Generally, four three-axis magnetometers (TAMs) are used to measure magnetic gradient tensor. But the measurement precision of magnetic gradient tensor is tightly connected with the imperfect performance of sensors such as non-orthogonality, different scale factors and biases among sensitive axes, misalignment between different TAMs. It is necessary to calibrate and correct the measurement device to obtain precise magnetic gradient tensor. A mathematical model for calibration of the device measuring magnetic gradient tensor is established, and the calibration algorithm and its steps based on functional link artificial neural network (FLANN) and least-squares method (LSM) are proposed. The numerical simulations prove the effectiveness and good convergence of calibration algorithm, which can improve remarkably the measurement precision of magnetic gradient tensor.
Intrinsic optical bistability (IOB) in Tm: YSGG (yttrium-scandium-gallium garnet) laser crystal pumped at 1um avalanche wavelength is predicted theoretically and studied numerically. The nonlinear rate equations of Tm: YSGG are given based on the nonlinear energy transfer processes. From the rate equations, the analytical formula of avalanche threshold condition is deduced in the steady-state approximation. Both the intrinsic bistability effect of population concentrations at Tm<sup>3+</sup> levels versus pumping power and the influence of system parameters on IOB avalanche threshold are studied numerically. The results show that the avalanche threshold power of IOB can be changed by adjusting experimentally adjustable parameters such as the Tm<sup>3+</sup> −dope concentration.