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Generation, measurement and optimization of a variable duration, short pulse, mode-locked cavity-dumped Nd:YAG laser
ARTICLE IN PRESS
Optics & Laser Technology 40 (2008) 427–434 www.elsevier.com/locate/optlastec
Generation, measurement and optimization of a variable duration, short pulse, mode-locked cavity-dumped Nd:YAG laser S. Chaurasiaa,, C.G. Muralia, L.J. Dhareshwara, R. Vijayanb, A.C. Shikalgarb a
Laser and Neutron Physics Section, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India Laser and Plasma Technology Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India
b
Received 23 April 2007; received in revised form 21 June 2007; accepted 22 June 2007
Abstract Spectral and temporal measurements undertaken on a single picosecond laser pulse from a flash lamp pumped, cavity dumped, active/ passive mode-locked Nd:YAG laser are presented in this paper. Optimization of several parameters of the resonator cavity produced a single pulse with 0.7 mJ energy and 102 contrast. The pulse duration was variable from 24 to 120 ps by using intra-cavity etalons of different thicknesses. The pulse width and spectrum of the pulse were simultaneously measured using a second harmonic autocorrelator and a spectrometer. The time bandwidth product was 0.445, which is close to theoretical limit for a bandwidth limited pulse. r 2007 Elsevier Ltd. All rights reserved. Keywords: Mode-locked lasers; Intra-cavity etalon; Time bandwidth product
1. Introduction Commercial picosecond lasers used for micro-material processing frequently consist of flash lamp pumped Nd:YAG oscillators followed by one or two amplifiers, due to their lower cost as compared to diode pumped systems. Large Nd:glass laser systems also use such oscillators at the front end. Stability requirements on the master oscillator at the front end are quite stringent in such laser systems since the characteristics of the processed material strongly depends on the laser pulse duration and pulse energy. In large laser systems, it is also a necessary prerequisite to have a stable and reliable mode-locked laser oscillator. Typically, laser pulse durations in the range of tens of picoseconds to a few nanoseconds with less than 5% fluctuation in pulse energy and pulse duration from shot to shot are used for these experiments. Short pulses of few picoseconds to few hundred picoseconds can be easily produced by the technique of active–passive mode-locking [1–4]. Single pulse energy of about a millijoule in a pulse of 20 ps duration has been obtained by the technique of active mode-locking with Q-switching in Nd:YAG lasers [5]. In Corresponding author. Tel.: +91 22 25590205; fax: +91 22 25505296.
E-mail address: [email protected] (S. Chaurasia). 0030-3992/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2007.06.007
Q-switched mode-locking or active/passive mode-locking using a saturable absorber dye along with the acousto-optic modulator (AOM), the laser output is typically in the form of a Q-switched envelope consisting of 8–10 mode-locked pulses [6]. The total output energy of about a millijoule is distributed amongst these pulses; and when an external pulse selector is used, the selected pulse has only a fraction of a millijoule energy. Gain control of an active/passive mode-locked Nd:glass laser has given very short laser pulses (about 10 ps) and diode pumped Nd:YAG lasers have generated mode-locked pulses as short as 8.5 ps [7]. We have used the technique of cavity dumping [8–10] to achieve more than a millijoule energy in the single modelocked pulse from a flash lamp pumped Nd:YAG laser. In this technique, internal gain switching has been done using a single intra-cavity electro-optic modulator to dump the single pulse which builds up in the cavity with a high peakto-background contrast. Cavity dumping in an active/ passive modelocked laser has several advantages, compared to active/passive mode locking with external pulse selection. These advantages are:
A 10-fold higher energy in the selected pulse. In case of external single pulse selection from mode-locked train,
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the energy in the pulse is about 0.05–0.2 mJ, whereas cavity dumping can yield more than a millijoule energy in the pulse. PC
M2
M1
PD1
AVPD
A DC
AOM
Etalon LH
P
ATC
Delay
TG
PD2
CRO
Fig. 1. Optical lay out of the cavity-dumped laser scheme. DC, dye cell; M1 and M2, resonator mirrors of maximum reflectivity; AOM, acoustoptic modulator; P, thin film polarizer; A, mode selecting aperture; LH, laser head; PC, Pockel’s cell; ATC, avalanche transistor circuit; TG, trigger circuit; AVPD, avalanche photodiode; PD1 and PD2, bi-planar photodiodes; CRO, oscilloscope.
Lower threshold of the laser due to high Q of the resonator (since both mirrors are of maximum reflectivity) during the pulse build-up which is an advantage while operating the laser at a higher repetition rate. Pockel’s cell needs a step of quarter wave voltage (3.4 kV) as compared to a half-wave voltage pulse of fast rise/fall time in case of external pulse selection and this is more difficult to achieve reliably. High peak to background contrast ratio, typically about 100:1. In external pulse selection, it is quite common to get prepulses due to jitter in the switching of Pockel’s cell. These pre-pulses could be detrimental in some laser applications.
