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An improved line narrowing technique for a dye laser excited by a nitrogen laser
Volume
4, number
2
OPTICS
AN
IMPROVED
A DYE
LINE
EXCITED
Perkin-Elmer
Corp.,
BY
S. A. MYERS Norwalk,
Received
to 0.9 A. The light reflected
New York
on an educational University.
from one prism
leave-of
A NITROGEN
FOR LASER
* Conwcticut
27 August
1971
06852,
USA
1971
high angle of incidence approaching 90° in the optical cavity of a rhoa pulsed nitrogen laser leads to improvement in linewidth reduction. 100 A to 2 A when the prism is placed in a broad-band cavity c‘onsisting When it is put in a mirror grating cavity, the reduction is from 11 li
face becomes
A simple modification of the cavity design of a dye laser optically pumped by a pulse nitrogen laser leads to significant reduction in the linewidth. In our experiment a 10m3 M/l solution of rhodamine 6-G in ethanol is transversely pumped by the focused 3371 A emission of an N2 laser. We measured a linewidth of 11 A with a dye laser optical cavity-consisting of a 304 transmittance mirror and a plane diffraction grating autocollimated in first order. The grating has 1800 l/mm and an effective reflectance of 60% in first order. Use of the grating provides both a means.of tuning the wavelength and narrowing the linewidth [l]. The linewidth is reduced to 0.9 A when a small prism is placed in this cavity between the dye cell and the grating as shown in fig. 1. Unlike conventional use of a prism in a laser cavity positioned at Brewster’s angle or the use of a Littrow prism in a dye laser cavity [2], the prism in this instance must be rotated so that the light which strikes the side of the prism facing the dye cell makes an angle of incidence very close to 90°. The reflected light from this surface of the prism becomes the output from the 1Zser. The wavelength is tuned by rotating either the prism or the grating; however, there is no angular deviation of the output beam if the grating rather than the prism is rotated. Similarly, if the prism is used in a broadband cavity made up of two high reflectance mirrors, the 100 A linewidth is reduced to 2 A. * Presently
TECHNIQUE
NARROWING
LASER
A prism positioned at a very damine G-G dye laser excited by The linewidth is narrowed from of two high reflectance mirrors.
October
COMMUNICATIONS
absence
at
the laser
output.
LASER OUTPUT BEAM
Fig. 1. Addition
of a prism
Tuning
is performed
closest
to the
to the optical laser. by
rotating
the
cavity
of a dye
mirror
prism.
The linewidth and average laser power were measured for different angles of incidence approaching 90’ caused by rotating the prism in the cavity. The prism is made of Schott SF’-55 glass (nD = 1.761) with an apex angle of 58.80 and a base 2.5 cm long. The end reflectors were separated by an optical path length of 10.7 cm with the prism in the cavity. Linewidths were measured with a 0.5 m Jarrell-Ash monochromator. The pulsed photomultiplier signal from the monochromator was integrated and amplified by a boxcar integrator and displayed on a chart 187
Volunle
4, number
. -I
2
OPTICS
COMMUNICATIONS
25
i
20
4
____
.
..-
: ‘. .
. .
PRISM-MIRROR PRISM-GRATING
‘.
15
77
-
%.
r
; LL
-
-
Y,
79
81
83
\
\
85
87
89
(a)
PRISM
INCIDENT
ANGLE
( DEGREES
I
Fig. 2. Change in laser performance as the prism is rotated. (al linewidth and (b) average laser po~ver versus the angle of incidcncc at the prism surface facing the dye cell.
