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The way lasers work
The enormous growth of laser technology has stimulated a broad range of scientific and engineering applications that exploit some of the unique properties of laser light. These properties derive from the distinctive way laser light is produced in contrast to the generation of ordinary light. In an ordinary sodium vapour street lamp for example, the atoms spontaneously emit in random directions and at irregular times.
There, wave fronts are produced in 'packets', by randomly born photons and, since the various births are unrelated, the result is a non-synchronised light emission, over a broad spectrum resulting in isotropic illumination of incoherent light. Laser light originates from atoms, ions, or molecule through a process of stimulated emission of radiation. The active laser medium is contained in an enclosure or cavity which organises the normally random emission process into an intense directional, monochromatic and coherent wave. The three main components of any laser device are:
- the active medium,
- the pumping source and
- the optical resonator.
It is known that an atom will emit radiation only at certain frequencies, which correspond to the energy separation between the various allowed states. Consider an atom with many allowed energy states, labelled E1 ,E2, E3,......., as Fig.1. When light is incident on the atom, only those photons whose energy, hf, matches the energy separation DE between two levels can be absorbed by the atom. A schematic diagram representing this stimulated absorption process is shown in Fig.2. At ordinary temperatures, most of the atoms are in the ground state. If a vessel containing many atoms of a gaseous element is illuminated with a light beam containing all possible photon frequencies (that is , a continuous spectrum), only those photons of energies E2-E1, E3-E1, E4-E1, E3-E2, E4-E2, and so on, can be absorbed. As a result of this absorption, some atoms are raised to various allowed higher energy levels called excited states.
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| Figure 1: Energy level diagram of an atom with various allowed states. The lowest energy state, E1 , is the ground state. All others are excited states. | Figure 2(a) and 2(b) represent the states before and after the stimulated absorption of a photon by an atom respectively. The dots represent electrons. One electron is transferred from the ground state to the excited state when the atom absorbs a photon whose energy hf= E |
Once an atom is an excited state, there is a certain probability that it will jump back to a lower level by emitting a photon, as shown in Fig.3. This process is known as spontaneous emission. In typical cases, an atom will remain in an excited state for only about 10-8s. Finally, there is another process, which is of importance in laser, known as stimulated emission (Fig.4). Suppose an atom is in an excited state E2, and a photon of energy hf=E2-E1 is incident on it. The incoming photon will increase the probability that the electron will return to the ground state and there by emit a second photon having the same energy, hf. This process of speeding up atomic transitions to lower levels is called stimulated emission. Note that there are two identical photons that result from this process, the incident photon and the emitted photon. The emitted photon will be exactly in phase with the incident photon. These photons can, in turn, stimulate other atoms to emit photons in a chain of similar processes. The many photons produced in this fashion are the source of the intense , coherent light in a laser.
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| Figure 3. Diagram representing the spontaneous emission of a photon by an atom that is initially in the excited state E2. When the electron falls to the ground state, the atom emits a photon whose energy hf = E2 - E1 | Figure 4.: Diagram representing the stimulated emission of a photon by an incoming photon of energy hf. Initially, the atom is in the excited state. The incoming photon stimulates the atom to emit a second photon of energy hf= E2 - E1 |
An incident photon can cause atomic transitions either upward (stimulated absorption) or downward (Stimulated emission). Both processes are equally probable. When light is incident on a system of atoms, there is usually a net absorption of energy because there are many more atoms in the ground state than in excited states when the system is in thermal equilibrium. That is, in a normal situation, there are more atoms in state E1 ready to absorb photons than there are atoms in states E2, E3,.... ready to emit photons. However, if one can invert the situation so that there are more atoms in an excited state than in the ground state, a net emission of photons can result. Such a condition is called population inversion. This, in fact, is the fundamental principle involved in the operation of a laser. The amplification corresponds to a build-up of photons in the system as a result of the chain reaction of events.
The three main components of any laser device are
- the active medium,
- the pumping source,
- the optical resonator.
The following conditions must be satisfied in order to achieve laser action (Fig.5):
Figure 5 : A schematic of a laser design. The tube contains atoms, which is the active medium.
An external source of energy (optical, electrical ) is needed to "pump" the atoms to excited energy states.
The parallel ends mirrors provide the feedback of the luminous wave.
- The excited state of the system must be a meta stable state, which means its lifetime must be long compared with the usually short lifetimes of excited states. When such is the case, stimulated emission will occur before spontaneous emission. (i.e., the requirement of active medium)
- The system must be a state of population inversion i.e., more atoms in an excited state than in the ground state (Therefore there is the need of pumping device).
- The emitted photons must be confined in the system long enough to allow them to stimulate further emission from other excited atoms. This is achieved by the use of reflecting mirrors at the ends of the system. one end is made totally reflecting, and the other is slightly transparent to allow the laser beam to escape (i.e., the optical resonator).
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