<title>Applications of optical fiber techniques to optical metrology</title>

The chemistry of sol-gel derived silica and refractive metal oxide has been systematically studied. Sol-gel processes have been developed for preparing porous silica and semiconductor metal oxide materials. Micelle/reversed micelle techniques have been developed for preparing nanometer sized semiconductor metal oxides and noble metal particles. Techniques for doping metal ions, metal oxides and nanosized metal particles into porous sol-gel material have also been developed. Optical properties of sol-gel derived materials in ambient and high temperature gases have been studied by using fiber optic spectroscopic techniques, such as fiber optic ultraviolet/visible absorption spectrometry, fiber optic near infrared absorption spectrometry and fiber optic fluorescence spectrometry. Fiber optic spectrometric techniques have been developed for investigating the optical properties of these sol-gel derived materials prepared as porous optical fibers or as coatings on the surface of silica optical fibers. Optical and electron microscopic techniques have been used to observe the microstructure, such as pore size, pore shape, sensing agent distribution, of sol-gel derived material, as well as the size and morphology of nanometer metal particle doped in sol-gel derived porous silica, the nature of coating of sol-gel derived materials on silica optical fiber surface. In addition, the chemical reactions of metal ion, nanostructured semiconductor metal oxides and nanometer sized metal particles with gas components at room temperature and high temperatures have also been investigated with fiber optic spectrometric methods. Three classes of fiber optic sensors have been developed based on the thorough investigation of sol-gel chemistry and sol-gel derived materials. The first group of fiber optic sensors uses porous silica optical fibers doped with metal ions or metal oxide as transducers for sensing trace NH{sub 3} and H{sub 2}S in high temperature gas samples. The second group of fiber optic sensors uses sol-gel derived porous silica materials doped with nanometer particles of noble metals in the form of fiber or coating for sensing trace H{sub 2}, NH{sub 3} and HCl in gas samples at for applications ambient temperature. The third classes of fiber optic sensors use sol-gel derived semiconductor metal oxide coating on the surface of silica optical fiber as transducers for selectively sensing H{sub 2}, CH{sub 4} and CO at high temperature. In addition, optical fiber temperature sensors use the fluorescence signal of rare-earth metal ions doped porous silica optical fiber or the optical absorption signal of thermochromic metal oxide materials coated on the surface of silica optical fibers have also been developed for monitoring gas temperature of corrosive gas. Based on the results obtained from this project, the principle of fiber optic sensor techniques for monitoring matrix gas components as well as trace components of coal gasification derived syngas has been established. Prototype sensors for sensing trace ammonia and hydrogen sulfide in gasification derived syngas have been built up in our laboratory and have been tested using gas samples with matrix gas composition similar to that of gasification derived fuel gas. Test results illustrated the feasibility of these sensors for applications in IGCC processes.

In-situ 3-D shape measurements with submicron shape uncertainty of fast rotating objects in a cutting lathe are expected, which can be achieved by simultaneous distance and velocity measurements. Conventional tactile methods, coordinate measurement machines, only support ex-situ measurements. Optical measurement techniques such as triangulation and conoscopic holography offer only the distance, so that the absolute diameter cannot be retrieved directly. In comparison, laser Doppler distance sensors (P-LDD sensor) enable simultaneous and in-situ distance and velocity measurements for monitoring the cutting process in a lathe. In order to achieve shape measurement uncertainties below 1 μm, a P-LDD sensor with a dual camera based scattered light detection has been investigated. Coherent fiber bundles (CFB) are employed to forward the scattered light towards cameras. This enables a compact and passive sensor head in the future. Compared with a photo detector based sensor, the dual camera based sensor allows to decrease the measurement uncertainty by the order of one magnitude. As a result, the total shape uncertainty of absolute 3-D shape measurements can be reduced to about 100 nm.

Optical techniques for water monitoring

Two-color one-way frequency transfer through one strand of optical fiber is an alternative method to two-way frequency transfer, and is useful if unknown asymmetries exist in the link due to different paths for each direction. The term “two-color” refers to the ability to send signals at two different wavelengths utilizing the same fiber in one direction. The method is suitable for implementation in existing urban Single Mode Fiber networks, for instance in networks that are utilized for data and television communication. It is therefore able to coexist with data channels in wavelength-division multiplexing (WDM) systems. It performs as a dynamical control of transit time and simultaneously enables a real-time phase stabilized output signal. This paper presents results from a comparison of two cesium beam frequency standards separated by about 3 km over an optical fiber network located in a metropolitan area in Sweden. The cesium standards were simultaneously compared to each other with a Global Positioning System (GPS) satellite link and over optical fibers, so that the optical fiber technique could be evaluated with respect to the GPS technique. The difference in frequency stability between the two methods is shown to be about 3 × 10-15 over an averaging interval of 10 000 s

