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We demonstrated ultra-wideband wavelength division multiplexing (WDM) transmission from 850 to 1550 nm in graded-index multi-mode fiber (GI-MMF) using endlessly single-mode photonic crystal fiber (ESM-PCF) as a launch device. Effective single-mode guidance is obtained in multi-mode fiber at all wavelengths by splicing cm-order length ESM-PCF to the transmission fiber. We achieved 3 × 10 Gbit/s WDM transmission in a 1 km-long 50-μm-core GI-MMF. We also realized penalty free 10 Gbit/s data transmission at a wavelength of 850 nm by optimizing the PCF structure. This method has the potential to achieve greater total transmission capacity for MMF systems by the addition of more wavelength channels.
©2012 Optical Society of America
1. Introduction
Multi-mode fibers (MMF) are widely used as short-reach, in-building transmission media because of their ease of use and overall cost efficiency. Driven by the rapid increase in internet traffic and the adoption of faster Ethernet standards, there is a growing demand for bandwidth in short reach applications, which MMF systems must handle. One major factor limiting the transmission capacity of MMF is signal degradation caused by mode dispersion. In MMF, light can propagate in a variety of modes, which usually have slightly different propagation velocities. To minimize the effect of mode dispersion most installed multimode fiber is designed with a graded index to equalize mode arrival times. By optimizing the index profile, state of the art GI-MMF can handle 10 Gbit data transmission up to a range of 300 meters [1]. However, it is difficult to completely equalize the velocity of all the modes of GI-MMF due to the great difficulty involved in its fabrication.
One effective way to overcome this problem is to reduce the number of excited transmission modes in the fiber. The propagation properties of an MMF are not only dependent on the characteristics of the fiber itself, but also on the quality of light source and its launch condition. Several recent studies have employed single-mode fiber (SMF) compliant with ITU-T Recommendation G.652 as a mode filtering device to achieve high-speed transmission in MMF at wavelengths of 1310 and 1550 nm [2–4]. However, these approaches are only effective for transmissions at wavelengths exceeding 1300 nm and not applicable to the shorter 850 nm band, which is commonly used in MMF transmission systems. If we could apply this technique to the short wavelength band, the capacities of MMF systems could be greatly improved by using the wavelength division multiplexing (WDM) technique.
In this paper, we describe ultra-wideband WDM transmission from 850 to 1550 nm in GI-MMF using ESM-PCF as launch device. PCF is a promising candidate as a mode-filtering device because it supports only fundamental modes at all working wavelengths. Moreover, the mode field diameter (MFD) of PCF can be adjusted using a tapering process [5]. Matching the MFD can greatly reduce the coupling loss of the fundamental mode and increase the tolerance for center launch adjustment precision. A schematic of our method is shown in Fig. 1 . PCFs are spliced to both sides of the transmission fiber as launch and filter devices for ultra-wideband WDM transmission. The light transmitted at different wavelengths is multiplexed using a WDM coupler. The existence of a higher order mode element in the launched light may result in the stimulation of higher order modes in the transmission fiber. By splicing a centimeter-order-length PCF between the SMF pigtail of the WDM coupler and the GI-MMF transmission fiber can solve this problem. The possible higher order mode element of the launch light will be eliminated before leaving the PCF since ESM-PCF supports only a fundamental mode. At the PCF and GI-MMF interface, the fundamental mode alone can be stimulated by optimizing the PCF structure. On the output side, another PCF is used to eliminate any possible higher order modes generated during the transmission. Using this method, we can simultaneously realize effective single-mode guidance in the 850, 1310 and 1550 nm bands. Moreover, this technique can be applied to other wavelengths such as 650 and 1000 nm [6,7]. We expect to use this technique to realize an MMF transmission system with both great flexibility and extensibility by using different wavelengths for specific applications to take advantage of both the original 850 nm band and the other wavelengths.
