Fiber Optic Intrinsic Sensors

About the fiber optic intrinsic sensors

Intrinsic sensors can modulate the intensity, phase, polarization, wavelength or transit time of light. Sensors which modulate light intensity tend to use mainly multimode fibers, but only monomode cables are used to modulate other light parameters. A particularly useful feature of intrinsic fiber optic sensors is that they can, if required, provide distributed sensing over distances of up to 1 meter.

Light intensity is the simplest parameter to manipulate in intrinsic sensors because only a simple source and detector are required.

Modulation of the intensity of transmitted light takes place in various simple forms of proximity, displacement, pressure, pH and smoke sensors. In proximity and displacement sensors (the latter are often given the special name Fotonic sensors), the amount of reflected light varies with the distance between the fiber ends and a boundary. In pressure sensors, the refractive index of the fiber, and hence the intensity of light transmitted, varies according to the mechanical deformation of the fibers caused by pressure. In the pH probe, the amount of light reflected back into the fibers depends on the pH-dependent color of the chemical indicator in the solution around the probe tip. Finally, in a form of smoke detector, two fiber optic cables placed either side of a space detect any reduction in the intensity of light transmission between them caused by the presence of smoke.

A simple form of accelerometer can be made by placing a mass subject to the acceleration on a multimode fiber. The force exerted by the mass on the fiber causes a change in the intensity of light transmitted, thus allowing the acceleration to be determined. The typical accuracy quoted for this device is ±0.02 g in the measurement range of ±5 g and ±2% in the measurement range up to 100 g.

A similar principle is used in probes which measure the internal diameter of tubes. The probe consists of eight strain-gauged cantilever beams which track changes in diameter, giving a measurement resolution of 20 µm.
A slightly more complicated method of effecting light intensity modulation is the variable shutter sensor. This consists of two fixed fibers with two collimating lenses and a variable shutter between. Movement of the shutter changes the intensity of light transmitted between the fibers. This is used to measure the displacement of various devices such as Bourdon tubes, diaphragms and bimetallic thermometers.

Yet another type of intrinsic sensor uses cable where the core and cladding have similar refractive indices but different temperature coefficients. This is used as a temperature sensor. Temperature rises cause the refractive indices to become even closer together and losses from the core to increase, thus reducing the quantity of light transmitted.

Refractive index variation is also used in a form of intrinsic sensor used for cryogenic leak detection. The fiber used for this has a cladding whose refractive index becomes greater than that of the core when it is cooled to cryogenic temperatures. The fiber optic cable is laid in the location where cryogenic leaks might occur. If any leaks do occur, light traveling in the core is transferred to the cladding, where it is attenuated. Cryogenic leakage is thus indicated by monitoring the light transmission characteristics of the fiber.

Yet another use of refractive index variation is found in devices which detect oil in water. These use a special form of cable where the cladding used is sensitive to oil. Any oil present diffuses into the cladding and changes the refractive index, thus increasing light losses from the core. Unclad fibers are used in a similar way. In these, any oil present settles on the core and allows light to escape.

The cross-talk sensor measures several different variables by modulating the intensity of light transmitted. It consists of two parallel fibers which are close together and where one or more short lengths of adjacent cladding are removed from the fibers. When it is immersed in a transparent liquid, there are three different effects which each cause a variation in the intensity of light transmitted. Thus, the sensor can perform three separate functions. First, it can measure temperature according to the temperature-induced variation in the refractive index of the liquid. Secondly, it can act as a level detector according to the depth of the liquid which changes the transmission characteristics between the fibers. Thirdly, it can measure the refractive index of the liquid itself when used under controlled temperature conditions.

The refractive index of a liquid can be measured in an alternative way by using an arrangement where light travels across the liquid between two cable ends which are fairly close together. The angle of the cone of light emitted from the source cable, and hence the amount of light transmitted into the detector, is dependent on the refractive index of the liquid.

The use of materials where the fluorescence varies according to the value of the measurand can also be used as part of intensity-modulating intrinsic sensors. Fluorescence-modulating sensors can give very high sensitivity and are potentially very attractive in biomedical applications where requirements to measure very small quantities such as low oxygen and carbon monoxide concentrations, low blood pressure levels, etc., exist. Similarly, low concentrations of hormones, steroids, etc., may also be measured (Grattan 1989).

Light phase, polarization, wavelength and transit time can be modulated as well as intensity in intrinsic sensors. Monomode cables are used almost exclusively in these types of intrinsic sensor.

Phase modulation normally requires a coherent (laser) light source. It can provide very high sensitivity in displacement measurement but cross-sensitivity to temperature and strain degrades its performance. Maintaining frequency stability of the light source and manufacturing difficulties in coupling the light source to the fiber are further problems. Various versions of this class of instrument exist to measure temperature, pressure, strain, magnetic fields and electric fields. Field-generated quantities such as electric current and voltage can also be measured. In each case, the measurand causes a phase change between a measuring and a reference light beam which is detected by an interferometer.

The principle of phase modulation has also been used in the fiber optic accelerometer (where a mass subject to acceleration rests on a fiber), and in fiber strain gauges (where two fibers are fixed on the upper and lower surfaces of a bar under strain).

Devices using polarization modulation require special forms of fiber which maintain polarization. Polarization changes can be effected by electrical fields, magnetic fields, temperature changes and mechanical strain. Each of these parameters can therefore be measured by polarization modulation.

Various devices which modulate the wavelength of light are used for special purposes. However, the only common wavelength-modulating fiber optic device is the form of laser Doppler flowmeter which uses fiber optic cables.

Fiber optic devices using modulation of the transit time of light are uncommon because of the speed of light. Measurement of the transit time for light to travel from a source, be reflected off an object and travel back to a detector is only viable for extremely large distances. However, a few special arrangements have evolved which use transit time modulation. These include instruments such as the optical resonator, which can measure both mechanical strain and temperature. Temperature-dependent wavelength variation also occurs in semi-conductor crystal beads (e.g. aluminium gallium arsenide). These are bonded to the end of a fiber optic cable and excited from an LED at the other end of the cable. Light from the LED is reflected back along the cable by the beads at a different wavelength. Measurement of the wavelength change allows temperatures in the range up to 200°C to be measured accurately. A particular advantage of this sensor is its small size, typically of 0.5 mm diameter at the sensing tip. Finally, to complete the catalogue of transit time devices, the frequency modulation in a piezoelectric quartz crystal used for gas sensing can also be regarded as a form of time domain modulation.

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