Encoding and decoding communications traffic in a pulse amplitude modulation format and optical apparatus for same

A method of decoding communications traffic bits carried on an optical carrier signal, the method comprising:

• receiving an optical signal comprising optical pulses having respective amplitudes and respective phases, and having a symbol period, T;
• directly detecting the optical pulses to generate a corresponding electrical received signal;
• oversampling the electrical received signal at a rate of two samples per symbol period to obtain even samples indicative of respective amplitudes of the pulses and odd samples indicative of respective phase differences between consecutive pulses;
• and
• decoding the even samples to obtain at least one even bit for each even sample and decoding the odd samples to obtain at least one odd bit for each odd sample, the even bits and the odd bits being interleaved according to the respective samples.

An optical transmitter comprising:

• an optical source configured to generate an optical carrier signal comprising optical pulses having an amplitude and respective phases;
• an optical modulator configured to receive the optical pulses and to perform pulse amplitude modulation and phase modulation on the optical pulses;
• and
• an encoding circuit configured to receive communications traffic bits to be transmitted and to:
• map at least one respective bit to one of a pre-set plurality of optical amplitudes;
• map at least one further respective bit to a phase difference between consecutive optical pulses;
• and
• generate and transmit a respective drive signal to the optical modulator, the optical modulator, in response to the drive signal, being configured to perform at least one of pulse amplitude modulation and phase modulation to thereby encode the bits onto respective optical pulses.

An optical receiver comprising:

• a photodetector configured to:
• receive an optical signal comprising optical pulses having respective amplitudes and respective phases, and having a symbol period, T;
• and
• directly detect the optical pulses to generate a corresponding electrical received signal;
• a sampling circuit configured to oversample the electrical received signal at a rate of two samples per symbol period to obtain even samples indicative of respective amplitudes of the pulses and odd samples indicative of respective phase differences between consecutive pulses;
• and
• a decoding circuit configured to decode the even samples to obtain at least one even bit for each even sample and decode the odd samples to obtain at least one odd bit for each odd sample, the even bits and the odd bits being interleaved according to the respective samples.