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We fabricate low-loss single-mode (SM) polymer optical waveguides using a photomask-free simple technique named the Mosquito method. The insertion losses of a 5-cm long SM polymer waveguide fabricated are 2.52 dB and 4.03 dB at 1310- and 1550-nm wavelengths, respectively. The coupling loss between a single-mode fiber and the waveguide is as low as 0.5 dB including the Fresnel reflection. The 0.5-dB misalignment tolerance in the radial direction is ± 2.0 μm at 1550 nm. The Mosquito method is promising for fabricating SM polymer optical waveguides compatible with silicon photonics chips.
© 2017 Optical Society of America
1. Introduction
Over the last several years, the data traffic in data centers has increased dramatically with the wide deployment of cloud computing services. In order to sustain the growth of data traffic, high bandwidth optical networks have been deployed in the data center network, while the power consumption of the whole data center has become an issue. Here, optical interconnects, in particular, silicon (Si) photonics technologies have been drawing much attention for solving the power consumption problem in data center networks [1]. So far, multimode fiber (MMF) links have been deployed in a large number of data centers. Although MMF links cooperate with vertical cavity surface emitting lasers (VCSELs) emitting at 850 nm, these VCSELs are not suitable for Si photonics technologies because Si exhibits high absorption loss at 850-nm wavelength. Alternatively, conventional laser diodes emitting at longer wavelengths such as 1310 nm and 1550 nm are adopted for the light source. In addition, single-mode fiber (SMF) based optical links are required for off-chip interconnects with Si photonics chips because Si waveguides have nanometer-scale cores to be operated as an SM waveguide, and longer and higher-bandwidth link distance that cannot be achieved by MMF links are required in large-scale data centers. These Si waveguides are mainly deployed within a chip, and so SM polymer optical waveguides are regarded as promising low-loss optical devices interoperable with low-loss Si waveguides and SMFs [2, 3].
Generally, SM polymer optical waveguides have been fabricated by means of several processes such as photolithography using photomasks [4, 5], UV imprinting [6], and direct-write lithography [7, 8]. Meanwhile, we have reported the “Mosquito method” to fabricate multimode graded-index (GI) polymer optical waveguides using a microdispenser system in order to apply the waveguides to on-board optical interconnects [9, 10]. The Mosquito method is a unique process: it is a photomask-free fabrication method in which circular GI cores are formed with a very simple process. In this paper, we demonstrate that the Mosquito method is very promising for fabricating even SM polymer waveguides because the core diameter is controlled accurately and easily. Although we already reported the possibility of SM waveguide fabrication using the Mosquito method in [11, 12], the optical loss and other characteristics of the fabricated SM waveguide have not been fully investigated.
Therefore, in this paper, we fabricate SM waveguides using the Mosquito method under various conditions, and then investigate their optical characteristics in more detail.
2. The Mosquito method
The Mosquito method that uses a microdispenser was originally developed for multimode polymer waveguide fabrication. The procedure of the Mosquito method is schematically shown in Fig. 1. First, a viscous (liquid) cladding monomer is coated on a substrate. Then, another viscous monomer for core is dispensed into the cladding monomer from a thin needle attached to a syringe. Finally, both the core and cladding monomers are cured under UV exposure, followed by post baking. The whole process shown in Fig. 1 takes just 15 minutes or less to fabricate a 5-cm long and 12-channel SM polymer waveguide.
As the waveguide material, silicate-based organic-inorganic hybrid resins named SUNCONNECT® (NP-005 for core: the monomer viscosity is 76,000 cps; NP-211 for cladding: the monomer viscosity is 3,900 cps) supplied by Nissan Chemical Ind., Ltd. are utilized, by which a core-cladding relative index difference (Δn) of 0.6% is achieved. The propagation loss spectrum of a SUNCONNECT® based multimode polymer optical waveguide is shown in Fig. 2 in which a conventional siloxane (FX-W712 for core, FX-W713 for cladding supplied by ADEKA Corporation) based multimode waveguide we previously fabricated using the Mosquito method [9] is compared. It is found from Fig. 2 that the propagation losses of the conventional siloxane based multimode waveguide are 0.34 dB/cm and 1.25 dB/cm at 1310 nm and 1550 nm, respectively, while those of the SUNCONNECT® based multimode polymer waveguide are 0.35 dB/cm and 0.58 dB/cm at 1310 nm and 1550 nm, respectively. Here, the material losses estimated are 0.28 dB/cm and 0.44 dB/cm at corresponding wavelengths, which are indicated by the material supplier [13]. Although the loss at 1310 nm is comparable between the two waveguides, remarkable loss reduction at 1550 nm is observed in the SUNCONNECT® based waveguide. However, the propagation loss values of the SUNCONNECT® based waveguide are slightly higher than the essential material losses. This is because the measured propagation losses include the excess loss inherent to the waveguide structure such as mode dependent loss for multimode operation. Hence, the propagation loss in single-mode waveguides is expected to be lower since the excess loss inherent to multimode waveguides could be reduced. The propagation loss of the single-mode polymer waveguides will be discussed in the later section. Furthermore, the SUNCONNECT® monomers have high thermal stability: they possess even solder-reflow capability. Therefore, the SUNCONNECT® resins are a promising material for SM polymer waveguide fabrication.
