In-line optical amplifier

An in-line optical amplifier for amplifying an input optical signal to produce an output optical signal, comprising:

• an input power detector for detecting the input power from the input optical signal;
• a rare-earth element doped fiber for optically amplifying the input optical signal after the input power is detected with a gain varying in accordance with an intensity of pumping light;
• a light source for supplying the pumping light to the rare-earth element doped fiber;
• an output power detector for detecting the output power from the output optical signal;
• a gain detector for detecting an overall gain of the in-line optical amplifier based on the input power and the output power;
• a gain regulator for adjusting the intensity of the pumping light such that the overall gain is kept at a predetermined gain;
• and
• a noise level detector for detecting an internal noise level based on a physical quantity associated with a deviation of the overall gain from the predetermined gain.

An in-line optical amplifier comprising:

• an input branching device for branching an input optical signal into a first input signal and a second input signal;
• an input detector for detecting an input intensity level from the first input signal;
• a rare-earth element doped fiber for optically amplifying the second input signal with a gain varying in accordance with an intensity of pumping light and producing a third signal;
• an optical coupler for coupling the third signal with the pumping light;
• a pumping source for supplying the pumping light to the rare-earth element doped fiber through the optical coupler;
• an output detector for detecting an output intensity level from an output optical signal passing through the optical coupler;
• a gain detector for detecting an overall gain of the in-line optical amplifier based on the input intensity level and the output intensity level;
• a gain regulator for adjusting the intensity of the pumping light such that the overall gain is kept at a predetermined gain;
• and
• a noise level detector for detecting an internal noise level based on a physical quantity associated with a deviation of the overall gain from the predetermined gain.

A method for detecting an internal noise in an in-line optical amplifier which optically amplifies an input optical signal to produce an output optical signal, the method comprising the steps of:

• detecting an input power from the input optical signal;
• amplifying the input optical signal after the input power is detected with a gain varying in accordance with an intensity of pumping light;
• detecting an output power from the output optical signal;
• detecting a resultant gain of the circuit based on the input power and the output power;
• adjusting the intensity of the pumping light such that the resultant gain of the in-line optical amplifier is kept at a predetermined gain;
• and
• detecting a physical quantity associated with a deviation of the resultant gain of the in-line optical amplifier from the predetermined gain.