Monolithic Dual Wavelength DFB Lasers Based on Sidewall Gratings for THz/MMW Signal Generation

We have developed a series of dual-wavelength DFB lasers (DWLs) based on different sidewall grating designs. These include DWLs utilizing uniform Bragg gratings (UBG), conventional sampled Bragg gratings (C-SBG), two-phase shifted sampled Bragg gratings (2PS-SBG), and four-phase shifted sampled Bragg gratings (4PS-SBG) for THz signal generation. Additionally, we have explored the use of 4PS-SBG combined with equivalent chirp technology, lateral modulation of the grating coupling coefficient κ, and four-phase-shifted sampled Moiré gratings (4PS-SMG) for millimeter-wave (MMW) signal generation. All the DWLs were fabricated in the AlGaInAs/InP system operating around 1550 nm. For DWLs using UBG, even employing e-beam lithography at its resolution limit of 0.5 nm, the smallest frequency separation is constrained to approximately 400 GHz. C-SBG designs allow precise control of the frequency separation, down to 1.1 GHz, but the effective grating coupling coefficient is only 1/π of that of UBG. Utilizing 2PS-SBG technology can automatically produce DWLs while maintaining the same sampling periods on both sides of the ridge waveguide. The 4PS-SBG demonstrates a higher effective κ (approximately 0.9x that of UBG) compared to the 2PS-SBG (approximately 0.64x that of UBG). To ensure single longitudinal mode operation and mitigate longitudinal mode competition, an equivalent π phase shift (EPS) is inserted at 1/3 of the DFB cavity length on one side, and another EPS is placed at 2/3 of the DFB cavity length on the opposite side. Using equivalent chirp methodology, the two peaks of the photon distributions of the two lasing modes are separated, significantly reducing the overlap region and ensuring stable dual-wavelength operation. Moreover, the wavelength separation can be adjusted by changing the chirp rate. Lateral modulation of the grating coefficient κ allows tuning of the dual-wavelength separation by adjusting the DWL cavity length and the κ value. We show that 4PS-SMG exhibits perfect apodization with a cosine profile and two π phase shifts in the cavity, eliminating the need for intentional insertion of two π phase shifts in the DWL cavity to achieve dual-wavelength operation. All the aforementioned DWLs serve as compact pumping sources for generating THz/MMW signals.