Find the pump power required to have zero gain/zero absorption. The pump is provided by green laser (up to 20 W), focused onto a 200 micron thick jet. The following parameters are given: gain molecule concentration: 6 10^17 cm3, relaxation time of upper lasing level: T1 = 2.5 ns, cross section of the focused beam: 30 m^2 (which is also the cross section of the waist of the cavity located in the jet), Absorption cross section of the pump beam p = 1. 10^16 cm^2, Gain cross section (for the lasing beam): = 5. 10^16 cm^2, Pump wavelength 514 nm, Lasing wavelength 590 nm, Reflectivity of the output mirror: 80%