An extrinsic fiber Fabry-Perot interferometer (EFFPI) is proposed and implemented for the characterization of waveguides inscribed by the femtosecond-laser direct writing technique. Various waveguides of 7.08 mm in length were inscribed in a soda-lime glass substrate by varying the laser scanning velocity in order to induce different refractive index (RI) changes. The measurement of the RI of the waveguide was carried out by means of an EFFPI formed between the end face of a multicore fiber (MCF) with seven coupled cores and the two polished surfaces of the inscribed waveguide. The end face of the MCF tip provided a broad beam and a wide effective area to ensure a large collection of the reflected light. The optical spectrum obtained by the interference of the multiple reflected beams was transformed to the Fourier domain and visualized in real-time. In the Fourier domain, the waveguides’ optical path length (OPL) was obtained and used to calculate its refractive index since its physical length was fixed and known. To obtain the value of the RI of the medium surrounding the waveguide, the fiber tip was displaced parallel to the polished surface, about 40 μm far from the center of the waveguide. The relative refractive index difference (Δn), which defines the light propagation and the insertion loss in a waveguide, was calculated for each waveguide inscribed at different velocities.
A multimode interference (MMI) sensor was designed and experimentally demonstrated for simultaneous measurement of curvature and temperature. A typical fiber structure of single-mode fiber – multimode fiber – single-mode fiber (SMS), mounted in a long and thin carbon steel sheet, and then coated with polydimethylsiloxane (PDMS) was manufactured and tested in curvature and temperature. Bending laboratory results showed that the proffered sensor has a curvature sensitivity of -0.9835 dB/m-1 over a range from 0 m-1 to 1.3652 m-1 , measurements were taken by keeping a constant temperature of 30°C. The laboratory temperature response was -119 pm/°C at a temperature range from 30°C to 60°C, showing an improvement in temperature response, temperature measurements were taken by keeping a constant bending of 0 m-1 . The results show that PDMS coatings are a good way to improve multimode interferometer sensitivity during temperature measurement while keeping a good curvature measuring response, moreover the device shows a linear response within the curvature and temperature ranges. Another advantage of the PDMS coating is that it makes the sensor insensitive to refractive index changes, it gives the sensor robustness and protection against dust.
We report on a contactless optical fiber refractive index (RI) sensor that can be used to measure the RI of solid or liquid samples. The sensor is simple to construct and consists of a Fabry-Perot interferometer (FPI) with long cavity. The cavity of our FPI consists of a tapered optical fiber tip and an external flat mirror. The output beam of the fiber tip is partially reflected from different interfaces of the sample present in the interferometer cavity. Each of such reflections interferes with the beam internally reflected by the fiber tip. Thus, a multiple-beam FPI is formed whose reflection spectrum is composed by the superposition of several two-beam interferences. The analysis of the multiple interference spectra was carried out in the Fourier domain. Several glass samples, water-sucrose and water-glycerol solutions were prepared and tested. Since the fiber tip is not in direct contact with the sample under test the measurement is simple and immediate. To our-knowledge, this is the first time that a fiber optic sensor can be used to measure the RI of solid and liquid samples without any modification.
Long-period fiber gratings (LPFGs) and tapered fibers are spectrally selective devices with important applications in sensing, especially for monitoring chemical and physical parameters. When combined, they can give rise to devices with enhanced characteristics that can be tailored by varying their fabrication parameters. We analyze the characteristics of arc-induced LPFGs written on the longer transition of asymmetric tapers, which provide a variable diameter section whose slope can be adjusted in order to tailor the device performance. In comparison with LPFGs fabricated in nontapered fibers, the grating inscription in the linear taper transition produces deeper notches (>20 dB ), whose bandwidth and separation increase with the transition slope. The simplicity of the electric arc technique to fabricate LPFGs, and the degrees of freedom added by the tapered fiber open up the possibility to realize thorough studies on the many possible combinations, to develop devices with characteristics designed on purpose.
An experimental study of the characteristics of arc-induced long-period fiber gratings inscribed in the longer transition of asymmetric adiabatic tapers is presented. Compared to the inscription in non-tapered fibers, the grating inscription in the linear taper transition resulted in deeper notch bands, whose bandwidth and separation increased with the transition slope. Changes in external refractive index from 1 to 1.606 resulted in variations in the notch bandwidth and spectral position.
