Simple Hertz-Level Laser Offset Locking by Using a Dual-Bandwidth Digital Band-Pass Filter

This letter presents a simple and easy-to-use self-referenced frequency discriminator for laser frequency offset locking. The key component of the “self-referenced” frequency discriminator is a dual bandwidth band-pass filter which is composed of two digital band-pass filters, one having a broader bandwith to provide a large capture range and the other having a narrow bandwidth for tight frequency locking. The digital band-pass filters are based on the PyRPL IQ modules running on a Red Pitaya STEMlab board. All the parameters of the IQ modules can be easily set and adjusted in the PyRPL GUI. To demonstrate the proposed scheme, we offset locked an external cavity diode laser to an iodine-stabilized external cavity diode laser. Their beat note was narrowed down from 500 kHz of the free running to 4 Hz as offset locked. The minimum Allan deviation of the locked offset frequency is 2.4 Hz at 100-s averaging.