Analysis of end-pumped Nd:Ti:LiNbO3 microchip waveguideFabry-Perot lasers

A self-consistent analysis of end-pumped Nd:Ti:LiNbO₃ microchip waveguide lasers based on the fast Fourier transform beam propagation method (FFT-BPM) has been proposed. The algorithm of the model allows one to describe the laser gain and pump absorption in terms of the complex atomic susceptibility for the case of the Nd³⁺ ions. Considering the interference effects between the forward and backward light waves, the population inversion longitudinal and transversal spatial effects can be simulated. The laser characteristics of the Nd:Ti:LiNbO₃ waveguide laser correlate well with the experimental data and theoretical results. The design rules for the optimized microchip laser are also developed by using the proposed model     

Ferroelectric integrated optics: recent developments

Advances in rare-earth-doped waveguide lasers in LiNbO₃, waveguided second harmonic generation through quasi-phase-matching in LiNbO₃ and LiTaO₃, and pulsed laser epitaxial growth of LiNbO₃ on Sapphire are reported. Efficient lasers operating in CW, Q-switch, and mode-lock modes were fabricated in Nd:LiNbO₃. In order to assess the potential of bulk-doped Er:LiNbO₃ samples, spectroscopic measurements have been carried out, the main limiting mechanism identified, the corresponding up-conversion coefficient evaluated, and a comprehensive gain model developed. First lasing action in this material, when pumped at 1.48 μm, is described. Quasi-phase-matched generation of blue light with conversion efficiencies up to 150%/Wcm² is also demonstrated, and finally we present the fabrication of low-loss LiNbO₃ single-crystal waveguiding layers     

Transversal mode competition in erbium-doped Ti:LiNbO3waveguide lasers

Theoretical modeling of erbium-doped LiNbO₃ waveguide lasers has been developed considering the competition between q-TM and q-TE signal modes. Two different approaches have been used: the first is based on the mean field approximation for signal and pump power and for population inversion, the second takes into account the longitudinal distribution of the pump power and of the population densities. Our formulation takes into account, for the first time, the competition between orthogonally polarized modes, evaluating the transversal overlap between the fields and the population inversion, the effects of double pass for the pump, the signal and pump saturation effects, the stimulated emission also at pump wavelength, the wavelength dependent cross sections and mirror reflectivities. A diffused dopant distribution in depth and uniform in lateral direction have been used. A q-TM or q-TE pump mode around 1.48 μm has been considered. The threshold pump power optimization has been performed changing the dopant profile diffusion depth and maximum surface concentration. The results show how, above threshold, various types of output power characteristics, experimentally found, are due to the different transversal saturation of q-TM and q-TE lasing modes     

Analysis of end-pumped and electrooptically tuned Nd,Ti,MgO:LiNbO3 microchip waveguide lasers

An end-pumped and electrooptically tuned Nd,Ti,MgO:LiNbO₃ microchip waveguide laser is studied by using the self-consistent model and the conformal mapping method. The complex atomic susceptibility is used to describe the laser gain and pump absorption by applying the fast Fourier transform beam propagation method (FFT-BPM) for simulation. Using the conformal mapping method, the index change distributions in the waveguide, caused by the applied electric field, were evaluated. The optimal configuration for the microchip waveguide laser with various cavity lengths was designed. The output performances of the end-pumped and electrooptically tuned Nd,Ti,MgO:LiNbO₃ microchip waveguide lasers were simulated and investigated     

Third harmonic generation of Nd:YAG laser light in periodicallypoled LiNbO3 waveguide

The authors propose and first demonstrate an LiNbO₃ waveguide device with cascading quasi-phase-matched second harmonic generation and quasi-phase-matched sum-frequency generation for generation of a third harmonic wave. Ultraviolet light of 355 nm wavelength, which was the shortest value ever reported for LiNbO₃ waveguide wavelength-convertors, was obtained with Nd:YAG laser light     

Ti:Er:LiNbO3 waveguide laser of optimized efficiency

The development of a Fabry-Perot-type Ti,Er:LiNbO₃ waveguide laser of optimized CW output power up to 63 mW (λ_s =1561 nm) at a pump power level of 210 mW (λ_p=1480 nm) and a slope efficiency of up to 37% is reported. The theoretical model for the waveguide laser is presented and applied to determine the optimum resonator configuration using waveguide parameters obtained from a detailed characterization of the laser sample. With pulsed pumping, waveguide laser pulses of up to 6.2 W peak power were observed. Apart from residual relaxation oscillations, the laser emission proved to be shot-noise limited     

Analysis of Nd:MgO:Ti:LiNbO3 waveguide lasers withnonuniform concentration distributions

We propose a theoretical analysis for the Ti-diffused Nd:MgO:LiNbO ₃ waveguide lasers with nonuniform concentration distributions by using the evolution equations of the pumping light and amplified spontaneous emission (ASE). The concept that ASE is the origin of the laser light is applied, and the evolutions of the pumping and ASE are calculated by considering the feedback from the reflectors at the waveguide ends. Furthermore, because this analysis also considers the spatial distributions of the Nd and Ti concentrations, pumping, and ASE light in the waveguide laser, it is appropriate for a case involving arbitrary Nd and Ti concentrations and waveguide profile. Also, the laser characteristics of the Ti-diffused channel waveguide structures in the Nd-diffused MgO:LiNbO₃ and Nd:MgO:LiNbO₃ crystals are analyzed and compared. Moreover, theoretical data correlate well with the experimental results reported previously     

