A new measurement technique, capable of quantifying the number and type of modes propagating in large-mode-area fibers is both proposed and demonstrated. The measurement is based on both spatially and spectrally resolving the image of the output of the fiber under test. The measurement provides high quality images of the modes that can be used to identify the mode order, while at the same time returning the power levels of the higher-order modes relative to the fundamental mode. Alternatively the data can be used to provide statistics on the level of beam pointing instability and mode shape changes due to random uncontrolled fluctuations of the phases between the coherent modes propagating in the fiber. An added advantage of the measurement is that is requires no prior detailed knowledge of the fiber properties in order to identify the modes and quantify their relative power levels. Because of the coherent nature of the measurement, it is far more sensitive to changes in beam properties due to the mode content in the beam than is the more traditional M(2) measurement for characterizing beam quality. We refer to the measurement as Spatially and Spectrally resolved imaging of mode content in fibers, or more simply as S(2) imaging.

Biological systems have, through the course of time, evolved unique solutions for complex optical problems. These solutions are often achieved through a sophisticated control of fine structural features. Here we present a detailed study of the optical properties of basalia spicules from the glass sponge Euplectella aspergillum and reconcile them with structural characteristics. We show these biosilica fibers to have a distinctive layered design with specific compositional variations in the glass͞organic composite and a corresponding nonuniform refractive index profile with a highindex core and a low-index cladding. The spicules can function as single-mode, few-mode, or multimode fibers, with spines serving as illumination points along the spicule shaft. The presence of a lens-like structure at the end of the fiber increases its lightcollecting efficiency. Although free-space coupling experiments emphasize the similarity of these spicules to commercial optical fibers, the absence of any birefringence, the presence of technologically inaccessible dopants in the fibers, and their improved mechanical properties highlight the advantages of the low-temperature synthesis used by biology to construct these remarkable structures.T he class Hexactinellida, of the phylum Porifera, the so-called glass sponges of the deep sea, consists of Ͼ500 extant species as well as a considerable number of extinct members extending from the early Cambrian period to the present (1). Their mineralized skeletons have been shown to be composed of spicules of amorphous hydrated silica deposited around a proteinaceous axial filament. Depending on the particular species, spicule morphology, and location within the sponge, the function of these spicules varies from a predation deterrent to providing the bulk of the sponge's mechanical rigidity (2, 3). In other instances, the spicules are modified into elaborate fibrous anchoring structures that permit the successful colonization on soft sediments (2, 3). More recent work has investigated potential optical roles for these fibers (4-7). The presence of apex type structures on the top of spicules from the hexactinellid sponge Rosella racovitzae, for example, was shown to enhance the light acceptance angles into these large fibers, thus considerably improving their ability to effectively channel ambient light (4).Recently, we suggested that spicules from another hexactenellid sponge, Euplectella aspergillum (Venus' flower basket), have fiber-optical characteristics similar to those of commercial telecommunications fibers (6). The Euplectella species live at depths ranging from 35 to 5,000 m in a cold environment, frequently with no ambient sunlight (8-10). The mineralized skeleton of E. aspergillum consists of an intricate cylindrical cage-like construction composed of a lattice of spicules imbedded in a secondarily deposited silica matrix that provides extended structural rigidity (Fig. 1a). The cage is typically inhabited by a pair of symbiotic shrimps. At the base of the sponge are the basalia, a...

No abstract

No abstract