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Distributed birefringence measurement with beat period detection of homodyne Brillouin optical time-domain reflectometry Yuangang Lu,1,2 Xiaoyi Bao,1,* Liang Chen,1 Shangran Xie,1 and Meng Pang1 1 2
Fiber Optics Group, Department of Physics, University of Ottawa, Ottawa, Ontario K1N6N5, Canada
Institute of Optical Communication Engineering, Nanjing University, Nanjing, Jiangsu 210093, China *Corresponding author:
[email protected] Received June 25, 2012; revised August 8, 2012; accepted August 9, 2012; posted August 9, 2012 (Doc. ID 171283); published September 17, 2012
We report a distributed optical fiber birefringence measurement method based on homodyne Brillouin optical time-domain reflectometry (BOTDR). Unlike conventional BOTDR, which requires scanning of the local oscillator to get the Brillouin spectrum, instead we propose the beat period measurement of fast and slow components of the backscattered Brillouin signal in single-mode fibers using homodyne detection. The beat period is measured by detecting the envelope of the Brillouin beat signal, which gives the beat length at different fiber locations, so that birefringence can be calculated accordingly. The distributed birefringence of a 1.7 km SMF-28 and a 4.3 km largeeffective-area fiber were measured with 0.6 m spatial resolution without frequency scanning of the Brillouin spectrum. © 2012 Optical Society of America OCIS codes: 060.2270, 060.2300, 190.4370.
Polarization mode dispersion (PMD) has limited the transmission capacity of optical telecommunication systems using single-mode fibers (SMFs). PMD arises from random fiber asymmetry induced by the manufacturing process and external perturbations. The intrinsic and extrinsic perturbations cause birefringence in fibers, and the local birefringence can be used to calculate PMD [1]. The methods for measuring distributed birefringence can be categorized into two groups: Rayleigh-scattering (RS)-based and stimulated-Brillouin-scattering (SBS)based techniques. The first RS-based technique is optical frequency-domain reflectometry, which provides high spatial resolution but with limited measurement range [2]. The second technique is the polarization optical time-domain reflectometry (POTDR) [3–6], which allows distributed measurements with up to 0.5 m resolution and tens of kilometers range. In SBS-based approaches, both Brillouin optical frequency-domain analysis [7] and Brillouin optical time-domain analysis [8,9] require the frequency scanning to get the Brillouin spectrum and need two-end measurement. Besides, the unexpected nonlinear birefringence induced by SBS should be considered [9]. In this Letter, we present what we believe to be the first measurement of fiber birefringence utilizing spontaneous Brillouin scattering, and report a simple single-end and fast measurement method based on the beat period detection of Brillouin optical time-domain reflectometry (BOTDR). The proposed method works for all types of birefringence as it measures beat length directly. If a square probe pulse with amplitude E 0 and width T launched at the fiber incident end (z 0) is expressed as E in t
E0 0
0≤t≤T ; otherwise
Z E BS t
tc ∕2n
t−Tc ∕2n
AzE in t − np nz ∕c
× expiωB t − nz ∕cdz;
(2)
where A is Brillouin scattering coefficient, np and n are the effective refractive index of fiber for probe and Brillouin scattered light, respectively. And c is the speed of light in vacuum and ωB is the angular frequency of Brillouin scattered light. At any time, the received scattering signal comes from the fiber within the spatial length of δz cT ∕2n. As the wavelength difference between probe light and its Brillouin scattering is less than 0.1 nm when the probe light is around 1550 nm, the np can be seen equal to n. As shown in Fig. 1, during the Brillouin backscattering process of SMFs with low birefringence, for the local fast and slow axes of each short fiber section, the electric field partition on the two axes changes along the fiber due to spatial polarization mode coupling, which results in the beat between the projection components corresponding to the fast and slow axes of each fiber section [1]. If we only consider the Stokes Brillouin scattering, the received scattering from two axes is Z E BS t E 0
tc ∕2n t−Tc ∕2n
fAf z expiωf t − nf z ∕c
As z expiωs t − ns z ∕cgdz;
(3)
(1)
at time t, the spontaneous Brillouin scattering from the pulse whose front propagates to location z is received at z 0 and can be expressed as 0146-9592/12/193936-03$15.00/0
Fig. 1. (Color online) Diagram of the beating Brillouin signal. © 2012 Optical Society of America
October 1, 2012 / Vol. 37, No. 19 / OPTICS LETTERS
where the lowercased subscript “f ” denotes fast axis and “s” slow axis. The value of Af z (or As z) equals zero if no Brillouin component is located in fast (or slow) axis at certain location z. The low frequency beat variations as a function of time received at z 0 will be Z I BS t 2RE 20 Re Z ×
tc ∕2n
t−Tc ∕2n
tc ∕2n t−Tc ∕2n
Af z expiωf t − nf z ∕cdz
As z exp−iωs t − ns z ∕cdz
