A phenomenological model describing the temperature dependence of the threshold current of 1.3 μm gain-coupled distributed feedback (DFB) lasers with periodically etched strained-layer quantum wells is presented and compared to experimental results. The model can predict the minimum threshold current as a function of temperature (infinite T0), which has been observed experimentally. The mechanism for the appearance of this minimum is explained by the decrease of the detuning of the DFB-mode wavelength from the material gain-peak wavelength as the temperature increases. The condition for single-mode operation is also discussed with use of the effective gain of DFB laser structures.

Effect of in-phase and antiphase gain-coupling on high-speed properties is studied for MQW DFB lasers with periodically truncated quantum-wells. The enhancement of modulation bandwidth due to antiphase gain-coupling is found significantly suppressed, and gain-coupled DFB lasers with high /spl kappa/L are preferred for large modulation bandwidth due to the presence of linear gain saturation in MQW lasers.<>

We present experimental results showing a reduction of relative intensity noise (RIN) in partly gain-coupled InGaAsP/InP multiquantum-well DFB lasers. By comparing with conventional index-coupled lasers, it is found that even a small gain-coupling improves significantly the feedback insensitivity of DFB lasers. The mechanism of the RIN reduction and the less feedback sensitivity are believed to be a combined effect of the high relaxation frequency and heavy damping rate, the stronger internal mode discrimination, the facet reflectivity immunity, and the flatter carrier distribution in the gain-coupling structure.

We have determined the junction temperatures of laser diodes mounted on ceramic submounts and hermetic transistor-outline (TO) packages using data from wavelength-shift measurement as well as finite-element modeling (FEM). We found that the difference in the junction temperature between chip/submount and chip/submount/TO measured at the free-standing room-temperature condition cannot be used as the reference data for the design of accelerated life-test condition at hot oven. For the free-standing condition, the heat spread from the active region of the chip to the chip substrate, submount, and TO. Hence, there was an additional thermal gradient in the chip/submount/TO arising from the thermal impedance of the TO header. For the oven condition, the heat dissipation via forced convection from the outer surface of the TO was important. Hence, the thermal gradient of the TO resulting from heat conduction became smaller. Using the FEM, we showed that the simulated junction temperature was in close agreement with the experimental value. The heating difference between the chip/submount and chip/submount/TO was smaller at the oven condition than at the free-standing condition.

The atmospheric noise widely present in very-low-frequency and low-frequency (VLF/LF) communication systems is usually considered as a type of impulsive noise, which can degrade the performance of signal processing methods based on Gaussian noise. To solve the problems of difficult model training and complex noise parameter estimation under impulsive noise in other demodulation schemes of minimum shift keying (MSK) signals, we use one-dimensional convolutional neural networks (1D-CNNs) to replace the integrator and decision module in coherent demodulation instead of the entire demodulation process. Our scheme preserves the operations that are beneficial for neural network training in the conventional coherent demodulation process, and the use of neural networks avoids the estimation of noise parameters. To further improve the symbol error rate (SER) performance, we remove the symbol conversion before output generation and add the symbol conversion before MSK modulation. The simulation results show that our scheme with fewer parameters and less calculation has better SER performance than other neural network demodulators. Our scheme’s SER performance exceeds the performance of the demodulation algorithm based on a myriad branch metric, whose performance is very close to the maximum likelihood (ML) performance. And, our scheme does not require complex noise parameter estimation.

This paper presents an optimized long-wave (10–300 kHz) wideband direction-finding system for scientific research. The antenna unit of the system comprises one vertical electric field sensor and two horizontal magnetic field sensors oriented in the north–south and east–west directions, respectively. The overall design prioritizes compactness, engineering feasibility, and ease of deployment, enabling the effective reception of long-wave radio signals within the 10–300 kHz range. The magnetic field sensitivity reaches $8fT/\sqrt{\mathrm{Hz}}@10\mathrm{kHz}$, while the electric field sensitivity achieves $3.2\mathsf{\mu }\mathrm{V}/\mathrm{m}/\sqrt{\mathrm{Hz}}@10\mathrm{kHz}$. The overall sensitivity of the receiver is $1\mathsf{\mu }\mathrm{V}$ (300 Hz bandwidth, 10 dB signal-to-noise ratio). The synchronization accuracy of the system is within 10 ns. Theoretically, with a baseline length of 5 km and a signal incidence angle ranging from 9.9° to 170.1°, the direction finding error is less than 2°. Additionally, direction-finding methods for MSK and ASK modulated signals are analyzed. To evaluate the system’s actual performance, initial measurements were conducted in Qingdao, Shandong.