Abstract
Widely tunable sampled-grating DBR (SGDBR) and super-structure grating DBR (SSG-DBR) lasers are regarded as major contenders for transmitters in dense wavelength division multiplexed (DWDM) systems.1,2 They can be tuned over 40-60-nm and show high side-mode suppression ratio (SMSR) >35 dB. It is required however that the frequency tuning is controlled with the accuracy greater than 10% and the SMSR be >30 dB without mode hops over a whole tuning range. These requirements are essential since a typical channel spacing in DWDM systems is at present 50-GHz and soon will narrow to 25-GHz. Practical implementation of these requirements is not simple mainly due to the fact that all three sections of a laser (front-, back-mirror and phase) are involved in the wavelength tuning, output power level and SMSR control. This task of controlling three tuning currents in practical devices is facilitated at present by the look-up table that is stored in a microprocessor. This approach has a major drawback associated with a periodic recalibration of a laser due to its aging3 and to keep the look-up table valid. Such operation is important as the look-up table is uniquely generated for each individual device. To reduce this costly and laborious task optoelectronic feedback techniques have been proposed.4,3 They are too cumbersome however since they require complex electronic circuitry. A much simpler approach based on the detection of transparency current in semiconductor laser amplifiers (SOAs) for wavelength monitoring and tracking was proposed lately.6,7 It relies on monitoring transparency current changes with wavelength. At each transparency point there is a noticeable drop in the bias current or the voltage induced across SOA. The voltage drop across SOA is too low a value however to yield accurate measurements (low SNR).
© 2002 Optical Society of America
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