What is CD?
Chromatic dispersion (CD) is a property of optical fiber (or optical component) that causes different wavelengths of a light source to propagate at different velocities, means if transmitting signal, from a LASER source, this LASER source having spectral width and emit different wavelengths apart from its center wavelength. Since all light sources consist of a narrow spectrum of light (comprising of many wavelengths), all fiber transmissions are affected by chromatic dispersion to some degree. In addition, any signal modulating a light source results in its spectral broadening and hence exacerbating the chromatic dispersion effect. Since each wavelength of a signal pulse propagates in a fiber at a slightly different velocity, each wavelength arrives at the fiber end at a different time. This results in signal pulse spreading, which leads two inter-symbol Interference between pulses and increases bit errors
What is cause of CD?
Chromatic dispersion is due to an inherent property of silica optical fiber. The speed of a light wave depends on the refractive index, n, of the medium within which it is traversing. In silica optical fiber, as well as many other materials, n changes as a function of wavelength. Thus, different wavelengths travel at slightly different speeds along the optical fiber. A wavelength pulse is composed of several wavelength components or spectra. Each of its spectral constituents travel at slightly different speeds within the optical fiber. The result is a spreading of the transmission pulse as it travels through the optical fiber.
What is unit of CD and CD Coefficient?
The chromatic dispersion (CD) parameter is a measure of signal pulse spread in a fiber due to this effect. It is expressed with ps/nm units, where the picoseconds refer to the Signal pulse spread in time and the nanometers refer to the signal’s spectral width. Chromatic dispersion can also be expressed as fiber length multiplied by proportionality
Coefficient. This coefficient is referred to as the chromatic dispersion coefficient and is measured in units of picoseconds per nanometer times kilometer, ps/(nm ⋅ km). It is
Typically specified by the fiber the cable manufacturer and represents the chromatic dispersion characteristic for a 1 km length of fiber.
Which are main factors for CD?
Chromatic dispersion affects all optical transmissions to some degree. These effects become more pronounced as the transmission rate increases and fiber length increases.
Factors contributing to increasing chromatic dispersion signal distortion include the following:
1. Laser spectral width, modulation method, and frequency chirp. Lasers with wider spectral widths and chirp have shorter dispersion limits. It is important to refer to manufacturer specifications to determine the total amount of dispersion that can be tolerated by the lightwave equipment.
2. The wavelength of the optical signal. Chromatic dispersion varies with wavelength in a fiber. In a standard non-dispersion shifted fiber (NDSF G.652), chromatic dispersion is near or at zero at 1310 nm. It increases positively with increasing wavelength and increases negatively for wavelengths less than 1310 nm.
3. The optical bit rate of the transmission laser. The higher the fiber bit rate, the greater the signal distortion effect.
4. The chromatic dispersion characteristics of fiber used in the link. Different types of fiber have different dispersion characteristics.
5. The total fiber link length, since the effect is cumulative along the length of the fiber.
6. Any other devices in the link that can change the link’s total chromatic dispersion including chromatic dispersion compensation modules.
7. Temperature changes of the fiber or fiber cable can cause small changes to chromatic dispersion. Refer to the manufacturer’s fiber cable specifications for values.
How to mitigate CD in a link?
1. Change the equipment laser with a laser that has a specified longer dispersion limit. This is typically a laser with a narrower spectral width or a laser that has some form of pre-compensation. As laser spectral width decreases, chromatic dispersion limit increases.
2. For new construction, deploy NZ-DSF instead of SSMF fiber.NZ-DSF has a lower chromatic dispersion specification.
3. Insert chromatic dispersion compensation modules (DCM) into the fiber link to compensate for the excessive dispersion. The optical loss of the DCM must be added to the link optical loss budget and optical amplifiers may be required to compensate.
4. Deploy a 3R optical repeater (re-amplify, reshape, and retime the signal) once a link reaches chromatic dispersion equipment limit.
5. For long haul undersea fiber deployment, splicing in alternating lengths of dispersion compensating fiber can be considered.
6. To reduce chromatic dispersion variance due to temperature, buried cable is preferred over exposed aerial cable.
