DBR ACTIVE OPTICAL FILTERS - TRANSFER-FUNCTION AND NOISE CHARACTERISTICS

An integrated distributed Bragg reflector (DBR) laser can be used as a narrow-band active optical filter if operated below threshold. A theoretical model of DBR active filters is presented and verified experimentally through measurements of the transfer function and the noise spectra. Our theory allows simple yet accurate evaluation of the transmission transfer function and of the noise properties of this class of filters. A conventional (rather than specifically designed for filter applications) DBR laser device used as a filter is shown, experimentally and theoretically, to have two drawbacks: a multilobe transfer function and small gain (less than 10 dB facet-to-facet). Both problems can be overcome via reduction of two key device parameters: the grating coupling coefficient and the physical length of the active and/or phase control sections. This technique can lead to devices with attractive properties, having a gain of 35 dB, sidelobe suppression of 32 dB, and bandwidth as narrow as 1 GHz. The noise spectrum at the output of the grating section has the same shape as the power transfer function while the noise spectrum at the output of the active section is generally different from that at the output of the grating section; it is also different from the transfer function. The tunability range of DBR active filters is likely to be limited by current-injection-induced losses. © 1990 IEEE