recorder. Average laser power was measured with an Eppley thermopile. The nitrogen laser is similar in design to that described by Leonard [3]. It was operated at a pulse repetition rate of 20 pulses ‘set at an average power of 6.0 mW. The pulse duration was 7.5 nscc with a corresponding peak power of approximately 40 kW. The output beam is focused by a quartz cylindrical lens into a line located just inside the wall of the 2.5 cm long dye cell in the manner described by Myer et al. [4]. Fig. 2a shows the decrease in linewidth as the incident angle increases. Multiple data points at a fixed prism angle represent linewidths measured from successive monochromator scans without change in the optical alignment. One point was repeated at the end of the experimental run by resetting the prism to i = 83O. The linewidth reproducibility is strongly dependent upon optical alignment for the prism-mirror cavity but much less so for the prism-grating configuration. Line narrowing comes about because the 188
October
1971
angular dispersion of the prism and end reflector (mirror or grating) markedly increases as the angle of incidence increases. In autocollimation the grating diffracts a ray incident upon it back along the same direction and the ray leaves the prism undeviated from the initial incident direction. As a result of prism and grating dispersion, a ray making the same angle of incidence on the prism, but at some slightly different wavelength will not satisfy the condition for autocollimation and will not return through the prism along the same path. Near the critical angle for total internal reflection a small change in incident angle makes a very large change in the refraction angle. When the prism incident angle, i, approaches 300, the condition for total internal reflection is approached and the slightly different angle of incidence at the final glass-air interface in the prism produces a large change in the refracted direction. When the angle of incidence approaches 90°, this dispersion is greater than that which would be observed for either the prism or the grating used alone. For example, the amount [5] of angular dispersion produced by the prism-grating combination is proportional to l,/cos i multiplied by a factor which is nearly equal to the dispersion of the grating alone. At i = 880 the angular dispersion exceeds that caused by the grating itself by a factor of about 20. When the grating is replaced by the mirror, the amouit of dispersion is reduced but still exceeds that due to the grating alone by a factor of 4. If the linewidth, AX, is approximately proportional to the reciprocal value of the dispersion then at large angles of incidence Ai will be proportional to the compiementary angle to i. The linewidths shown in fig. 2a behave approximately in this manner. The prism insertion method requires a very high gain laser medium such as rhodamine 6-G. The laser light is strongly polarized in the plane of incidence by the grating. As i a 90°, the prism reflectance for this state of polarization increases rapidly from a value of 200{, at i = 80’ toward unity at i = 90°. Only for a high gain laser can appreciable power be coupled out of the cavity as the reflected ray at the large angles of incidence required for enhanced dispersion. Zeroth order diffraction loss from the grating is minimized since the circulating power in the cavity between the prism and the grating is less than between the prism and the end mirror. The dependence of the average laser power on the angle of incidence of the light striking the prism is shown in fig. 2b. The laser power reaches a maximum for both the prism-mirror
Volume
4, number
2
OPTICS
COMMUNICATIONS
and prism-grating cavities at an incident angle between 850 and 8’7O. The corresponding reflectance is about 50% to 65%. At higher values of the incident angle the reflectance approaches unity and the cavity loss is so great that the power output rapidly declines. The power is slightly less for the grating cavity because of the extra zeroth order diffraction loss. This work was stimulated by our observation that the linewidths obtainable with N2 laser pumping exceed those reported by others using different mea!s of exciting the dye. Linewidths of lesf than 1A (compared with our linewidth of 11 A) are reported by other investigators [ 1,6] using similar grating cavity designs but different laser or flashlamp excitation. Results similar to ours with N2 laser excitation have been described by Kogelvik et al. [7]; they report linewidths of 12 A to 14 A with a 1200 l/mm grating used in first order for an N2 pumped exciplex 4-methylumbelliferone dye laser. The linewidth is narrowed to 4 A at reduced efficiency when the grating is used in second order to provide twice the angular dispersion. When they used holographic gratings with 3300 l/mm and 3700 l/mm, the disperion is sufficient to reduce the linewidth to less than 1 A. The increased difficulty in achieving narrow linewidths with diffraction grating tuning when pulsed N2 laser excitation of the dye is employed stems from the reduced number of cavity transits which are restricted by the shortness of the dye laser pulses and minimum intracavity distances of a few cm. We observe a 5 nsec pulsewidth
October
1971
for rhodamine 6-G; 2 nsec to 8 nsec pulsewidths depending upon the dye have been reported [4] for a 10 nsec N2 laser pulse. Dye laser pulses observed with other means of optical pumping vary upward from 10 nsec [I] to over 80 psec [8]. Such a small number of cavity transits reduces the difference in gain due to walk-off effects between an on-axis ray and a divergent ray. This impaired gain discrimination contributes strongly to line broadening. Such dependence of linewidth on pulse duration has been observed by Bradley et al. [6], where decreasing the ruby pumping pulse from 100 nsec to 35 nsec of a longitudinally excited DTCDCT dye laser lead to a corresponding increase in linewidth of 0.5 i to 4 A. The author gratefully acknowledges discussions with E. L. Kerr.
helpful
REFERENCES and B. B. McFarland, Appl. Phys. Letters 10 (19G7) 2GG. G. Yamaguchi and C. Yamanaku, [“I S. MurakaLva, Japan. J. Appl. Phys. 7 (1968) 681. Appl. Phys. Letters 7 (1965) 4. [31 D.A. Leonard, C. I,. Johnson, E. Kierstead, R. D. [41 J.A. Myrr, Sharma and I.Itzkan, Appl. Phys. Letters 16 (1970) 3. unpublished. [51 S. A. Myers, A.J.F.Durrant, G.hI.Gale, M.Moore [Gl D.J.Bradley, and P.D.Smlth, IEEE J. Quantum Electron. QE-4 (1968) 767. C. V.Shank. 1’. 1’. Sosnovski and [71 H.Koeelnik. A.Diknes, kppl. Phys. Letters 16 (1970) 499. and F. P. ScFTafer, Phys. Letters 28A [81 B. B.Snavely (19F9) 728.
