JPS6017084B2 - Omega type single polarization optical fiber - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a single polarization optical fiber used in a coherent transmission system, and in particular to material dispersion and waveguide over a wide wavelength band including the 1.5&m band, which is a low loss band of silica-based optical fiber. This invention relates to an omega-type single polarization optical fiber that minimizes signal distortion due to dispersion.

In the coherent transmission method, the plane of polarization of light emitted from the fiber must be kept stable.

In order to achieve such a purpose, it is necessary to solve the degeneracy of the modes in the optical fiber and to use HE, . X mode and HE,
．． By increasing the propagation constant difference of the y mode, HE, . X mode or HE, . Single polarization fibers that propagate only the y-mode are known to be effective. On the other hand, the range of single polarization fiber is HE, . 1 mode and HE,. In the absence of mode coupling between y-modes, it is limited by material dispersion and waveguide dispersion, similar to normal single-mode fibers.

A cross-sectional view of an example of a conventional stress-applying single-polarized fiber is shown in FIG. 1. In the figure, 1 is a core, 2 is a cladding, and 3 is a stress-applying portion.

When light polarized in the x direction is incident on a single polarization fiber having such a configuration, the propagation constant 8x of the HE, X mode is 8X=BX.

Ten K [c,. X+c2(〇y+.Z)]・--[Given by 11.

However, 6xo is the propagation constant of a normal single mode fiber without a stress applying part. X, oy,. z is stress, C,,C
2 is a photoelastic constant, and k (=2 strokes/input: input is wavelength) is the wave number of light. Also, dispersion of single polarization optical fiber (waveguide dispersion + material dispersion). T is for leaf = U. Group k (PS/Km
/skin).・It is given by ■.

However, C is the speed of light, which is 3×10-7 (Km/PS). Then, by substituting the above equation '11 into equation (2) and using the property that in the case of a stress-applied single polarization fiber, the contribution of stress to the propagation constant is wavelength-independent, that is, it has no wavenumber dependence, the dispersion is .

T is.

T=ma. K etc.・・・．・You can see that it will be on the side.

In other words, it was shown that the dispersion of the stressed single-polarization fiber is the same as that of a normal single-mode fiber. Here, as shown in FIG. 2, which is an explanatory diagram showing the refractive index distribution in FIG. 1, the refractive index of the core is dispersion of a conventional single-polarization fiber with a step-like refractive index. and the wavelength of light ^
The relationship is shown in Figure 3. In this Figure 3. M is the material dispersion and the state A. Obi (tower/Km/
nm). ...Represented by ■.

However, d in formulas ① to '4} is a differential symbol. on the other hand,.
w is waveguide dispersion, refractive index distribution, relative refractive index difference △(=
It is an amount that changes depending on n,2-~2)/(yo,2), etc. Also distributed. T is material dispersion, M and waveguide dispersion w
The transmission bandwidth f of the signal is given by the sum of
・・・・・・・Given by '5}. FIG. 3 is an explanatory diagram showing the relationship between the dispersion and the wavelength of light in this conventional single-polarized optical fiber.
．． 32%, core radius a = 3.0 French m, core refractive index n,
= 1.46319, but as is clear from the figure, in the conventional single polarization fiber, a specific wavelength (
In the same figure, it is dispersed at input o = 1.424〆m).

Even if T becomes zero, OT increases rapidly at other wavelengths, and the signal band width based on the above equation (5) decreases. On the other hand, in a silica glass optical fiber, the optical transmission loss is 1.5 to 1. It has been theoretically and experimentally shown that the minimum value is reached at &m (Miya et al., Ele.
ctron, Lett. Vol. 15, Ship. 10 19
79), dispersion at this wavelength castle.

By minimizing T, ultra-wideband and ultra-long distance coherent optical transmission will be realized. However, this kind of research on traditional single-polarized optical fibers has limited dispersion.

No consideration is given to the wavelength characteristics of T, and when the wavelength to be transmitted deviates from the input o, the dispersion OT increases rapidly, which is the low-loss wavelength castle of silica fibers = 1.5~
There was a drawback that a wide wavelength range including 1.6 rm could not be advantageously utilized. Furthermore, in conventional single-polarized optical fibers, the wavelength width at which the dispersion becomes less than ±IPS/Km nanometers is at most 0.0 m, and when used for wavelength division multiplexing transmission systems, etc. The drawback is that broadband signal transmission over a wavelength range is not possible. In view of the above points, the present invention has been made to solve these problems and eliminate these drawbacks.The present invention aims to provide an omega-type monomer that has an extremely wide wavelength range, minimal dispersion, and excellent polarization characteristics. The object of the present invention is to provide a monopolarized optical fiber.

