David H. Naghski

An integrated photonic Mach-Zehnder interferometer with no electrodes for sensing electric fields

A new configuration for a Mach-Zehnder interferometric electric field sensor device utilizing the electrooptic effect is reported. The lithium niobate device uses no metallic electrodes and operates solely by immersion in an electric field. Reverse poling of one arm of the interferometer provides opposing optical phase changes in the two interferometer arms when placed in an electric field. One fabricated device exhibits a measured minimum detectable field of 0.22 V/m/spl radic/Hz and a frequency response of greater than 1 GHz. Theoretical calculations show that detection of 0.11 V/m/spl radic/Hz is attainable while the upper frequency limit can exceed 6 GHz. >

Measurement of mode field profiles and bending and transition losses in curved optical channel waveguides

Curved, single-mode, silicon oxynitride optical ridge channel waveguides have been characterized by measuring mode field profile alteration, bending loss, and transition loss. The results were compared to theoretical calculations. Near field imaging of the channel mode field profiles showed the effect of bend radius on mode shape. Good agreement was obtained between the measurements and profiles obtained from two-dimensional (2-D) beam propagation method calculations. The exponential dependence of bending loss on bend radius and the variation of transition loss on the lateral offset between channels having different bend radius were successfully modeled by two simple theories.

Near field scanning optical microscopy measurements of optical intensity distributions in semiconductor channel waveguides

We present results of near field scanning optical microscopy measurements performed on single mode AlGaAs ridge channel waveguides. The optical intensity distribution just above the surface of the waveguide structure has been measured by scanning a tapered, aluminum‐coated, fiber probe transverse to the waveguide propagation direction. Experimental results are compared with model calculations performed using both the effective index and beam propagation methods. An accurate description of submicron features in the intensity profile near the ridge edges, as well as the magnitude of the field outside the channel, requires the use of the beam propagation method.

Near field measurements of optical channel waveguide structures

Abstract Near field scanning optical microscopy (NSOM) has been used to investigate the guided mode intensity distribution in optical channel waveguides, phase-matched directional couplers, and symmetric Y-junctions. A near field measurement of the guided mode intensity profile across the optical channel waveguide has been performed and compared with model calculations. The near field guided mode intensity profiles above the waveguides were measured as a function of distance along the propagation direction of both a directional coupler and a Y-junction, providing a near field view of the spatial evolution of optical power in these structures.

Potential for size reduction of AlGaAs optical channel waveguide structures fabricated by focused ion beam implantation and oxidation

Optical channel waveguides formed by focused ion beam (FIB) implantation-induced mixing of AlGaAs multiple quantum well structures and subsequent oxidation of the mixed regions have the potential of significantly reducing the size of integrated photonic waveguide structures. Since FIB implantation is a direct write process characterized by nanoscale precision, we suggest its use for forming channel waveguides having nanoscale (submicrometer) widths. Calculations presented for such channel waveguides show reductions in size by at least an order of magnitude are possible for directional couplers and other structures involving curved channel waveguide sections. Such size reductions would allow the realization of significantly higher levels of device integration than are now currently possible.

Use of near-field scanning optical microscopy (NSOM) to characterize optical channel waveguide structures

Near field scanning optical microscopy (NSOM) has been used to investigate the guided mode intensity distribution in channel waveguides, directional couplers, and Y-junctions. The intensity profile above the sample surface and transverse to the waveguide propagation direction has been measured using a tapered optical fiber to probe the guided evanescent field. The fiber probe was maintained at a constant height above the sample surface using feedback provided by performing these near field scanning measurements simultaneously with shear force microscopy topography measurements. Single mode channel waveguides were formed by etching a ridge in planar Si3N4/SiO2 structures and were excited with light of a wavelength of 830 nm. Measurements transverse to a channel waveguide revealed a cosine squared variation of intensity above the ridge and an exponential decay away from the ridge, as expected. Considerations for characterizing AlGaAs waveguides in this manner also are discussed. Multiple scans along the two waveguides of a directional coupler provided a detailed view of optical power transfer from one waveguide to the other and were in agrement with beam propagation method calculations. We anticipate that this type of measurement will provide a more detailed understanding of a central photonic structure, the channel waveguide, and its incorporation into a variety of device configurations.

Near-field measurements of optical channel waveguides

Near field microscopy has been used to investigate the guided mode intensity distribution in a variety of optical channel waveguide structures. We have studied the optical field intensity distribution in channel waveguides, directional couplers, and Y-branches above the surface of the structures scanning transverse to the waveguide propagation direction. Single mode channel waveguides are formed by etching a ridge in Si3N4/SiO2 structures and excited at a wavelength of 833 nm. Measurements of the intensity distribution transverse to a channel waveguide reveal a cosine squared variation of intensity above the ridge region and an exponential decay away from the ridge region, in agreement with theoretical expectations. For more complicated structures, for instance the directional coupler, measurements along the two waveguides of the coupler provide a detailed view of optical power transfer from one waveguide to the other. Measurements also provide a detailed view of the evolution of the optical power in the Y-junction.

Near field experiments and theoretical modeling on visible photonic bandgap structures

Summary form only give. We have obtained preliminary theoretical and experimental results on a visible photonic band gap (PBG) structures which consists of 20 rows of 146 nm air pores arranged in a triangular lattice with pitch of 260 nm etched into the center of a planar waveguide. The waveguide has a thermally grown 1.8 /spl mu/m thick silicon dioxide substrate buffer, followed by 250 nm thick silicon nitride waveguiding layer and a thin 75 nm silicon dioxide cladding layer. Near field scanning optical microscopy (NSOM) is employed to probe the PBG structures. NSOM achieves its subdiffraction spatial resolution by scanning an aperture of /spl sim/100 nm at a distance of /spl sim/10 nm above the surface of interest; thus NSOM can provide details of local photon density of states and mode structure as light propagates through both the planar region and PBG stripe.

Optical constants of thin-film gallium sulfide layers

Gallium sulfide (GaS) deposited by chemical vapor deposition (CVD) is known to passivate GaAs surfaces. In this paper we examine the thin film optical properties of GaS as they relate to the fabrication of optical waveguides. Spectroscopic ellipsometry was used to determine the index of refraction of GaS films deposited on various substrates. Results indicate that GaS has a high index of refraction suitable for waveguide structures. A gallium sulfide waveguide could provide both the optical interconnect and the passivating layer of GaAs integrated circuits. Progress toward fabricating GaS waveguides is also discussed.

An Electrode-Less Integrated Mach-Zehnder Interferometer Electric Field Sensor

Advances in the development of a newly configured Mach-Zehnder interferometric electric field sensor device utilizing the electro-optic effect are reported. The integrated optical lithium niobate device operates solely by immersion in an electric field, using no metallic electrodes. Reverse poling of one arm of the interferometer results in additive optical phase changes in the interferometer arms when the device is placed in an electric field. Recently fabricated devices have exhibited a measured minimum detectable field of 34 mV/m per √Hz and a frequency response of greater than 10 GHz.