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Acousto-Optic Programmable Dispersive Filter

Dans le document The DART-Europe E-theses Portal (Page 29-34)

1.4 Pulse shaping

1.4.2 Acousto-Optic Programmable Dispersive Filter

There exists an alternative method of pulse shaping different than 4f-line pulse shaper that is called Acousto-optic programmable dispersive filter (AOPDF) [40]. It has been invented in late 90th by Pierre Tournois in order to remove the residual phase of the chirped pulse amplifiers. Its principle finds an analogy with chirped mirrors [68] in which the various spectral components of the pulse experience different time delays by reflecting from different layers of the chirped mirror (fixed longitudinal grating).

In the AOPDF that is commercially called the DAZZLER, the grating is transiently generated by interaction of the acoustic beam and birefringent crystal [69]. It was initially designed for operating in IR region by using of a TiO2 crystal [70]; but later its application has been extended to the other optical regions covering mid IR [71] and UV [60,72] by using alternative crystals. In our lab, the temporal characterization of UV-AOPDF pulse shaper has been done by Weber [73,74]. Later, we have extended the characterization to the spatio-temporal domain [61]. In the following, I detail the performance concept and theory of the UV-AOPDF pulse shaper while its space-time characterization is detailed in chapter 2.

The pulse shaping in AOPDF is based on controlling the group arrival time of the pulse [69]. Using an anisotropic crystal helps to achieve this goal. In more detail, since the refractive index of the ordinary and extraordinary axis are different in anisotropic crystals, the spectral components propagating with polarization lying along the slow or fast axis will arrive at the end of the crystal with different delays. The group ar-rival time τ(ω) is the derivation of the spectral phase ∂φ(ω)/∂ω. Therefore, one may control the spectral phase by modifying the group arrival time of the spectral

compo-nents. Figure 1.9 shows schematically the performance concept of the AOPDF pulse shaper where the input pulse is polarized along the ordinary axis of the anisotropic crystal (slow axis). Depending on the form of the desired output pulse, the spectral components diffract at different places of the crystal z(ω) and hence propagate with polarization along the extraordinary axis of the crystal. Analogous to nonlinear

ef-z(ω )

Extraordinary (fast axis) Ordinary (slow axis)

Figure 1.9: Schematic demonstration of the AOPDF pulse shaper. Depending on the form of the desired output shaped pulse the spectral components of the incident pulse (along the ordinary axis) diffract at different positions of the birefringent crystal and hence propagate along the extra-ordinary axis. The diffraction occurs where the conser-vation of energy and momentum condition is satisfied between the acoustic and optical beam. Because of the difference of the refractive index along the two mentioned axes, spectral components experience different paths (group delay) and hence by controlling the group delays the output pulse takes the form of the desired shape.

fects, the diffraction occurs when the energy and momentum conservation condition are verified between the launched acoustic wave and the optical beam:

ωdi+ Ω

kdd) =kii) +K(Ω). (1.17) Here, d and i are the indices of diffracted and incident beam respectively. K and Ω are the wave-vector and the frequency of the launched acoustic wave. The group arrival time of the different spectral components τ(ω) is

τ(ω) = ∂φ(ω)/∂ω= z(ω)

vgo(ω) +L−z(ω)

vge(ω) (1.18)

whereLis the length of the crystal andvg is the group velocity along the ordinary (o) and extraordinary (e) axis. Knowing the form of desired shaped pulseφ(ω) yieldsτ(ω).

Referring to Eq. 1.18and knowing the values ofvgoandvge setsz(ω). Calculating the location of the interaction z(ω) reveals the K(Ω) from Eq. 1.17 [69]. The amplitude of each spectral component is controlled by the amplitude of applied acoustic wave at the place of diffractionz(ω).

In the frequency domain the output pulse can be written [40]:

Ed(ω)∝Ei(ω)·Sac(γω) (1.19) and equivalently in temporal domain:

Ed(t)∝Ei(t)⊗Sac(t/γ) (1.20) where γ = ω = |∆n|νc ac is the scaling factor between acoustic and optical frequencies.