A theoretical study of a Ti-diffused Er:LiNbO3 waveguidelaser

A detailed theoretical procedure of determining the mode sizes of fundamental mode field distribution in a coaxially pumped Ti-diffused Er:LiNbO₃ waveguide laser has been described. The mode sizes, effective refractive indexes, effective pump area, and coupling efficiency as a function of the initial Ti stripe width W were numerically calculated for the waveguide with 95- and 100-nm initial Ti stripe thicknesses, respectively. The results indicated that the threshold pump power is severely affected by the stripe width, while slope efficiency is hardly changed as W; both show little difference between 95- and 100-nm stripe thickness. In addition, the stimulated emission cross section of Er³⁺ in Er:LiNbO₃ channel waveguide versus the wavelength were calculated directly from its fluorescence spectra using the β-τ method. Subsequently, threshold pump power and slope efficiency were evaluated     

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

We propose a special lithium-niobate (LiNbO₃) 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 LiNbO₃ 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 LiNbO₃ waveguide structure could be further explored for the development of electrooptic gratings for high-speed applications.     

A novel method for laser-induced periodic domain reversal in LiNbO3 [nonlinear optical waveguide]

An intense laser light in conjunction with a strong localized field is used to reverse the polarity of the ferroelectric domain at the location of a laser beam. The focus beam from an argon laser raises the temperature of a moving z-cut LiNbO₃ plate close to the Curie point when a bipolar square wave electric field is applied. This technique has been used to construct a nonlinear periodic optical waveguide structure. This waveguide can be used for quasi-phase-matched second-harmonic generation. The applications of such waveguides are well documented in the literature     

Lithium niobate ridge waveguides fabricated by wet etching

The authors report a novel method for fabricating proton exchanged LiNbO₃ ridge waveguides by the wet etching of proton-exchanged LiNbO₃. To demonstrate a simple and easy application of the LiNbO₃ ridge waveguide, a Mach-Zehnder interferometric modulator with selfaligned electrode was fabricated     

Finite element analysis of LiNbO3 waveguides with Si orSi/SiO2 overlay

LiNbO₃ waveguides with Si overlays are emerging as a basic building block for a variety of integrated-optic components, including modulators, high-efficiency gratings, and narrowband WDM filters. However, the development and optimization of these devices are, in large part, hindered by the lack of understanding of the specifics of the Si-on-LiNbO₃ structure which appear to differ dramatically from those of the Si and LiNbO₃ waveguides, considered separately. In this work, we provide a specific insight into the waveguiding properties of vertically stacked Si-on-LiNbO₃ waveguides. In particular, we present a detailed theoretical analysis of the effect of the Si film on the modal characteristics (propagation constant and field distribution) of the structure. The vectorial finite element method (VFEM) is used to numerically investigate a step-index and graded-index single-mode channel waveguide in LiNbO₃, with a Si or Si/SiO₂ multimode overlay. We show that for ~70% of all Si thicknesses, in the range from 0 to 1.6 μm, the highest order normal mode of the entire structure has more than 99.9% of the total energy confined in the LiNbO₃ region, i.e., beneath the Si overlay. This fact is quite intriguing given the fact a planar Si layer of submicron thickness on bulk LiNbO₃ is already multimoded. Furthermore, we show that the effective mode index of the structure is considerably modified compared to that of the LiNbO₃ waveguide while the propagation loss is, on the other hand, practically unaffected (~0.3 dB/cm) even in the presence of the lossy Si film, as confirmed by our previous experimental results. Evidently, large modulation of the effective index and low-loss propagation provide an ideal combination of properties suitable for the fabrication of high-reflectance corrugated waveguide gratings, essential for a number of practical devices, in particular, WDM filters     

Electro-optic effect in proton exchanged LiNbO3 andLiTaO3 waveguides

Electro-optic properties of proton-exchanged (PE) waveguide layers in LiTaO₃ and LiNbO₃ are studied and related to their optical characteristics. The proton-exchange process induces a degradation of the electro-optic activity in both types of waveguides, PE LiNbO₃ and PE LiTaO₃. The measured electro-optic effect is close to the detection sensitivity even when the exchange regime is performed at low temperatures for short periods of time. The PE samples have been annealed (APE waveguides) and the changes of their r₃₃ electro-optic coefficient has been followed at successively higher temperatures and periods of time. Subjected to annealing at temperatures between 265-420°C, the LiTaO₃ layers show a partially recovered r₃₃ coefficient, the recovering being different for quick and slow cooling of the samples. In thin APE LiNbO₃ waveguiding layers a restoration of r₃₃ up to 75% of the bulk value is observed due to the annealing at temperatures between 200-340°C     

Blue light generated by frequency doubling of laser diode light ina lithium niobate channel waveguide

