Dispersion in optical fibers
In an optical medium, such as fiber, there are three types of dispersion, chromatic, modal, and material.
Chromatic dispersion results from the spectral width of the emitter. The spectral width determines the number of different wavelengths that are emitted from the LED or laser. The smaller the spectral width, the fewer the number of wavelengths that are emitted. Because longer wavelengths travel faster than shorter wavelengths (higher frequencies) these longer wavelengths will arrive at the end of the fiber ahead of the shorter ones, spreading out the signal.
One way to decrease chromatic dispersion is to narrow the spectral width of the transmitter. Lasers, for example, have a more narrow spectral width than LEDs. A monochromatic laser emits only one wavelength and therefore, does not contribute to chromatic dispersion.
Modal dispersion deals with the path (mode) of each light ray. As mentioned above, most transmitters emit many different modes. Some of these light rays will
travel straight through the center of the fiber (axial mode) while others will repeatedly bounce off the cladding/core boundary to zigzag their way along the waveguide, as illustrated below.
The modes that enter at sharp angles are called high-order modes. These modes take much longer to travel through the fiber than the low-order modes and therefore contribute to modal dispersion. One way to reduce modal dispersion is to use graded-index fiber. Unlike the two distinct materials in a step-index fiber, the graded-index fiber’s cladding is doped so that the refractive index gradually decreases over many layers. The corresponding cross-sections of the fiber types are shown below.
With a graded-index fiber, the light follows a more curved path. The high-order modes spend most of the time traveling in the lower-index cladding layers near
the outside of the fiber. These lower-index core layers allow the light to travel faster than in the higher-index center layers. Therefore, their higher velocity compensates for the longer paths of these high-order modes. A good waveguide design appreciably reduces modal dispersion.
Modal dispersion can be completely eliminated by using a single-mode fiber. As its name implies, single mode fiber transmits only one mode of light so there is no
spreading of the signal due to modal dispersion. A monochromatic laser with single-mode fiber completely eliminates dispersion in an optical waveguide but is usually used in very long distance applications because of its complexity and expense.
Material dispersion is caused by the wavelength dependence of the refractive index on the fiber core material, while the waveguide dispersion occurs due to dependence of the mode propagation constant on the fiber parameters (core radius, and difference between refractive indexes in fiber core and fiber cladding) and signal wavelength.
Material dispersion contributes to group delay distortion, along with waveguide delay distortion, differential mode delay, and multimode group delay spread.
Fiber Optic Dispersion Compensation Devices
Dispersion management is the process to design the fiber and compensating elements in the transmission path to keep the total dispersion to a small number. Typically, dispersion compensating elements are placed every 100 km or so.
The figure below shows the performance of a fiber path that has alternating lengths of (+D) NZ-DSF and (-d) NZ-DSF every 20 km. The first 20 km length of fiber is (+D) NZ-DSF, so the dispersion increases over that length to 60 ps/nm. The next 20 km length of fiber is (-D) NZ-DSF type, so the dispersion gradually decreases back to zero. This pattern repeats two more times. At the end of the 120 km fiber path, the dispersion has returned to near zero.
But in most reality applications, fiber is already in place and odds are that the fiber is NDSF type. More than 80% of all single-mode fiber worldwide is NDSF type. In these cases, a more common means of controlling dispersion is the use of DCM (Dispersion Compensating Modules) placed at periodic intervals.
DCM’s are usually one of two types. The first type is DCF or Dispersion Compensating Fiber. This is simply a spool of a special type of fiber that has very large negative dispersion. Typically DCF dispersion can be in the range of -80 ps/(nm∙km), so a 20 km length of DCF can compensate for the dispersion in a 100 km length of NDSF.
The second type of DCM is a FBG (Fiber Bragg Grating) type. Here, a series of FBG’s or one very long FBG is written into a tens of meter length of fiber to perform the dispersion compensation.
Both of these types of DCM’s have relatively high insertion loss. A 60 km compensator may exhibit 6 dB of loss or more. Because of this, DCM’s are usually co-located with EDFA’s.