Permanent narrow-band reflection holograms for infrared light recorded in LiNbO3:Ti:Cu channel waveguides

Permanent refractive-index gratings are generated by thermal fixing of holograms in photorefractive lithium niobate channel waveguides. The guides are fabricated by successive indiffusion of titanium stripes and thin layers of copper. After high-temperature recording with green light, refractive-index modulations exceeding Δn=8×10^-5 for light of the telecommunication wavelength 1550 nm appear without the need of any development process of the written holograms. The gratings are stable in the dark and no compensation mechanism via dark conductivity is observed. Thus this method may be well suited for long-time applications in holography and integrated optics. Received: 2 October 2000 / Revised version: 25 January 2001 / Published online: 22 March 2001     

Thermal fixing of holographic gratings in planar LiNbO3:Ti:Fe waveguides

3 :Ti:Fe waveguides are thermally fixed during hologram recording at elevated temperatures. Different guides are fabricated by titanium indiffusion using iron-doped and nominally pure y-cutLiNbO₃ substrates and characterization is performed by dark-mode spectroscopy. The refractive index modulation of gratings written and simultaneously fixed at 180 °C is investigated as a function of propagation depth, titanium and iron concentration. The experimental results are compared with those obtained for unfixed holograms recorded at room temperature. Received: 21. July 1997/Revised version: 13 October 1997     

Quasi-phase-matched difference-frequency generation in periodically poled Ti:LiNbO(3) channel waveguides

Mid-infrared radiation near 2.8 mum was generated by difference-frequency generation in an 80-mm-long periodically poled Ti:LiNbO(3) channel waveguide by pump radiation near 1.55 mum (tunable external-cavity laser) and a signal radiation of 3.391 mum (HeNe laser). We obtained a normalized conversion efficiency of 105% W(-1) , which is to our knowledge the highest value ever reported.     

Praseodymium-doped Ti:LiNbO3 waveguides

3 by diffusion doping is investigated by means of secondary neutral mass spectrometry and secondary ion mass spectrometry. The diffusion of praseodymium in LiNbO₃ can be described by Fick’s laws of diffusion with a concentration-independent diffusion coefficient and a limited solubility of praseodymium in LiNbO₃ increasing exponentially with rising temperature. The diffusion depends on the Li₂O content of the LiNbO₃ crystal. For LiNbO₃ crystals with a nominal slight difference in the congruent composition, the diffusion constants and activation energies for Z-cut LiNbO₃ are 3.28×10^-5 cm²/s and 2.27 eV, and 1.39×10^-5 cm²/s and 2.24 eV, respectively. Titanium-doped waveguides are formed in Pr:LiNbO₃ and characterised in relation to waveguide loss and absorption in the visible and near infrared. Received: 17 September 1998 / Accepted: 11 November 1998     

Ti:LiNbO3 optical waveguides

Various experiments on Ti diffused optical waveguides in LiNbO₃ have been carried out in order to determine precisely the character of the diffusion process. The required guide parameters and the effective mode indices could be controlled by adjusting only the diffusion time under fixed temperature and film thickness. Therefore the dependence of the guide characteristics on the diffusion time has been investigated in detail. On the basis of the data obtained, a two-stage diffusion model is proposed. In the first stage, the Ti diffusant profile is described by a erfc-function, and the second stage is characterized by a modified Gaussian form.     

Light-induced thermal gratings in LiNbO3: Fe

Holographic gratings are recorded in LiNbO³: Fe using frequency-doubled pulses of aQ-switched Nd : YAG laser (light wavelength = 532 nm, pulse durationt _p = 20ns). We monitor diffraction efficiencies of the holograms during and several milliseconds after exposure. Variations of Fe²⁺ concentration, light intensity, and fringe spacing show that different types of gratings are involved: photochromic, thermooptic, pyroelectric, and space-charge gratings. The influence of the internal pyroelectric field on the light-induced charge transport is discussed in terms of a two-center model. Pyroelectric and bulk photovoltaic effect are shown to partially compensate each other. In LiNbO₃: Fe, the photovoltaic effect is the dominant charge-driving force for our recording conditions.     

