A Cascaded Tunable DFB Semiconductor Laser With Compact Structure
A compact cascaded tunable distributed feedback semiconductor laser is proposed and theoretically analyzed. Each laser section (LS) is formed by two adjacent grating sections (GSs) with slightly different Bragg wavelengths and a <inline-formula> <tex-math notation="LaTeX">$\pi $ </tex-math></inline-formula> phase shift in the joint between them. A step-wise grating period profile is designed to realize multi-channel lasing. Since two LSs share a common GS, the total cavity length of the tunable laser is significantly reduced. As an example, a tunable laser with six GSs and five <inline-formula> <tex-math notation="LaTeX">$\pi $ </tex-math></inline-formula> phase shifts was designed and analyzed, resulting in a continuous tuning range of 12 nm. The total length of the laser is reduced by 40% compared with the conventional structure. Furthermore, an improved structure with apodized grating in each GS is also proposed. The single mode stability and fabrication tolerance are significantly improved. Due to the compact structure, the cost of the proposed laser can be highly reduced. Therefore, we think the proposed structure would benefit the practical applications to the low cost tunable lasers in wavelength-division multiplexing systems.