Blue light was generated in a LiNbO₃ channel waveguide by frequency doubling radiation from a laser diode in a guided to guided wave interaction, utilizing first-order quasi-phase-matching in a periodically domain inverted structure. A fabrication method that does not depend on the use of titanium was used. A periodic pattern of silicon oxide on the positive c-face of LiNbO₃ was used in combination with a heat treatment to achieve a periodic outdiffusion and domain reversal in the surface layer. A channel guide was subsequently formed by proton exchange     

Fabrication of magnesium-oxide-induced lithium outdiffusionwaveguides

A short-time high-temperature diffusion process for the fabrication of the lithium niobate (LiNbO₃) optical waveguide with a magnesium-oxide (MgO) strip-loaded structure is presented. The deposited MgO layer causes the lithium to diffuse out of the LiNbO₃ substrate. The secondary ion mass spectroscopy (SIMS) data of the fabricated waveguide show that the lithium diffuses out of the substrate and gets into the MgO cover layer, which results in an increase in the extraordinary refractive index of the substrate. In particular, it is found that the diffused atoms are piled up near the LiNbO₃ and MgO interface. Also, the electrooptic effect of the fabricated waveguide is better than that of the conventional titanium-indiffused waveguide. Moreover, single-mode and single-polarization waveguides can be easily obtained without any significant surface guiding or surface damage on the crystalline substrate     

Evaluation of absorption and emission cross sections of Er-dopedLiNbO3 for application to integrated optic amplifiers

The polarization-dependent absorption and emission spectra of the ⁴I_13/2-⁴I_15/2 transition (λ~1.5 μm) in single crystal bulk Er:LiNbO₃ have been measured. Low-temperature (10 K) measurements of the Stark split energy levels of these two manifolds indicate at least two Er³⁺ sites. McCumber theory is applied to determine the Er:LiNbO₃ absorption and emission cross sections. These values are used to calculate the gain characteristics of Er:LiNbO₃ channel waveguides. Calculations indicate that a gain of 10 dB is achievable in a waveguide of several centimeters using ~20-mW pump power     

Theoretical modeling of optical amplification in Er-doped Ti:LiNbO3 waveguides

An accurate theoretical analysis is presented describing optical amplification in Er-diffused Ti:LiNbO₃ channel waveguides. It follows as far as possible the theory already developed for Er-doped fibers. As optical pumping around λ_p≈1.48 μm is considered, a quasi-two-level model for the Er³⁺ ions is used with wavelength-dependent cross sections. The optical gain in the 1.53 μm<λ<1.64-μm wavelength range is evaluated. The characteristic parameters, as Er concentration profile, cross sections, pump, and signal mode distributions and waveguide (scattering) losses are taken from experiments. Examples of numerically calculated pump-, small-signal-gain-, and ASE-evolutions are presented. The model has been tested by comparing computed and experimentally observed gain characteristics for Xˆ- and Yˆ-cut LiNbO₃; an almost quantitative agreement has been obtained     

An optical integrated circuit for time-division 2-D velocitymeasurement

An LiNbO₃ optical integrated circuit pigtailed with two single-mode fibres, which allows time-division two-dimensional velocity measurement, is discussed. To detect time-division multiplexed beat signals corresponding to velocity components v_X and v_γ of a moving object, a waveguide switch is integrated on a Z-propagating LiNbO₃ substrate of 28×7 mm² in addition to a waveguide interferometer with a frequency shifter. In the optical IC, either v_X or v_γ could be measured selectively with signal-to-noise ratio of 20 dB by driving an electronic gate placed after a photodiode in synchronization with the waveguide switch     

Visible laser sources based on frequency doubling in nonlinearwaveguides

This paper reviews nonlinear quasi-phase-matching (QPM) waveguides and laser diodes with application to conversion of infrared laser diode wavelengths to the visible. The discussion of nonlinear QPM waveguides includes Ti-diffusion poled and E-field poled LiNbO₃ and KTP waveguides. Up to 25 mW of blue output power has been demonstrated for 120-mW infrared power injected into a nonlinear waveguide. Semiconductor laser sources suitable for frequency doubling are discussed, including distributed Bragg reflection (DBR) lasers operating at up to 200 mW output power, master oscillator power amplifiers with over 1 W output power, and wavelength-tunable flared semiconductor lasers with over 0.5 W output. A compact blue laser with 1-4 mW output power at 425-nm wavelength has been demonstrated based on a DBR laser frequency doubled in a nonlinear waveguide     

Buried optical waveguide polarizer by titanium indiffusion andproton-exchange in LiNbO3

Buried optical waveguide polarizers on LiNbO₃ have been realized by titanium indiffusion, followed by proton-exchange and annealing. The proton-exchange process decreases the ordinary refractive index and so modifies the index profile of the titanium indiffused waveguide. The measured intensity profile is in good agreement with calculation. An aluminum film absorbs the surface TM mode on z-cut LiNbO₃, leaving a buried nearly symmetric TE mode with lower optical loss than surface-guided TE modes. The extinction ratio obtained is estimated to be greater than 50 dB/cm at 0.633 μm