Pulsed solid-state lasers are among the most numerous and important commercial laser systems. They are by far the most popular laser for applications such as range finders, and share high utility with carbon dioxide lasers in the field of materials processing. Pulsed solid-state lasers are also used in many technical areas such as short-pulse holography and in research applications.

The three most common types of pulsed solid-state lasers are ruby, Nd:YAG, and Nd:glass. Ruby lasers operating at 694.3 nm were the first to be developed. Current use is in range finders, hole drillers, pulsed interferometry, holography, and retinal surgery. Nd:YAG lasers are the most popular for range finders, drillers, and welders. They are also used at high pulse repetition rates for scribing or cutting metal sheets into intricate patterns, as well as for laser markers. Nd:glass lasers may be made in large sizes and produce the highest pulse energies but suffer from lower duty cycle. Both Nd:YAG and Nd:glass operate at 1.06 m m.

Module 3-3, "Energy Transfer in Solid-State Lasers," discussed the theory of operation of solid-state laser materials and the concepts of optical pumping and spectral matching. A comparison was made of the various solid-state laser materials. This module is designed to follow Module 3-3 and to provide more detailed descriptions of individual systems. Topics included here are the basic components of pulsed solid-state lasers, output characteristics of pulsed solid-state lasers, and a discussion of system types and their applications. The safety hazards associated with pulsed solid-state lasers are discussed in some detail.

In the laboratory, the student will align a pulsed solid-state laser system and measure the output pulse energy and duration.

1. Compare output characteristics of the three most popular pulsed solid-state lasers.

2. Given a specific application and a description of the laser pulses required, discuss suitability of each of the three major pulsed solid-state laser types for the application.

3. Explain why xenon flashlamps are used with pulsed neodymium systems, even though krypton provides better spectral matching. Explain how the spectral match of xeon to neodymium may be improved.

4. Explain how pulse duration of a pulsed solid-state laser may be changed.

5. Explain how transmission of the output coupler is chosen for a pulsed solid-state laser.

6. Explain methods of rod and lamp cooling employed in pulsed solid-state lasers. Include a description of the two configurations used to deliver coolant to the rod and lamps in liquid-cooled systems.

7. Explain the importance of coolant temperatures in liquid-cooled pulsed solid-state lasers and the effects on laser performance if the coolant temperature is too high and if it is too low.

8. Draw and label a diagram of the output pulse of a typical pulsed solid-state laser. Explain the origin of spiking in the laser output.

9. Explain why precautions usually are taken to prevent shielding segments of a ruby rod from the flashlamp light, while such precautions are not used with neodymium systems.

10. Draw and label a master oscillator-power amplifier (MOPA) laser system and explain its operation. Include an explanation of why disks are used for larger-diameter beams in the Nd:glass systems.

11. Discus eye hazards and electrical hazards present in pulsed solid-state lasers.

12. Align a pulsed laser system in the laboratory, and measure its pulse energy and pulse duration.

Pulsed Solid-State Laser Components

Pulsed solid-state lasers use a rod of solid-state laser material as the active medium. This rod usually is composed of ruby, Nd:YAG, or Nd:glass, although other materials can be used. The rod is optically pumped by a flashlamp, with the light focused into the rod by a pumping cavity. The flashlamp is powered by a pulsed power supply that contains an LC circuit for shaping the input electrical pulse. The optical cavity usually is composed of two plane mirrors mounted external to the rod. Figure 1 is a simplified diagram of such a laser.

Fig. 1
Simplified schematic of a pulsed solid-state laser.

Laser Rod

The laser rod is a cylinder of solid-state laser material with a relatively smooth ground outer surface and optically polished ends. The ends of plane-parallel laser rods usually are coated with antireflection coatings for the laser wavelength, although this is not always done. The ends are usually plane parallel, but either or both ends may terminate with a Brewster’s-angle surface to minimize surface reflections and eliminate the requirement for antireflection coatings. Rods with Brewster’s-angle ends always produce output beams that are plane polarized in a plane perpendicular to the Brewster’s-angle surface. Brewster’s-angle rod ends also change with direction of the optical axis inside the laser cavity as shown in Figure 2. This can lead to greater problems with rod mounts and with alignment. For these reasons, the plane-parallel rods are much more popular.

