Straight Channel Waveguides Research Articles

Low-loss bent channel waveguides in lithium niobate thin film by proton exchange and dry etching

We propose and demonstrate an efficient method combining proton exchange with dry etching for the fabrication of low-loss bend channel waveguides in lithium niobate (LN) thin film. Our proposed method introduces the chemical etching caused by F+ ion to increase the etching rate. Our fabricated straight and bent channel waveguides have a trapezoid cross section with a top width of ~1.0 µm, a height of ~900 nm, and a slope of ~20° with respect to the vertical direction. To the best of our knowledge, this is the largest etching depth but with a small slope reported up to now. Mode intensity distributions and insertion losses were measured at 1.55 µm wavelength and bending losses were deduced. The results show that our fabricated bent channel waveguide with a radius of 20 μm can achieve low bending losses of 0.455 dB/90° and 0.488 dB/90° for the fundamental quasi-TE (qTE) and quasi-TM (qTM) modes, respectively. Compared with the fabrication methods reported so far, our method can realize a faster etching rate and a larger etching depth while maintaining a high etching quality.

Efficient Second Harmonic Generation in 3D Nonlinear Optical-Lattice-Like Cladding Waveguide Splitters by Femtosecond Laser Inscription.

Integrated photonic devices with beam splitting function are intriguing for a broad range of photonic applications. Through optical-lattice-like cladding waveguide structures fabricated by direct femtosecond laser writing, the light propagation can be engineered via the track-confined refractive index profiles, achieving tailored output beam distributions. In this work, we report on the fabrication of 3D laser-written optical-lattice-like structures in a nonlinear KTP crystal to implement 1 × 4 beam splitting. Second harmonic generation (SHG) of green light through these nonlinear waveguide beam splitter structures provides the capability for the compact visible laser emitting devices. With Type II phase matching of the fundamental wavelength (@ 1064 nm) to second harmonic waves (@ 532 nm), the frequency doubling has been achieved through this three-dimensional beam splitter. Under 1064-nm continuous-wave fundamental-wavelength pump beam, guided-wave SHG at 532 nm are measured with the maximum power of 0.65 mW and 0.48 mW for waveguide splitters (0.67 mW and 0.51 mW for corresponding straight channel waveguides), corresponding to a SH conversion efficiency of approximately ~14.3%/W and 13.9%/W (11.2%/W, 11.3%/W for corresponding straight channel waveguides), respectively. This work paves a way to fabricate compact integrated nonlinear photonic devices in a single chip with beam dividing functions.

Nano-Anatase TiO₂ for High Performance Optical Humidity Sensing on Chip.

An on-chip optical humidity sensor using Nano-anatase TiO2 coating is presented here. The coating material was prepared so that the result is in solution form, making the fabrication process quick and simple. Then, the solution was effortlessly spin-coated on an SU8 straight channel waveguide. Investigating the sensitivity and performance (response time) of the device revealed a great linearity in the wide range (35% to 98%) of relative humidity (RH). In addition, a variation of more than 14 dB in transmitted optical power was observed, with a response time of only ~0.7 s. The effect of coating concentration and UV treatment was examined on the performance and repeatability of the sensor. Interesting observations were found, and the attributed mechanisms were described. In addition, the proposed sensor was extensively compared with other state-of-the-art proposed counterparts from the literature and remarkable advantages were found. Since a high sensitivity of ~0.21 dB/%RH and high dynamic performances were demonstrated, this sensor is proposed for use in biomedical applications.

Integrated Young interferometer sensor with a channel-planar composite waveguide sensing arm

A simple integrated Young interferometer sensor was prepared on a glass substrate with a pair of single-mode straight channel waveguides 75 microm apart. A tapered thin film of TiO2 was locally deposited on one channel to form a composite waveguide sensing arm with a greatly enhanced evanescent field. With the prism-coupling method a single beam of laser light was simultaneously launched into the two channels to yield a high-contrast interference pattern. Refractive-index sensitivity of the interferometer and its response to protein adsorption were investigated by using a slit-photodetector assembly to interrogate the sensor signal. The factors affecting the fringe contrast were analyzed.

Ultraviolet writing of buried waveguide devices in epoxy-coated benzocyclobutene

We propose a simple ultraviolet (UV) writing technique for the fabrication of buried waveguide devices, which employs a UV lamp as the light source and commercial benzocyclobutene (BCB) and epoxy OPTOCAST 3505 as the core and cladding materials, respectively. We show that the light emitted from the UV lamp can induce a large refractive-index change (up to 0.012) in BCB, while a negligible index change in the epoxy, which makes possible the writing of buried waveguide devices in BCB by exposing an epoxy-clad BCB film to the UV light through a mask. We demonstrate this technique experimentally with straight channel waveguides, Y-junction waveguide branches, and long-period waveguide gratings. The propagation loss of a typical fabricated waveguide is 1.6 dB/cm at 1550 nm and the polarization-dependent loss is 0.4 dB/cm.

