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.
The recent interest in silicon based photonics, and the trend to reduced device dimensions in photonic circuits generally, has led to the need for mode converters to couple from optical fibres to such small devices. A range of structures have been proposed and in some cases demonstrated, including three dimensional tapers, inverted tapers and micromachined prisms. We have previously reported theoretical analyses of a Dual Grating Assisted Directional Coupler (DGADC), which promises high efficiency coupling over modest spectral linewidths. In this paper we report preliminary experimental results on the fabrication of such devices, together with an evaluation of the coupling efficiency. The approach has been to fabricate a demonstrator device for a particular arrangement of waveguide coupling parameters, i.e. we have fabricated a device that couples easily from fibre, because the input waveguide is approximately 5μm in cross sectional dimensions. The mode converter then couples to a 0.25μm silicon waveguide, primarily because comparisons exist in the literature. These results are compared with the predicted efficiency, and the results are discussed both in terms of the constituent parts of the DGADC, as well as the fabrication limitations. Whilst our device is not optimised we demonstrate that it has promise for very high efficiency coupling.
Recently there has been a strong trend to fabricate smaller photonic devices. In the literature, the problem of coupling optical fibres with thin semiconductor waveguides has not been solved sufficiently well to obtain both high coupling efficiency and good fabrication tolerances. This paper discusses a new approach, the Dual Grating-Assisted Directional Coupling (DGADC), which can result in a robust and very efficient device, with relaxed fabrication tolerances. Theoretical investigation of the coupler is presented. Coupling efficiency and device length are determined as functions of layer thicknesses and refractive indices, grating periods, depths and duty ratios, and finally wavelength. Fabrication of the coupler is also given, as well as preliminary experimental results.
In silicon based photonic circuits, optical modulation is usually performed via the plasma dispersion effect or via the thermo-optic effect, both of which are relatively slow processes. Until relatively recently, the majority of the work in Silicon-on-Insulator (SOI) was based upon waveguides with cross sectional dimensions of several microns. This limits the speed of devices based on the plasma dispersion effect due to the finite transit time of charge carriers, and on the thermo-optic effect due to the volume of the silicon device. Consequently moving to smaller dimensions will increase device speed, as well as providing other advantages of closer packing density, smaller bend radius, and cost effective fabrication. As a result, the trend in recent years has been a move to smaller waveguides, of the order of 1 micron in cross sectional dimensions. In this paper we discuss both the design of small waveguide modulators (of the order of ~1 micron) together with a presentation of preliminary experimental results. In particular two approaches to modulation are discussed, based on injection of free carriers via a p-i-n device, and via thermal modulation of a ring resonator.
Waveguide based Bragg grating devices have the potential of integration with passive or active optical components. A narrow bandwidth Bragg reflection filter or Fabry-Perot resonant structures can be realised using the approach of periodic refractive index modulation in waveguide gratings to form reflective structures. Most authors have considered 1st order Bragg gratings with periods of the order of 228nm operating at 1550nm but at the expense of complexity and high cost of fabrication. This paper describes the design of Silicon-On-Insulator (SOI) rib waveguides operating in the single mode regime that exhibit low polarisation dependence. A rigorous leaky mode propagation method (LMP) has been used to investigate the influence of etch depth in 3rd order Bragg gratings on the reflectance and bandwidth in the waveguides.
In silicon based photonic circuits, optical modulation is usually performed via the plasma dispersion effect, which is a relatively slow process. Until recently, most reserachers utilized Silicon on Insulator (SOI) waveguides with cross secitonal dimensions of the order of 5 microns. This limits the speed of devices based on the plasma dispersion effect due to the finite transit time of charge carriers. Consequently moving to smaller dimensions will increase device speed, as well as providing other advantages of closer packaging density, smaller bend radius, and cost effective fabrication. As a result, the trend in recent years has been a move to smaller waveguides, of the order of 1 micron in cross sectional dimensions. However, coupling light to such small waveguides is relatively inefficient. In the literature, the problem of coupling optical fibers to thin semiconductor waveguides has not been solved sufficiently well to obtain both high coupling efficiency and good fabrication tolerances, due to large difference between the fiber and the waveguide in both dimensions and refractive indices. In this paper, we discuss both the desing of small waveguide modulators (of the order of ~1 micron) together with a novel theoretical solution to the coupling problem. An example of coupling light to a thin silicon waveguide is given, as well as a discussion of a number of modulator design issues.
Silicon-based optical modulators are expected to be important components in some optical networks. The optical modulation mechanism can be achieved either via the plasma dispersion effect, or by thermal means. Both are relatively slow processes when utilized in large (multi micron) waveguide structures, which researchers tend to concentrate on for ease of coupling. Using large waveguide structures limits the operating speed and hence excludes the applicability of these devices in areas where higher speeds are required. This limitation could be overcomed by using smaller waveguides (of the order of 1Rm). In this paper, we present the basic operating mechanism, design, and fabrication details of an optimum three terminal p-i-n diode based optical phase modulator based on Silicon-On-Insulator (501). The device was optimised via electrical and optical modeling and is predicted to operated at 1 .3GHz with a power reduction of900%, as compared to previously published designs.
Silicon-on-insulator (SOI) technology offers tremendous potential for the integration of optoelectronic functions on a silicon substrate. In this research, we report on the fabrication process of a Mach-Zehnder interferometer on an SOI with 0.5μm wide waveguides in a Si layer of the order of ~1μm thick. These small dimensions increase the speed of these devices. However, with these small dimensions several fabrication difficulties such as alignment and thickness accuracy are present.
A simple valveless micropimp is fabricated using optical lithography and copper electroplating. The micropump is activated by magnetic force produced by two opposite spiral flat coils. Each coil consists of 30 tracks with 6mm diameter and 320m2 cross section. The micropump is fabricated between the two flat coils. By applying a square wave signal to the coils, they generate an alternative magnetic field, which contracts and expands the pumping chamber repeatedly causing the liquid in the pump to flow. Increasing the activating frequency, up to 300Hz, appears to increase the flow rate. Increasing the applied current, which is limited to the cross-section of the coil tracks, will increase the magnetic force and hence improve the pumping rate.
Silicon Carbide is a potentially useful compound for use in silicon based photonics because cubic silicon carbide (3C- SiC), possesses a first order electro-optic (Pockels) effect, something absent in pure silicon. This means the material is potentially suitable for high speed optical modulation. Furthermore, the wide bandgap (2.2 eV) of 3C-SiC makes the devices suitable for use over the visible and near infrared spectrum range as well as the longer communication wavelengths, and also means the material can tolerate high temperatures. However, relatively little work has been carried out in SiC for photonics applications. In this paper we will discuss design and fabrication of both SiC waveguides and modulators for silicon based photonics. The fabrication process utilizes ion implantation of oxygen into SiC to form the lower waveguide boundary. Subsequently, ribs are etched and contacts are added to form the optical modulators. Consideration of both Pockels modulators and plasma dispersion modulators has been made, and both will be discussed here. These devices have potential for optical modulation, but are also compatible with silicon processing technology. We have demonstrated waveguiding in 3C-SiC, established a processing recipe for the SiC wafers which enables fabrication of 3-dimensional devices, and demonstrated optical modulation. Performance of the resultant devices is compared to other silicon based devices in terms of operating speed and efficiency.