JP3450696B2 - Optical fiber - Google Patents

Description

Detailed Description of the Invention

[0001]

The present invention relates to relates to an optical fiber, in particular relates to a fiber optic which can be used as a transmission path in an optical communication by suppressing optical nonlinear phenomena.

[0002]

2. Description of the Related Art In a silica type single mode optical fiber (hereinafter referred to as SMF) which is generally used in optical communication, the wavelength region which gives the minimum transmission loss is 1.4 to 1.6 μm.
Since it is in the range of m, it can be said that it is preferable to use such a wavelength region for long-distance optical communication. But SMF
When an optical signal having the lowest transmission loss wavelength propagates through the inside, its waveform deteriorates due to the influence of chromatic dispersion, so that there is a problem that the transmission speed and the transmission distance are limited.

The wavelength dispersion in such an optical fiber is given by both material dispersion and structural dispersion. For example, in a normal SMF in which the core diameter is 10 μm and the relative refractive index difference Δ between the core and the clad is 0.3%, the material dispersion is more dominant than the structure dispersion. Therefore, the zero-dispersion wavelength appears in the 1.3 μm band reflecting the wavelength dispersion of quartz used as a raw material, so the SMF in the wavelength range of the 1.5 μm band used in large-capacity optical communication is +17 ps / nm. The wavelength dispersion will be about / km.

Incidentally, when described as +17 ps / nm / km, it means that the time waveform spreads by about 17 ps when an optical pulse having a spectral width of 1 nm (full width at half maximum) propagates through an optical fiber of 1 km. (See "Optical Fiber Communication Technology", Yamamoto, P.249 (pulse spread due to dispersion effect), published by Nikkan Kogyo Shimbun).

Therefore, it has been conventionally required to reduce the transmission limitation due to such wavelength dispersion and to increase the transmission speed and the transmission distance. In order to meet such demands, a dispersion-shifted fiber (hereinafter referred to as DSF) has already been developed as an optical fiber having the minimum chromatic dispersion at the communication wavelength of 1.5 μm band (Nobuo K .et al., 'Characteristics of dispersio
n-shifted dual shapecore single-mode fibers', JL
T., LT-5, No. 6, p. 792 (1987)).

The DSF has a different sign to the material dispersion in the design of the refractive index distribution between the core and the clad, and the structural dispersion is designed such that the material dispersion has the same absolute value. And its zero dispersion wavelength is 1.
It is provided in the 5 μm band. Further, in order to satisfy such conditions, the relative refractive index difference Δ between the core and the clad is set to 0.7 or more, that is, a method of increasing the structural dispersion is adopted. However, if Δ is large, it is necessary to reduce the core diameter in order to satisfy the below-mentioned single mode condition. As a result, the electric field distribution of light is narrowed, and the effective core area (hereinafter referred to as Aeff) is smaller than that of SMF. There is a problem of being small.

Here, the single mode condition will be described. In the case of an optical fiber having a step-type refractive index distribution, if the wavelength used is λ, the normalized frequency V at the wavelength used is given by the following equation. V = (2π / λ) · a · n ₁ (2Δ) ^0.5 (1) Δ = (n ₁ −n ₂ ) / n ₁ (2) where a is the core diameter, n ₁ is the refractive index of the core, and n is ₂ indicates the refractive index of the cladding, Δ indicates the relative refractive index difference between the core and the cladding, and the value of V must be 2.405 or less in order to satisfy the single mode condition.

Therefore, when the relative refractive index difference Δ is increased in order to increase the structural dispersion, it becomes necessary to design the core diameter a to be small in return. However, when the core diameter a is reduced and the relative refractive index difference Δ is increased, the effect of confining light in the core is increased, so that Aeff is larger than that of SMF.
Becomes smaller and the bending loss becomes smaller.

By using such a DSF for a transmission line and an erbium optical fiber amplifier (hereinafter referred to as EDFA) as a repeater, a transmission system having a regeneration repeat distance of 320 km and a transmission speed of 10 Gb / s. However, it has already been put to practical use.

As a method of expanding the transmission capacity,
2. Description of the Related Art Wavelength multiplex transmission (hereinafter referred to as WDM) has been attracting attention domestically and internationally, and in this WDM, a plurality of signal wavelengths can be mixed and used in one optical fiber for communication. Therefore, there is an advantage that a transmission system having a larger capacity than the conventional single wavelength transmission can be realized.

