Chinese Optics Letters

As a highly coherent light source, the soliton frequency comb generated on the microresonator has many applications thanks to its high integrity, low power consumption, and low phase noise. Especially, octave-spanning soliton frequency combs via dispersion engineering can realize a chip-scale 2f 3f or f-2f self-referencing scheme. This gives it the potential to become a new frequency standard. However, due to thermal effects or center frequency jitter induced by the pump, achieving a long-term stable soliton comb is still challenging, it often requires a complex feedback system, which hinders the minimization or operation of the device.

 

In recent years, thin-film lithium niobate (TFLN) photonic devices have been greatly developed and studied because of the broadband low-loss optical transparent window, excellent quadratic nonlinear effect, and high electro-optic tuning efficiency. Moreover, the TFLN has a considerable third-order nonlinear coefficient which can be used as a material to generate microcombs on the chip.

 

In particular, the optical intensity-dependent photorefractive effect of TFLN is just opposite to the thermo-optic nonlinear effect, so it enables a new mechanism for effectively generating soliton optical frequency combs. However, whether this mechanism of action can achieve long-term self-stability of solitons has not been verified.

 

To further study the characteristics of soliton optical comb generation in TFLN microresonators, Prof. Shi-Ning Zhu (academician of CAS), and Prof. Zhenda Xie's team at Nanjing University, experimentally report the preparation and characterization of High-Q microresonators on a TFLN platform. The generation of soliton optical frequency combs with long-term passive stability has been observed. The relevant results were published in Chinese Optics Letters, Vol. 22, No. 5, 2024.

 

In this paper, the microring resonator pattern is first generated on the photoresist by a standard lithography process, and then the microresonator is prepared by ion beam enhanced etching on the TFLN. The basic optical parameters and geometric characterization of the prepared device are carried out, and the Q-factor coefficient of the microresonator is greater than 1×106. The experimental results show that the dispersion distribution of the microresonator is consistent with the design. Subsequently, the frequency comb under different states was obtained by pumping frequency scanning method, and the spectrum and low-frequency phase noise of the optical comb were further characterized (as shown in Figure 1). Under the influence of the TFLN thermo-optical effect and light intensity-dependent photorefractive effect, the red detuning region in the microresonator becomes relatively heat-stable, which enables solitons to stay in this region for a long time (as shown in Figure 2). In particular, the researchers observed that the survival time of single-soliton optical combs generated directly by a single laser pump without any feedback is greater than 3 hours. Finally, this work demonstrates the generation of a 0.7-octave TFLN microresonator broadband Kerr optical frequency comb based on dispersion engineering. The research team pointed out that the experimental results illustrate the potential of the TFLN platform as a source of high-performance on-chip soliton optical comb. Further combined with domain engineering, electro-optical modulators, and other integrated photonic technologies, it is of great significance to realize the integrated application of coherent optical communication or microwave photon technology based on the TFLN platform.

 

Figure 1. The characterization of the long-term stable frequency comb

 

Figure 2. The schematic diagram of passively stable soliton in the TFLN resonator