There are three principal elements of a laser, which are (1) an energy pump, (2) an optical
gain medium, and (3) an optical resonator. These three elements are described
in detail below for the case of the HeNe laser .
(1) Energy pump.
A 1400 V high voltage, DC power supply maintains a glow discharge or plasma in a glass tube containing an optimal mixture (typically 5:1 to 7:1) of helium and neon gas, as shown in Fig. 1 and indicated in the diagram of Fig. 2. The discharge current is limited to about 5 mA by a 91 kW ballast resistor. Energetic electrons accelerating from the cathode to the anode collide with He and Ne atoms in the laser tube, producing a large number of neutral He and Ne atoms in excited states. He and Ne atoms in excited states can deexcite and return to their ground states by spontaneously emitting light. This light makes up the bright pink-red glow of the plasma that is seen even in the absence of laser action.
The process of producing He and Ne in specific excited states is known as pumping and in the HeNe laser this pumping process occurs through electron-atom collisions in a discharge. In other types of lasers, pumping is achieved by light from a bright flashlamp or by chemical reactions. Common to all lasers is the need for some process to prepare an ensemble of atoms, ions or molecules in appropriate excited states so that a desired type of light emission can occur.
(2) Optical gain medium.
To achieve laser action it is necessary to have a large number of atoms in excited states and to establish what is termed a population inversion. To understand the significance of a population inversion to HeNe laser action, it is useful to consider the processes leading to excitation of He and Ne atoms in the discharge, using the simplified diagram of atomic He and Ne energy levels given in Fig. 3. A description of the rather complex HeNe excitation process can be given in terms of the following four steps.
(a) An energetic electron collisionally excites a He atom to the state labeled 21S0 in Fig. 3. A He atom in this excited state is often written He*(21S0), where the asterisk means that the He atom is in an excited state.
(b) The excited He*(21S0) atom collides with an unexcited Ne atom and the atoms exchange internal energy, with an unexcited He atom and excited Ne atom, written Ne*(3S2), resulting. This energy exchange process occurs with high probability only because of the accidental near equality of the two excitation energies of the two levels in these atoms.
(c) The 3S2 level of Ne is an example of a metastable atomic state, meaning that it is only after a relatively long period of time - on atomic time scales - that the Ne*(3S2) atom deexcites to the 2P4 level by emitting a photon of wavelength 6328 Å. It is this emission of 6328 Å light by Ne atoms that, in the presence of a suitable optical configuration, leads to lasing action.
(d) The excited Ne*(2P4) atom rapidly deexcites to its ground state by emitting additional photons or by collisions with the plasma tube walls. Because of the extreme quickness of the deexcitation process, at any moment in the HeNe plasma, there are more Ne atoms in the 3S2 state than there are in the 2P4 state, and a population inversion is said to be established between these two levels.
When a population inversion is established between the 3S2 and 2P4 levels of the Ne atoms in the discharge, the discharge can act as an optical gain or amplification medium for light of wavelength 6328 Å. This is because a photon incident on the gas discharge will have a greater probability of being replicated in a 3S2-->2P4 stimulated emission process (discussed below) than of being destroyed in the complementary 2P4-->3S2 absorption process.
(3) Optical resonator or cavity.
As mentioned in 2(c) above, Ne atoms in the 3S2 metastable state decay spontaneously to the 2P4 level after a relatively long period of time under normal circumstances; however, a novel circumstance arises if, as shown in Fig. 2, a HeNe discharge is placed between two highly reflecting mirrors that form an optical cavity or resonator along the axis of the discharge. When a resonator structure is in place, photons from the Ne* 3S2-->2P4 transition that are emitted along the axis of the cavity can be reflected hundreds of times between the two highly reflecting end mirrors of the cavity. These reflecting photons can interact with other excited Ne*(3S2) atoms and cause them to emit 6328 Å light in a process known as stimulated emission. The new photon produced in stimulated emission has the same wavelength and polarization, and is emitted in the same direction, as the stimulating photon. It is sometimes useful for purposes of analogy to think of the stimulated emission process as a "cloning" process for photons, as depicted in Fig. 4. The stimulated emission process should be contrasted with spontaneous emission processes that, because they are not caused by any preceding event, produce photons that are emitted isotropically, with random polarization, and over a broader range of wavelengths.
As stimulated emission processes occur along the axis of the resonator a situation develops in which essentially all 3S2-->2P4 Ne* decays contribute deexcitation photons to the photon stream reflecting between the two mirrors. This photon multiplication (light amplification) process produces a very large number of photons of the same wavelength and polarization that travel back and forth between the two cavity mirrors. To extract a light beam from the resonator, it is only necessary to have one of the two resonator mirrors, usually called the output coupler, have a reflectivity of only 99% so that 1% of the photons incident on it travel out of the resonator to produce an external laser beam. The other mirror, called the high reflector, should be as reflective as possible. The small diameter, narrow bandwidth, and strong polarization of the HeNe laser beam are determined by the properties of the resonator mirrors and other optical components that lie along the axis of the optical resonator.