Comparison of the electro-optic coefficient r33 in well-defined phases of proton exchanged LiNbO3 waveguides

The electro-optic coefficient, r₃₃, of proton exchanged LiNbO₃ waveguides has been measured in well-defined phases of the exchanged layer. Namely, in two low index-jump α-phases, i.e. unannealed soft-exchanged (SPE) and annealed (APE) guides, and in two high index-jump phases, i.e. β₁ guides and, for comparison, ordinary proton exchanged (PE) guides (having a mixture of phases). The following values have been obtained (in pm/V): 22.1±0.6 for SPE; 20.9±0.7 for APE; 0.33±0.01 for β₁; and 0.76±0.04 for PE. Differences between these values are discussed in terms of the structure of the phase involved. Received: 18 May 2001 / Revised version: 22 August 2001 / Published online: 30 October 2001     

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     

Effect of patterns and inhomogeneities on the surface of waveguides used for optical waveguide lightmode spectroscopy applications

It has been found that patterns and inhomogeneities on the surface of the waveguide used for optical waveguide lightmode spectroscopy applications can produce broadening and fine structure in the incoupled light peak spectra. During cell spreading on the waveguide, a broadening of the incoupling peaks is observed, while regular microstructures on the incoupling grating produce shifts and splitting of the peaks. A theoretical model, based on the zigzag wave representation of light propagation in a planar optical waveguide has been developed in order to understand the physical background of the observed effects. Numerical results are given for the different cases observed, and they are compared with the experimental data. Several possible applications of these effects are considered. Received: 10 July 2000 / Revised version: 9 October 2000 / Published online: 21 February 2001     

Mode splitting in an optical slab waveguide filled with nematic liquid crystals

Mode splitting was observed when a He-Ne laser beam was reflected through a prism-coupled liquid-crystal slab waveguide under an applied electric field. The splitting manifests itself as the imposed voltage reaches a critical level, and diminishes when the voltage increases above a critical high value. If the applied voltage increases even further, mode splitting vanishes, attributing to the result that almost all the directors of the liquid crystal turn upright to the surface. The multi-layer matrix simulation can satisfactorily account for this phenomenon by exploiting the property of the anisotropic optical birefringence of a liquid crystal under applied voltages. Received: 2 December 2002 / Revised version: 24 January 2003 / Published online: 23 May 2003 RID="*" ID="*"Corresponding author. Fax: +886/3572-3052, E-mail: jtlue@phys.nthu.edu.tw     

X-ray diffraction study on precipitate of Er:LiNbO3 induced by vapor transport equilibration

An X-ray powder diffraction study was performed on vapor transport equilibration (VTE) treated Er:LiNbO₃ crystals with different doping levels (0.2, 0.4 and 2.0% Er per cation site), different cut orientations (X- and Z-cuts) and different VTE durations (120, 150 and 180 h). Their diffraction characteristics were compared with those of pure congruent LiNbO₃ and as-grown Er:LiNbO₃. The most significant characteristic is the appearance of additional weak and broad peaks around the 2θ angles 30° and 59° in the diffraction patterns of both X- and Z-cut 2.0 mol% doped VTE crystals, confirming that they precipitated. A further comparison of their diffraction data with the powder diffraction files indicated that the new phase in these precipitated crystals is ErNbO₄, which has an approximate concentration of 1.0%, 1.065%, 1.485% for 120, 150 and 180 h crystals, respectively. The crystalline grain sizes of the new phase are 132.2∼184.1?. The unit cell parameters of the as-grown and VTE crystals were also determined from diffraction data; the variation from pure LiNbO₃ to as-grown Er:LiNbO₃ was qualitatively explained according to the crystal structure of LiNbO₃ and using the concept of ionic radius. VTE brings the crystal closer to a stoichiometric composition, thus causing the contraction of the lattice constants. Finally, a tentatively qualitative explanation for precipitate formation is given on the basis of crystal structure. Received: 2 August 2000 / Accepted: 29 March 2001 / Published online: 20 June 2001     

