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British team uses direct laser modulation to generate QAM signals

Optical injection locking approach removes the need for external modulation schemes

Researchers from the Optoelectronics Research Centre (ORC) at the University of Southampton working with Eblana Photonics, in Ireland, have developed a way of directly modulating laser currents to generate spectrally efficient modulation format signals, which removes the need for external modulators.

Patented by the University of Southampton and licensed to Eblana Photonics, the method uses a technique called optical injection locking (OIL), which is explored in the journal Nature Communications in a paper entitled 'Modulator-free Optical QAM Signal Synthesis'. 

In OIL, a signal from a 'Master' laser (in this case a CW laser) is injected into the cavity of a 'Slave' laser (a directly modulated laser). Under certain conditions, the slave phase locks to the master, even when directly modulated. The bandwidth (that is, difference in frequency of the free-running lasers) over which this locking occurs is called the OIL bandwidth.

Once the directly modulated slave is locked to a CW master, its chirp is suppressed and its modulation bandwidth can be significantly increased (for example, up to 80GHz). Moreover, using the same master for two slave lasers allows mutual coherence between the three devices to be established and hence for the stable coherent superposition (interference) of their outputs. This mutual coherence is critical in the transmitter - it makes it possible to combine the two directly modulated slaves with a 90 degree shift, as well as to provide carrier suppression through destructive interference with a component of the master signal.

The diagram above explains the difference between a free-running and OIL assisted directly modulated semiconductor laser. Due to the large chirp associated with the directly modulated laser, the constellation consists of four rings rather than four points, making it unsuitable for IQ modulation. However, OIL causes the four circles to "˜collapse' into four distinct points in the IQ plane.

Radan Slavik, principal research fellow at the ORC said: "Our paper highlights the exquisite control that we have achieved over the optical field generated directly from a current-modulated semiconductor laser."

Direct current modulated lasers are of huge commercial relevance and are already widely used in optical communications, telecommunications and sensor and high power fibre laser systems. However, the inability to accurately control the full optical field emitted directly from such lasers has been a fundamental problem limiting applications. Radan explains: "The new capability we have demonstrated will be of relevance and could be of significant impact within many scientific and engineering communities that are directly concerned with or exploit laser radiation.

Rob Lennox, director of sales at Eblana Photonics, Dublin, said: "We are very pleased to have collaborated on this innovative development work performed by the ORC team and are looking towards making this new approach a commercial reality."

The paper was co-authored by Zhixin Liu, Radan Slavík, and David J. Richardson from the ORC, Brian Kelly, John O'Carroll, and Richard Phelan from Eblana Photonics Inc, and former Southampton PhD student Joseph Kakande, now at Alcatel-Lucent's Bell Labs in the USA. 

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