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Abstract
Multi-core fiber based on space division multiplexing technology provides a practical solution to achieve multi-channel and high-capacity signal transmission. However, long-distance and error-free transmission remains challenging due to the presence of inter-core crosstalk within the multi-core fiber. Here, we propose and prepare a novel trapezoid-index thirteen-core single-mode fiber to solve the problems that MCF has large inter-core crosstalk and the transmission capacity of single-mode fiber approaches the upper limit. The optical properties of thirteen-core single-mode fiber are measured and characterized by experimental setups. The inter-core crosstalk of the thirteen-core single-mode fiber is less than −62.50 dB/km at 1550 nm. At the same time, each core can transmit signals at a data rate of 10 Gb/s and achieve error-free signal transmission. The prepared optical fiber with a trapezoid-index core provides a new and feasible solution for reducing inter-core crosstalk, which can be loaded into current communication systems and applied in large data centers.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Currently, the transmission capacity of single-mode fiber (SMF) is approaching the upper transmission limit, which will hinder the future development of communication systems. With the in-depth research of space division multiplexing (SDM) technology, the research and application of multi-core fiber (MCF), few-mode fiber (FMF), and multi-core few-mode fiber (MC-FMF) have become one of the main ways to solve the shortage of communication capacity in the current communication system [1–5]. However, while MCFs provide more communication channels, they also introduce detrimental factors such as inter-core crosstalk (XT) and intra-core crosstalk, multi-mode dispersion, and other factors that are detrimental to signal transmission [6,7]. Thus, MCF with excellent transmission characteristics will be an important part of future communication systems.
At present, the proposed weakly coupled MCF based on SDM technology becomes a simple and effective method for reducing crosstalk between cores and expanding the number of channels [8,9]. Weakly coupled MCF has the advantage of multi-channel transmission, which can realize high-speed and high-capacity transmission in data centers and long-distance optical fiber transmission. For instance, the 36-core 3-mode fiber in the Ref. [10] achieves an extraordinary performance for high-density and high-capacity transmission. The 19-core 6-mode fiber achieves a transmission capacity of 2.05 Pbit/s while performing high-capacity transmission, according to Ref. [11]. In Ref. [12], the weakly coupled thirteen-core four-mode fiber achieved inter-core XT levels below −30 dB/100 km at a large core pitch when the wavelength is 1550 nm. To minimize substantial signal degradation caused by mode coupling and energy coupling between cores during multi-channel transmission, the optical fiber has to preserve low inter-core XT and intra-core XT. The structure of trench [5,13], air holes [7] and heterogeneous core arrangement [14,15] in the optical fiber can effectively reduce the energy superposition between the cores, which ensures the stable transmission of signals in the channel. In order to reduce inter-core XT and improve the signal quality, this paper presents a novel trapezoid-index thirteen-core single-mode fiber that can be prepared.
In this research work, we proposed and prepared a trapezoid-index thirteen-core single-mode fiber, and measured and characterized the optical properties of the optical fiber. According to the simulation analysis of the actual parameters of the prepared fiber, the LP01 mode of the thirteen-core fiber can transmit stably in the C + L band. The transmission loss of each core of the thirteen-core single-mode fiber is less than 0.5 dB/km, and the inter-core XT at 1550 nm is below −62.50 dB/km. Moreover, each core successfully demonstrated 10 Gb/s signal transmission, error-free transmission is achieved during the signal transmission of independent channels. The experimental results demonstrate that the proposed trapezoid-index thirteen-core single-mode fiber reliably prevents the energy coupling between the cores and maintains signal quality during long-distance transmission.
2. Design and preparation of optical fiber
2.1. Design and preparation of thirteen-core single-mode fiber
In the optical fiber practical application, the energy coupling between adjacent cores can seriously degrade the quality of the signal, which is not conducive to the long-distance transmission of signals. Furthermore, the transmission loss of optical fiber strictly controls the distance of optical fiber transmission, and the optical fiber with low transmission loss can support longer non-relay transmission of the optical signal [16]. Therefore, we propose and prepare a thirteen-core single-mode fiber with a trench structure. In the channel, the transmission of LP01 mode is highly stable and the light energy is concentrated in the center of the core, which is beneficial to reduce the energy coupling between the cores. The trench can hinder the energy coupling between the cores, which is beneficial to achieve low XT and low transmission loss of thirteen-core single-mode fiber. The cross-section of the proposed thirteen-core single-mode fiber is shown in Fig. 1(a), and core pitch (Λ) and cladding diameter (CD) are 42 µm and 250 µm, respectively. Each single-mode unit consists of trapezoid-index core, inner cladding, and trench. The refractive index distribution of the single-mode unit is shown in Fig. 1(b). The refractive index part of the trapezoid-index core consists of the refractive index flat area and the refractive index graded area, a and b are the radii of the refractive index flat area and the thickness of the refractive index graded area. c and d represent the thickness of the inner cladding and trench, respectively. Δn1 means the relative refractive index difference between the core and the inner cladding, Δn2 represents the relative refractive index difference between the inner cladding and the trench, and the inner cladding is composed of pure silica.