Fig. 2
Optical axis of a cavity with a Brewster’s angle rod.

The maximum energy available from a laser rod depends in large part on the volume of the rod. Rods used in pulsed solid-state lasers vary in size according to the type of material and energy per pulse required from the laser system. Ruby rods typically vary from one-quarter inch in diameter and three inches long to one-half inch in diameter and six inches long. Some larger ruby systems employ rods as large as an inch in diameter and 15 inches long. Nd:YAG rods for pulsed systems range in size up to three-eighths inch in diameter and six inches long, although smaller rods such as one-quarter-inch by five-inch rods are more common. Nd:glass laser rods may be fabricated in a variety of lengths. The diameter may be as great as an inch, but half-inch-diameter rods are far more common because of the more efficient cooling. Typical Nd:glass laser rods are from six inches to two feet in length.

Optical Pumping System

The optical pumping system for a solid-state laser consists of a flashlamp, a power supply to energize the flashlamp, and an optical pumping cavity to direct the flashlamp light into the laser cavity. Flashlamp and cavity design are discussed in Module 3-3, "Energy Transfer in Solid-State Lasers."

Xenon flashlamps provide the best spectral match for ruby lasers and are universally used as the optical source for pumping ruby rods. Krypton flashlamps provide a better spectral match for neodymium systems, but they are far more expensive and less frequently used. Nd:YAG and Nd:glass lasers usually are pumped with xenon flashlamps. A reduction of current density in these lamps shift their output spectrum toward the red and results in more efficient pumping.

Flashlamps used to pump solid-state laser rods may be either linear or helical in shape. When a linear lamp is used, dimensions of the discharge region inside the lamp are chosen to match the active rod dimensions. The lamp and rod are usually placed in an elliptical cavity with one at each focal point. The pumping cavity is totally coated with gold for neodymium systems and with chromium for ruby systems.

When helical flashlamps are used, the rod is inserted inside the helix. The lamp generally is chosen to have a total length equal to rod length. The lamp is constructed so that the central hole allows sufficient rod clearance but is no larger than necessary. The helical lamp is enclosed in a cylindrical pumping cavity to reflect all flashlamp light back into the rod. In some systems a white ceramic reflector may be used.

The power supply for the lamp is constructed as described in Module 3-2, "Pulsed Laser Flashlamps and Power Supplies." Energy-storage capability of the capacitor bank is matched to maximum energy input of the system. Most industrial systems are designed to provide the maximum pump energy that can safely be used for the rod size. In smaller systems the maximum design energy per pulse may be well below rod capabilities. The charging supply is designed to provide energy to the energy-storage capacitor at a rate that will allow it to be fully charged during the pulse repetition time of the laser. This may vary from a few shots per minute to as many as twenty per second.

The most common energy-storage system is a single capacitor and inductor in an RLC discharge circuit, but pulse-forming networks are also commonly used. Many systems have pulse-forming networks designed so the number of sections can be varied to change pulse duration.

Optical Cavity

Most pulsed solid-state lasers use plane-parallel optical cavities. This cavity configuration allows maximum use of large-diameter laser rods and has the added advantage of favoring the TEM₀₀ mode. The diffraction loss of the plane-parallel cavity is of no great concern in most solid-state lasers because amplifier gain is sufficient to overcome fairly high losses. The plane-parallel cavity also avoids focusing the laser light inside the cavity.

Both mirrors are usually external to the rod, but some systems employ rods with one of the mirrors deposited directly on the rod end. The high-reflectance mirror is typically more than 99.5 percent reflective at the laser wavelength. Transmission of the output coupler varies from system to system, with typical values of about 50% for ruby and Nd:glass and slightly less for Nd:YAG.

Selection of the transmission of the output coupler for any particular laser system generally is based on experimental data for system operation with several different output couplers. Figure 3 shows the variation of output pulse energy from a ruby laser as the transmission of the output coupler is changed. This curve is produced by changing output couplers and firing the laser with all other conditions the same for each mirror. The output coupler chosen generally falls slightly to the left of the peak of the curve. This provides more stable operation at lower pulse energies.

