JPS583206B2 - Single mode optical fiber with intermediate layer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a single mode optical fiber that minimizes signal distortion due to dispersion over a wide wavelength band.

Single mode optical fiber has less dispersion than multimode optical fiber and has a wider wavelength band, but due to signal distortion caused by material dispersion and waveguide dispersion, the usable wavelength band is naturally limited. In other words, light projected into the core of an optical fiber propagates within the core by repeating reflections at the boundary between the core and cladding, but the propagation constant that indicates the state of light propagation is determined by the angular frequency of the light. When the angular frequency increases, higher-order modes occur in the optical propagation mode.

Therefore, when using an optical fiber as a single mode, it must be used at an angular frequency of 0 to ωc, where ωC is the angular frequency at which a higher-order mode occurs.

In addition, material dispersion occurs because the refractive index of the glass that makes up the optical fiber exhibits a nonlinear change with respect to the wavelength of light. When C, material dispersion σ of a single mode optical fiber
M is represented by the following formula.

On the other hand, the waveguide dispersion is determined by the relationship between the propagation constant β and the angular frequency ω of light, and in the case of a single mode optical fiber, the waveguide dispersion σW is given by the following equation.

However, d in equations (1) and (2) is a differential symbol.

Note that the sum of the material dispersion σM and the waveguide dispersion σW is the total dispersion σ
T, and the frequency bandwidth f of light that can be propagated is given by the following equation.

Here, the refractive index difference between the core and the cladding is small, and
FIG. 1 shows the relationship between the dispersion σ and the wavelength λ of light in a conventional step type single mode optical fiber in which the refractive index of both fibers changes in a step manner.

However, the figure shows a refractive index difference Δ=0.32% and a core diameter of 2.
In this example, a = 6.0 μm and the refractive index of the core n1 = 1.46319, but as is clear from the figure, even if the total dispersion σT is zero at a specific wavelength, the total dispersion σT is zero at other wavelengths. increases rapidly, and the frequency bandwidth f based on equation (3) decreases.

On the other hand, it has been theoretically and experimentally clarified that the transmission loss of light is the minimum at λ = 1.5 to 1.6 μm (Miya, et al.: EIectron Lett. Vol. 15, pl.
06, 1979), if the total dispersion σT in this wavelength range can be minimized, ultra-wideband and ultra-long distance optical transmission will be realized.

Therefore, this type of research on conventional single-mode optical fibers has focused on the wavelength λo at which the total dispersion σT is zero at λ=1.
The main focus is on moving the thickness between 5 and 1.6 μm (Tsuchiya, et al.: Electron Lett).

Vol. 15, p. 476, 1979), various results have been shown.5 However, when the wavelength to be transmitted deviates from λ0, the total dispersion σT increases rapidly, which limits the bandwidth of the light that can be transmitted. This resulted in some drawbacks.

The present invention has the purpose of eliminating such drawbacks of the conventional technology at once, by providing an intermediate layer between the core and the cladding with a refractive index lower than that of the core and the cladding, specifying the refractive index difference between them, and By specifying the ratio of the outer diameter of the core and the thickness of the intermediate layer to the radius of the core, the present invention provides a single mode optical fiber with an intermediate layer that achieves extremely broadband transmission characteristics.

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

Figure 2 shows a cross-sectional view and the refractive index distribution of each part.
An intermediate layer 3, which is also mainly composed of silica glass, is provided between the core 1 and the core 1, and assuming that the refractive index of the core 1 is n4 and the refractive index of the layer 3 is 03, the refractive index is shown in the lower part of the figure. n
As shown by the outward radius from the center of one core 1 as r, n2 is smaller than n1, n3 is even smaller than n2, and the refractive index n3 of the intermediate layer 3 is the refractive index n1 of the core 1 and the cladding 2. It is smaller than n2.

In addition, in this example, when the radius of the core 1 is a, the diameter 2a is 7.2 μm, and the thickness t of the intermediate layer 3 is 1.0 μm.
, the outer diameter of the cladding 2 is 125 μm, and since the outer diameter of the cladding 2 is extremely thick compared to the core 1 and the intermediate layer 3, the cladding 2 is shown by a broken line.

