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Abstract
The coupling loss between optical devices is a critical factor affecting the performance of optical interconnect. This paper quantitatively studies the effectiveness of using a dye-doped-epoxy-based self-written waveguide (SWW) to reduce the coupling loss in optical interconnect caused by large mode-field mismatch and lateral offset. We formed SWW between single-mode fiber (SMF) with different mode-field diameters (MFD) and a 5 × 2 µm rectangular channel waveguide-under-test (WUT). For the case between a SMF with a mode-field diameter of 9.4 μm and the WUT, the coupling loss is -11 dB. After forming the SWW, the coupling loss is reduced by 8.34 dB. Using SWW, the lateral tolerance length between a SMF with a mode-field diameter of 4.5 μm and the WUT increases by 2.5 times. Under the above-mentioned situation, the coupling loss falls less than 0.20 dB over ± 2 μm lateral offset range. Our findings offer insights quantitatively for coupling loss reduction and relaxing the lateral tolerance under significant mode-field mismatch conditions.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Optical interconnect has received considerable attention in telecommunication networks due to its broadband bandwidth advantages compared with electrical connections. One main issue is how to achieve a low coupling loss connection between different optical devices using optical fibers. Mode-field mismatch is one key reason which causes coupling loss. Size and geometric difference are two critical factors for the mismatch. With the application of high refractive index contrast materials, optical devices tend to be integrated, which is an effective way to increase optical chip packing density in optical interconnect. However, there is a severe problem integrating optical devices/waveguides and optical fibers [1]. When they are butt-coupled, it causes a high coupling loss due to the large mode-field mismatch caused by the size difference. And conventional packaging approaches cannot effectively address the mode-field mismatch coupling issue caused by the waveguide's geometric difference [2–5]. The grating couplers and spot size converters have been used to solve these problems [6–11]. But the grating couplers approach is wavelength dependent. On the other hand, the spot size converters approach requires special designed tapers, which may increase the complexity of the overall design and the fabrication cost. Moreover, there usually is a slight offset under temperature changes between those aligned, packaged optical devices, which significantly causes optical loss because of the small core sizes of optical devices.
A passive alignment approach based on light-induced self-written waveguide (LISWW) techniques has been proposed to settle these issues [12–17]. The mechanism of the methods is to form an optical waveguide in photocurable epoxy by irradiating suitable laser light through an optical fiber. The SWW optical waveguide, once created, will not disappear even when the light source is removed, which acts as an optical link between cores of optical devices. Thus, the approach has many advantages: high mechanical strength, less fluctuation of optical coupling loss, and good alignment tolerance [18]. Besides, the LISWW technique can achieve self-alignment between optical devices during connection [19]. In this way, the costly active alignment process can be avoided. Since the approach requires only light irradiation through fibers and waveguides to achieve optical interconnect, it is simple and efficient compared with other methods. These advantages have attracted plenty of research upon LISWW for optical interconnect.
In previous studies [19–21], light-induced self-written optical waveguides were fabricated using a UV or visible laser light source in photocurable resins. Due to optical transparency constraints, the methods mentioned above do not apply to silicon photonics. Some research groups took advantage of the two-photon absorption (TPA) mechanism to cope with the problem [22–24]. Given the attractive optical properties like high optical nonlinearity and good electro-optical modulation ability, emerging materials such as lithium niobate [25–27] and chalcogenide glass [28,29] widely implement in recent photonics design. Compared with silicon, the materials mentioned above have good optical transparency ranging from visible to infrared regimes. As a result, we may not need to use the more tedious TPA method to achieve SWW. Among all the works mentioned above, only few studies have quantitatively discussed reducing coupling loss between large mode-field mismatch optical devices based on SWW in optical interconnect.
For the first time, this paper quantitatively investigates the impact of using dye-doped-epoxy-based SWW for coupling loss reduction and the relaxation of the lateral alignment of a SMF and the WUT with a large mode-field mismatch due to their size and geometric difference. Our study should provide a practical insight into other dye-doped epoxy material systems to address the coupling issue between fibers and waveguides with a large mode-field mismatch.
