Optical properties of waveguides fabricated in fused silica by femtosecond laser pulses

With tightly focused femtosecond laser pulses, waveguides are fabricated in fused silica. The guiding and attenuation properties of these waveguides at wavelengths of 514 nm and 1.5 microm are studied. We demonstrate that by changing only the writing speed, waveguides with a controllable mode number can be produced.     

Frequency conversion of femtosecond laser pulses in an engineered aperiodic poled optical superlattice

We theoretically propose a procedure based on a cascading genetic algorithm for the design of aperiodically quasi-phase-matched gratings for frequency conversion of optical ultrafast pulses during difference-frequency generation. By designing the sequence of a domain inversion grating, different wavelengths at the output idler pulse almost have the same phase response, so femtosecond laser pulses at wavelength 800 nm can be shifted to other wavelengths without group-velocity mismatch.     

XPS-investigation of proton exchanged waveguides in LiNbO3

An X-ray photoelectron spectroscopy (XPS) study of X-cut LiNbO₃ proton exchanged in pure and lithium benzoate buffered benzoic acid melt has been performed. Binding energies and full width at half maximum for the Li1s, O1s and Nb3d peaks in the depth range 0–10 nm were investigated by using Ar ion sputtering. The largest changes with increasing depth are established for the surface atomic layers between 0 and 1 nm. Li-enrichment of the surface is observed when both buffered and pure benzoic acid melts are used.     

Anomalous side-shifted multimode spectra in proton-exchanged LiNbO3 waveguides

We report the presence of a curious and highly reproducible effect in multimode lithium niobate waveguides fabricated by proton exchange (PE) in molten benzoic acid at temperatures ranging from 160 degrees C to approximately 250 degrees C. The spectral lines in the mode spectra of these guides (measured using a prism coupler) are anomalously side-shifted out of the expected geometrical plane. Transforming these measurements back into the plane of the waveguide, we find that the direction of scattering (relative to the crystal axis) is extremely precise (<1% deviation about a mean), and that the effect can be explained by postulating the existence of precisely oriented, stress-induced gratinglike structures (with irregular periods in the 10-70-microm range) in the guides.     

New optical configuration for nondestructive measurements of refractive index profiles of LiNbO(3) waveguides

A direct and nondestructive measurement technique for the determination of the refractive index profiles of one- and two-dimensional LiNbO(3) waveguides is presented for the first time to our knowledge. The technique generalizes the refracted near-field method, which is well known for optical fiber characterization. The spatial resolution and accuracy are 0.1 and 0.4 μm, respectively. The refractive-index calibration is done by an analysis of the near-field light power distribution, and its resolution is approximately 2 × 10(-4). The proposed experimental setup permits sample installation and data acquisition in a few minutes.     

Strong reduction of propagation losses in LiNbO3 ridge waveguides

We propose optimized processes for the reproducible production of LiNbO₃ ridge waveguides with propagation losses lower than 0.2 dB/cm. The performances are achieved for both Z-propagating and Y-propagating waveguides, TE and TM polarizations, in X-cut LiNbO₃ congruent substrates, and could be easily extrapolated to Z-cut substrates. The fabrication procedure is composed of three steps: titanium deposition, optical grade dicing and diffusion at high temperatures, so that lithography or cleanroom facilities are not required. The reproducible smooth waveguides with low propagation losses result from the diffusion step, which is performed after the production of the ridges. We also show how the losses can be evaluated from the Fabry–Perot effect in the waveguides, without any assumption on the input and output reflection coefficients.     

Characterization of alpha-phase soft proton-exchanged LiNbO3 optical waveguides

Waveguides in LiNbO3 are realized by a soft proton exchange (SPE) process with use of a melt of stearic acid highly diluted by lithium stearate. No phase transitions are formed when alpha-phase waveguides are obtained by SPE. The alpha-phase presents the same crystalline structure as that of pure LiNbO3 crystal, and it maintains the excellent nonlinear and electro-optical properties of the bulk material. The kinetics of the SPE method is studied by the use of secondary-ion mass spectrometry and prism-coupling techniques. The hydrogen effective diffusion coefficient as well as the self-diffusion coefficients of H+ and Li+ ions are determined as a function of the proton-exchange temperature for X-cut LiNbO3.     

Non-linear optical wavelength conversion in Ti:LiNbO3 waveguides

Phase matching temperatures for second harmonic generation were measured for fundamental wavelengths from 1.06 to 1.24 μm in a planar waveguide and compared with bulk crystal values. Furthermore, both second harmonic generation and sum frequency generation were achieved in a channel waveguide, the latter by mixing the Nd-YAG laser wavelength at 1.064 μm with Raman-shifted light from the same light source. A conversion efficiency for second harmonic generation in a channel waveguide of 4.2% at a fundamental power of 120 mW was obtained. Some material-related aspects on waveguides for this type of application are also discussed.     

Theoretical study of Ti:LiNbO3 strip waveguides in the Mid-infrared

Abstract

The transverse magnetic and transverse electric mode size and effective refractive index as functions of the Ti-strip initial width W, diffusion temperature T and Ti-strip initial thickness H in c-cut Ti-diffused LiNbO₃ waveguides have been theoretically studied at wavelengths of 3.0 and 3.39 μm on the basis of modified expressions for the waveguide surface index increment. The single-mode, multiple-mode and cut-off conditions have been identified from those effective refractive index plots. These mid-infrared numerical results are discussed comparatively with near-infrared data reported earlier.     