In order to achieve such an object, the present invention provides an intermediate layer having a lower refractive index than the core and the cladding between the core and the cladding, and a portion of the cladding that is heated by the cladding at opposing positions on both sides of the core. A stress applying portion having a thermal expansion coefficient different from the expansion coefficient is provided.Examples of the present invention will be described in detail below with reference to the drawings.

FIG. 4a is a sectional view showing an embodiment of the omega type single polarization optical fiber according to the present invention, and FIG. 4b is a refractive index distribution diagram of each part.

In the figure, 4 is a core whose main component is quartz glass, 5 is a similar cladding that surrounds this core 4, and 6 is an intermediate layer whose main component is quartz glass. is set between.

At opposing positions on both sides of a portion of the core 4 of the cladding 6, stress applying portions 7 having a thermal expansion coefficient different from the thermal tension coefficient of the cladding 5 are arranged. Now, the refractive index of the core 4 shown in a is expressed as n, . When the refractive index of the cladding 5 is n2, the refractive index of the intermediate layer 6 is ~, and the refractive index of the stress applying part 7 is ~, b is the refractive index distribution n, and radius r(X As shown in (axial direction), the refractive index & of the cladding 5 is much larger than the refractive index n of the core 4 by 4·m, and the refractive index ratio of the intermediate layer 6 is even larger than the refractive index n of the cladding 5 by 4·m, The refractive index of the layer 6 is smaller than the refractive index n, - of the core 4 and the cladding 5.

Note that the refractive index ~ of the stress applying portion 7 is set to be the same as or smaller than the refractive index − of the cladding 5 in order to prevent light from propagating to the same portion. In this example, the radius of the core 4 including the intermediate layer 6 (hereinafter referred to as
When the radius of the core 4 (abbreviated as core radius) is a, the core radius a is 3. The thickness of the intermediate layer 6 is 1.
0mm, the position of the stress applying part 7, and r2 are each 5a.
, 10a, and the outer diameter of the cladding 5 is 12 m. Since the outer diameter of the cladding 5 is extremely large compared to the core 4 and the intermediate layer 6, the outer diameter of the cladding 5 is not shown. Also,
The refractive index difference △ between the core 4 and the cladding 5 and the refractive index difference △2 between the intermediate layer 6 and the cladding 5 are expressed by the following formulas ■ and {7), respectively.
is expressed by △． =n・Kiu2×・oo(%)
‐‐‐‐‐‐'6}Muniヱdeath 2Xmo(%)...
In the embodiment shown in FIG. 4, in order to make the refractive index n of the core 4 larger than the refractive index n2 of the cladding 5, germanium is added to the quartz glass as the main component in the core 4. In order to make the refractive index n3 of the intermediate layer 6 smaller than the refractive index n2 of the cladding 5, fluorine is added to the quartz glass that is the main component in the intermediate layer 6. ), the ratio of Ge02 to Sj02 (silica glass) is set to 20%, the amount of fluorine added in the intermediate layer 6 is set to 4.2ho%, and the refractive index difference in equation (6) above is used. △, is 1.0% (The refractive index difference △2 of formula 7' is -1.0%.

In addition, in order to give the stress applying part 7 a coefficient of thermal expansion different from that of the cladding 5 and a refractive index that is the same as that of the cladding 5, boron and germanium are added to the quartz glass, which is the main component of the stress applying part 7. and the ratio of &03 (diboron trioxide) and Snake02 to Si02 was 18ho, respectively.
The coefficient of thermal expansion of the stress applying portion 7 is 3.8% of the coefficient of thermal expansion of the cladding 5.
%, and the refractive index ~ of the stress applying portion 7 has the same value as the refractive index peak of the cladding 5. And this stress applying part 7
Due to the main stress difference in the x and y directions caused by HE, . X
Mode HE, . A large difference occurs between the y-mode propagation constants, and the difference is shown by the following equation.

8x-By=k・(C,-C2)・(〇X-oy).・
・．． ...'81 Here, in the above example, the principal stress difference was (oX - oy) = 6.5 ([9/Iso)...■.

The bending stress caused by normal handling of optical fiber is 1ox-oil=0 when the bending radius is 1 arc.
．． 03 (k9/Iso) …,. ．． The value is approximately 00.

