Integrated optical source of polarization entangled photons at 1310 nm.

We report the realization of a new polarization entangled photon-pair source based on a titanium-indiffused waveguide integrated on periodically poled lithium niobate pumped by a CW laser at 655 nm. The paired photons are emitted at the telecom wavelength of 1310 nm within a bandwidth of 0.7 nm. The quantum properties of the pairs are measured using a two-photon coalescence experiment showing a visibility of 85%. The evaluated source brightness, on the order of 10(5) pairs s(-1)GHz(-1)mW(-1), associated with its compactness and reliability, demonstrates the source's high potential for long-distance quantum communication.

High-performance guided-wave asynchronous heralded single-photon source.

We report on a guided-wave asynchronous heralded single-photon source based on the creation of nondegenerate photon pairs by spontaneous parametric downconversion in a periodically poled lithium niobate wave-guide. We show that, by use of the signal photon at 1310 nm as a trigger, a gated detection process permits announcement of the arrival of single photons at 1550 nm at the output of a single-mode optical fiber with a high probability of 0.37. At the same time the multiphoton emission probability is reduced by a factor of 10 compared with Poissonian light sources. Furthermore, the model we have developed to calculate those figures of merit is shown to be accurate. This study can therefore serve as a paradigm for the conception of new quantum communication and computation networks.

Cascading phase shift and multivalued response in counterpropagating frequency-nondegenerate parametric amplifiers.

We propose and analyze a novel geometry for all-optical processing of low-power signals that is based on a frequency-nondegenerate counterpropagating parametric amplifier. The stationary response in the cascading regime exhibits multivalued solutions with enhanced nonlinear phase shifts. Implementation in a quasi-phase-matched LiNbO(3) waveguide is discussed.

Experimental observation of longitudinal modulation of mode fields in periodically segmented waveguides.

We present what is to our knowledge the first experimental observation by fluorescent visualization, of the evolution of the mode field profile in a periodically segmented waveguide. The experimental observations are then compared with the numerical results obtained by a finite-difference beam propagation method. Good agreement between experimental and numerical results is observed.

Ray analysis of parabolic-index segmented planar waveguides.

A ray analysis of periodically segmented waveguides with parabolic-index variation in the high-index region is presented. We carried out the analysis using ray transfer matrices, which is convenient to implement and which can be extended to study different types of graded-index segmented waveguide. Results of this ray tracing approach clearly illustrate the waveguiding properties and the existence of stable and unstable regions of operation in segmented waveguides. We also illustrate the tapering action exhibited by segmented waveguides in which the duty cycle varies along the length of the waveguide. This analysis, although restricted to multimode structures, provides a clear visualization of the waveguiding properties in terms of ray propagation in segmented waveguides.

Spectral Measurement of the Film-Substrate Index Difference in Proton-Exchanged LiNbO(3) Waveguides.

We report the spectral characterization of proton-exchanged lithium niobate (PE:LiNbO(3)) waveguides in terms of the variation of the refractive-index difference between the waveguiding layer and the substrate. The dispersion of the extraordinary refractive-index increase (deltan(e)) is measured from 405 to 1319 nm with several light sources. Two types of proton-exchanged waveguide, prepared under different conditions, are studied. These measurements should be of use in the optimization of PE:LiNbO(3) waveguides for nonlinear optical applications, particularly in second-harmonic generation in the blue-green wavelength region.

Modeling of parametric amplification in the Cerenkov-idler configuration in planar waveguides.

We present theoretical modeling of a novel configuration for quasi-phase-matched parametric amplification in which the pump and the signal fields form guided waves of a planar waveguide while the idler is radiated into the substrate. We follow a coupled-mode approach to study the parametric amplification in periodically domain-reversed proton-exchanged LiNbO(3) waveguides upon a Z-cut substrate and present numerical results relevant to the amplification process. In particular, for given pump and signal wavelengths we have studied the dependence of the gain coefficient on the periodicity of a grating formed by domain inversion. This configuration, which is referred to as the Cerenkov-idler configuration, is shown to provide a large signal gain bandwidth and is more tolerant to variations in pump frequency.

Fabrication parameter optimization of a low-threshold high-efficiency proton-exchanged waveguide laser in Nd:LiTaO(3).

Knowing the correlation between the Nd(3+) excited-state lifetime in proton-exchanged waveguides and the phase diagram of the H(x)Li(1-x)TaO(3) compound permits optimized waveguide fabrication parameters to be found. These have been used to produce a Nd:LiTaO(3) waveguide laser with a threshold of 2.9 mW and a slope efficiency of 33%, in good agreement with the best predicted values. Nevertheless this component suffers from instabilities because of the photorefractive effect.

Quasi-phase-matched parametric fluorescence in room-temperature lithium tantalate waveguides.

