Fabrication of channel waveguides from sol-gel-processed polyvinylpyrrolidone/ SiO(2) composite materials

Sol-gel-processed composite materials of polyvinylpyrrolidone (PVP) and SiO(2) were studied for optical waveguide applications. PVP is a polymer that can be crosslinked, so it is expected to have high thermal stability after crosslinking. However, thermal crosslinking and thermal decomposition of pure PVP take place around the same temperature, 200 °C, therefore pure PVP had a high optical propagation loss as a result of the absorption of the decomposed molecules after crosslinking. The incorporation of sol-gel-processed SiO(2) prevented the thermal decomposition of PVP and provided remarkably low optical propagation losses. The PVP/SiO(2)composite material also produced thick (>2-μm) crack-free films when the PVP concentration was 50% or higher. An optical propagation loss of 0.2 dB/cm was achieved at 633 nm in the 50% PVP/SiO(2) composite planar waveguide. Several aspects of the thermal stability of the waveguides were evaluated. The slab waveguide was then used for fabrication of channel waveguides with a selective laser-densification technique. This technique used metal lines fabricated with photolithography on the slab waveguide as a light absorbent, and these metal lines were heated by an Ar laser. The resultant channel waveguide had an optical propagation loss of 0.9 dB/ cm at 633 nm. This technique provides lower absorption loss and scattering loss compared with the direct laser-densification technique, which uses UV lasers, and produces narrow waveguides that are difficult to fabricate with a CO(2) laser.     

Absorption and emission cross sections of Er(3+) in Al(2)O(3) waveguides

Al(2)O(3) slab waveguide films were doped with erbium using ion implantation to a peak concentration of 1.5 at. %. Prism coupling measurements show absorption caused by (4)I (15/2) ?(4)I (13/2) intra-4f transitions in Er(3+) with a maximum at 1.530 mum of 8 dB/cm. The Er(3+) absorption cross section is determined as a function of wavelength. We used the McCumber theory to derive the emission cross section spectrum from the absorption results, which we then compared with the Er(3+) photoluminescence spectrum. The peak absorption and emission cross sections are found to be 6 x 10(-21) cm(-2). The results are used to predict the optical gain performance of an Er-doped Al(2)O(3) optical amplifier that operates around 1.5 mum.     

Optical waveguiding in epitaxial PbTiO(3) thin films

Epitaxial lead titanate (PbTiO(3)) thin films on SrTiO(3) (100) substrate were grown in situ by radio-frequency sputtering for optical waveguiding applications. The crystalline quality of the PbTiO(3) films deposited at 550 degrees C has been investigated through x-ray diffraction analysis. It indicates that thin films are completely c-axis oriented (rocking curve FWHM of 0.2 degrees for the 001 reflection). The transmission spectrum method has been used to measure the dispersion of the refractive index. At 632.8 nm, the PbTiO(3) film with an (001) orientation exhibits a refractive index of 2.61, which represents 98% of the bulk material. The prism-coupling technique has been also employed to determine the optical attenuation in the planar waveguide. In this study, we report a low propagation loss of 2.2 ? 0.2 dB/cm obtained in a PbTiO(3) optical waveguide.     

Ion-exchanged waveguide lasers in Er3+/Yb3+ codoped silicate glass

We investigated an Er(3+)/Yb(3+) codoped silicate glass as a host material for waveguide lasers operating near 1.5 mum. Spectroscopic properties of the glass are reported. Waveguide lasers were fabricated by K(+)-ion exchange from a nitrate melt. The waveguides support a single transverse mode at 1.5 mum. An investigation of the laser performance as a function of the Yb:Er ratio was performed, indicating an optimal ratio of approximately 5:1. Slope efficiencies of as great as 6.5% and output powers as high as 19.6 mW at 1.54 mum were realized. The experimental results are compared with a waveguide laser model that is used to extract the Er(3+) upconversion coefficients and the Yb(3+)-Er(3+) cross-relaxation coefficients. The results indicate the possibility of obtaining high-performance waveguide lasers from a durable silicate host glass.     

Waveguide-hologram-based wavelength-multiplexed pseudoanalog true-time-delay module for wideband phased-array antennas

A pseudoanalog true-time-delay (TTD) module based on substrate-guided waves and wavelength-division multiplexing is presented. A 1-to-32 (5-bit) even fan-out is demonstrated by use of a two-dimensional waveguide hologram array. This module has a packing density of 2.5 lines/cm(2) and very compact packaging (8 cm x 4 cm x 8 mm). It also reduces TTD system complexity by providing continuously tuned delay signals to parallel-control the whole phased-array antenna system. The device has a measured bandwidth of as high as 2.4 THz. The delay signal can range from tens of picoseconds to several nanoseconds.     

