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Double-frequency dye lasers with a continuously variable power ratio
Volume 31, number I
OPTICS COMMUNICATIONS
October 1979
DOUBLE-FREQUENCYDYELASERS WITHACONTINUOUSLYVARIABLEPOWERRATIO S.CHANDRA and A. COMPAAN Department Manhattan,
of Physics, Kansas State University, Kansas 66506, USA
Received 13 July 1979
Two designs are presented for independently tunable double-frequency dye lasers with continuously variable power ratio. Each incorporates a translatable dye cell and two tuning elements (gratings or mirrors) intercepting an expanded beam inside the laser cavity. The power ratio can be set at any desired value, regardless of relative positions of the two frequencies within the dye tuning range. The designs are easily modified to generate more than two independently tunable frequencies.
Tunable pulsed dye lasers with narrow bandwidth have proved to be extremely versatile research instruments. Most of these dye lasers use a grating as the frequency selecting element. A narrow bandwidth from such lasers requires beam expansion inside the cavity in order to illuminate a large number of grating lines. Hansch [ 1] used an intracavity telescope to expand the beam. However, intracavity telescopes tend to be expensive, introduce losses from lens surfaces, make laser cavity alignment difficult and require relatively long cavities. Alternative schemes for beam expansion utilize a grazing angle of incidence on a prism [2] or a grating [3] to expand the beam in one dimension. These designs all achieve narrow band and tunable dye laser output at one frequency. However, there are many applications in physics and chemistry where two tunable frequencies are highly desirable. Such applications include coherent excitation of three level atoms, doubly resonant two photon absorption and coherent anti-Stokes and Stokes Raman scattering (CARS and CSRS). Double frequency operations of the Hansch-design dye laser have previously been reported [4]. In these designs there is generally a problem of cross-talk between frequencies and the two output frequencies are emitted as physically separated beams. Furthermore, the power ratio is not easily controllable. Recently Prior [5] has reported a double frequency dye laser of the
grating expander [3] design by using two mirrors to reflect two distinct diffraction orders. In this design the problem of a cross talk persists. Furthermore, the bandwidths at the two frequencies are necessarily different due to the dispersion difference in the two orders; and the power ratio is fixed as determined by the dye gain curve and the relative diffraction efficiency of the grating in the two orders. This letter describes two designs (see fig. 1) for a double-frequency grazing-angle dye laser with easily controlled and continuously variable power ratio. Both designs use a fully reflecting mirror Mu and a dye cuvette [6] with a stirrer mounted on a translation stage. Design (a) uses an isosceles right angle prism set at 89.3’ to expand the transmitted beam into a 3 cm long line. The expanded beam is then intercepted by two 632 lines/mm gratings [7] G, and G, mounted in the Littrow configuration in the second order. The gratings provided most of the dispersion and could be rotated separately for independent tuning of each frequency. The double-frequency output was obtained from the grazing reflection from the prism. In design (b), we obtained [8] both the beam expansion and dispersion from a 2400 lines/mm holographic grating [7] G, which was set at an 89.5” angle of incidence. The grating diffracted in only one order besides the zeroth order reflection which was used for the output. The first order diffracted beam was 3.5 73
OPTICS COMMUNICATIONS
Volume 3 1, number 1
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DYE CWETTE
(Oi
Fig. 1. Two designs for an independently tunable doublefrequency dye-laser with variable power ratio. (a) Prismexpander design and (b) gratingexpander design. Gratings, Go, G1, G2 are described in the text. All mirrors, MO, Ml, Mz are fully reflecting.
