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     

Frequency-doubling in self-induced waveguides in lithium niobate

Efficient frequency-doubling is experimentally demonstrated in presence of beam self-trapping in congruent lithium niobate crystal. The self-trapping is induced by the generated second harmonic beam via photorefractive effect under an external applied field. The local space charge field distribution, formed by the second harmonic beam, is shown to efficiently trap both wavelengths. The dynamics of self-focusing is studied along with the power evolution of the second harmonic beam. Fast tuning of phase matching conditions in the written waveguide is realized by an externally applied voltage also used for the photorefractive confinement.     

Spatial modulational instability in one-dimensional lithium niobate slab waveguides

We report an experimental study of the breakup through modulational instability of broad fundamental beams near the phase-matching condition for second-harmonic generation in lithium niobate slab waveguides. Two mechanisms for initiating modulational instability, waveguide imperfections and noise on the input beam, are identified.     

Low-loss chalcogenide waveguides on lithium niobate for the mid-infrared

We demonstrate low-loss chalcogenide (As(2)S(3)) waveguides on a LiNbO(3) substrate for the mid-IR wavelength (4.8 μm). Designed for single-mode propagation, they are fabricated through photolithography and dry-etching technology and characterized on a mid-IR measurement setup with a quantum cascade laser. For straight waveguides, propagation loss as low as 0.33 dB/cm is measured and low-loss bends on the order of 100 μm are simulated, with measurement results showing <3 dB for a 250 μm bend radius. The coupling efficiency is estimated to be 81%. In addition, the influences of variations in width and bend radius are also investigated.     

Low-threshold spatial solitons in reverse-proton-exchanged periodically poled lithium niobate waveguides

Combining reverse proton exchange and uniform periodic poling in LiNbO3 planar waveguides, we demonstrate low-energy spatial optical solitons by second-harmonic generation at room temperature, with a threshold as low as 23 pJ/microm at 1.5 microm.     

Efficient second harmonic generation in silicon covered lithium niobate waveguides

We theoretically propose a hybrid lithium niobate(LN) thin-film waveguide that consists of an amorphous silicon stripe and etch-free z-cut LN for highly efficient wavelength conversion, circumventing the challenging etching on LN material.Profiting from the spatial symmetry breaking of the waveguide, the asymmetric hybrid modes can spontaneously achieve phase matching with small modal area and large spatial mode overlap, enabling enhanced second harmonic generation with a normalized conversion efficiency over 3900% W~(-1)· cm~(-2)(0.5-mm-long propagation distance). The choice of integrating silicon with LN alleviates the fabrication challenge, making the platform potentially compatible with silicon photonics.     

Generation of octave-spanning spectra inside reverse-photon-exchanged periodically poled lithium niobate waveguides

We demonstrate simultaneous octave-level spectral broadening and carrier-envelope-offset sensing of mode-locked Er- and Yb-doped femtosecond fiber lasers using constant-period and chirped reverse-proton-exchanged periodically poled lithium niobate waveguides. Chirped quasi-phase-matching gratings greatly improve spectral broadening of Yb-fiber lasers.     

Formation of dark spatial solitons in planar ion-implanted lithium niobate waveguides

The possibility of formation of dark photovoltaic spatial solitons in ion-implanted LiNbO₃:Cu-based planar waveguides by microwatt light beams has been investigated.     

Highly efficient polarization-entangled photon source using periodically poled lithium niobate waveguides

In our experiment, two channels of a 3 cm long periodically poled lithium niobate (PPLN) waveguide are used to generate the nondegenerate spontaneous parametric down conversion (NSPDC) photon pairs. We experimentally obtained the highly efficient polarization-entangled photon pairs by coherently combining two NSPDC processes, and keeping the phase stable over 30 min. We can also prepare two entangled photon pairs with different central wavelengths with the help of wavelength division multiplex.     

One-dimensional spatial soliton families in optimally engineered quasi-phase-matched lithium niobate waveguides

The advantage for quadratic soliton generation of engineering the quasi-phase-matching period near the input of lithium niobate slab waveguides is demonstrated. This approach allows members of one-dimensional quadratic soliton families with different values of the wave-vector mismatch to be cleanly excited and to be characterized by quantitative intensity-profile measurements of both the fundamental and the second-harmonic soliton components.     

Ultrabroadband tunable continuous-wave difference-frequency generation in periodically poled lithium niobate waveguides

We report, for what is the first time to our knowledge, widely tunable mid-IR light generation over a range of greater than 1000 nm in the 4 microm region by employing a single quasi-phase-matched periodically poled niobate waveguide at room temperature. The waveguide we used was fabricated by annealed proton exchange based on periodically poled lithium niobate. A peak conversion efficiency of 10%/W and a linewidth as small as 37 MHz were achieved. The developed mid-IR light generator may find wide applications in trace gas detection of multiple atmospheric species and high-resolution spectroscopy.     

