JPH0118401B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an ultra-wideband optical fiber transmission line.

In recent years, optical fibers have been used as optical transmission lines.
Optical communications using optical fibers are being put into practical use one by one, but when pulsed light is used as a communication medium, the temporal diffusion, or dispersion, of the pulsed light obtained from the receiving end of the optical transmission line causes problems. As a result, waveform distortion occurs, resulting in a phenomenon in which the transmission frequency band is restricted, and this has hindered the realization of broadband optical transmission lines using optical fibers.

That is, multimode dispersion occurs depending on the propagation mode when a light ray propagates in an optical fiber, material dispersion occurs when the refractive index of the material that makes up the optical fiber changes depending on the wavelength of the propagating light ray, Structural dispersion and the like occur because the group velocity of propagation in a particular propagation mode is not uniform with respect to the wavelength of the light beam, and these restrict the frequency band that can be transmitted by the pulsed light beam.

Note that material dispersion changes depending on the wavelength of the light beam,
Structural dispersion is collectively called chromatic dispersion, and in a single mode optical fiber that propagates only a single mode of light, there is no multimode dispersion and only chromatic dispersion exists, so the transmission band is relatively wide. The transmission band is limited by dispersion.

FIG. 1 shows an example of chromatic dispersion characteristics with respect to the wavelength of a transmitted light beam in a single mode optical fiber,
Fused silica is used as the core material, the relative refractive index difference between the core and the surrounding cladding layer is 0.25%, and the core diameter is 7.7μ.
This is an example of a uniform core optical fiber with a uniform relative refractive index of m and a uniform relative refractive index within the core.Since the wavelength dispersion value changes depending on the spectral wavelength distribution of the incident light beam, P _s ( The unit is picosecond)/K _n・Å.

Note that the wavelength at which chromatic dispersion becomes zero is approximately
1.326 μm, but chromatic dispersion does not occur for a light beam with this zero dispersion wavelength λ ₀ and the transmission frequency band becomes infinite, so when the wavelength of the light beam to be transmitted is λ _s , generally λ _s The material components and structure of the core of the optical fiber are determined so that = λ ₀ .

However, there are limits to strictly adjusting the material components and structure of the core, and _it is extremely difficult to maintain the above relationship at all times. Therefore, the disadvantage is that the zero dispersion wavelength λ ₀ cannot be made to match any given wavelength.

The purpose of the present invention is to fundamentally solve such drawbacks of the conventional methods, and the present invention has been developed to produce multiple types of single modes with different wavelength dispersion characteristics based on structural dispersion by varying structural parameters such as core diameter and relative refractive index Prepare optical fibers, connect them in cascade to form an optical transmission line, and establish a specific relationship between the total length of an arbitrarily selected specific type of single-mode optical fiber and other fibers to create an all-optical transmission line. The purpose of the present invention is to provide an ultra-wideband optical fiber transmission line that can easily match the zero-dispersion wavelength of the light beam to the wavelength of the transmitted light beam and realize an extremely wide transmission frequency band.

The details of the present invention will be explained below with reference to FIG. 2 and subsequent figures showing embodiments.

FIG. 2 is a configuration diagram of an optical transmission line, and shows two types of single mode optical fibers 1a to 1c with different wavelength dispersion characteristics based on structural dispersion by different mechanical configurations or structures based on refractive index. and 2
a and 2b are connected in cascade through a connecting portion 3 such as an optical connector to form an integrated optical transmission line. However, the total length of each single mode optical fiber (hereinafter referred to as optical fiber) 1a to 1c, 2a, and 2b is different, and the zero dispersion wavelength of the entire optical transmission line is λ ₀ with respect to the wavelength λ _s of the transmitted light beam. are assumed to match.