Cooling System

Cooling is an important factor in all pulsed solid-state lasers. All employ some coolant forced over the rod and flashlamp. In small systems such as those used for range finders, the coolant may be either air or nitrogen. Gases do not provide good thermal energy transfer, but for the relatively low pulse energies of small systems gas cooling is sufficient, and maintenance of gas-cooled systems is less troublesome. Larger pulsed laser systems may use gas cooling, but this limits the pulse repetition rate to a value much less than that available with liquid cooling.

Most solid-state laser systems use liquid cooling. Water and ethylene glycol are the most common coolants, but other liquids may be used. Two basic cavity designs are used for liquid-cooled systems. In flooded-cavity lasers the entire pumping cavity is flooded with coolant. The rod and flashlamps are sealed at the cavity ends, and coolant flows through a set of baffles that force it across the rod and lamp surfaces. The second cavity design involves the use of water jackets that enclose both the rods and the lamps. These water jackets generally are sealed with O-ring seals. Cooling water is routed to flow across the rod first, then across the lamps, and finally to any other water-cooled cavity elements such as reflectors. This provides the lowest temperature and maximum cooling effect for the rod.

Most systems use closed-loop cooling systems with a refrigeration unit to maintain proper coolant temperature. The size of the refrigeration unit necessary depends on the maximum continuous heat load produced when the laser is operated at its maximum average power for long periods of time. This may be calculated by the same method used for CW solid-state lasers in Module 3-4, "CW Nd:YAG Laser Systems."

Coolant temperature is often a critical factor in the operation of pulsed solid-state lasers. Higher temperatures result in lower laser efficiency. Thus, it is generally desirable to operate the system at the lowest temperature practical. If temperature is reduced below the dew point, condensation will begin to form on laser components exposed to the atmosphere. If there is some condensation on the rod ends, but not enough to prevent lasing, the laser beam can burn off the condensation layer and damage rod end surfaces in the process. Thus, solid-state laser systems should never be operated with coolant temperatures low enough to result in condensation.

Output Characteristics Of Pulsed Solid-State Lasers

Figure 4 shows the time history of the flashlamp and laser output pulses of a typical pulsed solid-state laser. The beginning of the flashlamp pulse establishes a population inversion in the active medium. When the loop gain reaches 1.0, lasing begins and continues as a series of closely spaced spikes for the duration of the flashlamp pulse. These spikes are produced by gain switching in the active medium. The amplifier gain rises quickly to a high value because of the intense pumping level. This results in a high loop gain and a high-intensity standing wave in the optical cavity. This quickly depletes the population inversion for that particular wavelength, and lasing stops. Thus, the laser switches itself off momentarily by using up all of its gain. This process is repeated many times for each of the modes of the laser cavity. Because solid-state lasers have relatively broad fluorescent linewidth, there are usually a large number of modes in the output. The result is output pulses that are composed of thousands of small spikes overlapping one another. Figure 5 is an expanded portion of a ruby laser pulse, showing details of the spiking.

Fig. 4
Output of typical pulsed solid-state laser compared to input pump light as a function of time.

Because of the spiking in the output, the peak power of a pulsed solid-state laser tends to be difficult to determine, and tends to vary from shot to shot, although pulse energy and overall pulse duration may remain constant. For these reasons, specifications of pulsed solid-state lasers usually do not include the maximum output power. Instead, pulse energy and pulse duration are specified. Peak output power may be approximated by dividing the energy of the output pulse by pulse duration as with other pulsed lasers.

Pulse duration available from pulsed solid-state lasers varies from as short as 50 m sec to as long as 8 msec. The usual pulse duration is about 1 msec. Shorter-duration pulses are often used in range finders by Q-switching to produce output pulses with durations of only a few to 10 nanoseconds. Only Nd:YAG systems are capable of pulse durations of much greater than 2.0 msec. Some Nd:YAG laser drillers use pulse durations of 5 to 8 msec.

