We present an interferometric displacement sensor based on a folded low-finesse Fabry--Perot cavity. The 10 fiber-optic sensor uses a quadrature detection scheme based on the wavelength modulation of a DFB 11 laser. This enables measuring position changes over a range of 1 m for velocities up to 2 m/s. The sensor 12 is well suited to work in extreme environments such as ultrahigh vacuum, cryogenic temperatures, or 13 high magnetic fields and supports multichannel applications. The interferometer achieves a repeatabil- 14 ity of 0.44 nm3$sigma$ at a working distance of 20 mm, a resolution of 1 pm, and an accuracy of 1 nm.     «

We present an interferometric displacement sensor based on a folded low-finesse Fabry--Perot cavity. The 10 fiber-optic sensor uses a quadrature detection scheme based on the wavelength modulation of a DFB 11 laser. This enables measuring position changes over a range of 1 m for velocities up to 2 m/s. The sensor 12 is well suited to work in extreme environments such as ultrahigh vacuum, cryogenic temperatures, or 13 high magnetic fields and supports multichannel applications. The interferometer...     »