Lasers, amplifiers and other optical apparatus based on optical fiber can provide flexible, rugged and relatively simple sources of optical energy. Accordingly, in many applications such optical fiber apparatus can often have one or more advantages as compared to counterparts based on a gas medium (e.g., CO2) or on a bulk solid-state medium (e.g., a Nd:YAG rod). For example, optical fiber lasers can have a smaller footprint, or can be more efficient, or can require less sophisticated cooling arrangements as compared to using a gas or bulk rod solid-state laser in a similar application. Often, however, it can be desirable to increase the output power of optical fiber apparatus, as certain gas and bulk solid-state lasers can readily produce high CW output powers or pulses of optical energy having high energy and/or high peak power.
Unfortunately, because of the high power density inherent in confining optical energy to the relatively small cross sectional area of an optical fiber, non-linear phenomena, such as Stimulated Raman Scattering (SRS) or Stimulated Brillouin Scattering (SBS), can severely limit scaling the output power of a fiber laser or amplifier to higher powers. Though these non-linear processes are complex, each can be addressed, at least in part, by reducing the power density in the core of the fiber. One way to reduce power density is to increase the diameter of the core of the fiber and/or reduce the numerical aperture (NA) of the core, such that the fiber has a larger mode field diameter (MFD). Reducing the power density in this manner can increase the power threshold for the onset of the undesirable non-linear phenomena.
This approach, however, is not without drawbacks. Fibers having larger core diameters can typically support higher order transverse modes (e.g., LP11, LP21, LP02 etc.) in addition to the fundamental mode (e.g., LP01). Such higher orders modes (HOMs) tend to degrade the quality of output optical energy provided by the fiber apparatus and hence raise the M2 parameter (lower M2 means better beam quality). In many applications a low M2 is desired. Forestalling the onset of non-linear effects while also maintaining good beam quality can present a challenge to the designer of optical fiber apparatus.
Some approaches to this challenge are known in the art. For example, U.S. Pat. No. 6,496,301, issued on Dec. 17, 2002 to Koplow, Kliner and Goldberg, teaches bending a multimode fiber having a larger core to substantially attenuate, via increased bend loss, higher order modes such that a fiber amplifier provides gain in substantially a single mode. See also U.S. Pat. No. 7,424,193, issued on Sep. 9, 2008 to Alamantas Galvanauskas, which teaches a composite waveguide having a central core and at least one side core helically wound about the central core and in optical proximity to the central core. According to the '193 patent, higher order modes of the central core selectively couple to the helical side core and experience high loss such that the central core is effectively single-mode.
Existing techniques, however, are not necessarily entirely satisfactory in all circumstances. Accordingly, it is an object of the present invention to address one or more of the deficiencies or drawbacks of the prior art.