The output beam of a pulsed solid-state laser is usually a TEM₀₀ beam with a diameter equal to the rod diameter. The beam profile is generally flattened on top rather than a true Gaussian beam shape. Because the laser aperture is usually fairly large, the divergence angle of the output beam is small. Typical values are a few tenths of a milliradian.

System Types And Applications

Pulsed solid-state lasers are available in a wide range of designs and sizes for many applications. The laser system chosen for any particular application depends upon details of the specific application. This section discusses a variety of pulsed solid-state laser systems for several applications and explains how the choices of the system were made.

Ruby Lasers

The ruby laser was the first solid-state laser demonstrated and the first to be used for industrial applications. Ruby lasers were once popular as hole drillers, but this application has largely been taken over by Nd:YAG lasers because of their higher efficiency and higher pulse repetition rates. Ruby lasers are used in a variety of laboratory applications, for pulsed holography, in range finding systems, and material processing.

Since ruby is a three-level system, the ruby material is a strong absorber of its own laser wavelength. This leads to a unique design feature of ruby rod holders. Holders are always designed to obscure as little of the ruby rod as possible from the flashlamp light to prevent absorption of laser light in unpumped portions of the active medium. This is not necessary in neodymium systems, it being a four-level laser, thus the unpumped rod will not absorb the laser wavelength.

A typical small ruby laser system has a rod 1/4 to 1/2 inch in diameter and three to four inches long. A helical lamp usually is used to pump the rod. The helical shape is chosen to give a longer discharge length than is possible for a linear lamp. This extra length increases electrical resistance of the lamp and allows the use of larger capacitors at lower voltages to store the same energy. Longer pulse durations are also more easily attainable with RLC circuits when flashlamp resistance is high.

The energy-storage system for such a laser typically consists of a single 500-m F capacitor with a maximum charge voltage of about 4000 V. Inductance for the RLC circuit is provided by the secondary of a series injection internal trigger transformer with an inductance of 100 to 200 m H. This results in a slightly overdamped system with a pulse duration of about 0.8 msec. Other pulse durations are attainable by replacing the RLC circuit with a pulse-forming network.

Nd:Glass Lasers

Nd:glass lasers are almost identical to ruby lasers in design. In fact most systems can be changed from ruby to Nd:glass operation by changing the rod and mirrors.

Nd:glass is used primarily where high-energy pulses are desirable. Nd:glass has efficiencies as high as 2% compared to ruby’s 0.5%. Thus, the same system typically produces about four times the output energy per pulse with an Nd:glass rod as with a ruby rod. Nd:glass rods also can be made in larger sizes for even higher energy pulses. The chief disadvantage of the glass system is that glass is not a good conductor of thermal energy. This means that a longer time is required to cool the glass rod between firings.

Nd:glass laser systems are sometimes arranged as master oscillator-power amplifier (MOPA) systems as illustrated in Figure 6. This system consists of an oscillator section that is a complete laser. In many cases, the laser oscillator includes either a Q-switch or a mode locker for pulse shaping. Output of the oscillator is directed into a rod amplifier. During the single pass through the rod the pulse is amplified and extracts a large portion of the available energy from the rod.

A second type of Nd:glass amplifier is constructed of disks of the laser glass mounted at Brewster’s angle. This allows the use of larger-diameter beams than may be used with rod amplifiers. If rods are larger than about an inch and a half in diameter, insufficient pump light reaches the center of the rod for adequate excitation of the active medium. The amplifier disks are produced in thicknesses up to about 1.5 inches and in several sizes. They are pumped by a ring of linear flashlamps that surround the disks. This configuration allows even excitation of the gain medium for larger-diameter beams by "face pumping" of the active media laser disks.

Nd:YAG Lasers

The Nd:YAG laser is by far the most popular pulsed solid-state laser. Small versions are widely used in many military applications such as range finders and target designators. These are designed for short pulse durations and usually employ Pockels cell Q-switches for pulse-duration control. They usually use a single linear flashlamp and a gold-coated elliptical cavity. Cooling may be by forced air or by a closed liquid cooling system with a liquid-to-air heat exchanger and a fan. Such systems are capable of only a fraction of a joule per pulse.

