Single-mode Channel Waveguides Research Articles

Dielectric slot-coupled half-Maxwell fisheye lens as octave-bandwidth beam expander for terahertz-range applications

We present a paradigm for integrated photonic devices based on broadband slab-confined collimated beams that are launched with half-Maxwell fisheye lenses. Although it is challenging to match to the low-index focus of the lens while maintaining adequate field confinement for a close approximation of a point source, integrated dielectric slot waveguides prove highly suitable, yielding collimators of 90% efficiency and bandwidth greater than one octave. Terahertz technology will benefit from such broadband slab-confined beams to replace free-space optics toward compact, mass-producible systems that do not require manual optical alignment. We present two prototype systems to demonstrate the versatility of this concept, namely, a diagonally set distributed Bragg reflector as a frequency-division diplexer for terahertz communications and an attenuated total reflection-based liquid sensor. Both are enabled by oblique in-slab reflections that are collected at a location other than the originating lens, which is not attainable using ordinary single-mode channel waveguides.

Watt-level Tm:LiYF4 channel waveguide laser produced by diamond saw dicing.

Low-loss surface channel waveguides with a cross-section of 30 × 30 μm2 are produced by diamond saw dicing of 6.2 at.% Tm3+, 3.5 at.% Gd3+:LiYF4 films grown by liquid phase epitaxy (LPE) on (001)-oriented bulk undoped LiYF4 substrates. Pumped by a Ti:Sapphire laser at 783 nm, a continuous-wave Tm:LiYF4 waveguide laser generated 1.30 W at 1880 nm (for π-polarization) with a slope efficiency of 80% with respect to the absorbed pump power. The laser threshold was at 80 mW. The waveguide morphology was studied revealing low roughness (3 ± 2 μm) as expressed by the propagation losses of <0.3 dB/cm. A combination of LPE and diamond saw dicing is a promising technology for multi-watt single-mode channel waveguide lasers and amplifiers.

All Integrated Lithium Niobate Standing Wave Fourier Transform Electro-Optic Spectrometer

In a standard stationary wave Fourier-transform spectrometer, a static interferogram obtained in a single-mode channel waveguide is sampled by scattering centers that are regularly spaced on top of a channel waveguide. Each scattering center is placed right below a pixel, so that the sampling step is limited by the pixel pitch to avoid crosstalk. For micron-size pixels, this implies under-sampling of the fringes, and therefore, a reduced spectral etendue. In this paper, we propose to overcome this subsampling limitation by using the electro-optical properties of lithium niobate. An integrated optic inversed Y-junction finished by a mirror is used to inject in the single waveguide output two waves characterized by an optical phase difference (OPD). The mirror reflection induces a static interferogram with an OPD null at the mirror (interference of each incident wave with its own reflection) and a dynamic interferogram coming from the interference between the one wave from one arm of the Y-junction and the wave from the other arm, after being reflected by the mirror. The dynamic feature is observed by the integration of an electro-optic phase modulator set on the two arms of the reversed Y-junction inputs. Using moderate control voltages (<100 V), a dynamic wideband fringe wave packet is shifted on several fringe periods under the sampling centers fabricated on the surface of the single waveguide output. We show that this solution can compensate the subsampling related to their large spacing and rebuild a complete interferogram, sufficiently sampled with a small acquisition time, thanks to the electro-optical performances of lithium niobate modulators. Moreover, we show that an equivalent long distance fringe scan can be obtained by superimposing the different fragments of the wideband spectrum seen by each individual sampling center for the same voltage scan. This paper opens the way to high resolution, large spectral etendue compact Fourier-transform spectrometers, where external optical path delay scan can be replaced by the internal phase-modulation using electro-optic properties of lithium niobate or similar materials.

Femtosecond laser writing of a depressed cladding single mode channel waveguide in high-purity tellurite glass

High-purity tungstate-tellurite glasses doped with molybdenum, lanthanum or bismuth oxides were investigated from a point of view promising medium for nonlinear optical devices fabrication. A tubular-like architecture was employed for fabrication of waveguides in the of tellurite glasses, in which refractive index change is negative under femtosecond laser writing at 1028nm. Thermal, nonlinear optical properties and optimal laser writing conditions were determined. The glasses revealed a very high stability against crystallization, high transparency in the visible, near- and mid-IR regions, large values of Kerr nonlinear coefficient that are within the range of (4.3÷5.4)·10−6cm2/GW. The highest values of the nonlinear coefficient were found for bismuth containing glasses. The single mode waveguide with propagation loss of 0.15dB/cm at 1064nm was inscribed in tungstate-tellurite glass.

