Wafer-fused optoelectronics for switching

Wafer fusion technique for realization of compact waveguide switches and three-dimensional (3-D) photonic integrated circuits is investigated theoretically and experimentally. Calculations based on beam propagation method show that very short vertical directional couplers with coupling lengths from 40 to 220 μm and high extinction ratios from 20 to 32 dB can be realized. These extinction ratios can be further improved using a slight asymmetry in waveguide structure. The optical loss at the fused interface is investigated. Comparison of the transmission loss in InGaAsP-based ridge-loaded waveguide structures with and without a fused layer near the core region, reveals an excess loss of 1.1 dB/cm at 1.55 μm wavelength. Fused straight vertical directional couplers have been fabricated and characterized. Waveguides separated by 0.6 μm gap layer exhibit a coupling length of 62 μm and a switching voltage of about 2.2 V. Implications for GaAs-based fused couplers for 850 nm applications will also be discussed     

Low-loss near-infrared passive optical waveguide components formedby electron beam irradiation of silica-on-silicon

Results are presented for a range of near infrared single-mode passive channel waveguide optical components fabricated in PECVD silica-on-silicon by electron beam irradiation. The devices include S-bends, Mach-Zehnder interferometers, Y-junction tree-structured splitters, and directional couplers. It is shown that low loss may be obtained through appropriate choice of waveguide bend radius and fabrication parameters; fiber-device insertion losses of ≈2 dB and ≈1 dB are achieved for 1×8 splitters and 3-dB directional couplers, respectively, at λ=1.525 μm     

Directional couplers using fluorinated polyimide waveguides

Optical properties of directional couplers using a fluorinated polyimide waveguide were demonstrated. At a wavelength of 1.3 μm, directional couplers with the refractive index difference of 0.3% between the core and cladding have a maximum power coupling ratio of more than 97 and 96% for TE and TM polarisations, respectively. The excess loss for a 3-dB coupler was as low as 0.3 dB. Moreover, the change in the coupling ratio of the 3-dB couplers remained within 0.5% between 25 and 100°C     

Ultralow sidelobe-level integrated acoustooptic tunable filtersusing tapered-gap surface acoustic wave directional couplers

We propose a new weighted-coupling scheme using tapered-gap surface acoustic wave directional couplers for realization of ultralow sidelobe-level integrated acoustooptic tunable filters (IAOTF's). Appropriate design analysis has been carried out for 30-mm-long filters operating at an optical wavelength of 1.55 μm in an X-cut Y-propagating LiNbO₃ substrate. New synthesized weighting functions have been used for the improvement of sidelobe level suppression over existing single-stage filters by as much as 27 dB. The -20 dB mainlobe width of the resulting IAOTF's varies from 2.7 to 3.9 nm only for the worst sidelobe levels ranging from -30.6 to -44.7 dB, respectively. It has also been shown that further suppression of sidelobe levels by 3-9 dB is possible if the filter is underdriven at 80% mode-conversion efficiency     

Design and fabrication of broad-band silica-based optical waveguidecouplers with asymmetric structure

Waveguide-type directional couplers with different guide widths in the interactive region are designed and fabricated with silica-based waveguides on silicon substrates. It is demonstrated that these asymmetric couplers have broadband coupling-ratios of, for example, 50%±5%, 25%±3%, or 5%±2% over a wide wavelength range from 1.3 to 1.55 μm, which are in good agreement with the results calculated using the beam propagation method (BPM). The excess loss of these couplers is as low as 0.7 dB, including input and output single-mode fiber coupling losses     

Design and realization of an integrated optic switch for crossbarswitching arrays

An electrooptic switch that is insensitive to parameter tolerances is proposed for use in crossbar switching arrays. It makes use of intrinsically mismatched and tapered directional couplers and allows the realization of switching arrays with the same switching voltage for each element. Computations are carried out to verify a realization with Ti:LiNbO₃ waveguides and to compare the latter with conventional directional couplers. Switching elements on Z-cut LiNbO₃ with a crosstalk of -27 dB for zero voltage and a switching voltage of 27.5 V have been realized for use at 1.3 μm     

