William K. Burns

Mode dispersion in diffused channel waveguides by the effective index method

The effective index method for calculating waveguide mode dispersion is reviewed and applied to uniform rectangular optical waveguides with both small and large index differences. The results are shown to be at least as accurate as other approximate techniques. The effective index method is then applied to channel waveguides assuming 1-D and 2-D diffusion. Channel waveguides without sideways diffusion are shown to be described by the method using a normalized notation and previously published universal dispersion curves. Two-dimensional diffusion theory is applied to treat the case of isotropic sideways diffusion. A new, normalized, 1-D universal chart is obtained which in conjunction with previous results defines waveguide mode dispersion in isotropically diffused 2-D channels.

Microwave-optical mixing in LiNbO/sub 3/ modulators

An investigation study of microwave-optical mixing in different configurations of LiNbO/sub 3/ Mach-Zehnder interferometric modulators is presented. In each case, models that describe mixer performance are developed and are shown to be in good agreement with measurements. For antenna remoting applications, a technique to down-convert RF signals is demonstrated by cascading in series a pair of Mach-Zehnder interferometric modulators. In general, it is shown that by virtue of their truly broadband characteristics, interferometric modulators can also by employed as microwave mixers at frequencies up to 40 GHz. >

Performance and modeling of broadband LiNbO/sub 3/ traveling wave optical intensity modulators

The design, fabrication and characterization of a traveling wave Ti:LiNbO/sub 3/ Mach-Zehnder interferometric modulator are discussed. The dependence of the velocity match condition on electrode thickness and wall angle is demonstrated experimentally and with finite element calculations. A set of test electrode structures is fabricated to study electrical losses in the modulator electrode. Loss coefficients are assigned to different sections of the device, and dielectric and radiative losses are shown to play an important role at high frequencies. This information is used in conjunction with finite-element calculations to develop accurate models for both the electrical and optical responses. The frequency dependence of the half-wave voltage is measured and shown to be in good agreement with a model. >

Fiber-optic gyroscopes with broad-band sources

The use of broad-band sources, polarized and unpolarized, in fiber gyroscopes with linearly birefringent fibers, is studied theoretically. Polarization-mode coupling in the fiber is modeled using one-mode coupling center. Gyroscope output equations are obtained which are useful in interpreting an origin of fiber noise in the limiting cases of low and high fiber birefringence. Interference effects in the output are shown to be related to mode coupling at particular locations in the fiber. The extent of these locations is governed by a depolarization length for which numerical estimates are given.

Ti diffusion in Ti : LiNbO3 planar and channel optical waveguides

Optical mode measurements have been used to determine the anisotropic diffusion coefficient and surface index changes for planar Ti : LiNbO3 waveguides at 0.633 μm. Measured values for the diffusion coefficient at 1000 °C are Dy=9.4×10−13 and Dz=1.4×10−12 cm2/sec. The ratio of the extraordinary to the ordinary surface index change (Δne/Δn0) was found to vary between 1.3 and 1.8, depending on diffusion temperature and Ti film thickness. Ti and Li concentration profiles were measured by secondary‐ion mass spectrometry. Additional high concentration peaks, 0.2–0.3 μm wide for both ions, were found superimposed on otherwise well‐behaved diffusion profiles. This observation is interpreted to result from a tendency towards Li‐Ti‐O compound (Li2TiO3 or Li2Ti3O7) formation at the diffusion temperature in a dilute mixture with LiNbO3. Such compound formation has the effect of impeding the Ti diffusion into the LiNbO3 substrate. Electron microprobe measurements were used to measure lateral diffusion from channel wa...

An analytic solution for mode coupling in optical waveguide branches

Mode coupling between local normal modes in branching and separating optical waveguides is treated. An analytic solution for power transfer is provided for structures whose shape has a particular functional form. Analytic results are compared to numerical calculations for linear branches. Numerical examples for the design of shaped structures fabricated with Ti:LiNbO3channel waveguides are given.

Optical waveguide parabolic coupling horns

We propose parabolic‐shaped coupling horns to provide adiabatic transition regions between wide‐ and narrow‐channel optical waveguides. The parabolic shape is shown to derive from a simple ray model of channel propagation and is consistent with diffraction theory in the wide‐channel limit. An approximate mode dispersion theory is used to convert the criteria into a horn design for indiffused channel waveguides. Experimental horn transmission measurements of nearly 90% are reported for transitions from 25 μm to 8, 6, and 4 μm in Ti:LiNbO3 waveguides at 0.6328 μm.

Active branching waveguide modulator

We report the observation of electrically controlled modulation in a single‐mode channel waveguide branch fabricated on Ti‐diffused LiNbO3. Over 55% intensity modulation has been observed for applied voltages of ±30 V with an active length of 2.9 mm. Experimental factors limiting device performance are discussed. A theoretical description of the device, based on power transfer between local normal modes, is presented and shown to be consistent with experimental results when sideways diffusion in the channels is approximately accounted for.

Polarization holding in birefringent single-mode fibers

Perturbations in highly birefringent single-mode fibers couple the two polarization modes and degrade the polarization-holding ability. With a broadband source we demonstrate wavelength averaging of the power in either mode, permitting a simple measurement of the power transfer to the cross-polarized mode as a function of fiber length. Using this technique, we confirm experimentally the theory of random mode coupling between the polarization modes.

A low-loss downconverting analog fiber-optic link

An analog fiber-optic link for concurrent detection and downconversion of microwave signals is reported. Optical amplification is employed in conjunction with electrical power combining of photodetectors to demonstrate link losses of 19.6 and 22.9 dB at RF carrier frequencies of 9 and 16 GHz, respectively. Analytic expressions validating the experimental observations are also developed. The link may be employed to detect phase sensitive or phase-modulated microwave signals and shows excellent potential for application in sensor systems involving remoting of an antenna element. >