Towards Distributed Measurements of Electric Fields Using Optical Fibers: Proposal and Proof-Of-Concept Experiment.

Nowadays there is an increasing demand for the cost-effective monitoring of potential threats to the integrity of high-voltage networks and electric power infrastructures. Optical fiber sensors are a particularly interesting solution for applications in these environments, due to their low cost and positive intrinsic features, including small size and weight, dielectric properties, and invulnerability to electromagnetic interference (EMI). However, due precisely to their intrinsic EMI-immune nature, the development of a distributed optical fiber sensing solution for the detection of partial discharges and external electrical fields is in principle very challenging. Here, we propose a method to exploit the third-order and second-order nonlinear effects in silica fibers, as a means to achieve highly sensitive distributed measurements of external electrical fields in real time. By monitoring the electric-field-induced variations in the refractive index using a highly sensitive Rayleigh-based CP-φOTDR scheme, we demonstrate the distributed detection of Kerr and Pockels electro-optic effects, and how those can assign a new sensing dimension to optical fibers, transducing external electric fields into visible minute disturbances in the guided light. The proposed sensing configuration, electro-optical time domain reflectometry, is validated both theoretically and experimentally, showing experimental second-order and third-order nonlinear coefficients, respectively, of χ(2) ~ 0.27 × 10-12 m/V and χ(3) ~ 2.5 × 10-22 m2/V2 for silica fibers.

Unusual properties of α7 nicotinic acetylcholine receptor ion channels in B lymphocyte-derived SP-2/0 cells.

This study demonstrates the presence of α7 nicotinic acetylcholine receptors (nAChR) in B lymphocyte-derived SP-2/0 cells by means of flow cytometry and immunocytochemistry. According to lectin and sandwich ELISA, the α7 subunits expressed in SP-2/0 cells are more glycosylated compared to those expressed in the brain or normal B lymphocytes and are combined with ß2 subunits. At zero and negative pipette potentials, either acetylcholine or α7-specific agonist PNU282987 stimulated the ion channel activity in SP-2/0 cells revealed by single channel patch-clamp recordings. The conductivity was within the range of 19 to 39 pS and reversal potential was between -17 mV and +28 mV, the currents were potentiated by α7-specific positive allosteric modulator PNU120596 and were partially blocked by α7-specific antagonist methyllicaconitine (MLA). However, they were oriented downwards suggesting that the channels mediated the cation outflux rather than influx. As shown by Ca2+ imaging studies, PNU282987 did not stimulate immediate Ca2+ influx into SP-2/0 cells. Instead, Ca2+ influx through Ca-release-activated channels (CRACs) was observed within minutes after either PNU282987 or MLA application. It is concluded that SP-2/0 express α7ß2 nAChRs, which mediate the cation outflux under negative pipette potentials applied, possibly, due to depolarized membrane or negative surface charge formed by carbohydrate residues. In addition, α7ß2 nAChRs may influence CRACs in ion-independent way.

Linear electro-optical effect in silica fibers poled with ultraviolet lamp.

A second-order nonlinearity was induced in silica fibers poled by exposure to ultraviolet (UV) radiation and simultaneous high voltage applied to internal electrodes. The UV light source was a tubular lamp with spectral peak at 254 nm. The highest second-order nonlinear coefficient measured through the linear electro-optic effect was 0.062 pm/V. The erasure of the recorded voltage with UV excitation was studied, and the stability of the poled fiber at a temperature exceeding ~400 K was investigated. By eliminating the use of a focused laser beam as excitation source, the technique enables poling many pieces of fiber in parallel.

Optical creation and erasure of the linear electrooptical effect in silica fiber.

We study the creation and erasure of the linear electrooptical effect in silicate fibers by optical poling. Carriers are released by exposure to green light and displaced with simultaneous application of an internal dc field. The second order nonlinear coefficient induced grows with poling bias. The field recorded (~108 V/m) is comparable to that obtained through classical thermal poling of fibers. In the regime studied here, the second-order nonlinearity induced (~0.06 pm/V) is limited by the field applied during poling (1.2 × 108 V/m). Optical erasure with high-power green light alone is very efficient. The dynamics of the writing and erasing process is discussed, and the two dimensional (2D) field distribution across the fiber is simulated.

Study of thermally poled fibers with a two-dimensional model.

A two-dimensional (2D) numerical model is implemented to describe the movement of ions under thermal poling for the specific case of optical fibers. Three types of cations are considered (representing Na(+), Li(+) and H3O(+)) of different mobility values. A cross-sectional map of the carrier concentration is obtained as a function of time. The role of the various cations is investigated. The assumptions of the model are validated by comparing the predictions to experimental data of the time evolution of the nonlinearity induced. A variational analysis of poling parameters including temperature, poling voltage, sign of the bias potential and initial ionic concentrations is performed for a particular fiber geometry. The analysis allows identifying the impact of these parameters on the induced second-order nonlinearity in poled fibers.

Temperature characteristics of the birefringence properties of filled side-hole fibers.

The temperature characteristics of the birefringence of side-hole fibers filled with liquids or metal are investigated, aiming at providing a basis for on/off temperature sensing. Short pieces of fiber are filled and the change in birefringence is registered using measurements in reflective mode of the transmitted power through a linear polarizer at 1550 nm. The rapid change in the birefringence behavior of the fiber at the temperature of the phase transition of the filler substance is shown, and from the measurement data the phase transition temperatures can be determined as well as an estimation of the birefringence change with temperature. The experimental results are supported by numerical simulations.

All-fiber Kerr cell.

An all-fiber nanosecond Kerr light gate is described that was constructed using microstructured fibers. The switching voltage for a 20 cm long device is as low as Vπ~85 V at a 1.06 µm wavelength. The device is fully spliced. The active element is a three-hole fiber provided with internal electrodes in the side-holes and a liquid core of nitrobenzene, which is fully enclosed. This work allows the exploiting of electrically driven liquid-core fibers and demonstrated the removal of the major limitations of Kerr cells in the past, allowing for integration, safe use, and relatively low switching voltage.

Pulse selection at 1 MHz with electrooptic fiber switch.

Two 78-cm long electrooptic fibers with nonlinear coefficient χ((2)) ~0.26 pm/V are used in a Sagnac loop for pulse selection at up to 1 MHz repetition rate. Laser pulses of 1.5 µm wavelength arriving at every 140 ns are selected with an extinction ratio as high as -30 dB. The arrangement is entirely based on silica fiber.

Soliton generation from an actively mode-locked fiber laser incorporating an electro-optic fiber modulator.

This work demonstrates an actively mode-locked fiber laser operating in soliton regime and employing an all-fiber electro-optic modulator. Nonlinear polarization rotation is utilized for femtosecond pulse generation. Stable operation of the all-fiber ring laser is readily achieved at a fundamental repetition rate of 2.6 MHz and produces 460 fs pulses with a spectral bandwidth of 5.3 nm.

Nanosecond monolithic Mach-Zehnder fiber switch.

An electrically controlled high-speed all-fiber switch is investigated. It is based on a monolithic Mach-Zehnder interferometer using a Gemini fiber. The fiber is provided with internal electrodes for active control of the phase using high-voltage electrical pulses. The demonstrated switching speed is 20 ns. The monolithic design guarantees that the off- and on-states are attained simultaneously for a broad range of wavelengths (50 nm). The interferometer can be switched-off using a second electrode, providing a 15 ns long optical pulse.