高掺锌富锂铌酸锂体抗光损伤能力的研究

以Czochralski法生长Zn(6mol%)：LiNbO3,Li/Nb=0.94,0.97,1,1.02。测试晶体的光损伤阈值。采用质子交换法制作高掺锌富锂铌酸锂晶体光波导,研究了它们的抗光损伤能力。     

Polarization dependence in Ti-diffused LiNbO3 directional couplers

The effect of design and fabrication parameters on the dependence on polarization of the splitting ratio in directional couplers produced by Ti diffusion in LiNbO3 has been investigated experimentally at a wavelength of 1.54 microm. The parametric study was carried out at a diffusion temperature of 1025 degrees C, which was found to suppress outdiffusion effects. The directional couplers were produced by use of various combinations of waveguide separation, Ti film thickness, and diffusion time. Of particular interest was the identification of parameter sets for which the sum of TE and TM splitting ratios equals unity, as required for electro-optic and acousto-optic tunable filter designs with relaxed beam-splitter requirements. Directional couplers that closely match this criterion were obtained by diffusion of 3.5-mm-long, 7-microm-wide Ti strips separated by 11 microm for 11 h. It was found that a bending angle of 0.6 degrees for input and output waveguides produces lower transmission loss for both polarizations than a 1.0 degree bend angle (>1-dB loss reduction).     

Narrowband Bragg reflectors in Ti:LiNbO3 optical waveguides

Bragg grating reflectors etched in amorphous silicon overlay films have been integrated with Ti:LiNbO3 optical waveguides to obtain a narrow (0.05 nm) reflectance spectrum with a > 20 dB dip in the transmittance spectrum. These results were realized at a wavelength of 1542.7 nm for TE polarization on an x-cut, y-propagating substrate with gratings etched to a depth of approximately 93 nm in a 105 nm thick silicon film over a length of 12.5 mm. The reflectance in the channel waveguides is found to be strongly dependent on the depth of the etched grating. The effect of the Bragg waveguide loss factor on the transmittance and reflectance spectra is investigated by using a model for contradirectional coupling that includes an attenuation coefficient. The values for coupling constants kappa and amplitude attenuation constants alpha of samples etched for different time durations to control the grating depths are obtained from the model through the use of the depth of the dips in the transmittance spectra and the spectral widths of the reflectance peaks. It is concluded that the corrugated Si overlay film increases the insertion loss by approximately 2.7 dB, and the loss is not significantly affected by the grating depth.     

Characterization of annealed, proton-exchanged optical waveguides in ZnO:LiNbO(3) crystal

We report a systematic study of the annealing process in proton-exchanged ZnO:LiNbO(3) optical waveguides. A z-cut multimode waveguide was subjected to annealing for different durations of time. A two-stage change in index profiles with annealing time was observed, which was consistent with the change in the m-line spectrum. A power-law relationship was established to correlate the optical parameters with annealing time. Annealed Li(+) concentration in the waveguide was solved based on the thermal diffusion equation. An analytical function was used to model the annealed-index profile of single-mode, proton-exchanged waveguides. Good agreement between the theoretical analysis and the experimental result was obtained.     

Intensity and phase mapping of guided light in LiNbO3 waveguides with an interferometric near-field scanning optical microscope

The design and implementation of a phase-sensitive near-field scanning optical microscope incorporating both heterodyne interferometric detection and a phase feedback mechanism are described. Using this microscope we demonstrate a new method for measuring the effective index of the guided mode of a waveguide from the phase images. Two types of LiNbO3 waveguide, defined by titanium diffusion or annealed proton exchange, were studied. Both the profile and the effective index of the mode were measured experimentally. For titanium-diffused waveguides, both agree well with values determined from numerical simulation. In annealed proton-exchanged waveguides that contain periodically poled domains, we find evidence for backreflection from the boundaries between neighboring regions of opposite pole directions, which could result in transmission loss in this type of waveguide.     

Annealing behavior of barriers in ion-implanted LiNbO3 and LiTaO3 planar waveguides

We formed planar waveguides in LiNbO(3) and LiTaO(3) crystals by megaelectron volt He-ion implantation. The dark modes of both waveguides are measured and their refractive-index profiles are described according to the parameterized index profile reconstruction method. The extraordinary indices of both ion-implanted waveguides exhibit quite different profiles. We compare the thermal stability of barriers in ion-implanted LiNbO(3) and LiTaO(3) waveguides by annealing at different temperatures. The results show that the barrier in a LiTaO(3) planar waveguide has higher thermal stability than that in a LiNbO(3) waveguide. The experiments also show that annealing at a temperature higher than 400 degrees C results in recrystallization of the barrier.     