Er–Yb Waveguide Amplifiers in Novel Silicate Glasses

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> A set of novel silicate glasses containing ZnO and co-doped with <formula formulatype="inline"><tex>${\hbox {Er}}^{3+}$</tex></formula> and <formula formulatype="inline"><tex>${\hbox {Yb}}^{3+}$</tex></formula> was designed as substrates for optical waveguide amplifiers. Characterized by exceptionally low up-conversion, minimum Er concentration quenching and high mechanical as well as chemical stability, the reported glasses can compete with phosphate-based materials typically used in the state-of-art active devices. Straight channel waveguides with propagation losses as low as 0.18 dB/cm were fabricated in these substrates using <formula formulatype="inline"><tex>${\hbox {Ag}}^{+} \Leftrightarrow {\hbox {Na}}^{+}$</tex></formula> and <formula formulatype="inline"> <tex>${\hbox {K}} ^{+} \Leftrightarrow {\hbox {Na}}^{+}$</tex></formula> thermal ion exchange. Net on-chip gain values of 6.7 dB at 1537 nm were measured and a net fiber to-fiber gain of 5 dB was achieved when pumped at 976 nm. A six-level spatially resolved numerical model of an Er–Yb co-doped active waveguide was developed to analyze and optimize the amplifier performance. Modification of the rare-earth dopant concentration and the channel waveguide geometry was proposed to increase the gain figure and improve the overall amplifier efficiency. </para>

Influence of chelating agents on the photopolymerization of hybrid Ti-based waveguides

Sol–gel derived inorganic–organic hybrid materials (HYBRIMERs) have shown a great potential for planar and channel waveguides due to their high photosensitivity, low-cost processing and good suitability for doping. These materials have been synthesized with a high content of titanium (40%) and with three different chelating agents: acetylacetone, methacrylic acid and trifluoroacetic acid. The degree of photopolymerization of the methacrylate part changes with the nature of the chelating agent. Trifluoroacetic acid and methacrylic acid are the chelating agents showing the highest photopolymerization conversion. All coatings were crack free (thickness ∼1 μm), transparent in the visible spectra range with refractive indexes from 1.54 to 1.58. Simple confined structures based on trifluoroacetic acid with well-defined shapes, smooth surfaces (RMS = 0.28 nm) and good optical properties were synthesized. Straight and bent channel waveguides and beam splitters were patterned by UV exposure and wet etching, and light confinement effects into the different channel waveguides were shown.

High-accuracy IR polarimetry of waveguide optical elements

This paper discusses the main physical factors that affect the accuracy with which the polarization characteristics of waveguide optical elements are measured: distortions of the polarization when radiation is being introduced (focused) into optical waveguides, elastic strains of fiber lightguides when they are being attached, and depolarization of the radiation at inhomogeneities and defects of waveguides. An experimental apparatus for polarimetric studies of fiber and integrated-optics devices is described, in which the relative power of the polarization noise can be estimated to within 10−5 at a wavelength of 1.32 µm. The distribution of the degree of polarization of IR radiation in the cross section of a light beam is measured at the output of a single-mode lightguide, and the results are presented of measurements of the depolarization of IR radiation in locally deformed single- and multimode lightguides, a fiber coil polarizer, and straight and bent integrated-optics channel waveguides.

Zn‐diffused LiNbO3 2 × 2 balanced‐bridge optical switch with post thermal‐treatment

AbstractA Zn‐diffused 2 × 2 balanced‐bridge optical switch is demonstrated on a Z‐cut LiNbO3 substrate for the first time. A post thermal‐treatment is used for precisely maintaining the 3‐dB‐power division of the two directional couplers in the device. The crosstalk ratio of both output waveguides is about −23 dB, while the measured propagation loss of a straight‐channel waveguide is kept within the range of 1.1–1.25 dB/cm. In the design, without using electrically controllable couplers as 3‐dB splitters or combiners, the total device length can be reduced to 18 mm. The switching voltage‐length product is 8.5 V‐cm for the TM polarized light at 1.32 μm. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 41: 493–496, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20182

Polarization-insensitive transition between sol-gel waveguide and electrooptic polymer and intensity modulation for all-optical networks

An intensity modulation using a hybrid electrooptic (EO) polymer/sol-gel straight channel waveguide, useful in the 1550-nm wavelength regime is demonstrated without using Mach-Zehnder interferometric waveguide. The sol-gel waveguide is selectively buried so that a vertical transition into and out of an EO polymer coated on the sol-gel waveguide is arranged. The throughput ratio for transverse electric (TE) and transverse magnetic (TM) modes of the light coupled out of the hybrid waveguide is improved up to 0.9 dB with the help of reduced birefringence of the EO polymer after corona poling. We show that the fabrication process of such hybrid-type waveguides enables production of a phase modulator operating at 1550-nm wavelength. The fabricated straight channel waveguide modulator exhibits stable- and high-intensity modulation efficiency (82%) using a simple cross-polarization setup after the polarization dependence is reduced. We demonstrate an all wet-etching process to fabricate polymeric EO modulators.

