| This version reflects the comments of the core participants as reviewed and incorporated in accordance with CORD's FIPSE-supported Curriculum Morphing Project. | |||||||||||||||||||||||||
MODULE 3-7 Module 3-6, "Energy Transfer in Ion Lasers" described the operation of argon ion laser tubes and the basic optical system for these lasers. This module discusses additional features of such laser systems and more complex optical components used in controlling laser wavelength. The first half of this module is a description of various subsystems present in argon lasers. These include the resonator frame structure, electrical systems, interlock and safety systems, gas and water systems, and additional notes on the optical cavity. The second half of this module describes the design and functioning of etalons used with argon lasers to achieve laser operation on a single longitudinal mode. An etalon acts as a fixed but tunable additional optical cavity inside the main resonator cavity. The only frequencies that will oscillate are those that are modes of both the main laser cavity and the etalon cavity. Desirability of single-mode operation and effects on laser output also are discussed. In the laboratory, the student will align the optical cavity of an argon laser and install the etalon to achieve single-mode operation.
Upon completion of this module, the student should be able to: 1. Explain types of resonator frame structures used in argon lasers and characteristics for each type. 2. Explain the interlock system of a typical argon laser and how it is used to protect both equipment and operator. 3. Describe operation of the two most common types of gas fill systems. 4. Explain the functions of an adjustable aperture in the laser cavity. 5. Explain, with diagrams, the function of an etalon in an argon laser cavity. 6. Given the length of an argon laser cavity and the laser linewidth, determine mode spacing of the laser and the approximate number of modes in oscillation. 7. Given the length of a laser cavity and the laser linewidth, determine the free spectral range and transmission bandwidth of an etalon for operating the laser in a single mode. 8. Given the free spectral range of an etalon and index of refraction of the material, determine thickness of the etalon. 9. Describe the two principal types of etalons used in argon lasers. Explain advantages, disadvantages, tuning methods, and applications of each type. 10. Explain operation of an optical spectrum analyzer. 11. Use materials and procedures listed in this module to align the optical system for an argon laser and install an etalon to achieve single-mode operation.
General Characteristics of Argon Ion Lasers Argon ion lasers are available in a range of sizes from air-cooled models, producing a few milliwatts of output power at 488 nm and 514.5 nm, to the largest commercial models with output of over 40 watts multiline. All but the smallest of these incorporate the same features of design and control. Module 3-6, "Energy Transfer in Ion Lasers," describes general characteristics of such lasers, construction of ion laser tubes, and design of the basic optical cavity. A list of output wavelengths of argon and krypton lasers is given in Table 1 of that module. This module describes design and operating characteristics of other subsystems in ion lasers.
Resonator Frame Design Stability of any laser depends on the geometrical (angular and length) stability of the resonator frame that supports the optical components and laser tube. Stability is often a prime consideration in argon laser applications. Argon lasers require more careful design for high stability than most other systems. Large temperature gradients and fluctuations as well as vibrations from water flow and magnetic effects associated with high-current operations all contribute to system instability. This section discusses how these problems are overcome by proper resonator engineering in argon and other ion lasers. The first factor to be considered is angular stability of the laser cavity. If the resonator frame is bent or flexed during laser operation, optical alignment will be affected, reducing output power. Long-term stability can be greatly reduced if one part of the resonator structure experiences temperature changes significantly different from others. A typical method of overcoming this is to make the resonator frame from a single aluminum extrusion with an L-shaped cross section. Mirror mounts are securely bolted to the ends of the frame; other optical components and the laser tube are mounted inside the frame on the horizontal surface of the "L." The high thermal conduction of the aluminum distributes heat evenly and prevents thermal distortion due to large thermal gradients. The structure is sufficiently rigid to minimize beam fluctuation due to mechanical vibrations. The extruded aluminum frame provides angular stability, but it does not provide length stability. As temperature of the frame increases with laser operation, cavity length increases and the wavelength of each laser mode decreases. If good, long-term frequency stability is required, cavity length must remain constant during laser operation. This is accomplished by mounting the laser mirror mounts on a quartz rod resonator structure that maintains an almost constant cavity length with typical increases in temperature. Three quartz tubes are mounted in an L-shaped aluminum frame inside protective aluminum tubes. Each end of each quartz tube is connected to a corner of one oft he mirror mounts. Other components are mounted directly on the aluminum structure. The aluminum surrounding the quartz tends to equalize the temperature along each rod. Plates connecting the aluminum tubgs equalize temperature between them. Because the thermal coefficient of expansion of quartz is very low, length of the laser cavity changes insignificantly with small changes in temperature. In such a system, the frequency shift of a single laser mode is typically around 60 MHz per centigrade degree of temperature change. Both types of resonator frames are shown in Figure 1.
