## 1 Introduction  
Four-wave mixing (FWM) has been investigated and used in the past few decades to generate multicolor laser emission in various wavelength regions. In 1981, more than 40 laser emission lines spanning from the deep ultraviolet (DUV) to the near infrared (NIR) were generated by focusing a two-color nanosecond pulse into hydrogen gas [1]. The generation of multicolor laser emission via FWM has hitherto been extensively studied in deuterium and hydrogen gases [2; 3; 4; 5] and extremely short optical pulses with durations shorter than 2 fs have been generated [3; 5] by Fourier synthesis of the emission lines [6; 7].  
The generation of multicolor laser emission via FWM has also been investigated in the femtosecond (fs) regime [8; 9; 10; 11; 12; 13; 14; 15; 16; 17]. Resonant and non-resonant FWM has been investigated in bulk media [11; 13; 15; 16; 17], leading to multicolor laser emission with a pulse energy of about 1 microjoule [17]. By employing the pre-compression technique, multicolor emission with short pulse durations less than 20 fs has been demonstrated [16]. Such short multicolor pulses can be used in ultrafast spectroscopy [22; 23; 24], as well as in nonlinear optical microscopy [25]. The efficiency of the multicolor generation can be improved by more than one order of magnitude using gaseous media rather than bulk media. In addition, the spectral range from the DUV to the NIR has been covered by employing Raman active gases as the nonlinear media for FWM [9; 10; 12; 14; 20; 21]. When pumping with short fs pulses with high intensities, self-phase modulation (SPM) and cross-phase modulation (XPM) are simultaneously induced, which results in a broad spectral width for each multicolor emission. The broad spectral width is advantageous for generating a short optical pulse after adequate dispersion compensation. The maximum spectral width of each multicolor emission is limited by the frequency separation between the center frequencies of the adjacent multicolor emission, which is determined by the Raman shift of the medium used. When using the vibrational transition of molecular hydrogen with a large Raman shift frequency of 4155 cm\({}^{-1}\)[26], it is possible to generate multicolor fs pulses with spectral widths supporting transform-limited pulse durations of about 10 fs. Multicolor generation via the vibrational transition has been investigated both in a hydrogen-filled hollow fiber using two-color fs pump pulses at 800 and 600 nm [12] and in a hydrogen-filled gas cell using shorter two-color NIR pump pulses emitting at 800 and 1200 nm [20]. These investigations have reported multicolor laser emission covering from the DUV to NIR. The use of a hollow fiber [12] and gas cell [20], however, has been investigated under independent experimental conditions. Therefore, a quantitative discussion of the efficiency of multicolor generation and the characteristics of the pulses has been hindered. Information related to the pulse energy of each multicolor emission has not been reported to date; these data are important for evaluating the multicolor laser pulses.  
We investigate here two approaches of FWM in a hydrogen-filled gas cell and a hydrogen-filled hollow fiber. We compare the results to analyze laser sources of multicolor fs pulses. By maintaining the experimental parameters at a constant--except for the gas pressure, which has been optimized in each experiment--the two schemes can be directly compared with one other. We find that the gas-filled hollow fiber with a longer propagation distance than that of the gas cell does not always lead to higher efficiencies in the multicolor generation. The gas cell gives rise to visible multicolor pulses with higher pulse energies than those generated in the hollow fiber. On the other hand, the latter produces higher-energy multicolor pulses in the ultraviolet range.