Avalanche photodiodes (APD) can amplify the photoelectric signal based on the avalanche multiplication effect of carrier to improve the sensitivity of detection. They have the characteristics of low noise and high gain, so they are suitable for long-distance optical communication. In this work, a multi-stage avalanche photodiode structure with SAGCM (Separated Absorption, Grading, Charge and Multiplication region) is proposed based on Impact Ionization Engineering (I2E). The photocurrent, dark current, electric field, gain and noise characteristics of InGaAs/InAlAs avalanche photodiodes are studied by optimizing the grading layer's thickness and doping concentration. According to the final simulation results, the optimized avalanche photodiodes has low excess noise. At 60 V voltage and 300 K temperature, the noise factor k value (the ratio of impact ionization coefficients) of the five-stage APD is 0.012, and the gain can reach 430.
InAs/GaSb Type II Superlattice (T2SL) has become a highly competitive material for Infrared (IR) detector. We designed a pπMn mid-wavelength infrared detector based on the InAs/GaSb T2SL materials and studied the influence of the detector structural parameters on the dark current density magnitude and the Quantum Efficiency (QE) from the theoretical simulation level. Through the simulation of the detector material and structure, the dark current density characteristics at various doping concentrations, the dark current density characteristics and QE at different superlattice material thicknesses, and the temperature dependence of the dark current level are calculated. The M-structure barrier is inserted in between the π and n layers of the T2SL infrared detector structure, and the overall dark current level can be effectively reduced. Under the condition of 77 K, according to the final simulation data results, the dark current density can achieve 9.46×10-8 A/cm2, and the peak QE can achieve 34.3%.
Laser-induced damage threshold (LIDT) is the key parameter in the transmission of high power laser. Research on LIDT can help us to determine the scope of use of optical devices and thereby optimize the performance. We use the finite element method (FEM) to study the LIDT of high-purity silica fiber with 1064 nm laser. We find that the results of temperature distribution calculated by ray optics and wave optics show similar tendency. With the increase of laser fluences through the incident surface, the thermal effects become higher and laser damage gradually appears at the surface. We calculate the temperature and thermal stress displacements distribution when the laser fluences surpass LIDT. Our research can provide systematic FEM simulations for the LIDT of fibers.
A novel training sequence is designed for the space division multiplexed fiber-optic transmission system in this paper. The training block is consisting of segmented sequence, which can be used to compensate time offset and distortion (such as dispersion) in the transmission link. The channel function can be obtained by one tap equalization in the receiver side. This paper designs the training sequence by adjusting the length of the training signals and implementing matrix transformation, to obtain the coefficient of equalizer for channel detect and equalization. This new training sequence reduces system complexity and improves transmission efficiency at the same time. Compared with blind equalization, the matrix transformation based training sequence can reduce system complexity, and perform targeted equalization to the mechanism of mode coupling in the space division optical fiber system. As a result, it can effectively improve signal transmission quality and reduce bit error rate.
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