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Simultaneous two-frequency oscillation in a dye laser system
Volume
7, number
3
OPTICS COMMUNICATIONS
SIMULTANEOUS
TWO-FREQUENCY Chung-Yung
Department
of Chemistry,
OSCILLATION
March
IN A DYE LASER
1973
SYSTEM
WU and John R. LOMBARD1 University of Illinois, Urbana, Illinois 61801,
Received
29 December
USA
1972
We describe a continuously tunable dye laser operating two-wavelengths simultaneously over a tuning range of about 250 A. With a beam splitter, two polarizers and two gratings the two output beams are spatially coherent and can be of any desirable polarizations with respect to each other.
A continuously tunable multiple-wavelength laser with perfectly spatial coherent output beams has been demonstrated in parametric oscillators [l] and second harmonic generation in dye laser systems [ 21. The former is made possible by using a strong pump wave at o3 causing the simultaneous generation of radiation in a non-linear crystal at WI and 02, where o3 = WI + 02. If the output coupling mirror is partially transmitting at those frequencies then one obtains three frequencies with o3 fured and WI, w2 tunable. The latter is produced by coupling a non-linear crystal with a broad-band dye laser. Due to the requirement of phase-matching in these two laser systems the
- ______ Et P2
Fig. 1. Experimental coating). C: gratings. laser.
arrangement. P: polarizers,
output beams are all linearly polarized and have the same polarization at all times. For many reasons we need two-frequency laser beams which are spatially collinear, collimated and tunable, yet having different polarizations. Recently, Zalewski and Keller [3] reported two tunable wavelength operation in a flashlamp pumped rhodamine 6G dye laser system using two gratings at both ends of the laser cavity. The laser output is from the zero order of a grating or from a beam splitter placed in the laser cavity. In this letter we describe a different design which can produce two continuously tunable wavelengths within the gain curve of the particular dye medium used. The two osM 8---
A,,
M: output mirror (50% R. aluminum coating). D: dye cell. B.S.: beam splitter (30% R. aluminum neutral density filters or beam expander. L: focusing lens (with f.1. = 15 cm). Na-P.L.: Nz-pulse
233
Volume
7, number
3
OPTICS COMMUNICATIONS
cillating wavelengths can either have the same polarization, orthogonal polarization or both can be unpolarized. During the course of this experiment, Pilloff [4] has reported a similar configuration in which a Clan-laser prism is used to decouple the two wavelengths from each other resulting in two mutually orthogonal polarized laser beams. A schematic diagram of the ap aratus is shown in ? fig. 1. The active medium is a I O-- - 10 3 M solution of rhodamine 6G in absolute methanol. The dye cell is made of a quartz cell with 2 cm optical length. The ultraviolet output of a pulsed nitrogen laser (AVCO Model C950, 1OOkW peak power with 10 nsec pulsewidth) is focused by a spherical quartz lens of I5 cm focal length onto a 1X 1.5 mm dimension at the imrer wall of the cell. The pumped volume is about lob2 cm3. A beam splitter (a glass slide with 30% R. aluminum coating) is utilized to split and direct the two beams to gratings 1 and 2. Grating 1 is a high dispersion echelle grating (blace angle 63”35’ , 3 16 grooves/ mm). Grating 3 has a large spectral bandwidth (600 grooves/mm, blaze wavelength at 4000 A, a grating designed for conventional spectroscopic usage). The spectra were recorded photographically on a low resolution spectrometer with a dispersion of = 10 8/mm in the visible region. The configuration can be operated as two independent laser cavities because of the large pumping volume. It is found that the deflection of the two output beams can be tuned within 20 mrad, a range of about five times the beam size. When a spatially coherent laser beam is desired, care must be taken to adjust the alignment. Polarized laser beams are obtained by simply inserting polarizers into the cavities. Thus, by setting the two polarizers with appropriate polarizations the two beams can be polarized parallel or perpendicular to each other. Furthermore, one or both of the beams can remain unpolarized by leaving out the appropriate polarizer. The laser output power is reduced slightly by introducing polarizers. Other characteristics of the laser output remain the same regardless of whether or nor the polarizers are inserted. Fig.2 shows the results obtained by setting grating 2 at a fixed angular position and tuning grating 1 from the long wavelength end to the blue end. In this picture the normal broadband emission is present at the same time from grating 2, but it disappears as the tuning wavelength X, approaches h,. Fig.3 shows the 234
March
1973
