Lithium niobate on insulator (LNOI) has become a research focus of photonic integrated circuits (PICs) due to its excellent material features, such as high electro optical and second-order nonlinear coefficients, extraordinary acousto-optic effects, piezoelectric effects and photoelastic effect. Plenty of on-chip optical devices have been achieved successfully on LNOI, for instance, modulators, frequency combs, nonlinear frequency converters. Recently, erbium-doped LN has been developed, and a series of research has been reported including waveguide amplifiers and microcavity lasers. Nevertheless, most of the reported lasers based on Er3+-doped microdisk are multi-modes. The on-chip integrated C-band single-mode laser is critical and needs to be developed to improve monochromaticity, stability, and beam quality.

We demonstrate an integrated tunable whispering gallery single-mode laser (WGSML) by making use of a coupled microdisk and microring on LNOI. The WGSML at 1560.40 nm m with a maximum 31.4 dB side mode suppression ratio is achieved with the 974 nm single-mode pump. The threshold pump power of WGSML is about 1.31 mW, and the corresponding slope efficiency is 4.41×10-5. By regulating the temperature, the output power of the WGSML increases, and the central wavelength can be changed from 1560.30 to 1560.40 nm. Furthermore, 1560.60 and 1565.00 nm WGSMLs have been achieved by changing the coupling gap width between the microdisk and microring.

Figure1. (a) Schematic diagram of the photonic molecule on Er3+-doped LNOI. (b) The scanning electron microscope (SEM) image of the fabricated sample (top) and the coupling regions among straight waveguide, microdisk, microring, and pulley-coupling waveguide(bottom). (c) Principle of the WGSML emission in the microring and microdisk. (d) The resonant wavelength of two coupled microresonators with different radii. Red solid lines and green dashed lines are numerically calculated resonant wavelengths of the microdisk with a radius of R1 = 150 µm and the microring with a radius of R2 = 165 µm. The pentagram points to the same wavelength resonant at 1560.20 nm.

By designing a relatively large radius (R1 = 150 µm), we ensure that the 974 nm single-mode pump light can resonate easily in the microdisk without extra adjustment, which reduced the threshold power of pump light and led to a lower experimental requirement. The single-mode laser in 1550 nm band (1560.40 nm) can be effectively achieved by designing suitable parameters of microring (R2 = 165 µm, waveguide width of 1.2um), and high side mode suppression ratio can be obtained. By regulating the temperature, the central wavelength of WGSML is tunable from 1560.30 to 1560.40 nm. In addition, coupled microdisks and microrings with different gap widths are fabricated, and 1560.60 and 1565.00 nm WGSMLs have been achieved with side mode suppression ratio of 22.60 dB and 22.38 dB respectively, which demonstrates prominent single-mode characteristics.

Figure 2. (a) Spectra of the WGSML under different wavelengths and pump powers. (b) The relationship between the WGSML 1560.40 nm power and the 974 nm pump power. (c) and (d) are the relationship between the temperature and the output power of the WGSML and central wavelength, respectively.

The above research has demonstrated a tunable single-mode laser in C-band on erbium-doped lithium niobate thin film. With the integrated C-band single-mode laser on erbium-doped LNOI, a series of on-chip optical devices and applications can be developed and shows great potential in photonic integrated circuits on LNOI.

The research is published by “Xiangmin Liu, Xiongshuo Yan, Yi’an Liu, Hao Li, Yuping Chen, and Xianfeng Chen, Tunable single-mode laser on thin film lithium niobate, Optics Letters, 46, 5505-5508 (2021)”。

Link： https://www.osapublishing.org/ol/fulltext.cfm?uri=ol-46-21-5505&id=462759