[ll R. H. Soffer
189
4, number
2
OPTICS
AN
IMPROVED
A DYE
LINE
EXCITED
Perkin-Elmer
Corp.,
BY
S. A. MYERS Norwalk,
Received
to 0.9 A. The light reflected
New York
on an educational University.
from one prism
leave-of
A NITROGEN
FOR LASER
* Conwcticut
27 August
1971
06852,
USA
1971
high angle of incidence approaching 90° in the optical cavity of a rhoa pulsed nitrogen laser leads to improvement in linewidth reduction. 100 A to 2 A when the prism is placed in a broad-band cavity c‘onsisting When it is put in a mirror grating cavity, the reduction is from 11 li
face becomes
A simple modification of the cavity design of a dye laser optically pumped by a pulse nitrogen laser leads to significant reduction in the linewidth. In our experiment a 10m3 M/l solution of rhodamine 6-G in ethanol is transversely pumped by the focused 3371 A emission of an N2 laser. We measured a linewidth of 11 A with a dye laser optical cavity-consisting of a 304 transmittance mirror and a plane diffraction grating autocollimated in first order. The grating has 1800 l/mm and an effective reflectance of 60% in first order. Use of the grating provides both a means.of tuning the wavelength and narrowing the linewidth [l]. The linewidth is reduced to 0.9 A when a small prism is placed in this cavity between the dye cell and the grating as shown in fig. 1. Unlike conventional use of a prism in a laser cavity positioned at Brewster’s angle or the use of a Littrow prism in a dye laser cavity [2], the prism in this instance must be rotated so that the light which strikes the side of the prism facing the dye cell makes an angle of incidence very close to 90°. The reflected light from this surface of the prism becomes the output from the 1Zser. The wavelength is tuned by rotating either the prism or the grating; however, there is no angular deviation of the output beam if the grating rather than the prism is rotated. Similarly, if the prism is used in a broadband cavity made up of two high reflectance mirrors, the 100 A linewidth is reduced to 2 A. * Presently
TECHNIQUE
NARROWING
LASER
A prism positioned at a very damine G-G dye laser excited by The linewidth is narrowed from of two high reflectance mirrors.
October
COMMUNICATIONS
absence
at
the laser
output.
LASER OUTPUT BEAM
Fig. 1. Addition
of a prism
Tuning
is performed
closest
to the
to the optical laser. by
rotating
the
cavity
of a dye
mirror
prism.
The linewidth and average laser power were measured for different angles of incidence approaching 90’ caused by rotating the prism in the cavity. The prism is made of Schott SF’-55 glass (nD = 1.761) with an apex angle of 58.80 and a base 2.5 cm long. The end reflectors were separated by an optical path length of 10.7 cm with the prism in the cavity. Linewidths were measured with a 0.5 m Jarrell-Ash monochromator. The pulsed photomultiplier signal from the monochromator was integrated and amplified by a boxcar integrator and displayed on a chart 187
Volunle
4, number
. -I
2
OPTICS
COMMUNICATIONS
25
i
20
4
____
.
..-
: ‘. .
. .
PRISM-MIRROR PRISM-GRATING
‘.
15
77
-
%.
r
; LL
-
-
Y,
79
81
83
\
\
85
87
89
(a)
PRISM
INCIDENT
ANGLE
( DEGREES
I
Fig. 2. Change in laser performance as the prism is rotated. (al linewidth and (b) average laser po~ver versus the angle of incidcncc at the prism surface facing the dye cell.
