Example 17: Raman amplifier

This is an example of a Raman amplifier. It is not a "real" problem but instead is highly stylized to illustrate some effects more simply.

This example illustrates Raman amplification of a seed beam by a shorter wavelength pump beam. The pump beam is at 1.06 microns and the seed beam is at 1.54 microns. Both the seed and pump beam are aberrated. The seed beam goes through a spatial filter which cleans up the beam somewhat. The seed and pump beams are combined and passed through a Raman amplifier. The amplifier depletes the pump beam and amplifies the seed beam. In this model none of the phase of the pump beam is imposed on the seed beam. The irradiance distribution of both the pump and seed beams show effects due to diffraction from the aperture edges and due to the aberration in the beams. The output of the seed beam is brought to a line focus by a cylindrical lens to illustrate the use of nonrotationally symmetrical optical elements and the use of rectangular arrays. Note that the code automatically determines the matrix units to achieve good resolution in both directions.

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The initial pump beam intensity was selected arbitrarily. The intensity of the pump beam will be significantly modified by propagation causing the phase aberrations to transform to intensity variations and by pump depletion in the Raman amplifier. (expand)

Starting pump intensity

Starting pump phase Initial pump phase distribution shows considerable aberration. Since there is nominally only intensity coupling, little of this aberration will be transfered to the seed beam. (expand)
An arbitrary initial starting intensity distribution for the seed beam. (expand) Starting seed intensity
Starting seed phase The initial seed beam is deliberately started with a lot of aberration to illustrate the cleanup effect of the spatial filter. (expand)
The spatial filter removes all higher order aberration from the seed beam, leaving only low order aberration -- astigmatism and defocus. (expand) Cleaned seed phase
Depleted pump beam Intensity of pump beam after Raman amplifier, showing depletion of the pump in the center due to energy extraction by the seed beam. (expand)
Some of pump nonuniform intensity (due to diffraction propagation) has been imprinted into the seed intensity. As the seed beam propagates the somewhat irregular intensity will be transformed into phase aberration. This is an indirect beams of coupling pump phase into seed phase, even though there is no direct phase coupling. (expand) Amplified seed beam
At line focus The final image of the seed beam shows significant elongation because of the cylindrical lens. The image shows little aberration because the seed was well cleaned up by the spatial filter and the pump beam provided amplification with little transfer of its aberrations. (expand)