2. Experiment
2.1 Material selection
Since we use dye-doped epoxy to form SWW, careful consideration of the materials used is necessary. In our study, the dye is dispersed into the epoxy. Thus, it is an important issue to select the proper dye and epoxy. Many types of epoxies are commercially available for optical interconnect. Compared with other packaging materials, the epoxy we used in our experiment is called MasterBond UV11-3. It is a commercially available material for UV-curable resin and is usually used in optoelectronics and packaging. The reason why we select this kind of epoxy is that it is optically transparent. It also has a large range of service temperatures (-50 °C to 120 °C). Thus, the packaging can keep in good condition when exposed to temperature changes. Another reason for selecting this epoxy is its relatively low viscosity (60 centipoises), which is good for filling the epoxy between two optical components. We used the most common xanthene dye in the experiment [30]. Among all xanthene dyes, the Rhodamine type has been viewed as the most important group. And Rhodamine 6G (R6G) was used in our experiment. The compound name of it is (9-(o-(ethoxycarbonyl)phenyl)-6-ethylamino-2,7-dimethyl-3-xanthenylidene) ethyl ammonium chloride. The chemical structure of R6G is shown in Fig. 1.
We chose R6G because of its absorption peak at a wavelength of around 530 nm allowing us to use green laser light at 532 nm to form the SWW by the photo-polymerization process. Besides, it has a large fluorescence spectrum, providing enough photon energy for photo-polymerization [31].
The polymerization process is a chemical reaction that converts together multiple monomer molecules to form a polymer chain. When exposed to green light, dye R6G absorbs the photon and exhibits donor properties. The pyran ring, a relatively weak bond in R6G, opens to release free radicals. The radicals are then pushed in their excited states and react with the monomer to form the bonding and initiate the polymerization process. The process of the photo-polymerization by radicals is shown by the following [32]:
2.2 Experiment demonstration
Figure 2 illustrates the schematic of our experiment process. We used a coupler to launch a 532 nm green laser and a 1550 nm infrared laser together to the waveguide through a single-mode fiber. Mode stripper was used to eliminating high-order modes of green laser. The output power of the fiber was measured by a power meter which is labeled as P0. A 10X lens was used to focus its power on a power meter for measurement. We applied a pin-hole placed before the photodetector to block the stray light. We used the working power P0 of green laser to control the shape and size of SWW. While the infrared laser is used to measure the coupling loss between fiber and waveguide. And we need to choose a suitable power of green laser to form SWW. Since a minimum threshold power is needed to initiate the photo-polymerization process. However, too intensive green light may excite multimode, which will affect the shape of SWW.
There are two types of single-mode fibers used in our experiment. SMF130V: single-mode at 1550 nm with 9.4 μm mode-field diameter (MFD). SM1500: single-mode at 1550 nm with 4.5 μm MFD. The waveguide-under-test (WUT) is a polymer channel waveguide with a 2.5 dB/cm transmission loss. The core size of the waveguide is 5 × 2 μm. It is an asymmetric waveguide with a significant shape difference with fiber. Figure 3 shows the cross-sections of SMF130V, SM1500, and polymer channel waveguide with their corresponding simulated mode-field profiles based on their parameters.
Fig. 3. (a) Cross-sections and (b) simulated mode-field profiles of SMF130V, SM1500, and polymer channel waveguide, respectively.
We employed Finite-difference-time-domain (FDTD) simulation to investigate the mode-field distributions of fibers and the WUT. By simulation, we found that the mode-field areas of SMF130V and the WUT are 70.18 and 18.78 μm2, respectively. As shown in Fig. 3(b), the mode-field mismatch between them is significant. Thus, when they are connected, there exists a large coupling loss. We created a SWW within dye-doped epoxy to test if our approach can effectively reduce the coupling loss between them.
