Tsinghua Physics Department Makes Important Progress in Laser High-frequency Phase Noise Cancellation Technology

Since the birth of the first laser in 1960, laser technology has been widely used in various fields of physics, such as laser communication, laser spectroscopy, laser particle gas pedals and laser interferometry. In experimental studies of atomic and molecular and optical physics, the performance of lasers determines one's ability to manipulate microscopic particles such as atoms and molecules. In recent years, with the development of quantum science and technology, the requirements of scientific experiments for lasers have been further improved. For example, in order to precisely manipulate the quantum state of atoms and realize high-precision quantum logic gates, people need lasers with stable absolute frequency and extremely low phase noise. In order to eliminate the frequency drift of lasers, three physicists, Robert Pound, Ronald Drever, and John Hall (PDH for short), developed a method based on a modulation-demodulation technique to lock the laser frequency to a Fabry-Perot optical cavity. The method is extremely effective in suppressing low-frequency (<100 kHz) phase noise in lasers. However, since it is a technique based on feedback control, it is inevitably limited by the feedback bandwidth, and thus cannot effectively suppress the high-frequency phase noise of the laser. On the other hand, the existing laser high-frequency phase noise elimination method means are more complicated, and there is still much room for improvement in stability and noise suppression.

Fig. 1 Schematic diagram of the experimental device as well as the effect diagram. (a) Typical PDH feedback frequency stabilization device (white background area) and PDH feed-forward phase noise cancellation device (blue shaded area). (b) Spectrum of the external difference between the output light and the transmitted light from the super-stabilized cavity. The red spectrum is the outlier spectrum when the PDH feedforward is not used, and the blue is the outlier spectrum when the PDH feedforward is used.
Recently, a new technique for laser phase noise cancellation has been developed by the Mung-Kun Cheng-Yuli group at the Department of Physics, Tsinghua University, which achieves up to four orders of magnitude suppression of high-frequency phase noise in the MHz range, which is one to two orders of magnitude better than all previous feedback/feedforward-based phase noise cancellation methods. The method is a clever extension of the classic PDH frequency stabilization technique. They found that when using PDH feedback to lock the laser frequency to the super-stabilized cavity, the remaining error signal is proportional to the real-time laser phase shift in the high-frequency region. As a result, they proposed a new feed-forward scheme by adding a ten-meter section of optical fiber to the original PDH technology setup to delay the laser light field, compensating for the time delay introduced by the PDH error signal generation process, and then applying this PDH signal to an electro-optical crystal that modulates the phase of the light field to remove the laser phase noise (Fig. 1).

Fig. 2 Variation curve of phase noise cancellation coefficient with noise frequency for the PDH feedforward method. The red dots in the figure show the experimental measured data, and the green solid line shows the theoretical value of the phase noise cancellation coefficient by the optical cavity filtering method.
The research team measured the suppression effect of the PDH feedforward method for phase noise at different frequencies and found that the method can efficiently suppress laser phase noise up to 20 MHz with a theoretical explanation (Figure 2). For lower frequencies, they found that the feedforward method is equivalent to the optical cavity filtering method, but does not limit the transmitted laser power as severely as the latter. For higher frequencies, the effectiveness of the method can be further enhanced by narrowing the dispersion effect of the electronics. It is worth mentioning that the circuits used to amplify the error signals to generate feedforward signals in this work were developed and designed by the team members, which lays a solid foundation for the work to be further popularized and applied.
The reviewer commented highly on this work: "The PDH technique is widely used in many fields, including time-frequency metrology, spectroscopy, and atomic physics. The study of suppressing laser phase noise in the MHz frequency range has received much attention and is important for numerous applications. Combined with the simple techniques presented in this paper, I believe that this research will have a broad impact and may drive the widespread use of feedforward schemes." The research was published under the title "Pound-Drever-Hall feedforward: laser phase noise suppression beyond feedback" on July 9, 2024 in the optics journal Optica.