Optimising the Raman red-shift of a soliton
Contacts
Simon Toft Sørensen (stso@fotonik.dtu.dk, bldg. 344/011 tel. 452-55140 )
Ole Bang (oban@fotonik.dtu.dk)
A soliton is an optical pulse that propagates through a dispersive medium completely without changing its shape, both in time and frequency. Normally, dispersion leads to a spectral broadening. However, when the parameters of the medium and pulse are exactly right, the pulse can propgate without changing it shape. In reality, higher-order effects distort the soliton and the so-called Raman effect causes the soliton to undergo a constant red-shift as it propagates. That is, the soliton shifts toward longer wavelengths as it propagtes through the fibre.
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A photonic crystal fibre (PCF) is a special type of optical fibre, that guides light by an array of closely spaced air-holes running along the length of the fibre. There is a great variety in the arrangement of the air-holes, leading to PCFs with very different properties. By tapering the fibre (gradually making it thinner), it is possible to control the fibre parameters seen by the soliton. If the fibre is tapered in a clever manner, it is possible to control the propagation of pulses through the fibre. In this project we are going to optimise the red-shift of the soliton using numerical simulations. Starting out with a soliton at 1064 nm, we will try to see how far it is possible to red-shift it by tailoring the fibre. Unfortunately, the red-shifted eventually stops, but designing the tapered fibre just right should give a red-shift to more than 2000 nm – around one octave! Lasers operating in this frequency range are very desirable. The project will give an insigt into fibre optics and you will learn how to model pulse propagation using the well-known and widely used split-step Fourier method.