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2                                                 Transport aspects


            7.1.2   External modulation

            External modulation creates the desired optical signal (RoF signal) using an external modulator coupled
            with  a  CW  light  source,  as  shown  in  Figure  7-2.  Figure  7-2-a  and  -b  represent  the  external modulation
            schemes  for  subcarrier  signal(s)  transmission  and  for  equivalent  low-pass  (baseband)  signal(s)  or  digital
            signal(s) transmission, respectively. Once again, a bias current or voltage may be added to the modulating
            electrical signal for a linear operation if needed. The power, phase, frequency or state-of-polarization of
            optical carrier can be modulated in proportion to the waveform of modulating electrical signal. Although
            this scheme requires an external modulator in addition to the light source, the modulated optical signal will
            be of higher quality, because the intrinsic imperfections of the light source are avoided.






                                     t     Bias       E/O converter
                                                                                                       t
                                                   f opt
                                         f ele
                   Modulating electrical signal  +  Light source  External modulator  Modulated optical signal (RoF signal)

                                                                                   ...               ...
                                 f                                                                     f
                       0  f ele                                                 0  f opt –2 f ele f opt – f ele  f opt + f ele f opt +2 f ele
                                                          a)

                   Modulating electrical signal  Bias  E/O converter             Modulated optical signal (RoF signal)
                                                   f opt
                                            +   Light source  External modulator

                                 f                                                                     f
                       0                                                        0           f opt
                                                          b)                                   G Suppl.55(15)_F7-2


             Figure 7-2 – Schematic block diagram of external modulation: a) for subcarrier signal(s) transmission; and
                         b) for equivalent low-pass (baseband) signal(s) or digital signal(s) transmission

            7.2     Optical-to-electrical conversion

            7.2.1   Optical incoherent detection
            Optical incoherent detection is a power detection scheme. The square-law function of photodetector is
            used for detecting the envelope of received optical signal (RoF signal), which corresponds to the power of
            optical signal. Therefore, if the optical signal power is modulated in proportion to the modulating signal,
            then  incoherent  detection  can  recover  the  original  signal,  as  shown  in  Figure  7-3.  Figure  7-3-a  and  -b
            represent  the  optical  incoherent  detection  schemes  for  subcarrier  signal(s)  transmission  and  for  digital
            signal(s) transmission, respectively. In most cases, a bias voltage should be added to the output port of
            photodetector for a linear operation. An electrical filter is used to extract only a desired electrical signal.























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