JPH10300966A - Optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the optical fiber which can easily design the suppression and induction of an optical nonlinear phenomenon. SOLUTION: The optical fiber has a core 1 with a refractive index n0, a 1st clad 2 which is formed around the core and has a refractive index n1, a 2nd clad 3 which is formed around the 1st clad 2 and has a refractive index n2, and a 3rd clad 4 which is formed around the 2nd clad 3 and has a refractive index n3, and the respective refractive indexes have relations n1>n2>n3 and n1>n0.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber, and more particularly to an optical fiber which can be used as a transmission line in optical communication by suppressing optical nonlinear phenomena, and an optical switch or wavelength converter by inducing optical nonlinear phenomena. The present invention relates to an optical fiber that can be used for a device.

[0002]

2. Description of the Related Art In a silica-based single mode optical fiber (hereinafter, referred to as SMF) generally used in optical communication, a wavelength region giving a minimum transmission loss is 1.4 to 1.6 .mu.m.
m, it is preferable to use such a wavelength region for long-distance optical communication. However, SMF
When an optical signal having the lowest transmission loss wavelength propagates through the inside, the waveform deteriorates due to the influence of chromatic dispersion, and thus there is a problem that the transmission speed and the transmission distance are limited.

[0003] The chromatic dispersion in such an optical fiber is given by both material dispersion and structural dispersion. For example, in a normal SMF having a core diameter of 10 μm and a relative refractive index difference Δ between the core and the cladding of 0.3%, material dispersion is more dominant than structural dispersion. Therefore, since the chromatic dispersion of quartz used as a raw material is reflected and a zero-dispersion wavelength appears in the 1.3 μm band, the SMF in the 1.5 μm band wavelength region used in large-capacity optical communication is +17 ps / nm. / Km wavelength dispersion.

[0004] When it is described as +17 ps / nm / km, it means that when a light pulse having a spectral width of 1 nm (full width at half maximum) propagates through an optical fiber of 1 km, a time waveform spreads by about 17 ps. (See “Optical Fiber Communication Technology”, Yamamoto, P.249 (pulse spread due to dispersion effect), published by Nikkan Kogyo Shimbun).

[0005] Therefore, it has been conventionally required to reduce the transmission limitation due to such chromatic dispersion and to increase the transmission speed and transmission distance. To meet such demands, a dispersion-shifted fiber (hereinafter, referred to as DSF) has been already developed as an optical fiber having a minimum chromatic dispersion at a 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)).

[0006] The DSF is a material in which the refractive index distribution between the core and the clad has a different sign with respect to the material dispersion, and the structural dispersion is designed so that the material dispersion and the absolute value are equal. And its zero-dispersion wavelength is 1.
It is provided in the 5 μm band. In order to satisfy such a condition, a 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, when Δ is large, it is necessary to reduce the core diameter in order to satisfy the single mode condition described later. As a result, the electric field distribution of light becomes narrow, and the effective core area (hereinafter, 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, assuming that the used wavelength is λ, the normalized frequency V at the used wavelength 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 ₂ is the refractive index of the cladding, Δ is 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 is necessary to design the core diameter a to be small as a price. However, when the relative refractive index difference Δ is increased by decreasing the core diameter a, the effect of confining light in the core is increased.
And the bending loss is reduced.

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

[0010] As a method of expanding the transmission capacity,
2. Description of the Related Art Conventionally, wavelength multiplex transmission (hereinafter, referred to as WDM) has attracted attention in Japan and overseas. 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 conventional single wavelength transmission can be realized.

[0011]

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

[0012] Since this optical non-linear phenomenon causes deterioration of the S / N ratio, it has been a problem in a transmission system using DSF as a major limiting factor of transmission capacity and transmission distance. Therefore, in an actual transmission system using DSF, there is a problem that transmission must be performed while suppressing the gain of the optical amplifier.

However, when the transmission speed is increased, the time slot per signal bit becomes shorter, 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. As a result, in order to suppress the optical nonlinear phenomenon, it is necessary to reduce the transmission power and limit the transmission speed.

