Surprisingly complex information processing systems can be built using appropriate combinations of roof mirrors and polarisers. In effect, we can construct Quasi-Optical Circuits using these (and some other) elements in just the same way as we can use capacitors, resistors, etc, to build conventional electronic circuits. It's a bit like the way digital logic engineers can use AND, OR, and NOT gates to build anything from a digital watch to a Cray supercomputer. Just as with conventional electronics we need to use Circuit Diagrams and Circuit Symbols as a language to describe the essential features of a quasi-optical circuit. Like normal circuit diagrams, these optical ones only show the essential features we need to know about to work out what the instrument is doing. Figure 11.5 shows the basic symbols.
The symbols illustrated in 11.5 assume that the optical circuit is build on the surface of a horizontal optical table — hence the references to ‘vertical’ and ‘horizontal’ wires. The ‘45°’ polariser has its wires aligned to appear at 45° to the vertical as seen by a beam striking the polariser. It has become conventional to assume that the roof mirror's roof-line is vertical unless otherwise indicated. In fact, the real optical system may have some other orientation. It may even be ‘folded up’ into a convenient 3D shape. However, the diagram is stylised just like a normal electronic circuit diagram to show the relationship between the optical elements which make it operate as intended.
Figure 11.6 shows two simple examples of quasi-optical systems which can be built and described using these types of symbols. 11.6a is an adjustable time delay or phase shifter. (This function is useful in interferometry and AM/FM conversion.) The input signal from the source has its E-vector polarised in the vertical direction so as to pass through the polariser. The roof mirror has its roof line at 45° to the vertical, hence the incident beam's polarisation is rotated by 2×45° upon reflection. The reflected beam is therefore horizontally polarised. It will therefore be reflected by the polariser and directed towards the detector. By altering the mirror-polariser separation, Z, by an amount we change the source-detector distance and hence change the time taken for the signal to pass through the system. For a fixed signal frequency this is equivalent to being able to adjust the phase of the signal reaching the detector.
The circuit illustrated in 11.6b uses the same optical elements as 11.6a, but the roof mirror is modified so that it's roof line can be rotated in the plane perpendicular to the beam axis. As a result, we can adjust the polarisation state returned to the polariser. This means that we can change the fraction of the signal which is reflected from the polariser towards the detector. So far as the detector is concerned, rotating the mirror's roof line makes it behave like a variable attenuator. We can therefore set any output level we wish, from zero up to full power, by adjusting the mirror angle. If required, we can build a circuit which combines both functions and we can adjust the output signal's amplitude and phase simply by adjusting the roof mirror. Most practical circuits are more complex than the examples shown in 11.6. We can, however, regard these as typical ‘building blocks’ for assembling more complicated systems.
Summary
You should now understand how we can couple signal sources and detectors or mixers via a suitable combination of waveguides and free space beams. That guides are useful for defining and controlling single-mode field patterns, but that free space has lower losses, lower dispersion, and is more convenient for wideband instruments. That one particularly convenient way to use quasi-optical circuits is to exploit polarisation state processing methods. That Gaussian Beam Mode techniques allow us to design systems easily taking diffraction effects into account. That we can build systems of arbitrary complexity using these methods in a manner analogous to conventional electronics
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University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.