Potassium-and silver-double-ion-exchanged slab glass waveguides: characterization and modeling

Double-ion-exchanged glass waveguides are made first by a potassium-ion exchange followed by a silver-ion exchange. The inverse WKB method is used to determine the index profile. It is shown that a combination of a truncated quadratic function and a complementary error function represents the refractive index best. A direct WKB method is employed to find the maximum index change and the diffusion coefficient of the second exchange process. The concentration of silver, potassium, and sodium ions are measured by an x-ray photoelectron spectroscopy technique. The results agree well with the index profile when a quadratic function is used.     

Non-linear optical wavelength conversion in Ti:LiNbO3 waveguides

Phase matching temperatures for second harmonic generation were measured for fundamental wavelengths from 1.06 to 1.24 μm in a planar waveguide and compared with bulk crystal values. Furthermore, both second harmonic generation and sum frequency generation were achieved in a channel waveguide, the latter by mixing the Nd-YAG laser wavelength at 1.064 μm with Raman-shifted light from the same light source. A conversion efficiency for second harmonic generation in a channel waveguide of 4.2% at a fundamental power of 120 mW was obtained. Some material-related aspects on waveguides for this type of application are also discussed.     

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.     

XPS-investigation of proton exchanged waveguides in LiNbO3

An X-ray photoelectron spectroscopy (XPS) study of X-cut LiNbO₃ proton exchanged in pure and lithium benzoate buffered benzoic acid melt has been performed. Binding energies and full width at half maximum for the Li1s, O1s and Nb3d peaks in the depth range 0–10 nm were investigated by using Ar ion sputtering. The largest changes with increasing depth are established for the surface atomic layers between 0 and 1 nm. Li-enrichment of the surface is observed when both buffered and pure benzoic acid melts are used.     

Simultaneous fabrication of optical channel waveguides and out-of-plane branching mirrors from a polymeric slab structure

Recently, polymeric multimode optical waveguides have been widely studied for low-cost interconnection use. We describe fabrication processes for a low-loss poly(methyl methacrylate) (less than 0.1 dB/cm at 675 nm) waveguide based on the reactive-ion-etching technique and for a slope (45 degrees mirror face) formed on the waveguide. To obtain a ridge core with a crack-free and extremely smooth surface, we applied a heating process at a temperature greater than the glass transition point and a smoothening process by solvent. Furthermore, to fabricate simultaneously both vertical and sloped sidewalls, we applied a unique phenomenon in the process that decreases the etching rate directly under a narrow opening of etching mask. By using the above fabrication techniques, we demonstrated an out-of-plane branching mirror.     

Anomalous side-shifted multimode spectra in proton-exchanged LiNbO3 waveguides

We report the presence of a curious and highly reproducible effect in multimode lithium niobate waveguides fabricated by proton exchange (PE) in molten benzoic acid at temperatures ranging from 160 degrees C to approximately 250 degrees C. The spectral lines in the mode spectra of these guides (measured using a prism coupler) are anomalously side-shifted out of the expected geometrical plane. Transforming these measurements back into the plane of the waveguide, we find that the direction of scattering (relative to the crystal axis) is extremely precise (<1% deviation about a mean), and that the effect can be explained by postulating the existence of precisely oriented, stress-induced gratinglike structures (with irregular periods in the 10-70-microm range) in the guides.     

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.     

Theoretical study of Ti:LiNbO3 strip waveguides in the Mid-infrared

Abstract

The transverse magnetic and transverse electric mode size and effective refractive index as functions of the Ti-strip initial width W, diffusion temperature T and Ti-strip initial thickness H in c-cut Ti-diffused LiNbO₃ waveguides have been theoretically studied at wavelengths of 3.0 and 3.39 μm on the basis of modified expressions for the waveguide surface index increment. The single-mode, multiple-mode and cut-off conditions have been identified from those effective refractive index plots. These mid-infrared numerical results are discussed comparatively with near-infrared data reported earlier.     

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.     

Strong reduction of propagation losses in LiNbO3 ridge waveguides

We propose optimized processes for the reproducible production of LiNbO₃ ridge waveguides with propagation losses lower than 0.2 dB/cm. The performances are achieved for both Z-propagating and Y-propagating waveguides, TE and TM polarizations, in X-cut LiNbO₃ congruent substrates, and could be easily extrapolated to Z-cut substrates. The fabrication procedure is composed of three steps: titanium deposition, optical grade dicing and diffusion at high temperatures, so that lithography or cleanroom facilities are not required. The reproducible smooth waveguides with low propagation losses result from the diffusion step, which is performed after the production of the ridges. We also show how the losses can be evaluated from the Fabry–Perot effect in the waveguides, without any assumption on the input and output reflection coefficients.     

