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3 Output Power Considerations
Output beam quality
The output beam from the AOTF crystal naturally includes side lobes (see figure
8-1). The side lobes have a different spatial distribution than the main peak. This
makes it possible to filter the beam spatially by inserting a small aperture or by
coupling to a single mode fiber.
Free space delivery In order to optimize the output beam quality, it is filtered through a small
aperture. Figure 23 shows the spectral output of a single AOTF channel as a
function of the size of the aperture. When there is no aperture, the main peak has
numerous side lobes on either side of the main peak. Decreasing the size of the
aperture decreases the power in the side lobes. Power decreases initially in the
outer side lobes followed by the inner side lobes and eventually the main peak.
In practice, the power in the first order side lobe and in the main peak is coupled,
so it is not possible to block the first order side lobes without significantly
decreasing the power in the main peak. The optimal aperture occurs when the
main peak and first order side lobes are transmitted but the higher order side
lobes are heavily damped.
Figure 23 Spectral output of a single AOTF channel
Fiber Delivery If a fiber delivery system
I
is mounted to the outputs of the SuperK SELECT, the
single mode coupling acts as a spatial filter. In this case, no aperture in the beam
path is required. Side lobe suppression will depend on the numerical aperture of
the delivery fiber.
I. Refer to the SuperK Fiber Deliver System Product Guide
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