Figure 4(a) shows the schematic of the SWW formation process between a fiber and a waveguide. First, we used a micropositioner to pre-align the fiber and the WUT and created a gap between them. Then, we filled the gap with 0.1 percentage weight (%-wt) dye-doped epoxy. Next, green laser light was launched through the fiber tip and reached the photo-polymerizable epoxy material. The light at the center of the core should be the most intensive. And the refractive index increase inside the dye-doped epoxy is proportional to the propagating green light intensity. Hence, the refractive index increase at the core regime should be higher than the regime away from the core [33].
Fig. 4. (a) Schematic diagram of the SWW formation process; (b) the microscope image of a SWW formed between a fiber (SMF130V) and the WUT. The inset in Fig. 4(b) shows the SWW formed at the edge of the fiber after removing the uncured epoxy.
Consequently, the region with a higher refractive index can confine and firmly guide the light in the following steps. Dye R6G is sensitive to the green light, absorbing the light photon and initiating the photo-polymerization process. When R6G is cured, it no longer absorbs green light, maintaining a low transmission loss. Eventually, the self-focusing effects continuously contribute to the growth of the optical waveguide along the light propagation direction. Since the length of the SWW is determined by the air gap between the fiber and the waveguide, its typical length is only a few tens of microns. In this case, we can neglect the propagation loss of the SWW when considering the coupling loss between the fiber and the waveguide. Once the SWW is formed, it can provide a relatively low-loss optical link between the cores of fiber and waveguide. Figure 4(b) shows the top view image of a SWW formed between a SMF130V and the WUT. The inset in Fig. 4(b) shows the SWW formed at the edge of the fiber after removing the uncured epoxy.
3. Result and discussion
In this section, we experimentally investigated the performance of the SWW in two aspects, including reducing coupling loss and improving lateral tolerance between large mode-field mismatched optical devices. First, we verified whether SWW can effectively reduce the coupling loss between two identical single-mode fibers. Then, we investigated the optimal gap distances for the formation of SWW between SMF and the WUT. Under the optimal gap distance, the performance of SWW in reducing the coupling loss and improving lateral tolerance between our structural cases were both investigated.
We defined a figure of merit (F) to quantify the performance of SWW in reducing the coupling loss, especially for those under large mode-field mismatch conditions. The figure of merit (F) is defined by,
where CLR is the measured coupling loss reduction, and Md is the mismatch degree of the SWW formed at the interface between two optical devices under investigation. Md is obtained by evaluating their electric field mode distribution's power overlap efficiency (η).The power overlap efficiency (η) is given as [34],
3.1 Butt coupling between single-mode fibers
In our experiment, one type of SMF used is SMF130V. An entire fiber was cut into two parts in the middle. Two fiber parts were cleaved in good quality and aligned in direct contact with each other. We first measured the transmission power under the direct contact condition as a reference for other coupling conditions. We defined that the coupling loss as zero under the reference condition. Then, we created a 100 μm air gap between two identical fibers by a micropositioner to create high mode-field mismatched conditions. After that, we filled the gap with dye-doped epoxy and formed a SWW by irradiating the green laser. The working power P0 of the green laser is -4.74 dBm. The experimental results are shown in Table 1.
Table 1. Butt coupling between single-mode fibers (SMF130V)
According to Table 1, when the SWW was formed between two fiber cores, we found that the coupling loss is reduced by 2.33 dB compared with the situation at 100 μm gap distance. It is because SWW can effectively confine and guide the light between two fibers. Therefore, SWW can reduce the coupling loss between optical devices with mismatched mode-field. Next, we investigated the performance of SWW under the condition of butt coupling between a SMF and the WUT with a large mode-field mismatch.
3.2 Optimal air gap distance for the formation of the SWW
Since SWW is fabricated within a gap between optical devices, it is necessary to study whether the gap distance affects the performance of SWW in reducing coupling loss. Therefore, we created SWW in different air gap distances (dG) between a SMF130V fiber and the WUT using a micropositioner. The working power P0 of the green laser is -4.74 dBm. Figure 5 shows the measured coupling loss reduction at different gap distances.
