Plate plastics optical waveguide

Abstract: A plate plastics optical waveguide comprises a core portion (3) composed of a polymer containing a deuteriumation or a halogen atom, and a clad portion (2,7) which surrounds the core portion and composed of a polymer having a refractive index lower than that of the core portion. The plastics optical waveguide which has a core portion comprising a deuterated or halogenated polyacrylate or polysiloxane shows an optical loss of 0.1 dB/cm or less for a light at a wavelength of 1.3 µm.

Description: The present invention relates to plastics optical waveguides, more particularly to a planar plastics optical waveguide useful as optical components such as optical couplers or multiplexers for optical communication or image transmission. As for a base material for optical components or optical fibers, generally inorganic substances such as silica glass and multicomponent glass are used because their propagation loss is low and their transmission band is wide. On the other hand, optical components using plastics as a base material have been developed. Plastics optical materials used therefore have a good fabrication flexibility and the controllability of optical properties compared with inorganic optical materials, and therefore, much attention has been paid thereon with an expectation that optical waveguides having relatively good characteristics could be produced with ease using them. A planar plastics optical waveguide made e.g. of PMMA, polycarbonate or polystyrene is disclosed in DE-A-3842480. Representative techniques for producing of planar plastics optical waveguides include a selective photopolymerization and a utilization of a photosensitive resin. The selective photopolymerization method is a technique in which monomers contained in a polymer are selectively polymerized to change the refractive index to make a pattern-like optical waveguide. More particularly, at first a mask having a predetermined pattern is mounted on a polymer sheet or substrate composed of a transparent polymer such as polycarbonate which contains a low refractive index monomer such as methyl acrylate, and the sheet or substrate is irradiated with ultraviolet rays through the mask to selectively polymerize the low refractive index monomer in accordance with the pattern. The photopolymerized portion of the polymer has a lower refractive index than the polymer matrix. Then the polymer sheet is heated in vacuum to remove unreacted monomers which remain in areas unexposed with ultraviolet rays. As a result, the unexposed portion of the polymer consists of the high refractive index polymer alone. Thus, a patterned sheet can be obtained in which a high refractive index portion serving as a core is formed in accordance with a predetermined pattern. Finally, the patterned sheet is sandwiched by a clad composed of a low refractive index polymer to produce an objective optical part. On the other hand, according to the method which uses a photosensitive polymer, a photosensitive polymer is pattern-wise exposed to a light to cause cross linking selectively, followed by development to remove unexposed portions to obtain a core pattern. More particularly, at first a polymer serving as a clad is coated on a substrate by dipping or spin coating. A polymer such as polyurethane containing a photosensitive crosslinking agent is coated on the clad in the same manner as above. Then, the coated polymer is irradiated with ultraviolet rays through patterned mask to selectively crosslink the polymer. Next, the substrate is immersed in a solvent to remove unexposed portions to obtain a core having a predetermined pattern. Finally, a clad material is provided on the core by dipping, spin coating or laminating to form a clad, thus an objective optical part is produced. In order to obtain practically useful optical parts which show a low optical loss, it is required that a film for transmitting light is of a good quality and patterns formed by microprocessing have a high reliability. That is, it is preferred that the material of the film itself show as low as possible an optical loss, and the thickness and refractive index of the film made of the material can be controlled with a high precision. And further, smoothness of side walls of the core, dimension stability and reproducibility are important factors for the microprocessing of the waveguide fabrication. In the case where a plastics material is used, optical parts produced by the selective photopolymerization method and the method using a photosensitive polymer, respectively, show a relatively low optical loss at a shorter wavelength (0.48 to 1.1 µm). However, they show a high optical loss as high as 0.5 to 10 dB/cm in an infrared region (1.3 to 1.55 µm), utilized in optical communication at present, because of higher harmonics of infrared vibrational absorptions due to the carbon-to-hydrogen bonds constituting the plastics, and therefore they are unsuitable for practical use. From the standpoint of fabrication flexibility of optical waveguides, both methods are simpler and easier than production of glass optical waveguides. However, the selective photopolymerization method has some problems, among others, that the content of the monomer varies depending on the conditions of evaporation of the solvent to cause subtle fluctuation of the refractive index difference. On the other hand, the method using a photosensitive polymer has a problem that resolution is poor and that protrusions and depressions tend to occur on the surface of the optical waveguide due to swelling upon development. These are the reasons why the conventional plastics optical waveguides show high optical losses. Under the circumstances, the present invention has been made in order to obviate the above-described defects of the conventional plastics optical waveguides. It is therefore an object of the present invention to provide a planar plastics optical waveguide which shows a low optical loss in the visible to infrared region. The present invention provides a planar plastics optical waveguide as set out in claim 1 of the accompanying claims. The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings. Figs. 1A to 1G are schematic cross-sectional views of a plastics optical waveguide according to the present invention for explaining an example of process for producing thereof; Fig. 2 is a characteristic graph illustrating the dependence of the refractive index of polyacrylate on the fluorine content thereof; and Fig. 3 is a characteristic graph illustrating the optical loss dependence of the plastics optical waveguide according to the present invention on the wavelength. The most important factor for optical loss of plastics is higher harmonics of infrared vibrational absorption due to the carbon-to-hydrogen bonds constituting the plastics: In the plastics optical waveguide of the present invention, the hydrogen atoms in the structure of the plastics are substituted with a halogen atom such as fluorine, chlorine and iodine or a deuterium atom in order to reduce the higher harmonics due to the carbon-to-hydrogen bonds and shift them to a longer wavelength region. The substitution enables reduction of optical loss of the material itself and makes optical waveguides produced using the material have a high performance. Such optical waveguides as described above can be produced by providing a substrate, forming a plastics film on the substrate, further forming a pattern of photoresist on the plastics film by lithography, and dry etching the substrate in an oxygen- or fluorine-containing gas using the patterned photoresist as a mask. More particularly, a photoresist is coated on a sheet of polymer to be processed and irradiated with actinic rays such as ultraviolet rays, electron beams, and X rays pattern-wise or image-wise. Next, the polymer sheet is immersed in a solvent to develop it to obtain a pattern. The resulting pattern is used as a mask when the underlying polymer is subjected to reactive dry etching in a fluorine- or oxygen-containing gas in order to transfer the pattern to the polymer sheet. While the plastics optical waveguides of the present invention can be produced by combination of the above-described steps, a representative processing procedure will be explained hereinafter referring to Figs. 1A through 1G. At first, a clad material is coated on a substrate 1 to form a layer-like clad 2 (Fig. 1A). Then, a core material composed of an organic polymer is coated on the clad 2 to form a core layer 3 (Fig. 1B). Next, as illustrated in Fig. 1C, a silicone-based photoresist 4 is coated on the core layer 3, and exposed to actinic rays through a mask 5, followed by development to obtain a mask pattern (Fig. 1D). Further, reactive ion etching using oxygen gas is carried out to remove the core layer 3 in a region other than the patterned portion (Fig. 1E). The photoresist is peeled off (Fig. 1F), and a clad material 7 which has the same composition as the clad 2 is coated or laminated thereon (Fig. 1G). This production method is advantageous in that patterns of a high resolution and high dimensional stability with steep and smooth side walls by the use of reactive ion etching can be obtained in fewer steps with excellent reproducibility. From a viewpoint of mass production, it is also advantageous to use a method in which a plurality of optical waveguides are made on a substrate of a large surface area simultaneously and fabricated by using a stamper or metal mold. As is well known, optical waveguides are designed to propagate light in the core utilizing difference in refractive index between their cores and clads. In the case of plastics optical waveguides, difference in refractive index can be generated by the use of different types of plastics in the core and clad, respectively. Alternatively, the refractive index of the core and/or clad can be controlled by the content of fluorine in the polymer or polymers. Fig. 2 illustrates variation of the refractive index of a copolymer of a deuterated heptafluoroisopropyl methacrylate with a deuterated methyl methacrylate depending on the fluorine content in the polymer. In Fig. 2, lines A and B indicate refractive indices for light at wavelengths of 0.6328 µm and 1.5230 µm, respectively. In both cases, the refractive indices decrease linearly with increase of the fluorine content. Now the present invention will be explained with reference to a specific embodiment. Embodiment Planar plastics optical waveguides each including a clad portion composed of a polyacrylate having a repeating unit represented by the following formula: wherein each of X 1 and X 2 is a deuterium atom or a halogen atom; R 1 is selected from the group consisting of a heavy hydrogen atom, a CD 3 group and a halogen atom; and R 2 is a halogenated or deuterated alkyl group of formula: C n Y 2n-1 (where Y is a halogen atom or a deuterium atom; n is one integer of 1 to 5), and a core portion composed of a deuterated or halogenated polystyrene resin having a repeating unit represented by wherein Y is a halogen atom or deuterium. Table 3 shows chemical structures of the polymers used as cores and clads of the optical waveguides and their optical losses for light having a wavelength of 1.3 µm. The optical loss of each optical waveguide was very low, i.e. as low as from 0.08 to 0.09 dB/cm. As described above the planar plastics optical waveguide of the present invention has superior light transmission characteristics in a region of from visible to near infrared region to the conventional ones. Particularly, because it shows a low optical loss in a wavelength region of from 650 to 1,600 nm, the planar plastics optical waveguide of the present invention can be used as being connected to multicomponent glass or silica optical fiber without resort to light/electricity conversion or electricity/light conversion. Therefore, it has an advantage that optical parts produced by using the optical waveguide of the present invention can be used for constructing highly economical optical signal transmission systems such a local area network and the like. The invention has been described in detail with respect to embodiments, and it will now be apparent from the foregoing to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and it is the intention, therefore, in the appended claims to cover all such changes and modifications as fall within the invention.

