JPS638707A - Dispersion shift type optical fiber - Google Patents

Abstract

PURPOSE:To decrease the chances of the incorporation of an OH group into a fiber in actual production and to obviate the increased absorption loss by the OH group by forming the fiber in such a manner as to have the low- refractive index part where the refractive index is not lower than the refractive index of the pure quartz glass sandwiched by plural high-refractive index parts and to have a clad part where the refractive index is lower than the refractive index of the pure quartz glass enclosing the core part. CONSTITUTION:This fiber has the high-refractive index part 2 doped with germanium in the central part of the core part 1, the low-refractive index part 3 around the same and further the high-refractive index part 4 doped with germanium around the low-refractive index part 3. The clad part 5 of the low refractive index is provided around the core part 1 so as to enclose the same. The low-refractive index part 3 sandwiched by the two high-refractive index parts 2, 4 in the core part 1 consists of the pure quartz glass or the quartz glass doped with a small amt. of the germanium and the refractive index thereof is not lower than the refractive index of the pure quartz glass. The clad part 5 has the refractive index lower than the refractive index of the pure quartz glass by doping fluorine thereto.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to a silica-based optical fiber that is optimal for long-distance, low-loss, broadband optical fiber transmission systems, and particularly relates to 1.4.
This invention relates to a dispersion-shifted optical fiber that has low chromatic dispersion over a wide wavelength range of wm=1.74 m.

2. Description of the Related Art As is well known in the past, the low loss wavelength range of silica-based optical fibers is from 1.4 μm to 1.4 μm. It is in the full m wavelength range (preferably in the wavelength range of 1.5 μm to 1.6≠m). In other words, this wavelength range is most suitable for long-distance non-repeater optical transmission; for example, non-repeater transmission over a distance of about 250 km can be achieved using a laser diode with an output of about 10 mW as a light source for an optical fiber transmission system. be.

On the other hand, regarding the transmission speed, since the wavelength of the light source is not a single spectrum, there is a problem of distortion of the transmission waveform due to so-called chromatic dispersion. The first most commonly used
In a single mode fiber with a core diameter of about 10 μm and a relative refractive index difference Δ=0.3% between the core and the craft, as shown in Figure 1,
The chromatic dispersion is like the curve shown by the solid line in FIG. 12, and becomes zero at a wavelength of 1.3 μm, but takes a large value at other wavelengths. For example, the chromatic dispersion value at a wavelength of 1.55 μm, where the transmission loss is lowest, is about 17 to 20 psec/km/nm in this conventional single mode fiber. This means that the spectral width of the light source is 1 nm, and the spread of the optical pulse per 1 km of fiber length is 17 to 20 ps.
This means that there is ec. In this case, for example, the spectral width of the light source is 4 nm (full width at half maximum), 1100k transmission,
The wavelength dispersion of optical fiber is 20 psec/Km/
nm, the optical pulse width is 8 nsec, which means that only about 60 Mbit/sec of information can be sent at most.

In order to improve this, a dispersion-shifted optical fiber was devised in which the wavelength dispersion curve was shifted as shown by the dashed line in FIG. 12. This dispersion shifted optical fiber has zero dispersion wavelength and low loss? In order to shift the tF to the 1.5 JLm band, the refractive index distribution must be devised, for example, as shown in FIGS. 13A to 13D.

By the way, the wavelength dispersion of an optical fiber is generally expressed as the sum of (1) material dispersion and (2) waveguide dispersion, as shown in FIG. 14, but (2) material dispersion is not greatly influenced by the refractive index distribution of the optical fiber. Therefore, as shown in Figure 14, around 1.55 pm, the waveguide dispersion is set to have a different sign and the same absolute value as the material dispersion, and the wavelength dispersion is set to zero around 1.55 μm (see Figure 14). solid line). For this purpose, in reality, if Δ is small, a sufficiently large waveguide dispersion cannot be obtained, so it is necessary to set Δ to 0.65% or more.
Similar parameters are used for the optical fiber having the refractive index distribution shown in FIG.

However, such a low-loss, low-dispersion optical fiber as described above cannot be used in practice by itself. That is,
In addition to these characteristics, as with general optical fibers, it is necessary to (1) be able to form a cable stably, and (2) be able to connect with low loss.

Optical fibers need to be able to be installed in the actual environment of use and are therefore cabled. That is, the optical fibers are assembled together with other cable constituent materials and provided with an outer sheath (protective sheath) to protect against pulling forces on the cable and lateral forces on the cable.

At this time, it may be bent. However, optical fibers originally have a small outer diameter, are flexible, and are easily bent.

Therefore, depending on the settings of the fiber parameters, when the fiber is bent once, the light may not be sufficiently confined within the core and may be emitted. Qualitatively speaking, in a dispersion-shifted optical fiber /<, when the zero dispersion wavelength is constant, the larger the relative refractive index difference between the core and the cladding, the smaller the increase in loss due to bending for the same bending radius.

On the other hand, when connecting optical fibers to each other, it is advantageous to have a larger mode field diameter.

