We propose three-dimensional (3D) polymer directional couplers operating at 1550 nm for on-board optical interconnects. The proposed directional coupler can realize free-position light coupling and splitting in three dimension and can be cascaded to serve as an effective and scalable device for multi-layer and multi-channel interconnects. The 3D directional coupler was fabricated with commercially available UV-curable epoxies by combining the use of a needle-type liquid micro-dispenser and a 3-axis robot stage. The main parameters of the 3D directional coupler such as the interaction length, coupling ratio, coupling position, and cascade numbers can be easily adjusted. In the experiment, single-mode two-dimensional (2D) polymer directional couplers in both horizontal and vertical directions operating at 1550 nm with different coupling ratios (CRs) were demonstrated at first. The experimental results agree with the simulated ones. A 3D directional coupler with coupling ratios of 58:23:19 at 1550 nm was then successfully fabricated using the mosquito method for the first time to the best of our knowledge. The CR varies with the wavelength as expected while the excess loss remains almost the same within C-band. The results imply that the proposed method may be applicable in the fabrication of functional devices in three dimension for high-density on-board optical interconnects with further improvement on fabrication process.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
High speed and large capacity on-board optical interconnects have advantages in bandwidth, power consumption and electromagnetic compatibility over their electrical counterparts [1,2]. In order to satisfy the growing demands in data throughput and in integration density, and to realize scalable and cost-effective on-board interconnects, 3-dimentional (3D) integration which provides a new dimension of integration, are under intensively investigation [3–5].
3D integration has been realized by employing inverse taper , vertical (through silicon vias, TSVs) optical waveguides , and cantilever couplers  for inter-layer connects purposes among multiple 2D photonic integrated circuits (PICs). However, these technologies are complex to fabricate and high in cost. On the other hand, 3D photonic devices, such as mode-division multiplexers [9,10], freeform photonic wire bonding from chip to chip , and fan-out devices for multicore fibers  have been successfully fabricated using femtosecond laser direct writing technique. Directional couplers in both two dimension [13,14] and three dimension [15,16] have also been fabricated by the femtosecond laser direct writing technique. Another technique as so-called mosquito method which is photomask-free and capable to fabricate waveguides with circular core by combining the use of a needle-type liquid micro-dispenser and a 3-axis robot stage has also been proposed [17–19]. Waveguides with various 3D patterns can be fabricated by a simple procedure [20, 21]. Compared to the femtosecond laser direct writing technique, the mosquito method has advantages such as high fabrication speed (up to 100 mm/s), large applicable size (up to a few hundreds of cm2), and low cost. Moreover, the index exponent g of the refractive index profile, which is dependent on the concentration of the core and cladding monomers, can be adjusted by varying the interim time before UV-curing process . Shorter interim time allows waveguides to have a profile with larger index exponent. By varying the needle-scan velocity, tapered waveguides can be fabricated easily . On the other hand, due to the liquid-state of the core and cladding monomer, the process of inscribing the neighboring core may cause unwanted turbulence of the fluid, which limits the position and the diameter accuracy of the core, especially when the cores become close to each other. This can be mitigated by reducing the monomer-flow effect, such as adopting a thinner needle , adjusting the needle-scan program , and compensating the position deviation according to the hydrodynamic analysis . We have fabricated 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 method .
In this paper, we propose the design and fabrication of the single-mode 3D directional coupler operating at 1550 nm which may serve as an effective and scalable device to realize inter-layer connection. First, we demonstrated 2D directional couplers in horizontal and vertical directions. Then, we successfully fabricated the 3D directional coupler. The device was fabricated using commercially available UV-curable epoxies (OrmoCore and OrmoClad). The characteristics of the device such as coupling ratio (CR) and excess loss (EL) in C-band as functions of the interaction length were evaluated. Besides, near field patterns (NFPs) at different wavelengths were observed using an infrared CCD camera. The CR dependence on the interaction length of the horizontal directional coupler was evaluated both numerically and experimentally. Moreover, we also investigated the requirement on fabrication tolerance and the method to satisfy it.
