Advertisment

Return of Raman

author-image
VoicenData Bureau
New Update

Highlights

  • EDFAs are more reliable and generally less expensive than electronic repeaters.
  • Next-generation EDFAs are focussing on L-band of 1570-1610 nm.
  • With Raman amplifiers, the transmission fibre itself becomes the amplifier medium. 
Advertisment

With the emergence of low-loss glass fibre as a broad bandmedium for transporting voice, video, and data traffic, fibre-optic technologyhas proven itself as a crucial milestone in the global telecommunications and ITrevolution. As technological advances in photonics and electronics permit moreand more wavelengths of light to be transmitted at ever-increasing data rate,the data carrying capacity of each strand of optical fibre is doubling everyyear. Transmission fibre has today evolved from a simple light pipe to a finelytuned medium that not only guides light but manipulates the optical pulses asthey propagate.

A typical loss spectrum for an optical fibre is shown infigure which indicates the minimum light attenuation of about 0.2 dB/km at 1550nm wavelength. This corresponds to loss of about 5 percent of the light afterpropagation through one kilometer of fibre, implying that only one in a trillionphotons leaving US would reach Japan. The optical losses due to absorption andscattering required electronic "repeaters"–placed at every tens ofkilometers–to amplify the signal along the fibre to compensate such losses.Each repeater detected signal, filtered it and retransmitted it using a newlaser diode.

Erbium Doped Fibre Amplifiers

Advertisment

A major revolution in the industry has been brought about byErbium Doped Fibre-optic Amplifiers (EDFA), invented in 1987. These amplifierspermitted the direct amplification of optical signals–without conversion tothe electronic domain–allowing propagation within a fibre for hundreds ofkilometers using periodic optical amplification (every 40 to 80 km) tocompensate for the fibre loss. These amplifiers are more reliable and generallyless expensive than electronic repeaters, with their performance independent ofbit-rates. They support more than one wavelength enabling WDM (WavelengthDivision Multiplexing) systems and networks, increasing their capacity manytimes.

These optical amplifiers are based on an optical fibre dopedwith a fraction of a percent of the rare-earth element "erbium". Theseerbium ions absorb light at 980 or 1480 nm (nano meter) and store this energy toamplify signals at wavelengths near 1550 nm. In a wonderful coincidence ofnature, this amplification band coincides with the minimum-loss wavelength ofoptical fibres. Moreover, EDFAs are easily pumped, have low noise, and unlikesemiconductor amplifiers, do not generate cross talk between channels.

Conventional band (C-band) EDFAs amplified 1530-1562 nm band,which is also quickly being consumed. The next-generation EDFAs are focussing onLong-wavelength band (L-band) of 1570-1610 nm.

Advertisment

Raman Amplifiers

While erbium amplifiers have been tremendously successful,they are only capable of amplifying 80 nm of the low-loss window. The spectralbandwidth of conventional EDFAs is beginning to be a capacity limitation. Tofully exploit the capacity of optical fibres, new amplifiers that can amplifysignals from 1270 to 1670 nm of wavelengths are needed. The only opticalamplifiers that work over this wavelength range are "Ramanamplifiers", which are now reviving after years of hibernation,particularly after the developments in the area of high power diode pump lasersand the optical grating technology. "Raman Scattering" being anon-resonant process, can provide amplification for both 1300 nm and 1500 nmwindows.

The advantage with these amplifiers is that any optical fibrecan serve as the amplifying medium. By simply injecting intense pump light intoan existing transmission fibre, signals propagating within the fibre can beamplified. Since Raman gain occurs in all optical fibres, even installedtransmission fibres can be made to amplify communication signals by injectingpump light into the fibre span. In essence, the transmission fibre itselfbecomes the amplifier medium. Launching pump light into the end of thetransmission span and allowing it to counter-propagate relative to the signals,triggers distributed Raman amplification, which can greatly enhance theperformance of communication systems.

Advertisment

Gain Across the Entire Window

In Raman amplification, an intense pump light couples tovibrational modes of the glass of fibre itself and is re-radiated at a longerwavelength, amplifying a signal if the pump is at an appropriately shorterwavelength than the signal. The gain spectrum is determined simply by the pumpwavelength rather than the fixed energy levels of a dopant like erbium as inEDFAs. Since the Raman gain spectrum is not locked to fixed energy levels likeerbium, a desired Raman gain can be generated at any wavelength in the infrared,as long as the requisite pump light is available. This feature allows Ramanamplification to be applied across the entire transmission window of silicaoptical fibres.

Raman amplifiers have been demonstrated at 1300, 1400 and1500 nm with gains as large as 40 dB and noise as low as 4.2 dB, and are theonly silica fibre amplifiers to demonstrate analog grade noise performance at1300 nm, which is of great interest for CATV signals. These amplifiers have beentested as high-gain, low-noise optical preamplifiers for Dense WavelengthDivision Multiplexing (DWDM) digital transmission systems and have demonstrated80 Gbps transmission for over 140 km of fibre as well as DWDM systems at 1400nm. These have also been used in combination with EDFAs to show transmission of40 wavelengths, each carrying 40 Gbps over 400 km of

fibre.

Advertisment

Ultimately, with the present ongoing developments, it wouldbe possible to generate a laser–with any light in the infrared between1100-1700 nm–and use it to pump a Raman amplifier over the entire transmissionwindow of optical fibres.

And that might be just the beginning of full realization of the capabilitiesof Raman’s gift to telecommunications.

Niraj K Gupta

www.telecombyNirajGupta.com

Advertisment