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.     

Characterization of Ti:LiNbO3 deep waveguides diffused in dry and wet oxygen ambient

The diffusivity of thicker than usual Ti films into LiNbO3 to create deep waveguides has been investigated as a function of crystal orientation and diffusion ambient for temperatures ranging from 1050 to 1100 degrees C. The diffusion parameters for substrates diffused in water vapor environment are always larger than ones diffused in dry ambient. Differences in the diffusion coefficient for the extraordinary mode as high as 90, 66, and 84% are measured for X-, Y-, and Z-cut crystals, respectively. Smaller but noticeable differences are detected for the ordinary mode. Similar enhancing effects of the water vapor are also obtained for the surface-index change. Severe rough surface finish and surface contouring, revealing an etchedlike appearance, are observed on Z-cut substrates diffused in wet atmosphere. In general, substrates diffused in dry O2 produced relatively smoother surfaces. No outdiffused modes are excited in Z-cut substrates when diffused in dry oxygen ambient for long times at high temperatures.     

Design, fabrication, and characterization of Si-based ARROW photonic crystal bend waveguides and power splitters

Silicon-based (Si-based) photonic crystal waveguide based on antiresonant reflecting optical waveguide (ARROW PCW) structures consisting of 60° bends and Y-branch power splitters were designed and first efficiently fabricated and characterized. The ARROW structure has a relatively large core size suitable for efficient coupling with a single-mode fiber. Simple capsule-shaped topography defects at 60° photonic crystal (PC) bend corners and Y-branch PC power splitters were used for increasing the broadband light transmission. In the preliminary measurements, the propagation losses of the ARROW PC straight waveguides lower than 2 dB/mm with a long length of 1500?μm were achieved. The average bend loss of 60° PC bend waveguides was lower than 3 dB/bend. For the Y-branch PC power splitters, the average power imbalance was lower than 0.6?dB. The results show that our fabricated Si-based ARROW PCWs with 60° bends and Y-branch structures can provide good light transmission and power-splitting ability.     

Second-order quasi-phase matched second harmonic generation in annealed proton-exchanged LiNbO3 channel waveguides

We report the first efficient blue second harmonic generation (SHG) in a second-order quasi-phase matching (QPM) LiNbO₃ waveguide. We show both theoretically and experimentally that the second-orderQPM grating is the optimum structure for waveguideSHG in LiNbO₃ waveguides. Using such a structure, we have demonstrated blue light generation with a conversion efficiency of 65%/W-cm², the highest reported so far in LiNbO₃ waveguides.     

Formation and properties of proton-exchanged and annealed LiNbO3 waveguides for surface acoustic wave

The proton-exchanged (PE) and annealed PE (APE) z-cut LiNbO₃ waveguides were fabricated using H₄P₂O₇. The positive strain, c-axis lattice constant change (Δc/c), was calculated to be about +0.43%, which was almost independent of the exchanged conditions. The penetration depth of H measured by secondary ion mass spectrometry (SIMS) exhibited a step-like profile, which was assumed to be equal to the waveguide depth (d). The surface acoustic wave (SAW) properties of PE and APE z-cut LiNbO₃ samples were investigated. The phase velocity (V_p) and electromechanical coupling coefficient (K ²) of PE samples were significantly decreased by the increase of kd, where k was the wavenumber (2π/λ). The insertion loss (IL) of PE samples was increased by the increase of kd and became nearly constant at kd>0.064. The temperature coefficient of frequency (TCF) of PE samples allowed an apparent increase with kd, reaching a maximum at kd=0.292, then slightly decreased at higher kd. The effects of annealing resulted in a restoration of V_p and an improvement of IL     

Optically transparent and non-linear LiNbO3: polyimide composite: preparation and characterization

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Planar optical waveguides obtained in Z-cut LiNbO3 and LiTaO3 by proton exchange in LiHSO4 vapors

A new proton source – LiHSO₄ vapors at low temperature – was used in forming optical waveguides in LiNbO₃ and LiTaO₃. The proton-exchanged layers were investigated by mode spectroscopy and infrared absorption spectroscopy. An estimate of the optical losses and phase contents of the waveguides was made. The diffusion parameters were determined for both types of crystal, and were compared to those obtained when benzoic acid melt was used as a proton source. The results presented could contribute to the realization of waveguides with controled phase compositions. The method proposed allows the use of a very simple and safe chamber construction, and the production of low-loss waveguides in a single technological step.