Low Loss Ultra-Small Core Pitch All-Fiber Fan-In/Fan-Out Device For Coupled-Core Multicore Fibers

We designed and fabricated all-fiber fan-in/fan-out (FI/FO) devices using a fused taper technique for square lattice structure coupled-core multicore fiber (CC-MCF). A vanishing core bridge fiber with a diameter of 80 μm was tailor-designed to minimize both the mode field mismatch and the required tapering ratio at the same time. In experiment, we succeeded in fabricating FI/FO devices with a core pitch as small as 17 μm and achieving practically low insertion losses of 1.43 dB and 2.28 dB for fan-in and fan-out devices, respectively. The fabricated FI/FO devices can well multiplex and demultiplex 1550 nm light into and out from the CC-MCF without obvious total power fluctuation even when an external perturbation is intentionally imposed on the CC-MCF. The proposed FI/FO device is very useful for both the study and application of CC-MCFs which usually have ultra-small core pitches.


I. INTRODUCTION
I N ORDER to satisfy the dramatically increasing demands for data transmission capacity, multicore fibers (MCFs) are considered to be promising transmission media. They have been intensively investigated to keep the system within a manageable power consumption and cost budget [1], [2], [3], [4], [5]. As vital interface devices to couple light between MCFs and traditional single-core fibers (SCFs), the fan-in/fan-out (FI/FO) devices are indispensable for applications using MCFs. Recently, coupledcore multicore fibers (CC-MCFs) have drawn a lot of attention due to their distinctive advantages such as reduced group delay spread [6] and mitigation of fiber nonlinearities [7] which make them particularly attractive for long-haul transmission applications [8], [9], [10], [11]. However, CC-MCFs usually require a core pitch design smaller than 30 µm to realize the desirable inter-core mode coupling, which is challenging for the design and fabrication of compatible FI/FO devices. Manuscript  Several FI/FO devices have been demonstrated based on allfiber [12], [13], [14], [15], [16], [17], [18], femtosecond laser inscription [19], [20], and lens optics [21], [22] method. The FI/FO devices fabricated by the femtosecond laser inscription technique can achieve high alignment accuracy and integration density. However, it is difficult to reduce their insertion losses due to the mode field mismatch and the nature of point-by-point fabrication. The lens type FI/FO devices can achieve low loss, while they require precise alignment and expensive free-space optics systems.
All-fiber type FI/FO devices have advantages such as low loss, low polarization-dependent loss, and high compatibility with MCFs and SCFs. Typical all-fiber type FI/FO devices include fiber bundle [12], [13] and fused taper types [14], [15], [16], [17], [18]. The fiber bundle type FI/FO devices are usually fabricated by inserting a bundle of etched fibers into the ceramic ferrule. This process is challenging for MCF with a core pitch less than 30 µm because the etched fiber is very fragile. Some fused taper type FI/FO devices designed for CC-MCF with a small core pitch of 20-30 µm were demonstrated with commercially available few-mode fiber [8], [9] or graded-index multi-mode fiber [18] as bridge fibers for tapering. However, these bridge fibers were difficult to simultaneously achieve low coupling loss with SCFs and MCFs due to the mode field mismatch.
On the other hand, using vanishing core fiber (VCF) as bridge fibers for fused taper type FI/FO devices, which was first proposed in [15], has been proved to be an effective way to realize low-loss connection between SCFs and MCF. The VCF design usually consists of three refractive index layers. Before tapering process, the inner two layers are used to minimize the mode field mismatch with the SCFs. After tapering process, the overall structure shrinks in size. The inner two layers constitute the new core and the outmost layer becomes the new cladding. The tapered VCF is used to realize mode field matching with MCFs. Several types of VCF designs [15], [16], [17] have been reported in FI/FO devices for weakly coupled multicore fibers with core pitches larger than 35 µm.
In this work, we demonstrate all-fiber FI/FO devices for a square lattice structure coupled-core four-core fiber (CC-4CF) with the smallest core pitch (17 µm), of its kind to the best of our knowledge. A new vanishing core bridge fiber (VCBF) was tailor designed with a thin cladding of 80 µm to minimize the mode field mismatch for this fused taper type FI/FO device. In experiment, the fabricated fan-in and fan-out devices achieved