On the other hand, when WDM is employed to increase the transmission capacity, four wavelength mixing (hereinafter referred to as FWM: Four Wave Mixing) occurs because a plurality of wavelengths exist in the optical fiber in WDM.
An optical nonlinear phenomenon called “xing” occurs, and transmission capacity and transmission distance are limited.

This FWM is a phenomenon in which interference occurs between signal wavelengths to generate new light due to a third-order nonlinear optical process, and the more the phase matching condition between wavelengths is satisfied, the more the FW becomes.
Since the generation efficiency of M is high, it is more likely to occur as the signal wavelength is closer to the zero dispersion wavelength and the interval between the signal wavelengths is smaller. 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 signal wavelength interval. However, since the amplification bandwidth of the EDFA is about several tens of nm, there is a problem that if the wavelength interval is widened, the number of signal channels decreases and the transmission capacity is limited.

The use of the DSF is not limited to the transmission path. Higher speed optical switches and wavelength conversion devices have been actively researched as transmission systems have become more sophisticated.However, these optical switches and wavelength conversion devices exhibit optical nonlinear phenomena, contrary to transmission lines. Switching and wavelength conversion are performed by using the method, and how to induce an optical non-linear phenomenon is an issue. Therefore, an example has already been reported in which such an optical switch and a wavelength conversion device are realized using a DSF having a small Aeff and easily causing an optical nonlinear phenomenon.

However, at a transmission speed of 20 Gb / s or higher, it is impossible to respond to a response signal required for signal processing electrically, and it is necessary to use an optical switch or a wavelength conversion device. When used for a switch and a wavelength conversion device, the conversion efficiency is poor, so that the length needs to be several tens km, and there is a problem that a large incident light power is required.

An object of the present invention is to solve such a problem, and an object of the present invention is to provide an optical fiber capable of easily designing suppression and induction of an optical nonlinear phenomenon.
Another object of the present invention is to reduce the electric field density of light in an optical fiber to suppress the occurrence of an optical nonlinear phenomenon, and to suppress the occurrence of FWM by disturbing the phase matching condition between wavelengths. An object is to provide an optical fiber. Another object of the present invention is to provide an optical fiber capable of inducing an optical nonlinear phenomenon by increasing the electric field density of light in the optical fiber.

[0019]

An optical fiber according to the present invention comprises:
In order to achieve the above object, 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 formed around the first cladding and having a refractive index of n ₁ Comprises a _second cladding of n _{2 and} a third cladding formed around the second cladding and having a refractive index of n ₃ , wherein the refractive indices are n ₁ > n ₂ > n ₃ and n those having a relationship of _1> n _0. With this configuration, the present invention can provide an optical fiber that suppresses or induces an optical nonlinear phenomenon. Therefore, an optical fiber that suppresses an optical nonlinear phenomenon can be used for a transmission line, and an optical fiber that induces an optical nonlinear phenomenon can be used for an optical switch or a wavelength conversion device.

[0020]

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 a refractive index distribution thereof. As shown in the figure, the refractive index at the center of the core of the DSF was reduced to suppress the occurrence of the optical nonlinear phenomenon. That is, in the present embodiment, a core 1 having a refractive index of n0, a cladding 2 having a refractive index of n1 (hereinafter, referred to as a first cladding),
A clad 3 having a refractive index of n2 (hereinafter, referred to as a second clad) and a clad 4 having a refractive index of n3 (hereinafter, referred to as a third clad), each having a refractive index of at least n
_1> n _2> n ₃ and those having a relationship of n _1> n _0. In particular, here, n _{1 is} used to suppress the optical nonlinear phenomenon.
> N ₂ > n ₃ ≧ n ₀ . By doing this,
The first cladding 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 Ae
ff increases, and the optical nonlinear phenomenon can be suppressed.

Although such an optical fiber for suppressing optical non-linear phenomena has a small electric field density of light, the material is the same as that of an existing optical fiber (for example, quartz or the like). Is possible. Therefore, the optical power in the transmission path can be increased, and by inserting the present fiber immediately after the optical amplifier in which the optical nonlinear phenomenon is easily induced, transmission deterioration due to the optical nonlinear phenomenon can be suppressed.