[0011]

As described above, D
When SF is used for the transmission line, the normal DSF has a small Aeff, so the electric field density of light in the optical fiber (that is, the optical power per unit area in the fiber cross section).
Becomes higher. On the other hand, as the optical intensity of the signal light increases, a symptom generally called an optical nonlinear phenomenon is likely to occur in the optical fiber. In particular, it can be said that it is easily induced in a DSF having a high electric field density of light.

Since this optical nonlinear phenomenon causes deterioration of the S / N ratio, it has been a problem as a major limiting factor of the transmission capacity and the transmission distance in the transmission system using the DSF. Therefore, in the actual transmission system using the DSF, there is a problem that the gain of the optical amplifier must be suppressed for transmission.

However, when the transmission speed is increased, the time slot per bit of the signal is shortened, and it is necessary to increase the signal light power per bit in order to secure a receivable light receiving level. This is contrary to the suppression of the optical nonlinear phenomenon, and as a result, in order to suppress the optical nonlinear phenomenon, the transmission power has to be reduced to limit the transmission speed.

On the other hand, when WDM is adopted to expand the transmission capacity, in WDM, a plurality of wavelengths are present in the optical fiber, so that four-wave mixing (hereinafter, FWM: Four WaveMis.
An optical nonlinear phenomenon called "xing" occurs, which limits the transmission capacity and transmission distance.

This FWM is a phenomenon in which interference occurs between signal wavelengths and new light is generated by a third-order nonlinear optical process, and the more the phase matching condition between wavelengths is satisfied, the FW.
Since the generation efficiency of M becomes high, it tends to occur as the signal wavelength becomes closer to the zero-dispersion wavelength and the interval between the signal wavelengths becomes narrower. Therefore, DS with zero dispersion wavelength in the signal wavelength band
In F, FWM is more likely to occur than in SMF, and it is necessary to widen the wavelength interval of signals. However, since the amplification bandwidth of the EDFA is about several tens of nm, there is a problem that the number of signal channels is reduced and the transmission capacity is limited if the wavelength interval is widened.

The application of the DSF is not limited to the transmission line. Higher-speed optical switches and wavelength conversion devices are being actively researched along with the sophistication of transmission systems.However, these optical switches and wavelength conversion devices are subject to optical nonlinear phenomena contrary to the case of transmission lines. It is used for switching and wavelength conversion, and how to induce an optical nonlinear phenomenon is an issue. Therefore, an example in which such an optical switch and a wavelength conversion device are realized by using a DSF having a small Aeff and easily inducing an optical nonlinear phenomenon has already been reported.

However, at a transmission rate of 20 Gb / s or more, the response signal required for signal processing cannot be electrically handled, so it is necessary to use an optical switch or wavelength conversion device. When it is used for a switch and a wavelength conversion device, there is a problem that its conversion efficiency is poor and its length must be several tens of km, and that large incident light power is required.

[0018] The present invention has been made to solve such problems, and an object thereof is to provide an optical fiber capable of suppression design pressure easily optical nonlinear phenomena. Another object of the present invention is to suppress the occurrence of an optical nonlinear phenomenon by reducing the electric field density of light in the optical fiber, and to suppress the occurrence of FWM by disturbing the phase matching condition between wavelengths. An object is to provide an optical fiber .

[0019]

The optical fiber of the present invention comprises:
To achieve the above object, a core having a refractive index of n _0, a first clad formed around the core and having a refractive index of n ₁ , and a refractive index formed around the first clad. Of n _{2 and} a third clad formed around the second clad and having a refractive index of n ₃ , the refractive index of n ₁ > n ₂ > n ₃ and n _It has a relation of ₁ > n ₀ . Thus the present invention by configuration can provide an optical nonlinear phenomena depressive push that optical fiber. Therefore, you can use a transmission path as long as the optical fiber for suppressing the optical nonlinear phenomena.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION Next, one embodiment of the present invention will be described with reference to the drawings. First, an optical fiber that can be used in a transmission line by suppressing an optical nonlinear phenomenon (hereinafter, referred to as an optical nonlinear phenomenon suppressing optical fiber)
Will be described.

