Descripción del sendero y la interpretación ambiental en el Parque

The observed spectra depicted in Fig.5.2reveal additional contributions that do not belong to the signal pulse or the Raman backscattered pump light. They represent pump light reflected from the walls of the chimney nozzle, in particular around the entrance holes for the laser pulses. There, the pump intensity is too small to burn the aluminum foil covering the sides. The pump light is reflected from the foil and enters the spectrometer together with the desired signal. Its contribution is measured separately injecting only the pump pulse without the signal pulse and without releasing gas. Subtracting this spectrum from the raw data yields the spectra of the amplified signal pulse and the RBS of the pump pulse in Fig.5.3. The same correction has been carried out for the energy measurement in the previous section.

The signal pulse is broadened significantly from 17 nm (FWHM) for the input to 30 nm for the amplified output. Furthermore on the long wavelength side, an additional wing extends from 830 nm up to 870 nm. The sharp Stokes line, that dominated the spectrum in the first experiments (Figs. 4.2 and 4.3) is absent, suggesting that SRBS contributes only little to the amplification. Instead, the amplification has switched to the SRA regime at some point accompanied by the pulse shortening and spectral broadening. The bandwidth theoretically estimated in section2.3.5ranges from the pump wavelength up to 865 nm, which corresponds very well to the end of the long-wavelength wing in the spectrum. Assuming the wing is due to SRA, the intensity ratio of the wing and the main spectrum suggests that SRA is still in an early stage.

It has to be excluded that SRBS causes the observed broadening. While SRBS is charac- terized by the narrow bandwidth of the backscattered light for small signal gain, two other regimes exist that entail broader spectra (cf. section 2.6): (i) The strongly coupled Raman regime[89,90] and (ii) thepump depletion regime[45]. Their spectra can have widths greater than the plasma frequency. The pump depletion regime cannot take place because it requires the pump pulse to be completely depleted from some point on during the interaction with the signal pulse. This is ruled out by the small energy transfer of less than 1 % from the pump pulse to the signal pulse.

The strongly coupled Raman regime is characterized by a growth rate γ exceeding ωpe

Figure 5.2: The unprocessed original spec- trum contains pump light reflected from the side walls of the chimney nozzle around the entrance holes. The black curve is taken when injecting only the pump pulse without gas and signal pulse. The spectra in Fig.5.3are ob- tained by subtracting this spectrum from the measured spectra of the signal output and Ra- man backscattered pump, respectively.

780 800 820 840 860 880

wavelength [nm]

spectral energy density

signal output pulse RBS of pump pulse light reflected from aluminum foil

5.3 Spectral Broadening of the Amplified Pulse

Figure 5.3: Spectra of the sig- nal input and output pulses and of the Raman backscattered pump light. The amplified spectrum is smoothed (black line) to infer its bandwidth. The bandwidth in- creases from 17 nm (FWHM) of the input signal pulse to 30 nm for the amplified output.

Figure 5.4:Integrated energy dis- tributions of the spectra for the in- put and output signal pulses show that 15 % of the energy of the am- plified pulse are contained in a long wing extending from 830 nm to 870 nm.

780 800 820 840 860 880

spectral energy density E(

λ

)

signal input pulse (x3) signal output pulse RBS of pump pulse (x3) running average with a binning width of 10 nm 780 800 820 840 860 880 wavelength [nm] 0 0.2 0.4 0.6 0.8 1 ∫ λ 400nm E( λ ’)d λ ’ / ∫ 1000nm 400nm E( λ ’)d λ ’

signal input pulse signal output pulse 30 nm

17 nm

substantially. The SRBS growth rate from Table2.1for our parameters is computed to

γ=1

2√ωpeωpuap=0.068ωpeωpe (5.1) using the approximation ωpumpωpe, ap =0.029 (from experiment), and pωpe/ωpump =

4

p

n/ncrit=√40.002=0.21. Therefore, the weakly coupled regime prevails with the gain band-

width

2γ0=0.14ωpe=0.0061ωpu (5.2)

corresponding to 5 nm. Including the 8.7-nm bandwidth of the chirped pump pulse, the total gain bandwidth in the Raman regime is still significantly smaller than the observed spectral broadening.

A special feature in Fig. 5.3 are the narrow peaks on top of the amplified spectrum. It appears that they might be caused by RBS, because they are narrow and they end around 830 nm on the long-wavelength side, where the Stokes line for the maximum electron density is expected. However, they are not very prominent in the pure RBS measurement when the signal pulse is blocked. Thus they have to be due to the interaction of the signal and pump pulses. This is only possible in the pedestal or a precursor of the signal pulse, because the intense maximum of the signal pulse is amplified in the SRA regime, thus the plasma wave is broken and Raman scattering ceases. It is not yet fully understood, whether the peaks in the spectrum could also be produced by SRA.

spatial direction − → -100 -50 0 50 100 autocorrelation time [fs] autocorrelation signal delay time−→ (a) (b)

Figure 5.5:(a) Single-shot autocorrelation of the amplified signal pulse captured by the CCD camera. (b) Line-out averaged over several rows with statistical deviation vs. the calibrated time axis. The rows were taken near the bottom of the image, where the 2ω-signal is maximal but not yet clipped.