Passive integrated-optical waveguide structures by Ge-diffusion insilicon

The realization of low loss optical channel waveguides and passive waveguide structures by a germanium indiffusion process into silicon will be discussed. Employing relatively simple technological processing like standard lithography, e-beam evaporation and diffusion, single-mode waveguides could be produced exhibiting polarization independent losses of as low as 0.3 dB/cm at wavelengths of λ=1.3 μm and λ=1.55 μm. S-bends fabricated with the same technology offered an excess loss of 1 dB at a radius of around 5 mm without any optimization. For a Y-junction designed with S-bends the maximum opening angle was determined to be 1.8° if an excess loss of 1 dB is allowed. The examination of directional couplers with various coupling lengths and distances revealed an excellent agreement between measurement and theory     

0.15 dB/cm loss in Unibond SOI waveguides

The optical loss of Unibond waveguides is measured and reported for the first time, using grating couplers. At a wavelength of 1.3 μm, a loss of 0.15±0.05 dB/cm is obtained for TE polarisation. This allows good quality low loss integrated optical circuits to be fabricated at low cost     

Dielectric mirror embedded optical fiber couplers

Dielectric mirrors in multimode and single-mode silica fibers have been produced by a fusion-splicing technique. Mirrors oriented normally and at a 45° angle to the axis have been studied. Fibers with angled internal mirrors serve as compact directional couplers with low excess optical loss (≈0.2 dB for multimode and 0.5 dB for single mode at 1.3 μm) and excellent mechanical properties. The reflectance is found to be wavelength dependent and strongly polarization dependent. Far-field scans of the reflected output power measured with a white-light source show a pattern that is almost circularly symmetric. The splitting ratio in a multimode coupler measured with a laser source is much less dependent on input coupling conditions than in conventional fused biconical-taper couplers. Spectral properties of multilayer mirrors oriented normally to the fiber axis have been investigated experimentally, and a matrix analysis has been used to explain the results     

Broad-band high-silica optical waveguide star coupler withasymmetric directional couplers

A broadband optical waveguide star coupler with asymmetric directional couplers is proposed. A coupling or splitting ratio of the coupler is flattened with respect to wavelength by choosing appropriate values for Δβ and complete power transfer length. The wavelength characteristics of uniform and alternating Δ&thetas;b directional couplers are investigated, and their broadband operation is shown theoretically. To demonstrate the feasibility of the new proposal, 8×8 star couplers composed of such asymmetric directional couplers are fabricated by using high-silica waveguides on a silicon chip. A star coupler composed of asymmetric directional couplers exhibited 2-dB loss variation in the wavelength range of 1.3-1.55 μm, and this value was almost one fourth of that of a star coupler composed of symmetric directional couplers     

Development of compact and low-crosstalk PLC-WDM filters for hybrid-integrated bidirectional optical transceivers

This paper reports on the development of planar-lightwave-circuit wavelength-division-multiplexing (PLC-WDM) filters based on double polynomial curve directional couplers, which will be integrated into compact and low-cost bidirectional optical transceivers for FTTx systems. Silica-based PLC-WDM filters smaller than 5 mm/spl times/100 /spl mu/m for Ethernet passive optical network (E-PON) applications were designed and fabricated to have low bidirectional crosstalk (lower than -40 dB) as well as low insertion losses (lower than 1 dB) within the operating temperatures of -40 to 80/spl deg/C. Several features and design guides that can reduce the size of directional couplers and improve the level of bidirectional crosstalk are discussed.     

Adiabatic 3-dB couplers, filters, and multiplexers made with silicawaveguides on silicon

The performance of adiabatic 3-dB couplers and V-branches is reported. These devices are broadband and divide power equally. They have no observable polarization dependence. Typical excess losses relative to a straight waveguide is 0.1-0.2 dB for the 3-dB couplers and about 0.4 dB for the V-branches. Fiber to fiber insertion loss of 0.31 dB was measured for a 2.5-cm straight waveguide. The devices were used to fabricate transmission filters peaked at 1.55 μm and power combiners having two channels at 1.48 and 1.55 μm. The device fabrication was improved by use of a flowable top cladding layer containing boron and phosphorous which easily filled-in between closely spaced waveguides     

Wavelength-selective fused fiber couplers utilizing fielddifference between core and cladding modes

It is shown that wavelength-selective fused taper couplers can be made by using the large difference in field distribution, between core and cladding modes. It is evident from the calculation results that fibers with a high-index difference and a thin outside diameter are necessary to fabricate the couplers. Fused taper couplers are made of thin fibers with a cladding diameter of 66.0 μm and an index difference of 0.84%. The splitting ratio of the coupler obtained is more than 20 dB in the 1.53-μm region and less than -20 dB in the 0.8 to 1.0-μm region. The loss is less than 0.2 dB and 0.4 dB in the 1.53-μm and the 0.8 to 1.0-μm regions, respectively     

