A 75-cm-long circular-core polymer waveguide compatible with standard 50-μm-core multimode fibers (MMFs) is designed and fabricated by using a direct inscribing method for high-speed and high-density optical interconnects. The fabricated waveguide has low loss (<0.044 dB/cm at 850 nm) and low crosstalk (<−34 dB with a core pitch of 62.5 μm) with a negligible coupling loss with the MMFs. It also exhibits a low bending loss (<0.08 dB/mm with a bending radius of 4 mm), which agrees well with calculated results. Error-free NRZ data transmission over the 75-cm-long waveguide at 25 Gb/s is demonstrated, and 4 × 25 Gb/s short wavelength division multiplexing (SWDM) is realized on a straight waveguide. Moreover, a two-layer waveguide and a 3-dimensional (3D) Y-splitter/combiner are also fabricated for 3D integration.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
The demands of short-reach interconnects for high-speed and high-density data transmission increase dramatically due to the rapid development of large-scale data centers and high-performance computers. For copper-based on-board electronic interconnects, the emergence of 4-level pulse amplitude modulation (PAM4) technique has extended the single-lane data rate from 25 Gb/s to 56 Gb/s. In the next-generation on-board applications with single-lane implementation with 112 Gb/s or beyond, however, the electronic interconnects suffer from inherent disadvantages in bandwidth, cost, integration density and power consumption . On the other hand, board-level optical interconnects have drawn significant attentions thanks to their advantages over electronic solutions in various aspects, such as broad bandwidth, low cost and low power consumption, as well as immunity to electromagnetic interference [2, 3]. Multimode polymer waveguides combined with 850-nm vertical cavity surface emitting lasers (VCSELs) and photodetectors (PDs) are considered a near-term solution for on-board optical interconnects. The waveguides have good compatibility with multiple substrates and can be laminated with printed circuit boards (PCBs) . The large core dimensions relax the alignment tolerance, and reduce the packaging cost effectively. Moreover, in order to satisfy the growing demand in integration density, and to realize scalable and cost-effective on-board interconnects, 3D integration is under intensive investigation for a new dimension of integration [5,6].
Polymer waveguides are usually fabricated by the lithography method, which is compatible with the fabrication technique of PCBs. The waveguides are with rectangular cores and step index (SI) profiles [7, 8]. Another technique as so-called mosquito method which combines the use of a needle-type liquid microdispenser and a 3-axis robot stage has been developed to fabricate polymer waveguides with circular cores [9–11]. Moreover, due to the diffusion between core and cladding monomers, graded index profiles can be achieved by adjusting waiting time before the UV-curving process [12, 13]. This method is photomask-free and can inscribe 3D waveguides with various patterns directly. Besides, the mosquito method has high fabrication speed, large applicable size and low cost. We have fabricated single-mode waveguides operating at 1550 nm [14, 15], 3D direction couplers  and wide-band mode (de)multiplexer  by using this method.
In this paper, we report the successful fabrication of a 75-cm-long multimode waveguide in spiral design using the mosquito method. Its insertion loss and interchannel crosstalk with different core pitches are examined. Bending losses of curved waveguides with different radius are evaluated both numerically and experimentally. NRZ 25 Gb/s transmission at 850 nm over the 75-cm-long waveguide is demonstrated, and 4 × 25 Gb/s SWDM is implemented on a straight polymer waveguide fabricated this way. Moreover, a two-layer waveguide and a 3D Y-splitter/combiner are fabricated successfully for 3D integration.
2. Fabrication and optical characteristics of multimode polymer waveguides
The process of the waveguide fabrication is the same as that proposed in [14, 16, 17]. First, the cladding monomer is coated on a glass substrate. Then the core monomer is dispensed into the cladding monomer with the mosquito method. Finally, the fabricated waveguides are UV cured. Commercially available UV curable monomers of OrmoCore and OrmoClad (Micro resist technology GmbH) are used as the core- and cladding-monomer, respectively. The relative index difference between the core and cladding is 1.3% at 850 nm. The core diameter is mainly determined by the needle diameter, the dispensing pressure, the scanning velocity and the viscosity of monomers. The main process parameters for the fabrication are summarized in Table. 2. In order to fabricate waveguides with designed parameters, both the temperature and the humidity are strictly controlled throughout the fabrication process.
Firstly, multiple samples of straight waveguides are fabricated. Cross-sectional micrograph of the fabricated waveguide with 8 channels is shown in Fig. 1. The measured average core diameter is 47 μm and the core pitch is 250 ± 2 μm. Moreover, we define the height of the core position as the core-center height from the substrate surface, and the measured maximum height difference of all channels is about 3 μm.
