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A high intensity titanium-dopedsapphire laserD. J. Fraser a & M. H. R. Hutchinson aa The Blackett Laboratory , Imperial College of Science,Technology and Medicine , London, SW7 2BZPublished online: 03 Jul 2009.

To cite this article: D. J. Fraser & M. H. R. Hutchinson (1996) A high intensity titanium-dopedsapphire laser, Journal of Modern Optics, 43:5, 1055-1062, DOI: 10.1080/09500349608233265

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JOURNAL OF MODERN OPTICS, 1996, VOL. 43, NO. 5 , 1055-1062

A high intensity titanium-doped sapphire laser

D. J. FRASER and M. H. R. HUTCHINSON The Blackett Laboratory, Imperial College of Science, Technology and Medicine, London SW7 2BZ

(Received 19 October 1995; revision received)

Abstract. A high-intensity titanium-doped sapphire laser system has been developed which employs the technique of chirped pulse amplification (CPA). Pulses of 1.50 fs duration are produced with energies up to 60 mJ allowing focused intensities of up to 10” W cm-* on target. This system has proved to be a useful tool in the study of the interaction of strong fields with matter such as high harmonic generation, above-threshold ionization and molecular dynamics.

1. Introduction Recent developments in the field of laser physics have led to the construction

of compact ultrashort pulse, high peak power lasers which are capable of producing intensities up to 1OI8 W cm-2 [l-31. These lasers have proved to be an ideal research tool for the study of multiphoton and strong-field physics, such as high harmonic generation, above-threshold ionization, molecular dynamics and laser- plasma interactions [4]. Laser systems used for these studies have been based around dye [S], excimer [6] and solid-state gain media. However solid-state laser materials such as Nd:glass, Cr:LiSAlF and Tisapphire offer several advantages over dye and excimer lasers, having broad gain bandwidths and large saturation fluences ( - J cm-2). However, when amplifying picosecond and femtosecond pulses to near the saturation fluence in solid-state materials, a critical intensity is reached where beam distortions occur, and in severe cases the damage threshold of the material may be exceeded. This problem, which is caused by the nonlinear refractive index (n2) in the gain material and other optical elements at high intensities can be overcome by using the technique of chirped pulse amplification (CPA) [7, 81. This technique has allowed the construction of compact table-top terawatt (T3) lasers. Here an initially short pulse produced from a mode-locked oscillator is stretched in time, producing a low-energy, long-duration chirped pulse. The pulse can then be amplified without encountering these deleterious nonlinear effects and finally compressed back to its original duration.

This technique was first implemented with neodymium-based sytems which typically used longer pulses which were spectrally broadened by self-phase modulation in an optical fibre. These systems suffered from low repetition rates and also from longer time-scale background (pedestal) on the final compressed pulse. With the development of ultrafast solid-state oscillators, such as those based on Ti:sapphire [9], many of the problems encountered with these earlier systems can be avoided. Tisapphire is a particularly convenient gain material, having a broad gain bandwidth ( - 200 nm) which allows the generation and amplification

0950-0340/96 $12.00 CQ 1996 Taylor & Francis Ltd

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1056 D. J . Fraser and M . H . R . Hutchinson

Spectrum , monitor and r

of ultrashort pulses. Therefore, amplification to only a modest energy is required in order to reach terawatt (10l2 W) output powers. T h e short upper state lifetime of Tisapphire ( - 3 ps) makes efficient flashlamp pumping difficult but it can be readily pumped with the frequency-doubled radiation from a Q-switched Nd:YAG laser [lo]. This paper reports on the development of a Tisapphire CPA laser system including its operation and characterization, as well as a brief review of some experimental work using this system.