νac is the acoustic wave speed and ∆n is the difference of the refractive index of the fast and slow axis. Working in UV region and hence the choice of KDP crystal, γ is calculated to be approximately 1.3 ×10−7 [60]. This scaling value lets the direct transfer of the phase and the amplitude of the acoustic wave to the optical wave.

The orthogonal polarization of diffracted and non diffracted beams makes it easier to distinguish the shaped pulse from the unshaped one.

Properties of our UV-AOPDF pulse shaper

Figure1.10shows the interaction geometry of our pulse shaper. The acoustic and the optical wave propagate in the plane consisting [100] and [001] axis of the crystal. The wave-vector of the acoustic waveKmakes an angleθawith the [100] axis. θdandθiare respectively the angles of the diffracted and input optical wave vectors (kdandki) with respect to [100] axis. The sound shear wave velocities along the [100] and [001] crystal axis are V100 =1650 m/s and V001 = 2340 m/s, respectively. The acoustic Poynting vector is aligned with the optical input optical beam to have optimized acousto-optic interaction length [70] which is realized when tanθa= (VV100001)2tanθo. This maximizes the spectral resolution and diffraction efficiency. Since the crystal is a birefringent medium, the diffracted beam experiences a refractive index nd which depends on the propagation angleθd through:

1/n2d= cos2θd/n2o+ sin2θd/n2e. (1.21)

Figure 1.10: Interaction geometry of our UV-AOPDF pulse shaper that consists of a KDP crystal with a negative anisotropy. The wave-vector of the acoustic wave K makes an angle θac with the [100] axis. kd and ki are the wave-vectors of the diffracted and incident beam respectively. no and ne are the ordinary and extraordinary refractive indices of the crystal. θi is the angle of the input optical wave-vector ki with respect to [100] axis. The sound shear wave velocities along the [100] and [001] crystal axis are indicated by V100 and V001 respectively.

In KDP crystal the optical anisotropy ∆n = no −ne is small compared to no which yieldsδn=nd−no= ∆n·cos2θi. Now according to the laws of energy and momentum conservation [Eq.1.17] the angles and the wave-vectors can be written

θd−θi = ∆n n0

cos2θitan (θi−θa) K =ki

∆n n0

cos2θi

cos (θi−θa).

(1.22)

The ordinary and extraordinary refractive index of KDP crystal for λ = 300 nm are 1.5451 and 1.4977 respectively. The optimum incident angle of the optical beam θi

for the case of maximum diffraction efficiency is calculated to be 48.5 [60]. The other calculated parameters of the KDP crystal is presented in Tab. 1.3[60]. The other im-portant parameters of the UV-AOPDF pulse shaper are spectral resolution, temporal

Parameter Value

Table 1.3: Optimum parameters of the KDP crystal for a pulse with central wavelength of 300 nm.

windows and the diffraction efficiency which can be calculated from the acousto-optic interaction theory that are addressed in reference [70]. For the specific case of KDP crystal, they are calculated in [60]. I only present the final equations and the calcu-lated values for the KDP.

Efficiency

The efficiency of the diffraction for particular frequency ν is given by Id(ν)

where M2 is the merit factor of the KDP crystal which depends on properties of the crystal and the speed of the acoustic beam in the crystal. Finally, ∆φ is the phase shift due to the phase mismatching which gives:

∆φ = ∆KL

π cos (θi−θa) (1.24)

Since ultrashort pulses contain broad spectral bandwidth, it is not straight-forward to extract the efficiency of the output shaped beam.

Spectral resolution

The resolution of the KDP crystalδλfor central wavelength of 270 nm and the crystal length of 75 mm is calculated to be 0.036 from [70]:

δλ= 0.8λ3

∆nLcos2θi

(1.25) when ∆φ = 0.8 and P =P0. In this particular case the efficiency is about 50%.

Temporal windows

Temporal windows of the AOPDF is determined by the crystal thickness and its anisotropy. It is equal to the maximum difference of the group time of a particular frequency propagating upon along ordinary or extraordinary axis:

L

Dans le document The DART-Europe E-theses Portal (Page 29-34)