A brief overview of fiber optic sensors

A few decades ago telecommunications had been revolutionized by fiber optic technology. This revolution became a route as mass production techniques coupled with technical improvements resulted in superior performance at lower costs than those of alternative approaches. Next to this revolution emerged another one as a result of combination of the fiber optic telecommunication product outgrowths with optoelectronic devices to create fiber optic sensors. These areas of opportunities include the potential of replacing the majority of environmental sensors in existence today as well as opening up entire markets where sensors with comparable capability do not exist. The figures 1.1 and 1.2 provide a brief classification of the fiber optic sensors that can be basically divided in two groups - extrinsic and intrinsic fiber optic sensors. Extrinsic fiber optic sensors are distinguished by the characteristic that sensing takes place in a region outside the fiber. There are also hybrid fiber optic sensors that are similar to extrinsic ones and can be thought of as a "black box" sensors for which fiber are used to carry light to the box and back. Frequently the two terms are applied interchangeably. A major distinction arises for the case of power by light sensors when a light beam is used to power an electronic sensor and data are carried back via a fiber optic data link. In this case the hybrid designation would appear to be more appropriate. Figure 1.2 shows a diagram illustrating many of the intrinsic or all-fiber optic sensors. "Intrinsic" and "all-fiber" indicate that the sensing takes place within the fiber itself. In this case the two designations can be and commonly are used interchangeably. A large and important subclass of intrinsic or all-fiber optic sensors are interferometric sensors. Many of the highest performance sensors fall into this group. The fiber optic sensors of figures 1.1 and 1.2 have been grouped into the categories that are representative of their most common current development and application. It is apparent that virtually any environmental effect that can be conceived of can be converted to an optical signal to be interpreted. The usual case that each environmental effect may be measured by dozens of fiber optic sensors approaches. The key is often to design the sensor so that only the desired environmental effect is sensed.

Initial penetration of fiber optic sensors into markets has been driven by performance advantages. Fiber optic sensors offer an all-passive dielectric approach that is often crucial to successful applications, inclusion the electrical isolation of patients in medicine, elimination of conductive path in high voltage environments, and compatibility within materials. The lightweight and small size of these devices are critical in such areas as aerospace and provide substantial advantages to many products. Coupled to the issue of size and weight is immunity to the electromagnetic interference. Conventional electrical sensors often require heavy shielding, significantly increasing the cost, size, and weight. Environmental ruggedness provides key opportunities for fiber optic sensors, including high-temperature operation and all solid state configurations capable of withstanding extreme vibrations and shock

levels. Completing these attributes are high sensitivity and bandwidth of fiber optic sensors. When multiplexed into arrays of sensors the large bandwidth of the optical fibers themselves offer distinct advantages in their ability to transport resultant data.

Fig. 1.1. Schematic classification of extrinsic fiber optic sensors

Fig. 1.2. Schematic classification of intrinsic fiber optic sensors

2. Intrinsic distributed sensors

Intrinsic distributed sensors are particularly attractive for use in applications where monitoring of a single measurand is required at a large number of points or continuously over the path of fiber. Examples of application areas include for example

Stress monitoring of a large structures such as buildings, bridges, storage tanks, and the like, and ships, oil platforms, aircraft spacecraft and so on Temperature profiling in electrical power transformers, generators, reactor systems, furnaces, press control systems, and simple fire detection systems Leakage detection systems in pipelines, fault diagnostics and detection of magnetic/electrical field anomalies in power distribution systems, and intrusion alarm systems Embedded sensors in composite materials for use in the real-time evaluation of stress, vibration, and temperature in structures and shells, especially in aerospace industry

In quasi distributed sensors configuration, the measurand is not monitored continuously along the fiber path, but at a finite number of locations. This is accomplished by sensing the fiber locally to a particular field of interest or by using extrinsic (bulk) sensing elements. This type of system can be used in single-measurand monitoring applications, but it is also capable of multimeasurand monitoring by sensitizing each sensor element to specific measurand field of interest. Application areas for this form of sensors include those listed above, and various multimeasurand monitoring applications such as those found in chemical, power and manufacturing industries. A crucial role in the distributed sensing play the backscattering method usually called Optical Time-Domain Reflectometry (OTDR). When light is guided by an optical fiber, loss occurs due to Rayleigh scattering which arises as a result of random microscopic variations in the index of refraction of the fiber core. A fraction of the light that is scattered in a direction 180o to the propagation axis of the light (backscattered light) is recaptured by the fiber aperture and returned to the input end of the fiber (source). By pulsing the input optical signal to a length of fiber, and monitoring the variation in the returned backscattered intensity, spatial variations in the fiber scattering coefficient or cross section, or attenuation, can be determined. This is the basic principle of the OTDR, which is a well established technique for fault/imperfection location and diagnostics in fiber communications applications. In sensing applications, OTDR can be used to detect localized measurand - induced variations in the loss or scattering coefficient of a continuous sensing fiber. A detail analysis and theoretical description of OTDR will be given later. The basic OTDR technique is essentially optical radar, and direct analogies between conventional radar systems and optical ranging in distributed sensors have

been recognized and utilized in the development of a number of variant techniques. Other optical ranging techniques suitable for use in sensing applications include

