An optical waveguide for optical communication transmitting light wave energy in a single mode composed of a core of a first transparent dielectric material and successive layers of a second and a third transparent dieletric material coaxially covering the core, the refractive index of the first dielectric material being higher than that of the third dielectric material and the refractive index of the second dielectric material being lower than that of the third dielectric material.
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1. An optical waveguide transmitting light wave energy in a single mode comprising:
a solid body of a first transparent dielectric material having a uniform refractive index n and a circular cross-section having a radius dimension a; a layer of a second transparent dielectric material formed coaxially on the solid body and having a refractive index qn and a thickness δa; a layer of a third transparent dielectric material surrounding said layer of the second transparent dielectric material and having a refractive index pn; and said three transparent dielectric materials having a following relationship: ##EQU17## where μ is an optical wavelength , p is smaller than 1 and q is smaller than p.
2. An optical waveguide according to
3. An optical waveguide according to
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The present application is a continuation-in-part application of an earlier application Ser. No. 476310 filed on June 4, 1974 now abandoned.
1. Field of the Invention
The present invention relates to an optical waveguide, and more particularly to an optical waveguide transmitting light wave energy in a single mode for optical communication made of transparent dielectric materials such as glass.
2. Description of the Prior Art
The important things for an optical waveguide for optical communication are that the reduction of light energy in transmission, i.e. the transmission loss of light energy is low, it has a wide band signal frequency response, and the launching of light in the waveguide, i.e. the connection between a light source and a waveguide or the connection between waveguides is easy.
To date various ones have been proposed as the optical waveguide among which a singly cladded optical fiber is known as an effective one as for optical communication. This optical fiber is composed of a transparent central dielectric body, i.e. a core having a high refractive index and a covering layer, i.e. a cladding consisting of a material having a lower refractive index.
The optical characteristic of this singly cladded optical fiber is determined mainly by the parameter expressed by the formula ##EQU1## where a is the radius of the core, λ is the wavelength of light, n1 and n2 are the refractive indices of the core and the cladding, respectively, and the parameter ν will hereinafter be referred to mainly as the normalized frequency though ordinarily it is called in various ways.
In the range of the value of v no larger than 2.40832.4083 2.40483 and j1.1 =3.8327 3.83171. As described above, when 1-p<<1-q, i.e., ##EQU13## j1.1 /B<<1. Thus, equation (7) result as follows: ##EQU14## Consequently, in the optical waveguide according to the present invention the cut off occurs for the lowest degree of mode HE11 and the single mode region becomes ν1 ' to ν2 ' to magnify the upper limit of the lowest degree of mode.
Next, the advantages resulting from the characteristics of the optical waveguide according to the present invention becoming as shown by the solid lines in FIG. 2 will be described.
First, the diameter of the core of the waveguide capable of performing single mode transmission can be extended. For example, since the cut off normalized frequency ##EQU15## changes from 2.408 2.40483 around 3.8317 3.83171, the radius a can be extended by about 60% when ##EQU16## is constant.
Second, when the single mode transmission is performed, the single frequency band width can be extended compared with the single mode transmission by a conventional optical waveguide.
The factor to determine the single frequency band width of the waveguide is the group velocity characteristic of light in the waveguide. The group velocity is expressed by dω/dβ as is well known. The group velocity of signal light generally varies depending on the frequency of the light. When the spectral width of the signal light is wide, a delay distortion occurs due to the difference in the group velocity therein to narrow the transmission capacity of the waveguide and hence the signal frequency band width.
The differential d(dω/dβ)/dω of the group velocity with respect to the frequency will hereinafter be referred to as the dispersion. For this dispersion there are the dispersion due to the refractive index characteristic of the material of the waveguide (hereinafter referred to as the material dispersion) and the dispersion due to the characteristic inherent in the propagation mode (hereinafter referred to as the mode dispersion). The combination of both dispersions will hereinafter be referred to as the overall dispersion.
The dielectric, the material of the ordinary waveguide, decreases in the refractive index with the increase in the waveguide of light at the wavelength range of from 0.6 to 1.06 micron. Consequently, the material dispersion depending on the refractive characteristic of the material is negative.
On the other hand, the mode dispersion is also negative due to the fact that the curve 3 shown in FIG. 2 is downwardly convex at the range 0 to ω1 corresponding to the range 0 to ν1, i.e. at the single mode region. Consequently, the negativity is further amplified for the overall dispersion. For example, the overall dispersion at the near infrared region of a wavelength of 1 micron for borosilicate glass is about -10-8 meter. If a light pulse having a spectral width of 100 angstroms is transmitted through such an optical waveguide, there occurs a pulse widening of about 5 ns per 1 km. That is, even for an optical waveguide of the single mode the signal frequency band width is considerably limited for the transmission of a light signal having a spectral width.
However, in the optical waveguide according to the present invention the characteristic of the single mode region has the cut off as shown at 3 in FIG. 2 so that the mode dispersion is positive. Consequently, the positivity of the mode dispersion and the negativity of the material dispersion compensate for each other to diminish the absolute value of the overall dispersion. For this reason the limitation due to the group velocity dispersion is moderated and the signal frequency band width is extended.
The optical waveguide according to the present invention has a further advantage compared with a conventional waveguide that the energy of light is more concentrated in the core.
As described above, since, in the optical waveguide according to the present invention, the upper limit of the single mode region is magnified, the eigen value u2 is augmented, so that the energy distribution is as shown in FIG. 5. In FIG. 5 the abscissa is the radial direction of the waveguide and the ordinate is the intensity of light which is normalized to 1 at the center of the waveguide. Such a concentration of the energy of light in the core provides the advantage that the loss due to the bending of the waveguide is reduced and the acceptable angle of the waveguide is increased. Considering the contribution of the transmission loss of the material of each layer composing the waveguide to the loss of the waveguide, the latter is almost determined by the loss of the core material. Consequently, as the layers other than the core, materials having a larger loss than conventional ones can be employed, so that the manufacture of the waveguide is easier than a conventional one.
Onoda, Seiichi, Kawakami, Shojiro, Nishida, Shigeo, Ohtaka, Yoichi
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