2. Design and fabrication
The ability for the mosquito method to fabricate freeform waveguides enables that the waveguides can distribute in various ways and the spacing between each layer can be adjusted accordingly. The schematic of the proposed 3D directional coupler is shown in Fig. 1(a) and the cross-sectional view is shown in the inset. It consists of three circular waveguides with the same core diameter and index-profile. In the input and output facet, the three waveguides distribute in an equilateral triangle manner. And in the coupling region L, the three waveguides are directed to one layer. S-bends are added at both input and output sides in order to obtain a spatial waveguide separation of 100 µm. The S-bend was designed with a radius of 20.0 mm that yielded negligible bending loss. The designed spacing D of parallel waveguides in the coupling region is about 20 µm. In order to guarantee the single-mode operation condition, the core diameter is designed to be 10 µm. The proposed device can be cascaded as demonstrated in Fig. 1(b) to serve as an effective and scalable device to realize multi-layer and multi-channel interconnects in three dimension.
We adopted the same method as proposed in  for fabrication. Commercially available UV-curable epoxy resins of OrmoClad and OrmoCore were used as cladding and core monomers, respectively. The relative index difference between core and cladding is adjusted to 0.26%. Using this method, waveguides with different core diameters can be easily fabricated by adjusting the dispensing conditions such as needle diameter, scan velocity, and dispensing pressure.
Schematics of the fabrication process are shown in Fig. 2. First, the cladding monomer is filled in a silicone-film frame on the glass substrate. Second, the core monomer is dispensed into the cladding by inserting and scanning the needle. Then, the monomers are UV cured which is followed by a post baking process. Last, the cured waveguide is peeled off from the substrate. In our experiment, the devices are fabricated with a constant velocity and pressure.
3. Experimental results
We first fabricated two-core couplers in horizontal and vertical directions as shown in Fig. 3(a) and 3(b), respectively. The inset shows the micrographs of the output facet. We evaluated their CR and EL using a tunable laser diode. NFPs at the output facet of directional couplers were observed using an infrared CCD camera.
Light was launched into the couplers and detected from the output ports using the single-mode fiber (SMF, Corning SMF28-e) with the index matching oil applied between the fibers and end facets. The CR and EL are defined as following:
The CR can be changed by varying the interaction length L. Two horizontal directional couplers (Coupler H1 and H2) and two vertical directional couplers (Coupler V1 and V2) with different interaction lengths have been successfully fabricated. We evaluated the wavelength dependence of the CR and EL in C-band. The measured results for horizontal couplers are shown in Fig. 4, and the insets represent the NFPs at 1530 and 1560 nm, respectively. Interaction lengths of Coupler H1 and H2 are 7.5 mm and 13.0 mm, respectively. It can be observed that the CR reasonably varies with the wavelength while the EL remains almost the same within the C-band. For Coupler H1 and H2, the measured CRs at 1550 nm are 0.66 and 0.55, and the average excess losses in C-band are 4.3 and 2.9 dB, respectively.
Figure. 5 shows the results for the two vertical couplers with different interaction length, and their NFPs at 1530 and 1560 nm are shown in the insets. For Coupler V1 with a interaction length of 10.0 mm and Coupler V2 with a interaction length of 7.0 mm, the measured CRs at 1550 nm are 0.13 and 0.48, and the average excess losses in C-band are 3.0 and 2.9 dB, respectively.
The total length of the device is approximately 30.0 mm. The relatively high EL of the coupler is mainly caused by the high absorption loss of the single-mode waveguide at 1550 nm (about 0.8 dB/cm) and the coupling loss (about 0.8 dB) . The possible reasons for the difference in measured losses between Coupler H1 and other couplers are the existing of defect of the waveguides, which may be caused by the bubbles and dust in the monomer, and the imperfections of the waveguides due to the inscription of the neighboring core in the coupling region. The inserted NFPs also demonstrate the different CRs of different couplers and the coupling-ratio variation with the wavelength. When the polarization of the incident light rotating by 90 degrees, we observed no obvious changes on both CR and EL.
We investigated the dependence of the CR on the interaction length both numerically and experimentally. We used a beam propagation method (BPM) for calculation and the calculation parameters are listed in Table. 1. Two critical parameters of core diameter and spacing were set to 10 µm and 13 µm as design values for horizontal two-core directional couplers. As shown in Fig. 6, the experimental results agree with the calculated ones, which can prove that the coupled mode equations are valid for the fabricated couplers. The deviation between them might be caused by the difference in fabrication process condition and monomer flow effect which resulted in the changes in parameters of the waveguides and caused phase mismatching between the two waveguides. From the calculation result with a coupling period of about 5.8 mm, we can estimate that the coupling coefficient κ is about 0.54 rad · mm−1.
