaining (PM) coupler is used to split the light from the TLS into the main and auxiliary interferometers. The 1% light is sent to a Faraday rotating mirrors (FRM) based auxiliary Michelson interferometer that provides an external clock (f-clock) to trigger the data acquisition to sample the main interference signals at uniform optical frequency points, which can be used to reduce TLS nonlinear tuning effects [24]. The length of delay fiber in this auxiliary interferometer is 250 m. The 99% light is sent to the main Mach-Zehnder interferometer. Then the light is separated by a 10:90 PM coupler, in which 10% of the light is used as a reference light, and 90% of the light is sent to a binary magneto-optic (MO) rotators based polarization state generator (PSG-001, General Photonics Inc.). In PSG, each binary MO rotator controlled by the microcontroller unit (MCU) can rotate the SOP to generate six distinctive polarization states [25]. Here we only use two orthogonal SOPs (horizontal and vertical states). Note that the pigtails of 1: 99 coupler and 10:90 coupler are made of PM fiber to avoid SOP fluctuations caused by external perturbations. And then the light is sent to the fiber under test (FUT) through a circulator. The Rayleigh backscattering from different positions of FUT is sent to interfere with the reference light directly from TLS in a 180 ° dual polarization hybrid (the dotted-line box in Fig. 1, Kylia Inc.). There are two free-space polarization beam splitters (PBSs) and a free-space polarization insensitive beam splitter (BS) in 180 ° dual polarization hybrid. A 10:90 PM coupler is used to align the axis of the PSG and dual polarization hybrid. In the test arm, the slow axis of the PM coupler is to align the axis of the polarizer in the PSG. In the reference arm, the axis of the PM coupler inputting to dual polarization hybrid is oriented 45 ° to the optical axis of two PBSs such that the reference light is equally split by two PBSs. The PSG and dual polarization hybrid both use the slow axis of 10:90 PM coupler as the reference axis. The interference signals of the H and V channels are detected by two balanced photo-detectors (BPD) and then the electronical signals are sent to data acquisition card (DAQ).
Fig. 1 P-OFDR system configuration. TLS is tunable laser source, which is linear tuning; PM is polarization maintaining; PC is Polarization controller; PSG is polarization state generator; FRM is faraday rotating mirror; BS is polarization insensitive beam splitter; PBS is polarization beam splitter; BPD is balanced photo-detector; FUT is fiber under test; DAQ is data acquisition card. Red line represents single mode fiber and blue line represents PM fiber.
2.2 Polarization determination method
We present a local birefringence determination method in Mueller matrix using two orthogonal input SOPs. By constructing the similarity between the measured matrices and FUT matrices, the local birefringence of FUT can be determined from the measured matrices. The idea of polarization determination method is similar to the Jones matrix diagonalization method in polarization sensitive optical coherence tomography [26]. Compared with the Jones matrix diagonalization method, the polarization dependent loss (PDL) and depolarization effects can be eliminated by a polar decomposition of Mueller matrix in this method. In this P-OFDR system shown in Fig. 1, the polarization properties of the backscattered light with two input SOPs are measured.