Further, the refractive index difference Δ1 between the core 1 and the cladding 2 and the refractive index difference Δ2 between the intermediate layer 3 and the cladding 2 are expressed by the following equation.

In the case of FIG. 2, in order to make the refractive index n1 of the core 1 larger than the refractive index n2 of the cladding 2, germanium is added to the quartz glass that is the main component in the core 1, and
In order to make the refractive index n3 of the intermediate layer 3 smaller than the refractive index n2 of the cladding 2, fluorine is added to the quartz glass that is the main component in the intermediate layer 3, but germanium, which is the additive in the core 1, is Using GeO2 (germanium oxide), the ratio of GeO2 to GiO2 (silica glass) is 1.
In the intermediate layer 3, the amount of fluorine added is 4.2 mol%, so that Δ1 in equation (4) is 1% and Δ2 in equation (5) is -1%.

In addition, various refractive index distributions can be considered as shown in Fig. 3 when the intermediate layer 3 is not provided and when the intermediate layer 3 is provided.
Since the power law distribution, the M-shaped distribution shown in C in the same figure, and the W-shaped distribution shown in D in the same figure are assumed, the relationship between the frequency of the light to be transmitted and the waveguide dispersion for these should be studied as a single mode optical fiber. When this was carried out, the results shown in FIG. 4 were obtained.

However, in this figure, the vertical axis represents the normalized waveguide dispersion CσW, which is the product of the waveguide dispersion σW and the speed of light C, and the horizontal axis represents the normalized frequency V determined by the following equation.

In addition, A-D in the same figure corresponds to A-D in FIG. 3, and E in FIG. 4 uses an intermediate layer 3 in which Δ1 and Δ2 shown in FIG. 2 are equal to each other and have opposite polarities. It has a symmetric W-shaped distribution,
The values of the refractive index differences Δ1 and Δ2 are as follows.

A-C…………Δ,=1.0% DΔ1=0.7%Δ2=-0.3%t=0.2aEΔ1
=0.5%Δ2−−0.5%t=0.3a, and as mentioned above, t is the thickness of the intermediate layer 3, a is the radius of the core 1, and in A-C, the intermediate layer 3 Since it is not provided with, only Δ1 is provided.

Here, when focusing on the normalized waveguide dispersion CσW, the fourth
E in the figure is the largest and the potency also changes in the negative direction, and since the total dispersion σT is minimized by canceling out the material dispersion σM and the waveguide dispersion σW, the E in the figure is the one shown in Figure 2. It is clear that a symmetrical W-shaped distribution is the most advantageous.

Figure 5 shows the changes in each dispersion σ with respect to the wavelength λ of the light to be transmitted for a single mode optical fiber with a symmetric W-shaped distribution, and shows the variation of each dispersion σ with respect to the wavelength λ of the light to be transmitted. It is clear that the total dispersion σT is within ±1 ps/km/nm over the entire range.

FIG. 6 shows the theoretical transmission loss in an optical fiber with a refractive index difference Δ1=1% (Miya, et al.: Electron Lett.
Vol 15, p 106, 1979) shows the change in photon energy PE of L and the wavelength λ of light, and λ
In the range of -1.32 to 1.69 μm, the transmission loss L is 0.
It is kept below 5dB/km.

Therefore, it is clear that the configuration shown in FIG. 2 is extremely effective for ultra-wideband and ultra-long distance transmission.

Based on these facts, the table below shows the relationship between the diameter 2a of the core 1, the thickness t of the intermediate layer 3, and the radius a of the core 1, which correspond to changes in the refractive index differences Δ1 and Δ2. The wavelength range λb in which the ratio and total dispersion σT are ±ps/km/nm or less is determined, and by using these values, a wide wavelength range λb can be obtained.

