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Position/Presence/Proximity

A Fluorescent Long-Line Fiber-Optic Position Sensor

March 1, 2005 By: Jonathan D. Weiss, Sandia National Laboratories Sensors


The device described in this article is intended to satisfy an industrial need for continuous position sensing over a measurement range varying from centimeters to many meters. As a fiber-optic sensor, it has the well-known advantages of immunity to EMI and an inability to create sparks in a potentially explosive environment. Furthermore, it is noncontact. Although a laboratory proof-of-principle has been accomplished, this patent-pending sensor would have to be engineered to satisfy a particular application. Sandia National Laboratories welcomes collaboration with an industrial partner to achieve that end.



Principle of Operation

In its simplest form the sensor (see Figure 1) consists of an optical fiber that is uniformly doped with fluorescers and a small light source that excites or "pumps" the fiber, thereby inducing fluorescence. It is assumed that the pump emits light in the vicinity of wavelength λ1 and that the fluorescence spectrum is significant in the vicinity of λ2, where λ2 > λ1. The pump source, which could be the end of another optical fiber, would be attached to a moving object that travels along the fluorescing fiber, although the fiber could also be in motion. It is the relative position between the pump source and the fiber that this sensor detects. How it does so is suggested by the lower right-hand corner of the figure, in which pump light impinges on a fluorescer within the fiber. Some of this optical power passes through the fiber without interaction, while some is absorbed by the fluorescer and is reemitted at the longer wavelength. A fraction of this radiation near λ2 travels at too large an angle to the axis of the fiber to be guided by it, but the rest of the radiation is guided to either end. Were it not for absorption within the fiber the optical power emerging at the two ends would be equal because of the symmetry of fluorescence emission, regardless of the position x of the pump source. However, absorption naturally exists or can be designed into the fiber, thereby producing the desired position sensitivity. Thus, fluorescence followed by absorption is at the heart of this sensor. High absorption implies high spatial resolution and small range; low absorption implies low spatial resolution and high range.

Figure 1. A mobile source of light can create in a doped optical fiber fluorescence radiation that initially travels with equal strength toward both ends from the point of generation. As a result of partial absorption in transit, the signals detected at either end will be different and their ratio, S1/S2, yields the location of the source.
Figure 1. A mobile source of light can create in a doped optical fiber fluorescence radiation that initially travels with equal strength toward both ends from the point of generation. As a result of partial absorption in transit, the signals detected at either end will be different and their ratio, S1/S2, yields the location of the source.

The basic equations governing the position sensitivity are shown in the lower left-hand corner of Figure 1, where a single extinction coefficient is assumed to be characteristic of the emission spectrum. We note that the logarithm of the ratio of the two signals S1 and S2 is linear in x and independent of the strength of the pump source. Thus, variations in source strength have no effect on sensor accuracy. In addition, variations in separation between the pump source and the fluorescent fiber have no effect on the ratio if the pump light is collimated, or (as can be shown) if it produces a distribution of power along the fiber that is symmetric at any separation. This latter limitation is minor, since most common optical sources produce symmetric distributions of power. 

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