Method and apparatus for monitoring noise figure of optical amplifier

An apparatus for monitoring a noise figure of a linear repeater including at least one optical amplifier for amplifying input signal light and a narrow bandwidth optical filter for removing amplified spontaneous emission (ASE) light generated at the optical amplifier, the apparatus comprising:

• gain detector means for detecting a gain of the optical amplifier;
• ASE optical power detector means for detecting an optical power of the ASE light generated at the optical amplifier, including:
• splitter means for splitting lights entering into the narrow bandwidth optical filter;
• local oscillator means for generating local light having a wavelength displaced from a wavelength of the input signal light;
• coupler means for coupling the lights split by the splitter means with the local light generated by the local oscillator means;
• optical power detection means for making a heterodyne detection of the optical power of the lights coupled by the coupler means;
• and
• electrical band-pass filter means for extracting the optical power of the ASE light generated at the optical amplifier from the optical power obtained by the optical power detection means;
• and
• noise figure calculation means for calculating the noise figure according to the gain detected by the gain detector means and the optical power of the ASE light detected by the ASE optical power detector means.

A method for monitoring a noise figure of a linear repeater including at least one optical amplifier for amplifying input signal light and at least one narrow bandwidth optical filter for removing amplified spontaneous emission (ASE) light generated at the optical amplifier, the method comprising the steps of:

• (a) detecting a gain of the optical amplifier;
• (b) detecting an optical power of the ASE light generated at the optical amplifier within a transmission bandwidth of the narrow bandwidth optical filter around a central transmission wavelength displaced from a wavelength of the input signal light;
• and
• (c) calculating the noise figure according to the gain detected at the step (a) and the optical power of the ASE light detected at the step (b), by approximating the optical power of the ASE light generated by the optical amplifier at an output side of the linear repeater within the transmission bandwidth of the narrow bandwidth optical filter around the wavelength of the input signal light by the optical power of the ASE light generated by the optical amplifier detected at the step (b) within the transmission bandwidth of the narrow bandwidth optical filter around the central transmission wavelength.

A method of monitoring a noise figure for an optical amplifier for amplifying input signal light, comprising the steps of:

• (a) measuring a gain G* in dB unit of the optical amplifier;
• (b) measuring an optical power P.sub.ASE * in dB unit of an amplified spontaneous emission (ASE) light generated by the optical amplifier at a wavelength .lambda.c2 displaced from a wavelength .lambda.s of the input signal light;
• and
• (c) calculating an approximate noise figure F*.sub.apr in dB unit by substituting the gain G* measured at the step (a) and the optical power P*.sub.ASE measured at the step (b) into an equation:
• F*.sub.apr .apprxeq.P.sub.ASE *(.lambda.c2)+K.sub.2 -K.sub.3 .multidot.G*
• where K.sub.2 and K.sub.3 are constant coefficients.

An apparatus for monitoring a noise figure for an optical amplifier for amplifying input signal light, comprising:

• gain detector means for measuring a gain G* in dB unit of the optical amplifier;
• ASE optical power detector means for measuring an optical power P.sub.ASE * in dB unit of an amplified spontaneous emission (ASE) light generated by the optical amplifier at a wavelength .lambda.c2 displaced from a wavelength .lambda.s of the input signal light;
• processing means for calculating an approximate noise figure F*.sub.apr in dB unit by substituting the gain G* measured by the gain detector means and the optical power P*.sub.ASE measured by the ASE optical power detector means into an equation:
• F*.sub.apr .apprxeq.P.sub.ASE *(.lambda.c2)+K.sub.2 -K.sub.3 .multidot.G*
• where K.sub.2 and K.sub.3 are constant coefficients determined in advance from known sets of the noise figures, the gain, and the optical power of the ASE light.