Lithium-niobate-on-insulator (LNOI) is a new material platform for integrated optics allowing for small bending radii,
high intensities and superior electro-optical and nonlinear properties. Ridge waveguides of different width are fabricated
on pure and Er-doped LNOI substrates using diamond-blade dicing, resulting in smooth side walls with lower roughness
when compared to dry etching techniques. Propagation losses for polarized modes are measured by the Fabry-Perot
method using a fiber coupling setup and a tunable laser at 1.5 μm. Loss values as low as ~1.4dBcm-1 were obtained for quasi-TM (qTM) modes, while losses for qTE modes are slightly higher. Characterization of Er:LNOI ridges is
performed using Raman and fluorescence spectroscopy. Spectral scans are obtained using a scanning confocal
microscope and a 488nm laser. Besides line broadening that may be attributed to internal strain in the bonded layer and
implantation induced defects, analysis of Raman spectra shows no significant difference between waveguide and bulk
material. However, Er emission of 2H11/2 and 4S3/2 to 4I15/2 contains small spatial differences across the layer thickness when compared to Er-doped bulk samples. While Raman intensity has a linear relationship with pump power, the intensity of the Er emission starts saturating already at pump levels of a few mW. To investigate fluorescence of the 4I13/2–4I15/2 transition inside the diced ridges, a fiber-coupled laser with wavelength 980nm is used for pumping. The emission is broadened and maxima are shifted to longer wavelengths, which may be attributed to defects induced by implantation, strain induced by the bonded LN-SiO2 interface, and re-absorption of fluorescence light.
As optical components continue to replace electronics in ultrafast signal processing applications, a growing interest in
further miniaturization and integration of photonic devices on a single chip is observed. Therefore, optical waveguides of
high refractive index contrast of core and cladding materials are developed since a couple of years. They can have a very
small cross section and also bending radius, enabling the development of ultra-compact photonic integrated devices and
circuits. Silicon-On-Insulator (SOI) waveguides ("photonic wires") and devices are the most prominent examples.
A corresponding technology for Lithium Niobate-On-Insulator (LNOI) waveguides is still in its infancy, though LN
offers - in contrast to SOI - excellent electro-optic, acousto-optic, and nonlinear optical properties. Moreover, it can be
easily doped with rare-earth ions to get a laser active material. Therefore, LNOI photonic wires will enable the
development of a wide range of extremely compact, active integrated devices, including electro-optical modulators,
tunable filters, nonlinear (periodically poled) wavelength converters, and amplifiers and lasers of different types.
The state-of-the-art of LNOI films as platform for high-density integrated optics is reviewed. Using a full-wafer
technology (3" diameter), sub-micrometer thin LN films are obtained by high-dose He+ ion implantations,
crystal-bonding to a low-index substrate (preferably SiO2) and cleaving by a special annealing step ("ion-beam-slicing").
Various LNOI structures, also combined with metallic layers, are presented. Based on such platforms, photonic wires
and micro-photonic devices are developed using different micro- and nano-structuring techniques. To be specific, the
fabrication and characterization of LNOI photonic wires with cross-section < 1 μm2, and periodically poled LNOI
photonic wires for second harmonic generation are reported in detail.
The development of wafer-scale (3'' diameter) smart-cut lithium niobate (LN) single-crystal films of sub-micrometer
thickness is reported. Z-cut LN wafers, implanted by high energy He-ions, are crystal-bonded to a SiO2 layer on another
Z-cut LN handle sample. The bonded pair of samples splits along the He-implanted layer by appropriate annealing. As
this fabrication method is similar to the process widely used for silicon-on-insulator (SOI) fabrication, the resulting
material is called LNOI.
Two different routes to develop periodically poled LNOI photonic wires are discussed. The first one starts with poling of
planar LNOI samples; the photonic wires are fabricated afterwards by ICP-etching. The second one starts with the
fabrication of LNOI photonic wires; they are "locally" poled afterwards. As both approaches were not yet successful, a
PPLN-substrate was ion beam sliced to generate a planar periodically poled LNOI sample directly.
Using planar LNOI samples as starting material, high-quality photonic wires have been developed. The smallest
structure has a cross-section of ~ 1 x 0.7 μm2 only. Its optical properties with mode distributions, waveguide propagation
losses, and group index were investigated. Moreover, the first periodically poled LNOI photonic wires were successfully
fabricated, but not yet investigated optically. They are of great potential for second order nonlinear integrated optics.