Kct ∕2n cosΔωt ∕2 Δnct ∕2nωs ∕c φ; (4) where the “Re” denotes “real part of” the superscript “” complex conjugate, R the photodiode responsivity, and φ the constant phase. The right side of the second equal sign is obtained by letting n nf , Δn ns − nf and Δω ωf − ωs . As shown in Fig. 1, Δω ∕2 describes the frequency spectrum of the beating Brillouin signal. If the 3 dB bandwidths of the spontaneous Brillouin scattering in fast and slow axes are δω, the 3 dB bandwidth of the beating Brillouin spectrum is δω. We should note that the strength of modal birefringence Bm jΔnj and Δβ jβf − βs j 2πΔn ∕λ where β denotes the mode propagation constant and λ is the wavelength of Brillouin scattering light. The Kct ∕2n 2RkE 20 Af ct ∕2nAs ct ∕ 2nc2 ∕ωf ωs nf ns is the slowly varying term that describes the loss along fiber (k is a nondimensional constant). As shown in Eq. (4), the beating Brillouin signal forms a spatial pattern with different beat periods in the detected time-domain trace, provided we use a homodyne BOTDR system. This beat period corresponds to the beat length of the fiber. The distributed beat length LB , therefore, can be measured by detecting the envelope of this changing pattern. By using envelope detection and letting z ct ∕2n, we can obtain the birefringence function curve (BFC) Fz D Kz cosΔβzz φ;
(5)
where the D is the term corresponding to direct current (DC) of the beating Brillouin signal. Therefore, the fiber birefringence can be retrieved as a function of fiber location. Once we obtain the BFC, Fz, we can detect the beat period and obtain the distributed beat length LB , and the fiber birefringence Bm is calculated by Bm λ ∕LB . As the Brillouin spectrum measurement is avoided, it provides a rapid measurement technique through beat period detection in homodyne BOTDR. The measurement accuracy depends on the signal-to-noise ratio (SNR) of the Brillouin beat signal, which is related to contrast of the fast and slow components of the backscattered Brillouin signal. Input state of polarization (SOP) of the probe pulse is chosen to make the power at fast and slow axes to be comparable along the fiber to ensure high precision of beat length measurement. The schematic diagram of the experimental setup is shown in Fig. 2. Different from conventional heterodyne detection BOTDR, the proposed BOTDR system is a simpler homodyne detection system, in which only sponta-
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Fig. 2. (Color online) Schematic diagram of the experimental setup for measuring distributed fiber birefringence.
neous Brillouin scattered lights reach the photodetector (PD). Besides, the measurement speed of the system is fast because it only needs to acquire the one time-domain trace of the broadband beat signal to complete one measurement without the requirement of frequency-scan. A 1550 nm distributed feedback laser with 2 MHz linewidth was modulated by an electro-optic modulator to generate a probe optical pulse. The probe pulse with a 10 ns pulse width was amplified by an erbium-doped fiber amplifier (EDFA), EDFA1, and then passed through a tunable fiber Bragg grating (FBG), FBG1 to filter the amplified spontaneous emission noise. The amplified pulse was injected into the fiber under test through a polarization controller (PC) and a circulator. After amplifying by EDFA2 and filtering by the 3 GHz bandwidth FBG2, only Stokes Brillouin scattering was detected by a low-noise PD with a bandwidth larger than 200 MHz. Then the detected beat signal was acquired and processed by a fast data acquisition (DAQ) system to obtain the BFC. The sampling rate of the DAQ was 175 MHz (the capture bandwidth is 140 MHz), which corresponds to the readout spatial resolution of 0.57 m. Taking into account the bandwidths of the detector, and DAQ and the rise time of the probe optical pulse, the spatial resolution is defined as 10%–90% rise time of a transition of measurand [10], which is 0.6 m. Thus, the minimum beat length can be measured is 1.2 m. Since the Rayleigh scattering is effectively filtered (its residual power is about 21 dB less than the Brillouin scattering power), the fading noises can be neglected. The random noise, such as thermal noise and shot noise, can be effectively eliminated by averaging over 20,000 waveforms. We measured the conventional fiber SMF-28 and the nonzero-dispersion-shifted large-effective-area fiber (LEAF) with 0.6 m spatial resolution. In 1.7 km SMF28 measurement, the first 50 m fiber was wrapped on a drum with 70 mm diameter, and the rest fiber was remained on a drum with 150 mm diameter. Thus, the bending induced birefringence of the 50 m fiber is 4.59 times of the rest fiber [11]. The peak power of the probe pulse was 220 mW. By adjusting PC, we obtained the BFC with the maximum contrast over the entire fiber. The repeated measurement times were 20,000 and entire measurement took 80 s. The obtained BFC of the entire fiber is shown in Fig. 3(a). Since the cycle of the trace corresponds to the fiber beat length [as derived from Eq. (5)], the distributed beat length can be obtained by calculating the distance between the neighbor peaks of the trace, and the obtained distributed beat length and birefringence are shown in Fig. 3(b). The BFC of the first 80 m fiber and its distributed beat length and birefringence are shown in Figs. 3(c) and 3(d), respectively. Figures 3(e) and 3(f), respectively, show the histogram of the beat length and birefringence of the entire fiber, where the
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Fig. 3. (Color online) Measurement results of SMF-28. (a) BFC of the entire fiber; (b) distributed beat length and birefringence of the entire fiber; (c) BFC of the first 80 m fiber; (d) distributed beat length and birefringence of the first 80 m fiber; (e), (f) histogram of the beat length and birefringence of the entire fiber.
