Influence of Antireflection Coating on the Performance of Broadly Tunable Quantum Dot External Cavity Laser

The influence of effective facet reflectivity of gain devices on the performance of broadly tunable quantum dot external cavity lasers has been investigated in detail. Firstly, the properties of the gain devices without coating, with λ/4 SiO₂ antireflection coating and with λ/4 ZrO₂ antireflection coating were studied. It is found that the reduction in facet reflectivity has three expected effects on the performance of the gain device, including a difference in the light output power between the two facets, an enhancement of threshold current, and a blueshift of the lasing peak. Grating-coupled external cavity lasers with the three gain devices were then fabricated and their properties were systematically studied. With decreasing facet reflectivity, the tunable range extends to the short wavelength significantly with a little sacrifice of long wavelengths due to the limited gain of quantum dots’ grounded state. As a result, the external cavity laser with ZrO₂-coated gain device exhibits a 149.6 nm-tuning bandwidth. Besides, the output power of external cavity laser is enhanced after the antireflection coating processing due to the allowed raised injection level induced by the elevated threshold current of the free-running gain device. Especially for the external cavity laser with the ZrO₂-coated gain device, the maximum output power of a single wavelength reaches up to 223 mW. For the external cavity laser with SiO₂-coated gain device whose minimum reflectivity wavelength is designed at the first excited state, a nearly-flat top profile of output power versus wavelength is realized.