Atmospheric absorption, scattering, and turbulence are impairments in practical high-speed free-space laser
communications. These atmospheric effects can be mitigated by choosing the proper transmission wavelength. It is
well known that the MWIR (~3.8 μm) has many low-absorption spectral lines suitable for low-loss propagation. Also,
MWIR can be more robust to turbulence in the weak-turbulence regime. Since high-speed laser transceivers are not
available in the MWIR, a 3.8-μm signal can be generated and detected using a 1.55-μm telecom transceiver via
wavelength conversion. Free-space transmission of optical homodyne RZ-QPSK through a turbulent channel at 3.8 μm
has been investigated. A pair of Ti:PPLN-based nonlinear wavelength converters were used to down- and up-convert
from 1.55 to 3.8 and back to 1.55 μm at the transmitter and at the homodyne receiver, respectively. The converted RZQPSK
signal was transmitted through a tabletop wind tunnel that produces a weak turbulent path. Comparison of 1.55
and 3.8 μm transmission through the wind tunnel shows that under weak-turbulence 3.8 μm transmission is more robust
than 1.55 μm. Under the same turbulence condition, the scintillation index measured at 3.8 μm is consistently lower
than that at 1.55 μm. Extrapolated scintillation indexes for 3.8 and 1.55 μm using the Rytov variance (~ λ-7/6 ) and
independent measurement at 632.8 nm are consistent with the RZ-QPSK scintillation data for 3.8 and 1.55 μm. Under
the most severe turbulence condition, the average bit-error-rate of 3.8-μm transmission is better than that of 1.55-μm
giving an estimated receiver sensitivity improvement of at least 6 dB.
In this paper, WDM transmission experiments are discussed showing simultaneous compensation of
nonlinear effects and chromatic dispersion through optical phase conjugation (OPC). The performance of
OPC and DCF for chromatic dispersion compensation are compared in a wavelength division multiplexed
(WDM) transmission link with 50-GHz spaced 42.8-Gb/s RZ-DQPSK modulated channels. The feasible
transmission distance for a Q-factor ~10 dB is limited to approximately 5,000 km and 3,000 km for the OPC
and the DCF based configuration, respectively. When the Q-factor as a function of the transmission distance
is observed, at shorter distances, the Q-factor of the OPC based configuration is about 1.5 dB higher than
that of the DCF based transmission system. Up to 2,500-km transmission a linear decrease in Q is observed
for both configurations. After 2,500-km transmission, the Q-factor of the DCF based configuration deviates
from the linear decrease whereas the OPC based performance is virtually unaffected.
We investigate the performance of two different all-optical wavelength conversion configurations: four-wave mixing in
highly nonlinear fiber and cascaded second harmonic and difference frequency generation in periodically poled Lithium
Niobate. Both configurations have the capability to convert phase-modulated signals with high data rates. Error free
wavelength conversion of up to 160 Gbit/s DPSK and 320 Gbit/s DQPSK data signals is demonstrated. The converter
using highly non-linear fiber can have advantages in network applications in which cascaded wavelength conversion are
requested due to its potentially higher conversion efficiency and OSNR. The Lithium Niobate converter generates no
phase distortion by wavelength conversion of phase-modulated signals. This could be useful for applications utilizing
PSK formats with 2 bit per symbol or more, like DQPSK or 8-PSK.
The propagation of solitons along the interface between two dielectric nonlinear media was investigated theoretically extensively in the 1980s but never realized experimentally. Recently we predicted that the required small index differences between the media and hence solitons can be created at the interface between continuous and periodic discrete media consisting of arrays of weakly coupled waveguides. Our theoretical analysis has predicted the existence of stable solitons with power thresholds both in the centre and at the edge of the Brillouin zone. We have observed both of these discrete surface solitons with power thresholds in both Kerr and quadratically nonlinear media. Spatial solitons with fields in neighboring channels either in phase or pi out of phase with one another have been identified.
A family of narrow linewidth integrated optical distributed Bragg reflector- (DBR-), distributed feedback- (DFB-), and DBR-/DFB-coupled cavity lasers with Er-doped LiNbO3 single-mode waveguide is reviewed. They have one or two photorefractive gratings in Fe-doped waveguide sections. Two types of DBR-lasers have been developed. The first type has a cavity consisting of one Bragg-grating in a Ti:Fe:LiNbO3 waveguide, a gain section with Ti:Er:LiNbO3 waveguide, and a multi-layer dielectric mirror deposited on one polished end face. The second DBR-cavity consists of two gratings in Ti:Fe:LiNbO3 on both sides of the Er-doped waveguide. Their power characteristics and spectral properties were investigated. Single-frequency operation could be achieved in the latter case at various wavelengths in the Er-band (1530nm <λ< 1575nm) with up to 1.12mW output power. A DFB-laser with two lowest-order modes has been demonstrated with a photorefractive grating in a Ti:Fe:Er:LiNbO3 waveguide; it is combined with an integrated optical amplifier on the same substrate. Moreover, an attractive DBR/DFB coupled cavity laser has been developed and investigated. Its cavity consists of a photorefractive Bragg grating in the Ti:Fe:Er:LiNbO3 waveguide section close to one end face of the sample, a Ti:Er:LiNbO3 gain section and a broadband dielectric multi-layer mirror of high reflectivity on the other end face.
