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Controlling the width of picosecond laser pulses
OPTICS COMMUNICATIONS
Volume 14, number 2
CONTROLLING
THE WIDTH OF PICOSECOND LASER PULSES
H. AL-OBAIDI*,
R.J. DEWHURST**, D. JACOBY, G.A. OLDERSHAW* and S.A. RAMSDEN
June 197.5
Department of Applied Physics, University of Hull, Hull, HU6 7RX, UK
Received 12 March 1975
The relaxation time rR of the saturable dye used to mode-lock a Nd:YAG laser has been changed using different dyes or dye solvent mixtures and the laser bandwidth Aw changed by the insertion of an etalon. The pulse duration 7p was approximately transform-limited for rR < 2n/Aw but increased to about twice this value when 2nlA.w < 7p < 7R. No significant increase in pulse duration was observed for rR > 2rr/Aw but multiple pulses were generated within each round-trip-transit-
The compound bis(4-dimethylaminodithiobenzil) nickel (BDN) has been shown to be a suitable saturable absorber for Q-switching the Nd:glass laser [l]. Moreover, the rate at which the molecule returns to the ground state following excitation at 1.06 pm is different in different solvents [2.-S], the relaxation time rR being typically a few nanoseconds in chloroalkanes but much shorter in solvents containing iodine or in others such as pyridine, methyl sulphoxide or ethyl sulphide. It has been suggested that it is possible to tailor the duration of mode-locked pulses by variation of the relaxation time of BDN through the choice of solvent or solvent mixture [2 1, and that transformlimited pulses without substructure can be produced by “tuning” the relaxation time of the saturable absorber to match the spectral bandwidth of the laser [3]. In the case of Nd:YAG laser pulses, the relaxation time of the dyes generally used for mode-locking, Kodak 9860 and 9740, fulfil the condition rR < 2n/Ao, where Aw is the lasing bandwidth. By using BDN dissolved in a suitable solvent, however, it is possible to reach a situation where rR > 2n/Ao. We have used a high speed streak camera to measure the effect of rR on the durations rp of mode-locked pulses produced from the Nd:YAG laser. rR was varied by using * Now with the Atomic Energy Commission, Bagdad, Iraq. ** Now at the Department of Natural Philosophy, University of Strathclyde, UK. * Department of Chemistry, University of Hull, UK.
BDN dissolved in mixtures of iodoethane and dichloromethane in different proportions, and the resulting val. ues of rp were compared with those obtained using Kodak dye 9860. The Nd:YAG laser used to generate the mode-locked train of picosecond pulses is shown in fig. 1. It consisted of a 2’ wedged 3” long, l/4” diameter Nd:YAG rod pumped by a linear flashlamp with a 60% reflectivity mirror forming the output coupler. The dye used for mode-locking, either Kodak dye 9860 or BDN, flowed through a 1 mm thick dye cell in contact with the 100% reflectivity mirror. A 2 mm aperture restricted the laser to single transverse mode operation, and the 4 m lens. together with repetitive pumping at approximately once every ten seconds, helped achieve reproducible mode-locking. Two-photon fluorescence (TPF) measurements taken previously had shown that around the peak of the train of pulses, the duration of the pulses from the Nd:YAG oscillator did not depend on the relative posi. tion of the switched-out pulse, in contrast to pulses from a Nd:glass laser [6,7]. Therefore, to minimize synchronisation difficulties, the Imacon 600 streak camera was triggered directly from a photodiode monitoring the whole mode-locked train. A high trigger level on the camera ensured that the camera streaked close to the middle of the laser train, the streaked pulses being recorded on 10,000 ASA Polaroid or 70 mm Eastman Kodak 2485 Film. To calibrate the 219
Volume 14, number 2
OPTICS COMMUNICATIONS
June 1975
CELL
Fig. 1. Experimental arrangement.
