Photonic-crystal fiber characteristics benefit numerous applications

Photonic-crystal fibers (PCFs) are among the most specialized optical lightguides. Ranging from fibers with low levels of nonlinearities supporting high-power pulses to highly nonlinear counterparts for supercontinuum generation, PCF is an attractive and versatile technology. It is based on a microstructured arrangement of low- and high-refractive-index materials. The high-index background material is typically undoped silica while the low-index region is usually provided by air holes along the fiber length. They can be made using a stack-and-draw fabrication process (see Figure 1), which is based on stacking glass capillaries and rods into a preform, allowing precise control of the core and cladding-index properties. Rare-earth and stress-element components are introduced simultaneously. Once the desired preform has been constructed, it is drawn into a fiber.

Figure 1. Stack-and-draw photonic-crystal-fiber (PCF) fabrication process.

PCFs are constructed using one of two basic design types, containing either a solid or hollow core. The former is typically made of silica—see Figure 2(left)—which, as for most conventional fibers, relies on total internal reflection. The latter typically contains air: see Figure 2(right). It relies on a photonic band gap to restrict guidance to the core (some solid-core fibers work similarly). These basic PCF types encompass a range of fiber designs suited for a variety of applications because they have properties not found elsewhere. For example, unique features of hollow-core fibers include small nonlinearities, low light loss, and the option to fill air cores with gases and liquids. As a result, they are being considered for use in many applications, including sensors, high power-pulse transmission, and medical use.

Figure 2. (left) Solid-core and (right) hollow-core fiber.

While many hollow-core fiber applications are in relatively early developmental stages, solid-core PCF applications are more mature. These designs also exhibit novel properties, such as enabling endlessly single-moded large-mode-area and highly nonlinear fibers. For active double-clad fibers, constructing secondary claddings with air holes instead of conventional low-index polymers provides PCF with a high power-handling advantage.

Lasers play a significant role in many industrial and material-processing applications. In recent years, fiber lasers have made advances, sometimes replacing more traditional lasers because of advantages in efficiency, beam quality, scalability, and operating cost. Achieving reliable high-power performance is a remaining challenge, since various components in fiber-laser systems struggle with power levels exceeding a few hundred watts. PCF solves this problem with design characteristics that are not available in conventional fibers (see Figure 3), including larger single-mode areas and the use of secondary cladding based on air holes, enabling higher damage thresholds than for the polymers used in conventional fibers. In addition, PCF offers similar benefits in fiber-based coupling and power-combining components within the high-power fiber-laser system.

Figure 3. (left) Polymer-coated conventional double-clad fiber and (right) air-clad microstructured double-clad PCF.

PCF can also be used in broadband supercontinuum devices in metrology,¹ optical-coherence tomography, and spectroscopy. Passive highly nonlinear PCFs can be pumped with short pulses to produce a supercontinuum of power distributed over a wide bandwidth, exhibiting the high brightness characteristics of fiber lasers and the broad spectral coverage of white-light sources. This is a combination not offered by other technologies (see Figure 4). Highly nonlinear PCFs can be tailored to support various pump wavelengths with flexible design of small core, high numerical aperture and dispersion to achieve superior supercontinuum generation.

Figure 4. Superior power and spectral coverage of a PCF-based supercontinuum (SC) source. ASE: amplified spontaneous emission, SLED: superluminescent LED.

We have translated PCF from the laboratory to commercial applications. Much of the progress achieved relied on a focus on making the fibers easy to use. The same unique physical characteristics that give PCF performance advantages in relation to conventional fibers could present challenges to those not accustomed to integrating fibers into their applications. However, we have developed components and methods to overcome those challenges. For example, one development offers subassemblies for high-power fiber lasers that include the active fiber, power combiners, and delivery fibers, with all the critical splicing, coupling, power handling, and thermal management already performed, so that one only needs to interface to standard fiber types. This same approach to making PCF products easy to use will be applied to other fibers as the applications and demands continue to develop, so that the many benefits of PCF can be fully realized.