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.

Based on the reconstruction-equivalent-chirp technique and tuning only by the injection current, a multisection DFB tunable laser is demonstrated experimentally. At the temperature of 20°C, each laser can be tuned at about 3.4-3.6 nm with a side-mode suppressing ratio (SMSR) of over 35 dB from 30 to 170 mA, and the tuning speed is about 10-30 ms, which is much faster than that by the thermoelectric cooler. The results may help to realize very low cost tunable lasers in the future.

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.

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 π 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 π 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.

We propose and theoretically analyze a hybrid transverse mode resonance DFB semiconductor laser which consists of one active section with π -phase shifted anti-symmetric Bragg grating ( π -ASBG) and two passive sections with uniform Bragg grating. The hybrid light resonance along with mode conversion between the fundamental transverse electric (TE ₀ ) mode and first order transverse electric (TE ₁ ) mode is formed by the π-ASBG. The emitted light from one facet with the TE ₀ or TE ₁ mode can be controlled by tuning the injection currents of the passive sections. As an example, a laser with ridge waveguide is designed and simulated. The transverse mode with the same wavelength is tuned by changing the injection currents of the passive sections. The suppression ratio between the two transverse modes reaches 18.4 dB and 18.7 dB when the TE ₀ and TE ₁ modes are emitted, respectively. A simple method for equivalently realizing the grating in the proposed laser was also analyzed. Therefore, as the transverse mode tunable light source, the proposed laser may benefit the future mode division multiplexing (MDM) systems for reconfigurability and more flexibility.