Fiber grating sensors may be used to monitor high-speed events that include catastrophic failure of structures,
ultrasonic testing and detonations. This paper provides insights into the utility of fiber grating sensors to measure
structural changes under extreme conditions. An emphasis is placed on situations where there is a structural
discontinuity. Embedded chirped fiber Bragg grating (CFBG) sensors can track the very high-speed progress of
detonation waves (6-9 km/sec) inside energetic materials. This paper discusses diagnostic instrumentation and
analysis techniques used to measure these high-speed events.
We proposed a novel intensity modulation based composite cavity optical fiber laser (CCFL) sensor, which is
suitable for the vibration measurement. The intensity modulation of the CCFL is caused by the modulation of the
weak feedback of the CCFL. The weak feedback of the CCFL is based on the end face reflection and the Rayleigh
scattering of optical fiber. An acoustic measurement of the CCFL sensor in anechoic water pool is demonstrated to
prove that the simple sensing scheme is feasible.
Spectral characteristics of fiber Bragg gratings are affected by both strain and temperature. While this makes gratings
useful for sensing, care must be taken to ensure adequate discrimination between spectral shifts associated with strain
and those due to changes in temperature. Recently, monitoring of cladding modes has been utilized for this purpose. In
this paper, such measurement capabilities are evaluated at high temperature by exploiting the characteristics of Type II
femtosecond infrared written gratings, achieving similar responsivity with significantly improved thermal durability.
We proposed a novel optical fiber hydrophone based on the feedback effect of a composite cavity optical fiber laser
(CCFL) and a corresponding intensity demodulation scheme. The feedback of the CCFL is introduced by the end face
reflection of the single mode optical fiber. The intensity modulation of the CCFL is caused by the modulation of the
CCFL feedback.
DFB Fiber laser based sensors, including strain sensing, temperature sensing and acoustic sensing, have attached a lot
interests because of high performance and small size.We demonstrate a simple DFB fiber laser hydrophone and its
demodulation is realized by a simple intensity based scheme. The reasons of the intensity modulation of DFB fiber laser,
including gain demodulation, feedback effect, and polarization demodulation, are discussed in detail based on rate
equations. The frequency response of the DFB fiber laser based hydrophone is measured in an anechoic water pool and
compared with a referenced B&K8104 hydrophone.
KEYWORDS: Fiber lasers, Single mode fibers, Sensors, Erbium, Ytterbium, Modulation, Photons, Chemical oxygen iodine lasers, Rayleigh scattering, Signal to noise ratio
We investigate an ultra-weak feedback effect of DBR fiber lasers based on the steady-state rate equations. The
ultra-weak-feedback effects of DBR fiber lasers are introduced by the fiber Rayleigh scattering and the scattering of the
fiber splicing point. The output powers of the DBR fiber lasers are modulated when their feedbacks are modulated. A
simple vibration sensing scheme based on this effect is proposed and demonstrated. The optimized design of the sensing
scheme is discussed at last.
Fiber grating sensors can be used to support a wide variety of high speed measurement applications. This
includes measurements of vibrations on bridges, traffic monitoring on freeways, ultrasonic detection to
support non-destructive tests on metal plates, and providing details of detonation events. This paper
provides a brief overview of some of the techniques that have been used to support high speed
measurements using fiber grating sensors over frequency ranges from 10s of kHz, to MHZ and finally
toward frequencies approaching the GHz regime.
Since its development in 2003, the technique of Bragg grating inscription using an ultrafast infrared laser and a phase
mask has proven to be far more versatile than the standard ultraviolet laser approach. The ultrafast IR laser-based
process allows for the creation of grating structures in glassy and crystalline material waveguides that are not typically
UV-photosensitive, thereby creating new applications for Bragg gratings where the use of UV-photosensitive silica
fibers is not possible. In this paper we will review the studies that have been performed at the Communications
Research Centre Canada on the applications of the ultrafast laser technique to fabricate gratings in various optical fibers
and waveguides.
In order to fully calibrate hydrocodes and dynamic chemistry burn models, initiation models and detonation models of
high explosives, the ability to continuously measure the detonation velocity within an explosive is required. Progress
on an embedded velocity diagnostic using a 125 micron diameter optical fiber containing a chirped fiber Bragg grating
is reported. As the chirped fiber Bragg grating is consumed by the moving detonation wave, the physical length of the
unconsumed Bragg grating is monitored with a fast InGaAs photodiode. Experimental details of the associated
equipment and data in the form of continuous detonation velocity records within PBX-9502 are presented. This small
diameter fiber sensor has the potential to measure internal detonation velocities on the order of 10 mm/μsec along path
lengths tens of millimeters long.
Short fiber lasers are increasingly studied due to their applications in communications and sensing1. These lasers require high concentrations of Erbium (Er) and Ytterbium (Yb) that are not compatible with the presence of Germanium (Ge) in the fiber core2. In stark contrast with more conventional fabrication methods, ultrafast lasers now allow for grating inscription within fibers having no Ge doping3. Normally for short gratings the reflected signal dispersion is small and relatively harmless to the operation of long cavities. As cavity length decreases however the signal will tend to travel more and more within the gratings, interacting with them proportionately more often. Hence a thorough understanding of the grating dispersion characteristics becomes even more important. As a result of their physical differences, the characteristics of ultrafast gratings can vary substantially from those produced using more conventional fabrication methods, and it is unknown whether these factors in combination with a high dopant concentration will significantly affect the dispersion properties of such gratings. In this study, Bragg gratings made with infrared (IR) femtosecond radiation and a first order phase mask were inscribed in fibers heavily doped with Er and Yb as well as a pure silica core fiber. Subsequent measurements of the power spectra, group delay and group delay ripple (GDR) are reported herein.
