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Additive-pulse modelocking of non-cw neodymium lasers
OPTICS COMMUNICATIONS
Optics Communications 97 (1993) 35-40 North-Holland
Additive-pulse modelocking of non-cw neodymium lasers P. Heinz, A. R e u t h e r a n d A. L a u b e r e a u Physikalisches Institut, UniversittitBayreuth, W-8580 Bayreuth, Germany
Received 20 October 1992
Passive modeloekingof several flash-lamp pumped neodymium lasers with electro-optic amplitude stabilization is demonstrated using a nonlinear Michelson interferometer.Improved performanceis reported for the GSGG- YLF- and glass-laseras compared to the nonlinear absorber, with shorter pulse durations and smalleramplitude fluctuations, e.g. 5 laJ pulses for 460 + 20 fs for Nd:glass. Evidence is obtained for multi-selfstabilityof the pulse energy.
1. Introduction Passive modelocking techniques are very efficient for the generation of ultrashort laser pulses. Over many years the standard method was the application of a saturable absorber [ 1 ]. For laser systems with small emission cross sections, e.g., Nd-lasers, this approach yields picosecond pulses. More shorter pulses of approximately 25 fs were obtained for dye lasers where the pulse shortening process is notably supported by population depletion of the laser transition. Besides the difficulty of finding suitable absorber materials with incidental frequency resonance to the laser transition an important shortcoming of the technique is the relatively long recovery time of the nonlinear absorber, usually in the picosecond range. As a consequence the shortening mechanism looses its efficiency for pulses in the subpicosecond and femtosecond range [ 2 ]. In the past years novel modelocking schemes termed Kerr lens modelocking [ 3-5 ] and additive pulse modelocking (APM) [6-9] have been demonstrated for cw-lasers that are based on the nonlinear refractive index n2 of the laser medium itself or of an additional glass fiber. The technique applies an auxiliary optical cavity or interferometer with nonlinear changes of the optical phase relative to the laser resonator leading to intensity-dependent cavity losses. One of the attractive features is the almost instantaneous response of the (electronic) n2-mechanism. Several solid-state lasers have shown success-
ful APM operation under cw conditions with single pulse energies in the nJ region and pulse durations in the range 0.1-10 ps [ 6-11 ]. In this letter we demonstrate for the first time APM laser operation under pulsed conditions. Several Ndlasers with fash-lamp pumping are investigated yielding single pulse energies up to 5 Id; pulses shorter than 500 fs are generated for the Nd:glass case, while crystalline Nd-systems yield durations of a few ps. These results refer to FCM-operation, i.e. electro-optic feedback-control [ 2,12-15 ] of the pulse amplitude.
2. General
A nonlinear Michelson interferometer (NMI) is considered that replaces a cavity mirror of the laser resonator [7]. The nonlinear medium is placed in one interferometer arm so that an intensity dependent phase shift A~ is generated between the two interfering beams via the nonlinear index of refraction n2. Figure 1 illustrates the situation. The effective reflectivity R of the interferometer with its well-known periodic dependence on the phase difference • between the two interferometer beams is shown in fig. I a. A suitable phase setting ~o of the NMI (referring to the low-intensity case) for passive modelocking is indicated. For higher intensity the phase position shifts to a value ~=¢Po+A~P with a corresponding reflectivity change. This behaviour is depicted in
0030-4018/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
35
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trol, i.e. FCM operation. With this technique the Qswitching effect inherently connected with passive modelocking is suppressed and the pulse amplitude is kept at a level where o p t i m u m nonlinear shortening occurs. We mention that the FCM technique is not required for cw-lasers since the pulse amplitude is stabilized there by gain saturation.
(a) 1
•.-> o . s
g
0
7r 2~T Interferomefrlc Phase
1
(b)
i
0.5
0 0.1
i Intensity I / I s
10
Fig. 1. Effective reflectivity of the nonlinear Michelson interferometer serving as a cavity mirror: (a) versus phase difference ¢~ of the interferometer arms; a suitable phase position ¢~o(at small intensity) is indicated; (b) versus peak intensity 1 in units of a characteristic intensity I, (solid curve); the behaviour of a nonlinear absorber is also shown (stationary case; broken line ). more detail in fig. lb. The reflectivity is plotted versus intensity in units of a characteristic intensity Is (solid curve). Is combines several material parameters, e.g. n2, length and wavelength. The phase setting ~o is assumed such that R o = R ( ~ o ) = 0 . 3 . The reflectivity first increases with rising intensity to a m a x i m u m value R = 1, then decreases to a m i n i m u m Rmln, with further oscillations for higher intensities. The value Rmin---0in the figure refers to a 50% beam splitter of the interferometer. For comparison the effective reflectivity of a cavity mirror combined with a nonlinear absorber is also shown (broken line in fig. I b). A quasi-stationary situation with saturation intensity I, is considered. The useful intensity range of the N M I for modelocking applications is obviously limited. Shortening of the circulating pulse in the laser cavity occurs if the peak of the pulse finds a higher reflectivity than the pulse wings. This condition can be fulfilled by additional amplitude con36
1 March 1993
3. Experimental
The laser system is depicted schematically in fig. 2. The cavity consists of the highly reflecting mirror M 1 and the Michelson interferometer made up by a 50% beamsplitter BS and two 100% mirrors (M2 and M3). One interferometer a r m (length approximately 20 c m ) is equipped with an acousto-optic modelocker AOM that is helpful for starting the modelocking. The second arm contains the nonlinear medium n2, a liquid cell (several m m of length) filled with CS2 or a block of heavy flint glass SF6 of 1-3 cm; in part of the investigations an inverted telescope was used in the interferometer arm to increase the peak intensity in the nonlinear medium. Mirror M2 is mounted on a piezo-drive with electronic control loop CL for the interferometric phase position @o (see below). The cavity axis is folded for practical reasons by an additional mirror M4. The cylindrical laser rod LR is p u m p e d by a linear flash lamp in an elliptical
M1
Pc
/
@++o,
~wP
AP
T
BS
HINI
Id5
Fig. 2. Schematic of the pulsed laser system with nonlinear Michelson interferometer for passive modelocking and electro-optic feedback control of the pulse amplitude; mirrors MI-M4, beam splitter BS, acousto-optic modelocker AOM, nonlinear medium n2, piezo-drive PZ, control loop CL of the interferometer phase, aperture AP, laser rod LR, wedged plate WP, dielectric polarizer Pol, double-Pockels cell PC, feedback control FC of the pulse energy.
