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Streak-camera studies of picosecond pulses from a mode-locked Nd: Glass laser
Volume 9, number 1
OPTICS COMMUNICATIONS
Septembe.r 1973
STREAK-CAMERA STUDIES OF PICOSECOND PULSES FROM A MODE-LOCKED Nd:GLASS LASER D.J. BRADLEY ~ and W. SIBBETT ~ Department of Pure and Applied Physics, The Queen's University o f Belfast, Belfast BT7 INN, Northern Ireland
Received 11 July 1973
Direct measurements with a picosecond streak-camera show that 1 mJ pulses, of transform limited durations (At ~ 3 psec, ~,tav ~ 0.5), are produced at the beginning of the pulse train of a mode-locked Nd:glasslaser. Pulse durations increase to ~10 psec at the end of the train, with self-phase-modulationfrequency broadening. The optimum length of the contacted saturable absorber cell was found to be ~50 urn,in agreement with earlier work using two-photon fluorescence measurements.
Earlier investigations [ 1-3 ], employing two.photon fluorescence (TPF) measurements of the pulse structure, showed that the most reliable mode.locking and the shortest pulse durations were obtained in a modelocked Nd:glass laser when the saturable absorber cell was in contact with one of the cavity mirrors. This result was explained theoretically using the maximum emission principle [2]. In these experhnents trains lasting 150 nsec, containing pulses of average durations 2.5-8 psec, were obtained with a dye cell length of ~ 30 tan. Simultaneous third harmonic measurements [l] did not indicate any sy,~tematic variation of the pulse durations along the train but the presence of quasi.random subpicosecond intensity fluctuations was established [3]. Direct measurements [4] of single second-harmonic pulses with a picosecond electrooptical streak camera [5], confirmed the TPF results and showed that there were no satellite pulses. Later Von der Linde [6] carried out TPF measurements of amplified single pulses and found that while bandwidth.limited pulses were present in the leading part of the pulse train, substantial frequency broadening developed as the pulse train intensity envelope reached its maximum. Pulse broadening to "-'l 1 psec occurred e, Present address: Applied Optics Section, Physics Department, Imperial College, London, S.W. 7, UK.
in the trailing part of the "-600 nsec train, with the formation of subpicosecond structure. More recently measurements on ring [7] and normal [8] Nd:glass resonators, with infra-red sensitive streak-cameras of ~2.5 psec time.resolution, have confirmed that the shortest pulses ( 5 - 1 0 psec) occur in the initial part of the train. However the authors of [7] found that near the peak of the 300-1000 nsec trains, pulse durations of ~100 psec, with complicated fine structure, were recorded. The pulses of [6] and [7] had an energy content of ~0.2 mJ. Because of the discrepancy between the results of these investigators we have again investigated the laser of [ 1-3] employing a streak-camera, with a S-1 image-tube (Instrument Technology Ltd.), of time-resolution 2.5 psec [9]. The experimental arrangement is shown in fig. 1. The mode.locked laser previously described [ 1-3] was modified by the addition of a l0 cm focal length, biconcave lens, L, placed 25 cm from the 100% reuectxvlty mirror, M 1, of 35 cxrt focal length. "--"' distortion in the laser rod could thus be easily corrected for with this generalized confocal cavity [ 10]. The saturable absorber dye cell, C (Eastman Kodak 9860 in dic~orethane with 70% low level light transmission) was in contact with the 70% refleclivity plane mirror M 2. A red filter, F 1 , t'rotected the dye from photodecomposition by stray flashlight. As before, corn17
Volume 9, number 1
OPTICS COMMUNICATIONS O.L. to
streak c
~
September 1973
mera instrumental time resolution limit of 2 £ ?~ec. Near the middle of the pulse train, usually about 250 nsec from the start, there was little increase in the 10
M~
L
FI
C MZ
(a)
RC. M3
Fig. 1. Mode-locked Nd:glass laser and optical delay line (D.L) arrartgement,
pletely mode-locked pulse trains were obtained with a better than 80% success rate. Fig. 2 shows a typical oscLllogram of a train with an average energy of ~ I nd
per pulse. About ten pulses were switched out of the train by a Pockels cell shutter, P.C., operated by a fast electronic switch [ 11 ]. A monitor signal from the streakcamera voltage ramp generator located on the oscillograms the position of the pulse Which was recorded by the camera. Otherwise the arrangement for operation and calibration of the streak-camera was that used in [4] and [ 11]. Initially the fifth pulse of the train was streaked. Microdensitometry of the streak photographs (l]ford HI'4 film) consistently gave pulse durations of 3-5 psec for a dye ceU thickness of 50 tan. With an additional delay of "-50 nsec (5 pulses further along the train) the recorded pulse durations had the distribution of fig. 3a. The streak photograph of fig. 4 shows two sub-pulses generated in the optical delay line [10] (D.L. of fig. 1), from a single 3 psec pulse, and separated by l0 psec. The microdensitometer trace shows a clear separation, expected from the ca-
I
0 3
,, I,,
4 5 6 PULSE DURATION p sec
I
,
(b)
0 m
o
8
W
'" ~D
4
:3 Z
il 6
1
S tO P~LSE DURATION p SOC
, Ii t2
Fig. 3. Distributions of measured pulse durations (a) 90 nsecs from start (10 pulses along trah~) (bj 500 asecs from start.
