Embedded Mirrors Improve Fiber Pumping

Daniel S. Burgess

A new approach for injecting radiation into double-clad fiber promises significant advantages in the production and performance of fiber lasers and amplifiers. Developed at the Naval Research Laboratory in Washington and at Sandia National Laboratories in Livermore, Calif., embedded-mirror side pumping enables the use of highly divergent pump sources, offers high coupling efficiency, displays low sensitivity to misalignment and is scalable to high output powers.

Dahv A.V. Kliner, a researcher at Sandia's combustion research facility, explained that double-clad fiber lasers are beginning to displace conventional bulk solid-state sources in a number of commercial applications. Moreover, the optical and physical characteristics of fiber lasers may enable new uses in applications such as materials processing and chemical detection.

Embedded-mirror side pumping improves the injection of radiation from diode laser sources into double-clad fiber. In the approach, a piece of glass with a high-reflection coating on one face is inserted into a channel cut into the inner cladding of the fiber. The pump beam is launched into the inner cladding by reflection from the mirror. Courtesy of Dahv A.V. Kliner.

In embedded-mirror side pumping, a channel is cut into the inner cladding of the fiber, and a transmissive element with one reflective facet is inserted into the groove. Radiation from the pump source is launched into the element and directed into the inner cladding, eliminating the need for pigtailing and leaving the ends of the fiber free for other purposes.

The researchers begin by mounting the fiber on a glass slide with UV-curable epoxy. They remove a section of the jacket and outer cladding with a razor blade, exposing the inner cladding and creating sufficient mechanical clearance for the pump source. They use a zirconia cutting tool to excise and polish a channel in the inner cladding to receive the mirrored element.

Using a common embedded mirror, the technique can launch the output of a diode bar into a coiled fiber at several points or into multiple fibers.

They fabricate the mirrors by drawing and cutting a shaped preform of glass, such as BK 7, to the necessary size using techniques that are similar to those used in fabricating conventional optical fiber. The hypotenuse of the triangular-shaped element is curved to reduce the divergence of the pump diode and is coated with a high-reflection film. The input facet may be treated with an antireflection coating. Optical-grade UV-curable epoxy affixes the mirror in the channel, and a pulsed 1064-nm laser may be employed to cleave unnecessary lengths overhanging the fiber.

To demonstrate the approach, the researchers fabricated diode-pumped amplifiers using double-clad fiber doped with ytterbium or co-doped with ytterbium and erbium. The maximum measured pump coupling efficiency was 80 percent, but they estimated that there was an excess loss of approximately 9 percent in the systems, which might have been caused by divergence of the pump beam after reflection, imperfections in the input face of the channel in the fiber, or contamination of the input and output facets of the optical element by the high-reflection coating.

Kliner said they are working to achieve higher coupling efficiencies and are investigating the use of embedded-mirror side pumping to couple the emission of a diode bar directly into the fiber, which would enable the scaling of fiber lasers to higher powers. One longer element, for example, could be embedded into a coiled length of fiber at many positions without introducing greater complexity or a higher parts count than a single-diode system.