Strain measurements on pipelines provide a nondestructive means to evaluate their in-service conditions. They have been proposed to detect abnormal operating pressures, pipe wall problems, or intrusive pipeline events. For such applications, optical fiber sensors are especially appealing because of their hazard and electromagnetic interference-free nature. In this work, two different types of optical fiber sensors are compared for use to monitor the hoop strain on a pressurized pipe, the well-developed fiber Bragg grating (FBG) sensor and a proposed simple fiber sensor based on multimode interference (MMI). The FBG sensor shows a better linearity of strain measurement, while the MMI sensor provides a larger wavelength shift with strain at relatively low pressures. An intensity-based detection of vibrations, simulating an intrusive pipe drilling event, was achieved using the proposed MMI sensor.
An intensity-based optical fiber sensor using single-mode-multimode-single-mode fiber sections concatenation is proposed and investigated for use as an intrusion detector. The sensor is composed with simple FC/PC connections between the fiber sections. It is found to be sensitive to intrusion disturbances applied to the multimode fiber section. The transmission spectra of the device under different conditions are measured and its operation as intrusion detector is demonstrated using a single-mode laser source. More than 1 dB variation in the detected power level was observed at a wavelength of 1543 nm. The device proposed is a simple intensity-based sensor, which can be used as a stand-alone device to detect intrusion events over its length or possibly interrogated in a quasidistributed intrusion detection system based on optical time domain reflectometry.
In optical lithography light diffracted from the mask has been customary assumed to have constant amplitude with the
angle of incidence of the light illuminating the mask. This approximation, known as constant scattering coefficient
approximation, has been successfully used at small NA. As the NA increases to unity and beyond, to cope with the
continuous demand for shrinking integrated circuits device dimensions and densities, the validity of this approximation
becomes questionable. In this paper, we study diffracted field variation with the angle of incidence using physical theory
of diffraction. An asymptotic theory like the physical theory of diffraction allows us to better understand, quantify, and
model using analytical formulae, induced effects of light diffraction from mask at oblique incidence. This paper presents
a semi analytical model that describes diffracted field variation with angle of incidence. The model accuracy is validated
by comparison with rigorous field simulations using Panoramic software.
The fiber optic gyroscope (FOG) is a single axis rotation sensor
which is currently employed in many advanced
navigation systems. A major contribution to the cost of an FOG is the price of components such as the polarizer, phase
modulator and associated detection electronics. As a lower cost realization of the device is of great importance for its
wide deployment in many applications, the possibility of rotation rate measurement with moderate accuracy using a
simplified FOG configuration is a very interesting issue. A low-cost simplified implementation of the open loop FOG
was carried out to investigate its performance in the absence of a polarizer and a phase modulator and observe the extent
to which it can usefully detect the rotation rate in the presence of polarization and phase fading. This paper reports on
the realization of the simplified FOG configuration and discusses the association of polarization and phase effects to the
measurement errors incurred. The results indicate that the error due to the absence of the polarizer and phase modulator
can be of the range of only few hundredths of the rotation rate. This is explained by noting that the phase changes in the
path affect both perpendicular polarizations approximately similarly leading to Φx being almost equal to Φy and hence
the polarizer importance appears when using a phase modulator which affects each polarization differently. Possible
practical uses of such a simplified gyroscope configuration are suggested for low accuracy automobile guidance
Multimode interference (MMI) couplers are important integrated optical components for the optical signal processing and routing. The realization of these components by ion exchange on glass substrates is particularly attractive for low cost integration. The design and analysis of MMI devices have generally been based on the self imaging principle in step-index waveguides, whereas waveguides fabricated by ion exchange on glass are practically graded-index due to the nature of the thermal diffusion of exchanged ions. In addition, the ion exchange process results in a guide with depth that depends on the mask opening (the guide width) which causes a high insertion loss at the interface between single mode and multimode sections of the MMI. To overcome these problems 3D simulation of the ion exchanged MMI structures is strongly required. In this work such 3D simulation is achieved on two levels. First the non-linear diffusion equation describing the ion exchange process is solved numerically using a finite-difference method with a modified algorithm to ensure solution stability for an extended range of nonlinearity. The resultant index distribution is used in a wide angle 3D BPM to simulate the optical field propagation in the structure. This allows accurate prediction of the structure performance under different fabrication and excitation conditions. Based on this simulation technique, 3 dB MMI splitter design with tapered access guides is optimized by both geometrical mask design and process parameter variations. The optimization shows that both the tapering and the use of annealing process can significantly improve the performance of the devices.