Abstract

We demonstrate monolithic 160-µm-diameter rare-earth-doped microring lasers using silicon-compatible methods. Pump light injection and laser output coupling are achieved via an integrated silicon nitride waveguide. We measure internal quality factors of up to 3.8 × 105 at 980 nm and 5.7 × 105 at 1550 nm in undoped microrings. In erbium- and ytterbium-doped microrings we observe single-mode 1.5-µm and 1.0-µm laser emission with slope efficiencies of 0.3 and 8.4%, respectively. Their small footprints, tens of microwatts output powers and sub-milliwatt thresholds introduce such rare-earth-doped microlasers as scalable light sources for silicon-based microphotonic devices and systems.

© 2014 Optical Society of America

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Figures (10)

Fig. 1
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 SiNx layer; (iii) patterning of the SiNx rings and bottom part of SiNx bus waveguide (at far right) followed by SiO2 encapsulation; (iv) deposition of the upper SiNx layer; (v) patterning and SiO2 encapsulation of the upper SiNx layer to define the trench etch stop and top part of the SiNx bus waveguide; (vi) microring trench etch and removal of SiNx 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
Fig. 2

(a) Top-view optical microscope image of an on-chip rare-earth-doped microlaser showing the integrated microring resonator and SiNx 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 SiNx and Al2O3:RE3+ microring resonator; (d) close-up view of the coupling region (indicated by the red box in (c)), showing the SiNx bus waveguide, waveguide width, w, and microring-waveguide gap, g.

Fig. 3
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
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, Qi, of 3.8 × 105 and 2.7 × 105.

Fig. 5
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, Qi, of 5.7 × 105 and 4.2 × 105 for the 1550 nm TE-like and TM-like modes, respectively.

Fig. 6
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
Fig. 7

Emission spectrum of an Er-doped microring laser with NEr,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
Fig. 8

Output power vs. on-chip 978.84-nm pump power for an Er-doped microring laser with NEr,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
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
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 SiNx waveguide, with a lasing threshold of 0.7 mW and double-sided slope efficiency of 8.4%.

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