Intra-cavity etalons of different thickness can be used easily to vary the duration of the dumped pulse [11]. In our laboratory, it was required to have a front-end oscillator
1100 V T1
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H.V. Power Supply
Pulse Amplifier
Laser Pulse
Avalanche Transistor Marx Bank Circuit
Pockel’s Cell 3.5kV
Fast Photodiode
Detector Fig. 2. (a) Avalanche transistor Marx bank circuit and (b) Pockel’s cell driver circuit arrangement.
R? 470
ARTICLE IN PRESS S. Chaurasia et al. / Optics & Laser Technology 40 (2008) 427–434
with a pulse duration variable from 20 to 120 ps for a 30 GW Nd:glass laser being used for several laser plasma experiments. A chirp free mode-locked laser pulse without sub-structure is essential for further amplification in amplifier stages to obtain a single amplified pulse with high peak to background contrast. In this paper, we present a laser scheme to generate short picosecond laser pulses which is chirp free and bandwidth limited. The scheme consists of an active/passive modelocked cavity dumped Nd:YAG laser oscillator. Several laser parameters have been optimized such as output energy per pulse, pulse-to-pulse stability in amplitude, and temporal width. The effect of the introduction of intracavity etalons of different thicknesses on temporal characteristic was studied. The laser pulse duration has been varied from 24 to 120 ps by inserting intra-cavity etalons of different thickness. Simultaneous optimization of several parameters such as gain, dye concentration, rf power on acousto-optic modulator, rise time of the electro-optic switch, resonator length has resulted in a reliable switching
429
of a single pulse of 0.7 mJ energy and 102 contrast. To the best of our knowledge, such an optimization with such a large number of parameters has not been presented before. 2. Experimental setup The optical schematic for the cavity dumped, active/ passive mode-locked laser oscillator is shown in Fig. 1. The laser resonator consisted of a plane mirror M2 and a concave mirror M1 with a radius of curvature 5 m, both mirrors have more than 99.9% reflectivity at 1.064 mm and are separated by 1.5 m (cavity round trip time is 10 ns, the cavity round trip frequency is 100 MHz). The curved mirror was within the contacted dye cell DC that had a dye path length of 0.5 mm. Eastman Kodak dye 9740 dissolved in 1,2-dichloroethane is used in the dye cell as the saturable absorber to give a 65% transmission for low intensity. The laser pump head consisted of a 5 mm diameter, 75 mm long Nd:YAG rod with a wedge of 11 between its two faces, placed along one of the foci of an elliptical gold-coated
Fig. 3. (a) Mode-locked train, (b) single dumped out laser pulse, (c) terminated mode-locked train within the cavity after the pulse is dumped out, (d) single pulse displayed along with the intra-cavity mode-locked train to show that pulse selection has occurred at the peak of the mode-locked train.
ARTICLE IN PRESS S. Chaurasia et al. / Optics & Laser Technology 40 (2008) 427–434
500V
A Charging Unit (Charging control, HV X
TO flash lamp C M 1KV
Dump
Trigger
Feedback
Comparator Set Voltage Safety Trip Safety
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aluminum cavity. A linear xenon filled flash lamp of 5 mm diameter and 75 mm arc length was placed along the other focus of the cavity. Electrical input to the flash lamp was 12–25 J. The cavity was cooled with pressurized cold air whenever the laser was operated in a repetitive mode. Active mode-locking was achieved by the acousto-optic modulator driven at 50 MHz (which is half of the cavity round trip frequency) and 14 W rf power. Depth of modulation was about 20%. Tuning of resonator length or the cavity round trip frequency was carefully done to match rf frequency, by moving the mirror M2 with an accuracy of 50 mm using a translational stage. Even a slight detuning of the cavity round trip frequency resulted in an irregular spiking of the mode-locked pulse train sampled by a vacuum photodiode PD1. Pulse-to-pulse reliability also improved when the resonator round trip frequency was exactly matched with rf frequency. The cavity dumping was achieved by the Pockel’s cell made of a KD*P crystal (INRAD model M/N 202-090) to which step pulse of 3.4 kV (quarter wave voltage) was applied at the desired instant of time when cavity dumping was required to be done. Initially, the voltage on the Pockel’s cell was zero. The high Q state of the cavity thus favors the build up of the mode-locked pulse within the resonator, which is continuously sensed by the avalanche photodiode (AVPD) which generates the 10 V trigger pulse for the Marx Bank circuit with 2N5551 transistors [12–14] shown in Fig. 2(a). The Pockel’s cell driver arrangement is shown in Fig. 2(b). After a delay of 18 ns (internal delay of the circuit), a 3.4 kV step voltage appears on the Pockel’s cell which rotates the polarization of the laser pulse by l/2 in double pass and hence was rejected by a thin film polarizer P kept at an angle of 571 with respect to the resonator axis. This is the cavity