Papers - Koike, Yasuhiro

Graded-index plastic optical fiber for needle-sized lens and rigid endoscope

Yamashita H., Koike I., Nakamura M., Koike Y.

Proceedings of SPIE - The International Society for Optical Engineering 13389 2025

ISSN  0277786X

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We have been developing a graded-index plastic optical fiber (GI-POF) for a needle-sized lens and a rigid endoscope. With highly accurate control of refractive index within a plastic lens, we can utilize it for an ultra-fine lens as a replacement for a relay lens or fiber-bundled lens in conventional rigid endoscopes. The 0.5-mm diameter lens performs high resolution with about 200-line pairs/mm. For endoscope imaging, we used optical and digital correction of chromatic aberration caused by material properties of GI-POF lens. The rigid endoscope with the GI-POF lens has a possibility to allow doctors to directly observe a patient's inner affected area before and after surgery in a minimally invasive procedure, enabling rapid and accurate assessment of the patient's condition and efficient post-operative followup.

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Randomizing polarization of displays for fundamental improvement of color mura caused by birefringence

Sasaki S., Koike Y.

Optics Letters 49 ( 22 ) 6501 - 6504 2024.11

ISSN  01469592

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Most existing displays utilize polarization technologies to produce images and improve image quality. However, polarized light from displays causes color mura because of the birefringence of the polymer films used. Thus, eliminating color degradation remains a challenge despite the incorporation of complex polarization technologies such as retardation films. Our proposed random depolarization film (RDF) addresses this issue by randomizing the polarization of light from displays. Chromaticity measurements demonstrate that the RDF effectively compensates for color mura due to low-cost polymer films. Notably, the RDF compensation mechanism is independent of the RDF position and light source spectrum. Therefore, the RDF could be an innovative solution for color degradation in existing displays.

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Random depolarization film design based on modulation transfer function measurements of displays with different pixel pitches

Sasaki S., Masaoka K., Koike Y.

Applied Optics 63 ( 11 ) 2900 - 2905 2024.04

ISSN  1559128X

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Polarized light emitted frommost displays causes blackout and color change problems when viewed through polarized sunglasses. These problems are addressed by random depolarization films (RDFs) doped with birefringent particles. These dopant particles, however, scatter light and degrade the sharpness of the displayed images.To maintain the image sharpness, we designed an RDF based on modulation transfer function measurements of displays with different pixel pitches. RDFs doped with larger particles achieved higher modulations; however, particles larger than a specific size caused undesirable sparkles on the displayed image. A display with a larger pixel pitch achieved higher modulations, suggesting that the RDF is particularly suitable for large-screen displays.

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Stabilized Plastic Optical Fiber Link Even Without a Physical-Contact Fiber Connection

Muramoto K., Minami H., Koike Y.

Journal of Lightwave Technology 42 ( 2 ) 875 - 881 2024.01

ISSN  07338724

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We propose a novel graded-index plastic optical fiber (GI POF) link that enables stable high-speed data transmission without the requirement for conventional coupling techniques to reduce reflection at fiber connection, as typified by physical-contact (PC) fiber connection, for short-reach multimode fiber (MMF) applications with vertical-cavity surface-emitting lasers (VCSELs). In the novel GI POF link, noise caused by light reflected at the fiber connection back into a VCSEL cavity can be suppressed through strong mode coupling in the fiber. It thereby improves the bit error rate of the data transmission compared with conventional silica and polymer MMFs. The capability of the novel GI POF link will be advantageous for optical interconnects in household and automotive applications, in which PC connection is not ideal to apply. The novel GI POF link will also be invaluable for multi-fiber interconnects based on multi-fiber connectors for data center applications, in which complete PC connections are difficult to obtain.

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Proposal of Novel Random Depolarization Film for Real-Color Displays with Sharp Images

Sasaki S., Koike Y.

Digest of Technical Papers - SID International Symposium 55 ( 1 ) 425 - 428 2024

ISSN  0097966X

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The random depolarization film (RDF), a polymer film doped with birefringent particles, converts linearly polarized light to randomly polarized light. This paper shows that a novel RDF with controlled birefringence resolves polarization problems of the display such as blackout, color change, and color mura problems. Furthermore, the novel RDF achieved a higher contrast ratio and a sharper image than a conventional RDF doped with calcite particles.

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Proposal of Error-Free GI POF for beyond 5G Society

Koike Y., Muramoto K.

