The generation of short pulses in fiber lasers using saturable absorbers made of graphene oxide (GO), focusing on film thickness, was studied and optimized. The saturable absorber comprised a GO thin film deposited onto a single-mode fiber using the spray coating technique. Water-dispersed GO with a concentration of 4 mg/mL, characterized by a high proportion of monolayer flakes, was employed. This thin film was integrated into a cavity ring laser featuring an erbiumdoped fiber amplifier (EDFA), resulting in a fiber laser emitting at a central emission wavelength of approximately 1564 nm and having a total cavity length of approximately 120 m. By controlling intracavity polarization, short-pulsed light was generated through mode-locking, Q switching, or a combination of both regimes.
This work presents a comprehensive characterization of the cavity ring laser operating under the mode-locking regime. It encompasses an analysis of the spectral behavior, focusing on the evolution of the Kelly’s sidebands with increasing pump power, as well as an assessment of its temporal stability. Moreover, the effects of the aging of the saturable absorber material were studied after a time period of 6 months after the fabrication. It was observed that the general characteristics of spectral signal of the laser were maintained, with long-term stability.
In this work, an erbium-doped fiber ring cavity based on a 3dB optical coupler for refractive index measurements is presented and experimentally verified. This interrogation system, based on a 1 x 2 optical coupler, uses one of these output ports to increase the reflected optical power by means of a fiber Bragg Grating (FBG), used as a reflector. Moreover, the other output port is used as a refractive index sensing head. A spectral analysis of this interrogator system as well as a fiber cavity ring refractive index sensor characterization are carried out. Finally, an experimental comparison of the refractive index sensor range when the 3dB coupler is replace by a 1x3 one is also presented.
In this work, two new interferometric sensors based on multicore optical fibers for the measurement of strain with the ultimate goal of traffic monitoring are presented. The operating principle of each sensor relied on the monitoring of the phase shift difference accumulated between the supermodes of the structure of the multicore segment in a full round trip. The strain characterization for both sensors resulted in a linear response, with sensitivities of -4.073·10-3 rad/με and - 4.389·10-3 rad/με for the aligned and V-shaped cases respectively, and one-hour instabilities below 4.6·10-3 rad with a 95% confidence level. These results suggest its feasibility in applications requiring high sensitivities over very wide strain ranges, such as heavy-vehicle traffic monitoring. To corroborate the hypothesis, both sensors were integrated into the pavement and their response to the traffic was analyzed.
In this work, we present an experimental measurement of temperature and strain sensitivities of a micro-drilled optical fiber (MDOF). The MDOF consisted of a quasi-randomly distributed reflector along a single mode fiber (SMF). A fiber cavity laser based on MDOF was experimentally studied, attaining a single-wavelength laser emission centered at 1568.6nm. The output power level obtained from this single-laser oscillation when pumped at 140mW was around - 9.6dBm, and an optical signal to noise ratio (OSNR) of around 45dB was measured. Although temperature sensitivities of fiber Bragg gratings used as sensors are similar to our MDOF, strain sensitivity is enhanced around one order of magnitude when the MDOF was used.
In this work we demonstrate the multiplexing capability of new optical fiber Fabry-Perot interferometers based on airmicrocavities using a commercial FBG interrogator. Three optimized air-microcavity interferometer sensors have been multiplexed in a single network and have been monitored using the commercial FBGs interrogator in combination with FFT calculations. Results show a sensitivity of 2.18 π rad/mε and a crosstalk-free operation.
In this work, a fiber ring laser sensor has been employed to analyze the influence of passive Er-doped fiber acting as saturable absorber. Lasing modes of fiber ring lasers can be reduced by properly locating a saturable absorber that narrows the spectral width of the FBG that selects the lasing wavelength. Employing commercial Er-doped fiber, different configurations have been evaluated, reaching the SLM regime on a ring cavity of several meters. Thus, the achieved strain response of the whole sensor behaves at its mirror FBG, exhibiting linear response to strain.
This work presents an experimental demonstration of a scheme based on an internally modulated fiber ring laser for high-sensitivity
temperature sensing. The attained temperature resolution has been as low as ± 2pm even when a commercial
FBG with a sensitivity of 10 pm/°C was used. Thus, a fivefold improvement in the temperature sensor resolution can be
achieved when compared to a simple FBG interrogation scheme. In addition to this, the measuring range could be selected
only by changing the frequency modulation of the fiber ring laser. This technology also allows to triple the photodiode
bandwidth unambiguously when temperature or strain measurements are carried out, which is a remarkable achievement
in term of cost reduction.
In this paper, a new sensor system for relative humidity measurements based on its interaction with the evanescent field of a nanowire is presented. The interrogation of the sensing head is carried out by monitoring the fast Fourier transform phase variations of one of the nanowire interference frequencies. This method is independent of the signal amplitude and also avoids the necessity of tracking the wavelength evolution in the spectrum, which can be a handicap when there are multiple interference frequency components with different sensitivities. The sensor is operated within a wide humidity range (20%–70% relative humidity) with a maximum sensitivity achieved of 0.14rad/% relative humidity. Finally, due to the system uses an optical interrogator as unique active element, the system presents a cost-effective feature.