FIG. 1 is a sectional view of an optical fiber for suppressing an optical nonlinear phenomenon and a graph showing its refractive index distribution. As shown in the figure, in order to suppress the occurrence of the optical nonlinear phenomenon, the refractive index at the center of the DSF core was lowered. That is, in the present embodiment, a core 1 having a refractive index n0, a clad 2 having a refractive index n1 (hereinafter referred to as a first clad),
A cladding 3 having a refractive index of n2 (hereinafter referred to as a second cladding) and a cladding 4 having a refractive index of n3 (hereinafter referred to as a third cladding), each of which has a refractive index of at least n.
_It has a relationship of ₁ > n ₂ > n ₃ and n ₁ > n ₀ . Particularly here, in order to suppress the optical nonlinear phenomenon, n ₁
> N ₂ > n ₃ ≧ n ₀ . By doing this,
The first clad effectively plays the role of the core, and the electric field distribution of the light spreads in the radial direction of the optical fiber and becomes Ae.
ff becomes large, and the optical nonlinear phenomenon can be suppressed.

Further, although such an optical nonlinear phenomenon suppressing optical fiber has a small electric field density of light, it is the same as an existing optical fiber (eg, quartz) in terms of material, so that it can be used as an existing transmission line. It is possible to perform fusion splicing. Therefore, the optical power in the transmission line can be made strong, and by inserting this fiber immediately after the optical amplifier in which the optical nonlinear phenomenon is likely to be induced, the transmission deterioration due to the optical nonlinear phenomenon can be suppressed.

By the way, as described above, when WDM is adopted to expand the transmission capacity, since a plurality of wavelengths are present in the optical fiber in WDM, an optical nonlinear phenomenon called FWM occurs and the transmission capacity is increased. And the transmission distance is limited. Therefore, the generation of FWM is suppressed by changing the chromatic dispersion of the optical fiber along the longitudinal direction thereof. That is, when the chromatic dispersion changes along the longitudinal direction of the optical fiber, the propagation speed of the signal light locally changes in the transmission path. Therefore, in WDM transmission, the phase matching condition between adjacent channels is disturbed, and it is possible to suppress the occurrence of FWM, which is a transmission limitation.

There are several methods for changing the chromatic dispersion, and for example, there are the following methods. That is, the chromatic dispersion at the wavelength of 1.55 μm can be continuously changed by continuously changing the relative refractive index difference α between the core and the third cladding in the longitudinal direction when manufacturing the optical fiber.

FIG. 2 shows an optical fiber according to FIG.
6 is a graph showing changes in chromatic dispersion at a wavelength of 1.55 μm when the relative refractive index difference α between the core and the third cladding is changed, and the relative refractive index difference β between the first and third claddings is 1 It is set to 0.5%. Note that α in the figure is given by the following equation. α = n ₀ −n ₃ / n ₀ Similarly, β is given by the following equation. β = (n ₁ −n ₃ ) / n ₁

Incidentally, +12 ps / nm / k shown in FIG.
An example of an optical fiber that varies from m to -12 ps / nm / km (chromatic dispersion variation width of 24 ps / nm / km) is not presented because the chromatic dispersion value must be ± 12 ps / nm / km. For example, only an example of how much the chromatic dispersion variation range can be obtained by changing α from 0% to −0.4% is shown.

By changing the relative refractive index difference β between the first cladding and the third cladding, the chromatic dispersion at the wavelength of 1.55 μm can be continuously changed. For example, FIG. 3 is a graph showing changes in chromatic dispersion at a wavelength of 1.55 μm when β is changed, and α is set to −0.4%.

Alternatively, the chromatic dispersion can be changed by changing the second cladding diameter along the longitudinal direction of the optical fiber. For example, FIG. 4 is a graph showing changes in chromatic dispersion at a wavelength of 1.55 μm when the diameter of the second cladding is changed, where α is −0.4% and β is 1.44%.