dumped laser pulse. Triggering of the Pockel’s cell is matched to occur at the peak of the Q-switched mode-locked train by adjusting the delay of triggering of the avalanche transistor circuit, which can be achieved by introducing the filter in front of AVPD. The strength of the laser signal incident on AVPD decides the instant at which the Pockel’s cell is triggered with respect to the peak of the mode-locked pulse train. A mode-selecting aperture (A) of 2 mm diameter was used to obtain operation of the laser in TEM00 mode. When AVPD is closed, the Pockel’s cell does not trigger, the laser operates in the normal mode giving the complete mode-locked pulse train detected by a biplanar photodiode (80 ps rise time) PD1 and oscilloscope, shown in Fig. 3(a). When the high-voltage step pulse is applied on the Pockel’s cell, cavity dumping takes place and the single ultra-short pulse ejected from P is detected by a bi-planar vacuum photodiode PD2 and is displayed on a 500 MHz oscilloscope as shown in Fig. 3(b). In this condition, since there is no radiation in the cavity after the pulse is dumped, the mode-locked pulse train is terminated at this instant and detected by PD1 is as shown in Fig. 3(c). A block diagram of laser power supply unit is shown in Fig. 4(a) and the trigger unit for single/repetitive operation
Charge
430
S Oscillator
Pulse Amp & Driver
A
Repetitive
Fig. 4. (a) Energy storage unit block diagram and (b) single/repetitive trigger circuit block diagram.
Fig. 5. High-voltage step pulse on Pockel’s cell displayed together with optical pulse to show the synchronization of both events.
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Fig. 6. Shifting of mode-locked pulse train along flash lamp pulse with change in energy to the flash lamps. Single switched out pulse energy is the highest when the mode-locked train is at the peak.
is shown in Fig. 4(b). The electrical energy input to the flash lamp is adjusted to obtain the mode-locked train at the peak of the flash lamp pulse. The step voltage pulse of 3.4 kV applied to the Pockel’s cell is detected by a 1000 high-voltage probe and is observed to have a rise time less than 7 ns (with cable 4 ns without cable) and is decided by the time constant of driver impedance and the Pockel’s cell capacitance. The output laser pulse synchronized with this voltage pulse is shown in Fig. 5. Pulse-to-pulse jitter in the high voltage pulse was less than 2 ns. Stringent precautions had to be taken to protect the high-voltage circuit against EMI. Shape of the pulse deteriorated with slight increase in cable length. The laser was operated in single shot as well as repetitive mode, with repetition frequency of 0.4 Hz. When operated in the higher repetition mode, the pulse-to-pulse variation in energy was found to be quite large, possibly due to insufficient cooling between the flash lamp pulses. 3. Results and discussions In order to improve the reliability of the laser in terms of peak amplitude of the pulse and its duration, effects of various parameters such as dye concentration, flash lamp input energy, rf power to acousto-optic modulator and finally length of the resonator, were studied. Initial setting of the laser was done with only the solvent in the dye cell. Resonator was aligned to give the lowest threshold and with the acousto-optic modulator ON. Tuning of resonator length gives a regular mode-locked pattern. Saturable absorber dye concentrations used were having low-level transmission between 50% and 90%. Concentration of the dye decides the laser threshold, duration of the Q-switched envelope, output pulse energy and also duration of the cavity dumped mode-locked pulse. Variation of single pulse energy with dye concentration was studied to get the maximum pulse energy of 1.5 mJ at a dye transmission of 55%. However, this dye concentration resulted in the power density in the cavity exceeding the damage threshold
Fig. 7. Effect of cavity length detuning on mode-locked pulse train uniformity.
of the thin film polarizer P, which has the lowest damage threshold. Hence, this constraint restricted the output to about 0.7 mJ (power density of 760 MW/cm2). Variation of flash lamp energy resulted in the shifting of mode-locked pulse with respect to flash lamp pulse peak as shown in Fig. 6. It is observed that, when the mode-locked train is positioned at the peak of the flash lamp pulse, energy of the single pulse dumped out of the cavity is the highest. Spatial, temporal and spectral diagnostics were used online to simultaneously characterize the mode-locked pulse. One of the temporal diagnostics consisted of a bi-planar photodiode with 80 ps rise time, which was mainly used to detect the intra-cavity pulse trains to determine the instant at which the Pockel’s cell had to be switched. Amplitude of the cavity-dumped pulse was monitored to observe
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Fig. 8. Effect of jitter in the voltage pulse on Pockel’s cell resulting in the variation in timing of dumped out pulse with respect to the voltage pulse and amplitude of the laser pulse.