We report what is to our knowledge the f irst observation of quasi-phase-matched parametric fluorescence in the 1.2-2.3-microm region, at ambient temperature, in lithium tantalate waveguides, using a pump wavelength between 805 and 845 nm. The signal fluorescent power depends strongly on the proton concentration in the waveguide, and signals of up to 7 pW have been observed for guided pump powers of 70 mW, which is 1 order of magnitude lower than the efficiency measured with lithium niobate guides.

Characteristics of hybrid modes in proton-exchanged lithium niobate waveguides.

A rigorous numerical model, verified by experimental results, gives an explanation of the particular electromagnetic behaviors observed in x-cut proton-exchanged lithium niobate waveguides. This approach, which allows an exact calculation of the weights of the coupled ordinary and extraordinarywaves that make up the hybrid modes, provides deeper insight into the study of the strains induced by the proton-exchange process in the waveguide itself, showing that the optical axis of the exchanged layer is not parallel to the waveguide plane.

1.2-µm transitions in erbium-doped fibers: the possibility of quasi-distributed temperature sensors.

We propose the principle of a high-dynamic, quasi-distributed temperature sensor based on the behavior of the 1.13- and the 1.24-µm emission lines in erbium-doped silica fibers. The ratio of fluorescent intensity of these lines presents a temperature dynamic of more than 11 dB between room temperature and 600 °C. As the lower level of these transitions is not the fundamental, the emission lines are absorption free, and no dependence of the intensity ratio of the two lines has been observed, with power and wavelength pump variations permitting the realization of self-calibrated quasi-distributed sensors.

Erbium-doped silica fibers for intrinsic fiber-optic temperature sensors.

The variation in the green intensity ratio ((2)H(11/2) and (4)S(3/2) energy levels to the ground state) of Er ions in silica fibers has been studied as a function of temperature. The different processes that are used to determine the population of these levels are investigated, in particular 800-nm excited-state absorption in Er-doped fibers and 980-nm energy transfer, in Yb-Er-codoped fibers. The invariance of the intensity ratio at a fixed temperature with respect to power, wavelength, and doped fiber length has been investigated and shown to permit the realization of a high-dynamic-range (greater than 600 °C), autocalibrated fiber-optic temperature sensor.

Thermalization effects between upper levels of green fluorescence in Er-doped silica fibers.

We present a spectroscopic study of the green fluorescence resulting from pump excited-state absorption in Er-doped silica fibers excited in the 800-nm range. The absorption and emission bands are selectively attributed to the (4)S(3/2) and (2)H(11/2) levels. The fluorescence response at two excitation wavelengths, the temperature behavior, and lifetime measurements demonstrate a fast thermalization between the (4)S(3/2) and (2)H(11/2) levels. This explains an important part of the (2)H(11/2) emission and the increase of the fluorescence intensity at high temperature.

Fluorescence in rare earth-doped fluorozirconate fibers.

Spontaneous fluorescence bands of erbium-, holmium-, and thulium-doped fluorozirconate fibers are studied experimentally and theoretically. From experimental data and for each trivalent ion we identify the set of optical transitions that gives rise to the observed linear fluorescence and upconversion process. Fiber perturbation theory and density matrix formalism are used to model fluorescence spectra with particular attention to modal structure, loss, and mode coupling in the fiber. The relationship between the experimental emission spectrum of thulium-doped fiber and the theoretical model is discussed.

Nd:MgO:LiNbO(3) waveguide laser and amplifier.

We report efficient operation of a channel waveguide laser and a channel waveguide amplifier in Nd:MgO:LiNbO(3). For the laser a cw output power of 2.9 mW was obtained for 23.6 mW of absorbed pump power. The absorbed pump power at threshold was 1.5 mW, and a slope efficiency of 13% was achieved. For the amplifier a small-signal gain of 7.5 dB was achieved for 22 mW of coupled pump power.

Fluorescence and saturated absorption in a neodymium doped silica fiber.

We report the study of absorption and fluorescence in neodymium doped silica fibers with excitation pulses much shorter than the fluorescence lifetime. Fluorescence and absorption saturation are observed for pump powers well below the damage threshold of the fiber. The saturation process is described using the density matrix method for the light absorption and the normal mode formalism for the propagation. The experimental results are in good agreement with the theory.

[Not Available]. / Effet Kerr optique dans une fibre optiquebirefringente multimode.

In this paper, we study the optical Kerr effect theoretically in multimode birefringent optical fibers in connection with the characteristics of the fiber and excitation conditions. We observe experimentally, for a pump power 2 W in the fundamental mode, 70% modulation depth for the probe light with multimodal coupling.

Independent control of index and profiles in proton-exchanged lithium niobate guides.

We demonstrate the possibility of controlling, practically independently, the form and indices of proton-exchanged lithium niobate guides by means of guide annealing and proton exchange in lithium-rich solutions. Experimental results are presented that indicate how one can realize specific guide designs.