Instrumentation J. C. Wright and M. J. Wirth Department of Chemistry University of Wisconsin Madison, Wis. 53706
Principles of Lasers In the REPORT, we learned about the spectroscopic fundamentals that lead to applications the laser has had in analytical chemistry. In this article, the characteristics of the individual lasers will be studied in order to explain their relationship to the applications. A laser consists of a medium in which there is a greater population in an excited state than in some lower state and a reflecting structure that traps the light within it (/). A medium that has a population inversion will cause a net increase in the intensity of light that passes through it because the stimulated emission caused by relaxation of the higher excited state exceeds the absorption from the lower state. It acts as an amplifier for light at a frequency that corresponds to the energy difference between the two levels that have the population inversion. Any light that is emitted by the laser medium, be it simple fluorescence or stimulated emission, is trapped in an optical cavity so it is fed back through the amplifier for further amplification. Lasing develops as the spontaneous emission is amplified by stimulated emission. If the lasing medium has a very small population inversion, it will have a low gain, and the cavity must be made highly reflective and free from losses so that light can pass through the amplifying medium many times. If the laser has a high gain, the cavity can be much less reflective or it can have greater losses. One "loss" that is important to us is the light which is permitted to "leak" from the cavity in the form of laser output. The desirable amount of this output coupling depends on optimizing the amount of light extracted from the cavity for an experiment and the amount of light left to circulate in the cavity. Low-gain lasers require small amounts of output coupling for optimum output power whereas high-gain lasers can have large amounts of output coupling. On the other hand, the 0003-2700/80/A351-1087$01.00/0 © 1980 American Chemical Society
intensities inside the highly reflective cavity of a low-gain laser are much higher than the output power. Any time waves are trapped within a structure, characteristic mode structures are established. Everyone is familiar with the characteristic modes of
a stretched string or a drum membrane where the mode structures are dictated by the points or edges of attachment. Light trapped within a cavity has similar mode structures that are defined by the cavity. In Figure 1(a), we have sketched examples of
Figure 1 . C a v i t y M o d e s (a) Several of the infinite number of cavity modes (b) Only one of the modes oscillates in a single mode laser (c) If the modes are all locked together in phase at one point so they constructively interfere, a short pulse develops as shown in the bottom trace
A N A L Y T I C A L CHEMISTRY, V O L . 5 2 , NO. 9, AUGUST
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the infinite number of longitudinal modes that are allowed for the reflec tive cavity shown. Since the separa tion between the mirrors is usually a substantial macroscopic distance and since the wavelength of light trapped inside the cavity is very short, many modes will be present within a typical cavity. For example, 0.5 μ green light in a 0.5 m cavity corresponds to 106 wavelengths of light, and the adjacent modes will have 1 000 001 and 999 999 wavelengths. In order for the modes to be stable, the length and alignment of the cavity must be extraordinarily stable. Although an infinite number of the longitudinal modes shown in Figure 1 are allowed, the only ones that will lase are those that have frequencies within the bandwidth in which the particular laser medium amplifies. At this point it is instructive to create our version of the ideal laser. It would, of course, be microprocessor controlled. The microprocessor would have com plete control of the amplitude, phase, and temporal behavior of each indi vidual mode in the laser. Software can now be used to create any kind of light source we want, as shown in Figures l(b-d). In Figures l(b-c) the ampli tudes of all but one mode have been reduced to zero. This corresponds to a single mode laser, which is extremely monochromatic. The wavelength can be changed simply by choosing anoth er mode. The laser can be operated continuously (CW) or pulsed simply by controlling the temporal behavior of that mode. In Figure 1(c), many modes are si multaneously oscillating but their phases have all been adjusted so that they constructively reinforce at one point. This situation is called modelocking. A large number of modes are locked in phase at this point, causing a short pulse which circulates in the cavity. The more modes that are locked, the shorter the pulse becomes. The pulse width is limited by the un certainty principle which requires Δω Δτ > 1, where Δω expresses the fre quency spread of the active modes and Δτ is the pulse width. The pulse width can be reduced into the subpicosecond regime and has given birth to the en tire field of picosecond spectroscopy. A more common situation is for the laser modes to be random or chaotic. Noise is seen on the output beam as the individual modes beat against each other. This mode noise actually has analytical applications (2). The fluorescence generated by a chaotic laser would have the same noise as the excitation source if the fluorescence lifetime were short compared to the noise frequencies. The fluorescence cannot follow noise frequencies higher than the relaxation rate of a fluoresc ing state and consequently is atten