Coherent OTDR (CO-OTDR) - The week returned backscattered signal is mixed with a strong coherent local oscillator optical signal to provide coherent amplification Correlation OTDR (COR-OTDR)

COR-OTDR based on pseudorandom signalCOR-OTDR based on Golay code signal

Low correlation OTDR (LC-OTDR) Photon-Counting OTDR (PC-OTDR) Optical Frequency-Domain Reflectometry (OFDR)

OFDR with the frequency scanning (OFDR-FS)OFDR with the synthesized coherence function (OFDR-SCF)

Polarization OTDR (PO-OTDR)The detail description of principles and characteristic features of the above listed variations of OTDR and OFDR will be given in the following chapters. Intrinsic distributed sensors based on Rayleigh backscatter utilize either the measurand-dependent loss coefficient α(z) or backscattering coefficient r(z) mechanism in a single length of optical fiber which forms an extended sensor. The sensitivity of the fiber may be enhanced to a particular measurand field, such as temperature, for example. Interrogation of this extended sensor element using OTDR permits the spatial variation of the measurand to be derived from the output information, thus allowing the measurand to be profiled within the OTDR spatial resolution along the fiber length. Figure 2.1 shows the basic mechanisms that can be used in Rayleigh backscatter-based intrinsic distributed sensors.

Fig. 2.1. Simplified diagram of mechanisms that can be used in OTDR based sensor systems

The most basic form of intrinsic distributed sensor relies on the detection of regions of localized excess loss, due to, for example, microbending. In the case of measurand-dependent loss, a localized region of increased loss (Δα) due to perturbation of the fiber by the measurand field (stress, temperature, external refractive index, etc.) causes a change in the slope of the detected backscattered signal versus time-delay curve (OTDR curve) at a time delay corresponding to the signal position of the perturbation. Polarization optical time-domain reflectometry (PO-OTDR) is a variant of OTDR which was the first to be proposed for use in sensing applications. Here the polarization of Rayleigh backscattered light in a monomode fiber is detected as a function of time. The birefringence parameters of monomode fiber are sensitive to a number of measurands, such as strain, pressure, and electric and magnetic fields. Consequently, the state of polarization (SOP) of the backscattered light varies with distance z along the fiber. This variant of the OTDR technique has been proposed for the distributed monitoring of electric (via the electro-optic Kerr effect) and magnetic fields (via the magneto-optic Faraday effect). The backscatter intensity in an OTDR system is proportional to the backscattered reflection coefficient r(z). This provides a means of distributed sensing via measurand dependencies in r(z). In general, this parameter is homogeneous along the fiber length

and has negligible dependence on external perturbations, such as bends lateral pressure and the like. The backscatter coefficient is, however, temperature dependent: in solid core fibers this effect is extremely weak, as the microscopic variations in refractive index in the core that give rise to the effect are essentially frozen in place. However, in liquids the Rayleigh scattering coefficient and refractive index are generally much more strongly dependent on temperature. Temperature sensing using the thermal dependence in liquid-core fibers was demonstrated at an early stage of research in this field. A number of specialty rare-earth-doped fibers have been researched for applications in optical fiber amplifiers and fiber lasers, and distributed temperature sensors based on temperature-dependent absorption in such fibers have been demonstrated. In particular, neodymium- and holmium-doped fibers have been shown to be suitable for use in distributed temperature-sensing applications. In these fibers the strong absorption bands associated with the dopants broaden and shift to slightly longer wavelengths with increasing temperature. OTDR measurements of the spatial variation of the fiber loss at a wavelength close to the absorption band can thus be used to perform the distributed temperature sensing. In practice, increased loss in doped fibers can occur due to effects other than temperature, such as bends and kinks, giving rise to the possibility of erroneous outputs. This necessitates the need for some means of referencing to provide correction or compensation of such effects. This can be accomplished very easily in this type of sensors using a second OTDR interrogating source of wavelength shifted sufficiently far enough from the absorption band to monitor non-temperature-dependent loss mechanisms in the fiber. Various other loss mechanisms can be utilized in distributed sensing, such as the temperature dependence of bending loss in plastic-clad silica fibers (PCS), evanescent field radiative loss due to external refractive index changes (due to, for instance, temperature variations in a fixed external medium or liquid leakage) or continuous microbending loss in fiber. For example, quasi-continuous microbending loss can be introduced in a fiber by stringing it along a zigzag path attached to a structure, or using a fiber sheathed with a spiral jacket to produce localized microbending when lateral pressure is applied.

The temperature dependence of fluorescence can also provide a means for distributed temperature sensing. To accomplish this, fibers dopants with short fluorescence lifetimes are required to provide a good degree of spatial resolution, normally attainable using OTDR based systems. Organic laser day dopant materials in polymer fibers have been proposed for distributed sensing by fluorescence.

