Acoustooptical delay lines for femtosecond pulse shaping...

Acoustooptical delay lines for femtosecond pulse shaping based on crystal materials with strong acoustic anisotropy

Vladimir Y. Molchanova, Sergey I. Chizhikova, Oleg Y. Makarovb, Efim A. Khazanovc, Vladislav N. Ginzburgc,

aNational University of Science and Technology “MISIS” (Russian Federation) bMolecular Technology GmbH (Germany)

cInstitute of Applied Physics of the Russian Academy of Sciences (Russian Federation)

The femtosecond pulse quality in a laser system is determined by a non-compensated high-order dispersion and spectral deformation of the amplifier. A dispersive device that modifies the spectral amplitude and phase of the femtosecond pulse should be used to improve the duration of the compressed pulse and suppress the prepulses. The application of acoustooptical dispersive delay lines is considered. Different modifications of delay lines are compared and the types of devices used to shape femtosecond pulses are discussed. Several experimental dispersive delay lines were fabricated and investigated. Keywords: femtosecond pulse shaping, acoustooptics, dispersive delay line.

1. I TRODUCTIO

In femtosecond laser systems, first created in the last two decades, energetic and temporal characteristics of laser

radiation are rapidly and vastly improving. Record power and impulse duration values have already been exceeded by several orders of magnitude. Lasers with peak impulse power of 1 petawatt can now be used to conduct fundamental research in new areas, used not only to create unusual matter states in laboratory conditions, but also to measure the dynamics of the changing properties of matter with temporal resolution of several femtoseconds, unattainable by any other method.

In most applications, the quality of compressed pulses (pulse duration, lack of prepulses, and high contrast ratio) are critical. The pulse quality is determined by a non-compensated high-order dispersions and spectral deformation in the amplifiers. An adaptive device that compensates for the induced high-order dispersion and spectrum distortion in the system by modifying the spectral amplitude and phase of the seed femtosecond pulse can be used to decrease the duration of the compressed pulse and suppress the prepulses. This conceptual solution helps increase femtosecond radiation power not by increasing the energy of the laser system, but by decreasing the duration of the pulse due to the correctly arranged spectral phase. Independent control of the spectral amplitude helps produce pulses of required shape, oriented at optimal conditions for amplifying the subsequent cascades.

Currently, three principal methods for controlling the spectral phase are known: chirped mirrors, a line of liquid crystal-based phase-modulated transparencies, and wave-electronics devices, i.e. adaptive time-delay lines based on the acoustooptical effect. The third method has a number of incontestable advantages, such as linear geometry that doesn’t require to be rigidly coordinated with stretcher geometry; continuous phase control (liquid crystal transparencies have finite-size borders between cells); the possibility of independently controlling not only spectral phase, but also amplitude; high-speed adaptation facilities.

Thanks to these advantages, acoustooptical time-delay lines (AODLs) have proven themselves to be the best among such devices.

A principal feature of AODLs stands out, compared to other types of acoustooptical devices. Acoustooptic interaction always performs consequent spectrum analysis. AODL is the only type among all other acoustooptical devices providing parallel but not consistent light spectrum processing. For example, acoustooptical modulators and deflectors control monochromatic laser radiation by multi-frequency sound (Table 1). An acoustooptical filter provides consistent light spectrum analysis by tunable monochromatic sound (Table 1). Light-sound interaction provides direct adaptive “light-to-light” spectral conversion determined by the transmission function of the AODL. Factually, AODL can be considered as a tunable acoustooptical filter, having an adaptive spectral transmission function (spectral window) synthesized by multi-frequency sound injected into the crystal according to the stated rules [1,2].

Laser Beam Shaping XI, edited by Andrew Forbes, Todd E. Lizotte, Proc. of SPIE Vol. 7789, 778909 · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.860698

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Table 1. The relation between light and sound spectra in acoustooptical devices

