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Acta Physica Sinica

The research on saturation of fast light in homogeneously broaden materials with gain

Qiu Wei, L uuml Pin, Ma Ying-Chi, Xu Xiao-Juan, Liu Dian, Chenghua Zhang

Published Jan 1, 2012• 0 citations
Publisher

Abstract

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.

DOI
10.7498/aps.61.104209

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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.
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.
Group velocity of the optical pulse propagation would be reduced based on coherent population oscillation effect in an absorbing medium. According to the absorption properties of erbium-doped optical fiber, there would appear a sharp absorption loss, which brought more difficulties to the actual measurement of slow light and its application in communications. From the ground rate equation of erbium-ion, the delay of optical pulse was simulated under controllable absorption loss， and experiments were implemented to verify the theoretical results. Moreover, we researched how the different pump power and fiber length effect the signal loss and the optical pulse delay.
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.
The slow light propagation in optical fiber has attracted much attention because of its practical applications. Use of coherent population oscillation to produce slow light propagation has many advantages over other methods. We investigate the reduction of the group velocity propagation by the coherent population oscillation in erbium-doped optical fiber, and also the relationship between the population of metastable state and the time delay by changing the doping concentration, the length of the fiber, the input power, and the pump power. The results show that we can select a high doping concentration of erbium in the fiber, increase the length of the fiber, decrease the pump power and select the appropriate power of the signal light to increases the time delay.
We experimentally and theoretically study the propagation of a 1536 nm light pulse superposed on a continuous wave background in a highly doped erbium fiber pumped at 977 nm. We observe a transition from subluminal to superluminal propagation with the pulse bandwidth. Furthermore, an improvement of the pulse delay and pulse distortion when increasing the pulse peak and keeping constant the background power is reported. These results are due to the relation between the pump-broadened transparency hole (induced by the coherent population oscillations) and the competition between gain and absorption along the fiber.
We analyze the propagation regime of an amplitude-modulated 1536nm1536\phantom{\rule{0.3em}{0ex}}\mathrm{nm}1536nm signal when traveling along a highly doped erbium fiber pumped at 977nm977\phantom{\rule{0.3em}{0ex}}\mathrm{nm}977nm as a function of the fiber length. A propagation-induced transition from superluminal to subluminal propagation takes place along the fiber length which allows a change in regime solely based upon increasing the signal modulation frequency. This peculiar behavior is due to the interplay between pump absorption and pump-power broadening of the spectral hole induced by coherent population oscillations. The effect of ion density on this frequency-dependent regime change has been investigated.