We report what is believed to be the first demonstration of optical fiber gratings written in photonic crystal fibers. The fiber consists of a germanium-doped photosensitive core surrounded by a hexagonal periodic air-hole lattice in a silica matrix. The spectra of these gratings allow for a detailed characterization of the fiber. In particular, the gratings facilitate coupling to higher-order leaky modes. We show that the spatial distribution and the effective index of these modes are determined largely by the design of the lattice and that the grating spectra are unaffected by the refractive index surrounding the fiber. We describe these measurements and corresponding simulations and discuss their implications for the understanding of such air-hole structures.

High-speed optical communication requires ultrafast all-optical processing and switching capabilities. The Kerr nonlinearity, an ultrafast optical nonlinearity, is often used as the basic switching mechanism. A practical, small device that can be switched with ~1-pJ energies requires a large Kerr effect with minimal losses (both linear and nonlinear). We have investigated theoretically and experimentally a number of Se-based chalcogenide glasses. We have found a number of compounds with a Kerr nonlinearity hundreds of times larger than silica, making them excellent candidates for ultrafast all-optical devices.

We have demonstrated the loss of transverse spatial coherence of an atomic wave function after a single spontaneous emission. He· atoms were both diffracted and excited by a standing light wave with a variable period. After the interaction. the excited atoms decay by a single spontaneously emitted photon. By changing the period of the standing light wave, we have mapped the loss of spatial coherence as a function of the transverse coordinate. By detecting the emitted photon one could "erase" the position information available and recover the transverse coherence in a correlation experiment, or realize a Heisenberg microscope.PACS numbers: 32.80.-t. 41.85.-p, 42.50.Vk A measurement apparatus influences a classical object in a deterministic way, such that in principle the disturbance by the measurement can approach zero. In the measurement of a quantum mechanical object, however, there is always a minimum amount of indeterministic disturbance of the object that is connected to the amount of extractable information. This difference was discussed by Heisenberg in 1927 [1] and subsequently by many authors using various gedanken experiments [2]. In this paper, we present a realization of such a gedanken experiment using the diffraction of atoms from a standing light wave.The scattering of a single photon is a fundamental dissipative process that can be used to measure the position of an object [1]. The scattering process couples the motional degrees of freedom directly to the continuum of radiation modes of the emitted photon. The spread in momentum space via the recoil of the photon on the object is directly coupled to the precision of the localization process. If the localization is not perfect, the spatial coherence of the object is not completely destroyed. In order to describe this partial loss of spatial coherence of an object, we use the transverse one-dimensional coherence function as in classical optics, where Iz denotes the Fourier transform with respect to kO . This is the two-point correlation function of the transverse atomic wave function c/J(z). The corresponding wave function in momentum space is ",(kO) = Iko{c/J(Z)}, where kO is the transverse momentum of the object, which in this paper will be an atom, and Jko denotes the Fourier transform with respect to z. This concept can be generalized for a statistical mixture of states described by a density matrix p to give [3] ( 2) where I(kO) is the momentum distribution of the object, which is experimentally accessible. A photon with a wave vector of length kP = IkP I, that is spontaneously emitted with a given radiation pattern, produces a spread of the transverse momentum distribution of the object kO via its recoil. The final momentum distribution of the object is then given by [4] (3) where ® denotes a convolution and P(kf) = P(-kf) is the probability of emitting a photon with the transverse component of the momentum kf; i.e., the projection of the radiation pattern on the z axis [see Fig. l(a)].

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Abstract-In this paper, we review the recent progress in transmission experiments by employing optical phase conjugation (OPC) for the compensation of chromatic dispersion and nonlinear impairments. OPC is realized with difference frequency generation (DFG) in a periodically poled lithium-niobate (PPLN) waveguide, for transparent wavelength-division multiplexed (WDM) operation with high conversion efficiency. We discuss extensively the principle behind optical phase conjugation and the realization of a polarization independent OPC subsystem. Using OPC for chromatic dispersion compensation WDM 40-Gb/s long-haul transmission is described. As well, transmission employing both mixed data rates and mixed modulation formats is discussed. No significant nonlinear impairments are observed from the nonperiodic dispersion map used in these experiments. The compensation of intrachannel nonlinear impairments by OPC is described for WDM carrier-suppressed return-to-zero (CSRZ) transmission. In this experiment, a 50% increase in transmission reach is obtained by adding an OPC unit to a transmission line using dispersion compensating fiber (DCF) for dispersion compensation. Furthermore, the compensation of impairments due to nonlinear phase noise is reviewed. An in-depth analysis is conducted on what performance improvement is to be expected for various OPC configurations and a proof-of-principle experiment is described showing over 4-dB improvement in Q-factor due to compensation of nonlinear impairments resulting from nonlinear phase noise. Finally, an ultralong-haul WDM transmission of 22 × 20-Gb/s return-tozero differential quadrature phase-shift keying (RZ-DQPSK) is discussed showing that OPC can compensate for chromatic dispersion, as well as self-phase modulation (SPM) induced nonlinear impairments, such as nonlinear phase noise. Compared to a "conventional" transmission link using DCF for dispersion compensation, a 44% increase in transmission reach is obtained when OPC is employed. In this experiment, we show the feasibility of using only one polarization-independent PPLN subsystem to compensate for an accumulated chromatic dispersion of over 160 000 ps/nm. Index Terms-Dispersion compensation, differential phaseshift keying (DPSK), differential quadrature phase-shift keying (DQPSK), duobinary, fiber-optics communications, nonlinear phase noise, phase conjugation, phase-shift keying, periodically poled lithium niobate (PPLN), spectral inversion.

Photon number squeezing of spectrally filtered optical solitons has been observed in the sub-picosecond regime, where the Raman effect changes the internal quantum structure of each pulse. 160 fs pulses were propagated down a 10 m optical fibre of anomalous group velocity dispersion. At energies below the soliton energy subsequent spectral filtering with an optical band-pass squeezed the directly detected photocurrent fluctuations to 3.2 ± 0.1 dB (52%) below the shot-noise limit. This implies 4.5 dB (65%) of soliton photon number squeezing, if detection losses are taken into account. Using a spectral edge filter the spectral components of the pulses are shown to contribute asymmetrically to the squeezing due to stimulated Raman scattering.

We report on 160-to IO-Gbitk demulripkring using a monolirhical[r. integrared Mach-Zehnder interfrometer nith semiconductor optical amplijiers. Bit error rates oJIess than IFiG mere achieved for aN tributary channels.

Single-mode planar waveguides were fabricated from chalcogenide glass compounds with large Kerr nonlinearities. Strong self-phase modulation of subpicosecond pulses along with low linear and nonlinear absorption losses demonstrates the potential for ultrafast, low-power, all-optical processing applications.