Fujitsu develops 40Gbps optical transmitter module, heralding next wave in high-speed optical communications systems

Fujitsu Limited announced today that it has developed a 40gigabit-per-second optical transmitter module, a major milestone in creating the next generation of high-speed optical communications systems. The company will now focus on commercialising the module and plans to begin shipping samples in March of this year.

As broadband Internet becomes more widely available, transmission volumes are expected to increase at a faster pace than ever. To help cope with these communication volumes, engineers around the world are pursuing the development of high-speed, high-capacity next-generation optical communications systems in such technology areas as DWDM.

In developing its new 40Gbps optical transmitter module, Fujitsu took advantage of its own ultra high-speed optical devices and electronic circuit technology, including its LiNbO3 (LN) Mach-Zehnder optical external modulator and double heterostructure HEMT MMIC. By eliminating the need to incorporate separate 40Gbps high-speed circuitry or complex control circuitry, Fujitsu's module will help speed the development of 40Gbps optical transmission systems.

Prototypes of the new module will be demonstrated at the 2nd Fiber Optics Expo starting January 16 at the Tokyo International Exhibition Center (Tokyo Big Site).

About the 40Gbps optical transmitter module

A key challenge in realising a 40Gbps optical transmission system has been developing an optical transmitter module (E/O) that can convert 40Gbps electrical signals into optical signals and transmit them. One problem has been the waveform distortions that occur when transmitting optical signals over fibre-optic cable for long distances. This problem has given rise to the need for characteristics (called dispersion-tolerant characteristics) that can control the wavelength deviations that cause distortions in the waveform.

Fujitsu was able to overcome these obstacles by employing its renowned optical device and electronic circuit technology, including the Mach- Zehnder LN optical external modulator, a four wave tunable LD module with wavelength locker, LN driver circuit, and control circuit block. Moreover, because the optical transmitter is in module form, customers need not worry about designing hard-to-handle 40Gbps high-speed circuitry and complex control circuitry, thereby accelerating the various testing processes and system development.

Features of the 40Gbps optical transmitter module

LN optical modulator

The Mach-Zehnder LN optical external modulator offers excellent dispersion-tolerant characteristics and high-speed optical modulation characteristics over a wide bandwidth to transmit optical signals over long distances. The lithium niobate (LiNbO3) optical crystal has the property of changing the phase of a lightwave when voltage is applied. This phase change can be used to switch the optical signal on or off, and makes for a faster electrical-optical conversion process. This optical modulator has excellent modulation properties, including a pulse reshaping effect, and, because the modulation spectrum (wavelength deviation) is narrow, it offers superior fibre transmission characteristics.

Four-wave tunable LD module with wavelength locker

As an optical source, the module incorporates a tunable LD module with wavelength locker that offers precise wavelength control, permitting any one of four ITU-standard 100GHz consecutively spaced wavelengths to be selected. A semiconductor laser module with a precisely tuned wavelength and stable output, this device varies the wavelength by changing the laser temperature. Because one device tunes four wavelengths-worth of light output, optical communications systems can be built with fewer spare devices.

Wideband, high-output LN driver circuit

While the LN optical modulator has excellent modulation and fibre-transmission characteristics, it requires a relatively high voltage of 5V. Developing a suitable ultra-high-speed, high-output modulator driver circuit was therefore a key technological challenge. Fujitsu was able to meet this challenge with an ultrawide-bandwidth distributed amplifier circuit using a double-heterostructure HEMT MMIC.

HEMTs are field-effect transistors that take advantage of the fact that electrodes created from the hetero-interface of different kinds of semiconductor material (such as GaAs and AlGaAs) operate at higher speeds than those within conventional silicon (Si) semiconductors. Fujitsu pioneered the development of these devices in 1980, and today they are used in nearly all satellite transceivers. Gate sensitivity is shorter, and, to ensure adequate gain and output voltage at high frequencies, a double heterostructure is used.

Control circuit block

It includes an automatic bias control circuit (which enables the LN optical modulator to run reliably for at least 20 years), a four-wave tunable LD frequency-selection circuit, and a precision oscillator control circuit, making the module especially easy to use.