Multi-wavelength distributed feedback laser array based tunable semiconductor lasers have been widely used owing to their high mode stability and simple tuning scheme. Comparing with the in-parallel laser array, the in-series one can avoid a large power loss caused by the optical combiner. However, the single-mode stability of an in-series laser array is vulnerable to the reflections of other lasers. Here we experimentally demonstrated a tunable in-series laser array with high single-mode stability and high uniformity of wavelength spacing. To reduce the impact of such reflections, the Bragg gratings in different lasers were designed in phase, and the grating phase error was reduced by utilizing the reconstruction-equivalent-chirp technique. Besides, a three-section laser structure was applied in each laser to increase the priority of the dominant lasing mode. Four lasers with small wavelength-spacings of 2.5 nm were integrated in the in-series laser array. According to the measurement results of 40 in-series laser arrays (160 lasers), wavelength deviations of 90.6% lasers were within ±0.2 nm, average wavelength spacings of 97.5% of the measured laser arrays were deviated from the design within ±0.2 nm, and SMSRs of 96.3% lasers were above 45 dB. The output power was above 25 mW and the relative intensity noise was below -130 dB/Hz. Like the in-parallel laser array, the proposed laser array can be applied in either continuous wavelength tuning or fast channel switching. For the continuous wavelength tuning applications, the 10-nm tuning range was obtained with a temperature tuning range of less than 20 °C. For the fast channel switching applications, 4 channels with 2.5-nm spacing are available, and the switching time between two channels was less than 300 ns.

We propose and experimentally demonstrate a novel multi-wavelength DFB laser with high mode stability and uniform spacing for the optical I/O technology. By employing the linearly chirped sampled grating and multiple equivalent π phase shifts among the cavity, the four-wavelength and six-wavelength lasers with 100 GHz spacing are implemented and experimentally validated. The proposed multi-wavelength DFB laser works with high mode stability, high uniform wavelength spacing, and high uniform output power with varied bias currents and operating temperatures. A semiconductor optical amplifier is monolithically integrated to amplify the output and the output power exceeds 25 mW when the current injected into the SOA is 120 mA. Besides, the relative intensity noise of the filtered wavelengths is below -130 dB/Hz for each wavelength, and clear 25 Gb/s NRZ eye grams are obtained for the filtered each wavelength with the external lithium-niobate Mach-Zehnder modulator, which indicates the capability of the proposed structure to be an alternative approach for the multi-wavelength optical sources in DWDM optical I/O links.

We proposed a new type of TE-polarized mode-order converter based on a deeply-etched polygonal slot on a silicon-on-insulator (SOI) waveguide. Along the transverse direction of the waveguide, two irregular boundary surfaces (BSs) of the slot can introduce high-contrast index modulation on guided modes, leading to multimode interference in the slot. Therefore, when the slot is optimized, we can achieve efficient mode conversions based on the multimode interference. As examples, mode converters from the fundamental transverse electric mode $\rm (TE_0)$ to the first-order transverse electric mode $\rm (TE_1)$ and to the second-order transverse electric mode $\rm (TE_2)$ have both been demonstrated with a short device length ( 97.6%), and a low modal crosstalk (< -20.0 dB) over a broad wavelength range from 1500 to 1600 nm (∼ 100 nm). In addition, based on different polygonal slots, other types of mode conversions such as from $\rm TE_1$ to $\rm TE_3$ and from $\rm TE_2$ to $\rm TE_1$ have be realized. Fabrication error tolerance of the proposed structure is analyzed. Owing to the high efficiency and compact size, the proposed structure could be applied to the on-chip mode division multiplexing (MDM) systems for high-density integration.

A four-channel wavelength-selectable in-series DFB laser array with good single-longitudinal-mode (SLM) property is proposed and theoretically analyzed. To realize good SLM property, a linearly chirped Bragg grating with multiple phase shifts are implemented in the laser cavity. In addition, two reflector sections with chirped Bragg grating are specially designed for reflection compensation at both ends. In our simulation, the split-step time-domain model with coupled-wave equations and carrier rate equations are used. Based on the simulation results, all the channels work with low threshold currents and the SLM properties are considerably enhanced compared with the conventional in-series laser array. Notably a high precision control of grating phase is required in the practical fabrication due to the ultra-small variation of grating period. Therefore, the reconstruction-equivalent-chirp technique is discussed to provide the fabrication of high-precision grating. In addition, the complex heterogeneous integration or butt-joint technique is not required during the whole fabrication process, which simplifies the fabrication and reduces the cost. The proposed wavelength-selectable in-series laser array is deemed to be applied in the dense wavelength division multiplexing technology, especially when fast wavelength switching is needed, such as in the application of NGPON2.

In this letter, we experimentally demonstrated a 1550-nm high-power single-longitudinal-mode (SLM) distributed feedback (DFB) semiconductor laser based on sampled moiré grating (SMG). By designing sampling structure with micrometer scale, moiré grating (MG) can be equivalently realized along laser cavity. Then, we can reduce coupling coefficient near laser facet so as to increase output power. The cavity length and ridge width of the fabricated laser are 1.0 mm and 3.0 μ m, respectively. The measured threshold current and the slope efficiency are 30.0 mA and 0.36 mW/mA at the heat-sink temperature of 25 oC, respectively. When the injection current is 800.0 mA, the maximum output power is about 183.0 mW. The saturation power is significantly improved compared with conventional DFB laser with uniform sampled grating (USG), which was fabricated on the same wafer. In addition, a four-channel DFB laser array based on SMG was also fabricated on the same wafer, showing good wavelength uniformity.

Most types of tunable lasers are sensitive to the ambient temperature and require thermoelectric coolers (TECs) for temperature control. For conventional tunable laser modules, high power consumption of cooling is required for stabilizing the laser temperature and complex hermetic packages are normally used to avoid moisture condensation problems, which are expensive and difficult to manufacture. To overcome these drawbacks, we demonstrated an uncooled tunable laser via the monolithically integrated multi-wavelength distributed feedback (DFB) laser array based on the reconstruction-equivalent-chirp technique. Totally sixteen DFB semiconductor lasers are densely integrated with a 4-by-4 matrix laser array. To ensure the uncooled laser operates properly, the active material of the laser chip is designed to work at high temperatures without TEC cooling. Therefore, an uncooled wavelength-tuning range of 33.8 nm is achieved during the whole commercial operating temperature range from 0 °C to 70 °C.