Abstract
A numerical model for a laser diode, weakly coupled to an external cavity, is presented. Using this model, the actual frequency noise spectrum is considered rather than a white frequency noise for estimating the minimum bit rate in a differential-phase-shift-keying (DPSK)-system. Despite possible linewidth reductions by up to 200 with long external cavities, the minimum bit rate (BER = 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> ) remains nearly unchanged. However, a longer laser cavity ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\approx600 \mu</tex> m) connected to an external cavity of moderate length ( ≈10 cm) yields a significant reduction of the minimum bit rate (BER = 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> ) for DPSK-systems at low feedback levels. If more stringent criteria are applied (e.g., receiver penalty < 1 dB) the minimum bit rate remains high also for longer laser cavities.
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