Er-Doped Integrated Optical Devices in LiNbO3

The state-of-the-art of Er-doped integrated optical devices in LiNbO3 is reviewed starting with a brief discussion of the technology of Er-indiffusion. This technique yields high-quality waveguides and allows a selective surface doping necessary to develop optical circuits of higher complexity. Doped waveguides have been used as single- and double-pass optical amplifiers for the wavelength range 1530 nm < < 1610 nm. If incorporated in conventional, lossy devices loss-compensating or even amplifying devices can be fabricated. Examples are an electrooptically scanned Ti:Er:LiNbO3 waveguide resonator used as an optical spectrum analyzer and an acoustooptically tunable filter used as a tunable narrowband amplifier. Different types of Ti:Er:LiNbO3 waveguide lasers are presented. Among them are free running Fabry–Perot lasers for six different wavelengths with a conitnuous-wave (CW)-output power up to 63 mW. Tunable lasers could be demonstrated by the intracavity integration of an acoustooptical amplifying wavelength filter yielding a tuning range up to 31 nm. With intracavity electrooptic phase modulation modelocked laser operation has been obtained with pulse repetition frequencies up to 10 GHz; pulses of only a few ps width could be generated.With intracavity amplitude modulation Q-switched laser operation has been achieved leading to the emission of pulses of up to 2.4 W peak power (0.18 J) at 2 kHz repetition frequency. Distributed Bragg reflector (DBR) lasers of emission linewidth 8 kHz have been developed using a dryetched surface grating as one of the mirrors of the laser resonator. Finally, as an example for a monolithic integration of lasers and extracavity devices on the same substrate, a DBR-laser/modulator combination is presented

Selectively Erbium Doped Titanium Diffused Optical Waveguide Amplifiers in Lithium Niobate

Selectively erbium (Er) doped titanium (Ti) in-diffused optical waveguide amplifiers on lithium niobate (LiNbO3) substrate have been fabricated and characterized in the wavelength regime around λ = 1.53μm using counter-directional pumping at λP = 1.48μm. LiNbO3 waveguide amplifiers are desirable for providing gain in optical circuit chips through integration with other optical elements on a single substrate. A prerequisite for achieving useful gain rests on the optimization of overlap between the incident guided optical signal mode distribution and the evolving emission from excited Er ions. The extent of overlap can be controlled by adjusting fabrication parameters. Fabrication parameters for Er-doped Ti in-diffused waveguide amplifiers of useful optical gain have been optimized by diffusing selective patterns of vacuum-deposited 17nm-thick erbium film at 1100˚C for 100 hours into LiNbO3, and integrating with 7μm-wide single mode straight channel waveguides formed by diffusing 1070Å thick titanium film into the LiNbO3. Small-signal gain characterization was carried out with a -30 dBm of transmitted input signal power at λS=1531nm with counter-directionally launched pump power ranging between 0 to 119mW at λP=1488nm, using TM polarization for both the signal and pump beams. At a maximum launched pump power of 119mW, a signal enhancement of 8.8dBm for 25mm-long erbium doped region, and 11.6dBm for 35mm-long erbium doped region were obtained. The corresponding calculated net gain values are 1.8dB and 2.8dB, for the 25mm-long and 35mm-long Er-doped regions, respectively

Gain Improvement of Er-doped Amplifiers for the Feedback Filters

The combination of the arsenic trisulfide (As2S3) waveguide and titanium diffused lithium niobate (Ti:LiNbO3) waveguide provide us compact and versatile means for transmitting and processing optical signals, which benefits from the high index contrast between these two materials and the electro-optical properties of Ti: LiNbO3. Furthermore, waveguide gain is introduced through selective surface erbium (Er) doping which yields high quality loss-compensated or even amplifying waveguides without disturbing the excellent electrooptical, acoustooptical and nonlinear properties of the waveguide substrate LiNbO3. The integration of these waveguides allows the development of a whole class of new waveguide devices of higher functionality and complexity. As one kind of the hybrid waveguide devices, a new configuration consisting of an As2S3 channel waveguide on top of an Er doped titanium diffused x-cut lithium niobate waveguide has been investigated by simultaneous analytical expressions, numerical simulations, and experimentation. Both simulation and experimental results have shown that this structure can enhance the optical gain, as predicted by the analytical expressions. An As2S3 channel waveguide has been fabricated on top of a conventional Er:Ti:LiNbO3 waveguide, where the higher refractive index As2S3 waveguide is used to pull the optical mode towards the substrate surface where the higher Er concentration yields an improved propagation gain. The relationship between the gain and As2S3 layer thickness has been evaluated and the optimal As2S3 thickness was found by simulation and experimentation. Side integration was applied to reduce the extra propagation loss caused by the titanium diffusion bump. The propagation gain (dB/cm) has been improved from 1.1 to 2 dB/cm. Another hybrid device which combines the As2S3 and LiNbO3 is to make an As2S3 racetrack ring resonator on top of an x-cut y-propagation Er:Ti:LiNbO3 waveguide which is the potential structure for integrated lossless all-path filter. The ring was side-coupled with the Ti:LiNbO3 waveguide and the optical gain was achieved when the 5mm long coupling region where has been diffused with Er in advance pumped by 144mW pump laser. The free spectral range (FSR) of the measured ring response for TM mode is 0.0587nm (7.33GHz) at 1550nm. The roundtrip loss are 4.4dB (2.60dB/cm) when pump on and 5.8dB (3.44dB/cm) when pump off. The optical gain in the Er diffused area is 0.72dB/cm

