An analysis of different cases of few-mode micro-optical fibers from 10 to 1 microns in diameter is performed based on solving the eigenvalue equation using both the weak guidance approximation (scalar LP modes) when the refractive index difference is small, and the exact full eigenvalue equation (vector TE, TM, HE and EH modes), when the refractive index difference is large, for example having air or a gas as the surrounding medium. One of the objectives of this analysis is to show at what point the propagation constant and optical field intensity of the fundamental modes LP01 and HE11 differ significantly depending of the refractive index difference, the other objective is to find out the evolution of the other modes along the final tapered section in a few mode fiber taper. The graphical behavior of the solutions of the eigenvalue equation is presented and the optical intensity distributions are calculated for different sizes, as for example in adiabatic tapers to evaluate the extent of the evanescent field. In general, the propagation constant and effective refractive index depends on the size of the core waveguide diameter, the refractive index difference and the wavelength. This analysis is useful to calculate the extension of the evanescent field in liquids or gases for optical fiber sensors that can be used to model, for example, fluorescent optical fiber sensors for biological or industrial applications. Additionally, the propagation characteristics of the few-mode micro optical fiber could be controlled or tuned by changing the refractive index of the surrounding media by changing, for example, its temperature.
The modeling of a temperature optical fiber sensor is proposed and experimentally demonstrated in this work. The suggested structure to obtain the sensing temperature characteristics is by the use of a mechanically induced Long Period Fiber Grating (LPFG) on a tapered single mode optical fiber. A biconical fiber optic taper is made by applying heat using an oxygen-propane flame burner while stretching the single mode fiber (SMF) whose coating has been removed. The resulting geometry of the device is important to analyze the coupling between the core mode to the cladding modes, and this will determine whether the optical taper is adiabatic or non-adiabatic. On the other hand, the mechanical LPFG is made up of two plates, one grooved and other flat, the grooved plate was done on an acrylic slab with the help of a computerized numerical control machine (CNC). In addition to the experimental work, the supporting theory is also included.
The modal characteristics of tapered single mode optical fibers and its strain sensing characteristics by using mechanically induced long period fiber gratings are presented in this work. Both Long Period Fiber Gratings (LPFG) and fiber tapers are fiber devices that couple light from the core fiber into the fiber cladding modes. The mechanical LPFG is made up of two plates, one flat and the other grooved. For this experiment the grooved plate was done on an acrylic slab with the help of a computer numerical control machine. The manufacturing of the tapered fiber is accomplished by applying heat using an oxygen-propane flame burner and stretching the fiber, which protective coating has been removed. Then, a polymer-tube-package is added in order to make the sensor sufficiently stiff for the tests. The mechanical induced LPFG is accomplished by putting the tapered fiber in between the two plates, so the taper acquires the form of the grooved plate slots. Using a laser beam the transmission spectrum showed a large peak transmission attenuation of around -20 dB. The resultant attenuation peak wavelength in the transmission spectrum shifts with changes in tension showing a strain sensitivity of 2pm/μɛ. This reveals an improvement on the sensitivity for structure monitoring applications compared with the use of a standard optical fiber. In addition to the experimental work, the supporting theory and numerical simulation analysis are also included.
Micro Optical Fibre Biosensors (MOFBs) are emerging as one of the most sensitive bio-detection system technologies which do not require of labelling or amplification of the analyte. In these devices, a short region of the fibre core is exposed to the external environment so that the evanescent field can interact with biological species such as cells, proteins, and DNA. In order to increase the sensitivity and selectivity, MOFBs are often used in combination with other optical transduction mechanisms such as changes in refractive index, absorption, fluorescence and surface plasmon resonance. In this work we present the full characteristics, analysis and design of a MOFBs for Flavin and Porphyrin detection.
A detailed description of a common path interferometer is given for CARS and SRS applications. In this
interferometer, both probe and reference arms are separated in time and polarization by a birefringent crystal.
In order to generate the pump pulse a Ti:Sa laser centered at 808 nm is used, as for the Stokes pulses a
Nd:YVO centered at 1064 nm is used, both lasers are synchronized in phase and frequency at a repetition rate
of 80MHz. Acetone is employed for analysis and detection in this experiment, with a extinction ratio of
1/250, a temperature heating control system is developed for the calcite birefringent crystals with a precision
of de ±0.01°C as well as an analysis of the contributions of both processes CARS and SRS by using a lock-in
amplifier.
The temperature sensitivity of a tapered Yb-doped fiber laser is numerically investigated. The laser rate equations are modified to analyze the output characteristics of the tapered fiber laser in the continuous wave regime under different temperature conditions. Numerical analysis shows that for different pump schemes, high sensitivity can be achieved when the pump power is reduced to close to the threshold value. Our results are reproducible and contribute new information to the development and optimization of tapered Yb-doped fiber lasers and temperature fiber laser sensors.
This work is focused on the design of a 250W high-intensity continuous-wave fibre optic laser with a 15μm spot size
beam and a beam parameter product (BPP) of 1.8 for its use on
Laser-assisted Cold Spray process (LCS) in the
micro-machining areas.
