Optical fiber faceplate; Charge coupled device; Optical detection; Coupling; Appressed imaging

Based on the high-resolution imaging property of the products of optical fiber material, a novel imaging system is proposed. The prototype of the system is made by coupling CCD chip (CMOS) and a large-scale optical fiber faceplate together.

Firstly, the working principle and manufacturing process of large-scale optical fiber faceplate is described.
Secondly, the effectiveness and practicability of the prototype of the system is verified by experiment.
Finally, the potential application prospect of the imaging system is discussed.

The theoretical analysis and experimental result show that the prototype works perfectly under high temperature and pressure condition and the resolution of the image that obtained by the imaging system is larger than 70 lp/mm, which can satisfy the basic imaging requirements in the research fields such as medical diagnosis, industrial detection and monitoring. Further, the novel imaging system provides a new approach for the application of optical fiber product in scientific research.

Industry camera is an useful equipment which has been widely used in medical imaging, industrial production and communication, with some attractive characteristics such as compact-sized, smaller exposure time and strong anti-interference capability. However, in most cases, the operating temperature of industry camera is limited from -30°C to 50°C. Besides, the major components of camera(such as camera lens or CCD chip) also must work in special humidity and stress environment.

Therefore, the application field of industry camera will be broader when these demands are reduced.

Optical fiber faceplate (OFP) is an image transmission component that has been widely used in night-vision technique. In general, OFP is made of thousands of optical fibers (i.e. mono fiber) and the diameters of the mono fibers are only 5 or 6 μm. There are many advantages of OFP for image transmitting:

• High numberical aperture (N.A.): The numberical aperture, or N.A., defines the angle of acceptance-beyond which light is lost out the sides of the fiber. In special cases, the N.A. of OFP can reach 1 or greater, which indicate that it can transmit the all the ray of which incident angle smaller than 90°.
• High resolution: Image resolution through an OFP is largely a function of fiber size. In general, an OFP with 5μm fibers will resolve about 100 lp/mm. Moreover, the image definition and contrast will be improved when the Extra-Mural absorption(EMA) is inserted.
• High coupling efficiency: Due to the zero optical-thickness property, OFP can couple to many kinds of photosensitive device with high coupling efficiency(greater than 70%).

In this paper, an imaging detection system based on the combination of OFP and industry camera is proposed. The probe of this system is a custom-made large-scale OFP that coupled with a CCD chip (CMOS), which can perfectly work in the harsh industrial environment. In theory, the coupling efficiency of this system can reach more than 35%. Experimental results show that the resolution of the image that obtained by the imaging system is larger than 70 lp/mm, which can satisfy the basic imaging requirements in the research fields such as medical diagnostic, industrial detection and monitoring.

The mono fiber, which consists of the assembly of the core and cladding, is the first state of OFP, as shown in Fig. 1.

Fig. 1 Ray transmission in optical fiber

Snell’s law gives us the relationship between the angle at which light enters the fiber α and the angle at which the light travels down the fiber θ₁. Simple geometry gives us the relationship between θ₁ and θ₂.

The rule of total internal reflection shows that the angle θ₂ must have a minimum value in order for reflection to occur. If θ₂ is smaller than this minimum value, the light will not be totally reflected. (Note that we can also relate the requirements for total internal reflection back to the angle α. There is a maximum angle for which light can be transmitted down the fiber. Light will be lost out of the sides of the fiber for the angles larger than α. The limiting angle α is called the “acceptance angle” of the fiber.) When total internal reflection occurs, it is 100% reflection. Therefore, the image of object can be transmitted from one side to the other.

Fig. 2 Flow chart of the production of optical fiber faceplate

From Fig.2, we know that the mono fiber needs to go through many processes during the production of OFP, such as assembling, muti-fiber drawing and pressing. It should be emphasized that the mono drawing process, which consist of the assembly of the core and cladding, as well as the drawing of these materials into a single clad fiber, is the key procedure to control the diameter of the mono fibers in OFP. The fiber drawing apparatus and the structure of input material(i.e. perform rod) are shown in Fig.3 (a) and Fig.3 (b) respectively. Since the volume of perform rod remains constant throughout the drawing, the diameter of the mono fiber can be expressed as

where
v₁ and v₂ are the feeding and pulling speeds respectively,
φ₁ and φ₂ are the internal and external diameters of the cladding tube respectively and
φ is the diameter of the core rod.
Therefore, if the diameter of perform rod is set in advance, we can conclude that the diameter of the mono fiber is determined by v₁ and v₂.

Fig. 3 Mono Fiber Drawing Process

In general, the length of the OFP is smaller than 50 mm, whereas the length of the large-scale OFP can reach 250 mm, as shown in Fig. 4. In this paper, the large-scale OFP of the imaging system is custom made.

