R. C. Alferness

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

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.

Low-loss Ti:LiNbO 3 waveguide bends at λ = 1.3 µm

Low-loss waveguide bends are necessary for many proposed integrated optical circuits. The bend loss associated with an S -shaped transition connecting offset 6 μm wide titanium-indiffused lithium niobate strip waveguides has been measured as a function of transition length and initial Ti metal thickness for 1.3 μm wavelength. Losses as low as 0.2 ± 0.2 dB have been achieved for a transition between offset parallel waveguides with a 0.1 mm lateral and 3.25 mm longitudinal separation. The bend loss is shown to be strongly dependent on the mode confinement and less sensitive to the shape of the transition curve.

Efficient waveguide electro‐optic TE⇄TM mode converter/wavelength filter

We report the first demonstration of efficient phase‐matched waveguide electro‐optic polarization conversion. Using a strip Ti‐diffused lithium niobate waveguide and periodic electrodes, we have achieved TE⇄TM conversion efficiency in excess of 99% with an applied voltage of 13 V. The conversion efficiency is strongly wavelength selective; filter bandwidths between 4.5 and 15 A have been obtained.

Polarization‐independent electro‐optically tunable narrow‐band wavelength filter

We propose and demonstrate the first polarization‐independent electro‐optically tunable wavelengh filter with single‐mode waveguides. The Ti:LiNbO3 filter utilizes narrow‐band electro‐optic TE ↔ TM conversion and employs TE/TM polarization splitters in the input and output waveguides. The filter operates at 1.52 μm with a bandwidth of only 12 A and can be electro‐optically tuned over at least 110 A at a tuning rate of 0.55 A/V.

Polarization‐independent optical directional coupler switch using weighted coupling

We report the first demonstration of a guided‐wave polarization‐independent electro‐optic switch. Using a specially designed weighted Ti‐diffused directional coupler with stepped electrodes, we have achieved crosstalk levels below −23 dB for both switch states for arbitrary incident optical polarization.

Narrowband grating resonator filters in InGaAsP/InP waveguides

We report the first demonstration of efficient narrowband optical wavelength filters using InGaAsP/InP passive waveguide grating resonators. Filter bandwidths as narrow as 1 A, centered about λ=1.55 μm with excess resonator loss as low as 1 dB, have been achieved.

Mode size and method for estimating the propagation constant of single-mode Ti:LiNbO 3 strip waveguides

We have formulated a model to calculate the mode size and propagation constant of single-mode titanium-lithium niobate diffused strip waveguides directly from controllable fabrication parameters and basic constants. The model is compared to measurements of the lateral and vertical mode width of Ti:LiNbO 3 waveguides for a variety of diffusion conditions. We show that the model accurately predicts the geometrical mean mode size of the two-dimensional waveguide. The model provides a simplified method for estimating the mode size and propagation constant of the guide, and is useful in designing waveguide devices having low fiber/waveguide coupling and bending losses.

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.

Optical directional couplers with weighted coupling

We have fabricated optical directional couplers using titanium‐diffused waveguides for which the coupling strength is carefully weighted along the device. With a Hamming function weighting, −25‐dB transfer response sidelobes have been obtained, a 17‐dB reduction over the uniform coupling case. Sidelobe reduction results in a significant increase in the performance of directional coupler filters and switches.

Waveguide electro‐optic polarization transformer

We propose and demonstrate a novel waveguide electro‐optic device which is capable of performing general polarization transformations under electrical control. The device, made with a Ti‐diffused lithium niobate waveguide, combines in a unique optical circuit an electro‐optic (e/o) TE⇄TM mode converter and two e/o phase shifters to provide general polarization transformations. This is the first report of a waveguide e/o device capable of performing arbitrary polarization transformations. As a linear rotator the device exhibits a rotation rate of 15°/V with better than 23‐dB fidelity. We also demonstrate transformation between circular and linear polarization.