We report the dynamics of multi-pulse in a ring cavity passively mode-locked fiber laser with a strict control of the polarization state. We study the relation between the polarization state of the pulses propagating in the cavity and the regimes of generation. We have found that small ellipticities, the laser generates one bunch of pulses in the cavity, while at higher ellipticities the laser generates multiple bunches. At constant ellipticity we rotated the polarization azimuth and observed a regime transition from the generation of a bunch of solitons to that of noise-like pulses (NLP).
The generation of clean solitons is important for a number of applications such as optical analog-to-digital conversion (ADC) based on soliton self-frequency shift. In real sources the quality of the pulses is deteriorated by dispersive waves, continuous wave (CW), amplified spontaneous emission (ASE). The dispersive waves appear in the spectral profile as side-lobe components that would limit the resolution of ADC. Spectral compression techniques cause the appearance of a pedestal on the spectrum. All of these imperfections of pulses have to be eliminated to improve the performance of alloptical systems. The nonlinear optical loop mirror (NOLM) is a good candidate for these tasks. In the present work we report the implementation of a polarization-imbalanced NOLM for soliton cleaning. The NOLM consists of a nearly symmetrical coupler with a 51/49 coupling ratio, 100 m of twisted OFS Truewave fiber whose dispersion value is 9 ps/nm/km at 1550 nm, and a tunable in-line fiber polarization controller (PC) asymmetrically inserted inside the loop. The use of the nearly symmetrical coupler allows very low transmission for low power components of radiation. At the same time adjustment of the PC allows the adjustment of the nonlinear characteristic to have a maximum transmission for solitons with different durations. We used two sources of pulses, SESAM based and a ring fiber laser. At the appropriate adjustment of PC, we obtained a rejection of ASE by 220 times, rejection of the dispersion waves and the pedestal by more than 200 times. The maximum transmission reached 70%.
We show that trapping and manipulation of microparticles can be achieved by Rayleigh convection currents using a low power lasers. Light absorbed by thin film of amorphous silicon (a:Si) creates the convection currents. In contrast to previous works, we show that multiple trapping can be achieved using solid microparticles without the creation of vapor bubbles. For low power (~1 mW), particles are trapped at the center of the beam, however at higher powers (~3 mW) particles are trapped on a ring around the beam due to two competing forces: Stokes and thermophoretic forces. Numerical simulations confirm that thermal gradients are responsible for the trapping mechanism.
Optical fiber Q-Switch lasers have been used in a variety of application areas in science as well as in industry owing to their multiple characteristics. A possible application is that owing to their high output power they can be used as pumping sources for supercontinuum generation. Such source can be employed in optical coherence tomography (OCT) focused to dermatology. Therefore it is important to develop sources with emission wavelength that are not injurious to human skin. In the present work erbium doped fiber (EDF) was used owing that its emission wavelength (1550 nm) is adequate for this purpose. The most efficient way of achieving high power in a Q-Switch laser is optimizing all the parameters involved in the pulses generation, such as pumping power, active medium length and modulation frequency. The results show that using a fiber length of 7 meters is possible to get 10 μJ of energy, a peak power of 140 W, an average power of 27.5mW with temporal widths of 500 ns. The laser uses an acousto-optic device to modulate the internal loses inside the cavity. As highly reflecting mirrors, a Sagnac Interferometer and a Fiber Bragg Grating was employed.
Pulsed lasers have become very important owing to the great amount of applications, from communications to diverse medicine areas. Many works have reported the development of these kinds of sources which uses quite complex cavity configurations and that present instabilities in the output signal. In this work the analysis of a pulsed laser that uses a ring cavity with a length of 16.5 m is presented. A phase modulator (LiNbO3) controlled by an RF generator operated at a frequency of 12.5108 MHz was used to perform the mode lock. The modulator input has a birrefringent fiber then the light polarization affects the mode lock. Therefore it was necessary to perform an analysis and characterization in the input and output signals of the modulator in order to obtain more stable output pulses without requiring a continuous adjustment. The laser implemented with 2 modes of operation, active mode-lock and passive mode-lock. The obtained pulses whit temporal width of 7 ns FWHM for the frequency fundamental 12.5108 MHz and 781 -261-120-116 ps for the harmonic 5-10-16-20 .The results for the passive mode-lock the obtained pulses whit temporal width 2 ps and average power 200 W.