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housing. Aperture AP controls the transverse mode pattern (see fig. 2). The feedback control device consists of a dielectric polarizer Pol, double-Pockels cell PC and a glass wedge WP that couples out the optical signal for the electronic control unit FC. The latter was described in previous publications [ 15 ]; it converts the detected laser amplitude by the help of a fast logic comparator into an electrical signal with strongly nonlinear amplification factor. Voltage pulses up to 500 V are generated and applied to PC. The control level of the laser amplitude is adjusted by the dc voltage at the second comparator input. The Pockels cell is biased at 1200 V close to the turning point of its transmission-voltage characteristics (in combination with polarizer Pol). Fast transmission changes up to a factor of 1.8 are generated within the delay time of the control unit of l0 ns, approximately equal to the cavity round trip time. The polarizer also serves as the output coupler (approximately 50%) of the laser emitting pulse trains of several microseconds. For passive modelocking the optical pass lengths of the two interferometer arms have to match within the coherence length of the laser radiation. Pulses of a few ps necessitate a (coarse) positioning of mirror M2 within less than 1 mm. In addition the interferometer phase ~o has to be adjusted by fine tuning of the piezo-element for M2. Long-time stability of ~o is maintained by a feedback control loop CL (see fig. 2). In an early part of the investigation an auxiliary cw-HeNe laser (4 mW, 633 nm) was used for this purpose. The resulting stability of the interferometric phase at 1.06 ~tm was _+0.07 tad. Later on the modelocked laser pulses served themselves to control the interferometer. To this end the second harmonic of the laser output is generated and detected (not shown in fig. 2) together with the laser fundamental. The signals are fed into a microcomputer with averaging over series of five consecutive laser shots; movements of the piezo-drive of mirror M2 in steps of a few nm are initiated on the time-scale of seconds to maintain optimum laser performance. For comparison the laser was tested also with passive modelocking by a nonlinear absorber. For this purpose the NMI in fig. 2 was replaced by a highly reflecting mirror and a thin glass cell filled with a solution of the dye Kodak 9860 in dichloroethane (thickness 0.1 mm, single pass transmission 70%).
1 March 1993
For the pulse analysis we use a conventional autocorrelation setup with off-axis second harmonic generation in a thin KDP crystal. The reported durations represent average values for sequences of approximately 50 pulses selected by an electro-optic shutter from the central part of the pulse train in front of the autocorrelator.
4. Results
For the generation of picosecond pulses two crystalline Nd-materials were investigated. The co-doped system Nd:Cr:GSGG (gadolinium scandium gallium garnet) was chosen because of its high slope efficiency, > 6%, for flashlamp pumping [16]. The broad and flat-top fluorescence spectrum of approximately 12 nm width [ 17 ] is attractive for modelocking applications [ 18 ]. A laser rod of 6 X 80 mm with doping by 1.65% Nd203 and 2.47% Cr203 was used allowing repetition rates of 15-40 Hz. The thermal lens for these pumping conditions was compensated by an intracavity telescope (not shown in fig. 2). Nd:YLF (yttrium lithium fluoride) excels by its small thermal lens effect. Because of the uniaxial birefringence the emission occurs with ~-polarization at 1053 nm and n-polarization at 1047 n m [ 19]. The latter transition was investigated for its larger emission cross section and favourable fluorescence bandwidth of 1.6 nm in a laser rod of 4 × 80 mm (doping 1%). The generation of subpicosecond pulses was studied in Nd:phosphate glass (Schott LG 760) with athermal properties and a fluorescence bandwidth of 19.5 nm. A laser rod of 5 x 105 mm (3% Nd2Oa) and pump energies <60 J allow a repetition rate of 5-10 Hz. With proper alignment of the NMI long and stable pulse trains are observed with properties superior to the nonlinear absorber case. The following discussion refers to the GSGG-laser; similar results are obtained for the two other laser materials. The length of the pulse trains is adjustable via the control of the amplitude level of the pulses in the range 4-30 ~ts with single pulse energies of 4.7-0.5 ~tJ; long pulse trains correspond to small pulse amplitudes and vice versa. A standard deviation of + 1.8% was measured 37
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OPTICS COMMUNICATIONS
for the total laser output of approximately 1 m J, while the single pulse energy was stable within _+ 1%. The diameter and the position of the aperture in the laser cavity strongly influence the pulse train envelope and single pulse stability. Frustrating the transverse mode pattern by a smaller aperture leads to improved modelocking with shorter pulses; this finding suggests that not only APM- but also Kerrlens modelocking [ 3-5 ] takes place. In addition, selfstabilisation of the laser amplitude is observed. To demonstrate the effect, the electro-optic feedback loop is electronically disabled after 3 ~ts of operation and the time evolution of the pulse train detected with a 400 MHz oscilloscope. Some results are shown in fig. 3 for the GSGG-laser with 1 cm of CS2 and an inverted telescope in the NMI. The onset of passive modelocking is indicated on the left hand side by a sharp initial spike of the train envelope with subsequent stabilization of the pulse energy due to the electro-optic amplitude control operative only for 3 ~ts. The cursor position of the oscilloscope display (vertical bars in the figure) marks the termination of the electronic control-loop. It is interesting to see that for both examples the laser
