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We have greatly increased the dynamic range of a synchronous multi-channel OTDR system for the mode coupling measurement of a multi-core fiber (MCF) by more than 20 dB by introducing an optical amplifier and an optical masking apparatus. We used the OTDR system to measure the mode coupling along 10 km-long MCFs with low crosstalks of less than −50 dB. Thus, we successfully measured the fiber structural irregularity dependence of mode coupling along the MCF.
© 2013 Optical Society of America
1. Introduction
Spatial division multiplexing (SDM) with multi-core fibers (MCFs) is a promising candidate technology with which to overcome the capacity limit of current optical communication systems [1–8]. Recently, a total capacity exceeding 1 Pbit/s has been successfully demonstrated by using 12- or 14-core fiber and a 32-multilevel modulation format [7, 8]. To increase the transmission capacity further, it is important to increase the spectral efficiency per single core with a higher order QAM signal as well as the core density in MCF. However, in an MCF transmission the mode coupling between adjacent cores distorts the signal waveform and limits the transmission performance [9]. Therefore, it is very important to diagnose the mode coupling along an MCF in detail.
We have already proposed a mode coupling measurement along an MCF using an optical time domain reflectometry (OTDR) technique [10]. With this method, the mode coupling coefficients between 7 cores can be simultaneously and nondestructively measured. We demonstrated a mode coupling measurement along a 2.9 km-long MCF with a 50 dB dynamic range and a 100 m spatial resolution [10]. Furthermore, we demonstrated a mode coupling measurement along a 9.7 km-long MCF with an improved OTDR system, which indicated a strong correlation between the change in the mode coupling ratio and the cladding diameter fluctuation along the MCF [11]. In this paper, we elaborate on our preliminary report in [11] and present a detailed comparison of the mode coupling behavior along MCFs with and without air hole markers. We have clarified that the amount of mode coupling strongly depends on the structural irregularity along the fiber.
2. Measurement setup for MCF mode coupling
A schematic diagram of the MCF mode coupling measurement setup is shown in Fig. 1. An optical pulse from ch. 1 of a multi-channel OTDR system operated at 1550 nm is coupled to core 1 through a fan-in device, and the backscattered light in core n, Pbsn, is detected by ch. n of a multi-channel OTDR system. The mode coupling ratio between cores 1 and n along the fiber can be obtained from the power ratio between Pbs1 and Pbsn:
where hn,1 is the mode coupling coefficient between cores 1 and n, L is the fiber length, and K is a constant determined by the Fourier transformation of the autocorrelation function of the mode coupling coefficient [12, 13] and the crosstalk caused by the fan-in device.An optical amplifier (EDFA) and an optical masking apparatus to remove the deadzone caused by the Fresnel reflections generated at both the input and output ends of the MCF were newly adopted in the OTDR system. The configuration of the optical amplifier and optical masking apparatus are shown in Figs. 2(a) and 2(b), respectively. By inserting an EDFA(wavelength range: C band, saturated output power: > 10 dBm, small signal gain: > 30 dB, noise figure < 4.2 dB) between two optical circulators, we can detect the backscattered signal from a multi-core fiber passing through a bypass. On the other hand, two optical switches are installed between two optical circulators in each channel of the optical maskingapparatus. Here, a 0.5 km SMF is also installed in front of the switches to adjust the timing between the electrical control signal and the backscattered optical signal. The Fresnel reflections generated at both the input and output ends of the multi-core fiber are removed by these optical switches, resulting in a reduction of the deadzone from 100 to 20 m.
Figure 3 shows the backscattered signals when a 100 ns optical pulse (corresponding to a spatial resolution of 10 m) was coupled into the center core of a 9.7 km MCF, which corresponds to Fiber A described later. As shown in Fig. 3(a), the optical dynamic range was 38 dB and we detected large Fresnel reflections before the OTDR system improved. Here, the measurement results for Pbs2-Pbs7 shown in Fig. 3(a) correspond to the noise level of the OTDR system. To reduce the noise, we adopted a 216 times averaging process in the OTDR measurement. By installing an optical amplifier, we increased the dynamic range to 60 dB as shown in Fig. 3(b). Furthermore, we removed the Fresnel reflections by installing an optical masking apparatus as shown in Fig. 3(c). A high dynamic range of more than 60 dB with a 10 m spatial resolution was obtained once the OTDR system had been improved.
Fig. 3 Backscattered signals when a 100 ns optical pulse was coupled into center core 1 of an MCF: (a) before improvement, (b) with optical amplifier, (c) with optical amplifier and optical masking apparatus.
3. Experimental results for mode coupling measurement along MCFs
We used two kinds of 10 km-long MCFs fabricated using the stack-and-draw method as test fibers. The fiber parameters and photographs of the fiber cross section are shown in Table 1 and Fig. 4, respectively. Fiber A has three air hole markers in its cross section and the cladding diameter changed along the MCF as a result of the collapse of the air hole markers [11]. On the other hand, Fiber B has a marker that employs glass material instead of air holes and the cladding diameter fluctuation is smaller than that of Fiber A. We measured the mode coupling along both MCFs and considered the fiber structural irregularity dependence on the mode coupling.
