JPS6160402B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a demultiplexer for optical fiber, and in particular,
The present invention relates to an angular dispersion type optical demultiplexer having a wide passing wavelength bandwidth.

[Background of the invention]

An optical demultiplexer decomposes the wavelength-multiplexed light transmitted through an input fiber into waves of multiple wavelengths and distributes them to separate output fibers. Conventionally, interference film type optical demultiplexers and angle dispersion type optical demultiplexers have been developed. The interference film type optical demultiplexer has the advantage that the center wavelength and bandwidth can be designed arbitrarily.
Forming an interference film requires extremely sophisticated technology and a great deal of work, and it also has the disadvantage that manufacturing becomes difficult as the number of demultiplexes increases, as it is necessary to combine interference films of the same number of demultiplexes. On the other hand, the angularly dispersive optical demultiplexer has 1
Although it has the advantage of being able to perform multiple demultiplexing with a single angular dispersion element, it has the disadvantage that the passing wavelength bandwidth is limited for the following reason. FIG. 1 is an example of a conventional angle dispersive optical demultiplexer. In Figure 1, 1 is an input fiber, 2 is a collimating lens, 3 is a diffraction grating, 4-a, 4-b
(Hereinafter, both will be referred to as 4) is the output fiber. The incident light from the input fiber 1 is converted into a parallel beam by the collimating lens 2 and is incident on the diffraction grating 3. The diffracted light is converged again by the collimating lens 2 and forms an image on the end face of the output fiber 4. Here, when the end faces of the input fiber 1 and the output fiber 4 are on the same plane, the optical system has a lateral magnification of 1. At this time, the relative positional relationship between the end face of the output fiber 4 and the image of the input fiber 1 is as shown in FIG. In Figure 2, 5 is the core end face of the input fiber 1, and 6 is the core end face of the input fiber 1.
is the clad end face of input fiber 1, 7-a, 7-
Including b, 7 is the core end face of the output fiber 4, 8-
Including a and 8-b, 8 is the clad end face of the output fiber 4, and 9, 10, 11, and 12 are the input fibers 1 when the wavelengths are λ ₁ , λ ₂ , λ ₃ , and λ ₄ respectively.
This is an image of the core end face 5 of FIG. In the wavelength band λ ₁ to _{λ 2} , the core end face image of the input fiber 1 is imaged inside the core end face 7-a of the output fiber 4-a, so that a coupling efficiency of 100% can be obtained. i.e. λ
₁ to λ ₂ is a wavelength band passing through to the output fiber 4-a. Similarly, λ ₃ to λ ₄ are connected to the output fiber 4-b.
This is the wavelength band that passes through. At this time, each passing wavelength bandwidth Δλ is given by the following equation.

Here, D _i is the core diameter of the input fiber 1, D ₀ is the core diameter of the output fiber 4, and f is the collimating lens 2.
The focal length of dθ/dλ is the angular dispersion of the diffraction grating 3. On the other hand, the passing wavelength center spacing λ _ab to the output fibers 4-a and 4-b is given by the following equation.

Here, C _ab is the core center spacing of the output fibers 4-a and 4-b. By the way, since the minimum core center spacing is equal to the cladding diameter D _c of the output fiber 4, the fractional band Δλ/λ _ab is given by the following equation.

Δλ/λ _ab ≦D ₀ −D _i /D _c ……(3) For example, D _c =125 μm〓, D _i =50 μm〓, D ₀ =
In the case of 50μm〓, the passing wavelength bandwidth Δλ, the fractional band Δ
Both λ/λ _ab become zero. Therefore, the condition of D ₀ >D _i is usually required. However, if D ₀ is brought closer to D _c , the image of the input fiber 1 becomes the image of the output fiber 4-a, 4-
Since the image is formed across the core end faces 7-a and 7-b of the beam, the wavelength range increases and the degree of separation deteriorates. The following conditions are required to obtain a sufficient degree of separation.

D _c −D ₀ ≧D _i (4) When D _c =125 μm and D _i =50 μm, D ₀ ≦75 μm from equation 4. At this time, the specific band Δ
The maximum value is λ/λ _ab =0.2. As mentioned above, in the conventional technology, if input and output fibers with the same parameters are used, the bandwidth becomes zero, making it difficult to insert a duplexer in the middle of the transmission line, and even if the core diameter of the input and output fibers is changed, the maximum The drawback was that the bandwidth was limited to about 0.2.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the above-mentioned drawbacks of the prior art and to provide an optical demultiplexer that can obtain a wide pass wavelength bandwidth without being limited by the optical parameters of the input fiber.

