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Физика и техника полупроводников

Комбинационное рассеяние света квазиоднофотонных импульсов в оптоволокне с накачкой

В.Г. Попов, V. G. Krishtop, C.А. Тарелкин, И.И. Корель

Published Jan 1, 2020• 0 citations
Publisher

Abstract

Processes of Raman scattering of quasi-single-photon pulses in a single-mode optical fiber with pumping are theoretically considered. The peculiarity of the scattering is that the pumping creates non-equilibrium molecular vibrations, which significantly increases the probability of Raman scattering in the optical fiber. Non-equilibrium vibrations are expected to be when the stimulated Raman scattering takes place for the pump pulse. As a result, the length of the optical fiber has been estimated where the probability of the Raman scattering is increased

DOI
10.21883/ftp.2020.08.49631.07

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Processes of Raman scattering of quasi-single-photon pulses in a single-mode optical fiber with pumping are theoretically considered. The peculiarity of the scattering is that the pumping creates non-equilibrium molecular vibrations, which significantly increases the probability of Raman scattering in the optical fiber. Non-equilibrium vibrations are expected to be when the stimulated Raman scattering takes place for the pump pulse. As a result, the length of the optical fiber has been estimated where the probability of the Raman scattering is increased
We theoretically and experimentally demonstrate the influence of dispersion on the temporal mode properties of the spontaneous Raman scattering pumped by picosecond pulses in single mode optical fiber. We model the process of the ultrafast spontaneous Raman scattering, calculate and measure the intensity correlation function of the Raman photons with different dispersion by varying the detuning between pump and Raman photons or changing the optical length. The intensity correlation function decreases with the increase of dispersion induced temporal walk-off both by increasing the detuning and by increasing the fiber length. We evaluate the dispersion parameter of the single mode fiber by fitting the measured data with the theoretical equation we deduced. The estimated value is close to the labelled one. Our study provides a new way to evaluate dispersion parameters of the media in which Raman scattering can occur and is beneficial for efficient distillation of entanglement in optical fibers.
The Raman effect is the inelastic scattering of photons by matter. When a monochromatic light beam (pump beam) propagates in an optical fibre, spon- taneous Raman scattering transfers some of the photons to new frequencies. The probability of a photon scattering to a particular frequency shift, depends on that frequency shift, forming a characteristic spectrum. The scattered photons may lose energy (Stokes) or gain energy (anti-Stokes). If the pump beam is linearly polarized, then the polarization of scattered photons may be the same (parallel scattering) or orthogonal (perpendicular scattering). If photons are already present at other frequencies then the probability of scattering to those frequencies is enhanced (stimulated scattering). In this thesis, the equations governing the Raman effect, both in bulk glass and single mode fibre, are restated. The probability of spontaneous scattering (parallel and perpendicular) in an optical fibre has been measured for frequency shifts between 4THz and 58THz. Particular attention has been given to those frequency shifts where the scattering is intrinsically weak. This has identified those frequency shifts where it might be possible to eliminate Raman noise from experiments designed to generate quantum states of light in optical fibres using four wave mixing. From the measured scattering probabilities, the Raman equations can be used to predict the scattering cross-section in bulk glass. The prediction is found to be in reasonable agreement with measurements of the bulk glass cross-section published by other authors. By increasing the pump power, the transition to stimulated scattering has been observed. In the case of Stokes scattering there is reasonable agreement between the Raman equations and experimental observations. In the case of anti-Stokes scattering, and in the limit of very high pump power, behaviour is observed which is not predicted by the Raman equations, indicating that other processes must be included to successfully describe such experiments.
It is shown that, in strong optical fields (~109 W/cm2), Raman lines may be excited due to transitions with changes by 3 and 4 in the rotational quantum number, which are forbidden in the linear approximation. This lifting of forbiddenness for Raman scattering is due to four-photon processes in which a pump photon is absorbed and a Stokes photon is emitted in an elementary event. Under quasiresonance pumping, this type of stimulated Raman scattering may be observed due to rotational transitions. Estimate are made of the conditions for observation of the spontaneous and stimulated processes in specific media.
By investigating the intensity correlation function, we study the temporal mode properties of spontaneous Raman photons generated in single mode fibers pumped by short pulses. We find that the intensity correlation function of the Raman photons is related to the dispersion induced temporal mismatch among the Raman photons generated in different subparts of the fiber. When no temporal mismatch exists, the intensity correlation function only depends on the ratio between the pulse width of the pump and the coherence time of the Raman photons. With the ratio decreasing, the intensity correlation function increase. Our study is helpful for easily creating high-quality true thermal light which will be a useful resource for second-order coherence based remote sensing.
We theoretically and experimentally investigate an all optical switch based on stimulated Raman scattering in optical fibers. The experimental setup consists of a Raman circuit of two stages connected in series through a bandpass filter. In the first stage, we have a saturated amplifier, in this stage the pump pulses are saturated when pump and signal are launched to the input or the pump pulses remain without saturation when pump only is launched at the input. The second stage works as the Raman amplifier; for this stage amplification is directly dependent on the pump power entering from the first stage. For the case when pump pulse only is launched at the input pass to the second stage without saturation and amplifies the signal entering in the second stage, very intense signal pulses appear at the output of this stage. For the case when both pump and signal pulses are launched to the input, the pump pulse is saturated in the first stage and the filter rejected the amplified signal, so that only low power pump enters the second stage and consequently no signal pulses appear at the output. We show that the contrast can be improved when using fibers with normal and anomalous dispersion connected in series in the first stage. The best contrast (the ratio of energies) obtained was 15 dB at 6 W pump peak power.
Stimulated Raman scattering of a randomly modulated pump is investigated by the method of successive approximations. This involves expanding solutions in terms of small parameters, which are ratios of the correlation scales of random effects to other characteristic dynamic scales of the problem. Systems of closed equations are obtained for the moments of the amplitudes of the Stokes and pump waves and of the molecular vibrations. These describe the dynamics of the process allowing for changes in the pump intensity and statistics due to a three-wave interaction. By analyzing equations in higher-order approximations, it is possible to establish the conditions of validity of the first (Markov) and second approximations. In particular, it is found that these are valid for pump intensities JL both above and below the critical value Jcr near which the gain begins to increase rapidly and reproduction of the pump spectrum by the Stokes wave is initiated. Solutions are obtained for average intensities of the Stokes wave and molecular vibrations in the first approximation in a constant pump field. It is established that, for JLJcr, the Stokes wave undergoes rapid nonsteady-state amplification which is associated with an increase in the amplitude of the molecular vibrations. The results of the calculations show good agreement with known experimental data.