streak speed an optical delay line, consisting of a quartz flat, was used to produce from each pulse in the mode-locked train two pulses separated by 130 ps. The pulses were then focused directly onto the Sl photocathode of the camera using a 3 cm focal length cylindrical lens, appropriate attenuation being provided by neutral density filters placed in front of the lens. Fig. 2 shows a streak photograph of such a pair of picosecond pulses produced using Kodak dye 9860 as the saturable absorber. Pulse durations were always much greater than the time resolution (- 3~s) of the camera. The results of the measurements of pulse duration are summarised in table 1. The shortest pulses were obtained using Kodak dye 9860 as the saturable absorber which corresponds to the case rR < 2n/Aw. Pulse durations of 29 * 8 ps were obtained, in agreement with earlier TPF measurements. Such durations correspond to a lasing bandwidth of 1.Of 0.3 A which is close to the expected bandwidth of Nd:YAG [8] and we therefore conclude that the pulses were practically transform-limited. Microdensitometer traces of the streaked-out pulses verified that they were symmetrical and without modulation. Rates of relaxation of bleaching in BDN solution 220
were examined using pulse trains mode-locked with Kodak dye 9860. The recovery of absorption at 1.06 cun following bleaching by each pulse in the train was monitored with a photodiode by using an optically delayed part of the bleaching pulse as a probe, and by varying the delay between bleaching and probe pulses. This yielded rR = 2&l ns for BDN dissolved in dichloromethane, and the other values in table 1 were calculated by linear interpolation of ril between this value and that for BDN dissolved in iodoethane [4,5], Using BDN in iodoethane as the saturable absorber in the Nd:YAG laser, rR was then made somewhat larger than 2n/Aw. The length of the laser pulses was increased to 55+15 ps, showing them to be temporally broadened to about twice the transform-limited duration, when 27r/Ao < rp < rR . Only one pulse was generated per round-trip-transit-time. When rR was increased further, the pulse duration did not increase significantly even though rR was made about 20 times larger than 2n/Ao by using BDN dissolved in a mixture of iodoethane and dichloromethane. Moreover, in this condition, rR % 271/Aw, multiple pulses were generated within each round-triptransit-time of the laser cavity, even though oscillograms of the mode-locked trains, monitored using an ITT F4000 photodiode and 5 19 oscilloscope, indicated
Volume 14, number 2
OPTICS COMMUNICATIONS
June 1975
good mode-locking. For cases in which multiple pulses were observed, the durations listed in table 1 are typical of each pulse. Their time separation varied. For example, for BDN dissolved in a mixture of 25% iodoethane and 75% dichloromethane, the time between pulses was sometimes as long as 500 ps, which we note is still within the relaxation time of the dye. Also shown in table 1 are values of pulse durations observed when a 6 mm thick quartz etalon was placed inside the laser resonator. The etalon served to reduce the laser bandwidth and therefore increase the pulse length. The same qualitative effect of rR on the duration of the laser pulses was observed as in the experiments without the etalon. For rR 4 2n/Aw, the time duration of the pulses was close to the bandwidth limited value of 140+20 psec. For rR m 27r/Ao some pulse broadening was observed whilst for rR > 2n/A~ no further broadening occurred and multiple pulses within each round-trip-transit-time were sometimes observed. These results are in qualitative agreement with the theoretical analysis carried out by Kryukov and Letokhov [lo] who showed that the probability of more than one pulse per round-trip-transit-time increases sharply when rR > 2n/Ao. Similar behaviour has been observed in the case of a mode-locked ruby laser [11,12]. SCALE
1
We would like to thank J. Hadland (P.I.) Ltd., for the use of an Sl Imacon 600 streak camera, and K.J. Toyne for the synthesis of BDN. Financial support was obtained under SRC grants B/RG/1937/3 and B/RG/195.
lOOps/div
References 111K.H. Drexhage and U.T. Miiller-Westerhoff, IEEE J. Quant. Electr. QE-8 (1972) 759.