Ion-exchange in Glasses and Crystals: from Theory to Applications

Since its first observation in 1850, ion-exchange (IEx) has become a fundamental process in many applications involving water treatment, catalysis, chromatography, and the food and pharmaceutical industries. Starting from the early 1900s, another relevant application of IEx has been in the glass industry, with the surface tempering of glass produced by a K+–Na+ ion exchange. Nowadays, photonics has greatly exploited IEx technology: graded-index microlenses, graded-index fibers and integrated optical waveguides and devices are examples of achievements made possible by the IEx process. Moreover, ion-exchange is possible in ferroelectric crystals, too, and has been fundamental for the development of many linear and nonlinear integrated optical devices in lithium niobate and tantalate.This volume collects articles published in the corresponding Special Issue of the Applied Sciences journal. Four review articles, written by internationally renowned experts in this field, provide complementary overviews of the history, fundamental aspects, designs and fabrications of devices, and technological achievements. Three articles describe original research in the fields of diffraction grating, photo-thermo-refractive glasses, and Yb-doped lithium niobate. This volume constitutes a valuable and updated reference for all students and researchers wishing to improve their knowledge and/or make use of ion-exchange technology and its applications

The Hybrid Integration of Arsenic Trisulfide and Lithium Niobate Optical Waveguides by Magnetron Sputtering.

It is well known that thermally evaporated a-As2S3 thin films are prone to oxidation when exposed to an ambient environment. These As2O3 crystals are a major source of scattering loss in sub-micron optical integrated circuits. Magnetron sputtering a-As2S3 not only produces films that have optical properties closer to their equilibrium state, the as-deposited films also show no signs of photo-decomposed As2O3. The TM propagation loss of the as-deposited As2S3-on-Ti:LiNbO3 waveguide is 0.20 plus/minus 0.05 dB/cm, and it is the first low loss hybrid waveguide demonstration. Using the recipe developed for sputtering As2S3, a hybrid Mach-Zehnder interferometer has been fabricated. This allows us to measure the group index of the integrated As2S3 waveguide and use it in the study of the group velocity dispersion in the sputtered film, as both material dispersion and waveguide dispersion may be present in the system. The average group index of the integrated As2S3 waveguide is 2.36 plus/minus 0.01. On-chip optical amplification was achieved through thermal diffusion of erbium into X-cut LiNbO3. The net gain measured for a transverse magnetic propagation mode in an 11 μm wide Er:Ti:LiNbO3 waveguide amplifier is 2.3 dB plus/minus 0.1 dB, and its on-chip gain is 1.2 plus/minus 0.1 dB/cm. The internal gain measured for a transverse electric propagation in an 7 μm wide Er:Ti:LiNbO3 waveguide amplifier is 1.8 dB plus/minus 0.1 dB and is among the highest reported in the literature. These gains were obtained with two 1488 nm lasers at a combined pump power of 182mW. In order to increase further the on-chip gain, we have to improve the mode overlap between the pump and the signal. This can be done by doping erbium into As2S3 film using multi-layer magnetron sputtering. The Rutherford backscattering spectroscopy shows that the doping of Er:As2S3 film with 16 layers of erbium is homogeneous, and Raman spectroscopy confirms no significant amount of Er-S clusters in the sputtered film. The deposition method was used to fabricate an Er:As2S3 waveguide, and the presence of active erbium ions in the waveguide is evident from the green luminescence it emitted when it was pumped by 1488 nm diode laser

Planar Waveguide Structures for Post-EDFA Broadband Near Infrared Optical Amplifiers

This thesis reports on optical gain of up to 5.7 dB from a planar waveguide with core made of tetravalent chromium-doped calcium germanate single crystal

