Coherent Fiber-Optic Sensor for Ultra-Acoustic Crack Emissions.

A coherent optical fiber sensor with adequate sensitivity for detecting the acoustic emission (AE) during the propagation of a crack in a ferrous material is presented. The proposed fiber optic sensor is successfully compared in terms of the SNR (Signal to Noise Ratio) and detectable AE energy levels to commercially available AE piezo-transducers sensors and is proven to be an effective and advantageous alternative for sensing and monitoring fatigue damage in structural applications.

Entropy, Carnot Cycle, and Information Theory.

The fundamental intuition that Carnot had in analyzing the operation of steam machines is that something remains constant during the reversible thermodynamic cycle. This invariant quantity was later named "entropy" by Clausius. Jaynes proposed a unitary view of thermodynamics and information theory based on statistical thermodynamics. The unitary vision allows us to analyze the Carnot cycle and to study what happens when the entropy between the beginning and end of the isothermal expansion of the cycle is considered. It is shown that, in connection with a non-zero Kullback-Leibler distance, minor free-energy is available from the cycle. Moreover, the analysis of the adiabatic part of the cycle shows that the internal conversion between energy and work is perturbed by the cost introduced by the code conversion. In summary, the information theoretical tools could help to better understand some details of the cycle and the origin of possible asymmetries.

Performance comparison between electrical copper-based and optical fiber-based backplanes.

A performance comparison between the electrical Cu-based backplane and a full-optical fiber-based backplane is presented in terms of capacity and power consumption. By means of systematic simulations we find the electrical configuration, which allows to optimize the Cu-based backplane by exploiting the best technologies available today. On the other hand, a fiber-based optical backplane is proposed by exploiting the most performing VCSEL sources. Limitations of the electrical and optical approaches are discussed, considering their capabilities to support up to about 25-Gb/s transmission and the possibility to evolve towards higher bit-rates.

Analog nonlinear MIMO receiver for optical mode division multiplexing transmission.

The complexity and the power consumption of digital signal processing are crucial issues in optical transmission systems based on mode division multiplexing and coherent multiple-input multiple-output (MIMO) processing at the receiver. In this paper the inherent characteristic of spatial separation between fiber modes is exploited, getting a MIMO system where joint demultiplexing and detection is based on spatially separated photodetectors. After photodetection, one has a MIMO system with nonlinear crosstalk between modes. The paper shows that the nonlinear crosstalk can be dealt with by a low-complexity and non-adaptive detection scheme, at least in the cases presented in the paper.

PDM-16QAM transmission performance over uncompensated fiber links.

The dependence of propagation performance on the signal baud-rate is investigated by simulations for WDM PDM-16QAM systems operating at a spectral efficiency of 4 b/s/Hz. We take into account the case of transmission over uncompensated links, both for standard single-mode fiber and non-zero dispersion-shifted fiber. Three baud-rates are tested: 16.25, 32.5 and 65 Gbaud, including overhead for soft-decision FEC. Hence, we compare the performance limited by the nonlinear impairments in case of 100, 200 and 400 Gb/s data rate transmission, respectively. We demonstrate that the trade-off between higher OSNR margin and nonlinear transmission penalty favors transmission at lower baud-rate (16.25 Gbaud) and narrower channel spacing (25 GHz), among the three different simulated baud-rates.

Microwave tunable dispersion compensator for optical fiber systems.

A tunable device based on chirped microstrip delay lines is proposed to precompensate at the transmitter; the chromatic dispersion accumulated during optical fiber propagation. Compensated dispersion is finely tuned by changing the effective dielectric constant of the microstrip line by means of moving dielectric perturbers. Compensation up to 51 ps/GHz necessary to propagate over 400 km uncompensated standard single-mode fiber at 10 Gb/s is demonstrated. The proposed solution does not require coherent detection and can find application in metropolitan and regional area networks, where the physical path traced by each channel can change owing to the traffic routing, requiring the dynamic compensation of different amounts of accumulated dispersion.

Gouy phase shift in nondiffracting Bessel beams.

The results of a theoretical and experimental investigation of the Gouy effect in Bessel beams are presented. We point out that the peculiar feature of the Bessel beams of being nondiffracting is related to the accumulation of an extra axial phase shift (i.e., the Gouy phase shift) linearly dependent on the propagation distance. The constant spatial rate of variation of the Gouy phase shift is independent of the order of the Bessel beam, while it is a growing function of the transverse component of the angular spectrum wave-vectors, originated by the transverse confinement of the beam. A free-space Mach-Zehnder interferometer has been set-up for measuring the transverse intensity distribution of the interference between holographically-produced finite-aperture Bessel beams of order from zero up to three and a reference Gaussian beam, at a wavelength of 633 nm. The interference patterns have been registered for different propagation distances and show a spatial periodicity, in agreement with the expected period due to the linear increase of the Gouy phase shift of the realized Bessel beams.

