Fibers deliver Q-switched pulses
Paula M. Noaker
A fiberoptic fabrication method developed by Japanese researchers may expand laser ablation applications for Q-switched pulsed Nd:YAG lasers. According to scientists at Tohoku University (Sendai, Japan) and the National Defense Medical College Research Institute (Tokorozawa, Japan), conventional silica glass fiber cannot deliver these pulsed beams efficiently because of durability problems related to nonlinearity in the refractive index of silica.1
To improve fiber durability, silver was mirror-plated on the internal surface of a hollow silica tube, followed by liquid-phase deposition of a thin dielectric polymer film. According to the scientists, the key to reducing loss in the fiber involved setting the internal dielectric coating thickness to an "optimum" thickness for the wavelength of transmitted light. The light would thus be highly reflected because of the interference effects of the dielectric film.
Experiments were done on silica capillary tubing with two different dimensions. One had an inner diameter of 700 µm, a wall thickness of 65 µm, and an outer diameter of 850 µm. The other had an inner diameter of 1000 µm, a wall thickness of 90 µm, and an outer diameter of 1500 µm. In each case, a plastic coating protected the 1.2-m-long fibers.
Ultrasonic mixing technique
With conventional mirror-plating conditions developed for fibers for mid-infrared lasers, the losses for the near-infrared region were excessive because the surface of the deposited silver film was too rough. This resulted from imperfections in the process of mixing silver nitrate with glucose. To improve the homogeneity of the mixture, the scientists incorporated an ultrasonic mixing technique into the coating process. Working with a conventional ultrasonic cleaner, they applied an ultrasonic wave directly after the solutions were mixed and then simultaneously deposited silver films on two fibers. This boosted both conductance and solution flow speed. To ensure uniform coating deposition along the full length of the fibers, the tubes were held vertically in a dark box to prevent the silver solution from deteriorating.
The next step was to coat the silver film with a dielectric film of cyclic olefin polymer (COP), its main advantages being high transmissivity in the near infrared and the capability of withstanding temperatures above 150°C. This involved pumping a polymer solution diluted with cyclohexane at a concentration of 5.5 wt% into the bores of silver-coated fibers. The tubes with the COp film were then cured in an electric furnace for about an hour at temperatures that gradually increased from 20°C to 180°C.
The light source was a pulsed Nd:YAG laser that produced 10-mJ pulses with a pulse width of 6 to 8 ns at a repetition rate of 10 Hp and a peak power of about 1.3 MW. With the addition of the ultrasonic technique, the researchers were able to reduce straight losses of the fibers to 0.3 dB for the 700-µm-bore fiber and 0.1 dB for the 1000-µm-bore fiber (see figure on p. 34).