II. PRINCIPLE AND DESIGN
The proposed all-fiber FI/FO device for interfacing SSMFs and a CC-4CF is shown in Fig. 1. It consists of a glass capillary and bridge fibers with a vanishing core design. The close-packed VCBFs are inserted in the capillary and tapered down to match the structure of CC-4CF. The taper ratio of the FI/FO device is calculated from the ratio of the VCBF diameter to the core pitch of the CC-4CF. Four SSMFs are spliced to VCBFs on the left and the end-face of the device consisting of a tapered VCBF bundle is spliced to a CC-4CF on the right, respectively. The FI/FO device is designed for a fabricated CC-4CF with a square lattice structure whose cross-sectional image is shown in Fig. 1. The core pitch and cladding diameter of the CC-4CF are 17 µm and 125 µm, respectively. The CC-4CF used in this paper was designed and fabricated by ourselves. Individual core design of the CC-4CF is in accordance with ITU-T recommendation G.652D. The optimum core pitch of 17 µm is used to realize low spatial mode dispersion of the designed CC-4CF.
In order to minimize the coupling loss of the FI/FO device with both the SSMF and CC-4CF, a VCF approach was used to realize mode field matching. Fig. 2 shows the designed VCBF with a four-layer step-index design. The trench structure with an index of n 3 is added to reduce the bending loss and prevent mode field leakage to outer cladding. Moreover, improvements of 80µm-diameter cladding have been made in our VCBF design to make it suitable for the FI/FO device with small core pitch. The small core pitch requires a large taper ratio. In general, the larger taper ratio makes the realization of the adiabatic taper condition more difficult. The designed thin cladding reduces the taper ratio to about 4.7, which decreases by 1.56 times compared with the  usual 125-µm cladding. Overall, the improved design facilitates the fabrication process. The parameters of each layer were tailor designed to make the designed VCBF suitable for the FI/FO with small core pitch of 17 µm. The parameters of the designed VCBF are listed in Table I. Fig. 2(c) shows the mode field diameter (MFD) of the designed VCBF at different taper ratios calculated by a finite element method. The coupling loss between the VCBF and the SSMF as a function of taper ratios was calculated by integral overlap of the mode fields. As shown by the red line in Fig. 2(c), theoretical coupling losses smaller than 0.1 dB at both sides of the FI/FO device can be achieved when the taper ratios of the VCBF are 1 and 4.7 (same for the FI/FO device), at which MFDs of the VCBF at 1550 nm before and after tapering are 9.9 µm and 10.1 µm, respectively.

III. FABRICATION
The VCBF was fabricated by a state-of-the-art plasma chemical vapor deposition (PCVD) method. Its cross-sectional micrograph and measured index profile are shown in Fig. 3. The measured MFD of the VCBF is 9.1 µm at 1550 nm, and both the measured index profile and MFD agree well with the  designed ones. The attenuation of the VCBF was measured to be 0.24 dB/km at 1550 nm using an optical time-domain reflectometer (OTDR). The splicing loss between the fabricated VCBF and the SSMF was measured to be 0.11 dB. Fig. 4(a) shows the fabrication schematic of the FI/FO device. First, four VCBFs were inserted into a fluorine-doped (F-doped) capillary and co-tapered to a ratio of about 4.7 using a Fujikura LZM-100 CO 2 laser processing station. In order to achieve adiabatic tapering condition and reduce the excess loss, the length of the taper transition region is set to be longer than 10 mm. Second, the tapered bundle was cleaved at the waist region using a high-precision fiber cleaver (Fujikura CT106), and the cross-sectional image of the taper end face is shown in Fig. 5(a). A perfect square lattice arrangement was successfully maintained even after tapering process with a ratio as large as 4.7 and the measured core pitch was about 17 µm with a deviation less than 0.5 µm. The near field profiles at the tapered end of the device with light input from different ports are shown in Fig. 4(b). It can be observed that the tapered bundle operates well at a wavelength of 1550 nm. To characterize its insertion loss of the individual channels, a large-area free-space photodetector was used to collect the optical power at the end face of tapered bundles. As shown in Table II, measured average insertion losses at 1550 nm are 0.21 dB and 0.23 dB for the fan-in and fan-out devices, respectively.
The tapered bundle was then spliced to a piece of 5-meterlong CC-4CF using a Fujikura FSM-100P+ splice machine to   Fig. 5(b), a uniform interface between the tapered bundle and the CC-4CF was achieved, indicating no deformation during the splicing. The insertion loss of the FI/FO devices was measured and the result is shown in Table III. A large-area free-space photodetector was adopted to receive optical power from all cores of the CC-4CF. The average insertion losses are 1.43 dB and 2.28 dB at 1550 nm for the fan-in and fan-out devices, respectively. The insertion losses of the fabricated fan-in and fan-out devices are mainly induced by the fiber deformation during the tapering process and the misalignment during the splicing process. These can be mitigated by further optimizing the fabrication process.