results when both gratings are tuned simultaneously. Grating 1 sweeps from red to blue while grating 2 tunes in the opposite direction. We have set the intensity of these two beams about equal in their initial angular position. This can be accomplished by either inserting suitable neutral density filters or elongating one “cavity”. The two tuning beams meet at 1 and cross at 2. This picture shows that the two beams can be tuned simultaneously over the range from 6070 to 5790 A if rhodamine 6G is the active medium. The superimposed sharp lines are low pressure Hg lines utilized as standard calibration. Fig.4 shows the output of the laser system when grating 2 is replaced by a total r-eflectance mirror aud grating 1 is tuned across the whole emission spectrum. The characteristics of homogeneous broadening of the dye media is quite obvious from the examination of these pictures. This can be understood on the basis of mode competition. When one of the oscillating wavelengths is made more intense than the others, for example, by means of a grating or Fabry--P&rot inter-
5q60
5 790 5770 /
G2
6,070A
Gl
Fig.2. Grating 1 tuned from red toward blue end. Grating 2 remains fixed. In addition to a narrow band emission from grating 2, a normal broadband laser emission is present at the same time. See text for explanation why the broad-band emission disappeared when the tuning frequency of hl approaches A?
Volume
5460 \
7, number
3
OPTICS
5790
5770
I
March
COMMUNICATIONS
6q7Oih
5n60
1973
5790
a, 4 c,
cz a2
c3 t3 Fig.3. Two simultaneous tunable wavelengths. This picture shows that the two beams can be simultaneously tuned over about 250 A. The active medium is rhodamine 6G.
ferometer or by injection of monochromatic radiation [S] , the laser media are forced to oscillate at this high gain wavelength and thus tend to suppress lasing actions at all other wavelengths. In fig.4 traces a are taken by blocking the total reflecting mirror and tuning the feedback from grating 1. Traces b are the output from the total reflectance mirror made by blocking grating 1. Traces c are when these two frequencies oscillate simultaneously. In c3 both wings of b, are eliminated because the high gain portion of the oscillating wavelengths is a3 plus the center portion of b,. Traces c1 and c2 show that the power output of al and a2 drops relative to that of a, and a2, respectively. All traces in this picture are taken with five pulses except al which is exposured eight pulses. The same effects can be seen in fig.2 in which grating 2 remains fixed and grating 1 is tuned from the red end limit of the tuning range toward the blue wavelength end. The further it is tuned the more the intense it gets, consequently the power of beam X2 is gradually attenuated as the power of h, increases. It whould be noted that the spectral width X2 produced
a3
Fig.4. Grating 2 is replaced by a total reflectance mirror. Traces a are taken when the total reflectance mirror is blocked. Traces b are taken when grating 1 is blocked. Traces c show both are lasing simultaneously as grating 1 is tuned across the broadband emission spectrum.
by the poor grating 2 narrows as its power decreases. Thus if we adjust the two desirable oscillating wavelengths to about equal intensity, the mode competition decreases, because they both have about the same gain in the cavity. This has been demonstrated in fig.3. The output spectral linewidth in this configuration is quite wide because of the kind of gratings employed. By inserting a beam expander [6,7] into one of the beams we are able to narrow the linewidth to 0. 1 A fwhm. Fig.5 shows the results. The broad band laser light is from the grating only. The tuned narrow light is from the beam expander. Its linewidth observed in this picture is instrument limited. However, an g-meter Czerny-Turner Mount high resolution spectrometer [8] shows the linewidth is about 0. 1 A fwhm. There is no doubt that we can get two simultaneous oscillating laser beams with linewidth on the order of 235
Volume
7. number
3
OPTICS COMMUNICATIONS
March
1973
be employed in two-photon absorption spectroscopy, photochemistry and photolysis. In non-linear opti-s two frequencies have been utilized to sum up or suotract in a non-linear crystal to produce tunable frequency up-conversion or tunable IR lasing. Tunable ultraviolet radiation in the region of 1800 A to 2500 A by mixing of visible laser light in alkali metal vapors has been proposed by Harris et al. [ 121. This device might be potentially useful in these areas. We acknowledge Science Foundation Society (PRF).
the financial support of National and the American Chemical
References
1:ig.S. The tuned narrow line is obtained by substituting Pa with a beam expander (40X). The broad output is from grating 1. The observed linewidth from the beam expander in this picture is instrument limited. A high resolution spectrometer shows it is about 0. I A fwhm.
