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Modern Physics Letters B

Saturation phenomenon research of fast light in a Tm3+-doped optical fiber at room temperature

Wei Qiu, Jianjun Liu, Yuda Wang, Yujing Yang, Pin Lv, Qiuli Jiang

Published Aug 31, 2017• 1 citation
Publisher

Abstract

The theoretical model and numerical simulations are established by the rate equation of thulium ion. Using the coherent population oscillation effect, we realize the controllable light velocity in the thulium doped fiber. We show that coherent population oscillations effect produces a very narrow spectral hole and anti-holes in the spectrum. The hole caused the amplitude modulation beam experience a large change of the group of refractive index, and makes us to achieve fast light propagation. Therefore, the pump intensity can be used as a control parameter to increase the fractional advance. In addition, we can confirm that the larger input intensity can lead to large time advancement, and saturation phenomenon of fast light exists in low frequency.

DOI
10.1142/s0217984917501925

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The theoretical model and numerical simulations are established by the rate equation of thulium ion. Using the coherent population oscillation effect, we realize the controllable light velocity in the thulium doped fiber. We show that coherent population oscillations effect produces a very narrow spectral hole and anti-holes in the spectrum. The hole caused the amplitude modulation beam experience a large change of the group of refractive index, and makes us to achieve fast light propagation. Therefore, the pump intensity can be used as a control parameter to increase the fractional advance. In addition, we can confirm that the larger input intensity can lead to large time advancement, and saturation phenomenon of fast light exists in low frequency.
According to the transition rates equation of the ground state population and the propagation equation,the numerical model of the group velocity slowdown was established. The relationship between the gain and pump power was analysized. In the absorption area of Tm3 +-dopedoptical fiber,the oscillation leads the pulse to experience absorption saturation,and the propagation delays.In the gain area of Tm3 +-doped optical fiber,this effect induces the pulse to experience gain saturation,and the propagation is in advance. According to the theoretical calculation,the group velocity slowdown is tunable due to coherent population oscillation in the Tm3 +-doped optical fiber amplifier at room temperature.
Abstract In this paper, we propose a general theory for the coherent population oscillation effect in an -doped optical fiber under dual-frequency laser pumping at room temperature. We aim at comparing the effect of dual pumping (1480 nm and 980 nm) with single pumping of 980 nm and single pumping of 1480 nm on time advancement. According to the numerical calculations, we analyze the relationships between relative modulation attenuation and modulation frequency, plus fractional advancement and modulation frequency under the dual-frequency pumping laser with co-propagation. Compared to single pumping, a greater change in the group refractive index can be found. The maximum time advancement of 0.24 ms under a dual-frequency pump laser with co-propagation is obtained. Moreover, we demonstrate the influence of the pump ratio M on time advancement with modulation frequencies of 40 Hz, 100 Hz and 200 Hz. The ability to control the group velocity of light could have an important application for photonics.
According to the analysis of gain theory, the absorption or gain of medium depends on pump power. The coherent population oscillation leads the pulse to experience absorption saturation and propagation delay in a medium with absorption. This effect induces the pulse to experience gain saturation and propagation advance in a medium with gain. Making use of the transition rate equation of metastable state poplation, we develop the model of fast light in erbium-doped optical fiber and obtain the analytic expression of time advance. We demonstrate that there is a saturation effect of fast light in low frequency region, but the fast light is enhanced with pump power increasing in high frequency region.
We perform a study of extremely slow pulse propagation speed in densely-doped erbium fibers by using a detuned signal. A quantum coherence effect, coherent population oscillations, produces a very narrow spectral "hole" in the homogeneous broadened absorption profile of the erbium-doped fiber. The resulting rapid spectral variation of the refractive index leads to a large value of the group index. The interparticle interaction between the nearest neighboring active particles due to the high density of erbium ions is considered. The result shows that the nonlinear refractive index can be greatly increased which leads to the smaller group velocity and the larger time delay.
The paper presents a critical analysis of publications on one of the methods of creating so-called slow light (light with an anomalously low group velocity) arising due to a high steepness of the refractive index dispersion curve. The method employs, for this purpose, the effect of coherent population oscillations accompanied by burning of a narrow spectral hole in the homogeneously broadened absorption spectrum. The interpretation of the experimental data in the studies under consideration is based on the analysis of the response of a nonlinear medium to a low-frequency intensity modulation of the propagating light beam. We show that all the observations of these papers can be easily interpreted in the framework of the simplest model of a saturable absorber and have nothing to do with the hole burning effect or group velocity reduction.
We investigate the reduction of the group velocity propagation resulting from the steep change of the refractive index by the coherent population oscillation in erbium-doped optical fibre. The largest delay is measured to be about 8.75ms corresponding to a group index of 1.312 × 10 6 . The time delay or advancement depends on the pump intensity. Influences of the ion density on the fractional delay and the group velocity reduction of light propagation are studied. Based on our discussion the optimization parameters should be selected in order to obtain more appropriate time delay and the slowdown of group velocity.