16th Pacific Rim Conference on Lasers and Electro-Optics, CLEO-PR 2024  2024

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We propose Error-free plastic optical fiber for 53.125 Gb/s PAM4 transmission without the need for forward error correction, contributing to energy saving and lower latency in high-speed interconnects, especially for data centers and high-performance computing.

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Design of Random Depolarization Films Based on Modulation Transfer Function Measurements

Sasaki S., Masaoka K., Koike Y.

Digest of Technical Papers - SID International Symposium 54 ( 1 ) 547 - 550 2023

ISSN  0097966X

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The random depolarization film (RDF), a polymer film doped with birefringent microparticles, solves polarization problems of displays. To design the RDF that realizes sharper images, the optimal dopant particle size and suitable display’s pixel size were investigated based on modulation transfer function (MTF) measurements. The MTFs indicated that the dopant particle size of the RDF and the pixel size of the display should be larger to prevent the sharpness degradation.

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Random depolarization film design for real-color displays

Sasaki S., Udono M., Koike Y.

Applied Optics (Applied Optics)  61 ( 3 ) 669 - 675 2022.01

ISSN  1559128X

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Liquid crystal displays and organic light-emitting diode displays emit polarized light through polarizers and quarter-wave plates, which cause blackout and color change problems when the displays are viewed through polarizers, such as polarized sunglasses. Random depolarization films (RDFs) are polymer films doped with calcite particles, which randomize the polarization of light emitted from displays to resolve polarization problems. To realize displays without any color change, we investigate the retardation values and fabrication conditions required for RDFs. Analysis of the transmittance spectra and interference colors of different RDFs suggested that retardation values of approximately 600–1200 nm are sufficient to realize real-color displays. In addition, the color change measurements of displays with the RDFs revealed that an optimal dopant particle size exists and that the RDFs doped with 3.6 and 7.3 µm calcite particles achieved chromaticity changes of less than 0.02 for all 24 colors in the Macbeth chart.

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Improvement of Color Mura by Using a Triple-Zero-Birefringence Polymer Film

Sasaki S., Koike Y.

Proceedings of the International Display Workshops 29   332 - 335 2022

ISSN  18832490

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This study proposes triple-zero-birefringence polymer (TZBP) to prevent color mura due to birefringence of polymer films used in displays. Chromaticity measurements of the display clarified the advantage of the TZBP film to prevent the color mura and reproduce an actual color of the displays.

Error-free PAM-4 transmission by a high-speed plastic optical fiber without forward error correction

Koike Y., Muramoto K.

Optics Letters (Optics Letters)  46 ( 15 ) 3709 - 3712 2021.08

ISSN  01469592

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This Letter reports, to the best of our knowledge, the first study in which error-free four-level pulse-amplitude modulation (PAM-4) transmission has been achieved using a graded-index plastic optical fiber (GI POF) without the use of forward error correction (FEC) techniques, at a high data rate (>50 Gb/s), for short-reach applications (10 m). The GI POF that is developed has the characteristic microscopic heterogeneities in the fiber core material, which provide distinct mode coupling and significantly stabilize PAM-4 data transmission. The results of the study in this Letter indicate that the GI POF achieves highly stable PAM-4 transmission without the need for the current FEC techniques, which seriously increase the communication delay and power consumption of the transmission systems. Our GI POF material is paving the way for “material changes the system” approaches for achieving unprecedented optical interconnects in real time at a high speed and with low power consumption.

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Real-color displays realized by randomized polarization

Sasaki S., Udono M., Koike Y.

Applied Optics (Applied Optics)  60 ( 11 ) 3108 - 3113 2021.04

ISSN  1559128X

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This study achieves real-color displays using a randomizing effect based on the concept of "natural light."At present, most displays emit linearly polarized light, which causes essential blackout and color degradation problems when the displays are viewed through polarizers such as sunglasses. To address this, complex polarization-control technologies are added to existing displays, but the problems remain unresolved. In contrast, this study randomizes the polarization using a polymer film called a random depolarization film (RDF) that is doped with specific birefringent crystal particles. The RDF placed on a display reproduces colors that are very close to the natural colors seen in reality without the need for complex polarization technologies.We believe that it has the potential to change the approach to color-reproducing technology for displays.

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Proposal of novel temperature-independent Zero–Zero-birefringence polymer with high heat-resistance

Watanabe K., Kobayashi Y., Koike Y.

