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Subpicosecond structure in Nd: Glass laser relaxation oscillations
Volume
28A.
number
7
PHYSICS
non scattering surface were determined in the region of the symmetry directions [OOl], [Oil] and [ill]. Points in these directions were derived
13 January 1969
LETTERS
Reference 1. F. Gompf, W. Reichardt.
kurts, Neutron inelastic Vienna. 1968). p.417.
by interpolation. The curves
W. Glaser scattering,
and K. H. BecVol. II, (IAEA
in fig. 2 represent a third neighbour Born- von K&man fit to the measured phonons in the symmetry directions. The interatomic force constants are given in table 1. A more complete discussion of the lattice dynamics of Ag will be published later.
Nd:
SUBPICOSECOND STRUCTURE IN GLASS LASER RELAXATION OSCILLATIONS
D. J. BRADLEY, G. H. C. NEW, B. SUTHERLAND, S. J. CAUGHEY Department
of Pure
and Applied Belfast
Physics? The Queen’s BT7 INN, N. Ireland
Received
Subpicosecond structure (3 x lo-I3 employing two photon fluorescence ported.
4 December
set) has been observed display inside the laser
We wish to report the observation of subpicosecond structure in the output of a Nd:glass laser operating essentially in the free running mode. The appearance of structure of -3 psec in duration, as measured by two photon fluorescence displays [1], in the relaxation oscillation spikes of a Nd:glass laser has recently been reported [2]. Our laser system consists of a 5” X 4” Brewster angled rod in a 70 cm cavity terminated by a 100% dielectric multilayer reflector and a plane parallel fused quartz plate coated on both sides by 30% and 60% reflectivity dielectric mirror respectively. Inserting a 2 cm long glass cell containing a solution of Rhodamine 6G in dichloroethane inside the laser cavity, adjacent to the 100% mirror, produced a weak Q-switching action. It was found that the effect was due not to the two photon fluorescence dye itself but to the organic solvent [3-51. The number of laser spikes was thus reduced from about 100 to 20 and the durations of individual spikes from 500 nsec to - 150 nsec. Regular structure was observed in the two photon fluorescence photographs when the laser 532
University
of Belfast,
1968
in the output spikes of a Nd:glass laser by cavity. Contrast measurements are also re-
was pumped about 50% above threshold. Both the widths of individual fluorescence peaks and the spacing between them varied for different filaments of the laser beam but regular structure, in the two photon fluorescence photographs (Ilford FP4 or Kodak Tri-X Pan films), of 50 pm width was consistently obtained. This corresponds to a time duration of 3 X lo-l3 set when allowance is made for the refractive index of the dye solut-
Fig. 1. Microdensitometer trace of two-photon-fluorescence display inside cavity of free running Nd:glass laser (Ordinate : arbitrary linear density scale).
Volume
28A.
number
7
PHYSICS
ion and the shortering inherent in the nonlinear two photon fluorescence display. A typical microdensitometer trace is shown in fig. 1. While the structure detail varied from shot to shot, the spacing could al\kays be related to the loop time in the 1.2 cm out@ut mirror flat, or to some submultiple of this time. This structure width is substantially shorter than any reported previously even from modelocked Nd:glass laser [6]. The corresponding Fourier transform frequency bandwidth of 100 cm-l agrees with the directly measured spectrum. This spectrum was strongly channelled, as expected, with a spacing corresponding to the spectral resonances of the output flat. Simultaneous two photon fluorescence displays outside the laser cavity in a dye cell with a 100% reflectivity mirror, were also recorded. Theoretically the two photon fluorescence structure should be indentiical, apart from intensity, in both records. However, while the filamentary structure was indentical, we have always found the two photon fluorescence display track definition to be much inferior for the cell outside the cavity and the fine subpiscosecond structure was practically unresolved. While our results definitely show that the whole bandwidth of the laser was oscillating simultaneously, it is not clear whether the structure originates from isolated subpicosecond pulses or from noise fluctuations. According to theory [7], the contrast ratip of the bright spots relative to that of the background should equal 3 for a fully separated pulse. For a frequency distribution of equally spaced modes of random phase the contrast ratio should be 1.5 [8]. As a check, contrast measurements were carried out by covering half the camera field with a neutral density filter chosen to give an i*ensity reduction, at the fluorescence wavelength, of 1.5. In all cases the measured contrast ratio was less than 1.5. However similar measurements we have made on modelocked pulse trains have so far yielded contrast ratios between 1.5 and 2, substantially less than the theoretical prediction but in agreement with results recently reported [9]. In addition, there are considerable practical difficulties involved in photographic recording of self-luminous struc-
LETTERS
13 January
1969
ture as fine as 50 pm. We find that accurate alignment and focusing of the camera is essential to obtain high definition photographs and small maladjustments lead to blurring to the image, with a consequent reduction in contrast. Finally, there is the uncertainty arising from the fact that the two photon fluorescence records represent the time integral of a succession of laser spikes. Time resolved two photon fluorescence photographs taken with an image-tube streak camera show that two thirds of the laser spikes do not produce a well-defined structure. Moreover there seems to be no correlation between the appearance of picosecond structure in the two photon fluorescence display and the appearance of nanosecond modulation in the corresponding oscillogram (Tektronix 519 travelling wave oscilloscope and coaxial photodiode of 1 GHz combined bandwidth). Since there is a considerable degree of overlap of filaments from different laser spikes in the time-integrated photographs, a reduction in contrast is to be expected. However we do not feel that this explains the low contrast in every case. In conclusion, while it can be said with certainty that some, if not all, of the laser spikes contain the entire bandwith of the laser medium, whether the subpicosecond structure originates from isolated pulses or from noise is still not clear. Neither can we say if the presence of the dye cell inside the laser cavity plays an appreciable role.