uated at very high frequencies. By ex amining the fluorescence intensity fluctuations as a function of frequen cy, the fluorescence lifetime can be de termined. Our ideal laser can be controlled for analytical applications by five primary specifications: • The range of output frequency over which it can operate; • The bandwidth of the output as determined by the number of active modes at any given time; • The temporal characteristics of the output; • The output power, either average or peak power; • The dimensions and character of the laser beam. The ideal laser is stable, noise free, re liable, cheap, lightweight, rugged, por table, and simple to operate with its push-button controls. Unfortunately, the characteristics of the ideal laser are not realized for any given real laser. In practice, one has to specify which ideal laser characteristics are most important for the experiment and then select the appropriate laser that best fits those characteristics. In the following section, the principles and characteristics of real lasers are discussed. Real Lasers There are three primary consider ations that determine a laser's proper ties: the gain medium, the pumping mechanism (i.e., the mechanism for producing an excited state population inversion), and the resonator design. There are three classes of gain media: gas, liquid, and solid. Gas lasers have only narrow wavelength regions where there is an appreciable optical gain since their spectroscopic transitions are sharp. Liquid lasers have broad re gions for optical gain corresponding roughly to their fluorescence. Solidstate lasers can have either-narrow or broad gain regions depending upon the nature of the fluorescence. There are two basic pumping mech anisms for lasers—optical and elec tron pumping. In optical pumping, ei ther a conventional incoherent black body source or a coherent laser can be used for exciting the laser medium to its excited state. Arc lamps and tung sten lamps are used in many continu ous lasers while flashlamps are used in many pulsed lasers. In electron pump ing, a discharge is created in the gain medium, which excites the population inversion. Both violent discharges like sparks and more gentle discharges like glow discharges are used. The pump ing mechanism determines in large part whether the laser is pulsed or continuous. The resonator provides the means by which an operator can control the laser. Control is achieved by adjusting
1088 A · ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
the losses experienced by the cavity— high losses are chosen for the modes that are not desired and low losses for the modes that are desired. If étalons, gratings, prisms, or other wavelength selective devices are in the cavity, one can provide low losses for a selected band of frequencies. The selection can even be made so stringent that only one mode has a low loss. Such single mode lasers have an ultra-narrow bandwidth and are capable of very high resolution. Q-switching is a technique that provides very high output powers from a pulsed laser. Cross polarizers are placed within the cavity to prevent any lasing. A Pockels cell is positioned in the cavity between the polarizers so that when an appropriate voltage is impressed, it will rotate the plane of polarization, allowing light to pass through the polarizers with low loss. In operation, the flashlamp, which is usually used in this technique, is turned on and the population inversion gradually grows. Lasing is prevented by the high cavity loss of the crossed polarizers. When the population inversion is at a maximum, the Pockels cell is pulsed so that lasing occurs. By this point, the excited population has become so great that a large burst of energy is emitted as the lasing rapidly depletes the population inversion. This technique is called Q-switching. A related technique is cavity dumping. The resonator cavity is made very highly reflective so the energy within the cavity becomes very large. At a desired time, an oscillating voltage is applied to a piezoelectric material which generates a traveling acoustic wave in a transparent material in the cavity. The acoustic wave pattern acts as a diffraction grating, which diffracts the light out of the cavity. A burst of intense light is emitted, corresponding to the light that was previously confined to the cavity. This technique prevents the situation where a partially transmitting mirror allows only a fraction of the light within the cavity to escape, but it does so at the expense of having a continuous laser. Cavity dumping and Q-switching are both methods of storing energy to increase the output power. In Q-switching, energy is stored in the form of excited states, while in cavity dumping the energy is stored in the form of intracavity power. Q-switching is applicable for laser materials having long-lived excited states, such as ruby and Nd:YAG crystals, while cavity dumping is applicable to materials with short excited state lifetimes, such as argon ions and dyes. Figure 1 shows how the phases of the cavity modes can be locked together to produce one large pulse in the cavity. Mode-locking is accom-
Argon Reservoir Prism
Plasma Tube Cooling Water
Figure 2. Argon ion laser Argon atoms are ionized in the water-jacketed plasma tube. The ends of the tube are sealed to Brewster windows, and a reservoir supplies argon gas. A prism is universally used as a wavelength selector. The positions of the states
pushed by modulating the gain (or the loss) in the cavity at the frequency of round trips for a photon. If the fre quency is matched to the cavity round trip time with a very high precision, each time an in-phase photon in any of the modes returns to the gain medi um, it arrives when the gain is high. Photons that are out of phase arrive at a time when the gain is small (remem ber the gain is modulated) and thus are not amplified. Consequently, all the cavity modes become locked by the requirement that these phases must be maximum at the one point in time and space where the gain is a maximum. In discussing lasers that have proved useful for analytical work, we will be concerned about their individ ual strengths and problems. We will be concerned specifically with their wavelength coverage, bandwidth, power, complexity and ease of usage, and reliability. In practice, the useful ness of the laser also depends upon the attitude and policies of the manu facturer. Potential laser buyers should be certain to establish the reputation of a company by speaking with its cus tomers. Gas Lasers
