Fiber Amplifiers

A suitably doped section of fiber can act as an optical amplifier when pumped by a specific wavelength. An erbium-doped fiber amplifier pumped by a 1480-nm wavelength will amplify a 1550-nm signal. The erbium transfers energy from the pumping light to the optical signal. Having the fiber as the amplifier rather than resorting to the expensive step of converting the optical signal to an electronic one for amplification and then reconverting it, greatly simplifies system designs, especially at high speeds where the design of electronic circuits is more difficult. Amplification, which can be in the range of 30 dB, occurs without regard to signal speed. With fiber amplifiers, transmission distances can reach several hundred kilometers or more.

Dense Wavelength-Division Multiplexing

Dense wavelength-division multiplexing (DWDM) allows the capabilities of fiber's information carrying capacity to be increased by carrying multiple channels. By separating channels by 0.8 or 1.6 nm, multiple channels can be carried by a fiber (most often in the 1550-nm optical window).

DWDM significantly increases the capacity of a fiber-optic system, essentially multiplying the capacity of a single channel by the number of channels. A telecommunications system can carry 10 Gb/s in a single optical wavelength. If there are 16 channels, the overall capacity is 160 Gb/s. Some equipment vendors have announced systems with capacities of 1.6 Tb/s (that's trillion bits!).

A DWDM device typically uses some form of optical filter to either separate or combine wavelengths. A one end, the DWDM device multiplexes or combines the different wavelengths onto a single fiber. At the other end, it demultiplexes or separates the individual wavelengths. A filter can do one of two things:

1. Pass one wavelength and reflect all others.
2. Reflect a single wavelength and pass all others.

There are several ways to construct suitable filters for DWDM applications. One popular method is to use a grating, such as a fiber Bragg grating [link to 1700]

Many DWDM designs are bidirectional. The example shown shows a demultiplexing application. If you simply reverse the arrows, you can see how the device serves to multiplex many wavelengths onto a single fiber. The bandpass filter passes the wavelength of interest in either direction. Other filters reflect the light so that the light is continuously combined at each stage of the device.

A variation of a DWDM is an add/drop multiplexer. This allows individual wavelengths to be added or extracted from a fiber. Suppose, for example, you have two cities 100 km apart and are sending 6 optical channels between them over a single fiber. What if you want to drop on of the channels or add another channel a some midpoint between the two cities? An add/drop multiplexer allows you to do this without disturbing the other channels.

All-Optical Systems

Systems today must at times convert the optical signal to an electrical signal to process it in some way. For example, if you want to switch the signal from one fiber to another or switch some wavelengths to one fiber and other wavelengths to another fiber, you must convert the optical signal to electrical.

A goal that telecommunications companies are working on is an all-optical systems that allow all switching, multiplexing/demultiplexing, and other needs to be performed solely on an optical signal, without conversion.