Low-insertion-loss femtosecond laser-inscribed three-dimensional high-density mux/demux devices

Abstract. Recently, transmitting diverse signals in different cores of a multicore fiber (MCF) has greatly improved the communication capacity of a single fiber. In such an MCF-based communication system, mux/demux devices with broad bandwidth are of great significance. In this work, we design and fabricate a 19-channel mux/demux device based on femtosecond laser direct writing. The fabricated mux/demux device possesses an average insertion loss of 0.88 dB and intercore crosstalk of no more than   −  29.1  dB. Moreover, the fabricated mux/demux device features a broad bandwidth across the C+L band. Such a mux/demux device enables low-loss 19-core fiber (de)multiplexing over the whole C+L band, showing a convincing potential value in wavelength-space division multiplexing applications. In addition, a 19-core fiber fan-in/fan-out system is also established based on a pair of mux/demux devices in this work.


Introduction
Over the last couple of years, the communication capacity of single-mode fiber (SMF)-based systems has been rapidly pushed toward its theoretical limit 1 due to the explosive growth in demand for optical communication traffic. Thus, spacedivision multiplexing (SDM) has been proposed and demonstrated to be a promising technology to further increase the capacity of a single optical fiber. 2,3 There are currently two kinds of fiber-based SDM communication systems. One propagates orthogonal spatial modes loaded with different signals in the single core of a multimode fiber [4][5][6][7] and the other transmits different signals in diverse cores of a multicore fiber (MCF). [8][9][10][11][12][13][14] Remarkably, high communication capacities with long-distance transmission have been accomplished by applying SDM transmission in MCFs. For instance, 2.15 Pb/s transmission was demonstrated applying a 22-core homogeneous single-mode MCF and a wideband optical comb. 10 105.1 Tb/s, 14,350-km transmission was achieved using a 12-core MCF. 15 One important component for such an MCF-based SDM system is a fan-in/fan-out (FIFO) device with broad bandwidth, low insertion loss, and low crosstalk, which acts as an SDM mux/ demux device. With the development of MCF-based SDM technology, several kinds of FIFO devices have been proposed and fabricated. Generally speaking, they can be divided into three kinds: traditional free-space FIFO couplers, 16,17 fiber-based FIFO couplers, [18][19][20][21] and on-chip FIFO couplers. [22][23][24][25][26][27][28] Traditional free-space FIFO couplers based on free-space optical devices have been widely utilized owing to their high efficiencies. However, they are too bulky. Fiber-based FIFO couplers can be divided into two kinds: tapered MCF connectors 18,19 and etched fiber bundles. 20,21 Such two kinds of fiber-based couplers feature low insertion loss, small size, and low crosstalk. Thus, they are capable of transmitting different signals in different spatial channels in long-haul MCFs. Compared to these methods, on-chip FIFO solutions are also preferred because, from an integration point of view, other functionalities can be integrated with them. Several kinds of integrated FIFO chips based on different platforms have been reported, such as on-chip grating coupler arrays on silicon-on-insulator (SOI) platforms 22,23 and silicon nitride platforms, 24 FIFO polymer waveguides, 25,26 and ultrafast laser-inscribed FIFO waveguides based on glass platforms. 27,28 Among them, inscribing waveguides in a glass chip is a competitive solution, [29][30][31] possessing the characteristics of low cost and broad bandwidth. Unfortunately, as the number of channels increases, the inside of the device will be more crowded, which will lead to a higher bend loss. A longer length of the device will provide more space; however, it means a higher path loss. Thus, a reasonable design of the device is of great significance because it aims to minimize the sum of path loss and bending loss. Moreover, a high-performance light-field matching of device and commercial SMF array also affects the loss.
