4. (a) Outline the energy levels involved in the operation of a CO2 laser and draw an appropriate energy level diagram. [10 marks] (b) CO2 has a bandwidth equal to Av = 50 MHz at 10.6 um. For simplicity, the spectral profile is assumed to be rectangular. The length of the cavity is equal to 1 m. Calculate the number of longitudinal modes that can oscillate in this cavity. [5 marks] (c) At what distance must the cavity mirror be placed so that at least one of the modes falls in the amplification band? How can this be achieved in practice? [10 marks]