We study numerically and experimentally multiple-wavelength operation of an erbium-doped figure-eight fiber laser
including a multiple-bandpass optical filter formed by two concatenated fiber tapers. Both continuous-wave and pulsed
operations are considered. In the continuous-wave regime, stable long-term operation at multiple closely spaced
wavelengths is only obtained if fine adjustments of the cavity losses are performed. Under these conditions,
simultaneous lasing at up to four wavelengths separated by 1.5 nm was observed experimentally. Tunable single-wavelength
operation over more than 20 nm is also observed in the continuous-wave regime. In the passive mode
locking regime, numerical simulations indicate that mechanisms involving the filter losses and the nonlinear
transmission characteristic of the NOLM contribute in principle to stabilize dual-wavelength operation, allowing less
demanding cavity loss adjustments. In this regime, the problem of synchronization between the pulse trains generated at
each wavelength adds an additional dimension to the problem. In presence of cavity dispersion, the pulses at each
wavelength tend to be asynchronous if the wavelength separation is large, however they can be synchronous in the case
of closely spaced wavelengths, if cross-phase modulation is able to compensate for the dispersion-induced walkoff.
Experimentally, fundamental and 2nd-order harmonic mode locking was observed, characterized by the generation of
noise-like pulses. Finally, a regime of multi-wavelength passive Q-switching was also observed. We believe that this
work will be helpful to guide the design of multiple-wavelength fiber laser sources, which are attractive for a wide range
of applications including Wavelength Division Multiplexing transmissions, signal processing and sensing.
We present the calculations of the higher order dispersion coefficients in a photonic crystal fiber. The dispersion
coefficient is obtained from the calculated effective index of the fiber structure by double differentiation with respect to
the wavelength (or frequency), or from the integral formula used to calculate the group index. Although both approaches
are equivalent we show that they lead to totally dissimilar results, like different zero dispersion wavelength and hence the
higher-order dispersion coefficients are different. In fact, the way in which the higher-order dispersion coefficients are
calculated will affect the prediction of the dynamics of the supercontinuum generation. This effect will be discussed in
this work.
We present the fabrication and characterization of a micro-displacement sensor using Mach-Zehnder
interferometer in conventional optical fiber SMF28-e. The Mach-Zehnder interferometer uses a
configuration of two long-period gratings (LPG) in series mechanically induced. The Mach-Zehnder
interferometers were made to operate in the region of 1300 nm. As a result the interferometers were
obtained with transmission bands with a bandwidth of 2 nm, extinction ratio of 12 dB and insertion loss of
2 to 3 dB. The characterization of the interferometer was found to be measured displacements up to 500
μm with a resolution of 7 microns, which envisions potential applications of micro-displacement sensor in
the measurement of micro-deformations.
Assuming that the imaginary part of the propagation constant of leaky modes is much smaller that its real part, we
describe a procedure for the calculation of the effective refractive index of multilayer optical fibers. In our method
there is no need to look for roots in the complex plane, since the differential equations for the transverse modal
distribution are expressed in terms of the absolute value of the propagation constant.
We present the temperature response of a mechanically-induced long-period fiber grating (MLPFG) made in photonic
crystal fiber (PCF) with and without the coating polymer. In both cases, we found a wavelength shift to shorter
wavelengths and a critical decrease of the attenuation peaks. A maximum wavelength shift of 6 nm at 1060 nm was
obtained when the temperature changed from 20 to 80 °C in PCF without the polymer. Whereas, the depth of the
attenuation peaks were dramatically reduced from 12 to almost 2 dB at 1060 nm when the temperature increase from 20
to 100 °C in both experiments. These results are important to consider when MLPFG are applied in a medium with room
temperature variation.
Optical microfibers decorated with PdAu nanoparticles are proposed for fast hydrogen sensing. The microfibers were
obtained by simply tapering conventional telecommunications fiber down to dimensions comparable to the wavelength
of the guided light. A few millimeters of the microfiber were coated with a PdAu layer in island form by depositing the
layer at low evaporation rate (0.1 Å/s). Then the islands were grown with a thermal annealing process until composite
nanoparticles were formed. The PdAu nanoparticles deposited on the optical microfibers experience optical and physical
changes when they exposed to hydrogen. This gives rise to reversible transmission changes with an unusual pulsed like
behavior which is attributed to scattering of the guided light. The devices are promising for detecting low concentrations
of hydrogen (up to 8%) at room temperature with response and recovery times on the order of seconds.
We demonstrate the minimization of background loss for arc-induced long-period fiber gratings in standard fiber by Taguchi's optimization method. We use Taguchi's method to determine the optimum values for parameters like electric-arc power, arc duration, and tensile strain applied over the fiber during the inscription process. With these optimal parameters, we minimize the background loss resulting from the geometrical deformations of the fiber. The experimental results show that background loss can be reduced from more than 1 dB to less than 0.3 dB at rejection bands with isolation >15 dB.