The input SOPs generated by PSG can be written as:
where ,, , and are the electric fields on the horizontal and vertical channels of two input SOPs. Here PSG is used to generate [1, 0]T and [0, 1]T as the two input SOPs. Any two of incident polarization states can be used as input SOPs, as long as in Eq. (1) has an invertible matrix.Since the PM pigtail fiber of 10:90 PM coupler inputting to dual polarization hybrid is oriented 45°, the power values of the reference light at the orthogonal polarization axes H and V are equal and the reference light inference matrix can be expressed as:
where and are the electric field on the horizontal and vertical channels of the reference light. is the phase difference between on the horizontal and vertical channels of the reference light. This phase mainly coms from the phase delay between horizontal and vertical channels of the dual polarization hybrid or PM fiber pigtail.Equations (1) and (2) can be converted into Mueller matrix using the relations as [27]:
where ⊗ represents the Kronecker tensor product. Submit Eqs. (1) and (2) to (3), Mueller matrices of Ein and Eref can be obtained as:The Mueller matrix represents round-trip polarization properties at the location of FUT from the start point to , and are the Mueller matrices of input and output paths in the P-OFDR system. The measured Mueller matrix at the location of FUT can be expressed as:The P-OFDR signals in the spatial domain can be converted from the signals in the optical frequency domain by a Fourier transform. In our P-OFDR system, since the PSG generates two input SOPs (SOP1 and SOP2), we can measure the signals,,, and at each location of H channel for SOP1, V channel for SOP1, H channel for SOP2 and V channel for SOP2. Based on Eq. (3), can be calculated as:In Eq. (5), consisting of single-trip Mueller matrices of local sites is given as:where (i = 1,2,3, · · ·) is the single-trip Mueller matrix of the local polarization properties from the location of FUT zi-1to zi.at the location of FUTcan be expressed as:
We construct a transform matrix as:Based on Eq. (9), the calculation of the local birefringence of FUT uses the polarization properties at the previous location as a reference point. This method essentially eliminate the polarization properties changing before the reference point.Submit Eqs. (5)- (8) to (9) as:
whereHere represents local round-trip polarization properties from to. Submitting Eq. (11) to (10), we can obtain as:AssumingEquation (12) can be written as:From Eq. (12), we find that and are unitarily similar matrices if A is unitary matrix. However, polarization properties of optical fiber in the system and FUT as , contain PDL and depolarization compositions, so A is not a unitary matrix and the matrix unitarily similarity condition is no longer satisfactory.Here we use a polar decomposition to process and remove PDL and depolarization compositions, is decomposed as [28]:
where is depolarization matrix. is PDL matrix. is birefringence matrix that can be used to measure the phase retardance of local birefringence. After a polar decomposition, Eq. (14) can be converted to:where, and only contain the compositions of birefringence. Since is unitary matrix, and are unitarily similar matrices.The total phase retardance of local birefringence can be written as [28]:
where the operator is the trace of a matrix. Since the traces of two unitarily similar matrices are equal, namely, , also can be written as:is calculated by Eq. (9) after a polar decomposition of Eq. (15), so can be measured.3. Experiments and discussions
3.1 P-OFDR system performance verification
In this P-OFDR system, the spatial resolution for each back-reflection data point is about 0.02 mm based on Δz = c/2nΔF, where ΔF is TLS tuning range (5 × 103 GHz), n is the effective index of fiber and c is the speed of the light in vacuum. The total FUT is divided to several sections with a ΔX length that contains N data points as the spatial resolution for phase retardance measurement. Namely, ΔX is the step size . To achieve a phase retardance measurement with high accuracy, the certain length of ΔX need ensure a sufficient measured Mueller matrix and variations at the step size beyond the noise level of the system. This method is also implemented in other P-OFDR systems [22]. In this system, we choose N = 10 and ΔX = 0.2 mm. In addition, to reduce the noise influence on calculation of phase retardance, the low-pass filtering and smooth filtering are applied for H and V channel signals.
In the experiments, we firstly verify the performance of our P-OFDR system using a standard SMF of 92 m shown in Fig. 2. The distributed signals of Rayleigh backscattering in the spatial domain are shown in Fig. 2(a). We use two weights of 4100 g to press on FUT at the locations of 89.3 m and 89.5 m as the two independently transverse stress points and the experimental results of phase retardance using our presented polarization determination method are shown in Fig. 2(b). The two phase retardance peaks at the locations of 89.3 m and 89.5 m reflect the transverse stress induced local birefringence changing. We also bend two loops on FUT with a diameter of 2.5 cm at the locations of 89.3 m and 89.6 m as the two independent bending points and the experimental results of phase retardance using our presented polarization determination method are shown in Fig. 2(c). The two phase retardance peaks at the locations of 89.3 m and 89.6 m reflect the bending induced local birefringence changing. From Fig. 2(b) and 2(c), as the 92 m FUT are winded on a disc, we find some low phase retardance peaks along the FUT, which is caused by transverse stress or twist in the fiber coil. The high peak retardance at the FUT end is caused by a fiber loop with a diameter of 5 mm that is used to reduce the reflection of FUT end. The experimental results verify the ability of measuring local birefringence using our presented polarization determination method in P-OFDR.