References P. M. Rentzepis, S. L. Shapiro and 1. J. A. Giordmaine. K. W. Wecht. App. Phys. Letters 11 (1967) 216. M. A. Lkguay and L. B. Kreuzer, App. 2. S. L. Shapiro, Phys. Letters 12 (1968) 36. A. Yariv, App. Phys. Letters 13 3. J. F. Laussade, (1968) 65. J. Katsenstein and A. C. Selden Opto4. &. Magyar, Electronics, to be published. 56 (1968) 1613. 5. F. Gires and B. Seep, Proc. I.E.E.E. and M. A. Duguay, Appl. Phys. Letters 6. P.M. Renzepis 11 (1967) 218. 7. H. P. Weber, Phys. Letters 27A (1968) 321. Phys. Letters 28A (1968) 8. S. K.Kurtz and S. L. Shapiro, 17. M. A. Duguay, J. A. Giordmaine and 9. J. R. Klauder. S. L. Shapiro, App. Phys. Letters 13 (1968) 174.
533
28A.
number
7
PHYSICS
non scattering surface were determined in the region of the symmetry directions [OOl], [Oil] and [ill]. Points in these directions were derived
13 January 1969
LETTERS
Reference 1. F. Gompf, W. Reichardt.
kurts, Neutron inelastic Vienna. 1968). p.417.
by interpolation. The curves
W. Glaser scattering,
and K. H. BecVol. II, (IAEA
in fig. 2 represent a third neighbour Born- von K&man fit to the measured phonons in the symmetry directions. The interatomic force constants are given in table 1. A more complete discussion of the lattice dynamics of Ag will be published later.
Nd:
SUBPICOSECOND STRUCTURE IN GLASS LASER RELAXATION OSCILLATIONS
D. J. BRADLEY, G. H. C. NEW, B. SUTHERLAND, S. J. CAUGHEY Department
of Pure
and Applied Belfast
Physics? The Queen’s BT7 INN, N. Ireland
Received
Subpicosecond structure (3 x lo-I3 employing two photon fluorescence ported.
4 December
set) has been observed display inside the laser
We wish to report the observation of subpicosecond structure in the output of a Nd:glass laser operating essentially in the free running mode. The appearance of structure of -3 psec in duration, as measured by two photon fluorescence displays [1], in the relaxation oscillation spikes of a Nd:glass laser has recently been reported [2]. Our laser system consists of a 5” X 4” Brewster angled rod in a 70 cm cavity terminated by a 100% dielectric multilayer reflector and a plane parallel fused quartz plate coated on both sides by 30% and 60% reflectivity dielectric mirror respectively. Inserting a 2 cm long glass cell containing a solution of Rhodamine 6G in dichloroethane inside the laser cavity, adjacent to the 100% mirror, produced a weak Q-switching action. It was found that the effect was due not to the two photon fluorescence dye itself but to the organic solvent [3-51. The number of laser spikes was thus reduced from about 100 to 20 and the durations of individual spikes from 500 nsec to - 150 nsec. Regular structure was observed in the two photon fluorescence photographs when the laser 532
University
of Belfast,
1968
in the output spikes of a Nd:glass laser by cavity. Contrast measurements are also re-
was pumped about 50% above threshold. Both the widths of individual fluorescence peaks and the spacing between them varied for different filaments of the laser beam but regular structure, in the two photon fluorescence photographs (Ilford FP4 or Kodak Tri-X Pan films), of 50 pm width was consistently obtained. This corresponds to a time duration of 3 X lo-l3 set when allowance is made for the refractive index of the dye solut-
Fig. 1. Microdensitometer trace of two-photon-fluorescence display inside cavity of free running Nd:glass laser (Ordinate : arbitrary linear density scale).
