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Observation of subpicosecond components in the mode-locked Nd:glass laser
Volume 28A, number 10
PHYSICS
4) For comparison with the experimental data, second order calculations for the L. J. potential are accurate enough (within less than 0.5%). while not much reliance can be placed on the ” rigid sphere potential calculations because of the simple nature of the potential. I am very grateful to Dr. K. Kumar for useful supervision and constant encouragement during the course of this work.
LETTERS
24 February 1969
References 1. M. P. Saksena, S. C. Saxena and S. Mathur. Trans.
Faraday Sot. 63 (1968) 591. 2. R. S. Devote, Phys. Fluids 9 (1966) 1230 (reference to earlier literature as well). 3. S. C. Gupta, Ph. D. Thesis (Australian National University: Canberra, 1968). 4. J. 0. Hirschfelder, C. F. Curtiss and R. B. Bird, The molecular theory of liquids and gases, Ch. 8 (John Wiley and Sons, New York, 1964, 2nd Ed.).
*****
OBSERVATION IN THE
OF SUBPICOSECOND COMPONENTS MODE-LOCKED Nd:GLASS LASER S. L. SHAPIRO * and M. A. DUGUAY
Bell Telephone Laboratories,
Incorporated,
Murray Hill, New Jersey,
07974, USA
Received 21 January 1969
The two-photon fluorescence pattern produced by ultrashort pulses from a dye mode-locked Nd:glass laser is shown to be a sharply spiked curve (spike width M 0.25 psec) with a peak contrast of 2.95+0.25. This removes a long-standing discrepancy between theory and experiment.
The duration of the light pulses generated by the dye mode-locked Nd:glass laser [l] has been measured by several authors [2-61 and has ranged from 4 to 16 psec. The two-photon fluorescence technique [3] has been widely used to display such pulses. Theory predicts [4,7] that the peak contrast ratios measured in such two-photon fluorescence experiments should be 3. The theory has found support in experiments done with a single mode ruby laser [4] and a Q-switched Nd:YaG laser [8]. In the case of the Nd:glass laser, a discrepancy has persisted between the theoretical peak contrast of 3 and experimental values which have ranged from 1.7 to 2.0 [4,6]. We report a measurement of the two-photon fluorescence display produced by the dye mode-locked Nd:glass laser in which a sharp component (or peak), N 0.25 psec wide, is observed, and displays a peak contrast of 3, thus resolving the discrepancy and implying the existence of subpicosecond components in the Nd:glass laser. The experimental arrangement is shown in the figure. The laser consists of a Brewster *Present address: General Telephone and Electronics Labs, Inc. Bayside, New York 11360, USA. 698
angled rod 20X 1.2 cm2 placed between two wedged and Q-switched by the Eastman Kodak #9860 dye. The ultra-short pulses emerge 3.5 nsec apart in a train - 100 nsec long. A beam splitter divides each ultra-short pulse in two which then cross in a cell containing a saturated solution of Rhodamine 6G in dichloroethane. In contrast to previous two-photon fluorescence experiments, the cell employed here is very thin; it consists of two antireflection coated optical windows spaced by a 28~ Mylar film. Thus, photomultiplier #l monitors the fluorescence yield produced in a 28 1 slice of the usual twophoton fluorescence display [3]. The second photomultiplier monitors the fluorescence due to a single passage of the short pulses in 5 mm thick cell placed just before the focus of a 40 cm focal lenght lens, thereby providing the usual reference signal. The two-photon fluorescence pattern is scanned by moving the thin cell along the direction M2 to Ml, and plotting the ratio of the pulse height from PM #l to that from PM #2 as a function of distance. The result is shown in the figure. Ratios obtained have been normalized to 1 in the wings to conform with units defined in ref. 4
Volume
28A, number
10
PHYSICS
LETTERS TPF
arrangement: the laser from the right.
0
beam
I
I
1
2
3
4 POSITION
where the two-photon fluorescence yield due to a single pass was assigned a value of f. We note that the peak two-photon fluorescence yield or contrast is now 2.95 f 0.25 in excellent agreement with the theoretical value of 3. The part of the curve where yields are >2 is very sharp, being only 75 f 25~ wide at the two-photon fluorescence level of 2.5, corresponding to a time interval of 0.25 psec. Such a sharp spike could easily have been blurred out in previous two-photon fluorescence experiments [3-61, all performed photographically. The shape of the two-photon fluorescence curve does not uniquely determine the shape of the laser pulse [4,7]. The distance over which two-photon fluorescence exceeds 1 corresponds, as in Armostrong’s work [2], to the overall length of the pulse, - 6 psec. The narrow two-photon fluorescence spike implies that within this 6 psec pulse (or burst) there is one or more components (or peaks) with widths 0.2-0.6 psec. For example, the pulse could consist of one sharp 0.3 psec peak followed by a long -6 psec tall. Other and more complex models also fit the same two-photon fluorescence curve, however. The narrow components account for a sizeable fraction of the 100-200 cm-l spectral widths which these pulses possess [1,3,5]. This is in contrast to recent work [9] where the spectral width was reported to be largely due to an FM modulation in the form of a frequency sweep. The present experiment does not exclude FM modulation because two-pho-
1969
YIELD
I
Fig. 1. Experimental comes
24 February
5
6
7
(mm)
Fig. 2. Two-photon fluorescence curve obtained for the dye mode-locked Nd:glass laser.
