Advisory Panel Jonathan W. Amy Glenn L. Booman Robert L. Bowman
INSTRUMENTATION Jack W. Frazer Howard V. Malmstadt William F. Ulrich
Tunable Lasers by R. G. Smith Bell Telephone Laboratories, Incorporated, Murray Hill, N. J .
Several techniques achieve tunable laser action, some of which yield large tuning ranges. Possible analytical uses will result when instrumentation of dye laser and parametric oscillator systems become readily available Τ Ν THE FIELDS of physics and chemis-
- 1 try the interaction of radiation with matter plays a key role in the research of most workers. Fundamental to these studies is the availability of an appropriate source of electromagnetic radiation. In many cases the de\relopment of sources lias opened up fruitful new areas of research or rejuvenated old ones. For example, following World War II developments in microwa\-e sources made possible great ad vances in microwave spectroscopy, and recently the laser has revived and ex tended the field of Ε aman spectrosco py. Sources in the microwave range and below are characterized by high spectral purity and high power levels, and have the desirable property of be ing tunable. It is the tunability, for ex ample, which makes possible microwave absorption spectroscopy. In the opti cal and infrared regions of the spectrum a combination of monochromator and broadband light source is used to obtain light of various wavelengths. The pri mary drawback of the monochromatorlight source combination is that the power available in a given frequency range is low, except for a small number of resonance lines. The power per unit wavelength range becomes smaller as one moves to the infrared. Conse quently, as one looks for higher resolu tion, for example in absorption spec troscopy, signals become weaker, ulti mately causing detection problems. The desirability of high power tunable optical and infrared sources is clear. The methods for achieving such tunable sources are at last in hand, and recent developments in tunable lasers suggest that their evolution into available labo ratory apparatus may not be too far off. In this article several techniques for achieving tunable laser action will be described, with emphasis on those
techniques which yield large tuning ranges. They include the stimulated Raman oscillator, the optical para metric oscillator, and the dye laser. Other techniques will be briefly men tioned. Stimulated Raman Oscillator ( I ) The Raman Effect, familiar to most chemists, involves the inelastic scatter ing of radiation by matter and is used as an analytic tool to permit the study of vibrational modes of molecules. In Raman scattering the difference in fre quency between the incident and scat tered radiation is characteristic of the scattering medium. Under normal illu mination conditions the amount of scat tered radiation is a small fraction, per haps one part in 10°, of the incident beam and is emitted in all directions. However, for sufficiently intense illumi nation, such as is afforded by high power pulsed lasers, the scattered radia tion becomes stimulated rather than spontaneous; it reaches power levels comparable to the incident pumping beam and it is emitted as a collimated beam. As such, it has the basic charac teristics of laser radiation, with the im portant fact that it is shifted in fre quency from the incident laser. Since large numbers of materials—including gases, liquids, and solids—have been made to emit stimulated Raman radia tion, the number of discrete shifted frequencies is large. Further, since ra diation both up-shifted and down-shift ed in frequency by integral multiples of the Raman frequency is usually ob served, a given Raman oscillator will produce a picket fence of frequencies given by the relation ν = vi ± η
Vr
(1)
where vi is the frequency of the laser, Vr
is the Raman frequency of the mole cule, and η is an integer. Useful powers are obtained for values of η up to 3 or so. By using the output of one Raman oscillator to pump another oscillator using a different material, large num bers of frequencies can be obtained. By employing various combinations of materials, as well as by using the pulsed ruby or neodymium lasers and their second harmonics, an extremely large number of discrete frequencies are pos sible. The primary drawback of this technique for achieving tunable laser action is that continuous tunability is not possible (one must in general change materials to obtain a new wave length). Further, some regions of the spectrum are difficult to cover with other than a large number of iterations, and operation has been achieved to date only on a pulsed basis. Its advantages are that a Raman oscillator is easy to build, requiring only a pulsed laser, and in the case of Raman active liquids, a simple absorption cell and a few cubic centimeters of chemicals, and it pro vides high powers reproducibly at a large number of discrete wavelengths. A recent advance reported only this year has produced a continuously tuna ble Raman oscillator with a tuning bandwidth of 200 cm" 1 . Continuous tuning was made possible by using ma terials which had transitions which were both Raman and infrared active. Because the transitions are infrared ac tive, they radiate the infrared, provid ing a powerful source in this frequency range. Observation of stimulated ra diation extending from 40 to 200 mi crons has recently been reported. Optical Parametric Oscillator (2) Another device for the generation of tunable coherent light is the optical VOL. 4 1 , NO. 10, AUGUST 1969 · 75 A
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INSTRUMENTATION parametric oscillator which also makes use of optical nonlinearities. The basic configuration of the optical parametric oscillator consists of a suitable non linear crystal surrounded by a p a i r of mirrors to provide feedback for the os cillation. The source of energy for this oscillator is usually derived from a sin gle frequency laser, called the p u m p , and the oscillation consists of two elec tromagnetic waves called the signal and idler. T h e frequencies of the p u m p , signal, and idler are related b y Vp — VS + Vi
