We constructed a three‐component combination seismometer and geodetic sensor suitable for borehole deployment. The instrument uses no electronics at depth in the borehole, rather it relies on optics. Two ∼40  cm pendulums suspended from two orthogonal axes with free periods slightly more than 1 s and a vertical mass‐spring suspension with a free period near 5 s are tracked with optical interferometers inside a borehole sonde. Laser light for the interferometers is provided by optical fibers from the surface; the interference fringe signals are transmitted from the sonde to photo detectors at the surface, also via optical fibers. The system was installed in a 60‐m‐deep borehole at the U.S. Geological Survey’s Albuquerque Seismological Laboratory (ASL) for 1 year. The seismic noise floor of the optical seismometer’s components compares well with those of a collocated KS‐54000 borehole seismometer. Because the optical sensors operate to zero frequency, they also provide useful geodetic records.

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