4. (a) A laser that emits a diffraction-limited beam (Ao = 632.84 nm), produces a light spot on the surface of the Moon a distance of 376000 km away. How big is the circular aperture of the laser, if the light spot has a radius of 58 km? Neglect any effects of the Earth's atmosphere.
4. (a) A laser that emits a diffraction-limited beam (Ao = 632.84 nm), produces a light spot on the surface of the Moon a distance of 376000 km away. How big is the circular aperture of the laser, if the light spot has a radius of 58 km? Neglect any effects of the Earth's atmosphere.
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Transcribed Image Text:4. (a) A laser that emits a diffraction-limited beam (Xo = 632.84 nm), produces a light spot
on the surface of the Moon a distance of 376000 km away. How big is the circular
aperture of the laser, if the light spot has a radius of 58 km? Neglect any effects of
the Earth's atmosphere.
(b) Consider a ruby crystal with two energy levels separated by an energy difference
corresponding to a free-space wavelength Xo = 694.3 nm, with a Lorentzian lines hape
of width Av=330 GHz. The spontaneous lifetime it tsp = 3 ms and the refractive
index of ruby is n = 1.76. What value should the population difference N₂ - №₁
assume to achieve a gain coefficient () = 0.5 cm-¹ at the central frequency?
(c) How long should the crystal be to provide an overall gain of 10 at the central frequency
when y() = 0.5 cm-¹?
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