V. Ramaswamy

Linear polarization in birefringent single‐mode fibers

Single‐mode fibers which maintain linear polarization are described. The fibers are birefringent and linear polarization is maintained along either of the two principal axes. The birefringence arises from the deliberate enhancement of anisotropic strains.

Elimination of out‐diffused surface guiding in titanium‐diffused LiNbO3

We describe a means of eliminating undesired surface guiding in titanium‐diffused strip waveguides on LiNbO3. Surface guiding for light of the extraordinary polarization, normally produced when the crystal is diffused in dry argon or oxygen, can be eliminated entirely by adding water vapor to the diffusion gases. Infrared absorption measurements show that hydrogen enters crystals diffused in a wet atmosphere. Although the mechanism for the prevention of surface guiding is not fully understood, we suggest that the presence of excess hydrogen reduces lithium mobility.

Lithium niobate ridge waveguide modulator

A narrow ridge was ion‐beam etched on an outdiffused LiNbO3 crystal to form a single‐mode waveguide confining the beam in both transverse dimensions. Electrodes were evaporated along the ridge to produce a very efficient electro‐optic phase modulator requiring a power of 20 μ W/MHz of bandwidth and a voltage of 1.2 V to produce a modulation index of 1 rad.

Birefringence in elliptically clad borosilicate single-mode fibers

Experimental results indicate that the stress-induced strain birefringence in borosilicate elliptically clad fibers arises from the expansion coefficient mismatch between the borosilicate elliptical cladding and the pure silica outer jacket. This strain birefringence cannot be annealed even after repeated thermal cycling. It is shown that, for a given dopant concentration, the birefringence depends primarily on the cladding ellipticity e and increases linearly with increasing ellipticity and expansion coefficient mismatch.

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.

Balanced bridge modulator switch using Ti‐diffused LiNbO3 strip waveguides

An experimental integrated optical version of the Mach‐Zehnder interferometer switch, analogous to the microwave balanced bridge, is reported. The bridge is formed by 3‐μm‐wide Ti‐diffused strip waveguides in LiNbO3 and the switch utilizes electro‐optical tuning to achieve 3‐dB operation of the directional couplers. The switching voltage required to switch between the states, corresponding to a π phase shift in one arm, equals 14.8 V. The cross talk between channels that corresponds to the extinction ratio when operated as an on‐off modulator is −21.6 dB.

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.

Single‐polarization optical fibers: Slab model

Calculations for a simple three‐layer slab model show that sufficient anisotropic strain birefringence can be introduced into germanosilicate, phosphosilicate, and borosilicate single‐mode fibers to reduce the coupling between orthogonally polarized modes to a level of practical interest for single‐mode single‐polarization propagation.

Efficient single-mode fiber to titanium diffused lithium niobate waveguide coupling for λ = 1.32 µm

We report detailed results on the achievement of very high optical throughput for titanium diffused lithium niobate waveguides coupled between input and output single-mode fibers. By determining appropriate diffusion parameters to obtain excellent dimensional match between the fiber and waveguide modes and simultaneously low propagation loss, we have achieved total measured fiber-waveguide-fiber insertion loss as low as 1 dB for a 1 cm long waveguide at \lambda = 1.32 \mu m. The relative contributions of coupling and propagation loss are determined. Very good correlation is found between the coupling loss and the match between the fiber and waveguide mode dimensions. Design data for diffusion parameters to obtain good mode match for arbitrary fiber dimension are presented.

Nb‐diffused LiTaO3 optical waveguides: Planar and embedded strip guides

Previous results on dielectric optical waveguides made by diffusing niobium into LiTaO3 have been extended. It is shown that the propagation constants of the thin‐film waveguide modes can be accurately calculated assuming that the refractive index decreases as the square of the distance into the film. Low‐loss single‐mode embedded strip guides made by electron resist masking techniques are also described.