Sensors - August 2002 - A Fiber-Optic Temperature Sensor

This fluorescence-decay temperature probe combines the advantages of optical fiber with a unique ratio measurement system that minimizes inaccuracies due to signal loss from the fiber and connector.

In the LumiTherm 500 temperature sensor, a fiber probe communicates optically with a temperature-sensitive phosphor at the probe tip (see Figure 1).

Figure 1. The fluorodecay sensor works by pulsing a temperature-sensitive phosphor that can be embedded or bonded to the fiber. The probe does not require a single-mode fiber, and it is resistant to annealing effects.

The phosphor is excited via the optical fiber by a low power source inside the instrument, and the resulting luminescence travels back to a detector. The source and detector, along with signal-processing electronics and control functions, are combined into a single compact module, to which the near end of the probe attaches by a standard, SMA fiber-optic connector. Excitation and emission light are separated in the module to enhance sensitivity.

The phosphor tip, at the far end, is either embedded in the medium to be measured or placed in contact with its surface. The probe is composed of silica fiber and various jacketing layers, all of which are stable over the full temperature measurement range of the instrument. The phosphor, which is typically stable to much higher temperatures than the glass itself, can respond to temperature in various ways: change of quantum efficiency, spectral shift of emission and/or excitation bands, and alteration of the fluorescence lifetime (decay time).

Of these temperature-dependent mechanisms, change in decay time provides the most robust approach to measuring temperature. The reason is that lifetime changes can be quantified in a manner independent of instrumental and environmental variables. To monitor lifetime, the excitation source is pulsed. Historically, this type of measurement has been made by sampling the decaying fluorescence intensity at two accurately separated times, t₁ and t₂. This yields signals:

*(T)*	= temperature-dependent fluorescence lifetime
C	= offset due to background crosstalk, ambient light, etc.

t = t₂ – t₁

= time interval from first to second reading of emission intensity

It is also possible to perform linear fits to the logarithm of the intensity in order to determine the decay constant from the slope of the trendline, but this approach requires a great deal of computation and high sampling rates to achieve acceptable accuracy. Sampling a decay signal with a timing uncertainty, t, and quantization error, I, means that the resulting lifetime will be determined with uncertainty given by:

Ipitek’s patented approach resolves all the issues of offsets (background, ambient, etc.) and stringent sampling requirements in a computationally efficient manner. Using phase-sensitive rectification of the signal, offsets can be eliminated and sampling errors reduced dramatically.

Fluorescent lifetime is an intrinsic property of the phosphor material, dependent exclusively on phosphor temperature, except for quite negligible pressure dependence. Knowing (T), then, the temperature is found from a look-up table established by prior calibration of the probe. Thus, the measurement process entails only the ratio of correlated readings combined with the accurate determination of a time interval, while all other instrumental and environmental variables automatically cancel out. Also, the simple form of the decay law permits accurate interpolation of data between calibration points. This leads to a temperature sensor requiring only one initial calibration but applicable to a variety of tasks in widely different environments.

The patented phosphor used in the standard LumiTherm 500, Y₃Al₅O₁₂:Cr³⁺ (Cr:YAG), is highly suited to measurements in the –50°C to 500°C range. The crystalline nature of the host, combined with its high melting point (1950°C), make it extremely stable over the measurement range, with properties that are reversible as temperature is cycled. The fluorescence decay measurement uses the well-known pair of narrow lines, or R lines, emitting red light near 694 nm, which can be pumped (excited) with reasonable efficiency by narrow or broadband green and yellow light. The fluorescence is detected high with quantum efficiency and speed by a standard Si photodiode, and the pulsed excitation light is provided by an off-the-shelf LED. The R lines obey a single exponential decay law over the range of interest, leading to precision of measurement and stability of calibration as explained above.

