Amplitude-squeezed solitons from an asymmetric fiber interferometer

Abstract

We experimentally demonstrate a new scheme for generating amplitude-squeezed solitons in an asymmetric fiber Sagnac loop. We measure by direct detection what is to our knowledge a record reduction of 5.7 6 0.1 dB (73%) and, with corrections for linear losses, 6.2 6 0.1 dB (76%) in the photon-number f luctuations below the shot-noise level. The same scheme is also shown to generate significant classical noise reduction and is limited by Raman effects in fiber. © 1998 Optical Society of America OCIS codes: 270.6570, 270.5530, 270.1670. Generation of amplitude-squeezed states by use of Kerr nonlinearity in optical fibers was recently demonstrated in a novel scheme pioneered by Friberg et al., who used soliton propagation followed by spectral f iltering. 1By launching a solitonlike pulse with energy slightly greater than the fundamental soliton energy ͑N . 1͒ into a fiber of length equivalent to several soliton periods followed by a spectral f ilter, it is possible to observe a reduction in photon-number f luctuations by direct detection. In subsequent experiments a reduction of as much as 3.8 dB was directly detected with femtosecond pulses. 2,3The f iltering action also introduces additional zero-point f luctuations into the system, limiting the highest achievable squeezing to approximately 8 dB below the shot-noise level for ideal fibers and bandpass filters. 4 Recently it was proposed that amplitude-squeezed pulses can be produced by interference between counterpropagating fields in an asymmetric fiber Sagnac loop. 5For the case of soliton squeezing the idea that interfering a high-energy ͑N . 1͒ soliton pulse with a weaker pulse or a dispersive wave can produce squeezing is also consistent with soliton perturbation treatment 6 as well as the general quantum theory of soliton propagation. 7,8 The geometry proposed in Ref. 5 is well suited for testing this idea in practice and was recently explored experimentally by Schmitt et al. 9Different geometries for achieving amplitude squeezing, such as polarization interferometry, were also demonstrated recently. 10In this Letter we experimentally demonstrate the asymmetric f iber Sagnac approach and report what is to our knowledge a record 5.7 6 0.1 dB (73%) directly detected photon-number squeezing below the shot-noise level. With correction for linear system losses, the actual amplitude squeezing is 6.2 6 0.1 dB (76%). We have also measured a significant reduction in the classical noise inherent in the optical signal.The experimental setup is shown in Fig. 1. A Spectra-Physics Opal optical parametric oscillator is used as a source of 182-fs (FWHM) sech-shaped optical pulses at a repetition rate of 82 MHz and centered at 1550 nm. The corresponding dispersion length ͑b 00 219 ps 2 ͞km͒ in the standard polarizationmaintaining (PM) fiber, Fujikura SM15-P-8 with an 8-mm core diameter, that was used in the experiments is ϳ54 cm, and the soliton period is 86 cm. The average power required for production of a fundamental ͑N 1͒ soliton, as determined by measurement of the ...

Photon-number squeezing in the normal-dispersion regime

“…4, to establish accurately the shot noise and the other relevant noise levels to within 0.1 dB. All the values were shown to be consistent with the ones reported in Ref.…”

“…All the values were shown to be consistent with the ones reported in Ref. 4. The photodiodes were independently calibrated to have their responsitivities matched and their saturation levels accurately established at 26 mW, which corresponds to approximately 70 mW of input optical power into the fiber Sagnac interferometer.…”

“…In an experimental demonstration of this scheme Krylov and Bergman recently reported a 5.7-dB (73%) reduction, or a 6.2-dB reduction (76%) after correcting for linear losses, in photon-number f luctuations below the shot-noise level, with 180-fs high-energy soliton pulses centered at 1550 nm in a highly asymmetric Sagnac interferometer geometry. 4 Soliton pulses have the distinctive property of acquiring a uniform nonlinear phase shift across the temporal envelope, which provides for the mosteff icient projection and measurement of squeezing. It is, however, not required by the quantum theory of nonlinear pulse propagation that one have a soliton pulse to observe amplitude-noise reduction, and thus the practical scope of the photon-numbersqueezing experiments in optical fibers can be significantly extended.…”

Soliton squeezing in a Mach-Zehnder fiber interferometer

“…To obtain a high degree of noise reduction, it is then crucial to control the relative amplitude and phase of the two pulses. In the Sagnac configuration [3,5] the pulses counter propagate in the same fiber, thus preventing the control of their relative phase.Moreover, the relative amplitude between the soliton-like pulse and the auxiliary dispersive pulse is unchangeable once one fixes the splitting ratio of the interferometer beamsplitter.To overcome these drawbacks we investigate the setup shown in Fig. 2.…”

“…To obtain a high degree of noise reduction, it is then crucial to control the relative amplitude and phase of the two pulses. In the Sagnac configuration [3,5] the pulses counter propagate in the same fiber, thus preventing the control of their relative phase.…”