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ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
Thermal Effects upon Spatial Mode Structure of a Linear Flashlamp-Pumped Dye Laser Sir: The ability of a double beam spectrometer to compensate for changes in pulse energy of a pulsed dye laser has been found to be influenced by the reproducibility of the spatial mode structure of the laser beam from one pulse to t h e next ( I ) . Reproducible cross section intensity patterns are important because the two beams of a double beam system usually do not respond with the same relative sensitivities to all corresponding cross-sectional regions of the two beams. This might be due, for instance, to detectors that have nonuniform responses across their surfaces, or to randomly located imperfections in the optical components through which the two beams pass. A pulse to pulse shift in the most intense region of the beam, for example, from the right hand side to the left hand side of the beam, could cause a significant change in the ratio of the signals produced b:i the two beams. even though t h e pulse energy did not, change. We have studied the influence of the temperature difference between the cooling water and dye solution upon the spatial mode structure and energy reproducibility of a flashlamp pumped dye laser beam, used in a double beam atomic absorption and fluorescence spectrometer ( I , 2 ) . The laser was a CMX-4 (Chromatix, Sunnyvale, Calif.) with a birefrigent filter for coarse wavelength selection and a FabryPerot etalon for fine wavelength selection. The dye tube and a linear flashlamp are located, respectively, a t the two foci of an ellipsoidal reflector. The flashlamp is cooled by dry nitrogen which passes through a tube coaxial with the flashlamp. Cooling water flows throughout the ellipsoidal reflector cavity and is therefore in direct contact with the dye tube. The laser cavity has a plane front mirror and a slightly concave rear mirror. The dye was 1.1 X l0-l M R640 (Exciton Chemical Co., Inc., Dayton, Ohio) in a solvent containing equal volumes of water and methanol. Because the cooling water in our laboratory was frequently above the maximum of 20 "C specified by the manufacturer, the cooling system of the laser was modified to include a refrigerated water bath. Consequently, the measurements discussed below were not made for the regular cooling system supplied with the laser. The temperature of the dye was controlled independently of the laser cooling water, and the temperatures of the dye and water were stable to within 0.1 "C for each set of measurements. T h e spatial mode structure of a horizont,al section of the laser beam was obtained by directing the 3-mm diameter laser beam through a 0.5-mm horizontal slit to a 500-channel, one-dimensional vidicon (Princeton Applied Research Corp., Princeton, N.J.). Beam attenuators were used to prevent saturation of the vidicon. Total laser energy and the response of each of the SO0 vidicon channels were transferred to a computer for 20 successive laser pulses and then stored on tape for later analysis. The laser pulse rate was 1 Hz. and was synchronized with the sweep of the vidicon. The reproducibility of the mode structure a t each dye temperature was observed by superimposing on an X-Y recorder the int,ensity vs. channel number plots for all 20 laser pulses. Examples for three different temperature differences are shown in Figure 1. Plots for each separate shot were also obtained. It was apparent from the superimposed plots that the mode structure within the horizontal sample of the beam was most reproducible when the dye temperature was from 0.5 "Ccooler than t h e ellipsoidal cavity water to 2.0 "C warmer. Reproducibilities within the most intense, and therefore most in0003-2700/7810350-1936$01 0010
C
Figure 1. Mode structure pattern (intensity vs. position) along a 0.5-mm slice of the flashlamp pumped laser beam. Typical pattern for a single shot (right)and 20 superimposed patterns from successive laser pulses (left). (a) Dye 3.3 O C cooler than water, (b) dye 0.3 O C cooler than water, and (c) dye 3.6 O C warmer than water
fluential, regions of the sampled section of the beam were studied. For this temperature range, the intensities within each of the most intense spatial regions deviated by only 10 to 20% from the mean value for the 20 shots in a data set. This was also the temperature range where the relative standard deviation of the energy of the laser pulses was the best, 4-50/0. Outside of the best temperature range, from a dye temperature 1.0 "C cooler to 4.9 "C cooler and from a dye temperature 3.0 "C warmer to 5.8 "C warmer than the cavity water, the mode structure varied dramatically from pulse to pulse. The intensities within narrow parts of the most intense regions typically varied by factors of 2 to 6 from one pulse to the next. The standard deviation of the total energy of the entire beam from pulse to pulse was worse in these temperature ranges, becoming as poor as 50% when the dye was 5.8 "C warmer than the water. These effects seem to be due to the influence of thermal focusing which is caused by a gradient in temperature from the outside of the dye to the center of the dye when the water flowing around the dye cell is a t a different temperature than the dye. T h e temperature gradient is accompanied by a gradient in refractive index, which causes the dye to act like a lens (3). When the dye is cool in the center, the thermal lens is convergent. Adding a convergent lens to the laser cavity degrades spatial selectivity and increases the divergence of the laser beam, producing a wider beam as shown in Figure 1a. Random optical fluctuations in the cavity may then allow the development of intense rays in random directions. causing dramatic changes in the intensity distribution pattern across the beam. When the dye is warmer in the center than around its circumference, the thermal lens is divergent. and the cavity increases its spatial selectivity by becoming niore like the c 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 13. NOVEMBER 1978
parallel plane mirror configuration. The parallel plane configuration boarders on being an unstable cavity resonator, and random optical fluctuations in the cavity can readily reduce the development of lasing in random areas of the beam by causing the rays to scatter or quickly walk out of the cavity. The more selective cavity produces a narrower beam, Figure IC,but the effects of random optical fluctuations become more critical causing larger shot-to-shot energy fluctuations. The dye temperature region corresponding to the generation of the most reproducible mode structure apparently produces a compromise cavity configuration that does not let random optical fluctuations generate too many widely divergent rays, when the thermal lens is too convergent on the one hand, or cause too many random cavity losses when the divergent thermal lens produces a less stable configuration on the other hand. In any case, we have found that careful control of the temperature difference between the dye and the water in the ellipsoidal cavity is necessary to obtain reproducible mode structure. Temperature difference control and knowledge about other experimental conditions that influence the noise level of a laser double beam spectrometer system (21, have
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helped to produce a reliable system for analytical measurements.