PI K.H. Drexhage and G.A. Reynolds, Opt. Commun. 10
SCALE
-
lOOps/div
Fig. 2. A streaked-out double pulse from the quartz flat, together with the corresponding microdensitometer trace.
l
(1974) 18. [31 D. Magde, B.A. Bushaw and M.W. Windsor, IEEE J. Quant. Electr. QE-10 (1974) 394. [41 D. Magde,B.A.Bushawand M.W.Windsor,Chem.Phys. Lett. 28 (1974) 263. [51 R.C. Greenhow and A.J. Street, IEEE J. Quant. Electr. QE-11 (1975) 59-60. [61 M.C. Richardson, IEEE J. Quant. Electr. QE-9 (1973) 768. (71 D.J. Bradley and W. Sibbett, Opt. Commun. 9 (1973) 17.
221
Volume 14, number 2
June 1975
OPTICS COMMUNICATIONS
Table 1 Summary of results Dye and solvents
Ko,dak dye 9860 in dichloromethane
Relaxation time, 7R
rP
Pulse duration with etalon,
(PS)
(Psi
rP (PSI
6-9
29t8
14Oi20
55*15
270+55
9oi35$
300*55 130*20* 145*20*
Pulse duration,
PI BDN in iodoethane
170tllO I4Sl
BDN in iodoethane:dichloromethane, BDN in iodoethane:dichloromethane, BDN in iodoethane:dichloromethane,
ratio 1: 1 by volume ratio 1: 3 by volume ratio 1:7 by volume
310*150 550*250 850* 300
55*25*
* Multiple pulses observed.
[E] W.A. Specht, J.K. Neeland and V. Evtuhov, IEEE J. Quant. Electr. QE-2 (1966) 537. [9] R.I. Scarlet, J.F. Figueria and H. Mahr, Lab. of Atomic and Solid-State Physics, Cornell Univ., Ithaca, Tech. Rept. (1968) 24. [IO] P.G. Kryukov and U.S. Letokhov, IEEE. J. Quant. Electr. QE-8 (1974) 766.
222
[ll]
P.G. Kryukov, Yu.A. Matvcev, S.A. Churilova and O.B. Shatberashvili, Soviet Physics JETP 35 (1972) 1062. [12] E.G. Arthurs, D.J. Bradley, T.J. Glynn, Opt. Commun. 12 (1974) 136.
Volume 14, number 2
CONTROLLING
THE WIDTH OF PICOSECOND LASER PULSES
H. AL-OBAIDI*,
R.J. DEWHURST**, D. JACOBY, G.A. OLDERSHAW* and S.A. RAMSDEN
June 197.5
Department of Applied Physics, University of Hull, Hull, HU6 7RX, UK
Received 12 March 1975
The relaxation time rR of the saturable dye used to mode-lock a Nd:YAG laser has been changed using different dyes or dye solvent mixtures and the laser bandwidth Aw changed by the insertion of an etalon. The pulse duration 7p was approximately transform-limited for rR < 2n/Aw but increased to about twice this value when 2nlA.w < 7p < 7R. No significant increase in pulse duration was observed for rR > 2rr/Aw but multiple pulses were generated within each round-trip-transit-
The compound bis(4-dimethylaminodithiobenzil) nickel (BDN) has been shown to be a suitable saturable absorber for Q-switching the Nd:glass laser [l]. Moreover, the rate at which the molecule returns to the ground state following excitation at 1.06 pm is different in different solvents [2.-S], the relaxation time rR being typically a few nanoseconds in chloroalkanes but much shorter in solvents containing iodine or in others such as pyridine, methyl sulphoxide or ethyl sulphide. It has been suggested that it is possible to tailor the duration of mode-locked pulses by variation of the relaxation time of BDN through the choice of solvent or solvent mixture [2 1, and that transformlimited pulses without substructure can be produced by “tuning” the relaxation time of the saturable absorber to match the spectral bandwidth of the laser [3]. In the case of Nd:YAG laser pulses, the relaxation time of the dyes generally used for mode-locking, Kodak 9860 and 9740, fulfil the condition rR < 2n/Ao, where Aw is the lasing bandwidth. By using BDN dissolved in a suitable solvent, however, it is possible to reach a situation where rR > 2n/Ao. We have used a high speed streak camera to measure the effect of rR on the durations rp of mode-locked pulses produced from the Nd:YAG laser. rR was varied by using * Now with the Atomic Energy Commission, Bagdad, Iraq. ** Now at the Department of Natural Philosophy, University of Strathclyde, UK. * Department of Chemistry, University of Hull, UK.
