The log ratio function is the optical power gain of the EDFA. This circuitry forms an automatic power level control

loop.

An alternate design of the system shown in Figure 7-14 is possible because the LOG114 inherently takes the log

ratio. Therefore, one log amp can be eliminated by connecting one of the photodiodes to the LOG114 I1 input,

and the other to the I2 input. The differential amplifier can then be eliminated.

The fast rise and fall times of the LOG114 are designed for most EDFA applications (typically less than 1μs for a

100:1 current input step). The device also measures a very wide dynamic range of up to eight decades.

1%

Tap

OPA569

PID

Diff

VCCS

IL

Power

Op Amp

Pump Laser

LOG114

Fiber

VERROR

VOUT2

LOG114

VOUT1

1%

Tap

Figure 7-14. Erbium-Doped Fiber Optic Amplifier (EDFA) Block Diagram

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LOG114

SBOS301B – MAY 2004 – REVISED MARCH 2025

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