Silicon photonic integrated circuits (PICs) at 2.2–2.5 μm wavelengths have great potential in applications of nonlinear optics, spectroscopy, and light detection and ranging, due to the merits of negligible two-photon absorption and moderate Rayleigh scattering from the waveguide sidewall roughness. As key components in silicon PICs, grating couplers at such wavelengths have been previously studied. However, previous devices were mainly developed based on the e-beam lithography (EBL) process, being challenging for high-volume and low-cost fabrication. Here, we demonstrated focusing subwavelength grating couplers at the spectral range of 2.2–2.5 μm based on a multi-project wafer (MPW) service. A mean coupling efficiency of −7.77 dB with a standard deviation of 0.5 dB at a center wavelength of 2.36 μm and a mean 3 dB bandwidth of 85 nm was experimentally achieved. Moreover, we studied the reproducibility of the fabricated grating couplers from die to die, as well as the grating design and fiber coupling tolerance. Our study is expected to provide a useful guideline for developing short-wavelength mid-IR devices based on MPW.

Mid-infrared (Mid-IR) Kerr frequency combs, which emit board, discrete, and evenly spaced laser frequencies in the spectral region of 2–20 μm, are expected to bring us tremendous applications from molecular spectroscopy to astronomy science. To develop mid-IR Kerr frequency combs, Ge has attracted significant attention due to its unique merits of a wide transparency window (2–14 μm), high refractive index (∼4), giant third-order nonlinear refractive index (∼10 ⁻¹⁶ m ² /W), and fully CMOS-compatible device fabrication. However, it is still debatable whether Ge is an excellent material for developing Kerr frequency combs in the short-wavelength mid-IR region due to the harmful multiphoton and free-carrier absorption effects. Here, we seek to provide physical insights into the feasibility of developing mid-infrared Ge-based Kerr frequency combs around 3-μm wavelengths. Our method is based on a comprehensive model developing from a modified Lugiato-Lefever equation via taking into account high-order dispersion, multiphoton absorption, free-carrier absorption, free-carrier dispersion, and thermal-optical phase shift effect. The results show that, only with the pump wavelength of above 3.5 μm, a frequency comb could be generated by adjusting the carrier lifetime and pump power appropriately. The study is expected to provide useful guidance for developing mid-IR Kerr frequency combs by using the CMOS technology.

On-chip Kerr frequency combs open up unprecedented opportunities for developing compact, low-cost, and high-density wavelength-division multiplexing (WDM) techniques for optical communications and optical interconnects. However, to achieve high-density WDM channels, a large-radius microring could be used to develop a Kerr frequency comb with a small free spectral range (FSR), inevitably increasing the comb's footprint and threshold. Herein, we theoretically study a dual-mode Kerr frequency comb based on a graphene-on-silicon (GOS) microring to overcome the limitation. Theoretical results show that a TE ₀ -mode comb with a 50-dB bandwidth of 200 nm and FSR of 170 GHz, and a TE ₁ -mode comb with a 50-dB bandwidth of 130 nm and FSR of 212.4 GHz could be generated in the single GOS microring. Moreover, the TE ₀ -mode and TE ₁ -mode combs could be selectively activated via modulating the Fermi level of the graphene nanoribbons integrated on the silicon microring. The study is expected to provide a useful technique for developing on-chip coherent light sources for high-speed optical communications.

Induction motors are widely applied in motor drive systems. Effective temperature monitoring is one of the keys to ensuring the reliability and optimal performance of the motors. Therefore, this paper introduces a multiplexed optical temperature sensing system for induction motors based on few-mode fiber (FMF) spatial mode diversity. By using the spatial mode dimension of FMF, fiber Bragg grating (FBG) carried by different spatial modes of optical paths is embedded in different positions of the motor to realize multipoint synchronous multiplexing temperature monitoring. The paper establishes and demonstrates a photonic lantern-based mode division sensing system for motor temperature monitoring. As a proof of concept, the system demonstrates experiments in multiplexed temperature sensing for motor stators using the fundamental mode LP₀₁ and high-order spatial modes LP₁₁, LP₂₁, and LP₀₂. The FBG sensitivity carried by the above mode is 0.0107 nm/°C, 0.0106 nm/°C, 0.0097 nm/°C, and 0.0116 nm/°C, respectively. The dynamic temperature changes in the stator at different positions of the motor under speeds of 1k rpm, 1.5k rpm, 2k rpm with no load, 3 kg load, and 5 kg load, as well as at three specific speed–load combinations of 1.5k rpm_3 kg, 1k rpm_0kg, 2k rpm_5 kg and so on are measured, and the measured results of different spatial modes are compared and analyzed. The findings indicate that different spatial modes can accurately reflect temperature variations at various positions in motor stator winding.

Mid-infrared (Mid-IR) Kerr frequency combs have great application potential in sensing and spectroscopy. To generate a Kerr frequency comb, a continuous-wave, wavelength-tunable, narrow-linewidth, low-noise laser is normally utilized to pump a high-nonlinearity micro-resonator to emit a large number of coherent and equally spaced modes. However, chip-based pump lasers in the mid-IR band, namely, quantum cascade lasers and interband cascade lasers, are usually challenging to develop with both high output powers and good wavelength tunability based on a single diode. To overcome the limitation, we theoretically study a mid-IR Kerr frequency comb generation technique based on a graphene-on-silicon micro-resonator by using a monochromatic mid-IR laser. The approach is based on the exploration of an electric-field-assisted resonance scanning technique and graphene-enhanced silicon Kerr nonlinearity. Our result shows that a soliton Kerr frequency comb with a spectral range of 3.23–5.26 μm, 3-dB bandwidth of ∼550 nm, and frequency spacing of 140 GHz could be generated under a pump wavelength of 4 μm. The study paves a promising way toward developing monolithically chip-integrated mid-IR Kerr frequency combs with cost efficiencies and ultrafast tuning speeds.