Suppression of sidelobe levels for guided-wave acousto-optic tunable filters using weighted coupling

An analysis of guided-wave acousto-optic tunable filters (AOTFs) that employ simple acousto-optic (AO) weighted coupling techniques for sidelobe reduction and the calculated and experimental results from a specific example that involves only variation of the width of a surface acoustic wave (SAW) slot waveguide are presented. The calculations on single- and multi-stage AOTFs consisting of an optical channel waveguide and a SAW slot waveguide in LiNbO(3) substrate show that waveguide width weighting using generalized Hamming functions would provide significant improvement in sidelobe suppression. Calculated results together with the design, fabrication, and measured performance characteristics of a single-stage AOTF that utilizes a weighted-aperture SAW slot waveguide in YX-LiNbO(3) substrate at the optical wavelength of 1.55 mum and the acoustic center frequency of 175 MHz are reported. The measured sidelobe level is -13.6 dB and the measured FWHM bandwidth is 26 A, as compared to the theoretical values of -15.0 dB and 15 A, respectively. The RF drive power was measured to be 1.0 W at a mode-conversion efficiency of 100%     

Compact optical-fiber variable attenuator arrays with polymer-network liquid crystals

Electrically controlled fiber variable-optical-attenuator arrays with polymer-network liquid crystals are shown to be compact and to have a large attenuation range (30-40 dB) and low residual loss (0.55 dB) at wavelengths from 1.3 to 1.6 mum. Their estimated power consumption is very low (<30 nW/channel), and arrays with more than ten channels can be made. The manufacturing process is simple: Trenches 30-100 mum wide are cut across parallel conductive-layer-coated optical fibers and are filled with a polymer-network liquid crystal. The attenuation properties depend on UV-curing conditions and on trench width.     

Baseband integrated acousto-optic frequency shifter/modulator module for fiber optic at 1.3 mum

A baseband integrated acoustooptic (AO) frequency shifter/modulator module that consists of a pair of titanium-indiffused proton-exchanged (TIPE) waveguide lenses and a pair of cascaded guided-wave AO Bragg cells has been realized in a Y-cut LiNbO(3) waveguide substrate 0.1 cmx1.0 cmx2.0 cm in size. A device module operating at the optical wavelength of 1.3 mum has provided a -3-dB tunable bandwidth of 120 MHz at baseband. The frequency-shifted or -modulated light propagates in a fixed direction, irrespective of the magnitude of frequency shift or modulation, and is focused into a spot (FWHM) of 6.2-mum size on the output edge of the waveguide. Accordingly, this optical frequency shifter/module can be directly interfaced with single-mode optical fibers to facilitate applications in fiber optic systems.     

Wideband true-time-delay beam former that employs a tunable chirped fiber grating prism

A fiber grating prism that consists of four tunable chirped-grating delay lines for wideband true-time-delay beam forming is proposed and demonstrated. The chirped gratings are produced by use of the grating bending technique in which a uniform grating is surface mounted on a simply supported beam. We obtained chirped gratings with different chirp rates by bending the uniform gratings with different beam deflections. Four linear chirped fiber gratings with identical spectral width but linearly increased grating length are fabricated. The spectra and time-delay responses of the tunable chirped gratings are measured. A chirped-grating prism for wideband true-time-delay beam forming by use of four chirped gratings is constructed and tested experimentally. We obtained different time delays by tuning the wavelength of the optical carrier. The proposed true-time-delay beam former with a four-element phased-array steerer is suitable for continuous beam forming at microwave frequencies up to 20 GHz.     

Low excess losses in a Y-branching plastic optical waveguide formed through injection molding

We have demonstrated low excess losses (1.9 dB at 660-nm wavelength) in a Y-branching plastic optical waveguide (POWG) that was fabricated using an injection-molding method. The waveguide had an amorphous vinyl polymer as the core and transparent polyolefin as the cladding. We then studied a method for isolating the excess loss in the Y-branching POWG, and with that method we estimated the lower limit of the loss to be 1.41 dB at 660 nm. The sample had a heat-resistant plastic optical fiber (POF) with a core composed of crosslinked poly (methyl methacrylate) (PMMA) copolymer, and a cladding composed of poly (tetrafluoroethylene-co-hexafluoropropylene). The POWG has sufficient reliability for ordinary uses below 100 °C. A model for a bidirectional wavelength-division multiplexing opticalcommunication system with the developed Y-branching POWG and the POF was also demonstrated.     

