Slow Light in Optical Fibers Using Stimulated Brillouin Scattering

过去几年的研究已经确定，有可能对光脉冲在材料系统中的传播速度进行非凡的控制。当群速度比光在材料中的相速度小得多时，就可以说是慢光。慢光效应通常利用在材料共振附近发生的折射率的快速变化来延迟光脉冲。对于经典的共振跃迁，问题是这伴随着极大的光吸收。已经提出并使用了各种方案来克服这一点。其中，电磁感应透明可能是最知名的。对慢光的研究既涉及现象的基础科学，也涉及其应用，特别是在光通信中。通信网络的基本构建块是可以缓冲或延迟信息到达的设备。对于超高速操作，希望使用全光学装置，其中信息用脉冲编码，对于该脉冲，需要脉冲持续时间的几倍的脉冲延迟，而延迟脉冲的吸收和失真应该相当小。慢光为实现这一目标提供了一种手段。目前，对慢光的研究主要集中在寻找与光通信技术更兼容的方案。2005年，利用受激布里渊散射（SBS）在光纤中演示慢光，因此在世界范围内引发了相当大的活动。沿着其明显的设备兼容性，它是一个放大而不是吸收过程，因此损耗可以忽略不计;通过改变泵浦波长，可以在任何波长处产生慢光共振;光纤允许长的相互作用长度，因此激光器的功率低，该过程在室温下运行，并且简单易行。与此相反，它被认为是受~20-50 MHz的SBS均匀增益线宽的限制，这大大低于所需的>1GHz。此外，从经典SBS理论可以容易地表明，可实现的最大脉冲延迟为脉冲持续时间的~1.4，而光通信系统中所需的值在2-4范围内。我们提出的研究利用了光纤中SBS的波导诱导光谱展宽现象，我们在2000年8月首次报道，以克服当前SBS慢光系统的分辨率和脉冲持续时间的经典限制。正如我们所示，加宽是光纤中受激布里渊散射的一般性质，并且与光纤的数值孔径成比例。它在常规高数值孔径光纤中可以大到~ 1GHz，在光子晶体光纤中可以大到几GHz。这对SBS中的慢光的后果是关键的：首先，由于加宽的不均匀性质，斯托克斯信号的脉冲延迟与脉冲持续时间的比率不再受经典极限1.4的限制，其次，由于加宽是巨大的，光脉冲的持续时间可以大大减少到亚纳秒，提供多Gb/s的操作速率。我们将在实验和理论上研究这一点，在赫瑞瓦特的一系列实验中，使用市售的光纤和高NA光纤在巴斯制造。

Research of the last few years has established that it is possible to exercise extraordinary control of the velocity of propagation of light pulses through material systems. One speaks of slow light when the group velocity is much smaller than the phase velocity of the light in the material. Slow light effects usually make use of the rapid variation of refractive index that occur in the vicinity of a material resonance to delay the optical pulse. For classical resonant transitions the problem is that this is accompanied by extremely large absorption of the light. Various schemes have been proposed and used to overcome this. Of these, electromagnetically induced transparency is perhaps the most well known. Investigations of slow light concern both the basic science of the phenomenon and its applications, particularly in optical communications. A fundamental building block of communications networks is a device that can buffer or delay the arrival of information. For ultra-high speed operation it is desirable to use all-optical devices where information is encoded with pulses for which pulse delays of a few times the pulse duration are required while absorption and distortion of the delayed pulse should be reasonably small. Slow light offers a means to this goal. Research on slow light is currently focused to the search for schemes which are more compatible with optical communications technology. The demonstration, in 2005, of slow light in optical fibre using stimulated Brillouin scattering (SBS) has for this reason sparked considerable activity world-wide. Along with its obvious device compatibility, it is an amplification rather than an absorption process so losses are negligible; the slow-light resonance can be created at any wavelength by changing the pump wavelength; optical fibre allows long interaction length and thus low power for the laser, the process runs at room temperature and is simple and easy to handle. Against this it is thought to be limited by the SBS homogeneous gain linewidth of ~20-50 MHz, which is considerably lower then the >1GHz required. Further it may be readily shown from classical SBS theory that the maximum pulse delay achievable is ~1.4 of the pulse duration, while the value needed in optical communication systems is in the 2-4 range.Our proposed research exploits the phenomenon of waveguide-induced spectral broadening of SBS in optical fibre, we first reported in August 2000, to overcome the classical limit to the resolution and pulse duration of current SBS slow light systems. As we have shown, the broadening is a generic property of stimulated Brillouin scattering in optical fibre and scales with the numerical aperture of the fibre. It may be as great as ~1 GHz in conventional high numerical aperture fibre and multi-GHz in photonic crystal fibres. The consequences of this to the slow light in SBS are critical: firstly because of inhomogeneous nature of the broadening the ratio of the pulse delay to the pulse duration for the Stokes signal is no longer restricted by the classical limit of 1.4, and secondly, because of the broadening is massive the duration of the optical pulses may be greatly reduced, to sub-nanoseconds, providing multi-Gb/s operational rate. We will investigate this experimentally and theoretically with a range of experiments at Heriot-Watt, using both commercially-available optical fibres and high NA fibres fabricated at Bath.