recorder. Average laser power was measured with an Eppley thermopile. The nitrogen laser is similar in design to that described by Leonard [3]. It was operated at a pulse repetition rate of 20 pulses ‘set at an average power of 6.0 mW. The pulse duration was 7.5 nscc with a corresponding peak power of approximately 40 kW. The output beam is focused by a quartz cylindrical lens into a line located just inside the wall of the 2.5 cm long dye cell in the manner described by Myer et al. [4]. Fig. 2a shows the decrease in linewidth as the incident angle increases. Multiple data points at a fixed prism angle represent linewidths measured from successive monochromator scans without change in the optical alignment. One point was repeated at the end of the experimental run by resetting the prism to i = 83O. The linewidth reproducibility is strongly dependent upon optical alignment for the prism-mirror cavity but much less so for the prism-grating configuration. Line narrowing comes about because the 188
October
1971
angular dispersion of the prism and end reflector (mirror or grating) markedly increases as the angle of incidence increases. In autocollimation the grating diffracts a ray incident upon it back along the same direction and the ray leaves the prism undeviated from the initial incident direction. As a result of prism and grating dispersion, a ray making the same angle of incidence on the prism, but at some slightly different wavelength will not satisfy the condition for autocollimation and will not return through the prism along the same path. Near the critical angle for total internal reflection a small change in incident angle makes a very large change in the refraction angle. When the prism incident angle, i, approaches 300, the condition for total internal reflection is approached and the slightly different angle of incidence at the final glass-air interface in the prism produces a large change in the refracted direction. When the angle of incidence approaches 90°, this dispersion is greater than that which would be observed for either the prism or the grating used alone. For example, the amount [5] of angular dispersion produced by the prism-grating combination is proportional to l,/cos i multiplied by a factor which is nearly equal to the dispersion of the grating alone. At i = 880 the angular dispersion exceeds that caused by the grating itself by a factor of about 20. When the grating is replaced by the mirror, the amouit of dispersion is reduced but still exceeds that due to the grating alone by a factor of 4. If the linewidth, AX, is approximately proportional to the reciprocal value of the dispersion then at large angles of incidence Ai will be proportional to the compiementary angle to i. The linewidths shown in fig. 2a behave approximately in this manner. The prism insertion method requires a very high gain laser medium such as rhodamine 6-G. The laser light is strongly polarized in the plane of incidence by the grating. As i a 90°, the prism reflectance for this state of polarization increases rapidly from a value of 200{, at i = 80’ toward unity at i = 90°. Only for a high gain laser can appreciable power be coupled out of the cavity as the reflected ray at the large angles of incidence required for enhanced dispersion. Zeroth order diffraction loss from the grating is minimized since the circulating power in the cavity between the prism and the grating is less than between the prism and the end mirror. The dependence of the average laser power on the angle of incidence of the light striking the prism is shown in fig. 2b. The laser power reaches a maximum for both the prism-mirror
Volume
4, number
2
OPTICS
COMMUNICATIONS
and prism-grating cavities at an incident angle between 850 and 8’7O. The corresponding reflectance is about 50% to 65%. At higher values of the incident angle the reflectance approaches unity and the cavity loss is so great that the power output rapidly declines. The power is slightly less for the grating cavity because of the extra zeroth order diffraction loss. This work was stimulated by our observation that the linewidths obtainable with N2 laser pumping exceed those reported by others using different mea!s of exciting the dye. Linewidths of lesf than 1A (compared with our linewidth of 11 A) are reported by other investigators [ 1,6] using similar grating cavity designs but different laser or flashlamp excitation. Results similar to ours with N2 laser excitation have been described by Kogelvik et al. [7]; they report linewidths of 12 A to 14 A with a 1200 l/mm grating used in first order for an N2 pumped exciplex 4-methylumbelliferone dye laser. The linewidth is narrowed to 4 A at reduced efficiency when the grating is used in second order to provide twice the angular dispersion. When they used holographic gratings with 3300 l/mm and 3700 l/mm, the disperion is sufficient to reduce the linewidth to less than 1 A. The increased difficulty in achieving narrow linewidths with diffraction grating tuning when pulsed N2 laser excitation of the dye is employed stems from the reduced number of cavity transits which are restricted by the shortness of the dye laser pulses and minimum intracavity distances of a few cm. We observe a 5 nsec pulsewidth
October
1971
for rhodamine 6-G; 2 nsec to 8 nsec pulsewidths depending upon the dye have been reported [4] for a 10 nsec N2 laser pulse. Dye laser pulses observed with other means of optical pumping vary upward from 10 nsec [I] to over 80 psec [8]. Such a small number of cavity transits reduces the difference in gain due to walk-off effects between an on-axis ray and a divergent ray. This impaired gain discrimination contributes strongly to line broadening. Such dependence of linewidth on pulse duration has been observed by Bradley et al. [6], where decreasing the ruby pumping pulse from 100 nsec to 35 nsec of a longitudinally excited DTCDCT dye laser lead to a corresponding increase in linewidth of 0.5 i to 4 A. The author gratefully acknowledges discussions with E. L. Kerr.
helpful
REFERENCES and B. B. McFarland, Appl. Phys. Letters 10 (19G7) 2GG. G. Yamaguchi and C. Yamanaku, [“I S. MurakaLva, Japan. J. Appl. Phys. 7 (1968) 681. Appl. Phys. Letters 7 (1965) 4. [31 D.A. Leonard, C. I,. Johnson, E. Kierstead, R. D. [41 J.A. Myrr, Sharma and I.Itzkan, Appl. Phys. Letters 16 (1970) 3. unpublished. [51 S. A. Myers, A.J.F.Durrant, G.hI.Gale, M.Moore [Gl D.J.Bradley, and P.D.Smlth, IEEE J. Quantum Electron. QE-4 (1968) 767. C. V.Shank. 1’. 1’. Sosnovski and [71 H.Koeelnik. A.Diknes, kppl. Phys. Letters 16 (1970) 499. and F. P. ScFTafer, Phys. Letters 28A [81 B. B.Snavely (19F9) 728.
[ll R. H. Soffer
189