Evanescent-field-induced two-photon fluorescence: excitation of macroscopic areas of planar waveguides

Two-photon excitation (TPE) of fluorescence is a powerful tool for separating a faintly emitted fluorescence signal from background excitation noise. Until now TPE has only been accomplished for very small excitation areas of a few square micrometre dimensions since they are readily available in the focal zone of high-power lasers. In this paper we demonstrate, to our knowledge for the first time, two-photon excited fluorescence with planar thin-film waveguide structures of macroscopic excitation areas of the order of square millimetres to square centimetres. Based upon our results, it can be envisaged that the new combination of planar waveguide technology with TPE will become a powerful tool for macroscopic surface-interaction studies. It can also open the way for further improving the sensitivity of biosensing platforms like genomic and proteomic microarrays based upon planar waveguides. Received: 7 November 2001 / Published online: 23 November 2001     

Ti:sapphire rib channel waveguide fabricated by reactive ion etching of a planar waveguide

We report, to our knowledge, the first active channel waveguide in Ti:sapphire. We have created ∼1.4-μm high ribs in a ∼10-μm thick Ti:sapphire planar waveguide by reactive ion etching. Following excitation by an Ar-ion laser, the rib structure showed channel-waveguide fluorescence emission. The mode profiles and the beam-parameter values (M²) were measured. The coupling efficiency of fluorescence emission into a single-mode fiber was an order of magnitude higher than for fluorescence from unstructured planar regions of the waveguide. Such devices are of interest as low-threshold tunable lasers and as broadband light sources in low-coherence interferometry. Received: 22 December 2002 / Revised version: 30 March 2002 / Published online: 8 August 2002     

Property study of Si + -ion-implanted Nd:YVO 4 waveguides

Planar waveguides were formed in Nd:YVO₄ crystals by 3.0-MeV Si⁺-ion implantation at doses of 1×10¹³–1.5×10¹⁵ ions/cm² at room temperature. The effective refractive indices of the waveguide propagation modes were measured by using a prism-coupling method. It was found that the number of the propagation modes is dependent on the doses for the waveguides in Nd:YVO₄. The atom displacement in the near-surface region (about 2 μm beneath the surface) of the Nd:YVO₄ crystal induced by the implantation was simulated by using the TRIM 98 (transport and range of ions in matter) code. The possible reasons for the waveguide formation are discussed in a primary way. Received: 17 July 2002 / Revised version: 20 September 2002 / Published online: 11 December 2002 RID="*" ID="*"Corresponding author. Fax: +86-531-8565167, E-mail: drfchen@sdu.edu.cn     

Loss-tolerant, narrow-band reflector in the UV using a grating-waveguide structure

We theoretically demonstrate the feasibility of using a grating-waveguide structure (GWS) as a loss-tolerant, narrow-band reflector in the UV. We simulate device operation using a Green-function technique. Our numerical simulations indicate that a GWS with a 25-nm grating can be used as a 95% reflector for 157-nm light, even in the presence of intensity absorptions of 50 cm^-1. Received: 23 July 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +49-551/5035-99, E-mail: mbader@llg.gwdg.de     

