Implementation of 20W high-power fiber optic frequency comb by the Institute of Physics, Chinese Academy of Sciences

High power optical frequency combs play a crucial role in nonlinear precision spectroscopy, extreme ultraviolet optical frequency comb generation, nuclear atomic clock research, and other fields. Fiber optic femtosecond lasers are the preferred solution for achieving high power optical frequency combs due to their 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 optical path in fiber laser amplification, the frequency stability of the optical comb is seriously affected. Therefore, obtaining high frequency stability while amplifying high power is a highly challenging task.

In response to the above problems, Group L07 of the Institute of Physics of the Chinese Academy of Sciences/Key Laboratory of Optical Physics of the National Research Center of Condensed Matter Physics in Beijing, based on the long-term research of optical frequency comb technology, has proposed in recent years the use of low-noise fiber seed sources, linear chirped pulse amplification, rapid phase modulation of intracavity electro-optic crystals and other technical solutions, And by using tapered photonic crystal fibers to generate high coherence supercontinuum spectra and combining various engineering designs such as structural integration, temperature control, and vibration isolation, the noise of optical frequency combs is effectively reduced.

Recently, associate researchers Han Hainian and postdoctoral fellow Shao Xiaodong of the group further achieved an external frequency stability of 10 ⁻ on a 20W high average power fiber optic frequency comb ¹ Results of the order of ⁹/1000s. It is known that this has achieved the frequency stability of the best optical atomic clock currently available, and is also the best result achieved by high-power optical frequency combs to date. Figure 1 shows the schematic diagram of two identical high-power fiber optic comb engineering prototypes and the measurement of frequency stability through the comparison of external dual combs.

Figure 1 (a) Photos of the prototype of the engineering principle of high-power optical fiber comb, (b) Schematic diagram of the measurement of frequency stability outside the high-power optical frequency comb ring

In the study, a self built nonlinear polarization rotation mode locked (NPR) fiber laser oscillator was used as a femtosecond seed pulse source. After linear fiber chirped pulse amplification (CPA), a high-power femtosecond laser output with an average power greater than 20 W and a pulse width of 75 fs was obtained. The locked frequency stability of the carrier envelope phase shift (CEO) frequency is 1.5 × 10 ⁻ ¹ ⁷/s. When locking the optical frequency, the femtosecond pulses output from different branches of the oscillator and amplifier and the ultra stable reference laser beat separately. If the beat signal of the amplifier is used for locking, the frequency stability inside the high-power amplification laser ring can be measured to reach 2 × 10 ⁻ ¹ ⁸/s, while in the case of only locking the femtosecond pulses output by the oscillator, if the amplifier is allowed to operate freely, the frequency stability can only reach 10 ⁻ ¹ Δ/s, three orders of magnitude difference. The noise power spectrum analysis shows that the amplifier introduces a large amount of low-frequency noise, which has a significant impact on the long-term stability of the optical frequency comb frequency. At a gate time of 1000 s, this difference can even reach four orders of magnitude, as shown in Figure 2 (a). In addition, the phase noise analysis results that characterize the short-term stability of the frequency also indicate 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.

Figure 2 (a) Frequency stability locked by oscillator and amplifier respectively, (b) Frequency stability compared to high-power optical frequency comb ring

In order to evaluate the frequency stability performance of the high average power optical frequency comb in practical applications, this study conducted the first measurement of external frequency comparison between two 20W high-power fiber laser frequency combs. The measurement results indicate that under 1 second integration time, the typical stability value of the out of loop frequency is 4.35 × 10 ⁻ ¹ ⁷/s, 1000 s integration time drops to 6.54 × 10 ⁻ ¹ As shown in Figure 2 (b), this will open up new doors for many applications that require high-power and high-frequency stability optical combs. The research progress has recently been summarized as "High power optical frequency comb with 10 ⁻ ¹ The topic 'frequency feasibility' was published in Optics Express. (Opt. Express 31 (20), 32813-32823 (2023)). The first author of the article is Shao Xiaodong, and the collaborating mentors are Wei Zhiyi and Han Hainian. The work was supported by the Chinese Academy of Sciences pilot projects (XDA150204004, XDB210104004) and the National Natural Science Foundation of China (60808007, 61378040, 11078022, 91850209).

Source: Institute of Physics, Chinese Academy of Sciences