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Shortening of excimer laser pulses with saturable absorbers
Volume 52, number 3
OPTICS COMMUNICATIONS
1 December 1984
SHORTENING OF EXCIMER LASER PULSES WITH SATURABLE ABSORBERS Ch.G. CHRISTOV, I.V. TOMOV, I.V. CHALTAKOV Faculty of Physics, Sofla Unfvmity, BG-1126 Sofm, Bulgaria
and V.L. LYuTsKAluov Institute of Solid State Physics, B&a&n
Academy of Sciences, BE1 IS4 So_& Bulgwia
Received 25 June 1984 Revised manuscript received 7 September 1984
Shortening by a factor of more than fwe of XeCl laser pulses by a single pass saturable absorber is recorded. The pulse shortening is attributed to the combined action of both nonlinear absorption and stimulated emission from the dye solution.
The rare gas halide excimer laser systems with their wide (l-2 nm) gain bandwidth have long been recognized as attractive candidates for amplifEation and generation of picosecond light pulses in the uv. So far pulses as short as 2 ps have been amplified to a power level of 20 GW [ 11. Attempts to apply the conventional mode-locking techniques for picosecond pulse production with excimer lasers have not given satisfactory results. The shortest pulses (300 ps), produced using a mode-locking technique [2], show that more detailed studies are necessary to determine the limitations of this approach. A more sophisticated approach adopted in ref. [3] has produced powerful pulses of 5 ps duration. For many applications, however, pulses of the order of 1 ns are sufficient and this allows simpler systems for pulse shortening to be used. The most straightforward approach is to use a saturable absorber for single-pass pulse shortening. This technique has been studied theoretically and experimentally for the Nd : glass laser [4] and the results show that a pulse shortening ratio of -4-5 may be achieved at optimum conditions. Recently Varghese [5] reported a similar experiment in which 5 ns pulses of a XeCl laser have been shortened to a00 ps. His suggestion, however, that the dye saturation is responsible for such shortening cannot be explained by Penzkofer’s 0 030-4018/84/$03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
theory. In order to obtain stronger shortening one may employ two nonlinear effects taking place in a saturable dye. Those are saturation of the dye absorp tion and stimulated emission from the same dye. Note, that this technique has been applied to the passively mode-locked dye lasers by Abakunov et al. [6] in order to decrease the effective relaxation time of the absorber. The basic idea is that when a laser pulse is passed through a saturable absorber the leading front of the pulse wiIl be shortened due to the dye absorption. If, after the saturation of the dye, conditions for stimulated emission are provided, it will depopulate in a very short time the upper level and the dye absorption will be of the order of the initial absorption. Thus, the leading front of the pulse will be formed by the saturation of dye absorption and the tail will be formed by the stimulated emission of the dye. The conditions for stimulated emission may be controlled by placing the saturable dye in its own cavity with proper losses. In this paper we present some results of the study of XeCl laser pulse shortening with saturable dyes employing stimulated emission. The experimental set-up used in the experiments is similar to that of Varghese and is shown in fa. 1. The pump laser beam, selected by an aperture A had a pulse duration 211
Volume 52, number 3
Fig. 1. Schematic of the experimental
1 December 1984
OPTICS COMMUNICATIONS
set up.
of 28 ns (FWHM) and an energy of 5 mJ and was produced from a discharge-pumped XeCl laser. The beam was focused with a 25 cm focal length lens LI into a dye cell made from two parallel quartz substrates separated by 2 mm. The outer surfaces of the substrates form a low Q resonator with estimated photon lifetime of -50 ps. A second lens L2 with a 15 cm focal length was used to recollimate the beam for further measurements. The variation in the density of the input radiation was achieved by changing the position of the dye cell between the lens and its focal point. The energies of the input and output pulses were measured by two RjP-735 pyroelectric probes and energy ratiometer “Laser Precision”-Rj 7200. The pulse shape was monitored by a FEK-15 vacuum photodiode and a Tektronix466 oscilloscope. The
temporal resolution of the detection system was 5 ns, measured with picosecond pulses from the third harmonic of a passively mode-locked Nd : glass laser. The dye cell was tilted relative to the XeCl laser beam in order to spatially separate dye emission from XeCl radiation. We have performed experiments with PTP, PBD, BPBD and PPO dyes, for which the peak absorption wavelengths lie near the pumping wavelength at h = 308 nm. These dyes are known as effective laser dyes with high quantum efficiency. Some relevant parameters of the dye solutions are given in table 1. In order to ensure a relatively high transmitted signal the measurements were carried out with such concentrations of the different dyes which provided a small signal transmission of the dye cell of To = 10F2. From table 1 we find that the actual concentration of the different dyes varies up to 8 times depending on their absorption cross section for h = 308 nm. This affects the gain for lasing in the dye and in our experimental conditions lasing was observed only in a PTP solution. Fig. 2 shows the transmitted pulse oscillograms obtained when the dye cell, filled with PTP solution, was translated from the lens L, towards the focal point. The PTP solution was associated with strong dye generation and highest shortening of the pump pulse. For an input pulse of 28 ns (FWHM) the output was -5 ns (FWHM) which was the resolution
Table 1. Parameters of the dyes used in the study
Dye
Solvent
T, (ns)
oa (A’)
PBD 2-phenyl-5-(4”-biphenyl)-1,3,4-oxadiazole
ethanol
1
1.75
3.6
BPBD 2-(4’-terbytyl-pheny1)-5~4”-biphenyl)-l,3,4-oxadiazole
ethanol
1
1.65
3.8
PPO 2,5-diphenyloxazole
cydohexane
1.6
1.3
3.1
PTP p-terphenyl
cydohexane
1
0.23
ra: effective lifetime of the St level of the dye. aa: absorption cross section of the dye for h = 308 mn. Is: saturation intensity of the dye, defined asZ, = hv/oaTa.