IEICE Transactions on Electronics (IEICE Transactions on Electronics)  E104C ( 2 ) 59 - 63 2021.02

ISSN  09168524

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Copyright © 2021 The Institute of Electronics, Information and Communication Engineers. Temperature-independent zero–zero-birefringence polymer (TIZZBP), which exhibits very small birefringence over the wide temperature range, is required to realize real-color images for displays, particularly vehicle-mounted displays. Previously, a TIZZBP was synthesized, but they did not put into practical use because of their too complex composition and low mechanical strength. In this paper, we propose a practical TIZZBP that has high heat resistance, high transparency and sufficient mechanical strength, using a simple binary copolymerization system. Our proposed novel polymer exhibits very low photoelastic birefringence and very low orientational birefringence. Both types of birefringence of this TIZZBP satisfy the negligible levels for displays, which are defined as follows: the absolute values of photoelastic coefficient and intrinsic birefringence are less than 1 ×10−12 Pa−1 and 1 ×10−3, respectively. In addition, temperature dependency of orientational birefringence was very low. Orientational birefringence satisfies the negligible level all over the temperature range from around −40◦C to 85◦C. This temperature range is important because it is the operational temperature range for vehicle-mounted display. Furthermore, our proposed novel TIZZBP showed high heat resistance, high transparency and sufficient mechanical strength. The glass transition temperature was 194◦C. The total light transmittance and the haze value is more than 91% and less than 1%, respectively. The tensile strength of non-oriented films was 35 ∼ 50 MPa. These results suggest our proposed novel TIZZBP has high practicality in addition to very low birefringence. Therefore, this TIZZBP film will be very useful for various displays including vehicle-mounted displays and flexible displays.

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Design of random depolarization film with random retardation for real-color displays

Sasaki S., Udono M., Koike Y.

Digest of Technical Papers - SID International Symposium (Digest of Technical Papers - SID International Symposium)  52 ( 1 ) 1283 - 1286 2021

ISSN  0097966X

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Our proposal, the random depolarization film (RDF) is highly efficient in preventing color change problem of LCD and OLED displays. We designed the RDF that realizes real-color displays by considering the retardation. The RDF with random retardation above 600 nm achieved much lower color change that human eyes cannot recognize.

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Random Depolarization Film Doped with Calcite Microparticles for Clear Real-Color Displays

Sasaki S., Udono M., Koike Y.

26th Microoptics Conference, MOC 2021 (26th Microoptics Conference, MOC 2021)   2021

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Random depolarization film, a polymer film doped with birefringent particles, can resolve blackout and color change problems of existing displays. This study optimized the dopant particle size with respect to image sharpness and color change of the display. Calcite microparticles improved the sharpness and color change, and realized real-color displays.

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Low-Noise Graded-Index Plastic Optical Fiber Achieved by Specific Copolymerization Process

Akashi T., Inoue A., Koike Y.

Journal of Lightwave Technology (Journal of Lightwave Technology)  39 ( 11 ) 3553 - 3559 2021

ISSN  07338724

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Ultra-high-definition (UHD) technologies have recently received attention on account of their practical applications in consumer electronics. UHD devices require uncompressed video transmission at a data rate exceeding 100 Gb/s; thus, an optical fiber connection is essential. In consumer applications, optical fibers are very short. Moreover, their connections must permit variations in the fiber alignment, which accommodates rough handling by consumers. Under this condition, the transmitted signal quality is significantly degraded owing to noise and instabilities that strongly depend on the fiber alignment conditions in optical modules and connectors. Therefore, graded-index plastic optical fibers (GI POFs) are promising optical cables for consumer applications because of their flexibility, safety, and high bandwidth. Recently, the authors experimentally demonstrated that GI POFs can reduce interferometric noise, such as modal noise and multipath interference noise, in a multimode fiber link based on a vertical-cavity surface-emitting laser (VCSEL). This noise reduction effect results from strong mode coupling of GI POFs, which is closely related to microscopic heterogeneous structures of a core polymer matrix. In this paper, a control method of mode coupling using the copolymerization process for fiber core materials is proposed. The formation of composition fluctuations by copolymerization increases the mode coupling in the GI POF core. It thereby enables highly stable and robust data transmission with a large fiber misalignment tolerance in a VCSEL-based multimode fiber link. The proposed method for mode coupling control is expected to contribute to household optical communication systems in the upcoming UHD era.

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Noise and Distortion Reduction in OFDM Radio-Over-Fiber Link by Graded-Index Plastic Optical Fiber

Muramoto K., Inoue A., Koike Y.

