Observation of twin-beam-type quantum correlation in optical fiber

We report generation of pulsed twin beams of light through optical parametr ic amplification in a fiber Sagnac loop. By pumping the Sagnac loop with pi cosecond pulses at a wavelength near the zero-dispersion wavelength of the fiber, we achieve phase-matched nondegenerate four-wave mixing with gain. F or a gain of 2.2, the intensity noises of the amplified signal and the gene rated idler (conjugate) pulses are found to be correlated by 5.0 dB, and th e subtracted noise drops below the shot-noise limit by 1.1 dB (2.6 dB when corrected for losses). We have investigated the gain dependence of the quan tum-noise reduction as well as of the intensity noises of the amplified sig nal and idler pulses. As the gain increases, we observe the onset of excess noise on the idler pulses. (C) 2001 Optical Society of America.