Seppo Nissilae

Fiber optic sensors for traffic monitoring applications

A novel fiber optic sensors for traffic monitoring applications are presented. They are useful for monitoring the traffic entering and leaving guarded areas, counting traffic on public roads as well as in determining lane occupancy at traffic lights. When installing sensor arrays, the sensors may also be used to provide additional information: measure the speed, wheel base of a vehicle and also to determine the number of axles and vehicle type. The sensors are based on optical fiber or cable, installed inside the asphalt layer of the road, to measure compression or vibrations near the road surface. Two sensor principles were used. One based on the microbending effect and another that utilizes the speckle phenomenon. In both cases the whole length of the fiber acts as a sensor. The microbending sensor requires a special fiber and special set-up, whereas in the speckle sensor a standard cable may be used. Both sensor types were tested in field applications where the harsh environment, especially the heat, sets great demands on installation. In these experiments, speed and vehicle type measurements were carried out with good results. In the paper, we will discuss the advantages and disadvantages of both sensor types and present some field test results. We will also show the benefits of these particular fiber optic sensors over traditional sensors.

Indicating cure and stress in composite containers using optical fibers

Long optical fibers embedded inside a composite material enable on-line monitoring of the manufacturing process and can also be used to assess damage by monitoring strain and cracks in the structure. We report on measurements of the microbending loss in long single-mode and multimode telecommunication fibers that were successfully installed inside the walls of composite containers during the normal manufacturing process. The work was largely empirical, and theoretical aspects were not investigated thoroughly. The measurements were carried out using the optical through power technique and a commercial optical time-domain reflectometer that gives not only the attenuation but also spatial information. We found that the coating material and its thickness have an effect on loading sensitivity and on the applicability of the method for cure monitoring. Although we discovered that repeatability issues are a problem, the measurement data indicate that the end-of-curing process of the composite can be determined by optical sensing fibers and damaging external loading of the structure can be detected.

Photoacoustic signal formation in absorbing and scattering liquids

The so-called photoacoustic technique combines optical properties, such as absorption and scattering, with acoustic properties, such as sound velocity and absorption, to monitor and measure the physical properties of materials. This paper comprises a theoretical study on the properties of acoustic pulses and a discussion on earlier theories presented in literature. It also describes and analyzes the results of simulation tests based on the Monte-Carlo method, undertaken to examine the effects that absorption and scattering co-efficients of two-layer media have on the shape of the acoustic transmitter. Finally, the theoretical results are corroborated with measurements using a CO2 laser and a two-wavelength diode laser PA system, developed during the study.

Use of optical fibers in the pulsed time-of-flight laser rangefinding technique

The problems associated with the fibres used in pulsed time-of-flight rangefinders were studied, and particularly errors due to the transit time disturbances of step and graded index fibres as a function of fibre length, input numerical aperture and temperature. The cladding modes and leaky modes ofa fibre can affect the transit time oflight pulses under suitable conditions, so that fibres become sensitive to environmental effects. The effect of temperature is smaller and more linear for acryl-coated fibres than for nylon-coated ones. The main reason for the non-linear effect of temperature on a nylon fibre is the non-linear Youngs modulus of nylon as a function of temperature. The increasing transit time of hard clad silica (HCS) fibres at lower temperatures (below +20°C), contrary to glass fibres, can be explained by the different thermal coefficient of the core and cladding, leading to increased non-homogenities on the core-cladding interface.

Skin temperature profiles on the human chest and in the wrist area

Skin temperatures on the chest and in the wrist area are interesting for continuous monitoring because they can be easily instrumented using an elastic belt or wristband which do not hamper movement in sports, for example. An infrared thermograph camera and NTC thermistors were used to take temperature profiles at these sensing points with a resolution of 0.1 degrees Celsius, and colored thermograms were used to analyze and compare the results. The effect of environmental changes on the skin temperature in the wrist area was studied by cooling and heating the fingers in water at 10 degrees Celsius and 40 degrees Celsius, and the effects of a loading situation on the chest area and wrist area were tested by means of a 30 min bicycle ergometer exercise. NTC thermistors were also used to measure wrist and chest temperatures in two environmental tests at minus 10 degrees Celsius and plus 60 degrees Celsius. Cooling of the fingers naturally reduces the skin temperature in the wrist area and heating increases it due to the venous circulation. The area of the radial artery in the wrist seems to be the most stable temperature point, altering by only about 2 degrees Celsius, whereas the temperature change at other points is up to 4 degrees Celsius. The bicycle ergometer exercise caused a decrease in skin temperature on the chest because of sweating. At the same time the skin temperature on the wrist decreased by about 1.5 degrees Celsius after the first 20 minutes and then returned to its previous level. The area of the radial artery in the wrist seems to be an attractive point for continuous temperature monitoring, especially under normal conditions, and also seems to reflect body temperature quite well upon loading and under different environmental conditions.

Optical fibers for strain and temperature measurement based on OTDR

Distributed fiber-optic sensing which implies monitoring of the magnitude of physical parameters and their variations along the length of continuous uninterrupted optical fiber is considered. OTDR technique and measurement results for various kinds of optical fibers are presented.

Cure and stress monitoring of composite containers using optical fibers

The monitoring of fuel tanks and toxic liquid containers with embedded long optical fibers makes it possible to determine their condition and to detect probable environmental risks. We present here measurements of microbending loss in long, single-mode telecommunication fibers installed inside the walls of composite containers during the manufacturing process. The measurements were performed by the optical through power technique and using a commercial optical time domain reflectometer. The measurement data indicate that the optical sensing fibers can detect the end of the curing process in the composite and the occurrence of external loading likely to damage the structure.

PC-compatible optical time domain reflectometer card based strain sensors

In this study distributed fiber optic sensors using commercial telecommunication and special sensor fibers have been studied and tested. A PC-compatible OTDR-card is used to obtain a portable measuring system. Especially the measuring of strain and stress in steel plates and bars is studied in comparison with strain gage measurements. The sensing of ice temperature with a special sensing fiber is also reported. Applications of the fiber sensors studied here are strain and stress measurements and crack detection in steel chambers and pipes especially at high temperatures (> 300 degree(s)C), and freezing detection of road structures.

Fiber laser as the pulse source for a laser rangefinder system

Active fibers, i.e. optically pumped doped fibers, have been developed and studied intensively during the last few years, and an optical amplifier based mainly on erbium-doped fibers has just been launched on the expanding telecommunications market. Fiber lasers have a market of their own in the sensor applications. The use of fiber lasers as pulse sources in laser rangefinder applications is studied here. The main advantages with respect to high energy pulses and a small emitting area are listed, the problems and disadvantages are discussed and some practical solutions to these problems are given. Possible Q-switching techniques for obtaining short, powerful pulses (> 10 W) of about 10 ns are studied as are liquid-crystal, PLZT crystal, acousto-optic and Pockels Cell modulators. Finally, the practicability of these modulators for laser pulsing in industrial environments is discussed.