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A randomly coupled 12-core erbium-doped fiber amplifier (EDFA) operating in the C- or L-band is investigated. We experimentally demonstrate that both C and L-band operation can be supported with the 12-core EDFA by adjusting the erbium-doped fiber (EDF) length. Evaluations of the amplification characteristics of the proposed EDFA showed that a modal gain of more than 20 dB or 15 dB, a noise figure of about 5 dB or 6 dB, and a mode-dependent loss penalty of less than 1 dB or 2 dB were respectively obtained over the whole C-band (1530–1565 nm) or L-band (1570–1605 nm) with 8- or 60-m EDF. Transmission experiments showed the applicability of our EDFAs for inline amplified transmission systems. We also examined the impact of MDL on the power efficiency of the randomly coupled multi-core EDFA for achieving power-efficient long-distance SDM transmission.

We realized a four-core fiber with 125 μm-cladding and four-holes using a novel fabrication technique, namely the over-cladding bundled rods (OBR) method. Four air holes were successfully introduced by using the OBR method without the need for a drilling process, and this enabled a 10 dB/km improvement in the crosstalk characteristic. As a result, we realized four-core fibers with a 125 μm cladding diameter and operating in both the single mode and 2 linear polarized (LP) modes in the C + L band were realized while maintaining an inter-core crosstalk of less than −30 dB/100 km. The OBR method is a promising technique for effectively realizing a multi-core fiber with ultra-low crosstalk and a high spatial density.

We briefly review the recent progress of the coupled multi-core fiber technologies in terms of the fiber design, cabling and amplification for high spatial density and energy efficient space division multiplexing (SDM) transmission system.

We propose a technique for reducing the differential modal gain (DMG) that occurs in a two-mode erbium-doped fiber (2M-EDF) by inscribing voids in the core center of a 2M-EDF with a femtosecond laser. We show that an empty void inscribed at the core center can attenuate the linearly polarized () mode selectively while suppressing excess loss for the mode. We also reveal that DMG can be controlled by means of void diameter. The longitudinal position dependence of the void in a 2M-EDF was also investigated considering its influence on gain and noise figure (NF) characteristics. Finally, we realize a sufficiently low DMG of less than 0.5 dB in the full C-band as well as a sufficient gain and NF by using the proposed technique.

We experimentally and theoretically investigate the effect of an air hole between cores in a multi-core fiber (MCF) on the suppression of crosstalk (XT). We clarify that XT decreases in proportion to the air hole diameter and that XT of about 20 dB in an MCF can be suppressed by employing just one air hole between cores. We confirm that the calculated XT suppression value is in good agreement with the measured value. We also clarify numerically that the minimum distance between cores satisfies the required XT without reducing the effective area by using an air hole. Next, we consider the influence of air holes on macro-bending loss and cutoff wavelength. We confirm that none are related to the arrangement of the air holes. Moreover, we discuss the relationship between the effective area and the core multiplicity factor (CMF) against several core arrangements. As a result, we show that the CMF in MCF with air holes can increase by 1 or more compared with that without air holes.

We demonstrate a ring-resonator-based gain-clamped 4-LP-mode erbium-doped fiber amplifier (EDFA). In this study, we show that a gain-clamped EDFA can reduce temporal modal gain variation simultaneously in all modes. We experimentally evaluate the transient response of a 4-LP-mode EDFA and succeed in reducing the modal gain variation for all modes at 1530 and 1550 nm from 4.0 to 1.1 dB and from 2.8 to 0.7 dB, respectively, by using a gain-clamping technique.

We demonstrate a randomly coupled 12-core erbium-doped fiber (EDF) based cladding pumped optical amplifier. In this paper, we show that a randomly coupled multicore EDF amplifier with a cladding pumped configuration can realize high amplification efficiency and a low mode-dependent gain (MDG). We experimentally evaluate the amplification characteristics and confirm a flat gain of more than 17 dB with a sufficiently low MDG of less than 1 dB over the full C -band and an average noise figure of 6.2 dB. In addition, we describe a transmission experiment in which we inserted a randomly coupled 12-core multicore fiber (MCF), and demonstrate that the proposed 12-core MCF can transmit 12 channels.

A multicore fiber design with more than 10 coupled cores is investigated numerically and experimentally. It is revealed that a randomly coupled 12-core structure with a square lattice core arrangement can be realized within a 125 μm cladding by employing fiber twisting of more than a few π rad/m. A 125 μm-clad twisting-rate-controlled 12-core fiber is then fabricated and a relative core density of 12 compared with conventional single-mode fiber is achieved while maintaining a low spatial mode dispersion characteristic of 8.4 ps/√km at 1550 nm. Finally, we experimentally evaluated the performance of 12 spatial channels by conducting a 24 × 24 MIMO transmission experiment.

