Xi'an Institute of Optics and Fine Mechanics has made progress in the field of integrated microcavity optical frequency comb

Recently, researcher Zhang Wenfu from the National Key Laboratory of Ultrafast Optical Science and Technology of Xi'an Institute of Optics and Mechanics, researcher Chen Wei from the academician team of Guo Guangcan from the Key Laboratory of Quantum Information of the Chinese Academy of Sciences of the University of Science and Technology of China, and professor Yang Jun from the School of Intelligent Science of the National University of Defense Technology have cooperated to make progress in the field of integrated microcavity optical frequency combs. The team has developed two sets of independently pumped "identical" microcavity soliton optical frequency combs based on techniques such as microwave injection, optical frequency reference, and thermal perturbation frequency tuning. Based on this, experiments have verified the high visibility Hong Ou Mandel (HOM) interference between 50 channel comb teeth that meet the ITU frequency spacing standard (50GHz), demonstrating the feasibility of using classical wavelength division multiplexing optical communication multiplexing to achieve large-scale parallel quantum communication.

The research team proposes to use an integrated microcavity dual optical frequency comb to solve the strict matching problem of non classical photon interference on independently generated photons in terms of frequency, spatiotemporal mode, polarization state, etc., breaking through the limitations of traditional atomic transition frequency reference lasers in terms of wavelength quantity and wavelength spacing. To achieve the above idea, the team developed an integrated microcavity optical frequency comb long-term stability and frequency alignment technology, realizing the generation of 50 channel "identical" comb pairs (photon pairs).

In terms of long-term stability of integrated microcavity optical frequency combs, it mainly involves pump frequency and repetition frequency locking. For optical frequency locking, the team used modulation transfer spectroscopy technology to lock the pump laser frequency to the transition frequency of rubidium atoms, achieving a two order of magnitude improvement in pump laser frequency stability (as shown in Figure 1 (a)); For repetitive frequency locking, high-order sideband microwave injection locking technology is adopted to reduce the repetitive frequency jitter from kHz level to Hz level (as shown in Figure 1 (b)). The full locking of optical frequency and repetition rate ensures the constant optical field inside the cavity, ensuring the long-term stable operation of the integrated microcavity optical frequency comb. As shown in Figure 1 (c), this full locking scheme successfully achieves the sustained stable existence of dissipative optical solitons in the microcavity for more than 120 hours (limited by experimental time). Compared with the free running microcavity optical frequency comb, the frequency stability of the comb teeth is improved by about 3 orders of magnitude.

Figure 1. Integrated microcavity optical frequency comb fully locked

In terms of frequency alignment of independently generated dual integrated microcavity optical frequency combs, it mainly involves achieving fine tuning of frequency under the premise of mode locking. Due to the inevitable material and structural errors in micro nano processing, there are slight differences in the free spectral range (FSR) and resonant frequency between different micro ring resonators, resulting in frequency differences between the comb teeth of independently generated micro cavity optical frequency combs in different locations, which accumulate positively with the mode order, seriously affecting the homogeneity of photons between the dual micro cavity optical frequency combs. Therefore, based on the stable frequency of the comb teeth of the fully locked microcavity optical frequency comb, it is necessary to further achieve the repetition frequency adjustment of soliton microcavities through physical means, in order to achieve frequency alignment of independent microcavity optical frequency combs generated by different microcavities in different locations. In the experiment, the thermal effect management inside the cavity was achieved through the auxiliary photothermal balance scheme of acousto-optic frequency shift, which extended the soliton step length by more than one order of magnitude, reaching 3GHz (as shown in Figure 2 (a)), effectively ensuring the stability of soliton mode locking under perturbation; Furthermore, by fine-tuning the three physical quantities of pump power, beat frequency between pump and auxiliary laser, and microcavity temperature, fine tuning of the repetition frequency of microcavity optical frequency combs exceeding 100kHz frequency range can be achieved, achieving frequency alignment between multiple comb teeth of two independently generated microcavity optical frequency combs in different locations (as shown in Figure 2 (b)).

Figure 2. Fine tuning and comb alignment of two sets of integrated microcavity optical frequency combs

Finally, based on two sets of independently generated frequency strictly aligned fully locked integrated microcavity optical frequency combs, HOM interference between 50 channel comb pairs was achieved, with an average interference visibility of over 46% (as shown in Figure 3 (a)), demonstrating the feasibility of using the classical wavelength division multiplexing optical communication multiplexing approach to achieve large-scale parallel quantum communication (as shown in Figure 3 (b)), laying the technical foundation for constructing more efficient and scalable quantum communication systems based on integrated optics.

Figure 3. HOM interference of 50 channels based on independently generated microcavity frequency comb

The relevant research results were published in the journal Science Advances under the title "Massive parallel Hong Ou Main interference based on independent solute microcoms" and were recommended by the editor as featured in this issue, as shown in Figure A1. Dr. Huang Long and Associate Researcher Wang Weiqiang from Xi'an Institute of Optics and Fine Mechanics, as well as Associate Researcher Wang Zhixiang from University of Science and Technology of China, are the co first authors of the paper. Dr. Wang Yang and Tang Linhan from Xi'an Institute of Optics and Fine Mechanics participated in the main experimental work, while Zhang Wenfu, Chen Wei, and Wang Guochao from National University of Defense Technology are co corresponding authors. Researcher Zhao Wei from Xi'an Institute of Optics and Fine Mechanics provided careful guidance on the work. This work has received support from the National Key Research and Development Program, the National Natural Science Foundation, and the Science and Technology Innovation 2030 Major Project.

Source: opticsky