FIG. 5 is a graph showing the relationship between the fiber length and the chromatic dispersion when the diameters of α, β and the second cladding are changed. In FIG. 5A, the wavelength is 1.55 μm, β is 1.5%, and in FIG. 5B, the wavelength is 1.
55 μm, α is −0.4%, and in FIG. 5C, the wavelength is 1.
55 μm, α is −0.4%, β is 1.44%. In any case, it can be seen that the chromatic dispersion continuously decreases as the fiber length increases, and the chromatic dispersion differs depending on the position in the optical fiber. Further, both the relative refractive index difference α between the core and the third cladding and the relative refractive index difference β between the first cladding and the third cladding are periodically varied along the longitudinal direction during the production of the optical fiber. By doing so, the phase matching condition may be disturbed and the generation of FWM may be suppressed.

The optical fiber for suppressing optical nonlinear phenomenon as described above may be used as follows. FIG. 6 is a block diagram showing a configuration of a transmission line when the optical fiber according to FIG. 1 is used and a graph showing a relationship between a transmission distance and an optical output level. As shown in the figure, the transmitter 5 is composed of a light source 5a and an optical amplifier 5b, and the receiver 7 is composed of a light receiver 7a and an optical amplifier 7b. An optical amplifier 6 is installed between the transmitter 5 and the receiver 7, and the transmitter 5 and the receiver 7 are connected by the optical amplifier 6, the optical nonlinear phenomenon suppressing optical fiber 8 and the existing optical fiber 9. It is connected.

Such a configuration is based on the following reasons. Since an actual optical fiber always has a transmission loss, the optical power of the optical signal being transmitted gradually weakens. That is, the strongest optical nonlinear phenomenon occurs in the optical fiber transmission line immediately after the optical amplifier (A and B in the figure). Therefore, the optical nonlinear phenomenon can be effectively suppressed by inserting the optical nonlinear phenomenon suppressing optical fiber according to FIG. 1 immediately after the optical amplifier in the existing transmission line.

Next, as a reference example, an optical fiber that can be used in an optical switch or a wavelength conversion device by inducing an optical nonlinear phenomenon (hereinafter referred to as an optical nonlinear phenomenon inducing optical fiber) will be described. FIG. 7 is a cross-sectional view of the optical nonlinear phenomenon inducing optical fiber and a graph showing its refractive index distribution. As shown in the figure, the refractive index n ₀ of the core for inducing the optical nonlinear phenomenon is higher than the refractive index n ₃ of the third cladding and lower than the refractive index n ₁ of the first cladding. The effect of confining the light in the center of the fiber was strengthened and Aeff was reduced (that is, n ₁ > n ₂ > n).
₃ > and n ₁ > n ₀ > n ₃ ). Therefore, the electric field density of the light in the optical fiber becomes high. Since the optical nonlinear phenomenon occurs according to the product of the electric field density (power) of light and the action length, this optical fiber can cause the nonlinear phenomenon with high efficiency.

[0033]

EXAMPLES Next, examples of the present invention will be described. FIG. 8 shows a mode field diameter (hereinafter referred to as MFD) when the relative refractive index difference α between the core and the third cladding is changed.
It is a graph which shows the change of. Here, MFD is a parameter indicating the spread of the electric field distribution of light in the fiber, and is known to be proportional to Aeff (Namihira et al., 'Nonlinear Kerr Coefficient Meas.
urements for Dispersion Shifted Fibersusing Self-P
hase Modulation method at 1.55 μm ', Oec '94, p.135,
(1994)).

As shown in FIG. 8A, in the optical fiber according to the present embodiment, when α has a larger absolute value in the negative direction, the electric field distribution of light spreads and the electric field density decreases, so that the optical nonlinearity is increased. An optical fiber for suppressing an optical nonlinear phenomenon that suppresses the occurrence of a phenomenon. From the viewpoint of manufacturing, since the amount of fluorine added when reducing the refractive index of the core is limited, the lower limit of α is actually -0.7 to -0.8.
It will be about%.

On the other hand, as a reference example, as shown in FIG. 8B , it can be seen that the value of MFD becomes smaller as α is increased in the positive direction. The MFD of a typical step-type dispersion shift fiber is effectively 7.4 to 8.4 μm (Ae
Since ff = 41 to 53 μm ² ), when α becomes positive, the electric field strength becomes narrower and the electric field density becomes higher than that of DSF.

