JPH0750722Y2 - Optical fixed attenuator - Google Patents

Description

[Detailed Description of the Invention] Overview Single-mode optical fibers having the same mode field diameter at a predetermined wavelength and having a refractive index distribution so that the wavelength dependence of the mode field diameter is different are fixed by connecting the end faces to each other. Configure an attenuator. The attenuation factor of this attenuator depends on the wavelength and can be substantially zero at the predetermined wavelength. Therefore, it can also be used as a simple filter that transmits light of a predetermined wavelength.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fixed optical attenuator formed by connecting optical fibers having different refractive index distributions, and more particularly to a structure of a fixed optical attenuator whose attenuation factor depends on wavelength.

In an optical communication system using an optical fiber as a transmission line,
Depending on the photoelectric characteristics of the light receiving section, it may be necessary to attenuate the transmitted signal light to an optimum level. In such a case, the optical fiber is usually passed through an appropriate fixed optical attenuator. I am trying to connect to. A single-mode optical fiber has a smaller core diameter than a multi-mode optical fiber and requires precision in mechanical connection. Therefore, an optical fixed attenuator that can obtain a desired attenuation rate with a simple configuration is available. Is requested.

On the other hand, it has been attempted to transmit a large number of signal lights in one direction or two directions by a single optical fiber by the wavelength division multiplexing method. When such a system spreads, a desired wavelength is used as a demultiplexer on the receiving side. A filter with a simple structure for separating the signal light is required.

2. Description of the Related Art A part of the structure of a fixed optical attenuator that has been proposed or put into practical use is shown below.

(1) A spacer is interposed at the end portion of the optical fiber, that is, at the connection portion with the light receiving portion.

(2) An attenuator having a low transmittance is provided at the end of the optical fiber or the joint between the optical fibers.

(3) A constriction is formed in the middle of the optical fiber.

(4) A bend is formed in the middle of the optical fiber.

(5) Another optical fiber having a different core diameter is connected to the middle or end of the optical fiber.

Further, as a wavelength separating filter, an interference filter in which a dielectric multilayer film or the like is laminated on a glass block is generally well used.

Problems to be Solved by the Invention Among the conventional fixed optical attenuators described above, with the structure (1), it is difficult to control the attenuation rate with high accuracy, and there is a practical problem. In the structure of (2), a lens system for condensing is required, and the configuration becomes complicated. (3) and (4)
In the above structure, strength problems arise when trying to obtain a practical attenuation factor. The structure of (5) utilizes the attenuation of signal light generated by the connection of optical fibers having different mode field diameters, and it can be said that the structure operates reliably with a simple configuration, but a certain attenuation factor It can only be used as a fixed optical attenuator.

On the other hand, the interference filter for wavelength separation requires a complicated film forming technique, is generally expensive, and is unstable with respect to environmental conditions such as temperature and humidity.

The present invention was created in view of these circumstances, and its purpose is to provide a simple fixed optical attenuation that can be used also as a filter for wavelength separation by making the attenuation factor have wavelength dependence. To provide a container.

Means for Solving the Problems The problems of the above-described conventional techniques are that in a fixed optical attenuator in which single mode optical fibers having different refractive index distributions are connected to each other at their end faces, the single mode optical fibers have the same wavelength at a predetermined wavelength. This can be solved by having a mode field diameter and changing the change of the mode field diameter with respect to the wavelength from each other.

Action Generally, the radial optical power distribution in the transmission mode of a single-mode optical fiber is approximately a Gaussian distribution,
The diameter at which the optical power is 1 / e ² of the peak value is called the mode field diameter. When optical fibers with different mode field diameters are connected, the signal light passing through these boundary surfaces becomes
Attenuation is performed with a predetermined amount of attenuation determined according to the difference in mode field diameter. On the other hand, it is known that the mode field diameter changes depending on the refractive index distribution of the optical fiber and the wavelength of the signal light.

In the optical fixed attenuator of the present invention, since the single mode optical fibers having the same mode field diameter at a predetermined wavelength and having different changes in the mode field diameter with respect to the wavelength are connected, Light of a predetermined wavelength is transmitted without being attenuated, but light of other wavelengths is transmitted with attenuation according to the wavelength.

Embodiment Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the drawings.

Referring to FIG. 1, there is shown an optical fixed attenuator constructed by applying the present invention. The optical fixed attenuator is constructed in such a manner that the central axes of single mode optical fibers 1 and 2 are aligned with each other. The end faces of are connected to each other by, for example, fusing. Each single mode optical fiber 1, 2 has the same mode field diameter at a predetermined wavelength,
Also, the changes in mode field diameter with respect to wavelength are different from each other. In order to satisfy such a condition, the refractive index distributions of the single mode optical fibers 1 and 2 are, for example, those shown in FIG. 2 where the refraction of (a) and (b) or (a) and (c) It suffices to combine the rate distributions.

