Integrated optical components based on ion-exchanged glass waveguides are finding applications in real systems. Recently, significant progress has occurred in the understanding of the ion-exchange process. It is now possible to model the thermal as well as electric field enhanced diffusion with reasonable accuracy and to design ion- exchanged waveguides and devices with prespecified characteristics. This paper describes the theory of the ion-exchange process and explains the diffusion profile for Ag+-Na+ and K+-Na+ cation pairs used in BK7 glass for planar and 2-D (channel) waveguide fabrication. Following this, we present the latest results in modeling and fabrication of several devices based on single-mode channel waveguides. These include couplers, power dividers, MUX/DEMUX, and polarization splitters for applications in optical communications. Finally, we point out the latest trends and novel emerging applications of ion-exchanged waveguides, such as all optical switching, waveguide lasers and amplifiers, and integration of passive waveguides on glass with optoelectronic devices.
The WKB relation and mode indices measurement have been used to determine the important surface index change and effective depth of planar K+-ion exchanged waveguides in soda-lime glass at 1.152 and 1.523 mm for any given set of fabrication temperature and time, whereas the substrate index can be determined by a multi-layer Brewster angle measurement. The data obtained are important in the design and fabrication of K+-ion exchanged waveguides in single-mode fiber systems at 1.3 and 1.55 pm.
Planar ion-exchange waveguides are fabricated by using diluted KNO3 molten salts with NaN03. The surface index increments are measured in the ranges of (7.6 — 1.6) x 10-3 and (9.1 — 1.8) x 10-3 for TE and TM modes with the change of the K+-ion concentration in the salt bath, respectively. Designing values of the guide width and depth for the single-mode channel waveguides by the diluted KN03 salts are presented, which are considerably wider compared with those by the pure salt. Experimental results of the single-mode channel waveguide by 50% KN03 salt are also shown.
We report the measurement of the damage threshold of optical waveguides made by ion exchange in glass subjected to high peak power pulses from a mode-locked/Q-switched YAG laser operating at a wavelength of 1.06 mm. We found that waveguides made from a potassium-sodium exchange could tolerate at least ten times more guided peak power than silver-sodium exchanged guides.
Silver ion exchange process is employed to make channel waveguides in neodymium and erbium doped phosphate and silicate glasses. In particular, we report on the first fabrication of ion exchanged waveguides in a new erbium doped phosphate glass. Stimulated emission is demonstrated in the fabricated waveguides.
A dry silver ion exchange process is described, in which silver evaporation and ion exchange are performed successively in a vacuum chamber. This makes possible the reproducible fabrication of large, nearly step index multimode waveguides. Asymmetric power dividers have been fabricated with the process.
Optical waveguides fabricated using proton exchange in LiNb03 are the basis of many optical components studied today. This paper reviews the history of proton exchange, our current understanding of the process, fabrication methods, and applications.
Low-loss proton-exchanged planar waveguides have been fabricated in z-cut LiNb03 using a new organic proton source: toluic acid. These waveguides exhibit a step refractive index profile with a surface index increase of 0.124 measured at 0.633/mm. The propagation loss was typically around 1 dB/cm. The diffusion constant D0 and the activation energy Q for the proton exchange process were found to be 7.02 x107 mm2/hr and 75.58 kJ/mol, respectively.
A generalized Gaussian function was used to accurately model the refractive-index profile of planar optical waveguides in z-cut LiNbOz fabricated by the annealed proton-exchange technique. The computed waveguide parameters were empirically correlated to the wide range of fabrication conditions through the use of a general power law. In addition, the validity of the conventional simple diffusion annealing model is investigated in detail
A two-step proton-exchange process is employed to produce waveguides and gratings in lithium niobate. In both steps mixtures of lithium benzoate and benzoic acid are used. The fabricated components are characterized at 0.633mm and 0.442/mm. In particular, we have observed that the light intensity diffracted out into the air by the grating is about 0.5% of that in the waveguide.
Efforts are made to correlate the hydrogen presence in proton-exchanged waveguides with their modal properties and vibrational spectra. The exchange process was performed by immersion Z-cut substrates in molten pyrophosphorlc acid at 240 °C for 8 h, thus realizing technological conditions similar to those for waveguides made previously by us from benzoic acid. The number of modes obtained in such type of waveguides was high enought (10-20) to ensure precise profile reconstructions by using the IWKB method. A surface refractive index change of 0.148 was obtained at y=632.8 nm. The exchanged samples were subjected to subsequent annealings for 1 h through 20° starting from 200 °C. Reflection High Energy Diffraction (RHEED), optical measurements and vibrational (IR and Raman) spectra were used at each stage of annealing in order to follow the guide characteristics. Seme correlations between optical profiles and vibrational spectra have been found which allow to make conclusions about the substitution degree and the structure of exchanged layers.
