Simultaneous tunable two-wavelength ultraviolet dye laser

Volume 22, number 3

OPTICS COMMUNICATIONS

September 1977

SIMULTANEOUS TUNABLE TWO-WAVELENGTH ULTRAVIOLET DYE LASER H. INOMATA* and A.I. CARSWELL Department of Physics and CR.E.S.S., York University, Toronto, Canada Received 3 June 1977

A dye laser system allowing simultaneous tunable two-wavelength oscillation in the ultraviolet has been developed. An intra-cavity dual beam system employs one diffraction grating and frequency doubling at both wavelengths is achieved with bidirectional illumination of a single KDP crystal. The system characteristics will be discussed with respect to application in a differential absorption lidar operating around 300 nm for atmospheric SO2 detection.

Several methods for operating dye lasers at a number of wavelengths simultaneously in the visible region of the spectrum have been reported [ 1 - 8 ] . In this paper we describe a convenient system capable of simultaneously generating two tunable ultraviolet wavelengths. This system has been designed as a light source for a differential absorption lidar for measuring atmospheric SO 2. As a result we have investigated its properties in the 300 nm region. The system configuration however will also be useful for a variety of multiple wavelength experiments in both the ultraviolet and visible spectral regions with the appropriate adjustments to the parameters. The absorption spectrum of SO 2 shows a rather regular array of absorption maxima and minima around the 300 nm region [9,10]. The typical separation between adjacent peaks is o f the order of 2 nm and their widths at atmospheric pressures are greater than 0.5 nm. For differential absorption nleasurements in the atmosphere it is essential to operate with the two wavelengths as closely spaced as possible to minimize the spectral dependence o f the atmospheric scattering. Thus for a differential absorption lidar for SO 2 the best arrangement would be to have laser wavelengths corresponding to an adjacent maximum and minimum. Thus the requirement is for a two-wavelength laser with line widths of the order of a few tenths of a nm and a wavelength separation of the or* Permanent address: Radio Research Laboratories, Ministry of Posts and Telecommunications, Tokyo, Japan.

278

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der of 1 nm in this region. Some lidar measurements of SO 2 have been reported [I 1,12] but these have involved the sequential measurement of the absorption at the two wavelengths. The simultaneous oscillation technique described here is highly desirable in order to reduce the error introduced by the time variation of the atmosphere during a measurement interval in the sequential technique. The configuration of the dye laser cavity and frequency doubling system is shown schematically in fig. 1. Pulses from a pump laser (both a nitrogen and a frequency doubled ruby laser have been employed) are focussed by a lens (L1) into a dye cell (DC)in a transverse pumping configuration. A 5 × 10 . 3 M solution of rhodamine B in ethanol is used as the active medium. A beam splitter (BS) (a dielectrically coated mirror with 50% reflectivity at 600 nm) is utilized to

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OPTICS COMMUNICATIONS

split and direct two cavity beams to a grating (G) (1800 grooves/ram, blaze wavelength 500 nm, 50 mm diameter). One beam travels directly to the grating through the beam splitter and the other indirectly via a totally reflecting mirror (M1) as shown. In order to obtain narrower line widths of the two beams a X 10 telescope (T) is mounted between the dye cell and the beam splitter. The cavity is closed by the totally reflecting mirror (M2). The output coupler (OM) is a broad band dielectrically coated plane mirror with 70% reflectivity at 600 nm. A Glanair prism polarizer (P) is mounted inside the cavity so as to efficiently obtain linearly polarized radiation for subsequent frequency doubling. The grating is first oriented so that radiation of some particular (fundamental) wavelength, say F1, is reflected back in a Littrow arrangement. The tuning of the second desired wavelength (F2) is obtained by adjusting the angle of M1 so as to satisfy the Littrow arrangement for F 2 as shown. With this spatial separation of the beams and the easy adjustment via M1 it is possible to derive a wavelength pair with the difference in wavelengths varying from 0 to about 10 nm. The optical arrangement shown ensures that the two beams are collinear and have parallel polarizations. The output pulses from the laser cavity are focussed onto a frequency doubler (FD), (KDP angle tuned crystal at 45°Z, 60°Y, 15 cm cube) by the lens (L2) which has a focal length of 21 cm. The beam (containing F 1 and F 2 superimposed) impinges on FD at an angle 61 chosen to satisfy the phase matching condition between radiations at the fundamental frequency F 1 and its doubled frequency H 1. After traversing the crystal the beams are then reflected back by the broadband total reflecting mirror (M3) so as to irradiate FD in the opposite direction at an angle 0 2 which satisfies the required phase matching conditions for F 2 and H 2, ( F i and H i represent the wavelengths of a fundamental beam and its corresponding second harmonic beam, respectively). In this way the harmonics of both fundamental waves are generated efficiently with the single doubling crystal. This bidirectional method is an extremely convenient method for simultaneous two frequency doubling compared to other possible approaches [ 13 ]. The four output wavelengths at F1, F2, H I and H 2 are taken via the steerable mirror M4. For UV operation the two fundamental wavelengths F 1 and F 2 are