shot-to-shot variation. The peak-to-background contrast ratio was measured on-line by a bi-planar photodiode and was observed to be 100:1. The build-up of mode-locked pulse train within the cavity and the timing at which the pulse is dumped out is shown in Fig. 3(d). In this figure, the signals of PD1 and PD2 are shown simultaneously. This helps to place the dumped out pulse at the peak of the pulse train. Fig. 7 shows the effect of resonator detuning on the mode-locked pulse train’s non-uniformity. Here, we see that even a slight detuning of resonator length by 0.1 mm produced satellite pulses. Effect of jitter in Pockel’s cell switching on the characteristics of the dumped output pulse is shown in Fig. 8(a)–(c). These photographs show that there is a jitter of about 2–3 ns in the high-voltage step pulse applied on the Pockel’s cell. This results in the shifting of the instant at which the Pockel’s cell is switched with respect to the build up of the mode-locked pulse train within the cavity and therefore there is a variation in the amplitude of the single switched out laser pulse. Ringing in the high-voltage pulse on Pockel’s cell which is caused by an impedance mismatch and connecting cable is shown in Fig. 9. Here, we see two or sometimes even three pulses dumped out of the cavity. This was taken care by proper impedance matching and keeping a very short connecting cable. Duration of the mode-locked pulse was measured using a high resolution (1 ps), background-free, scanning second harmonic autocorrelator working in a non-collinear geometry as shown in the setup of Fig. 10(a) and the single-shot commercial autocorrelator (Euroscan Make) autocorrelation signal is shown in Fig. 10(b). The pulse duration measured with scanning autocorrelator was 24 ps (full width at 1/e2 points) and with single shot autocorrelator, it was 26 ps, which is in good agreement. Spatial profile of the output laser was monitored using a beam profiler (Model-Coherent Auburn Division, USA)) and shown in Fig. 10(c). It is a TEM00 mode and beam
Fig. 9. Effect of ringing of voltage on Pockel’s cell resulting in two or three output pulses.
divergence is observed to be about 1 mrad. The calculated value for beam divergence for this setup is 0.77 mrad. The spectral characteristic of the laser pulse was recorded using a scanning type of monochromator having a resolution of 0.2 A˚. Fig. 11 shows the spectrum of the 24 ps pulse obtained without the intra-cavity etalon and bandwidth is observed to be 0.7 A˚, which gives the time bandwidth product 0.445, which is close to the theoretical value of 0.441. In order to vary the duration of the mode-locked pulse, glass etalons of 2, 3 and 5 mm thickness were
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pulse width measured by single shot auto correlator is (2Fl/ce2 = 24) ps.
1.2 BS 100% REFLEC TING MIRROR
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Fig. 10. (a) Second harmonic scanning autocorrelator setup, (b) autocorrelation signal and (c) spatial beam recorded by beam profiler and intensity plot of the beam using home made Promise software.
introduced within the laser resonator. Variation of laser pulse duration with each of these etalon thicknesses is shown in Fig. 12. 4. Conclusions Detailed studies and optimization of various parameters have been performed on a flash lamp pumped Nd:YAG active/passive mode-locked laser with cavity dumping scheme to generate a 24 ps (FWHM) duration near Gaussian laser pulse. Spectral bandwidth of the pulse was measured simultaneously to determine the time bandwidth
product. The time bandwidth product has been observed to be 0.445, which is very close to the product for a Gaussian Transform limited pulse. The energy per pulse was 0.7 mJ with a peak-to-background contrast of 102. Such a laser pulse is, therefore, free of frequency chirp and is ideally suited for being used at the front end of a large amplifier chain. It is also well suited as an optical probe pulse for interferometry experiments for laser plasma studies as in our experiments. Insertion of intra-cavity etalons of different thickness has resulted in laser pulses of increasing pulse duration; the highest was 120 ps for a 5 mm thick BK7 glass etalon.
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Acknowledgments The spectrum measured by spectrometer is 0.7 Å
0.6
The authors wish to acknowledge the leadership, continuing scientific support and encouragement given by Dr. S. Kailas, AD, Physics Group and Dr. V.C. Sahni, Director, Physics Group, towards the program of High Peak Power Solid State Lasers of which the present work forms a part.
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References
intensity (V)
1.0 0.8
[1] [2] [3] [4] [5]
0.2 0.0 1063.5
1064.0
1064.5
1065.0
wavelength (Å) [6] Fig. 11. Spectrum of the 24 ps laser pulse. [7] [8] [9] [10] [11] [12] [13]
120
100
Pulse width (ps)
[14]
80
60
40
20
0
1
2
3
4
5
Etalon thickness (mm) Fig. 12. Variation of laser pulse duration with etalon thickness.
