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This paper reports a MEMS displacement sensor with a sensitivity of 50 fm/radicHz at frequencies as low as 10 mHz. The sensor is composed of a near-field optical nano-grating that modulates the reflectivity of incident light by 10%/nm. A reference detection low noise readout circuit has been designed that cancels laser relative intensity noise (RIN) to frequencies as low as 0.7 Hz. The low frequency bandwidth of the position sensing microsystem is further reduced to below 10 mHz using a new MEMS correlated double sampling technique that cancels low frequency RIN, drift, and circuit 1/f noise by greater than 77 dB.