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Journal of the Optical Society of America B

Broadband Brillouin slow light based on multifrequency phase modulation in optical fibers

Yongkang Dong, Zhiwei Lü, Qiang Li, Yingfan Liu

Published Oct 6, 2008• 24 citations
Publisher

Abstract

We theoretically analyze and experimentally demonstrate broadband Brillouin slow light using multiple equal-amplitude spectral lines generated by multifrequency phase modulation. The theoretical analysis shows that 4n−1 equal-amplitude spectral lines can be obtained if the modulation signal comprises the fundamental frequency and the odd harmonic waves, where n denotes the number of frequency components in the modulation signal. In experiment, 7 and 11 equal-amplitude spectral lines are obtained and Brillouin bandwidths of 340 and 570 MHz are gained, respectively. The experimental results also indicate that the slope of the broadening factor for the delayed signal decreases in the case of broad and flattened Brillouin gain spectrum.

DOI
10.1364/josab.25.00c109

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Optics Express•Oct 15, 2008•2 citations
We theoretically analyze and experimentally demonstrate broadband Brillouin slow light using multiple equal-amplitude spectral lines generated by multifrequency phase modulation. The theoretical analysis shows that 4n−1 equal-amplitude spectral lines can be obtained if the modulation signal comprises the fundamental frequency and the odd harmonic waves, where n denotes the number of frequency components in the modulation signal. In experiment, 7 and 11 equal-amplitude spectral lines are obtained and Brillouin bandwidths of 340 and 570 MHz are gained, respectively. The experimental results also indicate that the slope of the broadening factor for the delayed signal decreases in the case of broad and flattened Brillouin gain spectrum.
We present a method to achieve flat-top gain spectrum through overlapping multiple gain lines, which can be used to increase slow light bandwidth and relative pulse delay. A tunable gain bandwidth can be realized by changing the number of spectral lines and frequency separation between adjacent spectral lines. We demonstrate the method in a SBS-based slow light system. A phase modulator is used to modulate the phase of the pump wave, generating a pump wave with multi-line spectrum and achieving a Brillouin gain bandwidth of ~ 330 MHz.
Slow light is generated via the gain mechanisms of nonlinear optical phenomena-- stimulated stimulated Brillouin scattering in optical fiber. Slow light is characterized in terms of Brillouin threshold, gain spectrum, Brillouin frequency shift by using optical heterodyne detection. According to the measured Brillouin frequency shift, we set our signal frequency in the gain region by manual matching method to get the maximum gain, and the gain bandwidth is broadened to 450MHz using a phrase-modulated pump. A delay of 32ns is achieved for the 500ns input signal when the gain is 15dB.
Slow light is generated via the gain mechanisms of nonlinear optical phenomena-- stimulated stimulated Brillouin scattering in optical fiber. Slow light is characterized in terms of Brillouin threshold, gain spectrum, Brillouin frequency shift by using optical heterodyne detection. According to the measured Brillouin frequency shift, we set our signal frequency in the gain region by manual matching method to get the maximum gain, and the gain bandwidth is broadened to 450MHz using a phrase-modulated pump. A delay of 32ns is achieved for the 500ns input signal when the gain is 15dB.
Dynamic control of the speed of a light signal, based on stimulated Brillouin scattering in optical fibers, was theoretically studied and also experimentally demonstrated as the core object of this thesis. To date, slow light based on stimulated Brillouin scattering has shown the unmatched flexibility to offer an efficient timing tool for the development of all-optical future router. Nevertheless, the seeming perfect Brillouin slow light suffered from three major obstacles: naturally narrow signal bandwidth, strong change of signal amplitude, and significant signal distortion. The essential contribution of this work has been mostly dedicated to resolve all those impairments so as to make Brillouin slow light a completely operating all-optical delay line for practical applications. Actually, high capability of tailoring the spectral distribution of the effective Brillouin resonance makes possible to resolve partially or completely all those problems. First of all, a broadband spectral window was passively obtained in between two Brillouin gain/loss resonances by simply appending two segments of fibers showing different Brillouin frequency shifts. The global Brillouin gain of the concatenated fibers manifests a gain/loss doublet resonance showing a broad window in between gain/loss peaks. In practice, this configuration has a crucial advantage that it removes