This version reflects the comments of the core participants as reviewed and incorporated in accordance with CORD's FIPSE-supported Curriculum Morphing Project.
1) Gas lasers can be grouped into three basic types, or classes: neutral atom, ion, or molecular.
(2) The importance of HeNe lasers for a growing number of commercial applications is well established; for example, HeNe laser systems are being used in surveying, pipeline laying, position sensing for earthmoving operations, vibration monitoring, and materials testing.
(3) The purpose of this module is to examine each of the four elements of the HeNe laser in greater depth, as well as some of the hardware associated with HeNe laser systems. Characteristics of power supplies and plasma tubes for offtheshelf HeNe lasers are discussed. In the laboratory, the student will examine the operating parameters of a HeNe laser and will draw characteristic operating curves.
Upon completion of this module, the student should be able to:
(4) The active medium of the HeNe laser is a mixture of helium and neon gas; in fact, HeNe lasers perhaps could be labeled more properly as "neon lasers" since neon atoms are the active elements and helium only serves as a buffer gas (enhances lasing by causing the pumping efficiency to be increased).
(5) The energy flow in the active medium of a HeNe laser is illustrated in Figure 1, which is a combined energylevel diagram for neon and helium. For simplicity, only a few of the many energy levels are indicated. Notice that the two levels Ss and 4s in neon have nearly the same energy as two excited states in helium.
Fig. 1 Energy-level diagram for the HeNe laser
This resonance between energy levels of helium and neon is the key to increased pumping efficiency. Electrons, released by the cathode, are accelerated toward the anode by the voltage applied across the plasma tube, resulting in energetic collisions between electrons and gaseous atoms. Since a helium atom weighs only about one-fifth as much as a neon atom, electrons transfer energy much more readily to helium atoms via collisions. This process results in helium excitation to Detestable levels. Thereafter, excitation of neon atoms to levels Ss and 4s occurs by means of collisions with excited helium atoms. The slight differences in energy between the helium and neon levels are accounted for by a change in the kinetic energy of the atoms during collision.
(6) The process of pumping the active laser gas indirectly by transfer of energy via collisions with another excited gas is called "resonant excitation" and has been utilized in several types of gas lasers. For example, although lasing action has been observed in carbon dioxide (CO2) alone, the pumping efficiency is greatly improved if a quantity of nitrogen (N2) is added as a buffer gas.
(7) If resonant transfer of energy from He metastable atoms to He atoms occurs at a rate faster than the decay rate from 5s and 4s to lower levels, then population inversion may be achieved. Stimulated emission of radiation in neon occurs at six different wavelengths as shown in Figure 1. Until 1985 lasing was practiced at only three of these lines: the familiar line at 632.8 nm in the red portion of the spectrum and two additional lines at 1.152 mm and 3.39 mm in the infrared. We will begin by discussing the operation of these three lines (primarily the 632.8-nm line) and conclude with the more recent operator at the other lines.
(8) The 3.39-mm line and the 632.8-nm line share the same upper lasing level and are in competition for the same excited atoms that sustain the lasing process. When such a condition exists, the line having the greatest gain will lose, and the weaker line will not. In most cases, the lower energy transition will predominate. With neon, the gain is so great for the 3.39 mm that lasing can be obtained at this wavelength without a pair of mirrors for feedback. The phenomenon of achieving an output from a laser with high gain without the use of a pair of mirrors is termed "superradiant lasing," which can occur in large HeNe lasers.
(9) The 1.15-mm line and the 632.8-nm line share the same lower lasing line. If the 1.15-mm line lases, atoms from 4s are transferred to level 3p, raising its population and reducing the population inversion for the 632.8-nm line.
(10) All three of these wavelengths may be present in the laser output simultaneously. If one is interested in a HeNe laser whose output is at the 632.8-nm wavelength only, positive steps must be taken to suppress oscillations at the other two wavelengths. Both the 3.39-mm and 1.15-mm lines have greater gain than the 632.8-nm lines and are capable of "stealing" power that otherwise would be available as output at the 632.8-nm wavelength.
(11) The most common method for the elimination of unwanted infrared lines in HeNe lasers is the use of mirrors that are highly reflective of 632.8 nm but highly transmitting at the other two lines. This method reduces the feedback for these lines so much that their loop gain is always less than one. An examination of most lowpower commercial HeNe lasers will reveal that no active steps have been taken to eliminate the infrared lines, other than use of selective mirror coatings that have a reflective bandpass strongly peaked at 632.8 nm.
(12) In the larger plasma tubes of higherpower HeNe lasers, selective mirror coatings will not prevent lasing on unwanted lines, and additional steps must be taken. The most common of these steps is the use of magnetic suppression, in which a row of magnets is placed beside the plasma tube. The weak magnetic field causes the energy levels of the neon gas to split into several closelyspaced levels, broadening the laser line as indicated in Figure 2.