Ti:Sapphire Nd:YAG Regenerative

1 OtW5Om J

2. Laser system A Ti:sapphire CPA laser has been constructed which is capable of producing

focused intensities up to 1017 W ern-'. It has been developed in order to study the interaction of strong fields with matter, and as such has to be a reliable and stable source of very intense optical pulses. It produces pulses with a duration of 150 fs and up to 60 mJ of energy and a far-field beam profile that is measured to be - 1.4 x the diffraction limit. A block diagram of the system is shown in figure 1 . It has four main stages, which are: (1) a mode-locked oscillator which is the source of ultrashort optical pulses, (2) a pulse stretcher, (3 ) amplifiers and (4) a pulse compressor. These various stages can be seen in figure 2, which shows the layout of the laser system on two optical benches. Each of these functional components will be described in turn.

CW Argon ion Laser Ti: Sap phi re

Single-Grating Pulse Stretcher

40mW enm/+ 250ps

Ti:Sapphire Multi-pass Amplifier

7nm’7 120mJ 250ps

Compressor Pulse Duration=l5Ofs Bandwidth=irnrn

Energy=GOrnJ RepRate=l OHz

I

Figure 1. Block diagram of the Tisapphire laser system.

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High intensity titanium-doped sapphire laser 1057

Figure 2. Optical layout of the Tisapphire laser system showing the different components of the system on two optical benches.

2.1. Laser system front end T o provide a source of ultrashort pulses, a Kerr lens mode-locked Ti:sapphire

oscillator has been developed. I t is pumped by 6.5 W from a CW argon-ion laser and produces nearly transform-limited 90 fs pulses at a repetition rate of 80 MHz and with an average output power of 400 m W (5 nJ per pulse, 50 kW peak power). T h e pulse duration is monitored continuously by a second-order scanning auto- correlator and typical day-to-day operation yields pulses of 90-1 00 fs (assuming a sech’ pulse shape). A grating spectrometer monitors the spectrum of the oscillator pulses, recording bandwidths ( F W H M ) of 8-10 nm, resulting in a time-bandwidth product of AVAT = 0.39. Intensity and interferometric autocorrelations are shown in figure 3 , as well as the spectral profile of the oscillator. T h e pump beam is focused onto a 20 mm Tisapphire crystal (0.05% by weight of Ti3+ in the host lattice) which is held in a water-cooled copper jacket to stabilize the temperature.

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1058 D. J . Fraser and M . H . R . Hutchinson

wavelength (nm)

-2'8 " " " " . ' '

-300 -200 -100 0 100 200 300

time (fs)

-a- I , , , , , , , . , , , , -1 .o -300 -200 -100 0 100 200 30

time (fs) Figure 3. (a) Spectrum of the oscillator, (b) interferornetric autocorrelation and (c) inten-

sity autocorrelation of oscillator pulses.

A 1.8 m folded cavity [l l] is used with a hard aperture to sustain the mode-locking process and an intracavity prism pair for dispersion compensation. Wavelength tunability is accomplished with the use of a slit after the intracavity prism pair where the beam is spatially dispersed. A tuning range of 740-820 nm is achieved but the output is typically set at a centre wavelength of 780nm. Stability is an important factor for CPA and it was found to be necessary to completely enclose the oscillator as air currents disrupt the mode-locking process.

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High intensity titanium-doped sapphire laser 1059

Figure 4. Schematic of (a) pulse stretcher and (6) pulse compressor,

The oscillator pulses are then stretched by a factor of -2500 to 250 ps in a four-pass grating stretcher, which acts as a dispersive delay line [12]. The stretcher consists of a single 2000 line per mm gold-coated holographic grating and a 750 m m focal-length plano-convex lens in a folded geometry, where the pulse makes four passes of the grating. The single-grating stretcher offers a number of advantages over the conventional two-grating design: (1) it is inherently simpler, (2) it offers easier alignment and (3) the angle of incidence is the same for each pass, whereas two-grating stretchers can suffer from an angular mis-match between the gratings. A schematic representation is illustrated in figure 4(a) . Due to the finite bandwidth of the pulses, different frequency components of the pulse will be diffracted through different angles by the first grating. T h e different frequency components then follow different paths through the stretcher, with the low-frequency components travelling a shorter distance than the higher-frequency components. Thus the pulse is stretched and frequency-coded (an approximately linear chirp) in such a way that a system acting on the stretched pulse in the opposite manner to the stretcher would reconstruct the pulse, returning it to its original duration. This process is carried out by a compressor as shown in figure 4(b).

