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I. The Helium-Neon Laser

Discussion

The bright, highly collimated, red ( = 6328 Å) light beam from the helium-neon (HeNe) gas laser is a familiar sight in the scientific laboratory, the industrial workplace, and even at the checkout counter in most supermarkets. HeNe lasers are manufactured in large quantities at low cost and with proper operation they can provide thousands of hours of useful service. Even though solid state diode lasers can now provide red laser light beams with intensities comparable to those obtained with HeNe lasers, it is anticipated that the HeNe laser will remain a common component in scientific and technical instrumentation in the foreseeable future.

In this experiment you will (a) assemble a 3 mW HeNe laser from readily available optical components, (b) align a HeNe laser cavity using two different cavity mirror configurations, (c) record photographically the transverse mode structure of the laser output beam, and (d) determine the linear polarization of the light produced by the HeNe laser. The principal goal of this experiment is to get hands-on experience with the various optical components of a working laser; however, to appreciate fully the role played by each of the components, it will be helpful to give here a brief overview of the principles of HeNe laser operation.

The three principal elements of a laser 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 used in this experiment.

(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 acceler- ating 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 2¹S^o in Fig. 3. A He atom in this excited state is often written He*(2¹S_o), where the asterisk means that the He atom is in an excited state.

(b) The excited He*(2¹S_o) 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(a). 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.

Procedure

A few words of caution are important before you begin setting up your HeNe laser. First, never look directly into a laser beam, as severe eye damage is likely to result. During alignment the laser beam can be observed safely by placing a small, white index card at the appropriate point in the optical path. Resist the temptation to lower your head down to the level of the laser beam to see where it is going. Second, high voltage is present at the HeNe discharge tube and you should avoid all possibility of contact with the bare electrodes of the HeNe plasma tube. Finally, the optical cavity mirrors and the Brewster windows of the laser tube have delicate optical surfaces that are easily scratched and should not be touched. If these surfaces need cleaning, ask the instructor to demonstrate the proper method for cleaning them.

To assemble the HeNe laser and investigate its properties, proceed with the following steps.

(1) You will first set up a confocal resonator configuration using two spherical mirrors with radii of curvature R = 0.500 m. Locate an output coupler mirror with a reflectivity of 99% and a high reflector mirror with ³ 99.7% reflectivity. The multilayer dielectric coating that provides the high reflectivity of these mirrors is on one side of the mirror only. Be sure to have the reflecting surfaces of both mirrors facing the interior of the optical cavity.

The focal length f of each mirror is given by f = R/2 = 0.250 m for both mirrors. The two spherical mirrors should be placed 0.500 m apart so that they share a common focus. Adjust the output coupler and high reflector mirror mounts until the reflecting surfaces of the output coupler and high reflector mirrors are 50.0 cm apart to within an uncertainty of 1 mm.

(2) To align the optical resonator for the HeNe laser it is easiest to use the beam of another HeNe laser. To do this, direct the alignment laser beam through the center of the output coupler mirror to the center of the high reflector mirror, with the HeNe discharge removed. With the room lights turned off, adjust the high reflector so that its reflected beam returns to the output coupler at the spot made by the alignment HeNe laser as it enters the cavity. Adjust the output coupler and high reflector mirrors until you observe concentric interference rings on the high reflector mirror that appear to slowly converge or diverge from the mirror center. Adjust the two mirrors until the rings appear perfectly circular. It may be necessary to adjust slightly the spacing between the two mirrors to achieve perfectly circular rings.

Now reinsert the HeNe plasma tube between the two mirrors of the optical cavity and adjust the plasma tube position so that the alignment beam passes through the center of the Brewster windows of the plasma tube. Be careful not to touch the Brewster windows or mirror surfaces during this process. With the HeNe plasma tube in place, it should be possible to see a spot at the center of the high reflector mirror that brightens and dims at approximately the same rate as the diverging and converging circular interference rings observed earlier.

Turn on the high voltage power supply to the HeNe plasma tube and (with luck) you will observe the HeNe lasing. If lasing does not occur, make small adjustments to the plasma tube and the two mirrors. If lasing still does not occur, turn off the high voltage supply, remove the HeNe plasma tube, and readjust the resonator mirrors for optimal interference rings. If after several attempts you do not achieve proper lasing action, ask the instructor for help in cleaning the Brewster windows and resonator mirrors.

Once lasing is achieved, record your alignment procedure in your laboratory notebook. Describe with a well-labeled sketch the nature of the concentric rings that you observed when aligning the optical cavity.

Investigate the range of distance d between the two mirrors over which lasing action can be maintained in the confocal resonator configuration. Do this in small steps, by increasing or decreasing the distance d by small increments, and making small adjustments to the two mirrors to maximize laser output.

(3) Repeat step (2) with a hemispherical resonator configuration. To do this, replace the spherical high reflector mirror with a flat (R = ), ³ 99.7% high reflectivity mirror. Set the resonator mirror spacing to d = 40.0 cm.

(4) With either the hemispherical or confocal resonator configuration, take photographs of the transverse mode structure of the HeNe laser output beam. By making small adjustments to the mirrors and the position of the HeNe plasma tube it should be possible to obtain transverse mode patterns such as those shown in Fig. 4(b). Mount your photographs in your laboratory notebook.

(5) Determine the linear polarization of the HeNe laser output beam using the large polaroid sheets. In your laboratory notebook explain your method for determining the polarization axes of the polaroid sheets. Explain the relationship between the linear polarization you observe for the external beam of the HeNe laser and the orientation of the Brewster windows of the HeNe plasma tube.

(6) When a linearly polarized light beam of intensity I_o passes through a linear polarizer with its axis rotated by angle from the light beam polarization, the emergent intensity I is given by

I = I_ocos²

which is known as Malus's law. Use the rotatable polarizer and photodiode detector to verify this law quantitatively. Make detector readings at several values of angle and record them in a neat table in your laboratory notebook. Graph your data to demonstrate the expected cos² dependence.

(7) Assume the HeNe laser produces 3 mW of laser output power as expected and that the electrical data given in the discussion section applies to the gas discharge tube you are using. Compute the efficiency, in percent, for converting electrical energy to red laser light energy with this HeNe laser. Discuss your result.

References
1. R.A. Serway, C.J. Moses, and C.A. Moyer, Modern Physics (Saunders College Publishing, Philadelphia, 1989), pp. 267-293.
2. M. Young, Optics and Lasers, 3rd rev. ed. (Springer-Verlag, New York, 1986), pp. 145-172.
3. J. Wilson and J.F.B. Hawkes, Optoelectronics, 2nd ed. (Prentice-Hall, Englewood Cliffs, NJ, 1989), pp. 155-215.