Optical Transmission Modes

There are three primary types of transmission modes using optical fiber. They are

1. Step Index
2. Graded Index
3. Single Mode.

Step index has a large core, so the light rays tend to bounce around inside the core, reflecting off the cladding. This cause some rays of take longer or shorter path through the core. Some take the direct path with hardly any reflections while others bounce back and fourth taking a longer path. The result is that the light rays arrive at the receiver at different times. The signal becomes longer than the original signal. LED light sources are used. A typical Core is 62.5 microns.

Graded index has a gradual change in the core's refractive index. This causes the light rays to be gradually bent back into the core path. This is represented by a curved reflective path in the attached drawing. The result is a better receive signal than with step index. LED light sources are used. Typical Core is 62.5 microns.

Both step index and graded index allow more than one light source to be used (different colors simultaneously), so multiple channels of data can be run at the same time.

Single index mode has separate distinct refractive indexes for the cladding and core. The light ray passes through the core with relatively few reflections off the cladding. Single mode is used for a single source of light (one color) operation. It requires a laser and the core is very small of 9 microns.

Frequency is not used to talk about speed any more, instead of that wavelengths are used. The wavelength of light sources is measured in nanometers or 1 billionth of a meter.

Indoor cable specifications:

• LED (Light Emitting Diode) light source
• 3.5 dB/Km attenuation (loses 3.5 dB of signal per kilometer)
• 850 nM - wavelength of light source
• Typically 62.5/125 (core dia/cladding dia)
• Multimode - can run many light sources.

Outdoor cable specifications:

• Laser Light Source
• 1 dM/Km Attenuation (loses 1 dB of signal per kilometer)
• 1170 nM - wavelength of light source
• Monomode (single mode).

1. Advantage of Fiber Optic
   • Noise immunity: RFI and EMI immune (RFI - Radio Frequency Interference, EMI - Electromagnetic Interference)
   • Security:cannot tap into cable.
   • Large Capacity due to BW (bandwidth)
   • No corrosion
   • Longer distances than copper wire
   • Smaller and lighter than copper wire
   • Faster transmission rate.
2. Disadvantages of Fiber Optic
   • Physical vibration will show up a signal noise.
   • Limited physical are of cable. Bend it too much and it will break.
   • Difficult to splice.

The cost of optical fiber is a trade-off between capacity and cost. At higher transmission capacity, it is cheaper than copper. At lower transmission capacity, it is more expensive.