Singly resonant optical parametric oscillation in periodically poled lithium niobate waveguides

We report quasi-phase-matched singly resonant optical parametric oscillation in electric-field-poled lithium niobate waveguides. Parametric gains as high as 250%/W, an oscillation threshold of 1.6 W (peak), idler output powers of 220 mW, and a tuning range of 1180-2080 nm for pump wavelengths of 756-772 nm have been observed. Pump depletion is limited to 40% because of the multiple launched transverse modes at the pump wavelength. We predict that fully optimized waveguide singly resonant oscillators can have thresholds of ~100 mW, accessible to cw diode pumping.     

Photonic band gap grating in He+-implanted lithium niobate waveguides

In this work, we report the investigation of 2D photonic crystals in lithium niobate associated with waveguiding structures fabricated by He⁺ implantation. From the material point of view, the investigation of PBG structures in lithium niobate is of great interest for optoelectronics technology since this crystal is one of the most important materials widely used in integrated and non linear optics. The choice of the implantation technique to produce the waveguide is motivated by the possibility of having both Transverse Electric (TE) and Transverse Magnetic (TM) guided modes in order to obtain a total photonic band gap (PBG).     

Aging effect in proton-exchanged optical waveguides based on lithium niobate crystals

The optical characteristics of planar waveguides produced by proton exchange in lithium niobate crystals in different acids (with and without additives) are unstable with time (“aging” effect). The aging effects in LiNbO₃ optical waveguides are investigated at room and elevated (50, 60, or 70°C) temperatures. In all cases, variations in the refractive index of the waveguide layer with time, i.e., for two or three years at room temperature and for several weeks at elevated temperatures, are determined for waveguides prepared in different media. The dependences of the optical characteristics of waveguides on the nature of proton sources and the aging conditions are obtained experimentally.     

Two-spatial-mode parametric amplifier in lithium niobate waveguides with asymmetric Y junctions

For optical parametric amplifiers in proton-exchange waveguide devices reported to date, both the signal and the idler photons are in the TM00 mode, causing difficulty in distinguishing signal from idler. We report a two-mode optical parametric amplifier. The key components, asymmetric Y junctions, successfully launch the pump wave in a pure TM10 mode and separate the signal and the idler in the TM10 and TM00 modes with a power ratio of as much as 27.5 dB. With high parametric gain, optical parametric generation with a threshold of 300 pJ/pulse was demonstrated for 1.8 ps long pump pulses near 780 nm. These periodically poled lithium niobate waveguides are efficient integrable photon sources.     

Metastable phases in proton-exchanged waveguides on an X cut of lithium niobate

The formation and decomposition of high-temperature phases in proton-exchanged waveguide layers of H_xLi_{1 ? x} NbO₃ on an X cut of lithium niobate are studied using IR spectroscopy and x-ray diffraction. It is shown that the phase transitions that occur in the proton-exchanged layers between the high-temperature phases fixed by quenching from T = 200°C and the phase that is equilibrium at room temperature are accompanied by changes in the frequencies and integrated intensities of the spectral components of the absorption bands characterizing OH groups and hydrogen bonds in the crystal. These changes imply two paths of proton redistribution during these completely reversible phase transitions: (i) transfer of some protons from substitutional sites to interstitial sites and (ii) proton transfer between substitutional sites having different ionic environments. The phase transition from the phase that is equilibrium at room temperature into a high-temperature phase is found to be accompanied by a more than twofold increase in strain in the H_xLi_{1 ? x} NbO₃ layer lattice (x ≈ 0.50). The experimental data obtained confirm complete reversibility and the diffusionless character of these phase transitions.     

Fast writing of soliton waveguides in lithium niobate using low-intensity blue light

We experimentally demonstrate the writing of soliton waveguides in lithium niobate with blue violet light, at λ?=?405?nm. This wavelength, obtained from a low-cost laser diode, allows very fast writing of soliton waveguides when comparing with the writing process using green light (532?nm). For the same writing time, there is a decrease of?~4 orders of magnitude of the light intensity necessary for writing soliton waveguides in blue. The writing process is investigated and characterized by tuning parameters, such as beam intensity, external field and beam polarization. The IR guiding capability of these soliton waveguides is tested by guiding femtosecond laser pulses at λ?=?1,550?nm.     

Direct ultraviolet writing of channel waveguides in congruent lithium niobate single crystals

We report the fabrication of optical channel waveguides in congruent lithium niobate single crystals by direct writing with continuous-wave ultraviolet frequency-doubled Ar+ laser radiation (244 nm). The properties and performance of such waveguides are investigated, and first results are presented.     

Simulation of refractive index profiles for titanium indiffused lithium niobate channel waveguides

A generally applicable method to simulate the two-dimensional profiles of Ti-concentration and refractive index of Ti:LiNbO₃, waveguides is presented. The influence of the fabricational parameters on the refractive index profiles has been studied in detail. The model parameters are extracted from the available experimental data. The computed values of surface refractive index change agree closely with the experimental results. The model is also extended for coupled waveguides.