Broderick, N.G.R., Taverner, D., Richardson, D.J., Ibsen, M. and Laming, R.I.
The optical pushbroom in action
At Bragg Gratings, Photosensitivity and Poling in Gladd Waveguides (BGPP), United States.
26 - 28 Oct 1997.
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The area of nonlinear pulse interactions in Bragg grating structures, although largely unexplored, contains many interesting and novel effects. Perhaps the simplest of these is the CW switching of a weak probe by a strong pump. Recall that a Bragg grating reflects strongly around the Bragg resonance frequency omega(0) which is inversely proportion to the average refractive index. In a nonlinear medium the presence of a strong pump alters the refractive index, and thus frequencies which were reflected (or transmitted) by the grating can be transmitted (or reflected). This effect was first seen by LaRochelle et al. in 1990 and to date this is the only experimental work done in this area. However since then considerable theoretical work has been done on pulse interactions in fibre Bragg gratings (FBGs). Also experimental reports of nonlinear propagation in FBGs have started appearing in the literature. A major factor in this upsurge of interest has been the development of techniques for writing long gratings at arbitrary wavelengths using the side illumination of fibres with UV light. This fact coupled with the latest generation of high power fibre sources allows the exploration of pulse interactions in FBGs in great detail. The CW switching of a probe beam can be generalised by considering the effects of a strong pump pulse on a weak CW probe - the so-called optical pushbroom effect, demonstrated experimentally here for the first time. The optical pushbroom utilises the frequency shift induced through the cross-phase modulation (XPM) of a CW probe by a strong pump beam to compress and sweep out the probe from the grating. As this frequency shift is proportional to the gradient of the pump beam it is intrinsically a pulse effect and although it compresses the probe it does not exchange energy between the two beams. The operation of the optical pushbroom has been explained in detail before and so we give only a brief description here.
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