Working Principle of A Laser.

To understand the fundamental principle of a laser, we first consider a passive optical resonator, such as a closed optical path formed by a set of mirrors. The simplest configuration is a passive resonator consisting of two mirrors, one being a flat mirror and the other a curved mirror.

Due to the curvature of the mirrors, a light beam with the appropriate radius of curvature can propagate back and forth inside the resonator without expanding after each round trip. However, the beam power gradually decreases because a portion of the optical energy is lost during every round trip. To sustain the beam energy, a gain medium can be introduced into the cavity. If energy is supplied to the gain medium through a pumping process, it can amplify the light during each round trip.

When the gain g is smaller than the cavity loss l, the optical power still decays, although at a slower rate. When g = l, the optical power remains constant. When g > l, the optical power increases continuously with each round trip. However, this situation cannot continue indefinitely. As the intracavity intensity increases, gain saturation eventually occurs. After a certain period, the laser reaches a steady-state condition in which the gain exactly compensates for the cavity losses, allowing the laser to operate in continuous-wave (CW) mode with a constant output power.

To extract a useful laser beam from the resonator, one of the mirrors can be designed as an output coupler, transmitting a certain percentage of the intracavity power (for example, 10%). To achieve optimal output power, the transmission of the output coupler must be selected according to the gain and all other losses present within the cavity.

As mentioned above, the gain medium requires pump energy. In most laser systems, the gain medium can be excited either by electrical pumping or optical pumping. An example of electrical pumping is the injection of current into a semiconductor structure, while optical pumping typically involves the absorption of pump light with a wavelength shorter than that of the generated laser radiation.