Type of the device Light spectrum Sound spectrum

AO Modulator

AO RF Analyzer

AO Deflector

AO Filter

AO Filter for WDM

AO Delay Line

For AODLs, collinear and quasicollinear acoustooptical interaction in crystals seems to be more suitable

because the spectral transmission function of the filter is defined by the spatial distribution of sound amplitude and frequency. Collinear acoustooptical interaction utilizes both the collinear phase and group velocities of light and sound. Collinear diffraction was demonstrated in crystal quartz and calcium molibdate single crystals by Harris at al in 1969 [3, 4]. Quasicollinear interaction takes place when phase velocity of incoming light is collinear to group velocity of sound. An acoustooptical tunable filter based on quasicollinear interaction in crystal quartz was first proposed by Kusters at al [5, 6]. According to available references, the collinear acoustooptical filter for the compression of ultrashort light pulses was first proposed in 1987 by Pustovoit and Pozhar [7-9]. In accordance with their approach, anisotropic acoustooptical interaction is used to convert each spectral compound from “fast” input polarization to “slow” diffracted polarization, with efficiency and position determined by sound spectral amplitudes and frequencies. The controlling of sound wave parameters provided pulse shaping both in spectral phase delay and amplitude. In 1997, Tournois [10-12] proposed the same technical solution for femtosecond pulse shaping, which he called an acoustooptic programmable dispersive filter (AOPDF). Presently this system, the DAZZLER, manufactured by Fastlite, France, is widely used in working with femtosecond systems in femtosecond laser equipment.

Despite the fact that AODLs seem to be the most effective means for increasing peak power of femtosecond pulses, they remain a virtually unstudied new generation of wave-electronics adaptive optical elements. Key parameters of AODLs are the same as key parameters of other acoustooptical devices: diffraction efficiency, spectral resolution, adaptive ability. However, for AODLs, these parameters should be considered separately for monochromatic and multi-spectral modes. Also, the unique parameter of AODLs is optical delay.

The goal of this paper is the development of new acoustooptical delay lines, optimized for varying system requirements.

2. ACOUSTOOPTICAL DELAY LI ES PRI CIPLES A D PARAMETERS

The basic concept of the AODL is based on dispersive properties of crystalline materials: the anisotropic acoustooptical interaction converts the fast (slow) input spectral component into a slow (fast) diffracted component. The amplitude and the phase of each spectral component are determined by the sound spectral amplitudes and frequencies. The control of sound waves provides laser pulse shaping both in the spectral phase delay and the amplitude; a concept that proved to be practical. As shown on Fig. 1, O-beam propagates with “fast” light velocity, E-beam propagates with “slow” velocity. AO interaction converts each spectral pulse component from “fast” O-polarization to “slow” E-polarization with spatial position and amplitude defined by sound spatial frequency and amplitude distribution. The maximum value of optical delay directly depends of the crystal length.



Fig.1. Ultrashort pulses compression based on anisotropic collinear light-sound interaction.

Consider a paratellurite single crystal. The main parameter that defines acoustooptical interaction geometry and properties is angle α between the incident light direction and crystallographic axis [110] (see [13]). System requirements should be taken into account to select the optimum value of angle α . The normalized dependences of acoustooptical figure of merit M2 (dashed line), as well as spectral resolution λλ Δ= /R (solid line) from α value [14] are presented in Fig.2. Here λ - optical wavelength, λΔ - spectral bandwidth of the filter. The normalized optical delay τ is represented by the dotted line. Fig.2 demonstrates clearly, that it’s impossible to create a quasicollinear AODL having simultaneously high resolution, low driving RF power and large optical delay.

Fig.2. Acoustooptical figure of merit M2 (solid curve), resolution λΔλ / (dashed curve) and optical delay τ versus angle α between incident light direction and [110] crystal axis.

The angular dispersion in adaptive dispersive devices is one of the most significant physical factors leading to angular chirp of femtosecond laser beam. This type of dispersion originates from the angular spectral dependence of Bragg synchronism and defines the chromatic aberrations of the device. The reduction of chromatic aberration can be done by adding a compensating prism at the crystal output having inverse dispersion. The compensation level can be next to perfect. In practice, it is convenient to employ acoustooptical devices when the diffracted beam propagates collinear to the input beam [15]. In this case, the angular compensation will be not so perfect. The typical angular chirp

0

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value γ outside the crystal is ~1 deg without the compensation, several angular seconds for compensation optimized for minimal angular chirp, and several angular minutes in case of “unbreaking of optical path” optimization. Even in the last case the angular beam spreading will be significantly less than typical diffractional divergence of laser beam in the crystal. The optical scheme of AODL with a compensation prism is shown in Fig.3. For a simpler illustration, the blue arrows, indicating light propagation direction, correspond to group light velocity direction while inside the crystal, and outside the crystal blue arrows show directions of phase velocities of light.