Fig. 2. Oscillogram of mode-locked laser, recorded with a fast photodiode and travelling wave oscilloscope. Time scale 200 nsec per major division.
18
Volume 9, number 1
OPTICS COMMUNICATIONS
,..
'~
c~ o
..° ,•q
/
Fig. 4. Top, streak photograph of two 3 psec sub-pulses, separated by 10 psec, generated in the optical delay line. Bottom: microdensitometet trace showing clear separation and confirming a deconvolved camera instrumental function of 2.5 psee. average pulse duration. Pulses selected from the trailing part of the train had durations of 5-13 psec, as shown in tag. 3b. Pulse spectra were recorded by using the street camera as a simple image converter. The exit slit of a l-m spectrograph, opened to 2 mm width, replaced the input slit of the camera. The spectral bandwidths of the pulses increased rapidly from the beginning of the train as foand by Duguay [13] and in [6]. For the fifth pulse of the train, an average of 20 measurements gave At = 3.8 +--0.8 psec with Av = 2.2 ± 0.5 X 1011 Hz (AX = 7.4 A). The corresponding time-bandwidth product [14] had au average value AvAt = 0.83:1: 0.3,
September 1973
showing that the initial pulses had practically transform. limited durations [6, 14]. The limiting value AvAt = 0.5 was obtained for several 3 psec pulses. It must be emphasized that unlike [6] and [13] these results were obtained by direct linear measurement. Along the train self-phase-modulation frequency broadening [ 13, 14] built-up at the rate of 10 cm-1 per round trip, and at the end of the train the pulse spectra had half-widths of ~50 A. Finally the effect of varying the saturable absorber cell length was investigated. As can be seen in fig. 5 the shortest pulse durations were obtained with a cell length of 50/am. The results plotted in the figure are in qualitative agreement with those of [2] but for the same cell thickness the pulse durations are now considerably shorter. This improvement could be due to the use of the generalized confocal cavity, which also gives more reproducible pulse trains. We conclude from these results that transformlimited pulses of ~ I mJ energy can be generated at the beginning of a mode-locked Nd:glass laser pulse train. Further along the train the spectral bandwidth increasesconsiderably because of self.phase-modulation but even at the end of the trainpulse durations are generally < 10 psec. These directmeasurements are similar to the TPF resultsof [6] where a contacted cellwas also employed but the pulses had considerably smallerenergies.The much shorter pulses produced throughout the pulse trainof our laserconfirm the superiority of the contacted dye cell cavity over
400
t
E 300 =t,
/
I ! I I I
!
-r
/ !
200
/
f
/
U
/
~oo
/-
0 L _ _ . . _ ~ t -
0
2
i,
, I
4
PULSE
'
,
~
DUrATiON
j
L
B"
psec
Fig. 5. Measured pulse durations for different values of the saturable absorber cell length.
19
Volume 9, number F
OPTICS COMMUNICATIONS
the ring laser [7], for the reliable production of ultrashort pulses. It should then be relatively easy, by era. ploying delay line techniques [4], to accurately buildup and shape Nd:glass laser pulses for compression experiments aimed at controlled fusion [ 15, 16]. We would like to thank Mr. W.E. Sleat for technical assistance with the streak.camera and Dr. G.H.C. New for useful discussions. Financial support was obtained from the Paul Instrumep~ Fund of the Royal Society.