The recent development of Nd3+- and Er3+?doped waveguide amplifiers and lasers in LiNbO3 with proton-exchanged or Ti-diffused channels is reviewed. Besides rare earth doped bulk crystals, also initially undoped substrates have been used for device fabrication. The latter were doped by ion-implantation, followed by thermal annealing, or by indiffusion of an evaporated (photolithographically defined) metallic (Er-) layer. Low threshold (a few mW of absorbed pump power), single transversal mode waveguide lasers of about 1 cm length and medium-gain (up to 7.5 dB) optical amplifiers have been developed in several versions in the Nd-doped material for ? = 1.08/?m (emission) wavelength. With Er a low-gain, but broadband optical amplifier for the wavelength range 1.53mm < ? < 1.62 ?m has been demonstrated as well as a 1 cm long laser of ? = 1.532 ?m emission wavelength.
Simply by the proper choice of the interaction length and number of cascaded three-waveguide directional couplers (3WC), appropriately connected, it is possible to design suitable spectral responses for filtering operations, tuning the passbands and achieving high stopband rejection. We report on the realization of low-loss, cascaded 3WCs for wideband filtering, based on high-silica waveguides on Si. The devices have been fabricated by a combination of Flame Hydrolysis Deposition, photolithographic patterning and reactive ion etching. The design has been directed towards the filtering of the amplified spontaneous emission in Nd-doped fluoride fibre amplifiers operating at 1.3 [?m], which limits the magnitude of the amplifier gain and requires wideband filtering operation to be suppressed. However, a similar arrangement would also find useful application in other devices such as a multi/demultiplexer for widely spaced channels or when wider rejection bandwidths are required to cover the wavelength variation of laser sources in practical conditions.
Using Beam Propagation Method (BPM), we show quantitatively how the asymmetry of Y- branches will affect the transmission ratio of the structure in the power dividing or mode splitting operating area. In the mode splitting area, radiation loss will increase due to conversion of fundamental mode to higher order modes. The mode splitting behavior can be retained even with large branching angle if the phase-front accelerator (PFA) design structure is employed.
A ten-channel single-mode wavelength division demultiplexer operating at 740, 750, 760, 770, 780, 790,800,810,820 and 830 nm with diffraction angles varying from 16° to 52° and using a graded index (GRIN) polymer waveguide is reported for the first time. Diffraction efficiency up to 60% was measured. The wavelength spreading of the Ti:Al2O3 laser (~ 4 nm, 3 dB bandwidth) causes an average crosstalk figure of -21 dB. The beamwidth of the diffracted signal as a function of the input beam width, the grating interaction length, and the diffraction angle are considered. Occurrence of maximum values are discussed. A waveguide lens is needed to efficiently couple the diffracted light into an output fiber whenever the diffracted beam size is beyond the core diameter of the fiber involved.
Passive polarization splitters with a dielectric cover layer patterned on the surface can be made on Ti:LiNb03 which are based on Two-Mode-Interference (TMI). In this paper a novel configuration is considered which doesn't require any external applied signal. The Y-branches, made of linear or curved waveguides, and the branching angles, have been designed in order to get low-loss and short-size devices.
Theoretical predictions and experimental results on two slightly different kinds of linearized optical modulators for use in wideband analog transmission is presented. Both modulators are based on a cascade coupling of phase-modulated parallel wave guide sections. In both cases, 95 dB suppression of the 3rd harmonic of a single test tone with an optical modulation depth of 20% is theoretically obtained. Some initial experimental results are presented for both modulators.
The PE technology has been applied in the fabrication of a wide bandwidth interferometric modulator in X-cut Lithium Niobate. The utilization of dilute melt of Benzoic Acid and Lithium Benzoate as source in the H-Li exchange process, and the introduction of a subsequent appropriate thermal annealing allow to obtain high quality channel waveguide characterized by high long term stability, high electrooptical efficiency , low propagation losses, low fiber-waveguide insertion losses and high resistance to the laser radiation induced damage. The main advantages of this modulator in comparison with the similar Ti-induffusion based device are the lower fabrication cost and the possibility to handle higher optical power, allowing higher dynamic range of the signal processing system.
