Published papers

This paper presents composite beam splitters realized with polymer materials for developing photonic integrated circuits. We used organic-inorganic hybrid polymer materials to form this composite beam splitter realized with step-index (SI) core profiles. We used the alternating direction implicit technique of the Rsoft CAD BeamPROP solver to design and analyze these beam splitters. We successfully examined and manipulated the beam splitter's polarization dependency to obtain a 99% output efficiency with a 50:50 splitting ratio. The SI beam splitter exhibits an excess loss of 0.014 dB. When we apply polarized light in this beam splitter, the excess loss increases to 2 dB, and this loss gradually decreases as the angle of incident light increases. The excess loss reduces to 0.05 dB at the 31-degree angles of the incident polarized light. We also investigated the crosstalk of this beam splitter by varying the wavelength, and it is evident that the lowest crosstalk is −19.77 dB at the polarized angle of 31°.

In this paper, we fabricate a polymer optical waveguide with 3-dimensional core arrangement applicable to a MUX device for mode-division multiplexing technology. We apply the Mosquito method that we developed as a fabrication technique to fabricate single-mode and multimode polymer optical waveguides with circular cores. Because of the unique fabrication procedures of the Mosquito method, 3-dimensional core arrangement is possible, and thus a polymer optical waveguide with a functionality of MUX devices that can launch all the propagating modes in a few-mode fiber (FMF) can be fabricated. We emphasize that the different higher-order modes, LP11aand LP11b in one core could be individually excited using the fabricated waveguide type MUX devices.

Using the Mosquito method, we fabricate waveguides with 3-dimensionally arranged cores to be applied to a MUX device for mode-division multiplexing technology. Applying the developed MUX, the different higher-order modes, LP11a and LP11b in one few-mode fiber core can be individually excited.

A Europium Aluminum Benzyl Methacrylate (Eu-Al/BzMA) integrated optical waveguide with 50 μm graded-index multimode circular core is fabricated by applying the Mosquito method for the potential application of optical interconnect. The parabolic index profile of the waveguide core is measured to confirm the profile exhibited by the waveguide. A preliminary experiment of the signal transmission is performed at 26 Gb/s to evaluate its capability in high dense and speed optical interconnections. According to the results, the waveguide successfully demonstrated free bit-error-rate through 5 cm waveguide transmission.

The gain performance of 50-iun core diameter graded-index (GI) multimode europium aluminum benzyl methacrylate (Eu-Al/BzMA) waveguide was investigated by numerically solving rate and propagation equations using MATLAB. At a fixed waveguide length of 10 cm and gain medium concentration of 13 wt.%, optical gain was found to be dependent on pump power and input signal power. This paper utilized a 532 nm wavelength pump with power varied from 100 mW to 500 mW, together with low (-30 dBm) and high (0 dBm) input signal powers, within the amplification range of 580 to 640 nm wavelength. With the highest pumping power of 500 mW and the lowest input signal power of -30 dBm, a 29-dB optical gain with wavelength of 617 nm was observed from forward pumping amplification. For comparison, an identical waveguide in terms of properties was fabricated through an innovative fabrication method for polymer waveguide - the Mosquito method. The fabricated waveguide was then experimentally tested for -30 dBm input signal power with 200 mW pump power in the attempt to realize future real-world applications of short reach networks such as in Local Area Network (LAN) and in-vehicle optical interconnects.

We experimentally demonstrate that the Mosquito method is capable of fabricating single-mode polymer optical waveguides with multiple cores while maintaining an identical mode-field diameter among them. The Mosquito method we developed is a technique to form circular cores in polymer waveguides using a commercially available microdispenser and multi-axis syringe scanning robot. However, the core shape tends to deteriorate from a circle because of the monomer flow due to the needle scan. We also demonstrate both theoretically and experimentally for the first time in this paper that the needles with a tapered outer form or just titled straight needles enable to form cores with a high circularity even if those cores are dispensed on any heights from the substrate surface.

We create a compact low-loss spot-size converter (SSC) which utilizes a tapered core polymer optical waveguide with circular cross-sectional graded-index (GI) core using the Mosquito method we developed. First, we theoretically analyze the mutual diffusion between the core and cladding monomers, which is a feature unique to the Mosquito method when forming GI cores. The monomer diffusion effect depends on the initial core diameter that is dispensed by a microdispenser and the diffusion time before UV curing: in a small core the monomer diffuses more rapidly than in a large core. Using this diffusion dependence on the initially dispensed core diameter, it is theoretically found that a tapered polymer waveguide based SSC can adiabatically convert the mode-field diameter between 4.0 and 8.6 µm at a 1550-nm wavelength waveguide as short as 4 mm. Next, the tapered waveguide based SSC with the designed structure is experimentally fabricated using the Mosquito method, and we confirm an 8-mm long tapered waveguide with an insertion loss of 1.83dB functions as a SSC that converts the MFD from 4.7 µm to 7.5 µm at 1550-nm wavelength.