Λ diagram of the argon or krypton ion lasers is shown in Figure 2. The heart of these lasers is the plasma tube where a very high current discharge passes down the barrel of the tube through argon or krypton gas. The outside is water-jacketed to cool the tube. The discharge performs two functions—it ionizes the Ar or Kr gas and it populates the excited ion states which are involved in the lasing. The important lasing transitions are listed in Table I along with representative powers for the high power versions of these lasers. The very high current densities required to both ionize and
involved in the major argon ion laser transitions are shown on the graph to the right of the schematic
excite the inert gas dictate large power consumption, big power supplies, and substantial cooling requirements. A big 18-W argon ion laser will typically require 38 kW of electrical power and comparable cooling requirements. The high current densities also heat the in side bore of the plasma tube so that appreciable amounts of incoherent black body radiation are also emitted by the laser. If the measurement in volves low light levels such as fluores cence or Raman experiments, this background must be removed by fil ters or monochromators. The optical cavity of commercial argon lasers is kinematically isolated from the outside and is designed to maintain the alignment and cavity length against mechanical or tempera ture changes in the environment. The light intensity is stable to ±3% under normal operation and can be con trolled to ±0.5% with appropriate feedback stabilization. The drawing in Figure 2 shows the prism that is in serted between the end mirrors to force the laser to oscillate at specific wavelengths. The prism can be re moved from the cavity, and the mir rors alone allow the laser to oscillate simultaneously on a number of laser transitions. This method of operation provides the highest output powers and is useful for exciting dye lasers. Single lines can be selected with a prism, which can be easily inserted into the cavity. If one examines the spectral struc ture of a single output line, one finds that it consists of many lines evenly spaced in frequency by c/2L where c is the speed of light and L is the cavity length. Typically this spacing is 50-200 MHz and the lines appear over a range of ca. 5 GHz—the total gain profile of the Doppler broadened lines. These individual lines correspond to the different longitudinal modes of
the cavity. An individual line has a width less than 3 MHz with a frequen cy stability determined by the stabili ty of the cavity length. A small étalon put into the cavity can be adjusted to select a single longitudinal mode, thereby converting the laser to a single mode laser with a very narrow bandwidth.
Table I.
Lasing Transitions
Laser Wavelength (nm)
Ion Laser Required
Power Available (watts)
799.3 752.5 676.4 647.1 568.2 530.9 528.7 520.8
krypton krypton krypton krypton krypton krypton argon krypton
0.30 1.2 0.90 3.5 1.1 Ί.5 1.0 0.70
"ΑΙΑ *ΐ Ο l*i.O
501.7 496.5 488.0 482.5 476.5 476.2 472.7 468.0 465.8 457.9 454.5 415.4 413.1 406.7 363.8 356.4 351.1 350.7 334
argon argon argon argon krypton argon krypton argon krypton argon argon argon krypton krypton krypton argon krypton argon krypton argon
A N A L Y T I C A L CHEMISTRY, V O L . 5 2 , NO. 9, AUGUST
7 Κ ι -Ο
1.5 2.5 6.5 0.40 27.0 0.40 1.2 0.30 0.75 1.35 1.1 0.27 0.50 0.9 1.0 1.1 0.3
1980 ·
1089 A
Laser Transition End Mirror 3
B II Electron Excitation
N 2 In H.V.
N 2 Out
Thyratron
Trigger
Figure 3. The N 2 laser A high voltage supply charges capacitor C,. A trigger pulse causes the thyraand a spark leaps longitudinally across the entire length of the laser tube excittron to conduct and switch the voltage onto capacitor C 2 . As C 2 charges rapid- ing the N 2 gas as shown at the top of the figure. The output window at the front ly, a voltage is reached where the electrodes across the laser tube break down of the tube and the end mirror at the back form the optical cavity
Ion lasers have reached a high de gree of engineering sophistication and reliability. A major disadvantage is that the plasma tubes must be re placed occasionally and these cost about one-fourth of the initial cost of the laser itself. In analytical applica tions, argon and krypton lasers have become standard sources for conven tional Raman spectroscopy. They are used extensively for pumping CW dye lasers and mode-locked dye lasers. Argon ion lasers provide high powers in the ultraviolet, blue, and green spectral regions that are effective in exciting most dye lasers, while kryp ton ion lasers provide power in the ul traviolet through blue regions and in the red region. The He-Ne laser is perhaps the most ubiquitous of the lasers. It is a simple, low power, low cost, and highly reliable laser that is conveniently used in the laboratory. The main output wavelength is 632.8 mm. Typical pow ers are 0.5-5.0 mW of continuous power. The analytical applications are limited to optical alignment of other systems or to specific experiments that are compatible with the single wavelength low power. The He-Cd laser is one of the first commercial metal vapor lasers. It is a convenient source for CW lasing in the blue and ultraviolet regions of the spectrum. It can provide tens of milli watts at 441.6 mm and several milli watts at 325.0 mm. The UV output has been used as an excitation for mo lecular fluorescence, particularly in a fluorescence detector for liquid chro matography. The output power is large enough to excite excellent fluo rescence signal levels while the high beam quality permits it to be focused to diffraction-limited spot sizes. The fixed wavelengths, however, restrict its use to samples with compatible ab sorption bands. The 325.0 mm wave length is not short enough to excite