Fig.3. Optical scheme of AODL with angular chirp γ compensation.

3. ACOUSTOOPTICAL DELAY LI ES SPECIFICASTIO S

In accordance with the theoretical investigations, several acoustooptical delay lines for phase and amplitude correction of femtosecond laser pulses were calculated, manufactured and tested. These different experimental types of AODLs were optimized for different characteristics. For visible and near infra-red light wavebands, TeO2 was used as the acoustooptical interaction media, while KDP crystal was used for the UV region.

Here are the specifications of the TeO2 based AODL, optimized for high diffraction efficiency: - central light wavelength – 1250 nm; - femtosecond pulse spectral widths – 120 nm; - central ultrasound frequency – 52 MHz; - optical aperture – 4x4 mm; - crystal length – 53 mm; - spectral resolution (monochromatic mode, -3 dB level) – 0.5 nm @ 1310 nm; - maximal optical delay at 1250 nm – 15 psec - controlling RF power (monochromatic mode, 90% efficiency) – 30 mW; - diffraction efficiency for 120 nm spectrum width – 70%.

The spectral transmission function bandwidth of another AODL, also TeO2 based, but optimized for high spectral

resolution, measured in a single frequency regime by optical spectrum analyzer at -3 dB level, is about 0.24 nm at 1550 nm. The crystal length is 67 mm, the optical aperture is 4x4 mm. Other specifications are as follows: central laser wavelength is 800 nm, maximum optical delay exceeds 20 psec. The frequency of Bragg synchronism at 800 nm is 105 MHz. Driving RF power in a single frequency regime was about 60 mW at 90% efficiency, which permitted to control the whole femtosecond pulse spectrum up to the 200 nm in a multifrequency mode.

An experimental device for UV light region, based on 21 mm long KDP crystal, had spectral resolution 1 nm at 532 nm at -3 dB level, 3x3 mm aperture and diffraction efficiency >30% at 532 nm for driving RF power 1 Watt. The spectral transmission function of the delay lines measured in a single frequency regime with optical spectrum analyzer using white light source is presented in Fig 4. Fig. 4a demonstrates spectral transmission for AODL optimized for high diffraction efficiency, while Fig.4b corresponds to delay line, optimized for high spectral resolution.

[001] [110]

incident light

α γ

incident light

diffracted light



Fig.4. Spectral transmission function of experimental AODL in a single frequency regime. a) – transmission function of AODL, optimized for high diffraction efficiency

b) – transmission function of AODL, optimized for high spectral resolution

The general view of an experimental acoustooptical delay line optimized for high spectral resolution and conservation of light propagation direction is presented in Fig. 5. All the experimental devices were fabricated by an original method, utilizing interdiffusion in indium-gold nanostructures in vacuum. The complex electrical impedance of the piezotransducer was matched with the standard 50 – Ohm output of the driver according to phenomenological wideband approach developed in [16].

Fig.5. Photo of manufactured AODL optimized for high spectral resolution and unbreaking of optical path.

4. ACOUSTOOPTICAL DELAY LI ES DEDICATED TESTI G To conduct a study of the AODLs, the devices were incorporated in a cutting-edge SubPW OPCPA laser

[17]system created at the Institute of Applied Physics of the Russian Academy of Sciences. The photo of the AODL experimental setup in a sub-pettawatt laser system is presented on Fig.6.

a) b)



Fig. 6. Experimental AODL in sub-pettawatt laser system (Institute of Applied Physics RAS, Nizhniy Novgorod, Russian

Federation).

The system was based on one double-pass optical parametric amplifier (OPA) and operated at a repetition rate of 1 Hz. The femtosecond master oscillator was a Cr:forsterite laser generating 40 fs pulses at a central wavelength of 1250 nm. The AODL was placed between the femtosecond oscillator and the stretcher. The non-diffracted beam was passed to photodiode. The diffracted beam has no deviation from the initial direction and was passed to the stretcher. The stretcher expanded the pulse duration up to 600 ps. The OPA were pumped by the second harmonic of a single-mode single-frequency c Nd:YLF laser with a wavelength of 527 nm, up to 1 J energy in a 1.2 ns pulse [17]. The nonlinear KD*P element in OPA was 95 mm in length. During the first pass, the OPA performed broadband conversion of chirped pulses at 1250 nm into pulses of signal radiation at 910 nm. During the second pass, the 910 nm radiation was amplified. After OPA part the signal radiation was fed to a custom-made spectrograph. The experimental setup is shown on Fig.7.