In this work, we design and fabricate 19-channel FIFO devices based on femtosecond laser direct writing. The insertion losses of 19 channels are evaluated to be no more than 1.2 dB at 1550 nm with an average value of 0.88 dB. In addition, insertion losses of specific channels are characterized to be almost the same over the whole C+L band, proving the broad bandwidth of this FIFO device. Intercore crosstalk of the FIFO device is also evaluated to be no more than −29.1 dB. Applying a pair of FIFO devices and a 1-km 19-core fiber, a 19-core fiber FIFO system is established. Remarkably, such a femtosecond laserinscribed 19-channel FIFO device, which features low insertion loss, broad bandwidth, and low crosstalk, may be able to promote applications in different areas, such as long-haul SDM data transmission, [8][9][10][11][12][13][14] quantum information processing, 32,33 MCFbased sensing, 34,35 and imaging. 36,37 . placed on a three-dimensional stage. The femtosecond laser beam is tailored by a slit and then vertically focused into the sample through a 50× objective (NA ¼ 0.42). The FIFO device is fabricated along the 20-mm side. Utilizing an LED lighting system and a visible CCD, the femtosecond laser direct writing process is monitored in real time. During the fabricating process, the glass sample is translated using a high precision XYZ air-bearing stage. The femtosecond laser beam is tailored by a slit so that approximately circular cross sections can be formed on the end facet of the glass. An approximately circular shape of the FIFO waveguide cross section will promote the mode field match between the FIFO device and 19-core fiber. 38 Moreover, through adjusting the parameters of the femtosecond laser direct writing system, the size of the approximately circular cross sections is adjusted so that high-performance light-field matching of the SMF array and the device can be achieved. As a result, the coupling loss will also be lower. (More details and parameters of the fabrication process can be found in Table S1 in the Supplemental Material.)

Design and Fabrication of the FIFO Device
For traditional ultrafast laser-inscribed FIFO devices fabricated in quartz glass, they are usually fabricated in the thermal fabrication regime (e.g., 5.1-MHz ultrafast laser). In such regimes, cumulative effects take place. The low pulse energy of the applied high-frequency lasers requires a high NA objective lens to achieve a few-hundred micrometers' vertical fabrication range. In these regimes, modifications are of a circular cross section due to the isotropic heat diffusion. In contrast, we choose a low-frequency (100 kHz) laser to accomplish the material modification produced by the individual pulses. The applied low NA objective lens enables the vertical fabrication range up to a few micrometers. Thus, waveguides inscribed in low-frequency regimes usually exhibit significant loss and strong core asymmetry with a large aspect ratio. However, we apply the slit beam shaping technique to fabricate waveguides with a circular cross section and low loss over a large depth range. In addition, the main parameters of the fabrication process are scanned, and the comprehensive optimal value is selected. In this way, we achieve both the ultralow propagation loss (∼0.1 dB∕cm) and high mode field matching performance of quartz glass FIFO devices.
The concept of a fabricated 19-channel FIFO device is indicated in Fig. 1(b). A side-by-side 19-channel Gaussian beam array that outputs from a SMF array is incident into the FIFO device. As a result, different beams of the beam array are confined to transmit along different channels in the FIFO device. The transmission of different beams in their corresponding channels is, in other words, a process of beam array redistribution in three dimensions. The redistributed beam array forms a distribution that matches a 19-core fiber. Therefore, light beams output from a SMF array can be accurately coupled into diverse cores of a 19-core fiber, as displayed in Fig. 1(b). By modulating different signals onto different beams transmitting in the SMF array, such a 19-channel FIFO device can be used as an SDM multiplexer that connects a commercial SMF array and a 19-core fiber. Figure 2(a) shows the designed end facets of the input and output of the 19-channel FIFO device, while Fig. 2(b) shows the captured output end facet by applying a microscope. The sideby-side channel distribution at the input of the FIFO device possesses a channel spacing of 127 μm to match a commercial SMF array. The channel distribution at the output end corresponds to the core distribution of a 19-core fiber so that light beams output from the FIFO device can be coupled into the 19-core fiber with high efficiency. To clarify the corresponding relationship between the input channels and output channels of the FIFO device, we number the channels of the FIFO device from 1 to 19, as displayed in Fig. 2(a). Figure 2(c) shows the captured cross section of the utilized 19-core fiber with size information marked.
The fundamental principle for fabricating such a 19-channel FIFO device relies on the direct writing of 19 waveguides in glass. The 19 waveguides inside the FIFO device get closer to each other along the track from its input to output, as indicated in Fig. 1(b). In what follows, we explain the design of the FIFO device in detail.
First, the design of the input and output channel distribution of the 19-channel FIFO device is required. The channel spacing of the input end of FIFO device is set to be 127 μm to match the distribution of applied SMF array. The channel distribution of the output end of the FIFO device is required to match the core distribution of the applied 19-core fiber. Figure 2(c) shows the captured cross section of the applied 19-core fiber, which possesses a 9-μm core diameter and a 125.32-mm cladding radius. The 19 cores can be divided into three types of cores located in three concentric circles: that is, one core in the middle of fiber, six cores in the second circle, and 12 cores in the third circle. A 42.655-μm spacing exists between adjacent cores in the second circle, while a 44.16-μm spacing exists between adjacent cores in the third circle. In addition, a 42.655-μm spacing exists between the middle core and cores in the second circle. An 85.31-μm spacing exists between the middle core and cores in the third circle.
