1 Digital intensity modulations: Alternate Mark Inversion, Duobinary Modulation, Carrier Suppressed Return to Zero
1.3 Transceivers for optical modulations

A transceiver is a device composed of a transmitter and a receiver, which are combined and share common circuits or/and common housing. Transceivers are placed at the input of a network to launch modulated, encoded information to transmission medium (e.g. optical fibres) – a transceiver works as a transmitter, and at the termination of the network to decode, demodulate the signal – it operates as a receiver.

In case of transmission in both directions (e.g. duplex transmission), each transceiver transmits and receives data (e.g. alternately or at the same time in duplex).

A transmitter part can use lasers as a source of symbols in optical networks; a receiver contains a photodiode that converts an optical signal to electrical signal, which is further processed e.g. by a digital signal processor (DSP).

Transceivers for CSRZ modulation

Each following binary symbol has phase shifted by π. A Mach-Zehnder modulator (MZM) is used to create the binary string, where pulses have the same shape and altered phase. The speed of this signal is the same as the speed of an information signal.

The information signal in a form of electronic pulses is sent to the modulator, and based on this “control” signal, the pulses with altering phase produced in the modulator are released or suppressed. The output signal is then a certain product of those two signals.

  • The NRZ transmitter is based on a continuous wave laser externally modulated with a Mach-Zehnder modulator. We set parameters, such as bit rate, laser frequency, full width at half maximum, output power of the laser, loss inserted by the modulator, etc.
  • Unlike NRZ, we set the RZ raised cosine format in the electrical signal generator.
  • For the CSRZ type, a NRZ optical generated signal is subsequently modulated by a second MZM, which is driven by a sinusoidal signal with a frequency half of the bit rate.
  • Any two adjacent bits will have a π phase shift and it is possible to suppress the carrier and generate a CSRZ modulated signal.
  • For NRZ, RZ and CSRZ, we use a compound receiver which models a photodetector, an electrical amplifier and an electrical filter.