IV. CHARACTERISTICS OF FI/FO DEVICES
The characteristics of a pair of FI/FO devices were investigated using an experimental setup as shown in Fig. 6. The fan-in and fan-out devices were spliced to the two ends of a piece of 5-m-long CC-4CF, respectively. We used an external-cavity laser (Osics T100) operating at a wavelength of 1550 nm as the light source. The light was coupled into the CC-4CF through the fan-in device and a 4-channel power meter (Santec MPM-200) was used to simultaneously receive the optical power of the four output ports of the fan-out device. Due to the intrinsic inter-core  mode coupling, the power in the cores of CC-4CF will exchange among all cores.
The insertion losses of the FI/FO devices are defined as where P i,j and IL i,j represent the received optical power and insertion loss, and the subscripts represent the output port i(i = 1, 2, 3, 4) and input port j(j = 1, 2, 3, 4), respectively. P in is the input power, which is set to be 1 mW in our experiments. The total received power P T,j is defined as the sum of four outputs for each input port, and the total insertion loss IL T,j is calculated from (2). Fig. 7 shows the measured output power at the fan-out ports when light is input from different ports in an observation time of 300 seconds. Although the light is mainly coupled into one of the cores of the CC-4CF at the fan-in side, it will be randomly coupled to the other cores due to the intrinsic inter-core mode coupling and fluctuations in the received power of individual cores can be observed. On the other hand, P T,j remains stable over time with a standard deviation of less than 0.022 dB. The total insertion losses for different input ports are shown in Table IV. The maximum IL T,j achieved is about 5.0 dB, which is practically low for both characterization and transmission  purposes. The wavelength dependence of the FI/FO link was investigated. The total insertion loss and the fluctuation of each port are lower than 5.2 dB and 1 dB over the C-band.
The characteristic of the FI/FO devices with perturbation was experimentally investigated and the results are shown in Fig. 8. Random perturbation was intentionally applied to the CC-4CF by continuously scratching the fiber. This perturbation includes not only vibration, but also the continuous deformation of the fiber such as twisting and bending. It can be observed that although there is a dramatic power coupling among all cores, the optical power in each core of the CC-4CF can still be well demultiplexed by the fan-out device with almost constant total received power. As shown in Fig. 8(b), the average total insertion loss during the perturbation is 4.71 dB with a standard deviation of about 0.151 dB, which probably resulted from the differences in insertion loss among individual ports of the FI/FO devices and can be minimized by further improving the fabrication process. After the external perturbation was stopped, the relative power in each port did not return to its initial level because the inter-core coupling state of the CC-4CF was changed due to the external disturbance such as changes in twisting and bending conditions. However, the total power of the 4 cores remains almost the same after the perturbation and the level of fluctuation in the power of each core would gradually return to its original value when the fiber was stable. In the case of real application, the coupling status of the CC-4CF will not change dramatically in a short time, and the gradual changes in coupling status which may result in changes in received power of individual core can be equalized by implementation of the digital signal process (DSP) technique.

V. CONCLUSION
We designed and fabricated all-fiber FI/FO devices for a square lattice structure CC-4CF with a core pitch as small as 17 µm, which shows practically low average insertion loss of 1.43 dB and 2.28 dB for fan-in and fan-out devices, respectively. It was experimentally demonstrated that the light can be well multiplexed and demultiplexed into and out from the CC-4CF with a small total power fluctuation even under strong random energy exchange among cores when external perturbation was imposed on the CC-4CF. As a result, the proposed FI/FO device is very useful for both the study and application of CC-MCFs with an ultra-small core pitch design.