That is, the wavelength dispersion value of the optical fibers 1a to 1c
S ₁ and wavelength dispersion value S ₂ of optical fibers 2a, 2b
However, if we seek the wavelength dependence that changes depending on the wavelength λ _s of the transmitted light, we can set λ = λ ₁ and λ ₂ , and
When f ₁ (λ) and f ₂ (λ), it is expressed by S ₁ = f ₁ (λ) ...(1) S ₂ = f ₂ (λ) ... (2). However, f ₁ and f ₂ have a relationship that depends on the wavelength λ of the light beam, and λ ₁ is the relationship between the optical fibers 1a and 1a.
1c is the zero dispersion wavelength, and λ ₂ is the zero dispersion wavelength of the optical fibers 2a and 2b. This functional form is well known (Reference: DG loge “Dispersion in Weakly
Guiding Fibers”Applied Optics, vol.10, No.
11, pp2442-2445, November 1971) It also depends on the core diameter of the optical fiber and the relative refractive index difference. Therefore, by changing the core diameter and relative refractive index difference, the wavelength dispersion characteristics based on structural dispersion can be changed. For example, if you change a normal single mode optical fiber with a core diameter of 10 μm and a relative refractive index difference of 0.2% to a core diameter of 4 μm and a relative refractive index difference of 0.7%, the zero dispersion wavelength will be
It varies from 1.3μm to 1.6μm.

Now, the sum of the lengths of optical fibers 1a to 1c, _l1a to _l1c , is the total length _l1 , and the length of optical fibers 2a and 2b is
If the sum of l _2a and l _2b is the total length l ₂ , then the optical fiber 1a~
The amount of chromatic dispersion at 1c is the chromatic dispersion value S ₁ and the total length l ₁
Since the amount of chromatic dispersion in the optical fibers 2a and 2b is determined by the product of the chromatic dispersion value S ₂ and the total length l ₂ , the amount S of chromatic dispersion of the entire optical transmission line is expressed by the following equation.

S=S ₁・l ₁ +S ₂・l ₂ ...(3) Therefore, substituting equations (1) and (2) into equation (3), and
If λ is the zero-dispersion wavelength λ ₀ , the total chromatic dispersion amount S at this time is zero, and the relationship is expressed by the following equation.

l ₁ · f ₁ (λ ₀ ) + l ₂ · f ₂ (λ ₀ ) = 0 ... (4) At this time, if the wavelength λ _s of the transmitted light ray is equal to the zero dispersion wavelength λ ₀ in equation (4), That is, if λ _s = λ ₀ ...(5), the transmission frequency band in this optical transmission line approaches infinity, and this is divided into optical fibers 1a to 1c.
From equations _{(4) and (5), l 1 /l 2 = -f 2 (} λ _s ) / f ₁ (λ _s ) ...(6) is obtained.

That is, if optical fibers 1a to 1c, 2a, and 2b are cascaded with total lengths _l1 and _l2 that satisfy the relationship of equation (6), an ultra-wideband optical fiber transmission line can be realized.

In addition, as shown in Figure 2, the same type of optical fiber 1
a to 1c and a different type of optical fiber 2
If a and 2b are divided and their respective total lengths are related to equation (6), the overall zero dispersion wavelength λ ₀ can be easily adjusted by cut-and-try, which is extremely convenient. The same is true if the fibers 1a to 1c are integrated, and the optical fibers 2a and 2b are integrated, and their lengths l ₁ and l ₂ are set to the relationship expressed by equation (6).

In addition, the above describes two types of optical fibers 1a to 1c and 2.
Although a and 2b were used, in general, the same result can be obtained using a similar theory even if a plurality of types, that is, n types of optical fibers are used.

In other words, if the zero dispersion wavelength of a specific type of optical fiber i among n types is λ _i , and the wavelength dispersion value similar to equations (1) and (2) is f _i (λ), then The zero dispersion wavelength λ ₀ of the entire optical transmission line composed of n types of optical fibers is expressed by the following equation as an extended form of equation (4).