0. 1 A, when the two beams are both coupled by means of beam expanders. Mode-locking of either flashlamp- or laser-pumped due lasers has been demonstrated [9, lo]. Therefore by coupling the system by means of a mode-locking unit, we can obtain picosecond pulse trains as well as a single picosecond pulse. One such beam with certain polarization is used as an excitation source while the other beam with appropriate polarization is used as a probing beam. In this way molecular diffusion motions or conformational changes in biological molecules [ 1 l] can be studied either in the picosecond or in the subnanosecond time scale. This design, of course, can also
236
[ I] CC. Wang and G.W. Racette, Appl. Phys. Letters 6 (1965) 169; J.A. Giordmaine and R.C. Miller, Phys. Rev. Letters 14 (1965) 973. [2] B.G. Huth, G.I. Farmer. L.M. Taylor and M.R. Kagdn, Spectry. Letters 1 (1968) 425. [ 31 E.F:. Zalewski and R.A. Keller, Appl. Opt. 10 (1971) 2773. ]4] 1l.S. Pilloff, Appl. Phys. Letters 21 (1972) 339. [ 51 L.E. Erickson and A. Szako, Appl. Phys. Letters 18 (1971)433. 161 T.W. Hansch, 4ppl. Opt. 11 (1972) 895. IEEE J. @anturn Electron [71 C.-Y, Wu and J.R. Lombardi, -IF to be published. ]81 J.R. Lombardi, J. Chem. Phys. 50 (1969) 3780. Appl. Phys. Letters 18 (1971) 481. I91 R.J. VonGutfeld,
[lOI 1111 1121
W. Schmidt and F.P. Schafer, Phys. Letters 26A (I 968) 558. P.M. Rentzepis, unpublished results. S.E. Harris and R.R. Miles, Appl. Phys. Letters 19 (1971) 385; U.S. Carson, S.E. Harris, A.H. Kung, R.B. Miles and J.F. Young, Section D.l of VI1 International Quantum Electronics Conference, Montreal, Canada (1972).
7, number
3
OPTICS COMMUNICATIONS
SIMULTANEOUS
TWO-FREQUENCY Chung-Yung
Department
of Chemistry,
OSCILLATION
March
IN A DYE LASER
1973
SYSTEM
WU and John R. LOMBARD1 University of Illinois, Urbana, Illinois 61801,
Received
29 December
USA
1972
We describe a continuously tunable dye laser operating two-wavelengths simultaneously over a tuning range of about 250 A. With a beam splitter, two polarizers and two gratings the two output beams are spatially coherent and can be of any desirable polarizations with respect to each other.
A continuously tunable multiple-wavelength laser with perfectly spatial coherent output beams has been demonstrated in parametric oscillators [l] and second harmonic generation in dye laser systems [ 21. The former is made possible by using a strong pump wave at o3 causing the simultaneous generation of radiation in a non-linear crystal at WI and 02, where o3 = WI + 02. If the output coupling mirror is partially transmitting at those frequencies then one obtains three frequencies with o3 fured and WI, w2 tunable. The latter is produced by coupling a non-linear crystal with a broad-band dye laser. Due to the requirement of phase-matching in these two laser systems the
- ______ Et P2
Fig. 1. Experimental coating). C: gratings. laser.
