C. H. Bulmer

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. >

Linear interferometric waveguide modulator for electromagnetic-field detection

A channel-waveguide interferometer with asymmetric arms, suitable for electromagnetic-field detection, is described. A π/2 intrinsic phase differential makes the output linearly proportional to the applied voltage (electromagnetic field). Horizontal and vertical electrode sets provide polarization-independent operation, with electrode lengths chosen to make the voltages approximately equal. In Ti-diffused devices in Z–X-cut LiNbO3, the TE- and TM-mode outputs are equally modulated with the horizontal voltage 43% of the vertical. Output linearity over a 40-dB range and operation up to 300 MHz are demonstrated.

Sensitive, highly linear lithium niobate interferometers for electromagnetic field sensing

An asymmetric Mach–Zehnder interferometer in Ti:LiNbO3 has been demonstrated to have a 84 dB linear dynamic range and 1.1 μV sensitivity, for a 3 kHz detection bandwidth and a 50 Ω resistance, at the 1.3 μm wavelength. This device is useful for electric and magnetic field sensing. Optimum linearity is achieved with a 90° intrinsic phase bias. The dependence of dynamic range and sensitivity on optical power, phase bias, and modulation voltage is reported. The reasons for, and magnitudes of, deviations from optimum linear behavior are described for many fabricated interferometers.

Pyroelectric effects in LiNbO3 channel‐waveguide devices

The thermal stability of Ti‐indiffused LiNbO3 channel‐waveguide devices is described. In Z‐cut interferometers the pyroelectric effect causes large output instabilities. However, both Z‐cut directional couplers and X‐cut interferometers show good thermal stability. We discuss the pyroelectric effect and explain device thermal behavior. We describe the decay of induced voltage or of change in interferometer phase bias after a temperature change and deduce that LiNbO3 resistivities can be electric field dependent.

Novel electrostatic mechanism in the thermal instability of z-cut LiNbO3 interferometers

Experimental data from z‐cut LiNbO3 Mach–Zehnder interferometers show that electrical contact to the device electrodes leads to the large thermal instability of the intrinsic phase bias. Withdrawing contact from the electrodes causes the thermal instability to virtually disappear. A model is proposed in which contact to the electrodes creates strong gradients in the local surface potential near the electrodes when unscreened pyroelectrically induced surface charges are present, independent of whether the probes are floating or grounded. The resulting differential electric field strength in the interferometer branches gives rise to the observed thermal instability, which is five orders of magnitude larger than the calculated instability due to the pyroelectric effect alone. The model of the instability mechanism is used to suggest methods of reducing the instability.

High‐efficiency flip‐chip coupling between single‐mode fibers and LiNbO3 channel waveguides

Coupling efficiencies of ≳70% have been consistently measured between single‐mode fibers and LiNbO3 channel waveguides for TE and TM polarizations, using a Si V‐groove/flip‐chip arrangement. Tapered fibers in deep transverse grooves provide precise altitudinal alignment of the coupling fibers. These efficiencies are as high as those measured with a micropositioner and compare well with theoretical estimates of maximum coupling limited only by mode field distribution mismatch. Fiber/channel/fiber throughput losses of ∼3 dB and a polarization‐independent interferometric waveguide modulator with fiber input and output are reported.

Linear 1×2 directional coupler for electromagnetic field detection

A 1×2 directional coupler which can be used as an integrated optical electromagnetic field detector is described. Modulators have been formed from titanium‐indiffused lithium niobate waveguides operating at the 1.3 μm wavelength. Linear dynamic ranges of 74.3 dB with 190 μW light and 76.3 dB with 760 μW light were observed for separate devices. Voltage sensitivity measurements are in good agreement with theoretical predictions. A 3.1 μV rms sensitivity was obtained for a device with a 10 mm electrode length.

Application of Li2O compensation techniques to Ti‐diffused LiNbO3 planar and channel waveguides

We report a novel technique to determine the index change in Li2O in‐diffused surface layer in LiNbO3 by observing the cutoff angle of the extraordinary substrated mode as it refracts through the surface‐layer–substrate interface. Surface‐layer index changes from 10−4 to 10−2 were measured in various samples which received compensation treatments for times of 10 min to 6 h, primarily in LiNbO3 powder. We also report the successful compensation of nearly single‐mode Ti : LiNbO3 channel waveguides.

Characterization of Ti:LiNbO3 deep waveguides diffused in dry and wet oxygen ambient.

The diffusivity of thicker than usual Ti films into LiNbO3 to create deep waveguides has been investigated as a function of crystal orientation and diffusion ambient for temperatures ranging from 1050 to 1100 degrees C. The diffusion parameters for substrates diffused in water vapor environment are always larger than ones diffused in dry ambient. Differences in the diffusion coefficient for the extraordinary mode as high as 90, 66, and 84% are measured for X-, Y-, and Z-cut crystals, respectively. Smaller but noticeable differences are detected for the ordinary mode. Similar enhancing effects of the water vapor are also obtained for the surface-index change. Severe rough surface finish and surface contouring, revealing an etchedlike appearance, are observed on Z-cut substrates diffused in wet atmosphere. In general, substrates diffused in dry O2 produced relatively smoother surfaces. No outdiffused modes are excited in Z-cut substrates when diffused in dry oxygen ambient for long times at high temperatures.