In this paper, we design a polymer waveguide to satisfy the single-mode condition and fabricate it using the Mosquito method. In order to apply the single-mode waveguides as a connection component with single-mode fibers (SMFs) or with other optical components such as laser / photodetector arrays, there are many issues such as the core shape, the mode-field diameter (MFD) variation, and the accuracy of the core position and pitch.Hence, we experimentally and theoretically demonstrate that the Mosquito method is capable of fabricating polymer optical waveguides with single-mode cores and the above issues on the single-mode connections are addressed in this paper.

We experimentally demonstrate that the Mosquito method is capable of fabricating polymer optical waveguides with single-mode cores. The Mosquito method we developed is a technique to form planar polymer waveguides using a commercially available microdispenser. In this article, the flow of the core monomer dispensed from the tip of the needle is simulated using fluid analysis software. The results show that the shape of the needle periphery significantly affects the fabrication of perfectly circular cross-sectional cores. In addition, we demonstrate that the Mosquito method is capable of reducing the MFD variation to 0.4 μm. We find such a small MFD fluctuation is comparable to that of typical single-mode optical fibers.

We experimentally investigate the polarization dependence of the optical properties of single-mode polymer optical waveguides. We compare two types of waveguides composed of the same polymer materials: Step-index square-core waveguides fabricated using the direct curing method and graded-index (GI) circular-core waveguides fabricated using the Mosquito method. We demonstrate that the GI circular-core single-mode waveguides exhibit remarkably small dependence of optical loss on the polarization angle, which is less than 0.1 dB polarization dependent loss (PDL) at both 1310 and 1550 nm wavelengths. This steady polarization property is attributed to the material stress free to the composing polymers during the fabrication process of the Mosquito method and to parabolic GI profile formed in symmetric circular core.

© 2019 IEEE. In polymer optical waveguides fabricated using the Mosquito method, the diffusion occurs between the core and cladding monomers during the interim time that is from the moment that the liquid core monomer is dispensed to when the UV curing begins, and a graded-index type refractive index profile is formed in the core. In this paper, the diffusion coefficient of monomer is measured. Then, a diffusion simulation model is created, which can calculate the concentration distribution of monomer using the diffusion coefficient that depends on the monomer concentration. Finally, the refractive index profiles formed in the cores are simulated and the output near-field patterns are theoretically estimated to evaluate the core diameter and numerical aperture dependences on the interim time.

© 2019 IEEE. In this paper, we propose a low loss and cost-effective assembly in optical transceiver for on-board applications by applying graded-index (GI) core polymer waveguides to effectively couple the signal light between light sources/photodetectors and multimode fibers (MMFs). In order to satisfy the requirements for higher bandwidth data transmission, on-board optical interconnects have been anticipated in datacenter networks. When applying active optical cables (AOCs) to the on-board interconnects, the assembly density between optical devices and fibers is an important issue. Hence, polymer optical waveguides with densely aligned cores are one of the promising candidates to be an interface between the active components and fibers. In this paper, GI circular core polymer optical waveguides are applied to the onboard wiring. We demonstrate the ability of GI core waveguide to achieve low-loss coupling with optical transceivers due to tightly confined optical field in the core. Furthermore, we propose to utilize the lens function of GI core in the transceiver assembly. We theoretically demonstrate that the GI core enables to improve the coupling efficiency by 0.6 dB as a cylindrical lens. Meanwhile, by just inserting a perpendicularly bent GI core waveguide between an optical transmitter and an MMF, we successfully transmit a 53.125-Gbit/s PAM4 optical signals experimentally.

© 2019 IEEE. We have proposed the Mosquito method for fabricating polymer optical waveguides. By applying the Mosquito method, three-dimensional multimode and even single-mode graded-index (GI) circular cores can be formed. In this paper, for fabricating low-loss fan-in/out waveguide using the Mosquito method, the fabrication conditions are investigated to improve the uniformity of optical characteristics in multiple cores. Furthermore, the hexagonally arrayed single-mode cores are formed under a precise temperature control.

© 2019 IEEE. We fabricate polymer optical waveguide amplifiers with europium (Eu)-aluminum (Al) polymer composite incorporated in their cores applying the Mosquito method. With decreasing the length of the waveguide with a step of 1 cm (cut-back method), the length dependence of the optical gain is measured under signal and pumping light is coupled to the core coaxially. The optical gain tends to increase as the waveguide length increases.

