Microresonator-based Kerr soliton microcombs are coherent light sources consisting of equally spaced and phase locked discrete optical frequency components, which are essential tools for practical applications in precision spectroscopy and data processing. While anomalous microresonator dispersion is mandatory for Kerr soliton microcomb formation, so far almost all dispersions are susceptible to manufacturing error and cannot be tuned once the microstructure is made. Moreover, microcomb formation in strongly Raman-active mediums like Lithium Niobate (LN) is challenging in the suppression of stimulated Raman scattering and mode crossing due to the existence of densely distributed multiple Whispering Galley Mode (WGM) families. Here, Kerr soliton microcombs were formed in a normal dispersion LN microdisk resonator by mode trimming. Despite that the fundamental WGM family is of normal dispersion and there are densely distributed high-order WGM families within the LN microdisk, high-Q square modes of anomalous dispersion and small mode volume are coherently formed by introducing weak perturbation for mode trimming. Under the optical pump of the square mode of 35-mW power, densely distributed WGM families are avoided to be excited, leading to the suppression of Raman scattering effects and mode crossing, and the formation of soliton microcomb with a spectrum spanning from 1450 nm to 1620 nm.
Mode trimming of suitable dispersion in a high-Q microresonator is of vital importance for lots of photonic applications ranging from Kerr soliton comb generation, single-frequency lasing to nonlinear frequency conversion. However, almost all the dispersion engineering and mode trimming cannot be tuned once the resonator structure is made. To overcome this difficulty, weak perturbation was introduced into a circular lithium niobate (LN) microdisk for the formation of polygon modes by a coupled tapered fiber. Various polygon modes with different spatial field distributions and star mode can be formed by tuning the pump wavelength and the coupling position. Thanks to the small spatial modal overlap between the polygon modes and the whispering gallery modes (WGMs), densely distributed WGM families were avoided to be excited under polygon-mode optical pump. Therefore, single-frequency/dual-wavelength microlasers have been demonstrated with narrow linewidths in weakly-perturbed erbium-doped LN microdisks, and Kerr soliton microcomb has been generated in the telecom band in a normal-dispersion LN microdisk by mode trimming through the excitation of the anomalous-dispersion polygon modes.
Single-frequency microlasers and photonic generation of microwave signal with a fast tuning are highly in demand for lots of applications, including high-resolution spectroscopy, precision metrology, coherent communication, and so on. Design and fabrication of narrow-linewidth microlasers and low-phase-noise microwave signal are challenging. Here, we fabricated high-Q erbium ion doped lithium niobate microcavities for single frequency lasing through simultaneous excitation of high-Q polygon modes at both pump and laser wavelengths. Tunable single-mode microlasers with linewidth as narrow as 454 Hz was demonstrated. Moreover, photonic generation of low-phase-noise microwave signals were synthesized from dual-wavelength microlasers on single active LNOI microdisks. The dual-wavelength microlasers were generated from high-Q nearly degenerate polygon modes with spatial intensity distributions almost the same but a -phase difference. Due to the suppression of the gain competition, dual-wavelength lasing and in turn the low noise microwave source are stable. The phase noise of the microwave signal was measured to -123 dBc/Hz.
Single-frequency ultranarrow linewidth on-chip microlasers with a fast wavelength tunability play a game-changing role in a broad spectrum of applications ranging from coherent communication, light detection and ranging, to metrology and sensing. Design and fabrication of such light sources remain a challenge due to the difficulties in making a laser cavity that has an ultrahigh optical quality (Q) factor and supports only a single lasing frequency simultaneously. Here, we demonstrate a unique single-frequency ultranarrow linewidth lasing mechanism on an erbium ion-doped lithium niobate (LN) microdisk through simultaneous excitation of high-Q polygon modes at both pump and laser wavelengths. As the polygon modes are sparse within the optical gain bandwidth compared with the whispering gallery mode counterpart, while their Q factors (above 10 million) are even higher due to the significantly reduced scattering on their propagation paths, single-frequency lasing with a linewidth as narrow as 322 Hz is observed. The measured linewidth is three orders of magnitude narrower than the previous record in on-chip LN microlasers. Finally, enabled by the strong linear electro-optic effect of LN, real-time electro-optical tuning of the microlaser with a high tuning efficiency of ∼50 pm / 100 V is demonstrated.
We present our latest progresses on the development of integrated photonic devices as well as microfluidic chips of unprecedented characteristics and performances using femtosecond laser micromachining. We demonstrate ultra-high Q microresonators in lithium niobate on insulator (LNOI), on-chip micro-laser and waveguide amplifier, and high-throughput micro-chemical reactor. The achievements are the result of persistent effort on improving the precision and efficiency in ultrafast laser processing.
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