Use of Dual-Clad Delivery Fiber for Isolators
Although convenient for carrying pump light within the laser, dual-clad fiber presents some challenges as a delivery fiber to an isolator. Unlike single-clad fiber, once light enters the cladding adjacent to the core, it does not escape. This extra light causes problems in several components within the isolator.
The adhesive used to strain relieve the fiber where it enters the ferrule can absorb cladding light when its index of refraction is greater than the cladding. Adhesives with properties most desirable for producing a robust strain relief have these higher indexes. Thus, dual-clad, fiber-coupled isolators either have inferior pull strength, or a damage threshold below 5W of cladding light power.
Cladding light that exits the ferrule exhibits higher divergence than the core light for which the collimating lens is designed to form a small diameter beam to pass through the isolating optics. As a result, the cladding light will not be properly collimated and hit apertures, thus scattering light onto metal components that convert it into heat. Since the magnets and rotating media have a strong temperature dependence, isolation drops as these components heat up, reducing the effectiveness of the optical isolator.
In the isolation direction, off-axis reflections can produce a condition where a portion of the reverse traveling light sneaks through the apertures and enters the fiber cladding. Since light remains in the dual-clad fiber, it is carried back to the laser where it could degrade performance.
During the collimation process, more care is required to precisely couple light only into the core. To minimize light entering the cladding, it is important that the customer very carefully align their laser fiber to the delivery fiber. Additionally, it is recommended that they add a cladding mode stripper after this splice. The fiber coil and bend radii must also be kept large to avoid re-introducing light into the cladding. All of these efforts are to ensure that the intended collimation is achieved without the influence of cladding light. The result of this is longer build time and higher cost relative to isolators using single-clad fiber.
To ensure optimal performance, better reliability, and lower cost, EOT recommends laser manufacturers use single-clad fiber for the delivery of laser energy to our isolators, with cladding light removed at the splice within the laser.