We present a novel amplified space-time coding technique which combines cyclic-Simplex and Simplex binary codes to overcome the main limitations of conventional coding for OTDR based ultra-long distance distributed temperature sensing applications. The decoding process is performed in two successive steps, addressing the main issue related to the computational complexity of conventional codes, which increases quadratically with the code lenght, seriously affecting their performance when dealing with extremely long code-words. A link control technique is also proposed to suppress gain transients induced by the EDFA dynamics, avoiding performance degradation due to nonlinear effects and codewords distortion. The proposed scheme provides significant coding gain enhancement and stable operations below the stimulated Raman scattering threshold, pushing the performance of Raman based distributed temperature sensors close to their physical limit using commercial off-the-shelf components.
We propose a flexible computational EDFA dynamic model for amplified modulated probes used in long-distance distributed sensors based on pulse coding, employing Rayleigh, Raman and Brillouin Scattering phenomena. The EDFA dynamics is studied numerically solving the differential equations of the reservoir model for Erbium ion concentrations. The model allows one to identify the optimal choice of gain parameters as well as the gain control mechanism needed for gain stabilization, minimizing both the code-words output power fluctuations and avoiding nonlinear effects which can degrade the distributed sensor performance when using pulse coding techniques such as Simplex, Golay and Cyclic- Simplex. The technique allows distributed sensors to be robust against fluctuations induced by long amplified code sequences, ensuring at the same time enough optical gain to enhance their performance, especially in terms of sensing distance, without overcoming the threshold of nonlinear propagation effects.
It is well known that interferometers make great sensors. This is because phase is a magnitude that can be measured with very accurate precision and with a great dynamic range at the same time. Integrating these devices on a chip is very appealing because it can make the device much smaller, lighter, and affordable. However, this technology also poses some challenges, which can degrade performance with respect to free-space or fiber-based devices. In this work we overview these challenges and possible strategies to tackle them.
Control systems for automotive applications have rapidly evolved introducing intelligence to address the increasing demand for pollution and oil consumption reduction. Fiber Bragg Grating (FBG) sensors are used in this work for monitoring Gasoline Direct Injectors (GDI) in order to optimize the engine performance and reduce the emissions. Several fast-acting solenoid injectors have been instrumented with FBG sensors and mounted in a test bench at the testing department of CPT Italy S.r.l to simulate the injector's behavior during the actuation phase. The FBG sensors, installed on the stem of the GDI, provide dynamic measurement of the strain variation during the injection process, pointing out the unwanted effects of the reopening, leading to injector tip wetting and consequent increased polluting emissions. The acquired data allows one to fully understand the GDI process and to optimize the injector design in order to reduce emissions, as required by recent European directives for the emission standards.
Distributed Acoustic Sensing (DAS) is a technology with interesting features for real-time safety and security monitoring applications, and constitutes a steadily growing share of the optical fiber sensing market. Recently, the quantitative measurement of disturbances using DAS schemes based on Phase-Sensitive Time Domain Reflectometry (Φ-OTDR) has become a focus of investigation. In this contribution, we propose and experimentally demonstrate a stable homodyne phase demodulation scheme in a fiber optic Φ-OTDR sensor using a double pulse probe and a direct detection receiver. We show that a carrier for the distributed dynamic phase change induced by an external perturbation can be generated by selective phase modulation of one of the probing pulses. The local phase is then retrieved from the backscattering signal using a demodulation technique robust against light intensity disturbances, which have been limiting factors in existing phase demodulation schemes. In addition, the method is independent of the phase modulation depth and does not require computationally costly multi-dimensional phase unwrapping algorithms necessary when using I-Q demodulation in DAS, and is a suitable candidate for analogue signal processing. We demonstrate the capacity of the sensor to measure the distributed dynamic phase change induced by a nonlinear actuator generating a 2 kHz perturbation at a distance of 1.5 km with an SNR of ~24 dB. The demodulated multi-frequency response is also shown to be consistent with one obtained using a point senor based on an FBG and a commercial reading unit.
In this paper, we experimentally demonstrate a hybrid distributed acoustic and temperature sensor (DATS) based on
Raman and coherent Rayleigh scattering processes in a standard singlemode fiber. A single commercial off-the-shelf
DFB laser and a common receiver block are used to implement a highly integrated hybrid sensor system with key
industrial applications. Distributed acoustic sensing and Raman temperature measurement are simultaneously performed
by exploiting cyclic Simplex pulse coding in a phase-sensitive OTDR and in Raman DTS using direct detection. Suitable
control and modulation of the DFB laser ensures inter-pulse incoherence and intra-pulse coherence, enabling accurate
long-distance measurement of vibrations and temperature with minimal post-processing.
We propose and experimentally demonstrate a Distributed Acoustic Sensor exploiting cyclic Simplex coding in a phase-sensitive OTDR on standard single mode fibers based on direct detection. Suitable design of the source and use of cyclic coding is shown to improve the SNR of the coherent back-scattered signal by up to 9 dB, reducing fading due to modulation instability and enabling accurate long-distance measurement of vibrations with minimal post-processing.
