We have observed the spatially-resolved optical amplification of femtosecond VUV pulses at 126 nm using an OFI Ar 2 * amplifier. The maximum amplification ratio of 2.57 was observed, which corresponded to the optical gain of 0.94 cm -1 .The progress of an ultrashort-pulse high-intensity laser is mainly based on solid-state laser media, such as a Ti:Sapphire laser. The lasing wavelengths are thus inevitably limited to the near infrared (IR) and visible spectral regions. In contrast, ultrashort-pulse lasers in the ultraviolet (UV) and vacuum ultraviolet (VUV) wavelength regions have not well been developed. The nonlinear wavelength conversion such as harmonic generation has been used to generate ultrashort UV and VUV pulses. The conversion efficiency is, however, low especially in the deep VUV region, since only available nonlinear media are low-density gases. As a result, observed pulse energy was limited to the order of nJ [1], which may be too small for applications.We have been developing new femtosecond (fs) VUV laser system at 126 nm [2], where we have used an argon excimer (Ar 2 *) amplifier to amplify a weak VUV ultrashort seed pulse generated by harmonic generation. The 7th harmonic emission of a fs Ti:Sapphire laser at 882 nm was optimized as a seed pulse for our laser system [3]. We also report the optical amplification of Ar 2 * at 126 nm, pumped by optical-field-induced ionization (OFI) created by an IR high-intensity laser [4,5]. An optical gain of 0.84 cm -1 was obtained by using multi-pass amplification using a VUV cavity [5]. From a practical point-of-view, this excitation method operates a tabletop laser at a high repetition rate, since a well-established IR high intensity laser is used as an excitation source.In this paper, we report the spatially-resolved optical amplification of the fs VUV seed pulse at 126 nm by using the OFI Ar 2 * amplifier. The maximum optical gain value of 0.94 cm -1 was consistent with that observed in previous experiments [5]. We measured the spatial distribution of the amplification ratio. The gain size of 220 μm (FWHM) was evaluated, which indicated the average plasma expansion temperature of 1.2 eV after 20 ns of the plasma production. Fig. 1. Experimental setup. Figure 1 shows a schematic diagram of the fs VUV laser system. This laser system was operated using one Ti:Sapphire laser operated at 882 nm, which produced an output energy of 1 mJ with a pulse width of 160 fs at a repetition rate of 1 kHz. The output pulse from the Ti:Sapphire laser was split into two beams using a 50:50 beam splitter (BS). One beam was focused into an Ar 2 * amplifier filled with argon gas up to 1 MPa to produce Ar 2 *. The other beam with a delay time 53 MD1.3 (Contributed) 9:15 AM -2:00 PM 978-1-4577-1507-5/13/$26.00 ©2013 IEEE