The continuous tuning range of a three-section DFB amplifier-filter with a passive centre section can be strongly enhanced by combining the carrier-induced Bragg wavelength shift with the conventional wavelength shift due to the phase shift between the two lateral gain sections. This principle is demonstrated by the fabrication of filters exhibiting a nearly constant gain of 15 dB as well as a 4 GHz filtering bandwidth over a 2.2 nm continuous tuning range.
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Nakajima et al. (1994) studied this question.
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