the need of the pump modulation, generally used to create a polychromatic pump source. Therefore, a broadband Brillouin slow and fast light was simple realized with a reduced distortion. Secondly, the signal amplification or attenuation associated to the signal delay was completely compensated by superposing Brillouin gain and loss resonances with identical depth but different width. As a result, the Brillouin gain led to effectively a spectral hole in the center of the broadband absorption and opened a transparent window while the sharp change in the refractive index was preserve. This way it makes possible to realize zero-gain Brillouin slow light. This configuration was also exploited to produce Brillouin fast light with a total absence of signal loss, simply by swapping the spectral position of the two pumps. At last, a signal was continuously delayed through a Brillouin fiber delay line without any distortion. Due to the strong induced dispersion, pulse broadening is a major difficulty in all slow light systems and it is impossible to compensate such broadening using a linear system. Therefore, a conventional Brillouin slow light system was combined with a nonlinear optical fiber loop mirror that gives a nonlinear quadratic transmission. Using this configuration, the inevitable pulse broadening was completely compensated at the output of the loop and a signal was delayed up to one symbol without any distortion. Brillouin slow light systems were further studied in the spectral domain. For a given Brillouin resonance the spectrum of a light pulse was optimized to better match the Brillouin bandwidth. When the time envelope of a pulse was properly shaped, it was clearly observed that the spectral width of the pulse became minimized while preserving the pulse duration. This way the maximum time delay through Brillouin slow light could be enhanced for a fixed pulse width. Brillouin fast light was even realized in total absence of any pump source, which is a plain requirement for the generation of Brillouin slow or fast light. This self-generated delay line, key contribution of this thesis, relies on both spontaneous and stimulated Brillouin scattering in optical fibers. In this implementation, a light signal was strongly boosted above the so-called Brillouin threshold, so that most power of the signal was transferred to a backward propagating wave, namely the generation of amplified spontaneous Stokes wave. Since the center frequency of the intense Stokes wave is below the signal frequency by exactly Brillouin shift of the fiber used, this wave led to a Brillouin loss band centered at the signal frequency. Consequently, the signal experienced fast light propagation and the propagation velocity of the signal was self-controlled, simply by varying the signal input power. This technique has many practical advantages such as its high simplicity of the configuration and an invariant signal power in the output of this delay line. Additionally, this system self-adapts the signal bandwidth as the spectrum of the amplified Stokes wave matches the spectral distribution of the signal. An alternative method to generate all-optical delay line was proposed instead of slow light. This scheme makes use of the combination of wavelength conversion and group velocity dispersion. This type of delay line was mainly aimed at improving the storage capability of delaying element. The wavelength of a signal was simply and efficiently converted at a desired wavelength using cross gain modulation in semiconductor optical amplifiers. Then the converted signal was delivered to a high dispersive medium and arrived at the end of the medium with relative time delay due to the group velocity dispersion. A fractional delay of 140 was continuously produced through this delay line for a signal with a duration of 100 ps, preserving signal bandwidth and wavelength. The effect of slow light on linear interactions between light and matter was experimentally investigated to clarify the current scientific argument regarding slow light-enhanced Beer-Lambert absorption. It was predicted that real slowing of the light group velocity could enhance the molecular linear absorptions so as to improve the sensitivity of this type of sensing. However, the experimental results unambiguously show that material slow light (slow light in traveling wave media) does not enhance the Beer-Lambert absorption.
It is an effective approach to use pump with multi-line spectrum to broaden the bandwidth of slow light based on stimulated Brillouin scattering (SBS). A novel method of generating multi-line spectrum is proposed and simulated, through which arbitrary numbers of spectral lines with arbitrary intervals can be obtained. The amplitude of each line is able to be tuned independently. These two merits enable to construct the shape of the gain spectrum of SBS as expected. The maximum broadened bandwidth of the pump is only limited by the bandwidth of related hardwares.