As in the previous table, Figures 7 to 9 show the interrelationships among the total dispersion σT, the wavelength λc where higher-order modes occur, the diameter 2a of the core 1, etc., and the range where the transmission loss is 0.5 dB/km or less. It is a diagram showing the wavelength λ and the diameter 2a of the core 1 on the vertical axis, and the total dispersion σT is ±1 ps/km/nm.
The following range is indicated by σTu and σTd, and the limit where the transmission loss is 0.5 dB/km is Lu and Ld.
It is a single mode, the total dispersion σT is ±1 ps/km/nm or less, and the transmission loss is 0.5.
It is clear that the condition of dB/km or less can be achieved within the shaded range in the figure, which indicates the usable wavelength λ band.

However, in Figure 7, Δ1 = 1.0%, t/a = 0.3 are fixed, and Δ2 is taken as a variable on the horizontal axis, and in Figure 8, Δ2 =
-1.0%, t/a = 0.3, and Δ1 is a variable. Figure 9 shows Δ1 = 1.0%, Δ2 = -1.0%, and t/a as a variable. These figures are used to calculate Δ1, Δ2, etc. under each condition, and
A specific diameter value is determined from a curve 2a showing the diameter 2a of the core 1.

Therefore, in order to mainly transmit light with a wavelength of around λ-1.5 μm, Δ1=1.0 to 1.5 as shown in the table above.
%, Δ2 = -0.5 to -1.0%, but depending on the conditions, Δ1, Δ2 may be obtained from Figures 7 to 9, and the wavelength range determined accordingly as shown in the previous table. 2a, t/ so that the total dispersion σT is 1 ps/km/nm or less over the entire range where λb is at least 1.54 to 1.55 μm.
It is only necessary to determine a, and thereby an ultra-wideband single mode optical fiber can be obtained.

Note that, of course, if the allowable total dispersion σT is made larger than ±1 ps/km/nm and the allowable transmission loss L is also made larger than 0.5 dB/km, optical transmission over a wider band can be performed.

As is clear from the above description, according to the present invention, the total dispersion can be reduced to ±1 ps/km/
WDM (wavelength division multiplexing)
This enables ultra-wideband and ultra-long distance transmission.

In addition, since it has an intermediate layer with a lower refractive index than the cladding, it has the advantage that there is less light leakage to the cladding side, and bending loss and microbending loss due to bending of the optical fiber are reduced. It exhibits remarkable effects for various optical transmission applications.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the relationship between dispersion and wavelength of light in a conventional single mode optical fiber, Fig. 2 is a cross-sectional view and refractive index distribution diagram of an optical fiber showing an embodiment of the present invention, and Fig. 3 is a diagram showing the relationship between dispersion and wavelength of light in a conventional single mode optical fiber. Figures showing various refractive index distributions; Figure 4 is a diagram showing the relationship between waveguide dispersion and light frequency for those shown in Figures 2 and 3; Figure 5 is for those shown in Figure 2. Figure 6 is a diagram showing the relationship between dispersion and wavelength of light, Figure 6 is a diagram showing the relationship between calculated transmission loss, wavelength of light, and photon energy, Figures 7 to 9 are wavelengths of light determined by simulation. FIG. 3 is a diagram showing the relationship between the refractive index difference, the core diameter, the ratio of the thickness of the intermediate layer, and the radius of the core. 1...Core, 2...Crat, 3...Middle layer, n1...
refractive index of core, n2... refractive index of cladding, n3... refractive index of intermediate layer, 2a... diameter of core, Δ1, Δ2... refractive index difference.

Claims (1)

[Scope of 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 the difference in refractive index between the core and the cladding is 1.0 to 1.5%, and , the refractive index difference between the intermediate layer and the cladding is set to -0.5 to -1.0%, and the total dispersion is at most ±1 ps/km/nm in the wavelength range of at least 1.54 to 1.55 μm. A single mode optical fiber with an intermediate layer, characterized in that the outer diameter of the core and the ratio of the thickness of the intermediate layer to the radius of the core are determined according to the respective refractive index differences. 2 In order to make the refractive index of the core larger than the refractive index of the cladding, the core is made by adding germanium to the quartz glass that is the main component, and the quartz glass that is the main component is used to make the refractive index of the intermediate layer smaller than that of the cladding. 2. The single mode optical fiber with an intermediate layer according to claim 1, wherein the intermediate layer is doped with fluorine.