Fig. 2 Line broadening by a magnetic field
(13) Instead of a single transition, each laser line now consists of a group of overlapping lines. This effect is much more pronounced for the lines of longer wavelength and, thus, reduces the gain at 3.39 mm and 1.15 mm below losing threshold, but has little influence upon 632.8 nm.
(14) Magnetic suppression can be tested on most HeNe lasers of 2 mW-output power or more if small magnets are placed near the plasma tube while the laser output power is being observed. Other methods of suppression include intracavity tuning with a prism or placement of a 3.39-mm absorber, such as methane gas, in the cavity.
(15) The optimum ratio of helium to neon for operation of a HeNe laser at 632.8 nm has been determined experimentally to be about 7.5:1. Helium atoms are so small that they gradually diffuse through the glass walls of the lasertube, lowering the partial pressure of helium. For this reason, most HeNe lasers are filled with an initial mix of 9:1. The range of gas mixture that will produce losing at 632.8 nm is from 3:1 to 20:1. The 9:1 ratio is chosen as the optimum for extended tube life, although the actual ratio of helium-to-neon drops with time.
(16) For maximum power output, the total gas-fill pressure in a HeNe plasma tube is dependent upon the capillary tube diameter, according to Equation 1.
*Pressure is defined as the force per unit area exerted on an object (Units: newtons/m2 or pounds/in2). The pressure exerted by the atmosphere upon all objects at sea level is approximately 14.7 lb/in2. Pressure also may be expressed in terms of the height of a column of liquid produced by a given pressure; for example, 1 atmosphere (14.7lb/in2) of pressure raises a column of mercury to a height of 760 mm at 00C. The unit of pressure known as the "torr"(T) is equivalent to a pressure of 1 mm of mercury, or 1/760 an atmosphere.
(17) Equation 1 for the tube pressure P should not be taken as a "magic recipe" but, merely, as a rule of thumb. Some HeNe lasers now in operation have either greater or lower fill pressure than that prescribed by Equation 1 for a given tube bore d. In most commercial HeNe laser tubes, the initial gas-fill pressure is about 1.5 times the optimum pressure for tube life extension.
(18) Example A illustrates the calculation of gas pressures in a HeNe laser tube.
(19) Figure 3 gives the relative populations of energy levels 3p and 5s in a HeNe laser as functions of tube current. At low currents, 5s is populated efficiently by He collisions. Its population increases with tube current but begins to level off at higher currents. The population of 3p depends, in part, upon gas temperature.
Fig. 3 Population inversion
(20) At low currents, the 3p population is low; but an increasing current heats the gas, its population rises rapidly, destroying the population inversion. Most HeNe lasers produce maximum output at 6-8 mA, and a few will lase at tube currents above 20 mA.
(21) Figure 4 is an experimental curve of output power versus tube current for a commercial 2mW HeNe laser.
Fig. 4 Power output of HeNe laser versus plasma-tube current (P-I curve)
(22) Notice the three regions in Figure 4. For tube currents less than 2 mA, the gas discharge is in the unstable region. Visually, the light emitted by the glowing gas flickers on-and-off intermittently. Coherent light output would not be seen with the eye. The light emitted by the tube during each brief "burst" is from incoherent spontaneous emission. Likewise, zero power output would be detected with an optical power meter. For currents greater than 2 mA, a stable discharge is observed; and measurable power outputs can be noted with a power meter. The usual operating range for the laser is between 3 and 6 mA, although many operate in the 5- to 6-mA range for added efficiency. Usable laser output at 632.8 nm in the power range 1-2.6 mW is obtained by varying the tube current.
(23) Beyond the maximum output power of approximately 2.6 mW at 5.7 mA, the P versus I curve begins to have a negative slope; that is, the power decreases for increased tube currents. When an increase in tube current no longer will result in further increase in output laser power, the plasma tube is said to be in the "saturation region." This phenomenon corresponds to the situation in which more electrical energy is being pumped into the gas than it actually can use for effective excitation. The gas heats up somewhat in this region and can result in a decrease in plasma-tube life; therefore, limiting current flow in the circuit to values that prevent saturation is desirable.
(24) Excitation of the gas is achieved in a HeNe laser by dc current flow through the gas. The power supply must be of a type that will provide a stable discharge at the desired current. The design of the power supply depends upon the electrical characteristics of the plasma tube.
(25) Figure 5 is a voltage-versus-current curve for an ordinary fixed resistor. The resistance is the slope of this curve. Current through the resistor can be controlled by controlling the applied voltage. An increase in voltage will produce a corresponding increase in current.