DEVLOPMENT OF NOVEL FUNCTIONAL

Rare-earth doped thin films are drawing increasing attention for their use in amplifiers and lasers and their suitability for integrated optics. The optical properties of rareearth ions in solids have been investigated widely and are well understood. Er3+-doped materials are attracting much attention because of the search for solid-state-laser devices operating in the green region, optical devices for 3D displays and for waveguides, which can work in telecommunication window. In this dissertation we researched and fabricated different novel functional thin films for photonics devices fabricated by RF-magnetron sputtering method as – Erbium-doped SiO2 Tantalum pentoxide [Ta2O5] Erbium-doped Tantalum pentoxide [Er-TaOx] Erbium- Ytterbium co-doped Tantalum pentoxide We fabricated different thin films using the RF-sputtering method and then annealed them at various temperatures and time durations. PL peaks were observed at wavelengths of 550 and 670 nm from the Er-TaOx films. We observed the strongest intensities of the 550 and 670 nm peaks from the samples with 0.96 and 0.63 mol% Er concentrations after annealing at 900° C for 20 min, respectively. To the best of our knowledge, this is the first report of light emission from Er-TaOx films fabricated by the RF-sputtering method. These results demonstrate that Er-TaOx films fabricated by RF sputtering can serve as high quality luminescent layers. These can easily be combined with other passive devices to realize novel active devices (e.g., a green-light-emitting photonic crystal), as only sputtering and annealing processes are needed for fabrication. Recent reports of optical waveguides fabricated on Ta2O5, higher nonlinear susceptibility χ3 of Ta2O5, and light emission from thin films makes Ta2O5 a promising material for novel photonic devices.学位記番号：工博甲39

Spectral slicing filters in titanium diffused lithium niobate (ti:linbo3)

A tunable guided-wave optical filter that performs spectral slicing at the 1530nm wavelength regime in Ti:LiNbO3 was proposed and fabricated. It is aimed at minimizing crosstalk between channels in dense wavelength division multiplexing (DWDM) optical network applications. The design utilizes a sparse grating allowing the selection of equally spaced channels in the frequency domain. Between selected channels, equally spaced nulls are also produced. The sparse grating is formed by using N coupling regions with different lengths along the direction of propagation of light in the waveguide, generating N-1 equally spaced nulls between adjacent selected channels. The distance between the centers of adjacent coupling regions is kept constant. The filtering is based on codirectional polarization coupling between transverse electric (TE) and transverse magnetic (TM) orthogonal modes in a waveguide through an overlay of strain-induced index grating, via the strain-optic effect. Two types of devices were fabricated. In the first type, the sparse gratings were produced on straight channel waveguides. Selected channels emerge from the device in a polarization state orthogonal to the input and a polarizer is needed to observe the filtered light. For the second type, an asymmetric Mach-Zehnder interferometer configuration was used to eliminate the need of the polarizer at the output, and yields an output response that is polarization independent. Both types of devices were fabricated on x-cut y-propagating LiNbO3 substrates, with N = 6 strain-induced coupling regions. The single mode channel waveguides were formed by Ti diffusion. Electrode patterns centered about the optical waveguide were defined by liftoff. In the straight channel devices, insertion loss was less than 2.5 dB on a 43 mm sample. The 3-dB channel bandwidth of the selected channels is approximately 1.0 nm. Devices were tuned thermally as well as by voltage application to surface electrodes resulting in tuning rates of 1.0 nm/oC and 0.04148 nm/V, respectively. In the polarization independent device the insertion loss for the phase-matched wavelength was 5.3 dB on a 53 mm long chip. The 3-dB bandwidth was also ~1.0 nm and the thermal tuning rate 1.0 nm/oC. The experimental results are in good agreement with design theory

Discrete Wave Propagation In Quadratically Nonlinear Media

Discrete models are used in describing various microscopic phenomena in many branches of science, ranging from biology through chemistry to physics. Arrays of evanescently coupled, equally spaced, identical waveguides are prime examples of optical structures in which discrete dynamics can be easily observed and investigated. As a result of discretization, these structures exhibit unique diffraction properties with no analogy in continuous systems. Recently nonlinear discrete optics has attracted a growing interest, triggered by the observation of discrete solitons in AlGaAs waveguide arrays reported by Eisenberg et al. in 1998. So far, the following experiments involved systems with third order nonlinearities. In this work, an experimental investigation of discrete nonlinear wave propagation in a second order nonlinear medium is presented. This system deserves particular attention because the nonlinear process involves two or three components at different frequencies mutually locked by a quadratic nonlinearity, and new degrees of freedom enter the dynamical process. In the first part of dissertation, observation of the discrete Talbot effect is reported. In contrast to continuous systems, where Talbot self-imaging effect occurs irrespective of the pattern period, in discrete configurations this process is only possible for a specific set of periodicities. The major part of the dissertation is devoted to the investigation of soliton formation in lithium niobate waveguide arrays with a tunable cascaded quadratic nonlinearity. Soliton species with different topology (unstaggered all channels in-phase, and staggered neighboring channels with a pi relative phase difference) are identified in the same array. The stability of the discrete solitons and plane waves (modulational instability) are experimentally investigated. In the last part of the dissertation, a phase-insensitive, ultrafast, all-optical spatial switching and frequency conversion device based on quadratic waveguide array is demonstrated. Spatial routing and wavelength conversion of milliwatt signals is achieved without pulse distortions

Active optical waveguides for lightwave applications

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