We demonstrate circular-core 1550 nm single-mode polymer waveguides with a graded-index profile fabricated by commercially available UV-curable epoxies using so-called mosquito method. The relative index difference ∆n of the waveguides was designed to be 0.46% in order to guarantee both single-mode operation and good compatibility with standard single-mode fiber. Accurate refractive index tuning of monomer for core construction was realized by mixing the core and cladding epoxies properly. The core pitch of the waveguides was chosen to be 50 μm to satisfy the requirements for high-density on-board optical interconnects. Both the optical characteristics and high-speed performances of the waveguides were comprehensively studied at 1550 nm. The measured transmission and coupling loss are 0.79 dB/cm and 0.78 dB, respectively. The waveguides exhibit an inter-channel crosstalk as low as −45 dB, and a 3 dB misalignment tolerance larger than ± 4 μm on the input and output facet in both horizontal and vertical directions. NRZ signal at a data rate of 25 Gb/s was transmitted over a 10 cm-long waveguide. There is no obvious degradation on the eye diagram due to the insertion of the waveguide and error free transmission was successfully obtained. Our results imply that the fabricated single-mode polymer waveguides have good potential in high-density and high-speed optical interconnects application.
© 2017 Optical Society of America
With the dramatic increase of data throughput within large data centers and high-performance computers (HPC), there are increasing demands for interconnects with higher speed and higher density [1–3]. Optical interconnects have drawn significant attentions thanks to their advantages in bandwidth, integration density, power consumption, and electromagnetic compatibility over conventional electronic solutions [4, 5]. Polymer waveguide is a cost-effective and power-efficient solution for optical interconnects owing to its low processing and material cost, as well as excellent compatibility with both printed circuit boards (PCBs) and fiber optics. Polymer waveguides can be embedded inside or put on the surface of PCBs . Presently, optical interconnects based on multimode waveguides and vertical cavity surface emitting lasers (VCSELs) at 850 nm have already been intensively investigated [7,8]. However, severe signal degradation due to their highly multimode nature limits both their transmission data rate and applicable distance [9–11]. Moreover, the large core dimension (typically 30~60 μm) also limits their density. Consequently, single-mode waveguides are attracting increasing attentions [12, 13].
On the other hand, high-function application-specific integrated circuits (ASICs), such as switch chips, require higher input/output (I/O) density and lower interface power, which are employing silicon photonics technology operating at wavelength of 1310 nm or 1550 nm for larger channel numbers and greater throughput that beyond 1 Tb/s . These single-mode silicon waveguides have a core size as small as several hundreds of nanometers and need an appropriate I/O interface with conventional fiber optics. To enable cost effective and scalable system-level integration of silicon photonics, single-mode polymer waveguides can be used as low-loss and high-density interposer that enable efficient coupling between standard single-mode fibers (SMFs) and silicon waveguides [15, 16].
Single-mode waveguides operating at 1310 nm have been widely studied due to their relatively low loss of about 0.3 dB/cm . The high absorption loss at 1550 nm limits the practical length of the waveguides and only few results have been reported. However, the 1550 nm single-mode waveguides are indispensable for the short distance applications of less than 10 cm such as chip-to-chip interconnects and polymer adiabatic coupler that connect the silicon waveguides with fiber array .
Polymer waveguides are usually fabricated by photolithography , laser direct writing  and nano-imprint lithography methods . There is another technique as so-called mosquito method which uses a needle-type liquid microdispenser to fabricate multimode polymer waveguides . The method has unique advantages such as photomask-free, able to fabricate waveguides with a circular core and waveguides in three-dimension . Single-mode polymer waveguides fabricated using this method have also been reported . However, the materials that used for the fabrication of single-mode waveguides, which usually have smaller relative refractive index differences ∆n compared to their multimode counterparts, are presently not commercially available, and the high-speed performance of the waveguides remains unknown.
In this paper, we demonstrate successful fabrication of circular-core single-mode waveguides operating at 1550 nm with a graded-index profile by using commercially available UV-curable epoxies (OrmoClad and OrmoCore) with the mosquito method. Owing to both of the similar chemical composition and the viscosity of the two epoxies, the desirable low ∆n can be obtained by mixing the original materials . Moreover, the refractive index of the mixed monomer are arbitrarily adjustable in the range between that of pure core and cladding monomers, which is valuable to the relative researches. In order to guarantee both single-mode operation and good compatibility with standard SMF, the relative index difference ∆n of the waveguides was designed to be 0.46%. The core pitch of the waveguides was chosen to be 50 μm for high-density on-board optical interconnects. We comprehensively evaluated both the optical characteristics and high-speed performances of the fabricated single-mode waveguides at 1550 nm.