Fig. 5 Voltage versus current for a fixed resistance.
(26) Figure 6 is a voltage-versus-current curve for a HeNe laser tube. The shape of this curve is characteristic of that for all gas discharges in which the conducting element is a plasma. The word "plasma" coined by Langmair in the 1920s, originally was defined as "a weakly ionized gas." Ionization is the process by which negatively-charged electrons (one or more) are removed from an atom, leaving it positively charged. The removal of an electron from the outermost shell of an atom requires a definite amount of energy that must be deposited; such energy is referred to as the "ionization potential energy," which usually is given in electronvolts (eV). The exact value of the ionization energy depends upon the structure of the atom; for example, helium (He) has an ionization energy of 24.6 eV, neon (He) of 21.5 eV, and argon of 15.8 eV. Notice that the ionization energy of Ne is less than that of He.
Fig. 6 Voltage-current characteristic curve
(27) As indicated by Figure 6, very little current flows through the tube until the applied voltage across the tube reaches a certain pointthe "hump" on the curve. This point is called the "breakdown voltage." For this particular tube, the breakdown voltage is determined experimentally to be about 3400 volts. When the breakdown voltage or potential is applied, some of the atoms are ionizeda neon plasma is createdresulting in a reduced tube resistance and an increase in tube current. As the current increases, more atoms are ionized because of the collisions of electrons with atoms; consequently, a negatively dynamic resistance of the laser tube is produced. In the negative resistance region, an increase in tube current results in a decrease in tube voltage, thus giving a negative slope for the voltage-versus-current curve.
(28) The current through a gas discharge cannot be regulated by control of the applied voltage because the resistance does not remain constant. At a constant voltage, the tube resistance will decrease until very large currents are reached unless some method of limiting the current is employed.
(29) The power supply of a HeNe laser tube must contain three functional elements dictated by the shape of the voltageversuscurrent curve in Figure 6:
(30) Figure 7 is a schematic diagram of a HeNe laser power supply. A detailed description of the operation of this supply is beyond the scope of this module. The following simplified description illustrates the three functional components of the power supply.
Fig. 7 HeNe power supply
(31) When the power switch is closed, current flows through the step-up transformer, which produces an ac output voltage of approximately 800. I remainder of the running supply consists of a voltage doubler-rectifier circuit that converts this ac voltage to dc voltage of approximately 2240. This voltage is not of sufficient value to ionize the tube, but it will sustain current flow once it has begun.
(32) The starter circuit consists of a voltage multiplier circuit capable of delivering several kilovolts but no current. Once the breakdown voltage is reached, the tube ionizes, and the diodes in this circuit conduct the current. The voltage difference across the starter circuit becomes nearly zero, and the voltage of the running supply is delivered to the tube and ballast resistor.
(33) The value of the ballast resistor depends upon the degree to which the current is to be limited. If this circuit is used with the tube whose characteristic curve is given in Figure 6, the ballast resistance value is 300 k xxx omega, which limits tube current to 4.5 mA, resulting in a voltage drop of 800 V across the ballast resistor and a drop of 1360 V across the laser tube.
(34) In recent years, many manufacturers have replaced ballast resistors with transistorized current regulators that ensure a constant tube current even as tube characteristics change with age.
(35) The positive electrode of the laser tube is the "anode." The anode is struck by electrons in the gas discharge. The light weight of the electrons causes them to have low kinetic energy; therefore, they transfer very little energy to the anode in the collision process, and the anode can be physically small. The anode of most HeNe laser tubes is a small nickel pin that extends through the glass tube.
(36) The negative electrode is the "cathode." It is struck by the massive ions in the gas discharge. These positive ions carry a large kinetic energy that heats the cathode; consequently, the cathode must have a surface area large enough to dissipate the thermal energy and to prevent overheating.
(37) Various types of cathodes have been used in HeNe plasma tubes. The most common is a hollow cylinder of aluminum alloy. Contaminants in a plasma tube should be kept to a minimum to avoid "poisoning" of the gas mixture. One type of contaminant comes from the material from which the cathode is fabricated. Some atoms in the metal cathode may be physically jarred loose by bombardment from positively charged ions attracted to it, a process known as "sputtering." Pure aluminum and certain of its alloys exhibit very low sputtering rates and are good cathode materials. Commercial plasma tubes that are equipped with cold aluminum cathodes have a useful lifetime of some 10,000 hours or more.
(38) Some of the various mirror configurations used in practical lasers were discussed in Module 17. In commercial HeNe gas lasers, the hemispherical resonator (Figure 8) usually is employed. It consists of one curved and one flat mirror, separated by a distance L that is equal to the radius of curvature of the curved mirror. The curved mirror normally is the output, or transmission, mirror; and the flat one is coated with a "highreflectance" (HR) coating.
