激光原理与技术--第二章 激光器的工作原理.ppt

* When a laser operates in several transverse modes, the total intensity profile is a superposition of all existing transverse modes. Figure 4.9 describes the intensity distribution of 3 lower modes, and their superposition. Figure 4.9: Laser radiation with a few transverse modes. Figure 4.9 shows that the lower transverse mode TEM00 have the smallest diameter compared to other modes. This gives a hint how to make a laser operate in a single basic transverse mode: Putting a pinhole with the proper diameter inside the optical cavity. By choosing the pinhole diameter equal to the diameter of the lower mode, only this mode can pass through the pinhole, and all higher modes are attenuated. Since radiation inside the optical cavity is moving many times, only the basic mode will be amplified, and appear in the output. A laser will produce an output for all TEM modes for which gain exceeds loss within the laser cavity. Some lasers will lase on several transverse modes at the same time, as indicated by Figure 4. Such simultaneous lasing produces a beam that has dark spots and hot spots, i.e., regions of low and high irradiance. Notice that in Figure 4, TEM00 has a smaller diameter than any other mode. All modes except TEM00 can be eliminated by a cavity-aperture diameter that produces little or no loss for the TEM00 mode, but that introduces greater loss for all higher-order modes. Optical cavities that exhibit high diffraction losses tend to oscillate in the TEM00 mode only. Thus, any cavity can be restricted to TEM00 by installation of a suitable aperture. For most gas lasers, the diameter of the laser tube is chosen only for the purpose of limiting oscillation to TEM00 TEM00 is termed the uniphase or pure Gaussian mode because it is the only transverse mode in which all the light is in one phase at any given time. This uniphase mode is the only mode in which all laser light is spatially coherent, resulting in the following three important characteristics of thi