Accuracy and Reliability
The small thermal mass of the phosphor element at the fiber tip of the LumiTherm 500 means rapid and accurate temperature measurement of the material of interest, whether the tip is in contact with the surface or embedded in the medium. Mounting the active phosphor at the end of a small optical fiber allows placement of the sensor in difficult-to-access locations. The materials in the fiber that communicates with the phosphor exhibit low thermal conductivity as well as a narrow cross-sectional area, minimizing heat flow to and from the active sensing element from outside the volume whose temperature is to be sampled. In contrast, standard RTDs usually have a large thermal mass and are highly conductive.

The accuracy of the fluorescent decay probe is not threatened by corrosion, as are many thermocouples. Probes can be manufactured with high chemical resistance. The probe requires no wires or other metal parts, with the result that it is electrically nonconducting, unlike both thermocouples and RTDs. It can therefore be deployed in high-field environments or in the presence of severe EMI.

Applications
Quantitative temperature measurements are a standard scientific method for characterizing physical and chemical properties. Temperature is a key indicator of the way various materials are interacting with one another. QA/QC procedures quite often include processing temperatures as part of accurate documentation on how materials are handled and products are manufactured. In biomedical laboratories, temperature is monitored and the data are used to optimize protocols and confirm experimental standards for sample preparation in situ—especially useful in drug uptake assays or pharmacokinetic studies. Some industrial processes entailing microwave curing or drying steps are using fiber-optic probes to measure temperatures and electric field strengths during operation.

Photo 1. The LumiTherm 500's fiber-optic probe can either be placed into surface contact with the material of interest or immersed in the sample vial. The experiment shown here monitored sample preparation over a 25 min. time course. Real-time data were exported and charted in Lab View from National Instruments.

Because optical fiber can withstand harsh chemicals, chemistry labs use fiber-optic probes to monitor the temperature of electrochemical syntheses and oxidation reactions (see Photo 1). Pharmaceutical manufacturers immerse the probes in bioreactor vessels, where real-time, continuous data logging helps pinpoint changes at precise times as indicators of reaction kinetics. The information can be extremely useful in scale-up protocols and calorimetry studies.

Fiber-optic cable is flexible, can be produced to any length, and can be autoclaved, attributes of considerable advantage to researchers in the field of animal physiology. The cable aids in capturing temperature measurements within organs, arteries, and veins, and even intracranially. The 100–450 micron fiber diameter allows insertion of the sensors into catheters. One proposed purpose of doing so is to measure temperature gradients in arterial walls at sites of plaque inflammation. The data may be used as an indicator of potential strokes or embolisms. Another promising application is brain temperature monitoring as it relates to memory and/or cognition, or as an early warning of seizures or aneurysms. Because of the immunity of fiber-optic probes to RF, stray optical fields, and even

Photo 2. For wind field and temperature profiling, LumiTherm probes are staggered above ground at 1, 5, 10, and 25 cm; and 1 and 2 m. (Photo courtesy of Michael Brown, Los Alamos National Laboratory.)

light sources, the technology is ideal for medical RF ablation instrumentation. Here, the sensors can provide precise temperature measurement to help prevent overburn of adjacent tissue and ensure minimal tissue damage, and, in the case of tumor ablation, confirm that the temperature end point was reached.

Fiber-optic cables are also used in wind field profiling (see Photo 2), an application that requires long-distance monitoring of temperature to predict shifts in wind direction based on thermal profiling. They are also an excellent solution for utility transformer substations, where temperature plays an important diagnostic role in detecting degradation of the internal windings and thus predicting the remaining life of costly transformers. Fiber is well suited to this application since it offers no conductive electrical path for current, whereas added wiring could cause a transformer malfunction. In addition, the oil used in transformers has a long-term destructive effect on many sensors, but fiber can resist this problem.

Gail Palmer is Sales Manager, Ipitek, 2330 Faraday Ave., Carlsbad, CA 92008; 888-447-4835 or 760-438-1010, fax 760-438-1069, gpalmer@ipitek.com.