High-stability offset-frequency locking of two lasers using a balanced filter discriminator

We demonstrate a high-stability frequency-only laser offset lock based on a balanced filter discriminator. The photodetected beat note between two 852 nm external-cavity diode lasers is divided before downconversion and processed in two branches driven by local oscillators placed symmetrically around the target offset. After low-pass filtering and RMS detection, subtraction of the two detector outputs produces an antisymmetric error signal: frequency deviations generate opposite responses in the two branches, whereas correlated beat-power fluctuations are reduced in the differential output. For an 8.653 GHz offset corresponding to the cesium repumping frequency difference used in our laser-cooling setup, the optimized SLP-1.9+ configuration reached a fractional instability of 4×10−15 at 10 s when referred to the 852 nm optical carrier and remains below approximately 6×10−15 up to 104 s without linear-drift removal. Because the lock point is primarily determined by the midpoint of the two microwave references, the offset can be tuned by shifting both references together. This architecture provides robust and tunable frequency-only locking for atomic-physics and precision-frequency-metrology applications.