High-power optical frequency combs play a very important role in nonlinear precision spectroscopy, extreme ultraviolet optical frequency comb generation, nuclear and atomic clock research, etc. Fiber femtosecond lasers are the preferred solution for realizing high-power optical frequency combs due to the advantages of simple structure, stable performance and easy amplification. However, due to the unavoidable spontaneous emission (ASE) noise, pump intensity noise, spectral coherence degradation, and phase jitter caused by light path in fiber laser amplification, the frequency stability of the optical comb is seriously affected. Therefore it is a challenging task to obtain high frequency stability while high power amplification.
To address the above problems, based on the long-term optical frequency comb technology research, L07 group of the Key Laboratory of Optical Physics, Institute of Physics, Chinese Academy of Sciences/Beijing National Research Center for Condensed Matter Physics, has successively put forward the technical solutions of adopting the low-noise optical fiber seed source, linear chirped pulse amplification, and intracavity electro-optical crystal fast phase modulation, etc., and generating the high-coherence supercontinuum spectrum by the pulling cone photonic crystal fiber and combining with the The integrated structural design, temperature control, vibration isolation and other engineering designs have effectively reduced the noise of the optical frequency comb. Recently, Hainian Han, an associate researcher in the group, and Xiaodong Shao, a postdoctoral fellow, have further achieved an out-of-ring frequency stability on the order of 10-19/1000s on a 20W high-average-power fiber optical frequency comb. This is known to have reached the frequency stability of the current best optical atomic clocks, and is also the optimal result achieved by high power optical frequency combs so far. Figure 1 shows the schematic diagram of the two identical high-power optical fiber comb engineering prototypes developed and the extra-annular dual optical comb comparison to measure the frequency stability.
A self-built nonlinearly polarized rotationally mode-locked (NPR) fiber laser oscillator was used as the femtosecond seed pulse source in the study, and a high-power femtosecond laser output with an average power greater than 20 W and a pulse width of 75 fs was obtained after linear fiber chirped pulse amplification (CPA). The locked frequency stability of the carrier envelope phase shift (CEO) frequency is 1.5×10-17/s. In the optical frequency locking, the femtosecond pulses output from different branches of the oscillator and amplifier are tapped with the super-stable reference laser separately, and if the amplifier's tapped signal is used for the locking, the measured frequency stability of the high-power amplified laser ring can reach 2×10-18/s, while in the case of only the CPA, the frequency stability can reach 2×10-18/s, and in the case of only the CPA, the frequency stability can reach 2×10-18/s. -18/s, while in the case of only locking the femtosecond pulse output from the oscillator, if the amplifier is allowed to run freely, the frequency stability can only reach 10-15/s, which is three orders of magnitude worse. The noise power spectrum analysis shows that the amplifier introduces a large amount of low-frequency noise, which has a significant effect on the long-term stability of the optical frequency comb frequency, and the difference is even as high as four orders of magnitude at a gate time of 1000 s, as shown in Fig. 2 (a). In addition, the results of the phase noise analysis, which characterizes the short-term stability of the frequency, also show that after a series of noise control measures, the high-frequency noise introduced by the amplifier is very small and does not affect the short-term stability of the system.
In order to evaluate the frequency stability performance of this high-average-power optical frequency comb in practical applications, two 20W high-power fiber laser frequency combs are measured for the first time in this study. The measurement results show that the typical stability value of the out-of-ring frequency is 4.35×10-17/s at 1 s integration time, and decreases to 6.54×10-19 at 1000 s integration time, as shown in Fig. 2(b), which will open up new doors for many applications that require high power and high frequency stability of optical combs. This research progress was recently published in Optics Express under the title "High power optical frequency comb with 10-19 frequency instability" (Opt. Express 31(20)). (Optics Express 31(20), 32813-32823 (2023)). The first author of the article is Xiaodong Shao, and the co-supervisors are Zhiyi Wei and Hainian Han. This work was supported by the Pilot Program of the Chinese Academy of Sciences (XDA1502040404, XDB210104004) and the National Natural Science Foundation of China (60808007, 61378040, 11078022, 91850209).
A 20W high-power optical fiber frequency comb with 10-19th order extra-annular frequency stability.
Fig. 1 (a) Photograph of the engineered principle prototype of the high power fiber optic optical comb, (b) Schematic diagram of the measurement of the extra-ring frequency stability of the high power optical frequency comb
Fig. 2 (a) Frequency stability of oscillator and amplifier locked separately, (b) Frequency stability of high power optical frequency comb with out-of-ring comparison
