Slow Light
Latest Research News, July 2007
Slow light in Fiber Bragg gratings without broadening
We have observed the excitation of gap solitons in a 30 cm silica fibre Bragg grating using 0.68 ns pulses, which emerge with a tunable delay of up to 3.2 ns, or almost 5 pulse widths, without broadening. These longer delays (x2) take advantage of the longer Bragg gratings (x3) with a higher index contrast (x2), as they could be fabricated in collaboration with Southampton University.
Figure 1: Transmitted (solid) and reference (dashed) pulse at various input peak powers.
Slow light switching in nonlinear Bragg-grating couplers
We have theoretically demonstrated switching of slow-light pulses in nonlinear couplers with phase-shifted Bragg gratings [1]. At low powers, a periodic tunnelling of the slow light pulses between two shifted Bragg grating waveguides is observed (Fig. 2(b)). However, the pulses are spectrally broadened due to dispersion. By contrast, at higher powers (Fig. 2(c-e)), this broadening can be compensated for by exploiting the nonlinear self-action of light. This manifests as the formation of gap solitons within the waveguides, which preserve a constant pulse width along their propagation. As the coupling length between the waveguides can be extended by increasing the input optical power, the device can produce all-optical switching of the slow pulses between the two output waveguides of the coupler (Fig. 2(c) and 2(d)). Most remarkably, the propagation velocity of such gap solitons and the corresponding pulse delay can be actively tuned by varying the input power (Fig. 2(e)). These results could be implemented in either two dimensional photonic crystals or 1D Bragg grating geometries.
Figure 2: (a) Schematic view of the waveguide coupler with phase-shifted Bragg gratings. (b-e) Pulse dynamics inside the nonlinear coupler for increasing peak input intensities, from top to bottom. Shown are the density plots of intensity in the first (first column) and second (second column) waveguides. Output intensity profiles normalized to peak input intensity at the first (blue solid line) and second (red dashed line) waveguides are shown in the third column.
Reference
[1] S. Ha, A. A. Sukhorukov, and Y. S. Kivshar, "Slow light switching in nonlinear Bragg grating couplers" Opt. Letters 32, 1429 (2007).
About the Slow Light Project
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| Project Manager: Christelle Monat | Science Leader: Martijn de Sterke |
Contributing staff: Ben Eggleton, Eduard Tsoy, Ian Littler (Sydney), Andrey Sukhorukov, Yuri Kivshar, Duk Choi, Steve Madden, Barry Luther-Davies (ANU), Graham Marshall, Mick Withford (Macquarie)
Students: Sangwoo Ha (ANU), Neil Baker, Joe Mok (Sydney)
Project Goals and Motivation
The generation and harnessing of slow light is a fundamental problem in physics that has a number of significant applications. Many of these are driven by a fundamental property of slow light: if a pulse of light slows down, and thus also the rate at which energy is transported, then the peak intensity of the pulse must go up in order to conserve the total energy flow. This increased intensity leads to increased nonlinear effects and hence lower input intensities for nonlinear photonic signal processing. A possible longer term application of slow light is as an optical buffer, an all-optical component that will be required in future optical telecommunications networks.
CUDOS strategy
CUDOS researchers are considering slow light in one-dimensional periodic media (gratings) and two dimensional ones (photonic crystal slabs). Though many groups are pursuing this line of research, we are the only ones in the world to exploit nonlinear effects for the control of slow light propagation in these structures. The nonlinearities allow us to eliminate dispersion-related pulse broadening which leads to the scrambling of information if adjacent pulses start to overlap. The combination of dispersion and optical nonlinearity leads to the formation of solitons which do not broaden upon propagation.