Table 1. Fiber parameters of two kinds of MCF
3.1 Mode coupling measurement of Fiber A
Figures 5(a)–5(d) show the measured mode coupling ratios when 100 ns optical pulses were coupled into cores 1, 2, 4, and 6 of Fiber A. Here, mode coupling offsets at the input end of the fiber (corresponding to K in Eq. (1)) were removed by subtracting the data at 0 km, and we considered only mode couplings that occurred in the fiber. The largest mode coupling wasobserved between cores 6 and 7, which were located near air hole markers as shown in Fig. 4(a). On the other hand, the smallest mode coupling was observed between cores 3 and 4, which were located far from the air hole markers.
Fig. 5 Changes in mode coupling ratio (a)-(d) when a 100 ns optical pulse was coupled into cores 1, 2, 4, and 6 of Fiber A, respectively.
Table 2 compares crosstalk values measured with the multi-channel OTDR and a conventional transmission method [14]. Here, the measurement error of both methods was approximately 1 dB. The difference between the two measurement values was within the measurement error range of 1 dB. This indicates that the measured values obtained with the multi-channel OTDR technique are in good agreement with those obtained with the conventional method.
Table 2. Comparison of crosstalk values measured with conventional transmission method.
Figures 6(a) and 6(b) show the change in the cladding diameter along Fiber A and the spatial frequency spectrum, respectively. The cladding diameter was changed along the fiber with an 8.4 μm fluctuation range and periods of 1330, 724-781, 555, and 425 m. This might be caused by the air hole diameter fluctuation along the fiber introduced by the temperature and pressure changes that occur during the fiber fabrication process. To consider the effect of cladding diameter fluctuation on the mode coupling along the fiber, we plot the detailed change in the mode coupling ratio between all adjacent cores along the fiber and the spatial frequency spectrum in Figs. 7(a)–7(l). Here, the spatial resolution was reduced from 10 to 100 m to increase the signal to noise ratio of the analysis data around the several hundred meter region. It was clarified that the mode coupling ratios between all adjacent cores changed along the fiber with the same periods as the cladding diameter. This result indicates that our OTDR system can successfully evaluate structural irregularity along an MCF in detail. In Fig. 6, the cladding diameter was measured during the fiber drawing process where the fiber was rotating very slowly in the radial direction. Therefore the radial axis for the diameter measurement was changing with time. This might reduce the correlation between the measurement results for the cladding diameter fluctuation and the mode coupling variation.
Fig. 7 Detailed changes in the mode coupling ratio along Fiber A and the spatial frequency spectrum. Between (a) cores 1 and 2, (b) cores 1 and 3, (c) cores 1 and 4, (d) cores 1 and 5, (e) cores 1 and 6, (f) cores 1 and 7, (g) cores 2 and 7, (h) cores 2 and 3, (i) cores 3 and 4, (j) cores 4 and 5, (k) cores 5 and 6, and (l) cores 6 and 7.
3.2 Mode coupling measurement of Fiber B
For comparison, we performed a mode coupling measurement on Fiber B, which had a marker made of a glass material instead of an air hole. The change in the cladding diameter along the fiber and the spatial frequency spectrum are shown in Figs. 8(a) and 8(b), respectively. The fluctuation range of the cladding diameter was as low as 1.9 μm with periods of 2-2.5 km.
Figure 9 shows the change in the mode coupling ratio along Fiber B when a 1 μs optical pulse was coupled into core 1. The mode coupling ratio between adjacent cores was one tenth of that obtained in Fiber A with a large cladding diameter fluctuation as shown in Fig. 5(a). Here, the optical pulse width for OTDR measurement was enlarged from 100 ns to 1 μs to increase the measurement dynamic range. The detailed change in the mode coupling ratio between all adjacent cores along the fiber and the spatial frequency spectrum are plotted in Figs. 10(a)–10(f). It was clarified that the fluctuation range of the mode coupling ratio was also reduced to one tenth and the mode coupling ratio was changed along the fiber with the same periods as the cladding diameter. Although Fiber B has a low crosstalk of less than −60 dB, the fiber structural irregularity dependence on the mode coupling can be obtained.
Fig. 9 Change in mode coupling ratio along Fiber B when a 1 μs optical pulse was coupled into core 1.
Fig. 10 Detailed changes in the mode coupling ratio along Fiber B and the spatial frequency spectrum. Between (a) cores 1 and 2, (b) cores 1 and 3, (c) cores 1 and 4, (d) cores 1 and 5, (e) cores 1 and 6, and (f) cores 1 and 7.
By comparing the mode coupling characteristics of Fiber A and B, it can be clarified that the amount of mode coupling strongly depends on the fluctuation range of the cladding diameter. This means that reducing the structural irregularity is very important if we are to reduce the crosstalk between two cores in the MCF.
4. Conclusion
We have succeeded in measuring precise mode coupling along two kinds of 10 km MCFs with low crosstalks of less than −50 dB by developing an improved multi-channel OTDR system. The new OTDR system introduced an optical amplifier and an optical masking apparatus, which could realize a high dynamic range of 60 dB. In both fiber measurements, we successfully measured the mode coupling changes along the MCF which were induced by the fiber structural irregularity. We then compared the mode coupling characteristics of the two MCFs and showed that the amount of mode coupling strongly depends on the cladding diameter fluctuation range. The present synchronous OTDR system becomes a powerful tool for monitoring local mode coupling in detail along an installed MCF transmission line.
Acknowledgment
This research was supported by the National Institute of Information and Communications Technology (NICT), Japan as part of their program “Research on Innovative Optical Fiber Technology”.
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