The insertion loss of each channel in a 9-cm-long waveguide sample is measured at 850 nm, 1310 nm and 1550 nm. The experimental setup is shown in Fig. 2. 50-μm GI MMFs are used as both input and output fibers with matching oil applied to the end facets to maximally reduce the coupling loss. The measured results of 8 channels are shown in Table. 2. The measured insertion loss (IL) are 0.40 dB, 2.75 dB and 7.55 dB at 850 nm, 1310 nm and 1550 nm, respectively. The average transmission loss that includes the coupling loss is 0.044 dB/cm at 850 nm, and the IL as low as 0.40 dB predicts that the coupling loss between waveguides and MMFs is negligible. Moreover, the IL deviation is defined by the maximal difference of the measured IL among 8 channels. The small IL deviation indicates the uniformity of all channels and the stability of the fabrication process.
Waveguide samples with lengths of 10 cm and different core pitches are fabricated for crosstalk evaluation. Micrographs of core pairs with different pitches are shown in Fig. 3. The input and output fibers are both 50-μm GI MMFs. The received optical power while scanning the output fiber in horizontal direction with a step of 10 μm is recorded as shown in Fig. 4(a) and the summarized crosstalk is shown in Fig. 4(b). Crosstalk of less than −34 dB with a core pitch of 62.5 μm is observed. For waveguides with core pitches of larger than 100 μm, the crosstalk is less than −58 dB. The results imply that the fabricated multimode polymer waveguides are promising for high-density optical interconnects.
Curved waveguides with different bending radii are fabricated and the bending loss is studied both numerically and experimentally. The bending loss is defined as the increased insertion loss in unit curved length compared with the neighboring straight waveguide in the same length. A laser diode at 850 nm is used as the light source and the experimental setup is shown in Fig. 2. We use the beam propagation method (BPM) for calculation and the parameters are exactly the same as those in the experiment. Figure 5 shows the calculated results and the measured results of two samples. The inset is the schematic of the designed curved waveguide which is combined by two circular arcs. The lateral offset is set to 2 mm. The bending loss of a 4-mm-radius bending is about 0.08 dB/mm and it can be ignored when the bending radius is larger than 4 mm, which shows good agreement between the calculation and the experiment.
We designed and fabricated a 75-cm-long spiral waveguide as shown in Fig. 6. The measured insertion loss at 850 nm is 4.3 dB and the average transmission loss including the coupling loss is 0.057 dB/cm. Compared with the straight waveguide mentioned above, the transmission loss of the spiral waveguide is higher, which is caused by the increased defects such as bubbles and dust in such long waveguide. Therefore, a cleaning room is essential for the fabrication and bubbles and dust in the cladding need to be removed carefully before fabricating the cores.
3. High-speed performances
Firstly, NRZ signals at 10 Gb/s and 25 Gb/s are transmitted over the 75-cm-long spiral waveguide. The experimental setup for both back-to-back and waveguide link are illustrated in Figs. 7(a) and 7(b), respectively. A distributed Bragg reflector laser (DBR-LD) at 850 nm is used as the light source and the bandwidth of the intensity modulator is 25 GHz. The NRZ data at rates of 10 Gb/s and 25 Gb/s are generated by a bit error rate tester (BERT). 50-μm GI MMFs are both applied as input and output fibers. A multimode variable optical attenuator (VOA) is used before the photodetector (PD) to adjust the received optical power. The demodulated electrical signal is amplified by an RF amplifier and then it is fed back to the BERT for the bit error rate (BER) analysis.
The measured BER curves at 10 Gb/s and 25 Gb/s are shown in Fig. 8. Error-free transmission is successfully obtained for both back-to-back link and waveguide link. It can be observed that the power penalty for BER of 10−9 due to the insertion of the waveguide are 0.4 dB and 0.6 dB at data rates of 10 Gb/s and 25 Gb/s, respectively. Figure 9 illustrates the eye-diagrams with the received optical power of −12 dBm and −6 dBm at 10 Gb/s and 25 Gb/s, respectively. The voltage and time scale are also recorded. Open eye-diagrams are observed after transmitting over the 75-cm-long waveguide. Compared with the back-to-back link, there is no obvious degradation of the eye-diagrams due to the insertion of the waveguide.
We further demonstrated SWDM transmission over a straight waveguide. The SWDM can be an alternative solution for 200 Gb/s and 400 Gb/s transmission besides increasing channel numbers. In the SWDM system, four VCSELs at 850 nm, 880 nm, 910 nm, and 940 nm are multiplexed into a single transmission channel. 4 × 25 Gb/s SWDM signal is transmitted over a 10-cm-long waveguide using a standard VCSEL-based SWDM transceiver. The NRZ electrical signal at 25 Gb/s is generated by a BERT to simulate the real data traffic. The optical signal is butt-coupled into and out of the waveguide via cleaved 50-μm GI MMFs. The demodulated electrical signal is directed to a sampling oscilloscope for the eye-diagram measurement and is fed back to the BERT for BER test.
