We report the waveguiding properties of a new type of low-loss optical waveguide. The photonic crystal fiber can be engineered to support only the fundamental guided mode at every wavelength within the transparency window of silica. Experimentally, a robust single mode has been observed over a wavelength range from 337 nm to beyond 1550 nm (restricted only by available wavelength sources). By studying the number of guided modes for fibers with different parameters and the use of an effective index model, we are able to quantify the requirements for monomode operation. The requirements are independent of the scale of the fiber for sufficiently short wavelengths. Further support for the predictions of the effective index model is given by the variation of the spot size with wavelength. # 1998 Optical Society of America [S0740-3232(98)00204-X] OCIS codes: 160.2290, 230.7370, 250.5300.
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