Fig. 2 Performance of our P-OFDR system using a standard SMF of 92 m. (a) Distributed signals of Rayleigh backscattering in the spatial domain. (b) Phase retardance results of transverse stress induced local birefringence caused by two weights pressing on FUT at the locations of 89.3 m and 89.5 m. (c) Phase retardance results of bending induced local birefringence caused by two loops on FUT at the locations of 89.3 m and 89.6 m.
3.2 External force induced birefringence measurements for spun HiBi fiber
The optical fiber suffering external forces such as bending, twist, or other transverse stress often exhibit local birefringence. For spun HiBi fiber of OFCS application, the key characteristic of the spun HiBi fiber is that the circular and residual linear birefringences are stable even through the spun HiBi fiber suffer varied external forces. Namely, the external forces induced birefringence of spun HiBi fiber with a high quality will be much smaller than that with a low quality under suffering the same external forces. We will measure distributed local birefringence caused by bending, twist and transverse stress of spun HiBi fibers from two different manufacturers (spun fiber A and B) and standard telecom SMF (G652.D) with a length of 2.5 m. The spun fiber A is SH130_125-5/250 of YOFC Ltd., with a spin pitch of 5 mm. The spun fiber B is SHB1250 (7.3/125) of Fibercore Co. Ltd., with a spin pitch of 4.8 mm. The standard telecom SMF is Full Band single-mode fiber of YOFC Ltd. By eight measurements averaging for each distributed phase retardance data of two kinds of spun HiBi fiber and standard SMF, we obtain their bending, twist and transverse stress induced local birefringences.
The bending induced local birefringence is generated by fiber loops on FUT shown in Fig. 1. We bent FUT as loops with different diameters of 2 cm, 2.5 cm and 3 cm in each experiment and measure the local birefringence of FUT. The fiber loops with different diameters are fixed on a paperboard using adhesive tapes. From the experimental results shown in Fig. 3, the phase retardances of bending induced local birefringences increase, when the diameters of fiber loops decrease for two kinds of spun HiBi fibers and standard SMF. The bending induced local birefringence of spun HiBi fibers is lower than that of standard SMF, which is an advantage for the spun HiBi fibers’ resistance to bending induced local birefringence changing. In addition, the bending induced local birefringence in the spun HiBi fibers of two different manufacturers are different. The bending induced local birefringence of spun fiber B is lower than that of spun fiber A.
Fig. 3 Phase retardances of bending induced local birefringences of standard SMF (a), spun fiber A (b) and spun fiber B (c) in different diameters of fiber loops.
From Fig. 3, the measured phase retardance curve exhibits a peak in a single fiber loop, representing that the bending induced linear birefringence change is not uniform within a loop, but a Gaussian-like distribution. This phenomenon is also observed in the bending induced birefringence measurements using PA-OFDR systems [12,22]. One reason is spread and superposition effects. Since the residual nonlinear phase, spectrum leakage, dispersion, and environment vibration in OFDR signals cause a widen back-reflection peak, the spatial resolution for phase retardance measurement are deteriorated. The bending induced phase retardance changing spread to both sides and a superposition effect is generated in the several points of a fiber loop, which exhibits a peak. The other reason is averaging of multiple phase retardance curves and de-noising processing, which also are illuminated in [22]. In the experiments, we apply an eight measurements averaging for each distributed phase retardance data. To reduce noise for P-OFDR system, the low-pass filtering and smooth filtering are applied for H and V channel signals, which also generates a smooth effect for phase retardance curves.
