Lasers are important components of many science and engineering applications. Generally, lasers emit light via the optical amplification process induced by the stimulation of the emitted electromagnetic radiation. The ability to control the stability of the frequency of the emitted radiation is of extreme importance in precision optical metrology systems, such as those used for inertial navigation, for atom interferometery and for atomic clocks aiming to keep time precisely for global positioning systems and geodesy.
Laser tones with fixed relative frequencies can be generated through numerous techniques, including direct stabilization of the frequency offset between lasers, shifting the laser frequency and splitting a single laser beam. Specifically, using the interference beat note to directly stabilize the frequency offset between lasers has drawn growing research attention because it offers a wide range of applicability while maintaining the spectral purity of the lasers. Typically, a very tight frequency lock can be achieved by phase locking two lasers with the help of an optical beat note. Unfortunately, this optical-phase-locking loop (OPLL) requires feedback-stabilization bandwidths that are relatively larger than the line widths of the lasers, making their applications challenging, especially for semiconductor diode laser systems.
In applications that do not require phase locking, beat-frequency discrimination is used to steer and lock laser frequencies. The commonly used discriminators are based on different architectures, including simple delay-line based architectures, digital counting architectures, integrated frequency-to-voltage-conversion circuits and side-of-filter-type architectures. While these architectures offer operation advantages, they have different drawbacks that limit their applications. Importantly, it is worth noting that the implementation of these architectures results in a trade-off between the locking stability and the capture range of the lock, leading to additional limitations. Therefore, developing effective frequency-based discrimination methods is highly desirable.
On this account, PhD candidate Vyacheslav Li, Dr. Fritz Diorico and Professor Onur Hosten from the Institute of Science and Technology Austria developed a new frequency-offset locking method for lasers. This new method was based on beat-frequency discrimination using hybrid electronic LC filters. It was specifically designed to decouple the tightness of the lock from the broadness of its capture range. The feasibility of the proposed method was validated in a system with two 780-nm miniature external-cavity diode lasers. Their work is currently published in the journal, Physical Review Applied.
The research team showed that the presented demonstration could effectively lock two 780 nm free-running diode lasers at an offset of 5.5 GHz from each other with an extremely low offset frequency instability of 12 Hz at 10s averaging time. Additionally, long-term stability below 55 Hz was achieved for more than 1000s. The results showed low beat-note-based offset locking instability free from optical phase locking. Through ramping the offset frequency (using a direct digital synthesizer), a laser tunability exceeding 200 kHz/µs, with a range of up to 1 GHz was demonstrated, and a 190-MHz lock-capture range was achieved. An error signal with broad tails and a 1.8-MHz-wide steep region generated by the implemented frequency discriminator resulted in the large capture range. Operation noise was reduced by the all-analog design of the offset circuitry.
In summary, the authors reported the design and implementation of a new frequency-offset locking method. This scheme offers the benefit of obtaining a single lockable point with remarkably higher locking stability, broad capture range and wide tunability, making it a promising practical application for stabilizing and tuning relative frequencies. In a statement to Advances in Engineering, Professor Onur Hosten, the lead and corresponding author explained the newly presented frequency-offset locking method would enable the generation of laser beams with stable and well-defined relative frequencies desirable in numerous applications.
Li, V., Diorico, F., & Hosten, O. (2022). Laser frequency-offset locking at 10-Hz-level instability using hybrid electronic filters. Physical Review Applied, 17(5).