Amplifiers and Regenerators

Three types of devices may be used to overcome attenuation:

• Electronic regenerator    This device regenerates signals by first converting optical signals to electrical signals. The electrical signal is regenerated, converted back to optical, and injected back into the fiber. Regenerators are too inefficient for modern high-speed optical networks due to electrical regeneration requirements. On WDM systems, each wavelength requires its own opto-electric amplifier, an expensive proposition if there are many wavelengths.
  
• EDFA (erbium-doped fiber amplifier)    This device is an amplifier rather than a signal regenerator. It directly amplifies optical signals without a need to do the optical-to- electrical conversion. An EDFA contains a short strand of erbium-doped fiber and two signal inputs. One input is the optical signal that needs to be amplified. The other is light from a pump laser that excites the erbium atoms so that they give up photons that amplify incoming optical signal photons as described later. Thank Albert Einstein for this "trick" of light magic.
  
• EDFA/Raman amplifier combination    Raman amplification is an add-on component that enhances EDFA optical amplifiers. At each EDFA amplifier, a Raman pump injects high-power laser light into the fiber in the opposite direction of the source signal. The injected photons boost the optical signal where it is needed most-at the far end of the laser signal where it is experiencing the most attenuation. Raman amplification can create signal gain of up to 10 dB, which allows longer distance cable runs. It also allows optical networks to obtain transmission rates as high as 40 Gbits/sec.

EDFAs are crucial to WDMA systems because a single amplifier boosts all the wavelengths simultaneously. With older electrical regenerators, one regenerator is required for each wavelength, meaning that an entire stack of regenerators is needed at each regeneration point.

The 1,550 range (C band) is often called "erbium window" because the energy level of erbium ions is close to the energy level of photons in the C band. Through the process of stimulated emission, erbium can be coaxed into releasing energy that amplifies light in the C-band. The process is as follows. The EDFA amplifier pumps photons into the erbium-doped cable. Erbium atoms absorb photons, which cause electrons to jump to a temporary excited state. When an electron decays, it releases a photon that is absorbed by the signal photons. Thus, the optical signal passing through the cable is amplified without any electrical conversion.

As light travels down a fiber, it loses power, and the sharp transitions (representing binary data - or 1's and 0's) of the digital signal become smoothed out and loses power. This is rectified by placing amplifiers and regenerators into series with the fiber cable.  Fiber can carry light pulses much farther than copper, and therefore the amp/regens can be spaced farther apart.  A typical Singlemode system is shown below, with ILA's (In-Line Amplifiers) placed every 100 km:

ILA's every 100 km - used with Intercity (long haul) SingleMode Fiber

An optic amplifier (or repeater), merely increases the power of the signal (i.e. makes the light brighter).  A regeneration station (called "regen") will reshape the digital signal into sharp, well-defined 1's and 0's.  In general, with metro fiber routes, there are about 4 or 5 amps for every regen, as shown below:

How Far can the Light Go before Requiring an Amp or Regen ??

Today the best of the best fiber allows transmission up to 400 km.  But the distance capability of an optical system varies greatly !!  It is dependant on the data rate, the type of transmitter & receiver, and the type of fiber.  It also depends on whether the system uses WDM (Wave Division Multiplexing), where, instead of the traditional, single white light, a number of different "colors" of light are transmitted, and therefore this allows multiple data channels.  Today, because of the huge expense of running fiber - most optical fiber networks use WDM, so that the provider can squeeze as much bandwidth out of each cable, as possible.

Distance Limitations and Controlling Cost through Concatenation

Very high data rates cannot travel far without regeneration.  For example, the new 10 Gigabit Ethernet (10 Gbps is approx OC-192, and 1 Gbps is approx OC-24) can only travel a few hundred meters max.  The newest fiber is called "laser-optimized fiber", and it can support 550 meters for 10Gbps, or 1100 meters for 1Gbps.  Of course, it requires excellent transmitter/receiver equipment as well to achieve those distances.  Think about that - only 550 meters !!  That means if a provider wants to build a nationwide, 10 Gbps backbone, they need to install thousands of expensive repeaters and regens !!  This gives you an idea, why fiber networks are so expensive.

Well, actually there is a way to reduce the cost dramatically.  Instead of installing pure OC-48 (2.5 Gbps) or OC-192 (10 Gbps) systems, the provider can install OC-48c or OC-192c systems.  The "c" stands for "concatenation", and this means that several fibers or several wavelengths on one fiber are used at lower speeds, and then concatenated at the endpoints.  For example 

Metro fiber systems (within a city) typically use CWDM (Course Wave-Division Multiplexing), and can only travel 5 km or so before requiring amplification and/or regeneration of the signal.  Long-haul, intercity fiber uses DWDM (Dense Wave Division Multiplexing), which is much more expensive because it is tightly controlled and high-quality and carries more channels (more colors of light) - and it can carry a signal quite a distance -- perhaps 42 to 60 miles (70 to 100 km). Undersea fiber systems use very expensive transmitters and cabling, and can travel up to 400 miles before requiring amplification/regeneration.  On a long distance line, there is an equipment hut every 40 to 60 miles. The hut contains equipment that picks up and retransmits the signal down the next segment at full strength.

"Launching" - the Light Source at the Transmitter and Receiving it

With fiber-optics, there are two types of light sources that launch light (shine light) into one end of the fiber: 

LED (Light Emmitting Diode) - these are going away, because their light power is rather weak and not concentrated into a small enough beam

Laser - this is now the light source of choice for fiber-optic systems.

For the receiving end, Photo-detectors are used.