6.5 The Schnupp modulation scheme

Similar to the Fabry–Pérot cavity, the Michelson interferometer is also often used to set an operating point where the optical gain of a direct light power detection is zero. This operating point, given by ΔL ∕λ = (2N + 1) ⋅ 0.25 with N a non-negative integer, is called dark fringe. This operating point has several advantages, the most important being the low (ideally zero) light power on the diode. Highly efficient and low-noise photodiodes usually use a small detector area and thus are typically not able to detect large power levels. By using the dark fringe operating point, the Michelson interferometer can be used as a null instrument or null measurement, which generally is a good method to reduce systematic errors [49].

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.

View Image

Figure 37: This length sensing scheme is often referred to as frontal or Schnupp modulation: an EOM is used to phase modulate the laser beam before entering the Michelson interferometer. The signal of the photodiode in the South port is then demodulated at the same frequency used for the modulation.

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 37View Image: 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 39View Image in Section 6.6.

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