Lithium Niobate Channel Waveguides Research Articles

Channel Waveguides in Lithium Niobate and Lithium Tantalate.

Low-loss photonic waveguides in lithium niobate offer versatile functionality as nonlinear frequency converters, switches, and modulators for integrated optics. Combining the flexibility of laser processing with liquid phase epitaxy we have fabricated and characterized lithium niobate channel waveguides on lithium niobate and lithium tantalate. We used liquid phase epitaxy with K2O flux on laser-machined lithium niobate and lithium tantalate substrates. The laser-driven rapid-prototyping technique can be programmed to give machined features of various sizes, and liquid phase epitaxy produces high quality single-crystal, lithium niobate channels. The surface roughness of the lithium niobate channels on a lithium tantalate substrate was measured to be 90 nm. The lithium niobate channel waveguides exhibit propagation losses of 0.26 ± 0.04 dB/mm at a wavelength of 633 nm. Second harmonic generation at 980 nm was demonstrated using the channel waveguides, indicating that these waveguides retain their nonlinear optical properties.

Design and Optimization of Proton Exchanged Integrated Electro-Optic Modulators in X-Cut Lithium Niobate Thin Film

In this study, we designed, simulated, and optimized proton exchanged integrated Mach-Zehnder modulators in a 0.5-μm-thick x-cut lithium niobate thin film. The single-mode conditions, the mode distributions, and the optical power distribution of the lithium niobate channel waveguides are discussed and compared in this study. The design parameters of the Y-branch and the separation distances between the electrodes were optimized. The relationship between the half-wave voltage length production of the electro-optic modulators and the thickness of the proton exchanged region was studied.

Waveguided Approach for Difference Frequency Generation of Broadly-Tunable Continuous-Wave Terahertz Radiation

The 1–10 terahertz (THz) spectral window is emerging as a key region for plenty of applications, requiring not yet available continuous-wave room-temperature THz spectrometers with high spectral purity and ultra-broad tunability. In this regard, the spectral features of stabilized telecom sources can actually be transferred to the THz range by difference frequency generation, considering that the width of the accessible THz spectrum generally scales with the area involved in the nonlinear interaction. For this reason, in this paper we extensively discuss the role of Lithium Niobate (LN) channel-waveguides in the experimental accomplishment of a room-temperature continuous wave (CW) spectrometer, with μW-range power levels and a spectral coverage of up to 7.5 THz. To this purpose, and looking for further improvements, a thought characterization of specially-designed LN waveguides is presented, whilst discussing its nonlinear efficiency and its unprecedented capability to handle high optical power (107 W/cm2), on the basis of a three-wave-mixing theoretical model.

Low-loss bent channel waveguides in lithium niobate thin film by proton exchange and dry etching

We propose and demonstrate an efficient method combining proton exchange with dry etching for the fabrication of low-loss bend channel waveguides in lithium niobate (LN) thin film. Our proposed method introduces the chemical etching caused by F+ ion to increase the etching rate. Our fabricated straight and bent channel waveguides have a trapezoid cross section with a top width of ~1.0 µm, a height of ~900 nm, and a slope of ~20° with respect to the vertical direction. To the best of our knowledge, this is the largest etching depth but with a small slope reported up to now. Mode intensity distributions and insertion losses were measured at 1.55 µm wavelength and bending losses were deduced. The results show that our fabricated bent channel waveguide with a radius of 20 μm can achieve low bending losses of 0.455 dB/90° and 0.488 dB/90° for the fundamental quasi-TE (qTE) and quasi-TM (qTM) modes, respectively. Compared with the fabrication methods reported so far, our method can realize a faster etching rate and a larger etching depth while maintaining a high etching quality.

Amplification combination of Er3+ and Tm3+ emissions in titanium-diffused lithium niobate strip waveguide

Erbium(Er)/thulium(Tm)-codoped active lithium niobate(LiNbO3) channel waveguides were fabricated by titanium diffusion following erbium/thulium co-diffusion doping in an X-/Z-cut LiNbO3 substrate with an initial congruent/Li-deficient composition. We show that the waveguide is in an initial congruent or Li-deficient composition environment, remains LiNbO3 crystalline phase, is monomode at the 1.5 µm wavelength region under either the transverse electric or the transverse magnetic polarization, and has a loss < 0.5 dB/cm. The X-cut/Z-propagation scheme can effectively suppress the photorefractive effect, while the Z-cut/Y-propagation scheme cannot. Codoping with thulium effectively promotes the resistance to optical damage. An optical gain of 0.8/0.1 dB/cm is attained for the 1547(erbium)/1450(thulium) nm small-signal, showing that erbium/thulium codoping enables to combine the emissions of the two ions and realize the S+C+L broadband amplification or lasing by using 980 and 795 nm laser diodes as the pump sources.

