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Suppose I have a light wave and I want to measure its amplitude, or check to see if it has an amplitude of a certain value: how would one go about doing this?

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  • $\begingroup$ Use a photodiode: en.wikipedia.org/wiki/Photodiode. There are very expensive calibrated ones, but even the uncalibrated ones are useful for most purposes and they are reasonably priced (in the $1-10 range) e.g. mouser.com/ProductDetail/Vishay-Semiconductors/BPW20RF/…. Be aware that you are really measuring intensity ($W/m^2$) by measuring the photocurrent. If you want to measure really large areas, use a solar cell. They are basically very large photodiodes, just not as precise. $\endgroup$ – CuriousOne Jun 2 '16 at 21:22
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Direct measurement of the amplitude of the optical field requires interferometric techniques. One that works is the FROG - Frequency Resolved Optical Grating. There are many variations on it today, including from the original developers: Trebino Research Group.

These devices were designed for ultrafast pulses. For CW one usually just measures intensity and polarization, followed by a computation.

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