When the electric field distribution has a Gaussian shape, Ae
ff = π × (MFD / 2) ² holds. Therefore,
Although Aeff increases as MFD increases, when the optical fiber according to the present invention has an optical nonlinear phenomenon suppression fiber structure (that is, α <0), the optical fiber deviates from the Gaussian shape, and thus the correction coefficient is added to the right side of the above equation. c (however, c>
It is necessary to multiply 1).

As a reference example, FIG. 9 is a graph showing the change in MFD when the ratio between the core diameter and the diameter of the first cladding is changed. In the figure, the black circles are when the relative refractive index difference α between the core and the third cladding is 0%, and the white circles are when α is + 0.3%. As shown in the figure, when α is 0%, when the ratio of the core diameter to the diameter of the first cladding is 0.4 or less, the DSF
It can be seen that the optical non-linear phenomenon inducing optical fiber is obtained because the MFD becomes smaller than that and the electric field density becomes higher. Further, when α is + 0.3%, if the ratio of the core diameter to the diameter of the first cladding is 0.5 or less, it is predicted that the optical fiber will induce an optical nonlinear phenomenon.

Next, the effect of suppressing FWM will be described. FIG. 10 is a graph showing the inhibitory effect of FWM,
It is shown that the FWM can be suppressed by changing the chromatic dispersion along the longitudinal direction of the optical fiber. This is the FWM generation efficiency (normalized with the generation efficiency when the dispersion variation is 0 as 1) in the chromatic dispersion variation fiber (when the length is 40 km) in which the chromatic dispersion increases linearly along the longitudinal direction of the optical fiber. FIG. 11 shows the dependence on the chromatic dispersion fluctuation width (the magnitude of the difference in chromatic dispersion between the entrance and the exit) (see FIG. 11).

When the fluctuation range of chromatic dispersion is 4 ps / nm / km, the generation efficiency of FWM is less than 1/10 of that of an ordinary optical fiber having a fluctuation range of chromatic dispersion of 0, and a sufficient suppression effect is obtained. You can see that Further, the suppression effect is enhanced by increasing the chromatic dispersion fluctuation range. Further, as can be seen from FIG. 10, when the fluctuation range of chromatic dispersion is 20 ps / nm / km, the generation efficiency of FWM is about 0.
Can be suppressed to about 03, 24 ps / nm / km
In this case, it can be expected that a larger FWM suppressing effect can be obtained.

FIG. 12 shows the core diameter satisfying the single mode condition and the relative refractive index between the first and third claddings when the relative refractive index difference α between the core and the third cladding is changed. It is a figure which shows the relationship with rate difference (beta). The shaded area in the figure is the region in which a single mode cannot be realized at a wavelength of 1.55 μm. It can be seen that the smaller α is, the more combinations of the core diameter and β that satisfy the single mode condition are.

Further, in the optical fiber shown in FIG. 13, by setting α to about −0.3% and β to about 1%, the zero dispersion wavelength can be expanded to 1.55 μm and Aeff to about 150 μm ^2. It was FIG. 14 shows changes in the zero-dispersion wavelength when the respective relative refractive index differences are changed while maintaining the relationship of the refractive indexes of n ₁ > n ₂ > n ₃ and n ₁ > n ₀ .
It can be seen from the figure that zero dispersion in the 1.4 μm to 1.5 μm band is realized. The zero-dispersion wavelength can be set to a 1.3 μm to 1.6 μm band and a longer wavelength band by changing the combination of the relative refractive index differences.

Considering the handling characteristics when the above optical fiber is actually used in a transmission line or the like, it is necessary that the optical fiber has a small bending loss. In the optical fiber of this example, the bending loss characteristic could be improved by increasing β. FIG. 15 shows the bending loss characteristics with respect to the bending radius of the optical fiber according to this example. It can be seen from the figure that the bending loss decreases as β increases. Moreover, in the optical fiber of the present embodiment,
By setting β to about 1.5%, it is about the same as conventional DSF (a few dB / m at a bending radius of 1 cm, but Aeff
Of about 50 μm ² ) was obtained, and the Aeff at that time was about 120 μm ² .