Fabrication of polymer waveguides by a replication method

We have developed a soft-lithography method to replicate polymer waveguides. In this method, the waveguides are produced by a two-step molding process where a master mold is first formed on a negative-tone photoresist and subsequently transferred to a polydimethylsiloxane (PDMS) mold; a PDMS silicone rubber mold is then used as a stamp to transfer the final waveguide pattern onto an UV cure epoxy. Initial results show good pattern transferring in physical shape. The optical performance is measured based on the propagation loss. In our design, the loss was measured at 0.28 dB/cm for 1.3 microm and 0.26 dB/cm for 1.55 microm.     

Heat-resistant flexible-film optical waveguides from fluorinated polyimides

Heat-resistant flexible-film optical waveguides were fabricated from fluorinated polyimides. These waveguides operated in single mode and had low optical loss (0.3 dB/cm) at a wavelength of 1.3 mum for TE and TM polarizations. They also had good flexibility: The optical loss did not significantly change above a minimum radius of curvature of less than 20 mm. The birefringence of 9 x 10(-5) between the TE and TM polarizations is 2 orders of magnitude smaller than that for a waveguide upon a substrate. Moreover, these waveguides had high thermal stability and moisture resistance: The optical loss and single-mode behavior changed little after heating the waveguides at 420 degrees C for 1 h or after their exposure to 85% relative humidity at 85 degrees C for more than 350 h.     

Broadband millimetre-wave passive spatial combiner based on coaxial waveguide

A broadband millimetre-wave passive spatial combiner using a microstrip probe array and an oversized coaxial waveguide is successfully designed and tested. The equivalent circuit approach of transverse electromagnetic (TEM) lines is adopted to synthesise the coaxial stepped impedance transformer from a K connector with the oversized coaxial waveguide and to analyse the microstrip probe array. The equivalent susceptances of the step discontinuities for the power divider are calculated. A four-way coaxial waveguide passive power combining circuit operating at the entire Ka-band is designed, fabricated and measured. Experiments on the four-way passive combiner show that a minimum insertion loss of 0.7 dB has been achieved at about 29.5 GHz. The combiner has shown a bandwidth of 26.5?40 GHz with 10 dB return loss and less than 2.5 dB insertion loss.     

Evaluation of the radiation pattern of a split aperture linear phased array for high frequency imaging

Developing transducer arrays for high frequency medical imaging is complicated because of the extremely small size and spacing of the array elements. For example, a 50 MHz linear phased array requires a center-to-center spacing of only 15 mum (one-half wavelength in water) to avoid the formation of grating lobes in the radiation pattern of the array. Fabricating an array with these dimensions is difficult using conventional technology. A split aperture design that permits much larger element spacing (3 to 4 times) while avoiding the formation of grating lobes is described. The 3-D radiation pattern of a 1.9x1.4 mm, 50-MHz split aperture linear phased array with 33 transmit elements and 33 receive elements has been evaluated theoretically. The azimuthal beam width is 90 mum at a distance of 4.0 mm. Grating lobes are suppressed by at least 60 dB at distances >4.0 mm (~f/2). The elevation beam width is 220 mum at 4.0 mm, and a useful depth of field over the axial range from 4 to 10 mm is obtained.     

Planar waveguide-based silica-polymer hybrid variable optical attenuator and its associated polymers

We have proposed a silica-polymer hybrid variable optical attenuator that we made by creating two trenches along the core of a straight silica planar waveguide and filling the trenches with a polymer. The polymer, which had a superhigh thermo-optic coefficient, was specially designed and developed in this study to lower the power consumption of the device. We achieved a maximum attenuation of -29 dB for the device by changing the polymer's temperature by only 7 degrees C. The device has a very low power consumption at 2-3 mW/channel, making it a good candidate for integration with arrayed waveguide gratings.     

Improved silica-zirconia sol synthesis for fabrication of a single-mode embedded dielectric channel waveguide with low transmission losses

A novel inorganic-organic hybrid silica-zirconia solgel material, which can generate 10 microm thick film in a single spin-coating process, has been developed and employed in the fabrication of an embedded dielectric channel waveguide on a silica buffer layer of a silicon substrate. The fabricated channel waveguide core had steep ridge walls, good smoothness, and high robustness, and the novel sol synthesis enabled a precise control of the geometrical and optical parameters of the embedded dielectric channel waveguide. In the 1.55 microm telecommunication window, the fundamental modes TE(00) and TM(00) in the embedded channel waveguide had low transmission losses of 0.40 +/- 0.03 dB/cm and 0.59 +/- 0.03 dB/cm, respectively.     