The precise phase-matching along the coupling region of the directional couplers requires inherently tight fabrication tolerance. The fabrication tolerance is dependent on the core diameter and spacing. As the calculation results shown in Fig. 6, couplers with different interaction length have different variation slopes and thus different tolerances. We numerically analyzed the tolerance dependence on the core diameter and spacing at the CR of 0.50 (interaction length of 7300.0 mm) and the CR of 0 (interaction length of 8800.0 mm), and the results are shown in Fig. 7(a) and 7(b), respectively. It can be observed that the couplers with a CR of 0 have a larger tolerance than those with a CR of 0.50. Couplers with the CR between 0.40 and 0.60 have tightest fabrication tolerances of about ± 0.2 µm in core diameter and ±0.1 µm in spacing. The fabrication tolerances are about ± 0.7 µm in core diameter and 0.3± µm in spacing for couplers with the CR between 0 and 0.20. In addition, the waveguide asymmetry which is defined by the core diameter deviation of waveguide 2 to waveguide 1 causes the difference in propagation constant between cores. When fixing the core diameter of waveguide 1 to 10 µm, the tolerance on the waveguide asymmetry is about ± 0.3 µm and ±0.2 µm for couplers with the CR between 0 and 0.2 and couplers with the CR between 0.4 and 0.6, respectively, as shown in Fig. 7(c). The waveguide asymmetry not only results in the deviation of the CR, but also limits the minimum CR that can be obtained, as shown in Fig. 7(d). The minimum CR that we have achieved is 0.11 of Coupler V1 at 1560 nm. Moreover, the difference in propagation constants, Δβ = β2 − β1, of two parallel waveguides as a function of waveguide asymmetry is shown in Fig. 7(d). The present fabrication process need to be improved to satisfy the tight fabrication tolerance of the directional coupler. We may adopt needles with smaller outer diameter and make compensations on needle-scan program in order to reduce the impact of the monomer flow according to the hydrodynamic analysis. We may also choose core and cladding monomer pairs with large viscosity difference, which is helpful to reduce both the core-diameter deviation and the non-circularity.
Then, we succeeded in fabricating the 3D directional coupler as shown in Fig. 1(a) using the same method. The interaction length L is set to 10.0 mm. The output-facet micrograph of the fabricated coupler is shown in Fig. 8(a). In order to verify the operation of the 3D directional coupler, broadband light was butt-coupled to the input port and then splitted to three waveguides with different ratios. NFPs of output facet at 1530 and 1560 nm are shown in Fig. 8(b). CRs of Port2 (P2/(P1 + P2 + P3)) and Port3 (P3/(P1 + P2 + P3)), and the excess loss in C-band were measured and the result is shown in Fig. 8(c). The dotted line shows the calculated results of CRs with a designed core diameter of 10 µm and a spacing D of 20 µm. The coupling ratios (P1: P2: P3) of 58:23:19 among the three waveguides at 1550 nm were experimentally obtained. The average EL in C-band is 3.2 dB. Theoretically, power coupled from input waveguide to the other two waveguides should be equal for a symmetric 3D directional coupler as calculated. However, small difference in the measured power in the two outer ports (P2 and P3) was observed, which might be caused by the differences in spacing D and different core diameters between the waveguides. The improvement on the fabrication process is necessary to realize complete symmetry control. We will further investigate the repeatability and variability of the process in fabricating 3D directional couplers.
We propose and demonstrate the design and fabrication of 3D polymer directional coupler for on-board interconnects. The couplers were fabricated by using commercially available UV-curable epoxies with the mosquito method. At first, 2D directional couplers in both horizontal and vertical directions were demonstrated. Horizontal couplers with CRs of 0.66 and 0.55 and vertical couplers with CRs of 0.13 and 0.48 at 1550 nm were demonstrated. The experiment results agree well with the simulated ones. Then, 3D directional coupler with coupling ratios of 58:23:19 at 1550 nm was fabricated successfully. The CR varies with the wavelength as expected while the excess loss remains almost the same within C-band. We also investigated the requirement on fabrication tolerance and the method to satisfy it. The 3D directional coupler can be cascaded accordingly to serve as an effective and scalable device for multi-layer and multi-channel interconnects purpose. The proposed method may be useful in the fabrication of functional devices in three dimension for on-board optical interconnects with further improvement on the fabrication process.
National Natural Science Foundation of China (NSFC) (61775138, 61620106015).
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