Fig. 8 Hemispherical laser cavity
(39) The multilayer dielectric coatings are deposited onto a substrate made of fused quartz and are designed to scatter and absorb as little of the laser radiation as possible in order that cavity losses can be minimized. The substrates are ground and polished to a high degree of surface accuracy. Small imperfections in mirror surfaces are additional sources of output power loss. Curves that relate percent transmission as a function of wavelength for typical multilayer dielectric coatings are displayed in Figure 9. At l = 632.8 nm, the reflectance R of the HR mirror is > 99.9%, and the transmittance T of the output mirror is ~1.2%.
Fig. 9 Transmission curves for typical multilayer
(40) The mirror separation L in lasers that contain a hemispherical mirror configuration usually is made a little less than the curvedmirror radius rim When L < r1, the wavefront that strikes the HR mirror contacts a larger area than the theoretical diffraction-limited spot area produced in the case r1 = L. The wavefront, in being reflected from a larger area, is less likely to undergo severe distortions caused by optical imperfections in the HR mirror. Such distortions could destroy the selfreproducing radiation field inside the cavity.
(41) One major advantage of the hemispherical cavity is its ease of alignment compared to other configurations, such as the plane-parallel mirror arrangement. The HR mirror normally is aligned first with respect to the capillary of the plasma tube. Such alignment is "the difficult part." Having the curved mirror as an output reflector facilitates the alignment procedure since there is more tolerance in the positioning of the curved mirror with respect to the plasma tube axis and the HR mirror.
(42) Figure 10 displays the components of a typical commercial HeNe laser tube. Lasing at 632.8 nm occurs in a capillary tube of approximately 1 mm inner diameter. The mirrors are epoxied warp directly onto the ends of the tube. Fine adjustments in the tube alignment can be made on this model: The support clamp can be adjusted in such a manner that the capillary is bent slightly. In other models, the mirrors are attached in accurate alignment, and no adjustment is possible.
Fig. 10 Typical helium-neon plasma tube
(43) The cathode is a large aluminum cylinder. The volume that encloses the cathode also serves as a gas reservoir to increase the volume of gas in the tube and, thus, to extend tube life. The anode is a nickel pin, as described earlier.
(44) This tube also contains an element called a "getter," which extends tube life by absorbing impurities from the gas.
(45) HeNe laser tubes have a limited useful lifetime. Aside from accidental breakage, the following mechanisms limit tube life.
(46) The useful operating life of a HeNe laser tube actually exceeds its shelf life. In order that the maximum lifetime of HeNe laser tubes can be ensured, the tubes should be turned on for several hours each week, even if the laser is not in use.
(47) Since 1985, HeNe lasers have become commercially available with outputs at 543 nm (green light) and 1.152 l (in the infrared).
(48) The green HeNe's (or GreNe's as they are sometimes called) are self-contained systems. They have randomly polarized outputs in the 0.5-mW range. The transition for the green line (543 nm) is from a 5s level to a 3p level, as shown in Figure 1. Lasing at this line is enhanced by altering the gas mixture and pressure, and changing the spectral reflection characteristics for the cavity mirrors.
(49) The green output wavelength of these lasers is especially useful in a number of new applications. While the available power output levels are somewhat lower than equivalent red HeNe lasers, the wavelength falls exactly in the middle of the peak of the eyeresponse curve. The result is that the laser beams are extremely visible in even adverse conditions. Some applications anticipated for these lasers follow.
(50) Helium-neon lasers, operating in the near IR at 1.523-micron wavelength are available at the 1.0-mW power levels with either randomly or linearly polarized outputs. The transition for this IR line is from a 4s level to a 3p level, as shown in Figure 1. Lasing at this line is also achieved by altering the ratio of helium to neon in the gas, the gas pressure and the sprectral reflectivity of the cavity mirrors.
(51) The operation of these HeNe lasers is somewhat less stable than the conventional red laser. Thermal expansion of the plasma tube causes enough movement to result in single-mode power sweeping of up to nearly 50%. Active control of an external cavity mirror can limit this value to less than 1% with suitable feedback. Applications requiring the stable output frequencies, such as communications applications, will require additional effort to supply adequate external stabilization. Most other applications will not be affected at all by this factor.
(52) Applications for and features of this new laser line include:
HeNe laser tube
In this laboratory, the student will measure tube voltage and output power as functions of tube current and will draw characteristic curves of a HeNe laser tube.
CAUTION: High-voltage terminals and components will be exposed during this experiment. Do not touch power components or leads during operation. Before touching any section of the power supply or leads, (1) turn off power switch; and (2) short output terminals of the supply with a clip lead to discharge capacitors.
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