Slow- and Fast-Light has been investigated during the last years as a powerful tool to reduce and increase the group velocity of light pulses by light. It has enjoyed much recent interest because of the practical applications for telecommunication and information systems. Among these are optical signal processing, radio frequency-photonics, nonlinear optics and time domain spectroscopy. Furthermore, it can be seen as a key technology for optical delay lines, buffers, equalizers and synchronizers in packet switched networks. For a realization, there are different methods and material systems possible. Among them the nonlinear effect of stimulated Brillouin scattering (SBS) is of special interest. This article gives an overview about the fundamentals and some experimental results of the Slow- and Fast-Light effect based on the SBS in optical fibers.
Slow- and Fast-Light has been investigated during the last years as a powerful tool to reduce and increase the group velocity of light pulses by light. It has enjoyed much recent interest because of the practical applications for telecommunication and information systems. Among these are optical signal processing, radio frequency-photonics, nonlinear optics and time domain spectroscopy. Furthermore, it can be seen as a key technology for optical delay lines, buffers, equalizers and synchronizers in packet switched networks. For a realization, there are different methods and material systems possible. Among them the nonlinear effect of stimulated Brillouin scattering (SBS) is of special interest. This article gives an overview about the fundamentals and some experimental results of the Slow- and Fast-Light effect based on the SBS in optical fibers.
Slow- and fast-light is the control of the velocity of light in a medium by light. As a fascinating new field in physics there is a fundamental interest on this effect on the one side, but on the other side there exist a lot of practical applications for telecommunication and information systems. Among these are optical signal processing, the radio frequency-photonics, nonlinear optics and spectroscopy in time domain. Furthermore, the slow- and fast-light effect can be seen as a key technology for optical delay lines, buffers, equalizers and synchronizers in packed switched networks. To realize the effect there are different methods and material systems possible. Beside these especially the nonlinear effect of stimulated Brillouin scattering (SBS) is of special interest because it has several advantages. This article gives an overview about the fundamentals and limits of the slow-and fast-light effect in general and based on the SBS in optical fibers. Some experimental results which were achieved so far are shown.
In today's information age demand for ultra-fast information transfer with ultra-high bandwidths has reached extraordinary levels. Hence, the transmission in the future internet-backbone will be increasingly constrained in the network nodes. At the same time, the power consumption of the network systems will increase to unsustainable levels.\nNowadays, optical signal processing and switching can be implemented relatively easily. However, the realization of optical bu ers and short-term memories is still an unsolved challenge. The slow- and fast-light e ect has been investigated as one solution for the optical bu ering over the last few years. It means the slowing down and acceleration of the group velocity of light pulses in a medium. To realize this, many di erent methods and material systems have been developed but due to its signi cant advantages the nonlinear e ect of stimulated Brillouin scattering (SBS) is particularly promising. However, it also su ers from disadvantages which limit the slow- and fast-light performance.
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.
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.
Frontiers in Guided Wave Optics and Optoelectronics 84 absorption is much too high to render the group effect useful.An exciting breakthrough happened in the early nineties when it was shown that group velocities of few tens of meters per second were possible with nonlinear resonance interactions (Hau et al., 1999).Two important features of nonlinear resonances make this possible: substantially reduced absorption, or even amplification, of radiation at a resonance, and sharpness of such resonances; the sharper a resonance, the higher dn/dω and so the stronger the enhancement of group index, and hence the greater the pulse is delayed.Widely ranging applications for slow light have been proposed, of which those for telecommunication systems and devices (optical delay lines, optical buffers, optical equalizers and signal processors) are currently of most interest (Gauthier, 2005).The essential demand of such devices is compatibility with existing telecommunication systems, that is they must be of wide enough bandwidth (≥10 GHz) and able to be integrated seamlessly into such systems.Of the various nonlinear resonance mechanisms and media, which allow sufficiently long induced delays, stimulated Brillouin and Raman scattering (SBS and SRS) in optical fiber are deemed to be among the best candidates.Currently SBS is the most actively investigated and many experimental and theoretical papers on pulse delaying via SBS in optical fiber have been published in the last few years, see the review paper (Thevenaz, 2008) and references therein.In this process the pulse to be delayed is a frequency down-shifted (Stokes) pulse.This is transmitted through an optical fiber through which continuous wave (CW) pump radiation is sent in the opposite direction to prime the delay process.It is supposed that the Stokes pulse is amplified by parametric coupling with the pump wave and a material (acoustic) wave in the medium (Kroll, 1965), and the amplification is characterised by a resonant-type gain profile.The dispersion of refractive index associated with this profile (which is similar to that in Fig. 1) can then be used to increase the group index for optical pulses at the Stokes frequency (Zeldovich, 1972).Along with obvious device compatibility, there are several other advantages of the SL via SBS approach for optical communications systems: slow-light resonance can be created at any wavelength by changing the pump wavelength; use of optical fibre allows for long interaction lengths and thus low powers for the pump radiation, the process runs at room temperature, it uses off the shelf telecom equipment, and SBS works in the entire transparency range of fibers and in all types of fiber.Currently a main obstacle to applications of this approach is the narrow SBS gain spectral bandwidth, (Thevenaz, 2008), which is typically ≈ 120-200 MHz in silica fiber in the spectral range of telecom optical radiation (~1.3-1.6 μm) (Agrawal, 2006).This chapter reviews our ongoing work on the physical mechanisms that give rise to pulse delay in SBS.In section 2 the theoretical background of the SBS phenomenon is given and the main working equations describing this nonlinear interaction are presented.In section 3 ways by which the SBS spectral bandwidth may be increased are addressed.Waveguide induced spectral broadening of SBS in optical fibre is considered as a means of increasing the bandwidth to the multi-GHz range.An alternative way widely discussed in the literature, (Thevenaz, 2008), is based on spectral broadening of the pump radiation.However it is shown through analytic analysis of the SBS equations converted to the frequency domain that pump radiation broadening by any reasonable amount has only a negligible effect on increasing the SBS bandwidth.Importantly in this section we show that, irrespective of the nature of the broadening considered, the SBS gain bandwidth remains www.intechopen.
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.