The twist induced local birefringence is generated by 3-paddle polarization controller (PC) on FUT shown in Fig. 1. FUT is winded on the one of paddles in PC and then we change the angles of the paddle to generate the twist effects on FUT. The different twist angles are chosen as 0°, 45°, 90°and 135° in each experiment. Here the local birefringences contain bending and twist effects. To differentiate bending and twist effects, we observe the variation of phase retardances between different twist angles. Since the diameter of fiber loop in PC is fixed, the phase retardances caused by bending effect are not changed. Whereas, the variations of phase retardances reflect the twist effect. We compare the maximum variation of phase retardances between different twist angles for two kinds of spun HiBi fiber and standard SMF to evaluate their resistance to the twist effect. From the experimental results shown in Fig. 4, the maximum variations of phase retardance in spun HiBi fibers is lower than that of standard SMF, which is an advantage for spun HiBi fibers’ resistance to twist induced local birefringence changing. In addition, the maximum variations of phase retardance in the spun HiBi fibers of two different manufacturers are similar.
Fig. 4 Phase retardances of twist and bending induced local birefringences of standard SMF (a), spun fiber A (b) and spun fiber B (c) in different twist angles of 0°, 45°, 90°and 135°in one paddle of a polarization controller. The maximum variations of phase retardances reflect local birefringences changing caused by twist effects.
The transverse stress induced local birefringence is generated by different weights pressing on FUT shown in Fig. 1. The different weights are chosen as 900g, 1100g and 1300g in each experiment. To ensure the transverse stress increasing along with the weights increasing, the contract areas between different weights and FUT are the same. The length of pressing fiber is a constant value of about 3.5 cm when applying different weights. From the experimental results shown in Fig. 5, the phase retardances of transverse stress induced local birefringence of FUT increase, when the weights increase. In Fig. 5(a), the peaks of phase retardances changing of standard SMF are minor in different weights, because the transverse stress induced local birefringence of SMF are very small. For a 900g weight pressing on FUT, we cannot observe phase retardance peaks of standard SMF. It is noteworthy that the transverse stress induced birefringences of spun HiBi fiber of two manufacturers are both much higher than that of standard SMF. The maximum possible reason is that spun HiBi fiber has been designed with an optimized coating package [29]. The coating materials of FUT can influence the transverse stress induced birefringence significantly.
Fig. 5 Phase retardances of transverse stress induced local birefringences of standard SMF (a), spun fiber A (b) and spun fiber B (c) caused by different weights.
To verify the FUT coating impact, we remove the coating of FUT and measure the phase retardance of pressing different weights. The length of pressing fiber is still a constant value of about 3.5 cm when applying different weights. We find that the transverse stress induced birefringence of FUT without coating is much more than that of FUT with coating. The accumulated phase retardation over the step size zi-zi-1 being more than π will cause a phase wrapping errors and this phenomena is also found in [22]. Here we choose lighter weights as 600g, 700g and 800g for the experiments of FUT without coating. The experimental results of transverse stress induced phase retardance of FUT without coating shown in Fig. 6. From Fig. 6, the phase retardances of transverse stress induced local birefringence of FUT increase, when the weights increase. Whereas, unlike Fig. 5, the transverse stress induced birefringences of standard SMF is slightly higher than those of spun fiber A and B. The results shown in Figs. 5 and 6 verify that the coating of FUT indeed influence the transverse stress induced birefringence significantly, which should be noticed by spun HiBi fiber and OFCS manufacturers. In addition, the transverse stress induced birefringences of spun HiBi fibers with and without coating from different manufacturers occur a slight difference.
Fig. 6 Phase retardances of transverse stress induced local birefringences of standard SMF (a), spun fiber A (b) and spun fiber B (c) without coating caused by different weights.