Larger pulsed Nd:YAG lasers are the most common solid-state lasers for materials processing applications. These applications include marking, hole drilling, scribing, and laser welding. The pumping scheme of such a laser usually includes a double elliptical cavity with a gold coating and two linear flashlamps. Pulse repetition rate is variable from single-shot to as high as 100 pulses per second in some systems. Pulse duration is adjustable by means of a variable pulse-forming network. Pulse duration can be varied from 0.5 msec to as long as 8.0 msec in some systems. Average power of such lasers is in the range of 100 to 400 watts.

Nd:YAG is chosen for most materials processing applications because of the high pulse repetition rates available. Power supplies of pulsed Nd:YAG laser machine tools are designed to produce a maximum average power from the system. At low pulse repetition rates higher pulse energies are available. At higher pulse repetition rates the same average power is available, but the energy per pulse is lower. One system, for example, can produce 20 pulses per second with 20 joules per pulse, or 200 pulses per second with 2 joules per pulse.

In such high pulse-rate systems the lamps are often "kept alive" to increase lamp lifetime. The greatest strain on the lamps occurs when the lamp is ignited. In a keep-alive or simmering system, a current of about 2 amps flows through the lamps continuously during laser operation. This maintains a discharge in the lamp and eliminates the requirement of a high-voltage trigger pulse for each laser pulse and the resultant lamp shock.

Safety With Pulsed Solid-State Lasers

Pulsed solid-state lasers present safety hazards that are among the most severe of any lasers. Peak output powers are so high that direct viewing of the beam or its reflections is an eye hazard even at great distances. The diffuse reflection of the beam from a roughened surface may also present a serious eye hazard. Such reflections are not focused to a small spot on the retina, but form an image with a size related to the apparent size of the focused spot. This is an extended source, but peak irradiance is so high that the damage threshold of the retina may be exceeded. Moving to a greater distance from such a diffuse reflection does not reduce irradiance on the retina, only the size of the image. Thus, viewing the diffuse reflection of the beam from a pulsed solid-state laser is usually hazardous. Safety goggles should always be worn by everyone present when such lasers are operated. Safety goggles for pulsed solid-state lasers are designed to protect against direct exposure to the laser beam. Optical density of these goggles is 14 to 16 at the laser wavelength. Nd:YAG and Nd:glass lasers are especially dangerous because their output wavelength is transmitted through the eye and focused onto the retina, but the output cannot be seen. Thus, there is no aversion response for protection as is the case with visible lasers.

Output pulses from larger pulsed solid-state lasers have sufficient energy to produce minor skin burns, and focused beams can cause small-area, deep burns. Thus, skin exposure should be avoided as well.

By far the most serious hazard associated with pulsed solid-state lasers has nothing to do with the laser beam. Electrical hazards present in the power supplies of pulsed solid-state lasers are the most dangerous of any laser system. Capacitors typically store several kilojoules at voltages of several kilovolts, producing discharge currents of any kiloamps. The power supply also contains other capacitors in the charging and trigger circuits, and larger laser systems are operated with input voltages of 220 V or higher. Although all systems are interlocked to prevent personnel from coming into contact with high voltages, service often requires that the interlock system be overridden, and some maintenance procedures must be performed near high-voltage terminals. No one has ever been seriously injured by a laser beam except for eye damage; however, several people have been electrocuted by the power supplies of pulsed solid-state lasers.

An associated hazard present with many pulsed solid-state laser systems is the respiratory hazard presented by material vaporized by the focused laser beam. Such material is always present when the laser is used in a material processing application. The vaporized material forms small particles, typically a micron in diameter, that are lodged in the lungs if inhaled. All vaporized material should be exhausted from the work area or trapped in fine filters.

Summary

Pulsed solid-state lasers are widely used for several applications. The most important of these are range finding and materials processing applications such as drilling and welding. The ruby laser was the first laser developed and was initially popular for both of these applications. Nd:YAG has become the most popular system in both applications in recent years because of its higher efficiency and higher pulse repetition rate. Nd:glass systems are employed in applications that require high-energy pulses that cannot be achieved with Nd:YAG or ruby. The greatest disadvantage of Nd:glass is the low pulse repetition rate resulting from poor thermal energy transfer in the glass.