Optical branching effect in Ti:LiNbO3 waveguides: near-field pattern studies

The paper presents a detailed study of a single optical beam splitting into several beams (the branching effect) in photorefractive sensitive Ti:LiNbO3 optical slab waveguides. The near-field patterns of the multibeam structures are presented for different values of optical power coupled into TE guided modes of different orders. The process of partial recovery of the optically damaged waveguide (the partial shrinking of the multibeam bundle created) is also described.     

Planar fabrication process of a high-coupling-efficiency interface between optical waveguides of large index difference

A high-coupling-efficiency interface connecting two optical waveguides of large index difference is reported. Both theoretical analysis and an experimental study were performed. High coupling efficiency is achieved by matching the mode profiles in two waveguides as well as connecting the waveguides with an antireflection (AR) section. Analysis shows a coupling efficiency as high as 98% between glass-SiO(2) and Ga(0.72)Al(0.28)As-Ga(0.59)Al(0.41)As waveguides with an AR section of TiO(2). An index difference of 1.8 for the TE(0) mode between these two waveguides is obtained. To implement the glass-SiO(2) waveguide with an AR section in the host AlGaAs waveguide, a new quadrilevel photomasking and lift-off process is developed. A coupling efficiency of 83% was measured for the TE(0) mode at the 1.15-μm wavelength, a very high value considering the large index difference (1.75) between the two waveguides that were connected. The design concept and the fabrication process developed should facilitate efficient integration of low- and high-index waveguides on the same substrate to produce a greater variety of photonic devices and modules.     

Surface periodic domain structures for waveguide applications

We report the results of fabrication and investigations of surface periodic domain structures created by a set of quasi-point e-beam irradiations both on the Y- and X-cuts of LiNbO(3), and on Ti:LiNbO(3) and Zn:LiNbO(3) planar waveguides. Domain gratings with spatial periods from 4.75 to 7.25 μm were formed by a 25-keV e-beam. Doses from 500 to 2000 μC/cm(2) were used for different structures to estimate optimal fabrication conditions. The investigations allowed the visualization of the formed surface domain structures, estimation of their uniformity, and determination of waveguide generation of the second optical harmonic. The surface structures can be used in optical devices for the realization of quasi-phase-matched frequency conversion, which includes the creation of compact radiation sources based on waveguides.     

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.     

低损耗、高消光比的Z切钛扩散铌酸锂调制器

为了获得低损耗、高消光比的光波导电光调制器,以钛扩散铌酸锂( Ti: LiNbO3 )波导折射率的分布和光波导的优化设计为基础,利用有限差分光束传播法( FD-BPM )和点匹配法,从理论上给出了一种光损耗为-0.53dB、消光比为30dB的Z切钛扩散铌酸锂调制器。并且,在10Gbit/s的系统中对其进行了测试,取得了在120 km长的光纤上传输无误码的效果和较清晰的眼图。     

A slab optical waveguide with negative effective thickness

Two new asymmetric slab optical waveguides with a left-handed media (LHM) cover or substrate are studied. The effective thickness is derived by using normalized waveguide parameters. An analytical method is then proposed to calculate the universal effective thickness. We show that negative effective thickness exists in the waveguide with a LHM substrate, unlike in conventional waveguide or other LHM waveguides studied previously. However, for the waveguide with a LHM substrate, the shape of high-order mode behaves like that of a traditional slab waveguide, and neither the fundamental mode nor the other higher order modes have the novel phenomena of negative effective thickness. Both TE and TM modes are discussed.     

Performance of submillimeter square hollow waveguides

Propagation losses are determined for 100 microm x 100 microm square, hollow waveguides constructed from glass capillaries. The small size makes it possible to observe optical effects not easily seen with larger waveguides. The depletion of higher-order even modes creates a large, nonlinear loss. Over a distance of a meter the loss approaches the smaller, linear value expected for the fundamental mode. Additionally, the lowest two even modes beat to produce an oscillatory loss with a period of approximately 2 cm. Making the focal radius 0.35 the waveguide width minimizes these two effects. In a related study, 50-microm waveguides embossed in polydimethylsiloxane are shown to have losses similar to glass capillaries.     

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.     

Collinear guided wave to leaky wave acoustooptic interactions in proton-exchanged LiNbO(3) waveguides

