NTT projects lower-cost fiber-to-the-home in 1997

Paul mortensen

Executives at Nippon Telegraph & Telephone (NTT) in Tokyo have disclosed a plan that details how the company plans to bring fiber-to-the-home (ftth) by 2010. Known as the new optical access system, the plan hinges on extensive cost-cutting to bring the costs of fiber installation to the same level as that of metallic cable installation (see Fig. 1, page 26).

Specifically, the system targets low-cost fiber installation to telephone poles near customers by using an optical passive double star (PDS) distribution method. The system will upgrade the conventional remote terminal system, which uses existing metallic cables for telephone service, by replacing them with fiber cables, thus accelerating deployment of fiber in the access network.

The PDS system, with an optical splitter, provides a low-cost access system because it allows multiple optical network units (ONUs) at or near the customer`s home to share a single subscriber-line terminal near an NTT building (see Fig. 2, page 26). The new optical-access system accommodates telephone, INS-Net 64 (an Integrated Services Digital Network service), low-speed open-computer network, and low-speed leased-line services. To cut costs, the system allows multiple customers to use a single optical fiber installed between an NTT operations building and an ONU.

Another ftth cost-cutting measure comes from Fujitsu Laboratories Ltd., which has developed a way to cut 90% from the manufacturing costs of modules that can link homes to Japan`s optical-fiber network, being laid by NTT.

These components send and receive optical signals, converting between the optical signals used over the fiber network and the electronic signals used by telephones and other types of information terminals in the home.

Fujitsu cut manufacturing costs by reducing the number of parts and by simplifying the process by which the photoelectric conversion element (photodetector and semiconductor laser), electronic parts and optical fiber are mounted on a single silicon substrate.

The transmitting component incorporates a photodiode so it is possible to monitor when optical signals are being sent. The receiving component can handle traffic volumes of 150 Mbits/sec.

Additional NTT cost-cutting measures come from the first Optoelectronics and Communications Conference held recently in Tokyo. At this meeting, NTT researchers revealed less-expensive manufacturing methods to reduce the cost of laser-diode modules, key components of ftth. Today, laser-diode-module production is relatively small (mostly for consumer CD devices), but it is expected to increase dramatically.

Japan`s ftth will need four million modules a year for 15 years to get 60 million fiber-optic telephone lines. The difficulties, however, include the large number of lenses, mirrors or filters that are essential for coupling or splitting light signals. Others include the small core size of singlemode fiber and the combination of optical components, which require complex optical coupling systems and precise optical axis alignment with submicron efficiency. Also, hermetically sealed packages have been considered necessary for reliable lasers and photodiodes.

To lower costs, the ftth solution will reduce the number of components and create innovative device structures and assembly technologies that can be adapted for mass production. For that, NTT has developed as key technologies lens-less optical coupling, passive alignment of the optical axis, simplified optical components and packages that are not hermetically sealed.

NTT has tested plastic (epoxy) molded laser diodes, which showed no significant degradation after more than 10,000 hours of constant-power-operation tests, at 25°C and 19 mW, and at 70°C and 5 mW. The results indicated the potential of stable operation of laser diodes in low-cost non- hermetically sealed packaging. A 10-year lifetime looks possible. NTT predicts a cost of $10 per module, but final cost will depend on volume.

As for lens-less, low-loss optical coupling, NTT has transformed the output from the laser diode by using an integrated spot-size converter. The company has developed three different types of spot-size converter-integrated laser diodes and checked their coupling efficiencies with optical fiber/ planar-lightwave-circuits (PLCs). With the large tolerances obtained, the focusing and coupling lenses are eliminated.

For the optical module, NTT researchers have achieved a 25% reduction (compared to the discrete module) in the number of components by using PLC-hybrid integration technologies (no lenses); a 40% reduction (compared to the discrete module) in assembly time; and a reduction in the number of optical connectors needed.

The prototype ONU module has a length of 5 cm; NTT is presently working on a monolithic type. Electronic devices related to the ONU include adjustment-free and low-voltage LSIs, long-life batteries and semiconductor lightning-surge protection devices.

Progress has also been made in low-cost fiber technology, including high-density and high-count optical fiber cables. NTT has achieved 3000-fiber cable-- claimed to be the world`s highest optical-fiber density--and reduced the fiber ribbon thickness from 0.4 to 0.3 mm. This cable permits more-efficient use of conduit. Preconnectorized optical-fiber cable and stacked 8- to 40-multifiber connectors result in a 60% reduction in splicing time.

Within the operations system, NTT`s Aurora system can perform maintenance measurements on in-service fibers. A dichroic filter, which reflects the test wavelength and transmits 1.3- and 1.55-micron communications wavelengths, provides a calibrated optical time-domain reflection. Initial fiber measurements are stored and compared to subsequent maintenance measurements to determine fiber performance. Individual fibers in 1¥N passive star networks, which split light into N fiber branches of different lengths, are terminated with a dichroic reflective filter. When a fault occurs, the problem fiber is easily identified because its peak value of reflector is lower. This method is a significant improvement over present methods that require adding an optical coupler to each branched fiber to gain direct optical access. Also, the company has implemented high-speed optical fiber switching from the operations center with an optical cable transfer splicing system.

For optical access networks, NTT will install fiber between feeder points (hundreds of meters away from customers` homes) and telephone poles near customers to enable access for economical 1.5-Mbit/sec high-speed digital leased circuit service by next year. NTT will also examine offering video transmission capability to cable-TV providers. Furthermore, by sharing optical-fiber cables and conduits for both high-speed broadband and conventional telephone services, NTT hopes to lower broadband service costs.

The deployment of an optical access system will also shorten the installation period for optical drop wire to customers. As the optical-fiber cables are already installed to telephone poles near customers, NTT will install optical drop wire from the telephone poles to customers who want high-speed broadband services. Such an approach, they say, will significantly shorten the installation period, which previously required one month.q

Paul Mortensen writes from Japan.