Thermal tuning of a fixed Bragg grating for IR light fabricated in a LiNbO3:Ti channel waveguide

We investigate a thermally tunable integrated narrow-bandwidth interference filter for light in the infrared wavelength region around 5=1550 nm fabricated in a channel waveguide in lithium niobate. The filter is based on a holographically recorded and thermally fixed refractive-index Bragg grating. The device is connected to optical single-mode fibers and works polarization-independent. We measure a peak reflectivity of 95% and a linewidth (FWHM) of 0.09 nm. By temperature-controlling the peak wavelength of the filter is tuned. For the linear thermal expansion coefficient of lithium niobate along the c axis the value !₃₃=(4.5ǂ.5)᎒^-6 K^-1 is obtained. Permanent illumination of the filter with incoherent light allows us to adjust the peak reflectivity. Dedicated to Prof. K. Ringhofer on the occasion of his 60th birthday.     

Optimal direct coupling from single-mode fibers to Ti:LiNbO3, channel waveguides

We investigate the optimal direct coupling from single-mode fibers to Ti:LihbO₃, channel waveguides using a very general formula and a heuristic optimization technique in this article. The coupling efficiency of the optical power depends on both the fiber positions and the modal sizes of the incident fields. From our numerical simulation, it is found that the optimal positions of the fiber axes are not in alignment with the peaks of the waveguiding modal fields and that the coupling efficiency can be improved by microlenses.     

Lifetimes of thermally fixed holograms in LiNbO(3):Fe crystals

The thermal erasure decay at different temperatures of holographic gratings thermally fixed in iron-doped lithium niobate has been exhaustively studied. An activation energy of 0.94 eV was found to be independent of the grating fringe spacing. At a given temperature the decay time is strongly dependent on the grating spacing. This dependence is well fitted to a square fringe spacing law, as predicted theoretically. From the experimental data the room-temperature hologram lifetime under illumination is extrapolated, giving as a result t(years)=(5.2+/-0.2)?(2) , with the grating spacing ? in micrometers.     

Refractive index profiles of LiNbO3: Ti waveguides

Eight LiNbO₃: Ti waveguides have been fabricated under differing conditions. The effective refractive indices of guided modes have been measured and successfully fitted to Gaussian index profiles. The change of refractive index and titanium concentration are related by a power law, the exponents being different for the extraordinary and the ordinary beam.     

Light-induced refractive index changes in LiNbO3:Ti waveguides

Holographic measurements of light-induced refractive index changes in LiNbO₃Ti waveguides are reported. The results demonstrate that Fe²⁺ centers are stabilized against oxidation in the region of large Ti concentration near the surface thus increasing the sensitivity to optical damage considerably. This undesired effect may be largely abolished by additional in-diffusion of protons.     

Spectral line broadening mechanism of Er3+ transitions in Er:Ti:LiNbO3 channel waveguides

In order to elucidate the interaction effects among the various defects present in a LiNbO₃-based integrated optical device, we investigated the change of the optical properties of Er³⁺ ions under the application of an external electric field and hydrostatic pressure. We obtained for stoichiometric bulk material a complete picture of the field-induced spectral shifts as a function of transition and site. As a first important application of these results we were able to clarify the mechanism of spectral broadening of the Er³⁺ transitions in Ti:Er:LiNbO₃ channel waveguides. By selecting different waveguide modes for excitation and using highly selective double-resonance excitation with two lasers, we found that the [Ti⁴⁺] concentration gradient caused by the indiffusion results in an internal E-field gradient. This translates, due to the averaging within the guided mode, into mode-dependent spectral line broadening. Received: 24 May 2001 / Published online: 23 October 2001     

