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[ Instrument Network Instrument Development ] Since the advent of the femtosecond optical frequency comb (optical frequency comb) at the beginning of this century and the 2005 Nobel Prize in Physics, its development has not only changed with each passing day, but also applications have emerged from optical frequency and precision spectroscopy. Basic research such as attosecond science and basic physics constants have been extended to high-tech fields such as absolute distance measurement, extraterrestrial planetary exploration, and microwave photonics, especially in recent years, in space high-precision frequency standards, high-precision time-frequency transmission of air-ground, and other major requirements. It also presents an increasingly broad and important application potential. In the research of optical frequency comb, an important problem is the precise locking of the carrier envelope phase shift frequency fceo, which not only determines the initial frequency drift of the entire frequency comb, but also affects the research of femtosecond laser and matter interaction. The effect and physical effects can be obtained. Therefore, how to precisely control and lock the fceo to obtain very low phase noise is one of the most challenging tasks in the research of optical frequency comb and ultrafast laser. An important sign that can reach the level.
The L07 group of the Institute of Physics of the Chinese Academy of Sciences/Beijing National Laboratory for Condensed Matter Physics has been working on low-phase noise and high-stability optical frequency comb technology and frequency locking for many years. It has proposed the technique of measuring the spectrum self-difference frequency measurement fceo. It proves a scheme to obtain high signal-to-noise ratio fceo, and combines electronic phase-locked loop feedback control technology to realize ultra-low phase noise titanium gem optical frequency comb with phase drift as low as 55mrad. In recent years, a new fceo feedback technology, the pre-feedback locking technology, has received widespread attention because it can directly compensate for the offset of fceo drift. The so-called pre-feedback locking technology inserts an Acousto-optic Frequency Shifter (AOFS) into the output laser, and directly modulates the measured fceo frequency onto the acousto-optic crystal. An output laser whose frequency is corrected by the body is obtained in the diffracted light. Compared with the traditional electronic phase-locked loop feedback technology, the pre-feedback lock has no proportional integral filtering (PID) link, which is equivalent to having a wide bandwidth, so it can respond to the error signal instantaneously, thus suppressing high-frequency phase noise well. It has extremely low noise. Most importantly, AOFS is inserted directly into the output laser and has no effect on the laser system, which greatly increases the utility of the technology. Recently, the group's associate researcher Han Hainian, Ph.D. student Zhang Ziyue and researcher Wei Zhiyi and others have achieved the first feedback lock of the all-solid Kerr lens-mode-locked Yb:CYA laser fceo on the basis of years of efforts, thus realizing a kind of Ultra-low phase noise all-solid-state new optical frequency comb. Figure 1 shows the experimental setup and results. In the experiment, the solid-state Kerr lens-mode-locked Yb:CYA laser has an output power of 200mW, a pulse width of 57fs, and a repetition rate of 84MHz, which passes through the zero-order and one-degree in AOFS. After the diffracted light, a set of f-2f interference devices is built as the inner and outer rings, and the fceo signal measured by the inner ring is directly fed back to the AOFS to control the drift of the fceo, and the fceo signal measured by the outer ring is used for analysis and locking. result. Thanks to the large increase in the bandwidth of the AOFS pre-feedback lock control, the fceo integral phase noise (1Hz-1MHz) of the optical frequency comb is measured as low as 79.3mrad, compared with the 316mrad result of the traditional phase-locked loop pump feedback method. The phase noise is reduced by 70%, which is the lowest fceo phase noise that has been achieved so far based on the all-solid-state laser in the 1um band. In addition, by analyzing the phase noise power spectral density and long-term frequency instability below 1 Hz, it shows that the technology they developed has an advantage in high-frequency phase noise suppression. The result of this work was published in the latest issue of Optical Express on the topic of Ultra-low-noise carrier-envelope phase stabilization of a Kerr-lens mode-locked Yb:CYA laser frequency comb with a feed-forward method (Optics) Letters Vol. 44 (22), 2019).
How to realize the coherent connection between the optical comb and the continuous laser is another important and challenging research content in the research of optical frequency comb and optical frequency measurement. The optical frequency comb locked to the ultra-stable laser is coherently connected with any continuous laser, and the frequency stability of the ultra-stable laser can be accurately transmitted to the continuous laser, thus becoming a perfect optical frequency synthesizer, which can be in the microwave to The optical frequency band provides any low phase noise and high stability frequency source. AOFS pre-feedback locking technology can also be used for coherent connection between continuous laser and optical frequency comb. Based on this patented invention (patent number: ZL 201811074118.3), Han Hainian, Ph.D. student Shao Xiaodong and researcher Wei Zhiyi and others have further realized frequency stability. Sexual coherent delivery. In the experiment, a 1064 nm continuous laser and an optical frequency comb are first beat frequency, and then the obtained beat frequency signal is mixed with the local oscillator signal to drive the AOFS to realize the pre-feedback, so that the first-order diffracted light after the AOFS is Locked onto the optical comb (left in Figure 2), this optical comb is locked to an ultra-stable 972nm continuous light source. Before the lock, the frequency drift of the 1064nm continuous laser is about several tens of kHz, and the frequency drift deviation of the first-order diffracted light after the lock is 10000 s is only 4.1 mHz, and the corresponding frequency stability is 1.5×10-17/ s (Fig. 2 right), the noise is greatly suppressed, and the long-term stability is also greatly improved, and the coherent connection between the optical frequency comb and the continuous laser is well realized. The main results were recently published on AIP Advances 9(11)2019 with the topic Precision locking CW laser to ultrastable optical frequency comb by feed-forward method.
The above research was supported by the Chinese Academy of Sciences Pilot Project (XDA1502040404, XDB21010400) and the National Natural Science Foundation of China (91850209, 11434016, 61575219).