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Digital Processing by Le Nguyen Binh

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3

External Modulators for Coherent Transmission and Reception

Under coherent transmission it is essential that the linewidth of the laser source remain as stable and narrow as possible so that the mixing with a local oscillator laser would give the baseband signals. Thus the modulation of the lightwaves generated by the light source must be done outside the lasing cavity, hence external modulators. This chapter deals with the modulation formats employed to manipulate the phase and amplitude of the electromagnetic fields of lightwave sources by electro-optic modulators.

The external modulation is implemented by using optical modulators within which the input is coupled with the output of a CW laser source. The laser is thus turned on at all ...

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