BDN dissolved in mixtures of iodoethane and dichloromethane in different proportions, and the resulting val. ues of rp were compared with those obtained using Kodak dye 9860. The Nd:YAG laser used to generate the mode-locked train of picosecond pulses is shown in fig. 1. It consisted of a 2’ wedged 3” long, l/4” diameter Nd:YAG rod pumped by a linear flashlamp with a 60% reflectivity mirror forming the output coupler. The dye used for mode-locking, either Kodak dye 9860 or BDN, flowed through a 1 mm thick dye cell in contact with the 100% reflectivity mirror. A 2 mm aperture restricted the laser to single transverse mode operation, and the 4 m lens. together with repetitive pumping at approximately once every ten seconds, helped achieve reproducible mode-locking. Two-photon fluorescence (TPF) measurements taken previously had shown that around the peak of the train of pulses, the duration of the pulses from the Nd:YAG oscillator did not depend on the relative posi. tion of the switched-out pulse, in contrast to pulses from a Nd:glass laser [6,7]. Therefore, to minimize synchronisation difficulties, the Imacon 600 streak camera was triggered directly from a photodiode monitoring the whole mode-locked train. A high trigger level on the camera ensured that the camera streaked close to the middle of the laser train, the streaked pulses being recorded on 10,000 ASA Polaroid or 70 mm Eastman Kodak 2485 Film. To calibrate the 219
Volume 14, number 2
OPTICS COMMUNICATIONS
June 1975
CELL
Fig. 1. Experimental arrangement.
streak speed an optical delay line, consisting of a quartz flat, was used to produce from each pulse in the mode-locked train two pulses separated by 130 ps. The pulses were then focused directly onto the Sl photocathode of the camera using a 3 cm focal length cylindrical lens, appropriate attenuation being provided by neutral density filters placed in front of the lens. Fig. 2 shows a streak photograph of such a pair of picosecond pulses produced using Kodak dye 9860 as the saturable absorber. Pulse durations were always much greater than the time resolution (- 3~s) of the camera. The results of the measurements of pulse duration are summarised in table 1. The shortest pulses were obtained using Kodak dye 9860 as the saturable absorber which corresponds to the case rR < 2n/Aw. Pulse durations of 29 * 8 ps were obtained, in agreement with earlier TPF measurements. Such durations correspond to a lasing bandwidth of 1.Of 0.3 A which is close to the expected bandwidth of Nd:YAG [8] and we therefore conclude that the pulses were practically transform-limited. Microdensitometer traces of the streaked-out pulses verified that they were symmetrical and without modulation. Rates of relaxation of bleaching in BDN solution 220
were examined using pulse trains mode-locked with Kodak dye 9860. The recovery of absorption at 1.06 cun following bleaching by each pulse in the train was monitored with a photodiode by using an optically delayed part of the bleaching pulse as a probe, and by varying the delay between bleaching and probe pulses. This yielded rR = 2&l ns for BDN dissolved in dichloromethane, and the other values in table 1 were calculated by linear interpolation of ril between this value and that for BDN dissolved in iodoethane [4,5], Using BDN in iodoethane as the saturable absorber in the Nd:YAG laser, rR was then made somewhat larger than 2n/Aw. The length of the laser pulses was increased to 55+15 ps, showing them to be temporally broadened to about twice the transform-limited duration, when 27r/Ao < rp < rR . Only one pulse was generated per round-trip-transit-time. When rR was increased further, the pulse duration did not increase significantly even though rR was made about 20 times larger than 2n/Ao by using BDN dissolved in a mixture of iodoethane and dichloromethane. Moreover, in this condition, rR % 271/Aw, multiple pulses were generated within each round-triptransit-time of the laser cavity, even though oscillograms of the mode-locked trains, monitored using an ITT F4000 photodiode and 5 19 oscilloscope, indicated
