This paper presents a design approach for developing MU-type single-mode miniature optical connectors featuring 1. 25 mm diameter zirconia ferrules. They are smaller and have a higher packaging density than conventional optical connectors. The ferrule pitch is 1/2, the plug volume 2/5 and the cross-sectional area 2/5 that of the SC connector. The aim of our approach is to reduce the ferrule size and to realize durable connectors. With 10/125 SM fibers, these MU connectors produced an average insertion loss of 0. 07 dB and an average return loss of 49. 4 dB, and there was no degradation during or after mechanical and environmental tests.

A novel flexible ferrule only 0.8 mm in diameter is proposed and demonstrated as a key element in optical fiber connectors. Miniature aggregated multiple fiber connectors with a fiber pitch of 2-5 mm are possible with this Flexible Fine ferrule" (FF-ferrule). This paper presents the fundamental concept of the FF-ferrule. Then, ferrule bending characteristics, a distinctive mechanical feature of this ferrule, are discussed in detail both theoretically and experimentally. The method of designing the FF-ferrule is also described based on the theoretical results. A prototype FF-ferrule was made by precision transfer molding. Connection loss was as small as 0.6 dB, because the ferrule bending can compensate for axial misalignments up to 0.5 mm. In addition, 20% of the measured connections showed a connection loss of less than 0.3 dB. The high pressure contact from the small cross-section of the ferrule makes physical contact" between the endfaces of the fiber.

The statistical properties of insertion losses and return losses for optical connectors are investigated theoretically using the probability theory and the Monte Carlo simulation. Our investigation is focused on an orientation method for reducing insertion loss by which a fiber-core center is adjusted in a region of within a certain angle to the positioning key direction. It is demonstrated that the method can significantly improve insertion losses, and that an adjusting operation angle of 90 degrees is sufficient to realize an insertion loss of less than 0.5 dB with 99% cumulative probability. Good agreement was obtained between the theoretical distribution and the experimental results for single-mode fiber connection. Consequently, it is indicated that the statistical distributions of insertion losses and return losses of optical connectors in the field can be predicted theoretically from the values measured in the factory by connection to a master connector.

We have proposed parallel optical interconnection technology, or ParaBIT, for high-throughput, low-cost optical interconnections and already developed a prototype parallel optical interconnect module called "ParaBIT-0," which has a total throughput of 28 Gb/s (700 Mb/s 40 channels). We are now developing a compact, high-throughput module called "ParaBIT-1," which has a total throughput of 60 Gb/s (1.25 Gb/s 48 channels) and is designed to achieve the highest-ever throughput density of 3.3 Gb/s/cc. In this paper, we describe the packaging structure, optical coupling structure and transmission characteristics of ParaBIT-1. We also discuss the technical prospect of realizing a parallel optical interconnect module with the bit rate of 2.5 Gb/s/ch.

We propose a compact multi-channel 90 optical deflection device for short-distance optical interconnection. The device consists of stacked bent multimode optical waveguides having reflecting mirrors with bending angles of 90. The structure of the bent multimode optical waveguide with a bending angle of 90 was designed by ray-tracing simulations. The simulated insertion loss for each channel of the device was 0.5 dB. We also propose a simple fabrication process using a pair of multi-channel linear optical waveguides with symmetrical 45 mirrors. An 8-channel 90 optical deflection device was fabricated using polymer materials and basic operation was confirmed. Our device has good potential for use as a high-density optical interconnection device.

We have developed a low-latency, error-correcting-code-(ECC-)adaptable skew-compensation technique, which is needed for high-speed and long-distance parallel optical interconnections. A new frame-coding technique called shuffled mB1C encoding, which requires no clock-rate conversion circuit and no data buffering, and a new skew-measurement method which is suitable for ECC adaptation have been developed for the compensation. Full-digital skew-compensation circuits using these new techniques were able to compensate for a two-clock-cycle skew, even when one transmission channel was removed. The maximum latency for skew compensation was only five clock cycles.

We have developed a low-latency, error-correcting-code-(ECC-)adaptable skew-compensation technique, which is needed for high-speed and long-distance parallel optical interconnections. A new frame-coding technique called shuffled mB1C encoding, which requires no clock-rate conversion circuit and no data buffering, and a new skew-measurement method which is suitable for ECC adaptation have been developed for the compensation. Full-digital skew-compensation circuits using these new techniques were able to compensate for a two-clock-cycle skew, even when one transmission channel was removed. The maximum latency for skew compensation was only five clock cycles.

Aiming at lower cost and further miniaturization, we developed a new optical coupling system for use as an optical interface of a parallel optical interconnect module, called ParaBIT-1. It consists of a new-structure 24-fiber bare fiber (BF) connector whose main parts are made of molded plastic and a 24-channel optical coupling component using new polymeric optical waveguide film. To prevent bare fibers from breaking, the BF connector plug has a fiber protector. This BF connector can be joined by direct physical contact between bare fibers in fiber guide holes with a 250-µm pitch. The buckling forces of the fibers themselves secure the physical contacts. The average measured insertion loss of the 24-fiber BF connector was 0.05 dB, and the return losses were over 35 dB. The optical coupling components are composed of a 24-ch polymeric optical waveguide film with 45 mirrors and the 24-fiber BF connector interface, and can be assembled by passive alignment. The high thermal stability of the film allows soldering, and the film is fabricated by direct photo patterning. The average insertion losses of the components for transmitter and receiver modules were 1.28 and 1.35 dB, respectively.

