External ring cavity interband cascade lasers for broadband gas sensing

A directional coupled emission external ring cavity (ERC) based on an interband cascaded laser (ICL) was designed and manufactured. The laser emitted from the front cavity facet of the Fabry-Perot ICL is vertically incident on the rear cavity facet through the blazed grating and three gold-plated reflection mirrors, thereby forming a closed-loop system. The laser emitted from the front and rear cavity facets form clockwise and counterclockwise traveling waves in ERC, respectively. Since the anti-reflection film coating only on the front cavity facet introduces different losses in two directions, resulting in directional coupling in the clockwise direction. And the 0th-order reflected light generated by the directional coupling light passing through the blazed grating is used as the output light. The emitted laser from the two cavity facets enter the ICL again through ERC, enhancing the gain of the ICL. Therefore, a higher output power and wider tuning range are achieved compared to the traditional external cavity with 0th-order diffracted light as output light. The output power of ERC-ICL is 75% of the output power of the corresponding Fabry-Perot ICL, which is much higher than the output power of Littrow external cavity ICL. And single-mode operation is observed within the tuning range of 163 cm-1 (220nm), which is twice the tuning range of Littrow external cavity.