Fig. 2 Propagation loss spectrum of a SUNCONNECT® based multimode polymer optical waveguide compared to a conventional siloxane polymer based waveguide [9].
3. Fabrication for single-mode waveguide
The core-diameter control is one of the large key issues in the SM polymer waveguide fabrication using the Mosquito method. In general, the single-mode condition is determined by the core diameter, operating wavelength, and Δn. Then, the single-mode condition for cores with a GI profiles at 1310 nm and 1550 nm is calculated using a commercially available mode solver (FIMMWAVE). Here, the refractive index profiles for simulation input are assumed to be a completely parabolic profile following to a power-law form with an index exponent g of 2.0 [9]. The wavelength dependence on the refractive indices of the core center and cladding is taken into account. In order to satisfy the single-mode condition, the core diameter of the waveguide with a GI profile has to be smaller than 8.9 μm and 10.6 μm at 1310- and 1550-nm wavelengths, respectively, as shown in Fig. 3.
In the Mosquito method, it has already been confirmed that the core diameter could be reduced by decreasing the dispensing pressure for the core monomer, increasing the needle-scan velocity, and using a thinner needle [9]. Figure 4 shows the core-diameter dependence on the needle scan velocity for single-mode polymer waveguide fabrication. The cores are dispensed from a needle with 100/230 μm inner/outer diameters and with a pressure of 500 kPa. Figure 4 indicates that the core diameter is inversely proportional to the square root of the needle-scan velocity, and a core diameter as small as 5 μm is realized with a sufficiently high scan velocity. Furthermore, a much smaller core, which should be highly compatible with Si photonics technologies, could be realized using a thinner needle such as one with 80/230 μm inner/outer diameters, as illustrated by green points and curve.
Fig. 4 Relationship between the core diameter and the needle-scan velocity (All the marks are the experimentally measured data).
Here, in order to satisfy the single-mode condition for both wavelengths, 1310 nm and 1550 nm, the needle-scan velocity should be set to 40 mm/s. Then, we fabricate a 12-ch., 250-μm pitch-parallel SM polymer waveguide using the SUNCONNECT® monomers [12]. A cross-section of the fabricated SM waveguide is measured by a digital microscope (KEYENCE CORPORATION, VHX-5000), as shown in Fig. 5. The core diameter is measured to be 7.9 μm on average for the 12 cores. The interchannel pitch of the fabricated waveguide is very accurately controlled to 250.17 ± 0.66 μm although the core monomer is dispensed into a liquid state cladding monomer.
Additionally, a narrow pitch SM waveguide compatible with Si photonics chips is realized with the Mosquito method by modifying the needle-scan program as shown in Figs. 6 and 7. The core shape shown in Fig. 5(b) looks rectangular, however, the actual core shape is not rectangle, as shown in Fig. 7.
4. Characterization
4.1. Near-field pattern
The optical characteristics of a fabricated 250-μm pitch SM waveguide are evaluated. The near-field patterns (NFPs) from an SMF (OZ Optics Ltd., SMJ-3S3S-1064-6/125-3-1) and the fabricated SM waveguide at 1310- and 1550-nm wavelengths are measured by an optical beam NFP measurement optics (SYNERGY OPTOSYSTEMS CO., LTD., M-Scope type S), as shown in Fig. 8 [12]. In addition, the normalized output intensity profiles obtained from the NFPs of fabricated SM waveguides at both wavelengths are also shown in Fig. 9. From these results, the output intensity profiles from the fabricated waveguide closely fit a Gaussian profile, and so an SM waveguide is successfully fabricated using the Mosquito method. Table 1 summarizes the mode field diameters (MFDs) calculated from the obtained NFPs (the diameter at 1/e2 of peak intensity), compared with the MFD of the SMF. The MFDs of the fabricated waveguide are as small as 8.27 μm and 9.47 μm at 1310- and 1550-nm wavelengths, respectively, which are comparable to those of the SMF (6.41 μm and 7.36 μm, respectively). Although there is a slight difference in MFDs between the waveguide and SMF, the MFD of the waveguide could be as close as that of the SMF, as indicated in Fig. 8.
Fig. 9 Normalized output intensity profiles in the radial direction from (a) an SMF and (b) the fabricated SM waveguide.
Table 1. Mode Field Diameter Calculated at 1310 nm and 1550 nm.