September 1977

blocked by a filter (Coming CS 7-54). The system has been operated with both plain and curved M3 mirrors and with some variation in the beam focussing geometry in the doubling crystal. The output beam characteristics are obtained with subsequent collimation optics. Once the system has been aligned to provide the single beam output at F 1 and H 1 the tuning and alignment for obtaining F 2 and H 2 by adjustments of mirrors M 1 and M3 is quite straightfor~ ard. Although it has not been attempted, it would be possible to mechanically interconnect mirrors M1 and M3 using techniques similar to those reported by Kuhl and Spitschan [10] to obtain single element tunability for both of these mirrors. Since the output beams are all accurately collinear in this system it is relatively simple to investigate their properties by using simple filter and spectrometer systems. Fig. 2 shows sample spectral profiles (with N 2 laser pumping at 337 iun) of the four output beams obtained with a 1/4 m Jarrell-Ash spectrometer having input and output slits of 25/lm. These curves were obtained for single beam operation, that is by blocking either F 1 or F 2 in front of the grating. The fundamental beams were observed in the first order of the grating and the UV beams were measured using the second order with appropriate blocking filters. Fig. 2 shows that the line width at the fundamental wavelengths (FWHM) is of the order of 0.6 nm. In this instance the fundamental wavelength separation was 2 nm and the two profiles are completely separated (i.e. the intensity drops effectively to zero at the midpoint). The line-

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Fig. 2. Sample spectral profiles of the two fundamental and two second harmonic beams in the single beam mode. 279

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OPTICS COMMUNICATIONS

widths o f the ultraviolet beams are of the order o f 0.2 nm. As shown in the figure the width of the H 1 curve is somewhat narrower than that o f H 2. This we feel is a result o f the geometric differences in the beam traverse path and could be corrected if necessary with further adjustments. However for our present requirements this difference is not important. The spectral linewidths have also been measured in both single and simultaneous operation modes using a Fabry-Perot etalon and sample patterns are shown in fig. 3. The etalon has a 50 ~m spacing and a finess of ~ 7 . The densitometer traces o f these photographs showed that the FWHM o f these beams was again of the order of 0.5 nm, Fig. 3 (a) shows the single beam operation at F 1 and 3 (b) shows the single beam at F 2. In fig. 3 (c) dual beam operation at F 1 and F 2 is illustrated. Fig. 4(a) and (b) shows the relative peak output powers at the centre of each of the fundamental and harmonic beams. In the diagram the values for both single beam and simultaneous dual beam operation are shown on the same scale. The outputs at F 2 and H 2 in the single beam mode are arbitrarily normalized to unit intensity. Each point in fig. 4 is obtained from a sequence o f five laser shots at a repetition rate o f about 1 Hz. The output intensity is quite constant, with the variability being attributable to the pulse to pulse variation in the N 2 pump laser output. In the example o f fig. 4 the o u t p u t intensity at F l i S about 15% below that at F 2, Again this could arise from geometric differences in the two beam paths and the gain properties o f the dye used. This can readily be rectified by insertion o f variable attenuators in either path in front o f the grating if desired. It was found that the intensity at b o t h F 1 and F 2 in the dual beam operating mode dropped to about 60% o f the intensity in the single beam mode. The outputs at the harmonics directly reflected the variation of the fundamental beam intensities and the reduction rates between the dual and single beam modes in the UV is very close to that expected on a square law intensity dependence of the fundamental beams. These results are typical o f those obtained at greater wavelength separations between F 1 and F 2. At smaller wavelength separations the interaction between the two fundamental beams in the dye medium became much more apparent and the coupling of the beams 280

September 1977

Fig. 3. Linewidth observation, using a Fabry-Perot etalon, of the two fundamental beams, F 1 and F 2. (a) and (b) in the single beam mode, (c) in the dual beam mode. increased. However since the ultraviolet beams are already separated by only 1 run, for an SO 2 detection system smaller separations are not required. In fact