Therefore, even if an external force such as bending is applied to the single polarization optical fiber shown in the embodiment of FIG. 4, HE, . X mode and HE,. There is almost no mode coupling between the y modes, and HE, . X mode or HE, . It can be seen that only the y mode is stably transmitted. Next, the relationship between waveguide dispersion and refractive index distribution of a single polarization optical fiber will be explained.

As shown in FIG. 5, which is an explanatory diagram showing various refractive index distributions, various refractive index distributions of a single polarization optical fiber can be considered, including a step-shaped distribution, b a square distribution,
Since c is assumed to have an M-shaped distribution and d to have a 0 (omega)-shaped distribution, the frequency (wavelength) of the light to be transmitted for these
and waveguide dispersion. When we investigated the relationship with W, we obtained the results shown in Figure 6. However, this figure 6 shows waveguide dispersion.

The normalized waveguide dispersion Cow, which is the product of W and the optical distance C, is plotted on the vertical axis, and the normalized frequency v obtained by the following equation is plotted on the axis. v=equal n, aノ2△・・….・
(11) Also, a to d in FIG. 6 correspond to a to d in FIG. 5, and d in FIG. Refractive index difference between 6 and cladding 5△
2 are mutually equal and have an opposite polarity intermediate layer 6, and the refractive index difference △ and the refractive index difference △
The value of 2 is as follows.

a~C...△,=1.0% d...△,=0.5%, △2 =-0.5%, t=
0.3 However, as mentioned above, t is the thickness of the intermediate layer 6, and a
is the radius of the core 4, and since the intermediate layer 6 is not provided in a to c, only Δ is shown.

Here, when focusing on the normalized waveguide approximate dispersion Cow, the sixth
d in the figure is the largest and changes in the negative direction, and the dispersion (=OM+.w) is the material dispersion. In order to minimize it by canceling M and waveguide dispersion ow, the one shown in Fig. 6d,
That is, it is clear that the O-shaped distribution shown in FIG. 4 is the most advantageous. FIG. 7 is an explanatory diagram showing the change in each dispersion ○ with respect to the wavelength input of the light to be transmitted for a single mode optical fiber with a ○-shaped distribution, where the input is 1.35 to 1.67 k.
Total dispersion OT is ±1Km/nm over a wide wavelength range of m
It is clear that it is within the range.

Figure 8 shows the refractive index difference △, = 1.0 between the core 4 and the cladding 5.
% Theoretical transmission loss in optical fiber (Miya, Ike: E
electron.戊TT, Vol. 15, P. 106,
1979) Photon energy of L (clothes/Km) - P
An explanatory diagram showing changes with respect to e and wavelength of light, where input =
Transmission loss L is 0.5 in the range of 1.32 to 1.69rm
It is kept below the old/Km.

Therefore, it is clear that the configuration shown in the embodiment of FIG. 4 is extremely effective as a single polarization optical fiber for an ultra-wideband and ultra-long distance coherent optical transmission system.

Based on this fact, Figures 9 to 11 correspond to changes in the refractive index difference △,, △2 and the ratio t/a between the thickness t of the intermediate layer and the core radius a, based on simulations using an electronic computer. total variance.

Find the wavelength range where T is ±IPS/Km/nm or less,
It is an explanatory diagram showing the wavelength input and the corresponding core diameter on the vertical axis, and total dispersion. The range in which T is ±IPS/Km/nm or less is: Ta and OTd represent the range where the transmission loss is 0.9 old/Km or less, and Lu and W represent the total dispersion. T is ±IPS/Km/nm
or less and the transmission loss is 0. It is clear that in order to satisfy the condition below the base B/Km, it is sufficient to fall within the shaded range in the figure, and this indicates the range of wavelengths that can be used. However, in Fig. 9, △, = 1.0%, t/a = 0.
3, and △2 is set as a variable on the horizontal axis. In Figure 10, △2 = -1.0%, t/a = 0.3, and △2 is set as a variable.
With △, as a variable, Figure 11 shows △, = 1.0%, △2 = -
It is fixed at 1.0%, and t/a is used as a variable. From these figures, △, △2, t/a, etc. under each condition can be determined, and a curve 2a indicating the diameter 2a of the core 4 can be obtained.
The specific diameter value can be found from

Here, in the wavelength division multiplexing transmission system, at least 0.
In a wavelength range of 1rm width, the dispersion is IR/Km/nm.
Must be less than or equal to

The refractive index difference between the core and intermediate layer that satisfies this condition △,,
△2 or the ratio t/a of the width t of the intermediate layer and the core radius a is 0.5 mm △, 32.5 (%), from Figures 9 to 11.
It can be seen that Δ2mi should be 0.2 (%) and 02mit/ami should be 0.6. Then, △2 should be -0.2% or less, but if F (fluorine) is added to reduce the refractive index of the intermediate layer, the refractive index difference should be △2 = -1.0.
It is difficult to make it less than %, and -1.0mi△2mi-0
．． 2 (%) is appropriate.

