Observation of fundamental mode cut-off at short wavelength in a holey optical fiber

We have observed that fundamental mode is cut-off at short wavelengths in a standard holey optical fiber (HOF), while the same fiber guides light at long wavelengths. This can be an another evidence that the guiding mechanism in a holey optical fiber is mostly due to average index effect rather than photonic bandgap effect. This strange property of a HOF can be potentially useful for device applications.