Optical Losses

Optical loss or attenuation can vary from 300 to 0.2 dBm/km for plastic or single-mode fibers, respectively. Optical fiber has different loss characteristics at different wavelengths. The optical windows, as mentioned earlier, are regions within the optical fiber spectrum with low loss.

The earliest fiber-optic systems operated in the first optical window in the 850 nm range. The second window is the 1310 nm range, which has zero dispersion. The third window is the 1550 nm window. A multimode fiber has an attenuation of about 4 dB/km at 850nm and about 2.5 dB/km at 1310 nm. The multimode fiber spectrum attenuation curve is shown in Figure 6.10-3. Note the high loss regions at 700, 1250, and 1380 nm. The single-mode fiber attenuation curve is shown in Figure 6.10-11. There are high-loss regions at 800, 1100, and 1490 nm regions. The high-loss region at about 1100 nm is called the mode transition region. This is where the fiber changes from multimode to singlemode characteristics.

In order to make use of the low-loss properties of a given region in the fiber, the optic light source must generate light at that wavelength. For multimode fiber, light sources are used in the 850 and 1310 nm wavelengths. In single-mode fiber, light sources are typically at 1310 and 1550 nm. CWDM lasers are in the 1470–1610 nm range. The curve in Figure 6.10-11 shows that the fiber has low loss and a flat spectrum at these wavelengths. Corning introduced a CWDM metro fiber that eliminated the high water peak or the high-loss region centered at about 1380 nm. Most single-mode fibers, on new installation, use this flatspectrum fiber with a usable spectrum from about 1270–1610 nm. The new fiber gives the ability to have up to 18 CWDM wavelengths on one single-mode fiber.

Most video fiber-optic systems take advantage of the 18 usable wavelengths. CWDM is far less expensive than its 42 wavelength counterpart, DWDM. With the fiber-optic systems available with up to 8 channels of video per wavelength, when combined with the capabilities of CWDM optical multiplexing, more than 144 channels of video can be transported over one fiber.

Plastic fiber is used over short distances due to high attenuation. The visible light region at around 650 nm is used over plastic fiber. Optical attenuation is constant at all bit rates and modulation frequencies. The attenuation in copper cable increases at higher bit rates and modulation frequencies. In a copper cable, a 100 MHz signal will be attenuated more per foot than a 50 MHz signal. This results in distances and bandwidth limitation. In a fiber cable, the 100 Mhz and 50 MHz signals are attenuated the same.

Figure 6-10-11 Singlemode Fiber Attenuation Curve

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End-To-End System Design

A common misconception is that it is very difficult to design a fiber optic system.  There are very simple calculations to be made using information from the fiber optic product data sheet.  When designing a fiber optic system we first need to know the number and type of signals to be sent through the fiber.  We also need to know the transmission distance or required optical budget.

Transmitter Launch Power

The data sheet of any fiber optic transport system will provide the transmitter’s  output optical power.  There may be different models with varying levels of output power,  since a more powerful transmitter can be chosen to reach a greater  distance.  A typical fiber optic transmitter has an output optical power of  – 8 dBm or 0.158 mWatts

Receiver Sensitivity

The receiver sensitivity is another parameter found on any fiber optic equipment datasheet.  The receiver sensitivity is the minimum optical signal or power required for the receiver unit to operate properly.  Many systems have a minimum receiver sensitivity of -28 dBm or 0.00158 mWatts.  The -28 dBm value represents an optical power that is 28 decibels below the 0 decibel or 1 mWatt reference point.

Optical Power Budget

The optical budget of a fiber-optic transport system takes into account the optical power of the transmitter, loss in the fiber for a given distance, receiver sensitivity, and signal-to-noise required. Optical power, like

electrical power, is measured in watts or milliwatts. Fiber-optic systems are typically designed using decibels referenced to 1 milliwatt or 0 dBm. The following formula shows the conversion from watts to decibels:

dBm = 10 × log(laser power in mW).The output power of an optical laser may be 1 milliwatt.

The equivalent power in dBm would be 10 * log (1mW) = 0 dBm. For 0.5 mW laser, output power would be 10 * log(0.5 mW) = –3 dBm. The optical attenuation of a multimode fiber at the 850 nm wavelength is about 3 dB/km. The attenuation on single-mode fiber at 1310 and 1550 nm is 0.5 and
0.2 dB/km, respectively. Using these numbers we can calculate how much optical power is required to reach
a certain transmission distance. For example, a 10 km run over single-mode fiber at 1310 nm would incur a
loss of 5 dB (10 km × 0.5 dB/km).
The optical budget that a fiber-optic system provides is the difference between the fiber-optic transmitter
optical output power and the receiver sensitivity. For example, if the transmitter power is –8
dBm and the minimum receiver level is –28 dBm, then the maximum loss the system can withstand is 20
dBm.
In many cases it may seem that a multimode or singlemode fiber run has optical power to reach 40–60
km. When transmissions exceed about 5 km in multimode systems and about 15 km in single-mode systems, other factors due to dispersion come into play and limit the transmission distance.

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