The BER as a function of the received optical power (ROP) for 4 × 25 Gb/s NRZ-SWDM transmission are measured in real time, which are shown in Figs. 10(a)–10(d). Because the measured BER fluctuates with time and the dispersion of the 10-cm-long waveguide is very small, the recorded two BER curves of back-to-back and waveguide link cut cross each other. For SWDM wavelengths of 850 nm, 880 nm and 910 nm, error-free transmission is obtained when the received optical power is larger than −6 dBm. Moreover, a noise floor of approximately 10−8 for both back-to-back and waveguide link can be observed at 940 nm due to the limited performance of the SWDM transceiver. It is worth noting that the absorption loss of the waveguide at 880 nm and 910 nm is about 5 times larger than that at 850 nm and 940 nm, and that is why we choose a short (10-cm long) waveguide for SWDM transmission test. Figure 11 shows the measured eye-diagrams at 25 Gb/s of the four wavelengths and the received optical power is −3 dBm. The voltage and time scale of the recorded waveforms are 150 mV/div and 6.66 ps/div, respectively. It can be observed that open eye-diagrams are obtained for both back-to-back and waveguide link and no obvious degradation is observed due to the insertion of the waveguide.
4. Two-layer waveguide and 3D Y-splitter/combiner for 3D integration
The high 3D flexibility of the mosquito method enables that waveguides in various 3D patterns can be fabricated easily. Multilayer waveguides and 3D splitter/combiners are essential for 3D integration.
Firstly, we fabricate a two-layer waveguide with 16 channels (8 channels × 2 layers) successfully, which is shown in Fig. 12. The length of the waveguide is 6.2 cm and the horizontal and the vertical core pitches are 250 μm and 125 μm, respectively. The measured average core diameter is 50.3 μm. We take the left channel in the lower layer as the origin and the relative position of all channels are measured as shown in Fig. 13(a). Due to the liquid state of the core and cladding monomers, the inscribing of the neighboring core causes unwanted turbulence of the fluid, which limits the accuracy of the core position. This can be mitigated by adopting a thinner needle and adjusting the needle-scan program to reduce the turbulence of the cladding monomer. Moreover, the position deviation can be compensated according to the hydrodynamic analysis . Insertion losses of all 16 channels at 850 nm are measured using 50-μm GI MMFs and matching oil is used at the end facets. The measurement results are shown in Fig. 13(b) and the average insertion loss of 16 channels is 0.39 dB.
Then, we designed and fabricated a 3D 1 × 4 (2 channels × 2 layers) Y-splitter/combiner. The schematic of the designed device and micrographs of end facets are shown in Fig. 14. The 3D splitter/combiner is composed of straight waveguides, horizontal S-bends and vertical S-bends. The S-bends are designed with bending radii of 20 mm to yield negligible bending loss. Using the mosquito method, each branch of the 1 × 4 3D Y-splitter/combiner needs to be fabricated separately, as a result, the output core is larger than the input cores. 50-μm GI MMFs are applied as both input and output fibers and the experimental results are shown in Figs. 15(a) and 15(b), respectively. The total length of the device is 4 cm, and the insertion loss of the neighboring straight waveguide in the same length is 0.31 dB. Compared with the straight waveguide, the excess loss of the 3D splitter is 0.79 dB and the maximum imbalance of 4 output ports is 1.24 dB. When working as a 4 × 1 combiner, the fabricated device shows an average insertion loss of 1.73 dB.
We demonstrate the design and fabrication of a 75-cm-long multimode polymer waveguide and 3D devices with circular cores for high-speed and high-density optical interconnects. The waveguide and the 3D device are fabricated using a direct inscribing method. Negligible coupling loss with MMFs, low loss and low crosstalk performance are achieved for the multimode waveguide. The bending loss is evaluated experimentally, which shows excellent agreement with the calculation result. Error-free NRZ data transmission of 25 Gb/s at 850 nm over the 75-cm-long waveguide is observed, and 4 × 25 Gb/s SWDM is demonstrated on a straight waveguide. Moreover, a two-layer waveguide and a 3D Y-splitter/combiner are fabricated successfully for 3D integration. High-uniformity power splitting and low-loss power combining are obtained for the 3D Y-splitter/combiner. The results imply that the proposed method is promising in fabricating polymer waveguides and 3D devices for high-speed and high-density optical interconnects.
National Natural Science Foundation of China (NSFC) (61775138, 61835006, 61620106015).
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