All pulsed solid-state lasers consist of a cylindrical rod of the laser material that is optically pumped by a flashlamp. The optical cavity is of the plane-parallel type almost without exception. Cooling of the laser material is always a critical factor. Forced air or nitrogen cooling may be used in small systems, but most pulsed solid-state lasers use liquid cooling. Pulsed solid-state lasers present serious eye hazards. Appropriate safety goggles should always be worn when these lasers are in operation.

1. Explain why xenon flashlamps are usually used with neodymium laser systems, even through krypton flashlamps would provide a better spectral match. Also describe how the spectral match of xenon lamps to neodymium may be improved.

2. State the range of pulse duration achievable from pulsed solid-state lasers and how pulse duration is controlled.

3. Draw a curve showing the variation in output pulse energy from a pulsed solid-state laser as the transmission of the output coupler is varied. Explain how this curve is used in mirror selection.

4. Explain the popularity of helical flashlamps for small ruby and Nd:glass laser systems.

5. Explain the process of lamp simmering and the advantages it offers in high-pulse-rate lasers.

6. Discuss problems that are often encountered if coolant temperature of a water-cooled pulsed solid-state laser system is too low or too high.

7. Explain the origin of spiking in the output pulse of pulsed solid-state lasers.

8. Discuss eye hazards present with pulsed solid-state lasers.

9. Explain electrical hazards present in pulsed solid-state lasers.

10. Explain precautions necessary in the construction of rod holders for ruby rods that are not necessary for Nd:glass or ND:YAG lasers.

11. For each of the following laser applications compare the three solid-state laser types for suitability. State the best choice for each application and explain why it is chosen.

a. A laser machine took is to be used for drilling holes. Pulse repetition rate must be at least ten pulses per second with a pulse energy of 5 joules.

b. A pulsed solid-state laser is to be used for studies involving the irradiance of materials. The system is to be fired only once every few minutes but must deliver a pulse energy of between 500 and 100 joules.

c. A range finder employs a laser that must produce two pulses per second with a pulse energy of only 0.25 joules.

12. The full angle beam divergence of a laser is given by

q =

where: q = Divergence angle in radians.

l = Laser wavelength.

d = Diameter of output beam.

Use this equation to determine beam divergence of the following systems:

a. A ruby laser with a 1/4-inch rod diameter ( assume that rod diameter and beam diameter are the same).

b. An Nd:YAG laser with a 2-mm rod diameter.

c. An Nd:YAG glass laser with a 1-inch rod diameter.

13. Explain why plane-parallel optical cavities are used for most pulsed solid-state lasers.

Calorimeter for measuring pulse energy

Photodiode for measuring pulse duration

Oscilloscope

Oscilloscope camera

HeNe laser

Adjustable table for HeNe laser

Three output couplers for the pulsed solid-state laser with different transmissions

Safety goggles for pulsed solid-state laser

Laboratory procedures for this module include general instructions for aligning a pulsed solid-state laser with a HeNe laser. These instructions are designed to be applicable to virtually any pulsed solid-state laser. The second part of the laboratory is a procedure for measuring pulse energy and pulse shape from a pulsed solid-state laser as the input energy from the capacitor bank and the transmission of the output coupler are changed.

Students should have completed the laboratory section of Module 3-3, "Energy Transfer in Solid-State Lasers," before beginning this lab.

1. Make an aperture of white cardboard for the HeNe laser with a hole just large enough for the beam to exit. Attach this aperture to the laser for easy viewing of reflected beams.