Optical phase conjugation in phase-modulated transmission systems: experimental comparison of different nonlinearity-compensation methods.

We experimentally compare the effectiveness of three different optical-phase-conjugation-based nonlinearity-compensation strategies on a transmission system employing phase-modulated signals, and hence affected by the Gordon-Mollenauer effect. We demonstrate that it is possible to obtain significant nonlinearity compensation, but that no improvement is obtained using configurations specifically aimed at the compensation of the nonlinear phase noise.

Roughness induced backscattering in optical silicon waveguides.

We report on the direct observation of backscattering induced by sidewall roughness in high-index-contrast optical waveguides based on total internal reflection. Our results demonstrate that backscattering is one of the most severe limiting factors in state-of-the art silicon on insulator nanowires employed in densely integrated photonics. We also derive the general relationship between backscattering and geometrical and optical parameters of the waveguide. Further, the role of roughness in polarization rotation and coupling with higher-order modes is pointed out.

Adjustable-chirp VCSEL-to-VCSEL injection locking for 10-Gb/s transmission at 1.55 microm.

1.55-microm single-mode VCSEL frequency chirp behavior is investigated in under-threshold and above-threshold operating conditions for different VCSEL-to-VCSEL injection locking configurations with respect to free-running case. We experimentally evaluated the capability of adjusting the frequency chirp, reducing its value and inverting the sign. The control over the frequency chirp is obtained changing the wavelength detuning and power injection ratio between the VCSEL master and the VCSEL slave. Advantages of the chirp inversion are demonstrated for 10 Gb/s error-free propagation at 1.55-microm over 40-km standard single mode fiber without any dispersion compensation.

Impact of Rayleigh backscattering on Stimulated Brillouin Scattering threshold evaluation for 10 Gb/s NRZ-OOK signals.

At bit rates comparable with the Brillouin shift, i.e. higher than 10 Gbit/s, the signal and the Brillouin backscattered spectra partially overlap. This implies an interaction between different scattering phenomena occurring through out the optical fiber. In particular we believe that an evaluation of how Rayleigh backscattered components of the modulated signal are subjected to Stokes gain is required. This interaction may lead to an increased backscattered power, which in turn will affect Brillouin threshold estimation. We experimentally verified a decrease of Stimulated Brillouin Scattering (SBS) threshold for 10 Gb/s NRZ-OOK signals with respect to theoretical predictions. Simulations carried out with a numerical model of SBS, accounting for Rayleigh contributions, well predict measured backscattered power levels. On the other hand we also experimentally verified that this SBS threshold decrease does not degrade transmission system performance. Indeed, measured BER curves put into evidence a penalty reduction for signal powers just before the saturation regime, which should be usefully taken into consideration in optical systems power budget planning.

Evidence of Raman-induced polarization pulling.

The interest towards passive control of the light polarization through nonlinear effects has been stimulated by recent works: in particular a polarization pulling effect has been obtained by means of stimulated Brillouin scattering. Here we investigate the condition for obtaining polarization pulling by exploiting the stimulated Raman scattering, which is most suitable for optical communications thanks to its large gain bandwidth. The role of the polarization-dependent Raman amplification and of the random fiber birefringence is clarified by theoretical considerations and numerical simulations starting from the vector theory of the Raman effect in optical fiber. Experiments carried out with a 1571-nm signal and high-power 1486-nm pump evidence the Raman-induced polarization pulling.

Error-free continuously-tunable delay at 10 Gbit/s in a reconfigurable on-chip delay-line.

A coupled-resonator optical waveguide (CROW) consisting of a chain of directly coupled ring-resonators (RRs) fabricated in 4.5%-indexcontrast silicon oxynitride technology is employed to control the delay of optical pulses with continuity and over several bit-slots. The moderate deterioration of the signal quality versus the delay is demonstrated by the observation of error-free transmission (BER < 10(-9)) at 10 Gbit/s for fractional delays of up to 3 bits, with fractional losses below 1 dB per bit-delay. The high storage efficiency of the device, exceeding 0.5 bit/RR, enables an easy management of the delay and the reduction of the footprint down to 7 mm(2). The presented reconfiguration scheme is hitless with respect to data transmission, since the CROW delay can be tuned without halting the data flow, while preserving the signal quality.

Measurement of PMD tolerance in 40-Gb/s polarization-multiplexed RZ-DQPSK.

We experimentally investigate the first-order polarization-mode dispersion (PMD) tolerance of two polarization-multiplexed (POLMUX) RZ-DQPSK signals at overall 40 Gb/s. The polarization demultiplexing is enabled by an automatic endless polarization stabilizer. Time-interleaving the two orthogonally polarized RZ-DQPSK signals minimizes the crosstalk due to the non-ideal polarization stabilization, while it represents the worstcase for the PMD-induced crosstalk. Bit-error rate measurements are performed both in back-to-back and after 25-km standard single-mode fiber. The PMD tolerance is evaluated as a function of the instantaneous differential group delay, introduced by a first-order PMD emulator. 40-Gb/s POLMUX RZ-DQPSK is more sensitive to PMD than single-polarization 20-Gb/s DQPSK, while it is more PMD-tolerant than 40-Gb/s NRZ-OOK. Besides, its chromatic dispersion robustness is similar to the single-polarization 20-Gb/s DQPSK. The combination of POLMUX and DQPSK is therefore very promising in view of transmission systems at high bit-rate.