Fig. 1. (a) Cross-section of thirteen-core single-mode fiber. (b) The refractive index distribution of single-mode unit.
Based on the existing plasma chemical vapor deposition (PCVD) technology, the thirteen-core single-mode fiber is prepared according to the fiber parameters of a = 3.7 µm, b = 1 µm, c = 3 µm, d = 6 µm, Δn1 = 0.34%, and Δn2=−0.68%. Figure 2 shows the refractive index distribution of the fabricated and designed fiber at 1632 nm. As shown in Fig. 3(a), the cross-sectional of the thirteen-core single-mode fiber is measured by optical microscope. It can be found from Fig. 3(a) that the core pitch is about 42–43 µm, which is consistent with the simulated core pitch. From Figs. 2 and 3, the actual parameter Table 1 of the thirteen-core single-mode fiber can be obtained. Compared with the optical fiber parameters before and after preparation, the results show that the optical fiber parameters before and after preparation are basically consistent. In addition, the order numbers of the thirteen cores are marked according to the position of Marked in Fig. 3(b). This is conducive to accurate measurement of the optical characteristics of each channel.
Fig. 2. Refractive index distribution of fabrication parameters and design parameters of single-mode unit.
Table 1. Actual parameters of thirteen-core single-mode fiber.
2.2. Single-mode transmission of thirteen-core single-mode fiber
The dispersion effect of multiple modes and the coupling between modes will expand the pulse width and greatly reduce the signal quality. Due to this, single-mode transmission is more beneficial to ensure signal quality than multi-mode transmission. According to the actual parameters of the thirteen-core single-mode fiber in Table 1, we are simulated and analyzed the mode transmission of the thirteen-core single-mode fiber in the C + L band. When the bending radius (R) is 140 mm, the relationship between the bending loss of the LP11 mode and the wavelength (λ) is shown in Fig. 4(a). As the wavelength increases, the bending loss of LP11 mode is increasing. It is worth noting that the bending loss of LP11 mode is 4.72 dB/m at λ=1530 nm and R = 140 mm, which is greater than 1 dB/m. Furthermore, the bending loss of the LP11 mode is greater than 4.72 dB/m in the whole C and L bands, which meets the cut-off condition of the LP11 mode in the Ref. [4,15]. In Fig. 4(b), the bending loss of LP01 mode increases with the increase of wavelength. When the bending radius is 30 mm and the wavelength is 1630 nm, the maximum bending loss of LP01 mode is 2.66 × 10−8 dB/m. However, the bending loss is still less than 0.5 dB/100turns (0.026 dB/m), which meets the loss standard for LP mode (wanted LP mode) stable long-distance transmission in Ref. [4,17]. This means that thirteen-core single-mode fiber can carry out independent transmission of single mode in the C + L band.
Fig. 4. (a) Relationship between the bending loss of the LP11 mode in the core and the wavelength at R = 140 mm. (b) Relationship between the bending loss of LP01 mode in the core and the wavelength at R = 30 mm.
When the thirteen-core single-mode fiber is performed for single-mode transmission, the inter-core XT and the effective mode field area (Aeff) are important factors that affect the signal transmission quality. The trapezoid-index core consists of the refractive index flat area and the refractive index gradient area. When the diameter of the refractive index flat area of the trapezoid-index core is equal to the diameter of the step-index core (the inner cladding and trench of the trapezoid-index core and the step-index core have the same optical fiber parameters), Fig. 5 shows the relationship between inter-core XT, Aeff and wavelength for both structures. As can be seen from Fig. 5(a), the inter-core XT of the LP01 mode in the trapezoid-index core is −64.00 dB/km at R = 80 mm and λ=1550 nm. At this time, the inter-core XT of LP01 mode in the trapezoid-index core at the same wavelength is 8 dB/km less than that of LP01 mode in the step-index core. The simulation results indicate that the trapezoid-index core can effectively prevent the energy coupling between the cores compared to the step-index core. As can be seen from Fig. 5(b), the Aeff of the LP01 mode of the trapezoid-index core is larger than that of the step-index core, which also indicates that the trapezoid-index core can better suppress nonlinear effects. Based on the above research results, the proposed and prepared trapezoid-index thirteen-core single-mode fiber in this paper provides a new design idea and scheme for reducing inter-core XT and expanding Aeff.