Dispersion is a quite important parameter in optical fiber systems, mainly in pulsed lasers where the temporal width of the pulses are affected by this factor. There are many interferometric arrangements to evaluate this parameter and generally, these systems vary the wavelength to obtain information about the refractive index dependency n(λ), which is contained in the interferogram phase. However, this makes the system quite slow and it requires tunable light sources with a narrow bandwidth. In this work we propose an alternative method to measure dispersion and we present the results of measurements of special optical fibers, such as microstructured, Er doped, Yb doped, and Er/Yb doped fibers, among others. The experimental arrangement consists of a Mach-Zehnder interferometer, where the fiber under test is located in one of its arms while the air is employed as the reference arm. In order to determine the n(λ) dependency, a wide spectrum light source in the wavelength range of interest was used together with an optical spectrum analyzer. The phase information was evaluated from the measured interferogram. The obtained dispersion values were in agreement with those reported by the fabricant and they were used to calibrate the system in order to obtain non reported values.
In this work an experimental setup that allows evaluating the group velocity dispersion (GVD) in optical
fibers with a hundreds of meters length is presented. The setup was composed by an optical fiber Sagnac
Interferometer, SI. The optical fiber under test and a phase modulator were placed inside of the SI loop. The
main idea is to place asymmetrically the phase modulator inside the SI loop. This means that the fiber length
in both sides of the phase modulator, L1 and L2, satisfy L1<<L2. This unbalance allows evaluating the GVD
due to the fact that contra-propagating beams travel with different propagation constants. With this technique
it was possible to measure GVD with a picosecond temporal resolution without using a fast detector or wide
band-width equipment. Results of GVD measurement performed with the developed algorithm are presented.
Fiber Bragg gratings (FBG) are important optical devices since they have been quite successful not only in the field of
communications but also in sensor systems and optical fiber lasers. In the sensors area they are generally used as
detection elements for different physical parameters such as temperature, strain, flow, etc. In the electronics and
optoelectronics laboratory at Benemérita Universidad Autónoma de Puebla (LEyO-BUAP), there are already
experimental setups of sensors as well as laser systems, where FBGs are fundamental elements for their adequate
performance. However, these FBGs are commercial devices and they present limited characteristics in their transmission
profiles, bandwidth and reflectivity. On the other hand, in some occasions, the delivery time from the fabricant to the
customer is quite long. Therefore, it is important for LEyO to implement a system to fabricate this kind of devices, which
would mean LEyO independence in the technological development. In this work, results of FBGs fabrication based on
the phase mask technique are presented. Such mask is optimized for UV and it has a period of 1060 nm. A Nd:YAG
pulsed laser with a 5 ns pulse length and an energy of 40 mJ was used as the UV source employing the 4th harmonic
generation to obtain a 266 nm wavelength. Ge-doped fiber was used to fabricate the devices.
A theoretical analysis of a multipoint sensor conformed by two Fabry-Perot cavities, both constructed with two fiber
Bragg gratings (FBG) is presented. Each cavity uses one FGB as reference and the other one as a sensing element. The
signals of the sensors were identified through their fundamental beating frequency (FBF), which is the result of the
interaction of two consecutive modes inside the cavity and its value depends upon the cavity length. The sensor response
was analyzed when there are two or more cavities in the system. In this work there is shown that is possible to determine
the magnitude of several variables from each sensor measuring the FBF signal intensity in the frequency domain using
FFT algorithm. The general conditions of the system operation are discussed. Some preliminary experimental results
about the optimal conditions to be able to distinguish the two sensor signals are presented.
The measurement of phase is important in multiple applications, such as in interferometry, radar applications,
tomography, among others. As a particular application, phase extraction can be used to evaluate the chromatic
dispersion coefficient in optical fibers. There are multiple setups that employ the phase of an interferometric
signal to evaluate this coefficient. Usually in those systems the phase shift evaluation of the sine-like
interferometric signal is performed measuring the shift of a single maximum. This means that only a few
points in the interferogram are used and many other are wasted. In this work a Fourier Transform method to
obtain interferogram phase shift to evaluate chromatic dispersion of an optical fiber is presented. Results of
the dispersion coefficient of fiber with a length of hundreds of meters evaluated by using a Sagnac
interferometer are shown.
Experimental studies of titania nanotubes (TiO2) and tungsten oxide nanospheres (WO3) as devices of saturable
absorption for a fiber laser in ring configuration to optical short-pulse generation are presented. A deposition technique,
based on optical pressure radiation generated from a coherent source at 1550 nm is used to deposit the nanostructured
materials. Since this nanomaterials can be deposited directly on the optical fiber, this proposal results very interesting for
applications of saturable absorbers. Experimental results, by using nanotubes TiO2 and nanospheres deposited on the
fiber as a saturable absorption device, show that the TiO2 nanotubes exhibit better saturable absorption properties than
WO3 nanospheres, generating pulses with a wavelength of 1550 nm, frequency of 10 MHz, temporally width of 4.5 ps
and an output power of 1 mW.
