William G. French

Polarization characteristics of noncircular core single-mode fibers.

It is well known that geometrical or dielectric imperfections in conventional graded-index single-mode fibers depolarize light after a few centimeters. A slight improvement in the polarization performance of these fibers is achieved by introducing noncircularity in the core shape. This is evident from the measurements on borosilicate fibers with dumbbell shaped cores. This result is correlated with Marcatilis analysis, which shows that changing the core geometry, from square to rectangular, does not appreciably alter the difference in the propagation constants of the two fundamental modes with orthogonal polarizations. Thus, the noncircular geometry and the associated increase in stress-induced birefringence introduced during the manufacturing process alone are not sufficient to improve the polarization performance, and the enhancement of the anisotropic birefringence is necessary to achieve single-polarization fibers.

Spectral losses of unclad vitreous silica and soda-lime-silicate fibers

Spectral transmission losses of unclad optical fibers drawn from various types of commercial vitreous silica and of soda-lime-silicate glasses were measured in the wavelength range between 0.5 and 1.12 µm. The automated technique employed was both convenient to use and sensitive enough to measure losses of 6 dB/km, i.e., 75% loss per km, with an estimated accuracy of ±2 dB/km. Bulk-loss measurements performed with calorimetric and bridge-type techniques were in good agreement with the fiber-loss measurements.

Single polarization optical fibers: Exposed cladding technique

A simple method is described for fabricating fibers with large strain birefringence starting with standard MCVD preforms. The method is illustrated using borosilicate fibers. The birefringence and polarization properties are measured in a long fiber and the birefringence is also measured using a fiber slice and polarizing microscope.

PREPARATION OF A LIGHT FOCUSING GLASS ROD BY ION‐EXCHANGE TECHNIQUES

A glass rod with a radially graduated refractive index was produced by replacing the lithium ions contained in the glass composition with sodium ions from a fused salt bath. Glass rods with such refractive index gradients are potentially useful as low resolution imaging devices.

Near-infrared sources in the 1–1.3 μm region by efficient stimulated Raman emission in glass fibers

Abstract Low-loss glass fiber waveguides are found to be excellent media for Raman lasers and amplifiers in the near-infrared region of the spectrum. Multiwavelength emission in the 1–1.3 μm range is readily obtained by efficient stimulated Raman scattering in single-mode silica fibers. With a 1.064 μm pulsed pump of 250 W in a 175-m, 6-μm diameter single-mode silica fiber we observed four orders of Stokes radiation at 1.12 μm, 1.18 μm, 1.23 μm and 1.3 μm, respectively. Our results imply that pulsed tunable stimulated Raman emission in this wavelength region is possible using kW tunable infrared dye lasers near 1 μm as pumps. These sources are useful for studying the dispersion of glass fibers as well as for other spectroscopic applications.

Low‐loss fused silica optical waveguide with borosilicate cladding

Optical waveguides consisting of a pure fused silica core and borosilicate cladding were produced by using chemical vapor deposition techniques. A typical guide showed an optical attenuation of 7.5 ± 1.5 dB/km, measured at 0.63 μm (He–Ne laser). In one of the guides whose loss spectrum was studied, the attenuation was less than 20 dB/km between 0.55 and 0.86 μm except for the third OH overtone at ∼ 0.72 μm. The lowest loss of this waveguide occurred at 0.80 μm, with other minima at 0.66 and 1.06 μm.

Automatic analysis of interferograms: optical waveguide refractive index profiles

Compensation of mode velocities in optical waveguides can be achieved by fabricating fibers having graded refractive index cores. One technique for measuring these index profiles is interference microscopy. We have developed a machine aided method for analysis of interference micrographs for the rapid determination of optical waveguide refractive index profiles. Our method consists of digitizing the interference micrograph with a scanning microdensitometer, followed by computer determination of the position of the center line of each fringe. The data obtained are then converted into refractive index and fiber radius information, which is used to calculate a best fit power law function.

Refractive-index profiling of single-mode optical fibers and performs

The application of the focusing method to measure the refractive-index profiles of single-mode optical fibers and preforms is described. In this automatic and nondestructive technique, the fiber or preform is immersed in index-matching oil, and collimated light is passed transversely through it. The intensity distribution of the transmitted light is detected with a video camera and processed with the aid of a computer-controlled video analysis system that provides the profile within a few minutes. The profiles of boron-doped and germanium-doped fibers and preforms with maximum Deltan varying from 0.0006 to 0.012 were measured with a repeatability of better than 1% and with a resolution better than 1 microm. The profiles of the preforms are in good agreement with those of the fibers and also with those of slab samples taken from the tips of the same preforms.

A cw tunable near-infrared (1.085–1.175-μm) Raman oscillator

This is the first report of a cw tunable Raman laser in the 1.1-microm region of the spectrum. A prism-tuned fiber Raman resonator with a 650-m-long, low-loss small-core single-mode silica fiber was pumped by a 5-W cw Nd:YAG laser at 1.064 microm. With two separate resonator mirrors for the first and the second Stokes oscillation, we have obtained first Stokes oscillation tunable from 1.085 to 1.13 microm and second Stokes oscillation tunable from 1.15 to 1.175 microm.

A near‐infrared fiber Raman oscillator tunable from 1.07 to 1.32 μm

We report a near‐infrared fiber Raman oscillator in the 1.1–1.3‐μm spectral region. The Raman medium is an 800‐m‐long 6.3‐μm‐core‐diam single‐mode fiber with a loss less than 2 dB/km near 1.2 μm and less than 4 dB/km over the 1–1.32‐μm spectral range. The oscillator is synchronously pumped by a cw mode‐locked Nd : YAG laser. With four separate resonator mirrors, four orders of Stokes oscillations, peaked near 1.12, 1.18, 1.24, and 1.31 μm, are obtained. With simultaneous tuning of all the four Stokes wavelengths we have achieved continuously tunable oscillation over 250 nm (1.07–1.32 μm), the first such broadly tunable laser in this spectral range.