High-contrast waveguides in sputtered pure TeO2 glass thin films.

We present a technological approach to the realization of channeled optical waveguides, starting from reactively sputtered tellurite glass thin films, grown on silica-coated 4" Si wafers. In particular, optical lithographic process and etching recipes have been developed to overcome the solubility of TeO(2) films in aqueous solutions, and to process them into high-index contrast structures with minimized post-etch roughness. Optical tests on preliminary rib waveguide geometries feature 6.3 dB/cm propagation loss for fundamental TE mode at lambda = 1.5 microm.

Strain Wave Acquisition by a Fiber Optic Coherent Sensor for Impact Monitoring.

A novel fiber optic sensing technology for high frequency dynamics detection is proposed in this paper, specifically tailored for structural health monitoring applications based on strain wave analysis, for both passive impact identification and active Lamb wave monitoring. The sensing solution relies on a fiber optic-based interferometric architecture associated to an innovative coherent detection scheme, which retrieves in a completely passive way the high-frequency phase information of the received optical signal. The sensing fiber can be arranged into different layouts, depending on the requirement of the specific application, in order to enhance the sensor sensitivity while still ensuring a limited gauge length if punctual measures are required. For active Lamb wave monitoring, this results in a sensing fiber arranged in multiple loops glued on an aluminum thin panel in order to increase the phase signal only in correspondence to the sensing points of interest. Instead, for passive impact identification, the required sensitivity is guaranteed by simply exploiting a longer gauge length glued to the structure. The fiber optic coherent (FOC) sensor is exploited to detect the strain waves emitted by a piezoelectric transducer placed on the aluminum panel or generated by an impulse hammer, respectively. The FOC sensor measurements have been compared with both a numerical model based on Finite Elements and traditional piezoelectric sensors, confirming a good agreement between experimental and simulated results for both active and passive impact monitoring scenarios.

Continuously tunable 1 byte delay in coupled-resonator optical waveguides.

A reconfigurable coupled-resonator optical waveguide made of a few directly coupled ring resonators is employed to control the delay of data streams modulated at tens of gigabits per second. A delay of 8 bit lengths (1 optical byte) with a small pulse broadening and 1 dB/bit fractional loss is achieved by using only eight rings. The limiting role of waveguide loss and spurious backreflections is experimentally investigated. The high storage efficiency (1 bit/ring) of the device enables an easy, reliable, hitless, and relatively low-power-consuming management of the delay. A higher storage efficiency is demonstrated to be associated to an unavoidable higher pulse distortion.

Optical coherence pulsed interferometry: shaping probe pulses in time-domain interferometry.

The autocorrelation of a modulated coherent light source is used as a probe pulse in a time-domain interferometry scheme. With respect to conventional techniques, higher flexibility in selecting the shape of the probe pulse can be achieved by simply acting on the modulation parameters. The complex amplitude of short pulses propagating through a generic optical device can be directly measured, with no need for fast sampling and time synchronization. The potentialities of the technique are shown by reporting measurements of amplitude distortion, group delay, and frequency chirp of optical pulses transmitted through integrated ring resonators.

Reflection photoelastic tomography for the detection of stress distribution in planar optical waveguides.

A novel method for measuring local stress distributions and birefringence of films on substrates and planar optical waveguides, with submicrometric resolution, is presented. The technique relies on a reflective tomographic configuration, applied in conjunction with a polarimetric setup, which processes the stress-induced change of the state of polarization of a laser probe beam reflected at the waveguide-substrate (film-substrate) interface. By this means, theoretically foreseen stress behavior can be experimentally verified and spurious or induced local stress variations in integrated optics components can also be brought into evidence. The feasibility of the proposed method has been verified by reconstructing the two-dimensional axial stress distribution in the 4 x 2 microm(2) core region of a doped silica-on-silicon optical waveguide.

Experimental confirmation of matched bends.

Modal conversion between the fundamental mode and the first-order (leaky) mode has been experimentally investigated in high-index-contrast integrated-optical bent waveguides. The matched bend condition has been experimentally confirmed on cascaded bends, and the effect of both matched and unmatched bends on Y branches has been investigated. Good agreement between theoretical predictions and experimental results for both monomode and multimode waveguides is achieved. It is demonstrated that, in bent waveguides, the modal conversion between the fundamental mode and the first (leaky) mode can be accurately controlled to avoid additional losses and transfer function impairments.