MARCH 28, 2007 -- This week at OFC/NFOEC 2007 in Anaheim, CA, Oki Electric Industry Co. Ltd. (search for OKI Electric) announced it has developed a gain-coupled (GC-) distributed-feedback (DFB) laser that is tolerant of optical reflection. By employing the GC-DFB laser, OKI says it has become the first to develop optical isolator-free optical transmitter modules for practical applications, enabling smaller, lower-cost optical modules for use in long-distance networks such as FTTH.
"The results we obtained from this technology confirm that it will contribute to achieving lower-cost, isolator-free optical subassembly modules. Because the manufacturing process of the GC-DFB laser is approximately equivalent to that of conventional index-coupled (IC-) DFB lasers, chip manufacturing costs are expected to be the same as well. We are developing samples for evaluation, planning to provide them to the market by June this year," states Harushige Sugimoto, senior vice president and chief technology officer at Oki Electric Industry. Sugimoto adds that OKI plans to commercialize other small, low-cost components for optical networking using its proprietary silicon lens technology.
For a normal FTTH light source, Fabry-Perot lasers that emit multiple modes are used for short-distance transmission (up to 10 km); IC-DFB lasers that emit a single mode are used for long-distance transmission (up to 20 km). In long-distance transmission, IC-DFB lasers can cause increased degradation in receiver sensitivity after sending data as the noise from unwanted optical reflection increases. An optical isolator is generally used to shut out reflection. However, these components are expensive and take up about several millimeters of real estate. Therefore, for optical sources, the industry has seen a need for semiconductor lasers that do not require optical isolators.
OKI says it achieved optical isolator-free capability using a GC-DFB laser with improved reflection tolerance and reduced degradation from relative intensity noise when impacted by reflected light. With this advance, the company conducted 25-km transmission experiments over the normal optical module operating temperature range (0° to ~70°C) and found that in a state where reflected light (-14 dB) was forcefully applied, there was almost no degradation of reception sensitivity. This confirmed a marked improvement in tolerance to reflected light, compared to conventional IC-DFB lasers.