Volume 22, number 3

OPTICS COMMUNICATIONS

1.0'

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Fig. 4. Relative peak output powers of the two fundamental and two second harmonic beams in the single beam operation mode and in the simultaneous dual beam mode. the centre wavelengths of the two ultraviolet beams described here were chosen to be located very close to the absorption minimum at 305.56 nm and the absorption maximum at 306.53 nm o f SO 2 which are reported by Wright et al. [14] to be the best choices for optimizing an atmospheric differential measurement system. Thus there is no requirement for smaller wavelength separations or narrower spectral widths for the ultraviolet beams. The output energy of the system has been measured at the fundamental wavelengths using a doubled ruby (347 mn) pumping pulse of 40 mJ. The 600 nm output from M4 was measured. In the dual beam mode b o t h F 1 and F 2 had energies o f about 0.4 mJ indicating an overall conversion from 347 nm to 600 nm of about 2%. This includes all extracavity losses as well as the doubling losses in FD. The ultraviolet output energy has not yet been measured but it is estimated that in the present system H I and H 2 are about 20/iJ in the dual mode. The system will be optimized for eventual lidar use when a 347 nm input of the order o f 100 mJ will be used. In addition to the points noted above the system has a number o f other very useful features. For example if it is necessary to have the polarizations o f the two fundamental beams perpendicular to one another

September 1977

the beam splitter BS could be replaced b y a polarizing beam splitter such as a Glan-air prism. In this case to achieve simultaneous doubling in the configuration of fig. 1 it would be necessary to insert a polarization rotator between FD and M3. It is also possible by realignment o f M4 to feed the output beam directly back to M3 so that the doubler now is inside a resonant cavity created by G and M4. Obviously in tiffs arrangement the appropriate curvatures and dichroic capabilities of the mirrors would be required. It is also possible to take energy out of the cavity via the mirror M2 if desired. In this case M2 would be partially transmitting. This output would be very convenient for providing auxiliary monitoring and if necessary controlling of the frequencies F 1 and F 2. The present configuration, as already mentioned, makes the relative intensity adjustment o f the two beams very simple by the insertion of suitable attenuators in front of the grating in either beam. In addition because of the spatial separation of the beams here it is very simple to run not only in the simultaneous but also in a sequential mode o f altering the output frequencies rapidly between F 1 and F 2 . This can be achieved by simply chopping the beams alternately in front o f the grating without any disturbance or adjustment to the optical and frequency stability of the cavity. The authors wish to acknowledge the assistance o f the National Research Council and the Atmospheric Environment Service o f Canada. One of the authors (H.I.) is grateful to the Science and Technology Agency o f Japan for the opportunity of studying abroad. The helpful assistance o f R. Bell, C. Dugan, S. Pal and G. Rose is gratefully acknowledged.

References [ 1] D.J. Taylor, S.E. Harris, S.T.K. Nieh and T.W. Hansch, Appl. Phys. Letters 19 (1971) 269. [2] E.F. Zalewski and R.A. Keller, Appl. Opt. 10 (1971) 2773. [3] H.S. Pilloff, Appl. Phys. Letters 21 (1972) 339. [4] Chung-Yung Wu and J.R. Lombardi, Opt. Commun. 7 (1973) 233. [5] A.A. Friesem, U. Ganiel and G. Nuemann, Appl. Phys. Letters 23 (1973) 249. [6] L.D. Hutcheson and R.S. Hughes, IEEE J. Quantum Electronics QE-10 (1974) 462. 281

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OPTICS COMMUNICATIONS

[7] A.J. Schmidt, Opt. Commun. 14 (1975) 294. [8] C. Kittrell and R.A. Bernheim, Opt. Commun. 19 (1976) 5. [9] P. Warneck, F.F. Marmo and J.O. Sullivan, J. Chem. Phys. 40 (1964) 1132. [10] J. Kuhl and H. Spitschan, Opt. Commun. 13 (1975) 6. [11] W.B. Grant and R.D. Hake Jr., J. Appl. Phys. 46 (1975) 3019.

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[12] J.M. Hoell Jr., W.R. Wade and R.T. Thompson Jr., Int. Conf. on Environmental sensing and assessment, Las Vegas, Nevada, (1975). [13] S. Saikan, Opt. Commun. 18 (1976)439. [14] M.L. Wright, E.K. Proctor, L.S. Gasiorek and E.M. Eistin, NASA CR-132724 (1975).