3. Optical Time-Domain Reflectometry (OTDR) - conventional approach

3.1 Basic principles of the method The backscattering method was invented by M. Barnoskim and M. Jensen in 1976 [1], in time when technology of the optical fiber manufacturing was at early stages. The precise and reliable measurement of local losses on the fiber was very important for further improvement of quality of fibers. In the paper cited above the authors describe a new method for the loss distribution along the fiber. The basic idea of the proposed method consisted in launching a rather short and high power optical impulse into the tested fiber and a consequent detection of back scattered optical power as a response of the fiber to the test impulse. The detected signal provides the detail picture about the local loss distribution or reflections along the fiber caused by any of the attenuation mechanisms or some other nonhomogeneities on the fiber. An important feature of the method is non-destructivity and the fact that the access to only input end of the fiber is needed. The measurement of the time delay of the detected signal from the fiber end or from any perturbation on the fiber allows to derive the information about the perturbation localization provided that the index of refraction in the fiber core or group velocity of light propagation is known. In any point on the fiber the magnitude of the backscattered optical power is proportional to the local transmitted optical power. Due to the nonzero losses this power is gradually attenuated along the fiber and consequently also the backscattered power is also attenuated. The measurement of the backscattered power as a function of time or position on the fiber gives the information about the local distribution of the attenuation coefficient along the fiber. In this way one can evaluate the space distribution and magnitude of various non-homogeneities along the fiber like optical connectors, splicings, micro- and macro-bend losses and others measurand-perturbances. The comparison of the losses closely before and after point of interest makes possible to evaluate insertion losses of the various optical components on the fiber link.

5. Optical Frequency-Domain Reflectometry (OFDR)

5.1 OFDR with the frequency scanning (OFDR-FS) 5.1.1 Introduction In OTDR techniques where the system is probed by the narrow pulses of optical radiation the spatial resolution can be improved as the pulses are shortened and the measurement band-width is broadened. However it results in the increase of noise level and consequently in the decrease of dynamical range (well-known trade-off between dynamical range and space resolution). To solve this problem other approaches to optical reflectometry in time domain were and are permanently being search for. To this time we can speak about the correlation OTDR based on the use of pseudorandom probe signal [2] or on the use of complementary Golay code probe signal [3], the photon-counting OTDR [4] and the low correlation OTDR [5]. Each of these OTDR modifications are characteristic by some advantages and drawbacks that determine the field of their utilization. For instance the low-correlation OTDR, which is characteristic with very high sensitivity (under - 180 dB) and space resolution (tens of μm) is predominantly used for the characterization of miniature integrated optical waveguides. Another approach is the optical frequency-domain reflectometry with the frequency scanning (OFDR-FS), called often also frequency-modulated continuous-wave reflectometry (FMCW) in which the probe signal is a continuous frequency modulated optical wave [6]. In contrast to OTDR the OFDR-FS systems, which use more energetic continuous wave probing, are characteristic by dynamical range that does not depend on the space resolution. This significant feature gives the OFDR-FS the potential to achieve high spatial resolution without the loss of dynamical range. Combination of this technique with the coherent detection scheme gains the additional advantage - the high sensitivity. Sensitivities down the - 100 dB and space resolution in millimeter range can be achieved. Comparing the performance parameters of classical OTDR and the low correlation OTDR one can state that OFDR-FS fulfils the gap between the two above mentioned extremes. It is determined for characterization of optical lines of medium length and with higher space resolution (LAN). A crucial element in the OFDR-FS is the optical source, which strongly influences the achievable spatial resolution and measurement range. High spatial resolution requires the highly coherent source having a broad, phase-continuous and simultaneously linear tuning range. However real sources depart from perfect coherence and produce the phase noise that limits the system performance. First - it limits the distance over which the discrete reflections can be measured and second - it decreases the dynamical range between the reflected signal of interest and the level of phase noise. For a long time the lack of suitable sources was the main objection for broader utilization of OFDR-FS in practice. Progress in technology of semiconductor laser diodes and fiber lasers during a few recent years brought new promises for the use of this technique. Special three electrode laser diodes made possible to achieve the space resolution 400 μm and to measure the back scattered signals on fibers 400 m

long [7]. The use of laser diodes with external fiber resonators allows to enhance the coherence length and to increase the measurement range to several hundred of meters [8]. OFDR-FS instrument based on the use of a tunable fiber laser with very narrow line-width (10 kHz) tuned mechanically by piezo-electric transducer makes possible to measure the reflectivity on the level - 110 dB with 80 dB dynamical range. The narrow line width of the source allows long-range measurement, at 150 m, with a spatial resolution of 16 cm [9]. In this section a detail theoretical analysis of the OFDR-FS for the recovery of distributed back scattered light and discrete Fresnel reflections along the optical fiber is given. The main limitation factors of the method such as the phase noise, the influence of polarization fluctuation, the distortion by frequency modulation non-linearity and the problems of the artefacts in output signal are discussed and the possible solutions are outlined.

5.1.2 Theoretical background of OFDR-FS

The basic idea of the OFDR-FS can be explained using the simplified block diagram of the OFDR-FS reflectometer as illustrated in the Fig. 5.1.2.1.

Fig. 5.1.2.1. Simplified block diagram of the OFDR-FS based reflectometer, FC - fiber coupler, FD - photodetector