2. Fabrication of single-mode waveguide using mosquito method
Schematics of fabrication process are shown in Fig. 1. First, the cladding monomer with lower refractive index is coated on a glass substrate with a silicone-film frame. Secondly, the core monomer with higher index is dispensed into the cladding by inserting and scanning the needle. Then the core and cladding monomer are UV cured which is followed by a post baking process. Finally, the fabricated polymer waveguide is peeled off from the glass substrate. Waveguides with a small core diameter can be easily fabricated by adjusting the dispensing conditions such as needle diameter, scan velocity, and dispensing pressure.
We used two kinds of commercially available UV-curable epoxy resins (OrmoClad and OrmoCore) as starting materials. The refractive indices of the epoxy resins at 1550 nm are 1.521 and 1.537, respectively. The OrmoClad was used directly as the cladding monomer. To guarantee single-mode operation and obtain a compatible mode field diameter with that of standard SMF, we adjusted the refractive index of the core monomer to obtain a ∆n of 0.46% by mixing the two original materials. The refractive index is linear with the mass ratio of OrmoClad to OrmoCore and the refractive index of the cured polymer can be calculated by the mass ratio of the two pure materials, as shown in . Owing to the diffusion between the core and cladding monomers before UV-curing, the fabricated waveguides have circular core shapes with graded index profiles .
We numerically analyzed the dependence of the mode numbers on the core diameters using the finite element method. The refractive index profile of the waveguide was set to be parabolic with an index exponent g of 2.0 . The refractive index of cladding and the ∆n of the waveguides were set to 1.521 and 0.46%, respectively, which coincided with that of the experimental condition. The calculated results are shown in Fig. 2. In order to obtain the single-mode waveguide, the core diameter should be smaller than 12 μm at 1550 nm. Micrographs of fabricated single-mode polymer waveguides are shown in Figs. 3(a) and 3(b). The measured core pitch of 8 channels is 50 ± 2 μm and the average core size is 10.7 μm.
3. Optical characteristics of the fabricated single-mode waveguide
The propagation loss and coupling loss of the fabricated waveguide were measured using a cut-back method. The experimental setup is shown in Fig. 4. Distributed feedback laser (DFB-LD) at 1550 nm was used as the light source. Standard SMF was used as both the input and output probes taking the real application of off-chip interconnects into consideration. Matching oil was applied to both the input and the output facet of the waveguide to maximally reduce the coupling loss. Using the cut back method, both the propagation loss of the waveguides and the coupling loss between waveguide and the optical fiber can be worked out. The measured propagation and coupling loss is 0.79 dB/cm and 0.78 dB, respectively, as shown in Fig. 5.
The crosstalk was measured using the same experimental setup as shown in Fig. 4 by fixing the input fiber and recording the optical power while scanning the output fiber in the horizontal direction with a step of 1 μm. The result is shown in Fig. 6. It can be observed that the inter-channel crosstalk of 10-cm-long waveguide is less than −45 dB. The measured low crosstalk indicates that the fabricated waveguides are suitable for high-density optical interconnects application.
Misalignment tolerance in the following all four possible situations was both numerically and experimentally studied: (a) horizontal direction at input side; (b) vertical direction at input side; (c) horizontal direction at output side; (d) vertical direction at output side. A beam propagation method was employed to calculate the received optical power at different offset positions using the parameters listed in Table 1. The received optical power at different offset positions was normalized to that without offset, and the results are shown in Figs. 7(a) and 7(b). It should be noticed that the calculated misalignment tolerance in horizontal and vertical direction is exactly the same due to the perfect circular symmetry of the calculation model. The experimental setup shown in Fig. 4 was also used for measuring the misalignment tolerance. For input alignment tolerance measurement, the input fiber scanned in both horizontal and vertical directions with a step of 0.5 μm and the output fiber was fixed at the same time, and vice versa for the output alignment tolerance measurement. It can be observed that the 3 dB misalignment tolerance is larger than ± 4 μm on the input and output facet in both horizontal and vertical directions. Due to the non-circularity of the waveguide at the output facet, the 3 dB misalignment tolerance in the vertical direction is larger than that of the horizontal direction. Moreover, the measured results are slightly larger than that of the simulated one due to the observational error of the core diameter. This is caused by the diffusion between the core and cladding monomers before UV-curing process, which makes the core-cladding boundary become blurry, and makes it difficult to precisely measure the core size.
