Three- and four-level lasers: (a) Explain briefly what is meant by the terms three-level and four-level laser. Discuss why there is a large difference in the threshold power which is required to achieve laser oscillation in these two classes of laser. (b) A laser cavity is formed by two mirrors of reflectivity 100% and 95%. Calculate the energy absorbed by the active ions which is necessary to achieve pulsed laser oscillation for rods of ruby and Nd:YAG each 50 mm long, 5 mm diameter, and each with an active ion concentration of 4 x 1019 cm. Take the pump bands to be at 20 000 cm and 12000 cm for the ruby and Nd:YAG laser respectively. For the Nd:YAG laser, you may assume that the peak optical gain cross-section is 21(0) = 6 x 10-19 cm. (c) How will these values compare with the electrical energy which must be supplied to the laser?