References I 1! D3. Bradley, G.H.C. New and S.J. Caughey, Phys. Letters 30A (1.969) 78. [2] D2. Bradley, G.H.C. New and S.J. Caughley, Opt. Commun. 2 ~1970) 41. [3] Dd. Bradley, G.H.C. New and S.J. Caughley, Phys. Letters 32A (1970) 313. [41 DJ. Bradley, B. Liddy and W.E. Sleat, Opt. Commun. 2 (1971) 391. [51 DJ. Bradley, British ~ovisional Patent Application 31167/70.
20
September 1973
16] D. Von Der Linde, IEEE J. Quantum Electrch. ',~E-8 (1972) 328.
[7] N.G. Basov, M.M. Butslov, P.G. Kryukov, Yu.A. Matveets, E.A. Smirnova, S.D. Fanchenko, S.V. Chekalin and R.V. Chikin, Lebedev Physical Institute Preprint No. 82 (1972). is] S.D. Fanchenko and B.A. Frolov, J.E.T.P. Letters 16 (1972) 101. [9] E.G. Arthurs, D.J. Bradley, B. Liddy, F. O'Neill, A.G. Roddie, W. Sibbett and W.E. Sleat, Proc. of 10th Intern. Conf. on High Speed Photography, Nice, September 1972 (in press). [101 D.J. Bradley, M.H.R. Hutchinson and H. Koetser, Proc. Roy. Soc. Lond. A329 (1972) 105. [11] D.J. Bradley, B. Liddy, A.G. Roddie, W. Sibbett and W.E. Sleat, Opt. Commun. 3 (1971) 426. [12l D.J. Bradley, B. Liddy, W. Sibbett and W,E. Sleat, Appl. Phys. Letters 20 (1972) 219. [13] M.A. Duguay, J.W. Hansen and S.L. Shapiro, IEEE J. Quantum Electron. QE-6 (1970) 725. [141 E.G. Arthurs, D.J. Bradley and A.G. Roddie, Appl. Phys. Letters 19 (1971)480. [151 J. Nuckolls, L. Wood, A. Thiesson and G. Zimmermann, Nature 239 (1972) 139. [161 J.S. Clarke, H.N. Fisher, R.J~ Mason, Phys. Rev. Letters 30 (1973) 89.
OPTICS COMMUNICATIONS
Septembe.r 1973
STREAK-CAMERA STUDIES OF PICOSECOND PULSES FROM A MODE-LOCKED Nd:GLASS LASER D.J. BRADLEY ~ and W. SIBBETT ~ Department of Pure and Applied Physics, The Queen's University o f Belfast, Belfast BT7 INN, Northern Ireland
Received 11 July 1973
Direct measurements with a picosecond streak-camera show that 1 mJ pulses, of transform limited durations (At ~ 3 psec, ~,tav ~ 0.5), are produced at the beginning of the pulse train of a mode-locked Nd:glasslaser. Pulse durations increase to ~10 psec at the end of the train, with self-phase-modulationfrequency broadening. The optimum length of the contacted saturable absorber cell was found to be ~50 urn,in agreement with earlier work using two-photon fluorescence measurements.
Earlier investigations [ 1-3 ], employing two.photon fluorescence (TPF) measurements of the pulse structure, showed that the most reliable mode.locking and the shortest pulse durations were obtained in a modelocked Nd:glass laser when the saturable absorber cell was in contact with one of the cavity mirrors. This result was explained theoretically using the maximum emission principle [2]. In these experhnents trains lasting 150 nsec, containing pulses of average durations 2.5-8 psec, were obtained with a dye cell length of ~ 30 tan. Simultaneous third harmonic measurements [l] did not indicate any sy,~tematic variation of the pulse durations along the train but the presence of quasi.random subpicosecond intensity fluctuations was established [3]. Direct measurements [4] of single second-harmonic pulses with a picosecond electrooptical streak camera [5], confirmed the TPF results and showed that there were no satellite pulses. Later Von der Linde [6] carried out TPF measurements of amplified single pulses and found that while bandwidth.limited pulses were present in the leading part of the pulse train, substantial frequency broadening developed as the pulse train intensity envelope reached its maximum. Pulse broadening to "-'l 1 psec occurred e, Present address: Applied Optics Section, Physics Department, Imperial College, London, S.W. 7, UK.
