Spectral broadening of femtosecond laser pulses using a hollow fiber with symmetric pressure gradient

We propose and demonstrate a pulse compression technique using a symmetric pressure-gradient hollow fiber. This technique improves the spatial and spectral qualities of multi-mJ femtosecond laser pulses spectrally-broadened by self-phase modulation. ©2007 Optical Society of America OCIS codes: (320.7110) Ultrafast nonlinear optics ; (320.5520) Pulse compression The hollow-fiber compression technique is a well-known method for generating high-peak power pulses with a duration less than 10 fs. Self-phase modulation is used to broaden the spectrum of the optical pulses, and the resulting phase is subsequently compensated by use of dispersive elements. In the conventional hollow-fiber technique, unexpected problems such as early self-focusing and defocusing by the generated plasma may arise in the hollow fiber, which limit the laser energy delivered into the fiber to below 1 mJ. When such phenomena occur, the spatial profile of the beam is degraded resulting in bad coupling between the beam and the hollow fiber. We demonstrated a procedure to avoid and/or postpone self-focusing using a so-called pressure-gradient hollow-fiber compression method (1). This method could provide spectral broadening of the pulses with higher energy and better spatial phase. We have also proposed a stiffer and more robust technique, so-called symmetric pressure-gradient hollow-fiber compression (2). In this technique, a pair of hollow fiber is connected to each other with a small gap between them, as shown in Fig. 1. The gas is provided from the gap and flows to both end of fibers where the pressure is kept at low. Thus, the gas pressure increasingly varies in the first fiber, and then decreases in the second fiber. According to numerical studies, unfavorable excess nonlinear processes before the pulse enters the hollow fiber and after exits the fiber can be prevented. Thus, high throughput and excellent spatial and spectral quality can be obtained by using the symmetric pressure gradient method. In this paper, we present an experimental result on the spectral broadening of multi-mJ femtosecond laser pulses in a symmetric pressure-gradient hollow fiber. The experimental setup is shown in Fig. 1. A pair of hollow fiber was connected to each other with a gap of 300 µm and installed in a vacuum cell. Argon gas was introduced to fibers through the gap in a manifold and the pressure was controlled by a needle valve. The inner diameter and entire length of the fibers were 500 µm and 200 cm, respectively. The energy of incident laser pulses from a Ti:Sapphire laser chirped-pulse amplification system was 5 mJ with a pulse duration of 25 fs at a repetition rate of 1 kHz (3). The beam was focused into a hollow fiber using a 450 cm focal-length lens. The beam diameter at the entrance of the hollow fiber was 350 µm, which was precisely adjusted in order to match the incident beam with the fundamental fiber mode. The output spectrum broadened by self-phase modulation in the symmetric pressure-gradient hollow fiber was observed using a fiber spectrometer. The output power was measured with a power meter.