Fiber optic technology for real-time monitoring of down-hole well conditions is advancing rapidly in the oilfield. Discrete fiber optic sensors or fiber optic for distributed temperature measurements are relatively easy to deploy in conjunction with the production tubing string. Incorporating fiber optics into a gravel pack assembly is considerably more complex and requires several specialized equipment components. A method to mount the fiber optic conduit to the screen and protect the conduit during screen deployment, gravel packing and production is required. A sand control packer with feed- throughs for the conduit and compatible with crossover tool gravel placement methods is also required. Finally, a method to reconnect the production tubing to the gravel pack packer and establish a continuous fiber optic conduit from the surface to the end of the screen is needed. This paper describes the development and application of a gravel packing system that incorporates a conduit for fiber optics. The primary components of the system and lessons learned from a full- scale stack-up test are discussed. Technology gaps relating to fiber optic deployment tools and techniques are outlined.

In satellite-to-ground optical communication, the effect of atmosphere turbulence to communication system is a very important problem. The effect of laser beam propagating in the turbulent atmosphere and the performance of communication system are analyzed in this paper. Adaptive optics (AO) technique can be used to solve this problem of turbulent atmosphere. There are emphasis research on the effect of communication link error rate on different turbulence and improvement of performance when using AO technique; the simulation results are presented in paper. AO technique plays an important role in satellite-to-ground laser communication.

With surrounding temperature changing, the shifting characteristics of the filtered spectrum of Panda high birefringence loop mirror are researched, according to the birefringence laws of Panda high-birefringence fiber and filter properties of the high-birefringence fiber loop mirror. Its sensitivity coefficient is up to 1.4219×10^-9m/°C. The transmission spectrum is shifted to short wave band, and the temperature sensitivity is 20 times higher than that of the fiber grating sensors, when the temperature rises up. This temperature sensing sensitivity is simulatated when the structural parameters are changed, such as the stress area's radius expressed as R, the high-birefringence optical fiber's length denoted as L and the distance between the core of the fiber and the stress area's central point marked as d. While the stress area's radius and the high-birefringence optical fiber's length are increased, the temperature sensitivity of the Panda high-birefringence loop mirror would become larger in its absolute value correspondingly. On the contrary, the temperature sensitivity would descend as the distance between the core of the fiber and the stress area's central point is increased. Furthermore, the influence on the temperature sensitivity, which is induced by changing the internal structural parameters of the high-birefringence optical fiber, is up to 10,000 times greater than the impacts resulted in by varying the fiber length L with the high-birefringence fiber loop mirror.

<div class="htmlview paragraph">A multi-optical fiber technique is presented, which enables one to detect the flame propagation during non-knocking and knocking conditions in real production engines. The measurement technique is appropriate to detect knock onset locations and to describe the propagation of knocking reaction fronts. With this knowledge, the combustion chamber shape can be optimized, leading to a better knock resistance and higher combustion efficiencies.</div> <div class="htmlview paragraph">Results of flame propagation under non-knocking and knocking engine operating conditions are presented. In addition, correlations between knock onset locations and areas in which knock damage occurs are shown for different engines. Presented are the effects of combustion chamber modifications on the combustion efficiency, based on the analysis of the optical fiber measurements.</div>

The metasurface based beam-steering devices with the advantages of large steering angles, arbitrary channels and ultra-compactness have played an important role for data allocation and exchange in the optical wireless communication. However, the current metasurface based optical wireless communication systems are mainly on intensity modulation and direct detection (IM/DD), which shows a relative lower transmission capacity, lower received optical signal-to-noise ratio (OSNR) and complexity of system. In this study, a bidirectional multichannel optical wireless system enabled by a polarization independent metasurface with coherent modulation and reception is designed and experimentally demonstrated, which exhibits exclusive 100 Gbps coherent optical signals to multiusers with their own wavelengths, 2 m free space distance and field of view of 20° × 20°. In addition, the proposed system can support optical broadcasting system with capacity of 900 Gbps. The demonstrated metasurface assisted optical wireless communication system merges the optical coherent communication techniques and emerging concept of metasurface, which reduces the complexity and cost of the system while contributing a high transmission capacity, opening a new avenue for high performance optical wireless communications.

