The light emitted from a conventional light source (like a sodium lamp) is said to be incoherent because the radiation emitted from different atoms do not, in general, bear any definite phase relationship with each other. On the other hand, the light emitted from a laser has a very high degree of coherence and is almost perfectly parallel. Because of the high degree of coherence associated with a laser beam, it finds important applications in many diverse areas like holography, spatial frequency filtering, communications, and so on.
The basic principle involved in lasing action is the phenomenon of stimulated emission which was predicted by Einstein in 1917. Einstein argued that when an atom is in the excited state, it can make a transition to a lower energy state through the emission of electromagnetic radiation; however, in contrast to the absorption process, the emission can occur in two different ways:
The first is referred to as spontaneous emission in which an atom in the excited state emits radiation even in the absence of any incident radiation. It is thus not stimulated by any incident signal but occurs spontaneously. Further, the rate of spontaneous emission is proportional to the number of atoms in the excited state.
The second is referred to as stimulated emission in which an incident signal of appropriate frequency triggers an atom in an excited state to emit radiation. The rate of stimulated emission depends both on the intensity of the external field and also on the number of atoms in the upper state. The consideration which led Einstein to the prediction of stimulated emission was the description of thermodynamic equilibrium between atoms and the radiation field. Einstein (1917) showed that both spontaneous and stimulated emission are necessary to obtain Planck’s radiation law.
The phenomenon of stimulated emission was first used by Townes in 1954 in the construction of a microwave amplifier device called the maser which is an acronym for Microwave Amplification by Stimulated Emission of Radiation. At about the same time a similar device was also proposed by Prochorov and Basov. The maser principle was later extended to the optical frequencies by Schawlow and Townes in 1958, which led to the realization of the device now known as the laser.
In fact, laser is an acronym for Light Amplification by Stimulated Emission of Radiation. The first successful operation of a laser device was demonstrated by Maiman in 1960 using a ruby crystal (see Sec. 23.2). Within a few months of the operation of the device, J a van and his associates constructed the first gas laser, namely the He-Ne laser (see Sec. 23.3). Since then, laser action has been obtained in a large variety of materials including liquids, ionized gases, dyes, semiconductors and so forth.
The three main components of any laser device are the active medium, the pumping source and the optical resonator`.
Active medium
The active medium consists of a collection of atoms, molecules or ions (in solid, liquid or gaseous form) which is capable of amplifying light waves. Under normal circumstances, there is always a larger number of atoms in the lower energy state than in the excited energy state. An electromagnetic wave passing through such a collection of atoms would get attenuated. Thus, in order to have amplification, the medium has to be kept in a state of population inversion, i.e. in a state in which the number of atoms in the upper energy level is greater than that in the lower energy level.
Pumping source
The pumping mechanism provides for obtaining such a state of population inversion between a pair of energy levels of the atomic system. The amplification process due to stimulated transition is phase coherent, i.e. [quoting Townes (1964)], “the energy delivered by the molecular system has the same field distribution and frequency as the stimulating radiation”.
We should mention that it is incorrect to associate phase with single photons, for example, the statement that “the stimulating photon and the emitted photon are in phase” is wrong because it would imply that the resulting amplitude is twice as large as the original and the energy four times as great, which is incorrect.
One can only say that the stimulated emission is into the same cavity mode as that in which the stimulating radiation exists, i.e., the energy emitted from a collection of excited atoms is added to the energy of the radiation in the cavity mode occupied by the stimulating radiation. We should also point out that the “directionality” of the laser beam depends on the form of the cavity mode. A typical laser beam is “directional” because the modes of the cavity which are excited lead to a more or less “directional” output.
A medium with population inversion is capable of amplification but in order that it act as an oscillator, a part of the output energy must be feedback into the system. Such a feedback is brought about by placing the active medium between a pair of mirrors facing each other.
The mirrors could be either plane or curved. Such a system formed by a pair of mirrors is referred to as a resonator. The sides of the cavity are usually open and hence such resonators are also referred to as open resonators. A resonator is characterized by various modes of oscillation with different field distributions and frequencies. If one chooses a closed resonator system, then the number of modes (which can get amplified and which can oscillate in a resonator of practical dimensions) becomes so large that the output would be far from monochromatic.
In order to overcome this problem, one uses open resonators where the number of modes (which can oscillate) are only a few and even single mode oscillation is possible; furthermore, the open sides of the resonator can be used for optical pumping as in ruby lasers. Because of the open nature of the resonator, all modes have a finite loss due to the diffraction spillover of energy at the mirrors.
In addition to this basic loss, scattering from the laser medium, absorption at the mirrors and output coupling at the mirrors also contribute to the cavity loss. One can visualize a mode as a wave having a well-defined transverse amplitude distribution which forms a standing wave pattern. In an actual laser, the modes that keep oscillating are those for which the gain provided by the laser medium compensates for the losses.
When the laser oscillates in steady state, the losses are exactly compensated for by the gain. Since the gain provided by the medium depends on the extent of population inversion, for each mode there is a critical value of population inversion (known as the threshold population inversion) below which that particular mode would cease to oscillate in the laser.
The onset of oscillations in a laser cavity can be understood as follows. Through a pumping mechanism, one creates a state of population inversion in the laser medium placed inside the resonator system. Thus, the medium is prepared to be in a state in which it is capable of coherent amplification over a specified band of frequencies. The spontaneous emission occurring inside the resonator cavity excites the various modes of the cavity.
For a given population inversion, each mode is characterized by a certain amplification coefficient due to the gain and a certain attenuation coefficient due to the losses in the cavity. The modes for which the losses in the cavity exceed the gain die out. On the other hand, the modes whose gain is higher than the losses get amplified by drawing energy from the laser medium.
The amplitude of the mode increases rapidly till nonlinear saturation depletes the upper level population to a value when the gain equals the losses, and the mode oscillates in steady state. When the laser oscillates in the steady state the losses are exactly compensated for by the gain provided by the medium, and the wave coming out of the laser can be represented as a continuous wave.
The ultimate monochromaticity is determined by the spontaneous emissions occurring inside the cavity because the radiation coming out due to spontaneous emission is incoherent. However, in practice, the monochromaticity is limited by external factors like temperature fluctuation, mechanical oscillation of the optical cavity, and so on.
