OPTICAL BRANCHING FILTER

Abstract: An optical branching filter branches or mixes lights of n-kind of wavelengths. A first input/output part (12) for the light mixed n-kind of wavelengths is provided to one of two surfaces of a transparent block (1) and second input/output parts (13, 14, 15, 16, 17, 18, 19) for forming optical filters allowing only the lights having the predetermined wavelengths to pass therethrough, respectively among the lights in n-kind of wavelengths, but reflecting the lights having other wavelenghts, respective­ ly is provided to any one of two surfaces. A first input/­ output port (COM) and n second input/output ports (A, B, C, D), respectively, are formed by an optical waveguide (3, 5, 7, 9, 11) and a lens (2, 4, 6, 8, 10) which optically connects the optical waveguide (3, 5, 7, 9, 11) and first input/output part (12) or a second input/output part (13, 14, 15, 16, 17, 18, 19). Positions of each second input/output port (A, B, C, D) are selected so that the lengths of optical paths formed between the lenses (2, 4, 6, 8, 10) of the first input/output port (COM) and the second input/output port (A, B, C, D) are in the reverse proportion to the lengths of wavelengths corresponding to the second input/output portions

Description: Background of the Invention The present invention relates to an optical branch­ ing filter which mixes or branches the lights in different wavelengths. An optical branching filter is a basic device for a wavelength division multiplexing optical communication system. For example, it is possible for using the lights mixed n-kinds of wavelengths λ₁, λ₂, ....,λ n to transmit n times amount of information as much as using the light of single wavelength λ₀ over a single line of optical fiber. And the n-channel bothway simultaneous optical communication can be realized by allocating the lights in different wavelengths for transmission and reception, namely the wavelengths λ₁, λ₃, ....,λ 2n-1 are allocated for transmission and wavelengths λ₂,λ₄, .....,λ 2n are allocated for reception. In order to realize such optical communication systems, an optical mixing filter which mixes the lights in different wave­ lengths and inputs the light to a single line of optical fiber is arranged at the sending side and an optical branching filter which branches respective lights in different wavelengths from the light mixed different wavelengths is arranged at the receiving side. The optical mixing filter and optical branching filter currently put into practical use generally mix and branch the lights by employing a dielectric materi­ al filter composed of the thin dielectric multilayer film laminated in a plurality of layers such as SiO₂ and TiO₂ as a bandpass filter which allows transmiss­ ion of the light having particular wavelength but reflects the lights having other wavelengths. The optical mixing filter can also be used as the optical branching filter by inverting the incoming and outgoing directions of light to/from said dielectric material filter. These mixing and branching filters have the same structure and therefore these filters are respectively called the optical branching filter. To realize this optical branching filter, there are further required various optical components in addition to said dielectric material filter. For example, these are a plurality of optical lenses which cause the light sent from the optical fiber to effectively input each di­ electric material filter and also causes the light passed the dielectric material filter to effectively input the optical fiber and a plurality of fixing mechanisms which fix the end face of optical fiber to the focus point of lenses. However, the optical branching filter deals with the lights having various wavelengths. A refractive index of lens generally changes in accordance with wavelength and thereby focus point of lens depends on the wavelength of an incident light. Therefore, since said fixing mechanisms require adjustment at focus points for each wavelength, it deteriorates the manufacturing yield of optical branch­ ing filter. This deterioration becomes distinctive with increase in kinds of wavelengths of lights to be mixed or branched. Particularly, various kinds of semiconductor lasers or light emitting diodes for long to short wavelengths have been developed and a kind of wavelengths dealt with an optical branching filter tends to increase and therefore a problem of adjustment at focus points is considered very serious. Summary of the Invention It is accordingly an object of the present invent­ ion to provide an optical branching filter which is capabale of improving manufacturing yield by realizing common adjustment of the distance between the lens and end face of optical fiber without depending on wavelengths. According to the present invention, it is provided an optical branching filter, comprising; a block which is optically transparent and has parallel two surfaces, a first input/output part for the light mixed n-­ kind of wavelengths provided to one of two surfaces, second input/output parts provided to any one of said two surfaces, for