In a previous work the monitoring of the fringes pattern shift produced by the interaction of a polymeric film with
volatile organic compounds (VOC), by a Pohl interferometric arrangement has been reported. Such fringes shift was
measured by a conventional photodetector. In the present work the optimization of an optoelectronic system for detection
and quantification of VOC is presented. In this new arrangement a charge coupled device (camera CCD) was used to
monitor such fringes shift. The CCD takes advantage of the fact that each pixel can be used as a single photodetector. A
program was realized in matlab for the acquisition and processing of the data, which allows to evaluate faster the
obtained results. Up to the moment results of the system performance by the measurement of the steady state response of
dimethylpolysiloxane (DMPS) to ethanol vapour are reported.
This work propose an array using a plate λ/4 as a way to get a phase difference equal to zero between the
interferometers. Experimental results show that adjusting the orientation angles of the axis of the plate inside the loop, is
possible to get the phase shift. Although exist a decreasing in the maxim transmission peak, is possible to reduce the time
and difficulty of adjusting and achieve by mean of the temperature the need conditions to the filter tuning.
The fringes projection technique allows evaluating 3D profiles from testing objects without contact. To obtain those
profiles it is necessary to get a photograph of the projected fringes. This image contains the information of the profile
testing object as a deformation of the projected fringes. In order to evaluate the profile it is necessary to extract the phase
from image fringes, usually this process requires an arctan()function. As is well known this function is limited by ±π
and as a result the called wrapped phase is obtain. Therefore in order to get the real phase an unwrapping technique must
be employed. In this work experimental and theoretical results of implementation of Itoh algorithm to unwrap phase are
shown. This algorithm was written in MatLab and results show that this technique is suitable to measure profiles using
fringes projection if images obtained are free of shadows or if the sampling satisfies Nyquist theorem.
The industrial processes need to control many variables such as temperature, flow, pressure, position, etc. For this
reason, one sensor is required for each signal to be controlled. Therefore, multipoint sensors that can measure different
signals in the same system have been proposed. In this work a multipoint sensor, which consists of a fiber laser with two
Fabry-Perot cavities based on fiber Bragg gratings was studied. The sensor operation principle is based on the
overlapping of the gratings reflection spectra, which produce a laser emission with a wavelength of 1536 nm. For this
kind of sensors, various detection methods to identify and quantify the signals from its intermodal frequencies, have been
used which employ some circuits like PLL (Phase Loop Lock), lock-in amplifiers and electrical spectrum analyzers.
However these equipments are quite expensive. Therefore in the present work signal analysis with Fourier discrete
transform to identify and quantify the sensor signals based on the laser intermodal frequencies is proposed. Such
frequencies were 200 and 800 kHz corresponding to cavities of 500 and 130 m length, respectively.
In the present work, automation of a dynamic system to measure response of gas sensors based on quartz crystal
microbalance (QCM) is presented. Automation was used to study transient response, which is necessary for sensor
characterization. A valves system, which controls the gas flow towards the sensor, was implemented to generate the step
function. The system was fully automated by using a PIC16F877 microcontroller. Results of sensor response
measurements are presented.
This work presents an optical fiber laser used as a sensor; the system consists of a laser formed by two Fabry-Perot type
coupled cavities, two gratings as references and two more gratings as sensors.
The wavelength division multiplexor (WDM) has a great importance in the technology of the communications by optical
fiber, is an economic and efficient form, to increase the capacity of transmission by several orders of magnitude, reason
why it is desirable to have laser sources of multiple wavelengths in a system WDM, which, according to reported works,
have been obtained using diverse types of filters. In this work we presented a laser of multifrequency emission, of
Erbium doped optical fiber, tuned with an optical fiber filter. The configuration of the optical filter, presents high
stability and low lost by insertion, independence to the changes of polarization, low powers of light entrance, and has an
useful spectral wide. It has the advantage to have a simple design and easy manufacture in addition to his low losses.
Some pH sensors based on optical fibers use organic dyes which are sensitive to the pH levels. Such dyes are deposited
over the fiber and they are supported by a matrix, (TiO2) in this case. In this work, we present the results obtained of the
fabrication and characterization of an optical fibers sensor with a thin film doped with two different organics dyes
(Rhodamine 6G and Coumarin), deposited in a little section of the optical fiber by the SOL - GEL process. It was
found that each dye is sensitive to different pH ranges (2 to 12). The fiber optics sensor can be used for the measured of
the pH level of aqueous testing solution, and the combination of the dyes is possible to cover a dynamic ranges from 2
to 12. It is also analyzed the sensitivity of the sensor with the Rhodamine 6G and Coumarin dye for different molar
concentrations in a range 0.01% to 0.08% and to determine this way the best concentration for the sensor.