The results of a detailed theoretical study on collinear guided wave to leaky wave acoustooptic (AO) interactions in proton-exchanged LiNbO(3) (PE:LiNbO(3)) planar waveguides are presented. The guided-to-leaky mode conversion for an input optical beam at the wavelength of 632.8 nm by the induced diffraction grating from a collinear surface acoustic wave (SAW) is analyzed using a generalized multimode formulation of the coupled mode theory. Mode conversion efficiency and AO bandwidth have been calculated as functions of acoustic frequency, interaction length, guiding layer thickness, and acoustic drive power density for three cuts of the LiNbO(3) substrate. High performance configurations that are desirable for application to demultiplexing and switching in optical communication systems are identified, and the corresponding channel capacity and frequency resolution are determined. For example, it was shown that the X-cut configuration features the highest mode conversion efficiency. However, a relatively small AO bandwidth is associated with this configuration. Both high mode conversion efficiency and large AO bandwidth can be accomplished at the guiding layer thickness of 1.0 mum. A TM(o)-->TE(nu) mode conversion efficiency as high as 42% together with an AO bandwidth of approximately 70 MHz can be achieved in the Z-cut waveguide at the guiding layer thickness of 1.0 mum, acoustic drive power density of 50 mW/mm, interaction length of 40 mm, and acoustic frequency of 460 MHz. The corresponding channel capacity and frequency resolution are 745 and 0.09 MHz, respectively. Measured mode conversion efficiencies as high as 90 and 78% obtained at the acoustic frequencies of 107 and 367 MHz using the X-cut substrate and the Y-propagation SAW have verified the theoretical prediction on the mode conversion efficiencies.     

TM and TE propagating modes of photonic crystal waveguide based on honeycomb lattices

The waveguide based on the honeycomb photonic crystal has propagating modes for both the TE and TM polarizations. The group index-normalized frequency curves are U-shaped for the two polarizations. The average group index of the TE mode is approximately 3, while the average group index of the TM mode is over 10, which implies that the TM mode is a slow light mode. With the shift value 0 ≤ δx ≤ 0.025a, the group index is over 10 and the normalized delay-bandwidth product is from 0.316 to 0.349, which is ideal for the slow light mode of the TM polarization. In the group velocity dispersion of the waveguide, there is a very large "zero" dispersion region for both the TM and TE modes, which is far larger than that of other photonic crystal waveguides. The TM mode of this kind of waveguide structure is a slow light mode with wide bandwidth and a large "zero" dispersion region.     

玻璃基掩埋式光波导堆栈的制作与表征

在硅酸盐光学玻璃基片上制作了光波导堆栈, 这种光波导堆栈通过Ag⁺/Na⁺熔盐离子交换和电场辅助离子扩散技术顺次制作了两层掩埋式光波导. 对光波导堆栈的横截面显微结构进行了观察, 并对堆栈中两层波导的损耗特性进行了测试. 所获得的光波导堆栈中的上、下两层波导芯部分别位于玻璃表面以下14和35 μm处; 上层光波导芯部尺寸约为12 μm×7 μm; 下层光波导芯部尺寸约为9 μm×8 μm. 通光测试显示两层波导在1.55 μm工作波长下均为单模光波导, 且两者之间没有相互耦合. 损耗测试分析结果显示: 堆栈中两层光波导的传输损耗均约为0.12 dB/cm,与单模光纤之间的耦合损耗分别为0.78和0.73 dB. 分析表明, 这种光波导堆栈在玻璃基集成光芯片的高密度集成方面具有很好的应用前景.     

Simple method of measuring propagation properties of integrated optical waveguides: an improvement

In this paper we report a simple method of measuring mode propagation losses of integrated optical waveguides with small scattering. The basic technique, reported previously, uses a microcomputer-assisted video camera; here we have improved our observation scheme by coating the optical waveguide with a thin film of fluorescent dye (Nile Blue A perchlorate) thus permitting observation of the optical fields. The Nile Blue A perchlorate absorbs light centered at 0.63 microm and emits light centered at 0.69 microm. We have measured a mode propagation loss of the order of 0.1 dB/cm of a potassium ion-exchanged glass waveguide and confirmed the damped oscillatory behavior of the attenuation vs a slightly lossy thin-film thickness curve for the ion-exchanged waveguide coating with an indium tin oxide film.     

Planar LiTaO(3) waveguides fabricated by proton exchange in concentrated and diluted pyrophosphoric acid with annealing

Annealed proton-exchanged z-cut LiTaO(3) planar waveguides fabricated with pyrophosphoric acid have been characterized. For proton exchange, the extraordinary index increase Δ n(e) ranged from 0.0119 to 0.0141, depending on the exchange temperature. The effective diffusion coefficient D(e) ranged from 0.1325 μm(2)/h at 240°C to 0.545 μm(2)/h at 280 °C. Single-mode propagation losses were α = 0.7 dB/cm. Compared with benzoic acid, pyrophosphoric acid produces waveguides with a higher Δ n(e) and a lower propagation loss. For proton exchange in lithium phosphate-diluted pyrophosphoric acid, a lower Δ n(e) was obtained, but D(e) and propagation losses were not reduced. After proton exchange, the waveguides were annealed. The surface index initially increased, peaked, and then decreased. The waveguide depth d and the surface index n(s) were measured at regular intervals. Figures and empirical formulas relating the waveguide depth d and the surface index increase Δ n(s) to the anneal time, anneal temperatures, and the waveguide depth after proton exchange are given. An example is given in which, the desired waveguide parameters d and Δ n(s), the fabrication conditions could be calculated with the previously derived formulas. Propagation losses decreased to 0.4 dB/cm after prolonged annealing.