Hollow-Fibers Deliver High-Power IR Pulses

To eliminate absorption and optical damage issues associated with fiber delivery of high-power beams, researchers have developed robust, silver-coated hollow fiber that offers low-loss performance.

By: Yvonne Carts-Powell

Researchers in Japan have proposed a beam delivery method that can make pulsed near-IR light easier to use. Infrared 1.06-µm light from neodymium-doped yttrium aluminum garnet (Nd:YAG) lasers are used for both industrial and medical applications, and are gaining in popularity. Q-switching the lasers generates pulses with very high peak powers, which are useful for a variety of applications. In much the same way that the same amount of force can be applied either by hammering a nail once or pushing the nail with a finger for minutes, high-peak-power pulses can ablate surfaces and cause other useful effects that CW beams cannot provide.

This high intensity complicates beam delivery, however. Continuous Nd:YAG light is routinely transmitted using silica fibers, but the high peak powers in pulses cause a nonlinear self-focusing effect in the center of the fiber, which can damage the core. To solve this problem, Yuji Matsuura and others at Tohoku University (Sendai, Japan) and the National Defense Medical College Research Institute (Tokorozawa, Japan) have adapted and refined a method used to transmit mid-infrared (IR) laser emission using a hollow-core fiber with a mirrored coating deposited on the inside to guide the light.

Hollow-core fiber
In conventional fiber, light propagates down a solid core via total internal reflection from the refractive index mismatch between the fiber core and cladding. In hollow-core fibers, the light transmits through air, eliminating difficulties associated with absorption and damage to the core.

Loss through the mirror coating is, however, still an issue. Carbon-dioxide lasers, emitting at 10.6 µm, and erbium-doped YAG lasers at 2.94 µm use mirror coatings optimized to reduce absorption at those wavelengths. In making the hollow-core fiber for the near-IR laser, the researchers also tried to optimize the silver mirror plating for this wavelength. However, as the wavelength shortens, the smoothness requirements for a mirror become more stringent. Methods used for plating mid-IR fibers resulted in losses too high for use in the near-IR. To achieve the desired smoothness, Matsuura and coworkers developed new methods of depositing the silver.

The team produced the fiber by mirror plating silver onto the inner surface of a 1.2-m-long silica tube and then coating the silver with a liquid-phase-deposited polymer to protect the silver. They discovered that the main cause of roughness in the silver film was imperfect mixing of the silver nitrate and glucose solutions used for mirror plating. To improve the process and lower absorption losses, the researchers used an ultrasonic mixer and increased the flow speed by coating two tubes at a time rather than one.

Performance
For a 1-m-long fiber with a 700-µm bore, the loss was measured at 0.3 dB, and for a similar 1000-µm-bore fiber the loss was 0.1 dB. Other measurements showed attenuation loss in the fiber as low as 0.1 dB/m. In tests, the fibers survived pulse energies as high as 30 mJ, which corresponds to 4 to 5 MW of peak power. While the attenuation increased for the high-power measurements, possibly due to the lower quality beam, the fiber was not damaged.

The team also evaluated bending loss. A 1-mm-bore 1-m-long fiber with both the silver and polymer coatings was bent with uniform bending radii and 20 cm along each end were held straight. While the 10-mJ pulses were increasingly attenuated with decreasing bending radius, the increase was relatively small. Loss for a 30-cm bending radius was as low as 0.5 dB. The output from the end of the fiber also has low-enough divergence that a collecting lens is not necessary for many applications.

References
Yuji Matsura and Konosuke Hanamoto, "Hollow-fiber delivery of high-power pulsed Nd:YAG laser light," Optics Letters 23[23], 1998.

About the author…
Yvonne Carts-Powell is a freelance science writer based in Belmont, MA.