CUDOS@USyd Research

Optical Waveguide Gratings and Slow Light

Irena Kabakova, Falk Eilenberger, Neil Baker, Martijn de Sterke, Benjamin Eggleton

Optical Grating Writing
Many of the nonlinear materials exploited in CUDOS exhibit a degree of photosensitivity, allowing the modification of the refractive index using lasers. By interfering light in complex ways, detailed resonant structures can be created in waveguides and fibres, which underpin many of the experimental programs in CUDOS. We have established a facility to produce complex grating structures in optical fibres at 1 µm as well as 1.5 µm. These have been used as filters and complex dispersion compensators for nonlinear experiments as well as specialized long gratings for gap soliton studies. In addition, value added grating based devices have been demonstrated, including the variable bandwidth dispersionless filter for reconfigurable networks. More recently, we have demonstrated strong gratings in chlacogenide waveguides. The high nonlinearity and high refractive index modulation achievable in this material (dn/n=1%) open up the possibility of fully integrated waveguide components, such as photonic switches or optical regenerators on a chip.

Figure 1. A Sagnac based interefometer was used to write a Bragg resonance into a chalcogenide waveguide with FWHM ~10nm.

Figure 2. Apodised fibre Moire grating 120mm - Index modulation made visible via sidescan spectroscopy

Slow light in optical gratings
Even though the phase velocity of light is more or less fixed, the group velocity can vary widely. The group velocity (dw/dk) determines the time it takes an optical pulse to travel through an optical waveguide is. In dispersive media like gratings and photonic crystals the group velocity can become much less than the phase velocity c/n, so the transit time of the pulse in the waveguide stretches proportionally. If the range of frequencies over which this dispersive behaviour exists encompasses the spectrum of the pulse and the group velocity is constant, then the pulse will be slowed with minimal distortion in its shape – hence the term “slow light”. Slow light is of fundamental interest and may also be useful in applications such as optical delay lines and ultra-low threshold optical devices.

The ability to control the group velocity of light, the velocity at which the energy travels, has important applications in all-optical network routers and other devices where buffereing is required. In these applications it is important that the light can be delayed in time by a tunable amount longer than the pulse length. Currently this is done electronically, but electronics will not be able to deal with the short pulses used in the next generation of networks.

Slow Gap Solitons
In applying grating based slow light to the problem of optical bufferes, one of the problems faced is that group velocity in general is not constant and so pulse distortion occurs. To address this, we have developed a slow-light approach based on the excitation of temporal gap solitons (Fig 1.), which are intense pulses that can propagate inside the bandgap of a nonlinear periodic structure. Pulses are reflected at low incident intensities, but are transmitted at intensities above a certain threshold, with the transmitted pulse travelling at low group velocity. Unlike linear systems, slow light by exciting gap solitons is not limited by the delay-bandwidth trade-off imposed by the Kramers-Kroning relation.

Fig. 3 Experimental setup for demonstrating temporal gap solitors within a fiber Bragg grating. Grating resonance can be tuned by moving one stage.

An apodized fibre Bragg grating is used as the medium to observe such behaviour. Experiments show a nonlinear transmission of the device, which is characteristic of gap soliton formation (Fig. 2). Pulse measurement reveals that the transmitted pulse is delayed by as much as 1.6 ns, corresponding to a group velocity of 0.23 c/n (Fig. 3). Results also show that one can achieve some tunability in the delay by controlling the incident intensity.

Fig. 4 Observation of nonlinear transmission for different offsets of the grating bandgap.

Fig. 5 Measured behaviour of the slow light progation for different pulse powers. (red dotted line is when the resonance is detuned from the pulse while the blue solid line is when the pulse is slowed by the resonance)

Publications

Authorised by: Prof. Ben Eggleton
Maintained by: Bill Corcoran
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