In this work, a new multiplexing scheme for ultra-long range measurements (up to 200 km) is presented. Time and wavelength sensor multiplexing is achieved by means of a modulated random distributed feedback laser (DFB). The direct modulation of the laser’s cavity allows the interrogation of sensors by measuring the reflected power for different wavelengths and distances. As a proof of concept, nine fiber Bragg gratings (FBGs) placed at different fiber locations and wavelengths have been interrogated.
It is presented an optical fiber sensing system projected to operate in the demanding conditions associated with coal waste piles in combustion. Distributed temperature measurement and spot gas sensing are requirements for such a system. A field prototype has been installed and is continuously gathering data, which will input a geological model of the coal waste piles in combustion aiming to understand their dynamics and evolution. Results are presented on distributed temperature and ammonia measurement, being noticed any significant methane emission in the short time period considered. Carbon dioxide is also a targeted gas for measurement, with validated results available soon. The assessment of this technology as an effective and reliable tool to address the problem of monitoring coal waste piles in combustion opens the possibility of its widespread application in view of the worldwide presence of coal related fires.
The combustion of coal wastes resulting from mining is of particular environmental concern and therefore the importance of the proper management involving real-time assessment of their status and identification of probable evolution scenarios is recognized. Continuous monitoring of combustion temperature and emission levels of certain gases opens the possibility to plan corrective actions to minimize their negative impact in the surroundings. Optical fiber technology is well-suited to this purpose and in this work it is described the main attributes of a fiber optic sensing system projected to gather data on distributed temperature and gas emission in these harsh environments.
In this work, a wavelength division multiplexed fiber ring laser, based on optical add-drop multiplexers to interconnect intensity sensors has been experimentally demonstrated. Three different laser lines were obtained simultaneously all with an optical signal to noise ratio higher than 30dB. This proposed configuration is based on commercial devices and is adapted to the ITU channels normative. By using this configuration each sensor was associated with a different wavelength directly offered by each OADM and a reference wavelength was also included in order to distinguish between power variations induced by the transducer or to detect a fiber failure. This sensor system has been experimentally verified by using microbending sensors obtaining experimental slope sensitivity as good as -0.327dB/μm.
In this work, an all-fiber loop mirror using a clover microstructured fiber for the simultaneous measurement of
temperature and strain is presented. The sensing head is formed by a short piece of clover microstructured fiber with 35
mm length. The geometry of the fiber allowed observing different interferences created by the microstructured fiber core
section. Different sensitivities to temperature and strain were obtained and, using a matrix method, it is possible to
discriminate both physical parameters. Resolutions of ±2ºC and ±11 με, for temperature and strain, respectively, were
attained.
We experimentally demonstrate a resilient wavelength division multiplexed (WDM) fiber ladder network, based on
optical add-drop multiplexers (OADMs), to interconnect sensors. It recovers operation after failures and it enables "selfdiagnosis",
the identification of the failed element(s) from the patterns of surviving end-to-end connections at all
operating wavelengths. A theory for such topologies, the "binary state connectivity analysis", used a matrix formalism to
predict the surviving channels at the receiver node after damage occurs in one or more sites. We present experimental
measurements for double failures in the network, demonstrating complete agreement with theoretical predictions.
In this work, a novel single-longitudinal-mode (SLM) four-wavelength laser configuration for sensing applications in
L-band is proposed and experimentally demonstrated. This spectral region presents some advantages for the detection of
dangerous gases. The sensor system presented here is based on ring resonators, and employs fiber Bragg gratings to
select the operation wavelengths. The stable SLM operation is guaranteed when all the lasing channels present similar
output powers. It is also experimentally demonstrated that when a SLM behavior is achieved, lower output power
fluctuations are obtained.
A hybrid Erbium-Brillouin fiber laser sensor to measure the temperature along 22 km fiber is proposed and
experimentally demonstrated. A multi-line laser oscillation is induced by the Brillouin gain of different concatenated
transducer fiber sections placed in the ring cavity. Integral temperature measurements of each fiber section are obtained
through each laser line. This sensor can be used to monitor the temperature of off-shore wind energy power cables.
An experimental comparison of the stability performance of two different optical multiwavelength lasers is shown based
on erbium highly doped fiber amplification. The two laser structures compared were: a star configuration using a 2x4
coupler to extract the signal from the ring to 4 FBGs and a structure with 4 active ring resonators that use both the same
pump laser. Both lasers use Fiber Bragg gratings (FBG) to select the operation wavelengths within the resonant ring
cavity.
The stability of this new laser is compared with the achieved by a single ring configuration. The improvement of both
stabilities depending on the time and temperature for the new structure was experimentally demonstrated.