Albrecht GF, Lund L, Smith D. Appl Opt 1983;22:1276. Vanherzeele H, VanElck JL, Siegman AE. Appl Opt 1981;20:3484. Seka W, Bunkenburg J. J Appl Phys 1978;49:2277. Kuizenga DJ. Opt Commun 1977;22:156. Kuizenga DJ, Phillon DW, Lund T, Siegman AE. Opt Commun 1973;9:221. Dhareshwar LJ, Gopi N, Murali CG, Bapna RC. Bhabha Atomic Research Centre Report BARC/2001/I/011. Liu KX, Flood CJ, Walker DR, Van Driel HM. Opt Lett 1991; 17:1361. Glotz M, Eichler H-J. J Phys E Instrum 1978;20:1493. Heinz P, Fickenscher M, Laubereau A. Opt Commun 1987;62:343. Maydan D, Chesler RB. J Appl Phys 1971;42:1031. Magnante PC. J App Phys 1969;40:4437. Jethwa J, Marinero EE, Muller A. Rev Sci Instrum 1981;52:989. Oak SM, Bindra KS, Narayan BS, Khardekar RK. Rev Sci Instrum 1991;62:308. Christiansen H, Frank K, Hartmann W. Nucl Instrum and Methods Phys Res A 1987;256:539.
Optics & Laser Technology 40 (2008) 427–434 www.elsevier.com/locate/optlastec
Generation, measurement and optimization of a variable duration, short pulse, mode-locked cavity-dumped Nd:YAG laser S. Chaurasiaa,, C.G. Muralia, L.J. Dhareshwara, R. Vijayanb, A.C. Shikalgarb a
Laser and Neutron Physics Section, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India Laser and Plasma Technology Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India
b
Received 23 April 2007; received in revised form 21 June 2007; accepted 22 June 2007
Abstract Spectral and temporal measurements undertaken on a single picosecond laser pulse from a flash lamp pumped, cavity dumped, active/ passive mode-locked Nd:YAG laser are presented in this paper. Optimization of several parameters of the resonator cavity produced a single pulse with 0.7 mJ energy and 102 contrast. The pulse duration was variable from 24 to 120 ps by using intra-cavity etalons of different thicknesses. The pulse width and spectrum of the pulse were simultaneously measured using a second harmonic autocorrelator and a spectrometer. The time bandwidth product was 0.445, which is close to theoretical limit for a bandwidth limited pulse. r 2007 Elsevier Ltd. All rights reserved. Keywords: Mode-locked lasers; Intra-cavity etalon; Time bandwidth product
1. Introduction Commercial picosecond lasers used for micro-material processing frequently consist of flash lamp pumped Nd:YAG oscillators followed by one or two amplifiers, due to their lower cost as compared to diode pumped systems. Large Nd:glass laser systems also use such oscillators at the front end. Stability requirements on the master oscillator at the front end are quite stringent in such laser systems since the characteristics of the processed material strongly depends on the laser pulse duration and pulse energy. In large laser systems, it is also a necessary prerequisite to have a stable and reliable mode-locked laser oscillator. Typically, laser pulse durations in the range of tens of picoseconds to a few nanoseconds with less than 5% fluctuation in pulse energy and pulse duration from shot to shot are used for these experiments. Short pulses of few picoseconds to few hundred picoseconds can be easily produced by the technique of active–passive mode-locking [1–4]. Single pulse energy of about a millijoule in a pulse of 20 ps duration has been obtained by the technique of active mode-locking with Q-switching in Nd:YAG lasers [5]. In Corresponding author. Tel.: +91 22 25590205; fax: +91 22 25505296.
E-mail address: [email protected] (S. Chaurasia). 0030-3992/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2007.06.007
Q-switched mode-locking or active/passive mode-locking using a saturable absorber dye along with the acousto-optic modulator (AOM), the laser output is typically in the form of a Q-switched envelope consisting of 8–10 mode-locked pulses [6]. The total output energy of about a millijoule is distributed amongst these pulses; and when an external pulse selector is used, the selected pulse has only a fraction of a millijoule energy. Gain control of an active/passive mode-locked Nd:glass laser has given very short laser pulses (about 10 ps) and diode pumped Nd:YAG lasers have generated mode-locked pulses as short as 8.5 ps [7]. We have used the technique of cavity dumping [8–10] to achieve more than a millijoule energy in the single modelocked pulse from a flash lamp pumped Nd:YAG laser. In this technique, internal gain switching has been done using a single intra-cavity electro-optic modulator to dump the single pulse which builds up in the cavity with a high peakto-background contrast. Cavity dumping in an active/ passive modelocked laser has several advantages, compared to active/passive mode locking with external pulse selection. These advantages are:
A 10-fold higher energy in the selected pulse. In case of external single pulse selection from mode-locked train,
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the energy in the pulse is about 0.05–0.2 mJ, whereas cavity dumping can yield more than a millijoule energy in the pulse. PC
M2
M1
PD1
AVPD
A DC
AOM
Etalon LH
P
ATC
Delay
TG
PD2
CRO
Fig. 1. Optical lay out of the cavity-dumped laser scheme. DC, dye cell; M1 and M2, resonator mirrors of maximum reflectivity; AOM, acoustoptic modulator; P, thin film polarizer; A, mode selecting aperture; LH, laser head; PC, Pockel’s cell; ATC, avalanche transistor circuit; TG, trigger circuit; AVPD, avalanche photodiode; PD1 and PD2, bi-planar photodiodes; CRO, oscilloscope.
