Spectrally based fiber optic sensors depend on a light beam modulated in wavelength by an environmental effect. Examples of these types of fiber sensors include those based on blackbody radiation, absorption, uorescence, etalons, and dispersive gratings.

One of the simplest of these sensor types is the blackbody sensor of Figure 1. A blackbody cavity is placed at the end of an optical ber. When the cavity rises in temperature, it starts to glow and act as a light source.

Detectors in combination with narrow-band filters are then used to determine the prole of the blackbody curve and, in turn, the temperature, as in Figure 2. This type of sensor has been successfully commercialized and used to measure temperature to within a few degrees Celsius under intense radio frequency (RF) fields. The performance and accuracy of this sensor are better at higher temperatures and fall off at temperatures on the order of 200°C because of low signal-to-noise ratios. Care must be taken to ensure that the hottest spot is the blackbody cavity and not on the optical ber lead itself, as this can corrupt the integrity of the signal.

Another type of spectrally based temperature sensor, shown in Figure 3, is based on absorption. In this case a gallium arsenide (GaAs) sensor probe is used in combination with a broadband light source and input/output optical fibers. The absorption profile of the probe is temperature dependent and may be used to determine temperature.

Figure 1. Blackbody fiber optic sensors allow the measurement of temperature at a hot spot and are most effective at temperatures of higher than 300°C.

Figure 2. Blackbody radiation curves provide unique signatures for each temperature.

Figure 3. Fiber optic sensor based on variable absorption of materials such as GaAs allows the measurement of temperature and pressure.

Figure 4. Fluorescent fiber optic sensor probe configurations can be used to support the measurement of physical parameters as well as the presence or absence of chemical species. These probes may be congured to be single ended or multipoint by using side etch techniques and attaching the fluorescent material to the fiber.

Fluorescent-based fiber sensors are widely used for medical applications and chemical sensing and can also be used for physical parameter measurements such as temperature, viscosity, and humidity. There are a number of configurations for these sensors; Figure 4 illustrates two of the most common. In the case of the end-tip sensor, light propagates down the ber to a probe of €uorescent material. The resultant €uorescent signal is captured by the same fiber and directed back to an output demodulator. The light sources can be pulsed, and probes have been made that depend on the time rate of decay of the light pulse.

In the continuous mode, parameters such as viscosity, water vapor content, and degree of cure in carbon fiber reinforced epoxy and thermoplastic composite materials can be monitored.

An alternative is to use the evanescent properties of the fiber, etch regions of the cladding away, and rell them with €uorescent material. By sending a light pulse down the fiber and looking at the resulting €uorescence, a series of sensing regions may be time division multiplexed.

It is also possible to introduce €uorescent dopants into the optical ber itself. This approach causes the entire optically activated fiber to €uoresce. By using time division multiplexing, various regions of the fiber can be used to make a distributed measurement along the fiber length.

In many cases, users of fiber sensors would like to have the fiber optic analog of conventional electronic sensors. An example is the electrical strain gauge widely used by structural engineers. Fiber grating sensors can be configured to have gauge lengths from 1 millimeter to approximately 1 centimeter, with sensitivity comparable to conventional strain gauges.

This sensor is fabricated by “writing” a fiber grating into the core of a germanium-doped optical fiber. This can be done in a number of ways. One method, illustrated by Figure 5, uses two short-wavelength laser beams that are angled to form an interference pattern through the side of the optical fiber. The interference pattern consists of bright and dark bands that represent local changes in the index of refraction in the core region of the fiber. Exposure time for making these gratings varies from minutes to hours, depending on the dopant concentration in the fiber, the wavelengths used, the optical power level, and the imaging optics.

Figure 5. Fabrication of a fiber grating sensor can be accomplished by imaging to short-wavelength laser beams through the side of the optical ber to form an interference pattern. The bright and dark fringes imaged on the core of the optical fiber induce an index of refraction variation resulting in a grating along the fiber core.

Figure 6. Fiber grating demodulation systems require very high-resolution spectral measurements. One  way to accomplish this is to beat the spectrum of light re€ected by the ber grating against the light transmission characteristics of a reference grating.

Other methods that have been used include the use of phase masks as well as interference patterns induced by short, high – energy laser pulses. The short duration pulses have the potential to be used to write fiber gratings into the fiber as it is being drawn.

Substantial efforts are being made by laboratories around the world to improve the manufacturability of fiber gratings because they have the potential to be used to support optical communication as well as sensing technology.

Once the fiber grating has been fabricated, the next major issue is how to extract information. When used as a strain sensor, the fiber grating is typically attached to, or embedded in, a structure. As the fiber grating is expanded or compressed, the grating period expands or contracts, changing the grating’s spectral response.

For a grating operating at 1300 nanometers, the change in wavelength is about nanometers per microstrain. This type of resolution requires the use of spectral demodulation techniques that are much better than those associated with conventional spectrometers. Several demodulation methods have been suggested using fiber gratings, etalons, and interferometers . Figure 6 illustrates a system that uses a reference fiber grating. The reference fiber grating acts as a modulator fiber. By using similar gratings for the reference and signal gratings and adjusting the reference grating to line up with the active grating, one may implement an accurate closed-loop demodulation system.

An alternative demodulation system would use fiber etalons such as those shown in Figure 7. One fiber can be mounted on a piezoelectric transducer and the other moved relative to a second fiber end. The spacing of the fiber ends as well as their re€ectivity in turn determines the spectral filtering action of the fiber etalon, illustrated by Figure 8.

The fiber etalons in Figure 7 can also be used as sensors for measuring strain, as the distance between mirrors in the fiber determines their transmission characteristics. The mirrors can be fabricated directly into the fiber by cleaving the fiber, coating the end with titanium dioxide, and then resplicing. An alternative approach is to cleave the fiber ends and insert them into a capillary tube with an air gap. Both of these approaches are being investigated for applications where multiple in-line fiber sensors are required.

Figure 7. Intrinsic ber etalons are formed by in-line re€ective mirrors that can be embedded into the optical fiber. Extrinsic fiber etalons are formed by two mirrored fiber ends in a capillary tube. A fiber etalon-based spectral lter or demodulator is formed by two re€ective ber ends that have a variable spacing.

Figure 8. The transmission characteristics of a fiber etalon as a function of finesse, which increases with mirror re€ectivity.

For many applications a single point sensor is adequate. In these situations an etalon can be fabricated independently and attached to the end of the fiber. Figure 9 shows a series of etalons that have been configured to measure pressure, temperature, and refractive index, respectively.

In the case of pressure, the diaphragm has been designed to de€ect. Pressure ranges of 15 to 2000 pounds per square inch can be accommodated by changing the diaphragm thickness with an accuracy of about 0.1% full scale. For temperature the etalon has been formed by silicon – silicon dioxide interfaces. Temperature ranges of 70 to 500 kelvins can be selected, and for a range of about 100 kelvins a resolution of about 0.1 kelvin is achievable. For refractive index of liquids, a hole has been formed to allow the €ow of the liquid to be measured without the diaphragm de€ecting. These devices have been commercialized and are sold with instrument packages.

Figure 9. Hybrid etalon-based fiber optic sensors often consist of micromachined cavities that are placed on the end of optical fibers and can be configured so that sensitivity to one environmental effect is optimized.