forming optical filters allowing only the lights having the predetermined wavelengths to pass therethrough, respectively among the lights in n-­ kind of wavelengths, but reflecting the lights having other wavelengths, respectively; a first input/output port and n second input/out­ put ports, respectively formed by an optical waveguide and a lens which optically connects the optical wave­ guide and the first input/output part or a second input/output part, wherein positions of each second input/output port is selected so that the lengths of optical paths formed between the lenses of the first input/output port and the second input/output port are in the reverse proport­ ion to the lengths of wavelengths corresponding to the second input/output portions. Brief Description of the Drawings Figs. l(a),(b) are schematic plan views of an optical branching filter of the present invention; Fig. 2 is a pictorial presentation for explaining the distance between lenses and distance between lens and optical fiber end face; Fig. 3 is a sectional view of the assembled fixing mechanism of optical fiber end face and lens; Fig. 4(a) is a plan view of practical optical branching filter; Fig. 4(b) is a perspective view of practical optical branching filter; Fig. 5 is a pictorial presentation for explaining relation of the distance between lenses and distance betwteen lens and optical fiber end face; Fig. 6 is a pictorial presentation for explaining wavelength characteristics of dielectric material filters; and Fig. 7 is also a pictorial presentation for ex­ plaining relation of distance between lens and optical fiber end face and loss increase. The like elements are given the like reference numerals throughout the drawings. Description of the Preferred Embodiments Figs. l(a),(b) are schematic plan views illustrat­ ing a structure of optical branching filter of the present invention. In Figs. l(a),(b), four kinds of different wavelengths λ₁, λ₂, λ₃, λ₄ are mixed and branched. In a glass block l consisting of an opti­ cally transparent substance such as BK7 having a pair of parallel surfaces, the parallel surfaces thereof are provided with a bandpass filter l3 which allows only the light of the wavelength λ₁ to pass, a bandpass filter l5 which allows only the light of wavelength λ₂ to pass and a bandpass filter l7 which allows only the light of wavelength λ₃ to pass. The light mixed four kinds of wavelengths λ₁, λ₂, λ₃, λ₄ is emitted from the end face of optical fiber 3, converted to a parallel light by a spherical lens 2, and is input to the glass block l through an anti-reflection film l2 provided for eliminating the reflected lights. In the case of branching this mixing light to four kinds of wave­ legths wavelengths respectively, this mixing light is input from the end face of optical fiber 3. Among this mixing light, only the light of wavelength λ₁ is able to pass the band-pass-filter l3 provided to the oppo­ site surface of glass block l, is output to the outside of glass block l through the anti-reflection film l4, is focused by spherical lens 4 and is then input to the optical fiber 5 effectively. The other lights of wavelengths λ₂, λ₃, λ₄ are reflected. Moreover, only the light of wavelength λ₂ is able to pass the band­ pass filter l5, is output to the outside of glass block l through the anti-reflection film l6, is focused by spherical lens 6 and is then input to the optical fiber 7. The lights in the wavelengths λ₃, λ₄ are reflected. In the same way, the light in the wavelength λ₃ is able to pass the bandpass filter l7 and is then input to the optical fiber 9 by the spherical lens 8 through the anti-reflection film l8. Here, the light reflected by the bandpass filter l7 is only the light of wavelength λ₄ and therefore the light of the wavelength λ₄ is input to the optical fiber ll by the spherical lens l0 via only the anti-reflection film l9. The bandpass filters and anti-reflection films are formed by the dielectric material multilayer film of SiO₂,TiO₂ obtained by vacuum depositon. In an example explained above, each light of wave­ lengths are branched from the light mixed four kinds of wavelengths. On the other hand, the light mixed four wavelengths can be input to the optical fiber 3 by reversing the travelling direction of the lights of the wavelengths λ₁,λ₂,λ₃,λ₄ emitted from the optical fibers 5, 7, 9, ll. Here, it will become a problem that the spherical lenses deal with the lights in the different wave­ lengths. Namely, a refractive index of lens changes in accordance with wavelength and therefore the focus point of spherical lenses 4, 6, 8, l0 are also differ­ ent in the input/output ports A, B, C, D. In order to have the lights of the wavelengths λ₁, λ₂,λ₃,λ₄ input most effectively to the optical fiber 5, 6, 9, ll, the end faces of optical fiber 5, 6, 9, ll must be provided at the focus points of the spherical lenses 4, 6, 8, l0 at respective input/output ports. Therefore, since the end faces of each optical fiber must be arranged at focus point at each input/­ output port A, B, C, D, it is required to adjust the distances between the