cm wide and was intercepted by two mirrors, M, and M,, Again the mirrors could be rotated separately for independent tuning of each frequency. Note that in this design the mirrors M, and M, could be replaced by gratings to obtain even narrower bandwidths as discussed for a single frequency laser in ref. [9]. Both the prism-expander and grating-expander designs were pumped with a 250 kW, 10 ns pulse-width nitrogen laser running at 15 Hz. The pump-laser beam was adsorbed in approximately 0.3 mm inside the cuvette...The cuvette waspositioned in such-away that the dye fluorescence fell equally on the two tuning elements (gratings G, and G, in design (a) and mirrors M, and M, in (b)). Two different dyes were tried with each dye-laser design: 5 X 10V3 molar rhodamine 6G in ethanol and a coumarin dye 7D4TMC, 1O-2 molar in p-dioxane. Both dye laser designs and both dyes yielded approximately the same total output power of i 5 kW in the double frequency beam. The spectral widths were measured with a Fabry-Perot and were found to be typically Q.4-0.6 cm -I for the prism-expander design and 0.2-0.4 74
October 1979
cm-l for the grating expander design at each of the two output frequencies. The smaller spectral width of the second design appears to be due to the higher dispersion of the holographic grating G,. The power ratio in the double frequency beam could be set at any desired value regardless of the relative positions of the frequencies within the dye tuning range by translating the cuvette perpendicular to the dye laser axis. The cell translation had the effect of feeding different proportions of dye fluorescence to the two tuning elements. There appears to be some spatial overlap_of the two beams within the dye laser cell since blocking one of the tuning elements leads to an increase in the intensity of the other frequency from 7.5 kW to 11 kW. However, the output power ratio at the two frequencies once set was highly stable and did not fluctuate from shot to shot. We attribute this absence of cross talk to the fact that the two beams probe rather distinct regions of the active dye volume (see fig. 1). Absence of cross talk remained even for small separation between the frequencies. With design (a) we tuned the two frequencies as close as 0.24 nm or 10 cm-’ using the coumarin dye with no deterioration in the power ratio stability. A similar result was observed with design (b) using rhodamine 6G when we tuned the frequencies as close as 0.1 nm or 3 cm-l. Closer frequency spacing is possible but we could not separate the two beams sufficiently to visually check for amplitude fluctuations. For all frequency separations the two output frequencies were not physically separated within the 3 milliradian beam divergence. The ability of our dye laser designs to control the power ratio in the double frequency beam is important for maximizing signal in many experimental situations. Thus for cascaded doubly resonant transitions~ one would Eke equaI power at the two frequencies. On the other hand for maximum coherent Raman scattering signals one wants a two to one ratio in the laser power [ 101. These dye laser designs are especially useful for applications requiring the two frequencies to be either copropagating or counterpropagating. The grating expander dye laser of fig. 1 (b) was used by us to generate coherent anti-Stokes Raman scattering (CARS) using three laser beams in a geometry which required two of the laser frequencies to be counter propagating [ 111. This was achieved by reflect. ing the double frequency beam back on itself, Exact
Volume 31, number 1
OPTICS COMMUNICATIONS
retroreflection was easily determined from the readily visible enhancement in the dye laser power due to the resulting feedback. For applications requiring the two laser frequencies to be incident from different directions, the frequencies may be separated physically by using an external dispersive element. Finally it should be noted that more than two independently tunable discrete frequencies may readily be obtained from the prism and grating expander dye lasers. By using five independently mounted mirrors in the grating expander design in place of the two shown in fig. 1 (b) we obtained up to five different frequencies in the one output beam. In conclusion two dye lasers have been demonstrated which each have independently tunable double frequency output with a stable but continuously variable power ratio. Simultaneous generation of more than two frequencies was also demonstrated.
References [ 11 T.W. Hansch, Appl. Opt. 11 (1972) 895. [2] S. Meyers, Optics. Comm. 4 (1971) 187; E. Stokes, F. Dunning, R. Stebbings, G. Walters and R. Rundel, Optics Comm. 5 (1972) 267;
October 1979
D. Hanna, P. Karkainen nad R. Wyatt, Opt. Quantum Electron 7 (1975) 115. [31 0. Shoshan, N. Danon and U. Oppenheim, J. Appl. Phys. 48 (1977) 4495; M.G. Littman and H.J. Metcalf, Appl. Opt. 17 (1978) 2224. 141 A.J. Schmidt, Optics Comm. 14 (1975) 294; H. Lotem and R.T. Lynch, Jr., Appl. Phys. Lett. 27 (1975) 344; H. Lotem, R.T. Lynch and N. Bloembergen, Phys. Rev. A 14 (1976) 1748; A. Compaan, E. Wiener-Avnear and S. Chandra, Phys. Rev. A 17 (1978) 1083. [51 Y. Prior, Rev. Sci. Instrum. 50 (1979) 259. [61 Dye cuvette was obtained from Molectron Corporation, 177 North Wolfe Road, Synnyvale, CA 94086. 171 Gratings were obtained from PTR Optics, 145 Newton Street, Waltham, MA 02154. [81 S. Chandra and A. Compaan, Bull. Phys. Sot. Amer. 24 (1979) 472. PI I. Shoshan and U. Oppenheim, Optics. Comm. 25 (1978) 375; M.G. Littman, Optics. Lett. 3 (1978) 138. [lo] W.M. Tolles, J.W. Nibler, J.R. McDonald and A.B. Harvey, Appl. Spectr. 31 (1977) 253. [ 111 S. Chandra and A. Compaan, Bull. Phys. Sot. Amer. 24 (1979) 66; A. Compaan and S. Chandra, Optics. Lett. 4 (1979) 170.
