A.M.P. Leite

Optical fiber sensing with a low-finesse Fabry–Perot cavity

A detailed comparison between a low-finesse Fabry-Perot cavity and a typical two-beam interferometer is developed and checked experimentally. The consequences of approximating the true Fabry-Perot function by the two-beam function are evaluated for commonly used signal-processing schemes in order to quantify the final error introduced in various fiber sensing schemes employing this configuration.

Chromium diffusion in lithium niobate for active optical waveguides

A method to fabricate chromium‐doped lithium niobate substrates in which the active ions are introduced by thermal diffusion from a film is reported. Chromium concentration depth profiles have been obtained by secondary‐ion‐mass spectrometry and the relevant diffusion parameters have been derived. Fluorescence spectrum and upper laser level lifetime of chromium diffused proton‐exchanged and chromium/titanium‐diffused lithium niobate waveguides have been measured. A simple model has been used to estimate the performance of such structures as waveguide optical amplifiers and lasers.A method to fabricate chromium‐doped lithium niobate substrates in which the active ions are introduced by thermal diffusion from a film is reported. Chromium concentration depth profiles have been obtained by secondary‐ion‐mass spectrometry and the relevant diffusion parameters have been derived. Fluorescence spectrum and upper laser level lifetime of chromium diffused proton‐exchanged and chromium/titanium‐diffused lithium niobate waveguides have been measured. A simple model has been used to estimate the performance of such structures as waveguide optical amplifiers and lasers.

Optical modulation at around 1550 nm in an InGaAlAs optical waveguide containing an InGaAs/AlAs resonant tunneling diode

We report electroabsorption modulation of light at around 1550 nm in a unipolar InGaAlAs optical waveguide containing an InGaAs/AlAs double-barrier resonant tunneling diode (RTD). The RTD peak-to-valley transition increases the electric field across the waveguide, which shifts the core material absorption band edge to longer wavelengths via the Franz–Keldysh effect, thus changing the light-guiding characteristics of the waveguide. Low-frequency characterization of a device shows modulation up to 28 dB at 1565 nm. When dc biased close to the negative differential conductance region, the RTD optical waveguide behaves as an electroabsorption modulator integrated with a wide bandwidth electrical amplifier, offering a potential advantage over conventional pn modulators.

Optical modulation in a resonant tunneling relaxation oscillator

We report high-speed optical modulation in a resonant tunneling relaxation oscillator consisting of a resonant tunneling diode (RTD) integrated with a unipolar optical waveguide and incorporated in a package with a coplanar waveguide transmission line. When appropriately biased, the RTD can provide wide-bandwidth electrical gain. For wavelengths near the material band edge, small changes of the applied voltage give rise to large, high-speed electroabsorption modulation of the light. We have observed optical modulation at frequencies up to 14 GHz, associated with subharmonic injection locking of the RTD oscillation at the fundamental mode of the coplanar transmission line, as well as generation of 33 ps optical pulses due to relaxation oscillation.

Frequency multiplexing of remote all-fiber Michelson interferometers with lead insensitivity

A scheme to frequency multiplex a group of sensors based on all-fiber Michelson interferometers is presented. Lead insensibility is obtained by using the two fiber leads of the configuration as an extra Michelson interferometer whose differential phase is kept constant by active compensation. Topics concerning the system design, sensor sensitivity, and crosstalk between sensors are investigated. Experimental and numerical computational results are presented. >

Optical fiber sensor technology in Portugal

Abstract A general overview of the R&D activity in fiber optic sensing developed over the last fifteen years in Portugal is given. Different topics are addressed, including interferometric, intensity and Bragg grating based fiber optic sensors, signal processing and multiplexing techniques, optical current sensors, together with some references to field trials and applications. Possible guidelines for present and future national R&D activity on this subject are outlined.

Spectroscopy of doped lithium niobate

The values of measured and calculated spectroscopic quantities of lithium niobate doped with rare earth and transition metal ions, such as polarized emission and absorption cross sections, variation of fluorescence life time with temperature and concentration of the dopant, Judd- Ofelt coefficients, non-radiative transition probabilities and energy levels are presented. Wherever published data is available, comparison with measured or calculated data presented in this work is carried out. The theories utilized in the interpretation of the experimental results, such as Judd-Ofelt theory, Fuchtbauer-Lademburg relation and McCumber theory are summarily presented.

Resonant tunneling diode electroabsorption waveguide modulator operating at around 1550 nm

Summary form only given.The InGaAlAs/InP RTD-electroabsorption modulator (EAM) consisted of a unipolar InGaAlAs (1520 nm) ridge optical waveguide containing a double-barrier resonant tunneling diode (6 nm wide InGaAs quantum well and 2 nm thick AlAs barriers). The experimental set-up for measuring the DC electroabsorption response of the RTD-EAM consisted of an external cavity 1.5 /spl mu/m tunable diode laser which was fibre coupled to the RTD-EAM; the output from the RTD-EAM was fibre coupled to an optical power meter. DC characterisation of the electroabsorption response of the modulator shows a change in absorption of 5 dB for a 1 mV change in bias voltage within the NDR region of the RTD. This is an exceptionally low drive voltage for this amount of modulation. A total change in absorption of 13 dB was observed as the RTD-EAM biasing voltage was swept through the NDR region.

High speed electroabsorption modulator by integration of a RTD with an optical waveguide

High speed Optoelectronic modulation using the resonant tunnelling characteristics by interaction of a resonant tunnelling diode (RTD) in the middle of an unipolar ridge optical waveguide based on the GaAs/AlGaAs system is investigated. This interaction of an RTD within an optical waveguide introduces high non-linearities into the electrical characteristics of the waveguide by inducing large electric field changes on the collector region which give rise to a band-edge shift via the Franz-Keldysh effect that can be used for electroabsorption modulation up to millimeter-wave frequencies of light at photon energies close to the bandgap energy of the semiconductor, in this case around 900 nm. The device allows efficient high speed intensity modulation requiring a few hundreds of mV as drive voltage. Streak camera studies have shown around 4 dB modulation depth at 14 GHz for an applied voltage at 1 GHz of around 0.4 V, implying a bandwidth-to-voltage ratio of 33 GHz/V.

Optical amplification in localized doping of Er:Ti:LiNbO3 waveguides

An investigation on optical amplification in Ti waveguides in LiNbO3 doped with Er ions by thermal diffusion of thin metallic stripes is presented. The possibility of fabricating efficient optical amplifiers in LiNbO3 substrates realized by localization of the dopant on surface areas of the crystals was theoretically evaluated and the feasibility of fabricating efficient amplifiers in such doped structures was experimentally verified. It was concluded that the localized doping technique allows optimization of amplifier performance through adjustment of the active region geometry to the mode intensity profile.