Sub-shot-noise measurement with fiber-squeezed optical pulses.

A squeezed vacuum can enhance the sensitivity of an interferometric measurement limited by shot noise. By injecting a squeezed vacuum into the unexcited port of a phase-measuring device, and by orienting the squeezed vacuum so that its quiet quadrature is along the signal direction, the signal-to-noise ratio is improved. This was shown theoretically' 2 and demonstrated experimentally for measurements of phase modulation with a Mach-Zehnder interferometer,3 polarization rotation,4 and index modulation.5 The generation of squeezed light was first demonstrated in 1985 by Slusher et al.,6 and they were closely followed by other researchers. 7 8 Squeezing in optical fibers with the Kerr effect [X(3)] was pioneered by Shelby et al.9 Squeezing with pulses within an interferometric geometry, first proposed by Shirasaki and Haus,10"' naturally separates the pump from the generated squeezed vacuum. This scheme leads to two important consequences: the pump power may be reused in full, and the classical noise of the pump may be subtracted successfully even at low frequencies. 2 When the squeezed vacuum is injected into the measurement device that uses all the original pump power, the signal-to-noise ratio is improved absolutely. Although squeezing has been demonstrated with a fiber ring interferometer,12 "3 it has been limited by the classical noise termed guided-acoustic-wave Brillouin scattering"4 -'6 (GAWBS). In this process the propagating light in the fiber is scattered by thermally induced vibrations of the fiber cylinder, thereby acquiring phase-noise sidebands. The vibrational modes range in frequency from approximately 20 MHz to 1 GHz. Even if the measurement window is chosen at lower frequencies, the noise may be convolved into the window through coherent mixing in the balanced homodyne detection normally used to observe the squeezed vacuum noise along its quadrature directions.' -' 8 The measurements reported in Ref. 12 were performed with a fiber that exhibited particularly low GAWBS at the measurement window. Since the level of GAWVBS can be much higher with other fibers and at different measurement windows, it is essential that means be developed to eliminate its effect on the squeezing. The GAWBS noise may be overcome by employing a method proposed by Shirasaki and Haus,19 the experimental results of which are described in this Letter. The fiber ring is excited by two consecutive pulses separated by a time interval shorter than the inverse bandwidth of the GAVVBS. Before detection, one of the local-oscillator pulses is vr phase shifted with respect to the second pulse. Since the two pulses experience the same phase noise, each pulse contributes to the detection noise current but with an opposite sign. The GAVVBS phase-noise contribution to each of the two detectors is nulled. Following the balanced subtraction the laser classical noise is canceled, and only the quantum noise remains. In the experimental configuration shown in Fig. 1 the laser source is a 1.32-,ttm mode-locked Nd:YAG laser emitting 100-ps pulses at a 100-MHz repetition rate. Each input pulse is separated into two pulses delayed by 500 ps with respect to each other, which are coupled into the fiber ring interferometer composed of 50 m of polarization-maintaining fiber and a 3-dB coupler. The coupling loss to the fiber is approximately 10%. A portion of the reflected dualpulse pump is picked off to be used as the dual-pulse local oscillator by an 85/15 fiber coupler (but all the pump power could be reused, in principle, with a nonreciprocal optical circulator). A ir phase shift is then imposed on one of the local-oscillator pulses with a polarization-maintaining fiber phase modula-