The utilization of the new configuration for multiplexing optical fiber sensors has also been carried out. The sensors
provide amplitude modulation in response to the curvature introduced by a temperature-sensitive element. Each sensor is
identified by a different wavelength generated by each FBG. The signal generated by the grating firstly is used as
reference signal ant secondly is launched towards the sensors and the transmitted signal is measured. The sensors show a
stable and linear response when measurements are carried out using simultaneously all the outputs of the structure for
transmitting the multiple wavelengths to the sensors and utilizing the serial referencing output of the laser.
We propose a hybrid network that combine point and distributed Brillouin sensors in an architecture that also deploys
remote distributed Raman amplification to extend the sensing range. A 46-km proof-of-concept network is
experimentally demonstrated integrating point vibration sensors based on fiber-optic tapers, with distributed temperature
sensing along the network bus. The sensor network with a double-bus topology offers a higher optical signal to noise
ratio and dynamic range than a single-bus for intensity point multiplexed sensors. In this network, we include low-cost
intensity sensors that are able to measure vibrations in the 0.01 to 50 Hz frequency range, which are important in the
monitoring of large infrastructures such as pipelines.
This paper shows a long-distance remote sensing system using a multiwavelength Raman laser. The sensor network is
based on a 50 km of standard single-mode fiber (SMF) and is composed by a simple cavity based on a loop mirror and
four Fiber Bragg Gratings (FBGs) arranged in a star configuration. FBGs are used for both the sensing function and the
selection of the lasing wavelengths. The system is designed to be inherently resilient to fiber failures. The
multiwavelength laser has been characterized for temperature measurements showing a good stability performance.
In this work, we present and experimentally demonstrate a switchable Erbium-doped fiber laser for remote sensing
applications. The laser uses four Fiber Bragg Gratings (FBGs) for wavelength selection and for temperature sensing and
a 2x4 optical switch. By adjusting the switch combinations, the laser can be switched among the four different
wavelength lasing configurations. Stable one- and two- wavelength oscillations were achieved based on the use of this
device. An output power instability analysis with the temperature for two different multi-wavelength configurations, one
of them using the 2x4 optical switch and the other using a 1x4 coupler, was performed, for a cavity length of 50 km. A
comparison between the performances of these topologies is carried out, and their temperature sensitivity and
stabilization time are shown.
In this work, a novel CWDM (Coarse Wavelength Division Multiplexing) network that multiplexes intensity modulated
optical fibre sensors is presented. These transducers are based on a sensing material that suffers reversible changes in its
optical properties in presence of organic vapours. This compound is fixed onto cleaved ended single mode optical fibres
pigtails using Electrostatic Self-Assembly method. As their response depends on intensity changes, a referencing signal
is simultaneously generated to eliminate the effect of optical source power fluctuations. Thus, a parallel array of phaseshifted
fibre Bragg gratings (PS-FBGs) is included to obtain the referencing signal. Looking forward to remote sensing
applications, an Erbium doped fibre amplifier (EDFA) is included in the header of the network to reach up to 4 km of
distance. Four new sensors have been multiplexed and exposed to saturated atmospheres of methanol and ethanol
individually, showing different response and recovery times in each case. Finally, we checked that our sensors were able
to distinguish between pure and kerosene polluted water probes in terms of response and recovery times as well.
This paper presents the application of a new erbium-doped fiber ring laser configuration for the multiplexing
of intensity sensors. The proposed scheme exploits two separated branches to generate simultaneous emission
at four different wavelengths. Fiber Bragg gratings are used to select the operating wavelengths within the
erbium-doped fiber ring. The first branch presents a serial configuration to generate the reference lasers. The
second branch exploits a tree configuration and includes the optical intensity temperature sensors. A comparison
between both branches of the same setup is carried out, and their temperature and time stability is shown by
varying the erbium doped fiber.
A new amplified CWDM (Coarse Wavelength Division Multiplexing) self-referencing sensor network using phase-shifted
fiber Bragg gratings (PS-FBGs) is experimentally demonstrated in this work. The network uses the PS-FBGs to
address intensity sensors in a transmissive configuration, obtaining simultaneously in reflection a wavelength encoded
reference signal. In order to enable the remote operation of the sensors, we have introduced optical amplification at the
interrogation header of the network, using highly doped erbium fiber.
We report on the use of a frequency-reflectometry domain technique for referencing optical intensity sensors located
between two fibre Bragg gratings. The experiment combines the concept of frequency modulated continuous wave with
the spectrally selective mirror properties of fibre Bragg gratings to interrogate with referencing properties intensity based
sensors. Multiplexing two of these sensors using this technique in a parallel topology system was also experimentally
demonstrated. Due to the use of different fibre lengths (delay lines), signals from the fibre Bragg gratings located at
different positions in an array are separated in the frequency-domain.
An amplified optical fiber double ring network for the wavelength division multiplexing (WDM) of fiber Bragg grating
(FBG) sensors is demonstrated experimentally. The network is inherently resilient to fiber failures due to the
simultaneous interrogation of all the sensors using both rings. Furthermore, power transparency at the rings is obtained
by means of Raman amplification. We optimize the performance of the network with the launched pump power that
shows low levels of amplifiers' noise. We also demonstrate how the topology allows the received powers from the
sensors to be equalized.
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