We report the design, fabrication, and evaluation of a cladding pumped two-LP-mode six-core moderately coupled erbium-doped fiber (EDF) amplifier. In this study, we show that moderately coupled multi-core EDF can realize a high pump power density in a cladding by allowing core-to-core power coupling. We also experimentally evaluate the amplification characteristics and confirm an average mode-core gain of 18 dB, a differential mode-core gain of 4 dB, and an average noise figure of 7 dB in the C-band. Moreover, we confirm an average mode-core gain of 15 dB, a differential mode-core gain of 5 dB, and an average noise figure of 7.5 dB in the L-band.

A coupled 12-core fiber amplifier with the highest optical power conversion efficiency of 10.2% is achieved among the reported C-band cladding-pumped amplifiers. Potential as full C-band inline amplifier is confirmed using full coupled-core SDM link

We report a 5-mode erbium-doped fiber (EDF) amplifier operating in the L -band (1570–1605 nm) designed to extend the bandwidth of mode division multiplexed transmission. We design an EDF with a center depressed core index and a ring-doped erbium concentration profile to obtain a low differential modal gain (DMG) and a low splice loss with a graded-index fiber as a transmission line. Then, we fabricate a center depressed core EDF and realize a gain of 20 dB for all the modes and a low DMG with a deviation of less than 3 dB in the 1570–1605 nm range with a 48-m-long erbium-doped fiber when the pump mode is ${\rm{LP}}_{{01}}$ mode.

We investigate a few-mode multicore fiber structure suitable for realizing a high spatial density while maintaining a feasible cladding diameter. First, we discuss the impact of cladding diameter on mechanical reliability, and set a target cladding diameter of less than 250 μm to maintain a comparable reliability to conventional fiber. We then optimize the 6-mode core profile to obtain a low differential mode delay (DMD) characteristic and reveal that a hexagonally arranged 19-core structure can realize more than 100 spatial channels with a cladding diameter of less than 250 μm. Finally, we fabricate 6-mode 19-core fiber with a 246 μm cladding diameter and realize a relative core multiplicity factor of more than 60 with a low loss and a DMD of less than 0.24 dB/km and 0.33 ns/km, respectively.

We designed and fabricated a low loss hole-assisted few-mode fiber (HA-FMF) taking the surface imperfections of air holes into consideration. We employed a simulation model with a “defective layer” to estimate the modal loss dependence on the surface imperfections of air holes. We confirmed that the experimental result agrees well with the theoretical calculation. The theoretical studies suggest that the radius of an inscribed circle linking the air holes C has the greatest influence on the additional loss due to the surface imperfections of the air holes in the HA-FMF. We designed and fabricated the hole-assisted two-mode fibers (HA-TMF) according to our theoretical study. We obtained an HA-TMF with losses of 0.20 dB/km in the $\rm{LP}_{\rm{01}}$ mode and 0.26 dB/km in the $\rm{LP}_{\rm{11}}$ mode in the 1550 nm band. The additional loss induced by surface imperfections can be reduced by controlling the parameter C and this improves the bending properties of the $\rm{LP}_{\rm{11}}$ mode even without employing a polishing process for the air holes. Our results show the great potential of the HA-FMF for space-division multiplexing applications.

We demonstrate a cladding-pumped L-band randomly-coupled 12-core EDFA for the first time. Flat spectra and low mode dependent loss of 1.9 dB or less in the 1570 to 1605 nm range are achieved.

We review our recent work on gain-clamped multimode erbium doped fiber amplifier to suppress temporal modal gain variation. We show a concept of a gain-clamped EDFA and report amplification characteristics for all modes.

Digital coherent transmission systems with multilevel and multicarrier modulation formats have been moving beyond 100-G systems. In this paper, we report a one-chip 400-G coherent receiver founded on a silica-based planar lightwave circuit (PLC) integration platform. To integrate a polarization beam splitter (PBS), two polarization rotators, four-optical hybrids, and 16 high-speed photodiodes (PDs) in one chip, we employed a 5%-Δ waveguide with a minimum bending radius of 300 μm, and the heterogeneous integration of InP-PDs. To achieve a compact receiver without any degradation in optical passive performance, we also investigated a three-branched spot size converter with a low loss and a high-fabrication tolerance and a waveplate type PBS with a high-polarization extinction ratio. Using these techniques, we fabricated a dual-carrier dual-polarization (DP) 16-level quadrature amplitude modulation (QAM) receiver PLC, and successfully demonstrated 32-Gbaud DP-16QAM signal demodulation for each channel.

A world record capacity 118.5 Tbit/s transmission through optical fiber with a 125 micron cladding diameter was achieved using a low loss and 316 km-long full space division multiplexing link incorporating multiple vendor technologies.