(a)
-~2 ps~-
(b)
Fig. 3. Oscilloscopetraces for the microsecondpulse train of the GSGG laserwith active amplitude stabilization (FCM) switched offat the cursor position; one observesself-stabilization(a) and multi-selfstability (b). 38
1 March 1993
amplitude continues on a similar level for at least 2 Ixs. In fig. 3a the pulse energy remains constant for further 16 tts without active feedback control indicating self-stabilization. The more frequent behaviour is shown in fig. 3b: the train envelope switches in several steps to different amplitude levels representing multi-selfstability. Transition from one stability regime to another is accompanied by a sudden signal overshoot. The normal FCM operation (without external termination) is obviously supported by the self-stabilisation effect, leading to the improved reproducibility of the pulse energy noted above. A summary of the properties of the different lasers is presented in table 1. The laser material and the repetition rate used in the investigation are listed in the first two columns. The latter is frequently chosen just for practical reasons and does not represent a limiting value. The measured data for the single pulse energy, duration and bandwidth-duration product are listed in columns 3-6. The upper part of the table refers to APM with the nonlinear interferometer, while the lower part presents some results for passive modelocking with the nonlinear absorber dye 9860; here pulse durations are also indicated in brackets for laser operation with an additional etalon in the cavity. The latter is tuned at anti-resonance to enlarge the effective spectral width of the stimulated amplification curve; details of the etalon were discussed recently [ 15 ]. If not stated differently in the table, a gaussian shape was consistent with the measured autocorrelation curves and used to derive the pulse duration. Table 1 shows that the NMI achieves shorter pulses than the nonlinear absorber. The pulses are approximately bandwidth-limited for all the investigated examples although deviations of several ten percent occur from the expected Fourier transform limit. Of special interest are the results for Nd:glass. An example for the measured autocorrelation curve is depicted in fig. 4. The exponential slopes are noteworthy extending over three orders of magnitude with a 1/e decay time of 150 fs. The solid line in the figure is calculated for a sech 2 shape yielding the pulse duration of t p = 4 6 0 + 2 0 fs. The circulating pulse in the laser approaches a constant duration in the latter part of the pulse train, obviously due to a balance between shortening and stretching mechanisms. Some information on the
1 March 1993
OPTICS COMMUNICATIONS
Volume 97, number 1,2 Table 1 Modeloeking results.
Rep. rate (Hz)
Energy (ILd)
Nd:Cr:GSGG Nd:YLF Nd:glass
20 10 5
5 2 6
Nd:Cr:GSGG no. 9860 etalon b) Nd:YLF no. 9860 etalon b) Nd:glass no. 9860
30
5
70
4
10
10
Duration (ps) 3.2+0.1 4.7+0.1 0.46 + 0.02 a) 4.2_+0.1 (3.0_+0.1) 7+0.5 (2.9_+0.1) b) 0.8_+0.1
A~,X tp 0.78__+0.03 0.45 + 0.05 0.63+0.03 0.5_+0.05 0.6_+0.05
") For sech2 shape, b) Ref. [ 15 ].
I
I
estimated. It is believed that spectral gain narrowing represents the opposing effect [2 ] determining the measured d u r a t i o n o f 460 fs.
I
u v
10_ I
5. C o n c l u s i o n s
Ur-
~ u
We have d e m o n s t r a t e d A P M operation o f flashl a m p p u m p e d Nd-lasers with i m p r o v e d properties as c o m p a r e d to nonlinear absorbers. Pulses o f several ~tJ energy and duration 0.5-5 ps are generated close to the F o u r i e r transform limitation. Self-stabilization o f the pulse a m p l i t u d e and a reproducibility o f the single pulse energy within _+ 1% are reported.
10 - 2
o
'5 lO. 3 .<
10-4 -1.6
I -o.a
I 0.0
I 0.8
1.6
Delay Time ( p s ) Acknowledgements
Fig. 4. Autocorrelation data for the Nd:glass laser with passive modelocking by the NMI with 2 mm of CS2; a pulse duration of 460_ 20 fs is measured assuming a sech2 shape (calculated solid c u r v e ).
shortening potential o f the N M I is available from autocorrelation m e a s u r e m e n t s o f the m o d i f i e d pulse decoupled from the b e a m splitter BS o f the interferometer (compare fig. 2). This " t r a n s m i t t e d " pulse o f the N M I is found to be longer by a factor o f 1.2 than the reflected pulse in the resonator suggesting a shortening o f the latter o f a few ten percent per cavity r o u n d trip. These n u m b e r s give a d d i t i o n a l supp o r t to the larger efficiency o f the N M I for passive modelocking as c o m p a r e d to the nonlinear absorber where shortening by only 1-2% per r o u n d trip was
The authors thank Dipl. Phys. T. D a h i n t e n for cooperation in the investigation o f the N d : Y L F laser. Stimulating discussions with Prof. A. Seilmeier are gratefully acknowledged.