Now, using the core diameter a (μm) and the normalized frequency V, the mode field diameter ω (μm) of the single mode optical fiber is expressed as ωa (logeV) ^{−1 ] 2} . The normalized frequency V is the refractive index of the core part.
It is represented by V = πn ₁ a (2Δ) ^{1 ] 2} λ ⁻¹ by using n ₁ , the relative refractive index difference Δ, and the wavelength λ. Here, the relative refractive index difference Δ is Δ = (n ₁ ² −n ₂ ² ) / 2n ₁ ^{2 when} the refractive index of the cladding portion is n ₂ .

Explaining each refractive index distribution in FIG. 2, first, in (a), the core diameter d ₁ = 8.7 μm, the relative refractive index difference Δ between the core portion and the clad portion is shown.
₁ = 0.3% of uniform distribution type. (B) shows the core diameter d ₂ = 1.3 μm, the relative refractive index difference Δ ₂ between the core and the clad.
= 2.15%, and (c) shows core diameter d ₃ = 1.55μ
m, the relative refractive index difference Δ ₃ = 2.15% between the core part and the clad part is also a uniform distribution type.

FIG. 3 shows how the mode field diameter of the single mode optical fiber having the respective refractive index distributions shown in FIG. 2 changes according to the wavelength, and the wavelength dependence of the mode field diameter depends on the fiber. It is clear that they are different. The mode field diameters of (a) and (b) match at the wavelength λ = 1.3 μm, and the mode field diameters are different in the wavelength region other than the predetermined wavelength. In addition, the mode field diameters of (a) and (c) have a wavelength λ = 1.
They match at 55 μm and have different mode field diameters in the wavelength range other than this predetermined wavelength. Therefore, by connecting two single-mode optical fibers having such a combination of refractive index distributions, a predetermined wavelength (1.
It is possible to construct an optical fixed attenuator having an attenuation rate of 0 for light of 3 μm and 1.55 μm) and a predetermined attenuation rate for light of other wavelength regions.

If the mode field diameters are ω ₁ and ω ₂ , the attenuation rate α caused by the difference in the mode field diameter is α20log
_{Since 10} {(ω ₁ ² + ω ₂ ² ) / 2ω ₁ ω ₂ }, the characteristics of the optical fixed attenuator configured by combining the above-described refractive index distributions are as shown in FIG. In the combination of (a) and (b), the attenuation rate is zero at the wavelength of 1.3 μm, and the attenuation rate increases as the wavelength deviates from 1.3 μm. This optical fixed attenuator can be used as a filter capable of transmitting light in the wavelength band of 1.3 μm and attenuating light in other wavelength bands. If the amount of attenuation is insufficient, a desired attenuation rate can be obtained by connecting a plurality of fixed optical attenuators in series. In the combination of (a) and (c), the attenuation factor is zero at the wavelength of 1.55 μm and the wavelength is 1.55 μm.
The attenuation rate increases with the deviation from μm. In this case, it can be used as a filter capable of transmitting light in the wavelength band of 1.55 μm and attenuating light in other wavelength bands.

As described above, since the attenuation factor has wavelength dependence only by connecting at least two single-mode optical fibers, a fixed optical attenuator that can be used as a wavelength selective filter with an extremely simple configuration. Can be provided. Further, when a single-mode optical fiber is spliced, for example, the variation in splicing loss due to the splicing work is approximately 0.1 dB or less, so it can be said that the stability is practically sufficient. Further, since this optical fixed attenuator has an extremely solid structure in which a single mode optical fiber is fused, it is extremely stable against environmental conditions such as temperature and humidity.

In the above embodiments, the combination of the refractive index distributions is set so that the predetermined wavelength at which the attenuation factor becomes zero becomes 1.3 μm and 1.55 μm, which are becoming the mainstream of the transmission wavelength. The present invention is not limited to the embodiment, and the refractive index distribution may be freely set by changing the structural parameters of the single mode optical fiber and used as a filter for light of other predetermined wavelengths.

Advantageous Effects of Invention As described above in detail, according to the present invention, it is possible to provide a fixed optical attenuator having a simple structure that can be used as a filter for wavelength separation by making the attenuation factor have wavelength dependence. Has the effect that it becomes possible.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an optical fixed attenuator showing a preferred embodiment of the present invention, and FIGS. 2 (a), (b) and (c) are diagrams of a single mode optical fiber constituting the optical fixed attenuator. FIG. 3 is a diagram showing the refractive index distribution, FIG. 3 is a diagram showing the relationship between the mode field diameter ω and the wavelength λ, and FIG. 4 is a diagram showing the relationship between the attenuation factor α and the wavelength λ. 1,2 …… Single-mode optical fiber.

Claims (3)

[Scope of utility model registration request] 1. A fixed optical attenuator comprising single-mode optical fibers (1,2) having different refractive index distributions connected to each other at their end faces, wherein the single-mode optical fibers (1,2) have the same wavelength at a predetermined wavelength. An optical fixed attenuator having a mode field diameter and different changes in the mode field diameter with respect to wavelength. 2. The fixed optical attenuator according to claim 1, wherein the predetermined wavelength is in the 1.55 μm band. 3. The fixed optical attenuator according to claim 1, wherein the predetermined wavelength is in the 1.3 μm band.