D. Fluck

Modeling of refractive index profiles of He+ ion‐implanted KNbO3 waveguides based on the irradiation parameters

Planar optical waveguides were fabricated by He+ ion implantation in KNbO3 crystals with doses from 2.5×1014 to 1×1016 cm−2 and energies from 1 to 3.5 MeV, and the index profiles were analyzed. A comparison with theoretical radiation damage profiles calculated by the trim code indicates that the index change is mainly caused by nuclear collisions. However, at higher implantation doses the influence of the electronic excitation on the refractive index profiles cannot be neglected. A method is derived to predict the index profile of nb from the He+ implantation parameters energy and dose. The measured mode spectra of waveguides produced by single and dual energy implantation are in excellent agreement with the predicted spectra.

Optical strip waveguides in KNbO3 formed by He ion implantation

Permanent optical strip waveguides in single crystals of KNbO3 were produced with MeV He ion implantation. Guided TE00 modes were observed and the propagation losses were characterized at the wavelengths of 514, 632, and 860 nm. Waveguide propagation losses of 1.4 dB/cm were measured at 632 nm.

Nonleaky optical waveguides in KNbO3 by ultralow dose MeV He ion implantation

Planar optical waveguides in single crystals of KNbO3 were produced by 1 and 2 MeV He ion implantation with doses between 5×1013 and 5×1014 cm−2. We observe nonleaky waveguiding due to an implantation induced increase of nc, the smallest of the three refractive indices of KNbO3. For 2 MeV He ions and a dose of 1014 cm−2 waveguides with typical propagation losses of ∼3 dB/cm have been fabricated. The possibility of producing nonleaky waveguides by ultralow dose implantation promises the creation of complicated channel waveguide structures within minutes.

Phase-matched second harmonic blue light generation in ion implanted KNbO3 planar waveguides with 29% conversion efficiency

Abstract We report for the first time to our knowledge, noncritical phase-matched second harmonic generation in an ion-implanted KNbO 3 planar waveguide. The guided TM 0 mode of the fundamental wave (868 nm) is converted into the second harmonic TE 1 mode (434 nm). From 1.3 kW of internal fundamental peak power 385 W second harmonic blue light is generated giving a conversion efficiency of 29%. A comparison of the measured and the theoretical efficiencies implies that phase-matching is achieved over the full waveguide length. Based on these first experimental results we estimate that 100 mW of blue light can be generated from 400 mW of input power using an optimized KNbO 3 planar waveguide of 1 cm length.

Blue‐light second‐harmonic generation in ion‐implanted KNbO3 channel waveguides of new design

We report phase‐matched second‐harmonic generation in KNbO3 channel waveguides. The guides were fabricated by one single homogeneous He+ ion irradiation and specially structured photoresist on the KNbO3 crystals serving as an implantation mask. The mask enabled the formation of channel guides with a trapezoidal‐shaped cross section providing simultaneous confinement of both TE and TM modes. 2.6 mW second‐harmonic blue‐light at 441 nm was generated in a 5.8‐mm‐long guide for an incident fundamental power of 280 mW.

Second-harmonic generation in potassium niobate waveguides

We report on our progress in the formation of waveguides in potassium niobate (KNbO/sub 3/) using techniques such as ion implantation and ion sputtering. Different methods for the structuring of channel waveguides are presented, and their advantages and disadvantages are discussed in terms of their optical and nonlinear optical properties. The excellent power-handling capability of KNbO/sub 3/ waveguides is compared to other waveguide materials, and we highlight the influence of postimplantation annealing and repoling on the waveguide attenuation and the nonlinear optical coefficient. We also review recent results on second-harmonic generation in KNbO/sub 3/ waveguides focusing on blue light generation.

Low‐loss optical channel waveguides in KNbO3 by multiple energy ion implantation

Permanent optical channel waveguides in single crystals of KNbO3 are formed with low‐dose MeV He ion irradiation. A single energy implantation forms a confinement barrier at the projected range of the ions whereas multiple energy implantations combined with a positive shielding mask form the side walls. Guiding modes are produced and propagation losses as low as 1.0 dB/cm are measured without the need for any annealing. The irradiation induced refractive index changes that define the boundaries of the waveguiding channels are made visible with cross‐polarized light microscopy.

Nondestructive waveguide loss-measurement method using self-pumped phase conjugation for optimum end-fire coupling

We present a new nondestructive method for measuring the propagation loss coefficient in waveguides based on optimum end-fire coupling by self-pumped phase conjugation in a photorefractive BaTiO3 crystal. In a first pass through the waveguide, the laser beam is adapted to the waveguide mode profile, and hence the phase-conjugated beam is automatically coupled back into the guide with the optimum coupling efficiency. The propagation loss coefficient is determined by measurement of the transmittance of the phase-conjugated beam in the second pass through the wave-guide. We apply this method to measure the propagation loss coefficient of ion-implanted KNbO3 channel waveguides with an accuracy of better than 5%.

Photorefractive effect in crystals with a nonlinear recombination of charge carriers: theory and observation in KNbO 3

Photorefractive and photoconductivity steady-state as well as time-resolved measurements have been performed in strongly reduced KNbO3. The recorded photorefractive gratings show a nonexponential dark decay and an intensity-dependent two-wave mixing gain. The measured photoconductivity depends sublinearly on light intensity. The observed behavior is interpreted in terms of a band transport model that uses a single donor level with only electrons as free charge carriers, taking the nonlinear recombination of free charge carriers (owing to variation of the trap density) and the time dependence of the free-charge-carrier density into account. An analytical solution for the nonexponential dark decay of the photorefractive grating is derived. As an application of this model the results of the photorefractive and the stationary and transient photoconductivity experiments are used to determine the model parameters (thermal excitation rate, photoexcitation cross section, recombination coefficient, acceptor and donor densities, and electron mobility).

Photorefractive effect in proton-implanted Fe-doped KNbO 3 waveguides at telecommunication wavelengths

We report on photorefractive two-beam coupling in proton-implanted Fe-doped KNbO3 waveguides at wavelengths from 632.8 to 1550 nm. Exponential gain coefficients of 11 and 0.9 cm-1 were measured at wavelengths of 632.8 nm and 1550 nm, respectively. Photorefractive response times as low as 120 µs are reported at 632.8 nm for a pump power of 4 mW. It is shown that proton irradiation increases the effective trap density, decreases the response time, and extends the photorefractive sensitivity of Fe-doped KNbO3 crystals toward near-infrared wavelengths.