An optical amplifier amplifies a light signal inputted to an optical amplifier
fiber and outputs an amplified light signal, by exciting the fiber with an excitation
light. The optical amplifier can keep its output light level constant and stable
and reduce the excitation light when the light inputted to the fiber is cut off,
just by detecting the optical output level at a post-stage of the fiber. The optical
amplifier comprises a first control circuit, a second control circuit and a selector
circuit. The first control circuit amplifies the difference between a first predetermined
voltage and an o/e-converted voltage of the optical amplifier fiber output light
to output an amplified voltage. The second control circuit amplifies the difference
between the o/e-converted voltage of the excitation light and a selected one of
the first control circuit output voltage and a second predetermined voltage to
output an amplified voltage and controls the excitation light based on the second
control circuit output voltage. The selector circuit detects a change in the first
control circuit output voltage and depending on the change, selects the first control
circuit output voltage or a second predetermined voltage.
