Alice C. Liu

Electro-optic phase modulation in a silica channel waveguide.

We report what is to our knowledge the first demonstration of electro-optic phase modulation in a fused-silica channel waveguide. The no linearity is induced through elevated temperature poling of an electron-beam-irradiated waveguide. A phase shift of 32 mrad was measured at λ = 633 nm for a device interaction length of 4.8 mm and an applied electric field of 7.3 V/μm.

Absolute measurement of the second-order nonlinearity profile in poled silica.

Measurement of the thin nonlinearity profile of poled silica by the Maker fringe technique has been impossible because of total internal reflection (TIR) at the back surface of the sample. We demonstrate that this limitation can be removed by placing a prism against each face of the sample, thus avoiding TIR. This novel technique allows, for the first time to our knowledge, the nonlinearity profile of a thin film to be inferred by the Maker fringe technique. Applied to a silica sample thermally poled under standard conditions (275 degrees C and 5.3 kV for 30 min), it suggests a Gaussian profile with a 1/e width of 8 mum and a maximum d(33) of 0.34 pm/V.

Spectrum thermal stability of Nd- and Er-doped fiber sources

The spectral thermal stability of broadband rare-earth-doped fiber sources makes them attractive for fiber sensor applications. We quantify the mean wavelength variation of both Nd- and Er-doped fiber sources operating as superfluorescent fiber lasers. Besides the intrinsic variation of such sources the effects of pump power and pump wavelength are also considered since both the power and wavelength of conventional laser diode pump sources are temperature sensitive. Other types of rare-earth-doped sources are also briefly considered.

DC Kerr coefficient of silica: theory and experiment

We report the first measurement of the frequency and polarization dependence of the low-frequency third-order nonlinear susceptibility (chi) (3)(-(omega) ;(omega) ,0,0) of silica. In the frequency range tested (0.5 - 19 MHz), we observed sharp resonances with a complex dependence on polarization. Observations are quantitatively well explained by a theoretical model that assumes the presence of two contributions to (chi) (3), namely the Kerr effect and electrostriction. The model shows that our measurements are consistent with (1) a DC Kerr constant of 1.9 X 10-22 m2/V2 with a 3:1 polarization dependence, comparable to the Kerr constant at optical frequencies, and (2) an electrostriction modulation that is greatly enhanced by mechanical resonances of the sample and exhibits a polarization dependence of 2.3:1, in agreement with an elasto-optic model. This work suggests new means of producing a low-voltage, high-frequency phase modulator by operating at a fundamental resonance of the structure. It also lends credence to the general belief that DC rectification does not fully account for the large second- order nonlinearity that occurs in poled silica.

Measurement of the electrostrictive constants of silica and their impact on poled silica devices

In an electro-optic poled-silica switch, modulation arises from both the electro-optic effect and electrostriction. To investigate the magnitude of these two contributions, we measured the phase shift induced in a thin slab of unpoled silica subjected to a strong dc plus a small, low-frequency (0 - 19 MHz) ac voltage. The frequency responses for light polarized parallel and perpendicular to the applied field exhibit a constant term due to electronic dc Kerr, and strong peaks due to electrostriction enhanced by mechanical resonances of the slab. A theoretical model is presented that gives good quantitative agreement with these observations. From this comparison we infer the values of the dc Kerr and electrostrictive constants of silica for each polarization. For the perpendicular polarization, electrostriction largely dominates in the frequency range under study. A potential electrostrictive modulator and the impact of electrostriction on the polarization dependence of poled-silica devices are discussed.

DC Kerr measurement in a silica channel waveguide

We describe interferometric measurements of the DC Kerr susceptibilities in an SiO2-GeO2 channel waveguide. The measured magnitude of (chi) 1111(3)(-(omega) ; (omega) , 0,0) equals 2.44 +/- 0.2 X 10-22 m2/V2 is comparable to the previously measured optical Kerr susceptibility. However, the ratio of nonlinear tensor elements (chi) 1111(3)/(chi) 1122(3) equals 1.1 for the DC Kerr effect differs from that observed in the optical Kerr effect where (chi) 1111(3)/(chi) 1122(3) equals 3. We provide theoretical interpretation for this difference in the polarization dependence.

Frequency shifting in optical fiber using a SAW horn

The authors present an in-fiber frequency shifter for optical fiber which uses flexural acoustic waves to induce mode coupling between the two optical modes of a dual-mode, elliptical-core fiber. A sharply tapered surface-acoustic-wave horn is used to excite these waves in the fiber, providing a robust, efficient acoustic source. Optical mode filters at the input and output ensure spectral purity of the frequency-shifted light. A 6.3-MHz frequency upshift in 1.3- mu m light with 400-mW input power, with a 35-dB sideband suppression, was demonstrated. Recently studied methods for improving the sideband suppression through mode-filtering techniques are also discussed.<<ETX>>

Characterization of the second-order nonlinearity in poled fused silica

Second harmonic generation is used to characterize the second order nonlinearity induced by thermal electric field poling in fused silica. The uniformity of the poled region is improved by using an evaporated rather than a machined poling electrode configuration. The effective thickness L and strength of the nonlinear coefficient d33 of the nonlinear region is estimated using Maker fringes excited through a prism pair.

ELECTRO-OPTIC PHASE MODULATION IN A SILICA CHANNEL WAVEGUIDE

We report what is to our knowledge the first demonstration of electro-optic phase modulation in a fused-silica channel waveguide. The nonlinearity is induced through elevated temperature poling of an electron-beamirradiated waveguide. A phase shift of 32 mrad was measured at lambda = 633 nm for a device interaction length of 4.8 mm and an applied electric field of 7.3 V/microm.