Clasificación de compañías del sector “B”

Since the temporal structure of the DFG pulse from the PPLN in the last section was not ac- ceptable, an alternative approach was needed. As reported in [55], DFG in a BBO crystal was achieved with high efficiency and an octave-spanning phase-stable spectrum. While their ap- proach contained two arms, using a strong pump and a weaker (OPA-amplified) broadband seed, the idler in this slightly non-collinear scheme can only be kept from experiencing angular dis- persion to a certain amount.

Therefore the broadband white light was employed for pump and signal in our scheme (see also [34]). This offers many advantages like perfect pump-seed synchronization and no angular dis- persion on the idler output. At the same time it can be very effective at over 10% [34] conversion efficiency. As type-I DFG in the 1 mm thick BBO (θ= 29.2 deg, φ = 90 deg) requires the pump in the extra-ordinary and the signal in the ordinary axis, we simply tilted the crystals to about

36 2. Seeding OPCPA delay (fs) SF wavelength (nm) −400 −200 0 200 400 450 500 550 600 0 500 1000 1500 2000 2500 3000

Figure 2.11: XFROG spectrogram of the DFG idler generated in a BBO crystal. Also compare to fig. 2.10. The inset shows the beam profile.

45 degrees between respective polarizations and optical axis (angle as projected onto the crys- tal surface). Rotation of this axis allows changing of the projections in both axes and more pump/signal amplitude to be used. The generated idler shares its polarization with the signal. Using a periscope at about 45 degrees allows the polarization to be rotated back for the AOPDF. With the much larger damage threshold, the lower n2 of BBO (see table B.1) and the shorter

crystal compared to section 2.4.1, an overall output of about 500 nJ could be achieved, while SPM remained very low. This output energy is far beyond the level reached using PPLNs or PPLTs like in section 2.4.1.

The same XFROG setup as in the last section was used to characterize the pulses. However, it was soon discovered that without using the AOPDF (which is quite inefficient when self- compensating, as discussed in section 4.2.2), the pulses were intense enough to be characterized directly without pre-amplification. The spectrogram is shown in fig. 2.11 and shows an ex- ceptionally well-defined phase that is curved around the zero-dispersion point (ZDP) at 540 nm (SFG) (u 1.7µm) and reaches from 420 nm (SFG) (u 880 nm) to 600 nm (SFG) (u 2400 nm),

spanning significantly more than one octave. The ZDP of BBO is lower (see table B.2), how- ever several meters of additional air path (from the XFROG setup) can shift this point to longer wavelengths.

While the selected spectrogram was the most broadband, angle tuning allowed moving its cen- ter more into the long-wavelength regime, where the OPCPA amplification spectrum is located. Also spectral clipping by limited SFG phase-matching and IR BBO absorption cannot be ex- cluded. Still the OPCPA of chapter 5 proved to be sufficiently seeded by this source. The inset

2.4 Phase-stable infrared seed generation 37 wavelength 1000 1005 1010 1015 1020 5000 10000 15000 0 200 400 600 800 1000 1200 1400 1600 1800 2000 -3 -2 -1 0 1 2 3 shot phase (r ad)

Figure 2.12: f-2f spectrogram of the DFG idler generated in BBO

of Figure 2.11 shows the beam profile of the generated idler. Due to spatial constraints, the colli- mating mirror had to be used at a slightly larger angle, leading to some astigmatism in the beam profile. Diffraction effects on the visible beam like in the inset of fig. 2.9 could be observed as well. However, these occurred at much larger intensities and without negative effect on the idler output in time or space.

In order to confirm the phase-stability of the DFG output, f-2f interferometry was used (see section 1.1.1). For f, the fundamental is taken from the broadband DFG idler around 1µm (reflected off the silicon stretcher of the OPCPA). The second harmonic (2f) is amplified by a first OPCPA stage (as described in chapter 5) to be intense enough for frequency doubling in a BBO crystal. As the OPCPA setup makes the optical path lengths long (7 m respectively), the linear change in path also contributes to jitter in this large interferometer. Figure 2.12 shows the interferogram, recorded over 2000 shots in 245 seconds with an integration time of 60 ms. No measures were taken to optimize the fringe contrast. This should not be necessary, as we do not expect fast jitter components from an inherently CEP stabilized source. Rather, the fast jitter components should be the result of temporal jitter between interferometer arms (see eq. (1.2)). The CEP jitter is evaluated to be 760 mrad, which is surprisingly little given the large dimen- sions of the interferometer. As we shall see in the f-2f measurement of the OPCPA itself (see section 5.5), the delay jitter is dominant here, while the CEP stability from DFG is excellent. Therefore this measurement rather proves the phase-stability of the short-wave tail of the spec- trum, which is not used any further in this manuscript.

38 2. Seeding OPCPA

However, a second OPCPA (pumped with the second harmonic of the pump laser) from 800 nm to 1500 nm could be seeded CEP stable from this spectral tail (which, according to the spectrogram of fig. 2.11, contains several 100 nJ of pulse energy). As the interferogram of fig. 2.12 indicates jitter of only 760 mrad around 1µm (that we can safely assume to be dominated by timing jitter) this is equivalent to no more than 0.7 fs of jitter between both arms. As the main contributions of the jitter even appear at low frequency and the beam paths for the measurement were not covered yet, this should be easy to compensate and allow sub-cycle OPCPA synthesis as described in [58] (where the NIR OPCPA was still actively CEP-stabilized).

It should be noted that type-II phase-matching of the DFG process in BBO is also possible. While being a bit less broadband and efficient, it offers the advantage of signal and pump both being polarized on the extraordinary axis, so the idler can be separated by polarization. For seeding NIR OPCPA this is a great advantage in terms of contrast.

Chapter 3

Pumping OPCPA

In order to reach the required spectral bandwidth for sub-two-cycle pulses around 2.1µm using LiNbO3crystals, short pump pulses on the order of few picoseconds have to be used to keep the

required pump intensities below the damage threshold (see section 1.2.2 and [132]). A higher pump intensity allows shorter nonlinear crystals and thereby helps loosen the phase-matching constraints.

Because of the relatively high energies required for an OPCPA system to drive high harmonics, the pump laser needs to be able to produce multi-mJ pulses. This requires high peak-power pump pulses that can only be dealt with using the CPA scheme [131] to keep unwanted nonlinearities in the amplifier low.

A further challenge is the low seed energy. Since the OPCPA is optically synchronized, both pump and seed amplifier derive their respective seed pulses from the same broadband oscillator. In our case this Ti:Sa based oscillator emits only pJ pulse energies around the amplification spectrum of Yb:YAG. Therefore a total amplification of ∼ 109 _{is required for the pump laser,}

leading to unwanted (chaotic) amplification dynamics. Employing these chaotic behavior then implies consequences for the synchronization of the OPCPA that will be described in chapter 4 in more detail.

This chapter is based on [88] and the PhD work of Tom Metzger [87] who was joined by the author later. Therefore the work published in [88] will only be summarized very briefly for completeness, in order to see the implications in the following chapters. Beyond that, later changes and characterizations performed by the author are described and the implications for OPCPA are elaborated.

40 3. Pumping OPCPA

3.1

Pump laser for OPCPA