212
Zs (MW cm-?)
28
Volume 52, number 3
1 December 1984
OPTICS COMMUNICATIONS
of the upper level. During the measurements care was taken to avoid breakdown in the dye cell and the examination of the far f*ld of the transmitted radiation did not indicate any effect of self-focusing or microbubbles formation. A possible distortion of the transmitted XeCl pulse recorded by the photodiode due to the dye superfluorescence was checked by introducing a colour filter CC-20 in front of the photodiode. The measured transmission of the filter at X = 308 nm was 2% and for the wavelength range 330400 nm it was less than 1%. The insertion of the filter resulted only in the decrease of the signal but no change of the pulse shape was recorded. Simultaneously with the transmitted XeCl laser pulse we recorded the dye generation pulse. The dye laser pulse was with a strong leading peak followed by a tail. The leading peak duration was below the resolution limit of the detection system but the overall dye laser pulse duration was comparable with that of the XeCl laser pump pulse. We assume that the dye laser pulse has a spiking nature, which is not observed because of low time resolution of the detection system. The time correlated measurements
Fig. 2. Successive oscilloscope traces of the transmitted pulse obtained when the dye cell is translated towards the focus of the lens (5 ns/small div.): a) no saturation of the dye; b) intermediate position; c) lasing in the dye. PTP solution with To = 10-Z.
limit of our registration system. The input pulse energy was 5 mJ and the corresponding transmitted pulse energy, when the dye cell was near to the focal point, was a250 d. The observed shortening of the pulse cannot be explained only by the dye saturation, if we apply the approach used in ref. [4]. The shortening of the pulse is a result of the collapse of the input pulse tail which qualitatively agrees with the concept of increased dye absorption due to stimulated emission depopulation
WAVELENGTH
cl4
Fig. 3. Ftiorescent emission spectrum of the PPO dye solution (To = 10”): a) a dye cell close to the lens; b) dye cell in the focus of the lens.
213
Volume 52, number 3
OPTICS COMMUNICATIONS
have shown that the dye laser leading peak coincides with the collapse of the XeCl pump pulse. For PBD, PPO and BPBD we did not register dye lasing, but the temporal behaviour of the transmitted pulse was similar to PTP and pulse shortening was about 2.5-3 times. In order to clarify the reason for this shortening, the fluorescent radiation from the dye cell in the direction of the pump beam was studied with an optical multichannel analyser OSA500. The evolution of the dye fluorescence when the dye cell was moved towards the focus of L, is shown in fig. 3. Fig. 3a presents the spectrum when the cell was close to L1 and fg 3b when the cell was in the focus. The spectrum narrowing is more than twice (FWHM) indicating an amplification of spontaneous emission, which may be responsible for the observed pulse shortening. In conclusion, we have observed excimer laser
214
1 December 1984
pulse shortening in a single pass dye cell. The recorded shortening of >5 times was attributed mainly to the increased absorption of the dye due to the onset of stimulated emission. Development of a theoretical model in which both saturation of absorption and stimulated emission in the dye are taken into account is underway.
References [I] P. Corkum and R.S. Taylor, IEEE J. Quant. Electron. QE-18 (1982) 1962. [ 2) M. Watanabe, S. Watanabe and A. Endoh, Optics Lett. 8 (1983) 638. [3] S. Szatmari and F.P. Safer, Optics Comm. 48 (1983) 279. [4] A. Penzkofer, Opto-Electronics 6 (1974) 87. [ 51 T. Varghese, Appl. Phys. Lett. 41(1982) 684. [6] GA. Abakumov, A. Antipov, A. Simonov, A. Sinitsyn and V. Fadeev, Sov. J. Quant. Electron. 7 (1977) 1394.
