Fiber optic sensors have been used to measure temperatures in hostile environments. When a rare earth doped refractory oxide phosphor such as europium or fluorescent crystal such as chromium doped aluminum oxide is excited by a modulated light, it glows. The phenomenon of glowing or fluorescence arises when some of the energy of the photons absorbed and converted to vibrational energy while the remaining energy is emitted as photons of lower energy. An electron in the ground state is excited by the capture of an incident photon into a higher energy band. An incident photon may very quickly lose some energy through collisional nonradiative processes to occupy the lowest vibrational energy level in a higher energy band. If the quantum efficiency is relatively high very few excited electrons lose their energy through collisional processes and the electrons decay to the ground state with the simultaneous emission of a lower energy photon.

The lifetime of the excited electron varies from tens of nanoseconds to seconds. The luminescent decay time of the phosphor material is a function of the temperature of an environment. With known sensors, pulses of laser light are communicated through a fiber bundle to a remote tip located in a hostile environment. The tip contains a phosphor material, which is illuminated by the laser light. The luminescent decay time varies with the temperature of the tip. The glow or litiht emitted by the phosphor material travels back through ne same fiber

bundle to a coupler where it is diverted to a photodetector. The measurement of the decay time is an indication of the temperature of the tip.

Typically the fiber bundle is divided in the middle to establish first and second separate arrays. The first array carries the laser pulses to the phosphor material while the second array carries the light emitted by the phosphor material to the photodetector. This type fiber bundle is easy to manufacture and offers mechanical strength since the individual fibers are parallel to each other. However, it has been observed that most of the light emitted by the phosphor material occurs adjacent the first array with a proportional reduction of the light emitted by the phosphor material adjacent the second array through which the output is carried to the photodetector. In addition, a portion of the light emitted by phosphor material never enters the second array because the angle of entry is too oblique to enter the individual fibers.

In an effort to improve the communication of the output signal transmitted to the photodetector, it was decided to randomly distribute the first array and second array of fibers in the bundle. In this arrangement there is a greater possibility of having a receiver fiber near an exciter fiber and thus more of the fluorescent activity or output of the phosphor material could be communicated to the photodetector. While this arrangement does provide for a greater communication of the output signal from the phosphor material, the manufacture of such structure is more difficult. When the fibers cross over each other, internal void occurs and the outside diameter of the fiber bundle increases as compared with a bundle for the same number of fibers where the fibers are parallel to each other. In addition, during interweaving of the fibers, stress is placed on the fibers were they engage the other fibers and in some instance actual breakage in the individual fiber occurs. Once a break in a fiber occurs, the efficiency is proportionally reduced.

In the present invention, it was discovered that a lens system placed adjacent the end of the fiber bundle could collimate the beam of modulated light onto the entire surface of the phosphor material. Each fiber in the first array acts as a point source of light to illuminate the phosphor material. Since the entire surface of the phosphor material is simultaneously excited by each point source of light created by the modulated light carried by the first array, the fluorescent output of the phosphor material is raised to a higher level. Correspondingly, each of the atoms or molecules in the phosphor material also acts as a point source of light to develop an output signal. The lens system focuses the output created by phosphor material onto the entire end surface of the fiber bundle. Since the second array comprises approximately one half of the fiber bundle, a corresponding level of the output signal of the phosphor material is communicated by the second array to the photodetector. It is an object of this invention to provide an optic sensor system with means to collimate a modulated light beam into a phosphor material to improve the fluorescent output thereof and focus the fluorescent output onto a fiber carrier of transmission to a photodetector.

It is a further object of this invention to provide an optic sensor system with lens means for collimating a beam of light onto the entire surface of a phosphor material to improve the fluorescent output thereof.

It is another object of this invention to provide an optic sensor system with a lens system which collimates a beam of light carried by a first array of fibers in a bundle onto the entire surface of a phosphor material and focuses the output glow of the phosphor material back onto the fiber bundle where a second array communicates a signal corresponding to the glow to a photodetector. The signal having an intensity level equal up to one half of the level of the glow created by the phosphor material.

These objects and the advantages offered from the use of this invention should be apparent from reading this specification and viewing the drawings wherein:

Figure 1 is a schematic illustration of an optic sensor system embodying the present invention; and

Figure 2 is an enlarged sectional view taken along lines 2-2 of Figure 1 illustrating a fiber optic bundle; and

Figure 3 is a sectional view illustrating a lens system for communication a light beam carried on a first array of a fiber optic bundle to a phosphor material and for focusing a fluorescent output of the phosphor material to a second array of the fiber optic bundle for transmission to a photodetector. The sensor system 10, shown in Figure 1 for sensing the temperature in an environment, is based on the temperature dependence of the luminescent decay time of a phosphor material. Short pulses of light from a source 12 are carried on a fiber cable 14 to a connector 16. A bifurcated fiber bundle 18, see Figure 2, connected to the connector 16 has a first array 20 made up of a series of individual fibers 22, 22'...22 ⁿ for communicating the pulses of light from source 12 to a probe 28 and a second array 24 made of a series of individual fibers 26, 26'...26 ⁿ for communicating an output signal from the probe 28 to the connector 16.

The probe 28, as best shown in Figure 3, has a housing 32 for positioning a tip in an environment whose temperature is to be monitored. Tip 30 which is made of a chromium doped yttrium oxide known as Alexandrite although it is felt that any phosphor material such as europium doped yttrium would also respond in a satisfactory manner to pulse of light from source 12, produces a output signal corresponding to the temperature of the environment. Tip 30 is located in a housing 32 a fixed distance from the end 34 of the bifurcated fiber bundle 28. End cap 36 attached to housing 32 acts on the fiber bundle 28 to hold the end 34 adjacent a lens arrangement 38.

The lens arrangement 38 is a graded index lens having a first function of collimating the light from the laser 12 that is carried on the first array 20 onto the surface 36 of the phosphor or fluorescent material of the tip 30 and a second function of focusing the glow or fluorescent output of the fluorescent material onto the second array 24. The light carried in each individual fiber 22, 22'...22 ⁿ exits therefrom in the shape of a cone. The lens arrangement 38 focuses the individual cones into a collimated beam to uniformly illuminate the surface 36. The light from source 12 carried in fiber 22 is illustrated in Figure 3 where the cone shaped beam is changed into a collimated beam 40 by lens arrangement 38 while light carried by fiber 22 is changed into a collimated beam 40 ⁿ . The collimated beams 40,

40*...40 ⁿ are combined at the surface 36 to provide the phosphor material with an excitation signal to correspondingly develop a glow or fluorescent output signal through the activation of the atoms or molecules in the phosphor material 30. Activation of the atoms or molecules in the phosphor material 30 functions as a point source of light 42, 42'...42 ⁿ . The lens arrangement 38 focuses the point source of light 42, 42'...42 ⁿ back onto end 34 to completely illuminate the entire fiber bundle array 18 where the second array 24 carries the fluorescent signal to connector 16 for communication to detector 44 . The fluorescent signal is evaluated in processor 46 and the resulting temperature corresponding to the luminescent decay time supplied to an operator on display 48.

The lens arrangement 38 which collimates the light from the first array 20 to illuminate the entire surface 36 and focuses the fluorescent activity created by the illumination back onto the entire bundle of individual fibers in the second array 24 assures the optimum utilization of the sensor system 10 in evaluation of the temperature in the hostile environment where the probe 28 is located.