Single-mode channel waveguide at 1540 nm in Er-doped ZnO thin film

Er-doped ZnO thin film is fabricated by radio frequency magnetron sputtering technology. It is found that the Er:ZnO film has good film quality with excellent single-mode characteristic. When the film is stimulated by 532nm laser, one waveguide mode at 1540nm can be excited. By confining the lateral width of planar waveguide, a single-mode channel waveguide at 1540nm is fabricated by focused-ion-beam etching. This kind of waveguide meets the requirements of single-mode and low loss in optical communication, which could be a choice for future optical design.

Pump-and-probe optical transmission phase shift as a quantitative probe of the Bogoliubov dispersion relation in a nonlinear channel waveguide

We theoretically investigate the dispersion relation of small-amplitude optical waves superimposing upon a beam of polarized monochromatic light propagating along a single-mode channel waveguide characterized by an instantaneous and spatially local Kerr nonlinearity. These small luminous fluctuations propagate along the waveguide as Bogoliubov elementary excitations on top of a one-dimensional dilute Bose quantum fluid evolve in time. They consequently display a strongly renormalized dispersion law, of Bogoliubov type. Analytical and numerical results are found in both the absence and the presence of one- and two-photon losses. Silicon and silicon-nitride waveguides are used as examples. We finally propose an experiment to measure this Bogoliubov dispersion relation, based on a stimulated four-wave mixing and interference spectroscopy techniques.

Conditions of a single-mode rib channel waveguide based on dielectric TiO2/SiO2

In this paper, we propose conditions for the design of a single-mode rib channel waveguide based on dielectric materials such as titanium dioxide (TiO2) and silicon dioxide (SiO2) for the 0.633-µm visible light. We also design Y-splitter structures, which show high-degree optical confinement and low bend losses at various radii of curvatures. Small radii of curvatures are extremely desirable in integrated photonics as they permit decreasing the dimensions but can also potentially reduce power consumption in the active devices.

Conditions of a single-mode rib channel waveguide based on dielectric TiO2/SiO2

In this paper, we propose conditions for the design of a single-mode rib channel waveguide based on dielectric materials such as titanium dioxide (TiO2) and silicon dioxide (SiO2) for the 0.633-µm visible light. We also design Y-splitter structures, which show high-degree optical confinement and low bend losses at various radii of curvatures. Small radii of curvatures are extremely desirable in integrated photonics as they permit decreasing the dimensions but can also potentially reduce power consumption in the active devices.

A SiON Microring Resonator-Based Platform for Biosensing at 850 nm

In this paper, we report on the design, fabrication, and characterization of a photonic circuit for biosensing applications. Silicon oxynitride with a bulk refractive index of 1.66 is the core-layer material. The photonic circuit is optimized for a wavelength of ∼850 nm, which allows on-chip integration of the light source via cost effective vertical-cavity surface-emitting lasers and of the detector by using standard silicon photodetectors. Design as well as fabrication processes are explained in details. The best characteristics for the single optical components in the photonic circuit are: for single-mode channel waveguides with dimensions of 350 nm × 950 nm; propagation losses of 0.8 dB/cm; bending losses of 0.1 dB/90°-bend (radius of curvature 100 μm); 49/51 splitting ratio for 3-dB power splitters (directional couplers); quality factors up to 1.3 × 105 for microring resonators. Volumetric sensing yields a bulk sensitivity of 80 nm/RIU and a limit of detection of 3 × 10−6 RIU. Therefore, SiON-based photonic circuits represent a reliable material platform for biosensing in the short-wave near infrared region.

Time and Temperature Dependent Study of Zn and Ni Codiffused LiNbO&lt;sub&gt;3&lt;/sub&gt; Waveguides

Zn and Ni codiffused lithium niobate planar and channel waveguides are fabricated at various temperatures for different times. Compositional and optical properties of the waveguides are studied. Single-mode channel waveguides are measured for 2-D refractive index profiling using a modified end-fire coupling method at excitation wavelengths of both 633 and 1550 nm. Planar waveguides with the same metallic thicknesses as those of channel waveguides are analyzed using secondary-ion-mass-spectrometry to obtain the diffused metal ion concentration profiles. The relationship between the diffusion depths and index depths is discussed based on the completeness of the diffusion process.

