as u03b4 n 1 u2212 n 2 n 1 where n 1 is the maximum refractive index of the glass region under consideration and n 2 is a reference index in this example taken to be that of silica when the preform using these clad rods is drawn into a waveguide fiber the surrounding clad glass layer becomes the matrix glass of the fiber clad and the central portion of the clad rods becomes the glass columns embedded in the matrix the glass columns in this example are chosen to be silica the clad rods are assembled about the core body to form a periodic array having pitch l for example 56 is fig8 the diameter of the glass columns is denoted d the v eff which is descriptive of the number of modes the waveguide can support is calculated for several values of the ratio d l as measured in the fiber drawn from the preform the chart of fig2 shows v eff as a function of inverse light wavelength scaled by the pitch of the array l u03bb curves 46 and 48 show that for d l ratios of 0 2 and 0 4 the associated v numbers approach about 2 5 and 4 respectively as the wavelength decreases because the second mode of the example waveguide fiber cuts off at v of about 4 curves 46 and 48 show the waveguide to be single mode at essentially all wavelengths this is the endlessly single mode condition the geometry of the waveguide fiber is reasonable in that the pitch is of the order of several microns about 1 5 u03bcm to 39 u03bcm at an operating wavelength of 1550 nm the.