Fig. 1 Both resonator frame structures require isolation from mechanical forces applied to the laser case. This is accomplished with a kinematic mount that prevents stresses on the case from being transmitted to the resonator frame. This system is shown in Figure 2. Mirror mounts and other optical mounts of ion lasers are also designed and constructed to provide maximum stability.
Fig. 2 Electrical Systems Electrical systems of argon lasers are more complex and sophisticated than those of most laser types. A detailed discussion of the electrical systems is beyond the scope of this module. Module 3-1, "Power Sources for CW Lasers," contains a section describing general requirements and design features of argon ion laser power supplies. This section describes the function and control of important electrical subsystems. (Refer to instruction manual for detailed electrical schematics for a specific system.)
Power Supply The power supply for argon lasers is a high-current, low-voltage supply with an LC filter section and a transistor pass bank current regulator. Input power typically is either 208 V, three-phase, or 460 V, three-phase, depending on laser power. The regulated power supply provides a constant current for both the laser tube discharge and the solenoid. Tube discharge current is controlled by a front panel adjustment that controls voltage on the bases of transistors in the pass bank and, thus, current flowing through the bank. In most models, a separate current regulator and control are used for magnet current. Two other major components are necessary for supplying power to the argon laser tube. A filament transformer provides a large current at low voltage to heat the cathode to the proper temperature. This current is preset and is not adjustable. A trigger transformer activated by a push-button switch provides a high-voltage pulse that initiates laser discharge. The control circuitry includes a time delay that prevents application of the start pulse until the cathode has time to achieve proper operating temperature.
Interlock Systems Argon lasers have a series of interlock and safety features for protection of both equipment and operator. Interlocks on the covers of the power supply and the laser head automatically turn laser power off if covers are removed. An input terminal is usually provided to the interlock system to shut off laser power if some external circuit is broken. In many cases, this circuit is part of a room interlock system that turns the laser off if the door is opened during laser operation. Other safety systems are designed primarily to protect laser equipment. These include sensors that turn off the laser if any of the following conditions exist:
Auxiliary Circuits A number of other electrical subsystems are included in most argon lasers. These include power supplies necessary for operation of interlock circuits, the thermocouple gage, the gas fill system, and control and monitoring circuitry. Most argon ion lasers have a front panel meter that serves several measuring functions. These typically include tube current, output power on two scales, tube gas pressure, and power supply regulation range. Indicator lights often are used to indicate conditions of several laser subsystems.
Gas Fill System Two types of gas fill systems are commonly used in argon lasers. Both are shown in Figure 3. Figure 3a shows a two-valve system in which two solenoid-operated valves are separated by a short length of tubing with a measured volume. Actuating the "ready" valve allows a measured amount of gas to enter the volume between the two valves. Actuating the "fill" valve then allows this gas to enter the laser tube. The single-valve system shown in Figure 3b uses a single solenoid-operated valve that has a definite cycle time. The output end of this valve is connected to a stainless steel capillary tube that leads to the laser tube. When the valve is opened momentarily, gas flows into the capillary. Resistance to gas flow is such that the proper amount of gas flows into the laser tube during the cycle of the single valve. Both systems usually are designed to increase gas pressure about 10 millitorr at a time.
Fig. 3 Some lasers employ additional elements in the gas fill system. In some cases, a warning buzzer sounds continuously if gas pressure is too low. Some also have automatic gas fill systems that add gas until the proper pressure is reached when the gas fill system is actuated.