[Summary of the invention]

A feature of the present invention is that the diffracted light from the angle dispersion element is made to travel backwards through a right angle prism and enters the angle dispersion element again to obtain re-diffracted light. The object of the present invention is to form a structure in which an incident fiber image is formed on the fiber.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 3 is a perspective view for explaining one embodiment of the present invention, in which 13 is an input fiber, 14 is a gradient index rod lens, 15 is a glass block, and 16 is a perspective view for explaining an embodiment of the present invention.
is a diffraction grating, 17-a and 17-b are right angle prisms, and 18-a and 18-b are output fibers. This embodiment operates as follows. The incident light beam from the input fiber 13 is converted into a parallel beam by a graded index rod lens 14, and is incident on a diffraction grating 16 placed at a predetermined angle by a glass block 15. After the diffracted light in the wavelength range λ ₁ to _{λ 2} converges within the reflection surface of the right-angle prism 17-a, it becomes retrograde divergent light, passes through the distributed index rod lens 14, and is converted into a parallel beam of light, which is then reflected by the diffraction grating 16. will be guided again. The parallel light beam diffracted again by the diffraction grating 16 is transmitted to the distributed index rod lens 1
4 and converges on the end face of the output fiber 18-a. Similarly, the diffracted light in the wavelength range λ ₃ to _{λ 4} is reflected by the right angle prism 17-b and sent to the output fiber 18-b.
converges to. This relationship will be explained in detail with reference to FIG. FIG. 4 shows the relative positions of the input/output fiber and the right angle prism on the fiber side end face of the distributed index rod lens in FIG. 3. In FIG. 4, 19 is the core end face of the input fiber 13, 20 is the clad end face of the input fiber 13, 21-a is the input side reflective surface of the right angle prism 17-a, 22-a is the output side reflective surface of 17-a, 23
-a, 24-a is the reflection surface 21 where the diffracted light of wavelength λ ₁
-a, area reflected by 22-a, 25-a, 2
6-a is the reflection surface 21-a, 2 where the diffracted light of wavelength λ ₂
This is the area reflected by 2-a. Similarly 21-
23-b and 24-b are areas where the diffracted light of wavelength λ ₃ is reflected by the reflecting surfaces 21-b and 22-b, 25 -b and 26-b are regions where the diffracted light of wavelength λ ₄ is reflected by the reflecting surfaces 21-b and 22-b. In addition, 27-a and 28-a are output fibers 18
-A core and clad end face, 27-
b, 28-b are the core and clad end faces of the output fiber 18-b.

The operation of this embodiment will be described below with reference to FIG. The light beam emitted from the core end face 19 of the input fiber is diffracted by the diffraction grating 16, and is divided into λ ₁ -λ ₂
In the wavelength range of , the light is converged and reflected near the region extending from 23-a to 25-a on the incident side reflective surface 21-a of the right-angle prism 17-a. Wavelength range Δ
λ _a =λ ₂ −λ ₁ is approximately given by the following equation.

Here, L _a is the side length of the rectangular prism 17-a, and D _i
is the core diameter of the input fiber 13, f is the focal length of the collimating lens 14, and (dθ/dλ) is the angular dispersion of the diffraction grating 16. Strictly speaking, the focal point is set on a plane bisecting the vertical angle of both reflecting surfaces 21-a and 22-a of right-angle prism 17-a, but from that surface, both reflecting surfaces 21-a and 22-a are given approximately. -a,22-
Since the distance to a is short and the angular spread of the diffracted light is a small value determined by the NA (numerical aperture) of the input fiber 13, both reflective surfaces 21-a, 22-
This is because the image on a and the image on the focal plane are considered to be approximately equal.

