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Chirp reduction and on/off contrast enhancement via optical injection locking and coherent carrier manipulation

The most cost-effective solution for modulating data onto an optical carrier is via direct modulation of a semiconductor laser. Unfortunately, this approach suffers from high chirp. The chirp can be reduced by reducing the on/off modulation contrast ratio (i.e. by keeping the signaling laser well above threshold when generating both logical ‘0’ and ‘1’ bits), but the low contrast ratio itself compromises performance. Other techniques can better suppress chirp, e,g., based on selfinjection or optical injection locking of the directly-modulated laser (slave) to another laser (master) that emits CW light. However, this technique although very efficient at eliminating chirp, also requires the slave laser be operated well above threshold. We show however that the issue of the limited on/off modulation contrast can be addressed in this instance by subtraction of the carrier using a component of the master beam and an interferometric arrangement.