References
[ 1] For a review, see C.V. Shank, in: Ultrashort laser pulses and applications, ed. W. Kaiser, Topics in Applied Physics, Vol. 60 (Springer, Berlin, 1988) p. 5. [2]P. Heinz and A. Laubereau, J. Opt. Soc. Am. B 6 (1989) 1574. [3] D.E. Spence, P.N. Kean and W. Sibbett, Optics Lett. 16 ( 1991 ) 42; J. Opt. Soc. Am. B 8 ( 1991 ) 2053. 39
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[4] F. Salin, J. Squier and M. Piche, Optics Lett. 16 (1991) 1674; G.P.A. Malcolm and A.L. Ferguson, Optics Lett. 16 ( 1991 ) 1967. [5] D.K. Negus, L. Spinelli, N. Goldblatt and G. Feuget, OSA Proc. on Advanced Solid State Lasers 10 ( 1991 ) 120; Ch. Spielmann, F. Krausz, T. Brabec, E. Wintner and A.J. Schmidt, Optics Lett. 16 ( 1991 ) 1180; 17 (1992) 204. [6] L.F. Mollenauer and R.H. Stolen, Optics Lett. 9 (1984) 13; K.J. Blow and B.P. Nelson, Optics Lett. 13 ( 1988 ) 1026; K.J. Blow and D. Wood, J. Opt. SOc. Am. B. 5 ( 1988 ) 629. [7] F. Ouellette and M. Piche, Optics Comm. 60 (1986) 99; Can. J. Phys. 66 ( 1988 ) 903. [8] P.N. Kean, X. Zhu, D.W. Crust, R.S. Grant, N. Langford and W. Sibbett, Optics Lett. 14 (1989) 39; J. Mark, L.Y. Liu, K.L. Hall, H.A. Haus and E.P. Ippen, Optics Lett. 14 (1989) 48; E.P. Ippen, H.A. Haus and L.Y. Liu, J. Opt. SOc. Am. B 6 (1989) 1736. [9] J. Goodberlet, J. Wang, J.C. Fujimoto and P.A. Schulz, Optics Lett. 14 (1989) 1125; U. Keller, G.W. 't Hooft, W.H. Knox and J.E. Cunningham, Optics Lett. 16 ( 1991 ) 1022; H.A. Haus, U. Keller and W.H. Knox, J. Opt. Soc. Am. B 8 (1991) 1252. [10] F. Krausz, Ch. Spielman, T. Brabec, E. Wintner and A.J. Schmidt, Optics Lett. 15 (1990) 737, 1082; Appl. Phys. Lett. 58 (1991) 2470. [ 11 ] J. Goodberlet, J. Jacobson, J.G. Fujimoto, P.A. Schulz and T.Y. Fan, Optics Lett. 15 (1990) 504; L.Y. Liu, J.M. Huxley, E.P. Ippen and H.A. Haus, Optics Lett. 15 (1990) 553;
40
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J.M. Liu and J.K. Chee, Optics Lett. 15 (1990) 685; G.P.A. Malcolm, P.F. Curley and A.I. Ferguson, Optics Lett. 15 (1990) 1303. [12] K.P. Komorov, A.S. Kuyanov and V.D. Ugozhaev, Sov. J. Quantum Electron. 16 (1986) 520; A.M. Varshin, V.M. Gordienko, S.V. Krayuskin, V.T. Platonenko and V.K. Poppy, SOv. J. Quantum Electron. 16 (1986) 1125. [ 13 ] K. Burneika, R. Grigoris, A. Piskarskas, G. Sinkyavichius and V. Sirukaitis, Sov. J. Quantum Electron. 18 (1988) 1034. [ 14 ] P. Heinz, W. Kriegleder and A. Laubereau, Appl. Phys. A 43 (1987) 209. [15] P. Heinz and A. Laubereau, J. Opt. Soc. Am. B 7 (1990) 182; K. Wolfrum and P. Heinz, Optics Comm. 84 ( 1991 ) 290. [ 16] E.V. Zharkiov, N.N. ll'ichev, V.V. Laptev, A.A. Malyuthin, V.G. Ostroumov, P.P. Pashinin, A.S. Pimenov, V.A. Smirnov and I.A. Sherbakov, Sov. J. Quantum Electron. 13 (1983) 82. [ 17 ] A. Reuther and P. Heinz, unpublished data. [18] R. Danelyus, I. Kuratev, A. Piskarskas, V. Sirukaitis, E. Shvom, A. Yuozapavichyus and A. Yankauskas, Sov. J. Quantum Electron. 15 ( 1985 ) 1160; A.V. Babushkin, N.S. Vorob'ev, E.V. Zharikov, S.P. Kalitin, V.V. Osiko, A.N. Prokhorov, Yu.N. Serdyuchenko, M.-Ya. Shchelev and I.A. Shcherbakov, Sov. J. Quantum Electron. 16 (1986) 428. [ 19 ] A.L. Harmer, A. Linz and D.R. Gabbe, J. Phys. Chem. Solids 30 (1969) 1483; J.E. Murray, IEEE J. Quantum Electron. QE-19 ( 1983 ) 488.