Poling-induced birefringence phase matching for 1.5-μm-band wavelength conversion in a polymeric rib waveguide

A birefringence phase-matching (BPM) scheme for second harmonic generation, difference frequency generation (DFG), and their cascading is developed in a polymer rib waveguide. Poling-induced birefringence (n_TM–n_TE) of PMMA:DANS side-chain polymer films showed quadratic increase upon applied DC poling fields, and was controlled for satisfying the BPM. The three conversion processes using the nonlinear coefficient of the d₁₅ were simultaneously phase matched at the poling field around 150 V/μm when the fundamental guiding modes of a pump (DFG: 0.775 μm, cascading: 1.55 μm) and a signal (∼1.55 μm) waves were used. In addition, a numerical simulation of the DFG including the waveguide losses showed high conversion efficiencies around 0 dB on the conditions: the pump power of 100 mW, the waveguide length around 2 cm, and the waveguide losses of 2 dB/cm, demonstrating that the poling-induced BPM is an effective phase-matching method in the wavelength conversion processes.     

Superhigh Electromagnetic Interference Shielding of Ultrathin Aligned Pristine Graphene Nanosheets Film

Ultrathin, lightweight, high-strength, and thermally conductive electromagnetic interference (EMI) shielding materials with high shielding effectiveness (SE) are highly desired for next-generation portable and wearable electronics. Pristine graphene (PG) has a great potential to meet all the above requirements, but the poor processability of PG nanosheets hinders its applications. Here, efficient synthesis of highly aligned laminated PG films and nacre-like PG/polymer composites with a superhigh PG loading up to 90 wt% by a scanning centrifugal casting method is reported. Due to the PG-nanosheets-alignment-induced high electrical conductivity and multiple internal reflections, such films show superhigh EMI SE comparable to the reported best synthetic material, MXene films, at an ultralow thickness. An EMI SE of 93 dB is obtained for the PG film at a thickness of ≈100 µm, and 63 dB is achieved for the PG/polyimide composite film at a thickness of ≈60 µm. Furthermore, such PG-nanosheets-based films show much higher mechanical strength (up to 145 MPa) and thermal conductivity (up to 190 W m⁻¹ K⁻¹) than those of their MXene counterparts. These excellent comprehensive properties, along with ease of mass production, pave the way for practical applications of PG nanosheets in EMI shielding.     

Epitaxial grown K1−xRbxTiOPO4 films with extremely flat surfaces for waveguiding

We report on epitaxial {1 0 0} K_1−xRb_xTiOPO₄ waveguide films for the visible spectral range grown on KTiOPO₄ substrates by liquid phase epitaxy. Using the m-line technique a refractive index increase of Δn_x≈0.007 and Δn_z≈0.004 for TM and TE polarisation has been determined for a K_0.78Rb_0.22TiOPO₄ film. Optical transmission and nearfield distribution are comparable to conventional ion-exchanged waveguides. Typical attenuation of about 1 dB/cm for both TM and TE polarisation was obtained at λ=532 and 1064 nm. Energy-dispersive X-ray spectrometry reveals solid-solution films with graded rubidium composition profiles. X-ray rocking curve analyses confirm the epitaxial growth process and indicate perfect and relaxed K_1−xRb_xTiOPO₄ films. Atomic force microscopy investigations reveal regular step structures with step heights Δh<1.3 nm resulting in rms-roughness values of ≈0.4 nm.     

Fabrication and analysis of a low-loss in-fiber active polymer waveguide

We present a method for fabricating an in-fiber electro-optic polymer waveguide within a D-shaped optical fiber. A combined process of selective chemical etching and spin coating creates a 2-cm in-fiber poly(methyl methacrylate)-DR1 dye polymer waveguide section with an overall insertion loss of micro 1.6 dB at 1550 nm. Numerical simulations show that, for in-fiber polymer waveguides to have low loss, the polymer layer's thickness must be kept below a certain value so that it will not support slab waveguide modes. Long transition regions between the unetched fiber and the polymer waveguide section also reduce loss. We analyze the efficiency of an in-fiber polymer waveguide by simulating its theoretical performance as an electro-optic modulator.