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Proceedings of the Japan Academy Series B

Dispersion-controlled slow light in photonic crystal waveguides

Toshihiko Baba, Jun ADACHI, Norihiro Ishikura, Yohei Hamachi, Hirokazu Sasaki, Takashi KAWASAKI, Daisuke MORI

Published Jan 1, 2009• 28 citations
Publisher

Abstract

Slow light with a markedly low group velocity is a promising solution for optical buffering and advanced time-domain optical signal processing. It is also anticipated to enhance linear and nonlinear effects and so miniaturize functional photonic devices because slow light compresses optical energy in space. Photonic crystal waveguide devices generate on-chip slow light at room temperature with a wide bandwidth and low dispersion suitable for short pulse transmission. This paper first explains the delay-bandwidth product, fractional delay, and tunability as crucial criteria for buffering capacity of slow light devices. Then the paper describes experimental observations of slow light pulse, exhibiting their record high values. It also demonstrates the nonlinear enhancement based on slow light pulse transmission.

DOI
10.2183/pjab.85.443

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Slow light with a markedly low group velocity is a promising solution for optical buffering and advanced time-domain optical signal processing. It is also anticipated to enhance linear and nonlinear effects and so miniaturize functional photonic devices because slow light compresses optical energy in space. Photonic crystal waveguide devices generate on-chip slow light at room temperature with a wide bandwidth and low dispersion suitable for short pulse transmission. This paper first explains the delay-bandwidth product, fractional delay, and tunability as crucial criteria for buffering capacity of slow light devices. Then the paper describes experimental observations of slow light pulse, exhibiting their record high values. It also demonstrates the nonlinear enhancement based on slow light pulse transmission.
Slow light with a markedly low group velocity is a promising solution for optical buffering and advanced time-domain optical signal processing. It is also anticipated to enhance linear and nonlinear effects and so miniaturize functional photonic devices because slow light compresses optical energy in space. Photonic crystal waveguide devices generate on-chip slow light at room temperature with a wide bandwidth and low dispersion suitable for short pulse transmission. This paper first explains the delay-bandwidth product, fractional delay, and tunability as crucial criteria for buffering capacity of slow light devices. Then the paper describes experimental observations of slow light pulse, exhibiting their record high values. It also demonstrates the nonlinear enhancement based on slow light pulse transmission.
Slow light with a markedly low group velocity is a promising solution for optical buering and advanced time-domain optical signal processing.It is also anticipated to enhance linear and nonlinear eects and so miniaturize functional photonic devices because slow light compresses optical energy in space.Photonic crystal waveguide devices generate on-chip slow light at room temperature with a wide bandwidth and low dispersion suitable for short pulse transmission.This paper rst explains the delay-bandwidth product, fractional delay, and tunability as crucial criteria for buering capacity of slow light devices.Then the paper describes experimental observations of slow light pulse, exhibiting their record high values.It also demonstrates the nonlinear enhancement based on slow light pulse transmission.
Light showing extremely slow propagation (known as "slow light") can produce various effects such as compression of optical signals, buffering, convolution integral calculation, beam forming, enhancement of optical absorption, gain, non-linearity, and so on. To generate such light, very large material and structural dispersions are used. Photonic crystal waveguides are good candidates for many device applications since they can easily generate slow light at room temperature. This paper discusses the potential of slow light devices while explaining some critical issues, such as the delay-band product and the dispersion compensation.
This paper reports two advances in a slow light device consisting of chirped photonic crystal slab coupled waveguide on SOI substrate. One is concerning the delay-bandwidth product, indicating the buffering capacity of the device. We experimentally evaluated a record high value of 57 (a 40 ps delay and a 1.4 THz bandwidth). We also observed ~1 ps wide optical pulse transmission in the cross-correlation measurement. Regarding the pulse as a signal and considering the broadening of the pulse width due to the imperfect dispersion compensation in the device, storage of more than 12 signal bits was confirmed. The other is a wide-range tuning of the pulse delay. We propose a technique for externally controlling the chirping to permit variable delay. We demonstrate tuning of the pulse delay up to 23 ps, corresponding to a ~7 mm extension of the free space length.
Paradoxically, slow light promises to increase the speed of telecommunications in novel photonic structures, such as coupled resonators [1] and photonic crystals [2,3]. Apart from signal delays, the key consequence of slowing light down is the enhancement of light-matter interactions. Linear effects such as refractive index modulation scale linearly with slowdown in photonic crystals [3], and nonlinear effects are expected to scale with its square [4]. By directly observing the spatial compression of an optical pulse, by factor 25, we confirm the mechanism underlying this square scaling law. The key advantage of photonic structures over other slow light concepts is the potentially large bandwidth, which is crucial for telecommunications [5]. Nevertheless, the slow light previously observed in photonic crystals [2,3,6,7] has been very dispersive and featured narrow bandwidth. We demonstrate slow light with a bandwidth of 2.5 THz and a delay-bandwidth product of 30, which is an order of magnitude larger than any reported so far.
We demonstrate ultrafast delay tuning of a slow-light pulse with a response time <10 ps. This is achieved using two types of slow light: dispersion-compensated slow light for the signal pulse, and low-dispersion slow light to enhance nonlinear effects of the control pulse. These two types of slow light are generated simultaneously in Si lattice-shifted photonic crystal waveguides, arising from flat and straight photonic bands, respectively. The control pulse blueshifts the signal pulse spectrum, through dynamic tuning caused by the plasma effect of two-photon-absorption-induced carriers. This changes the delay by up to 10 ps only when the two pulses overlap within the waveguide and enables ultrafast tuning that is not limited by the carrier lifetime. Using this, we succeeded in tuning the delay of one target pulse within a pulse train with 12 ps intervals.