We observe from Table 1 that SWW can reduce coupling loss between two optical devices. Therefore, we can determine the roughly optimal air gap distance according to the coupling loss reduction for evaluating the performance of SWW. For example, in Fig. 5, the coupling loss reduction between the SMF130V fiber and the WUT is the most significant at 30 μm air gap distance. After forming the SWW, we broke the SWW away from the waveguide and removed the uncured epoxy surrounding the SWW.
Figure 6 shows that the SWW is like an optical taper. The mechanism behind was explained in the Section. 2.2. And its upper end, as shown in the inset, is circular in the shape with diameter D ∼ 5 μm. It should be noted that the diameter of the bottom part of the SWW is about 9 μm. It means that the SWW acts like an optical taper to converge the mode-field at the fiber end to a smaller mode-field diameter at the waveguide end. Under the situation, we roughly estimate the mode-field area as 19.63 μm2 (by πD2/4). The figure is very close to the mode-field area of the WUT (18.78 μm2). Based on the simulation and Eq. (7), the power overlap efficiency (η) is 0.81. SWW can now effectively match the mode-fields between the SMF130V fiber and the WUT. That is why the SWW can give the largest coupling loss reduction at 30 μm air gap distance. We observe that the coupling loss reduction is less than the maximum loss reduction value showing a more significant mismatch at other air gap distances. Consequently, the power overlap efficiency will be lower accordingly. Under optimal air gap distance condition, SWW can achieve the best performance in reducing the coupling loss. And we can obtain the optimal air gap distance for SM1500 fiber and the WUT using the same method. The working power P0 of the green laser is -7.78 dBm. The experimental results are shown in Fig. 7.
According to Fig. 7, the optimal air gap distance for SM1500 fiber is around 10 μm. In the subsequent experiments, we compared the performance of SWW in reducing coupling loss between two types of fibers and the WUT at their corresponding optimal air gap distances, i.e. SM1500 @ 10 μm, SMF130V @ 30 μm, respectively.
3.3 Butt coupling between the fiber (SM1500) and WUT
We created a SWW between the SM1500 fiber and the WUT within the optimal gap distance of 10 μm. The working power P0 of the green laser is -7.78 dBm. We set the aligned direct contact between fiber and waveguide as a reference condition to study the performance of SWW. The experimental results are shown in Fig. 8.
The red horizontal dotted line indicates the average value of measured coupling loss reduction (CLR) of test samples. According to Fig. 8, when SWW was formed between fiber and waveguide, the coupling loss is reduced by 3.22 dB compared with the reference condition. The measured coupling loss under the reference condition is -4.27 dB. After SWW interconnection, the fiber-to-waveguide coupling loss can be reduced to -1.05 dB. Based on simulation and Eq. (7), we can obtain the power overlap efficiency (η) as 0.82. Thus, Md between the SM1500 fiber and the WUT is 0.18. Since CLR is 3.22 dB, F is calculated by Eq. (5) as 0.58 dB (0.18×3.22).
3.4 Butt coupling between the fiber (SMF130 V) and WUT
To demonstrate the performance of SWW in reducing coupling loss under significant mode-field mismatch conditions, we created a SWW between the SMF130V fiber and the WUT within the optimal gap distance of 30 μm for comparison. In comparison with SM1500 fiber, SMF130V fiber has a larger MFD of 9.4 μm. Thus, Md between SMF130V fiber and the WUT is more significant than SM1500 fiber and the WUT. The working power P0 of the green laser is -4.74 dBm. The experimental results are shown in Fig. 9.
The red horizontal dotted line indicates the average value of measured coupling loss reduction (CLR) of test samples. According to Fig. 9, the coupling loss between the SMF130V fiber and the WUT is reduced by 8.34 dB. The measured coupling loss under the reference condition is -11 dB. After forming the SWW, fiber-to-waveguide coupling loss is reduced to -2.66 dB. Using a similar analytical way, we obtain the power overlap efficiency (η), mismatch degree (Md), and figure of merit (F) as 0.60, 0.40, and 3.34 dB, respectively. In comparison with Section. 3.3, F is more prominent under more significant mode-field mismatch conditions, which means SWW can effectively reduce the coupling loss, in particular, under a large mode-field mismatch condition. Furthermore, we observed a more significant reduction in coupling loss when the mode-field mismatch is more significant after SWW interconnection. Our findings further demonstrate the excellent performance of SWW in addressing coupling issues of large mode-field mismatch optical interconnect.