ton fluorescence is insensitive to it [4,7]. When the Q-switching dye was removed from the Nd:glass laser (free-running case), we found a Gaussian shaped peak = 300~ wide with twophoton fluorescence yields of 2.8 f 0.2 at the peak and 2.0 f 0.2 far out in the wings. This is in agreement with what the theory predicts for a collection of randomly phased modes covering the spectral width observed in this case, viz. -20 cm -I. We would like to thank I. Freund, J. K. Galt, A. Giordmaine, J. R. Klauder and Robert C. Miller for stimulating discussions.
J.
References D. A.Stetser and H. Heynau, Appl. 1. A. J.DeMaria, Phys. Letters 8 (1966) 176. Appl. Phys. Letters 10 (1967) 16. 2. J. A. Armstrong, P. M. Rentzepis, S. L. Shapiro and 3. J. A. Giordmaine, K. W. Wecht. ADIJ~.Phvs. Letters 11 (196’7) 216. J. A. Giordmaine and 4. J. R. Klauder, M. A. D&ray, S. L. Shapiro, Appl. Phys.Letters 13 (1968) 174. 5. A. J. DeMaria, W. H. Glenn, M. Brienza and M. E. Mack. to be published in IEEE J. Quant. Elect. January 1969. 6. G. Kachen, L. Steinmet:< and J. Kysilka, Appl. Phys. Letters 13 (1968) 229. 7. H. P. Weber, Phys. Letters 27A (1968) 321. 8. S. K. Kurtz and S. L. Shapiro, Phys. Letters 28A (1968) 17. Phys. Letters 28A (1968) 34. 9. E. B. Treaty,
*****
699
PHYSICS
4) For comparison with the experimental data, second order calculations for the L. J. potential are accurate enough (within less than 0.5%). while not much reliance can be placed on the ” rigid sphere potential calculations because of the simple nature of the potential. I am very grateful to Dr. K. Kumar for useful supervision and constant encouragement during the course of this work.
LETTERS
24 February 1969
References 1. M. P. Saksena, S. C. Saxena and S. Mathur. Trans.
Faraday Sot. 63 (1968) 591. 2. R. S. Devote, Phys. Fluids 9 (1966) 1230 (reference to earlier literature as well). 3. S. C. Gupta, Ph. D. Thesis (Australian National University: Canberra, 1968). 4. J. 0. Hirschfelder, C. F. Curtiss and R. B. Bird, The molecular theory of liquids and gases, Ch. 8 (John Wiley and Sons, New York, 1964, 2nd Ed.).
*****
OBSERVATION IN THE
OF SUBPICOSECOND COMPONENTS MODE-LOCKED Nd:GLASS LASER S. L. SHAPIRO * and M. A. DUGUAY
Bell Telephone Laboratories,
Incorporated,
Murray Hill, New Jersey,
07974, USA
Received 21 January 1969
The two-photon fluorescence pattern produced by ultrashort pulses from a dye mode-locked Nd:glass laser is shown to be a sharply spiked curve (spike width M 0.25 psec) with a peak contrast of 2.95+0.25. This removes a long-standing discrepancy between theory and experiment.