High spatial multiplicity fiber designs are presented for homogeneous and heterogeneous 4LP-mode multicore fibers (MCFs) that support six spatial modes per core. The high-spatial-density 4LP-mode MCF design methodology is explained in detail. The influence of the number of cores on the cladding diameter (Dsubcl/sub) and relative core multiplicity factor (RCMF) is investigated. The optimal core designs and MCF layouts with square and triangular lattices maintain glass fiber reliability (maximum Dsubcl/sub= 250 μm). For homogeneous 4LP-mode MCFs, a 19-core triangular-lattice fiber gives the highest RCMF of 61.7. For heterogeneous 4LP-mode MCFs, an RCMF of 65.4 is obtained for a 21-core square-lattice fiber.

Randomly coupled multi-core fiber (MCF) with a uniform twist realized lower spatial mode dispersion (SMD) and bending-radius dependence. The SMD-macrobending relationship was examined numerically and experimentally using MCFs fabricated with a preform rotation mechanism.

125-pm cladding randomly-coupled 12-core fiber is realized with the highest core density of any single-mode multi-core fiber. A spatial mode dispersion coefficient of 8.4 ps/√km is achieved by controlling the twisting rate along the fiber.

We demonstrate a cladding-pumped 2-LP mode coupled 6-core EDFA with a 125-µm cladding diameter. A differential mode-core gain of less than 4-dB and a 6.5-dB average noise figure are successfully achieved in the C-band.

We demonstrate optimized design of 4LP-mode multicore fibers (MCFs) with low differential mode delay (DMD) for high spatial multiplicity and find that 4LP-mode 19-core fiber can achieve the highest reported relative core multiplicity factor (RCMF) with a cladding diameter (CD) of less than 250 pm.

In this paper, we propose a novel method for controlling the gain spectrum of a thulium-doped fiber amplifier (TDFA) in the S-band. The conventional gain spectrum control method used for a silica erbium-doped fiber amplifier (EDFA) cannot be applied to TDFAs because of the complicated fluctuation of the TDFA gain spectrum. Our proposed method controls the gain spectrum by a combination of gain clamping and pump power adjustment. The algorithm for the method is as simple as that for the conventional EDFA gain spectrum control method. Furthermore, we describe a function for correcting the gain excursion generated by the incorporation of amplified spontaneous emission (ASE) at a signal monitoring photodiode (PD). We achieved a gain excursion of 0.35 dB against a total input signal power of 32 to 2 dBm.

We experimentally investigate an L-band 5-spatial-mode EDFA. A low differential modal gain of less than 3 dB and flat gain properties are achieved in the 1570–1600 nm range by optimizing a depressed core structure.

In the paper, we describe in detail a highly efficient S-band thulium-doped fiber amplifier (TDFA) employing a high-thulium-concentration doping technique. A TDFA with a dual-pass configuration is superior to a TDFA with a single-pass configuration with regard to power-conversion efficiency, whereas it is inferior with regard to noise figure (NF). We propose a cascaded configuration consisting of a single- and double-pass configuration, which provides high efficiency and a low NF. The configuration generated no interference noise, which can result from multipass interference (MPI) with a double-pass configuration. We achieved gains exceeding 26 dB and NFs of less than 7 dB in the 1480-1510-nm wavelength region (30-nm bandwidth). Furthermore, an 8 times 10-Gb/s transmission experiment also confirmed that there was no MPI-induced excess noise.

We studied both theoretically and experimentally the additional loss in photonic crystal fiber (PCF) that results from inner surface imperfections such as contamination and the surface roughness of air holes. We estimated the modal loss dependence of these imperfections using a model with a “defective layer” for the first time. The theoretical studies suggest that higher order modes have a larger loss due to imperfections in the air holes. By minimizing the inner surface imperfections of the six innermost air holes, we can theoretically expect any additional loss to be reduced to a negligible level. Moreover, we examined our theoretical prediction experimentally. We fabricated few-mode PCFs by employing a suitable inner surface treatment for just the six innermost holes. As expected theoretically, the transmission loss was greatly reduced by employing these processes. The lowest transmission losses in the 1550 nm band were 0.31 dB/km for the LP01 mode and 0.43 dB/km for the LP11 mode. Our theoretical model will be useful with a view to realizing few-mode PCF with a loss comparable to that of conventional fibers.

This paper details an approach for shifting the gain of an S-band thulium-doped fiber amplifier (TDFA) that employs a high thulium concentration doping technique. The technique can shift the gain from the conventional gain band to the promising longer wavelength region by forming a suitable population inversion between the 3H4 and 3F4 levels that results from cross relaxation between thulium ions. It has been shown that, unlike with an erbium-doped fiber amplifier, the interaction between thulium ions caused by the high concentration doping increases the efficiency up to a thulium concentration of 6000 ppm. The authors also describe the particular characteristics of Tm ions that appear when the fiber length and pump power ratio between the forward and backward pump powers in bidirectional pumping are varied. Furthermore, the authors optimized the thulium concentration, fiber length, and pump power ratio between the forward and backward pump powers, so that a high gain and a low noise figure (NF) in the S-band are obtained. As a result, the TDFA achieved a gain of 22 dB and NFs of < 6 dB in the 1477- to 1507-nm wavelength region at a thulium concentration of 6000 ppm