(1) Laser working substance is the substance used to emit laser in laser. As the working medium of laser, it must be an active medium, that is, under the excitation of external energy, the number of particles in the medium can be reversed (if the medium destroys the thermal balance under the excitation of external energy, the number of particles at high energy level is greater than that at low energy level, and this state is called particle number inversion state. In this state, the light is amplified after passing through the medium, which is called optical gain, and the medium at this time is the optical gain medium. The working substance of ruby laser is ruby containing chromium ions, the working substance of He-Ne laser is gaseous neon (helium is an auxiliary working substance), and the working substance of ordinary argon ion laser is gaseous argon.
(2) The excitation energy can excite the particles in the ground state to the required excited state, thus resulting in the inversion of the number of particles. Ruby laser adopts optical excitation mode, and usually uses pulsed xenon lamp as excitation light source. In order to improve the efficiency of large-scale energy utilization, xenon lamp tube and ruby rod are usually placed on two focal lines of elliptical column concentrator respectively. He-Ne laser is usually excited by branch gas discharge.
(3) There are two mirrors at both ends of the gain medium, one of which is a total mirror with a reflectivity of about1; The reflectivity of the other block is less than 1, which is a polarizer, and the laser will be output from this end of the polarizer. It is usually necessary to adjust these two mirrors to be strictly parallel and perpendicular to the axis of the gain medium. Therefore, a resonant cavity is formed. Among the stimulated radiation generated by the laser working substance, the light propagating along the axial direction of the laser working substance will be set back and forth between the two mirrors of the resonant cavity and amplified continuously, and part of the light will be emitted from one end of some mirrors to become laser. On the one hand, the resonant cavity can "extend" the gain medium, improve the light energy density, and at the same time control the propagation direction of the output light. In addition, the laser output wavelength can be selected as long as the length of reflector and resonant cavity is properly selected.
Finally, it should be pointed out that the gain generated by light traveling back and forth in the gain medium should be enough to compensate for its various losses (including medium absorption, diffraction and laser output). ), otherwise the laser output cannot be formed. The condition that it must meet is called threshold condition.