3.5 Lateral tolerance of butt coupling between the SM1500 fiber and WUT based on SWW
In optical interconnect, due to the coefficient of thermal expansion (CTE) mismatch of different materials [35], there usually exists a slight lateral offset (dL) between aligned optical devices under temperature changes. Since the core sizes of optical devices are generally in the micron or submicron range, a slight offset causes high coupling loss. Since SWW functions as an optical link between two optical devices, we expect that SWW should relax a certain degree lateral offset. So, we also quantitatively investigated the effect of SWW on improving the lateral tolerance in optical interconnect.
We created a SWW between SM1500 fiber and the WUT within a 10 μm gap distance under a good lateral alignment condition. After forming the SWW, we applied some lateral offsets between them using a micropositioner. We then measured the coupling loss for each misalignment condition. The schematic of lateral offset and experimental results are shown in Fig. 10 and Fig. 11, respectively.
Fig. 11. Lateral tolerance of butt coupling between fiber (SM1500) and the WUT based on SWW connection.
We set the aligned direct contact between fiber and waveguide as the reference condition and set coupling loss under the condition above as zero. A positive value of coupling loss means an improved coupling condition. That is to say, the coupling loss between fiber and waveguide is further reduced compared with the reference condition. According to Fig. 11, when the fiber and waveguide directly align with each other, the lateral tolerance is stringent. A slight lateral offset leads to a high coupling loss. A gap distance can release the lateral tolerance to some extent. Based on SWW, the coupling condition can be improved and remain in good status even there exists some lateral offsets. We successfully maintained the coupling loss within 0.20 dB in ± 2 μm lateral offset range. To further intuitively illustrate the performance of SWW in improving the lateral tolerance between fiber and waveguide, we defined a conceptual lateral tolerance range (LTR) which refers to the range at which the coupling loss variation is within 1 dB from its maximum value. Under the direct contact condition, LTR is ± 1 μm (2 μm in total). When the SWW was formed between fiber and waveguide, LTR is around ± 2.5 μm (5 μm in total), which is improved by 2.5 times (3 μm). Therefore, SWW can effectively help to extend the lateral tolerance between two optical devices in optical interconnect. The findings are of great significance since the lateral offsets in optical interconnect are usually small but can cause high coupling loss due to temperature variation.
4. Conclusion
This paper provides a quantitative study on the coupling loss problem due to the mode-field mismatch and lateral offset between two optical devices using a dye-doped-epoxy-based SWW. Based on our findings, we found that the more significant the mode-field mismatch between two optical devices, the greater coupling loss can be reduced by SWW. However, a larger mode-field mismatch condition will lead to a larger initial insertion loss. We defined a figure of merit and studied the coupling loss between two types of single-mode fibers coupled with an asymmetric WUT. Our study found that the coupling loss is reduced by 8.34 dB from -11 dB and by 3.22 dB from -4.27 dB, respectively, between a SMF130 V & SM1500 and the WUT. We believe that the present work is worth extending to the investigation of more significant mode-field mismatch platforms such as Lithium Niobate-on-Insulator (LNOI). Moreover, we need to investigate various mismatch cases to see whether a net reduction in the absolute gain can be obtained. Nevertheless, the application of other material systems with higher index contrast against irradiation is also worth considering in the future work. Finally, by applying SWW, we also demonstrated the lateral tolerance range (LTR) was extended by 3 μm between SM1500 fiber and the WUT. Within ± 2 μm lateral offset range, the coupling loss fluctuation can be kept within 0.20 dB. The above findings are of great significance since a slight lateral offset due to CTE mismatch under temperature changes usually causes high coupling loss in optical interconnect. In conclusion, we should effectively relax the lateral tolerance constraint and maintain a good connection quality between two different optical devices with large mode-field mismatch by using SWW techniques appropriately.
Funding
City University of Hong Kong (SRG-Fd 7004826).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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