As described above, when WDM is employed to increase the transmission capacity, an optical nonlinear phenomenon called FWM occurs because the WDM has a plurality of wavelengths in the optical fiber, 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. That is, when the chromatic dispersion changes along the longitudinal direction of the optical fiber, the propagation speed of the signal light locally differs in the transmission path. Therefore, in WDM transmission, the phase matching condition between adjacent channels is disturbed, so that it is possible to suppress the occurrence of FWM that limits transmission.

There are several methods for changing the chromatic dispersion. For example, there are the following methods. That is, by continuously changing the relative refractive index difference α between the core and the third cladding in the longitudinal direction at the time of manufacturing the optical fiber, it becomes possible to continuously change the chromatic dispersion at the wavelength of 1.55 μm.

FIG. 2 shows an optical fiber according to FIG.
FIG. 9 is a graph showing a change in chromatic dispersion at a wavelength of 1.55 μm when a relative refractive index difference α between the core and the third cladding is changed, and a relative refractive index difference β between the first and third claddings is 1; 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

It should be noted that +12 ps / nm / k shown in FIG.
The example of an optical fiber that varies from m to −12 ps / nm / km (chromatic dispersion variation width 24 ps / nm / km) is not presented in the sense that the chromatic dispersion value must be ± 12 ps / nm / km. For example, when only α is changed from 0% to −0.4%, only an example of how much the chromatic dispersion variation width can be obtained is shown.

By changing the relative refractive index difference β between the first clad and the third clad, the chromatic dispersion at a wavelength of 1.55 μm can be continuously varied. For example, FIG. 3 is a graph showing a change in chromatic dispersion at a wavelength of 1.55 μm when β is changed, where α is -0.4%.

Alternatively, the chromatic dispersion can also be varied by changing the diameter of the second cladding along the longitudinal direction of the optical fiber. For example, FIG. 4 is a graph showing a change in chromatic dispersion at a wavelength of 1.55 μm when the diameter of the second clad is changed. 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 α, β, and the diameter of the second cladding are respectively changed. FIG. 5 (a) has a wavelength of 1.55 μm and β of 1.5%, and FIG.
55 μm, α is −0.4%, and FIG.
55 μm, α is −0.4%, and β is 1.44%. In each case, the chromatic dispersion decreases continuously as the fiber length increases, and it can be seen that the chromatic dispersion differs depending on the position in the optical fiber. In addition, 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 periodically fluctuate along the longitudinal direction during the manufacture of the optical fiber. By doing so, the occurrence of FWM may be suppressed by disturbing the phase matching condition.

The above-mentioned optical fiber for suppressing the optical nonlinear phenomenon is preferably used as follows. FIG. 6 is a block diagram illustrating a transmission path configuration when the optical fiber according to FIG. 1 is used, and a graph illustrating a relationship between a transmission distance and an optical output level. As shown in the figure, the transmitting section 5 is composed of a light source 5a and an optical amplifier 5b, and the receiving section 7 is composed of a light receiver 7a and an optical amplifier 7b. An optical amplifier 6 is provided between the transmitting unit 5 and the receiving unit 7, and the transmitting unit 5 and the receiving unit 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 decreases. 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 especially in the existing transmission line immediately after the optical amplifier.

Next, an optical fiber which can be used for 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 a refractive index distribution thereof. As shown in the figure, the refractive index n ₀ of the core is higher than the refractive index n ₃ of the third cladding and lower than the refractive index n ₁ of the first cladding to induce an optical nonlinear phenomenon. Aef enhances the effect of confining light to the center of the fiber,
f (ie, n ₁ > n ₂ > n ₃ and n ₁
> N _0> n _3). Therefore, the electric field density of light in the optical fiber increases. Since the optical nonlinear phenomenon occurs in accordance with the product of the electric field density (power) of light and the operating length, this optical fiber can cause the nonlinear phenomenon with high efficiency.

