Dispersion also referred as pulse spreading, it signifies the spreading out of pulses of light waves when it is propagated through the optical fiber. It occurs mainly because of the difference in time taken by the propagation of optical rays in different paths within the fiber core.
The different dispersion mechanisms include:
- Chromatic or intramodal dispersion
- Intermodal or mode dispersion.
- Polarization mode dispersion.
In other words, Intramodal and intermodal dispersion are terms commonly used in the field of optical communication to describe the distortion or spreading of optical signals as they travel through fiber optic cables.
Intermodal or mode dispersion
Intermodal dispersion is found in multimode optical fibres. Multimode fiber are the fibres that allow various modes to propagate through it. Therefore it is not observed in single mode fibers as only a single mode is allowed to propagate through the single mode fibre. But single mode fibres suffer from the intramodal dispersion (chromatic dispersion).
The intermodal dispersion results due to propagation delay difference between various modes propagating through the optical fibre.
Intermodal dispersion is found more in case of multimode step index fibers in comparison to graded index fibers. As in case of multimode step index fibers, the refractive index of the core is uniform. Because of this same refractive index throughout the core of the multimode step index fiber, different modes propagating through the core travel with same speed. Because of this same speed, different light rays launched into the optical fiber at different angles at the transmitting end takes different times to reach to the other end of the optical fiber as their propagation path (path length) changes with change in angle while launching.
Chromatic or Intramodal Dispersion
This type of dispersion results from the finite spectral linewidth of the optical source. Propagation delay differences between the different spectral components of the transmitted light signal causes broadening of each transmitted mode and hence collectively known as intramodal dispersion. It may occur in all types of optical fibers (single-mode as well as multimode, step-index as well as graded-index). It is caused by the dispersive properties of the fiber material (material dispersion) and also guidance effects within the fiber structure (waveguide dispersion).
Intramodal vs Intermodal Dispersion: Key Differences
| Basis | Intramodal Dispersion | Intermodal Dispersion |
|---|---|---|
| Definition | Dispersion within a single mode (single type of signal). | Dispersion between multiple modes (different types of signals). |
| Causes | Caused by various factors within a single mode, such as material dispersion, waveguide dispersion, and modal dispersion. | Caused by differences in propagation characteristics between different modes, like modal dispersion in multimode fibers. |
| Types | Includes material dispersion, waveguide dispersion, and modal dispersion. | Primarily refers to modal dispersion in the context of optical fibers. |
| Impact on Signals | Results in spreading of different wavelengths or modes of light over time, affecting signal integrity. | Leads to differential delay in various modes, causing signal distortion. |
| Mitigation | Can be mitigated by using dispersion-compensating fibers, equalization techniques, or using single-mode fibers. | Typically addressed by using single-mode fibers or mode-division multiplexing. |
| Frequency Dependency | Dispersion varies with wavelength in intramodal dispersion. | Not applicable as it deals with different modes of propagation. |
| Applicability | Relevant in both single-mode and multimode fiber optics. | Mainly relevant in multimode fiber optics. |
| Measurement Methods | Measured using techniques like time-domain reflectometry (TDR) and optical time-domain reflectometry (OTDR). | Measured using mode coupling and modal delay measurement techniques. |
| Impact on Bandwidth | Can limit the bandwidth and data transmission capabilities of a fiber optic system. | Can limit the distance over which multimode signals can be transmitted without significant distortion. |
| Examples | Examples include chromatic dispersion (material dispersion), waveguide dispersion, and mode dispersion in optical fibers. | Example primarily includes modal dispersion in multimode optical fibers. |
Intramodal vs Intermodal Dispersion: Key Takeaways
Description
- Intramodal Dispersion: Intramodal dispersion, also known as chromatic dispersion, occurs within a single mode (mode refers to a specific path or trajectory) of an optical fiber due to variations in the refractive index for different wavelengths of light.
- Intermodal Dispersion: Intermodal dispersion, also called modal dispersion, arises from the fact that different modes of light propagation within a multimode optical fiber have different propagation velocities.
Fiber Type
- Intramodal Dispersion: Primarily affects single-mode optical fibers, as they support only one mode of light propagation.
- Intermodal Dispersion: Primarily affects multimode optical fibers, which support multiple modes of light propagation.
Cause
- Intramodal Dispersion: Caused by the difference in the speed of light at different wavelengths (chromatic dispersion) or material properties in a single-mode fiber.
- Intermodal Dispersion: Caused by the varying propagation velocities of different modes within a multimode fiber due to their different paths.
Wavelength Dependency
- Intramodal Dispersion: Highly wavelength-dependent, meaning it varies with the wavelength of light used.
- Intermodal Dispersion: Less wavelength-dependent and primarily depends on the number of modes in the fiber.
Dispersion Effects
- Intramodal Dispersion: Leads to the spreading of different wavelengths over distance, resulting in signal distortion.
- Intermodal Dispersion: Causes spreading of light pulses due to different modes traveling at different speeds, leading to signal distortion.
Frequency
- Intramodal Dispersion: More pronounced in higher-frequency, or shorter-wavelength, optical signals.
- Intermodal Dispersion: Becomes more significant with increasing fiber length, making it a concern for longer-distance communications.
Mitigation
- Intramodal Dispersion: Can be compensated for with devices like dispersion-compensating fibers or dispersion-shifted fibers.
- Intermodal Dispersion: Typically managed by using graded-index multimode fibers or choosing single-mode fibers for longer-distance communication.
Applicability
- Intramodal Dispersion: Mainly relevant in long-haul and high-data-rate single-mode optical communication systems.
- Intermodal Dispersion: Mainly a concern in shorter-distance, lower-data-rate multimode optical communication systems.