many compounds of interest but it is adequate for a large number. Nitrogen and excimer lasers are both pulsed sources. A diagram of a representative nitrogen laser is sketched in Figure 3 along with the N2 electronic levels that are involved in the lasing (3). Electrodes are mounted along the entire length of a long tube. A flow of N2 gas passes through the tube at ca. 40 torr. A high voltage sup ply charges capacitor Ci to a voltage of 20-30 kV and a thyratron then can be triggered to rapidly charge capacitors C2, the dumping capacitors. The la ser's electrodes are in parallel with C2 and will break down as C2 charges. The energy in the dumping capacitors at that point is transferred into the spark discharge. The N2 C 3 Π state is preferentially populated by electron impact because of a more favorable Franck-Condon factor, and a popula tion inversion results relative to the Β 3 Π. Light emitted at 337.1 nm along the discharge path will be amplified as it travels down the tube. A mirror is located at one end to add a second pass for the light while an uncoated window defines the other end of the cavity. The optical gain of this laser is so high that the highly reflective dou ble mirror cavity of the other gas la sers is undesirable. The lasing is selfterminating because the Β 3 Π level has a long lifetime and quickly builds up a population greater than the C 3 Π level. The laser pulse widths are only 5-10 ns. The key to the efficiency of N2 lasers is achieving a very fast "ringup" of the voltage across the laser tube so the C 2ΓΙ level population is established rapidly. The energy of each pulse is ca. 1-10 mj in 5-10 ns resulting in a peak power of 100 kW-1.5 MW. The pulse repetition rates typically extend to 50-100 Hz, giving average powers of 20-150 mW. The beam does not look at all like the familiar laser. It is more
1090 A · ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
rectangular (dimensions ca. 6 X 25 mm) and much more divergent than other CW gas lasers. The short resi dence times of photons in the cavity do not allow the normal mode struc tures needed for good beam quality to develop. N2 lasers are one of the stur diest and most reliable lasers because of their simplicity. They do not need the same care as the more sophisticated ion lasers. Nitrogen lasers are a very effective excitation source for tunable dye la sers and they are used widely in that application. They are also a conve nient source for fluorescence excita tion. The 337.1 nm output wavelength can excite many molecules effectively. Poor beam quality prevents one from focusing the laser down to diffractionlimited spot sizes but this generally isn't a problem for most applications since the resulting power densities would damage the samples anyway. The N2 laser, or any of the pulsed la sers for that matter, are possible sources of severe radiofrequency inter ference that can overwhelm any elec tronics or computer instrumentation that is also used in the experiment. If rf could be a problem in an experi ment, it is important to select a com mercial laser designed to contain the rf. Generally, rf elimination requires that the manufacturer put the entire power supply and discharge electron ics within the same enclosure as the laser tube, that he filter all lines in and out, and that he provide an rftight seal for the enclosure. With these provisions, the rf can be eliminated. Excimer lasers are quite similar to the nitrogen laser in construction (4). A gas mixture of helium, fluorine, and one of the rare gases—argon, krypton, or xenon—is flowed through the dis charge channel where the rare gas is electronically excited. In its excited state, it combines with F2 to form a XeF, KrF, or ArF excimer (a molecule
Mirror
Output 6940 Â
Pump
Flashlamp -I Ruby Rod
Flashlamp
Mirror
Pulsed High Voltage
Figure 4. Ruby laser The ruby rod can be placed at the focus of a double ellipsoidal cavity to pump efficiently with two flash lamps. The three-level ruby laser process is shown at the upper left of the schematic
stable only in the excited state). Since the excimer ground state is unstable, relaxation results in the rapid disso ciation of the molecule. Thus, there is a population inversion while the ex cimer exists and lasing can be achieved. The excimer laser produces high energy pulses in the UV—XeF lases at ca. 351 nm with ca. 50 mJ, XeCl at 308 nm with ca. 60 mJ, KrF at 248 nm with ca. 150 m J, and ArF at 193 nm with ca. 50 mJ. The output pulse widths are ca. 10-25 ns and the lasers typically can be operated at 20 Hz repetition rates. The beam quality of the excimer lasers is similar to the nitrogen laser. The gas handling system is an im portant consideration in operating ex cimer lasers. Closed system operation has not been possible, and the gas mixture must be changed periodically or continuously flowed. One can usu ally obtain 5 Χ 104 laser pulses before the output power has fallen by a factor of two. Provisions must be made for introducing, circulating, and venting F2 gas. The requirement for changing the rare gas can become expensive if
there isn't a way of recovering and pu rifying the gas (5). The excimer lasers are an excellent source of UV, provid ing higher energies and shorter wave lengths than other lasers. They are an effective excitation source for dye la sers. They can be expected to play an important role in photoionization or photodissociation experiments in the future. An important class of gas lasers that will not be discussed is IR gas lasers including CO2, CO, and many other molecules. The lasers provide a large number of discrete and usually very powerful wavelengths throughout the IR. They are operated in both pulsed and CW modes. Analytical applica tions of these lasers require methods that can produce a coincidence be tween the molecular absorption and the discrete laser line. Solid State Lasers The ruby laser was the first laser in vented and it remains an important one. A typical solid-state laser is shown in Figure 4. The ruby rod is lo cated at one focus of a single or double
