51-nm Uniform-Intensity Tuning of Superstructure Grating Active-DBR Laser for 2-μm Wavelength Band

To suppress the fluctuation of light intensity during wavelength tuning caused by carrier induced optical loss, we utilized a single quantum well (SQW) inserted into the bulk core layer of distributed Bragg reflectors (DBRs), called an active DBR structure, for a 2-μm-wavelength superstructure grating (SSG-) DBR laser. In the active DBR structure, some injected currents in the DBR regions are consumed for an optical gain in the inserted SQW, which suppresses the optical loss in the bulk core layer induced by the free-carrier absorption. The SSG-DBR laser fabricated with the active DBR structure exhibited a more than 8-nm lasing wavelength shift of a single SSG-DBR reflection peak. In addition, the reduction of light intensity was less than 2.7 dB when the total injection current of the front and rear DBR regions was 200 mA. This is significantly smaller than that of a conventional SSG-DBR laser that has the same device structure except for the SQW in the DBR regions, although a relatively large current was required for the wavelength shift. Evaluations of the lasing characteristics under various combinations of the front and rear DBR currents demonstrated that all seven SSG-modes could be selectively oscillated for wavelength tuning. In addition, the measured fluctuation of light intensities was less than 5 dB for almost all conditions even when the front and rear DBR currents were individually changed from 0 up to 100 mA. Furthermore, the active-DBR type laser simultaneously attained the quasi-continuous tuning range of 51.2 nm and the fluctuation of light intensity across the whole range of less than 2.5 dB which represents a significant suppression compared to the conventional SSG-DBR laser with the same laser cavity design.