Information can be transmitted in an optical communication system in a form of optical symbols that are created by modulating a source of optical radiation.
Goals
- The primary goal of modulation is to suit the information signal to a form that is acceptable for transmission.
- Another goal is to reduce chromatic dispersion causing different speed of each frequency components contained in an optical pulse.
- And finally the goal is to avoid the problem of long string of the same binary symbols, i.e. long strings of logical 0s or logical 1s that can potentially cause problems with timing.
Classification
There are many modulation formats used in fibre-optic communication systems. They can be classified based on whether they modulate the signal’s amplitude or phase:
- Intensity modulation formats:
- OOK – On-Off Keying
- AMI – Alternate Mark Inversion
- DB – Duobinary Modulation
- CRZ – Chirp Return to Zero
- CSRZ – Carrier Suppressed Return to Zero
- Frequency modulation formats:
- OFDM – Orthogonal Frequency-Division Multiplexing
- VDMT – Vectored Discrete Multi-Tone
- Phase modulation formats:
- PSK – Phase Shift Keying
- BPSK – Binary Phase Shift Keying
- DPSK – Differential Phase Shift Keying
- QPSK – Quadrature Phase Shift Keying
- Formats combining amplitude, phase, polarization of a signal:
- PM-QPSK – Polarization Multiplexing QPSK
- QAM – Quadrature Amplitude Modulation
- 16-QAM, 64-QAM, etc.
- Multi-Carrier Modulations (MCM):
- OFDM – Orthogonal Frequency Division Multiplexing
- DMT – Discrete Multi-Tone
- VDMT – Vectored Discrete Multi-Tone
These are selected modulation formats (among many others) that are widely used in optical communications and are promising for high-speed transmission systems.
One of the most used formats is OOK, where binary 1 is assigned certain power level of a laser. Binary 0 is represented by absence of laser power. An optical symbol can last the entire clock period dedicated for a bit, then it is a Non-Return to Zero (NRZ) signal or some part of the bit slot, then it is a Return to Zero (RZ). One of the examples is that a symbol starts with the rising edge and returns to zero during the clock period, e.g. in the half of the bit slot.
However, this is just one of the cases of NRZ and the symbols do not have to last exactly half of the period and start/end exactly at the edge.
The main benefit of having shorter symbols is greater immunity to their dispersion (spread due to different speed of particular components of which a laser pulse is composed, i.e. frequencies, modes, etc.) that leads to Inter-Symbol Interference (ISI).