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We present a monolithic integrated low-threshold Raman silicon laser based on silicon-on-insulator (SOI) rib waveguide ring cavity with an integrated p-i-n diode. The laser cavity consists of a race-track shaped ring resonator connected to a straight bus waveguide via a directional coupler which couples both pump and signal light into and out of the cavity. Reverse biasing the diode with 25V reduces the free carrier lifetime to below 1 ns, and stable, single-mode, continuous-wave (CW) Raman lasing is achieved with threshold of 20mW, slope efficiency of 28%, and output power of 50mW. With zero bias voltage, a lasing threshold of 26mW and laser output power >10mW can be obtained. The laser emission has high spectral purity with a side-mode suppression of >80dB and laser linewidth of <100 kHz. The laser wavelength can be tuned continuously over 25 GHz. To demonstrate the performance capability of the laser for gas sensing application, we perform absorption spectroscopy on methane at 1687 nm using the CW output of the silicon Raman laser. The measured rotationally-resolved direct absorption IR spectrum agrees well with theoretical prediction. This ring laser architecture allows for on-chip integration with other silicon photonics components to provide an integrated and scaleable monolithic device. By proper design of the ring cavity and the directional coupler, it is possible to achieve higher order cascaded Raman lasing in silicon for extending laser wavelengths from near IR to mid IR regions.

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