Novel Approach for Design and Fabrication of Single Mode Microstructured Optical Fibers

Researchers at the University of Arizona College of Optical Sciences have developed a means of increasing power output in single mode fiber lasers by significantly increasing the mode area available for doping, thus providing higher power in shorter fiber lasers. Using a novel geometry of "holey fibers", also called "photonic crystal fibers" (PCF), the invention is a low-cost approach to the design and manufacture of waveguides for optical switching, high-speed data transmission, laser surgery, and welding applications.

Instead of the conventional refractive index relationship between the cladding (outer) region and the core (inner) region of the fiber, a material of lower refractive index is used in the core region. This approach significantly lowers control and fabrication tolerances, thus lowering the total cost of production. In the presence of air holes, the unique properties of the PCF allow for light propagation in the fiber core even if the fiber core has a smaller index than that of the index in the surrounding cladding region. The exact guiding properties of such a PCF depend on the distance between air holes, the doping arrangement, the pitch, and air hole diameter. With the negative index step approach, single mode fibers of a wide variety can be designed to include multiple fibers and polarization maintaining PCFs. This novel process is compatible with existing technologies such as MSVD or batch glass, to make both passive and active single mode fibers with large mode diameters.

Background:
The field of high-power fiber lasers hits limitations with conventional single-mode step-index fibers because the high ion doping concentrations needed for high power require a long length of fiber, making it difficult to maintain a single mode. Shorter lengths of fiber can remain single mode, but can only achieve powers in the milliwatt range because the short length cannot support sufficient ion doping concentrations to achieve higher power. Part of the problem is due to the small mode area of single-mode step-index fibers.

Applications:

• Compact high-power fiber lasers and amplifiers
• Optical sensors
• Pump sources
• Passive single mode fibers with core sizes >600 µm2

Advantages:

• Continuous high-power single mode operation with narrow linewidth
• Compact
• Lower manufacturing costs- Fabrication tolerances eased
• Compatible with existing technologies
• Passive and active fibers with large mode diameters
• Reduces optical non-linear effects and instability

Status: Stage of Development: 600 square micron mode areas have been produced and the method is available for production application immediately.