A laser communications link

A laser communications link

To study optimization using OSLO, you will work with a simple system that might be used to make a laser communications link. This example is typical of the type of problem that can be readily solved using catalog lenses. Here's the scenario:

You are working on an advanced communications system that includes a 100-meter free-space propagation path between two optical fibers. The system uses a helium-neon laser operating at 0.6328 micrometers. The diameter of the fiber core is 5 micrometers. The beam emitted from the end of the transmitter fiber is adequately characterized as a single-mode Gaussian having a numerical aperture of 0.2.

You are to design the transmitting and receiving optics for the system. For mechanical packaging reasons, the transmitter and receiver heads must be less than 75 mm long by 40 mm diameter.

To meet the numerical aperture requirement without truncating the beam too much, the lenses should operate at f/2 (N.A. 0.25) or faster. To leave room for mounting the lenses, the lens diameter should not exceed 30 mm. This means that the focal length must be about 60 mm. The transmitting and receiving optics are chosen to be the same. The general layout is shown below.

Optical systems should usually be analyzed from long to short (to minimize numeric round-off error), so we will design the receiver lens. The transmitter lens will be the same.

The question becomes one of how much complexity in the lens is needed to achieve the required image quality. Although specific data are not given, it is clear that the spot size (radius) formed by the lens must be less than 2.5 micrometers, to match the fiber core diameter. The design task then is to find a lens with a focal length of about 60 mm and a diameter of about 30 mm, and evaluate its image quality.

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