In December of 1960, Bell Telephone Laboratories produced the first laser to emit visible radiation: the He-Ne laser. (Shimoda 7)

How a Laser works. . .

A large potential difference (about 2000V) is placed across the cavity.  This potential strips electrons off the conductor, giving the electrons a large amount of kinetic energy.  Some of these electrons collide with the Helium atoms in the laser.  In this collision, the kinetic energy of the electrons excites the Helium atoms to the metastable 2¹S or 2³S state.  This newly excited atom then collides with a Neon atom in the ground state.  This collision causes the Neon atom to be excited into the 2s and 3s levels (Shimoda 8).  This occurrence is due to the striking similarity between the energy at the 2s and 3s state in the Neon atom and the 2¹S and 2³S in the Helium atom.  It is also in part due to the metastability of the 2¹S or 2³S in the Helium atom - the states are transient yet relatively long-lived.  Because the state is metastable there is a greater probability that it will collide with the Neon atom (Siegman 64).  The Neon atom then decays down to lower levels (3p or 2p) releasing light with a wavelength 632.8nm or 115.23nm. (Shimoda 8).  This light is almost completely reflected by the mirror at the end of the cavity.  When this light is reflected back, it has the opportunity to cause the stimulated emission of more Neon atoms.  The small percentage of unreflected, transmitted light is emitted as a laser beam.

What is stimulated emission?  Take an atom with two energy levels, ground state 1 and excited state 2.  Consider a photon of energy equal to the energy difference between these two states in an atom.  If this photon collides with an atom in the excited state, the atom will decay from the excited state into the less excited state emitting another photon identical to the original (Eastham 34).   In the He Ne Laser, if there are enough electrons in the 2s and 3s states and if the photons emitted are reflected back into the cavity of the laser, an increasing number of photons will be emitted until a coherent beam of light forms. 

Why are more electrons in the excited 2s and 3s states than in the 2p and 3p states?  Through population inversion.  Due to the potential difference placed across the cavity, electrons will continue to excite Helium atoms into metastable states.  These atoms will transfer their energy to the Neon atom.  Thus, more Neon atoms tend to be in the 3s and 2s states.  If there are neon atoms in the 2p or 3p states, then they will quickly leave these states by decaying into the 1s state and the ground state.  Once they are in these states they will collide with a Helium atom to be excited back into the 3s and 2s states.  If the Neon atoms in the 2p or 3p states have not yet decayed into the 1s state or ground state, they can be excited back into the higher energy states by absorbing an emitted photon.  So, in general, there are more Neon atoms in the excited 2s and 3s states than in the 2p or 3p states (Siegman 62).  Due to this population inversion, the “fuel”  needed to create stimulated emission is provided.

What does the light emitted by the He-Ne laser look like?  Contrary to popular belief, light emitted by a laser is not truly monochromatic.  This is due to Doppler spreading.  If an atom is moving away from or toward an instrument measuring the frequency of the light, then that light will either be seen as having a higher frequency (if it is moving toward) or lower frequency of light (if it is moving away).  On average, the frequency emitted is that of an atom standing still.  The result is a Gaussian curve of intensity vs. frequency.

Frequency is affected not only by the Doppler effect, but also by the length of the cavity.  In order for light to be reflected back and forth between the two mirrors (there must be a node at either mirror), its wavelength must be related to the distance between the two mirrors.  The relationship is given by

  ^{(Allen 89)}

where d is the cavity length.  

Combining these two factors produces the following relationship:

Light emitted from the laser is not monochromatic.  Furthermore, it has different polarizations - some frequencies are vertically polarized while others are horizontally polarized. 

The above discussion pertained mainly to the longitudinal waves emitted by our laser.  However, we know waves in our laser can also move normal to the cavity length.  Due to the three- dimensional aspect of our laser cavity, transverse waves are also emitted.  One can approximate the transverse mode structure by repeated application of Huygens’ principle (Eastham 111).  The solutions for the rectangular case have the general form

where H_m and H_n are the Hermite polynomials of order me and n, and w is a parameter with the dimensions of length related to the cavity geometry (Eastham 116). 

Works Cited

A. Siegman, Lasers.  (University Science Books, Mill Valley, 1986). 

B. Lengyel, Introduction to Laser Physics.  (John Wiley, New York, 1967).

D. Eastham, The Physics of Lasers.  (Taylor, London, 1986).

K. Shimoda, Introduction to Laser Physics. (Springer-Verlag, Berlin, 1984).

L. Allen, D. Jones, Principles of Gas Lasers.  (Butterworth, London, 1967).

S. Orazio, Principles of Lasers.  (Plenum, New York, 1976).

Burleigh, Spectrum Analyzer and Accessories Instruction Manual.