Raman study on vapor-phase equilibrated Er:LiNbO3 and Er:Ti:LiNbO3 crystals

Raman spectra of Er:LiNbO₃ crystal and Ti-diffusedEr:LiNbO₃ strip waveguide, in which the Li/Nb ratio was altered using a vapor-phase equilibration (VPE) technique, were measured at room temperature in the wave-number range 50–3500 cm^-1. Both 488 and 514.5 nm radiations were used to excite Raman scattering, A₁(TO) and E(TO) modes were recorded at backward scattering geometry. The results indicated that the lattice vibrational spectra of the as-grown Er:LiNbO₃ are almost the same as those of pure LiNbO₃ except for the little shift of the peak position and the change of relative intensity of some peaks. In comparison with the spectra of as-grown Er:LiNbO₃ crystal the vapor-phase equilibrated Er:LiNbO₃ and Er:Ti:LiNbO₃ crystals in the lattice vibrational region exhibit the following features: firstly, Raman peaks become narrow, indicating that the VPE process has brought Er:LiNbO₃ and Er:Ti:LiNbO₃ crystals closer to a stoichiometric composition; secondly, relative intensity of some peaks varies with the VPE time; and finally, slight blue shifting in peak position was observed. Some of these features were correlated with the NbO₆ octahedra and with the site distribution of the doped Er ions. In addition, green fluorescence peaks and/or bands associated with the electron transitions ² H _11/2?⁴ I _15/2 and ⁴ S _3/2?⁴ I _15/2 of the doped Er³⁺ were also observed. For 488 nm excitation they appear in the wavenumber range of 1200–3000 cm^-1 and are well separated from lattice vibrational region; for 514.5 nm excitation, however, these fluorescence peaks shift towards the low wavenumber region and overlap partially with the lattice vibrational spectra. Received: 24 May 2000 / Accepted: 29 May 2000 / Published online: 13 September 2000     

Effect of stresses on the OH libration bands in proton-exchanged LiNbO3 optical waveguides

i -H_xLi_1-xNbO₃ phases generated on x-cut LiNbO₃ the new anisotropic libration mode at 890–940 cm^-1 is observed in addition to the known isotropic mode at 960–970 cm^-1, whereas the same phases on z-cut exhibit only latter isotropic libration mode. The observed phenomena are explained as a result of anisotropic stresses generated in the x-cut waveguides, whereas the stresses in the structures on z-cut are isotropic. The OH-bond librations are clearly anisotropic in the α, κ₁, and κ₂ phases generated on both z- and x-cuts, which indicates a fundamental difference of the local symmetry of the proton’s position in the β_i phases compared to the α, κ₁, and κ₂ H_xLi_1-xNbO₃ phases. Received: 4 May 1998/Revised version: 26 June 1998     

Reconstruction of extraordinary refractive index profiles of optical planar waveguides with single or double modes fabricated by O ion implantation into lithium niobate

A method named intensity calculation method (ICM), which is based on beam propagation method (BPM) and image processing, was carried out to reconstruct the extraordinary refractive index profile (RIP) of single-mode planar waveguide in lithium niobate (LiNbO₃), which was fabricated by multi-energy megaelectron-volt (MeV) O²⁺ ion implantation. In addition, it has been proved reasonable that the alternation of extraordinary refractive index induced by ion implantation into LiNbO₃ is mainly due to the degradation of polarization and reduction of material physical density. As a result, the possible extraordinary RIP of the double-mode planar waveguide could be reconstructed using BPM according to such a hypothesis and the calculated guiding mode values. The end-fire coupling and m-line arrangements were carried out to obtain the near-field modal patterns and dark-mode spectra of waveguides, respectively.     

Compositional characterisation of Zn-diffused lithium niobate waveguides

Rutherford backscattering (RBS) and secondary-ion mass spectrometry techniques have been used to investigate the two-step process involved during waveguide fabrication in LiNbO₃ using Zn-vapour diffusion. Compositional analysis (O, Nb, Li and Zn) in the two steps has been characterised. RBS analysis reveals that the first step, involving a heating of the substrate under a metallic Zn atmosphere, gives rise to a partial exchange between the Nb and Li ions from the crystals and the Zn from the vapour source. The second treatment at higher temperature in an open atmosphere diffuses the Zn deeper into the substrate, thus forming an optical waveguide, while the Nb and Li ions recover their bulk values. Received: 16 May 2001 / Revised version: 7 September 2001 / Published online: 30 October 2001     