_o 〓 ⁱ⁼¹ l _i・f _i (λ ₀ )=0 ...(7) Also, the optical fiber required to make the entire zero dispersion wavelength λ ₀ equal to the wavelength λ _s of the transmitted light beam is The ratio between the total length l _i of i and other items can be obtained by substituting equation (5) into equation (7) and using _o 〓 ⁱ⁼¹ l _i・f _i (λ _s )=0... (8) shown.

Therefore, when using n types of optical fibers, it is sufficient to select an arbitrary type of optical fiber i as the specific type of optical fiber i and determine the total length l _i that satisfies the relationship of equation (8).

FIG. 3 is a configuration diagram of an embodiment corresponding to the second invention, in which two types of optical fibers 1 and 2 with slightly different wavelength dispersion characteristics based on structural dispersion are used, and for the sake of simplicity, each This shows the case where they are integrated.

The wavelength dispersion values of optical fibers 1 and 2 are generally
Expressed by equations (1) and (2), the chromatic dispersion value changes almost linearly within the range of change for a slight change in wavelength λ, and is a linear function f(λ) = aλ + b
(a, b are constants), so equations (1) and (2) are f ₁ (λ)=a ₁ (λ−λ ₁ ) ……(9) f ₂ (λ)= a ₂ (λ−λ ₂ ) ...] However, a ₁ and a ₂ are the first-order coefficients of the chromatic dispersion values in the optical fibers 1 and 2 when λ=λ ₁ and λ ₂ .

Now, by substituting equations (9) and (10) into equation (4), the zero dispersion wavelength λ ₀ of the entire optical transmission line is expressed by the following equation.

λ ₀ =a ₁・l ₁・λ ₁ +a ₂・l ₂・λ ₂ /a ₁・l ₁ +a ₂・l ₂ …
...(11) Also, optical fibers 1 and 2 constituting an optical transmission line in which the wavelength λ _s of the transmitted light and the zero dispersion wavelength λ ₀ match
From equation ( ₆ ) _{, the ratio of each} total _length l ₁ and l _{2 of} It will be done.

In addition, when using n types of optical fibers within the range where the same approximation holds, arbitrarily select a specified type of optical fiber i from among them, set its zero dispersion wavelength as λ _i , and If the first-order coefficient of the chromatic dispersion value at the zero-dispersion wavelength is a _i and the total length of the optical fiber i is l _i , then the following relationship is obtained by expanding equations (11) and (12).

That is, if the total length l _i of the optical fiber i is determined according to the relationship of equation (14), an ultra-wideband optical fiber transmission line can be obtained.

However, in the configuration shown in Fig. 3, if the optical fiber 2, which has a shorter total length _l2 , is placed on the side connected to a repeater inserted in the middle of the optical transmission line, the delay equalizer can be reduced. Although it is convenient because the optical fibers 1 and 2 can also be used, the optical fibers 1 and 2 may be divided and connected in cascade for use, as in FIG.

As described above, multiple types of optical fibers with different wavelength dispersion characteristics based on structural dispersion can be expressed using equations (8) and (14).
An ultra-wideband optical transmission line can be easily realized by cascade connection according to the relationship shown in the equation, and a specific numerical example thereof is as follows.

First, an example will be described in which fibers having the same relative refractive index difference but different core diameters are connected. For two uniform core optical fibers A and B, the relative refractive index difference is
The same value as 0.25%, but the core diameter of A is 6.9μm,
By selecting the core diameter of B as 8.5 μm and changing the structural parameters, the zero dispersion wavelength λ ₁ of A is 1.355 μm and the zero dispersion wavelength of B is 1.310 μm, as shown in Figure 4.
Make it different from m. As a result, the first-order coefficient of the chromatic dispersion value at λ=λ ₁ and λ ₂ is 0.00080Ps/K _n・
Å ² and B become 0.00089Ps/K _n Å ² , and let these be a ₁ and a ₂ respectively, and the wavelength λ _s of the transmitted light beam is
If it is 1.33μm, then from equation (12), l ₁ /l ₂ = −a ₂ (λ _s − λ ₂ )/a ₁ (λ _s − λ ₁ ) = −8.9
×10 ^-4 /8.0×10 ^-4 × (1.33−1.310) ×10 ^{-6 /} (1.33−1.355) ×10
^-6 = 0.89 Therefore, when the total length l ₁ of optical fiber A is 890 m, the total length l ₂ of optical fiber B is 1000 m, and the two can be connected as shown in FIG.