[0033]

Next, an embodiment 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 clad is changed.
6 is a graph showing a change in the graph. Here, the 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 1.55μm ', Oec '94, p.135,
(1994)).

As shown in FIG. 8A, in the optical fiber according to the present embodiment, when the absolute value of α is increased in the negative direction, the electric field distribution of light is widened and the electric field density is reduced, so that the optical nonlinearity is increased. It becomes an optical nonlinear phenomenon suppressing optical fiber for suppressing the occurrence of the phenomenon. In addition, 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.
%.

On the other hand, as shown in FIG. 8B, it can be seen that the value of MFD decreases as α increases in the positive direction. MFD of general step-type dispersion-shifted fiber
Is effectively 7.4 to 8.4 μm (Aeff = 41 to 5
Since it is about 3 μm ² ), when α becomes positive, the electric field distribution becomes narrower than that of DSF and the electric field density becomes higher.

When the electric field distribution is Gaussian, Ae
ff = π × (MFD / 2) ² holds. Therefore,
Aeff increases as the MFD increases. However, when the optical fiber according to the present invention has an optical non-linear phenomenon suppressing fiber structure (ie, α <0), it deviates from a Gaussian shape, so that the correction coefficient is c (however, c>
It is necessary to multiply 1).

FIG. 9 is a graph showing a change in the MFD when the ratio between the core diameter and the diameter of the first clad is changed. The black circles in the figure indicate the relative refractive index difference α between the core and the third cladding of 0%, and the white circles indicate the case where 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 clad is 0.4 or less, MFD is larger than DSF.
Is smaller and the electric field density is higher, which indicates that the optical fiber is an optical fiber inducing an optical nonlinear phenomenon. Also, if α is +
When the ratio is 0.3%, if the ratio of the core diameter to the diameter of the first clad becomes 0.5 or less, it is expected that the optical fiber will be an optical nonlinear phenomenon-inducing optical fiber.

Next, the effect of suppressing the FWM will be described. FIG. 10 is a graph showing the suppression effect of FWM,
This shows that FWM can be suppressed by changing the chromatic dispersion along the longitudinal direction of the optical fiber. This is the FWM generation efficiency (normalized assuming that the generation efficiency when dispersion fluctuation is 0 is 1) in a chromatic dispersion fluctuation fiber (when the length is 40 km) in which chromatic dispersion increases linearly along the longitudinal direction of the optical fiber. 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).

Assuming that the chromatic dispersion fluctuation width is 4 ps / nm / km, the generation efficiency of FWM is 1/10 or less as compared with a normal optical fiber whose chromatic dispersion fluctuation width is 0, and a sufficient suppression effect can be obtained. It is understood that it is possible. Furthermore, the suppression effect is increased by increasing the chromatic dispersion fluctuation width. Further, as can be seen from FIG. 10, when the chromatic dispersion fluctuation width is 20 ps / nm / km, the generation efficiency of FWM is reduced to about 0.5.
03ps, 24ps / nm / km
In the case of (1), it can be expected that a greater FWM suppression effect can be obtained.

FIG. 12 shows the core diameter satisfying the single mode condition when the relative refractive index difference α between the core and the third cladding is changed, and the relative refraction between the first cladding and the third cladding. FIG. 6 is a diagram illustrating a relationship with a rate difference β. The shaded area in the figure is an area where a single mode cannot be realized at a wavelength of 1.55 μm. It can be seen that the smaller the α is, the more combinations of the core diameter and β satisfying the single mode condition.

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 extended to 1.55 μm and the Aeff can be extended to about 150 μm ^2. Was. FIG. 14 shows a change in the zero-dispersion wavelength when the relative refractive index difference is changed while maintaining the refractive index relationships 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 band of 1.3 μm to 1.6 μm and a longer wavelength band by changing the combination of the relative refractive index differences.