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     

Three-dimensional reversible laser micromachining with subnanojoule femtosecond pulses based on two-photon photochromism

Three-dimensional reversible laser micromachining of polymer materials based on two-photon photochromism is demonstrated. Unamplified 60-fs, 0.5-nJ pulses of 790-nm Ti:sapphire laser radiation are used to induce a refractive-index change in a polymethyl methacrylate (PMMA) sample doped with spiropyran molecules through a two-photon absorption process. Waveguides are written in the bulk of spiropyran-doped PMMA samples by scanning these samples with respect to a tightly focused Ti:sapphire laser beam. Laser-induced fluorescence is used for on-line monitoring of the laser-micromachining process. The structures written in photochromic samples can be erased and reconfigured due to the reversibility of the photochromic effect. Received: 23 April 2003 / Published online: 6 June 2003 RID="*" ID="*"Corresponding author. Fax: +7-095/939-51-74, E-mail: zheltikov@top.phys.msu.su     

Photorefractive LiNbO3 waveguides fabricated by combining He-implantation and copper exchange

We report on the fabrication and properties of He-implanted waveguides in lithium niobate, which are additionally doped with copper by using an ion-exchange process. We show that the photorefractive sensitivity in the waveguides is increased by a factor of 3000. Received: 23 November 1998 / Revised version: 15 February 1999 / Published online: 12 April 1999     

M-waveguide structures for direct phase matching in AlGaAs

In this paper the potential of M-waveguide structures for direct phase matching in AlGaAs is investigated by numerical simulations. Principal waveguiding characteristics are discussed. The impact of the optical layer thicknesses is analyzed with respect to phase matching and conversion efficiency for second harmonic generation (SHG). An optimization of the M-waveguide parameters yields a normalized conversion efficiency of 153 or 214 %/W respectively. Received: 16 May 2001 / Revised version: 2 August 2001 / Published online: 23 October 2001     

Raman characterization of H:LiNbO3 waveguides

In this work we present polarized Raman measurements of z-cut multimode H:LiNbO₃ waveguides in the κ_i- and β_i-phase states. A spectrum with unexpected form and behaviour, originating from the layer at the very surface, was observed in samples with various degrees of Li–H substitution. Possible reasons for the presence of this strange spectrum are considered. Received: 16 May 2001 / Revised version: 7 August 2001 / Published online: 23 October 2001     

Coherent thermal emission by microstructured waveguides

Following Greffet et al. [Coherent emission of light by thermal sources. Nature 2002;416:61-4], we study in this article, the possibility to engineer thermal coherent sources with waveguides. The idea is to rule a grating on a waveguide made of a slice of germanium deposited on a participating media such as glass. The guided waves, thermally excited are coupled to the far field by the grating and increase the system emissivity in certain directions and wavelengths. We numerically compute the diffraction of a plane wave by the grating by a rigorous coupled waves algorithm (RCWA). The reflected, transmitted and absorbed energy calculated allows to obtain the system emissivity by means of the Kirchhoff law.     

High-efficiency optical compression of Ti:sapphire laser pulses at 800 nm using a silver-coated hollow fiber

A 50 cm silver coated hollow fiber with inner diameter of 250 μm and filled with argon has been used to compress optical pulses from a Ti:sapphire laser at 800 nm. Input pulses with energy of 250 μJ and duration of 110 fs were used and compressed pulses with energy of 220 μJ and duration of 20 fs were generated by using a prism compressor. Numerical and experimental results are compared. There is good agreement between the measured beam diameters of the hollow-fiber output pulse and the calculated values obtained from propagation of the HE₁₁ mode into free space. For comparison, a similar uncoated fused-silica hollow fiber was also used to obtain 20 fs compressed pulses with an energy of 190 μJ. Received: 7 September 2002 / Published online: 26 March 2003 RID="*" ID="*"Corresponding author. Fax: +1-780/492-1811, E-mail: mohebbi@ee.ualberta.ca