3.3 Discussions
We use a commercial product PA-OFDR (OFDR-1000, General Photonics Inc.) to cross-validate our experimental results. We utilize PA-OFDR [23] to test the spun fiber B (SHB1250 (7.3/125) of Fibercore Co. Ltd.) as FUT and bent FUT as loops with different diameters of 2 cm, 2.5 cm and 3 cm in each experiment and measure the local birefringence of FUT shown in Fig. 7, which is similar to the results in Fig. 3(c). The measured bending induced local birefringences with different diameters of 2 cm, 2.5 cm and 3 cm of PA-OFDR and our system are shown in Table 1 as below. We convert the values of phase retardance in Fig. 3(c) to the values of birefringence shown in Table 1 as below using this relation as where is the center wavelength. In this system, is 0.2 mm and the factor 2 is a result of the round-trip propagation at local site of FUT. From Table 1, we find that the maximum difference of birefringence is 1.5 × 10−7 and the experimental results of PA-OFDR and our system basically are close. Since the effective of birefringence measurements using PA-OFDR (OFDR-1000) have been theoretically and experimentally validated [22], our experimental results are validated.
Fig. 7 Measured bending induced local birefringences of the spun fiber B (SHB1250 (7.3/125) of Fibercore Co. Ltd.) with different bending diameters using PA-OFDR (OFDR-1000, General Photonics Inc.)
Table 1. Measured bending induced local birefringences of the spun fiber B with different bending diameters of PA-OFDR and our system
In our presented local birefringence determination method, the total phase retardance contains the linear and circular birefringence, which can be expressed as [28]:
where is local phase retardance of linear birefringence and is local optical rotation of circular birefringence. and can be expressed as: where and is the matrix element of ith row and jth column of . Based on Eq. (16), and are unitary similar matrices. The traces of two matrices are equal. We only obtain the trace of and no information about matrix elements of , so we cannot decompose the linear and circular birefringence form the total phase retardance based on Eqs. (19)-(21). Different matrix decomposition methods are need to be explored to solve this issue in the future.Based on the white paper of the spun HiBi manufacturers, it has been designed with an optimized coating package for micro-bending [29]. From the experimental results show in Fig. 3, we find that the bending induced birefringence of spun fiber A and B are much lower than that of standard SMF by optimized coating. Whereas, the transverse stress induced birefringences of spun fiber A and B are much higher than that of standard SMF shown in Fig. 5. By comparing with the transverse stress induced birefringence results of FUT with and without coating shown in Figs. 5 and 6, we find that the coating material influence the transverse stress induced birefringence of spun HiBi fiber significantly. Spun HiBi fiber manufacturers should notice this issue and improve the technologies of coating package to reduce the influence from transverse stress induced birefringence.
4. Summary
In summary, we develop a local birefringence determination method of distributedly measuring external force induced birefringence in spun HiBi fiber using P-OFDR. By constructing the similarity between the measured Mueller matrices and FUT matrices using two input SOPs, the total phase retardance caused by the local birefringence of FUT can be determined from the trace of the measured matrices. We measure the local birefringence for spun HiBi fibers from two different manufacturers and standard SMF caused by bending, twist and transverse stress using our presented P-OFDR system. From the experimental results, we find that bending and twist induced birefringence of spun HiBi fiber is much lower than that of standard SMF. Whereas, more remarkably, the transverse stress induced birefringence of FUT with and without coating are different. With coating, the transverse stress induced birefringence of spun HiBi fiber is much higher than that of the standard SMF. Without coating, the transverse stress induced birefringence of spun HiBi fiber is slightly lower than that of the standard SMF. The coating package influences the transverse stress induced birefringence of spun HiBi fibers significantly, which is needed to be noticed for spun HiBi fiber and OFCS manufacturers. In addition, the bending and transverse stress induced birefringence of spun HiBi fibers from different manufacturers are also different. Above all, these experimental results verify that our presented method is beneficial to evaluate and improve the quality of the spun HiBi fibers.
Funding
National Natural Science Foundation of China (NSFC) (61505138, 61635008, 61475114, 61735011); Tianjin Science and Technology Support Plan Program Funding (16JCQNJC01800); China Postdoctoral Science Foundation (2015M580199, 2016T90205); National Instrumentation Program (2013YQ030915); National Key Research and Development Program (2016YFC0100500).
Acknowledgments
We thank Dr. X. Steve Yao of General Photonics Inc. for his technical assistance and Prof. Luca Palmieri and Prof. Andrea Galtarossa of University of Padova for fruitful discussions.
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