A coherent dual-frequency lidar architecture for long-distance high-precision ranging and velocity measurement is proposed, using the method of optical heterodyne detection, which can avoid complex optical coherence configurations. The system uses a dual-frequency laser with a beat frequency of 200MHz as the dual-frequency light source, and performs measurement through the principles of phase ranging and Doppler velocimetry. The experimental verification shows that the operating distance of the system reaches 3200m, the distance resolution is less than 0.5m, and the speed measurement accuracy range is ±0.25m/s. The results show that the system can realize long-distance high-precision single-point ranging and speed measurement.
Spatial patterns are a significant characteristic of lasers. The knowledge of spatial patterns of structured laser beams is rapidly expanding, along with the progress of studies on laser physics and technology. Particularly in the last decades, owing to the in-depth attention on structured light with multiple degrees of freedom, the research on spatial and spatiotemporal structures of laser beams has been promptly developed. Such beams have hatched various breakthroughs in many fields, including imaging, microscopy, metrology, communication, optical trapping, and quantum information processing. Here, we would like to provide an overview of the extensive research on several areas relevant to spatial patterns of structured laser beams, from spontaneous organization to multiple transformations. These include the early theory of beam pattern formation based on the Maxwell–Bloch equations, the recent eigenmodes superposition theory based on the time-averaged Helmholtz equations, the beam patterns extension of ultrafast lasers to the spatiotemporal beam structures, and the structural transformations in the nonlinear frequency conversion process of structured beams.
The efficiency of solar-pumped lasers (SPLs) is limited when the length of the laser medium is unsuitable. This is because superfluous regions in the laser medium introduce losses and contribute slightly to the stimulation of radiation in the laser resonator. Before designing an SPL, an appropriate length of laser medium is critical. We present a method to calculate the optimal length of the gain medium in SPLs by exploring the relationship between the absorbed solar power and material loss for different laser medium lengths. Thus, the lengths of Nd:YAG crystals with diameters of 3 to 6 mm were optimized, and the output characteristics were calculated numerically. The maximum collection efficiency (CE) (40.1 W / m2) was obtained for the 5.5-mm diameter Nd:YAG crystal rod of length 21.1 mm, which was 1.7 W / m2 higher than the previous numerical record. The optimal length of the 6-mm diameter Nd:YAG crystal rod was found to be 21.9 mm. For a laser rod of this length, a CE of 36.3 W / m2 is expected. This value is 1.13 times greater than the existing experimental record for the Nd:YAG crystal of the same diameter, which highlights the importance of optimizing the length of the laser rod.
Nonlinear frequency conversion of structured beams has been of great interest recently. We present an intracavity second harmonic generation (SHG) of laser beams in transverse mode locking (TML) states with a specially designed sandwich such as a microchip laser. The intracavity nonlinear frequency conversion process of a laser beam in a TML state to its second harmonic is theoretically and experimentally investigated, considering different relative phase and weight parameters between the basic modes in the TML beam. Comparison between the far-field SHG beam patterns of fundamental frequency transverse modes in coherently locked and incoherently superposed states demonstrates that the SHG of TML beams can carry more information. Various rarely observed far-field SHG beam patterns are obtained, and they are consistent with the theoretical analysis and numerical simulations. With the obtained SHG beams, the characteristics of the structured fundamental frequency beams can also be conversely investigated or predicted. This work may have important applications in optical 3D printing, optical trapping of particles, and free-space optical communication areas.
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