Single Clad and Dual Clad Optical Fibers for Laser Power Delivery
Joe Zhou, PhD
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Introduction:

For nearly two decades, hard polymer clad silica optical fibers (HPCF) have been commercially available and have increased in popularity. The optical fibers are easy to use and are cost effective. There are two common designs of hard polymer clad silica fibers widely used in medical and industrial applications for laser power delivery:

Fiber Specifications:

These fibers are of step index, multimode design and have been successfully employed for laser power delivery in the wavelength range from 450 to 2100 nm¹. The fibers are typically provided with a mechanically strippable extruded buffer (jacket) material, such as ETFE (Tefzel^TM), nylon, or PFA. Each design is capable of withstanding environmental temperature from -65 to 125°C.  A review of the typical sizes for each type follows in Tables 1 and 2 below. 

Fiber Selection:

While fiber optic assemblies for laser power delivery are known to many, it is worthwhile to discuss fiber parameters and mechanical issues that may impact the performance in practical use. In addition to operating wavelength and specific power handling requirements, the following are usually considered and evaluated in selection of an optical fiber:

1. Fiber configuration - For a specific application one must decide on fiber configuration. In general, single clad HPCF offers a higher NA, reduced bend sensitivity, and lower cost while dual clad fiber is capable of handling greater laser power and a wider wavelength range.
2. Fiber geometry– Key geometry considerations include core/clad diameters, concentricity, and geometric tolerances. These parameters are of particular interest in selection of dual clad fibers. Tighter tolerances are indicative of higher quality fiber and provide ease of alignment and improved coupling of laser light into the fiber.

3. Fiber proof test level - Fiber proof test level is a good indicator of maximum fiber strength and strength consistency. Proof testing is often performed in-line during the fiber draw process. The proof test level reflects the degree of quality control a fiber manufacturer achieves during manufacture of its optical fiber. It is directly related to the capability of the fiber, and resulting assemblies, to endure physical abuse such as pulling, flexing, and bending that occur in the real world.  The higher the proof test level, the more durable the fiber and resulting assemblies. Estimated short term and long term minimal bend radii are also related to the proof test level. A calculated value based on the proof test level is more realistic in predicting minimal bend radius than a simple, rudimentary estimate such as a radii multiple.
4. Cost - The single clad is less expensive than the dual clad. As a trade-off, the dual clad provides a significant performance improvement in power handling capability.
5. Adhesion of hard polymer clad to the silica glass - Good adhesion eliminates the effect of fiber “pistoning” after termination and avoids “connector pull-off” in practical use.
6. Buffer – A high quality extrusion of the buffer is critical in maintaining the high strength properties of the fiber.  The buffer must have good concentricity and be able to strip off the fiber mechanically with minimal damage to the underlying hard polymer.  The stripping properties may need to be adjusted for the specific application and termination process being used.  The selection of the buffer material depends largely on the termination technique.  In general if the connector is adhered (with epoxy) to the buffer, then a nylon jacket is preferred. Vendor knowledge and experience with buffer extrusion is critical for fiber performance and therefore is significant in the fiber selection criterion.

Summary:

Single clad and dual clad HPCF have become common solutions for laser power delivery in industrial and medical applications in the wavelength range from 450 to 2100nm. For these applications, a thoughtful consideration of multiple factors is needed in selection of proper fiber. Soft factors such as adhesion of hard clad to the glass and of extruded buffer to the clad can not be neglected, specifically in high power delivery, while hard factors such as proof test levels, NA, geometry, and so on have an impact on fiber and assembly performance in the real world.

¹For 2.9 µm and 10.6 µm infrared transmission, Polymicro Technologies provides hollow waveguides that offers ease of use and lower cost than solid core infrared optical fibers.

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