Formation of reconfigurable singlemode channel waveguides in LiNbO/sub 3/ using spatial solitons

The formation of reconfigurable singlemode channel waveguides using photovoltaic dark spatial solitons is demonstrated in a titanium-diffused slab LiNbO/sub 3/ waveguide. The generated mode profile is shown to be influenced by the presence of titanium.     

Numerical calculation of the small signal gain in Er:Ti:LiNbO/sub 3/ channel waveguides

The overlap integral describing the small signal gain coefficient in Er:Ti:LiNbO/sub 3/ channel waveguides is numerically evaluated, and the results are used to discuss the polarization dependence and the influence of the erbium ion distribution. It is shown that bleaching is obtained at a substantially lower pump power for an extraordinarily polarized mode. A study of the influence of the overlap and the mode confinement at different pump bands indicates that the poorer overlap between the pump and signal mode at shorter pump wavelengths can be compensated for by an improved overlap with the erbium ions.<>     

A 54-GHz distributed amplifier with 6-V/sub PP/ output for a 40-Gb/s LiNbO/sub 3/ modulator driver

We have developed a distributed amplifier for a LiNbO/sub 3/ modulator driver using double-doped AlGaAs-InGaAs-AlGaAs pseudomorphic high electron mobility transistors (p-HEMTs). By using a stabilization and negative resistance control technique with source inductance and grounded coplanar waveguided lines, we obtained a gain of 15 dB, a bandwidth of 54 GHz, and 6-V/sub PP/ output. These results indicate that our circuit is a leading candidate for use as a LiNbO/sub 3/ modulator driver in 40-Gb/s fiber-optic communication systems.     

Optical and structural characterization of Z-cut LiNbO/sub 3/ optical waveguides formed in a mixed proton source

In this paper, the optical and structural characteristics of Z-cut LiNbO/sub 3/ optical waveguides are experimentally investigated. The waveguides have been fabricated by fully immersing the samples in a closed beaker containing a mixture of benzoic and adipic acid at different concentration ratio. The influence of the dilution ratio on crystallographic phases, wavelength dispersion, diffusion coefficient, and index profile has been determined by using infrared absorption spectroscopy and optical characterization. The control of the thickness of guiding layers corresponding to /spl beta//sub 1/ and /spl beta//sub 2/ phases can be realized by varying the dilution ratio of the melt.     

First order quasiphase matching in LiNbO/sub 3/

First order quasiphase matched second harmonic generation in LiNbO/sub 3/ channel waveguides is experimentally demonstrated and compared with theoretical estimation. Conversion efficiencies of 11%/W for 1 cm long interaction length were measured for the generation of 435 and 450 nm blue light, which is the same order of magnitude as calculated for the structure.<>     

Index profile of proton-exchanged waveguides in LiNbO/sub 3/ using pyrophosphoric acid

A systematic study was carried out on proton-exchanged LiNbO/sub 3/ optical waveguides using pyrophosphoric acid. A maximum surface index increase of 0.145 was measured at 0.633 mu m for z-cut substrates. The index profile could be accurately modelled by a polynomial expression of simply by a 'truncated' step-index profile.<>     

Stimulated emission in Er:Ti:LiNbO/sub 3/ channel waveguides close to 1.53 mu m transition

Fabrication and characterisation of Er:Ti:LiNbO/sub 3/ channel waveguides is reported. A lifetime of 2.4+or-0.2 ms has been measured for the /sup 4/I/sub 13/2/ level. In a pump and probe experiment using a high power InGaAsP laser and a tunable DBR laser, a signal increase of 0.75 dB for a transmitted pump power of 4.8 mW was demonstrated.<>     

Proton-exchanged LiNbO/sub 3/ optical waveguides using stearic acid

Proton-exchanged optical waveguides have been fabricated in z-cut LiNbO/sub 3/ using a new proton source: stearic acid. These waveguides were characterized optically and were found to exhibit a step index profile with Delta n=0.118 measured at 0.633 mu m. The propagation losses were typically around 1.5 dB/cm, and the diffusion constant and the activation energy for the proton-exchange process were measured to be 5*10/sup 6/ mu m/sup 2//h and 69 kJ/mol, respectively.<>     

