Published papers

In this paper, the conditions for fabricating Y-branched optical waveguides using the Mosquito method are discussed, focusing on the viscosity of the monomer and the branch angle. With increasing the viscosity of core monomer, the Y-branching structures tend to be distorted particularly when the branching angle is larger than 0.5 degree. After finding out the appropriate fabrication conditions, Y-branched optical waveguides in which two-port cores are vertically stacked are fabricated, taking the advantage of the Mosquito method: the ability to form three-dimensional core wiring patterns. Single-mode and multimode cores are successfully formed in which we could confirm the light propagation.

Using the Mosquito method, we fabricate asymmetric single-mode Y-branched waveguides applicable to MUX/DEMUX devices for MDM technology. In order to reduce the insertion loss of Y-branched waveguides, we reexamine the fabrication conditions of symmetric single-mode Y-branched waveguides.

This paper demonstrates step-index (SI) and graded-index (GI) core-based multimode interference (MMI) splitters realized with polymer materials for photonic integrated circuits (PICs). These SI and GI index-based MMI splitters guide the input light through the multimode waveguide following the self-imaging principles and demonstrate the multiple outputs with the same uniformity when the self-imaging length is obtained. We numerically design and develop (1 ×5) MMI splitters using the beam propagation method (BPM), and we use the RSoft CAD BeamPROP solver for the structural design and waveguide characterization. The structures of the SI and GI MMI splitters are optimized by investigating the structural parameters, particularly the width and length of the multimode waveguide and the core spacing between the output ports with the maximum uniform intensity at the self-imaging length. We also compare the structural design and optical properties of the SI and GI MMI waveguides. The excess losses of the SI MMI splitter are 1.09 and 1.12 dB for transverse electric (TE) and transverse magnetic (TM) modes, respectively, while for the GI MMI splitter, the losses are 0.57 dB and 0.56 dB for TE and TM modes, respectively at 1.55 µm. The power splitting efficiency for the SI MMI splitter is 77%, and for the GI MMI splitter, it is 88%. We also investigate the polarization dependence loss (PDL) for both MMI splitters, and the losses for the SI and GI MMI splitters are 0.024 dB and 0.004 dB, respectively.

We theoretically calculate the coupling loss between single-mode fibers (SMFs) and polymer optical waveguides with different refractive index profiles. Temperature resistance is one of the concerns when integrating polymer optical waveguides near the electronics and photonics chips where they are highly likely to dissipate heat. Hence, the calculation focuses on the core size variations due to changes in operating temperature. The calculated results show that the refractive index variation with respect to temperature has a greater impact on coupling loss compared to the temperature-dependent core size variation. Then, polymer optical waveguides are experimentally fabricated and characterized. The temperature dependence of the output light intensity distribution from the fabricated waveguides is observed, as it is estimated by the theoretical calculation. In addition, it is also found that graded-index cores in polymer waveguides demonstrate slightly higher temperature stability in connection with SMF than their step-index counterparts. Such a low coupling loss with SMF could be maintained even in an environment at 100 °C, making polymer optical waveguides promising for the application of co-packaged optics.

Mosquito method is used to fabricate a 50 μm core diameter graded index (GI) multimode Eu-Al doped polymer composite optical waveguide amplifier. In this paper, waveguide length and pump power are varied with an input signal of - 30 dB m. Under a 532 nm laser excitation, 16.27 dB optical gain under an optimal 10 cm waveguide length is observed from forward pumping amplification for a threshold pump power of 23 dB m. Also, the waveguide has been characterized to be 0.83 dB/cm at 617 nm wavelength.

To achieve a subminiature low-loss adiabatic coupler to silicon photonics chips, we design a PMT connector-integrated polymer waveguide device with circular single-mode cores to fabricate it using the Mosquito method.

To realize a subminiature low-loss adiabatic coupler to silicon photonics chips, we design a PMT connector- integrated polymer waveguide device with circular single- mode cores to fabricate it using the Mosquito method.

In this paper, we present a low-loss optical assembly utilizing a 90°-bent graded-index (GI) core polymer optical waveguide on vertical cavity surface emitting laser (VCSEL) based optical transceivers. The proposed assembly can replace conventional components such as mirrors and lenses for realizing subminiature optical engines applicable to on-board integration. To minimize the total insertion loss of the waveguide when connected to a high-speed VCSEL and a GI-core multimode fiber (MMF) at each end, the characteristics of the beam emitted from VCSELs are measured and taken into consideration for the waveguide design. In order to confirm the effect of insertion loss reduction by the waveguide numerical aperture control, 90°-bent GI-core polymer waveguides are fabricated applying the Mosquito method. The fabricated waveguide exhibits a total insertion loss as low as about 2 dB at 850-nm wavelength, which includes the coupling losses at both ends, bending, and propagation losses. We also investigate a way to reduce the insertion loss when a gap exists between the waveguide and VCSEL chip. We theoretically and experimentally confirm that filling the gap with a high index resin can reduce the coupling loss by 5 dB.

Compact and low-loss fan-in/-out waveguides are successfully fabricated using the Mosquito method, in which four cores vary their arrangement three-dimensionally from one end to the other and they are precisely aligned to couple to multicore fibers.

In this paper, a compact (10-mm long) and low-loss fan-in/-out polymer waveguide is successfully fabricated using the Mosquito method, in which four cores three-dimensionally vary their arrangemnent from one end to the other and they are precisely aligned to couple to a multicore fiber.