Lower threshold of the laser due to high Q of the resonator (since both mirrors are of maximum reflectivity) during the pulse build-up which is an advantage while operating the laser at a higher repetition rate. Pockel’s cell needs a step of quarter wave voltage (3.4 kV) as compared to a half-wave voltage pulse of fast rise/fall time in case of external pulse selection and this is more difficult to achieve reliably. High peak to background contrast ratio, typically about 100:1. In external pulse selection, it is quite common to get prepulses due to jitter in the switching of Pockel’s cell. These pre-pulses could be detrimental in some laser applications.
Intra-cavity etalons of different thickness can be used easily to vary the duration of the dumped pulse [11]. In our laboratory, it was required to have a front-end oscillator
1100 V T1
R? 10M
2
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200pF 2
3
2
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Fast Photodiode
Detector Fig. 2. (a) Avalanche transistor Marx bank circuit and (b) Pockel’s cell driver circuit arrangement.
R? 470
ARTICLE IN PRESS S. Chaurasia et al. / Optics & Laser Technology 40 (2008) 427–434
with a pulse duration variable from 20 to 120 ps for a 30 GW Nd:glass laser being used for several laser plasma experiments. A chirp free mode-locked laser pulse without sub-structure is essential for further amplification in amplifier stages to obtain a single amplified pulse with high peak to background contrast. In this paper, we present a laser scheme to generate short picosecond laser pulses which is chirp free and bandwidth limited. The scheme consists of an active/passive modelocked cavity dumped Nd:YAG laser oscillator. Several laser parameters have been optimized such as output energy per pulse, pulse-to-pulse stability in amplitude, and temporal width. The effect of the introduction of intracavity etalons of different thicknesses on temporal characteristic was studied. The laser pulse duration has been varied from 24 to 120 ps by inserting intra-cavity etalons of different thickness. Simultaneous optimization of several parameters such as gain, dye concentration, rf power on acousto-optic modulator, rise time of the electro-optic switch, resonator length has resulted in a reliable switching
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of a single pulse of 0.7 mJ energy and 102 contrast. To the best of our knowledge, such an optimization with such a large number of parameters has not been presented before. 2. Experimental setup The optical schematic for the cavity dumped, active/ passive mode-locked laser oscillator is shown in Fig. 1. The laser resonator consisted of a plane mirror M2 and a concave mirror M1 with a radius of curvature 5 m, both mirrors have more than 99.9% reflectivity at 1.064 mm and are separated by 1.5 m (cavity round trip time is 10 ns, the cavity round trip frequency is 100 MHz). The curved mirror was within the contacted dye cell DC that had a dye path length of 0.5 mm. Eastman Kodak dye 9740 dissolved in 1,2-dichloroethane is used in the dye cell as the saturable absorber to give a 65% transmission for low intensity. The laser pump head consisted of a 5 mm diameter, 75 mm long Nd:YAG rod with a wedge of 11 between its two faces, placed along one of the foci of an elliptical gold-coated
Fig. 3. (a) Mode-locked train, (b) single dumped out laser pulse, (c) terminated mode-locked train within the cavity after the pulse is dumped out, (d) single pulse displayed along with the intra-cavity mode-locked train to show that pulse selection has occurred at the peak of the mode-locked train.