Directly Connected Image Guide 3-dB Couplers with Very Flat Couplings

The design and evaluation of the directly connected image guide 3-dB directional coupler is described. These couplers have several useful features as a component for millimeter-wave integrated circuits because of broadband and flat coupling characteristics, mechanical stability, and compactness. The bandwidth of the directly connected image guide coupler with proper dimensions and the nearly optimixed value of h extends to about 28 percent under the tolerance limits of +-0.25 dB of deviation in coupling from 3 dB. Furthermore, experimental verification has been performed, and, hence, the usefulness of the proposed directly connected couplers with appropriate tapered sections was confirmed, even at frequencies where higher modes could be excited.     

Ultracompact multimode interference 3-dB coupler with stronglateral confinement by deep dry etching

Extremely short InP-InGaAsP MMI 3-dB couplers (L~15-50 μm) with conventional structure have been successfully fabricated. The significant size reduction was achieved by deep dry etching and tapered access waveguide with large S-bend design. Optical transmission measurements on these devices have been performed. An optical bandwidth about 80 nm with a limit of 2-dB excess loss was obtained for most of the couplers. During the whole tunable wavelength range (λ~1485-1580 nm), the imbalance is within ±0.5 dB. Fabrication tolerance and polarization independence are also investigated and the results demonstrated the applicability of these ultracompact devices in high-density photonic integrated circuit     

Interdigitated Stripline Quadrature Hybrid (Correspondence)

lnterdigitated microstrip couplers consist of three or more parallel striplines with alternate lines tied together. A single ground plane, a single dielectric, and a single layer of metallization are used. Thus the approach is eminently suited for monolithic or hybrid thin-film microwave integrated circuitry. Tight coupling is achieved much more easily than with noninterdigitated edge-coupled lines. Fabrication and tolerance problems make it almost impossible to build noninterdigitated 3-dB edge couplers. Also, current crowding at the edges, which can result in high loss, is much less severe for the interdigitated coupler. Previously, tight coupling in directional couplers for microwave integrated circuits has been achieved by broadside coupling, reentrant sections, tandem sections, or branch-line couplers. Some of the disadvantages of these approaches are narrow bandwidth, large substrate area, and the need for multilayer circuitry. A 3dB directional coupler (quadrature hybrid) for S band has been fabricated in microstrip on 40-mil alumina. A single quarter-wave section was used. The hybrid showed a directivity of over 27 dB, a return loss of over 25 dB, an insertion loss of less than 0.13 dB, and an imbalance of less than 0.25 dB over a 40 percent bandwidth.     

Millimeter-wave wide-band amplifiers using multilayer MMICtechnology

This paper describes millimeter-wave wide-band single-ended and balanced amplifiers using novel multilayer monolithic microwave/millimeter-wave integrated circuit (MMIC) technology. The fundamental characteristics of thin-film transmission lines and a 50-GHz-band multilayer MMIC directional coupler are described through measurements up to 60 GHz. A single-ended amplifier fabricated in a 1.1 mm×0.8 mm area, shows a gain of about 12 dB with a noise figure of better than 5 dB around 50 GHz. A balanced amplifier fabricated using the multilayer MMIC directional couplers and single-ended amplifiers, shows a gain of 10-17 dB with input and output return losses of better than 14 dB from 33 to 53 GHz. The transmission lines and directional couplers can be effectively combined with millimeter-wave active circuits without degrading the circuit performance or increasing the circuit area. To our knowledge, these are the first millimeter-wave active circuits employing multilayer MMIC technology     

Waveguide polarization-independent optical circulator

We fabricated a new type of waveguide polarization-independent optical circulator which does not need a polarization-beam splitter. The circulator is based on a non-reciprocal Mach-Zehnder interferometer which consists of two waveguide Faraday rotators, two thin-film half-waveplates and two planar lightwave circuit-type 3-dB couplers. The fabricated circulator provides a 14.0-23.7-dB isolation and a 3.0-3.3 dB insertion loss at λ=1.55 μm. This circulator presents a new possibility for developing non-reciprocal devices in the field of integrated optics     

Ti:LiNbO3 interferometric activated X-switch forhigh-speed applications: IAX

High-speed switching is accomplished by using a balanced bridge interferometric switch with X-switches as electro-optic tunable 3 dB couplers. A device with a 3-dB bandwidth of 6·5 GHz at a switching voltage of 7·0 V and an insertion loss of 4 dB has been realised for λ=1·3 μm