Alkaline aluminum phosphate glasses for thermal ion-exchanged optical waveguide

Alkaline aluminum phosphate glasses (NMAP) with excellent chemical durability for thermal ion-exchanged optical waveguide have been designed and investigated. The transition temperature Tg (470°C) is higher than the ion-exchange temperature (390°C), which is favorable to sustain the stability of the glass structure for planar waveguide fabrication. The effective diffusion coefficient De of K+–Na+ ion exchange in NMAP glasses is 0.110μm2/min, indicating that ion exchange can be achieved efficiently in the optical glasses. Single-mode channel waveguide has been fabricated on Er3+/Yb3+ doped NMAP glass substrate by standard micro-fabrication and K+–Na+ ion exchange. The mode field diameter is 9.6μm in the horizontal direction and 6.0μm in the vertical direction, respectively, indicating an excellent overlap with a standard single-mode fiber. Judd–Ofelt intensity parameter Ω2 is 5.47×10−20cm2, implying a strong asymmetrical and covalent environment around Er3+ in the optical glasses. The full width at half maximum and maximum stimulated emission cross section of the 4I13/2→4I15/2 are 30nm and 6.80×10−21cm2, respectively, demonstrating that the phosphate glasses are potential glass candidates in developing compact optoelectronic devices. Pr3+, Tm3+ and Ho3+ doped NMAP glasses are promising candidates to fabricate waveguide amplifiers and lasers operating at special telecommunication windows.

Waveguides in single-crystal lithium niobate thin film by proton exchange.

The proton exchanged (PE) planar and channel waveguides in a 500 nm thick single-crystal lithium niobate thin film (lithium niobate on insulator, LNOI) were studied. The mature PE technique and strong confinement of light in the LN single-crystal thin film were used. The single mode and cut-off conditions of the channel waveguides were obtained by finite difference simulation. The results showed that the single mode channel waveguide would form if the width of the PE region was between 0.75 μm and 2.1 μm in the β(4) phase. The channel waveguide in LNOI had a much smaller mode size than that in the bulk material due to the high-refractive-index contrast. The mode size reached as small as 0.6 μm(2). in simulation. In the experiment, the refractive index and phase transition after PE in LNOI were analyzed using the prism coupling method and X-ray diffraction. Three different width waveguides (5 μm, 7 μm and 11 μm) were optically characterized. Near-field intensity distribution showed that their mode sizes were 3.3 μm(2).,5 μm(2). and 7 μm(2). The propagation losses were evaluated to be about 16 dB/cm, 12 dB/cm and 11 dB/cm, respectively. The results indicate that PE is a promising method for building more complicated photonic integrated circuits in single-crystal LN thin film.

Resonantly pumped single-mode channel waveguide Er:YAG laser with nearly quantum defect limited efficiency

We demonstrated the continuous-wave operation of a resonantly pumped Er:YAG single-mode channel waveguide laser with diffraction-limited output and nearly quantum defect limited efficiency. Using a longitudinally core-pumped, nearly square (61.2 μm×61.6 μm) Er3+:YAG waveguide embedded in an undoped YAG cladding, an output power of 9.1 W with a slope efficiency of 92.8% (versus absorbed pump power) has been obtained. To the best of our knowledge, this optical-to-optical efficiency is the highest ever demonstrated for a channel waveguide laser.

Nitroxide mediated synthesis of low dispersity random copolymers for low-loss optical waveguides

Candidate materials for low-loss optical waveguides based on poly(glycidyl methacrylate-ran-pentafluostyrene) [P(GMA-ran-PFS)] copolymers were synthesized by nitroxide mediated polymerization (NMP) initiated with BlocBuilder® [N-(2-methylpropyl)-N-(1-diethlphosphono-2,2-dimethylpropyl)-O-(2-carboxylprop-2-yl) hydroxylamine] bearing a succinimidyl ester group (NHS-BlocBuilder) at 90 °C in 1,4 dioxane. The copolymerizations yielded copolymers with low dispersity Mw/Mn between 1.2 and 1.4. The core structure of single-mode channel waveguides was fabricated by direct UV lithographic patterning. The copolymers with low Mw/Mn resulted in line width roughness (LWR) of about 0.16 μm, whereas LWR of copolymers with Mw/Mn=3.5 but similar compositions was about 0.5 μm. The improvement in microstructural control allotted by NMP permitted finer pattern replication for copolymers desired for optical waveguides, as suggested for photoresist polymers. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2970–2978

Fabrication of single-mode channel polymer waveguides using vacuum assisted microfluidic soft lithography

We report on the fabrication of single-mode channel waveguide structures produced by the vacuum assisted microfluidic soft lithography technique. The soft lithographic technique in conjunction with a specially designed photomask pattern and a synthesized UV curable epoxy resin can result in a high yield, cost-effective method for fabrication of channel waveguides. In particular, the use of the microfluidic technique allows for the production of freestanding high quality single-mode channel waveguides on various substrates. Notably, the introduction of sectional flow tapers allows for proper uniform filling of long length small cross-sectional waveguide structures that would ordinarily succumb to clogging at shorter distances during channel filling. The fabricated polymer waveguide was 30 mm in length with a cross section dimension of 7 × 7 µm. The single-mode waveguide propagation was confirmed with beam profiler measurement, and the measured propagation loss for this polymer waveguide was about 0.55 dB/cm.