In this paper, the statistical distributions of polarization mode dispersion (PMD) for a pulse and pulse broadening due to
PMD for short pulses are examined by simulations using the waveplate model with strong mode coupling. It is found
that, like the case of differential group delay (DGD) for a single frequency, the distribution of PMD for a pulse is still
Maxwellian with a smaller mean value than the mean DGD for one specific frequency. The statistical distribution of
pulse broadening with various pulse widths and PMD values were simulated. The distribution of pulse broadening due to
PMD for short pulses fits the modified Rayleigh dis tribution with lower probability of pulse broadening than that for
long pulses in the small pulse broadening region.
Wireless technology is a cost-effective means to bring broadband communications to both mobile users and home consumers; however, deploying next generation, multi-GHz wireless systems is currently too expensive. For these systems, photonic technologies can bring cost reduction as well as an increase in performance, mainly due to the ultra low-loss property of optical fibers. One approach to signal distribution is to capitalise on the vast fibre-optic distribution networks deployed within and between cities. A microwave carrier can be optically deployed from central offices to remote antenna sites using these optical links. This paper will discuss the generation of such a microwave carrier using a dual-wavelength, external-cavity laser (ECL).
Two different dual-wavelength ECL's, constructed with fiber-Bragg-gratings (FBG's), have been investigated. One uses a semiconductor gain chip with a dual-FBG acting as an external reflector. The other uses two similar dual-FBG reflectors on each side of a semiconductor optical amplifier (SOA). In both cases the wavelength separation between the gratings is 0.25 nm.
We will demonstrate that a dual-wavelength emission can be temporarily stabilized in the gain-chip ECL if a specific phase relation, between the external feedback from the FBG's and the residual feedback from the gain chip, is satisfied at both lasing wavelengths. The power of the RF beat signal generated by the dual-wavelength optical signal was typically 25 dB above the noise floor. The 3-dB linewidth of the RF signal was approximately 2 MHz and it can be tuned over a frequency range of 200 MHz. The physical mechanisms underlying the observed laser instability will be briefly discussed.
The dispersion curves of tapered fibers at various geometries are characterized by using white-light interferometry. In this presentation, the white-light interference patterns at frequencies near the taper’s second zero-dispersion frequency are measured and discussed in detail. A more convenient formula is proposed to fit the experimental data at these frequencies. As a result, small dispersion values can be calculated more efficiently.
Traditionally in the process of writing fiber Bragg gratings with a phase mask the fiber is placed near or in close contact with the mask. With low coherence excimer sources this is necessary because the fringe visibility is greatly reduced beyond 500 μm. As a result of these limitations there has been increased interest in understanding the interference phenomena associated with a phase mask. During the past year we studied the beam interference phenomena associated with ultrafast gratings. We observed that with these coherent sources it was possible to write gratings remotely (Phase Mask-Fiber distance of ≈1 cm). In addition to this we observed evidence of walk-off between mask orders that significantly affected the interference patterns. In this paper we demonstrate that a frequency doubled Argon-ion laser, being a coherent source, can be used to inscribe fiber Bragg gratings at large distances from the phase mask (> 15 mm). We will demonstrate that walk-off between mask orders will change the interference profile along the grating length. We show that the spectral profile correlates with the calculated interference pattern. Beam walk-off effects play a role in the inscription of any photonic device with a phase mask. This remote writing technique can be used to tailor the index modulation pattern in the fiber and could potentially be used to produce two beam interference gratings even in the presence of a significant zero order amplitude.
It has been known for sometime that the tap angles associated with slanted, tilted and blazed Bragg grating structures can be affected by the guided mode's state of polarization (SOP). Recently this polarization dependent out-coupling has been employed in order to develop a number of useful devices including in-line polarimeters and PDL equalizers. Although a variety of tools are available to model blazed fibre Bragg grating (FBG) characteristics, a simplified explanation of the fundamental dependencies and potential behaviour has never been fully presented in the literature, making the optimization of these devices difficult and elusive at times. In this submission we present a thorough, intuitive discussion of these trends and possibilities as observed through an extensive theoretical analysis rooted in the Volume Current Method (VCM). In addition to discussing the potential limitations and shortcomings of this formulation, some rough guidelines for the manufacture of various devices are also disclosed.
The system outage probabilities (probabilities that BER>10-12) due to polarization mode dispersion (PMD), polarization dependent loss (PDL) and their combination are calculated for 10 and 40 Gb/s systems. It is found that the combination of PMD and PDL has a stronger impact on 40 Gb/s system than on 10 Gb/s system.
A new waveplate model of polarization mode dispersion (PMD) is proposed. Using finite number of polarization maintaining fiber (PMF) sections in the new model, it can generates any differential group delay (DGD) values, while the conventional model produces zero possibility beyond certain DGD range. Importance-sampling technique is used to simulate the DGD probability density at large DGD range. Simulation results show that the DGD probability density function generated by the new model is Maxwellian even when the probability density is as low as 10-35.
A novel method of measuring the polarization dependent loss (PDL) and the polarization mode dispersion (PMD) of fiber Bragg gratings (FBGs) is proposed and demonstrated experimentally. This method eliminates the influence of components (e.g. circulator) on the PDL and PMD characterization of FBGs. The experimental results are compared with those measured using other methods.
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