Optics Communications 97 (1993) 35-40 North-Holland
Additive-pulse modelocking of non-cw neodymium lasers P. Heinz, A. R e u t h e r a n d A. L a u b e r e a u Physikalisches Institut, UniversittitBayreuth, W-8580 Bayreuth, Germany
Received 20 October 1992
Passive modeloekingof several flash-lamp pumped neodymium lasers with electro-optic amplitude stabilization is demonstrated using a nonlinear Michelson interferometer.Improved performanceis reported for the GSGG- YLF- and glass-laseras compared to the nonlinear absorber, with shorter pulse durations and smalleramplitude fluctuations, e.g. 5 laJ pulses for 460 + 20 fs for Nd:glass. Evidence is obtained for multi-selfstabilityof the pulse energy.
1. Introduction Passive modelocking techniques are very efficient for the generation of ultrashort laser pulses. Over many years the standard method was the application of a saturable absorber [ 1 ]. For laser systems with small emission cross sections, e.g., Nd-lasers, this approach yields picosecond pulses. More shorter pulses of approximately 25 fs were obtained for dye lasers where the pulse shortening process is notably supported by population depletion of the laser transition. Besides the difficulty of finding suitable absorber materials with incidental frequency resonance to the laser transition an important shortcoming of the technique is the relatively long recovery time of the nonlinear absorber, usually in the picosecond range. As a consequence the shortening mechanism looses its efficiency for pulses in the subpicosecond and femtosecond range [ 2 ]. In the past years novel modelocking schemes termed Kerr lens modelocking [ 3-5 ] and additive pulse modelocking (APM) [6-9] have been demonstrated for cw-lasers that are based on the nonlinear refractive index n2 of the laser medium itself or of an additional glass fiber. The technique applies an auxiliary optical cavity or interferometer with nonlinear changes of the optical phase relative to the laser resonator leading to intensity-dependent cavity losses. One of the attractive features is the almost instantaneous response of the (electronic) n2-mechanism. Several solid-state lasers have shown success-
ful APM operation under cw conditions with single pulse energies in the nJ region and pulse durations in the range 0.1-10 ps [ 6-11 ]. In this letter we demonstrate for the first time APM laser operation under pulsed conditions. Several Ndlasers with fash-lamp pumping are investigated yielding single pulse energies up to 5 Id; pulses shorter than 500 fs are generated for the Nd:glass case, while crystalline Nd-systems yield durations of a few ps. These results refer to FCM-operation, i.e. electro-optic feedback-control [ 2,12-15 ] of the pulse amplitude.
2. General
A nonlinear Michelson interferometer (NMI) is considered that replaces a cavity mirror of the laser resonator [7]. The nonlinear medium is placed in one interferometer arm so that an intensity dependent phase shift A~ is generated between the two interfering beams via the nonlinear index of refraction n2. Figure 1 illustrates the situation. The effective reflectivity R of the interferometer with its well-known periodic dependence on the phase difference • between the two interferometer beams is shown in fig. I a. A suitable phase setting ~o of the NMI (referring to the low-intensity case) for passive modelocking is indicated. For higher intensity the phase position shifts to a value ~=¢Po+A~P with a corresponding reflectivity change. This behaviour is depicted in
0030-4018/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
35
Volume 97, number 1,2
OPTICS COMMUNICATIONS
trol, i.e. FCM operation. With this technique the Qswitching effect inherently connected with passive modelocking is suppressed and the pulse amplitude is kept at a level where o p t i m u m nonlinear shortening occurs. We mention that the FCM technique is not required for cw-lasers since the pulse amplitude is stabilized there by gain saturation.
(a) 1
•.-> o . s
g
0
7r 2~T Interferomefrlc Phase
1
(b)
i
0.5
0 0.1
i Intensity I / I s
10
Fig. 1. Effective reflectivity of the nonlinear Michelson interferometer serving as a cavity mirror: (a) versus phase difference ¢~ of the interferometer arms; a suitable phase position ¢~o(at small intensity) is indicated; (b) versus peak intensity 1 in units of a characteristic intensity I, (solid curve); the behaviour of a nonlinear absorber is also shown (stationary case; broken line ). more detail in fig. lb. The reflectivity is plotted versus intensity in units of a characteristic intensity Is (solid curve). Is combines several material parameters, e.g. n2, length and wavelength. The phase setting ~o is assumed such that R o = R ( ~ o ) = 0 . 3 . The reflectivity first increases with rising intensity to a m a x i m u m value R = 1, then decreases to a m i n i m u m Rmln, with further oscillations for higher intensities. The value Rmin---0in the figure refers to a 50% beam splitter of the interferometer. For comparison the effective reflectivity of a cavity mirror combined with a nonlinear absorber is also shown (broken line in fig. I b). A quasi-stationary situation with saturation intensity I, is considered. The useful intensity range of the N M I for modelocking applications is obviously limited. Shortening of the circulating pulse in the laser cavity occurs if the peak of the pulse finds a higher reflectivity than the pulse wings. This condition can be fulfilled by additional amplitude con36
1 March 1993
3. Experimental
The laser system is depicted schematically in fig. 2. The cavity consists of the highly reflecting mirror M 1 and the Michelson interferometer made up by a 50% beamsplitter BS and two 100% mirrors (M2 and M3). One interferometer a r m (length approximately 20 c m ) is equipped with an acousto-optic modelocker AOM that is helpful for starting the modelocking. The second arm contains the nonlinear medium n2, a liquid cell (several m m of length) filled with CS2 or a block of heavy flint glass SF6 of 1-3 cm; in part of the investigations an inverted telescope was used in the interferometer arm to increase the peak intensity in the nonlinear medium. Mirror M2 is mounted on a piezo-drive with electronic control loop CL for the interferometric phase position @o (see below). The cavity axis is folded for practical reasons by an additional mirror M4. The cylindrical laser rod LR is p u m p e d by a linear flash lamp in an elliptical
M1
Pc
/
@++o,
~wP
AP
T
BS
HINI
Id5
Fig. 2. Schematic of the pulsed laser system with nonlinear Michelson interferometer for passive modelocking and electro-optic feedback control of the pulse amplitude; mirrors MI-M4, beam splitter BS, acousto-optic modelocker AOM, nonlinear medium n2, piezo-drive PZ, control loop CL of the interferometer phase, aperture AP, laser rod LR, wedged plate WP, dielectric polarizer Pol, double-Pockels cell PC, feedback control FC of the pulse energy.
