Datasheet

"#$$%
SBOS301A − MAY 2004 − REVISED MARCH 2007
www.ti.com
20
ERBIUM-DOPED FIBER OPTIC AMPLIFIER
(EDFA)
The LOG114 was designed for optical networking sys-
tems. Figure 13 shows a block diagram of the LOG114
in a typical EDFA application. This application uses two
log amps to measure the optical input and output power
of the amplifier. A difference amplifier subtracts the log
output signals of both log amps and applies an error
voltage to the proportional-integral-derivative (PID)
controller. The controller output adjusts a voltage-con-
trolled current source (V
CCS
), which then drives the pow-
er op amp and pump laser. The desired optical gain is
achieved when the error voltage at the PID is zero.
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 13
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 I
1
input, and the other to the I
2
input. The differential
amplifier would then be eliminated.
The LOG114 is uniquely suited for most EDFA
applications because of its fast rise and fall times
(typically less than 1µs for a 100:1 current input step).
It also measures a very wide dynamic range of up to
eight decades.
V
CCS
Pump Laser
Power
Op Amp
Fiber
V
ERROR
V
OUT2
R
REF2
R
REF1
I
REF1
I
REF2
I
2
I
1
V
OUT1
PID
Diff
REF
DAC
I
L
Tap
OPA569
LOG114LOG114
1%
Tap
1%
Figure 13. Erbium-Doped Fiber Optic Amplifier (EDFA) block diagram.