50 GHz velocity-matched broad wavelength LiNbO/sub 3/ modulator with multimode active section

A velocity-matched LiNbO/sub 3/ modulator is described which achieves an electrical 3 dB bandwidth of 44 GHz and an optical 3 dB bandwidth in excess of 50 GHz. The modulator also uses multimode waveguides in the active section to achieve excellent loss, drive voltage, and contrast ratio characteristics over a 1.3-1.55 mu m wavelength range.<>     

Multichannel wavelength conversion exploiting cascaded second-order nonlinearity in LiNbO/sub 3/ waveguides

Ten-channel wavelength conversion is achieved by exploiting cascaded second-order nonlinearity in periodically poled LiNbO/sub 3/ waveguides. No external pump input is required in the converter and conversion efficiency is improved due to the enhancement of pump and signal interaction. Approximate 70-nm conversion bandwidth is realized under pump power less than 25 mW at 1550-nm band.     

Laser-induced fracturing: an alternative to mechanical polishing and patterning of LiNbO/sub 3/ integrated optics chips

We report a method for cutting z-oriented LiNbO/sub 3/ substrates into nearly arbitrary shapes using a pulsed UV laser to controllably fracture the crystal. The end surfaces appear optically smooth. Ti-diffused waveguides terminated using this technique have fiber-waveguide-fiber insertion losses which are equal to the lowest values reported for waveguide end faces prepared using conventional mechanical polishing techniques.     

A novel wet-etching method using joint proton source in LiNbO/sub 3/

By mixing benzoic and adipic acids as the source of proton exchange, wet etched ridge waveguides are fabricated in Z-cut LiNbO/sub 3/. Under appropriate adipic-benzoic acid concentration ratios, the joint proton source can suppress the proton diffusion in the lateral directions, and thus, the sidewalls are much more vertical than the usual. The etched depths are also enhanced for more than 30%. And meanwhile, the fabricated ridge waveguides possess smooth surfaces and the scattering losses are lower than that of the conventionally produced samples.     

A novel wet-etching method using electric-field-assisted proton exchange in LiNbO/sub 3/

This paper presents a novel wet etching method for LiNbO/sub 3/ using electric-field-assisted proton exchange. By applying voltage with appropriate polarity on designed electrodes placed on both sides of substrate, the induced electric-field distribution can effectively suppress or enhance proton diffusion in the lateral and depth directions. Thus, the proton-exchanged range in LiNbO/sub 3/ can be expertly manipulated. Because the proton-exchanged region can be removed by using a mixture of HF/HNO/sub 3/ acids, the proposed wet-etching method can effectively control the shape of the etched region in the LiNbO/sub 3/ substrate. Under appropriate electrode and proton-exchange parameters, a vertical sidewall with smooth surface is successfully produced, which makes fabricating reflection mirrors and T-junctions in LiNbO/sub 3/ possible. By utilizing the proposed wet-etching method, optical integrated circuits with higher integration density can be fabricated in LiNbO/sub 3/.     

Unbroadened-spectrum chirped Modulation: effects of the chirp-inducing mechanism on the spectral broadening in LiNbO/sub 3/ Modulators

The spectrum of a chirped optical nonreturn-to-zero data stream obtained by an external Mach-Zehnder modulator (MZM) is analyzed. We demonstrate that it is possible to induce chirp without broadening the optical spectrum by employing an MZM with optical power unbalancing in the two waveguides. For standard chirped MZMs (different electric fields on the two arms), such as z-cut LiNbO/sub 3/ modulators, the broadening as a function of chirp factor is measured for comparison.     