© 2019 IEEE. In this paper, microchannels with circular cross-sections are fabricated using the Mosquito method in order to mimic microvascular networks. We experimentally confirm that microchannels with straight and Y-branched channel patterns are fabricated by applying photo curable PDMS(poly dimethyl siloxane) to the Mosquito method. Compared to our previous trials with heat-curable PDMS, the photo-curable PDMS allows to fabricate the microchannels in one-third of curing time. Finally, liquid flow in the fabricated Y-branched microchannels is confirmed by flowing water with red-colored pigment: the water with pigment is injected to a channel from the two-port side and the state of the liquid flow in the microchannel is observed.

© 2019 IEEE. In this paper, we evaluate the optical properties of low-loss Y-branched multimode polymer optical waveguide couplers with graded-index (GI) cores fabricated by different methods: the Mosquito method and the imprint method in order to apply them to coarse wavelength division multiplexing (CWDM) transmission. We confirm the low insertion loss of those waveguides regardless of fabrication method in a wavelength range from 850 to 1060 nm for CWDM. Furthermore, we succeeded in transmitting a 26 Gbps signal through the GI waveguide at 850-nm wavelength.

© 2019 IEEE. The polarization-dependent optical properties of single-mode polymer optical waveguides are experimentally investigated. We apply the two fabrication methods for single-mode polymer optical waveguides: the direct curing method and the Mosquito method, and compare the optical properties of step-index square-core waveguides and graded-index circular-core waveguides fabricated by the above two methods, respectively.

Polymer optical waveguides fabricated using the Mosquito method are expected to realize high bandwidth density 3- dimensional (3-D) on-board wiring. In the Mosquito method, the waveguides are fabricated by dispensing a liquid core monomer into a liquid cladding monomer using a microdispenser. Hence, for the on-board applications particularly 3-D wiring, the core position and alignment accuracies are important to couple the waveguides with the other optical components with high efficiency. We already succeeded in fabricating graded-index core multimode polymer optical waveguides with low propagation loss using the Mosquito method. However, the positions of the formed cores tended to deviate from the original design, since both the core and cladding monomers are in the liquid state during the Mosquito process. In this paper, we apply a fluid analysis simulation using a COMSOL Multiphysics® in order to theoretically simulate the influence of several fabrication parameters on the core position. The calculated core height deviation from the designed height is dependent on the needle-tip height, because the core positions are influenced by the pressure distribution of cladding monomer caused by the monomer flow. Meanwhile, we find that the monomer wetting on the needle outer wall also affects the core height. When the effect of monomer wetting is taken into account, the simulated core heights are different from the results without the effect of monomer wetting and we can theoretically predict the height of the formed core. Finally, we confirm that the core height can be controlled by adjusting the needle-tip height setting in which the effect of the monomer flow and wetting theoretically calculated is taken into account in the Mosquito method.

The IP traffic in datacenter has rapidly increased over the last couple of years because of the wide deployment of cloud computing services. Hence, multimode fiber (MMF) links are already deployed in the rack-to-rack networks particularly in large-scale datacenters to sustain the rapid growth of data traffic. In recent years, the network topology in datacenters shifts from the legacy tree-type to fabric-type: the leaf and spine structure is requiring higher-bandwidth distant product for the MMF links to maintain low latency. However, the modal dispersion has been a large issue to realize high bandwidth links.To address the problem, we focus on the restricted mode launch (RML) approach, by which optical power is coupled to limited numbers of propagating modes in MMFs. Key parameters for the RML are the numerical aperture (NA) and the spot size on the MMF launch end: both of them should be small to reduce the modal dispersion. These are difficult to achieve with small-size launch systems composed of microlenses. Thus, in this paper, a new compact RML device without using microlenses is designed and fabricated: an axially tapered graded-index (GI) core polymer optical waveguide is applied to the RML exciter. Firstly, the launch condition for a 1000-m long OM3 fiber is investigated by calculating the effective modal bandwidth (EMBc). To realize OM4-grade EMBc, we find that the NA and the spot size of the launching beam should be lower than 0.118 and 33.2 m, respectively. Also, the polymer waveguides are designed to meet the above conditions. The optimum refractive index profile of the tapered core is calculated by solving the Fick’s diffusion equation, and then using the beam propagation method (BPM), the spot sizes and NAs from the tapered polymer waveguides are simulated. Finally, we fabricate GI core tapered waveguides using the imprint method.

In this paper, crossed polymer waveguides with graded-index (GI) square cores are fabricated using the soft-lithography method. We experimentally demonstrate that the fabricated GI-core crossed waveguides exhibit a much lower insertion loss than conventional step index (SI)-core counterparts, which is almost independent of the cross angle. We also show in this paper that the crossed waveguides fabricated applying organic-inorganic hybrid resins show remarkably high thermal resist ance compared to the waveguides fabricated utilizing an acrylate resin and a dopant system we previously reported.