O. Eknoyan

Traveling-wave photodetectors

Designs for traveling wave photodetectors in semiconductor materials are presented, and advantages over conventional photodetectors are discussed.

Low‐loss strain induced optical waveguides in strontium barium niobate (Sr0.6Ba0.4Nb2O6) at 1.3 μm wavelength

Low‐loss optical waveguides have been produced in z‐cut Sr0.6Ba0.4Nb2O6 (SBN:60) and electro‐optic modulation has been demonstrated at a wavelength of 1.3 μm. The refractive index increase responsible for waveguiding results from a strain produced by a SiO2 film which has been deposited on the surface of the substrate at 320 °C. The waveguides are formed in the crystal by dry etching of channels in the strain film. The resulting optical waveguides support both polarizations. Propagation loss values of 0.7 dB/cm for TM polarization and 1.6 dB/cm for TE polarization were measured. Electro‐optic modulation up to 22 MHz was performed on repoled samples using coplanar electrodes.

Rapidly tunable optical add-drop multiplexer (OADM) using a static-strain-induced grating in LiNbO/sub 3/

A rapidly tunable, polarization-independent, low-loss optical add-drop multiplexer (OADM) for the 1550-nm wavelength regime is reported. The four-port device consists of two input waveguides, a polarization beam splitter (PBS), two polarization conversion/electrooptic tuning waveguide sections, a second PBS and two output waveguides. The waveguides, which are single mode for both TE and TM polarizations, are fabricated on a LiNbO/sub 3/ substrate by Ti diffusion. Spectral selection is based on phase-matched polarization conversion due to shear strain induced by a dielectric grating overlay film. An applied voltage tunes the device by changing the waveguide birefringence and hence the optical wavelength at which most efficient polarization conversion occurs. Tuning of 0.1 nm/V with a maximum range of 24 nm has been obtained. A tuning speed of 50 ns, corresponding to a rate of 0.128 nm/ns, has been achieved. Channel isolation is better than 24 dB. Fiber-to-fiber insertion loss <5.4 dB has been obtained and polarization independent loss (PDL) of 0.24 dB for both drop and through ports have been realized over the tuning range. Thermal tuning has also been demonstrated.

Guided wave acousto-optic and electro-optic tunable filter designs with relaxed beam-splitter requirements.

New designs for acousto-optic tunable filters and electro-optic tunable filters with the Mach-Zehnder configuration are proposed. The new designs differ from conventional designs in three respects: (1) polarizing beam splitters are not required, (2) the optical path difference for the waveguides between the beam splitters is a half-wavelength, and (3) the relative positions of the polarization coupling regions in the two waveguides are displaced in the propagation direction by half of the spatial period of the perturbation responsible for the coupling. Because the new designs provide an additional degree of freedom in achieving the required beam-splitter performance, they are expected to be much easier to fabricate than conventional filter designs with polarizing beam splitters.

Characterization of vapor diffused Zn:LiTaO/sub 3/ optical waveguides

Diffusion and optical properties of planar waveguides produced in LiTaO/sub 3/ by a recently developed technique that utilizes Zn diffusion from vapor phase are presented. The waveguides are obtained by 6-h diffusion at temperatures as low as 800 degrees C. The results indicate that Zn is a fast diffusant in LiTaO/sub 3/ and has activation energies that are slightly less than those for Ti diffusion into LiTaO/sub 3/. The diffusion temperatures for Zn are much lower than the 1150-1200 degrees C require for Ti metal indiffusion. The diffusion coefficient of Zn at 800 degrees C, is comparable to that of Ti at 1200 degrees C. The resulting waveguides support both ordinary and extraordinary modes of polarization. The low temperature diffusion slows out diffusion and has been shown to be advantageous for making low-loss optical waveguides in LiTaO/sub 3/. >

Electro‐optic modulation and self‐poling in strain‐induced waveguides in barium strontium titanate niobate

Characterization of bulk single crystals and optical waveguides in Ba1−xSrxTiyNb2−yO6 (BSTN) indicate it to be a promising new ferroelectric material for electrooptic devices. The electro‐optic coefficient r33 is measured to be 218±12 pm/V, a factor of 7 greater than LiNbO3. Data on refractive indices, dielectric constant, and Curie temperature Tc in bulk samples are also presented. Strain‐induced waveguides in Z‐cut samples exhibited low losses (1.8 dB/cm for TM polarization and 2.5 dB/cm for TE polarization) at a wavelength of 1.3 μm. Electro‐optic modulation was demonstrated in these waveguides to frequencies ≳100 MHz. A ‘‘self‐poling’’ effect was found, whereby strong electro‐optic modulation is observed in the strain waveguides without repoling the crystal after processing at temperatures far above Tc.

Optical waveguide Fabry–Perot modulators in LiNbO 3

Fabry-Perot-type electro-optic modulators have been produced with Ti:LiNbO(3) optical waveguides. The end mirrors are formed by depositing six pairs of SiO(2) and TiO(2) layers at the waveguide input and output faces. A V(π) of approximately 4 V was measured at 0.81 µm wavelength for the extraordinary polarization in a 7-mm interaction length. Comparison of the modulation performance in the linear region with the modulation performance of a polarization intensity modulator of similar geometry indicates a sensitivity for the Fabry-Perot modulator that is ~3 times larger. The sensitivity can be further improved by using higher reflectance mirrors and reducing waveguide losses.

Comparison of photorefractive damage effects in LiNbO3, LiTaO3, and Ba1−xSrxTiyNb2−yO6 optical waveguides at 488 nm wavelength

Photorefractive-induced damage effects at 488 nm wavelength in LiNbO3, LiTaO3, and Ba1−xSrxTiyNb2−yO6 (BSTN) optical waveguides are compared for the extraordinary mode of polarization. The damage effects are strongest in LiNbO3 and weakest in BSTN. At an input power density of 3000 W/cm2, the output power from passive channel waveguide in LiNbO3 is more than 12 dB lower than in BSTN. Optically induced index changes measured using a Fabry–Perot interferometric method show a nonlinear behavior with increasing input intensity for all these materials. For incident power densities greater than 200 W/cm2 on the waveguides, the optically induced index-change damage increases at a rate of 2.8×10−3 cm2/μW in LiNbO3, 4.9×10−4 cm2/μW in LiTaO3, and 3.1×10−4 cm2/μW in BSTN.

Compact Bends for Achieving Higher Integration Densities for LiNbO

A new waveguide platform is demonstrated that allows the bend radii to be substantially decreased for titanium-diffused lithium-niobate (LiNbO3) waveguides using vertically integrated arsenic-trisulfide (As2S3) overlay waveguides. Power is transferred from a Ti-diffused waveguide into the overlay waveguide using tapers, guided by the As2S3 waveguide through the S-bend region and transferred back into another Ti-diffused waveguide. This structure also behaves like a polarization beam splitter. We present simulation results as well as measurements to show the feasibility of achieving low loss and reduced bend radii for electrooptic waveguides.

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We report a photonic frequency discriminator built on the vertically integrated As₂S₃-ring-on-Ti:LiNbO₃ hybrid platform. The discriminator consists of a Mach Zehnder interferometer (MZI) formed by the optical path length difference (OPD) between polarization modes of Ti-diffused waveguide on LiNbO₃ substrate and a vertically integrated As₂S₃ race-track ring resonator on top of the substrate. The figures of merit of the device, enhancement of the signal-to-3rd order intermodulation distortion (IMD3) power ratio and corresponding 3rd order intercept point (IP3) over a traditional MZI, are demonstrated through device characterization.