2. Remove both mirrors from the pulsed solid-state laser. Wrap them in clean lens tissue and set them aside.

3. Place the HeNe laser on the adjustable table, at a distance of at least 2 m from the pulsed laser, and direct its beam through the center of the solid laser rod. This is the most critical step in the alignment process. It establishes the optical axis of the system and all other alignment is made relative to it. For large-diameter rods, centering the beam may be accomplished more easily if rod ends are fitted with apertures slightly larger than the HeNe laser beam. Correct alignment of plane-parallel rods results in a spot of light reflected to the HeNe output aperture from the rod ends. Presence of two spots indicates that the rod ends are not exactly parallel. Neither the HeNe laser nor the solid-state laser should be moved from its present position until the end of the of experiment.

4. Replace the solid-state laser mirror on the opposite end of the laser rod from the HeNe laser, and adjust its mount to reflect the HeNe beam back through the laser rod to the output aperture to the HeNe laser. Because laser mirrors have some wedge, there will be two reflected spots. For ruby lasers, the brighter spot is from the high-reflectance surface and should be placed on the HeNe aperture for proper alignment. In neodymium systems the two reflected spots often have about the same intensity because reflective coatings for 1.06 microns do not work well at 632.8 nm. If this is the case, there is no sure way to tell which spot is the proper one. A process of trial and error must be used. If the wrong spot is chosen the first time, the laser will not lase or will not produce an even output spot.

5. Replace the other mirror of the pulsed solid-state laser and align it to reflect the HeNe beam back into the HeNe output aperture. Again, two spots of light will be present, and the proper one must be chosen. When the solid-state laser is very near the proper alignment, multiple reflections will occur between its cavity mirrors and the HeNe output coupler. These reflections may be observed on the aperture of the HeNe laser. The final step in alignment is to superimpose all of these reflected beams centered on the HeNe output aperture. The pulsed solid-state laser should now be properly aligned.

6. Block the solid-state laser beam to protect the HeNe laser, and fire the pulsed solid-state laser following operating instructions in the laser manual. All personnel present must wear appropriate safety goggles whenever the laser is in operation. Verify laser operation and observe the burn pattern produced by the beam on exposed photographic paper or film. Alter alignment slightly and observe changes in the beam shape.

7. Set up the photodiode to monitor pulse shape, and connect the photodiode to the oscilloscope. Place the calorimeter to intercept the output beam of the solid-state laser for an energy measurement. Refer to Module 3-13, "Measurement of Laser Outputs," for descriptions of pulse measurement techniques. This beam monitoring system will vary from laser to laser and should conform to individual requirements specified by the lab instructor. Have your instructor check your setup before firing the laser.

8. Fire the laser system, and measure the energy of the output pulse. Display the pulse shape on the oscilloscope, and estimate pulse duration.

9. Attach the camera to the oscilloscope, and photograph the time history of a laser pulse. Determine approximate peak power of the pulse by dividing pulse energy by pulse duration.

10. Use the above procedures and equipment to perform the following experiment:

Determine the threshold for lasing for the system experimentally. Make a series of measurements ranging from threshold to the maximum pulse energy. In each case measure pulse energy and pulse duration and calculate the approximate peak power. Repeat this procedure with two other output couplers with different transmissions.

11. Upon completion the experiment return the original output coupler to the laser and align the laser for operation. Return all equipment to its original condition, and secure the laboratory.

LABORATORY REPORT

The laboratory report for this experiment is a report of variations in pulse energy, pulse duration, and peak power as the input energy to the flashlamp and the transmission of the output coupler are changed. The report should include a list of all equipment used and specifications of the pulsed solid-state laser system. Experimental procedures used should be outlined.

All data should be arranged in a logical order. Data for each shot should include energy stored in the capacitor bank, measured pulse energy, pulse duration, and calculated peak power. Photographs of pulses should be included if available. The data is to be used to prepare the following graphs:

1. Pulse energy versus pump energy for each output from threshold to maximum (three graphs).

2. Pulse duration versus pump energy for each mirror (three graphs).

3. Peak power versus pump energy for each mirror (three graphs).

4. Pulse energy versus output coupler transmission for maximum input energy.

5. Pulse duration versus output coupler transmission for maximum input energy.

6. Peak power versus output coupler transmissions for maximum input power.

Harry, John E. Industrial Laser and Their Applications. Maidenhead-Berkshire, England: McGraw-Hill Book Company (UK) Limited, 1974.