2. Elimination of higher order modes using ESM-PCF launch device
In this section, we study the elimination of higher order modes in launched light using PCF. For this purpose, it is desirable for the PCF launch device to have a sufficient higher order mode extinction ratio while maintaining a low coupling loss for the fundamental mode. As a result, the structural parameters of PCF such as d/Λ, and the number of air hole layers are of great importance. First, we studied the mode filtering effects of three different types of PCFs. Figure 2 shows cross-sectional images of the PCFs obtained with a microscope. Their parameters are shown in Table 1 , where the listed MFDs were calculated at a wavelength of 850 nm. Type A PCF has 7 layers of air holes and a d/Λ of 0.46, Both type B and C PCFs have 5 layers of air holes and d/Λs of 0.41 and 0.25, respectively. Therefore, these two types of PCFs support only one fundamental mode at any wavelength according to the endlessly single-mode transmission condition of d/Λ<0.43 [8].
Table 1. Parameters of PCF types A, B and C.
A beam propagation method (BPM) was used to study the mode coupling efficiency between the SMF pigtail of the WDM coupler and PCFs [9]. First, we calculated the mode profiles of the SMF. At a wavelength of 850 nm, there are two guided modes in SMF: the LP01 mode and the LP11 mode. The profiles of each calculated mode are then center launched to PCF numerical models. The parameters obtained from the microscope images shown in Fig. 2 are used for calculation. The coupling loss is calculated by normalizing the output and input powers of the PCF model, and is used to evaluate the coupling efficiency of each mode. The results for the LP01 and LP11 modes are shown in Fig. 3(a) and 3(b), respectively. With the LP01 mode, the coupling losses of all three types of PCF are less than 0.5 dB. And the type B PCF with a d/Λ of 0.41 achieved a coupling loss as low as 0.1 dB. This is because type B has a similar MFD to SMF, which is 7.4 μm at a wavelength of 850 nm. With the LP11 mode, it is desirable for PCF to achieve a sufficiently large coupling loss within a cm-order length. The coupling loss of type A PCF is saturated in the first 5 cm at 2 dB. In contrast, the coupling losses of type B and C PCF with a length of 5 cm are 23 and 38 dB, respectively. This becomes 34 and 50 dB when the propagation length along the PCFs increases to 10 cm. As a result, by using an ESM-PCF launch device, we can achieve a sufficiently high coupling loss in the LP11 mode while maintaining a low coupling loss in the LP01 mode.
We next examined the mode filtering effect using the experimental setup shown in Fig. 4 . We chose a wavelength of 850 nm for the experiment. This is the shortest wavelength we used for the WDM transmission experiment. If a PCF can eliminate higher order mode at this wavelength, it works well for 1310 and 1550 nm wavelengths since a fiber support fewer modes at longer wavelengths. First, we used an offset launch technique to deliberately stimulate the LP01 and LP11 modes with a power ratio of 1:1 in a 20 m-long SMF. The observed near field pattern (NFP) of the output facet is shown in Fig. 5(D) . The existence of higher order modes can be clearly observed. Then 10 cm-long PCFs of types A, B and C were center spliced to the output facet and their NFPs were observed. The results are shown in Fig. 5(A), 5(B) and 5(C), respectively. The LP11 mode was not eliminated in type A with a d/Λ of 0.46. Moreover, a cladding mode-like field element was observed because the fiber was short and the number of air hole layers was large, thus making it difficult for the cladding mode to leave the fiber. In contrast, the NFPs from types B and C were LP01 mode shaped and remained stable even when a small bend was introduced into the fiber. This result agrees well with the simulation results.
3. Ultra-wideband WDM transmission in GI-MMF
We carried out an ultra-wideband WDM transmission experiment using the type B PCF discussed above. Figure 6 shows a schematic of our experimental setup for WDM transmission. A 50 μm-core graded-index multi-mode fiber (GI-MMF) with a length of 1 km was used as the transmission medium. Two 10 cm-long type (B) PCFs were used to obtain the desired single-mode launch and to remove any high-order modes that could be generated during the transmission in GI-MMF. The PCFs were sandwiched between the GI-MMF and SMF pigtails on both the input and output sides. On the transmitter side, distributed feedback laser diodes operating at 850, 1310 and 1550 nm were used as light sources. They were modulated at 10 Gbit/s in a non-return-to-zero modulation format with a 231-1 pseudorandom binary sequence by a lithium niobate intensity modulator. The signals were then multiplexed with a polyimide WDM module and fed to the transmission medium through the SMF pigtail. On the receiver side, the transmitted optical signals were demultiplexed by the same type of WDM module. The signals were then amplified by optical amplifiers, detected and examined.