Water Cooling System All but the smallest argon lasers require water cooling with flow rates in the range of 2.2 to 7.0 gallons per minute, depending on laser power and size. The water flow system normally includes an input filter to remove any contaminants. Water flows through the laser power supply pass bank and the laser tube. Sensors mentioned earlier monitor flow rate and exhaust water temperature and turn the laser off if there is a problem. The water flow sensor should be connected at the output water port of the laser system so it will turn the system off if a large leak or disconnection occurs. If the switch is located on the water input, the system will continue to operate even if a hose is broken and no water reaches some components. The water flow switch should be checked periodically to ensure proper operation and flow setting. All water should be drained from the system for storage or shipment.
Optical System Basic design of the optical cavity for argon lasers, characteristics of laser mirrors, and operation of the prism wavelength selector were discussed in Module 3-6, "Energy Transfer in Ion Lasers." Several additional optical elements and subsystems are often included in argon lasers. One common additional element in the optical cavity is an adjustable aperture near the laser output coupler. This aperture is used to change the effective aperture of the laser cavity. With the aperture opened all the way, the limiting aperture is provided by the walls of the laser tube. In this condition, some of the laser lines, particularly those at shorter wavelengths, may oscillate in TEM modes other than TEM00. This may be desirable to achieve the greatest output power. Closing the aperture part way will result in TEM00 mode of any laser line selected by increasing the diffraction losses for higher-order modes. (This phenomenon is discussed further in Module 3-9, "CO2 Laser Systems," where it is applied to the design of optical cavities for molecular lasers.) Another common optical subsystem is a built-in optical power meter for monitoring laser output power. This consists of a beam splitter on the laser output aperture that reflects a small percentage of the laser output to a photocell that is connected through an amplifier to the front panel meter of the laser power supply. The meter is calibrated to indicate true output power of the laser. Many systems employ a light regulation mode in which this optical signal is used to control tube current and, thus, output power of the laser. This assures a constant output power even if some laser parameters change during operation.
Etalons for Controlling Longitudinal Modes An etalon is an optical component inserted in a laser cavity to eliminate all longitudinal modes but one. This reduces spectral linewidth of the laser output and increases coherence length of the laser. In many cases, this is highly desirable, particularly in argon lasers. A review of the mode structure of a laser is presented below as a foundation for understanding the operation of an etalon.
Longitudinal Modes in Argon Lasers Figure 4 shows the longitudinal mode* structure of a laser. Lasing will occur for any mode for which the gain is above the threshold line. Frequency spacing between two adjacent modes is given by Equation 1.
Fig. 4
Example A illustrates the use of this equation in determining mode spacing of an argon laser.
The number of longitudinal modes present in the laser output is approximately equal to total laser linewidth divided by mode spacing. In an argon laser, linewidth depends upon which line is lasing, the current, and the magnetic field strength. Linewidth for the 488-nm line is typically around 8 GHz. If the laser in Example A has this linewidth, there will be approximately 67 modes in its output. Ion lasers typically have broad laser linewidths because of the Doppler effect in the high-temperature laser gas. In many uses, this is not a desirable quality. Many spectrographic applications require narrow linewidths and stable wavelength operation or control. Applications involving interference of light often require long coherence lengths. Coherence length of a laser is givgn by Equation 2.
Thus, a narrower linewidth results in a greater coherence length, as illustrated by Example B.
Installing an etalon in the optical cavity of an argon laser can reduce linewidth by a factor of 1000 and increase coherence length proportionately. Characteristics of Etalons An etalon is an optical cavity consisting of two parallel surfaces that are partially reflective. Characteristics of an etalon are similar to those of the optical cavity of a laser, and optical standing waves form in the etalon just as in the laser cavity. Figure 5 shows the transmission of an etalon as a function of incident light frequency. This curve has the same characteristics as the mode structure of a laser. Mode spacing of the etalon (also called the "free spectral range" or FSR) is the frequency spacing between two modes. It depends on the index of refraction of the etalon material and its thickness. The transmission bandwidth (sharpness or finesse) of one of the etalon modes is dependent upon reflectivity of the etalon surfaces.
Fig. 5 When light of the frequency of one of the transmission modes is incident upon the etalon, some of the energy is transmitted into the etalon, where it forms a standing wave. Intgnsity of this standing wave increases because of the incoming light, and a portion of the light is transmitted out the other side of the etalon. If light frequency is not coincident with one of the etalon modes, there is no buildup of a standing wave and thus very little transmission.