3. Transmission loss experiment of thirteen-core single-mode fiber
The losses of optical fiber are mainly divided into transmission loss and additional loss. Moreover, the transmission loss of optical fiber is unavoidable due to the scattering and absorption of light energy in the material. The added losses in the bending and fusion of the fiber are called additional losses, and these losses can be avoided as much as possible. Consequently, low transmission loss plays a crucial role in the long-distance transmission of optical signals [16].
In this paper, the thirteen-core single-mode fiber is winded on a fiber spool with a bend radius of 80 mm, and the transmission loss of the cores is measured by optical time domain reflectometer (OTDR). The transmission loss of each channel can be measured with the help of fan-in device (space division multiplexer). The fan-in device (fan-out device) is prepared by using a single-mode fiber. The experimental setup for transmission loss of thirteen-core single-mode fiber is shown in Fig. 6. The OTDR and the thirteen-core space division multiplexer are connected together, and then the transmission loss of each core of the 10.3 km-long thirteen-core single-mode fiber can be repeatedly measured with the help of the experimental optical path. The transmission loss of all cores in the thirteen-core single-mode fiber is shown in Fig. 7. Furthermore, it can be found from Fig. 7 that the transmission loss of the thirteen-core is less than 0.5 dB/km. The geometric deformation of the core rod is large due to the extension of the core rod. Meanwhile, the outer layer cores (core 8-core 13) and inner layer cores (core 1-core 7) are subjected to different stresses and temperatures during optical fiber preparation, which results in different ellipticities for the outer layer and inner layer cores. Due to this, the transmission loss of thirteen-core single-mode fiber fluctuates between 0.32 dB/km and 0.48 dB/km. The measurement results show that although there are some differences in the transmission loss of each core, the loss characteristics of the optical fiber meet the fundamental requirements of the communication system for transmission loss.
Fig. 6. The experimental setup for measuring the transmission loss of thirteen-core single-mode fiber by OTDR.
4. Inter-core XT experiment of thirteen-core single-mode fiber
MCF provides more transmission channels than SMF, but the signal quality and signal transmission distance can be seriously affected when the inter-core XT of MCF is too large. During signal transmission, the reduction of inter-core XT in multi-core fiber is the key to maintaining signal transmission over long distances [18]. This paper prepares a novel trapezoid-index thirteen-core single-mode fiber to reduce inter-core crosstalk while ensuring signal quality. The experimental setup for measuring the inter-core XT is shown in Fig. 8. The amplified spontaneous emission (ASE) light source provides the light energy in each channel of the thirteen-core single-mode fiber. Meanwhile, the light energy enters each individual channel of the 18 km-long thirteen-core single-mode fiber through fan-in and fan-out devices (space division multiplexer and spatial division demultiplexer). The output energy spectral lines of the core can be obtained on the optical spectrum analyzer (OSA) by adjusting the input and output ports of light energy. The inter-core XT can be calculated from the two-output energy spectral lines of adjacent cores [16].
The variation of inter-core XT with wavelength is shown in Fig. 9. As shown in Fig. 9(a), when core 1 inputs light energy, the XT of adjacent cores is less than −60 dB/km in the measurement wavelength range. The experimental results show that the maximum inter-core XT is −66.99 dB/km at 1550 nm, which indicates that the trench-assisted structure and the trapezoid-index core can reduce the inter-core XT. For core input light energy from core 2 to core 7, the variation of the inter-core XT curve with wavelength is shown in Fig. 9(b). The inter-core XT is in the range of −60 dB/km to −80 dB/km for the whole measurement wavelength range. Meanwhile, the inter-core XT at 1550 nm is below −66.12 dB/km, which is helpful for the long-distance transmission of optical fiber. Under the same measurement conditions, when the light energy is injected into the core from core 8 to core 13, the inter-core XT data is shown in Fig. 9(c) and (d). At 1550 nm, the inter-core XT of the thirteen-core single-mode fiber is below −62.50 dB/km. The crosstalk characteristics of various multi-core single-mode fibers are shown in Table 2. From Table 2, the trapezoid-index thirteen-core single-mode fiber has lower inter-core XT than the weakly coupled twelve-core single-mode fiber with a trench-assisted structure in Ref. [19]. In contrast to the six-core single-mode fiber in Ref. [20], the proposed thirteen-core single-mode fiber with a trench-assisted structure avoids the collapse problem of air holes during optical fiber preparation. Moreover, the thirteen-core single-mode fiber provides more transmission channels and has lower crosstalk than the four-core single-mode fiber in the previous research work [21]. Based on the above-mentioned, the prepared trapezoidal-index thirteen-core single-mode fiber has considerable application prospects.