arrangement. P: polarizers,
output beams are all linearly polarized and have the same polarization at all times. For many reasons we need two-frequency laser beams which are spatially collinear, collimated and tunable, yet having different polarizations. Recently, Zalewski and Keller [3] reported two tunable wavelength operation in a flashlamp pumped rhodamine 6G dye laser system using two gratings at both ends of the laser cavity. The laser output is from the zero order of a grating or from a beam splitter placed in the laser cavity. In this letter we describe a different design which can produce two continuously tunable wavelengths within the gain curve of the particular dye medium used. The two osM 8---
A,,
M: output mirror (50% R. aluminum coating). D: dye cell. B.S.: beam splitter (30% R. aluminum neutral density filters or beam expander. L: focusing lens (with f.1. = 15 cm). Na-P.L.: Nz-pulse
233
Volume
7, number
3
OPTICS COMMUNICATIONS
cillating wavelengths can either have the same polarization, orthogonal polarization or both can be unpolarized. During the course of this experiment, Pilloff [4] has reported a similar configuration in which a Clan-laser prism is used to decouple the two wavelengths from each other resulting in two mutually orthogonal polarized laser beams. A schematic diagram of the ap aratus is shown in ? fig. 1. The active medium is a I O-- - 10 3 M solution of rhodamine 6G in absolute methanol. The dye cell is made of a quartz cell with 2 cm optical length. The ultraviolet output of a pulsed nitrogen laser (AVCO Model C950, 1OOkW peak power with 10 nsec pulsewidth) is focused by a spherical quartz lens of I5 cm focal length onto a 1X 1.5 mm dimension at the imrer wall of the cell. The pumped volume is about lob2 cm3. A beam splitter (a glass slide with 30% R. aluminum coating) is utilized to split and direct the two beams to gratings 1 and 2. Grating 1 is a high dispersion echelle grating (blace angle 63”35’ , 3 16 grooves/ mm). Grating 3 has a large spectral bandwidth (600 grooves/mm, blaze wavelength at 4000 A, a grating designed for conventional spectroscopic usage). The spectra were recorded photographically on a low resolution spectrometer with a dispersion of = 10 8/mm in the visible region. The configuration can be operated as two independent laser cavities because of the large pumping volume. It is found that the deflection of the two output beams can be tuned within 20 mrad, a range of about five times the beam size. When a spatially coherent laser beam is desired, care must be taken to adjust the alignment. Polarized laser beams are obtained by simply inserting polarizers into the cavities. Thus, by setting the two polarizers with appropriate polarizations the two beams can be polarized parallel or perpendicular to each other. Furthermore, one or both of the beams can remain unpolarized by leaving out the appropriate polarizer. The laser output power is reduced slightly by introducing polarizers. Other characteristics of the laser output remain the same regardless of whether or nor the polarizers are inserted. Fig.2 shows the results obtained by setting grating 2 at a fixed angular position and tuning grating 1 from the long wavelength end to the blue end. In this picture the normal broadband emission is present at the same time from grating 2, but it disappears as the tuning wavelength X, approaches h,. Fig.3 shows the 234
March
1973
results when both gratings are tuned simultaneously. Grating 1 sweeps from red to blue while grating 2 tunes in the opposite direction. We have set the intensity of these two beams about equal in their initial angular position. This can be accomplished by either inserting suitable neutral density filters or elongating one “cavity”. The two tuning beams meet at 1 and cross at 2. This picture shows that the two beams can be tuned simultaneously over the range from 6070 to 5790 A if rhodamine 6G is the active medium. The superimposed sharp lines are low pressure Hg lines utilized as standard calibration. Fig.4 shows the output of the laser system when grating 2 is replaced by a total r-eflectance mirror aud grating 1 is tuned across the whole emission spectrum. The characteristics of homogeneous broadening of the dye media is quite obvious from the examination of these pictures. This can be understood on the basis of mode competition. When one of the oscillating wavelengths is made more intense than the others, for example, by means of a grating or Fabry--P&rot inter-
5q60
5 790 5770 /
G2
6,070A
Gl
Fig.2. Grating 1 tuned from red toward blue end. Grating 2 remains fixed. In addition to a narrow band emission from grating 2, a normal broadband laser emission is present at the same time. See text for explanation why the broad-band emission disappeared when the tuning frequency of hl approaches A?
Volume
5460 \
7, number
3
OPTICS
5790
5770
I
March
COMMUNICATIONS
6q7Oih
5n60
1973
5790
a, 4 c,
cz a2
c3 t3 Fig.3. Two simultaneous tunable wavelengths. This picture shows that the two beams can be simultaneously tuned over about 250 A. The active medium is rhodamine 6G.
