## 3 Results and Discussion
### Energies of the Multicolor Emission  
For estimation of the energy of the anti-Stokes emission, the gas cell was first evacuated. Only P2 was focused into the gas cell and the pulse energy of P2 emerging from the cell was measured using a power meter. The spectrum of the output P2 was measured using a sensitivity-calibrated spectrometer, the position of which was kept unchanged during the subsequent experimental procedure. The measured pulse energy was divided by the integral of the spectral intensity of P2. The resulting value is hereafter referred to as the calibration factor. The gas cell was then filled with hydrogen gas and both P1 and P2 were focused into the gas cell. The spectrum of the output beam from the cell was measured using the spectrometer at different gas pressures. The integral of the spectral intensity for each anti-Stokes emission was multiplied by the calibration factor to estimate the pulse energy of the emission. The same procedure was repeated for the hydrogen-filled hollow fiber. Because of the nearly continuous structure of the spectrum observed at pressures higher than 1 atm, the pulse energy could not be estimated for the hydrogen-filled hollow fiber at high pressures. The results of the energy estimation are shown in Figure 5. In both the experiments using the gas cell and the hollow fiber, the energy of P2 was notably depleted by the generation of the anti-Stokes emission. The energies of the low-order anti-Stokes emission, _i.e._, AS1 and AS2, generated in the gas cell were 1.8 times higher than those generated in the hollow fiber. This result indicates that the gas cell is superior to the gas-filled hollow fiber in generating high-energy visible pulses. The conversion efficiency from P2 into the anti-Stokes emission was similar in these two cases (see the observed conversion efficiencies shown in Figure 5). This fact suggests that if there are no propagation and coupling losses for the hollow fiber, the anti-Stokes emission of AS1 and AS2 would have energies close to those for the gas cell. The energies of the higher-order anti-Stokes emission, on the other hand, were larger for the hollow fiber than those for the gas cell. The conversion efficiencies from P2 to the anti-Stokes emission of AS5 and AS6 were three times and four times larger than those generated in the gas cell, respectively, as shown in Figure 5c,f. The energies of P1 and P2 for the gas cell were reduced until the output energies were the same as those in the hollow fiber experiment (140 \(\upmu\)J for P1 and 160 \(\upmu\)J for P2). For the energy reduction, the neutral density filters placed in the input beam paths were employed (see Figure 1). The filters with thicknesses of 2 mm would not have stretched the pump pulses appreciably; each filter stretches a 35-fs pulse at 800 nm by only 0.5 fs. The resultant spectrum of the output beam from the gas cell pressurized at 2 atm is shown in Figure 6, together with the spectrum measured using the hollow fiber without a reduction in the pump energy. No notable difference was observed in the energies of the first- and second-order anti-Stokes emission under these conditions, although the spectral intensities of the emission were higher for the gas cell. This result suggests that the pulse energy of the visible emission from the hollow fiber does not exceed that from the gas cell, even under the ideal condition that there are no propagation and coupling losses for the hollow fiber. **Figure 5.** The pressure dependence of the energy for the output beams. (**a-c**) The gas cell and (**d-f**) the hollow fiber. The maximum energy obtained for each anti-Stokes emission is indicated together with the conversion efficiency from P2 to the anti-Stokes emission. For P2, the ratio of the output energy measured at a specified pressure and the output energy measured at 0 atm is listed in each panel. **Figure 6.** Spectra of the output beam from the gas cell (red solid line) and the hollow fiber (black solid line) measured at a pressure of 2 atm.