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End-to-End System Design – Part 10

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Published on Oct 20, 2014 | News

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

Transmitter Launch Power

The datasheet of any fiber-optic transport system will provide the transmitter unit’s output optical power. There may be different models with varying levels of output power. A more powerful transmitter can be chosen to reach a further transmission distance. A typ- ical fiber-optic transmitter has an output optical power of –8 dBm or 0.158 mW.

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 mW. The –28 dBm value represents an optical power that is 28 decibels below the 0 dBm or 1 mW 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 single-mode 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.

Bandwidth

The optical losses and usable bandwidth of a fiber-optic system have to be taken into account. As men- tioned previously, multimode fibers have greater losses and less bandwidth compared to single mode. Single mode has lower losses and very high band- width 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), where:

  • Disp is the dispersion of the fiber at the operating wavelength with units seconds per nanometer per kilometer
  • SW is the spectral width (rms) of the light source in nanometers
  • 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 long distance 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. See Table 6.10-2, which shows the typical fiber-optic cable losses, and Table 6.10-4, which shows the band-width for different types of fiber cable.

Optical Losses

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

The earliest fiber-optic systems operated in the first optical window in the 850 nm range. The second win- dow is the 1310 nm range, which has zero dispersion. The third window is the 1550 nm window. A multi- mode fiber has an attenuation of about 4 dB/km at 850 nm 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 single-mode characteristics.

TABLE 6.10-4: Typical Fiber-Optic Bandwidth

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 flat-spectrum 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.

FIGURE 6.10-11 Single-mode fiber attenuation curve. (Courtesy of Corning Glass Works.)

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.

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