in the trailing part of the "-600 nsec train, with the formation of subpicosecond structure. More recently measurements on ring [7] and normal [8] Nd:glass resonators, with infra-red sensitive streak-cameras of ~2.5 psec time.resolution, have confirmed that the shortest pulses ( 5 - 1 0 psec) occur in the initial part of the train. However the authors of [7] found that near the peak of the 300-1000 nsec trains, pulse durations of ~100 psec, with complicated fine structure, were recorded. The pulses of [6] and [7] had an energy content of ~0.2 mJ. Because of the discrepancy between the results of these investigators we have again investigated the laser of [ 1-3] employing a streak-camera, with a S-1 image-tube (Instrument Technology Ltd.), of time-resolution 2.5 psec [9]. The experimental arrangement is shown in fig. 1. The mode.locked laser previously described [ 1-3] was modified by the addition of a l0 cm focal length, biconcave lens, L, placed 25 cm from the 100% reuectxvlty mirror, M 1, of 35 cxrt focal length. "--"' distortion in the laser rod could thus be easily corrected for with this generalized confocal cavity [ 10]. The saturable absorber dye cell, C (Eastman Kodak 9860 in dic~orethane with 70% low level light transmission) was in contact with the 70% refleclivity plane mirror M 2. A red filter, F 1 , t'rotected the dye from photodecomposition by stray flashlight. As before, corn17
Volume 9, number 1
OPTICS COMMUNICATIONS O.L. to
streak c
~
September 1973
mera instrumental time resolution limit of 2 £ ?~ec. Near the middle of the pulse train, usually about 250 nsec from the start, there was little increase in the 10
M~
L
FI
C MZ
(a)
RC. M3
Fig. 1. Mode-locked Nd:glass laser and optical delay line (D.L) arrartgement,
pletely mode-locked pulse trains were obtained with a better than 80% success rate. Fig. 2 shows a typical oscLllogram of a train with an average energy of ~ I nd
per pulse. About ten pulses were switched out of the train by a Pockels cell shutter, P.C., operated by a fast electronic switch [ 11 ]. A monitor signal from the streakcamera voltage ramp generator located on the oscillograms the position of the pulse Which was recorded by the camera. Otherwise the arrangement for operation and calibration of the streak-camera was that used in [4] and [ 11]. Initially the fifth pulse of the train was streaked. Microdensitometry of the streak photographs (l]ford HI'4 film) consistently gave pulse durations of 3-5 psec for a dye ceU thickness of 50 tan. With an additional delay of "-50 nsec (5 pulses further along the train) the recorded pulse durations had the distribution of fig. 3a. The streak photograph of fig. 4 shows two sub-pulses generated in the optical delay line [10] (D.L. of fig. 1), from a single 3 psec pulse, and separated by l0 psec. The microdensitometer trace shows a clear separation, expected from the ca-
I
0 3
,, I,,
4 5 6 PULSE DURATION p sec
I
,
(b)
0 m
o
8
W
'" ~D
4
:3 Z
il 6
1
S tO P~LSE DURATION p SOC
, Ii t2
Fig. 3. Distributions of measured pulse durations (a) 90 nsecs from start (10 pulses along trah~) (bj 500 asecs from start.
Fig. 2. Oscillogram of mode-locked laser, recorded with a fast photodiode and travelling wave oscilloscope. Time scale 200 nsec per major division.
18
Volume 9, number 1
OPTICS COMMUNICATIONS
,..
