Published papers

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

© 2018 The Author(s). We demonstrate that novel graded-index plastic optical fibers significantly reduce nonlinear distortion of transmitted signals in optical links because of strong mode mixings caused by microscopic heterogeneities in polymers, enabling high-quality multilevel 4K/8K video transmission.

© 1983-2012 IEEE. We have developed a novel optical interconnect for graded-index plastic optical fibers based on the use of ballpoint-pen connectors, where ball lenses are precisely mounted on fiber end faces by a simple and low-cost ballpoint-pen production technology. This report demonstrates that the ballpoint-pen interconnect allows us for significantly robust data transmission with sufficient misalignment tolerance and mating durability for consumer applications, where very short optical fibers are frequently connected and disconnected to the consumer electronics in a manner similar to metal cables. In the ballpoint-pen interconnect, power penalties are barely affected even by misalignments with a lateral offset of 50 μm or a connector separation of 1.0 mm, allowing for optical fiber connections without the requirements of precise alignment or high manufacturing accuracy. Moreover, it is suggested that modal noises due to dusts and scratches on fiber end-faces are generated for conventional butt-coupling based on physical contact in applications that require frequent connection and disconnection of optical fibers. This modal noise generation can be prevented using nonphysical contacts with the ballpoint-pen connectors. We achieve high-speed data transmission that is barely influenced by 2500 mating cycles. These results suggest that the ballpoint-pen interconnect allows us for consumer-friendly optical fiber connections without the requirement of any cleanings in a manner similar to electrical interconnect.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement We introduce a graded-index plastic optical fiber (GI POF) design for very short-distance household applications, in which the transmission quality is predominantly determined by system noise rather than the loss and bandwidth. The developed GI POF has strong mode coupling with low accompanying scattering loss, which is closely related to the specific microscopic heterogeneities in the core material. Such characteristic mode coupling significantly decreases reflection noise, improving the transmission quality compared with silica GI multimode fiber (MMF) for lengths below 30 m. Moreover, in the GI POF link, the transmission quality tends to improve with increasing fiber length, despite the increased loss and decreased bandwidth. This feature suggests that the system noise can be controlled by the microscopic heterogeneous properties of the GI POF for a very short MMF link, where the fiber loss and bandwidth are sufficiently low and high, respectively. This unconventional concept for optical-fiber design can advance fiber-optic communication in emerging applications in households located near optical network terminals.

© 2018 The Author(s). We demonstrate that propagating light through graded-index plastic optical fibers is significantly depolarized by intrinsic mode mixing because of microscopic core heterogeneities. This depolarization effect allows for stable optical link through reduction of interference-induced noises.

© 2018 IEEE. We develop plastic optical fibers (POFs) with microscopic material properties designed for stable data transmissions for ultrahigh-definition applications in households (<30m). In the POF link, transmission quality tends to be improved with increase in link length despite loss increase.