A quantum dot (QD) mode-locked laser as an active comb generator takes advantage of its small footprint, low power consumption, large optical bandwidth, and high-temperature stability, which is an ideal multi-wavelength source for applications such as datacom, optical interconnects, and LIDAR. In this work, we report a fourth-order colliding pulse mode-locked laser (CPML) based on InAs/GaAs QD gain structure, which can generate ultra-stable optical frequency combs in the O-band with 100 GHz spacing at operation temperature up to 100°C. A record-high flat-top optical comb is achieved with 3 dB optical bandwidth of 11.5 nm (20 comb lines) at 25°C. The average optical linewidth of comb lines is measured as 440 kHz. Single-channel non-return-to-zero modulation rates of 70 Gbit/s and four-level pulse amplitude modulation of 40 GBaud/s are also demonstrated. To further extend the comb bandwidth, an array of QD-CPMLs driven at separate temperatures is proposed to achieve 36 nm optical bandwidth (containing 60 comb lines with 100 GHz mode spacing), capable of a total transmission capacity of 4.8 Tbit/s. The demonstrated results show the feasibility of using the QD-CPML as a desirable broadband comb source to build future large-bandwidth and power-efficient optical interconnects.
Direct epitaxial growth of III-V quantum dot (QD) lasers on Si (001) substrates is recognized as a promising and low-cost method for realizing high-performance on-chip light sources in silicon photonic integrated circuits (PICs). Recently, the CMOS-compatible patterned Si (001) substrates with sawtooth structures have been widely implemented to suppress the lattice mismatch induced defects and antiphase boundaries (APBs) for heteroepitaxial growth of high-quality III-V materials on Si. Considerable progresses have been made on high-performance 1300 nm InAs/GaAs QD lasers on Si (001). Here, we report a thermal stress-relaxed (111)-faceted silicon hollow structures by homoepitaxial method for reliable InAs/GaAs QD lasers growing on Si (001) substrates. Both simulation analysis and experimental results indicate that the voids buried below the sawtooth structures can release about 9% of the accumulative thermal stress of the III-V/Si system during the cooling process. Furthermore, electrically pumped InAs/GaAs QD narrow ridge lasers are grown and fabricated on the specially designed Si (001) platforms with a maximum operation temperature up to 90 ℃ under continuous-wave (CW) operation mode. Additionally, an extrapolated lifetime of over 5300 hours is calculated from the reliability test at 65 ℃. These results lead toward high-yield, scalable, and reliable III-V lasers on Si (001) substrates for PICs.
We have studied the anisotropic dispersion properties of hyperbolic metamaterials having a graphene/dielectric periodic structure and proposed a sensitive chemical potential sensor. It is found that the equifrequency contour of the structure will transit from an ellipse to a hyperbola as the chemical potential of graphene increases over a certain critical value. Interestingly, as the chemical potential increases close to the critical value, the transmittance varies very sensitively and nearly linearly in a wide range but with a small change of chemical potential. Based on these unique properties, we design a sensitive chemical potential sensor with advantages of fast response and high sensitivity simultaneously. Additionally, the measurement range of chemical potential can be expanded by adjusting the working frequency conveniently. These results have potential in many application fields such as beam control and sensing.
Metamaterials have shown potential for next generation optical materials since they have special electromagnetic responses which cannot be obtained in natural media. Among various metamaterials, hyperbolic metamaterials (HMMs) with highly anisotropic hyperbolic dispersion provide new ways to manipulate electromagnetic waves. Besides, graphene has attracted lots of attention since it possesses excellent optoelectronic properties. Graphene HMMs combine the extraordinary properties of graphene and the strong light modulation capability of HMMs. The experimental fabrication of graphene HMMs recently proved that graphene HMMs are a good platform for terahertz optical devices. The flexible tunability is a hallmark of graphene-based HMMs devices by external gate voltage, electrostatic biasing, or magnetic field etc. This review provides an overview of up-to-now studies of graphene HMMs and an outlook for the future of this field.
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