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Journal of Lightwave Technology

Quasi-Light Storage: A Method for the Tunable Storage of Optical Packets With a Potential Delay-Bandwidth Product of Several Thousand Bits

Thomas Schneider, Kambiz Jamshidi, Stefan Preußler

Published Jul 19, 2010• 31 citations
Publisher

Abstract

We investigate and discuss the possibilities and limits of a method for the tunable storage of optical packets called Quasi-Light Storage (QLS). The QLS is simply based on a sampling in the frequency domain. Hence, it is independent of the data rate, the modulation and the wavelength of the optical packets. It works at room temperatures in standard fibers with off-the-shelf telecom equipment. As we will show in theory, the method has the potential to store packets with several thousand Bits of data from Zero to up to 100 ns. In the experiments we have stored several 8 Bit packets with a data rate of 1 Gbps for up to 100 ns, which corresponds to a delay-bandwidth product of 100 Bit.

DOI
10.1109/jlt.2010.2056355

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We investigate and discuss the possibilities and limits of a method for the tunable storage of optical packets called Quasi-Light Storage (QLS). The QLS is simply based on a sampling in the frequency domain. Hence, it is independent of the data rate, the modulation and the wavelength of the optical packets. It works at room temperatures in standard fibers with off-the-shelf telecom equipment. As we will show in theory, the method has the potential to store packets with several thousand Bits of data from Zero to up to 100 ns. In the experiments we have stored several 8 Bit packets with a data rate of 1 Gbps for up to 100 ns, which corresponds to a delay-bandwidth product of 100 Bit.
In this paper, we compare two methods to delay optical signals based on quasi-light storage (QLS). These methods are very simple and offer high-tunable storage times up to several thousand bits. Furthermore, they are transparent to the bit rate and modulation format of the signal. We explore the potentials and limitations of both methods. Fiber-based QLS is useful when high absolute delay is required and no loss or very small amount of attenuation can be tolerated. Frequency-to-time-conversion-based QLS can be used for the delay of very high bandwidth signals.
An all-optical telecommunications network requires all-optical routers. These routers would be able to process optical signals directly, without the transformation to the electrical domain. Thus, optical routers have the potential to accelerate telecommunications, i.e. to enhance the data rate and to minimize the latency times. Furthermore, since the double transformation at each network node can be omitted, they could be able to reduce the energy consumption of telecommunications. Key elements of optical routers are optical buffers, which are able to store the optical packets prior to processing in order to avoid congestion. Therefore, many different concepts for the realization of optical buffers have been presented so far. Most of these can just store or delay on-off keyed (OOK) signals. However, in optical telecommunications advanced modulation formats will be increasingly used because they are able to improve the transmission properties and to take advantage of the capacity of optical fibers. Thus, advanced modulation formats can drastically increase the transmissible data rates. Most of these advanced formats modulate the carrier’s phase or are a combination of amplitude and phase modulation. Here the storage of amplitude and phase modulated data packets with the so called quasi-light-storage (QLS) method will be shown. The QLS relies on the time-bandwidth coherence of each signal. Thus, as long as the sampling theorem in the frequency domain is not violated, a distortion-free storage of optical data packets with a delay-bandwidth product (DBP) of several thousand bits is possible. Here we will discuss the applicability of the QLS method to advanced modulation formats and we show experimental results of the tunable alloptical storage of OOK modulated signals with a DBP of 160 bit and the storage of phase shift keyed signals with a DBP of 60 bits.
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.
Today's telecommunication is based on optical packets which transmit the information in optical fiber networks around the world. Currently, the processing of the signals is done in the electrical domain. Direct storage in the optical domain would avoid the transfer of the packets to the electrical and back to the optical domain in every network node and, therefore, increase the speed and possibly reduce the energy consumption of telecommunications. However, light consists of photons which propagate with the speed of light in vacuum. Thus, the storage of light is a big challenge. There exist some methods to slow down the speed of the light, or to store it in excitations of a medium. However, these methods cannot be used for the storage of optical data packets used in telecommunications networks. Here we show how the time-frequency-coherence, which holds for every signal and therefore for optical packets as well, can be exploited to build an optical memory. We will review the background and show in detail and through examples, how a frequency comb can be used for the copying of an optical packet which enters the memory. One of these time domain copies is then extracted from the memory by a time domain switch. We will show this method for intensity as well as for phase modulated signals.