One approach to make use of the advantages of the dark fringe operating point is to use an operating point very close to the dark fringe at which the optical gain is not yet zero. In such a scenario a careful trade-off calculation can be done by computing the signal-to-noise with noises that must be suppressed, such as the laser amplitude noise. This type of operation is usually referred to as DC control or offset control and is very similar to the similarly-named mechanism used with Fabry–Pérot cavities.
Another option is to employ phase modulated light, similar to the Pound–Drever–Hall scheme described in Section 6.3. The optical layout of such a scheme is depicted in Figure 37: an electro-optical modulator is used to apply a phase modulation at a fixed (usually RF type) frequency to the (monochromatic) laser light before it enters the interferometer. The photodiode signal from the interferometer output is then demodulated at the same frequency. This scheme allows one to operate the interferometer precisely on the dark fringe. The method originally proposed by Lise Schnupp is also sometimes referred to as frontal modulation.
The optical gain of a Michelson interferometer with Schnupp modulation is shown in Figure 39 in Section 6.6.
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