Etalons in the Laser Cavity Figure 6 shows an etalon installed in the cavity of an argon laser. The wavelength selgctor first is installed to obtain lasing on a single line. Then the etalon is inserted and adjusted (by tilt) to allow lasing only on one of the modes within that line. The etalon introduces significant losses into the laser cavity for any light frequencies that are not modes of the etalon. Thus, any laser modes falling outside the etalon modes will not oscillate. Laser modes that fall within the etalon modes will continue to oscillate with low loss. Thus, only those frequencies that are modes of both laser cavity and etalon cavity will be present.
Fig. 6 Figure 7 shows the longitudinal mode pattern of Figure 4 with the etalon mode pattern of Figure 5 superimposed on it. In this case, only the single laser mode inside the etalon mode will lase. Etalons for laser mode selection are designed so their free spectral range is greater than the laser linewidth. Thus only on etalon mode will be within the laser linewidth at any one time. Transmission bandwidth of the etalon is designed to be on the order of the mode spacing of the laser. Thus, only one laser mode can be within the etalon transmission mode at any one time. Single-mode operation is achieved by simply inserting the etalon in the optical cavity and adjusting its alignment properly.
Fig. 7 The etalon should not be aligned so that reflections from its surfaces are exactly along the optical axis. If this is the case, unwanted modes will lase between the etalon surfaces and the laser mirrors. The etalon first is aligned to the position and then misaligned slightly to achieve single-mode operation. The exact frequency of any etalon transmission mode depends on cavity length of the etalon. Fine tuning of the frequency of the etalon mode is accomplished by changing cavity length slightly. Tuning methods for two etalon types are discussed in the next section. Frequency of any laser mode depends on cavity length of the laser. When the etalon mode is positioned for the proper frequency, the laser mode may be tuned to a particular frequency range under the etalon curve by proper positioning of one of the laser cavity mirrors with a piezoelectric mirror mount. This is required only if a specific output frequency is required, as in spectroscopic application.
Etalon Types Figure 8 shows the two types of etalons commonly used with ion lasers. Figure 8a is a solid quartz etalon. Its surfaces are plane and parallel and are coated with partially-reflective coatings. This type of etalon is mounted in a mount with angular adjustments between the laser tube and the wavelength selector. Tuning is accomplished by changing the angle of the etalon with respect to the optical axis. A very slight angular change changes the optical path through the etalon enough to shift the etalon modes through a full free spectral range, allowing selection of any mode in the laser gain profile.
Fig. 8 Advantages of the solid etalon include low cost and low insertion loss in the system. Solid etalons are best in low-gain systems. A serious disadvantage of a solid etalon is temperature instability. If temperature of the etalon changes by only a fraction of a centigrade degree, wavelength of the laser will also change. If the only concern is to extend coherence length of the laser, this is of no consequence. If close frequency control is required, the etalon must be placed in a temperature-controlled environment. This increases system cost and is inconvenient because a length warm-up time of ~30 minutes is required before the system stabilizes in frequency. For higher-gain systems, temperature stability may be achieved by using the air space etalon shown in Figure 8b. It is composed of two optically flat, thin windows. Each is antireflective coated on its outer surface and has a partially-reflective coating on its inner surface. The two windows are bonded to the ends of a hollow spacer made of titanium silicate glass. The thermal coefficient of expansion of this material is only 0.03 ´ 10-6/° C. This means that etalon length is virtually unaffected by temperature changes normally encountered in ion lasers. The air space etalon is mounted in an adjustable mount in the same location as the solid etalon. It is tuned by applying a compression force to change the cavity length slightly. The major disadvantage of the air space etalon is that the two additional optical surfaces introduce more loss in the system, even though they are antireflection coated. This type of etalon cannot be used in low-gain systems. Placing an etalon in the laser cavity reduces output power, but in most cases, the maximum output power available from any argon laser line in single-mode oscillation is over half the multimode power. Most of the energy that is not being used for lasing by other modes is available to the single mode that does lase.
