Graded-index plastic optical fiber (GI-POF) is expected to become the universal transmission medium for low-cost data links in home, office, and factory-floor environments. To speed deployment of GI-POF, several of Japan’s major electronics companies, most notably NEC and Hitachi (both in Tokyo), are directing an increasing amount of research at the fiber, related light sources, and detectors.
Researchers at NEC have already confirmed an available transmission capacity about 30 times greater than that of conventional step-index plastic optical fiber (SI-POF) by demonstrating mode-dispersion-limited, 2.5-Gbit/s, 100-m transmission with GI-POF using a 647-nm diode laser. The company is now addressing development of light sources and detectors for the POF low-loss windows at 570 and 650 nm. At 570 nm, the only candidates are light-emitting diodes (LEDs), but at 650 nm both LEDs and diode lasers are available.
Given their operating costs, performance, and eye safety, 650-nm LEDs are currently regarded as the best choice for links up to speeds of 156 Mbit/s; at higher speeds, however, diode lasers are more promising. Unfortunately, though, the frequency response of semiconductor optical devices operating in the visible-wavelength region is poorer than that of the long-wavelength devices currently used in telecommunications systems. Hence, company researchers have specially developed a 650-nm, AlGaInP multiple-quantum-well (MQW) diode laser and a GaAs/AlGaInP PIN photodiode.
The 300-µm cavity of the MQW diode is fabricated on a GaAs substrate using metal-organic vapor-phase epitaxy (MOVPE). A pair of channels is formed beside a 30-µm-wide current-injection region to reduce the parasitic capacitance at p-n-p-n current-blocking structures. The estimated laser capacitance is ~15 pF, about one-fourth that of conventional all-surface-metal-contact diode laser of similar cavity length. The 24-mA threshold current is the lowest recorded for an edge-emitting diode laser, while the maximum CW operating temperature is 80°C.
The PIN photodiode is fabricated with a 90-µm-diameter mesa structure as a high-speed monitoring detector (see figure). The layers—grown on a GaAs substrate by MOVPE—consist of a 5-µm-thick GaAs absorption layer, a 10-µm AlGaAs intermediate layer, and a 0.3-µm AlGaInP window layer (580 nm). The capacitance at 10-V bias is 0.87 pF, indicating a bandwidth of more than 4 GHz.
The researchers tested pulsed modulation and detection at rates up to 4 Gbit/s, and the results indicate that the devices have adequate high-frequency characteristics for application to gigabit-per-second POF transmission. Furthermore, the bandwidth of the photodiode (more than 4 GHz) raises the possibility of expanding its 90-µm receiver diameter to the several-hundred micrometers required for GI-POF transmission.
Meanwhile, the ATM (asynchronous transfer mode) Forum--a group of firms in Japan and the USA promoting application of ATM local-area networks (LANs)—is now considering the use of a plastic-optical-fiber-based physical-media-dependent (PMD) sublayer to cut costs for users who install private ATM LANs with a new transmission line. This proposed PMD sublayer will use a POF (either step-index or graded-index) for the transmission medium and a red LED or diode laser as the transmitter light source.
An advantage of POF is its low total-system cost. The desktop LAN environment is characterized by relatively short transmission distances (typically less than 100 m) with a large number of connections. Hence, the total link cost is dominated by connectivity and transceiver costs. To date, glass fiber has not significantly penetrated the desktop LAN market because of its high transceiver cost and high termination cost. Thus, category 3/5 unshielded twisted pair remains dominant.
In Japan a multivendor environment already exists for LEDs, diode laser, and silicon photodetectors, and another is emerging for POF. Mitsubishi Rayon Co. Ltd. (Tokyo), for example, has recently developed the two types of high-bandwidth POF (graded index and step index). The graded-index technology was licensed from the developer, Dr. Koike of Keio University (Tokyo), and has reached the stage of sample production. In November, the company plans to provide samples to LAN vendors and manufacturers so they can review the characteristics required for data-link design and determine final specifications. The company says its step-index plastic fiber, when combined with visible LEDs, will meet market demand in the LAN market; commercial production of the fiber was started in June.