n
i>vr> — nsvs +
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(2)
T h e nonlinear crystal in which the in teraction between the three waves takes place must have several properties: it must have a second-order optical non linear coefficient relating the polariza tion to the electric field b y a t e r m Ρ = dE2 ; it must be transmitting a t the three frequencies involved, and it must possess sufficient birefringence to allow synchronous propagation of the three waves. The latter condition is expressed mathematically as niVi
(3)
where the n's are the indexes of refrac tion at the three frequencies. Birefrin gence is required of the crystal to over come the effects of dispersion, limiting the number of suitable nonlinear m a t e rials. Tunability of the optical p a r a metric oscillator is accomplished by varying the indexes of refraction of the crystal in a controlled manner, altering the propagation constants and ulti mately the frequencies vs and vi which satisfy Equations 2 and 3. Tuning has been achieved b y varying the angle of propagation through the crystal, its temperature, or b y applying an external
electric field. Materials in which op tical parametric oscillation has been achieved are A D P (ammonium dihydrogen p h o s p h a t e ) , K D P (potassium dihydrogen phosphate), L i N b 0 8 , (lith ium niobate) and B a 2 N a N b 5 0 1 5 (bar ium sodium niobate) nicknamed "ba nanas." A number of optical parametric oscil lators have b y now been constructed. Some of the salient features of oscilla tors reported to date are tunability from 0.7 μ to 2 μ, and efficiencies of u p to 4 5 % . Most of the oscillator work has been pulsed and peak powers of several hundred kilowatts have been reported. One of the features t h a t makes the optical parametric oscillator particularly interesting is the fact t h a t it is capable of continuous operation. Two continuously pumped tunable os cillators, one using L i N b 0 3 and the other Ba2NaNb 5 O 1 0 were constructed last year, one operating in the visible and the other in the infrared. An ex ample of one of the continuous optical parametric oscillators is shown in Figure 1. In very recent work, an ef ficiency of 30% with an o u t p u t power of 45 M W has been achieved using a " b a n a n a " crystal only 5 m m long. These results clearly point to the p r a c ticality of tunable oscillators. Another feature of the optical parametric oscil lator is the fact t h a t it is capable of op eration in the infrared out as far as the nonlinear material used is transmitting. Although great strides have been made in the development of optical parametric oscillators during the past year all the problems are not solved. Particularly troublesome with both pulsed and continuous oscillators is the lack of wavelength reproducibility, in some cases excessively broad emission spectra, and amplitude instabilities. Considerable improvements have re cently been made both in amplitude stability and spectral width and further
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76 A ·
ANALYTICAL CHEMISTRY
improvements appear feasible with improved designs. I t is not unreasonable to expect that progress in optical parametric oscillators will parallel that of the laser in the past several years where reasonably-priced reliable devices have resulted.
Worth
looking
into
Dye Lasers
In contrast to the Raman oscillator and the optical parametric oscillator which use nonlinear optical effects, the dye laser makes use of the basic laser technique of inverted population to achieve gain. The active species of the dye laser is an organic dye in solution. It is pumped optically, either with the output of another laser or by a flashlamp. The laser configuration consists of the optically-pumped dye cell and a pair of reflectors to provide feedback for the oscillation. Its simplicity is one of the main features of the dye laser. The wavelength range covered by dye lasers is large, extending from 4000 Â to 1 micron. This broad range is covered because a large number of dyes can be made to exhibit laser action, with different dyes being used to cover different regions of the spectrum. Familiar compounds such as Rhodamine 6G and Fluorescein are examples of dyes which have been made to lase. The output spectrum of a dye laser is usually broad, varying from the order of 50 to several hundred angstroms. The central wavelength can be shifted by varying such parameters as the solvent, the concentration, the cavity length, and cavity losses. The most successful method of tuning that has been found to date employs a diffraction grating as one of the reflectors. When used as a Littrow reflector (reflected wave directed back toward incident wave) the output wavelength can be adjusted by simply rotating the grating. With different dyes and a grating, most of the visible can be covered. Further narrowing of the spectrum to the order of 10^2 A bandwidth can be done through the use of other frequency selective elements within the cavity. The features of narrow linewidth, ease of tunability, simplicity, and high efficiency (50% has been reported for laser pumping) make the dye laser an attractive choice as a tunable source. Although to date dye lasers have been made to lase only on a pulsed basis, it is the opinion of many workers in the field that continuous operation can be achieved by using highintensity arc lamps as the pumping source. At present the major limitation of the dye laser is that it does not appear suitable for operation in the infrared due to the lack of dyes which have efficient fluorescence bands in that region. Three principal types of tunable laser have been described. The simplest, be-
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VOL. 4 1 , NO. 10, AUGUST 1969