length of each fiber section is 0.57 m. The distribution of birefringence follows Rayleigh distribution as in [1]. The beat length results agree well with that of the 24.63 km SMF-28 in [9]. According to the equation in [9], the total differential group delay (DGD) of the 1.7 km SMF-28 is 0.28 ps, which is almost equivalent to the 1.13 ps DGD for the 24.63 km SMF-28 in [9]. The average beat length of the first 50 m fiber under a large drum was 2.2 m. Thus, the birefringence of the first 50 m fiber is 4.77 times larger than that of the rest fiber. For small bending diameter, the bending induced birefringence dominates the intrinsic birefringence [11]. For large bending diameter, the bending induced birefringence and intrinsic birefringence are comparable, and thus the intrinsic birefringence cannot be neglected. In the 4.3 km LEAF measurement, the last 43 m fiber was wrapped on a drum with 60 mm diameter, and the rest fiber was remained on a drum with 150 mm diameter. Therefore, the bending induced birefringence of the 43 m fiber is 6.25 times that of the rest fiber [11]. The peak power of the probe pulse was 280 mW. We also adjusted the maximum contrast over the entire fiber. The total measurement time was 140 s when the repeated measurement times were 20,000. The measurement results are shown in Fig. 4. The random birefringence on the fiber follows Rayleigh distribution. The DGD is 0.44 ps. The average beat length of the last 43 m fiber was 2.1 m, while the average beat length in the large drum was 13.7 m. Therefore, the birefringence of the last 43 m fiber is 6.52 times larger than that of the rest fiber, which agrees well with the theoretical value of 6.25. By sinusoidal fitting the sampling data of the far end of LEAF on the 150 mm diameter drum, we obtained the beat length (12.0 m) and its measurement uncertainty (0.2 m). The 1.7% beat length measurement uncertainty is better than that of POTDR, which is 10% [1,4]. The uncertainty of the BOTDR system changes with SNR and bandwidth of the system, and the 1.2 m minimum measurable beat length is shorter than the 4 m in the POTDR method [4]. In conclusion, a simple and fast BOTDR-based distributed fiber birefringence measurement method was
Fig. 4. (Color online) Measurement results of LEAF. (a) BFC of the entire fiber; (b) distributed beat length and birefringence of the entire fiber; (c) BFC of the last 75 m fiber; (d) distributed beat length and birefringence of the last 75 m fiber; (e), (f) histogram of the beat length and birefringence of the entire fiber.
proposed and demonstrated. Different from the conventional heterodyne and frequency-scanning BOTDR, the proposed method directly measures beat length by acquiring the time-domain trace of the broadband beating Brillouin signal, which is equivalent to only one frequency component comparing with the conventional BOTDR. Thus, that significantly simplifies the BOTDR system. It will allow distributed fiber birefringence measurement with 0.6 m resolution for uncertainty of 0.2 m. Y. Lu is thankful for the support by the National Basic Research Program of China (No. 2010CB327803), the National Natural Science Foundation of China (No. 61027017), and the Suzhou Application Research Program (No. SYG201106). We acknowledge the financial support of NSERC Discovery Grants and Canada Research Chair program. References 1. A. Galtarossa and C. R. Menyuk, Polarization Mode Dispersion (Springer, 2005). 2. M. E. Froggatt, D. K. Gifford, S. Kreger, M. Wolfe, and B. J. Soller, J. Lightwave Technol. 24, 4149 (2006). 3. A. Galtarossa, L. Palmieri, M. Schiano, and T. Tambosso, Opt. Lett. 25, 384 (2000). 4. A. Galtarossa and L. Palmieri, J. Lightwave Technol. 22, 1103 (2004). 5. B. Huttner, B. Gisin, and N. Gisin, J. Lightwave Technol. 17, 1843 (1999). 6. M. Wuilpart, P. Megret, M. Blondel, A. Rogers, and Y. Defosse, IEEE Photon. Technol. Lett. 13, 836 (2001). 7. T. Gogolla and K. Krebber, J. Lightwave Technol. 18, 320 (2000). 8. L. Thévenaz, M. Facchini, A. Fellay, M. Niklés, and P. Robert, in Conference Digest OFMC’97 (NPL Publication, 1997), p. 82. 9. S. Xie, L. Chen, and X. Bao, Appl. Opt. 51, 4359 (2012). 10. X. Bao and L. Chen, Sensors 12, 8601 (2012). 11. R. Ulrich, S. C. Rashleigh, and W. Eickhoff, Opt. Lett. 5, 273 (1980).
