D. C. Johnson

Photosensitivity in optical fiber waveguides: Application to reflection filter fabrication

The observation of photosensitivity in Ge‐doped core optical fibers is reported. The photosensitivity is manifested by light‐induced refractive‐index changes in the core of the waveguide. Narrowband reflectors in a guide structure have been fabricated using this photosensitivity and the resulting DFB reflectors employed as laser mirrors in a cw gas laser in the visible.

Bragg gratings fabricated in monomode photosensitive optical fiber by UV exposure through a phase mask

A photolithographic method is described for fabricating refractive index Bragg gratings in photosensitive optical fiber by using a special phase mask grating made of silica glass. A KrF excimer laser beam (249 nm) at normal incidence is modulated spatially by the phase mask grating. The diffracted light, which forms a periodic, high‐contrast intensity pattern with half the phase mask grating pitch, photoimprints a refractive index modulation into the core of photosensitive fiber placed behind, in proximity, and parallel, to the mask; the phase mask grating striations are oriented normal to the fiber axis. This method of fabricating in‐fiber Bragg gratings is flexible, simple to use, results in reduced mechanical sensitivity of the grating writing apparatus and is functional even with low spatial and temporal coherence laser sources.

cw three‐wave mixing in single‐mode optical fibers

Strong continuous three‐wave mixing of 514.5‐nm argon laser light in a single‐mode fiber is reported. The effect, due to the third‐order nonlinearity of silica, has been observed for light whose frequency spectrum consists of either a few discrete monochromatic frequency components separated by ∼1 GHz or a quasicontinuous distribution of frequencies having a spectral envelope ∼4 GHz wide. We show that the effect provides a simple and effective method for measuring the nonlinearity of silica. In the first manifestation of the effect, the nonlinearity mixes the frequency components to produce new frequencies. In the second, multiple mixing occurs that broadens the quasicontinuous spectrum. This manifestation of the effect is large; broadening by a factor of 4 has been observed with lower intensity levels than are required to produce stimulated Brillouin scattering in the same fiber. A theoretical model is presented to describe spectral broadening by three‐wave mixing for the case of small broadening. The ef...

An all-fiber dense wavelength-division multiplexer/demultiplexer using photoimprinted Bragg gratings

A wavelength multiplexing/demultiplexing device is fabricated and used to drop/insert a single wavelength channel from/into a multiple wavelength transmission link with 100 GHz channel-spacing at 1550 nm. The device consists of an all-fiber Mach-Zehnder interferometer with photoinduced Bragg gratings. The following performances were measured: extraction/coupling efficiency =99.4%, excess loss <0.5 dB, adjacent channel-isolation >20 dB, and return loss >23 dB.<<ETX>>

Low loss access coupler for multimode optical fiber distribution systems

The low loss access coupler includes two multimode optic fibers, each having a biconical taper section. The biconical taper sections of the fibers are fused together to provide optical coupling between the fibers. The fused fibers may also be twisted around one another to enhance mode mixing. The access couplers may be produced by fusing two fibers together along a small length, then heating the fused length and pulling the fibers to form the biconical tapers; or by twisting a portion of each of the fibers around one another, applying a tensile force to the twisted portions of the fibers and heating a region of the twisted fibers to soften and fuse a predetermined length of twisted fibers. If the fibers already have biconical taper sections, the access coupler may be produced by twisting the fibers together along their taper sections and heating a region of the taper sections to fuse them together.

Chirped in-fiber Bragg gratings for compensation of optical-fiber dispersion.

A linearly chirped in-fiber Bragg grating is reported that can compensate at 1549 nm for the dispersion [ approximately -19 ps/(nmkm)] of standard telecommunications optical fiber optimized for 1300-nm operation.

Narrow-band Bragg reflectors in optical fibers

The formation and characterization of narrow-band-waveguide reflection filters in Ge-doped silica optical fibers is described. The filters can have complex response profiles and are tunable in frequency by mechanical strain.

cw Brillouin laser

The observation of efficient continuous‐wave Brillouin laser action in an optical‐fiber ring resonator is reported. Internal laser conversion efficiencies of 50% and a Brillouin‐shifted output of 20 mW have been achieved.

Photosensitization of optical fiber and silica-on-silicon/silica waveguides

Localized heating with a flame is shown to be a simple and effective method for substantially augmenting the photosensitivity of high-silica optical waveguides to (UV) light. The method increases the photosensitivity of standard (Ge-doped core) telecommunications fiber by a factor greater than 10 (photoinduced Deltan(uv) > 10(-3)) and renders strongly photosensitive the cores of high-quality Ge:SiO(2)-on-Si and Ge:SiO(2)-on-SiO(2) planar waveguides that were negligibly photosensitive before treatment. We have written large-modulation-depth Bragg gratings, in both fiber and planar optical waveguides photosensitized by our method, using KrF (249-nm) radiation incident upon the waveguides through a zero-order-nulled phase mask. It is noteworthy that photosensitization by our method is achieved with a negligible increase in loss at the three principal optical communication windows.

Single-excimer-pulse writing of fiber gratings by use of a zero-order nulled phase mask: grating spectral response and visualization of index perturbations.

Optical fiber Bragg reflectors have been written by irradiating the fiber from the side through a phase mask with a single pulse of high-power 249-nm excimer-laser light. Efficient tapping of light to the radiation modes has been achieved for light at wavelengths shorter than the Bragg wavelength. The photoinduced periodic refractive-index perturbations have been observed directly with an optical microscope and are shown to have the same period as the phase mask and to be highly localized on one side, the irradiated side of the fiber core-cladding boundary.