We give a tutorial review and brief summary of the present status of waveguide electrooptic modulators.

Waveguide Electrooptic Modulators

We review the current status of guided-wave optical communication devices. The operation and presently demonstrated performance characteristics of waveguides switches, modulators, filters, and polarization transforming devices are discussed. Emphasis is given to recent developments with very high speed modulators and polarization insensitive devices.

Guided-wave devices for optical communication

We present a new ultrafast optical waveguide gate based on a nonlinear optical interaction in a waveguide interferometer. A train of signal pulses is gated by control pulses which change the refractive index in only one arm of the interferometer. The principle of operation is demonstrated using a modified Mach-Zehnder interferometer, 2 cm long, fabricated in a LiNbO 3 substrate. The experiments were performed with a near-infrared dye laser which produced 5 ps pulses at \lambda = 840 nm. A maximum modulation on the order of 10-3was obtained for a peak control power of 2 W. Experimentally, we determined the appropriate nonlinear coefficient of LiNbO 3 to be n_{2} = 3 \times 10^{-9} (MW/cm2)-1. As possible applications of the structure we describe an all-optical inverter, an XOR gate, and an AND gate. We conclude with a brief description of a random number generator encoder and decoder based on a few all-optical logic gates.

An ultrafast all-optical gate

We propose and analyze a new technique for achieving velocity match between the traveling wave electrical drive and guided optical signal for modulators in substrates for which there is an inherent mismatch. The traveling wave electrodes are laterally shifted periodically to reverse the direction of the applied electric field within the optical waveguide which exactly compensates for the polarity reversal caused by the microwave-optical walkoff. Consequently, the electrooptically induced phase shifts of each section add in phase and several sections can be used to reduce the required drive voltage at the design frequency. This artificial velocity-matching technique moves the mismatch-limited bandwidth to an arbitrarily high design frequency. In addition, we extend the new concept of phase reversal and the previously suggested technique of intermittent interaction by proposing electrode structures with large inactive to active aspect ratios. This generalization provides increased flexibility for manipulating the total available bandwidth to, for example, allow efficient modulation by a train of arbitrarily short electrical pulses. These techniques are ideally suited for several proposed integrated optic devices, including picosecond samplers and gates, which require strong overmodulation at a single high frequency.

Velocity-matching techniques for integrated optic traveling wave switch/modulators

Coupling between two uncoupled waveguides in proximity can be affected by introducing a third waveguide between them. One advantage of this scheme is that it can eliminate the need for bends. Another advantage is the resulting improvement in the shape of the transfer characteristic. Improved sampling and tunable filter operation of this new structure are also shown. An analysis of a uniform filter structure is presented. The "sidelobe" analysis of Alferness is adapted to the present case and tapers are investigated for further sidelobe suppression.

Three-waveguide couplers for improved sampling and filtering

A high speed sampling technique is proposed using a cascade of optical waveguide interferometers driven by CW sinusoidal traveling or standing microwaves. Each interferometer multiplies the optical signal by a function closely approximated by \cos^{4} \omegat , where ω is the frequency of its microwave drive. A cascade of four interferometers with 2.6 V microwave drives at 10, 20, 40, and 80 GHz and a total length of 2 cm can sample an optical signal with 2 ps resolution and 50 ps between samples. The sampler might be used in a picosecond sampling oscilloscope or in experiments requiring a jitter-free train of identical pulses.

Picosecond optical sampling

A high speed optical sampler is proposed that is driven by cw microwaves. The sampler is composed of waveguide interferometers of the type originally proposed by Martin. A voltage applied to one (or both) arms of the interferometer modulates the interference of the optical radiation at the output. Such an interferometer can be driven at high frequencies if synchronism is maintained between the optical radiation and the applied voltage pattern. Spatially periodic exciting structures can serve this purpose. Deviations from synchronism can be compensated (within limits) by adjustment of the peak amplitude of the microwave voltage. A cascade of four interferometers on LiMb03, driven at 10 GHz, at multiples of (appr.) 3V (3, 6, 12, 24) have a predicted sampling-function width of 2 ps at 20 GHz repetition rate. Experiments on single section and double-section interferometers will be reported.© (1981) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

F. J. Leonberger

Papers

Picosecond optical sampling

Picosecond Optical Sampling