Nevion EDFA-B-C 17dBm - Manual (Page 13)

EDFA-B-C 17dBm Rev. B
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6
Operation
6.1 Introduction
The EDFA-B-C 17dBm will typically be used as a midway booster when there are many
signals on the fiber, or directly after the transmitter / DWDM multiplexer if there are few
signals. The reason for this is that optimum input power on the EDFA for DWDM
applications is -6dBm.
A full 40-channel system based on the flashlink DWDM-40C together with 0dBm flashlink
transmitters will already have approximately +14dBm
1 at the output, thereby requiring a
20dB attenuator to get +17dBm (just 3dB up) with a flat gain response out of the EDFA-B-C
17dBm.
On the other hand, a narrow band DWDM system (up to 8 channels) will not experience
problems with the gain/frequency response of the EDFA-B-C 17dBm, and can therefore
directly benefit from the +17dBm output power (compared to the app. +7dBm
1 from 8
channels in a DWDM-8C).
6.2 EDFA theory
Understanding the way the erbium doped fiber amplifier works can lead to easier handling
of problems that occur, such as wideband noise, signal dependent noise (bit errors under
certain signal conditions) etc.
The pump laser at 1480nm (980nm would be used for a low-noise preamp with low output
power) and a short piece of fiber with a small amount of SiO
2
(glass) molecules in the
structure replaced by Er
2
O
3
are the central elements of an EDFA. The photons at 1480nm
emitted from the laser excite electrons belonging to the erbium atoms. The excited state
(called 11/2, actually a broad range of sub states and thermic variations in energy) has a
limited life span, but if a photon at the appropriate wavelength (energy) comes close to this
electron, it will collapse down to the ground state (15/2) with a new photon emitted at the
excact same phase and direction as the original photon. If no photon passes by within the
life span of the excited state, the electron will collapse by itself, and a photon will be
generated at the wavelength matching the energy level, in a random direction. Some of
these photons will have a direction along the signal path of the fiber, and will therefore be
amplified by other excited electrons at the same energy level. This is called amplified
spontaneous emission (ASE) and is the primary source of noise introduced in an EDFA.
6.3 Operation modes
An EDFA can normally be operated in one of three modes: AGC, ACC or APC. The EDFA-
B-C 17dBm has dip switch and GYDA settings for two of these, AGC (referred to in this
document as “Constant Gain”) and APC (“Constant Power”).
In “Constant Power” mode, the output power is regulated independently of input power. In
“Constant Gain” mode, the output power varies with input power.
In addition, there are two different versions of “Constant Power”. For use with a relatively
low number of SDI links which might transport a pathological signal
2
, there is a special SDI
mode. When using this mode, the EDFA should not be used near saturation (full output
power). Recommended output power in “SDI optimised Constant Power” mode is +14dBm.
There are two reasons for this. First, the regulation loop is slower in this mode, therefore
adding or subtracting optical channels from the fiber can lead to bit errors in other channels
1 Example values, not for performance indications.
2 Specifically the EQ stressing 19-1 sequence that comes from displaying a uniform purple colour over several whole video
lines.