An FSO system can provide a solution to the problem of last-mile connectivity while also offering a high degree of security and a large capacity for high-speed information transmission, making it a versatile and powerful option for data communication. Despite the above-mentioned advantages, some factors have to be considered when establishing long-range communication, one such factor is atmospheric turbulence (haze, light fog, moderate fog) which degrades the ideal characteristics of the FSO channel and causes a loss in the signal’s power. This loss of power during transmission can be improved by using suitable amplifiers (hybrid optical amplifier, semiconductor optical amplifier) to provide better performance. The next-generation networks must be capable of supporting a significant amount of backhaul data traffic, accommodating a larger number of users, and increasing channel capacity to meet the demands of modern data communication. By implementing wavelength division multiplexing (WDM) and free space optical (FSO) communication techniques, the capacity of the channel can be increased, and more users can be accommodated. The maximum link length offered by the FSO–WDM system has been investigated with hybrid optical amplifier considering the minimum bit error rate, which can provide reliable and long-distance communication.

Clinical Imaging with Optical Coherence Tomography

Even in this digital age, SAW and Optical devices remain indispensable analogue signal processing technologies whose operation and success derive from common wave propagation phenomena. In this review I hope to explain why this is so, and will remain so for the foreseeable future. Historically, the development and exploitation of these technologies arose during the Cold War primarily from the military need for an unsurpassed signal processing capability, although both are now widely exploited in civil applications as well. Following the Introduction, this paper briefly reviews the construction, operation, attractions, and selected applications of the SAW devices developed since the invention of the Interdigital Transducer (IDT) in 1965. These devices include delay lines, bandpass filters, and the first SAW device used in anger, the dispersive delay line which was vital to the realisation of pulse compression radar. I shall also review the attractions of SAW oscillators, which I believe still enjoy an opportunity to contribute to the insatiable military need for improved overall stability. Following this I introduce some components and techniques devised by the optics community for signal and image processing since the invention of the laser in 1960, beginning with techniques for pattern recognition and the ingenious technique devised to realise real-time Synthetic Aperture Radar, (SAR) image formation. More recently, the invention and development of laser diodes and fibre optics by the telecommunications industry has led to the realisation of optical signal processing techniques whose functionality is in many respects complementary to that of SAW devices. In particular, optical techniques are now useful for the generation, delay, sampling, and distribution of signals at higher rf frequencies (and bandwidths) than SAW devices, ie at microwave and mm- wave frequencies and above, leading to the modern field of activity known as Microwave Photonics. Some current research topics in this field are described, including stable microwave generation, phased array antenna beamforming, and A/D conversion, each being relevant to future military and civil systems.

Today, the laser, with its ability to generate coherent light, has become an indispensable tool in optics and not only in optics. Lasers come in a variety of types, hardly to be counted. Coherent light can be generated from the far infrared to the near ultraviolet. It is hard to imagine optical diagnostics or communication techniques without the laser. In huge plants they serve for fusion research, and as pin-sized light sources they are found in CD players. They serve as a scalpel in the hands of surgeons for medical operations and for the cutting, drilling, and hardening of materials of all kinds. A vast literature exists where this variety is documented [10.1].KeywordsBifurcation DiagramSpontaneous EmissionChaotic AttractorPhoton NumberSaturable AbsorberThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Preliminary results obtained in developing a numerical model of laser light intensity transverse distribution into undeformed/deformed step index optical fiber are presented. The main purpose of the presented preliminary numerical modelling results consists in developing a simple method of fiber optical sensors interrogation, especially concerning strain and pressure measurements. It is a potential important matter for aeronautical research and industry because of the more extended use of fibre optic sensors in aircraft manufacturing. The developed numerical model relies on solving the equations of electromagnetic waves propagation into optical fibers by using the finite element method technique (FEM). The results of numerical simulation obtained by considering single mode or multimode and various laser wavelengtsh are presented. One important achievement reported in this paper consists in preliminary experimental results concerning the modification of laser intensity transverse distribution observed for multimode optical fiber with and without perpendicular mechanical load. The reported preliminary experimental results confirm to some extent the predictions of numerical simulations regarding laser intensity distribution under low and medium transverse mechanical load. One important conclusion of this paper consists in the future development of fiber optic sensor interrogation techniques based on the reported preliminary experimental and numerical simulation results.

The paper considers methods for assessing the reliability of FOCL during operation and analyzes methods for diagnosing an optical fiber cable. The main factors affecting the reliability parameters of optical cables and leading to signal losses in optical cables are presented. Reliability indicators are listed. A calculation method for assessing the reliability of digital information transmission is described. The analysis of the calculation of the service life of the optical fiber methods for testing fiber-optic equipment and cables, the procedure for checking an optical fiber, and checking a fiber-optic communication line using an optical time domain reflect meter are shown.