elliptical cavity with linear flashlamps at the other focus or focuses. Energy stored in a capacitor bank is dumped into the flashlamps, and Cr 3+ ions in the ruby crystal are excited first into the intensely absorbing 4Fx and 4F2 bands from which they quickly relax to the 2 E excited state. A population inversion is established between the 2 E state and the ground state, causing stimulated emission at 693.4 nm. Op tical feedback is provided either by aligning the ruby crystal between two mirrors to form an optical cavity or by using the ends of the ruby rod itself to act as the mirrors. The ends can be ei ther covered with reflective coatings or a 90° roof can be cut on an end to retroreflect the light. A Q-switch is often used in the cavity to control the output by concentrating all of the en ergy into a single, intense pulse with a duration of ca. 25 ns. A single pulse can have 10 J or more of energy. The ruby rod is strongly heated during the excitation, and the cooling require ments normally restrict the pulse rep etition rates to 0.03-1 Hz, a rate which can try the patience of an experiment er. The very high peak powers can be easily frequency doubled to 346.7 nm with an external nonlinear crystal. A common method of operating the ruby laser is to use an oscillator-amplifier combination. The output from an os cillator like that in Figure 4 is passed through a second flashlamp pumped ruby rod housed in another elliptical cavity. The output power is amplified in a single pass through the second rod. This configuration keeps the intracavity powers lower in both the os cillator and amplifier so the combina tion is less sensitive to optical damage. The Nd:YAG (where YAG repre sents yttrium aluminum garnet) laser is very similar to the ruby laser in con struction (see Figure 5). These lasers have become very popular recently be cause of their very high output ener gies, repetition rates, and wavelength outputs. A Nd:YAG rod is placed within an elliptical cavity where flashlamps excite a broad range of Nd 3 + excited states. The energy is rapidly channeled to the 4F3/2 state where a population inversion builds up rela tive to the 4 In/2 state. It is much easi er to achieve a population inversion in Nd:YAG because it is measured rela tive to another excited state instead of to the ground state as in ruby lasers. The gain of the Nd: YAG laser is suffi ciently high that it is often operated with an unstable resonator configura tion that can extract energy from the sides of the rod as well as the center (6). One such mirror configuration is shown in Figure 5 where a convex and a concave mirror are used. This con figuration does not allow multiple passes of light through the cavity but it does produce feedback over the en-
A N A L Y T I C A L CHEMISTRY, V O L . 5 2 , NO. 9, AUGUST
1980 ·
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h
3/2
Flashlamp Excitation
Laser Transition M 5/2
13/2 11/2 9/2
Laser Rod
Back Mirror
Q Switch
Flashlamps
Laser Rod
Output Mirror
Flashlamps
Figure 5. Cavity configuration for a Nd:YAG laser The oscillator is on the left side of the figure and is defined by the output mirror and the back mirror, which are arranged in an unstable resonator configuration. The energy levels of the Nd 3+ are shown on the top of the figure. The flashlamps cause an excitation of a number of excited levels that relax to the 4 F 3 / 2 manifold from which the lasing occurs
tire volume of the Nd:YAG rod. The oscillator is often Q-switched to obtain very high, reproducible peak powers that can be efficiently doubled, tri pled, and quadrupled in frequency. An amplifier can boost the output powers so that ca. 700 mJ of energy are avail able at 1.06 μ. If the output is passed through a series of nonlinear crystals installed on commercial lasers, one ob tains 200 mJ at 532 nm, 100 mJ at 355 nm, and 50 mJ at 266 nm. The pulse widths and repetition rates are typi cally 10-20 ns and 10 Hz respectively. These characteristics are almost ideal for a number of applications. The multiplicity of fixed wavelengths with high energies makes the laser attrac tive for fluorescence or ionization exci tation in situations involving broad band absorbers. The 532 and 355 nm wavelengths are well situated for dye laser excitation, which is their most widespread application. Semiconductor diode lasers are also solid-state lasers but they are excited by current passed through a rectifying junction (7). Emission occurs in the junction region of the device because of electron-hole recombination. Com mercial diode lasers are based upon the Pb salts Pb!. x Cd x S, PbS^Se,,, Pbi- x Sn x Se, or Pbi_ x Sn x Te. They can provide CW powers up to ca. 500 μ\Υ at any frequency from 380-3500 c m - 1 simply by altering the doping levels of the appropriate Pb salt. Any given
diode laser can be tuned only over ca. 50-200 c m - 1 . The optical cavity of the diode laser is defined by the ends of the semiconductor, which are cut and polished. The entire crystal is only ca. 400 X 200 X 100 μ\ν and must be well heat-sinked to prevent excessive heat ing. Since the cavity is very short, the longitudinal modes are separated by ca. 1 c m - 1 , and several modes will lase simultaneously. The linewidth of any one mode is ca. 10~4 c m - 1 and is limited by the current and temperature con trol available. Diode lasers must oper ate at temperatures below 50 Κ and are usually cooled with closed-cycle He refrigerators. The diode lasers can be roughly tuned over their 50-200 c m - 1 range by changing the crystal temperature and then finely tuned by changing the current through the diode, again changing the tempera ture. Since several longitudinal modes are normally oscillating, a small monochromator is used to select a specific mode. A diode laser can only be tuned over ca. 0.5-2 cm""1 continuously be fore a new mode must be selected. The beam from a diode laser diverges sharply because it is emerging from a spatially small region and because the cavity reflectors, which are actually the polished sides of the crystal, do not provide a high quality beam pro file. The small wavelength coverage of a given diode laser and the difficulty of
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continuously tuning the wavelength over wide intervals limit the versatili ty of these lasers. Nevertheless, for a dedicated application where particu lar vibrational lines of gas phase mole cules must be monitored, diode lasers are very attractive and are being used for environmental analyses and pro cess monitoring or control. Scan rates of 104 cm _ 1 /s permit real-time display of spectra on oscilloscopes, a very sig nificant advantage over other possible methods. The F-center laser is one of the newest developments in laser technol ogy (8). The gain medium consists of an alkali halide crystal that has been treated to contain a high concentra tion of color centers. The color centers exhibit broadband luminescence in the near-infrared when they are excit ed by visible light. The F-center laser system typically consists of a krypton ion pump laser to populate the excited states and a cavity to provide optical feedback through the crystal. A cryo stat is built into the laser cavity to cool the crystal and stabilize the color centers. A grating is presently used for wavelength tunability. Linewidths of 0.05 cm""1 are achieved with the grat ing, and single mode oscillation with a linewidth of 1 MHz is achieved with an étalon and the grating. The F-center laser can be tuned from 2.2-2.9 μ if an argon laser is used as the excitation source or from 2.2-3.3 μ if a krypton or dye laser is used. Conversion effi ciency of excitation power to infrared power is usually several percent. A 1-W input power provides between 3 and 20 mW of output power. Dye Lasers