Transmission properties of two-dimensional photonic crystal channel waveguides

We study the transmission properties of straight channel waveguides designed in a two- dimensional (2D) photonic crystal patterned into an AlGaAs heterostructure. 2D dispersion calculations show the existence of small gaps occurring along the dispersion branch of the fundamental mode. We show that their location agree very well with mini-stop bands observed on the transmission spectra.

A design for improving the sensitivity of a Mach–Zehnder interferometer to chemical and biological measurands

Integrated optical Mach–Zehnder interferometers (MZIs) composed of graded-index channel waveguides are often used as chemical/biological sensors. Such MZIs have a relatively low sensitivity because the graded-index active arms have a weak evanescent field. To improve the sensitivity, a channel-planar composite optical waveguide (COWG) is proposed as a substitute for the graded-index active arm. An actual channel-planar COWG was fabricated by sputtering a tapered TiO 2 film onto a straight glass channel waveguide prepared by the potassium ion exchange method. Measurement of the evanescent absorption of the dye solution demonstrated a significantly enhanced evanescent field over the TiO 2 film region caused by adiabatic transition of the guided mode between the channel waveguide and TiO 2 film. Theoretical calculations show that the sensitivity of a glass-based MZI can increase 71 times when the COWG active arm contains a 25 nm thick and 5 mm long tapered TiO 2 film. The use of a COWG as the active arm of a glass-based MZI also allows for elimination of the low-index buffer layer because of a large difference in evanescent field between the COWG and channel waveguide.

Single-mode/multimode waveguide electro-optic grating coupler modulator

A waveguide electro-optic grating coupler modulator based on a single straight channel waveguide and an interdigital modulation electrode structure on an electro-optic substrate, has been reported. Both single-mode and multimode operations are possible. Preliminary experiments have demonstrated the modulation on a proton exchanged LiNbO3 waveguide device. Modulation performed on other electro-optic waveguides, such as GaAs, LiTaO3, and EO polymers, are also possible.

Photorefractive effects in proton exchanged LiTaO/sub 3/ optical waveguides

A circuit model is proposed to describe photorefractive effects in LiNbO/sub 3//LiTaO/sub 3/ channel waveguides at any intensity level. Capacitive charge storage at the waveguide boundaries is assumed to be provided by trapping states associated with photoconductivity. A consequence of this model is that photoconductive transients are independent of optical intensity at low intensity levels. Photovoltaic and photoconductive effects in proton exchange LiTaO/sub 3/ channel waveguides were experimentally investigated. Dark conductivities of 2*10/sup -15/ to 2 *10/sup -14/( Omega -cm)/sup -1/ were extrapolated from photoconductivities up to 2*10/sup -13/ ( Omega -cm)/sup -1/ for power levels of 0.1 to 3 mW. Large DC voltage dependent effects on the conductivity were observed. Straight channel waveguides were observed to be free of photovoltaic effects for output power levels below 35-75 mW.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Electro-optic analog-to-digital converter using channel waveguide Fabry–Perot modulator array

A new electro-optic analog-to-digital converter that makes use of an array of channel waveguide Fabry–Perot modulators is reported. Since this converter utilizes only straight channel waveguides, it should possess the inherent advantages of small substrate size, simple geometrical layout, and low optical insertion loss. Experimental results obtained with a four-bit converter in a X-cut LiNbO3 substrate have demonstrated some of these desirable features.

Crosstalk characteristics of Ti-LiNbO&amp;lt;inf&amp;gt;3&amp;lt;/inf&amp;gt;intersecting waveguides and their application as TE/TM mode splitters

Crosstalk characteristics of an intersecting waveguide are presented. Two straight channel waveguides which intersect at an angle of a few degrees on y -cut LiNbO 3 were fabricated by in-diffusion of Ti. Experimental results show that the crosstalk characteristics are determined by the refractive index change profile and the geometry of intersection associated with guided wave modes. In a special case, a TE/TM mode splitter was obtained by using the intersecting waveguide which provides adequate anisotropy by the change in refractive indices. Splitting ratio was 17 and 14 dB for the TE and TM modes, respectively.

Crosstalk Characteristics of Ti-LiNbO/sub 3/ Intersecting Waveguides and Their Application as TE/TM Mode Splitters

Crosstalk characteristics of an intersecting waveguide are presented. Two straight channel waveguides which intersect at an angle of a few degrees on y-cut LiNbO/sub 3/ were fabricated by in-diffusion of Ti. Experimental results show that the crosstalk characteristics are determined by the refractive index change profile and the geometry of intersection associated with guided wave modes. In a special case, a TE/TM mode splitter was obtained by using the intersecting waveguide which provides adequate anisotropy by the change in refractive indices. Splitting ratio was 17 and 14 dB for the TE and TM modes, respectively.