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Optics Letters

Bandwidth reduction in a multistage Brillouin system

Stefan Preußler, Thomas Schneider

Published Sep 28, 2012• 51 citations
Publisher

Abstract

We investigate the gain bandwidth of stimulated Brillouin scattering (SBS) in multistage systems. As we will show, the bandwidth is reduced significantly by the number of stages, which can increase the distortions in cascaded slow-light systems, for instance. However, other applications, which are limited by the minimum bandwidth achievable with SBS, can benefit from the reduced bandwidth. Since the peak value of the SBS gain spectrum is not reduced by this method, the reduced bandwidth can drastically enhance the performance of many different applications, such as the quasi-light storage (QLS). The minimum bandwidth of 10.3 MHz for a one-stage system was reduced down to 5.8 MHz for three stages. With this reduced bandwidth, we achieved a storage time advancement of the QLS method by 60%.

DOI
10.1364/ol.37.004122

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Increasing the storage time of Quasi-light storage by reducing the Brillouin gain bandwidth in a feedback system

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Different Slow-light systems have been discussed and developed as a powerful tool to reduce the group velocity of optical signals during the last years. It has enjoyed much recent interest because of the potential system applications in optical telecommunication and information systems. Especially the nonlinear effect of stimulated Brillouin scattering (SBS) has become important for fiber based Slow-light systems. But, the SBS also has two challenges. First, the maximum time delay is limited due to the saturation of the Brillouin amplification process and second, the delay of the pulses is accompanied by pulse distortions (pulse shape broadening). For these aspects the shape of the Brillouin spectrum is of special interest. In this article a system is presented which allows the optimisation of the Brillouin spectrum. Simulations and first measurements show how the time delay can be enhanced with a simultaneous reduction of the pulse distortions.

Irish Signals and Systems Conference•Jan 1, 2008•3 citations
We investigate the gain bandwidth of stimulated Brillouin scattering (SBS) in multistage systems. As we will show, the bandwidth is reduced significantly by the number of stages, which can increase the distortions in cascaded slow-light systems, for instance. However, other applications, which are limited by the minimum bandwidth achievable with SBS, can benefit from the reduced bandwidth. Since the peak value of the SBS gain spectrum is not reduced by this method, the reduced bandwidth can drastically enhance the performance of many different applications, such as the quasi-light storage (QLS). The minimum bandwidth of 10.3 MHz for a one-stage system was reduced down to 5.8 MHz for three stages. With this reduced bandwidth, we achieved a storage time advancement of the QLS method by 60%.
The core elements of optical packet-switched networks rely on optical routers. A key technology for the realization of such components is the variable delay and storage of optical data packets. Optical Memories are used for tasks, such as packet buffering for collision avoidance and synchronization, as well as processing of headers in optical routers. The quasi-light storage (QLS) is a method for the variable and almost distortion free optical data storage, which can be realized within the entire transparency range of optical fibers with standard telecommunication components. QLS is based on the time-frequency-coherence and utilizes stimulated Brillouin scattering (SBS) for filtering the spectrum of the data signal. Due to the natural bandwidth of the SBS, the maximum storage time of the QLS is limited to 100 ns. By reducing the bandwidth, the storage time can be significantly increased. There exist some approaches for reducing the SBS bandwidth. Firstly, the Brillouin gain can be superimposed with two losses and secondly, an aperture in the frequency range can be used. Both methods achieve a SBS bandwidth of about 3 MHz, but have some disadvantages when applied to the QLS. For the superposition method, the overall gain is too low, and for the other method the aperture cannot be separated from the signal. In this paper we show the reduction of the Brillouin gain bandwidth through a feedback system. The sampled signal passes several times through the system, so that the Brillouin gain bandwidth is reduced. This leads to a drastic increase of the QLS storage time. In a first experiment with single pulses, a maximum storage time of 600 ns was achieved, which corresponds to a sixfold increase compared to a normal system QLS.
The manipulation of the group velocity of optical signals (slow and fast light) has many possible applications in telecommunications, sensing, nonlinear optics, and so on. The exploitation of the effect of stimulated Brillouin scattering (SBS) is of special interest for the alteration of the group velocity in optical fibers, since it just requires standard telecom equipment and low-to-moderate optical power. However, each delay in one single, low-gain SBS-based slow-light system is accompanied by a pulse broadening. This is a result of the inherent Kramers–Kronig relations among the gain, the phase change, and the accompanied group velocity. For an ideal flat gain, the phase change is nonideal, and for an ideal phase change the gain curve leads to a broadening. Furthermore, if the gain bandwidth is adapted to the signal by broadening, the delay will be reduced. Thus the signal broadening can be reduced by several methods for one single low-gain slow-light system, but it cannot be zero. Here, we will show how a zero-broadening SBS-based slow light can be achieved with several methods. The basic idea is a reshaping of the original pulse in a second stage by saturation of the SBS system, or by the superposition of two gains. As will be shown, all of these systems show an almost ideal overall gain and phase function over the bandwidth of the pulses. Thus, SBS-based slow light with a delay bandwidth product of 1 bit and zero distortion is possible.
We show a simple method to reduce the distortions in SBS based slow light systems. The distortion reduction is simply based on a broadening and adaptation of the gain bandwidth. However, a broadened gain reduces the achievable fractional delay which cannot be compensated by higher pump powers. Here we will show that this compensation can be done by additional loss spectra. With the presented method low distortions for high fractional pulse delays are possible. We show the theory and experimental verifications of our method. For Gaussian pulses with a fractional delay of 1 Bit we achieved a distortion reduction of around 23%.
To change the group velocity of optical signals has a lot of possible applications in telecommunications, sensing, nonlinear optics and so on. Especially the exploitation of the effect of stimulated Brillouin scattering (SBS) in optical fibers is of special interest since it just requires standard telecom equipment and low to moderate optical power. However, each delay in one single, low-gain SBS based slow-light system is accompanied by pulse broadening. This is a result of the inherent Kramers-Kronig relations between the gain, the phase-change and the accompanied group velocity. For an ideal flat gain the phase-change is non-ideal, and for an ideal phase-change the gain curve leads to a broadening. Furthermore, if the gain bandwidth is broadened in order to adapt it to the signal, the delay will be reduced. Thus, for one single low-gain slow-light system the broadening can be reduced by several methods but it cannot be zero. Here we will show how a zero-broadening SBS based slow-light system can be achieved by two different methods. The basic idea is a reshaping of the original pulse by an adapted gain in a second stage. This adaptation is achieved by the superposition of two Gaussian gain profiles or by a single saturated gain. As will be shown, these systems show an almost ideal over-all gain and phase function over the bandwidth of the pulses. Thus, SBS based slow-light with a delaybandwidth product of more than 1 bit and zero distortion is possible.
Different Slow-light systems have been discussed and developed as a powerful tool to reduce the group velocity of optical signals during the last years. It has enjoyed much recent interest because of the potential system applications in optical telecommunication and information systems. Especially the nonlinear effect of stimulated Brillouin scattering (SBS) has become important for fiber based Slow-light systems. But, the SBS also has two challenges. First, the maximum time delay is limited due to the saturation of the Brillouin amplification process and second, the delay of the pulses is accompanied by pulse distortions (pulse shape broadening). For these aspects the shape of the Brillouin spectrum is of special interest. In this article a system is presented which allows the optimisation of the Brillouin spectrum. Simulations and first measurements show how the time delay can be enhanced with a simultaneous reduction of the pulse distortions.