1. Ultrahigh-speed optical soliton transmission and nonlinear optics in optical fibers

What is a "soliton"?

The chromatic dispersion of optical fibers broadens optical pulses, and this means that signals may not be detected correctly due to dispersion-induced inter-bit interference, thus limiting transmission capacity and distance. However, pulse compression due to fiber nonlinearity (Kerr effect) is found to balance dispersion-induced pulse broadening. Optical fibers support the propagation of a stable optical pulse referred to as a "soliton" without any change in the waveform even in the presence of dispersion and nonlinearity (see Fig. 1). We take advantage of this phenomenon and use solitons as an information carrier in long distance high-speed optical transmissions.

We carry out research designed to improve soliton-based optical transmission. This research includes increasing the channel bit rate to 100 Gbit/s and beyond (with a speed of over 100 Gbit/s, it takes just a few seconds to transmit all the data in a commercial hard disk).

Fig. 1 Waveform evolution of optical pulse signals during optical propagation.

Evolution of soliton transmission systems

As a result of a number of technical advances such as the development of lasers, fibers, and optical amplifiers, optical solitons have been evolving in the direction of optical transmission applications since 1973 when solitons in optical fibers were first discovered. In the 70's and 80's, the transmission of "ideal solitons" was considered in which solitons propagate in optical fibers with constant dispersion and nonlinearity values and where their amplitude and width remain identical over propagation distance. However, optical fibers exhibit loss, which reduces nonlinearity. This breaks the balance between dispersion and nonlinearity, preventing soliton propagation over long distances.

By using erbium doped fiber amplifiers (EDFAs), which were developed in the late 80's to 90's, stable soliton propagation was found to be possible in transmission lines where the fiber nonlinearity (and thus the soliton amplitude) varies rapidly due to loss and lumped amplification. This type of soliton, namely a soliton propagating in a fiber with constant dispersion in the presence of a nonadiabatic variation in amplitude with its pulse width invariant, is called a "dynamic soliton" or "averaged soliton".

Since the late 90's, research interest has focused on soliton transmission in a fiber with periodically varying nonlinearity as well as dispersion. In such a fiber, both soliton amplitude and pulse width vary, yet the original pulse shape is recovered precisely at every period. This periodically stationary pulse can be considered as a soliton in a broad sense and is referred to as a "dispersion managed soliton". Interestingly dispersion managed solitons have superior properties to conventional solitons in terms of practical communication applications. These properties include a better tolerance to power margin and dispersion variation. Research on dispersion managed solitons has accelerated the application of optical solitons to real communication systems and enabled soliton venture businesses such as Algety, Solstice, and Corvis to develop in the U.S. and Europe.

Fig. 2 Historical evolution of soliton communication.

Applications of soliton technology

Optical solitons are applicable not only to communication but also to laser sources and all-optical signal processing. For instance, a soliton has a uniform temporal phase everywhere within the pulse. This property may be beneficial for all-optical switching based on optical interference. Furthermore the stability of solitons makes it possible to equalize the waveform automatically without the help of electronics and thus leads to an all-optical 3R (re-amplification, reshaping, and retiming) regenerator. These applications of solitons to all-optical signal processing, which will play an important role in ultrafast photonic networks, are another research target of our group.