After finishing the design of the input and output channel distributions of the 19-channel FIFO device, tracks of the 19 waveguides inside the FIFO device should be determined, with the principle of independent transmission. Ultraviolet (UV) optical quartz glass with a size of 20 mm × 50 mm × 1 mm is utilized as the substrate material. Its 20-mm side is chosen as the waveguide length. The channel number of input end facet and output end facet is shown in Fig. 2(a). The channel 10 here is set as a straight waveguide, while other channels are designed to be bending waveguides. Tracks of 18 bending channels should be designed obeying the following rules: (i) confirm the plane on which the track is located. Assuming that input and output beam of the FIFO device propagates along horizonal lines, as shown in Fig. 2(d), a specific plane is determined that contains both the input line and output line; (ii) plotting the following 2D track on this specific plane: where L denotes the horizonal length of the track, h corresponds to the vertical length of the track, and x denotes the horizonal coordinate. Then the radius of curvature of the waveguide can be expressed as where R denotes the radius of curvature of the waveguide. (iii) After plotting such a 2D track [like the orange track in Fig. 2

Experimental Setup of 19-Channel FIFO System
To achieve the characterization of the 19-channel FIFO device, we use a pair of 19-channel FIFO devices to establish a 19-channel FIFO system. The schematic of the 19-channel FIFO system is shown in Fig. 3(a). A C+L band tunable laser is connected to an optical coupler (OC) so that light beam output from the laser is split into 19 beams transmitting in an SMF array. The 19-channel fiber-guided beams then output from the SMF array and enter the FIFO device. During the transmission in the FIFO device, the 19-channel beam array is redistributed to form a distribution that matches the 19-core fiber. Then the 19channel beam array is coupled into a 1-km 19-core fiber. The applied 19-core fiber possesses a measured transmission loss of 0.26 dB/km. To demultiplex the 19-channel beams output from the 1-km 19-core fiber, another FIFO device is reversely inserted into the experimental setup, as shown in Fig. 3(a). Finally, the demultiplexed side-by-side 19-channel beams that output from the FIFO device are incident into another SMF array, thereby demultiplexing the 19-channel beams into 19 different SMF channels.
In the experiment, all the coupling processes are monitored in real time by a visible CCD. To accomplish accurate coupling alignment, the two SMF arrays and the 1-km 19-core fiber are placed on six-axes stages, which enable precise control of three-dimensional angles and displacements. In addition, two rotational fiber holders are utilized to rotate the two ports of the 19-core fiber to finish the channel distribution match between the 19-core fiber and the FIFO devices. Once the 19channel FIFO system is successfully established, all the devices (i.e., the two SMF arrays, the two FIFO devices, and the 1-km 19-core fiber) can be integrated by applying a UV glue so that the applied six-axes stages, rotational fiber holders, and monitoring CCD are freed.

Characterization of the FIFO Device
To obtain the performance of the fabricated 19-channel FIFO device, a comprehensive characterization is carried out, including capturing the output light field, measuring the insertion loss, characterizing the bandwidth, and evaluating the crosstalk of the 19-channel FIFO device.
The intensity profile of the output light field of the 19channel FIFO device is displayed in Fig. 4(a). It is captured after the FIFO device output by using a 50× objective lens and a 1550 nm CCD. Considering that all the 19 channels possess the same input power, the output intensities of 19 different channels are similar, as illustrated in Fig. 4(a). Such a uniform intensity profile proves that accurate coupling is achieved. The intensity profile of the 19-channel beam array obeys a distribution that matches the 19-core fiber, thereby coupling the 19-channel beams into a 1-km 19-core fiber. As a result, the captured intensity profile is indicated in Fig. 4(b). In Fig. 4(b), relatively obvious intensity disparity exists between different channels. This is mainly due to the imperfect coupling process. Higher-precision coupling devices (e.g., better rotational fiber holders, six-axes stages) may be a potential solution to this power disparity problem.
