Eric Lantz

Spectral broadening of a partially coherent CW laser beam in single-mode optical fibers

The nonlinear propagation of a partially coherent continuous-wave laser beam in single-mode optical fibers is investigated both theoretically and experimentally, with a special attention to the zero-dispersion wavelength region where modulation instability is expected. Broadband asymmetric spectral broadening is reported experimentally and found in fairly good agreement with a numerical Schrödinger simulation including a phase-diffusion model for the partially coherent beam. This model shows in addition that the underlying spectral broadening mechanism relies not only on modulation instability but also on the generation of high-order soliton-like pulses and dispersive waves. The coherence degradation which results from these ultrafast phenomena is confirmed by autocorrelation measurement.

Impact of pump phase modulation on the gain of fiber optical parametric amplifier

In practice, fiber optical parametric amplifiers are generally driven by a phase-modulated pump wave to avoid stimulated Brillouin back-scattering. We show both analytically and numerically that the phase modulation of the pump can induce strong parametric gain modulation and that this detrimental effect depends both on the rise-fall time of the phase modulator and on the fiber dispersion slope.

Broadband and flat parametric amplifiers with a multisection dispersion-tailored nonlinear fiber arrangement

We describe a simple scheme to allow for the achievement of flat gain over ultrabroad bands with a single-pump fiber-optic parametric amplifier operating in the zero-dispersion wavelength region. The proposed method, based on a multisection dispersion-tailored in-line nonlinear fiber arrangement, is demonstrated by both modulational instability theory and numerical simulations of the nonlinear Schrodinger equation. The results show that the design can be adjusted to generate gain bands that exceed either 100 nm with a ripple of less than 0.2 dB and for a pump power of only 500 mW, or even 200 nm when a pump power of 5 W is used. In addition, the robustness of this gain-flattening technique has been numerically checked against random fluctuations of the zero-dispersion wavelength in each of the fiber sections.

Impact of pump OSNR on noise figure for fiber-optical parametric amplifiers

Electrical measurements of the noise figure of a fiber-optical parametric amplifier are presented and compared with optical measurements. The transfer of pump noise by four-wave mixing was clearly demonstrated. A numerical model was developed to simulate the transfer of pump noise and validated by these measurements. Using this model, we determine, for practical systems, a minimum required pump optical signal-to-noise ratio of 65 dB.

Oscillator noise analysis: multivariance measurement

Since the noise altering the output signal of oscillators may be modeled as power laws in the spectral density of frequency deviation, oscillator noise analysis is the measurement of the level of each power law noise. The principle of this new multivariance method consists of obtaining the noise-type contributions with different variances and different integration time values. All the data obtained from the different variances with the different integration times are then operated simultaneously. Thus, the most probable measurement, in the sense of least squares, is obtained for each type of noise. This method lends itself to an estimation of the uncertainty of the noise-type contribution measurement, taking into account the dispersion of the variance results. >

Spatiotemporal behavior of periodic arrays of spatial solitons in a planar waveguide with relaxing Kerr nonlinearity

We report picosecond pulsed experiments and numerical simulations of spatially induced modulational instability, which we used to form stable periodic arrays of bright soliton beams in a planar Kerr-like CS2 waveguide. We have found that the generation stage of these arrays is accurately described by the usual nonlinear Schrodinger wave equation, whereas the temporal dynamics of the nonlinearity is beneficial for subsequent stable propagation of the soliton arrays. In the picosecond regime the finite molecular relaxation time of the reorientational nonlinear index inhibits the Fermi–Pasta–Ulam recurrence predicted for an instantaneous Kerr nonlinearity. Moreover, the inhibition is associated with a novel spatiotemporal dynamics confirmed by numeric and streak-camera recordings.

Parametric amplification of a polychromatic image

The picosecond parametric amplification of a polychromatic image with a wavelength bandwidth of 140 nm and a gain of 15 dB has been obtained in a lithium triborate type-I crystal (LBO). Approximately 8 lines/mm in both lateral dimensions were resolved in the crystal plane. This resolution value is in good agreement with a numerical study of the phase-matching conditions near collinear degeneracy, where phase matching is noncritical for the signal beam in angle as well as in wavelength. The parametric amplification of a monochromatic image in LBO is also studied. Because their polarizations are identical, the signal and the idler can interfere, leading to phase-sensitive amplification.

Raman-assisted parametric frequency conversion in a normally dispersive single-mode fiber.

We demonstrate efficient frequency conversion with large frequency shifts of an anti-Stokes signal into a parametrically seeded Stokes idler, which is generated by a highly mismatched three-wave mixing interaction and subsequent Raman amplification in a normally dispersive single-mode fiber. The use of non-phase-matched waves in Raman-assisted three-wave mixing interactions overcomes the strict spectral limitations imposed by phase-matching conditions in parametric frequency-conversion processes.

Two-dimensional time-resolved direct imaging through thick biological tissues: a new step toward noninvasive medical imaging.

We report an original two-dimensional time-resolved direct imaging method for transillumination optical tomography that combines the time-gating and forward phase-conjugation properties of type II degenerate parametric amplification. An object with subcentimeter resolution embedded in 4-cm-thick chicken breast tissue was imaged with a signal-to-noise ratio of 2.

Multi-imaging and Bayesian estimation for photon counting with EMCCDs

A multi-imaging strategy is proposed and experimentally tested to improve the accuracy of photon counting with an electron multiplying CCD (EMCCD), by taking into account the random nature of its on-chip gain and the possibility of multiple photodetection events on one pixel. This strategy is based on Bayesian estimation on each image, with a priori information given by the sum of the images. The method works even for images with large dynamic range, with more improvement in the low light level areas. In these areas, two thirds of the variance added by the EMCCD in a conventional imaging mode are removed, making the physical photon noise predominant in the detected image.