Devices have been formed in Ti-diffused and proton-exchanged lithium niobate for applications in optical communications, sensing and signal processing. Most active devices make use of the electrooptic effect. The design and operation of various modulators, couplers and Y-branch power dividers will be described. Sometimes, device characteristics are tailored to meet specific requirements, e.g. phase-bias trimming of a linear modulator sensor. Proton-exchanged waveguides are generally used where high optical powers are transmitted.
For the use in cost-effective optical communication systems, two types of guided-wave devices have been studied using ion exchange in glass: (1) A microlens-loaded coupler module composed of a polarization-maintaining fiber array, a directional coupler and a twin-microlens array for coherent balanced receivers has been proposed and fabricated with an excess loss of 1.0 dB and an extinction ratio of _> 20 dB for 0-60 degC operation. (2) An optimum mask pattern for splitters has been devised theoretically and verified experimentally, which has reduced the scattering loss to < .0.1 dB and also suppressed their possible bimodal behavior in the branching region.
Construction of a four node prototype, multi-wavelength, multi-hop, photonically switched packet network is under way at the University of Colorado. The data will occupy approximately 20 nm
bandwidth centered on the erbium doped fiber amplifier gain window, 1.55 mm. The wavelength behavior of the directional couplers to be used in the switching nodes of this network is an important consideration. We describe the wavelength dependence of a series of directional couplers fabricated using the annealed, proton exchange (APE) technique. Measurements of crosstalk in the cross state, i.e. no voltage applied,
versus wavelength for the range, 1.50 mm -1.65 mm are presented. It is shown that the data fit a simple expression for the crosstalk over the entire 150 nm range, using a simple first order wavelength dependence for the phase difference parameter.
To characterize z-cut Ti:LiNb03 Y-branch optical modulators, numerical simulations were performed showing that high on/off ratios are attainable without special Y-junction asymmetries or long electrodes. The effective index and 2-D finite difference beam propagation methods were used for the simulations. A modulator with a 2° Y-branch was fabricated. The measured on/off ratios were 5:1 at 25 V, 12:1 at 50 V, and 60:1 at 75 V, corresponding to 4:1, 12:1, and 62:1 for the simulations for y= 632.8 nm.
In this paper, we present the design of a 4x4 complete interconnection network, including a new switching element configuration which allows to reduce the overall dimensions. It is based on the asymmetric passive 3-branch power divider, which allows the beam to be splitted with half power in the central waveguide and 1/4 power in either lateral waveguide. This structure, which can be inserted in various switching network architectures, has been analyzed by using the Beam Propagation Method, in order to determine as the various design parameters affect the system performances. The total area of the switching element is about 0.6 mm2 , for the control voltage of 9.7 V. The total insertion losses are 2.1 dB in the cross state and 1.2 dB in the bar state.
A potassium ion-exchange process is employed to make integrated optical Mach-Zehnder interferometers in a glass substrate. An extensive study is carried out to determine the contribution of different components of the interferometer in its performance. Propagation properties of Mach-Zehnder interferometers, straight and S-shape waveguides, and Y-junctions are investigated.
The solution of the Helmholtz wave equation requires the application of an exponentiated square root operator to an initial field. This operation is greatly facilitated by the introduction of a representation in which the aforementioned operator is diagonal. The Lanczos method allows this diagonalization to be performed in a low dimensional space, e.g., of the order of 4-6, if one is interested in advancing the field over a limited propagation step of length Az. Although some boundary conditions may be ill-posed for the unapproximated Helmholtz equation, in the sense that certain plane wave components cannot propagate in the forward direction, the Lanczos method damps all of these components exponentially, thus guaranteeing the correctness of the solution.
The Beam Propagation Method (BPM) combined with the Effective Index Method (EIM) has been used to simulate and design the performances of planar integrated optical waveguide devices. To achieve high performances of optical devices, design optimization is very important. For accurate simulations and designs, detailed information on the characteristics of slab and channel guides made in specific substrates must be known in relation to their fabrication conditions since these simple waveguides often form the basic units in more complicated waveguide structures used in devices. This paper will discuss the characterization and modelling procedures of such waveguides, and then present the methodology of design optimization of waveguide devices, employing the BPM. Several illustrative examples will be given.
