Contributors: Mai Vinh Nguen
Table of Contents
Background
Topic of fiber optics is divided into small sub-topic. Our group chooses to work on dispersion.
What questions did you pick up for further investigation?
- What is dispersion?
- How many types of dispersion in optical fibers?
- In which situation certain type of dispersion dominates others?
Our explanation (original hypothesis)
Dispersion based on our own knowledge is a phenomenon relating to light traveling from one medium to another and light with different wavelength will bend at different angels and produce dispersion. One typical example will be a white light passing through a transparent prism will divided in to a spectrum of light with blue light bend at highest angle and red light at lowest angle. Hence, blue light is in the bottom and red light is at the top of the spectrum
Critical evaluation
Our explanation is only based on our secondary knowledge regarding light dispersion. More studies need to be done to understand dispersion in details and how it relates to fiber optics.
Results
Dispersion is defined as the spreading of light pulse when they travel a fiber. This phenomenon is due to the fact that speed of light depends on its wavelength and propagation mode. In case of travelling long distances, slight differences in speed accumulate. As a result, bit errors will occur.
Like attenuation, dispersion shortens the distance that signal travels inside optic fibers. Unlike attenuation, dispersion does not weaken the signal, but it blurs the signal. For example, a pulse of 1 nanosecond at the transmitter will be spread out to 10 nanoseconds at the receiver. Hence, signals are not properly received and decoded.
Basically there are 4 types of dispersion in fiber optics:
- Modal dispersion: different modes propagate at different group velocities
- Material dispersion: the index of refraction of the medium changes with wavelength
- Waveguide dispersion: refractive index change across waveguide means that different wavelengths have different delays
- Polarization mode dispersion: if waveguide is birefringent
1. Modal dispersion happens only in multimode fiber optics. This is due to the fact that each mode has its own characteristic velocity through optical fiber as it there were light rays coming to the fiber at different angels. As a result, pulse spreads put as they travel along the fiber. The more modes the fiber transmits, the more pulses spread out.
It is important to note that modal dispersion is the largest type of dispersion in multimode fibers. For example, with a confinement angel of 8 degree, the pulse spreads out to 30 ns in 1 km of fiber and hence limits the transmission speed since pulses need to be separated as least 30ns to avoid pulse overlap.
Maximum data rate can be estimated using pulse spreading through a formula: Max data rate = 0.7 / pulse spreading
2 & 3. Material and waveguide dispersions are added together to form a chromatic dispersion.
Material dispersion is caused by a change in fiber optic material’s refractive index with different wavelength. The higher the index, the slower the light travels.
Waveguide dispersion is due to the distribution of light between core and cladding. Since waveguide properties depend on wavelength, changing wavelength affects how light is guided along the fiber.
A practical use of material and waveguide dispersion is to combine them in a way that produces zero chromatic dispersion at a desirable operating wavelength (normally between 1530 and 1620 nm). This can be done by changing waveguide dispersion since material dispersion is usually undesirable to change due to desirable intrinsic properties of chosen material for optical fiber (most likely silica). The following figure shows how material dispersion, waveguide dispersion and chromatic dispersion vary with wavelength in nonzero dispersion-shifted fiber and indicates zero chromatic dispersion at 1.5-micrometer wavelength.
4. Polarization mode dispersion: Since in multimode optical fiber, the effect of polarization mode dispersion is relatively small compared with modal and chromatic dispersion; it is usually ignored. In single-mode fiber, pulses are transmitted in two distinct polarization modes and the electric fields of two modes are perpendicular to each other. In ideal situation where all forces acting on optical fiber are perfectly symmetrical, these two modes cannot be distinguished. However, in real world, stress during manufacturing process as well as environmental stresses due to factors as temperature, sea water loading and low-level vibration cause slight differences in the refractive index experienced by light pulses in two polarization modes. This phenomenon is called birefringence.
If the birefringence is uniformly distributed along the fiber, light pulse in faster polarization mode will travel one wavelength ahead of the slower one in every 10 meters. However, due to the causes of changes fluctuates in random manner, producing polarization mode dispersion (PMD). The below picture illustrates the effect of PMD.
In case of short distance transmission or low data rates (2.5 Gb/s or less), polarization mode dispersion is less compared with chromatic dispersion. However, adequate control is critical for long distance transmission at higher speed.
Since these dispersions are independent of each other, they are added to form the total dispersion. Hence, total pulse spreading is give by the formula:
In multimode fibers, polarization mode dispersion is negligible and the formula is given as:
In single-mode fibers, there is no modal dispersion hence the formula becomes:
In the above formula, dispersion is in the unit of time per unit distance (normally nanosecond or picosecond per kilometer) and pulse spreading is in the unit of time (usually nanosecond or picosecond).
Conclusions
Our initial theory regarding dispersion in fiber optics is only about chromatic dispersion. By doing research and further study, dispersions are understood in more details which include four major type of dispersion in optical fibers. In what situation each type of dispersions dominate are also well understood.
Studied material
Hecht, J. (2011). Understanding fiber optics (5th ed.). Pearson Prentice Hall
https://www.youtube.com/watch?v=ZhTEhPJqha0
https://www.youtube.com/watch?v=kLDQLkiPfZQ&list=PL3585AC23FCCEBAAD