Detection of glucose concentration using ion-exchange channel waveguide

An integrated sensor structure based on channel waveguide is constructed. The single-mode channel waveguide is prepared using 0.1% AgNO3–99.9% NaNO3 mixed molten salts under the condition that the diffusion coefficient can be assumed as constant. An experimental method is proposed and implemented to determine the equivalent diffusion coefficient and surface refractive index increment. The refractive index distribution of the channel waveguide is fitted and inferred. Ten glucose samples with different concentrations are tested at different wavelengths. The imaginary part of the propagation is determined by the Kramers–Kronig diversification. The correlation coefficient is designated as R2, and it is within the range of 0.9850–0.9961. The lowest detection limit of 0.1μM is realized in the trace test.

Fabrication and characteristics of low loss and single-mode channel waveguides based on DNA-HCTAC biopolymer material

A novel biopolymer, deoxyribonucleic acid-hexadecyltrimethylammonium chloride (DNA-HCTAC), is used as the core layer material in optical waveguide, and the cleanroom technology is successfully applied to fabricate the single-mode channel waveguides with low propagation loss. The prepared DNA-HCTAC material shows high optical quality at the optical telecommunication wavelengths, such as high transparency, relatively high refractive index and low birefringence. In the fabrication approach, polymethyl methacrylate (PMMA) is used as a barrier layer to protect the DNA-HCTAC material from the corrosive of photoresist developer, and the etching conditions are optimized to form the smooth wall and sharp cross-section of the waveguide. Lastly, the optical characteristics of DNA-HCTAC channel waveguides are measured. The results show that the DNA-HCTAC waveguide operates with single-mode propagation and has a low optical loss.

Determination of Refractive Index Profile and Mode Index from the Measured Mode Profile of Single-Mode LiNbO3-Diffused Waveguides

A refractive index profile and a mode index of single-mode LiNbO3 channel waveguides are simultaneously estimated from the measured near-field intensity distributions. A third-order low-pass Butterworth filter is used to filter the measured amplitude and its double derivative to remove high-frequency spatial noise and impulses. The mode index or effective index of the propagating mode is extracted by matching the index change profile equal to zero at the waveguide boundary. The proposed technique is applicable to any slowly varying, single-mode, integrated-optic waveguide for TE and TM modes and for any operating wavelength within the single-mode regime.

Direct laser writing of near-IR step-index buried channel waveguides in rare earth doped YAG

A new (to our knowledge) ultrashort laser pulse irradiation regime that allows us to directly modify and increase the refractive index of rare earth doped YAG polycrystalline ceramics has been identified. Single-mode buried channel waveguides in both Ho:YAG and Er:YAG ceramics at the near-IR wavelengths of 1.55 μm and 1.95 μm are demonstrated by fabricating positive square step-index cores. Minimum propagation losses of 1.5 dB cm(-1) at a 1.51 μm wavelength have been preliminarily obtained. Confocal microluminescence mapping reveals that the increased refractive index regions retain the near-IR spectral properties of Er3+ ions in the YAG crystalline matrix.

Differential Rayleigh scattering method for measurement of polarization and intermodal beat length in optical waveguides and fibers

We propose a modification of the Rayleigh scattering method, which allows for measurement of polarization and intermodal beat length in single-mode and few-mode channel waveguides and optical fibers. A significant sensitivity increase is achieved by taking two high-resolution photographs in oblique scattered light of π-shifted intensity distributions produced by interference of polarization or spatial modes and applying Fourier analysis to the differential image. In the case of polarization beat length measurements, the π-phase shift is obtained by switching the polarization state at the fiber input, while in intermodal measurements, the π-phase shifting is realized by changing the excitation conditions. The usefulness of the method for characterization of channel waveguides and optical fibers is demonstrated in several examples. Moreover, we show that the combination of the spectral interferometry method with the proposed method allows for broadband measurements of differential phase and group effective indices.