OPTICS COMMUNICATIONS
1 December 1984
SHORTENING OF EXCIMER LASER PULSES WITH SATURABLE ABSORBERS Ch.G. CHRISTOV, I.V. TOMOV, I.V. CHALTAKOV Faculty of Physics, Sofla Unfvmity, BG-1126 Sofm, Bulgaria
and V.L. LYuTsKAluov Institute of Solid State Physics, B&a&n
Academy of Sciences, BE1 IS4 So_& Bulgwia
Received 25 June 1984 Revised manuscript received 7 September 1984
Shortening by a factor of more than fwe of XeCl laser pulses by a single pass saturable absorber is recorded. The pulse shortening is attributed to the combined action of both nonlinear absorption and stimulated emission from the dye solution.
The rare gas halide excimer laser systems with their wide (l-2 nm) gain bandwidth have long been recognized as attractive candidates for amplifEation and generation of picosecond light pulses in the uv. So far pulses as short as 2 ps have been amplified to a power level of 20 GW [ 11. Attempts to apply the conventional mode-locking techniques for picosecond pulse production with excimer lasers have not given satisfactory results. The shortest pulses (300 ps), produced using a mode-locking technique [2], show that more detailed studies are necessary to determine the limitations of this approach. A more sophisticated approach adopted in ref. [3] has produced powerful pulses of 5 ps duration. For many applications, however, pulses of the order of 1 ns are sufficient and this allows simpler systems for pulse shortening to be used. The most straightforward approach is to use a saturable absorber for single-pass pulse shortening. This technique has been studied theoretically and experimentally for the Nd : glass laser [4] and the results show that a pulse shortening ratio of -4-5 may be achieved at optimum conditions. Recently Varghese [5] reported a similar experiment in which 5 ns pulses of a XeCl laser have been shortened to a00 ps. His suggestion, however, that the dye saturation is responsible for such shortening cannot be explained by Penzkofer’s 0 030-4018/84/$03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
theory. In order to obtain stronger shortening one may employ two nonlinear effects taking place in a saturable dye. Those are saturation of the dye absorp tion and stimulated emission from the same dye. Note, that this technique has been applied to the passively mode-locked dye lasers by Abakunov et al. [6] in order to decrease the effective relaxation time of the absorber. The basic idea is that when a laser pulse is passed through a saturable absorber the leading front of the pulse wiIl be shortened due to the dye absorption. If, after the saturation of the dye, conditions for stimulated emission are provided, it will depopulate in a very short time the upper level and the dye absorption will be of the order of the initial absorption. Thus, the leading front of the pulse will be formed by the saturation of dye absorption and the tail will be formed by the stimulated emission of the dye. The conditions for stimulated emission may be controlled by placing the saturable dye in its own cavity with proper losses. In this paper we present some results of the study of XeCl laser pulse shortening with saturable dyes employing stimulated emission. The experimental set-up used in the experiments is similar to that of Varghese and is shown in fa. 1. The pump laser beam, selected by an aperture A had a pulse duration 211
Volume 52, number 3
Fig. 1. Schematic of the experimental
1 December 1984
OPTICS COMMUNICATIONS
set up.
of 28 ns (FWHM) and an energy of 5 mJ and was produced from a discharge-pumped XeCl laser. The beam was focused with a 25 cm focal length lens LI into a dye cell made from two parallel quartz substrates separated by 2 mm. The outer surfaces of the substrates form a low Q resonator with estimated photon lifetime of -50 ps. A second lens L2 with a 15 cm focal length was used to recollimate the beam for further measurements. The variation in the density of the input radiation was achieved by changing the position of the dye cell between the lens and its focal point. The energies of the input and output pulses were measured by two RjP-735 pyroelectric probes and energy ratiometer “Laser Precision”-Rj 7200. The pulse shape was monitored by a FEK-15 vacuum photodiode and a Tektronix466 oscilloscope. The
temporal resolution of the detection system was 5 ns, measured with picosecond pulses from the third harmonic of a passively mode-locked Nd : glass laser. The dye cell was tilted relative to the XeCl laser beam in order to spatially separate dye emission from XeCl radiation. We have performed experiments with PTP, PBD, BPBD and PPO dyes, for which the peak absorption wavelengths lie near the pumping wavelength at h = 308 nm. These dyes are known as effective laser dyes with high quantum efficiency. Some relevant parameters of the dye solutions are given in table 1. In order to ensure a relatively high transmitted signal the measurements were carried out with such concentrations of the different dyes which provided a small signal transmission of the dye cell of To = 10F2. From table 1 we find that the actual concentration of the different dyes varies up to 8 times depending on their absorption cross section for h = 308 nm. This affects the gain for lasing in the dye and in our experimental conditions lasing was observed only in a PTP solution. Fig. 2 shows the transmitted pulse oscillograms obtained when the dye cell, filled with PTP solution, was translated from the lens L, towards the focal point. The PTP solution was associated with strong dye generation and highest shortening of the pump pulse. For an input pulse of 28 ns (FWHM) the output was -5 ns (FWHM) which was the resolution
Table 1. Parameters of the dyes used in the study
Dye
Solvent
T, (ns)
oa (A’)
PBD 2-phenyl-5-(4”-biphenyl)-1,3,4-oxadiazole
ethanol
1
1.75
3.6
BPBD 2-(4’-terbytyl-pheny1)-5~4”-biphenyl)-l,3,4-oxadiazole
ethanol
1
1.65
3.8
PPO 2,5-diphenyloxazole
cydohexane
1.6
1.3
3.1
PTP p-terphenyl
cydohexane
1
0.23
ra: effective lifetime of the St level of the dye. aa: absorption cross section of the dye for h = 308 mn. Is: saturation intensity of the dye, defined asZ, = hv/oaTa.