4.2. Loss measurement
The propagation loss of the fabricated waveguide is evaluated employing the cut-back method. The propagation losses are 0.29 dB/cm and 0.45 dB/cm at 1310- and 1550-nm wavelengths, respectively, as shown in Fig. 10 [12]. Compared to a conventional siloxane-based waveguide with 1-2 dB/cm losses at 1550 nm, the loss reduction at 1550 nm in the SUNCONNECT® based waveguide is remarkable because the absorption loss at the near-infrared region inherent to the carbon-hydrogen bonding is reduced in the SUNCONNECT® resin. Moreover, compared to the SUNCONNECT® material loss potential of 0.44 dB/cm, the excess loss caused due to the scattering at the core-cladding boundary is as low as 0.01 dB, which means that the core-cladding boundary formed by the Mosquito method is sufficiently smooth.
The insertion loss and coupling loss for the obtained SUNCONNECT® based waveguide are measured. The measurement setup is shown in Fig. 11. Here, the length of the waveguide is approximately 5.0 cm. In this measurement, two different laser diodes emitting at 1310 nm and 1550 nm are used as the light source, and an SMF (the same as the one shown in Figs. 8 and 9) is used for coupling the light into the waveguide core. We adopt two types of fibers for the detection probe to evaluate the coupling loss. First, a 1-m long 105-μmø step-index (SI) MMF is used for collecting all the light from the waveguide output end. Next, the SI MMF for the detection probe is replaced for an SMF (the same one as the launch probe) assuming an actual application of the SM waveguide for off-chip interconnects: the waveguides should connect to the SMFs. For the coupling loss measurement, the waveguide is butt-coupled to two SMFs at both ends, where no index-matching fluid is used. The insertion loss and coupling loss measurement results are summarized in Fig. 12. As shown in Fig. 12, the insertion losses of the 5.0-cm long waveguide with two SMFs are as low as 2.52 dB and 4.03 dB at 1310 nm and 1550 nm, respectively. The coupling loss between the waveguide and the SMF is estimated as low as 0.5 dB at 1310 nm on one end, including the Fresnel reflection loss. Compared to be the measurement setup using an MMF as the detection probe, the excess coupling loss caused by the SMF for the detection is as low as 0.29 dB at 1310 nm. This is because the MFD of the waveguide is controlled to be almost the same value as the SMF.
4.3. Misalignment tolerance
The misalignment tolerance in SMF and SM waveguide connections is important to the evaluation of the pitch accuracy of the waveguide fabricated using the Mosquito method. In the misalignment tolerance measurement, the SMF at the waveguide output side is scanned along the horizontal direction, as shown in Fig. 13. Figure 14 shows the obtained misalignment tolerance curve at both 1310- and 1550-nm wavelengths. It is found that the 0.5-dB misalignment tolerances are approximately ± 1.5 μm and ± 2.0 μm at 1310 nm and 1550 nm, respectively. From this result, the interchannel pitch of the fabricated waveguide observed as 250.17 ± 0.66 μm is sufficiently accurate.
Furthermore, in order to investigate the pitch accuracy of the SM waveguide fabricated using the Mosquito method, a 127-μm pitch, 1x16-channel optical splitter is connected to a 254-μm pitch, 12-channel SM waveguide. In this measurement, the pitch accuracy of the fabricated SM waveguide by measuring the intensity profile using an optical beam NFP measurement optics: M-Scope type S (SYNERGY OPTOSYSTEMS CO., LTD.). As shown in Fig. 15, it is found that all the cores show the high output intensity even at 1550-nm wavelength, which means the core position accuracy is high enough to be connected to the optical splitter with a high coupling efficiency in all the cores.
4.4. Interchannel crosstalk
The optical properties of the fabricated 5-cm long, narrow-pitch SM waveguides are evaluated. In very narrow-pitch waveguides, the interchannel crosstalk due to mode coupling is a large concern. The interchannel crosstalk of the fabricated waveguides at 1310- and 1550-nm wavelengths is measured, as shown in Fig. 16. The crosstalk in the waveguides with a 30-μm pitch is lower than −30 dB at both wavelengths. On the other hand, the crosstalk in the 20-μm pitch waveguide is significantly high: the main cause of such a high crosstalk should be mode coupling. Figure 16 also indicates the simulated results of the crosstalk at 1310- and 1550-nm wavelengths with respect to the interchannel pitch of the waveguide. It is found that good agreements are observed with the experimental results. From these results, it is experimentally confirmed that the crosstalk of waveguides with a pitch narrower than 30 μm tends to be too high because the effect of mode coupling is quite large.
5. Conclusion
We successfully fabricated single-mode polymer parallel optical waveguides using a simple fabrication technique named the Mosquito method. We investigated the optical properties of the fabricated SM waveguide. The MFDs of the waveguide were 8.27 μm and 9.47 μm at 1310- and 1550-nm wavelengths, respectively. The insertion losses of a 5.0-cm long SUNCONNECT® based waveguide were as low as 2.5 dB and 4.0 dB at 1310- and 1550-nm wavelengths, respectively, by reducing both propagation loss and coupling losses. The 0.5-dB misalignment tolerance in the connection between the fabricated waveguide and an SMF was ± 2.0 μm at 1550 nm. Therefore, the SM waveguides fabricated using the Mosquito method are very promising for off-chip interconnects.
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