Rami Arieli: "The Laser Adventure" Section 6.2.1 page 2
Energy Levels of Ruby Laser
The energy level diagram of a Ruby laser is described in figure 6.13.
Figure 6.13: Energy Level Diagram of a Ruby Laser

This system is a three level laser with lasing transitions between E2 and E1.
The excitation of the Chromium ions is done by light pulses from flash lamps (usually Xenon).

The Chromium ions absorb light at wavelengths around 545 [nm] (500-600 [nm]). As a result the ions are transferred to the excited energy level E3.
From this level the ions are going down to the metastable energy level E2 in a non-radiative transition. The energy released in this non-radiative transition is transferred to the crystal vibrations and changed into heat that must be removed away from the system.

The lifetime of the metastable level (E2) is about 5 [msec].

Ruby laser has another absorption band which can be used for pumping, in the spectrum range: 350-450 [nm].

It is difficult to achieve continuous operation of a Ruby laser since it is a three level laser. However, in 1962, by using very intensive pump, using arc lamp with high pressure Mercury vapor, a continuous wave Ruby laser was build.

If you want to play with an interactive simulation about the energy level diagram of a Ruby Laser, click here. (VQM Simulation - Ruby) (Requires Shockwave)