Question

4.8 He-Ne Laser The He-Ne laser system energy levels can be quite complicated, as shown in Figure 4.58. There are a number of lasing emissions in the laser output in the red (632.8 nm), green (543.5 nm), orange (612 nm), yellow (594.1 nm), and in the infrared regions at 1.52 μm and 3.39 μm, which give the He-Ne laser its versatility. The pumping mechanism for all these lasers operations is the same, energy transfer from excited He atoms to Ne atoms by atomic collisions in the gas discharge tube. There are two excited states for the He atom in the configuration 1s 2s2. The configuration with electron spins parallel has a lower energy than that with opposite electron spins as indicated in Figure 4.58. These two He states can excite Ne atoms to either the 2p34s1 or 2p35s1. There is then a population inversion between these levels and the 2p33p' and between 2p35s1 and 2p34p1, which leads to the above lasing transitions. Generally, we do not have a single discrete level for the energy of a many-electron atom of given n,e configuration. For example, the atomic configura- tion 2p'3p has ten closely spaced energy levels resulting from various different values of m and ms that can be assigned to the sixth excited electron (3p') and the remaining electrons (2p) within quantum mechanical rules. Not all transitions to these energy levels are allowed as photon emission requires quantum number selection rules to be obeyed. (a) Table 4.8 shows some typical commercial He-Ne laser characteristics (within 30-50%) for various wave- lengths. Calculate the overall efficiencies of these lasers TABLE 4.8 Typical commercial He-Ne laser characteristic 1523 Infrared 594.1 Yellow 612 Orange 4 6.5 2070 632.8 Red Wavelength (nm) 543.5 Green Optical output power (mW) Typical current (mA) Typical voltage (V) 6.5 1910 6.5 2750 6.5 2070 3380 Lasing emissions (1s'2s1) (2p 3.39 um 20.6 eV Collisions (2p34p') 2n4s 632.8 nm (1s12s') 543.5 nm 19.8 eV 1523 nm 1152 nm 118 nm (2p33p') 600 nm Electron impact Fast spontaneous decay (2p33s1) Collisions with the walls (1s 0 Ground states Various lasing transitions in the He-Ne laser. FIGURE 4.58

Answer 1

Consider the formula for overall efficiency of the laser Optical power output x 100% Electrical power input Overall efficiency 594.1 543.5 Green 1.5 632.8 1523 Wavelength (nm) 612 Yellow Orange Red Infrared Optical output power 6.5 2750 17875 6.5 2070 13455 6.5 2070 13455 6.5 1910 12415 Typical current (mA) Typical Voltage Electrical input power 3380 20280 Overall efficiency 0.00839% | 0.0148% | 0.0297% | 0.0402% | 0.0049% 노트 1x10090 02