Volume97, number 1,2
OPTICS COMMUNICATIONS
housing. Aperture AP controls the transverse mode pattern (see fig. 2). The feedback control device consists of a dielectric polarizer Pol, double-Pockels cell PC and a glass wedge WP that couples out the optical signal for the electronic control unit FC. The latter was described in previous publications [ 15 ]; it converts the detected laser amplitude by the help of a fast logic comparator into an electrical signal with strongly nonlinear amplification factor. Voltage pulses up to 500 V are generated and applied to PC. The control level of the laser amplitude is adjusted by the dc voltage at the second comparator input. The Pockels cell is biased at 1200 V close to the turning point of its transmission-voltage characteristics (in combination with polarizer Pol). Fast transmission changes up to a factor of 1.8 are generated within the delay time of the control unit of l0 ns, approximately equal to the cavity round trip time. The polarizer also serves as the output coupler (approximately 50%) of the laser emitting pulse trains of several microseconds. For passive modelocking the optical pass lengths of the two interferometer arms have to match within the coherence length of the laser radiation. Pulses of a few ps necessitate a (coarse) positioning of mirror M2 within less than 1 mm. In addition the interferometer phase ~o has to be adjusted by fine tuning of the piezo-element for M2. Long-time stability of ~o is maintained by a feedback control loop CL (see fig. 2). In an early part of the investigation an auxiliary cw-HeNe laser (4 mW, 633 nm) was used for this purpose. The resulting stability of the interferometric phase at 1.06 ~tm was _+0.07 tad. Later on the modelocked laser pulses served themselves to control the interferometer. To this end the second harmonic of the laser output is generated and detected (not shown in fig. 2) together with the laser fundamental. The signals are fed into a microcomputer with averaging over series of five consecutive laser shots; movements of the piezo-drive of mirror M2 in steps of a few nm are initiated on the time-scale of seconds to maintain optimum laser performance. For comparison the laser was tested also with passive modelocking by a nonlinear absorber. For this purpose the NMI in fig. 2 was replaced by a highly reflecting mirror and a thin glass cell filled with a solution of the dye Kodak 9860 in dichloroethane (thickness 0.1 mm, single pass transmission 70%).
1 March 1993
For the pulse analysis we use a conventional autocorrelation setup with off-axis second harmonic generation in a thin KDP crystal. The reported durations represent average values for sequences of approximately 50 pulses selected by an electro-optic shutter from the central part of the pulse train in front of the autocorrelator.
4. Results
For the generation of picosecond pulses two crystalline Nd-materials were investigated. The co-doped system Nd:Cr:GSGG (gadolinium scandium gallium garnet) was chosen because of its high slope efficiency, > 6%, for flashlamp pumping [16]. The broad and flat-top fluorescence spectrum of approximately 12 nm width [ 17 ] is attractive for modelocking applications [ 18 ]. A laser rod of 6 X 80 mm with doping by 1.65% Nd203 and 2.47% Cr203 was used allowing repetition rates of 15-40 Hz. The thermal lens for these pumping conditions was compensated by an intracavity telescope (not shown in fig. 2). Nd:YLF (yttrium lithium fluoride) excels by its small thermal lens effect. Because of the uniaxial birefringence the emission occurs with ~-polarization at 1053 nm and n-polarization at 1047 n m [ 19]. The latter transition was investigated for its larger emission cross section and favourable fluorescence bandwidth of 1.6 nm in a laser rod of 4 × 80 mm (doping 1%). The generation of subpicosecond pulses was studied in Nd:phosphate glass (Schott LG 760) with athermal properties and a fluorescence bandwidth of 19.5 nm. A laser rod of 5 x 105 mm (3% Nd2Oa) and pump energies <60 J allow a repetition rate of 5-10 Hz. With proper alignment of the NMI long and stable pulse trains are observed with properties superior to the nonlinear absorber case. The following discussion refers to the GSGG-laser; similar results are obtained for the two other laser materials. The length of the pulse trains is adjustable via the control of the amplitude level of the pulses in the range 4-30 ~ts with single pulse energies of 4.7-0.5 ~tJ; long pulse trains correspond to small pulse amplitudes and vice versa. A standard deviation of + 1.8% was measured 37
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for the total laser output of approximately 1 m J, while the single pulse energy was stable within _+ 1%. The diameter and the position of the aperture in the laser cavity strongly influence the pulse train envelope and single pulse stability. Frustrating the transverse mode pattern by a smaller aperture leads to improved modelocking with shorter pulses; this finding suggests that not only APM- but also Kerrlens modelocking [ 3-5 ] takes place. In addition, selfstabilisation of the laser amplitude is observed. To demonstrate the effect, the electro-optic feedback loop is electronically disabled after 3 ~ts of operation and the time evolution of the pulse train detected with a 400 MHz oscilloscope. Some results are shown in fig. 3 for the GSGG-laser with 1 cm of CS2 and an inverted telescope in the NMI. The onset of passive modelocking is indicated on the left hand side by a sharp initial spike of the train envelope with subsequent stabilization of the pulse energy due to the electro-optic amplitude control operative only for 3 ~ts. The cursor position of the oscilloscope display (vertical bars in the figure) marks the termination of the electronic control-loop. It is interesting to see that for both examples the laser