We have been working on a project called ParaBIT (for parallel inter-board optical interconnection technology) to achieve large-capacity switching systems. The ParaBIT module being developed as the first step in this project is a front-end module with 40 channels providing throughput of 28 Gb/s, cost-effectiveness and compactness. To realize the module, this project has developed five novel technologies: (1) 850-nm 10-ch Vertical-cavity Surface-emitting laser (VCSEL) arrays as very cost-effective light sources, (2) new high-density multiport bare fiber connectors that do not need a ferrule and spring, (3) passive optical alignment using polymeric optical waveguide film with a 45-degree mirror for coupling to the optical array chips and the waveguide, (4) transferred multichip bonding to mount optical array chips on a substrate with a positioning error of only a few micrometers, and (5) simple electronic circuits with a fixed-decision-level receiver and an APC-less transmitter, and low power consumption. Experimental results show that the design targets of throughput of 700 Mb/s per channel and a compact and cost-effectiveness structure were met. Thus, ParaBIT is a promising technology for large-capacity switching systems.

We have been working on a project called ParaBIT (for parallel inter-board optical interconnection technology) to achieve large-capacity switching systems. The ParaBIT module being developed as the first step in this project is a front-end module with 40 channels providing throughput of 28 Gb/s, cost-effectiveness and compactness. To realize the module, this project has developed five novel technologies: (1) 850-nm 10-ch Vertical-cavity Surface-emitting laser (VCSEL) arrays as very cost-effective light sources, (2) new high-density multiport bare fiber connectors that do not need a ferrule and spring, (3) passive optical alignment using polymeric optical waveguide film with a 45-degree mirror for coupling to the optical array chips and the waveguide, (4) transferred multichip bonding to mount optical array chips on a substrate with a positioning error of only a few micrometers, and (5) simple electronic circuits with a fixed-decision-level receiver and an APC-less transmitter, and low power consumption. Experimental results show that the design targets of throughput of 700 Mb/s per channel and a compact and cost-effectiveness structure were met. Thus, ParaBIT is a promising technology for large-capacity switching systems.

A high-density multi-port optical connector that exploits the flexibility of bare optical fibers has been developed for use as an optical interface of a parallel optical interconnection module. In the BF (Bare-Fiber) connector, 24 multimode-fibers are mated by direct physical contact in micro-glass-capillaries with a 250-µm pitch. The buckling forces of the optical fibers themselves secure the physical contact. Optical fiber buckling is investigated theoretically and experimentally. A new design method to optimize the span length l and the longitudinal displacement ΔL for the buckling is also proposed based on the requirements afor optical characteristics, mechanical reliability, and dimensional tolerances, etc. A prototype BF connector with l 10 mm and ΔL of 50 µm was designed and fabricated for multimode fiber connections. This connector provides high optical performance: an average insertion loss of 0.05 dB and a return loss of over 35 dB at 850 nm. The optical performance remained stable after a durability test with ten connection-repetitions.

We have studied the basic optical and physical characteristics of polymeric optical waveguide films with S-shaped waveguides and 45 mirrors applied as multimode optical interconnection components. The core and cladding of the waveguide films were made of deuterated-polymethylmethacrylate (d-PMMA) and UV-cured resin, respectively. We evaluated the insertion losses of the waveguides, the crosstalk and the 45-mirror losses in these waveguide films and demonstrated that they have low propagation loss. The shrinkage and thermal expansion of the polymeric optical waveguide films are also discussed because of the interest in improving module packaging.

This paper describes the optical characteristics and static fatigue reliability of a zirconia alignment sleeve, which is a component part of an optical connector with zirconia ferrules. This combination of sleeve and ferrules hardly generates any wear debris during connector insertion and removal cycles. This has reduced the cleaning frequency of the ferrule endface during cycles and greatly improved the return loss stability of the optical connectors. The zirconia alignment sleeve enables stable return loss characteristics to be achieved over a wide temperature range as it has the same thermal expansion coefficient as the zirconia ferrule. Furthermore, the gauge retention force for the zirconia alignment sleeve is defined with a view to its practical use. This force must be between 2.0 and 3.9 N to allow stable optical connections to be made under various mechanical and environmental conditions. We also clarify the conditions for a proof test by which to prevent the occurrence of static fatigue fractures in the sleeve, and we confirm the validity of the test. The static fatigue parameters for zirconia ceramics and derived from the static fatigue theory for brittle materials and fracture testing. We use these static fatigue parameters to predict the lifetime of a zirconia sleeve under working stress. An appropriate stress level for the proof test which eliminates weak sleeves, is about 3 times greater than working stress. The strength of the sleeve as demonstrated in the proof test is confirmed by accelerative stress aging. The performance of this sleeve is superior to that of a conventional copper alloy sleeve and the proof test confirms its reliability; under 0.1 FIT for 20 years of use.