Abstract

To try to demonstrate a quantum nondemolition (QND) measurement of photon number,¹ we built a nonlinear interferometer system with a 10-km polarization-maintaining fiber. (In theory, this fiber length is nearly optimum for 1.5-μm operation.) The optical Kerr effect allows a signal beam intensity to change a probe beam phase, which can then be read out by the interferometer. A modified ring interferometer structure and feedback stabilization enabled the 1,32-μm probe to read the intensity of the 1.06-μm signal beam. We measured a χ⁽³⁾ value in this fiber of3.6× 10^-33(1.4×10-³³)MKS when the polarization of the signal beam is coincident with (perpendicular to) that of the probe beam. The interferometer output spectrum displayed large noise peaks due to guided acoustic-wave Brillouin scattering (GAWBS)² in the fiber. GAWBS was also observed in 5-km conventional single-mode fibers in various interferometer configurations. The coexistence of two different kinds of GAWBS (polarized and depolarized) makes it particularly difficult to obtain QND measurements. We conclude that a QND measurement system free from GAWBS noise requires differential phase detection of two pulses having the same polarization state.

© 1988 Optical Society of America