spherical lenses and optical fiber end faces, ℓ₁,ℓ₂,ℓ₃,ℓ₄ respectively. However, the input/output ports A, B, C, D and the mixing light input/output port COM (optical fiber 3 and spherical lens 2) are assembled as shown in Fig. 3. Fig. 3 is a sectional view of the structure of the assembly of spherical lens and optical fiber at the respective input/output ports A, B, C, D, COM. The end part of optical fiber cord 23, for examle, NYLON, is removed and the ferrule is formed by the optical fiber 24 (core and clad) and the sleeve 22 to which said optical fiber 24 is inserted. An assembled holder 20 consisting of stainless provides a cylindrical hole for fixing such ferrules and a cylindrical hole for fixing a drum lens 2l, which is fixed to the assembled holder 20 by the glass solder 25, in different dia­ meters. Moreover the ferrule is fixed to the assembled holder 20 by laser welding of the part 26. The drum lens 2l is formed by grinding a spherical lens because of fixing it easier. The assembled input/out­ put ports A, B, C, D, COM are fixed on the substrate 27 via an oblique spacer 28 as shown in Figs. 4(a),(b). Moreover, the glass block l providing bandpass filters is fixed onto the substrate through a fixing member 29 by screwing. Here, the input/output ports A, B, C, D, COM are fixed through the oblique spacer 28 in order to make match the optical axis easier. However, as mentioned above, it is possible to fix the optical components easily with good accuracy onto the substrate 27 that the spherical lens and optical fiber, which form respective input/output ports, are assembled to be a unit, it is very troublesome to adjust the distances between the optical fiber end and lens, shown in Fig. 3, corresponding to respective input/output ports A, B, C, D, COM (corresponding to wavelength). Accordingly, it is apparent that manufacturing yield can be very much improved by making the distances between optical fiber end and lens,ℓ₁,ℓ₂,ℓ₃,ℓ₄, equal without depending on wavelength. A method for making the distances between optical fiber end and lens,ℓ₁,ℓ₂,ℓ₃,ℓ₄, equal will then be explained hereunder. Fig. 2 is a pictorial presentation for explaining the distance L = L₀ + L a + L₁ between lenses in the input/output port COM and input/output port A in Fig. l and the distance ℓ= ℓ₁ between optical fiber end and lens at the input/output port A. When a focus point of lens 4 for the light of wavelength λ₀ is considered as f₀ when in the relation of λ₀ < λ₁, the focus point of lens 4 for the light of wavelength λ₁ becomes f₁. Namely, since a refraction index decreases when wavelength becomes longer, the focus point of lens 4 shifts in such a direction as being far from lens 4. An example of BK7 used as the lens 4 is further explained in detail. A refraction index of BK7 is l.5l45 for the He-Ne laser beam in the wavelength λ₀ = 0.63 µm and when a radius r of spherical lens is 2.5 mm, the focal dis­ tance f₀ is l.l8 mm. Fig. 5 shows relation between the lens-to-lens distance L and increment Δℓ of focal distance for each wavelength λ= l.3 µm, l.2 µm, 0.89 µm and 0.8l µm with reference to f₀ = l.l8 µm. As is apparent from Fig. 5, when the lens-to-lens distance L increases, increment Δℓ of focal distance also increases. Moreover, it can also be understood a rate of increase is larger as the wavelength is longer. In case the light emitted from the optical fiber end is converted to the perfectly parallel light by the sphe­ rical lens 2, increment Δℓ only depends on the wavelength without depending on the lens-to-lens distance L. However, the spherical lens 2 cannot perfectly convert the light to the perfectly parallel light and converts it actually to the light which is a little diverging. Accordingly, when the lens-to-lens distance L increases, increment also increases. Further investigation will be made for Fig. 5. A longer wavelength generally results in a larger incre­ ment Δℓ. On the other hand, a shorter wavelength results in a shorter increment Δℓ. Therefore, when the lens-to-lens distance L of shorter wavelength is set larger and the lens-to-lens distance L of longer wave­ length is set smaller, the difference between the distances ℓ₁,ℓ₂,ℓ₃,ℓ₄ can be smaller. Considering these points, difference of distance between lens and optical fiber end resulting from difference of wavelengths can be alleviated to such a degree as an aberration by setting the lens-to-lens distance in reverse proportion to the length of wavelength. In more concrete, the lens-to-lens distance between the lens 2 of the mixing light input/output port COM and the lenses 4, 6, 8, l0 of the input/output ports A,B,C,D of respective wave­ lengths can be set in such a manner that it is reversely proportional to wavelength by setting the wavelengths of the input/output ports A,B,C,D in such a manner as λ₁ > λ₂ > λ₃ > λ₄ in Fig.l. In the following explanation, practical values are given. The wavelengths are set as follow; λ₁ = l.3 µm, λ₂ = l.2 µm, λ₃ = 0.89 µm, λ₄ = 0.8l µm and the wavelength characteristics of filters for each wavelength are set to that of bandpass filters shown in Fig. 