When an optical signal travels through the core of an optical fibre, the speed at which it travels is inversely proportional to its wavelength. Shorter wavelengths travel faster than longer wavelengths, and so arrive at the detector sooner.
Each optical pulse in a train of pulses has a finite optical bandwidth, or group of wavelengths and amplitudes all contained within a wavelength envelope, centred on a specific wavelength. When such an optical pulse travels along an optical fibre, the pulse broadens as the shorter wavelengths travel faster than the longer wavelengths. The longer the distance travelled, the greater the broadening. This is defined as chromatic dispersion.
As an optical signal comprises trains of such pulses, if the delay causes pulses in a pulse train to overlap, signal integrity is lost and the network capacity degraded.
Test and measurement
Optical fibres can be tested for chromatic dispersion by measuring the time delay between multiple discrete wavelengths over a range of wavelengths. This is achieved in a number of ways, including:
- The time delay is directly measured using optical time domain reflectometry (OTDR). The outputs of several lasers with different centre wavelengths are launched into optical fibre and reflected back to the source. The OTDR measures the optical travel time (“time-of-flight”) for each wavelength, and hence the time delay.
- The time delay is inferred from the relative phase shift of a modulated broadband source over the entire wavelength range. The phase of the broadband light is measured at the end of a fibre and compared with the signal used to phase-modulate the light.
Chromatic dispersion can cause a train of optical pulses that form a signal to become indistinguishable, particularly for signals with large number of pulses per second (“bit rate”). This can result in decoding errors at the receiving end of a fibre, and therefore a greater bit error rate (BER).
Dispersion-compensating fibre can be installed in a network to counteract the effect of chromatic dispersion by slowing the high-velocity wavelengths, allowing the other wavelengths to catch up. In this way, optical pulses that may have broadened can now be recognised by the receiver.