We propose and numerically analyze a broadband and high extinction ratio mode converter to achieve the mode conversion between the fundamental transverse electric mode (TE0) and the first-order transverse magnetic mode (TM1). The mode converter is achieved by using the tapered hybrid plasmonic waveguide which the metal is directly covered on the Si waveguide without a thin low-index layer. After optimizing the structure of the tapered hybrid plasmonic waveguide, a wide operation bandwidth of 100 nm is obtained with the extinction ratio of the TM1 mode and TE0 mode larger than 20 dB in the output Si waveguide for the forward propagation. And for the back propagation, the operation bandwidth over 65 nm is obtained with the extinction ratio larger than 20 dB in the output Si waveguide. The whole footprint of the mode converter is only 0.8×11 μm2. Fabrication tolerance analysis is also demonstrated

Room-temperature continuous-wave operation of InGaAs/AlGaAs quantum well lasers directly grown on on-axis silicon (001) has been demonstrated. A 420 nm thick GaAs epilayer completely free of antiphase domains was initially grown on the silicon substrate in a metal-organic chemical vapor deposition system and the other epilayers, including four sets of five-period strained-layer superlattices and the laser-structural layers, were successively grown in a molecular beam epitaxy system. The lasers were prepared as broad-stripe Fabry–Pérot ones with a stripe width of 21.5 μm and a cavity length of 1 mm. Typically, the threshold current and the corresponding threshold current density are 186.4 mA and 867 A/cm², respectively. The lasing wavelength is around 980 nm, and the slope efficiency is 0.097 W/A with a single-facet output power of 22.5 mW at an injection current of 400 mA. This advancement makes the silicon-based monolithic optoelectronic integration relevant to quantum well lasers more promising with an enhanced feasibility.

Optical microfiber-based refractometer, which relies on modal interference operating near the dispersion turning point (DTP), has attracted a high level of research interest. The ultrahigh sensitivity can be achieved by mitigating the group effective refractive index difference between the modes in the coupling. Through modifying the microfiber diameter, the group effective refractive index difference can be modified to zero which is defined as DTP. However, all the DTP-based sensors report low fabrication tolerance and narrow operation bandwidth of probing wavelength for the high-sensitivity region where the refractive index (RI) sensitivities higher than 10 ⁴ nm/RIU can be achieved, which extremely limits the practical applications. To address the challenge, we manipulate and optimize the mode dispersion by using a tapered specially-designed single stress-applying fiber (SSAF). By introducing the stress-applying part (SAP) into the cladding, the wavelength sensitivity of the group effective RI difference can be significantly reduced through the flexibly engineerable control over the material index. In this way, the operation bandwidth is dramatically broadened for 500 nm in comparison with that of conventional microfiber with the same diameter based on DTP, indicating a broad range of optional probing wavelength and high fabrication tolerance. We experimentally verify the ultrahigh sensitivity and broadband operation around the DTP using an SSAF-based microfiber with a waist diameter of about 2.3 μm. The maximum RI sensitivity of 30563 nm/RIU is obtained.

In this paper, physically-based simulations are carried out to investigate and design uni-traveling carrier photodiode (UTC-PD) for high-power sub-terahertz wave generation at zero- and low-bias operation. The reliability of the physically-based simulation is demonstrated by comparing with our experimental result. Both the bandwidth and RF output power of the proposed UTC-PD is significantly improved by careful design the built-in electric field distribution under high-power input. For the optimized UTC-PD with the mesa diameter of 5 μm, its 3dB bandwidth large than 100 GHz even if the photocurrent reaches 6 mA under zero-bias operation. The device can reach a high bandwidth of 92.4 GHz, 105 GHz, and 119.5 GHz under the reverse bias of 0.5 V, 1 V, and 2 V, respectively, even the input photocurrent as high as 18.2 mA. The peak output-power of the device has enhanced at least 7 dB even at 170 GHz and zero- or low-bias operation. Besides, a novel design of mushroom-mesa UTC-PD (MM-UTC-PD) is proposed which with 4.3% improved high-speed performance. The MM-UTC-PD can trade-off between the external quantum-efficiency and bandwidth when miniaturized junction size is required.

An interval-sweeping pulse equivalent sampling (ISPES) method based on compressed sensing technique is proposed to enhance the frequency response of phase-sensitive optical time-domain reflectometry (Φ-OTDR). The proposed method achieves an equivalent high-sampling-rate acquisition of Rayleigh backscattered signals along each sensing point of the fiber by the interval-sweeping pulse sampling and compressed sensing reconstruction. Meanwhile, based on the Whittaker-Shannon interpolation formula, the observation matrix corresponding to the interval-sweeping pulse sampling is successfully constructed and proved, which is easy to implement in hardware. In experiments, an ultrasonic signal with a frequency up to 28 kHz at the end of a 4.4-km sensing fiber is accurately identified by using this method, which outperforms other compressed-sensing-based methods. And multi-frequency vibration signals are also accurately detected. Besides, this method performs signal compression while sampling. The number of samples is only 13.3% of the reconstructed data, significantly reducing the amount of data acquisition/transmission/storage. Therefore, the proposed method can effectively enhance the frequency response of Φ-OTDR to sparse-wideband vibration signals without any hardware modification. It also has great advantages in reducing the hardware burden and improving the response speed.