212
Zs (MW cm-?)
28
Volume 52, number 3
1 December 1984
OPTICS COMMUNICATIONS
of the upper level. During the measurements care was taken to avoid breakdown in the dye cell and the examination of the far f*ld of the transmitted radiation did not indicate any effect of self-focusing or microbubbles formation. A possible distortion of the transmitted XeCl pulse recorded by the photodiode due to the dye superfluorescence was checked by introducing a colour filter CC-20 in front of the photodiode. The measured transmission of the filter at X = 308 nm was 2% and for the wavelength range 330400 nm it was less than 1%. The insertion of the filter resulted only in the decrease of the signal but no change of the pulse shape was recorded. Simultaneously with the transmitted XeCl laser pulse we recorded the dye generation pulse. The dye laser pulse was with a strong leading peak followed by a tail. The leading peak duration was below the resolution limit of the detection system but the overall dye laser pulse duration was comparable with that of the XeCl laser pump pulse. We assume that the dye laser pulse has a spiking nature, which is not observed because of low time resolution of the detection system. The time correlated measurements
Fig. 2. Successive oscilloscope traces of the transmitted pulse obtained when the dye cell is translated towards the focus of the lens (5 ns/small div.): a) no saturation of the dye; b) intermediate position; c) lasing in the dye. PTP solution with To = 10-Z.
limit of our registration system. The input pulse energy was 5 mJ and the corresponding transmitted pulse energy, when the dye cell was near to the focal point, was a250 d. The observed shortening of the pulse cannot be explained only by the dye saturation, if we apply the approach used in ref. [4]. The shortening of the pulse is a result of the collapse of the input pulse tail which qualitatively agrees with the concept of increased dye absorption due to stimulated emission depopulation
WAVELENGTH
cl4
Fig. 3. Ftiorescent emission spectrum of the PPO dye solution (To = 10”): a) a dye cell close to the lens; b) dye cell in the focus of the lens.
213
Volume 52, number 3
OPTICS COMMUNICATIONS
have shown that the dye laser leading peak coincides with the collapse of the XeCl pump pulse. For PBD, PPO and BPBD we did not register dye lasing, but the temporal behaviour of the transmitted pulse was similar to PTP and pulse shortening was about 2.5-3 times. In order to clarify the reason for this shortening, the fluorescent radiation from the dye cell in the direction of the pump beam was studied with an optical multichannel analyser OSA500. The evolution of the dye fluorescence when the dye cell was moved towards the focus of L, is shown in fig. 3. Fig. 3a presents the spectrum when the cell was close to L1 and fg 3b when the cell was in the focus. The spectrum narrowing is more than twice (FWHM) indicating an amplification of spontaneous emission, which may be responsible for the observed pulse shortening. In conclusion, we have observed excimer laser
214
1 December 1984
pulse shortening in a single pass dye cell. The recorded shortening of >5 times was attributed mainly to the increased absorption of the dye due to the onset of stimulated emission. Development of a theoretical model in which both saturation of absorption and stimulated emission in the dye are taken into account is underway.
References [I] P. Corkum and R.S. Taylor, IEEE J. Quant. Electron. QE-18 (1982) 1962. [ 2) M. Watanabe, S. Watanabe and A. Endoh, Optics Lett. 8 (1983) 638. [3] S. Szatmari and F.P. Safer, Optics Comm. 48 (1983) 279. [4] A. Penzkofer, Opto-Electronics 6 (1974) 87. [ 51 T. Varghese, Appl. Phys. Lett. 41(1982) 684. [6] GA. Abakumov, A. Antipov, A. Simonov, A. Sinitsyn and V. Fadeev, Sov. J. Quant. Electron. 7 (1977) 1394.