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Chinese Science Bulletin (Chinese Version)

Cascaded superluminal propagation via Brillouin lasing resonance in optical fibers

Liang Zhang, Li Zhan

Published Sep 28, 2017• 6 citations
Publisher

Abstract

All-optical group velocity manipulation of light, so-called slow and fast light, has drawn extensive attention due to its potential applications in the fields of optical signal processing, light storage, light-matter interaction enhancement, hypersensitive sensing and temporal cloak. Tremendous approaches of slow and fast light generation have been widely demonstrated by the creation of the sharp spectral resonances in the past decade. Specifically, superluminal propagation (larger-than-light velocity c) through the optical media has been proposed and validated in anomalous dispersion media. One effective approach is employing stimulated Brillouin scattering to harness the narrowband spectral resonance and thus generate slow and fast light in the optical fiber. In particular, Brillouin lasing oscillation has been experimentally demonstrated to generate fast and superluminal propagation in ten-meter optical fibers. Afterwards, the propagating distance over hundreds of meters has been experimentally demonstrated by the suppression of multiple-longitudinal-mode operation. However, the delay-bandwidth product or the fractional advancement is still fundamentally limited in conventional fast and slow light system. For example, the maximum advancement as well as the fractional temporal advancement are practically restricted due to the Brillouin gain saturation and multiple longitudinal mode lasing operation under high pump power in the Brillouin lasing resonance-based fast and superluminal system. In this paper, we experimentally validate the extension of time advancement of superluminal propagation based on Brillouin lasing oscillation for the first time. By employing two stages of single-frequency Brillouin lasing fiber ring resonators, Stokes lasing resonance was effectively created in each fiber ring cavity to deliver Brillouin-induced loss resonance at the center frequency of pump, which essentially leads to fast light and superluminal propagation of the pump light. Consequently, Gaussian signal pulses with the pulse width of 500 ns experiences cascaded Brillouin-induced fast light and even negative group-velocity superluminal propagation. The temporal advancement was basically extended to 365.8 ns which is twice than that of single stage fast light and superluminal system. Correspondingly, the fractional advancement was increased from 0.36 to 0.73. The maximum group velocity was 6.3 times larger than the light speed c in vacuum while negative-group- velocity propagation can be found in the range of −9.9 to −0.2 by increasing the total input power from 593.2 to 812.0 mW. Furthermore, the group index can be manipulated in the range of 1.5 to −4.0 by simply varying the total input pump power. It indicates superluminal signals can be cascaded to essentially extend the propagation distance and enlarge the temporal advancement. The scheme exhibits robustness in cascading Brillouin spectral loss resonance for temporal advancement enhancement as well as propagating distance extension. More importantly, the cascading design could basically eliminate the fundamental limitation introduced by Brillouin gain saturation and multiple longitudinal lasing mode under high power operation in single stage Brillouin fast and superluminal system. Further optimization could also be implemented by utilizing the nonstandard specialty fibers as Brillouin gain medium to achieve an efficient Brillouin lasing resonance for an enhanced advancement efficiency. Additionally, the combination with the cascading technique and long-cavity single-frequency Brillouin lasing resonance could enable the feasibility of distance-limitless and large-advancement superluminal propagation, paving a way for promising applications in sensing and temporal cloak.