Claims: A planar plastics optical waveguide comprising: a core portion (3) composed of a polystyrene having a refractive index, and a clad portion (2,7) surrounding the core portion (3) and being composed of a polymer having a refractive index which is lower than the refractive index of the core portion (3); wherein said planar plastic optical waveguide is characterised in that said clad portion (2,7) is composed of a deuterated and/or halogenated polyacrylate having at least one kind of repeating unit represented by formula (I): wherein each of X 1 and X 2 is a deuterium atom or a halogen atom; R 1 is selected from the group consisting of a deuterium atom, a CD 3 group and a halogen atom; and R 2 is a halogenated or deuterated alkyl group of formula C n Y 2n+1 in which Y is a halogen atom or a deuterium atom and n is an integer of 1 to 5, and further wherein deuterium atom(s) and halogen atom(s) coexist in said repeating unit, and in that the polystyrene of said core portion (3) is a polystyrene having a repeating unit represented by wherein Y is a halogen atom or deuterium.

Inventors: IMAMURA SABURO, IZAWA TATSUO, IMAMURA, SABURO, IZAWA, TATSUO
Applicants: NIPPON TELEGRAPH & TELEPHONE, NIPPON TELEGRAPH AND TELEPHONE CORPORATION
Application year: 1990
Main classification: G02B6
IPC codes: G02B6, G02B1