## 4 Signal Transmission of Ultra-Bend-Resistant 4-Core SXC  
Finally, the signal transmission experiments of the 4-core SXC link were conducted by using a 100 GBASE-LR4 transceiver as the transmitter (Tx) and receiver (Rx). Figure 7 is the schematic diagram of the signal transmission system. In the Tx, four channels satisfying the Lan Wavelength Division Multiplexing (LWDM) standard were excited, where the central wavelengths (\(\lambda\)) are around 1295 nm, 1300 nm, 1305 nm and 1309 nm, respectively [24]. By using the direct modulation method, the laser diode was modulated by a pseudo-random binary sequence (PRBS31) with 2\({}^{31}\)-1 bits (25 Gb/s/\(\lambda\)). The signal propagated through the fan-in device was launched into the 1.2 km long 4-core SXC. After passing through the fan-out device, the signal was received by the corresponding photo detectors (PD) in Rx. By properly choosing the FIFO device ports and central wavelength of incident light, the eye diagram and bit error rate (BER) performance of each core at different wavelengths over the 4-core SXC were both obtained. The inset figure in Figure 7 is the eye diagram of the back-to-back transmitted signal. When the signal light is injected into different cores, the measured BER curves at four wavelengths are shown in Figure 8. It can be seen that the 4# core, which is the nearest neighbor to the marker core, shows a relatively larger deterioration of the BER curve. Based on the above-mentioned experimental results, the higher XT level and larger transmission loss degradation due to cabling would cause this deterioration of the BER curve. Additionally, the changes of polarization-mode dispersion (PMD) inside the fabricated 4-core SXC should not be neglected, which could be attributed to the differences in the intrinsic birefringence parameters of each core. Significantly, the 4# core which is the first nearest neighbor to the marker core would suffer from a severer PMD due to a larger core ellipticity that can also lead to this deterioration of the BER curve [25]. In the future, the optimization of the fiber drawing and cabling process around the marker core can provide a feasible and reliable way to improve the BER performance of the 4-core SXC. Furthermore, the inset figures in Figure 8 are the eye diagrams with the most obvious deteriorations of the BER curve, which are 1295 nm of the 1# core, 1309 nm of the 2# core, 1295 nm of the 3# core and 1309 nm of the 4# core, respectively. Compared with the eye diagram of the back-to-back case, there are no significant distortions in the eye diagrams and they are clearly open after propagating through the 1.2 km long 4-core SXC. Actually, the used length of an indoor SXC in terminated cable assemblies generally varies from several to tens of meters. Thus, the impacts caused by XT and transmission loss will be  
Figure 6: XT between adjacent cores test. (**a**) Experimental setup and (**b**) test results. Figure 7: A schematic diagram for the signal transmission experiment. further reduced due to the shorter transmission distance in short-reach OI. It is worth noting that, compared with the conventional SC-SXC and duplex cable, this 4-core SXC can, respectively, obtain 75% and 50% space reduction in existing short-reach OI infrastructure, which can achieve the hybrid unidirectional and bi-directional transmission by using the 4-core SXC. In this case, the induced propagation constant mismatch due to co-propagation will further suppress the XT level in the SDM transmission system [26; 27; 28]. Additionally, once the fabricated 4-core SXC with a tight design is installed in practice, the environmental effects, such as temperature, moisture and mouse biting, would have significant impacts on transmission performance [21; 29]. For example, the polymer materials used as the different layers of the 4-core SXC have individual coefficients which are quite different from that of the silica fiber.