In this paper we demonstrate distributed Raman temperature sensing (RDTS) in a loop scheme employing anti-Stokes
light intensity only. Using a single-channel receiver and anti-Stokes traces measured in loop configuration, we
implement RDTS with inherent compensation of fiber wavelength-dependent losses, as well as local external
perturbations. Experimental results show a signal-to-noise ratio enhancement with respect to a standard RDTS in loop
configuration, providing a robust and reliable high-performance sensor for long sensing ranges.
In this paper we propose the use of optimized bi-directional distributed Raman amplification to enhance the operating
range of Brillouin optical time-domain analysis (BOTDA) sensors. In particular by combining high-power fiber-Raman
lasers and polarization-multiplexed Fabry-Pérot lasers operating at 1450 nm with low relative-intensity-noise (RIN), we
demonstrate distributed sensing (using first-order Raman amplification) over 120 km of standard single-mode fiber with
2 meter spatial resolution and with a strain/temperature accuracy of 45με/2.1°C respectively.
KEYWORDS: Erbium, Ions, Absorption, Waveguides, Silicon, Optical amplifiers, Amplifiers, Simulation of CCA and DLA aggregates, Laser optics, Signal attenuation
We present an extensive study of an Er doped Silicon Rich Silicon Oxide (SRSO) based material used for the realization of
optical waveguide amplifiers in which Si-nanoclusters (Si-ncls) are formed by thermal annealing. In particular we focus our
attention on the confined carrier absorption (CCA) mechanism within the Si-ncls and on the fraction of Er ions coupled to
them. Experimental data are used for accurate modeling of Si-ncls sensitized EDWAs (Erbium Doped Waveguide
Amplifiers) longitudinally pumped by visible broad area lasers.
Although the material requires further optimization to be effectively deployed, accurate numerical simulations of Si-ncls
sensitized EDWAs, based on this material and longitudinally pumped by visible broad area lasers at 660 nm, point out
significant benefits provided by the nanoclusters sensitization. Our model, based on the Finite Element Method, performs the
modal analysis of the guiding structure, and then allows to study the propagation of pump and signal electric fields along the
waveguide amplifier; the rate equations for the coupled Er/Si-ncls system account for their coupling ratio.
Numerical results, based on measured material parameters, point out that resonant pumping at 660 nm provides significant
benefits in terms of gain enhancement, with respect to standard EDWAs, even at low Er/Si-ncls coupling ratio. This feature
suggests that a careful design can lead to the realization of compact integrated amplifiers and lasers, compatible with CMOS
technology.
KEYWORDS: Waveguides, Amplifiers, Erbium, Absorption, Simulation of CCA and DLA aggregates, Optical amplifiers, Ions, Silicon, Silica, Refractive index
We present a multimode longitudinal pumping scheme for integrated rare-earth doped waveguide amplifiers which
allows an efficient use of low cost multimode pump sources. The scheme is based on evanescent pump light coupling
from a multimode low loss waveguide, which is gradually transferred to a single mode Si-nc sensitized Er3+ doped active
core. Population inversion is ensured along the whole amplifier length, thus overcoming the main limitation of
conventional single mode pump butt-coupling in case of strongly absorbing active materials. Great flexibility in
controlling the pump power intensity values within the active core is also provided.
We propose this pumping scheme at 477 nm for Si-nanocluster sensitized Erbium doped waveguide amplifiers, in
which top pumping by LED arrays is limited by the low pump intensity values achievable within the active region.
The coupling between the multimode waveguide and the active core has been numerically studied for slab waveguide
structures using a 2D split-step finite element method.
Numerical simulation results, based on propagation and population-rate equations for the coupled Er3+/Si-nanoclusters
system, show that high pump intensities are indeed achieved in the active core, ensuring good uniformity of the
population inversion along the waveguide amplifier.
Although longitudinal multimode pumping by high power LEDs in the visible can potentially lead to low-cost integrated
amplifiers, further material optimization is required. In particular, we show that when dealing with high pump intensities,
confined carrier absorption seriously affects the amplifier performance, and an optimization of both Si-nc and Er3+
concentrations is necessary.
KEYWORDS: Raman spectroscopy, Wavelength division multiplexing, Fiber lasers, Optical amplifiers, Interference (communication), High power lasers, Single mode fibers, Signal to noise ratio, Amplifiers, Polarization
In this paper we describe advanced bi-directional Raman pumping schemes for extending the maximum reach achievable in long-span unrepeated WDM transmission systems operating at 10 Gb/s. We describe both first- and higher-order bi-directional Raman pumping schemes, pointing out that co-pumping, although critical in terms of noise transfer from pump to WDM signals, can be very effective in extending the maximum system reach. New co-propagating Raman pump lasers, characterized by high power levels and low relative intensity noise, are also proposed to further increase the maximum achievable span loss, avoiding transmission penalties induced by relative intensity noise transfer.
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