Integration improves performance and cost
Roland Roux
With an eye on the optical networks of the future, the research concern France Telecom (Paris) has oriented its optoelectronic-components research and development toward integration of multiple functions on a single substrate. Anticipating the evolving needs of telecommunications networks, the Centre National d`Etudes des Telecommunications Paris (CNET) is also investigating deployment of monolithically integrated devices into network subsystems, such as switching and mixing in the network nodes. In so doing, the organization expects to significantly reduce fiberoptic network distribution costs and to improve transmission performance.
Among the new components developed by CNET is a fast multiple-quantum-well (MQW) electroabsorption modulator with integrated semiconductor laser. The device was one of several optoelectronic components shown by CNET at the recent Intertronic show (June 12-16, at the Parc Des Expositions de Paris Nord Villepinte). External modulation of the semiconductor laser provides a very large modulation bandwidth and eliminates wavelength variations normally induced by direct modulation of a semiconductor laser. This device is based on the Stark effect, by which the optical absorption of a material can be influenced by an applied electric field (see Laser Focus World, June 1995, p. 84), and has been successfully tested at data rates of 20 Gbit/s over a 108-km experimental link without amplification.
In the overmodulation regime, the device produces ultrashort soliton-type pulses that are able to propagate themselves over about 1 million km without deformation. A 38-GH¥optical signal has been generated, which confirms the potential of the device for use in radiocommunication networks.
Monolithic integration of the single-frequency laser and fast electroabsorption MQW modulator is made possible by a new approach to the technology. The same active layer (InGaAsP/ InGaAsP) is used for both devices in constrained multiple quantum wells. The modulator and 1.55-µm output laser source combine the advantages of external laser modulation with the high output power of distributed-feedback (DFB) lasers. The device is rated for high transmission rates (10 Gbit/s) over long-distance fiberoptic networks and also enables soliton propagation and high multiwavelength transmission rates (see figure).
To meet the needs of both local networks and future subscriber distribution networks, other low-cost optoelectronic components must be developed. Optoelectronic integration with new materials has been developed such that an organic polymer optical waveguide can be fabricated on the same substrate as a semiconductor laser. A buried-ridge striped-laser structure integrated with a polymer-based waveguide, for example, can result in a coupling loss of less than 1 dB.
For wavelength multiplexing and switching, multiple-wavelength laser sources are key components. Two types of single-frequency laser sources have been developed by CNET: the DFB laser and the distributed-Bragg-reflector (DBR) laser. Fabricated onto arrays, these lasers emit several wavelengths simultaneously, leading to more compact, stable systems. These sources have been developed for integration in a coupler to simultaneously provide all the required wavelengths. An array of six DFB lasers with buried-ridge structures has been successfully made with a spacing of 2.16 ۪.2 nm between each emitted wavelength.