PCT No. PCT / RU97 / 00278 Sec. 371 Date Jul. 9, 1998 Sec. 102(e) Date Jul. 9, 1998 PCT Filed Sep. 3, 1997 PCT Pub. No. WO98 / 28643 PCT Pub. Date Jul. 2, 1998The invention relates to
fiber optics and can be employed in
fiber radiation collimators, optical
waveguide couplers, spectral filters, optical isolators, long-period gratings, dispersion compensators,
cascade mode field diameter conversion fibers on simulated Raman effect, physical value sensors,
radiation suppression units for predetermined wavelengths, and for
smoothing the
gain spectrum in
erbium fiber amplifiers. The invention facilitates fabrication of optical waveguides and apparatuses based on them. To produce preforms for optical waveguides (1) by a
plasma chemical vapor deposition method, molecular gaseous agents, fed to a substrate tube (24), are mixed so that less than five atoms of
oxygen fall on every atom of
silicon and more than one atom of
nitrogen falls on every 1000 atoms of
oxygen. The
refractive index is locally changed by heating a length of an optical
waveguide (1). This causes a local thermal
diffusion of elements contained in a core (3) into a cladding (2), or vice versa. The length of the optical
waveguide (1) is heated by current of an
electric arc (10) or by
radiation (16) of an
infrared laser (15). The core (3) is doped with
nitrogen at concentration from 0.01 at. % to 5 at. %. In the
mode field diameter conversion fiber, a
diameter of the core (3) changes along the length of the optical waveguide (1), increasing towards its end (4).