Two kinds of multi-air-core optical fibers were designed and fabricated by extruding optical grade PMMA pellets and drawing to fiber. The imaging function of the fibers was investigated with home-made specialized microscopy. This new type of fiber provides strong potential for applications in endoscopy, chemical sensing, biosensors, fiber-optical faceplates.
© 2008 Optical Society of America
It has been reported that microstructured polymer optical fibers (MPOFs) are able to transmit images in two distinctly different ways: using islands array of high-index material embedded in air holes as the pixel array, and /or air channels as the pixel array through an anti-guiding mechanism . In that paper, imaging ability of 112 holes MPOF has been well demonstrated using islands array of high-index material embedded in air holes as the pixel array. After that, no further reports can be found about it. We are especially interested in the imaging ability of MPOF to guide light in air holes array. It could have huge usability in clinical diagnoses, chemical and biochemical sensing fields, fiber-optic faceplate and fiber-taper if a thousand array air-holes-contained MPOF can be successfully fabricated in the low-cost material and low-cost processing method. The multi-air-core array fiber proposed here can be also considered to use not only for their image transmission, but also a bi-functional fiber-sensing probe to view bio-sample morphology, and to measure its chemical concentrations as a similar way employed a coherent imaging fiber bundle . This imaging fiber used in sensors could exhibit high material flexibility and greatly enhance specific surface area of sensing compared with conventional sensing fiber. Prof. David R. Walt’s group has demonstrated a series of combined imaging and chemical sensing technique as strong bioanalytical tool for studying and mapping cellular dynamics in diverse microenvironments .
With drilling-stretching of PMMA rod and stacking-stretching of PMMA capillary, fabricating methods for MPOF have been reported by M. Large and other groups [4-5]. In 2005, our group started to explore a set of methods for mass-production of MPOF, and now we have designed and fabricated various structured MPOF with the extrusion-stretching process for many applications [6-12].
In this paper, we report the design and fabrication of two kinds of MPOFs, with 547 holes hexagonal array and 525 holes rectangular array for image transmission, respectively. It demonstrates that the mass-production of high quality imaging MPOFs can be realized with a commercial single screw extruder, home-made specialized mould, and a home-made specialized MPOF draw tower. The extrusion of thermoplastic optical polymer materials is one of the most promising fabrication methods for mass production of complex structured MPOF preforms.
2. Design of preform structures of MPOFs for image transmission
These two kinds of preform structures of MPOFs with 525 holes rectangular array and 547 holes hexagonal array were designed as shown in Fig. 1. Both the diameter of preforms is 70 mm, the air-hole diameter is 1.5 mm, the holes spacing is 1.9 mm. High precision moulds are elaborated to ensure high regularity of the holes in preforms, which is very difficult to get by other methods.
3. Extrusion of MPOF preforms
Firstly, high quality metal moulds are designed and prepared according to the predesigned structure. The moulds are made of a stainless steel tube with 7 cm inner diameter, two pieces of stainless steel discs with 525 or 547 small holes and lots of stainless thin steel rods. These thin steel rods are inserted into the two pieces of holey discs, and then inserted into a thick-wall steel tube. In this way, a full mould is finished. The length of the extruded preform depends on the length of the steel tube. In order to allow the molded preforms to be easily removed, all parts of mould surfaces are treated with fluoropolymer solution.
In order to study the direct extrusion process to mass production of the designed preforms, a commercial single-screw extruder, stainless steel mould, and a mould-preheating furnace are connected together as shown in Fig. 2. The length and diameter of the screw is 112 cm and 4.5 cm, respectively. The diameter of the nozzle was 70 mm. Before extrusion, optical grade PMMA pellets are dried for 8 hours at 85 °C.
According to the experiment run with this equipment, the optimal temperature of extrusion at nozzle was 175 °C. The extrusion pressure was 17 MPa. The extruded PMMA viscous liquid was directly put into a mould (150 °C) heated by a preheater. The PMMA-filled mould was removed from the extruder after extruding, and then slowly cooled to room temperature. After pulling out all thin steel rods and two steel discs, a preform with 525 or 547 air holes was formed as shown in Fig. 3(a). The designed holes arrays are ideally retained in the preforms. The length of the resultant preform could be about 30 to 40 cm, and the diameter is 7 cm, the holes diameter is 1.5 mm and the holes spacing is 1.9 mm. It is large enough to produce more than a hundred kilometers fibers with diameter of 150 µm. The total time consumed for fabricating a full preform is about 2 hours. During the extruding process, little impurity is induced and the decomposition of PMMA molecules is avoided by strictly controlling extruding conditions.
4. Stretching of MPOF preforms
A specialized drawing tower with 12 m height was designed and constructed so as to obtain high quality MPOF. It consists of the feeding part of preform, a furnace (1.2 meter height, inner diameter 10 cm), a laser diameter gauge, a capstan, a dancer, and a taking up equipment. The diameter of the resultant MPOF is mainly controlled by changing drawing speed and temperature. The drawing temperature is controlled from 175 °C to 200 °C. The drawing speed could be changed in a range from 1 to 50 m/min. When the drawing tower reached the steady state the resultant MPOF’s diameter fluctuation was less than 20 µm as shown in Fig. 4. A photo inserted in Fig. 4 shows the microscopic image of the cross section of the MPOF with 310 µm diameter. The holes diameter is about 7 µm, and holes spacing is about 9 µm. In addition, while the preforms were being stretched, some tapered and thicker secondary preforms were also obtained as shown in Fig. 3(b). It demonstrates that they have certain prospect for making new type fiber-optical faceplates and optical taper. Compared with conventional fiber-optical faceplates and optical taper fabrication methods which have difficulty in controlling the position of individual cores (pixels), the method proposed by us provide a suitable solution for this problem. The position and size of cores (pixels) could be controlled well since the fiber was drawn by a monolithic highly regular preform. Moreover, they have advantages including low cost and light weight.