A method for tuning the transmission characteristics of arc-induced long-period fiber gratings by local heating is
presented. A traveling burner, produced by the combustion of a mixture of oxygen and butane, locally heats the longperiod
grating producing a wavelength shift of the resonant peaks. We have found that the resonant wavelengths are
blue-shifted during the first 7 round trips of the flame, but when heating process continues the notches shifts toward
longer wavelengths. A fine and long range tuning of the resonant wavelengths up to 120 nm can be achieved without
substantial degradation on the grating characteristics. The process is repeatable and only takes a few minutes.
We propose a temperature sensor based on dispersion shifted long period fiber grating (LPFG) fabricated with the
fattening method, using electric arc discharges from a commercial fusion splicer machine. The fattened LPFG are
centered at 1524 and 1550 nm, respectively. A temperature characterization in terms of the transmitted spectrum is
realized with a pumped Er+3 doped fiber as the light source. The less than 3 mm in length fattened LPFG reached a
sensitivity of 72 pm/°C between a temperature range of (25-500°C). Temperature stability was observed among the
temperature range of (25-200°C), result in agreement with our previous studies. From up to 200°C sensing is observed
as temperature increases. To apply this LPFG as a temperature sensor, sensing was measured as voltage signal outputs
with repeatable results. This sensor is a high temperature resistant device because non deterioration of its properties was
observed, and can find useful applications when temperature stability may be needed or the sensing requirements are
desirable.
By using two optical fibers and a capillary it is possible to measure the refractive index of liquids. Light
leaving a fiber is sent transversally to a capillary that behaves as a cylindrical lens when liquids are
inserted in it. Focused light is collected by a second fiber and sent to a detector.
We report an all-optical fiber hydrogen sensor based on absorption changes of evanescent
fields caused by an annealed Pd/Au thin film. The sensor consists of a small piece of standard
single-mode fiber (SMF) coated with a Pd/Au thin film sandwiched between two multimode
fibers (MMFs). Due to core diameter mismatch the SMF cladding guides light. When the
device is exposed to hydrogen the layer refractive index diminishes and causes attenuation
changes of the evanescent fields. Adding gold to palladium allows the fabrication of fast,
durable, and reliable sensors suitable for the detection of hydrogen concentration below the
critical 4%.
The fabrication of a compact all-fibre modal interferometer that can be used to sense different parameters is reported.
The device consists of a tapered large-mode-area microstructured optical fibre with collapsed air holes over a localized
region. The tapering is carried out by slowly elongating the fibre while it is heated with a high-temperature oscillating
flame torch. This non adiabatic method allows the collapsing of the air holes and transforms a section of the
microstructured fibre into a solid one. As a consequence the fundamental HE11 mode is coupled to the HE1m cladding
modes which can beat or interfere. This makes the transmission of the device versus the wavelength to exhibit an
oscillatory pattern. Such a pattern shifts with strain, high temperature, or refractive index. The device is compact, can
operate in a broad range of wavelengths, and can be fabricated in a few minutes which makes it attractive for optical
sensing.
Metal-coated singlemode tapered optical fibers are demonstrated for high-resolution refractive index sensing. The fundamental fiber mode can excite the different hybrid surface plasmon modes supported by the semi-cylindrical metallic shell. The transmission spectrum of our devices exhibits multiple resonance peaks with either TE- or TM-polarized light. The number of resonance peaks as well their depth depend on the taper diameter and also on the external refractive index. Experimentally we have found that the first two or three resonance peaks are the most sensitive peaks to refractive index changes. We estimate that a resolution of about 2x10-7 can be achieved with our sensors. Our devices can be used for high-resolution biosensing.
The sensor is based on a chirped fiber grating made in a fiber tapered by fusion. The grating has a metallic jacket and acts as sensor element by comparison with the second grating that provides a reference signal. The sensor is interrogated by measuring the power reflected by the two gratings. The system has an accuracy of 0.05°C over a linear operation range of more than 10°C that can be adjusted in the fabrication process.
A nonlinear spectrally selective Sagnac interferometer based on a special configuration of a birefringent fiber is demonstrated. Spectral resolution less than 1 nm can be easily obtained. The theoretical and experimental characteristics of the mirror are derived. The applications of this fiber reflector as a multichroic mirror and passive mode-locking and Q-switching element are discussed.
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