Narrowband Bragg reflectors in Ti:LiNbO3 optical waveguides

Bragg grating reflectors etched in amorphous silicon overlay films have been integrated with Ti:LiNbO3 optical waveguides to obtain a narrow (0.05 nm) reflectance spectrum with a > 20 dB dip in the transmittance spectrum. These results were realized at a wavelength of 1542.7 nm for TE polarization on an x-cut, y-propagating substrate with gratings etched to a depth of approximately 93 nm in a 105 nm thick silicon film over a length of 12.5 mm. The reflectance in the channel waveguides is found to be strongly dependent on the depth of the etched grating. The effect of the Bragg waveguide loss factor on the transmittance and reflectance spectra is investigated by using a model for contradirectional coupling that includes an attenuation coefficient. The values for coupling constants kappa and amplitude attenuation constants alpha of samples etched for different time durations to control the grating depths are obtained from the model through the use of the depth of the dips in the transmittance spectra and the spectral widths of the reflectance peaks. It is concluded that the corrugated Si overlay film increases the insertion loss by approximately 2.7 dB, and the loss is not significantly affected by the grating depth.     

Investigation of multiconversion processes in periodically poled LiNbO3-based optical parametric oscillators

The multiconversion processes in optical parametric oscillators based on periodically poled LiNbO3 are investigated. Interpretations based on simultaneous quasi- and birefringent phase matching are presented. Three parametric and three harmonic generation processes in a multigrating periodically poled LiNbO3 crystal were observed. Two of the parametric processes and two of the harmonics were quasi-phase matched, and the other conversion processes were phase matched through birefringence in the crystal. The primary parametric process (omegap --> omegas + omegai) was obtained through first-order quasi-phase matching. The other quasi-phase-matched processes occurred within higher orders. The existence of even-order quasi-phase matching in the crystal is due to other than a 50% duty-cycle grating periods. The tuning range for each of the generated waves is obtained and compared with theoretical fittings.     

Effect of MgO doping of periodically poled lithium niobate on second-harmonic generation of femtosecond laser pulses

We theoretically analyze type-I broadband second-harmonic generation (SHG) of femtosecond laser pulses based on a quasi-phase-matching configuration in periodically poled congruent LiNbO3 (LN) andperiodically poled MgO:LiNbO3 (PPMgLN) (5% and 7%). Group-velocity matching (GVM) can be achieved at the fundamental waves of 1.59, 1.56, and 1.55 microm for SHG when the three types of crystals have grating periods of 22.31, 20.07, and 23.45 microm, respectively. It is found that the central wavelength of the fundamental wave for GVM will increase with the decrease of MgO doping in LN. It is concluded that the shift of the GVM central wavelength is due to the difference of MgO doping, which changes the dispersion of the crystal. Therefore, tunable and high efficiency broadband SHG of femtosecond laser pulses in a long crystal can be realized by selecting different doping rates of PPMgLN.     

Generation of 6.6-microm optical parametric oscillation with periodically poled LiNbO3

Greater than 6-mum-oscillation was demonstrated by means of optical parametric oscillation with periodically poled LiNbO(3) (PPLN). The interaction length and thickness were 40 mm and 500 mum. The pump source used was a Q-switched Nd:YAG laser with a pulse duration of 120 ns and a repetition rate of 1 kHz. The tuning ranges of the idler waves were 6.576.56, 6.226.12, and 6.065.94 mum for PPLN wafers of 20-, 21.3-, and 22-mum periods, respectively.     

Annealing behavior of barriers in ion-implanted LiNbO3 and LiTaO3 planar waveguides

We formed planar waveguides in LiNbO(3) and LiTaO(3) crystals by megaelectron volt He-ion implantation. The dark modes of both waveguides are measured and their refractive-index profiles are described according to the parameterized index profile reconstruction method. The extraordinary indices of both ion-implanted waveguides exhibit quite different profiles. We compare the thermal stability of barriers in ion-implanted LiNbO(3) and LiTaO(3) waveguides by annealing at different temperatures. The results show that the barrier in a LiTaO(3) planar waveguide has higher thermal stability than that in a LiNbO(3) waveguide. The experiments also show that annealing at a temperature higher than 400 degrees C results in recrystallization of the barrier.     

Duty cycle dependence of a periodically poled LiNbO3-based electro-optic Solc filter

We demonstrate that the performance of a periodically poled LiNbO3- (PPLN-) based electro-optic Solc filter is dependent on the duty cycle of the crystal. This may limit the performance of the device for applications such as add-drop filtering and switching, owing to the deterioration of the extinction ratio. It is shown that by adding a retarder to the Solc filter it is possible to improve the extinction ratio; thus the dependence of the filter on the duty cycle can be reduced. Using Jones calculus, we analyzed the effect of a variable retarder that can also be rotated on the extinction ratio. We experimentally observed a 6 dB increase in the extinction ratio when we used a half-wavelength retarder.     

Planar optical waveguides obtained in Z-cut LiNbO3 and LiTaO3 by proton exchange in LiHSO4 vapors

A new proton source – LiHSO₄ vapors at low temperature – was used in forming optical waveguides in LiNbO₃ and LiTaO₃. The proton-exchanged layers were investigated by mode spectroscopy and infrared absorption spectroscopy. An estimate of the optical losses and phase contents of the waveguides was made. The diffusion parameters were determined for both types of crystal, and were compared to those obtained when benzoic acid melt was used as a proton source. The results presented could contribute to the realization of waveguides with controled phase compositions. The method proposed allows the use of a very simple and safe chamber construction, and the production of low-loss waveguides in a single technological step.