| Summary: | We design a unique external cavity configuration and present an ultra-narrow linewidth external cavity diode laser (ECDL) employing a wide interference filter and diffraction grating. The interference filter is for suppressing the noise floor, and diffraction grating is used for the mode selection. The combination of the filter and diffraction grating can effectively use a narrow linewidth without a significant reduction in optical gain. The Lorentz linewidth reaches 13.6 kHz at a wavelength of 1555 nm. The ECDL can be tuned by rotating the diffraction grating, and a wide tunable range of 40 nm is achieved with a maximum output power of 25.6 mW at an injection current of 200 mA. In addition, the ECDL shows an excellent side mode suppression ratio (SMSR) performance with a maximum of 57 dB at 1555 nm. Additionally, the SMSR changes gently throughout the tuning range at the injection current of 200 mA.
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