6. Where, a bandpass filter for the wave­ length λ₄ is unnecessary. In case an incoming angle ϑ₁ of the light at the input/output port COM set to 23°, outgoing angle ϑ₂ to l5°, refractive index n for the reference light of glass block l (He-Ne laser beam wavelength = 0.63 µm) to l.5l54, distance between the lens and glass block at each input/output port COM, A, B, C, D to L₀ = L₁ = L₂ = L₃ = L₄ = 5 mm, the distance between the lens 2 of the input/output port COM and the lens 4 of input/output port A is expressed as follow; L₀ + L₁ + L a × l/(COSl5°) × (l/n) = 2l.7 mm In the same way, distance between the lens 2 of the input/output port COM and the lens 6 of input/­ output port B is exprssed as follow; L₀ + L₂ + L a × l/(COSl5°) × (l/n) ≒ 33.5 mm Concerning the input/output port C, L₀ + L₃ + L a × 3 × l/(COSl5°) × (l/n) ≒ 45.2 mm Concerning the input/output port D, L₀ + L₄ + L a × 4 × l/(COSl5°) × (l/n) ≒ 56.9 mm Where l/COSl5° is used for converting La to the distance in the air. In this case, deviation Δℓ of focal distance at each input/output port can be obtained from Fig. 5 as follow; Input/output port A; Δℓ = 38 µm Input/output port B; Δℓ = 46 µm Input/output port C; Δℓ = 32 µm Input/output port D; Δℓ = 34 µm Therefore, since the focal distance f₀ of spheri­ cal lens for the reference light is l.l8 mm, the optimum distances ℓ₁,ℓ₂,ℓ₃,ℓ₄ between the optical fiber end and lens at each input/output port A, B, C, D are indicated as follow; At the input/output port A, ℓ₁ = l.l8 + 0.038 (mm) At the input/output port B, ℓ₂ = l.l8 + 0.046 (mm) At the input/output port C, ℓ₃ = l.l8 + 0.032 (mm) At the input/output port D, ℓ₄ = l.l8 + 0.034 (mm) A mean value of the maximum and minimum values for all input/output ports can be obtained as follow; (0.046 + 0.032)/2 = 0.040 Therefore, in case a common deviation Δℓ at each input/output port is set to 0.040, following can be obtained. ℓ= ℓ₁ = ℓ₂ = ℓ₃ = ℓ₄ = l.l8 + 0.04 = l.22 mm. Namely, a common value = l.22 mm can be obtained as the distance between the optical fiber end and lens. Fig. 7 is a graph which indicates loss increase by deviation Δz from the optimum position of lens at an input/output port, corresponding to Fig. 2. As is obvious from Fig. 7, loss is 0.l dB or less when Δz < l3 µm. Therefore, in case Δz < l3 µm, influence of loss is as small as can be neglected. As explained above, when Δℓ is set to 40 µm, the maximum Δz for respective input/output ports A, B, C, D is equal to 40 - 32 = 8 µm < l3 µm. Accordingly, distance between the optical fiber end and lens at each input/output port can be set in common. If Δℓ cannot be determined as small as an aberrat­ ion, the lens-to-lens distance ℓ₁,ℓ₂,ℓ₃,ℓ₄ between the input/output ports can be changed by adjusting L₀, L₁, L₂, L₃, L₄ of input/output ports A, B, C, D, COM, in view of the relation betrween L and Δℓ.

Claims: 1. An optical branching filter for branching or mix­ ing lights of n-kind of wavelengths, comprising: a block (1) which is optically transparent and has parallel two surfaces, a first input/output part (12) for the light mixed n-kind of wavelengths provided to one of said two surfaces, second input/output parts (13, 14, 15, 16, 17, 18, 19), provided to any one of said two surfaces for forming optical filters allowing only the ligths having the predetermined wavelengths to pass therethrough, respectively among the lights in n-kind of wavelengths, but reflecting the lights having other wavelengths, respectively, and a first input/output port (COM) and n second input/output ports (A, B, C, D), respectively formed by an optical wave­ guide (3, 5, 7, 9, 11) and a lens (2, 4, 6, 8, 10) which optically connects said optical waveguide (3, 5, 7, 9, 11) and said first input/output part (12) or a second input/­ output part (13, 14, 15, 16, 17, 18, 19), wherein positions of each second input/output port (A, B, C, d) are selected so that the lengths of optical paths formed between the lenses (2, 4, 6, 8, 10) of said first input/output port (COM) and said second input/output port (A, B, C, d) are in the reverse proportion to the lengths of wavelengths corresponding to said second input/output portions. 2. An optical branching filter according to claim 1, wherein distances between said optical waveguides (5, 7, 9, 11) and said lenses (2, 4, 6, 8, 10) are unified to a mean value of the maximum and minimum values among the optimum values of respective second input/output ports (A, B, C, D). 3. An optical branching filter according to claim 1, wherein said first and second input/output port (COM, A, B, C, D) comprises a cylindrical holder (20) which holds unitarily said optical waveguide (23, 24) and said lens (21) therein. 4. An optical branching filter according to claim 2, wherein said optical paths can be changed by adjusting the corresponding distance between each input /output port (COM, A, B, D, D) and said block (1), thereby said mean value can be set to the desired value.

Inventors: ASANUMA, KAZUSHI, ISONO, HIDEKI, TERAI, KIYOSHI, YOKOTA, TAKASHI
Applicants: FUJITSU LIMITED
Application year: 1987
Main classification: G02B6
IPC codes: G02B6