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INSTRUMENTATION cause they make use of readily available organic liquids, are the stimulated Raman oscillator and the dye laser. Provided one has a high power pulsed laser (and perhaps the ability to generate the second harmonic of this laser) either of these two types of laser is simple to construct—the Raman oscillator giving a large number of fixed frequencies and the dye laser a more continuous coverage. Continuous operation of neither has been demonstrated. The optical parametric oscillator is the most difficult to construct, primarily because it requires crystals of high optical quality which are not yet readily available. The characteristics of the optical parametric oscillator which make it interesting are continuous tunability, operation in the infrared, and
operation on a cw basis. We have mentioned only three methods of achieving tunable laser action; a number of others exist (4). They include the direct tuning of lasers by the application of electric or magnetic fields, by variation of temperature, or by the application of pressure. An interesting example of the last technique was the demonstration of tunable laser action from 7.5 to 25 microns by the application of hydrostatic pressure up to 14 kbar to a lead selenide diode laser. By using any of the techniques described to generate new wavelengths it is possible to further extend the spectrum covered by mixing various combinations of frequencies in suitable nonlinear media to obtain sum and difference frequencies. As we have seen, a number of techniques exist to achieve tunable laser action. At present, product oriented companies are working on dye lasers and optical parametric oscillators and, in fact, one company is ad-
vertising a pulsed dye laser for sale. In the next year or two many competitive systems should be available, and it is fair to assume that a large number of interesting experiments will result.
References
(1) A good summary of the Stimulated Raman Effect is given by R. W. Terhune and P. D. Maker in "Lasers," Vol. II, ed Albert K. Levine, Marcel Dckker, Inc., New York, 1968. (2) A review of optical parametric oscillator theory is given by S. A. Akhmanov and R. V. Khokhlov in Soviet Physics Uspekhi, 9 (2), 210-222, September-October 1966. (3) For details of dye lasers see "Organic Dye Lasers" by M. R. Kagan, G. I. Farmer, and B. G. Huth, Laser Focus, pp 26-33, Sept., 1968. and "Organic Lasers" by P. P. Sorokin, Scientific American 220, pp 30-40, February, 1969. (4) A more complete paper by the author including a detailed bibliography will appear in the Annals of the New York Academy oj Sciences.
COMMENTARY by Ralph H. Müller
V S T E FIND Dr. Smith's discussion of
' ' Tunable Lasers most provocative, not solely as a promise for new, versatile and useful light sources but for the multitude of electro-optical phenomena which will undoubtedly be investigated in the course of these studies. In the comparatively short time that the laser has been used, it has found important uses in communications radar, holography, surges, time standards, metal cutting and welding, alignment instrumentation, high temperature studies, and in ignition systems. In the forty-one years since its discovery, the Raman effect has been a source of information on molecular structure supplementary and complementary to infrared spectroscopy, but until the advent of laser excitation of Raman spectra, it was rarely used by the analytical chemist, largely because it was too insensitive. Now that excitation by a high power pulsed laser can cause the scattered radiation to become stimulated rather than spontaneous, we have the high power Raman oscillator. This is an accomplishment that transcends mere improvement in detection 78 A ·
ANALYTICAL CHEMISTRY
sensitivity—it presents innumerable possibilities. The number of systems which can be used and the multiplicity of frequencies which can be generated would seem to be limited only by the number of substances which the chemist can find on his reagent shelf. It is of major importance that the radiation so produced is coherent and collimated. Dye lasers are likely to be of greatest interest to chemists even though, as the author points out, they are not particularly useful in the infrared. Fluorescence spectrophotometry is a highly developed field and elaborate instruments are available for the accurate delineation of both the absorption (excitation) and fluorescence spectrum. Sensitivities are of the order of parts per million or parts per billion and hundreds of substances have been precisely characterized, many of them of biochemical and medical importance. Is it not reasonable to assume that many of these substances can be excited to the point where laser action ensues? If high levels of output can be attained, it would seem that detection and identification could be achieved by relatively simple
optical methods of abridged spectrophotometry. The physicist and engineer will continue to study and improve tunable lasers and, at each stage of development, the chemist will benefit from the results, but he may do well by considering the implications of Raman and dye laser action. He should not merely regard them solely as high intensity, tunable light sources to be employed in conventional spectrophotometric techniques. In these thoughts, we may do well to keep the meaning of the acronym LASER, constantly in mind— "light amplifications b}' stimulated emission of radiation." To the extent that an array of molecules can, by this optical feedback and ultimate oscillation, become a relatively powerful little broadcasting station and of stable frequency, it becomes relatively simple to establish its identity. To switch for a moment to the region of much lower frequencies—i.e., in the microwave region—the ammonia maser (microwave amplification by stimulated emission of radiation) can be excited