Volume
28A.
number
7
PHYSICS
ion and the shortering inherent in the nonlinear two photon fluorescence display. A typical microdensitometer trace is shown in fig. 1. While the structure detail varied from shot to shot, the spacing could al\kays be related to the loop time in the 1.2 cm out@ut mirror flat, or to some submultiple of this time. This structure width is substantially shorter than any reported previously even from modelocked Nd:glass laser [6]. The corresponding Fourier transform frequency bandwidth of 100 cm-l agrees with the directly measured spectrum. This spectrum was strongly channelled, as expected, with a spacing corresponding to the spectral resonances of the output flat. Simultaneous two photon fluorescence displays outside the laser cavity in a dye cell with a 100% reflectivity mirror, were also recorded. Theoretically the two photon fluorescence structure should be indentiical, apart from intensity, in both records. However, while the filamentary structure was indentical, we have always found the two photon fluorescence display track definition to be much inferior for the cell outside the cavity and the fine subpiscosecond structure was practically unresolved. While our results definitely show that the whole bandwidth of the laser was oscillating simultaneously, it is not clear whether the structure originates from isolated subpicosecond pulses or from noise fluctuations. According to theory [7], the contrast ratip of the bright spots relative to that of the background should equal 3 for a fully separated pulse. For a frequency distribution of equally spaced modes of random phase the contrast ratio should be 1.5 [8]. As a check, contrast measurements were carried out by covering half the camera field with a neutral density filter chosen to give an i*ensity reduction, at the fluorescence wavelength, of 1.5. In all cases the measured contrast ratio was less than 1.5. However similar measurements we have made on modelocked pulse trains have so far yielded contrast ratios between 1.5 and 2, substantially less than the theoretical prediction but in agreement with results recently reported [9]. In addition, there are considerable practical difficulties involved in photographic recording of self-luminous struc-
LETTERS
13 January
1969
ture as fine as 50 pm. We find that accurate alignment and focusing of the camera is essential to obtain high definition photographs and small maladjustments lead to blurring to the image, with a consequent reduction in contrast. Finally, there is the uncertainty arising from the fact that the two photon fluorescence records represent the time integral of a succession of laser spikes. Time resolved two photon fluorescence photographs taken with an image-tube streak camera show that two thirds of the laser spikes do not produce a well-defined structure. Moreover there seems to be no correlation between the appearance of picosecond structure in the two photon fluorescence display and the appearance of nanosecond modulation in the corresponding oscillogram (Tektronix 519 travelling wave oscilloscope and coaxial photodiode of 1 GHz combined bandwidth). Since there is a considerable degree of overlap of filaments from different laser spikes in the time-integrated photographs, a reduction in contrast is to be expected. However we do not feel that this explains the low contrast in every case. In conclusion, while it can be said with certainty that some, if not all, of the laser spikes contain the entire bandwith of the laser medium, whether the subpicosecond structure originates from isolated pulses or from noise is still not clear. Neither can we say if the presence of the dye cell inside the laser cavity plays an appreciable role.
References P. M. Rentzepis, S. L. Shapiro and 1. J. A. Giordmaine. K. W. Wecht. App. Phys. Letters 11 (1967) 216. M. A. Lkguay and L. B. Kreuzer, App. 2. S. L. Shapiro, Phys. Letters 12 (1968) 36. A. Yariv, App. Phys. Letters 13 3. J. F. Laussade, (1968) 65. J. Katsenstein and A. C. Selden Opto4. &. Magyar, Electronics, to be published. 56 (1968) 1613. 5. F. Gires and B. Seep, Proc. I.E.E.E. and M. A. Duguay, Appl. Phys. Letters 6. P.M. Renzepis 11 (1967) 218. 7. H. P. Weber, Phys. Letters 27A (1968) 321. Phys. Letters 28A (1968) 8. S. K.Kurtz and S. L. Shapiro, 17. M. A. Duguay, J. A. Giordmaine and 9. J. R. Klauder. S. L. Shapiro, App. Phys. Letters 13 (1968) 174.
533