Recent Achievements and Highlights
The key experimental result during 2006 was the experimental observation of slow gap solitons in gratings written in the core of silica fibres [1]. The solitons travelled, without broadening, at a velocity of 0.23v where v is the propagation velocity in the fibre in the grating’s absence, giving a total delay of approximately 1.5 ns, as shown in Figure 1. Since the incident pulses had a width of 0.68 ns, this corresponds to a fractional delay of well over 2 pulse widths. This large fractional delay illustrates the key advantage of using solitons, rather than low intensity pulses. We also showed that the delay is tunable, both by varying the intensity (see Figure 1) and by varying the strain on the grating. This work was published in Nature Physics and was featured in Nature itself.
Figure 1. Measured nonlinear transmission versus input power (top) and associated simulations (bottom). The dotted curve gives the transmission in the absence of the grating. Note the delay of 1.61 ns at an input power of 1.75 kW, and that the delay can be tuned by varying the input power.
What limits the delay that can be achieved? Using a theoretical and numerical approach we studied the delay that can, in principle, be achieved in fibre gratings [2]. As illustrated in Figure 2, we found that the delay that was observed is at the lower end of the range of possibilities and that significantly larger delays should be observable in stronger gratings although the transmission would be lower.
Figure 2. Calculated delay in nanoseconds (left-hand side) and fractional delay (right-hand side) versus the grating strength for different values of the strain on the fibre. Note that for strong gratings the delay can in principle be well over 10 pulse widths. The inset shows the power transmission versus the strain.
With the weak nonlinearities in silica, high peak power pulses are required (1.5-2 kW) to generate gap solitons. For this reason we are preparing for experiments in chalcogenide glass, which is a hundred to a thousand times more nonlinear than silica glass. This should reduce the peak power of the incident pulses to about 10 W. Much stronger gratings can be written in chalcogenide glass compared to silica, and this should lead to very large delays. As a first step we fabricated complex gratings in a chalcogenide waveguide [3]. The spectrum of this grating (Figure 3) clearly demonstrates the very deep gratings that can be written in these glasses.
Figure 3. Experimental and theoretical normalized transmission spectra of a strong grating fabricated in a 5 cm long As2S3 rib waveguide (W=4 m, H=2.37 m, h=1.25 m). Inset: the grating profile used for modeling.
We demonstrated [4] that the propagation direction and velocity of optical pulses can be independently controlled in structures with multi-scale modulation of refractive index in the transverse and longitudinal directions (see Figure 4a). In arrays of waveguides with phase-shifted Bragg gratings, the refraction angle does not depend on the speed of light, allowing for efficient spatial steering of slow light. In this system spatial diffraction and temporal dispersion can be designed independently, and it is possible for slow light to be self-collimated when diffraction is suppressed for some propagation directions (see Figure 4b). Moreover, the broadening of pulses in space and time can be completely eliminated in nonlinear media, supporting the formation of slow light bullets that remain localized irrespective of propagation direction, as illustrated in Figure 4c. Efficient all-optical switching of slow-light pulses can be realized in nonlinear Bragg-grating couplers.
Figure 4. (a) Waveguide array with phase-shifted Bragg gratings; integer n counts the waveguides; (b) Iso-frequency contours featuring wavelength independent refraction indicated by arrows; (c) Example of a slow light bullet exhibiting nonlinear self-trapping in space and time.
References
[1] JMok JT, de Sterke CM, Littler ICM, Eggleton BJ Dispersionless slow light using gap solitons NATURE PHYSICS 2, 775-780 2006
[2] Mok JT, de Sterke CM, Eggleton BJ Delay-tunable gap-soliton-based slow-light system OPTICS EXPRESS 14, 11987-11996 2006
[3] Baker NJ, Lee HW, Littler IC, de Sterke CM, Eggleton BJ, Choi DY, Madden S, Luther-Davies B Sampled Bragg gratings in chalcogenide (As2S3) rib-waveguides OPTICS EXPRESS 14, 9451-9459 2006
[4] Sukhorukov AA, Kivshar YS Slow-light optical bullets in arrays of nonlinear Bragg-grating waveguides PHYSICAL REVIEW LETTERS 97, 233901 2006
