EL MUNICIPIO DEL CERCADO FUERTE AL FINAL DE LA LLANURA

The pressure of the Ne in HCF1 was adjusted in a way to get maximum broadening in a sta- ble output mode. Very fine adjustment of the programmable dispersion from the DAZZLER, which optimizes the chirp of the incident pulse, improved the throughput as well as the spectral broadening [139]. At an optimum pressure of 2.3 bar, an overall throughput of 650 µJ with

the central mode containing more than 85% of the total energy was obtained. The broadband spectrum is shown in Fig. 4.16(a). The output of HCF1 was collimated by a 0.75 m focal length silver coated mirror (FM1) and then compressed using the CMC. The CMC consists of specially

4.3. Broadband seed generation using cascaded spectral broadening

spectrum generated in HCF1. Since we demonstrated the implementation of these DADM’s for few-cycle pulse compression for the first time, therefore before showing the pulse compression a brief introduction of these mirrors is presented as was reported in [140].

Double-angle dispersive mirror compressor for few-cycle pulses

Owing to their high control of GDD for an unprecedented bandwidth, chirped mirrors (CM) [103] are generally used to produce few-cycle pulses. The conventional CM, however, suffers from unavoidable spectral oscillations of the GDD which may adversely affect the quality of the few- cycle laser pulses being controlled with the CM. Generation of few-cycle nearly TL pulses us- ing CM demands suppression of such oscillations as well as fine control of the dispersion for the whole spectral range of the chirped pulses. Among many schemes developed to dampen such oscillations [105, 141, 142, 143], or to minimize their influence on the pulse quality, the complementary-pair approach [105] is commonly employed. The mirrors in this approach are used in pairs with one having the GDD oscillations anti-phase to the other, hence resulting in a smooth dispersion when equal number of reflections are applied. Such complementary-pairs are manufactured in two separate coating runs, therefore a high precision of layer deposition is required during both runs in order to achieve the desired characteristics of the CM’s. This drawback makes this approach more time consuming for manufacturing and costly compared to the mirrors for pulse compression that could be manufactured in a single run. In addition these complementary-pairs require very high accuracy of angle of incidence during pulse compression. The newly developed DADM technology [140] rely on a single coating run for manufactur- ing. The GDD oscillations of a DADM changes their phase in spectral domain with respect to the angle of incidence [140, 144]. In this way two identical DADM’s can be aligned at two dif- ferent angles of incidence to achieve anti-phase GDD oscillations. Moreover a DADM relaxes the strict requirement of a very precise angle of incidence for pulse compression, as imposed by the complimentary-pair design, because here a slight deviation of the angle of incidence of one pair can be compensated by fine-tuning the angle of incidence of the other. For detailed realization of the design of these mirrors see [140]. The schematic sketch of this approach is depicted in Fig. 4.15(a), whereas the design curve of the DADM’s is shown in Fig. 4.15(b). As indicated by the blue and red curves in Fig. 4.15(b), the phase oscillations of the same mirror for two different anglesθ1=5◦ and θ2=20◦ exhibit a phase-shift ofπ, while the reflectivity

remains more than 99% for the entire required spectral range. The effective GDD and reflectance of the DADM is the geometric mean of the GDD and reflectances, respectively for the described incidence angles. The optimum effective GDD exhibits residual oscillations which are an order of magnitude lower than those of a single reflection for each incidence angle. These residual oscillations show an insignificant influence on the pulse durations. The mirrors were designed

Chapter 4. Broadband seed generation for OPCPA chain

Figure 4.15: (a) Schematic of the double-angle dispersive mirror compressor. (b) Designed dispersion the DADM: Red and blue curves correspond to angles of incidenceθ1=5 andθ1=

20 degrees, respectively. Green curves show the effective GDD and reflectance. The inset depicts the target GDD of the double-angle DM, picture courtesy of Pervak et al. [140].

to roughly compensate the dispersion of two 0.5 mm fused-silica (FS) Brewster windows of the HCF1, 0.5 mm of (FS) wedges, and 1.7 m of air (or gas). The target design curve of the DADM for one bounce is shown in the inset of Fig. 4.15(b), which corresponds to the GDD and TOD of

−35 fs2, and−30 fs3, respectively, at a central wavelength of∼770 nm.

Figure 4.16(a) depicts the output spectrum of HCF1 for a Ne-pressure of 2.3 bar. In order to compress this spectrum we used a total of eight reflections on the DADM’s. The compressor exhibited an overall throughput of >85%. The pulses after the compressor were characterized by a second-order interferometric autocorrelator (Femtolasers GmbH). The measured autocor- relation function is presented in Fig. 4.16(a). This corresponds to a FWHM of 4.3 fs, for sech2 pulse shape, and is very close to the transform limited (TL) pulse duration of 4.2 fs, as calculated from the measured spectrum for the range of 550-1100 nm. The good agreement between the measured and calculated widths of the compressed pulses indicates that this new scheme of pulse compression is capable of delivering high quality few-cycle pulses by compensating their disper- sion in a more simple and accurate way than other conventional techniques [105, 141, 142, 143]. These high quality few-cycle pulses after the chirped mirror compressor were used to gener- ate the broadband supercontinuum by i) filamentation in argon, and ii) broadening in a second HCF. We call the former scheme “HCF-filament” since it follows prior broadening in an HCF (HCF1), similarly the latter is termed “cascaded-HCF”. In the following the details of our ex- perimental findings for these two schemes are presented.

4.3. Broadband seed generation using cascaded spectral broadening

Figure 4.16: Few-cycle pulse generation: (a) HCF1 output spectrum at 2.3 bar. (b) measured and calculated autocorrelation of the compressed pulse.