Site-selective spectroscopy of Er3+:Ti:LiNbO3 waveguides

Starting from previous investigations in LiNbO₃ bulk crystals, we studied the optical properties of Er³⁺ ions in Ti:LiNbO₃ channel waveguides and investigated the waveguide-specific lattice environment of the Er³⁺ ions (“sites”) caused by the doping method used and the presence of a large number of Ti⁴⁺ ions. For that purpose the method of combined excitation–emission spectroscopy was applied for the first time to waveguides at low temperatures. Comparing the spectroscopic results obtained for the green, red, and near-IR luminescence (λ≈550, ≈650 and ≈980 nm) under direct (450 nm), 2-step (980 nm), and 3-step (1.5 μm) laser excitation, we found several distinguishable Er³⁺ sites which in terms of energy levels and relative numbers are similar to those in bulk material, but exhibit significantly different up-conversion efficiencies and strongly inhomogeneously broadened transitions. Moreover, we were able to distinguish isolated and cluster Er³⁺ sites by their characteristic excitation and emission transition energies and studied the respective excitation/relaxation channels. The cluster sites are most efficient in the up-conversion process, especially under 3-step excitation. Using accepted microscopic models for Er³⁺ and Ti⁴⁺ incorporation into the LiNbO₃ crystal lattice, the site distribution and up-conversion mechanisms are elucidated and their consequences for laser applications in different spectral regions are discussed. Received: 16 November 2000 / Published online: 21 March 2001     

LiNbO3:Fe晶体中光写入波导时折射率的变化规律

研究了利用光辐照法制作光折变波导时LiNbO3:Fe晶体中折射率变化的规律.分别采用波长为6328nm和532nm的寻常偏振和非常偏振的细激光束和片状激光束，在LiNbO3:Fe晶体中进行了写入波导实验.研究表明，制作波导的写入光宜采用寻常偏振光.在利用由光束辐照LiNbO3:Fe晶体形成的正折射率变化区域作为波导结构时，必须严格控制辐照时间.否则，由于长时间光辐照会带来较强的噪音栅以及折射率变化区域会发生扩展，而难以形成优 质波导.利用片光在“三明治”方式辐照下，以小曝光量制作波导时，可以避免噪音栅的 关键词： 光致折射率变化 光折变波导 光辐照法 LiNbO3:Fe晶体     

Second-harmonic generation in Zn-diffused periodically poled LiNbO3 channel waveguides

In this work second-harmonic generation by quasi-phase matching (QPM) in Zn-diffused periodically poled lithium niobate channel waveguides is presented. A stable TM?TE conversion by QPM has been found. The results are in good accordance with theoretical estimations obtained by the phase-matching condition, either for the polarisation character of the second-harmonic wave as well as for the spectral range, taking into account the periodicity of the domains. Received: 16 May 2001 / Revised version: 7 September 2001 / Published online: 30 October 2001     

Fixed index gratings in LiNbO(3):Fe upon long-term exposure to an intense laser beam

Permanently fixed noisily recorded refractive index gratings are discovered in single crystals of LiNbO(3):Fe upon long-term exposure to a single laser beam with high intensities of I=100 kW/m(2) and ambient temperature. The fixing process is explained by considering the effect of laser-induced heating of the sample and the well-known simultaneous thermal fixing procedure.     

Seeded and spontaneous light hexagons in LiNbO3:Fe

The first observation of optical hexagons in a medium with dominating photovoltaic charge transport – in LiNbO₃:Fe – is reported. The optimum conditions for hexagon observation are revealed and discussed. The dynamics of hexagon formation are also studied. Received: 9 February 2000 / Published online: 8 March 2000     

Design and demonstration of interferometric integrated-optic sensors in Ti:LiNbO3 waveguides

The design of an integrated optic Michelson interferometer sensor for stress, temperature, and microdisplacement measurements consisting of a semi-asymmetric X-junction made by Ti in-diffused LiNbO₃ waveguides is reported. The possibility of performing simultaneous measurements of strain and temperature is investigated, and different calibration schemes are proposed. Experimental results of displacement-only measurements show a 64% intensity modulation with 33-dB optical loss.     

Thermooptical characteristics of H+:LiNbO3:Ti proton-exchanged optical waveguides

An experimental investigation was carried out on the temperature coefficients for phase angles during excitation of output waves in proton-exchanged planar optical waveguides based on lithium niobate. The functional dependence of these coefficients on the time following annealing was studied for various waveguide modes and light propagation directions. On the basis of experimental results and the analytical expressions obtained for the temperature coefficients of input angles, conclusions were drawn about the significantly lower temperature coefficients for the extraordinary refractive index in the waveguide region.Tomsk State Academy of Control Systems and Radioelectronics. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 84–90, July, 1995.