In addition, as an example of using two types of optical fibers with different wavelength dispersion characteristics based on structural dispersion, see Figure 4A.
For example, an optical fiber having wavelength dispersion characteristics shown in FIG. 5 may be used.

Figure 5 shows the recently proposed cadmium C _d
This is the wavelength dispersion characteristic of an optical fiber made of tellurium and T _e as materials, and since the zero dispersion wavelength is 5 μm or more, it can be calculated by applying equation (6), which is the premise of equation (8). can.

Now, if the wavelength λ _s of the transmitted light beam is 1.5 μm, the chromatic dispersion value f ₁ at the wavelength 1.5 μm in Figure 4A is
(λ _s ) is approximately −0.98P _s /K _n ·Å, and the chromatic dispersion value f ₂ (λ _s ) in FIG. 5 is approximately +107P _s /K _n ·Å, so from equation (6), l ₁ / l ₂ = −f ₂ (λ _s ) / f ₁ (λ _s ) = −+107/−0.98
≒109 Therefore, the total length of optical fiber A shown in Fig. 4 is
If l ₁ is 50K _n , then l ₂ = l ₁ /109 = 50 x 10 ³ /109≒0.460 x 10 ³ , that is, the total length l ₂ of the optical fiber shown in Fig. 5 is
460m and connect as shown in Figure 3.

However, the combination of FIGS. 4A and 4B and the combination of FIGS. 4B and 5 may be constructed by dividing and connecting each of them as shown in FIG. 2.

As is clear from the above description, according to the present invention, if a plurality of types of single mode optical fibers each manufactured individually and having different structures are connected in cascade with the total length of each determined depending on the wavelength of the light beam to be transmitted, Since the wavelength of the transmitted light and the zero dispersion wavelength of the all-optical transmission line match, and an ultra-wideband transmission line is obtained, the material composition,
Compared to adjusting the zero dispersion wavelength by structure etc.
You can achieve your goals with great ease and precision.

Therefore, it can be applied to optical transmission lines such as optical communication, which will continue to develop in the future, and will exhibit remarkable effects such as improved transmission characteristics and reduced optical transmission line manufacturing costs.

[Brief explanation of drawings]

Fig. 1 shows an example of wavelength dispersion characteristics of a single mode optical fiber, Fig. 2 and subsequent figures show embodiments of the present invention, Fig. 2 is a configuration diagram of an optical transmission line corresponding to the first invention, and Fig. Fig. 4 shows the wavelength dispersion characteristics of the single mode optical fiber used in the second invention, and Fig. 5 shows the wavelength dispersion characteristics of the single mode optical fiber used in the first invention. be. 1, 1a to 1c, 2, 2a, 2b...Optical fiber (single mode optical fiber), 3... Connection portion.

Claims (1)

[Claims] 1. An optical transmission line is constructed by cascade-connecting a plurality of n types of single-mode optical fibers with constant material dispersion and different core diameters and relative refractive index differences between the core and the cladding. In addition, the chromatic dispersion value f _i (λ _s ) for the wavelength λ _s of the light beam transmitted in a specific type of single mode optical fiber i among the plurality of n types and the total length l _i of the single mode optical fiber i are expressed as _o An ultra-wideband optical fiber transmission line characterized by having the following relationship ^{: i=1} l _i ·f _i (λ _s )=0.