The most important lasers from an analytical viewpoint are the dye lasers. A dye solution will lase at wavelengths controlled by its fluorescence spec trum, typically a 30-100 nm region. Different dyes allow lasing from ca. 340 nm-1 μ. There are many different kinds of dye lasers with widely differ ing characteristics (.9). They differ from each other primarily in the method of optical pumping and wave length selection. There are three major classes—N2, excimer, or Nd: YAG pumped; flashlamp pumped; and CW dye lasers pumped by argon or krypton ion lasers. The N 2 , excimer, or Nd:YAG pumped dye lasers are the simplest, most reliable, and the most easily used (10, 11). A diagram of such a laser is shown in Figure 6. The exciting laser is focused along an axis perpendicular to the dye laser axis (transverse pumping) with a cylindrical lens so that a thin line of fluorescence ap pears across the front of the dye cell. Dye concentrations are chosen so the excitation light is totally absorbed within a short distance of entering the
Quartz Plate Pump Laser
„ Cylindrical Lenses
Grating
Prism Beam Expander
Dye Cell
Wedged Output Window
Dye Cell
Figure 6. Construction of a high-power tunable dye laser A quartz plate reflects a small portion of a powerful pump laser beam through a cylindrical lens onto a dye cell where a dye solution circulates. A set of four prisms expands the beam in one dimension onto the diffraction grating, which selects the laser wavelength. The output from this oscillator is sent through a second dye cell which is pumped by the main portion of the pump laser beam and amplifies the output to high power levels
solution. The excitation lasers are so intense during their pulse that an almost total population inversion can be obtained for the dye molecules within that small excited region. A diffraction grating diffracts light at a specific wavelength determined by the angle of the grating back through the excited region of the dye cell. A beam expander is used between the grating and the dye cell to decrease the bandwidth of the laser by magnifying the angles of diffraction leaving the grating and filling the grating. Either a set of prisms or a Galilean telescope (i.e., a telescope with one diverging lens and one converging lens) is used to expand the beam (12). The set of prisms has the advantage that it magnifies only in the plane of the grating's dispersion. The angular adjustments of the grating in the other plane are not magnified, and the laser output is not overly sensitive to grating alignment. As in any high-resolution instrument, one must make certain that the design of the grating mount and drive is rigid and rugged enough to ensure wavelength stability. An étalon can be placed between the beam expander and the grating if a very narrow bandwidth is desired. The bandwidth of a typical dye laser is 0.01 nm with just the grating and beam expander but reduces to ca. 0.001 nm with an étalon. The étalon reduces the power somewhat and makes continuous scanning of the wavelength more difficult because the étalon scanning must be synchronized with the grating motion. The other end of the cavity is an un-
coated glass or quartz wedge. A small reflection back into the cavity is all that is desirable for these very high gain lasers. There are differences in dye laser design that depend upon the pump laser used. Excimer and Nd:YAG pump lasers as well as some N2 lasers have so much power available that the oscillator in Figure 6 saturates (10). Under these conditions, a second dye cell is added as an amplifier, and the major portion of the pump beam is directed through this cell along with the output of the oscillator. With the Nd: YAG pump lasers, it is possible to obtain 35% conversion efficiencies with
the oscillator-amplifier combination. The pulse width of the pump laser must also be compatible with the length of the dye laser cavity. If the pump laser has a pulse width of several nanoseconds, light may not have time to go from the dye cell to the grating and back during the pump laser pulse. Typical pulse energies are 0.4 mJ, 5 mJ, and 50 mJ for a N2, excimer, and Nd:YAG pump laser respectively. Flashlamp pumped dye lasers have a tube filled with a flowing dye solution and optically coupled to a flashlamp with an elliptical cavity (9, 13). In Figure 7, a prism is in the optical cavity to tune the wavelength, but diffraction gratings and biréfringent filters are also commonly used. The output mirror of the cavity must have a reasonably high reflectivity because the gain of the flashlamp pumped lasers is lower. As a result, the mirror alignment also becomes a more important parameter than the N2, excimer, or Nd:YAG pumped lasers. The flashlamp pumped dye lasers have several strengths. They are the cheapest dye laser because they do not require a second laser to excite them. They have the largest energy/pulse—between 0.025 and 2 J—although their peak power is usually lower than the Nd: YAG pumped dye lasers because the pulse width is ca. 0.2-1 μδ. The repeti tion rate depends upon how rapidly the dye can be flowed and the heat dissipation. Two exchanges of dye vol ume must be accomplished between laser pulses, otherwise thermal distor tion of the beam occurs and the power falls. The rates are typically 1-20 Hz for designs where the dye flow is longi tudinal through the dye tube and up to 500 Hz for transverse flow geome tries. The average powers generated can be as high as 10 W for some of the standard models.