Fig. 7. Experimental femtosecond setup.

D-diffracted beam; ND – non-diffracted beam;1 – femtosecond oscillator λi; 2 – AODL; 3 – stretcher; 4 – OPA λs; 5 – photodiode; 6 – oscilloscope; 7 – synchronization system; 8 – pumping laser Nd:Ylf, 1 Hz λp=527 nm; 9 – spectrograph.

The investigation of AODLs was made to provide control of the amplitudes of the spectral components of the femtosecond pulses in order to form an amplitude-frequency response optimized for further amplification [18]. In OPA, one of the pulses diffracted in AOPDF transformed to signal radiation with wave number determined by the equation 1/λs=1/λp-1/λi. Pump pulse energy was about 0.4 J to avoid saturation of parametric amplification, which can noticeably

λp

λs λi DND

λi 1

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a) b) c)

distort the shape of signal spectrum. The bandwidth of signal spectrum was about 12 nm FWHM (see Fig. 8a). This value was smaller than the bandwidth of injected pulses due to temporal non-uniformity of pump pulse (wings of signal spectrum amplified in smaller pump intensity). In order to control spectral component amplitudes, sound amplitude distribution was formed as: 1-d·exp[-(λ-λo)2/w2], producing corresponding modifications in signal spectrum (See Fig.8b). Proper selection of values for the gap’s depth and width made it possible to enlarge signal spectrum bandwidth by a factor of two (Fig. 8c).

Fig. 8. Signal pulse spectrums a) - without AODL; b) - with AODL forming crevasse with depth=1, width=4 nm; c) - with AODL forming spectral π -shaping with depth=0.6 width=28 nm. Values of width are converted to signal wave number.

A two-stage synchronization design [18] provided the simultaneous (to within ∼50 ps) passage of the pump and amplified radiation pulses through the nonlinear OPA crystals. Nevertheless the laser system has optical jitter ±12 μs with regard to the trigger pulse that started the AODL. The delay line must have a wide efficiency valley in order not to lose optical pulse. Fig. 9 shows coincidence of pump pulse with valley in consequence of non-diffracted femtosecond pulses. The AODL was tuned to transform whole spectrum of femtosecond pulses (the sound amplitude was constant, the sound frequency distribution was linear function) [19]. The duration of valley was about 50 μs, this value was enough in order not to lose injected in OPA pulses. The depth of valley corresponds to AODLs efficiency in 70 %. As demonstrated by the obtained results, the AODL completely outperformed the DAZZLER, which could not correctly work with ±12 μs optical jitter and provided efficiency ~30%.

Fig. 9. The jitter problem solution.



The designed AODL has demonstrated high efficiency and flexibility of femtosecond pulse control, and proved to be easy to operate. It opens the way for the optimization of the whole OPCPA laser system by increasing efficiency of parametric amplification, enlarging spectrum bandwidth, and improving the parameters of compressed pulse.

5. CO CLUSIO S

The performance of the first alternative dispersion acoustooptical system was analyzed and compared to DAZZLER system results. Our system showed higher efficiency, a possibility to control the injected femtosecond pulses over a wide range of jitter, and proved itself to be more ergonomic in installation and operation. Another advantage over DAZZLER is that the diffracted beam is parallel to the incident beam. The experimental AODL demonstrated a possibility of adaptive control of the femtosecond pulse spectrum, opening up new opportunities for optimizing the entire OPCPA laser system by increasing the efficiency of parametric amplification, expanding signal spectrum, and improving compressed pulse parameters.

After modernizing, the system is to be used at the IAP RAS for suppressing In the future the device will be used in the sub PW OPCPA laser system to suppress parasitic parametric reverse conversion into a pump wave as well as to reduce pre-pulses of the laser system.

Further research is possible in the area of increasing the adaptation flexibility of AODLs, with a perspective of creating intellectual control of femtosecond radiation in superpowerful systems.

REFERE CES

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