(1) Field of the Invention
The present invention relates to a method of manufacturing an optical filter adapted to selectively transmit a light of specific wavelength, and more particularly to a method of manufacturing an optical filter formed with dielectric multilayer coatings.
(2) Related Art
There have been known a variety of optical filters for extracting a light of specific wavelength. As representative ones, such as those adopting a dielectric multilayer coating are utilized mostly. Generally, these optical filters adopting a dielectric multilayer coating are manufactured by alternately evaporating, onto a substrate glass, materials having different refractive indices such as TiO2 and SiO2, thereby forming a multiple interference coating (dielectric multilayer coating) by 50 to 100 layers. As the number of layers in the dielectric multilayer coating is increased, this type of optical filter is to have a relatively flat transmission characteristic and a sharply changing cutoff characteristic.
Further, as optical filters having a high resolving power, there have been known those elements such as called xe2x80x9cFabry-Perot interferometer (Fabry-Perot etalon, or simply etalon)xe2x80x9d. Generally, etalon is manufactured by applying coatings having high reflectance onto the side faces of a plane-parallel plate.
Meanwhile, as a method of achieving large-capacitance of optical transmission system, attention has been directed to a wavelength-division multiplexing (WDM) optical transmission system in which two or more optical signals having wavelengths different from each other are multiplexed and transmitted in a single transmission path. In this WDM optical transmission system, since there are transmitted a plurality of WDM optical signals having different wavelengths arranged with very narrow wavelength intervals, there is required an optical filter having a high resolving power with a relatively flat transmission characteristic and a sharply changing cutoff characteristic, so as to select an optical signal of a specific wavelength from these WDM optical signals.
However, although the aforementioned conventional optical filters adopting dielectric multilayer coatings are supposed to realize a flat transmission characteristic and a high cutoff characteristic, by increasing the number of the layers as mentioned above, the increase in the number of layers undergoes a predetermined limit such as due to restriction in actual manufacturing. Thus, it has been practically difficult to manufacture an optical filter adopting a dielectric multilayer coating, having both of a flat transmission characteristic and a high cutoff characteristic.
Further, the conventional etalons generally have a higher resolving power as compared to the optical filters adopting dielectric multilayer coatings. However, such etalons are problematic in that they have less flatness of transmission characteristic and have a deteriorated cutoff characteristic relative to adjacent bands. It has been also problematic that the yield of etalons having a high resolving power is low, since it has been difficult to form reflective coatings having uniform characteristics on the side faces of the plane-parallel plate.
The present invention has been carried out in view of the aforementioned points, and it is therefore an object of the present invention to provide a manufacturing method of readily realizing an optical filter having a flat transmission characteristic and a superior cutoff characteristic relative to adjacent bands.
To this end, with one embodiment of the present invention, there is provided a method of manufacturing an optical filter comprising two band-pass filters consisting of dielectric multilayer coatings, the optical surfaces of the filters being substantially parallelly arranged to each other, comprising: a dielectric multilayer coating forming step of forming a band-pass filter coating consisting of a dielectric multilayer coating, on one plane of a transparent substrate; a dividing step of dividing said substrate formed with said band-pass filter coating consisting of the dielectric multilayer coating, into a plurality of segments; and an adhering step of adhering those two segments which are selected from said plurality of segments of said substrate, by means of a transparent adhesive, so that respective surfaces of said band-pass filter coatings consisting of the dielectric multilayer coatings of said two segments are faced to each other and are rendered to be substantially parallel to each other with a predetermined distance.
According to such a manufacturing method, the band-pass filter coating consisting of a dielectric multilayer coating is formed on the substrate in a single step, the substrate is divided, and the surfaces of the segments applied with the dielectric multilayer coating are adhered to manufacture an optical filter. This optical filter corresponds to what is obtained by forming band-pass filters instead of reflective coatings to be applied to a conventional etalon, thereby enabling provision of an optical filter having a flat transmission characteristic and a superior cutoff characteristic.
It is preferred that the adhering step in the abovementioned manufacturing method selects those adjacent two segments from the plurality of segments of the substrate divided by the dividing step. By selecting and adhering those adjacent segments of dielectric multilayer coatings formed in the same step, there are combined two band-pass filters having uniform characteristics.
Further, the abovementioned manufacturing method may further comprise an antireflection coating forming step of forming an antireflection coating on the other plane of the substrate, wherein the dividing step divides the substrate formed with the band-pass filter coating consisting of the dielectric multilayer coating and formed with the antireflection coating, into the plurality of segments. As a concrete example, the adhering step may adhere the selected two segments with a transparent thin plate interposed therebetween; and also it is preferable to set the distance between the band-pass filter coatings consisting of dielectric multilayer coatings of the selected two segments, to be equal to or less than 100 xcexcm.
With another embodiment of the present invention, there is provided a method of manufacturing an optical filter constituted of two band-pass filters consisting of dielectric multilayer coatings, the optical surfaces of the filters being substantially parallelly arranged to each other, comprising: a dielectric multilayer coating forming step of forming on a front surface of a transparent substrate a band-pass filter coating consisting of a dielectric multilayer coating rendered to have a grading characteristic concerning a transmission wavelength, and forming on a back surface of the transparent substrate a band-pass filter coating consisting of another dielectric multilayer coating rendered to have a grading characteristic in a direction perpendicular to that of the grading characteristic of the band-pass filter coating formed on the front surface; and a cutting step of cutting out a segment located at a specific area of the substrate formed with the band-pass filter coatings consisting of the dielectric multilayer coating at the front and back surfaces thereof, respectively.
According to such a manufacturing method, two band-pass filter coatings consisting of two dielectric multilayer coatings having grading characteristics concerning transmission wavelengths are formed on the front and back surfaces of the substrate, respectively, such that the grading directions of the band-pass filter coatings are rendered to be perpendicular to each other. As a result, those segments of band-pass filters of uniform characteristics formed on the front and back surfaces, are distributed along a specific direction. Thus, there can be obtained an optical filter having an appropriate transmission characteristic, by cutting out a segment located at the specific area.
Preferably, the dielectric multilayer coating forming step in the abovementioned manufacturing method forms the band-pass filter coatings consisting of dielectric multilayer coatings on the front and back surfaces of the substrate having a thickness of equal to or less than 100 xcexcm.
Further objects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments when read in conjunction with the accompanying drawings.