What is Bandwidth of an optical fiber why there is a big demand?

Posted on January 7, 2012. Filed under: fiber optic | Tags: , , |

Typical Fiber Optic Bandwidth(Click 2 Enlarge)

Today, while reading through the history of Internet, interestingly it came to the notice that the name given to Internet was ARPAnet when it was invented in the 1970s. U.S department of Defense used this ARPAnet to link research computers. SONET system was there in the 1970s as redundant ring network. The first telephone networks with optical fibers came in the 1980s.

The first fiber optic telephone network was advertised with adjectives that it is so quiet that we could ‘hear a pin drop’. The optical fiber has gigantically pushed the growth of internet by supporting with limitless bandwidth options to the network provider. Optical fiber has always been ready to transmit whatever data, image, voice the humankind may develop in the recent future.

Internet growth has doubled every year if not exaggerated. After the telecommunication growth collapse in 2000, the internet and the network bandwidth has been growing a realistic figure of 10 to 30 percent per annum. Since the price of bandwidth continues to drop, the revenue growth has been limited to 10 %.

So, why bandwidth is more important in optical communication is evident from the cost of the bandwidth. Bandwidth will continue to grow faster than the population growth. More and more people get reliable access to the internet due to drop in prices of computers and access networking. The growth in optical fiber communication ensures delivery of high quality video and high data intensity services that calls for high bandwidth.


The optical losses and usable bandwidth of a fiberoptic system have to be taken into account. As mentioned
previously, multimode fibers have greater losses and less bandwidth compared to single mode.
Single mode has lower losses and very high bandwidth than does multimode.

Most manufacturers of multimode fiber-optic cable do not specify dispersion. They will provide a figure
of merit known as the bandwidth-length product or just bandwidth with units of MHz-kilometer. For
example, 500 MHz-km translates to a 500 MHz signal that can be transported 1 km. The product of the
required bandwidth and transmission distance cannot exceed 500:

BW × L ≤ 500

A lower bandwidth signal can be sent a longer distance.
A 100 MHz signal can be sent

L = BW – product/BW
= 500 MHz-km/100 MHz
= 5 km

Single-mode fiber typically has a dispersion specification provided by the manufacturer. The dispersion
is specified in picoseconds per kilometer per nanometer of light source spectral width or ps/km/nm. This
loosely translates to the wider the spectral bandwidth of the laser light source, the more dispersion. The analysis of dispersion of a single-mode fiber is very complex. An approximate calculation can be made with
the following formula:

BW = 0.187/(disp × SW × L),


disp is the dispersion of the fiber at the operating wavelength with units seconds per nanometer per

SW is the spectral width (rms) of the light source in

L is the length of fiber cable in kilometers.

For example, with a dispersion equal to 4 ps/nm/km, spectral width of 3 nm, and a transmission length
of 20 km, then:

BW = 0.187/(4 × 10–12 s/nm/km) × (3 nm) × (20 km)
BW = 779,166,667 Hz or about 800 MHz.

If the spectral width of the laser light source is doubled to 6 nm the bandwidth will drop to about 390
MHz. This shows how significant the spectral width of the laser source is on the usable bandwidth of a fiber.
If a laser light source with a narrow optical spectral width is used, or a fiber with a lower dispersion figure,
the bandwidth and transmission distance will increase.

In single-mode fiber communications, there are two basic types of laser light sources. The first type is the
less expensive laser that uses Fabre-Perot laser diode (FP-LD) technology. The FP-LD is an inexpensive
choice for digital fiber-optic communication. With a spectral width of typically 4 nm or more, it is primarily
used for lower bandwidth or short-distance applications. The second is the distributed feedback
laser diode (DFB-LD) technology. These light sources are more expensive and are widely used for longdistance fiber-optic communications. The typical spectral width for a DFB laser is about 1 nm. When a DBF laser is used in combination with a low dispersion fiber, the transmission bandwidth and distance can be significantly higher.

Typical Fiber Optic LossBandwidth is nothing but the spectral width expressed in terms of nanometers of the signal or device under test measured at a specific power level below the minimum loss. Bandwidth is a critical parameter for all wavelength selective components. Optical power level must be indicated as part of the bandwidth measurement as expressing the bandwidth without the power relative to the central wavelength is useless. Bandwidth will give the device’s bandpass, which is also useful to describe the shape of the band edges. One of the ways to express bandwidth is by using the unit dBc.

Bandwidth = 0.80 nm at -3 dBc

The bandwidth is 0.80 nm at a 3 dB power level than the center of the filter’s bandpass. Knowing the bandwidth at two or more levels will be good to get the shape of the filter’s edge. The bandwidth at -30dB gives an indication of the crosstalk in an adjacent channel.

Today’s telecommunication, data, video and image transportation applications require a bandwidth as indicated below. Please note that these figures are an approximation and can be used only for academic purpose.

Uncompressed High definition Television – 1200 Mbps
60 Hz video with 1280 x 960 pixel image quality – 600 Mbps
3 Hz video with 640 x 480 pixel quality – 75 Mbps
Compressed High definition video – 20 Mbps
15 Hz video with 320 x 240 pixel quality – 9 Mbps
Digital video standard and NTSC video – 6 Mbps
Compressed VHS video – 3 Mbps
Internet games and appliances – 2 Mbps
Video conferencing sessions and CD-quality audio – 1.2 Mbps
Web browsing through broadband – 300 kbps (256 Kbps is common in many countries)
High quality Audio sessions – 125 Kbps
Voice communication – 64 Kbps

From above we can conclude, which medium can well support us now and in the future. It is nothing but our Optical fibers!

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