'~
c~ o
..° ,•q
/
Fig. 4. Top, streak photograph of two 3 psec sub-pulses, separated by 10 psec, generated in the optical delay line. Bottom: microdensitometet trace showing clear separation and confirming a deconvolved camera instrumental function of 2.5 psee. average pulse duration. Pulses selected from the trailing part of the train had durations of 5-13 psec, as shown in tag. 3b. Pulse spectra were recorded by using the street camera as a simple image converter. The exit slit of a l-m spectrograph, opened to 2 mm width, replaced the input slit of the camera. The spectral bandwidths of the pulses increased rapidly from the beginning of the train as foand by Duguay [13] and in [6]. For the fifth pulse of the train, an average of 20 measurements gave At = 3.8 +--0.8 psec with Av = 2.2 ± 0.5 X 1011 Hz (AX = 7.4 A). The corresponding time-bandwidth product [14] had au average value AvAt = 0.83:1: 0.3,
September 1973
showing that the initial pulses had practically transform. limited durations [6, 14]. The limiting value AvAt = 0.5 was obtained for several 3 psec pulses. It must be emphasized that unlike [6] and [13] these results were obtained by direct linear measurement. Along the train self-phase-modulation frequency broadening [ 13, 14] built-up at the rate of 10 cm-1 per round trip, and at the end of the train the pulse spectra had half-widths of ~50 A. Finally the effect of varying the saturable absorber cell length was investigated. As can be seen in fig. 5 the shortest pulse durations were obtained with a cell length of 50/am. The results plotted in the figure are in qualitative agreement with those of [2] but for the same cell thickness the pulse durations are now considerably shorter. This improvement could be due to the use of the generalized confocal cavity, which also gives more reproducible pulse trains. We conclude from these results that transformlimited pulses of ~ I mJ energy can be generated at the beginning of a mode-locked Nd:glass laser pulse train. Further along the train the spectral bandwidth increasesconsiderably because of self.phase-modulation but even at the end of the trainpulse durations are generally < 10 psec. These directmeasurements are similar to the TPF resultsof [6] where a contacted cellwas also employed but the pulses had considerably smallerenergies.The much shorter pulses produced throughout the pulse trainof our laserconfirm the superiority of the contacted dye cell cavity over
400
t
E 300 =t,
/
I ! I I I
!
-r
/ !
200
/
f
/
U
/
~oo
/-
0 L _ _ . . _ ~ t -
0
2
i,
, I
4
PULSE
'
,
~
DUrATiON
j
L
B"
psec
Fig. 5. Measured pulse durations for different values of the saturable absorber cell length.
19
Volume 9, number F
OPTICS COMMUNICATIONS
the ring laser [7], for the reliable production of ultrashort pulses. It should then be relatively easy, by era. ploying delay line techniques [4], to accurately buildup and shape Nd:glass laser pulses for compression experiments aimed at controlled fusion [ 15, 16]. We would like to thank Mr. W.E. Sleat for technical assistance with the streak.camera and Dr. G.H.C. New for useful discussions. Financial support was obtained from the Paul Instrumep~ Fund of the Royal Society.
References I 1! D3. Bradley, G.H.C. New and S.J. Caughey, Phys. Letters 30A (1.969) 78. [2] D2. Bradley, G.H.C. New and S.J. Caughley, Opt. Commun. 2 ~1970) 41. [3] Dd. Bradley, G.H.C. New and S.J. Caughley, Phys. Letters 32A (1970) 313. [41 DJ. Bradley, B. Liddy and W.E. Sleat, Opt. Commun. 2 (1971) 391. [51 DJ. Bradley, British ~ovisional Patent Application 31167/70.
20
September 1973
16] D. Von Der Linde, IEEE J. Quantum Electrch. ',~E-8 (1972) 328.
[7] N.G. Basov, M.M. Butslov, P.G. Kryukov, Yu.A. Matveets, E.A. Smirnova, S.D. Fanchenko, S.V. Chekalin and R.V. Chikin, Lebedev Physical Institute Preprint No. 82 (1972). is] S.D. Fanchenko and B.A. Frolov, J.E.T.P. Letters 16 (1972) 101. [9] E.G. Arthurs, D.J. Bradley, B. Liddy, F. O'Neill, A.G. Roddie, W. Sibbett and W.E. Sleat, Proc. of 10th Intern. Conf. on High Speed Photography, Nice, September 1972 (in press). [101 D.J. Bradley, M.H.R. Hutchinson and H. Koetser, Proc. Roy. Soc. Lond. A329 (1972) 105. [11] D.J. Bradley, B. Liddy, A.G. Roddie, W. Sibbett and W.E. Sleat, Opt. Commun. 3 (1971) 426. [12l D.J. Bradley, B. Liddy, W. Sibbett and W,E. Sleat, Appl. Phys. Letters 20 (1972) 219. [13] M.A. Duguay, J.W. Hansen and S.L. Shapiro, IEEE J. Quantum Electron. QE-6 (1970) 725. [141 E.G. Arthurs, D.J. Bradley and A.G. Roddie, Appl. Phys. Letters 19 (1971)480. [151 J. Nuckolls, L. Wood, A. Thiesson and G. Zimmermann, Nature 239 (1972) 139. [161 J.S. Clarke, H.N. Fisher, R.J~ Mason, Phys. Rev. Letters 30 (1973) 89.