IEEE Photonics Technology Letters (IEEE Photonics Technology Letters)  32 ( 13 ) 835 - 838 2020.07

ISSN  10411135

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© 1989-2012 IEEE. We demonstrate that a graded-index plastic optical fiber (GI POF) enables higher-quality radio-over-fiber (RoF) transmission of an orthogonal frequency-division multiplexing (OFDM) signal, compared with a conventional multimode fiber, for short-distance household applications (< 100 m). It is shown that the improvement in OFDM signal transmission, using the GI POF, can be attributed to the reduction in noise and intermodulation distortion, owing to the external optical feedback from the transmission fibers. The low noise and low distortion in the signal transmission are mainly due to the strong mode coupling in the GI POF, which is closely related to the microscopic heterogeneous structures in core polymer materials. The developed GI POF will be invaluable for RoF networks in households as a solution for indoor broadband wireless communications in the upcoming 5G era.

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Low-noise radio over graded-index plastic optical fiber

Inoue A., Koike Y.

Optics Letters (Optics Letters)  45 ( 12 ) 3192 - 3195 2020.06

ISSN  01469592

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© 2020 Optical Society of America We developed a graded-index plastic optical fiber (GI POF) that enables lower-noise radio frequency (RF) transmission than conventional multimode fibers for short-distance household applications (<100 m). It is shown that reflection noise degrades RF transmission, regardless of the carrier frequency, through the spurious generation that accompanies the RF modulation of a vertical-cavity surface-emitting laser. The GI POF with distinctive mode coupling, which is closely related to its microscopic polymer structure, suppresses noise and spurious generation to improve transmission quality. Our low-noise radio-over-GI-POF technology will offer significant advantages for optical wiring systems for broadcast and communication in small- and medium-scale buildings.

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Reflowable optical connector with glass-ceramic ferrule for advanced pluggable transceiver packaging

Shikama K., Koike Y.

Optical Fiber Technology (Optical Fiber Technology)  54 2020.01

ISSN  10685200

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© 2019 Elsevier Inc. We describe a new physical-contact optical fiber connector/receptacle that can withstand a solder-reflow process with a maximum temperature of 260 °C for advanced pluggable transceiver packaging such as the co-packaging of photonic and electronic chips using the reflow process. We experimentally clarify the problems that occur in the conventional simplex optical connector through the reflow process while discussing the glass-transition behavior of the adhesive used in the connector. We then present an improved receptacle structure with glass-based ferrules whose coefficients of thermal expansion are closer to that of the fiber compared to the conventional zirconia ferrule. We demonstrate through testing that our proposed receptacle results in a dramatic decrease of permanent fiber withdrawals after the reflow process. The proposed connector, which is composed of a glass-ceramic ferrule, a polyimide tube, and a mechanical clip, achieved physical-contact connection and maintained satisfactory connection and return losses of below 0.3 dB and over 45 dB, respectively, even after six reflow processes. We examined the withdrawal behaviors for both the zirconia and the glass-ceramic ferrules during reflow based on the online measurement of connection and return losses, and observed the multiple reflections caused by the change in the air-gap width between the mated connectors. We also confirmed that the developed simplex optical connectors based on the glass-ceramic ferrule yielded sufficient environmental durability even after multiple reflow processes.

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Proposal of novel polymer materials with no birefringence over wide temperature range

Watanabe K., Yuma K., Koike Y.

Digest of Technical Papers - SID International Symposium (Digest of Technical Papers - SID International Symposium)  51 ( 1 ) 1600 - 1602 2020

ISSN  0097966X

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© 2020 SID. We propose two types of temperature-independent zero–zero-birefringence polymers (TIZZBPs), which have different Tgs of 194°C and 156°C. These TIZZBPs also exhibit sufficient mechanical strength and high transparency. These TIZZBPs will achieve real-color images not only for vehicle-mounted displays but also for flexible displays.

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Analysis of random depolarization films with birefringent micro particles for liquid-crystal and oled displays

Sasaki S., Udono M., Koike Y.

Digest of Technical Papers - SID International Symposium (Digest of Technical Papers - SID International Symposium)  51 ( 1 ) 1596 - 1599 2020

ISSN  0097966X

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© 2020 SID. Our proposal, the random depolarization film (RDF), prevents blackout and color change of the display image viewed through polarized sunglasses. We demonstrated that the RDF doped with micro-sized particles realized the real-color display without the blackout and the color change, and the clear display.

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