Effect of Waveguide Inhomogeneity in a χ(2)-Based Pulsed Optical Parametric Amplifier

The effect of waveguide inhomogeneity in transmitted pulses of a χ(2)-based optical parametric amplifier is investigated. Periodically poled lithium niobate (PPLN) channel waveguides are used to achieve optical parametric amplification (OPA) via cascaded second harmonic generation and difference frequency generation (cSHG/DFG) process at telecommunication bandwidth. To access high input power, a Q-switched laser of low duty cycle was used for the fundamental source. The ns-pulsed fundamental generates the pump for the DFG (OPA) process via SHG. The signal amplification and the wavelength converted idler generation are achieved via DFG process between the SH and the signal. It is shown that inhomogeneity of the waveguide leads to change in the relative phase between interacting waves along the waveguide. Consequently, transmitted pulses are disturbed for an inhomogeneous waveguide. Experimental and theoretical results of transmitted pulses from a homogeneous and an inhomogeneous PPLN channel waveguides are presented. In the homogeneous channel guide with Gaussian-shaped transmitted pulses, more than 20 dB of parametric gain was achieved in an 88-mm-long waveguide when ∼1.7 W of coupled fundamental peak power is launched to the waveguide.

Enhanced electro-optic response in domain-engineered LiNbO3 channel waveguides

Substantial enhancement (36.7%) of the intrinsic electro-optic coefficient (r33) has been observed in lithium niobate channel waveguides, which are made to overlap with a pole-inhibited ferroelectric domain. The waveguide and the overlapping ferroelectric domain are both produced by a single UV irradiation process and are thus self-aligning. The enhancement of the electro-optic coefficient effect is attributed to strain, which is associated with the ferroelectric domain boundaries that contain the channel waveguide.

Time and Temperature Dependent Study of Zn and Ni Codiffused LiNbO&lt;sub&gt;3&lt;/sub&gt; Waveguides

Zn and Ni codiffused lithium niobate planar and channel waveguides are fabricated at various temperatures for different times. Compositional and optical properties of the waveguides are studied. Single-mode channel waveguides are measured for 2-D refractive index profiling using a modified end-fire coupling method at excitation wavelengths of both 633 and 1550 nm. Planar waveguides with the same metallic thicknesses as those of channel waveguides are analyzed using secondary-ion-mass-spectrometry to obtain the diffused metal ion concentration profiles. The relationship between the diffusion depths and index depths is discussed based on the completeness of the diffusion process.

Theoretical analysis of electromagnetic field distribution and Cerenkov second harmonic generation conversion efficiency based on lithium niobate ion-implanted channel waveguide

Cerenkov type second harmonic generation (CSHG) is proposed in ion-implanted Z-cut lithium niobate (LiNbO3) channel waveguides. We have obtained exact analytical solutions for the field distribution and the power of second-harmonic waves. Conditions for attaining higher conversion efficiency are discussed on the basis of numerical calculations.

Theoretical analysis of the TE mode Cerenkov type second harmonic generation in ion-implanted X-cut lithium niobate channel waveguides

Optical second harmonic generation (SHG) in the form of Cerenkov radiation from ion-implanted lithium niobate (LiNbO 3) channel waveguides is analyzed by directly resolving the wave equations. Useful formula of the SHG efficiency is derived and expressed in terms of waveguide parameters. Numerical examples are plotted for LiNbO 3 crystals. The results enable the optimization of waveguide design for efficient second harmonic generation in the Cerenkov configuration.

Electro-optic guided-to-radiation mode conversion in annealed proton-exchanged PPLN waveguides

We report the design and experimental demonstration of electro-optically active TM-guided to TE-radiation mode converters in annealed proton-exchanged (APE) periodically poled lithium niobate (PPLN) channel waveguides in telecom S-C-L bands (1495-1640 nm). A maximum mode conversion efficiency of >95%/cm was obtained at 1520 nm from a 24-μm-period APE PPLN waveguide under an electro-optic (EO) field of ~6.3 V/μm at 35°C. This efficiency has been enhanced by a factor of >4.6 over a waveguide built in the single-domain (unpoled) LiNbO3; it is also to the best of our knowledge the most efficient guided-to-radiation (GTR) mode converter ever reported based on LiNbO3 on-axis waveguides. A conversion bandwidth of ~250 nm was also observed from this EO GTR mode converter.

Periodically Poled Titanium-Diffused Lithium Niobate Wavelength Filters Utilizing Šolc-Type TE↔TM Mode Converters

Šolc-type wavelength filters have been demonstrated in a 52-mm long periodically poled titanium-diffused lithium niobate channel waveguide with a domain period of 16.6 μm. At room temperature, the center wavelength and the full-width at half maximum of the filter were about 1,272.49 nm and 0.23 nm, respectively. The nearest side-lobe is about 7 dB. New structures of flat-top bandpass filters and an optical add/drop multiplexer utilizing Šolc-type TE↔TM mode converters are proposed for the first time.

