Monolithic erbium- and ytterbium-doped microring lasers on silicon chips

Fig. 1 (a) Silicon-compatible microring laser fabrication steps: (i) deposition of the SiO₂ bottom-cladding layer on a silicon substrate; (ii) deposition of the lower SiN_x layer; (iii) patterning of the SiN_x rings and bottom part of SiN_x bus waveguide (at far right) followed by SiO₂ encapsulation; (iv) deposition of the upper SiN_x layer; (v) patterning and SiO₂ encapsulation of the upper SiN_x layer to define the trench etch stop and top part of the SiN_x bus waveguide; (vi) microring trench etch and removal of SiN_x etch stop; (vii) deposition of the Al₂O₃:RE³⁺ gain medium into the trench. (b) Illustration of the resulting monolithic rare-earth-doped microring laser structure.

Fig. 2 (a) Top-view optical microscope image of an on-chip rare-earth-doped microlaser showing the integrated microring resonator and SiN_x bus waveguide; (b) scanning electron micrograph (SEM) image of the Al₂O₃:RE³⁺-filled trench on top of the silicon chip; (c) SEM cross-section image of the edge of the SiN_x and Al₂O₃:RE³⁺ microring resonator; (d) close-up view of the coupling region (indicated by the red box in (c)), showing the SiN_x bus waveguide, waveguide width, w, and microring-waveguide gap, g.

Fig. 3 980-nm pump coupling in undoped microrings with w = 0.4 µm and microring-waveguide gaps ranging from 0.1 to 1.0 µm. Maximum coupling for both TE and TM polarizations occurs at gaps near 0.5 µm, where the internal and external quality factors of the resonator are matched.

Fig. 4 980-nm transmission measurements in an undoped microring with w = 0.4 µm and g = 0.7 µm. The top plots were measured over a scan range of 4 nm (10 pm step size) and show coupling to multiple TE-like (left) and TM-like (right) resonances. The optimal calculated Er- and Yb-doped microring pump modes (TE1 and TM1) are shown in the insets and their resonances indicated on the plots. Bottom: high resolution (1 pm step size) scans of single TE1 (left) and TM1 (right) resonances. By fitting the data using a Lorentzian function we determined internal quality factors, Q_i, of 3.8 × 10⁵ and 2.7 × 10⁵.

Fig. 5 1550-nm transmission measurements in an undoped microring with w = 0.9 µm and g = 1.0 µm. The top plots were measured over a scan range of 4 nm (1 pm step size) and show coupling to multiple TE-like (left) and TM-like (right) modes. The calculated optimum 1550-nm Er-doped microring laser modes (TE1 and TM1) are shown in the insets and their corresponding resonances in the undoped microrings indicated on the plots. Bottom: high resolution (0.1 pm step size) scans of single TE1 (left) and TM1 (right) resonances. By fitting the data using a Lorentzian function we determined internal quality factors, Q_i, of 5.7 × 10⁵ and 4.2 × 10⁵ for the 1550 nm TE-like and TM-like modes, respectively.

Fig. 6 Top-view of an Er-doped microring laser injected with 980-nm pump light, showing the characteristic spontaneous green-light emission from excited Er³⁺ ions.

Fig. 7 Emission spectrum of an Er-doped microring laser with N_Er,peak = 3 × 10²⁰ cm^-3 and g = 0.3 µm and pumped using a laser diode centered at 976 nm. The output is single-mode at 1559.82 nm with a side-mode suppression of > 30 dB.

Fig. 8 Output power vs. on-chip 978.84-nm pump power for an Er-doped microring laser with N_Er,peak = 2 × 10²⁰ cm^-3 and g = 0.5 µm. The laser threshold is 0.5 mW and the double-sided slope efficiency is 0.3%.

Fig. 9 Emission spectrum of an Yb-doped microring laser with g = 0.4 µm and under resonant pumping at 970.96 nm. The output is single-mode at 1042.74 nm with a side-mode suppression of > 40 dB (inset).

Fig. 10 Output power vs. on-chip 970.96-nm pump power for an Yb-doped microring laser with g = 0.4 µm. The laser emits > 100 µW power into the integrated SiN_x waveguide, with a lasing threshold of 0.7 mW and double-sided slope efficiency of 8.4%.