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Wavelength determining apparatus, method and program for thin film thickness monitoring lightWavelength determining apparatus, method and program for thin film thickness monitoring light description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070223008, Wavelength determining apparatus, method and program for thin film thickness monitoring light. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATIONS [0001] The present application is a divisional application of U.S. patent application Ser. No. 10/330,936, filed on Dec. 27, 2002, the entire contents of which are incorporated herein by reference. FIELD OF THE INVENTION [0002] The present invention relates to an apparatus, a method and a program for determining a wavelength of a monitoring light which monitors a thin film thickness while an optical thin film with an optical profile such as an optical filter and the like used for optical transmission is deposited. [0003] The present invention also relates to a multi-layer thin film filter used for optical transmission, and a method and a system to produce the multi-layer thin film filter. [0004] In addition, the present invention relates to a multi-layer thin film filter used for optical transmission, and an apparatus and a method to design the multi-layer thin film filter. RELATED ART [0005] It is desired to increase a present capacity of data transmission because a broadband era has arrived. Under such circumstances, it is expected to realize WDM (Wavelength Division Multiplexing) transmission systems to transmit plural light beams with different wavelengths in a wavelength-multiplexed light signal. [0006] As one of the key devices supporting such WDM transmission systems, there are optical filters that optically affects on an incident light. Such optical filers include optical band pass filters (BPF) for transmitting only a light with a desired preset wavelength out of the incident light obtained by wavelength-multiplexing light beams with different wavelengths, and gain flattening filters (GFF) for flattening an output of an optical fiber amplifier such as EDFA (Erbium-doped Fiber Amplifier) or the like. [0007] A multi-layer film filter consisting of plural optical thin films with different refractive indices is known as one of the above mentioned optical filters. This multi-layer thin film filter makes it possible to obtain a desired wavelength loss profile by setting a thin film thickness and a refractive index of each layer suitably. [0008] For example, as a thin film deposition method and a thin film deposition apparatus both for producing a multi-layer thin film filter, or a multi-layer thin film, there is known a method and apparatus in which optical thin films (filter films) are deposited sequentially on a deposition substrate by utilizing vacuum deposition technique or sputtering technique. [0009] As mentioned above, it is required to design and produce a thin film thickness of a multi-layer thin film filter precisely because desired filtering profiles (target loss-wavelength profile) are obtained by a precise thickness of each layer which composes multi-layer thin films. [0010] Therefore it is required to control production of a optical thin film such as always monitoring an optical film thickness during the thin film deposition and stopping the thin film deposition precisely when the monitored optical thin thickness reaches a target thickness. For example, one type to control production of an optical thin film is a B/A control method and the like. [0011] According to the B/A control method, it is required to project a monitoring light to optical thin films under a deposition and monitor a change of a transmission rate of the monitoring light that is transmitted through or reflecting from the optical thin films. As shown in FIG. 45, it is possible to illustrate theoretically the B/A which is a ratio between "B" which is a distance from a maximum value for a optical intensity of a stopping light and "A" which is a distance between a top and a bottom of a locus of the transmission rate change because this locus of the transmission rate change has a definite amplitude, and a maximum and a minimum value. [0012] Therefore a desired film thickness is obtained with stopping the thin film deposition when the B/A value based on an actual change of a transmission rate and the B/A value based on a theoretical change for a transmission rate of a desired film thickness becomes the same value. [0013] However there are the following problems when an optical thin film is produced with using a optical thin film thickness monitoring method such as the above mentioned B/A control method or the like. [0014] For multi-layer thin film filters, more layers of optical films are required to obtain a transmission loss wavelength profile with a sharper transmission loss change. However it becomes more difficult to design a multi-layer thin film filter when a number of layers increase because of the above mentioned reason. Therefore it takes long time to optimize a optical thin film thickness to obtain a target optical profiles. [0015] In addition, a target transmission loss wavelength profile cannot be obtained when a difference between a designed optical thin film thickness of each layer and a optical thin film thickness actually deposited becomes larger than an allowable maximum difference. [0016] When a number of layers of a multi-layer thin film filter is increased, an allowable maximum difference to obtain a profile close to the target transmission loss wavelength profile becomes smaller and a precise control for a thin film deposition is required. Therefore it is difficult to produce a multi-layer thin film filter with a target transmission loss wavelength profile. [0017] On the other hand, FIG. 46 is a graph illustrating an example of a result of a thin film deposition using a conventional control method of a thin film thickness. [0018] Data as shown in FIG. 46 is data of a gain flattening filter (GFF) produced by a method that two kinds of film materials (Ta.sub.2O.sub.5, SiO.sub.2) are deposited sequentially. A large difference is found out between a design value (design data) and actual optical profile data in a wavelength band of optical signals (1529 nm to 1561 nm in FIG. 46) in this data. A flatness to express a difference of the transmission loss is a value calculated by a maximum difference minus a minimum difference, and the value shown in FIG. 46 reaches 2.514 dB. [0019] However, when the GFF is produced, a desired flatness (a difference of transmission loss) varies, the GFF with flatness of 1.0 dB or less can be used as a commercial product. Therefore the GFF as shown in FIG. 46 has 2.541 dB as a difference of transmission loss, and it cannot be used as a commercial product. [0020] The reason of such large difference between a profile of the optical transmission loss based on the design data and a profile of the optical transmission loss of the thin film filter actually deposited is explained below. [0021] The difference between the desired thin film thickness and the thin film thickness actually deposited is hereinafter called an error. Continue reading about Wavelength determining apparatus, method and program for thin film thickness monitoring light... 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