Therefore, in order to mainly transmit light in the vicinity of wavelength input = 1.5 to 1.6 mm, △,, △2
, t/a and determine 2, t accordingly,
As a result, a single polarized optical fiber with minimum dispersion and a stable plane of polarization can be obtained over a wide range of wavelengths.

Note that the allowable total dispersion OT is greater than ±IPS/Km/nm, and the allowable transmission loss L is also 0. Of course, if it is larger than the base B/Km, optical transmission over a wider wavelength range will be performed.

As is clear from the above description, according to the present invention, the total dispersion is ±1 false/Km/
nm or less, and by applying asymmetric stress to the core, HE, . X mode and HE,. By increasing the propagation constant difference between the y modes and suppressing mode coupling between both modes, HE, . X mode or HE, . Since only the y-mode can be transmitted, this has a very large advantage in the wavelength division multiplexing coherent transmission system, and therefore has a very large practical effect.

In addition, since it has a refractive index of 4.0 and an intermediate layer than the cladding, there is less light leakage to the cladding side, and
It has advantages such as reducing bending loss and microbending loss due to bending of the optical fiber, and is extremely advantageous in that it exhibits remarkable effects in various optical transmission applications.

[Brief explanation of drawings]

Figure 1 is a cross-sectional view showing an example of a conventional single polarization optical fiber, Figure 2 is an explanatory diagram showing the refraction distribution of the core in Figure 1,
The figure is an explanatory diagram showing the relationship between dispersion and wavelength of light in a conventional single-polarization optical fiber, and FIG. 4 is a cross-sectional view and refractive index distribution diagram showing an example of an omega-type single-polarization optical fiber according to the present invention. , Fig. 5 is an explanatory diagram showing various refractive index distributions, Fig. 6 is an explanatory diagram showing the relationship between waveguide dispersion and normalized frequency of the one shown in Fig. 5, and Fig. 7 is an explanatory diagram showing the implementation shown in Fig. 4. An explanatory diagram showing the relationship between each dispersion and the wavelength of light in an example. Figure 8 is an explanatory diagram showing the relationship between calculated transmission loss, wavelength of light, and photon energy. Figures 9, 10, and 11 The figure shows the refractive index difference between the core and the intermediate layer obtained by simulation, the ratio t/a of the intermediate layer width to the core radius, the dispersion of ±IPS/Km/nm or less, and the loss of 0. Old mother/
FIG. 3 is an explanatory diagram showing a relationship with a wavelength range that is equal to or less than Km. 4...core, 5...crud, 6...middle layer,
7... Stress applying part, △,, △2... Refractive index difference, a... Core radius including intermediate layer, t...
...The width of the middle layer. Figure 1 Figure 2 Figure 3 Figure 5 Figure 4 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11

Claims (1)

[Claims] 1. An intermediate layer having a refractive index lower than that of the core and the cladding is provided between the core and the cladding, and an intermediate layer having a coefficient of thermal expansion different from that of the cladding is provided at opposing positions on both sides of the core of a part of the cladding. A stress applying portion having a coefficient of thermal expansion is arranged, and the refractive index difference between the core and the cladding is set to 0.5 to 2.5%,
The refractive index difference between the intermediate layer and the cladding is -0.2 to 1.0.
%, and at least 0.1 around the wavelength of 1.55 μm
Total dispersion is ±1PS/Km/nm in the μm wide wavelength range
An omega type single polarization optical fiber, characterized in that the core and the intermediate layer have diameters and widths that correspond to the refractive index difference as follows. 2 When the core radius including the intermediate layer is a and the width of the intermediate layer is t, the ratio of the width of the intermediate layer to the core radius t/a
2. The omega type single polarization optical fiber according to claim 1, wherein the omega type single polarization optical fiber is 0.2 to 0.6.