Fig. 4 Large-scale OFP

The schematic diagram of coupling system is shown in Fig. 5 and it can be seen that the large-scale OFP is coupled with CMOS by optical glue. The image of object can be transmitted from one side of OFP to the CMOS of camera, and, finally, displayed on the computer screen. Due to the appressed-imaging property of OFP , it should be noted that high quality image can be transmitted only if the OFP is close enough to the CMOS and the object. (In practice, the distance between OFP and CMOS must be shorter than 1 mm)

Fig. 5 The schematic diagram of coupling system

Assume that the intensity of incident ray at the surface of OFP satisfies Lambertian distribution. If αc is the acceptance angle which is shown in Fig.1, the coupling efficiency at the incident side of OFP is:

where
I(α) is the intensity of the incident ray,
φT is the luminous flux emitted by the light source,
φF is the luminous flux received by the OFP and

On the other hand, the light loss due to Fresnel reflection in OFP should be considered. During the process of light transmission, the number of reflection in OFP can be expressed as:

Where
L is the length of OFP and
D is the diameter of mono fiber.
If the reflection index is s, the transmitted efficiency of OFP is P₂= S^η.
At last, it also should be noted that light loss will be occurred at the surface of CMOS. Fortunately, since the OFP and CMOS are completely wrapped by the optical glue, this light loss can be kept to a minimum. If the light receiving efficiency at the surface of CMOS is P₃, the coupling efficiency of the imaging system is

The parameters of the large-scale OFP is shown in Fig. 6. According to our calculation, the coupling efficiency of the imaging system is 38.2%, which can satisfy the detection requirements in general.

Fig. 6 Parameters list of the large-scale OFP

The industry camera in the imaging system is produced by Huaray Technology, model A7500M/CU75, offer 2448*2048 pixel images at up to 75 frames/s. The size of CMOS is 2/3” and the diameter of the cross section of the large-scale OFP is 6.9 mm. The prototype and the pictures obtained by the imaging system are showed in Fig. 7. and Fig. 8., respectively.

Fig. 7 Coupled imaging system

Fig. 8 Pictures obtained by the imaging system

The object in the short-distance imaging test is a scale-plate which is placed close to the input side of OFP, as shown in Fig.8 (a). The width of each line, the interval between two lines and the diameter of the imaging area are 0.4 mm (116 pixel), 0.4 mm (116 pixel) and 6.9 mm (2000 pixel), respectively.

If we assume that it needs at least 4 pixels to distinguish a line pair, then the resolution of the system will be greater than 72 lp/mm. The result of long-distance imaging test is showed in Fig. 8 (b). Compared with the short-distance test, the imaging system also works well, although the image definition and contrast is lower.

Due to the special imaging properties, the system can be applied in many fields of space-imaging detection:

• Industry detection: In industry field, many products need to be processed under high temperature and high pressure environment. For instance, during the preparation of cement, the temperature of the boiler can reach 300° or greater. However, the industry camera can only work in the low temperature environment (no more than 50°.) Therefore, it is hard to realize the real time monitoring of industrial manufacture. Since the performance of OFP will not change within 500°, the large-scale OFP can be used as a probe to monitor the reaction in the boiler, which has significance in the field of optical detection.
• Medical diagnosis: According to the analysis in section 2, the resolution of optical fiber product can reach 100 lp/mm or greater, which is higher than other optical imaging product. When the large-scale OFP comes into contact with the parts of person or animal, such as skin, hair and organ, the details of these regions will be clearly displayed on the computer screen, which is very helpful in Medical diagnosis.
• Acidic-alkaline environment detection: Some creatures must live in the acidic or alkaline environment. In order to monitor their living habits, the industry camera has to be placed into water or soil. Since optical fiber has excellent corrosion resistance, the OFP can be used as a probe to detect the creatures in corrosive environments.
• Nuclear matter detection: Due to the radioactivity of nuclear reactor, many nuclear detecting task still rely on robots today. The new imaging system can be used in radioactive environment to accomplish special tasks and reduce the cost.
• The deep-see and space exploration: In the fields of aviation and navigation, detector must has the ability that work in the weightless and high pressure environments. Large-scale OFP can be applied in these domains due to the excellent pressure resistance.

A novel imaging detection system based on the coupling of large-scale OFP and CMOS is proposed, with some attractive characteristics such as high temperature resistance, nondestructive and long distance detecting. The coupling efficiency of the system can reach 35% in theory and the experimental results show that the resolution of the system is greater than 72 lp/mm, which can satisfy the detection requirements in many fields. This system has wide application prospect on industry, biology and medicine domains. Further, it provides a new approach for the application of optical fiber product in scientific research.