Shanghai Institute of Microsystems makes important progress in self-referencing terahertz dual optical comb

Recently, the terahertz (THz) photonics research team led by Juncheng Cao and Hua Li at the Shanghai Institute of Microsystems and Information Technology, Chinese Academy of Sciences, in collaboration with Professor Heping Zeng's team at East China Normal University, has made important research progress in highly stable self-referencing terahertz dual optical combs.

 

The project team proposed a self-referencing method to completely eliminate the common carrier noise of the THz dual optical comb, while suppressing the repetitive frequency noise, compressing the THz dual optical comb comb tooth linewidth from the unstabilized 2-3 MHz order of magnitude to 14.8 kHz, and significantly improving the stability of the THz dual optical comb light source. The results were published in Laser & Photonics Reviews on February 3, 2023 under the title "Terahertz Semiconductor Dual-comb Source with Relative Offset Frequency Cancellation". Photonics Reviews, and was selected as the cover paper.

 

The dual-comb consists of two optical frequency combs with slightly different repetition frequencies, and maps the spectral information directly in the microwave band through multiple outlier sampling. This time-delayed structure, which does not rely on mechanical scanning, gives the dual-comb a natural advantage of high speed and high resolution, and has important applications in high-precision spectroscopy, imaging, ranging, and high-capacity high-speed communication.

 

In the THz band, an electrically pumped semiconductor quantum cascade laser (QCL) is an ideal vehicle to realize THz optical frequency combs with dual optical combs. Currently, THz QCL dual optical combs usually operate in free-running mode with high phase noise, which limits their high-precision applications. The main idea to improve the frequency stability of dual optical combs is to control the two optical frequency comb fundamental frequency components, i.e., carrier envelope offset frequency and repetition frequency, separately. To fully lock the THz QCL dual optical comb requires locking four different frequencies simultaneously, namely two carrier envelope offset frequencies and two repetition frequencies.

 

Although the project team has achieved locking of one tooth of the THz dual optical comb with a phase-locked loop and improved the stability of the dual optical comb, complete hardware locking of the THz dual optical comb has not yet been achieved. To achieve full locking of the four frequencies in the laboratory would involve a very complex hardware system.

 

In this work, the researchers propose a self-referencing "soft locking" method to manipulate the overall signal of the dual-comb without any hardware locking module to achieve a highly stable self-referencing THz QCL dual-comb light source. The noise of the dual comb teeth comes from the carrier envelope offset frequency and repetition frequency noise of the two unlocked optical combs, and each comb tooth of the dual comb generated by multi-outlier tapping enjoys the same carrier envelope frequency and noise. By eliminating the common carrier envelope frequency noise, the stability of each dual comb tooth can be significantly improved.

 

As shown in Fig. 1(a), one of the comb teeth is filtered out and mixed with the whole dual-comb signal by a narrowband filter, which completely eliminates the common carrier noise of the dual-comb teeth and suppresses the repetitive frequency noise, resulting in a zero-bias dual-comb with no carrier envelope offset frequency and significantly improving the long-term stability of the dual-comb signal. Figure 1(b) shows the spectrum of an unstabilized THz dual optical comb with a measured "maximum hold" line width of 2 MHz for 15 s. Figure 1(c) shows the spectrum of a THz dual optical comb after applying a self-referenced stabilization frequency. The measured "maximum hold" linewidth is 14.8 kHz within 60 s, which is more than 130 times higher than the linewidth of the unstabilized THz dual optical comb.

 

The self-referenced frequency stabilization method proposed in this work does not rely on any locking element and can be easily transferred to other laser systems, providing a simple and effective method for improving the stability of various applications such as spectroscopy and imaging.

 

 

Figure 1(a) Self-referenced frequency stabilization principle. The "rainbow" spectrum represents the down-converted dual-comb signal in the MHz range, and one of the combs is filtered out through a bandpass filter (dashed box), thus achieving a zero-bias self-referencing frequency by mixing dual optical combs. (b) Unstabilized THz two-comb "max hold" spectrum with a measurement time of 15 s. (c) Self-referenced two-comb "max hold" spectrum with a measurement time of 60 s. (d) Unstabilized THz two-comb "max hold" spectrum with a measurement time of 60 s.

 

 

Figure 2 Paper cover

 

The co-first authors of this paper are Ziping Li, associate researcher, and Xuhong Ma, Ph.D. student of Shanghai Institute of Microsystems, Chinese Academy of Sciences, and the co-corresponding authors are Hua Li, Juncheng Cao, and Heping Zeng. Meanwhile, Associate Professor Min Li from Shanghai University of Technology and Researcher Ming Yan from East China Normal University also made important contributions to the work. This work was supported by the Key Project of National Natural Science Foundation of China (62235019), the National Excellent Youth Science Foundation of China (620022084), the Young Team Program for Stable Support of Basic Research of Chinese Academy of Sciences (YSBR-069), and the Original Innovation Project of Chinese Academy of Sciences (ZDBS-LY-JSC009). LY-JSC009), CAS Research Instrument Development Project (YJKYYQ20200032), Shanghai Outstanding Academic Leaders Program (20XD1424700), etc.

Links to papers:

https://doi.org/10.1002/lpor.202200418

 

Cover link:

https://doi.org/10.1002/lpor.202370016

 

Reference link:

https://mp.weixin.qq.com/s/QKOx9vpJj2ByrBpCk5J_AQ