Fig. 9. (a) When core 1 input light energy, the relationship between inter-core XT and wavelength. (b) When any core input light energy from core 2 to core 7, the relationship between inter-core XT and wavelength. (c) When any core input light energy from core 8 to core 10, the relationship between inter-core XT and wavelength. (d) When any core input light energy from core 11 to core 13, the relationship between inter-core XT and wavelength.
Table 2. Crosstalk performance comparison of various multi-core single-mode fibers.
5. Signal transmission measurement of thirteen-core single-mode fiber
The signal transmission measurement of the prepared thirteen-core single-mode fiber is carried out at 1550 nm. Figure 10 shows the schematic diagram of the experimental setup. At the transmitter side, we use the mach-zehnder modulator (MZM) to modulate the 10 Gb/s on-off keying (OOK) signal. The signal passes through the fan-in device into the 20 km-long thirteen-core single-mode fiber. At the end of the fan-out device, the optical signal is subjected to dispersion compensation and amplification, and then the optical signal enters the receiving end. At the receiving end, the transmitted optical signals are first attenuated by the variable optical attenuator (VOA) and then detected by a photodiode (PD). After the electrical signal generated by PD is amplified, the clock and data recovery (CDR) module performs clock recovery and data decision, and the bit error rate tester (BERT) measures the bit error rate (BER) in real time on each core.
Fig. 10. The experiment setup. MZM: mach-zehnder modulator; PPG: pulse pattern generator; PC: polarization controller; MCF: multi-core fiber; DCF: dispersion compensating fiber; EDFA: erbium doped fiber amplifier; PM: power meter; VOA: variable optical attenuator; PD: photodiode; Amp: amplifier; CDR: clock and data recovery; BERT: bit error rate tester.
When thirteen cores are injected energy one by one, the relationship between the BER curve and the received power is shown in Fig. 11. In the back-to-back case, the input signal of the thirteen-core single-mode fiber is directly analyzed. Compared with the eye diagram of back-to-back transmitted signals, there is no significant distortion in the eye diagram after long-distance transmission. However, the BER curve does deteriorate after long-distance transmission. The BER curves of each core have different degrees of deterioration, and the BER curve of the outer layer cores has a greater degree of deterioration than that of the inner layer cores. This is due to the different optical characteristics of each core (including insertion loss, dispersion, and polarization mode dispersion, etc.) and the preparation differences of the inner layer and outer layer cores, and the signal quality of each core will be degraded to different degrees after long-distance transmission.
Core 1, core 2, and core 8 are in different locations in the three-layer cores of the thirteen-core single-mode fiber and they are adjacent to each other. Therefore, we choose these three cores as representative cores to measure the BER curves. Figure 12 shows the variation of the BER curve with received power when selected cores and their adjacent cores are injected energy. It is worth noting that when energy is injected into the selected core and its adjacent cores, the energy coupling between the cores affects the quality of signal transmission. At this point, the quality of the eye diagram after long-distance transmission will further deteriorate. The degree of eye-opening is reduced in the eye diagram. For example, core 1 is affected by the energy coupling of six adjacent cores, so core 1 has a smaller eye opening than core 2 and core 8. Nevertheless, core 1, core 2, and core 8 continue to keep error-free transmission under the influence of energy coupling of adjacent cores. In conclusion, the prepared optical fiber has excellent transmission characteristics that could be applied to future multi-core communication systems.
Fig. 12. The BER performance and eye diagram of core 1, core 2 and core 8 of 20 km-long thirteen-core single-mode fiber.
6. Conclusion
In summary, this paper demonstrates the experiments of the prepared trapezoid-index thirteen-core single-mode fiber for low XT and error-free long-distance transmission. Based on the actual parameters of the prepared optical fiber, we successfully verified that the thirteen-core single-mode fiber can maintain single-mode transmission in the C + L band range. With the help of fan-in and fan-out devices, optical characteristics such as transmission loss, inter-core XT, and BER can be efficiently measured. It is especially worth noting that the inter-core XT of thirteen-core single-mode fiber is below −62.50 dB/km at 1550 nm and the transmission loss is less than 0.5 dB/km. This facilitates the transmission of optical signals over long distances. In the experiment, the single-mode operation over 20 km-long thirteen-core single-mode fiber at 1550 nm is achieved at a data rate of 13 × 10 Gb/s with OOK signals. With the above optical properties, the prepared thirteen-core single-mode fiber can provide an effective way for applications of large data centers and terrestrial and undersea communication systems.
Funding
National Natural Science Foundation of China (12074331); National Key Research and Development Program of China (2019YFB2204001).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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