[0043]

According to the optical fiber of the present invention, the electric field distribution is not concentrated at the center of the optical fiber even if the zero dispersion wavelength is designed to be in the 1.55 μm band. Aeff while maintaining the same loss characteristics
Could be more than doubled. That is, since the electric field density of light is reduced and the optical nonlinear phenomenon is suppressed,
The deterioration of the signal waveform is reduced, and the transmission speed and the transmission distance can be expanded. Further, by changing the zero-dispersion wavelength in the 1.5 μm band along the longitudinal direction of the optical fiber, the phase matching condition between wavelengths is disturbed, the generation efficiency of FWM can be lowered, and the WDM wavelength interval can be reduced. The number of channels can be increased .

[Brief description of drawings]

FIG. 1 is a cross-sectional view of an optical fiber for suppressing an optical nonlinear phenomenon according to the present invention and a graph showing its refractive index distribution.

FIG. 2 is a relative refractive index difference α between the core and the third cladding,
It is a graph which shows the relationship with chromatic dispersion.

FIG. 3 is a graph showing the relationship between the relative refractive index difference β between the first cladding and the third cladding and the wavelength dispersion.

FIG. 4 is a graph showing the relationship between the diameter of the second cladding and chromatic dispersion.

FIG. 5 is a graph showing the relationship between fiber length and chromatic dispersion.

FIG. 6 is a block diagram showing a configuration of a transmission line using the optical nonlinear phenomenon suppressing optical fiber according to FIG. 1 and a graph showing a relationship between a transmission distance and an optical output level.

7A and 7B are a cross-sectional view of an optical nonlinear phenomenon inducing optical fiber and a graph showing its refractive index distribution.

FIG. 8 is a relative refractive index difference α and M between the core and the third cladding.
It is a graph which shows the relationship of FD.

FIG. 9: Ratio of core diameter to diameter of first cladding and MFD
It is a graph which shows the relationship of.

FIG. 10 is a graph showing the relationship between the chromatic dispersion fluctuation range and the FWM generation efficiency .

FIG. 11 is a graph showing the relationship between fiber length and chromatic dispersion.

FIG. 12 is a graph showing the relationship between the relative refractive index difference β between the first cladding and the third cladding and the core diameter.

FIG. 13 is a graph showing the relationship between the relative refractive index difference β between the first cladding and the third cladding and Aeff.

FIG. 14 is a graph showing the relationship between wavelength and chromatic dispersion.

FIG. 15 is a graph showing the relationship between bending radius and bending loss.

[Explanation of symbols]

1 ... Core, 2 ... 1st clad, 3 ... 2nd clad,
4 ... Third cladding, 5 ... Transmitter, 5a ... Light source, 5b,
6, 7b ... Optical amplifier, 7 ... Receiver, 7a ... Photo receiver, 8 ...
Optical non-linear phenomenon suppressing optical fiber 9, existing optical fiber.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshiaki Miyajima 3-19-2 Nishishinjuku, Shinjuku-ku, Tokyo Nihon Telegraph and Telephone Corporation (56) References JP-A-1-163707 (JP, A) Special Features Kaihei 8-160241 (JP, A) JP 9-26517 (JP, A) JP 7-209539 (JP, A) JP 8-194123 (JP, A) JP 9-159856 ( (58) Fields surveyed (Int.Cl. ⁷ , DB name) G02B 6/22 G02B 6/16

Claims (2)

(57) [Claims] _1. A core having a refractive index of n _0, a first cladding formed around the core and having a refractive index of n ₁ , and a refractive index of n ₂ formed around the first cladding. Second cladding and a third cladding formed around the second cladding and having a refractive index of n ₃ , the refractive index being n ₁ > n ₂ > n
₃ and n ₁ > n _0, the refractive index further has a relationship of n ₃ ≧ n ₀ , and the relative refractive index difference β between the first cladding and the third cladding is An optical fiber, which is configured to continuously increase or decrease monotonically along the longitudinal direction of the optical fiber. 2. A core having a refractive index of n _{0 and} a first core formed around the core and having a refractive index of n ₁ .
With a refractive index of n ₂ formed around the first cladding .
A second cladding and a refractive index n ₃ formed around the second cladding .
A third cladding, wherein the refractive index is n ₁ > n ₂ > n
₃ and has a relationship of n _1> n _0, wherein the refractive index is further have a relationship of n ₃ ≧ n _0, the second cladding diameter in the longitudinal direction of the optical fiber
Configured to continuously monotonically increase or decrease along
An optical fiber characterized by being used .