In this paper, we experimentally introduce a low-loss 3-dimensional (3D) symmetric crossover multimode optical waveguides realized with parabolic graded-index (GI) profiles using the Mosquito method. This compact crossover multimode waveguide is comprised of two S-bend core groups (6 Ch. × 6 Ch.), where the last six cores cross over the first six cores, maintaining a height gap at the crossover section to avoid the smudging from needle scan. The suitable height gap for this crossover multimode structure is 0.7 mm. The insertion loss of this multimode crossover waveguide is 1.64 dB at 850 nm wavelength. The insertion losses of this multimode crossover waveguide are compared with separately fabricated 3D S-bend waveguides, and it indicates that multimode crossover structures do not exhibit remarkable excess loss. The interchannel crosstalk of this waveguide is less than -45 dB. This crossover multimode waveguide demonstrates sharply open optical and electrical eye diagrams and transmits error-free data at 26 Gb/s.

In this paper, Y-branched circular core single-mode/few-mode polymer optical waveguides are designed and fabricated using the Mosquito method we have developed. They comprise a low loss multiplexing (MUX) device for mode division multiplexing. In the Mosquito method, since a liquid core monomer is dispensed into another liquid cladding monomer while the needle scans along the path of the wiring patterns, it was difficult to form in-plane core crossings and core branches. In this paper, to form single-mode/few-mode Y-branched cores, we apply a unicursal needle-scan path for the Y-branch structure with the Mosquito method. For MUX device applications, cores satisfying the single-mode condition are successfully formed on the two-port side while the one-port side has a few-mode core.

In this paper, circular cross-section microchannels with 3-D lattice arrangements are designed and fabricated using the Mosquito method in order to construct on-off valves. The 3-D microchannels with on-off valves consist of two types of lines: the flow lines for chemical liquid flow and the control lines to activate the valves. We confirmed that both a circular cross-section and a PDMS with low elastic modulus used as the microchannel material contribute to a valve that can be closed with a lower pressure. Then, we demonstrated liquid flow to evaluate the functionality of the valve. Fluorescein solution was flown into a flow line. We found that the fluorescence intensity decreases at the intersection between the flow and control lines when the flow line is closed by the inflation of the control line, experimentally confirming the functionality of the valve microchannels fabricated via the Mosquito method.

This paper describes the characteristics of graded-index (GI) core Y-branched polymer optical waveguides as optical couplers for multimode fibers (MMFs) in coarse wavelength division multiplexing links with a restricted mode launch function. In this paper, we compare two GI-core Y-branched waveguides fabricated using different methods: the Mosquito method and the imprint method. Error free transmission is confirmed when transmitting a 25.6125-Gbps NRZ optical signal (PRBS31) at 850-nm wavelength through a 100-m long OM3 fiber using the waveguide as an optical coupler. We also discuss the function of a restricted mode launch exciter for a 1-km long MMF. From the effective modal bandwidth analysis, realizing higher bandwidth-distance product in long MMFs should be possible by reducing the core size of the waveguide coupler.

The Mosquito method, a polymer optical waveguide fabrication method invented by Professor Ishigure at Keio University, enables the three-dimensional (3D) formation of core patterns in cladding using a commercially available micro dispenser and a multi-axis syringe-scanning robot. This fabrication method is expected to be used to realize devices required for 3D waveguides such as Fan-in/Fan-out (FIFO) devices, which are essential for the introduction of multicore fibers (MCF). On the other hand, prior studies have mainly examined multimode waveguides, and several issues need to be solved for making single-mode devices. In particular, the core shape tends to deteriorate from circular due to monomer flow caused by needle scanning in the cladding, which is a factor that increases the connection loss with the optical fiber. This paper presents a theoretical and experimental study on the fabrication of single-mode waveguides by the Mosquito method and the formation of circular cores using the method to reduce connection loss with single-mode optical fibers.

We present unique polymer optical waveguide coupler devices fabricated using the Mosquito method to apply for adiabatic coupling with Si photonics chips, three-dimensional fan-in/fan-out devices for multicore fibers, and MUX devices for mode division multiplexing.

We present unique polymer optical waveguide coupler devices fabricated using the Mosquito method to apply for adiabatic coupling with Si photonics chips, three-dimensional fan-in/fan-out devices for multicore fibers, and MUX devices for mode division multiplexing.

In this paper, we design and demonstrate polymer-based polarization beam splitters realized with parabolic graded-index (GI) directional couplers at 1550 nm. We design and investigate these beam splitters by using the beam propagation method (BPM) and the RSoft CAD BeamPROP solver is used for model design and characterization. We study the splitting ratio of 50:50 and 100:0 of these beam splitters for both transverse electric (TE) and transverse magnetic (TM) modes of the polarized light. In particular, the 50:50 beam splitters exhibit an output efficiency of 95% due to the absence of polarization. However, in the presence of the polarization effect, the polarization dependency is analyzed and manipulated successfully, and the output efficiency increases by approximately 98% at an incident angle of 31° due to the polarized launch field. Moreover, the 100:0 beam splitters act as the switching devices realized with parabolic GI polymer directional couplers. The insertion losses of the GI beam splitters are 0.162 dB and 0.186 dB, while the excess losses are 0.148 dB and 0.149 dB for TE and TM modes, respectively. Also, the polarization extinction ratio (PER) of the 100:0 beam splitters is >25 dB and >20 dB for TE and TM modes, respectively.

We simulate the optical properties of polymer optical waveguides with different refractive index profiles in their cores as coupling components (edge couplers) between single-mode fiber and SiOx waveguides. In this paper, we focus on the single-mode operation of graded-index (GI) core polymer waveguides, for which we previously demonstrated low propagation loss under multimode operation. We design the optimum core structure (size and index contrast) for different refractive index profiles, and then demonstrate the unique optical properties of GI waveguides contributing to the low optical loss compared to the step-index counterparts, in particular, mode field diameter variation and taper angle tolerance.