Fredrik Laurell

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

Blue light was generated in a LiNbO/sub 3/ 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 LiNbO/sub 3/ 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.<<ETX>>

Electric field poling of flux grown KTiOPO4

A method to periodically pole conductive ferroelectric materials has been developed. With this method we have fabricated high quality domain inverted crystals of flux grown KTiOPO4 suitable for efficient quasi-phase matched frequency conversion. Samples have been evaluated by second harmonic generation and normalized conversion efficiencies up to 6.9%/W cm2 have been obtained.

Transmission measurements in KTP and isomorphic compounds

The optical biaxial nature of crystals in the potassium titanyl phosphate (KTP) family result in anisotropic transmission that depends on the polarization direction of the transmitted radiation with respect to the fundamental crystal axes. Knowledge of the polarization-dependent crystal transmission is important for all wavelength-conversion applications and in particular is the only limitation on possible combinations of wavelengths when one uses the quasi-phase-matching techniques recently developed for the KTP family materials. In this study, polarized transmission spectra of KTiOPO(4) (KTP), RbTiOPO(4), RbTiOAsO(4) (RTA), and KTiOAsO(4) were measured over the 0.3-6-mum wavelength range with a spectrophotometer and a Fourier-transform infrared spectrometer. Seven crystal samples were studied, including four samples of KTP crystals of different origins. Variations in spectral transmission on the short- and long-wavelength edges, as well as visible-wavelength transmission and OH(-) absorption properties, are presented and discussed. The transmission of one sample of KTP and of RTA was also measured before and after periodic electric field poling.

Improved photodarkening resistivity in ytterbium-doped fiber lasers by cerium codoping

We show that the photodarkening resistivity of ytterbium-doped fiber lasers can be greatly improved by cerium codoping. It is suggested that the coexistence of the redox couple Ce(3+)/Ce(4+) in the glass provides means for trapping both hole- and electron-related color centers that are responsible for the induced optical losses in Yb-doped fiber lasers.

Fabrication of periodically domain-inverted channel waveguides in lithium niobate for second harmonic generation

A method to produce periodically domain-inverted channel waveguides in LiNbO/sub 3/ is reported. With this type of waveguide, using quasi-phase matching (QPM), arbitrary wavelengths can be frequency doubled by choosing the appropriate period. It is concluded that, with improved fabrication methods, it will be possible to generate milliwatts of blue light with a diode laser as pump, which is of considerable interest. >

Generation of 740??mW of blue light by intracavity frequency doubling with a first-order quasi-phase-matched KTiOPO 4 crystal

We report on efficient intracavity frequency doubling of a cw diode-pumped Nd:YAG laser on the (4)F(3/2)?(4)I(9/2) laser transition at 946 nm. The nonlinear crystal used in the experiments was a first-order quasi-phase-matched flux-grown KTiOPO(4) crystal (period, 6.09mum ; thickness, 1 mm; length, 9 mm). The fluctuations in the generated second-harmonic wave were lower than 3% at output powers of as much as 500 mW. The overall optical-to-optical efficiency was 5.7%. A maximum output power of 740 mW of blue light was generated, which was stable for only 0.5 min. The decrease the output power at this power level was attributed to heating and thermal lensing in the periodically poled KTiOPO(4) crystal. The short-term behavior of the second-harmonic wave exhibited switching between a cw mode and chaotic intensity fluctuations.

Narrowband bulk Bragg grating optical parametric oscillator

A narrowband, pulsed optical parametric oscillator (OPO) whose output coupler is a bulk glass Bragg grating is demonstrated. The OPO is based on periodically poled KTiOPO4 and is pumped by a pulsed, frequency-doubled Nd:YAG laser at 532 nm, generating a signal at 975 nm with a total efficiency of 35%. This novel and compact device shows a spectral bandwidth of 0.16 nm (50 GHz), a decrease by a factor of 20 compared with that when a conventional mirror is used. By using a folded cavity, we demonstrate tunable radiation with a tuning range of 60 nm and maintained spectral bandwidth.

Wet etching of proton-exchanged lithium niobate-a novel processing technique

It is known that in proton-exchanged wave-guides in lithium niobate,1 the crystal structure is modified. We demonstrate that proton-exchanged regions on the c+-face can be etched away selectively using a mixture of HF and HNO3. In this way it is possible to form, for example, a groove with a smooth bottom surface. The unexchanged c+-face of lithium niobate is only very slowly etched by this acid.

Second-order nonlinearities in the domain walls of periodically poled KTiOPO4

The domain wall regions in periodically poled KTiOPO4 crystals were examined and found to give rise to phasematched second harmonic generation in the Cerenkov directions. This phenomenon is caused by the nonlinear coefficients d11 and d12, which are not present in single domain regions, but are nonzero at and close to domain walls. The appearance of these nonlinearities is attributed to strain, produced by the domain inversion process and results in the creation of a dc piezoelectric field.

Interferometric study of poled glass under etching

An interferometric technique was developed to characterize in real time the etching rate of glasses with <100-nm resolution. The interferometer can be used to help reveal the charge distribution in poled glass.