2.2. Amplification and pulse compression In order to amplify the low-energy stretched pulse by greater than six orders

of magnitude, two stages of amplification are used. Firstly, a Tixapphire regenera- tive amplifier (RGA) is used, which provides the large small-signal gain. The RGA comprises a stable cavity consisting of a 20-mm Brewster-cut Tisapphire crystal (0.1 wt% doping) between two dielectric mirrors, 3 m radius of curvature (ROC) concave and 4 m ROC convex. This has the advantage that the output pulse will have the temporal characteristics of the input pulse but the spatial characteristics

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1060 D . J . Fraser and M . H . R . Hutchinson

of the RGA cavity, i.e. a TEM,, Gaussian mode. The Ti:sapphire crystal is pumped at 10 Hz by 50 mJ from frequency-doubled radiation of a Q-switched Nd:YAG laser (Spectron Laser Systems SLlQ/129) with the pump beam being imaged by a 50-cm focal length lens onto the Ti:sapphire crystal with a pump fluence on the first surface of - 1.7 J cm-2. The cavity also contains a Pockels cell between two thin-film plate polarizers and this combination acts as an electro-optic polarization switch. A high-voltage pulse generator drives the Pockels cell to its half-wave voltage of about 6 kV trapping a single pulse in the RGA cavity. The high-voltage electrical signal then propagates through a delay-line before returning to the Pockels cell to switch out the pulse after amplification close to saturation. The single switched-out pulse has now been amplified by a factor of 5 x lo6 to - 5 mJ. After magnification and collimation by a 2 x Galilean telescope to a diameter of 6 mm it is sent to the second stage of amplification. This is a Ti:sapphire multipass power amplifier (MA) which uses angular multiplexing, whereby the pulse propagates through the gain medium at small angles so that successive passes are spatially separated, but retains a good overlap with the pumped volume. A single antireflection-coated 1 cm3 Ti:sapphire crystal is pumped by a total of 700mJ at 10Hz from a Q-switched frequency-doubled Nd:YAG (BMI 503). A lens system is used to image the field distribution at the second harmonic crystal onto the Tisapphire crystal to provide spatially uniform pump beams of about 5 mm diameter. T h e Ti:sapphire crystal is pumped longitudinally from both sides so as to avoid damage to the coatings, and the pulse makes five passes of the crystal with a total gain of -20, taking the energy to 130 mJ. T h e beam is then up-collimated to a diameter of 10 mm before being sent to the compression stage. This consists of two parallel 2000 line per mm gold-coated holographic gratings, matched to that of the stretcher, which are double-passed, imparting negative group velocity dispersion (GVD) on the pulse and compressing it to near its original duration. The final pulse duration is monitored on a shot-to-shot basis by a second-order single-shot autocorrelator. A typical trace of a compressed pulse is shown in figure 5 , with a duration of 145 fs (assuming a sech2 pulse shape). This is somewhat longer than the original oscillator pulse duration due to bandwidth limiting elements and also due to the higher-order dispersion imparted on the pulse from the amplifier materials that cannot be fully compensated for by the compressor. T h e bandwidth of the compressed pulse is measured to be 6.8 nm, giving a time-bandwidth product of 0.49, which is approximately 1.5 x transform-limited. The total throughput of the compressor for p-polarized light is about 50%. The energy of the compressed pulse is measured to be -60 mJ, which corresponds to a peak power of 0.4 T W . The spatial beam profile was obtained by splitting off some of the main beam and imaging the focal spot of a 0.8-m focal length lens and using a charge-coupled device (CCD) camera and frame-store system. Figure 6 shows the smooth Gaussian profile of the beam at the focus with a l / e 2 radius of 56 pm, indicating a beam that is 1.4 times diffraction-limited.