The insertion losses of a FIFO device result in lower received power of an SDM communication system, thereby affecting the bit error rate (BER) performance of the SDM communication system. Here, we evaluate the insertion losses of 19 different channels of the FIFO device at 1550 nm. Light beams output from the FIFO device are coupled into a short SMF for insertion loss characterization. During the loss characterizations, the input power only exists in the channel that is under test. First, the input intensity of a specific channel is measured by recording its power at the output of the OC using a fiber power meter. Then, the output intensity of this channel is measured at the output of the SMF by applying the same fiber power meter. Finally, the insertion losses of the 19 different channels are calculated by means of subtracting the input intensities from their output intensities. The measured insertion losses of 19 different channels are evaluated to be no more than 1.2 dB at 1550 nm, which can be found in Fig. 5(a). It is worth mentioning that the measured insertion losses contain coupling losses and linear propagation loss of the FIFO waveguide. Thus, a coupling loss about 0.3 dB/facet and a linear propagation loss of about 0.1 dB/cm are evaluated for channel 10 at 1550 nm. Central channels feature lower insertion losses due to their lower bending losses and shorter paths. Compared to central channels, the low increase of channel path length of the farthest channel will not affect the insertion loss as much, due to the linear propagation loss of ∼0.1 dB∕cm. That is, the bending loss is the main reason that causes the higher insertion loss of the farthest channel.
The wavelength-division multiplexing (WDM) technology is an efficient approach to improve the optical communication capacity. Hence, the bandwidth of an SDM FIFO device is required to be large enough so that the SDM technology can be used together with WDM to further increase the capacity of the optical network. Fortunately, the fabricated 19-channel FIFO device on a glass chip has the advantage of broad bandwidth. Therefore, we measure the insertion losses of three different channels (central channel, channel 10; farther channel, channel 14; the farthest channel, channel 19) over the whole C+L band to prove it. Insertion losses of these three different channels are evaluated at diverse wavelengths from 1528 to 1625 nm with a measurement interval of 1 nm, as illustrated in Fig. 5(b). Insertion losses at different wavelengths almost retain the same value, demonstrating the C+L bandwidth of the fabricated FIFO device. Thus, the femtosecond laser-inscribed 19-channel FIFO device can be utilized as a (de)multiplexer in wavelengthspace division multiplexing (WSDM) applications.
The interchannel crosstalk of an SDM system describes how much power in the input channel couples to other channels. In addition to insertion losses, the interchannel crosstalk is another factor that determines the performance of an SDM communication system. The interchannel crosstalk can be evaluated by measuring the output power of the input channel and the output power of other channels. Here, we characterize the intercore crosstalk of the 19-channel FIFO device (for details about the crosstalk characterization, refer to Fig. S1, Fig. S2, and   Table S2 in the Supplemental Material). For a specific channel, we choose the biggest crosstalk between it and other channels as the crosstalk of this channel, as shown in Fig. 6. All the channels possess crosstalk of no more than −29.1 dB. Such a low crosstalk enables high-speed signal transmission in a 19-core fiber.

Conclusion and Discussion
We designed and fabricated a 19-channel FIFO waveguide based on femtosecond laser direct writing. The insertion losses of 19 channels are evaluated to be no more than 1.2 dB at 1550 nm, with an average value of 0.88 dB. Intercore crosstalk of the FIFO device is also evaluated to be no more than −29.1 dB. In addition, the broad C+L bandwidth of the 19-channel waveguide is characterized and demonstrated by measuring the insertion losses of different channels at diverse wavelengths. Applying a pair of waveguides and 1-km 19-core fiber, a 19-core fiber FIFO system is established, showing the practical value of the fabricated FIFO device.
Compared to free-space FIFO solutions 16,17 and fiber-based solutions, 18,19 our FIFO mux/demux devices have the advantages of small size and low cost. In contrast to on-chip FIFO solutions based on other material platforms, [22][23][24][25][26] our FIFO mux/demux devices based on glass combines the advantages of low insertion loss and broad bandwidth. Even compared to other ultrafast laser inscribing methods, 27,28 our FIFO devices achieve the lowest propagation loss of ∼0.1 dB∕cm. Such a femtosecond laserinscribed 19-channel FIFO device may pave the way for many applications. For instance, the FIFO device can be applied as a (de)multiplexer in large-capacity long-haul WSDM transmission based on MCF 8-13 due to its broad bandwidth and low interchannel crosstalk. In addition, multiport beam splitters can be achieved by a specially designed FIFO device, which is a cornerstone device for high-dimensional quantum information tasks. 39 Such a FIFO device is also capable of establishing integrated FIFO systems, so it may be helpful to fabricate integrated MCF-based sensors and endoscopes. In addition, a photonic lantern can also be fabricated based on femtosecond laser direct writing, [40][41][42][43][44][45] which is desirable for SDM applications based on higher-order modes.