Search results for "MODULATION"
showing 10 items of 948 documents
Grating compensated dispersion-managed systems incorporating nonlinear optical loop mirrors
2006
In this work, we investigate the use of nonlinear optical loop mirrors (NOLMs) in DM fiber systems compensated by CFGs with GDR by launching a 128-bit Gaussian-shaped pseudo-random bit sequence having pulse width of 5 ps for simulating a 40 Gb/s system. The dispersion map contains a fiber segment of length 10.3 km and a CFG with dispersion of -15.6 ps/nm. We consider a lossless grating and the GDR is modeled by a sinusoidal function for simplicity. We have shown that the use of NOLMs can substantially improve the transmission performance in a grating-compensated DM fiber system even with presence of amplifier noise and random variations of GDR, parameters in CFGs along the propagation dista…
Imaging Static Charge Distributions: A Comprehensive KPFM Theory
2018
We analyze Kelvin probe force microscopy (KPFM) for tip-sample systems that contain static charges by presenting a rigorous derivation for the respective KPFM signal in all common KPFM modes, namely amplitude modulation, frequency modulation, or heterodyne detection in the static, open-loop or closed-loop variant. The electrostatic model employed in the derivation is based on a general electrostatic analysis of an arbitrary tip-sample geometry formed by two metals, and which can include a static charge distribution and dielectric material in-between. The effect of the electrostatic force on the oscillating tip is calculated from this model within the harmonic approximation, and the observab…
Interpretation of negative birefringence observed in strong-field optical pump-probe experiments: High-order Kerr and plasma grating effects
2013
The analysis of negative birefringence optically induced in major air components (Loriot et al., [1, 2]) is revisited in light of the recently reported plasma grating-induced phase-shift effect predicted for strong field pump-probe experiments (Wahlstrand and Milchberg, [3]). The nonlinear birefrin- gence induced by a short and intense laser pulse in argon is measured by femtosecond time-resolved polarimetry. The experiments are performed with degenerate colors, where the pump and probe beam share the same spectrum, or with two different colors and non-overlapping spectra. The in- terpretation of the experimental results is substantiated using a numerical 3D+1 model accounting for nonlinear…
Optical kerr effect in the strong field regime
2013
The optical Kerr response of hydrogen atom submitted to a strong and short near infrared laser pulse excitation is studied by solving the full 3D time-dependent Schro¨dinger equation. The nonlinear polarization evaluated at the driving field frequency is compared to the canonical expression derived from perturbation theory. A discrepancy between the two models is observed at large intensity affecting the nonlinear propagation of short and intense laser pulses.
Incoherent modulation instability in instantaneous nonlinear Kerr media
2005
We demonstrate theoretically and experimentally in an optical fiber system that partially temporally incoherent light exhibits modulational instability during its propagation in an instantaneous response nonlinear medium. We show that the modulation frequency and gain are substantially increased with respect to the corresponding values of coherent modulational instability.
Transition from plasma-driven to Kerr-driven laser filamentation.
2011
While filaments are generally interpreted as a dynamic balance between Kerr focusing and plasma defocusing, the role of the higher-order Kerr effect (HOKE) is actively debated as a potentially dominant defocusing contribution to filament stabilization. In a pump-probe experiment supported by numerical simulations, we demonstrate the transition between two distinct filamentation regimes at 800 nm. For long pulses (1.2 ps), the plasma substantially contributes to filamentation, while this contribution vanishes for short pulses (70 fs). These results confirm the occurrence, in adequate conditions, of filamentation driven by the HOKE rather than by plasma.
Modulation transfer function of a toric intraocular lens: evaluation of the changes produced by rotation and tilt.
2011
PURPOSE: To evaluate the changes in optical quality when toric intraocular lenses (IOL) are rotated or tilted and to demonstrate that IOL rotation produces an increasing effect of aberrations. METHODS: Modulation transfer function (MTF) and average modulation were used to analyze the image quality of a toric IOL. The axis of the toric IOL was rotated 5°, 10°, 15°, 20°, 25°, and 30° in successive MTF measurements. The tilt values were 0° to 5°, in increments of 1°, plus a tilt of 15°. Pupil diameters of 3 and 5 mm were used. RESULTS: The MTF decay due to aberrations was more sensitive to rotation than tilt. The main decrement in the average modulation, of approximately 50% in both pupils, o…
A Coherent Master Equation for active mode locking in lasers
2015
We present the derivation of a new master equation for active mode locking in lasers that fully takes into account the coherent effects of the light matter interaction through a peculiar adiabatic elimination technique. The coherent effects included in our model could be relevant to describe properly mode-locked semiconductor lasers where the standard Haus’ Master Equation predictions show some discrepancy with respect to the experimental results and can be included in the modelling of other mode locking techniques too.
The optical counterpart to SAX J1808.4-3658 in quiescence: Evidence of an active radio pulsar?
2003
The optical counterpart of the binary millisecond X-ray pulsar SAX J1808.4-3658 during quiescence was detected at V = 21.5 mag by Homer et al. (2001). This star shows a 6% semi-amplitude sinusoidal modulation of its flux at the orbital period of the system. It was proposed that the modulation arises from X-ray irradiation of the intrinsically faint companion by a remnant accretion disk, and that the bulk of the optical emission arises from viscous dissipation in the disk. The serious difficulty in this scenario lies in the estimate of the irradiating luminosity required to match the observational data, that is a factor 10-50 higher than the quiescent X-ray luminosity of this source. To over…
Observation of induced modulational polarization instabilities and pulse-train generation in the normal-dispersion regime of a birefringent optical f…
1998
Four-photon mixing in a low-birefringence fiber is strongly influenced by the orientation of the pump and signal waves with respect to the fiber axes. We experimentally investigated the dependence of the modulational gain spectra on pump power and polarization by mixing orthogonal pump and probe light beams in a birefringent optical fiber. With a pump on the fast fiber axis, a cascade of sidebands was generated in the regime of normal fiber dispersion. These sidebands are shown to correspond to 0.2–0.3-THz trains of pulses with complex polarization profiles. The analysis reveals that, at particular values of the input pump and probe powers and signal frequency detuning, trains of dark-solit…