In this paper we studied an asymmetrically cladded Y-Junction in Ti:LiNb03 in order to investigate the passive polarization splitting behaviour related to the action of a buffer layer supposed to cover only one branch.
Different widths of the two arms and appropriate dividing angle are chosen to improve the expected performance. We analysed the Y-structure by utilizing a bounded step approximation involving the mode-conversion between guided and radiation modes. To evaluate the effective indexes of both modes in each step, we utilized the effective index method by considering the two dimensional refractive index profiles.
A simple method is developed to analyze the bent waveguide, which is described in the cylindrical coordinate system. By means of this method based on the Galerkin method, the sampling spacing can be chosen arbitrarily and it is possible to treat narrow beams. In addition we introduce the absorber using the graded lossy medium. As this lossy absorber can remove the radiation wave from the bend, so we can use the finite computational window. The lightwave propagating phenomena in the uniformly bent slab waveguide and in the nonlinear slab waveguide are demonstrated
A reflected light by a substrate becomes zero when H-polarized light is incident at the Brewster angle. Stacking many identical substrates together forms a multi-layer structure. The valley of this reflected intensity pattern centered at the Brewster angle becomes narrower due to the overlapping of these similar reflections from many parallel interfaces of air and substrates. Thus, measuring the Brewster angle of this multi-layer structure leads to an accurate determination of the substrate index.
We present the analysis of a semi-leaky waveguide with the help of 3 different methods. Since the resulting numerical values still show some differences, the validity of these methods is discussed. Both theoretical and experimental studies of a semi-leaky waveguide show a maximum of the losses for an index of the superstate slightly greater than the index of the waveguide. We finally apply these results to design and characterize a polarizer based on the deposition of an anisotropic layer on an otherwise lossless waveguide.
In this paper, the modeling of the acoustooptic interaction in multilayered guiding structures is presented. The mathematical analysis has been performed on the basis of the generalized coupled-mode theory and applied for the first time to ZnO/AIGaAs/AIGaAs/GaAs complex structures.
The quantum noise is well known as a physical final noise in optical communication system. No correction method for the error caused by the quantum noise has been known. In this paper, a correction method to the quantum noise is presented. The correction is in principle based on the polarization signal and polarization split detection. Because the method requires two polarization channels for signal transmission, it is not advantageous. Then a more efficient method for the transmission and correction which is based on the PSK signal and homodyne detection is presented. As result of application of the correction, it is shown that the error probability is brought to 0.
Advances in electro-optical polymer research for guided wave devices have taken place at a rapid rate. Presented here is an overview of this progress. Included is a comparison of material types and fabrication techniques. Also described are device design issues including poling, electrodes, and cladding layers. Future directions and applications will also be discussed.
We present a model for a single polarization optical waveguide structure and demonstrate experimental verification of the model. The optical «waveguide structure involves a silicon nitride film deposited onto an oxidized silicon wafer to form a single mode planar waveguide. We achieve single polarization by choosing layer thicknesses so that the attenuation due to substrate coupling for the TM mode is much larger than that for the TE mode. Calculations are presented defining ranges of design parameters fa- which TM mode attenuation is 3-4 orders of magnitude higher than that for die TE mode. Measured data is presented for which the ratio of TM mode loss to TE mode loss is over 750 for a TE loss of 0.28 dB/cm at a wavelength of 632.8 nm.
Preliminary experimental results on thermo-optic induced modulation in a silicon etalon at 1.5 mm are reported. The measures have also allowed an accurate determination of the thermo-optic effect in silicon at this wavelength. Finally, a new class of all-silicon integrated Fabry-Perot modulators at 1.3 and 1.5 mm, based on thermo-optic effect, is proposed.
Polymeric materials suitable for the formation of both passive and electro-optically active waveguide devices have been produced. We have developed methods for delineating channel waveguides in thin polymer films via a photopatteming technique. A description of the performance characteristics of devices incorporating these materials will be presented. An analysis of the advantages and drawbacks of polymeric systems will also be included.
Novel dielectric properties are achieved when semiconductor crystallites are emedded in polymeric, passive host material. Upon illumination which photon energy above the semiconductor band-gap, a dielectric constant change will be observed for a below band-gap, propagating photon.