The necessity of detection and recognition of different types of gases, such as volatile organic compounds, which are
frequently found in food and beverage industries among others, requires the development of different types of sensors. In
this work, an application of optical fiber for the detection of volatile organic compounds, particularly ethanol is
presented. The sensor was constructed removing a portion of the cladding and depositing instead a sensing titanium
dioxide (TiO2) film doped with an organic dye (rhodamine 6G) by the sol-gel method. The sensor response was
measured in a Teflon chamber where the sample to be measured was injected. A He-Ne laser beam was coupled to the
fiber and the variation in the output power was measured which indicates the gas presence. The difference between the
output power with and without gas gives a measure of the concentration that exists in the chamber. The experimental
results showed that for an ethanol concentration range from 0 to 10500 ppm, the response of the sensor was
approximately linear with a correlation coefficient of 0.9924.
The coherent light communication systems have several advantages compared to the intensity-modulation direct-detection systems (IM-DD). The most important advantages are that offer increases transmission capacity and in principle they will limit only by the shot noise. However, there are important conditions for these systems, are that the beams must be mutually coherent, with the same polarization and, mechanisms to compensate the slow shift phase. Therefore the construction of these systems is generally most expensive that IM-DD systems. Nevertheless the use of adaptive photodetectors allows an easy implementation of this type of systems. In this work the results of a system that evaluates distances by means of an ultrasonic sensor and a coherent light communication system to send the information, with an adaptive detector based on the photo-EMF effect, are shown.
In this work an interferometric experimental setup that allows measuring the oscillation frequencies of vibrating objects is presented, using the photo-EMF effect. Due to the high sensibility of the interferometers this system can detect oscillations in the order of some nanometers. At the moment, the system can work in a frequency interval from 500 Hz to 5 kHz. However the bandwidth can be enlarged in a very simple way by changing some electronic elements, such as resistor and capacitors.
KEYWORDS: Multimode fibers, Single mode fibers, Fiber optics sensors, Signal attenuation, Sensors, Neodymium, Light wave propagation, Laser optics, Transportation security, Fiber to the x
We present results of measurements of fiber bend losses induced by periodic distortions in a single-mode and a multimode fiber having step and graded index profile. These bends are caused by soft and swellable materials with various moduli of Young. We found that the attenuation increases for longer periods of perturbations. The transition losses and pure bend losses have been calculated. Our experimental conditions show that the purebend loss is higher than the transition loss, i.e. the mode coupling mechanism of the loss is low in our experiments. These results may be used to choose suitable materials and construction for assembling a fiber optics sensor based in bend losses.
We present results of measurements of fiber bend loss caused by soft and swellable materials with perturbation periods of 1 mm to 20 mm. We used swellable materials that have different moduli of Young. We show that the swell of soft materials causes the pure bend loss that increases when the period rises. We show that the higher modulus of Young the higher the loss caused by swellable materials. These results may be used to choose suitable materials and construction for assembling a fiber optics sensor based in the bend loss.
In this work we show an education experimental setup for a single-mode fiber interferometric temperature sensor based on an Mach-Zehnder array. The used fiber has a refractive index core of 1.456 for wavelength of 633 nm. The sensor calibration was done by using ice and phase change was measured by counting of the fringes.
A novel type of distributed fiber optic sensor for petroleum hydrocarbon leak detection based on the measurement of transmitted and Rayleigh backscattered power is presented. The sensor structure includes a sensitive polymer, which reversibly expands in hydrocarbon presence and induces the bending losses in the fiber. The location of the loss region is determined from unique relationships between normalized transmitted and Rayleigh backscattered powers for different positions of the disturbance along the test fiber. The localization of a strong disturbance with an estimated accuracy of +/- 1 m along a 2.844 km-length single-mode fiber was demonstrated.
In this work we decided to show the behavior about the bend loss caused by soft and swellable materials. The principal measurements of fiber bend loss caused by soft and swellable materials were done with perturbation periods of 1 mm to 20 mm, because we needed to be close in the range of small curture. We used soft materials that have different moduli of Young. To accomplish this measure, we used single mode and multimode fibers. We had to watch the behavior over range of wavelength 1400nm to 1600nm with different radii of curvature. We would like to point out that the material that has the highest modulus of Young causes the highest loss. Ours results show that the highest attenuation and fastest sensor operation can be achieved at respectively long periods of perturbation, more than 10 mm in our experiments.
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