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Simplifying what and why of Raman Amplifier.

August 23, 2017

 

It's always a wondering situation when we discuss Raman Amplifier;its benefits , requirement and application.I have tried to make it simpler to understand here.

Hope it will help the readers.

 

Introduction

  • The Raman amplifier is typically much more costly and has less gain than an EDFA amplifier. It, therefore, it is used only for specialty applications.
  • The main advantage this amplifier has over the EDFA is that it generates very little noise and hence does not degrade span OSNR as much as the EDFA.
  • Its typical application is in EDFA spans where additional gain is required but the OSNR limit has been reached.
  • Adding a Raman amplifier may not significantly affect OSNR, but can provide up to a 20 dB signal gain.
  • Another key attribute is the potential to amplify any fiber band, not just C band as is the case for the EDFA. This allows for Raman amplifiers to boost signals in O, E, and S bands (for CWDM amplification application).
  • The amplifier works on the principle of stimulated Raman scattering  (SRS), which is a nonlinear effect.
  • It consists of a high-power pump laser and fiber coupler (optical circulator).
  • The amplification medium is the span fiber in a distributed type Raman amplifier (DRA).

Common type of Raman amplifier 

  • The lumped or discrete type Raman amplifier internally contains a sufficiently long spool of fiber where the signal amplification occurs.
  • The DRA pump laser is connected to the fiber span in either a counter pump (reverse pump) or a co-pump (forward pump) or configuration.
  • The counter pump configuration is typically preferred since it does not result in excessively high signal powers at the beginning of the fiber span, which can result in nonlinear distortions,

  •  The advantage of the co-pump configurations is that it produces less noise.

Principle

As the pump laser photons propagate in the fiber, they collide and are absorbed by fiber molecules or atoms. This excites the molecules or atoms to higher energy levels. The higher energy levels are not stable states so they quickly decay to lower intermediate energy levels releasing energy as photons in any direction at lower frequencies. This is known as spontaneous Raman scattering or Stokes scattering and contributes to noise in the fiber. 

Since the molecules decay to an intermediate energy vibration level, the change in energy is less than the initial received energy during molecule excitation. This change in energy from excited level to intermediate level determines the photon frequency since Δ f = Δ E / h . This is referred to as the Stokes frequency shift and determines the Raman gain versus frequency curve shape and location. The remaining energy from the intermediate level to ground level is dissipated as molecular vibrations (phonons) in the fiber. Since there exists a wide range of higher energy levels, the gain curve has a broad spectral width of approximately 30 THz. 

During stimulated Raman scattering, signal photons co-propagate frequency gain curve spectrum, and acquire energy from the Stokes wave, resulting in signal amplification.

Theory of Raman Gain

The Raman gain curve’s FWHM width is about 6 THz (48 nm) with a peak at about 13.2 THz below the pump frequency. This is the useful signal amplification spectrum. Therefore, to amplify a signal in the 1550 nm range the pump laser frequency is required to be 13.2 THz below the signal frequency at about 1452 nm.

Multiple pump lasers with side-by-side gain curves are used to widen the total Raman gain curve. 

where fp = pump frequency, THz  fs = signal frequency, THz Δ f v = Raman Stokes frequency shift, THz 

Raman gain is the net signal gain distributed over the fiber’s effective length.It is a function of pump laser power, fiber effective length, and fiber area.

 

 

For fibers with a small effective area, such as in dispersion compensation fiber, Raman gain is higher. Gain is also dependent on the signal separation from the laser pump wavelength,Raman signal gain is also specified and field measured as on/off gain. This is defined as the ratio of the output signal power with the pump laser on and off.In most cases the Raman ASE noise has little effect on the measured signal value with the pump laser on. However, if there is considerable noise, which can be experienced when the measurement spectral width is large, then the noise power measured with the signal off  is subtracted from the pump on signal power to obtain an accurate on/off gain value.The Raman on/off gain is often referred to as the Raman gain.

Noise sources

Noise created in a DRA span consists :-

  • Amplified spontaneous emissions (ASE)
  • Double Rayleigh scattering (DRS)
  • Pump laser noise.

ASE noise is due to photon generation by spontaneous Raman scattering.

DRS noise occurs when twice reflected signal power due to Rayleigh scattering is amplified and interferes with the original signal as crosstalk noise.

The strongest reflections occur from connectors and bad splices.

Typically DRS noise is less than ASE noise, but for multiple Raman spans it can add up. To reduce this interference, ultra polish connectors (UPC) or angle polish (APC) connectors can be used. Optical isolators can be installed after the laser diodes to reduce reflections into the laser. Also span OTDR traces can help locate high-reflective events for repair.

Counter pump DRA configuration results in better OSNR performance for signal gains of 15 dB and greater. Pump laser noise is less of a concern because it usually is quite low with RIN of better than 160 dB/Hz.

Nonlinear Kerr effects can also contribute to noise due to the high laser pump power. For fibers with low DRS noise, the Raman noise figure due to ASE is much better than the EDFA noise figure. Typically the Raman noise figure is –2 to 0 dB, which is about 6 dB better than the EDFA noise figure.

Raman amplifier noise factor is defined as the OSNR at the input of the amplifier to the OSNR at the output of the amplifier.

Noise figure is the dB version of noise factor

The DRA noise and signal gain is distributed over the span fiber’s effective length.

 

Counter pump distributed Raman amplifiers are often combined with EDFA pre-amps to extend span distances. This hybrid configuration can provide 6 dB improvement in the OSNR, which can significantly extend span lengths or increase span loss budget. Counter pump DRA can also help reduce nonlinear effects by allowing for channel launch power reduction.

 

  Functional Block Diagram for CoPropagating and Counter Propagating Raman Amplifier

                                                         

                       Co-Propagating                                                                                                                                                Counter-Propagating

 

Field Deployment architecture of EDFA and RAMAN Amplifiers

 

Interesting to know

 

 

Reference:-

  1. Planning Fiber Optic Networks by Bob Chomycz
  2. https://www.cisco.com/c/en/us/products/collateral/optical-networking/ons-15454-series-multiservice-provisioning-platforms/data_sheet_c78-658538.html

 

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