ferometer or by injection of monochromatic radiation [S] , the laser media are forced to oscillate at this high gain wavelength and thus tend to suppress lasing actions at all other wavelengths. In fig.4 traces a are taken by blocking the total reflecting mirror and tuning the feedback from grating 1. Traces b are the output from the total reflectance mirror made by blocking grating 1. Traces c are when these two frequencies oscillate simultaneously. In c3 both wings of b, are eliminated because the high gain portion of the oscillating wavelengths is a3 plus the center portion of b,. Traces c1 and c2 show that the power output of al and a2 drops relative to that of a, and a2, respectively. All traces in this picture are taken with five pulses except al which is exposured eight pulses. The same effects can be seen in fig.2 in which grating 2 remains fixed and grating 1 is tuned from the red end limit of the tuning range toward the blue wavelength end. The further it is tuned the more the intense it gets, consequently the power of beam X2 is gradually attenuated as the power of h, increases. It whould be noted that the spectral width X2 produced
a3
Fig.4. Grating 2 is replaced by a total reflectance mirror. Traces a are taken when the total reflectance mirror is blocked. Traces b are taken when grating 1 is blocked. Traces c show both are lasing simultaneously as grating 1 is tuned across the broadband emission spectrum.
by the poor grating 2 narrows as its power decreases. Thus if we adjust the two desirable oscillating wavelengths to about equal intensity, the mode competition decreases, because they both have about the same gain in the cavity. This has been demonstrated in fig.3. The output spectral linewidth in this configuration is quite wide because of the kind of gratings employed. By inserting a beam expander [6,7] into one of the beams we are able to narrow the linewidth to 0. 1 A fwhm. Fig.5 shows the results. The broad band laser light is from the grating only. The tuned narrow light is from the beam expander. Its linewidth observed in this picture is instrument limited. However, an g-meter Czerny-Turner Mount high resolution spectrometer [8] shows the linewidth is about 0. 1 A fwhm. There is no doubt that we can get two simultaneous oscillating laser beams with linewidth on the order of 235
Volume
7. number
3
OPTICS COMMUNICATIONS
March
1973
be employed in two-photon absorption spectroscopy, photochemistry and photolysis. In non-linear opti-s two frequencies have been utilized to sum up or suotract in a non-linear crystal to produce tunable frequency up-conversion or tunable IR lasing. Tunable ultraviolet radiation in the region of 1800 A to 2500 A by mixing of visible laser light in alkali metal vapors has been proposed by Harris et al. [ 121. This device might be potentially useful in these areas. We acknowledge Science Foundation Society (PRF).
the financial support of National and the American Chemical
References
1:ig.S. The tuned narrow line is obtained by substituting Pa with a beam expander (40X). The broad output is from grating 1. The observed linewidth from the beam expander in this picture is instrument limited. A high resolution spectrometer shows it is about 0. I A fwhm.
0. 1 A, when the two beams are both coupled by means of beam expanders. Mode-locking of either flashlamp- or laser-pumped due lasers has been demonstrated [9, lo]. Therefore by coupling the system by means of a mode-locking unit, we can obtain picosecond pulse trains as well as a single picosecond pulse. One such beam with certain polarization is used as an excitation source while the other beam with appropriate polarization is used as a probing beam. In this way molecular diffusion motions or conformational changes in biological molecules [ 1 l] can be studied either in the picosecond or in the subnanosecond time scale. This design, of course, can also
236
[ I] CC. Wang and G.W. Racette, Appl. Phys. Letters 6 (1965) 169; J.A. Giordmaine and R.C. Miller, Phys. Rev. Letters 14 (1965) 973. [2] B.G. Huth, G.I. Farmer. L.M. Taylor and M.R. Kagdn, Spectry. Letters 1 (1968) 425. [ 31 E.F:. Zalewski and R.A. Keller, Appl. Opt. 10 (1971) 2773. ]4] 1l.S. Pilloff, Appl. Phys. Letters 21 (1972) 339. [ 51 L.E. Erickson and A. Szako, Appl. Phys. Letters 18 (1971)433. 161 T.W. Hansch, 4ppl. Opt. 11 (1972) 895. IEEE J. @anturn Electron [71 C.-Y, Wu and J.R. Lombardi, -IF to be published. ]81 J.R. Lombardi, J. Chem. Phys. 50 (1969) 3780. Appl. Phys. Letters 18 (1971) 481. I91 R.J. VonGutfeld,
[lOI 1111 1121
W. Schmidt and F.P. Schafer, Phys. Letters 26A (I 968) 558. P.M. Rentzepis, unpublished results. S.E. Harris and R.R. Miles, Appl. Phys. Letters 19 (1971) 385; U.S. Carson, S.E. Harris, A.H. Kung, R.B. Miles and J.F. Young, Section D.l of VI1 International Quantum Electronics Conference, Montreal, Canada (1972).