Bulk Modification In An Optical Fiber Using a High-Intensity Femtosecond Laser

The solid-density plasma formation and induced modification have recently been a major concern since the advent of a high-intensity femtosecond laser.' The solid-density plasma formation is attributed to contributions from both self-focusing from the intensity-dependent refractive index and self-defocusing resulting from ionization.2 Recently, laser-induced bulk modification in transparent materials has been found to yield new tools for ultra-precise three-dimensional micromachining of small, complex-shaped components of transparent dielectrics and polymers for optoelectronic, micro optic, and fiber technological application^.^ This has also provided useful microstructuring technology for application of waveguide and memory in transparent material because refractive index change of induced modification is increasing. Although experiments for bulk modification in solids using high-powr femtosecond laser pulses have been reported: the process of laser-induced modification in a transparent material for number of input pulses have not been clarified yet in detail. In this paper, for the first time to our knowledge, the experimental observation of laser induced bulk modification process in a multimode optical fiber excited by a high intensity (above lo'* W/cm') femtosecond Ti:sapphire laser. In our experiment, a multimode optical fiber was used, which enabled one to easily in situ study the process of laser-induced modification for various number of laser pulses. The schematic diagram of the experimental setup for laser-induced modification in an optical fiber is shown in Fig. 1. A multimode optical fiber (step index) with 200/220 pm corekladding diameters (Newport F-MCC-T) was used. The core of the optical fiber is composed of pure silica. Moreover, the optical fiber is a high-temperature one in order to increase the fiber damage threshold when powerful laser pumping is employed. Ti:sapphire oscillator amplifier laser system (h,=790 nm) with 110 fs pulse duration, 1 W average output power, and 1 kHz repetition rate was used as a pumping source. The number of laser pulse was controlled by electric shutter. The 5-mm-diameter beam was focused through a focusing lens ( f = 60 mm ) and injected into the optical fiber which was located further away than the breakdown point. The average input intensity of the laser beam at the optical fiber location was controlled using neutral-density filters that were inserted between the laser and the focal lens. The length of optical fiber was 10 cm. When the input intensity exceeded 1 . 5 ~ 1 0 ' ~ W/cm2, self-channeled plasma formation was observed with a length of 8 10 mm from the input end of optical fiber. The side views of the plasma formation were observed in situ using a microscope and recorded using a CCD camera. (Fig. 2) When self-channeled plasma formation occurred at the input intensity of 1 . 5 ~ 1 012W/cm', the process of laser-induced modification was obsevered using a microscope by controlling the number of laser pulses. The microscopic side views of the modified area are shown in Fig. 3. It was observed that the bulk modification had a diameter of approximately 5 pm and the length of 6 mm in the optical fiber with 200/220 pm corekladding diameters after 5 minutes ( 3 . 0 ~ 1 0 ~ shots) irradiation of the laser induced modification at different input intensities. The first modification spot, which has a diameter of approximately 5 pm, was located 72 pm distant from the input end of optical fiber, due to the self-focusing effect. The diameter variation of laser induced modification for different input intensity are shown in Fig. 4 after 3x105 shots irradiation. The diameter of modification, 4 7 pm was controlled by the laser input intensity. In conclusion, the modification in optical multimode fiber having the structure of double cladding fiber is useful method in various applications for fiber lasers and fiber amplifiers. The femtosecond laser-induced modification technologies using the self-channeled plasma formation provides sophisticated tools for ultra-precise three dimensional modification and micromachining in transparent materials without disruption of the remnant materials.