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OPTICS COMMUNICATIONS
October 1979
DOUBLE-FREQUENCYDYELASERS WITHACONTINUOUSLYVARIABLEPOWERRATIO S.CHANDRA and A. COMPAAN Department Manhattan,
of Physics, Kansas State University, Kansas 66506, USA
Received 13 July 1979
Two designs are presented for independently tunable double-frequency dye lasers with continuously variable power ratio. Each incorporates a translatable dye cell and two tuning elements (gratings or mirrors) intercepting an expanded beam inside the laser cavity. The power ratio can be set at any desired value, regardless of relative positions of the two frequencies within the dye tuning range. The designs are easily modified to generate more than two independently tunable frequencies.
Tunable pulsed dye lasers with narrow bandwidth have proved to be extremely versatile research instruments. Most of these dye lasers use a grating as the frequency selecting element. A narrow bandwidth from such lasers requires beam expansion inside the cavity in order to illuminate a large number of grating lines. Hansch [ 1] used an intracavity telescope to expand the beam. However, intracavity telescopes tend to be expensive, introduce losses from lens surfaces, make laser cavity alignment difficult and require relatively long cavities. Alternative schemes for beam expansion utilize a grazing angle of incidence on a prism [2] or a grating [3] to expand the beam in one dimension. These designs all achieve narrow band and tunable dye laser output at one frequency. However, there are many applications in physics and chemistry where two tunable frequencies are highly desirable. Such applications include coherent excitation of three level atoms, doubly resonant two photon absorption and coherent anti-Stokes and Stokes Raman scattering (CARS and CSRS). Double frequency operations of the Hansch-design dye laser have previously been reported [4]. In these designs there is generally a problem of cross-talk between frequencies and the two output frequencies are emitted as physically separated beams. Furthermore, the power ratio is not easily controllable. Recently Prior [5] has reported a double frequency dye laser of the
grating expander [3] design by using two mirrors to reflect two distinct diffraction orders. In this design the problem of a cross talk persists. Furthermore, the bandwidths at the two frequencies are necessarily different due to the dispersion difference in the two orders; and the power ratio is fixed as determined by the dye gain curve and the relative diffraction efficiency of the grating in the two orders. This letter describes two designs (see fig. 1) for a double-frequency grazing-angle dye laser with easily controlled and continuously variable power ratio. Both designs use a fully reflecting mirror Mu and a dye cuvette [6] with a stirrer mounted on a translation stage. Design (a) uses an isosceles right angle prism set at 89.3’ to expand the transmitted beam into a 3 cm long line. The expanded beam is then intercepted by two 632 lines/mm gratings [7] G, and G, mounted in the Littrow configuration in the second order. The gratings provided most of the dispersion and could be rotated separately for independent tuning of each frequency. The double-frequency output was obtained from the grazing reflection from the prism. In design (b), we obtained [8] both the beam expansion and dispersion from a 2400 lines/mm holographic grating [7] G, which was set at an 89.5” angle of incidence. The grating diffracted in only one order besides the zeroth order reflection which was used for the output. The first order diffracted beam was 3.5 73
OPTICS COMMUNICATIONS
Volume 3 1, number 1
.
DYE CWETTE
(Oi
Fig. 1. Two designs for an independently tunable doublefrequency dye-laser with variable power ratio. (a) Prismexpander design and (b) gratingexpander design. Gratings, Go, G1, G2 are described in the text. All mirrors, MO, Ml, Mz are fully reflecting.