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500V
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aluminum cavity. A linear xenon filled flash lamp of 5 mm diameter and 75 mm arc length was placed along the other focus of the cavity. Electrical input to the flash lamp was 12–25 J. The cavity was cooled with pressurized cold air whenever the laser was operated in a repetitive mode. Active mode-locking was achieved by the acousto-optic modulator driven at 50 MHz (which is half of the cavity round trip frequency) and 14 W rf power. Depth of modulation was about 20%. Tuning of resonator length or the cavity round trip frequency was carefully done to match rf frequency, by moving the mirror M2 with an accuracy of 50 mm using a translational stage. Even a slight detuning of the cavity round trip frequency resulted in an irregular spiking of the mode-locked pulse train sampled by a vacuum photodiode PD1. Pulse-to-pulse reliability also improved when the resonator round trip frequency was exactly matched with rf frequency. The cavity dumping was achieved by the Pockel’s cell made of a KD*P crystal (INRAD model M/N 202-090) to which step pulse of 3.4 kV (quarter wave voltage) was applied at the desired instant of time when cavity dumping was required to be done. Initially, the voltage on the Pockel’s cell was zero. The high Q state of the cavity thus favors the build up of the mode-locked pulse within the resonator, which is continuously sensed by the avalanche photodiode (AVPD) which generates the 10 V trigger pulse for the Marx Bank circuit with 2N5551 transistors [12–14] shown in Fig. 2(a). The Pockel’s cell driver arrangement is shown in Fig. 2(b). After a delay of 18 ns (internal delay of the circuit), a 3.4 kV step voltage appears on the Pockel’s cell which rotates the polarization of the laser pulse by l/2 in double pass and hence was rejected by a thin film polarizer P kept at an angle of 571 with respect to the resonator axis. This is the cavity dumped laser pulse. Triggering of the Pockel’s cell is matched to occur at the peak of the Q-switched mode-locked train by adjusting the delay of triggering of the avalanche transistor circuit, which can be achieved by introducing the filter in front of AVPD. The strength of the laser signal incident on AVPD decides the instant at which the Pockel’s cell is triggered with respect to the peak of the mode-locked pulse train. A mode-selecting aperture (A) of 2 mm diameter was used to obtain operation of the laser in TEM00 mode. When AVPD is closed, the Pockel’s cell does not trigger, the laser operates in the normal mode giving the complete mode-locked pulse train detected by a biplanar photodiode (80 ps rise time) PD1 and oscilloscope, shown in Fig. 3(a). When the high-voltage step pulse is applied on the Pockel’s cell, cavity dumping takes place and the single ultra-short pulse ejected from P is detected by a bi-planar vacuum photodiode PD2 and is displayed on a 500 MHz oscilloscope as shown in Fig. 3(b). In this condition, since there is no radiation in the cavity after the pulse is dumped, the mode-locked pulse train is terminated at this instant and detected by PD1 is as shown in Fig. 3(c). A block diagram of laser power supply unit is shown in Fig. 4(a) and the trigger unit for single/repetitive operation
Charge
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S Oscillator
Pulse Amp & Driver
A
Repetitive
Fig. 4. (a) Energy storage unit block diagram and (b) single/repetitive trigger circuit block diagram.
Fig. 5. High-voltage step pulse on Pockel’s cell displayed together with optical pulse to show the synchronization of both events.
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Fig. 6. Shifting of mode-locked pulse train along flash lamp pulse with change in energy to the flash lamps. Single switched out pulse energy is the highest when the mode-locked train is at the peak.
is shown in Fig. 4(b). The electrical energy input to the flash lamp is adjusted to obtain the mode-locked train at the peak of the flash lamp pulse. The step voltage pulse of 3.4 kV applied to the Pockel’s cell is detected by a 1000 high-voltage probe and is observed to have a rise time less than 7 ns (with cable 4 ns without cable) and is decided by the time constant of driver impedance and the Pockel’s cell capacitance. The output laser pulse synchronized with this voltage pulse is shown in Fig. 5. Pulse-to-pulse jitter in the high voltage pulse was less than 2 ns. Stringent precautions had to be taken to protect the high-voltage circuit against EMI. Shape of the pulse deteriorated with slight increase in cable length. The laser was operated in single shot as well as repetitive mode, with repetition frequency of 0.4 Hz. When operated in the higher repetition mode, the pulse-to-pulse variation in energy was found to be quite large, possibly due to insufficient cooling between the flash lamp pulses. 3. Results and discussions In order to improve the reliability of the laser in terms of peak amplitude of the pulse and its duration, effects of various parameters such as dye concentration, flash lamp input energy, rf power to acousto-optic modulator and finally length of the resonator, were studied. Initial setting of the laser was done with only the solvent in the dye cell. Resonator was aligned to give the lowest threshold and with the acousto-optic modulator ON. Tuning of resonator length gives a regular mode-locked pattern. Saturable absorber dye concentrations used were having low-level transmission between 50% and 90%. Concentration of the dye decides the laser threshold, duration of the Q-switched envelope, output pulse energy and also duration of the cavity dumped mode-locked pulse. Variation of single pulse energy with dye concentration was studied to get the maximum pulse energy of 1.5 mJ at a dye transmission of 55%. However, this dye concentration resulted in the power density in the cavity exceeding the damage threshold
Fig. 7. Effect of cavity length detuning on mode-locked pulse train uniformity.
of the thin film polarizer P, which has the lowest damage threshold. Hence, this constraint restricted the output to about 0.7 mJ (power density of 760 MW/cm2). Variation of flash lamp energy resulted in the shifting of mode-locked pulse with respect to flash lamp pulse peak as shown in Fig. 6. It is observed that, when the mode-locked train is positioned at the peak of the flash lamp pulse, energy of the single pulse dumped out of the cavity is the highest. Spatial, temporal and spectral diagnostics were used online to simultaneously characterize the mode-locked pulse. One of the temporal diagnostics consisted of a bi-planar photodiode with 80 ps rise time, which was mainly used to detect the intra-cavity pulse trains to determine the instant at which the Pockel’s cell had to be switched. Amplitude of the cavity-dumped pulse was monitored to observe
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Fig. 8. Effect of jitter in the voltage pulse on Pockel’s cell resulting in the variation in timing of dumped out pulse with respect to the voltage pulse and amplitude of the laser pulse.