We have successfully demonstrated an In203:Sn semiconductor thin film waveguide. The energy gap of the film can be manipulated from 3.1 eV (0.4 pm) to 3.7 eV (0.335 mm) by changing the ratio of In203 and Sn02. Waveguide propagation losses of 3 dB/cm for transverse magnetic (TM) and 8 dB/cm for transverse electric (TE) guided waves were experimentally confirmed at the wavelength of 632.8 mm. A phase modulator containing an indium tin oxide waveguide, two holographic mirrors, two microprisms, and two ohmic contacts was fabricated. Electro-optic (current injection) and all optical modulations were conducted. A modulation depth of 18% was experimentally confirmed for the current injection device, using 15-volt applied voltage, and a modulation depth of 15% using 250 mW 355 nm UV light as the activation sources. An In203:Sn waveguide device working at the cutoff boundary was made. A modulation depth of 26 dB was measured with an applied voltage of 30 volts. An array of applications, including use in current sensors, ozone UV sensors, attenuated total reflection (ATR) modulators, delay lines for phased array antennae and multi-quantum wells are highly feasible.
The ability to produce changes in the refractive index of a glass of the order of 0.01 to 0.001 allows a laser beam to be guided through glass. However, to perform the functions required for passive elements demands the formation of highly resolved gradient index patterns. Porous glass impregnated with photosensitive organometallic compounds exhibits optical changes when exposed to light. Changes in transmission and refractive index can be induced and are permanent after consolidation of the glass. Photolithographic techniques for making channel waveguides of the order of 1-10 mm, and more complex optical circuits composed of active and passive components have been developed.
The acousto-optic transducer (AOT) is a fundamental component of a number of guided-wave devices and circuits suitable to the signal processing and optical computing. Most of such devices has been fabricated by using the LiNb03 technology, but some effort has been also spent on components based on lll/V semiconductor technology, i.e. GaAs and related compounds. However, because of the weak piezoelectric effect in these materials, it needs to analyze complex multilayer structures, including an additional ZnO superstrate, which shows an acoustic mode dispersion, due to the dependence of the surface acoustic wave (SAW) propagation speed on the acous.ic frequency applied to the transducer . In this paper a generalized model of the SAW propagation into multilayered structures has been applied to some typical acoustic waveguides, a simple semi-infinite layer of AlxGa-1 _xAs, as a function of the Al title x, a ZnO/AIGaAs structure, and a more complex ZnO/Alx1Ga-|_xiAs/Alx2Gai_X2As/GaAs structure, as a function of the intermediate layer thicknesses, frequency and Al titles x1 and x2.
A semiconductor laser diode with a nominal relaxation oscillation frequency of 6 GHz with an anti-reflection coating on one facet is coupled to an external high Q cavity with the fundamental resonance set to 5 GHz. The laser diode is mounted in a custom microstrip fixture and is driven by a HP-8510A network analyzer. The experimental and theoretical results show enhancement greater than 25 dB at the external cavity resonance frequencies. The results also show holes near the resonance frequencies which may affect the useful bandwidth.
Based on a non-linear coupling between the etchant species and the photo-induced carriers during photo-electrochemical etching of semiconductor surfaces, we propose that the reaction residue, the oxide layer, regulates the reaction.
Annealing properties of proton-exchanged optical waveguides in LiNbO3 fabricated using stearic acid have been studied systematically. The effects of annealing on the index profile, the waveguide depth, the surface index change and the propagation loss were measured as a function of annealing time.
A simple photonic integrated circuit consisting of a ring laser, a passive waveguide and a detector has been fabricated in GaAs/AlGaAs. A ring laser is employed as a source in this circuit because it is easily integrated with other components on the same substrate. The detector monitors the counterpropagating mode of the ring laser and shows no leakage current. The structure is easily fabricated by straightforward lithography and does not require overgrowth.
The conditions for the practical use of integrated optic gratings as high efficiency flat optical antennae are fulfilled. Some examples are reviewed. A CAD tool for the dimensioning and the optimization of real coupling structures based on Rayleigh approximation is proposed.
We compared WDS and inverse WKB measurement of Thallium/Potassium exchanged waveguides in glass. Their good agreement allowed us to use the WDS technique for testing numerical simulations in case of complex structures.
Modified scheme of Mach-Zehnder interferometer is proposed that allows post-fabrication wide-range tuning of intrinsic phase bias and extinction ratio improvement by adjustment of single-mode fiber position at the modulator input.
A vector beam propagation method based on finite-difference (FD-VBPM) is developed and described. The polarization property and the hybrid nature of the propagating waves are considered. The assessment and the applications of the FD-VBPM are presented.