Study On Several Key Issues Of Laser Sources For Detection

Laser sources are the core devices in laser detection, and we studied several key issues in single longitudinal mode(SLM) diode pumped solid-state lasers used in detection, including the analysis on spectrum of the laser oscillation, study of thermal effect on laser performance under rough surface heat conduction, the systematical study on self-injection-seeding in acousto-optic modulated ring laser, and a scheme for multiple sub-beams coherently emitting.In the first part, based on the multiple longitudinal mode oscillating model, mode oscillation is discussed. Results show that the spatial hole burning is formed when the photons grow largely at the first time of relaxation, and the modes coexist after stabilizing in a standing wave cavity, while only the mode with maximum emission cross section exists in a travelling wave cavity. The changing cavity length leads to the oscillating wavelength drifting, and it shifts smoothly under a tiny change while with relaxation under a larger one.In the second part, in order to study the thermal effect on fundamental mode laser under rough surface heat conduction, a surface heat sources adjusting approach is presented. In the approach, virtual surface heat sources are introduced at the boundary of the object, which are self-adaptively adjusted to satisfy the boundary condition, and then the temperature filed is calculated using the Green function with all the heat sources included. Moreover, a contact model of rough surfaces with heat conduction connection is established. On this basis, the randomly distributed boundary conditions are established. The calculated results show that the temperature field presents some random fluctuation due to the untight contact between the surfaces, especially at the areas near the surface. In addition, the smaller effectively contacted induces more intense fluctuation. Phase difference near the surface presents a sizable fluctuation, and this fluctuation becomes greater with the increasing pump power, while the fluctuation as well as its depth reduces with the increasing contact area. The thermotropic diffraction loss increases linearly with the pump power, and a greater fundamental beam size causes a larger slope. Furthermore, a smaller contacted area causes a greater thermotropic diffraction loss.In the third part, the self-injection-seeding in the ring resonator by the acousto-optic modulation is studied. The diffraction efficiency is calculated by solving the acousto-optic interaction coupled wave equations numerically. Results show that the maximum diffraction efficiency difference occurs at an incident angle deviated from Bragg angle by an amount, and the deviation becomes smaller with increasing sound frequency, and the maximum difference increases with the sound power with a decreasing rate. Moreover, a higher sound frequency provides a greater maximum diffraction efficiency difference. Based on the calculated diffraction loss, we study the self-injection-seeding process by numerically solving the rate equations in ring resonator. Results show that at the end of the high loss stage, the photon density ratio to the counter-propagating directions increases sharply up and then keeps constant. Similarly, the ratio increases rapidly with the sound frequency while stays unchanged beyond 10 MHz, and increases with the pump power with a decreasing rate. The ratio is a dozen orders of magnitude under parameters easily implemented in experiment, which ensures the injection seeding. The pulse peak power almost increases linearly with the sound power, and the maximum repetition rate can be applied increases near linearly with the pump power, and the repetition rate reaches several kilohertz at pump power of dozens of watts.With fiber-coupled laser diode end pumping configuration, we experimentally demonstrated self-injection-seeding output in a Nd:YAG four mirror ring resonator. At an average power of 1 W, an output of repetition rate of 2 k Hz and pulse width of 475 ns representing a peak power of 1.1 k W is experimentally demonstrated, which is the best result reported for self-injection-seeding. The beam intensity profile is close to a Gaussian distribution and the beam quality factor M2 is 1.3.In the fourth part, a method for multiple sub-beams coherently emitting is put forward to improve the ability to detect small targets in a long distance. The laser beam is split into many sub-beams and parallelly emitted to the target. The sub-beams produce interference pattern on the target plane due to the fact that they are from the same single-longitudinal-mode laser source and have good coherence. The detection is realized by using the interference peak, and it will benefit especially to the detection of small targets. Calculated results show that an interference peak is produced. Ideally, the maximum intensity would be subN times as large as the intensity of the single beam emitted directly, where subN is the number of the sub-beams. Beam spaces in system design are given, and the beam divergence should be reduced to let a greater power to the target. In order to control the light intensity instability and the spot distortion, the jitters of the emitting mirrors must be controlled as well. Results show that a position accuracy of 0.1 wavelength and an angle accuracy of 0.2 divergence angle are needed to have a stable and ideal interference peak. In addition, the size of the peak could be one order of magnitude smaller than direct emitting, which could improve the spatial resolution of detection.