Scanning Optical Spectrum Analyzers An optical spectrum analyzer* is a device used to examine spectral output of a laser. It is essentially an etalon with reflecting spherical optical elements in the cavity. Mirrors of a scanning optical spectrum analyzer are separated by a cylinder of piezoelectric material. Applying a voltage to the cylinder changes its length and, thus, shifts the transmission peaks of the analyzer. These devices are designed to have very narrow transmission bandwidths (high finesse) for resolving narrow spikes in the laser output. Transmitted light is sensed by a photocell connected to an oscilloscope. Mirrors of the spectrum analyzer move apart at a constant rate and then return to their original position and repeat the sweep. This motion is synchronized to the oscilloscope sweep to produce a display of laser output power vertically as a function of frequency horizontally on the scope face. Optical spectrum analyzers are commonly used to monitor the output of argon lasers in single-mode operation.
Summary Argon ion lasers are among the most complex systems in common use. Subsystems necessary for operation of an argon laser include the laser tube, a stable resonator frame, a sophisticated and versatile optical system, a gas control system, water cooling system, and an electrical system that includes the power supply and a wide range of monitoring and control circuitry. Module 3-6, "Energy Transfer in Ion Lasers," and this module discuss essential elements and functions in ion lasers. Argon lasers are often constrained to operate on a single longitudinal mode to improve spectral purity of the beam and coherence length of the laser. An etalon is an additional small optical cavity inserted inside the main laser cavity. The only laser cavity modes that will oscillate are those that are also modes of the etalon. With proper etalon design and tuning, any laser mode may be selected for single-mode operation.
1. Explain the two types of resonator frames used in argon lasers and give examples of the applications of each. 2. Describe the operation of two types of gas fill systems. 3. Describe two types of etalons used in argon lasers, and explain advantages, disadvantages, tuning methods, and applications of each. 4. Sketch the front panel of the argon laser power supply and state the function of each control and indicator. 5. Refer to the argon laser manual and describe all interlock and safety features of the laser. 6. Refer to the argon laser manual and draw a schematic diagram of the cooling system. 7 An argon laser has a linewidth of 7.5 GHz and a cavity length of 1.5 m. Determine mode spacing and the number of modes in operation. 8. The above laser is to be operated as a single-mode laser by placing a quartz etalon in the cavity. Determine the following:
9. A HeNe laser has a linewidth of 1.5 GHz and a cavity length of 1.45 m. Determine the thickness of a solid quartz etalon for a single-mode operation of this laser. 10. Explain the operation of an optical spectrum analyzer. 11. List at least three manufacturers or argon and krypton ion lasers and the products offered by each company. Include a list of all optional equipment offered for ion lasers. 
Argon ion laser system with wavelength selector Instruction manual for argon laser Optical power meter compatible with laser HeNe laser Beam block Etalon for argon laser Optical spectrum analyzer Instruction manual for spectrum analyzer Oscilloscope Adjustable table Argon laser safety goggles
In this laboratory, the student will align the optical system of the argon laser, install the etalon for single-mode operation, and observe the output mode structure with and without the etalon in the cavity by means of a spectrum analyzer. The student should read all procedures before beginning. 1. Read the sections of the argon laser manual on system alignment and etalon operation. Follow appropriate safety precautions. 2. Remove the mirrors from the argon laser. Wrap the mirrors in clean lens tissue and set them aside. Clean Brewster windows and mirror only if necessary. 3. Place the HeNe laser on the adjustable table at a distance of about one meter from the output aperture of the argon laser. Align HeNe laser beam to be coaxial with the bore of the argon laser. This alignment is the most critical of the argon laser alignment process and must be accomplished with great care. Proper alignment is indicated by the appearance of the HeNe beam as it strikes a surface after passing through the argon laser bore. If the HeNe beam is centered on the argon bore and in line with it, the beam exiting the argon laser will consist of a central bright spot and a series of symmetrical bright and dark rings surrounding it. If this pattern does not exist, alignment is not correct and further adjustment is required. When the HeNe laser is properly aligned, it should not be moved until the argon alignment procedure is complete. 4. Please the HR mirror of the argon laser in its mount. The wavelength selector should not be used during system alignment. 