In order to investigate the supported modes of the fabricated waveguide, the near field patterns (NFPs) under different launching conditions were investigated using an infrared CCD camera. NFPs of two waveguides with different core diameters are shown in Fig. 8. One is a single-mode waveguide with a core diameter of 10.6 μm, and the other is a few-mode waveguide with a core diameter of 20.0 μm for comparison. A light beam at 1550 nm was butt-coupled into the core of fabricated waveguides through a SMF and the output NFPs of the waveguides were recorded at different offset lunching positions. For the single-mode and few-mode waveguides, the input light was offset launched by a step of 2 and 4 μm and the observed NFPs are shown in Figs. 8(a) and 8(b), respectively. In the case of single-mode waveguide, the Gaussian-like beam profile remains almost unchanged with the increase of the offset except for the decreasing in received power. In the case of the few-mode waveguide, however, NFPs of higher order modes can be observed with the increase of the offset. Therefore, we can conclude that the fabricated waveguides operate in a single-mode fashion.
4. High-speed performances
To evaluate the high-speed data transmission performance of the waveguide, we measured the BER curves and eye diagrams of NRZ data at 25 Gb/s for both back-to-back and 10-cm-long single-mode waveguide transmission, respectively. The experimental setup for back-to-back and waveguide link are shown in Figs. 9(a) and 9(b), respectively. The pulse pattern generator (PPG, KEYSIGHT N4951B) generated electrical NRZ signal with a pseudo-random binary sequence (PRBS) length of 27-1 and the data rate was set to 25 Gb/s. Light from a DFB-LD at 1550 nm was applied to the intensity modulator with a bandwidth of 20 GHz together with the NRZ signal and a direct current (DC) driving signal. The modulator operating conditions (DC driving current and modulation amplitude) were carefully set to obtain the optimum performance. The light beam was butt-coupled into the single-mode waveguide through a standard SMF after modulation. An identical fiber was used as the probe to collect the light from the waveguide. Matching oil was applied on both input and output side to minimize the coupling loss. A single-mode variable optical attenuator was inserted into the link to adjust the received power of a 12 GHz photodiode. After amplifying by a 50 GHz RF amplifier, the electrical signal was link to the bit error ratio tester (BERT) to measure the BER.
We measured the BER in real time, which fluctuated even when the received optical power remained same. We measured the BER curves for seven times, and the severest power penalty observed was 1.1 dB. The acquired BER curves by averaging the results of the seven times measurement are shown in Fig. 10. It can be observed that the average power penalty for BER of 10−9 is about 0.4 dB. The received eye diagrams of both back-to-back and waveguide link were measured using a sampling oscilloscope (KEYSIGHT DCA-X 86100D), as shown in Figs. 11(a) and 11(b). The voltage and time scale of the recorded waveform was 100 mV/div and 6.66 ps/div, respectively. The average received optical power (Pre) was 1.9 dBm. Open eye diagrams are obtained for both back-to-back and waveguide link. Moreover, the insertion of the fabricated waveguide does not cause obvious degradation on the eye diagram due to the small dispersion of it.
We successfully designed and fabricated circular-core single-mode waveguides with a graded-index profile by commercially available UV-curable epoxies using the mosquito method. The single-mode waveguides operated at 1550 nm and the core pitch was chosen to be 50 μm to satisfy the requirements for high-density optical interconnects. By mixing the core and cladding monomers properly, the relative index difference ∆n of the waveguides was set to be 0.46% to guarantee the single-mode operation and the compatibility with standard SMF at the same time. We investigated the optical characteristics and high speed performances of the waveguides at 1550 nm. The measured transmission and coupling loss are 0.79 dB/cm and 0.78 dB, respectively. The waveguides exhibit a low inter-channel crosstalk (< −45 dB) and large 3 dB misalignment tolerance (> ± 4 μm) on the input and output facet in both horizontal and vertical directions. NRZ signal at a data rate of 25 Gb/s was transmitted over a 10-cm-long waveguide. Error free transmission and open eye diagrams were successfully obtained for both back-to-back and waveguide link. The fabricated single-mode polymer waveguides are promising for high-density and high-speed optical interconnects application.
This work was supported by the National Natural Science Foundation of China (NSFC) under Grant 61405113, 61620106015.
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