Here, if a plane reflecting mirror with the same side length L _a is provided in the same place instead of the right angle prism 17-a, the diffracted light in the wavelength range λ ₁ to λ ₂ will travel backwards, and the diffraction grating 1
It is obvious that the light is diffracted again at the input fiber core 6 and imaged on the end face 19 of the input fiber core. By the way, in this embodiment using the right angle prism 17-a, the incident side reflecting surface 21
The light beam incident on -a is reflected at the regions 23-a to 25-a and guided to the output-side reflecting surface 22-a, and then reflected at the regions 24-a to 26-a above this and guided to the diffraction grating. It will be destroyed. That is, while the diffracted light travels backward through the right-angle prism, the image position moves by a distance T within a plane including the input/output fiber and the right-angle prism. The amount of movement T is determined by the position of the diffracted light incident on the right angle prism 17-a. The light flux that has gone backwards from the output-side reflective surface 22-a is diffracted again by the diffraction grating 16,
An image is formed on the core end surface 27-a of the output fiber 18-a, which is provided at a predetermined position moved by a distance T from the input fiber core end surface. Similarly, the wavelength range λ ₃ to λ
The diffracted light of _{No. 4} is reflected by the right angle prism 17-b, diffracted again by the diffraction grating 16, and is imaged on the core end surface 27-b of the output fiber 18-b. The wavelength range Δλ _b =λ ₄ −λ ₃ is given by the following equation.

Here, L _b is the side length of the right angle prism 17-b. On the other hand, the passing wavelength center spacing Δλ _ab of the output fibers 18-a and 18-b is given by the following equation.

Here, G is the distance between the right angle prisms 17-a and 17-b. G≧D to obtain sufficient resolution
Since the relationship of _i is necessary, the fractional bands Δλ _a , Δλ _b /Δ
λ _ab becomes the following formula.

Δλ _a /Δλ _ab = L _a - D _i /(L _a + L _b )/
2+D _i Δλ _b /Δλ _ab =L _b −D _i /(L _a +L _b )/
2+D _i ...(8) In equations (5) to (8), L _a and L _b are quantities independent of the input and output fibers, so even if the same input and output fibers are used, any passband width can be obtained. Δλ _a , Δλ _b and large comparison ranges Δλ _a /Δλ _ab and Δλ _b /Δλ _ab can be obtained. L _a =500 μm, L _b =500 μm, D _i
= 50 μm, Δλ _a /Δλ _ab = Δλ _b /Δλ _ab =0.8 (9), which is four times the fractional band of the conventional technology.

〔Effect of the invention〕

As explained above, according to the present invention, it is possible to obtain a wide passband and a large fractional band without depending on the optical parameters of the input and output fibers, and it is extremely effective in configuring a demultiplexer for wavelength division multiplexing optical communications. It is.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of a conventional example, FIG. 2 is a diagram of the relative positional relationship between the end face of the output fiber and the image of the input fiber in FIG. 1, FIG. 3 is a perspective view of an embodiment of the present invention, and FIG. 4 4 is a diagram showing the relative positional relationship between the input/output fiber and the corner cube in FIG. 3. FIG. Explanation of symbols: 1, 13...Input fiber, 2...Collimating lens, 3, 16...Diffraction grating, 4-a,
4-b, 18-a, 18-b... Output fiber, 1
4...Gradient refractive index rod lens, 15...Glass block, 17-a, 17-b...Right angle prism, 1
9... Core end face of the input fiber 13, 20... Clad end face of the input fiber 13, 21-a, 22-a
...Incidence side and exit side reflective surfaces of right angle prism 17-a, 21-b, 22-b...Right angle prism 17-b
Incident side, output side reflective surface, 23-a, 24-a...
A region where the diffracted light of wavelength λ ₁ is reflected by 21-a, 22-a, 25-a, 26-a...A region where the diffracted light of wavelength λ ₂ is reflected by 21-a, 22-a, 27
-a, 28-a... core and clad end face of the output fiber 18-a, 27-b, 28-b... core and clad end face of the output fiber 18-b.

Claims (1)

[Claims] 1 In an optical demultiplexer that separates wavelength-multiplexed light transmitted through an input fiber into waves of multiple wavelengths and distributes them to separate output fibers, the incident light from the input fiber is guided to an angular dispersion element and diffracted. A plurality of rectangular prisms each having a reflecting surface larger than the real image of the incident fiber covering each passing wavelength band and returning the incoming diffracted light to antiparallel are placed near the imaging position of this diffracted light at a fixed interval. The optical demultiplexer is characterized in that the diffracted light is guided to the angular dispersion element again, and the obtained re-diffracted light is respectively focused on the end faces of a plurality of output fibers installed near the end faces of the input fibers. .