Few-mode fibers for spatial division multiplexing

Few-mode fibers for spatial division multiplexing

The pursuit for efficient and high-speed data transmission has drawn the attention of researchers towards the less exploited spatial dimensions in fibers. In this context, SDM that uses either FMF or MCF as the medium for data transmission forms the essential part of next-generation fiber telecommunication systems as summarized by Richardson et al. (Igarashi, Tsuritani et Morita, 2016; Kitayama et Diamantopoulos, 2017; Richardson, Fini et Nelson, 2013). The spatial spectral efficiency parameter termed “spatial multiplicity” has been brought forth keeping in view the space required for channel transmission.

The aggregate spectral efficiency (ASE) can be measured if the spectral efficiency and spatial multiplicity are known. In the transmission experiments conducted earlier, the spatial multiplicity started from the value of 3 for the number of modes (Salsi et al., 2012) denoted by ‘M’ and a value of 7 for the number of cores (Zhu et al., 2011) which is denoted by ‘N’ in multimode and multicore transmissions respectively. Later, the number of cores, ‘N’ was maximized to 19 thus increasing the multiplicity (Sakaguchi et al., 2012b). Spatial multiplicity above 30 could be attained by introducing the region of dense SDM, so as to improve the scalability (Mizuno et al., 2014). The results of various studies using MCF, MMF or FMF indicate great strides in the progress of data transmitting capacity of a single fiber link in the recent years (Igarashi, Tsuritani et Morita, 2016; Kitayama et Diamantopoulos, 2017; Minardo, Bernini et Zeni, 2014; Nikles, Thevenaz et Robert, 1997; Rademacher et al., 2018a; Rademacher et al., 2018b; Richardson, Fini et Nelson, 2013; Soma et al., 2018b; Song et Kim, 2013).

Few-mode multicore fiber for SDM transmission 

Several techniques have been employed with a view to derive maximum benefit from SDM which can be accomplished in two different ways. Firstly, the SDM can be implemented by parallel propagation of several spatial channels in the same fiber. Another approach to introduce SDM systems is to insert multiple cores within a multicore fiber (MCF). Initial studies on transmission using a 7-core MCF reported a capacity increment of 100 Tb/s equal to the maximum SMF limit (Sakaguchi et al., 2012a). Certain studies have reported using around 19 independent optical cores inside a single clad fiber. Transmission experiments conducted with a 19-core fiber, reported a transmission rate of 305 Tb/s that is 3 times the SMF limit (Sakaguchi et al., 2012b). One of the studies has reported 1.01 Pb/s across 52 km distance in the transmission using a MCF having 12-cores (Takara et al., 2012). The highest data transmission rate of 1 Pb/s was reported with an ASE of 91.4 b/s/Hz by employing a 12 core fiber with a one-ring structure (Takara et al., 2012). A long haul transmission rate of 1 Eb/s could be accomplished using a 7 core, 12 core MCF over 7326 km 1700 km transmission distances (Igarashi et al., 2013). The second approach uses a guiding medium supporting multiple modes, like in a FMF that is capable of guiding only around 6 to 12 distinct modes inside a single core and is designed slightly larger than that of a SMF.

FMF transmission has been exploited to demonstrate ASE of 32 bits/s/Hz making use of six spatial modes (Ryf et al., 2013). While ASE of 109 bits/s/Hz has been accomplished by employing 12 cores in MCF transmission (Qian et al., 2012). The highest transmission rate reported for a 3 mode fiber transmission is 57.6 Tb/s over a distance of 119 km FMF (Sleiffer et al., 2012). Studies on hybrid multiplexing of multiple modes of MMF and multiple cores of MCF reported an ASE of 247.9 bits/s/Hz, which is 3 times greater than the 1 Pb/s reported in 12 core transmission (Takara et al., 2012). Uden et al., utilized WDM with 50 wavelength channels in a few-mode multicore fiber to demonstrate a data transmission of 255 Tb s-1 over a 1km length of fiber (Van Uden et al., 2014). Further, spatial modes up to 15 have been reported for FMF transmission (Fontaine et al., 2015), and the length of fiber with 3 modes over 3500 km (Rademacher et al., 2018b) and 6 modes over 1600 km (Weerdenburg et al., 2017) have been reported. Most recently Soma et al., demonstrated 10.16-Peta-B/s ultradense SDM transmission over 6-mode 19-core fiber across the C+L band, which is one of the highest record fiber capacity reported in recent time (Soma et al., 2018b).

Mux/demux: a crucial component for SDM system

The combinations of LP01 and modes including polarization and degeneracy are used to attain MDM in either two mode fiber (TMF) or FMF. The possibility of using MDM and multiple input multiple output (MIMO) to transmit data using FMF has been reported by different groups (Fontaine et al., 2015; Huang et al., 2015; Luís et al., 2018; Ryf et al., 2012; Shibahara et al., 2018; Sleiffer et al., 2012; Takara et al., 2012; Winzer et Foschini, 2011). Intense research is in progress on various aspects like fiber design (Grüner-Nielsen et al., 2015), amplification (Genevaux et al., 2015), transmission rate (Chen et al., 2015a) etc. for further advancements in FMF transmission technology. Some of the already existing FMF designs have used either with the step-index profile or graded-index profile. Fabrication of step-index profile is easy and does not require advanced processes such as stack and draw technique (needed for some micro structured and hollow core fibers) whereas, in graded index profile, differential modal delay (DMD) within the modes can be reduced to a great extent. Desirable FMF designs will be engineered to lower the DMD to a much lower level, decrease the non-linearity in fiber and also to minimize the propagation near levels currently possible with SMF(Ferreira, Fonseca et da Silva, 2014; Yaman et al., 2010).