The metal-powder deposition process LCS, is a novel method based on Cold Spray technique (CS) assisted by laser
technology. The LCS accelerates metal powders by the use of a
high-pressure gas in order to achieve flash welding
of particles over substrate. In LCS, the critical velocity of impact is lower with respect with CS while the powder
particle is heated before the deposition by a laser beam. Furthermore, LCS does not heat the powder to achieve high
temperatures as it happens in plasma processes. This property puts aside cooling problems which normally happen in
sintered processes with high oxygen/nitrogen concentration levels.
LCS will be used not only in deposition of thin layers. After careful design, proof of concept, experimental data, and
prototype development, it should be feasible to perform micro-machining precise work with the use of the highintensity
fibre laser presented in this work, and selective deposition of particles, in a similar way to the well-known
Direct Metal Laser Sintering process (DMLS).
The fibre laser consists on a large-mode area,
Yb3+-doped, semi-diffraction limited, 25-m fibre laser cavity,
operating in continuous wave regime. The fibre shows an arguably high slope-efficiency with no signs of roll-over.
The measured M2 value is 1.8 and doping concentration of 15000ppm. It was made with a slight modification of the
traditional MCVD technique. A full optical characterization will be presented.
The critical power level provides an objective tool for the determination of the maximum power available in a fiber laser based
on physical parameters as: core diameter, temperature, and absorption and emission cross section for both the pump and laser
wavelengths. This work presents a theoretical study of critical power levels when Ytterbium-doped fibers are exposed to
changes of temperature. We found that critical power curves extend their wavelength dependence, ranging from 1 μm to 1.2 μm
when fibers were heated up 300°K. Also we found that critical power values are large than those obtained in conditions of room
temperature. While low critical powers were obtained at lower temperatures (around 77°K) with a reduced interval of
wavelengths from 1 μm to 1.1 μm.
High power, high-energy, rare-earth-doped fiber laser sources are now considered a
suitable option for a number of applications in Medicine, Telecom, LIDAR and industrial
applications. Regarding the generation of high-energy pulses, there is a compromise between gain
volume and M2 value or beam parameter product for the generation of pulses with useful energy
content, for the aforementioned applications and good pulse shape and even a Gaussian output.
In this paper, we will present a set of large mode area fiber designs along with full optical
characterization data will be presented. Results on energy per pulse from 4 to 20KHz repetition
rate and up to 1.3mJ energy for a 350um inner cladding LMA fiber will be discussed among other
fiber designs. All fibres were made from proprietary procedures and will be employed in micromachining
applications such as metal deposition in applications where laser-assisted cold spraying
processes are used. A full review of our LMA fibre designs will be presented.
In the article, the design and stimulation is presented of an integrated circuit for the amplification and
modulation of an electrical signal proceeding from a Micro-Electro-Mechanical Systems (MEMS)
arterial pressure sensor. The signal consists of voltage ranking from 0-10 mV, 1 mA and frequency of 50-
500 Hz. This simple but effective design consists of an operational amplifier (op-amp) configured as a
differential amplifier, which amplifies the signal (up to 1V and 10 mA), originating from a Wheatstone
bridge in the MEMS sensor, and then this signal is modulated by Pulse width modulation (PWM). The
technology employed in this circuit is MOSIS AMIS 1.5 um. The circuit was designed with a two-state
op-amp, which is utilized in diverse stages of the system. The use of a differential amplifier, the op-amp,
and PWM simplifies the design and renders this compact due to the employment of few components (40
transistors). The use of the PWM facilitates the signaling process at later stages. Results comprise the
design of the circuit and the simulation. This consists of a schematic diagram of the layers of all the rules
specified in the MOSIS AMIS 1.5 um. Electric and LTSpice software was employed for the design and
simulation of the circuit. We present a complete description of the design philosophy, design criteria,
figures, and final results.
Results on a designed piezo resistive transducer (PZR) are presented in this work. The PZR will be specially
manufactured for accurately measuring human blood pressure levels. Such transducer consists of four indifussed piezoresistors
within a 10-μm Si membrane. The voltage signal response (VSR) is predicted when pressure is applied to the
membrane, using a MEMS design tool that includes Finite Element Analysis (FEA). This transducer has the purpose of
serving as a basis for the integration of an implantable Bio-MEMS BP sensor.
MEMS technology could be an option for the development of a pressure sensor which allows the
monitoring of several electronic signals in humans. In this work, a comparison is made between the
typical elasticity curves of several arteries in the human body and the elasticity obtained for MEMS
silicon microstructures such as membranes and cantilevers employing Finite Element analysis tools. The
purpose is to identify which types of microstructures are mechanically compatible with human arteries.
The goal is to integrate a blood pressure sensor which can be implanted in proximity with an artery. The
expected benefits for this type of sensor are mainly to reduce the problems associated with the use of bulk
devices through the day and during several days. Such a sensor could give precise blood pressure readings
in a continuous or periodic form, i.e. information that is especially important for some critical cases of
hypertension patients.
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