Feedback Control fine-tunes wdm lasers
Paula Noaker Powell
When controlling tunable lasers for DWDM applications, two issues must be addressed. First, a look-up table (to be stored in EEPROM) is necessary to tell the microprocessor what currents should be applied to the different sections to produce a specific output power and frequency. Second, some form of feedback control is required to produce long-term stability and frequency accuracy. Researchers from the University of Gent (Gent, Belgium) have resolved both issues with a new feedback control scheme for tunable super-structure-grating distributed-Bragg-reflector (SSG-DBR) lasers that offers frequency accuracy exceeding ±0.5 GHz and a side-mode suppression ratio greater than 35 dB. With this method, the active section is monitored to maintain mode stability, and a highly stable Fabry-Perot etalon is used as a reference to lock the laser frequency to a specific ITU (International Telecommunication Union) channel. As a result, the scientists were able to maintain stability even when directly modulating the laser at 1.244 Gb/s.
A characterization scheme that uses the voltage monitoring to generate a look-up table of operation points also was demonstrated. The above stated accuracies were reached for all points, which indicates that applications in DWDM systems with channel spacings of 25 GHz or less are feasible.
By including power equalization at the characterization stage, power variations across the tuning range were reduced from almost 8 dB to 0.8 dB. The total characterization time dropped from two to three days to 100 to 150 minutes, but the scientists believe they may be able to reduce this to 20 to 30 minutes by integrating the control circuitry on a single board. Once the feedback scheme is used in practice, though, some form of screening will be necessary to filter out defective devices before starting the characterization process.
In principle, the feedback control scheme also will work for other three- or four-section DBR-type lasers with minor adjustment. According to the researchers, care must be taken to ensure that the lasers are properly designed. For more information, contact Roel Baets at email@example.com.