The duration of the light pulses generated by the dye mode-locked Nd:glass laser [l] has been measured by several authors [2-61 and has ranged from 4 to 16 psec. The two-photon fluorescence technique [3] has been widely used to display such pulses. Theory predicts [4,7] that the peak contrast ratios measured in such two-photon fluorescence experiments should be 3. The theory has found support in experiments done with a single mode ruby laser [4] and a Q-switched Nd:YaG laser [8]. In the case of the Nd:glass laser, a discrepancy has persisted between the theoretical peak contrast of 3 and experimental values which have ranged from 1.7 to 2.0 [4,6]. We report a measurement of the two-photon fluorescence display produced by the dye mode-locked Nd:glass laser in which a sharp component (or peak), N 0.25 psec wide, is observed, and displays a peak contrast of 3, thus resolving the discrepancy and implying the existence of subpicosecond components in the Nd:glass laser. The experimental arrangement is shown in the figure. The laser consists of a Brewster *Present address: General Telephone and Electronics Labs, Inc. Bayside, New York 11360, USA. 698
angled rod 20X 1.2 cm2 placed between two wedged and Q-switched by the Eastman Kodak #9860 dye. The ultra-short pulses emerge 3.5 nsec apart in a train - 100 nsec long. A beam splitter divides each ultra-short pulse in two which then cross in a cell containing a saturated solution of Rhodamine 6G in dichloroethane. In contrast to previous two-photon fluorescence experiments, the cell employed here is very thin; it consists of two antireflection coated optical windows spaced by a 28~ Mylar film. Thus, photomultiplier #l monitors the fluorescence yield produced in a 28 1 slice of the usual twophoton fluorescence display [3]. The second photomultiplier monitors the fluorescence due to a single passage of the short pulses in 5 mm thick cell placed just before the focus of a 40 cm focal lenght lens, thereby providing the usual reference signal. The two-photon fluorescence pattern is scanned by moving the thin cell along the direction M2 to Ml, and plotting the ratio of the pulse height from PM #l to that from PM #2 as a function of distance. The result is shown in the figure. Ratios obtained have been normalized to 1 in the wings to conform with units defined in ref. 4
Volume
28A, number
10
PHYSICS
LETTERS TPF
arrangement: the laser from the right.
0
beam
I
I
1
2
3
4 POSITION
where the two-photon fluorescence yield due to a single pass was assigned a value of f. We note that the peak two-photon fluorescence yield or contrast is now 2.95 f 0.25 in excellent agreement with the theoretical value of 3. The part of the curve where yields are >2 is very sharp, being only 75 f 25~ wide at the two-photon fluorescence level of 2.5, corresponding to a time interval of 0.25 psec. Such a sharp spike could easily have been blurred out in previous two-photon fluorescence experiments [3-61, all performed photographically. The shape of the two-photon fluorescence curve does not uniquely determine the shape of the laser pulse [4,7]. The distance over which two-photon fluorescence exceeds 1 corresponds, as in Armostrong’s work [2], to the overall length of the pulse, - 6 psec. The narrow two-photon fluorescence spike implies that within this 6 psec pulse (or burst) there is one or more components (or peaks) with widths 0.2-0.6 psec. For example, the pulse could consist of one sharp 0.3 psec peak followed by a long -6 psec tall. Other and more complex models also fit the same two-photon fluorescence curve, however. The narrow components account for a sizeable fraction of the 100-200 cm-l spectral widths which these pulses possess [1,3,5]. This is in contrast to recent work [9] where the spectral width was reported to be largely due to an FM modulation in the form of a frequency sweep. The present experiment does not exclude FM modulation because two-pho-
1969
YIELD
I
Fig. 1. Experimental comes
24 February
5
6
7
(mm)
Fig. 2. Two-photon fluorescence curve obtained for the dye mode-locked Nd:glass laser.
ton fluorescence is insensitive to it [4,7]. When the Q-switching dye was removed from the Nd:glass laser (free-running case), we found a Gaussian shaped peak = 300~ wide with twophoton fluorescence yields of 2.8 f 0.2 at the peak and 2.0 f 0.2 far out in the wings. This is in agreement with what the theory predicts for a collection of randomly phased modes covering the spectral width observed in this case, viz. -20 cm -I. We would like to thank I. Freund, J. K. Galt, A. Giordmaine, J. R. Klauder and Robert C. Miller for stimulating discussions.
J.
References D. A.Stetser and H. Heynau, Appl. 1. A. J.DeMaria, Phys. Letters 8 (1966) 176. Appl. Phys. Letters 10 (1967) 16. 2. J. A. Armstrong, P. M. Rentzepis, S. L. Shapiro and 3. J. A. Giordmaine, K. W. Wecht. ADIJ~.Phvs. Letters 11 (196’7) 216. J. A. Giordmaine and 4. J. R. Klauder, M. A. D&ray, S. L. Shapiro, Appl. Phys.Letters 13 (1968) 174. 5. A. J. DeMaria, W. H. Glenn, M. Brienza and M. E. Mack. to be published in IEEE J. Quant. Elect. January 1969. 6. G. Kachen, L. Steinmet:< and J. Kysilka, Appl. Phys. Letters 13 (1968) 229. 7. H. P. Weber, Phys. Letters 27A (1968) 321. 8. S. K. Kurtz and S. L. Shapiro, Phys. Letters 28A (1968) 17. Phys. Letters 28A (1968) 34. 9. E. B. Treaty,
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699