ACKNOWLEDGMENT The authors wish to express appreciation to Princeton Applied Research Corp. for their loan of the vidicon system.
LITERATURE CITED (1) J. W. Hosch and E. H. Piepmeier, Appl. Specfrosc., Sept./Oct. (1978). (2) J. W. Hosch and E. H. Piepmeier, Appl. Specfrosc., Sept./Oct. (1978). (3) J. M. Drake and R . I. Morse, Opt. Commun., 12, 132 (1974).
'
Present address, Los Alamos Scientific Laboratory, P.O. Box 1663, Los Alamos, N.M. 87545.
John R. F i t z P a t r i c k ' E d w a r d H. Piepmeier* Department of Chemistry Oregon State University Corvallis, Oregon 97331 RECEIVED for review May 4, 1978. Accepted August 14, 1978. This research program was supported by National Science Foundation Grant Number CHE73-05031.
Exchange of Comments on Interferometric Concentration Determination of Dextran after Gel Chromatography Sir: Hagel (1) has described a method to determine the dextran concentration in solution from the relation between its concentration and refractive index. He measures a signal from a Multiref 901 (2) that is presumably proportional to the retardation difference between a dextran solution cell and a reference cell. The signal reading is calibrated against the values of dextran concentration obtained by the anthrone method (3). The method of measuring the refractive index using the principle outlined in the theory presented by Hagel is capable of accurately measuring a small difference in retardations, i.e., less than one wavelength, between the sample and reference cells. Therefore, it is a very good method for determining a small refractive index such as that of a gas. Likewise, it is suitable for the detection of very small concentration differences between solutions. However this method is not capable of determining the order of the retardation differences. If one dextran solution produces a retardation difference of a fraction of one wavelength and another produces an equal retardation plus a number of whole wavelengths, the Multiref 901 reads the same signal. Therefore, the retardation difference between the sample and reference cells, and hence the refractive index of the sample, cannot be unambiguously determined unless the order is known. Hagel did not offer a solution to resolve this ambiguity. This note is written to point out that erratic results may be obtained unless this matter is resolved, and also to offer the solution. Fortunately there are ways to determine the order of retardation difference. The use of a compensator with white light can determine the order and may resolve this problem. The number of orders contained in the retardation difference can also be determined by measuring the retardation
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a t two wavelengths that are slightly different ( 4 ) . The magnitudes of the measured retardation difference can be used to calculate the number of orders. Suppose that the number of orders contained in a retardation difference is n, the additional fraction of retardation is AI when using light with wavelength AI, and for light with wavelength A*. The path difference of the light between the sample and reference cells is the same for the two wavelengths, provided that the wavelengths are close enough to neglect the effect of dispersion. Therefore,
(n +
lJX1
= (n
+ A,)&
(1)
From this it can be shown that
Thus, the retardation difference can be readily determined as ( n + 1 ) A .
LITERATURE CITED (1) Lars Hagel, Anal. Chem., 50, 569 (1978). (2) "Multiref 901, Instruction Manual", Optilab AB, Box 138, S162 12 Vallingby 1, Sweden. (3) J. R. Burt, Anal. Blochem., 9, 293 (1964). (4) F. El-Hosseiny, J . Opf.SOC.A m . , 65, 1279 (1975)
F. El-Hosseiny* R. D. Gilbert Fiber Sciences Department Weyerhaeuser Company 3400 - 13th Avenue S W Seattle, Washington 98134 RECEIVED for review May 19, 1978. Accepted August 7, 1978.
Q 1978 American Chemical Society