(a)
-~2 ps~-
(b)
Fig. 3. Oscilloscopetraces for the microsecondpulse train of the GSGG laserwith active amplitude stabilization (FCM) switched offat the cursor position; one observesself-stabilization(a) and multi-selfstability (b). 38
1 March 1993
amplitude continues on a similar level for at least 2 Ixs. In fig. 3a the pulse energy remains constant for further 16 tts without active feedback control indicating self-stabilization. The more frequent behaviour is shown in fig. 3b: the train envelope switches in several steps to different amplitude levels representing multi-selfstability. Transition from one stability regime to another is accompanied by a sudden signal overshoot. The normal FCM operation (without external termination) is obviously supported by the self-stabilisation effect, leading to the improved reproducibility of the pulse energy noted above. A summary of the properties of the different lasers is presented in table 1. The laser material and the repetition rate used in the investigation are listed in the first two columns. The latter is frequently chosen just for practical reasons and does not represent a limiting value. The measured data for the single pulse energy, duration and bandwidth-duration product are listed in columns 3-6. The upper part of the table refers to APM with the nonlinear interferometer, while the lower part presents some results for passive modelocking with the nonlinear absorber dye 9860; here pulse durations are also indicated in brackets for laser operation with an additional etalon in the cavity. The latter is tuned at anti-resonance to enlarge the effective spectral width of the stimulated amplification curve; details of the etalon were discussed recently [ 15 ]. If not stated differently in the table, a gaussian shape was consistent with the measured autocorrelation curves and used to derive the pulse duration. Table 1 shows that the NMI achieves shorter pulses than the nonlinear absorber. The pulses are approximately bandwidth-limited for all the investigated examples although deviations of several ten percent occur from the expected Fourier transform limit. Of special interest are the results for Nd:glass. An example for the measured autocorrelation curve is depicted in fig. 4. The exponential slopes are noteworthy extending over three orders of magnitude with a 1/e decay time of 150 fs. The solid line in the figure is calculated for a sech 2 shape yielding the pulse duration of t p = 4 6 0 + 2 0 fs. The circulating pulse in the laser approaches a constant duration in the latter part of the pulse train, obviously due to a balance between shortening and stretching mechanisms. Some information on the
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OPTICS COMMUNICATIONS
Volume 97, number 1,2 Table 1 Modeloeking results.
Rep. rate (Hz)
Energy (ILd)
Nd:Cr:GSGG Nd:YLF Nd:glass
20 10 5
5 2 6
Nd:Cr:GSGG no. 9860 etalon b) Nd:YLF no. 9860 etalon b) Nd:glass no. 9860
30
5
70
4
10
10
Duration (ps) 3.2+0.1 4.7+0.1 0.46 + 0.02 a) 4.2_+0.1 (3.0_+0.1) 7+0.5 (2.9_+0.1) b) 0.8_+0.1
A~,X tp 0.78__+0.03 0.45 + 0.05 0.63+0.03 0.5_+0.05 0.6_+0.05
") For sech2 shape, b) Ref. [ 15 ].
I
I
estimated. It is believed that spectral gain narrowing represents the opposing effect [2 ] determining the measured d u r a t i o n o f 460 fs.
I
u v
10_ I
5. C o n c l u s i o n s
Ur-
~ u
We have d e m o n s t r a t e d A P M operation o f flashl a m p p u m p e d Nd-lasers with i m p r o v e d properties as c o m p a r e d to nonlinear absorbers. Pulses o f several ~tJ energy and duration 0.5-5 ps are generated close to the F o u r i e r transform limitation. Self-stabilization o f the pulse a m p l i t u d e and a reproducibility o f the single pulse energy within _+ 1% are reported.
10 - 2
o
'5 lO. 3 .<
10-4 -1.6
I -o.a
I 0.0
I 0.8
1.6
Delay Time ( p s ) Acknowledgements
Fig. 4. Autocorrelation data for the Nd:glass laser with passive modelocking by the NMI with 2 mm of CS2; a pulse duration of 460_ 20 fs is measured assuming a sech2 shape (calculated solid c u r v e ).
shortening potential o f the N M I is available from autocorrelation m e a s u r e m e n t s o f the m o d i f i e d pulse decoupled from the b e a m splitter BS o f the interferometer (compare fig. 2). This " t r a n s m i t t e d " pulse o f the N M I is found to be longer by a factor o f 1.2 than the reflected pulse in the resonator suggesting a shortening o f the latter o f a few ten percent per cavity r o u n d trip. These n u m b e r s give a d d i t i o n a l supp o r t to the larger efficiency o f the N M I for passive modelocking as c o m p a r e d to the nonlinear absorber where shortening by only 1-2% per r o u n d trip was
The authors thank Dipl. Phys. T. D a h i n t e n for cooperation in the investigation o f the N d : Y L F laser. Stimulating discussions with Prof. A. Seilmeier are gratefully acknowledged.