Biréfringent Filter
Flashlamp
Dye Cell
Dye Solution
Figure 7. Flashlamp pumped dye laser A flashlamp is used to pump a flowing dye solution over a long pathlength. The biréfringent filter is one of several commonly used tuning elements
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980 ·
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Biréfringent Filter
Jet S t r e a m
Pump
Figure 8. Continuous wave dye laser A free-flowing jet stream containing the dye solution is pumped by a CW laser. A three-mirror folded cavity with a biréfringent tuning element is the most common configuration
The CW dye laser is sketched in Figure 8. An argon or krypton ion laser is used as the excitation source depending upon whether one is interested in the blue to red spectral region or the deep red to infrared region. A mirror focuses the excitation beam onto the dye to a spot size of ca. 10 μ in order to achieve the very high in tensities required to reach threshold. The dye solution is flowed through a nozzle to form a high optical quality, free-flowing jet which eliminates the need for windows. The gain of the CW dye laser is low and requires a highly reflective cavity with low losses. The small excited region of the dye is at the focus of one end mirror while a second mirror collimates the beam and directs it to the output mirror. Di electric coatings are used on each of the three mirrors in the cavity to en sure optimum reflectivities. A given set of mirrors can typically be used over the wavelength ranges of several dyes before they must be changed. Wavelength tuning is performed by ei ther a biréfringent filter or an interference wedge. Typical bandwidths with either element are 0.03 nm. If narrow bandwidths are desired, étalons, which are capable of providing single mode oscillation with a 1 MHz linewidth, can be inserted into the cavity. The étalons can be scanned over a very limited wavelength range. The biréfringent filter or interference wedge can be mechanically scanned over the range of the dye although commercial manufacturers generally do not supply this capability. The CW dye laser is a very convenient laser to mode-lock by synchronous pumping (14-16). The argon or krypton ion is mode-locked by an acousto-optic modulator driven by a frequency synthesizer. A train of 100 ps pulses conies from the ion laser with a period that corresponds to the roundtrip time of a photon in the ion laser cavity. This train of pulses excites a dye laser that has been modified by 1094 A ·
moving the output coupling mirror so that the length of dye laser cavity matches the length of the ion laser cavity. With this situation, photons emitted from the dye on one excitation pulse will arrive back at the dye each time a new excitation pulse arrives and thus will be amplified. Photons that arrive at other times are out of phase and will not receive the same amplification. The pulse width of the dye pulses depends upon the bandwidth of the output through the uncertainty principle between energy and time. For the shortest tunable pulses, the bandwidth must be increased by decreasing the selectivity of the tuning element. Typically, 5 ps pulses of 3 nJ energies are attained at repetition rates of ca. 80 MHz giving 250 mW of average power. All of the dye lasers must contend with photo-induced dye disintegration. Flashlamp pumped dye lasers are particularly sensitive to this problem because much of the broadband excitation energy can cause more decomposition than excitation. The UV and blue dyes are most susceptible to decomposition, some degrading in less than 20 min of usage. The problem becomes increasingly severe as the pump laser energy increases. Other Methods of Generating Coherent Radiation
Although the lasers described provide continuously tunable operation over the near-UV, visible, and nearIR, it is necessary to have coherent light at other frequencies in the UV and IR. A number of nonlinear techniques are presently available for providing extension of the tuning range (17). The simplest of these is frequency doubling where the high intensity of light propagating through a noncentrosymmetric crystal causes distortions of the induced polarization at twice the input frequency. For the most efficient conversion, the power