From these gross parameters we can infer a maximum focused intensity when focusing with an f / l O lens of mid-10”W cm-2. The contrast ratio (defined as the ratio of the intensity of the main pulse to that of any pre- or post-pulse) is measured to be better than lo3 using a fast photodiode.

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High intensity titanium-doped sapphire laser

1 .oo-

0.80- e ? 0.60- (d

0.40- W

z bD 0.20- .3

1061

/ :. (b) \ \

1 \

a

;

! i

! \ :

1 .oo 0.80 -

3 0.60 -

0.40 -

I t . 8 . # . 0 . ,

-400 -200 0 200 400

time delay (fs)

wavelength (nm) Figure 5 . ( a ) Single-shot autocorrelation trace of an amplified and compressed pulse and

(6) corresponding spectrum.

Figure 6. Intensity distribution of an amplified and compressed pulse using a 0.8-m focal length lens.

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1062 H i g h intensity t i tanium-doped sapphire laser

Tunability of the system is achieved by changing the oscillator centre wave- length and adjustment of the grating angles in the stretcher and compressor. In principle, a tuning range of 740-820nm should be possible, but so far the laser has only been operated in the range 770-790 nm.

3. Experiments The ability to produce picosecond and femtosecond optical pulses and amplify

them to high peak powers has led to the ability to subject atoms to field strengths comparable to the coulomb field experienced by the atomic electrons. This has led to the discovery of many exciting new phenomena including above-threshold ionization (ATI) and high-order harmonic generation (HHG). This laser system has been used in a number of experimental studies, including HHG in atomic gases and organic molecules [13], and the dynamics of diatomic molecules [14].

4. Summary An ultrashort pulse, high peak power Ti:sapphire laser system has been

developed which is based on the technique of chirped pulse amplification. The system runs at 10 Hz and produces pulses with durations of - 150 fs and up to 60 mJ of energy, resulting in a maximum output power of -0.4 TW. The laser exhibits a smooth beam profile allowing good focusability, with a far-field distribution 1.4 times the diffraction limit. With tight focusing, intensities of greater than lo” W cm-* are achieved. Creating such high intensities is important for studying the non-perturbative behaviour of atoms and molecules in strong fields, and this laser system has proved to be an ideal research tool in the study of high-order harmonic generation, above-threshold ionization and molecular dynamics.

5. Acknowledgment

is gratefully acknowledged. Financial support by the Engineering and Physical Sciences Research Council

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[3] BARTY, C. P. J., GORDON 111, C. L., and LEMOFF, B. E., 1994, Opt . Lett . , 19, 1442. [4] See for example, Gavrila, M., (editor), 1992, Atoms in Intense Laser Fields (San Diego:

[S] MURNANE, M. M., and FALCONE, R. W., 1988, J. opt. SOC. A m . B, 5 , 1573. [6] WATANABE, S., ENDOH, A., WATANABE, M., SARUKURA, N., and HATA, K., 1989, J . opt.

[7] STRICKLAND, D., and MOUROU, G., 1985, Optics Commum., 56, 219. [8] MAINE, P., STRICKLAND, D., BADO, P., PESSOT, M., and MOUROU, G., 1988, IEEE J.

[9] FRENCH, P. M. W., 1995, Rep. Prog. Phys., 58, 169.

BLAIR, R. J., and FALCONE, R. W., 1991, O p t . Lett . , 16, 1406.

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[lo] MOULTON, P. F., 1986, J. opt. SOC. A m . B, 3, 125. [11] SPENCE, D. E., KEAN, P. N., and SIBBETT, W., 1991, O p t . Let t . , 16, 42. [12] MARTINEZ, 0. E., 1987, IEEE J. quant. Electron., 23, 59. [13] FRASER, D. J., HUTCHINSON, M. H. R., MARANGOS, J. P., SHAO, Y. L., TISCH, J. W. G.,

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