Measurements of damage and nonlinear thresholds for large-core multimode optical fibers

The beam transport of high-power laser pulses by an optical fiber is an important issue in high-power laser application. For CW lasers, the power of 100's kW can be transmitted by an optical fiber, and 10-MW power could be theoretically possible. However, mirror systems are used for high-power pulse lasers in many fields because the limitation of incident pulse energy in the fiber is not understood well. Our purposes are to make it clear the limitation of incidence and the causes of the limitation, and that develop the techniques and the conditions, which are possible to transmit high-power laser pulses with optical fibers. The limitations of incident energy to large-core multimode optical fibers were estimated experimentally with core size of 600, 800, 1000, and 1200 microns. A Nd:YAG pulse laser system with a 10-ns pulse width at 1064nm was used. The incident energy of the laser pulse was adjusted using a half-wave plate and a polarizer, and focused on the fiber surface with a lens of 3000-mm focal length. The focused beam size was measured with a CCD camera, and adjusted to be 80% of each core size. The transmission efficiency was measured with the input and the output energy, and the damage on the input fiber core was monitored with a microscope for each laser pulse. In the 1000-microns core fiber, the surface damage was happened at 150mJ. Also, the efficiency was decreased with increasing input energy more than 50mJ because stimulated Brillouin scattering was occurred. The dependences of surface damage and stimulated Brillouin scattering on the laser and focusing parameter will be presented.