Dual-wavelength lasers are widely used in lidar, microwave photonic systems, and optical sensing systems. Traditional dual-wavelength lasers usually consist of two discrete lasers. Due to the influence of external temperature and environmental vibration, the operation stability of traditional dual-wavelength lasers is poor, which limits their application. To solve the above problems, multiple integrated dual-wavelength distributed feedback (DW-DFB) lasers have been developed, including integrated Y-waveguide DW-DFB lasers, multi-section DW-DFB lasers, and transverse coupling DW-DFB lasers. Monolithic integrated DW-DFB lasers boast a compact structure and stable performance. However, they usually require complex manufacturing processes and control circuits, which indicate high costs. Therefore, a dual-wavelength laser with a simple structure and good stability is urgently needed.

In this paper, a monolithic integrated two-section DW-DFB (TS-DW-DFB) laser is proposed experimentally. The TS-DW-DFB laser consists of a DW-DFB laser section and a grating reflector (GR) section. The two sections use the same epitaxial layer structure and share the same waveguide. Both facets of the TS-DW-DFB laser are deposited with anti-reflection coatings. The total length of the TS-DW-DFB laser chip is 1000 μm, and the lengths of the DW-DFB laser section and GR section are both 500 μm. There is also an electrical isolator between the two sections. The gratings of the two sections are fabricated by the reconstruction-equivalent chirp (REC) technique with the same seed grating. The grating in the DW-DFB laser section is a linearly chirped sampling grating with two π-phase-shifts at its 1/3 and 2/3 length, and the chirp ratio of the grating is 50 nm/mm. The grating in the GR section is a uniform sampling grating, and the sampling period of the grating is equal to the sampling period at the center of the grating in the DW-DFB laser section. To eliminate the side lobes in the reflection spectrum of the GR section and to decrease their reflection to the side modes of the DW-DFB laser section, we equivalently apodize the sampling grating in the GR section by changing the sampling duty cycle. As a consequence, the two main modes of the DW-DFB laser section are symmetrically located at the stop-band center of the reflection spectrum of the sampling grating in the GR section, while the side modes lie outside of the stop-band. The reflectivity of the grating in the GR section for the main modes is much higher than that for the side modes, which can lead to a much higher side-mode suppression ratio (SMSR). Due to the reflection of the GR section, the threshold current and power difference of the main modes (PDM) of the TS-DW-DFB laser are reduced.

Using the transfer matrix method, the output characteristics of TS-DW-DFB lasers with different grating structures are analyzed. The simulated results show that the proposed TS-DW-DFB laser has a lower threshold current and higher output power [Figs. 4 and 5(a)] compared with the single-section DW-DFB laser. The PDM can also be optimized by changes in the length of the GR section [Fig. 5(b)]. Given these results, the TS-DW-DFB laser is fabricated. In contrast, a single-section DW-DFB laser with the same structure of the DW-DFB laser section as the TW-DW-DFB laser is also fabricated. The optical spectra of the single-section DW-DFB laser and TS-DW-DFB laser are measured under the same biased currents (Fig. 7). It is obvious that the TS-DW-DFB laser has a smaller PDM and a larger SMSR than the single-section DW-DFB laser. The influence of the biased current of the GR section on the PDM and SMSR of the TS-DW-DFB laser is also discussed (Fig. 8), and the current of the GR section is adjusted to optimize the PDM and SMSR simultaneously. In addition, the operation stability is a figure of merits for dual-wavelength lasers. When the temperature of the TS-DW-DFB laser is tuned from 16 ℃ to 40 ℃, both the wavelengths of the main modes shift toward long wavelength [Fig. 10(b)]. However, the PDM, SMSR, and wavelength spacing of the TS-DW-DFB laser are not changed significantly (Fig. 11). Their changes in one hour are observed when the operation temperature is kept at 24 ℃, and the bias currents of the DW-DFB laser section and GR section are set at 80 mA and 12 mA, respectively. The results reveal that the PDM, SMSR, and wavelength spacing remain stable within an hour.

The in-cavity mode competition of dual-wavelength lasers is fierce, and thus, mode stability is a key figure of merit for dual-wavelength lasers. To reduce the power difference between the two main modes and improve SMSR, we propose a DW-DFB laser integrated with GR. The grating structure in the dual-wavelength laser is simulated by the transfer matrix method. The influence of GR on the threshold and PDM of the laser is analyzed. After that, a monolithic integrated TS-DW-DFB semiconductor laser is fabricated and experimentally demonstrated. The measured results show that the proposed method can improve the operation stability, enhance the SMSR, and reduce the PDM of the DW-DFB laser. In a stable operation state, the PDM of the dual-wavelength laser is less than 0.3 dB, and the SMSR is larger than 35 dB.