Advisory Panel Jonathan W. Amy Glenn L. Booman Bowman Robert
L,
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 field large t i ning ranges. Possible analytical uses will result when instrumentation of dye laser and para metric osci Ilator systems become read i Iy ava i I able u
THE FIELDS
of physics and chemis-
I try the interaction of radiation with
matter plays a key role in the research of most workers. Fundamental to these studirs is the availability of a n appropriate source of electromagnetic radiation. I n many cases the development of sources l i : ~opened u p fruitful new areas of research or rejuvenated old ones. For example, following World K a r I1 developments in microwave sources mlide possible great adm m e s in microwave spectroscopy, and recently tlie laser has revived and extended the field of Raman spectroscopy. Sources in the microwave range and below are characterized by high spectral purity and high pon-er levels, a n d Iinvr the clesirnhle property of being tunahle. It is the tunability, for example, n-hirh makes possible microwave absorption spectroscopy. I n the optical and infrnred regions of tlie spectrum a combination of monocliromator and liroadband light source is used to obtain light of Txrious wavelengths. The primary drnwback of the monocliromatorlight source combination is that the goner available in Igiven frequeiicy range is low, except for a small number of resonance lines. The power per unit wavelength range becomes smaller as one mows to the infrared. Consequcntly, :IS one looks for higher resolution, for example in ahsorption spectroscopy, signnls become v-eaker, ultimately causing detection problems. The dc+-ability 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 laboratory apparatus may not be too far off. In this article several tecliniqiies for acliieving tunable laser action will he clcicrihetl, with emphasis on those
techniques which yield large tuning ranges. They include the stimulated Raman oscillator, the optical parametric oscillator, and the dye laser. Other techniques will be briefly menti oned . Stimulated Raman Oscillator (1)
The Rainan Effect', familiar to most chemists, involves the inelastic scattering of radiation by matter and is used as a n analytic tool to permit the study of vibrational modes of molecules. I n Raman scattering the difference in frequency between the incident and scattered radiation is charncteristic of the scattering medium. Under normal illuinination conditions the amount of scattered radiation is a small fraction, perhaps one part in lo6, of the incident benm nnd is emitted in all directions. However, for sufficiently intense illumination, such as is afforded by high pon-er pulsed lasers, tlie scattered radiation 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 characteristics of laser radiation, with the important fact that it is shifted in frequency from the incident laser. Since large numbers of mnterials-including gases, liquids, and solids-have been made to emit stimulated Raman radiation, the number of discrete shifted frequencies is large. Further, since radiation both upshifted and down-shifted in frequency by integral multiples of the Raman frequency is usually observed, a given Ramnn oscillator will produce a picket fence of frequencies given by the relation v=vl*nv,
(1)
where v 1 is the frequency of the laser, u,.
is the Raman frequency of the molecule, and n is an integer. Useful powers are obtained for values of 12 up to 3 or so. By using the output of one Raman oscillator to pump another oscillator using a different material, large numbers 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 possible. 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 wavelength). 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 provides high powers reproducibly at a large number of discrete wavelength. A recent advance reported only this year has produced a continuously tunable Raman oscillator with a tuning bandwidth of 200 em-l. Continuous tuning was made possible by using materials n-hich had transitions which n-ere both Ramnn and infrared active. Because the transitions are infrared active, they radiate the infrared, providing a powerful source in this frequency range. Observation of stimulated radiation estending from 40 to 200 microns has recently been reported. Optical Parametric Oscillator (2)
Another device for the generation of tunable coherent light is the optical VOL. 41, NO. 10,AUGUST 1969
75A
electric field. Materials in which optical parametric oscillation has been achieved are ADP '(ammonium dihydrogen phosphate), KDP (potassium dihydrogen phosphate), LiNbO,, (lithium niobate) and BazNaNh5OI5 (barium sodium niobate) nicknamed "bananas." A number of optical parametric oscillators have by now been constructed. Some of the salient features of oscillators reported to date are tunability from 0.7 p t o 2 p, and efficiencies of u p to 45%. Most of the oscillator work has been pulsed and peak powers of several hundred kilowatts have been reported. One of the features that makes the optical parametric oscillator particularly interesting is the fact that it is capable of continuous operation. Two continuously pumped tunable oscillators, one using LiNbO, and the other Ba,NaNb5015 were constructed last year, one operating in the visible and the other in the infrared. An example of one of the continuous optical parametric oscillators is shown in Figure 1. I n very recent work, an efficiency of 30% with an output power of 45 MW has been achieved using a "banana" crystal only 5 mm long. These results clearly point to the practicality of tunable oscillators. Another feature of the optical parametric oscillator is the fact that it is capable of operation 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 recently been made both in amplitude stability and spectral width and further
I INSTRUMENTATION
)arametric oscillator which also makes ise of optical nonlinearities. The basic :onfiguration of the optical parametric ,scillator consists of a suitable nonh e a r crystal surrounded by a pair of nirrors t o provide feedback for the os:illation. The source of energy for this xcillator is usually derived from a sin
