Optical fibers can be used to transmit a laser beam from a laser source to a desired location. The use of an optical fiber to transmit a laser beam from a laser source to a desired location is a significant enabler in a number of laser applications because the optical fiber offers a flexible transmission path that involves no free-space optics and which can be re-routed in real time. High power fiber lasers (e.g., having an output power in the range of about 1-10 kW or higher) are useful for a wide variety of applications, including military applications and in the industrial fields of welding, high-speed cutting, brazing, and drilling. However, the use of optical fibers for delivery of laser beams from high power fiber lasers has been limited by numerous challenges. For example, a significant limitation to the use of high-power fiber lasers in industrial applications is power loss due to non-linear effects, such as stimulated Brillouin scattering (SBS) or stimulated Raman scattering (SRS), as the beam propagates through the delivery fiber from the fiber laser source to the work area.
To reduce degradation of the laser beam by these non-linear effects and accommodate high peak or average power, conventional optical fibers have a large core area, for example having core diameters as large as 0.5 to 1 mm, and large numerical aperture (NA), for example, in the range of 0.1-0.2. As a result, these conventional fibers are highly multi-mode. Due to the multimode aspect of these conventional optical fibers, bending of the optical fibers results in a strong mode coupling to the higher order transverse electromagnetic modes that are guided along with the fundamental and other lowest order transverse electromagnetic modes. If the lowest-order transverse electromagnetic mode is launched in these conventional optical fibers, the lowest-order mode will lose most of its power as it feeds higher-order transverse electromagnetic modes. As a result, even if the input beam is nearly diffraction-limited and the output beam suffers only minor power loss, the output beam quality is typically greater than 50-100 times diffraction limited (XDL). Therefore, these conventional optical fibers can meet power-delivery requirement, but not beam-quality delivery requirements.
To overcome the above deficiency with high beam quality transportation, a large mode area (LMA) optical fiber design can be implemented. LMA optical fibers can guide a few higher-order transverse electromagnetic modes while still maintaining beam quality at or better than approximately 1.3 times diffraction-limited (XDL). An LMA optical fiber differs from the standard large-core delivery fibers by having a relatively small core diameter, between about 20 μm and about 30 μm for signal wavelengths of about 1 μm, and a reduced NA of approximately 0.06. Generally, LMA fibers must be properly coiled to maintain good beam quality. In particular, for LMA fibers, a mode-dependent loss may be created by forming the fiber laser into a coil with a predetermined bend radius. Coiling imposes radiation losses that are highly dependent on mode order, with the loss rate increasing rapidly with increasing mode order. Hence, using a proper coiling radius the higher-order modes can be stripped out leaving only the lower-order modes, thereby “cleaning up” the laser beam. However, even though proper coiling can reduce the number of supported higher-order modes, LMA fibers cannot generally be made to operate only in a single mode operation (i.e., with the optical energy substantially in the fundamental fiber mode while having relatively small or negligible optical energy in higher fiber modes), as is desired in many applications, at least in part because the numerical aperture required for single-mode operation at large core sizes is lower than can be reliably achieved. For conventional LMA fibers having relatively large core diameter (e.g., exceeding 50 μm), as may be necessary for very high power applications, mode discrimination via coiling becomes inadequate between the increased number of competing modes and the diminishing separation in loss rates between neighboring modes to reliably select the lowest-order mode while operating at a low transmission loss.