Impact Of Two-photon Absorption Research Articles

Impact of two-photon absorption on two-dimensional solitons in silicon waveguides with a nonstationary potential*

We study the effects of weak two-photon absorption on two-dimensional (2D) solitons interaction in a silicon-based waveguide under a framework of coupled perturbed D nonlinear Schrödinger equations with a pure Kerr nonlinearity and a nonstationary potential. We extend the perturbative analysis for collisions of two one-dimensional solitons in silicon waveguides to derive the theoretical expressions for the amplitude evolution of a single perturbed soliton and for the abrupt amplitude downshift of a colliding soliton due to a fast interaction between two 2D solitons in silicon lattices. The theoretical calculations are then validated by the extensive simulations of the coupled perturbed D nonlinear Schrödinger model with periodic and non-periodic potentials. The simulations are implemented by using the accelerated imaginary-time evolution scheme and the split-step Fourier method. These results allow us to accurately capture the abrupt energy downshift of a colliding soliton in silicon-based two-dimensional waveguides.

Impact of two-photon absorption and free-carrier effects on time lens based on four-wave mixing in silicon waveguides

We investigate the influences of two-photon absorption (TPA) and free-carrier effects on a time lens via four-wave mixing in silicon waveguides. It is found that the output waveform is severely modulated and the conversion efficiency is sharply reduced in temporal imaging owing to the impacts of the nonlinear absorption. A silicon–organic hybrid slot waveguide with negligible TPA is proposed for the time lens, which could realize 600× magnification of femtosecond pulses. These results can increase the signal bandwidth from gigahertz to terahertz and give rise to important potential applications for ultrafast optical signal processing in integrated optics, ultrafast optics, and optical communications.

Impact of two-photon absorption on soliton trapping in silicon-on-insulator waveguides

In this paper, we present a numerical simulation study of the effects of two-photon absorption on soliton trapping occurring between two components decomposed from a soliton pulse. The two pulse components polarize along the polarization directions of the quasi-TE mode and quasi-TM mode in a silicon-on-insulator waveguide. The effects of free carriers generated by two-photon absorption are also investigated, leading to free-carrier absorption and free-carrier dispersion. The simulation image of the propagation of solitons is provided by solving the coupled nonlinear Schrödinger equation satisfied by the quasi-TE mode component and the quasi-TM mode component utilizing the split-step Fourier method with MATLAB. The simulation shows that two-photon absorption and free carriers will hinder the formation of soliton trapping. As the incident pulse power increases, the soliton trapping will occur, inspite of the presence of two-photon absorption and free carriers. The simulation also indicates that if the value of the free-carrier’s lifetime is comparable to the value of the pulse width, the formation of soliton trapping is susceptible to the lifetime of free carriers. With the incident pulse width increasing, soliton trapping forms between the mean peak of two polarization components, whereas it originally formed between the mean peak of TE mode and secondary peak of TM mode at small pulse width.

The theoretical investigation of the impact of two-photon absorption and free-carrier absorption on phase-sensitive amplification in silicon waveguides

We investigate the influence of two-photon absorption (TPA) and free-carrier absorption (FCA) on phase-sensitive amplification (PSA) process in a dispersion engineered silicon strip waveguide at the telecom wavelengths. The phase-dependent gain and phase-to-phase transfer functions as well as the phase regeneration capacity of PSA are numerically analysed. It is found that TPA and FCA will suppress the phase-sensitive extinction ratio (PER) and the efficiency of phase squeezing. Moreover, the phase regeneration capability of silicon waveguide-based PSA is investigated by comparing the regenerated signal waveforms at a different signal power with/without TPA and FCA. Our results have potential application in all-optical signal regeneration.

Characterization of efficient wavelength conversion by four-wave mixing in sub-micron silicon waveguides

We characterize silicon waveguide based wavelength converters using a commercial semiconductor optical amplifier (SOA) based wavelength converter as a benchmark. Conversion efficiency as high as -5.5 dB can be achieved using a 2.5 cm long sub-micron silicon-on-insulator rib waveguide. Comparison with the SOA reveals that silicon offers broader conversion bandwidth, higher OSNR, and negligible channel crosstalk. The impact of two-photon absorption and free carrier absorption on the conversion efficiency and the dependence of the efficiency on the rib waveguide dimensions are investigated theoretically. Using a nonlinear index coefficient of 4x10(-14) cm(2)/W for silicon, we obtain good agreement between simulations and measurements.

Noise Figure of Silicon Raman Amplifiers

The noise figure of silicon Raman amplifiers in the presence of nonlinear losses is calculated. The impact of two-photon absorption (TPA) and free-carrier scattering on the noise figure is quantified using the quantum formulation of the Langevin approach. It is found that TPA-induced free-carrier loss degrades the noise figure by an amount that depends on the carrier lifetime. For example, in a 1-cm-long waveguide pumped at 200 MW/cm2, the noise figure is 5.2 dB for a lifetime of tau = 1.6 ns and is reduced to 3.7 dB for tau = 0.1 ns. The reduction in the noise figure along with a concomitant increase in Raman gain from 2 to 8 dB suggests that lifetimes on the order of 0.1 ns or less are needed to create a useful silicon Raman amplifier that operates in the continuous-wave mode. It is also shown that in devices that use a p-n junction for carrier sweep-out, the screening of the junction field by generated free carriers results in a sharp increase in the noise figure at high-pump intensities. These results apply to operation in the near-infrared communication wavelengths. For mid-infrared wavelengths above the two photon absorption band-edge (2.3 nm), the absence of TPA and pump-induced free-carrier absorption ensures that the amplifier has a low-noise figure.

All-optical regeneration based on a nonlinear long period grating

We theoretically investigate a nonlinear all-optical switching scheme based on a long period grating written in a highly nonlinear chalcogenide fiber. We investigate the impact of two-photon absorption on the device and show that the influence of a modest amount of two photon absorption is actually beneficial to device performance as quantified by the power transfer function and improvement of the Q-factor of an incident signal.

Impact of two-photon absorption on self-phase modulation in silicon waveguides

We study the effects of two-photon absorption on the self-phase modulation (SPM) process in silicon waveguides while including both free-carrier absorption and free-carrier dispersion. An analytical solution is provided in the case in which the density of generated carriers is relatively low; it is useful for estimating spectral bandwidth of pulses at low repetition rates. The free-carrier effects are studied numerically with emphasis on their role on the nonlinear phase shift and spectral broadening. We also consider how the repetition rate of a pulse train affects the SPM process.