We proposed and experimentally demonstrated a broadband terahertz (THz) metamaterial absorber based on a symmetrical L-shaped metallic resonator. The absorber structure produces two absorption peaks at 0.491 and 0.73 THz, with the absorption rates of 98.6% and 99.6%, respectively. Broadband absorption was obtained from 0.457 to 1 THz, achieving a > 90 % absorption bandwidth of 0.543 THz. By analyzing the distributions of the electric and magnetic field at the two resonance frequencies, electric and magnetic dipole resonances were proposed to explain the broadband absorption mechanism. Furthermore, various widths and lengths of the symmetrical L-shaped metallic resonator on the absorption characteristics were investigated. Moreover, the broadband absorption characteristic can be maintained with an incident angle of up to 45° for transverse-electric and 30° for transverse-magnetic polarization. Finally, we experimentally observed a > 70 % broadband absorption characteristic from 0.42 to 1 THz. This proposed absorber has the potential for bolometric imaging, modulating, and spectroscopy in the THz region.

In this study, a perfect metamaterial absorber (PMMA) based on an indium antimonide temperature-sensitive material is designed and investigated in the terahertz region. We demonstrate that it is an ideal perfect narrow-band absorber with polarization-insensitive and wide-angle absorption properties. Numerical simulation results show that the proposed PMMA can be operated as a temperature sensor with a sensitivity of 21.9 GHz/K. A graphene layer was added to the PMMA structure to improve the sensitivity, and the temperature sensitivity was increased to 24.4 GHz/K. Owing to its excellent performance, the proposed PMMA can be applied in thermal sensing, detection, and switching.

We developed two kinds of erbium (Er)-bismuth (Bi) co-doped silica fibers with different erbium doping concentrations. After investigation of its spectral characteristics, it was found that the co-doping of the bismuth could alleviate the clustering of the erbium ions in the silica fiber and thus improve the fluorescence efficiency. However, the near-infrared (NIR) fluorescence spectrum attributed to the bismuth ions was different from that reported before and needs further comprehensive investigation. For the heavy Er doped erbium-bismuth (Er-Bi) co-doped fiber (about 8000 ppm Er concentration), the optical absorption coefficients at the wavelengths of 976 and 1530 nm are more than 40 and 90 dB/m, respectively. A couple of fiber Bragg gratings with proper reflectivity were chosen to be spliced to 2 cm long of this fiber forming a compact linear cavity fiber laser. Upon the excitation of the 976 nm laser diode, stable single frequency fiber laser operation was realized with a maximum side mode suppression ratio more than 60 dB and a line-width less than 1 kHz. By splicing a segment of the Er fiber to the output end of the fiber laser, the fiber laser worked in a master oscillator pumping amplifier (MOPA) structure and emitted an increased laser power of more than 100 mW.

Fiber Bragg gratings (FBGs) have been widely employed as a sensor for temperature, vibration, strain, etc. measurements. However, extant methods for FBG interrogation still face challenges in the aspects of sensitivity, measurement speed, and cost. In this Letter, we introduced random speckles as the FBG’s reflection spectrum information carrier for demodulation. Instead of the commonly used InGaAs cameras, a quadrant detector (QD) was first utilized to record the speckle patterns in the experiments. Although the speckle images were severely compressed into four channel signals by the QD, the spectral features of the FBGs can still be precisely extracted with the assistance of a deep convolution neural network (CNN). The temperature and vibration experiments were demonstrated with a resolution of 1.2 pm. These results show that the new, to the best of our knowledge, speckle-based demodulation scheme can satisfy the requirements of both high-resolution and high-speed measurements, which should pave a new way for the optical fiber sensors.