DOI
10.1360/n972017-00652

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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. 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Applied Physics Letters•Dec 16, 2013•15 citations
All-optical group velocity manipulation of light, so-called slow and fast light, has drawn extensive attention due to its potential applications in the fields of optical signal processing, light storage, light-matter interaction enhancement, hypersensitive sensing and temporal cloak. Tremendous approaches of slow and fast light generation have been widely demonstrated by the creation of the sharp spectral resonances in the past decade. Specifically, superluminal propagation (larger-than-light velocity c) through the optical media has been proposed and validated in anomalous dispersion media. One effective approach is employing stimulated Brillouin scattering to harness the narrowband spectral resonance and thus generate slow and fast light in the optical fiber. In particular, Brillouin lasing oscillation has been experimentally demonstrated to generate fast and superluminal propagation in ten-meter optical fibers. Afterwards, the propagating distance over hundreds of meters has been experimentally demonstrated by the suppression of multiple-longitudinal-mode operation. However, the delay-bandwidth product or the fractional advancement is still fundamentally limited in conventional fast and slow light system. For example, the maximum advancement as well as the fractional temporal advancement are practically restricted due to the Brillouin gain saturation and multiple longitudinal mode lasing operation under high pump power in the Brillouin lasing resonance-based fast and superluminal system. In this paper, we experimentally validate the extension of time advancement of superluminal propagation based on Brillouin lasing oscillation for the first time. By employing two stages of single-frequency Brillouin lasing fiber ring resonators, Stokes lasing resonance was effectively created in each fiber ring cavity to deliver Brillouin-induced loss resonance at the center frequency of pump, which essentially leads to fast light and superluminal propagation of the pump light. Consequently, Gaussian signal pulses with the pulse width of 500 ns experiences cascaded Brillouin-induced fast light and even negative group-velocity superluminal propagation. The temporal advancement was basically extended to 365.8 ns which is twice than that of single stage fast light and superluminal system. Correspondingly, the fractional advancement was increased from 0.36 to 0.73. The maximum group velocity was 6.3 times larger than the light speed c in vacuum while negative-group- velocity propagation can be found in the range of −9.9 to −0.2 by increasing the total input power from 593.2 to 812.0 mW. Furthermore, the group index can be manipulated in the range of 1.5 to −4.0 by simply varying the total input pump power. It indicates superluminal signals can be cascaded to essentially extend the propagation distance and enlarge the temporal advancement. The scheme exhibits robustness in cascading Brillouin spectral loss resonance for temporal advancement enhancement as well as propagating distance extension. More importantly, the cascading design could basically eliminate the fundamental limitation introduced by Brillouin gain saturation and multiple longitudinal lasing mode under high power operation in single stage Brillouin fast and superluminal system. Further optimization could also be implemented by utilizing the nonstandard specialty fibers as Brillouin gain medium to achieve an efficient Brillouin lasing resonance for an enhanced advancement efficiency. Additionally, the combination with the cascading technique and long-cavity single-frequency Brillouin lasing resonance could enable the feasibility of distance-limitless and large-advancement superluminal propagation, paving a way for promising applications in sensing and temporal cloak.
We have experimentally demonstrated superluminal propagation through a 500 m optical fiber using stimulated Brillouin scattering (SBS), which is the longest superluminal propagating distance reported so far, to the best of our knowledge. Brillouin-induced loss resonance for fast light generation of pump signals is achieved in a single frequency Brillouin lasing oscillator with a highly nonlinear fiber as the SBS medium. The single frequency operation is realized by embedding the saturable absorber fiber loop with an unpumped erbium-doped fiber. Consequently, signals with a pulse width of 710 ns experience a maximum advancement of 1330 ns. Furthermore, the effects of pulse width and the duty cycle of pump signals are also investigated.
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.
We demonstrated the superluminal propagation at negative group velocity using stimulated Brillouin scattering (SBS) in optical fibers. In the scheme, the Stokes wave is circulated back into the ring cavity for Brillouin lasing to enhance the peak loss. The experiment demonstrated the superluminal light propagation can be realized by increasing the input signal power. The largest time advancement of 221.2ns has been observed. Correspondingly, the negative group velocity is -0.15c and the group index is -6.636.
Superluminal propagation at negative group velocity was demonstrated in a highly nonlinear fiber embedded in a Brillouin laser ring cavity. A maximum advancement of 369 ns and strong Stokes lasing power of 482 mW were achieved when the cavity was pumped with a 1 MHz sinusoidal wave modulated signal at power level of 1 W. The frequency dependence of fast light in this fiber ring cavity was examined with modulation frequencies of 1 kHz to 15 MHz. a maximum fractional advancement of 0.54 was achieved at 10 kHz and a maximum negative group index of - 9480 was demonstrated at 1 kHz.
We report superluminal propagation in optical fibers using Brillouin lasing oscillation in a ring cavity. Negative group velocity propagation through a 10-m single mode fiber has been experimentally demonstrated. An advancement of 221.2 ns was observed before the input signal, which was achieved with a very high slope efficiency of 211.3 ns/dB. This indicates that this way is suitable for long-distance low-loss superluminal propagation via optical fibers. Correspondingly, the group velocity is -0.151c and the group index is -6.636-the highest group velocity ever reported for optical fibers.
Superluminal propagation at negative group velocity was demonstrated in a highly nonlinear fiber embedded in a stimulated Brillouin scattering laser ring cavity. A maximum advancement of 369 ns and strong Stokes lasing power of 482 mW were observed when the cavity was pumped with a 1 MHz sinusoidal wave modulated signal at the power level of 1 W. The frequency dependence of fast light in this fiber ring cavity was examined with modulation frequencies of 1 kHz to 15 MHz, a maximum fractional advancement of 0.54 was achieved at 10 kHz, and a maximum negative group index of −9480 was demonstrated at 1 kHz.