Proton-exchanged optical waveguides in LiNbO/sub 3/ using octanoic acid

Low-loss proton-exchanged planar waveguides in z-cut LiNbO/sub 3/ were fabricated and characterized optically using octanoic acid as a proton source. The waveguide exhibited a step-index profile with an index change of 0.118 measured at 0.633 mu m. The lowest waveguide propagation loss measured was 1.2 dB/cm, and it was reduced further to 0.4 dB/cm after annealing. The diffusion rate and the activation energy using this acid were found to be lower than those reported using other acids.<>     

Numerical analysis of optical amplification in Er/sup 3+/-Yb/sup 3+/ codoped Ti:LiNbO/sub 3/ strip waveguides

Optical amplification at 1530 nm in 980-nm pumped Er/sup 3+/-Yb/sup 3+/-diffused Ti:LiNbO/sub 3/ strip waveguides was numerically analyzed on the basis of rate equation model of Er/sup 3+/-Yb/sup 3+/ system and pump/signal power propagation equations. The model includes total eight energy levels of the Er/sup 3+/-Yb/sup 3+/ system and simultaneously takes into account the excited state absorption (ESA) and upconversion processes within the Er/sup 3+/ ions as well as possible energy transfer processes between Yb/sup 3+/ and Er/sup 3+/ ions. A comparison of numerical results from the eight-level model and highly simplified five-level model has indicated that the /sup 4/S/sub 3/2/ manifold of Er /sup 3+/ ion must be included in the rate equation model. Dependences of threshold pump power and amplification gain on pump power (for gain only), Yb/sup 3+/ surface concentration and waveguide length were calculated and discussed. The characteristics of pump power evolution along the waveguide axis and population density distribution, as well as the influences of energy transfer coefficient from Yb/sup 3+/ to Er/sup 3+/, 550-nm (Er/sup 3+/), and 1060-nm (Yb/sup 3+/) fluorescence lifetime and above-mentioned detrimental processes including ESA, upconversion, and cross-relaxation processes on numerical result were studied. The role of Yb/sup 3+/ is demonstrated. A novel design idea that Yb/sup 3+/ is only incorporated at the end of the waveguide is proposed.     

Broad-band distributed Bragg reflectors with photorefractive Ti : Cu : LiNbO/sub 3/ waveguide gratings

We have realized broad-band distributed Bragg reflectors with photorefractive gratings recorded at 441.6 nm in channel Ti : Cu : LiNbO/sub 3/ waveguides. Proton-assisted copper exchange is used to enable a high level of copper doping and, thereby, achieve an extremely large modulation of refractive index (/spl ges/ 5/sup */10/sup -4/) within a photorefractive grating. Experimental structures demonstrate reflectivities up to 17% with full-width at half-maximum bandwidths in excess of 1.2 nm at center wavelengths around 1.55 /spl mu/m.     

Hybrid Q-switched laser with Ti-indiffused LiNbO/sub 3/ and Er-Yb co-doped glass waveguides

We report the design and characterization of a novel Q-switched laser configuration based on a composite Er/sup +3/-Yb/sup +3/ co-doped ion-exchanged glass and Ti-indiffused LiNbO/sub 3/ waveguides. This 1536 nm Q-switched laser provides stable 200 ns wide optical pulses for a cavity length of 7 m with a peak power of 13.5 W and a pulse energy of 2.7 /spl mu/J at a 2-kHz repetition rate and 180 mW of 980-nm pump power. Also, we examine the dependence of the pulsewidth and peak power on the laser cavity length to estimate the performance of the integrated version of such a laser.     

Polarization dependent loss in a Ti:LiNbO/sub 3/ polarization scrambler/controller

We have investigated the polarization dependent loss (PDL) of a waveguide-type polarization scrambler/controller consisting of a half-wave plate (HWP) and a quarter-wave plate (QWP) in series fabricated in x-cut/z-propagating Ti-diffused LiNbO/sub 3/ optical waveguides. Based on the polarization dependences of both the propagation loss coefficient and mode-mismatch loss with fiber at 1.55 /spl mu/m combined with a numerical analysis, the PDL of the fabricated device is estimated to be 0.063 dB. It is shown that the low PDL is a direct consequence of a special anisotropy relation between the propagation loss and waveguide-fiber mode-mismatch loss.     

Electro-optically tunable wavelength demultiplexer using depressed index waveguides

A novel configuration of an integrated optical tunable wavelength demultiplexer with a stable channel spacing is proposed. The structure consists of a three-mode depressed index diffused waveguide with single-mode input and output waveguides. The refractive index depression has been designed using experimental data of double diffusion of MgO on Ti:LiNbO/sub 3/ waveguides. The BPM simulation of the structure supports the feasibility of the device and shows a good wavelength tunability.<>