7. Use adjustments of the argon HR mirror mount to bring the HeNe beam reflected from the HR mirror into the output aperture of the HeNe laser. Several indicators may be used to assist alignment. If the mirror mount is far out of adjustment, the HeNe beam may not be reflected from the argon HR mirror back into the bore of the argon tube. This may be checked by passing a piece of lens tissue through the beam near the Brewster window of the argon laser tube. When the HeNe beam is directed into the bore of the argon laser, rings of reflected light will be visible around the output aperture of the HeNe laser. The argon HR mirror mount is then adjusted to minimize the size of the rings. When rings are minimized, the reflected beam will be seen near the HeNe output aperture. This beam should be aligned to enter the HeNe aperture. If alignment is correct, the light around the HeNe aperture and the spots reflected from the argon tube Brewster windows will flicker. At this point, the HR mirror is properly aligned and should not be moved until the final stages of fine adjustment. 6. Place the output coupler in its mount and adjust the beam reflected from it back into the HeNe aperture. This does not assure that the argon laser will be in proper alignment for lasing, but it will be close. 7. Place a beam block between the two lasers and turn on the argon laser. If lasing is not present, the front mirror mount should be adjusted in small increments until lasing is observed. One effective way to accomplish this is to oscillate the mirror on one axis while adjusting through a linear range on the other. Normally, only small adjustments will be necessary to achieve lasing. 8. Adjust the output coupler mount to achieve the highest laser output power. 9. Adjust the HR mirror to achieve the highest output power. 10. At this point, the beam may not be passing down the center of the tube bore. This may be ensured by "walking" the beam across the bore. This is accomplished by moving the beam vertically and horizontally in the optical cavity for centering. Note the output power of the laser before beginning. Then adjust the vertical adjustment of the output coupler mount to reflect the beam slightly upward, lowering the laser power. Adjust the vertical adjustment of the HR mirror mount to achieve the maximum output power. If this power is higher than the original power noted, the beam is more centered. If it is lower, the first alignment was better. This process is continued to maximize the power through adjustments in the vertical plane. Then it is repeated independently for the horizontal plane. 11. Make fine adjustments of both mirror mounts to ensure that alignment is maximized. 12. Install the wavelength selector and achieve lasing on the argon line selected to be operated single mode. 13. Turn off the argon laser and install the etalon, following directions in the laser manual. 14. Turn on the argon laser and adjust the etalon mount to align beams reflected from the etalon surface with the main beam of the argon laser cavity. This should produce lasing between the etalon and the output coupler. This is evidenced by a change in apparent color of the beam for weaker lines or by a weak beam at 514.5 nm that is present even though the HR mirror is out of adjustment or removed. 15. Adjust the etalon to a slight misalignment to achieve single-mode operation. 16. Turn off the argon laser and review the instruction manual for the spectrum analyzer. 17. Set the argon laser power so it is barely lasing and install the optical spectrum analyzer to intercept the argon laser beam. 18. Align the optical spectrum analyzer and place it in operation as indicated in the instruction manual. Have the lab instructor verify proper operation. 19. Use the optical spectrum analyzer, according to the instructions in its manual, and change the argon laser optical system and control settings as appropriate to answer the following questions:
20. Upon completion of this experiment, return all equipment to its original condition and secure the laboratory. 21. Prepare a report on spectral measurements taken with the spectrum analyzer. The report should include a list of specifications of all equipment used, an outline of the experimental procedure, all pertinent data (including sketches or photographs of the laser spectral output), and answers to the questions asked in Step 19 of the procedure. 
Bloom, Arnold. L. Gas Lasers. New York: John Wiley & Sons, Inc., 1968. Bridges, W.B. and Chester, A.N. "Ionized Gas Lasers," in Handbook of Lasers. Presley, R.J., editor. Chemical Rubber Co., 1971. O’Shea, Donald C.; Callen, Russell W.; Rhodes, William T. Introduction to Lasers and Their Applications. Reading, MA: Addison-Wesley Publishing Co., 1977. Ready, John F. Industrial Applications of Lasers. New York: Academic Press, 1978.
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(assume n = 1.0 for gas laser)
= 1.2 ´ 108/sec