Laser-induced-forward-transfer: a rapid prototyping tool for fabrication of photonic devices

We propose a rapid prototyping method for the fabrication of optical waveguides based on the direct laser-printing method of ultrafast Laser-Induced Forward Transfer (LIFT) followed by further processing. The method was implemented for the fabrication of titanium in-diffused lithium niobate channel waveguides and X-couplers by LIFT-depositing titanium metal followed by diffusion. Propagation loss as low as 0.8 dB/cm was measured in preliminary experiments.

Šolc-Type Wavelength Filters Based on TE↔TM Mode Conversion Utilizing Periodically Poled Ti-Diffused Lithium Niobate Channel Waveguides

We have demonstrated the Šolc-type wavelength filters in a 52 mm long periodically poled Ti-diffused lithium niobate channel waveguide which has a domain period of 16.6 μm. At room temperature, the center wavelength and the full-width at half maximum of the filter were about 1272.49 nm and 0.23 nm, respectively. The nearest side-lobe is about 7 dB. New structure of optical add/drop multiplexer (OADM) utilizing Šolc-type TE↔TM mode converters was proposed for the first time.

Inverse Algorithm With Built-In Spatial Filter to Obtain the 2-D Refractive Index Profile of Optical Waveguides From the Propagating Mode Near-Field Profile

We propose a modified inverse regularization algorithm for computing the 2-D refractive index profiles of integrated-optic waveguides from measured near-field data. The algorithm involves embedding of a spatial mean filter to remove impulses and high frequency noises from the near-field intensity as well as from its second derivative. The proposed filtering scheme is built-in to the inverse algorithm and does not require extra computation for filtering. The effectiveness of the proposed method is demonstrated by reconstructing the refractive index profile of a single mode lithium niobate channel waveguide from the propagating mode near-field intensity pattern.

Optimisation of the proton-exchange technology for fabricating channel waveguides in lithium niobate crystals

We have studied the effect of process parameters on the mode profile in lithium niobate channel waveguides produced by low-temperature proton exchange. A model has been proposed for the processes underlying waveguide formation, and has been used to numerically calculate the mode profile for various fabrication parameters. We have identified low-temperature proton exchange conditions which ensure high electro-optical performance of the waveguides and stability towards fabrication errors. The obtained results may be helpful in optimising integrated optical devices and matching them to various types of optical fibres.

Mode-selective coupler for wavelength multiplexing using LiNbO3:Ti optical waveguides

Abstract A mode-selective directional coupler based on titanium in-diffused channel waveguides in lithium niobate is investigated. This coupler may be utilized as a key part of an add-drop multiplexer for dense wavelength division multiplexing in optical network nodes. The proposed coupler is based on evanescent coupling of the fundamental mode of a single-mode channel to the first higher mode of a parallel bi-modal waveguide. Our experimental results show that a compact directional coupler with coupling efficiencies larger than 90%, large bandwidth around 1550 nm, and with negligible polarization dependence can be realized using electro-optic lithium niobate substrates.

Lithium–Niobate Channel Waveguide for the Realization of Long-Period Gratings

We propose a special lithium-niobate (LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) single-mode waveguide for the realization of long-period gratings, which consists of a channel core embedded in a thin slab cladding. We fabricated the waveguide on a z-cut LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> substrate with a two-step proton-exchange process and demonstrated its suitability for grating application with a number of removable photoresist long-period gratings deposited on the waveguide surface. The waveguide fabrication process and the LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> waveguide structure could be further explored for the development of electrooptic gratings for high-speed applications.

Guided-wave frequency doubling in surface periodically poled lithium niobate: competing effects

We carried out second-harmonic generation in quasi-phase-matched \alpha-phase lithium niobate channel waveguides realized by proton exchange and surface periodic poling. Owing to a limited ferroelectric domain depth, we could observe the interplay between second-harmonic generation and self-phase modulation due to cascading and cubic effects, resulting in a nonlinear resonance shift. Data reduction allowed us to evaluate both the quadratic nonlinearity in the near infrared as well as the depth of the uninverted domains.

Light-enhanced electro-optic spectral tuning in annealed proton-exchanged periodically poled lithium niobate channel waveguides

We report the observation of light-enhanced electro-optic spectral tuning in annealed proton-exchanged, asymmetric domain-duty-cycle periodically poled lithium niobate (PPLN) channel waveguides for second-harmonic generation. The spectral tuning rate was increased rapidly from 0.07 nm/(kV/mm) to a saturated value of 0.32 nm/(kV/mm) in a 30%/70% domain-duty-cycle PPLN waveguide when the fundamental pump power near 1534 nm was increased from 0.6 to 46 mW. The second-harmonic laser power at 767 nm was identified to be the source enhancing the spectral tuning.