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density in the crystal should be as large as possible without reaching the region where damage can result. High power Nd:YAG and ruby lasers can be doubled, tripled, and quadrupled in frequency with 30-50% conversion efficiencies simply by sending their output through a nonlinear crystal. Lower power lasers such as a flashlamp pumped dye laser can be frequency doubled by putting the crystals into the optical cavity itself where the circulating powers are much higher than the output power. Lasers with lower powers can be frequency doubled by focusing the beam into the crystal. The angle and temperature of the crystal are very important in frequency doubling because of the need to match the phases of the induced polarization and the doubled output. The optimum angle and/or temperature is wavelength dependent and needs to be changed as the wavelength is changed. If the wavelength is scanned, there must be appropriate provisions for scanning the doubling crystal parameters as well. Tunable infrared radiation can be generated by a related process in a parametric oscillator (18). A nonlinear polarization will generate the sum frequency (ω; + ω8) from input waves at ω; and ws. Similarly, in the reverse process a beam at ω ρ can generate two frequencies such that (ω, + a>s) = ω ρ . In practice, this is accomplished by fo cusing the output from a near-IR laser like a Nd:YAG laser to a diffractionlimited spot size in a LiNbOa crystal. The crystal is placed within an optical cavity so that the two wavelengths that are generated can build up to large values. The phase matching re quired will restrict this parametric process and only a particular set of frequencies, ω; and ωΒ, will be ampli fied. The wavelength can be tuned by changing the phase matching condi tions to favor a different set of ojj and cos. Despite the strangeness of this technique, it is a surprisingly efficient method with ca. a 10% conversion effi ciency. It can be used in nonlinear crystals that can be properly phasematched out to the limits where the crystals transmit. In LiNbOs, the most commonly used crystal, tunable in frared radiation can be obtained out to ca. 4.5 μ. The nonlinear methods require care and knowledge to make them perform reliably. Another alternative that has become very attractive with the suc cessful commercialization of the Nd: YAG pumped dye laser is to Raman shift the laser output by stimulated Raman scattering in H 2 or D2 gas at 10-20 atmospheres (19). The very high peak powers of these dye lasers can be gently focused into the gas (where one does not have to worry about optical damage), and a series of
new beams are generated at frequen cies t h a t differ by the vibrational fre quency of the gas (4155 c m - 1 for H2). Frequencies up to the eighth antiStokes and third Stokes frequencies can be generated in a H2 cell to pro vide continuously tunable o u t p u t from the vacuum UV to the mid-IR. T h e method is simple and straightforward, requiring only the very high peak pow ers and good beam quality available in ruby or Nd:YAG pumped dye lasers. A very exciting laser under develop ment now is the free electron laser (20). All of the lasers discussed thus far are restricted in their wavelength range by the characteristic resonance frequencies of bound electrons in ei ther atoms, ions or molecules. A free electron laser functions by firing elec trons through periodic magnetic fields t h a t force the electron to oscillate at a frequency determined by the periodic ity of the field and the electron veloci ty. Such a device could be tunable from the far IR to the vacuum UV if suitable periodic magnetic fields are available. T h e efficiency from such la sers is remarkable, reaching values in excess of 40%. Such lasers could have a remarkable impact on all areas of science, certainly including analytical chemistry.
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References (1) B. A. Lengyel, "Introduction to Laser Physics," John Wiley, New York, N.Y., 1966. (2) G. M. Hieftje, G. R. Haugen, and J. M. Ramsey, Appl. I'hys. Lett., 30, 463 (1977). (3) B. W. Woodward, V. J. Ehlers, and W. C. Lineberger, Rev. Sci. Instrum., 44, 882 (1973). (4) M. H. R. Hutchinson, Appl. Phys., 21, 95 (1980). (5) P. M. Johnson, N. Keller, and R. E. Turner, Appl. Phys. Lett., 32, 291 (1978). (6) R. L. Herbst, H. Komine, and R. L. Byer, Opt. Commun., 21, 5 (1977). (7) J. F. Butler and J. O. Sample, Cryogen ics, December 1977, ρ 661. (8) L. F. Mollenauer, Opt. Lett., 1, 164 (1977). (9) F. P. Schâfer, "Dye Lasers," Vol. 1, Springer-Verlag, Berlin, 1977. (10) J. E. Lawler, W. A. Fitzsimmons, and L. W. Anderson, Appl. Opt., 15,1983 (1976). (11) T. W. Hansen, Appl. Opt., 11, 895 (1972). (12) M. A. Novikov and A. D. Tertyshnik, Sou. J. Quant. Electron., 5, 848 (1975). (13) B. B. Snavely, Proc. IEEE, 57,1374 (1969). (14) J. M. Harris, R. W. Chrisman, and F. E. Lytle, Appl. Phys. Lett., 26,16 (1975). (15) C. K. Chan and S. O. Sari, Appl. Phys. Lett., 25, 403 (1974). (16) A. I. Ferguson, J. N. Eckstein, and T. W. Hansen, J. Appl. Phys., 49, 5389 (1978). (17) Y. R. Shen, Rev. Mod. Phys., 48, 1 (1976). (18) S. E. Harris, Proc. IEEE, 57, 2096 (1969). (19) V. Wilke and W. Schmidt, Appl. Phys., 18,177 (1979). (20) L. R. Elias, Phys. Rev. Lett., 42, 977 (1979).
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