The development of a whole family of near and mid-IR quasi- phase matched parametric frequency converters with periodically poled in Ti:(Er:)LiNbO3 waveguides is reviewed. Due to high quality waveguides with very low losses and excellent homogeneity unprecedented conversion efficiencies have been achieved for second-harmonic generation, difference-frequency generation, optical parametric fluorescence and doubly as well as singly resonant optical parametric oscillation.
A whole family of waveguide lasers (fixed frequency, acousto- optically tunable, modelocked, Q-switched) has been developed in Er-diffusion doped LiNbO3 substrates. By periodically poling the Ti:Er:LiNbO3 waveguides quasi-phase-matched nonlinear interactions can be achieved in the same structure. In this way the development of self-frequency doubling lasers, of laser/difference frequency generator combinations, and of optical parametric oscillators with intracavity pump laser becomes possible.
The state-of-the-art of Er-doped integrated optical lasers in LiNbO3 is reviewed. They are fabricated in Er- diffusion doped substrates with Ti-diffused channel guides of high quality. The laser resonators are formed by dielectric mirrors vacuum-deposited on the polished waveguide end faces. Five different types of Ti:Er:LiNbO3 waveguide lasers are presented.Among them are free running Fabry-Perot lasers of six different wavelengths in the range 153nm < (lambda) < 1610nm with a cw-output power up to 63mW. They have a shot noise limited relative intensity noise at frequencies above 50MHz. Tunable lasers have been developed by the intracavity integration of an acoustooptical amplifying wavelength filter yielding a tuning range up to 31nm. With an intracavity electrooptic phase modulator modelocked laser operation has been obtained with pulse repetition frequencies up to 10GHz; pulses of only a few ps width could be generated. With an intracavity amplitude modulator Q-switched laser operation has been achieved with up to 2.4W pulse peak power at 2kHz repetition frequency. Moreover, distributed Bragg reflector lasers of emission linewidth < 8kHz have been developed using a dry- etched surface grating as one of the mirrors of the laser cavity.
The recent development of Nd3+- and Er3+?doped waveguide amplifiers and lasers in LiNbO3 with proton-exchanged or Ti-diffused channels is reviewed. Besides rare earth doped bulk crystals, also initially undoped substrates have been used for device fabrication. The latter were doped by ion-implantation, followed by thermal annealing, or by indiffusion of an evaporated (photolithographically defined) metallic (Er-) layer. Low threshold (a few mW of absorbed pump power), single transversal mode waveguide lasers of about 1 cm length and medium-gain (up to 7.5 dB) optical amplifiers have been developed in several versions in the Nd-doped material for ? = 1.08/?m (emission) wavelength. With Er a low-gain, but broadband optical amplifier for the wavelength range 1.53mm < ? < 1.62 ?m has been demonstrated as well as a 1 cm long laser of ? = 1.532 ?m emission wavelength.
We report Erbium-doped Ti-indiffused single-mode optical waveguides of good quality in LiNbO3 fabricated by Erimplantation followed by annealing. Besides ground state absorption also xcited-tate-bsorption (ESA) has been investigated indicating that even at 1. 48 pm pump wavelength ESA occurs as a three stage process. The guided-wave spontaneous fluorescence was investigated. In a " pump and probe" experiment for the first time wavelength ranges could be identified where the stimulated emission overcomes the Er-absorption.
To allow an extremely sensitive measurement of optically induced changes of the index of refraction in a Ti:LiNbO3 stripe waveguide a specific two wavelengths " excite and probe" technique has been recently developed [1J taking advantage of the large phase sensitivity of a waveguide resonator. A resolution of Ln 5 x iO has been achieved which is nearly two orders of magnitude better than demonstrated with conventional methods. For both polarizations and for several wavelengths in the visible and near infrared the most important parameters characterizing the photorefractive effect could be determined. They were evaluated by analyzing the measured index changes using a widely accepted theoretical model adopted to the waveguide geometry.
A periodic, electric field induced reversal of ferroelectric microdomains on (+Y)- and (—Y)- LiNbO3 surfaces was achieved for the first time at temperatures (< 400 °C) well below the Curie point. Contrary to the recently developed domain inversion processes on Z-cut substrates, no modification of the material (e.g. by a Ti-doping) nor an electron bombardment was necessary. The pyroelectric effect was used to generate an electrical field between photolithographically structured, periodic Al-electrodes on the surface of the crystal concentrating the electric field in the small volume of the microdomains to be inverted. As (+Y)- and (—Y)-LiNbO3 faces have strongly different etching rates in HF/HNO3, a successful domain reversal could be identified by selective etching. Microdomains of different dimensions (of typical 6x 15xO.25tm3), number of periodicities (100 and 1500) and periodicities (3Opm, 6.5,um and 7.6im) have been fabricated not only in undoped LiNbO3 surface layers, but also in Ti:LiNbO3 optical strip waveguides. The chosen periodicities will allow quasi-phase matched optical second harmonic generation.