Considering the handling characteristics when the optical fiber is actually used in a transmission line or the like, the optical fiber needs to have a small bending loss. In the optical fiber of this example, the bending loss characteristics 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 the present embodiment. It can be seen from the figure that the bending loss decreases as β increases. In the optical fiber of the present embodiment,
By setting β to about 1.5%, the same as the conventional DSF (about several dB / m at a bending radius of 1 cm, but Aeff
Can obtain a bending loss characteristic of about 50 μm ² ), and the Aeff at that time is about 120 μm ² .

[0043]

According to the optical fiber of the present invention, the electric field distribution does not concentrate on the center of the optical fiber even if the zero dispersion wavelength is designed to be in the 1.55 μm band. While maintaining the loss characteristics at the same level, the Aeff
Was 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 increased. Also, by varying the zero dispersion wavelength in the 1.5 μm band along the longitudinal direction of the optical fiber, the phase matching condition between the wavelengths is disturbed, the generation efficiency of FWM can be reduced, and the wavelength interval of WDM can be reduced. Thus, the number of channels can be increased. On the other hand, by reducing the Aeff, the electric field density in the optical fiber can be increased, and the optical nonlinear phenomenon can be efficiently generated. Therefore, an optical fiber suitable for an optical switch or a wavelength conversion device is provided. be able to.

[Brief description of the drawings]

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

FIG. 2 shows a relative refractive index difference α between a core and a third cladding;
4 is a graph showing a relationship with chromatic dispersion.

FIG. 3 is a graph showing a relationship between a relative refractive index difference β between a first clad and a third clad and chromatic dispersion.

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

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

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

FIG. 7 is a cross-sectional view of an optical nonlinear phenomenon-inducing optical fiber according to the present invention, and a graph showing a refractive index distribution thereof.

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

FIG. 9 shows the ratio of the core diameter to the diameter of the first clad and the MFD.
6 is a graph showing the relationship of.

FIG. 10 is a graph showing the relationship between the chromatic dispersion fluctuation width 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 a relationship between a relative refractive index difference β between a first clad and a third clad and a core diameter.

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

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

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

[Explanation of symbols]

1 ... core, 2 ... first cladding, 3 ... second cladding,
4 Third clad 5 Transmitter 5a Light source 5b
6, 7b: optical amplifier, 7: receiver, 7a: light receiver, 8 ...
Optical nonlinear phenomenon suppressing optical fiber, 9 ... Existing optical fiber.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Yoshiaki Miyajima 3-19-2 Nishishinjuku, Shinjuku-ku, Tokyo Nippon Telegraph and Telephone Corporation

Claims (9)

[Claims] 1. A core of refractive index n _0, a first cladding having a refractive index of n ₁ is formed around the core, the refractive index is formed around the first cladding n ₂ And a third cladding formed around the second cladding and having a refractive index of n ₃ , wherein the refractive index is n ₁ > n ₂ >
an optical fiber characterized by having a relationship n ₃ and n _1> n _0. 2. The optical fiber according to claim 1, wherein the refractive index further has a relationship of n ₃ ≧ n ₀ . _3. The optical fiber according to claim 1, wherein the refractive index further has a relation of n ₀ > n ₃ . 4. The optical fiber according to claim 1, wherein a relative refractive index difference α between the core and the third cladding changes continuously along a longitudinal direction of the optical fiber. An optical fiber. 5. The optical fiber according to claim 1, wherein a relative refractive index difference β between the first cladding and the third cladding changes continuously along a longitudinal direction of the optical fiber. An optical fiber, comprising: 6. The optical fiber according to claim 1, wherein a diameter of the second cladding is configured to change continuously along a longitudinal direction of the optical fiber. 7. The optical system according to claim 4, wherein a relative refractive index difference α between the core and the third cladding is configured to monotonically increase or decrease along a longitudinal direction of the optical fiber. Characteristic optical fiber. 8. The optical fiber according to claim 5, wherein the relative refractive index difference β between the first cladding and the third cladding monotonically increases or decreases along the longitudinal direction of the optical fiber. An optical fiber. 9. The optical fiber according to claim 6, wherein the diameter of the second cladding monotonically increases or decreases along the longitudinal direction of the optical fiber.