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arXiv:0904.1779 · IEEE Journal of Quantum Electronics

Slow Light of an Amplitude-Modulated Gaussian Pulse in Cesium Vapor

Wenzhuo Tang, Bin Luo, Yu Liu, Hong Mei Guo

Published Mar 1, 2010• 1 citation
arXivHTMLPDF

Abstract

Slow light of an amplitude-modulated Gaussian (AMG) pulse in cesium vapor is demonstrated and studied, since the appropriate amplitude modulation to a single pulse can enrich its spectrum and even enhance the bandwidth utilization efficiency of the slow light system. In a single- ¿ type electromagnetically induced transparency (EIT) system, the slowed AMG pulse experiences severe distortion, which is mainly owing to the frequency dependent transmission of medium. Additionally, due to its spectral distribution, the frequency dependent dispersion of the medium causes simultaneous slow and fast light for different spectral components and thus a certain dispersive distortion of the AMG pulse. Further, a post-processing method is proposed to recover the slowed (distorted) pulse, which indicates that, by introducing a linear optical system with desired gain spectrum, the optical pulse can be recovered in an ¿all-optical¿ way. Finally, we discuss the limitations of this compensation procedure in detail. Although this method is demonstrated in the cesium vapor using EIT, it is also applicable to a wide range of slow light systems.

DOI
10.1109/jqe.2009.2034122

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Slow light of an amplitude-modulated Gaussian (AMG) pulse in cesium vapor is demonstrated and studied, since the appropriate amplitude modulation to a single pulse can enrich its spectrum and even enhance the bandwidth utilization efficiency of the slow light system. In a single- ¿ type electromagnetically induced transparency (EIT) system, the slowed AMG pulse experiences severe distortion, which is mainly owing to the frequency dependent transmission of medium. Additionally, due to its spectral distribution, the frequency dependent dispersion of the medium causes simultaneous slow and fast light for different spectral components and thus a certain dispersive distortion of the AMG pulse. Further, a post-processing method is proposed to recover the slowed (distorted) pulse, which indicates that, by introducing a linear optical system with desired gain spectrum, the optical pulse can be recovered in an ¿all-optical¿ way. Finally, we discuss the limitations of this compensation procedure in detail. Although this method is demonstrated in the cesium vapor using EIT, it is also applicable to a wide range of slow light systems.
The slow light effects of an amplitude modulated Gaussian (AMG) pulse in a cesium atomic vapor are presented. In a single-ΛΛΛ type electromagnetically induced transparency (EIT) medium, more severe distortion is observed for an AMG pulse than a Gaussian one. Using Fourier spectrum analysis, we find that the distortion, as well as the loss, is dominantly caused by linear absorption than dispersion. Accordingly, a compensation method is proposed to reshape the slow light pulse based on the transmission spectrum. In addition, we find a novel way to obtain simultaneous slow and fast light.
This thesis investigates the result of modulation schemes in electromagnetically induced transparency (EIT) in rubidium vapor. Through the process of EIT, a control beam can be used to change the optical properties of a medium experienced by a probe field, generating a transparency window. The corresponding change in index of refraction results in slowed group velocity while the beam propagates through the medium. Modulation is used to transmit a message signal through a medium. In classical systems, this entails converting data into a radio frequency by adding a message to an electronic or optical carrier signal. The properties of EIT, light-slowing, and their impacts on a probe pulse with modulation will be confirmed through a detailed semi-classical analysis and by experimental observation.
This report shows that the electromagnetically-induced transparency (EIT) window can be used as a time-to-frequency filter for the probe pulse whose spectrum is much broader than the width of the EIT window. This study demonstrates that the narrow spectral region of the pulse, coincident with the EIT window, develops in a long Gaussian pulse with slow group velocity. An analytical theory is developed describing the propagation of a small amplitude pulse through an optically dense absorptive medium with a narrow EIT window.
We study analytically pulse distortion in linear slow light systems, and provide some useful limits on these devices. Additionally, we also show that the contributions of phase and amplitude broadening can be de-coupled and quantified. It is observed that phase broadening is generally smaller than amplitude broadening in conventional slow light media (lorentzian gain profile) except for very large fractional delays, where it becomes larger. Upon these expressions, we may envisage new strategies to minimize the distortion in the delaying of pulses, depending on the specific application and the required fractional delay. To overcome the residual distortion, we show that nonlinear systems can lead to a resharpening of the pulses and a re-generation of the filtered frequencies.
The dispersion experienced by a signal in a slow light system leads to a significant pulse broadening and sets a limit to the maximum delay actually achievable by the system. To overcome this limitation, a substantial research effort is currently being carried out, and successful strategies to reduce distortion in linear slow light systems have already been demonstrated. Recent theoretical and experimental works have even claimed the achievement of zero-broadening of pulses in these systems. In this work we obtain some physical limits to broadening compensation in linear slow light systems based on simple Fourier analysis. We show that gain and dispersion broadening can never compensate in such a system. Additionally, it is simply proven that all the linear slow light systems that introduce a low-pass filtering of the signal (a reduction in the signal root-mean- square spectral width), will always cause pulse broadening. These demonstrations are done using a rigorous shape-independent definition of pulse width (the root-mean-square temporal width) and arguments borrowed from time-frequency analysis.
We present undergraduate-friendly instructions on how to produce light pulses propagating through warm Rubidium vapor with speeds less than 400 m/s, i.e., nearly a million times slower than c. We elucidate the role played by electromagnetically induced transparency (EIT) in producing slow light pulses and discuss how to achieve the required experimental conditions. The optical setup is presented, and details provided for preparation of pump, probe, and reference pulses of the required size, frequency, intensity, temporal width, and polarization purity. EIT-based slow light pulses provide the most widely studied architecture for creating quantum memories. Therefore, the basic concepts presented here are useful for physics and engineering majors who wish to get involved in the development of cutting-edge quantum technologies.