The optical fiber delivery laser power technology in laser flexible manufacturing system is introduced.
The characteristics of optical fiber delivery Nd:YAG pulsed laser power through multimode silica fiber
are experimentally researched, which include beam spatial characteristics of fiber exit beam and the
capacity of optical fiber delivery high power laser. The effects of laser injection condition and optical
fiber bending radius on optical fiber delivery laser beam system are analyzed. The morphology
characterization of laser induced damage to optical fiber end-faces were measured and investigated.
The model of laser induced damage to fiber end faces is presented.
For laser detonator application, high-peak power pulsed Nd:YAG laser is transmitted through all-silica optical fiber. The
transmission properties of step-index fibers are investigated, using a high-peak power pulsed Nd: YAG rod laser with
beyond 1MW power and Q-switch mode. The fibers are step-index multimode fibers with 400 or 600 μm core diameters,
440 or 660 μm cladding diameters. The power delivery characteristics were studied by theory and experiments. The
results show that the fiber core diameter, NA, length and so on affect the transmission efficiency for high power laser.
When the laser power is beyond a certain threshold, the SRS and SBS will be serious; the quantity of fiber end-face
limits to the raising of laser power passing through fibers; the zero-probability damage threshold is calculated according
to ISO/DIS standard 11254-1.2, which is 58.6J/cm<sup>2</sup>. Energy distribution of output beam from fibers will be uniform.
Even the fiber end-face was partly damaged, laser power is still deliverable, and the transmission efficiency is related to
the fiber damage grade.
A novel coupling method for injecting a high peak power laser into a multimode optical fiber through cone-channel condenser is introduced. The novel coupler is investigated by experiments and theories. The design minimizes the irradiance on the fiber input face and reduces its dependence on the system alignment. A simple lens and a special designed cone-channel condenser operate together to transform a laser beam with 5 mm diameter into a smaller one that fits on the 400 μm or 600 μm diameter fiber face. The method resolves the problem that laser induce damage to fiber input end faces. The design principle and method of cone-channel condenser are described by the light transmission theory. The prototype was fabricated without anti-reflection coatings on the end faces. The experimental results show that the transmission efficiency of cone-channel condenser is up to 90%. Though there was 1 mm gap between the cone-channel condenser and a fiber, the coupling efficiency of cone-channel condenser to fiber reach 73%. The maximum transmitted energy before front-face of cone-channel condenser breakdown is 84.5mJ. The transmission capacity of fiber increases by 2-3 times comparing with the traditional method. The interest in this new coupling method is related to the development of transmitting high peak powers through multimode fibers applied to laser-based firing systems for initiating explosives and driving flyer, et al.
The properties of high peak pulsed laser induced damage to fused-silica fibers are investigated using damage
experiments. The laser source is Q-switched Nd:YAG pumped dye laser system, the pulse width is 15.2 ns, and the
wavelength is 1064 nm. The experimental results show that all the damaging scenes are fiber entry faces.
Damaging patterns can be classified as three types: pit damage, fusion damage and sputtering damage. Pit damage
occurs most frequently. Scaffolding defects come into being pits when the laser irradiation is lower. Fusion
damage is related to the laser energy, but sputtering damage occurs frequently when the laser energy or power
density is very high (>10<sup>7</sup>W/cm<sup>2</sup>~109W/cm<sup>2</sup>, ns pulse). The damage photos illustrate that fiber end damage is
mainly due to laser ablation and gasification. Impurities of fiber material or contaminant particles adhere to fiber
end are stress-raisers, which absorbed enough laser energy, make local temperature rises up quickly, ulterior fusion
or gasification, and finally strong tensile stress. When stress goes beyond the tensile strength of fused-silica,
damage occurs. The main damage mechanisms appear to be thermal effect and plasma ionization. The origin for
the decline of laser induced-damage to fibers threshold appears to be extrinsic defect. The damage criterion and
damage threshold test method are presented. The zero probability damage threshold is calculated by linear fitting,
that is 58.6J/cm<sup>2</sup>. The damage process of fiber end faces could be divided into six steps. The origin for the decline
of laser induced-damage to fibers threshold appears to be extrinsic defects which are closely related to the