cm wide and was intercepted by two mirrors, M, and M,, Again the mirrors could be rotated separately for independent tuning of each frequency. Note that in this design the mirrors M, and M, could be replaced by gratings to obtain even narrower bandwidths as discussed for a single frequency laser in ref. [9]. Both the prism-expander and grating-expander designs were pumped with a 250 kW, 10 ns pulse-width nitrogen laser running at 15 Hz. The pump-laser beam was adsorbed in approximately 0.3 mm inside the cuvette...The cuvette waspositioned in such-away that the dye fluorescence fell equally on the two tuning elements (gratings G, and G, in design (a) and mirrors M, and M, in (b)). Two different dyes were tried with each dye-laser design: 5 X 10V3 molar rhodamine 6G in ethanol and a coumarin dye 7D4TMC, 1O-2 molar in p-dioxane. Both dye laser designs and both dyes yielded approximately the same total output power of i 5 kW in the double frequency beam. The spectral widths were measured with a Fabry-Perot and were found to be typically Q.4-0.6 cm -I for the prism-expander design and 0.2-0.4 74
October 1979
cm-l for the grating expander design at each of the two output frequencies. The smaller spectral width of the second design appears to be due to the higher dispersion of the holographic grating G,. The power ratio in the double frequency beam could be set at any desired value regardless of the relative positions of the frequencies within the dye tuning range by translating the cuvette perpendicular to the dye laser axis. The cell translation had the effect of feeding different proportions of dye fluorescence to the two tuning elements. There appears to be some spatial overlap_of the two beams within the dye laser cell since blocking one of the tuning elements leads to an increase in the intensity of the other frequency from 7.5 kW to 11 kW. However, the output power ratio at the two frequencies once set was highly stable and did not fluctuate from shot to shot. We attribute this absence of cross talk to the fact that the two beams probe rather distinct regions of the active dye volume (see fig. 1). Absence of cross talk remained even for small separation between the frequencies. With design (a) we tuned the two frequencies as close as 0.24 nm or 10 cm-’ using the coumarin dye with no deterioration in the power ratio stability. A similar result was observed with design (b) using rhodamine 6G when we tuned the frequencies as close as 0.1 nm or 3 cm-l. Closer frequency spacing is possible but we could not separate the two beams sufficiently to visually check for amplitude fluctuations. For all frequency separations the two output frequencies were not physically separated within the 3 milliradian beam divergence. The ability of our dye laser designs to control the power ratio in the double frequency beam is important for maximizing signal in many experimental situations. Thus for cascaded doubly resonant transitions~ one would Eke equaI power at the two frequencies. On the other hand for maximum coherent Raman scattering signals one wants a two to one ratio in the laser power [ 101. These dye laser designs are especially useful for applications requiring the two frequencies to be either copropagating or counterpropagating. The grating expander dye laser of fig. 1 (b) was used by us to generate coherent anti-Stokes Raman scattering (CARS) using three laser beams in a geometry which required two of the laser frequencies to be counter propagating [ 111. This was achieved by reflect. ing the double frequency beam back on itself, Exact
Volume 31, number 1
OPTICS COMMUNICATIONS
retroreflection was easily determined from the readily visible enhancement in the dye laser power due to the resulting feedback. For applications requiring the two laser frequencies to be incident from different directions, the frequencies may be separated physically by using an external dispersive element. Finally it should be noted that more than two independently tunable discrete frequencies may readily be obtained from the prism and grating expander dye lasers. By using five independently mounted mirrors in the grating expander design in place of the two shown in fig. 1 (b) we obtained up to five different frequencies in the one output beam. In conclusion two dye lasers have been demonstrated which each have independently tunable double frequency output with a stable but continuously variable power ratio. Simultaneous generation of more than two frequencies was also demonstrated.
References [ 11 T.W. Hansch, Appl. Opt. 11 (1972) 895. [2] S. Meyers, Optics. Comm. 4 (1971) 187; E. Stokes, F. Dunning, R. Stebbings, G. Walters and R. Rundel, Optics Comm. 5 (1972) 267;
October 1979
D. Hanna, P. Karkainen nad R. Wyatt, Opt. Quantum Electron 7 (1975) 115. [31 0. Shoshan, N. Danon and U. Oppenheim, J. Appl. Phys. 48 (1977) 4495; M.G. Littman and H.J. Metcalf, Appl. Opt. 17 (1978) 2224. 141 A.J. Schmidt, Optics Comm. 14 (1975) 294; H. Lotem and R.T. Lynch, Jr., Appl. Phys. Lett. 27 (1975) 344; H. Lotem, R.T. Lynch and N. Bloembergen, Phys. Rev. A 14 (1976) 1748; A. Compaan, E. Wiener-Avnear and S. Chandra, Phys. Rev. A 17 (1978) 1083. [51 Y. Prior, Rev. Sci. Instrum. 50 (1979) 259. [61 Dye cuvette was obtained from Molectron Corporation, 177 North Wolfe Road, Synnyvale, CA 94086. 171 Gratings were obtained from PTR Optics, 145 Newton Street, Waltham, MA 02154. [81 S. Chandra and A. Compaan, Bull. Phys. Sot. Amer. 24 (1979) 472. PI I. Shoshan and U. Oppenheim, Optics. Comm. 25 (1978) 375; M.G. Littman, Optics. Lett. 3 (1978) 138. [lo] W.M. Tolles, J.W. Nibler, J.R. McDonald and A.B. Harvey, Appl. Spectr. 31 (1977) 253. [ 111 S. Chandra and A. Compaan, Bull. Phys. Sot. Amer. 24 (1979) 66; A. Compaan and S. Chandra, Optics. Lett. 4 (1979) 170.
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