5. Imaging capabilities of multi-air-core MPOF samples with rectangular and hexagonal array
The MPOF’s imaging capabilities were observed by using a set of home-made assay system as shown in Fig. 5. The main components of the imaging system are twin LED white light sources, twin collimating lenses, a home-made air-core array fiber sample, objective lenses, and a CCD camera. A thin aluminum screen with an “OE” shape is placed in front of a objective lens to reduce “OE” size suitable to enter fiber sample. When lighting is turned on, the “OE” screen is imaged onto another end face of the fiber sample. And then the second objective magnifies the “OE” into CCD camera. In this way, imaging function of the fiber samples with 525 and 547 air holes array were investigated. The results are shown in Fig. 6. Figure 6(a) is the image transmitted through a 35 cm length fiber with 525 air holes rectangular array, 3 mm outer diameter, and 64 µm holes diameter. A similar result was also obtained from 547 holes hexagonal array with 2.8 mm outer diameter, 20 cm length and 60 µm holes diameter. This image is shown in Fig. 6(b). These photos (Fig. 6) clearly demonstrated that the structure of air-core array is regular and integrated. All air cores have no any damage. The slight irregularities in the transmission pattern originate from the imperfections of the razor-blade fiber cutting method. Metal screen “OE” is clearly transmitted by these tow kinds of fiber in a coherent way and obviously transmit through the air holes, but not the solid-cores. It proves that hollow channels (or channels of low refractive index) in the fiber samples can guide pixel individually through an anti-guiding mechanism . Because the solid core’s attenuation of this fiber is higher than air core, the light through 20 cm length solid core is too weak to be observed. Image could simultaneously transmit through air core and solid core when the fiber length shorter than 7-8 cm. The light almost has no loss when the fiber with diameter of 2.8 mm down to a bending radius of 60 mm which is the minimum bend radius of its flexibility. It demonstrates that this imaging fiber has ability to guide the image through the bends of the fiber.
A tapered air-core array fiber sample with 1.3 m length, 7 mm diameter of big end face, 3 mm diameter of small end face was also used to observe image transmitting ability. When the 7 mm diameter of big end face is used as the image enter face, the “OE” could be transmitted to small end face, because the big end face of the fiber sample could provide larger light flux in the fiber. The present results indicates that the monolithic fiber with hundreds of air-core pixels can provide the same amount of information with hundreds of conventional multimode fiber or imaging fiber bundle in a short distance. Therefore, the monolithic multi-air-core MPOF will have extensive application in image transmission and optical interconnection, etc.
6. The multi-air-core array polymer fiber-based new type endoscope.
Currently, fiber-optic endoscopes use bundles of thin glass fibers to transmit light to and from the objectives being viewed. These fibers use the principle of total internal reflection to transmit the light entering one end to the other end. Fiber-optic endoscopes are delicate and expensive items. The fibers have to be made of special glass, and each fiber has to be coated with a layer of glass of a different refractive index. In addition, the orientation of fibers in a bundle used for endoscopy has to be coherent. That is, the spatial orientation of each one of thousands of fibers has to be constant. Each endoscope has one set of fiber bundles to transmit light inside, and another set to transmit reflected light out to the eye of the viewer. In newer endoscopes the viewing fiber bundle is replaced by a miniature CCD video camera chip which transmits signals via wires. But in medical analysis, one need to know information not only images, but also chemical and biochemical changes [2-3]. In this case, the multi-air-core array polymer fiber produced by us will be powerful. Here, the prototype of a new type of air-core array imaging fiber based endoscope is proposed and designed in Fig. 7. This new type endoscope only uses a monolithic fiber with 525 or 547 air-cores as the imaging fiber. The length and diameter of a fiber sample in a prototype device made from 525 air-core array fiber sample was 17 cm and 3 mm, respectively; and another one with 12.5 cm length and 3.5 mm diameter [Fig. 7(b)]. In order to avoid the disturbance of outside light, fibers were covered by black film. When the image entrance of endoscope is closed to the sample with no lens between them, by illuminating the light entrance of light fiber, the small sample “OE” was seen from the imaging fiber’s exit face of this prototype, though the resolution of the image fiber was not high. The results are shown in Fig. 7(c). It indicated that MPOF has strong potential for application in medical endoscopes.
In summary, to the best of our knowledge, PMMA-based air-core array imaging fibers with 547 holes and 525 holes was successfully fabricated by extruding-stretching process for the first time. The fabrication method of the holey preforms described in this work is simple and cheap, and the preform size is large enough to produce MPOFs over 50 km. The present research has opened the door of fabricating low-cost high-volume new type of imaging fibers for low cost endoscope, chemical and biochemical sensing applications.
In order to realize mass-production of PMMA-based air-core array imaging fiber for practical application in chemical and biochemical analysis, we are designing and fabricating a new multi-air-core and multi-liquid-core PMMA optical fiber with 1027 holes, holes diameter of 1.5 mm and holes spacing of 1.8 mm The good results about imaging, chemical and biochemical sensing combined applications will be reported in future.
This work was supported by the National Natural Science Foundation of China (project number: 60437020) and the High-tech Research and Development Program of China (No. 2007AA032452).
References and links
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