shot-to-shot variation. The peak-to-background contrast ratio was measured on-line by a bi-planar photodiode and was observed to be 100:1. The build-up of mode-locked pulse train within the cavity and the timing at which the pulse is dumped out is shown in Fig. 3(d). In this figure, the signals of PD1 and PD2 are shown simultaneously. This helps to place the dumped out pulse at the peak of the pulse train. Fig. 7 shows the effect of resonator detuning on the mode-locked pulse train’s non-uniformity. Here, we see that even a slight detuning of resonator length by 0.1 mm produced satellite pulses. Effect of jitter in Pockel’s cell switching on the characteristics of the dumped output pulse is shown in Fig. 8(a)–(c). These photographs show that there is a jitter of about 2–3 ns in the high-voltage step pulse applied on the Pockel’s cell. This results in the shifting of the instant at which the Pockel’s cell is switched with respect to the build up of the mode-locked pulse train within the cavity and therefore there is a variation in the amplitude of the single switched out laser pulse. Ringing in the high-voltage pulse on Pockel’s cell which is caused by an impedance mismatch and connecting cable is shown in Fig. 9. Here, we see two or sometimes even three pulses dumped out of the cavity. This was taken care by proper impedance matching and keeping a very short connecting cable. Duration of the mode-locked pulse was measured using a high resolution (1 ps), background-free, scanning second harmonic autocorrelator working in a non-collinear geometry as shown in the setup of Fig. 10(a) and the single-shot commercial autocorrelator (Euroscan Make) autocorrelation signal is shown in Fig. 10(b). The pulse duration measured with scanning autocorrelator was 24 ps (full width at 1/e2 points) and with single shot autocorrelator, it was 26 ps, which is in good agreement. Spatial profile of the output laser was monitored using a beam profiler (Model-Coherent Auburn Division, USA)) and shown in Fig. 10(c). It is a TEM00 mode and beam
Fig. 9. Effect of ringing of voltage on Pockel’s cell resulting in two or three output pulses.
divergence is observed to be about 1 mrad. The calculated value for beam divergence for this setup is 0.77 mrad. The spectral characteristic of the laser pulse was recorded using a scanning type of monochromator having a resolution of 0.2 A˚. Fig. 11 shows the spectrum of the 24 ps pulse obtained without the intra-cavity etalon and bandwidth is observed to be 0.7 A˚, which gives the time bandwidth product 0.445, which is close to the theoretical value of 0.441. In order to vary the duration of the mode-locked pulse, glass etalons of 2, 3 and 5 mm thickness were
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pulse width measured by single shot auto correlator is (2Fl/ce2 = 24) ps.
1.2 BS 100% REFLEC TING MIRROR
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Fig. 10. (a) Second harmonic scanning autocorrelator setup, (b) autocorrelation signal and (c) spatial beam recorded by beam profiler and intensity plot of the beam using home made Promise software.
introduced within the laser resonator. Variation of laser pulse duration with each of these etalon thicknesses is shown in Fig. 12. 4. Conclusions Detailed studies and optimization of various parameters have been performed on a flash lamp pumped Nd:YAG active/passive mode-locked laser with cavity dumping scheme to generate a 24 ps (FWHM) duration near Gaussian laser pulse. Spectral bandwidth of the pulse was measured simultaneously to determine the time bandwidth
product. The time bandwidth product has been observed to be 0.445, which is very close to the product for a Gaussian Transform limited pulse. The energy per pulse was 0.7 mJ with a peak-to-background contrast of 102. Such a laser pulse is, therefore, free of frequency chirp and is ideally suited for being used at the front end of a large amplifier chain. It is also well suited as an optical probe pulse for interferometry experiments for laser plasma studies as in our experiments. Insertion of intra-cavity etalons of different thickness has resulted in laser pulses of increasing pulse duration; the highest was 120 ps for a 5 mm thick BK7 glass etalon.
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Acknowledgments The spectrum measured by spectrometer is 0.7 Å
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The authors wish to acknowledge the leadership, continuing scientific support and encouragement given by Dr. S. Kailas, AD, Physics Group and Dr. V.C. Sahni, Director, Physics Group, towards the program of High Peak Power Solid State Lasers of which the present work forms a part.
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References
intensity (V)
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[1] [2] [3] [4] [5]
0.2 0.0 1063.5
1064.0
1064.5
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wavelength (Å) [6] Fig. 11. Spectrum of the 24 ps laser pulse. [7] [8] [9] [10] [11] [12] [13]
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[14]
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1
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Etalon thickness (mm) Fig. 12. Variation of laser pulse duration with etalon thickness.
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