References
[ 1] For a review, see C.V. Shank, in: Ultrashort laser pulses and applications, ed. W. Kaiser, Topics in Applied Physics, Vol. 60 (Springer, Berlin, 1988) p. 5. [2]P. Heinz and A. Laubereau, J. Opt. Soc. Am. B 6 (1989) 1574. [3] D.E. Spence, P.N. Kean and W. Sibbett, Optics Lett. 16 ( 1991 ) 42; J. Opt. Soc. Am. B 8 ( 1991 ) 2053. 39
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[4] F. Salin, J. Squier and M. Piche, Optics Lett. 16 (1991) 1674; G.P.A. Malcolm and A.L. Ferguson, Optics Lett. 16 ( 1991 ) 1967. [5] D.K. Negus, L. Spinelli, N. Goldblatt and G. Feuget, OSA Proc. on Advanced Solid State Lasers 10 ( 1991 ) 120; Ch. Spielmann, F. Krausz, T. Brabec, E. Wintner and A.J. Schmidt, Optics Lett. 16 ( 1991 ) 1180; 17 (1992) 204. [6] L.F. Mollenauer and R.H. Stolen, Optics Lett. 9 (1984) 13; K.J. Blow and B.P. Nelson, Optics Lett. 13 ( 1988 ) 1026; K.J. Blow and D. Wood, J. Opt. SOc. Am. B. 5 ( 1988 ) 629. [7] F. Ouellette and M. Piche, Optics Comm. 60 (1986) 99; Can. J. Phys. 66 ( 1988 ) 903. [8] P.N. Kean, X. Zhu, D.W. Crust, R.S. Grant, N. Langford and W. Sibbett, Optics Lett. 14 (1989) 39; J. Mark, L.Y. Liu, K.L. Hall, H.A. Haus and E.P. Ippen, Optics Lett. 14 (1989) 48; E.P. Ippen, H.A. Haus and L.Y. Liu, J. Opt. SOc. Am. B 6 (1989) 1736. [9] J. Goodberlet, J. Wang, J.C. Fujimoto and P.A. Schulz, Optics Lett. 14 (1989) 1125; U. Keller, G.W. 't Hooft, W.H. Knox and J.E. Cunningham, Optics Lett. 16 ( 1991 ) 1022; H.A. Haus, U. Keller and W.H. Knox, J. Opt. Soc. Am. B 8 (1991) 1252. [10] F. Krausz, Ch. Spielman, T. Brabec, E. Wintner and A.J. Schmidt, Optics Lett. 15 (1990) 737, 1082; Appl. Phys. Lett. 58 (1991) 2470. [ 11 ] J. Goodberlet, J. Jacobson, J.G. Fujimoto, P.A. Schulz and T.Y. Fan, Optics Lett. 15 (1990) 504; L.Y. Liu, J.M. Huxley, E.P. Ippen and H.A. Haus, Optics Lett. 15 (1990) 553;
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J.M. Liu and J.K. Chee, Optics Lett. 15 (1990) 685; G.P.A. Malcolm, P.F. Curley and A.I. Ferguson, Optics Lett. 15 (1990) 1303. [12] K.P. Komorov, A.S. Kuyanov and V.D. Ugozhaev, Sov. J. Quantum Electron. 16 (1986) 520; A.M. Varshin, V.M. Gordienko, S.V. Krayuskin, V.T. Platonenko and V.K. Poppy, SOv. J. Quantum Electron. 16 (1986) 1125. [ 13 ] K. Burneika, R. Grigoris, A. Piskarskas, G. Sinkyavichius and V. Sirukaitis, Sov. J. Quantum Electron. 18 (1988) 1034. [ 14 ] P. Heinz, W. Kriegleder and A. Laubereau, Appl. Phys. A 43 (1987) 209. [15] P. Heinz and A. Laubereau, J. Opt. Soc. Am. B 7 (1990) 182; K. Wolfrum and P. Heinz, Optics Comm. 84 ( 1991 ) 290. [ 16] E.V. Zharkiov, N.N. ll'ichev, V.V. Laptev, A.A. Malyuthin, V.G. Ostroumov, P.P. Pashinin, A.S. Pimenov, V.A. Smirnov and I.A. Sherbakov, Sov. J. Quantum Electron. 13 (1983) 82. [ 17 ] A. Reuther and P. Heinz, unpublished data. [18] R. Danelyus, I. Kuratev, A. Piskarskas, V. Sirukaitis, E. Shvom, A. Yuozapavichyus and A. Yankauskas, Sov. J. Quantum Electron. 15 ( 1985 ) 1160; A.V. Babushkin, N.S. Vorob'ev, E.V. Zharikov, S.P. Kalitin, V.V. Osiko, A.N. Prokhorov, Yu.N. Serdyuchenko, M.-Ya. Shchelev and I.A. Shcherbakov, Sov. J. Quantum Electron. 16 (1986) 428. [ 19 ] A.L. Harmer, A. Linz and D.R. Gabbe, J. Phys. Chem. Solids 30 (1969) 1483; J.E. Murray, IEEE J. Quantum Electron. QE-19 ( 1983 ) 488.