The ubiquitous role of optical fibres in modern photonic systems has stimulated research to realize slow and fast light devices directly in this close-to-perfect transmission line. Recent progress in developing optically controlled delays in optical fibres, operating under normal environmental conditions and at telecommunication wavelengths, has paved the way towards real applications for slow and fast light. This review presents the state-of-the-art research in this fascinating field and possible outcomes in the near future.

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Slow and fast light in optical fibres

Recent research has established that it is possible to exercise extraordinary control of the velocity of propagation of light pulses through a material system. Both extremely slow propagation (much slower than the velocity of light in vacuum) and fast propagation (exceeding the velocity of light in vacuum) have been observed. This article summarizes this recent research, placing special emphasis on the description of the underlying physical processes leading to the modification of the velocity of light. To understand these new results, it is crucial to recall the distinction between the phase velocity and the group velocity of a light field. These concepts will be defined more precisely below; for the present we note that the group velocity gives the velocity with which a pulse of light propagates through a material system. One thus speaks of “fast” or “slow” light depending on the value of the group velocity vg in comparison to the velocity of light c in vacuum.

Slow and Fast Light

A recent renaissance in Brillouin scattering research has been driven by the increasing maturity of photonic integration platforms and nanophotonics. The result is a new breed of chip-based devices that exploit acousto-optic interactions to create lasers, amplifiers, filters, delay lines and isolators. Here, we provide a detailed overview of Brillouin scattering in integrated waveguides and resonators, covering key concepts such as the stimulation of the Brillouin process, in which the optical field itself induces acoustic vibrations, the importance of acoustic confinement, methods for calculating and measuring Brillouin gain, and the diversity of materials platforms and geometries. Our Review emphasizes emerging applications in microwave photonics, signal processing and sensing, and concludes with a perspective for future directions. Acousto-optical interactions within integrated optics platforms are reviewed with a discussion of the useful chip-based devices such as lasers, amplifiers, filters, isolators and more besides that can result.

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Brillouin integrated photonics

Sinc-shaped Nyquist pulses possess a rectangular spectrum, enabling data to be encoded in a minimum spectral bandwidth and satisfying by essence the Nyquist criterion of zero inter-symbol interference (ISI). This property makes them very attractive for communication systems since data transmission rates can be maximized while the bandwidth usage is minimized. However, most of the pulse-shaping methods reported so far have remained rather complex and none has led to ideal sinc pulses. Here a method to produce sinc-shaped Nyquist pulses of very high quality is proposed based on the direct synthesis of a rectangular-shaped and phase-locked frequency comb. The method is highly flexible and can be easily integrated in communication systems, potentially offering a substantial increase in data transmission rates. Further, the high quality and wide tunability of the reported sinc-shaped pulses can also bring benefits to many other fields, such as microwave photonics, light storage and all-optical sampling.

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Optical sinc-shaped Nyquist pulses of exceptional quality

"Slow and fast light" constitute a broad class of
science and technology that
 can dramatically change the group
index of a medium over a certain wavelength
 range. This thesis is
composed of studies regarding both fundamental aspects and
applications of slow light.
 The thesis starts with some discussion
on two fundamental questions. The first
 one is how much momentum a
photon carries within a slow-light medium, and what
 kind of force
is experienced by a slow-light medium when a photon enters or
leaves it.
 The second issue is how the noise properties of an
optical field change as it propagates
 through a slow-light
medium.
 The second part of the thesis deals with the applications
of slow light for tunable
 time delays. For such applications, one
of the key figures of merit is the maximum
 fractional delay that a
slow-light element can achieve. I first present a method with
experimental demonstrations for improving the maximum fractional
delay using a
 multiple-gain-line medium. Second, I present a
design with experimental demonstration for how to achieve
simultaneous tunable delay and advancement using slow and
 fast
light in a single module. I then propose a design of a digitally
tunable module
 using channelized slow light, which can be useful
for optical packet delays, etc.
 The third part of the thesis
studies the use of slow light to enhance the performance of
spectroscopic interferometers. I start with the derivation of the
spectral
 sensitivity of two-beam and multiple-beam interferometers
with slow-light media incorporated in them. I show both
theoretically and experimentally that the spectral
 sensitivity is
proportional to the group index of the medium inside the
interferometers.
 Second, I propose and demonstrate experimentally
a new type of Fourier-transform
 interferometer using tunable slow
light. I then analyze the performance of three types
 of slow-light
media for interferometry applications. Lastly, I present a design
of an on-chip slow-light spectrometer as well as some studies on
slow-light waveguides using
 photonic crystal
structures.

Fundamentals and Applications of Slow Light

We show a method for distortion-free quasi storage of light which is based on the coherence between the spectrum and the time representation of pulse sequences. The whole system can be considered as a black box that stores the light until it will be extracted. In the experiment we delayed several 5 bit patterns with bit durations of 500ps up to 38ns. The delay can be tuned in fine and coarse range. The method works in the entire transparency range of optical fibers and only uses standard components of optical telecommunications. Hence, it can easily be integrated into existing systems.

Quasi-Light-Storage based on time-frequency coherence.

We compare two simple mechanisms for the enhancement of the time delay in slow light systems. Both are based on the superposition of the Brillouin gain with additional loss. As we will show in theory and experiment if two losses are placed at the wings of a SBS gain, contrary to other methods, the loss power increases the time delay. This leads to higher delay times at lower optical powers and to an increase of the zero gain delay of more than 50%. With this method we achieved a time delay of more than 120ns for pulses with a temporal width of 30ns. To the best of our knowledge, this is the highest time delay in just one fiber spool. Beside the enhancement of the time delay the method could have the potential to decrease the pulse distortions for high bit rate signals.

Comparison of delay enhancement mechanisms for SBS-based slow light systems

A technique for the distortion reduction of delayed pulses in single and cascaded slow light delay lines is demonstrated. The method is based on the overlap of different independent Brillouin gains. With three Brillouin lines a distortion reduction of around 30% in a two-segment delay line was achieved.

Distortion reduction in cascaded slow light delays

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 1Bit we achieved a distortion reduction of around 23%.

Distortion reduction in Slow Light systems based on stimulated Brillouin scattering.

We show in theory and experiment that in a SBS based delay line pulses can be delayed to more than a bit period without broadening. Zero-broadening is possible since the broadening due to the narrow Brillouin gain bandwidth can be compensated by the group velocity dispersion accompanied with the pulse delay. We achieve compensation by a superposition of a broad gain with two narrow losses at its wings. In our experiments 1.9ns pulses were delayed to around 1.5Bit, at the same time its FWHM width was compressed to 80%. Therefore, besides slowing down pulses, the method could have the potential to compensate the fiber dispersion.

Ronny Henker

Papers

Quasi-Light-Storage based on time-frequency coherence.

Comparison of delay enhancement mechanisms for SBS-based slow light systems

Distortion reduction in cascaded slow light delays

Distortion reduction in Slow Light systems based on stimulated Brillouin scattering.

Zero-broadening and pulse compression slow light in an optical fiber at high pulse delays.