Figure 7 shows the measured BER curves before and after the transmission. The filled and open symbols show the results under back-to-back and transmission conditions, respectively. Error free transmission was successfully obtained for all the wavelengths. Thus, we have achieved ultra-broadband WDM transmission using ESM-PCF as a launch device. Moreover, the power penalty is lowest at a wavelength of 850 nm. We calculated MFDs of type B PCF and GI-MMF at all the operation wavelengths. The results are shown in Table 2 . Compared with GI-MMF’s wavelength dependence on MFD, which of PCF is negligible. This is an inherent characteristic of PCFs. The smaller MFD mismatch between type B PCF and GI-MMF at 850 nm wavelength may be the reason for the lower power penalty.
Table 2. MFDs of Type B PCF and GI-MMF.
4. Structural optimization of ESM-PCF launch device
To investigate the dependence of the PCF structure on higher order mode depression, we fabricated three PCFs with various air hole diameters but similar pitch values. We then studied their LP11 mode elimination abilities. Their cross-sectional images and parameters are shown in Fig. 8 and Table 3 , respectively. The listed MFDs were calculated at a wavelength of 850 nm. Unlike the PCFs listed in Table 1, all of these PCFs have the same simplified three air hole layer structure, and this greatly facilitated the investigation. Moreover, it is much easier to fabricate this structure.
Table 3. Parameters of PCF sample 1, 2 and 3.
We calculated the mode coupling efficiency between SMF and PCFs. Figure 9 shows the mode coupling loss of the LP11 mode as a function of device lengths at a wavelength of 850 nm. The coupling loss of the LP11 mode increases as the d/Λ value decreases. Compared with the type C PCF in Table 1, although sample 3 has a similar d/Λ, the coupling loss of LP11 is 20 dB higher. This is because the type C PCF in Table 1 has 5 layers of air holes, which makes it more difficult for the unguided LP11 mode to leave the fiber core within a short distance. On the other hand, the MFD of the PCF in Table 3 is almost twice that of SMF, and the coupling loss of the fundamental LP01 mode also increased owing to the MFD mismatch. However, the aim of this experiment was to investigate the effect of the PCF structure on higher order mode depression, and we did not adjust the MFD of the PCF.
We compared the mode elimination ability of the three types of PCF discussed above by carrying out a transmission experiment using the same 1 km 50 μm-core GI-MMF at a single 850 nm wavelength. Since the fiber is wound on a bobbin and there is no connector in the transmission fiber, the effect of the higher modes generation due to the fiber distortion and offset between fiber connector is negligible. We spliced each of the three types of PCF at only the input side to function as launch devices and omitted PCF at output side to reduce the coupling loss of fundamental mode. The lengths of three PCFs were adjusted to 20 cm to ensure a sufficient elimination ratio for the higher order mode. The transmission experiment setup is shown in Fig. 10 and the measured BER is shown in Fig. 11 . When launching a signal without the ESM-PCF launch device, we observed a power penalty of 1 dB, which was probably due to the existence of higher-order modes. This value may increase greatly when the transmission system becomes more complex with the addition of WDM couplers and fiber connection points. This may cause a more serious center mismatch problem and stimulate a larger portion of the higher order modes. On the other hand, using a PCF mode filter can reduce the power penalty as expected. This effect is theoretically independent of the complexity of the transmission system. Figure 11 shows that the power penalty decreases with the d/Λ of the PCF. And by using a PCF with a d/Λ of 0.24 power penalty free transmission is realized. This result implies that PCF with an optimized d/Λ is highly effective for higher order mode elimination. Further matching of the MFDs using a tapering process can largely reduce the coupling loss of the fundamental mode and increase the tolerance of center launch adjustment precision.
5. Conclusion
In summary, we achieved ultra-wideband WDM transmission (3 × 10 Gbit/s) in a 1 km-long GI-MMF by using ESM-PCF as a launch device. We realized penalty free 10 Gbit/s data transmission at a wavelength of 850 nm by optimizing the PCF structure. This method has the potential to achieve an even greater total transmission capacity for MMF systems through the addition of more wavelength channels.
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