Volume 14, number 2
OPTICS COMMUNICATIONS
June 1975
good mode-locking. For cases in which multiple pulses were observed, the durations listed in table 1 are typical of each pulse. Their time separation varied. For example, for BDN dissolved in a mixture of 25% iodoethane and 75% dichloromethane, the time between pulses was sometimes as long as 500 ps, which we note is still within the relaxation time of the dye. Also shown in table 1 are values of pulse durations observed when a 6 mm thick quartz etalon was placed inside the laser resonator. The etalon served to reduce the laser bandwidth and therefore increase the pulse length. The same qualitative effect of rR on the duration of the laser pulses was observed as in the experiments without the etalon. For rR 4 2n/Aw, the time duration of the pulses was close to the bandwidth limited value of 140+20 psec. For rR m 27r/Ao some pulse broadening was observed whilst for rR > 2n/A~ no further broadening occurred and multiple pulses within each round-trip-transit-time were sometimes observed. These results are in qualitative agreement with the theoretical analysis carried out by Kryukov and Letokhov [lo] who showed that the probability of more than one pulse per round-trip-transit-time increases sharply when rR > 2n/Ao. Similar behaviour has been observed in the case of a mode-locked ruby laser [11,12]. SCALE
1
We would like to thank J. Hadland (P.I.) Ltd., for the use of an Sl Imacon 600 streak camera, and K.J. Toyne for the synthesis of BDN. Financial support was obtained under SRC grants B/RG/1937/3 and B/RG/195.
lOOps/div
References 111K.H. Drexhage and U.T. Miiller-Westerhoff, IEEE J. Quant. Electr. QE-8 (1972) 759.
PI K.H. Drexhage and G.A. Reynolds, Opt. Commun. 10
SCALE
-
lOOps/div
Fig. 2. A streaked-out double pulse from the quartz flat, together with the corresponding microdensitometer trace.
l
(1974) 18. [31 D. Magde, B.A. Bushaw and M.W. Windsor, IEEE J. Quant. Electr. QE-10 (1974) 394. [41 D. Magde,B.A.Bushawand M.W.Windsor,Chem.Phys. Lett. 28 (1974) 263. [51 R.C. Greenhow and A.J. Street, IEEE J. Quant. Electr. QE-11 (1975) 59-60. [61 M.C. Richardson, IEEE J. Quant. Electr. QE-9 (1973) 768. (71 D.J. Bradley and W. Sibbett, Opt. Commun. 9 (1973) 17.
221
Volume 14, number 2
June 1975
OPTICS COMMUNICATIONS
Table 1 Summary of results Dye and solvents
Ko,dak dye 9860 in dichloromethane
Relaxation time, 7R
rP
Pulse duration with etalon,
(PS)
(Psi
rP (PSI
6-9
29t8
14Oi20
55*15
270+55
9oi35$
300*55 130*20* 145*20*
Pulse duration,
PI BDN in iodoethane
170tllO I4Sl
BDN in iodoethane:dichloromethane, BDN in iodoethane:dichloromethane, BDN in iodoethane:dichloromethane,
ratio 1: 1 by volume ratio 1: 3 by volume ratio 1:7 by volume
310*150 550*250 850* 300
55*25*
* Multiple pulses observed.
[E] W.A. Specht, J.K. Neeland and V. Evtuhov, IEEE J. Quant. Electr. QE-2 (1966) 537. [9] R.I. Scarlet, J.F. Figueria and H. Mahr, Lab. of Atomic and Solid-State Physics, Cornell Univ., Ithaca, Tech. Rept. (1968) 24. [IO] P.G. Kryukov and U.S. Letokhov, IEEE. J. Quant. Electr. QE-8 (1974) 766.
222
[ll]
P.G. Kryukov, Yu.A. Matvcev, S.A. Churilova and O.B. Shatberashvili, Soviet Physics JETP 35 (1972) 1062. [12] E.G. Arthurs, D.J. Bradley, T.J. Glynn, Opt. Commun. 12 (1974) 136.