Here, the mode field diameter is an index indicating how light energy spreads within the cross section of an optical fiber, but its size does not match the core diameter. In so-called single-mode fibers, the propagating energy does not propagate only in the core, but 10% to 50% of the energy propagates in the cladding, so in principle, unlike dispersion-shifted optical fibers, the propagating energy does not propagate only in the core. In the case of optical fibers that tend to have weak light confinement, the mode field diameter is larger than the core diameter. However, in the case of a dispersion-shifted fiber, the core diameter is set from the beginning to be smaller than that of a normal 1.3 pm zero-dispersion fiber, so the final mode field I''13 cannot be made that large. What is the most common connection between optical fibers?

This is the eccentricity of the core, and the amount of eccentricity is usually expressed by how far the center of the core deviates from the center of the outer diameter of the optical fiber. If the amount of eccentricity is large, when two optical fibers are butted together with the same outer diameter, the centers of the cores will not match, and the light from one optical fiber will not be transmitted to the other optical fiber and will be radiated outside. This is connection loss. Further, even if the amount of eccentricity is the same, if the mode field diameter of the optical fiber that continues for tS is small, the amount of light transmitted will be relatively reduced, resulting in a large splice loss. That is, in order to connect dispersion-shifted optical fibers, it is better to have a larger mode field diameter for the same zero dispersion wavelength. In the case of the same zero dispersion wavelength, it is effective to reduce the relative refractive index difference between the core φ and the cladding in order to increase the mode field diameter.

In this way, on the one hand, in order to reduce bending loss, it is necessary to increase the relative refractive index difference between the core and cladding, and on the other hand, in order to increase the mode field diameter and reduce connection loss, it is necessary to increase the relative refractive index difference between the core and cladding. The two contradict each other, as it is necessary to reduce the relative refractive index difference between them.

Therefore, as shown in FIG. 15, a structure having a refractive index distribution in which the core part has a high refractive index part near the center and a high refractive index part exists around it with a low refractive index part sandwiched therebetween. is proposed. In other words, when we examine the relationship between the bending loss characteristics and mode field diameter of dispersion-shifted optical fibers with various refractive index distributions, we find that in so-called unimodal refractive index distributions, both triangular and step-type refractive index distributions have almost the same curve. It was found that as long as the zero dispersion wavelength is the same and the mode field diameter is the same, the bending loss characteristics are essentially the same even if the peak refractive index is different (see Figure 2). Here, the mode field diameter is "(1/e) diameter of the light beam where the light is excited most efficiently when the optical fiber is excited with a light beam having a Gaussian intensity distribution in the radial direction." is defined in In Figure 2, α is a parameter that determines the refractive index distribution of the optical fiber, α = 1 is a triangular refraction distribution, α =
(2) corresponds to a so-called step-type refractive index distribution. In this figure, the bending loss characteristics of the optical fiber are representatively shown as the loss per optical fiber in when the optical fiber is bent to a diameter of 20 mm. From the inventors' past experience, the bending loss at a straight line of 1120 mm must be approximately 30 dB/m in order to be able to form a cable relatively easily.
Must be below. In other words, from the data of the unimodal refractive index distribution shown in Figure 2, conventionally, when the refractive index at the center of the core is high and the refractive index around the core is low, as in the case of a triangular or square distribution, the mode It was thought that it would be possible to increase the mode field diameter even when the electromagnetic field distribution was widened and the zero dispersion wavelength was constant, but in fact, contrary to this intuitive perception, the mode field diameter is unimodal. It turns out that the limit is 50 steps or 100 steps.

Therefore, instead of a single-modal refractive index distribution like this,
This is why a bimodal refractive index distribution as shown in FIG. 15 was proposed.

Problems to be Solved by the Invention However, in the case of a refractive index distribution as shown in FIG. 15, there are the following problems. In other words, in order to achieve such a refractive index distribution, the low refractive index part sandwiched between the high refractive index parts and the surrounding low refractive index part are made of pure silica glass,
Either the high refractive index part is made of quartz glass doped with germanium, or the low refractive index part sandwiched between the high refractive index part and the surrounding low refractive index part are made of fluorine-doped quartz glass. The part must consist of germanium-doped quartz glass. However, in case (2), the high refractive index portion is doped with a large amount of germanium, and this dopant increases concentration fluctuations in the glass, increasing Ray scattering, which is disadvantageous in terms of loss in the optical fiber.

In addition, in the case of (2), it is effective as a method to reduce such Ray scattering, but on the other hand, when considering actually producing an optical core ino, it is necessary to dope each layer with germanium and fluorine alternately. Not only is this not so easy, but it is also likely to lead to an increase in OH group absorption loss.

To explain this case (2) a little more, first, when fluorine and other dopants are mixed between each layer, the increases and decreases in the refractive index cancel each other out, so it is not easy to obtain the desired refractive index. That is, when producing an optical fiber using fine glass powder, fluorine mixed into the fine glass powder is extremely unstable and moves relatively easily through the gaps between the particles of the fine glass powder. When fluorine moves to areas already doped with germanium, the effects of germanium, which is a dopant that increases the refractive index, and fluorine, which is a dopant that decreases the refractive index, cancel each other out, so that the desired refractive index difference can be finally obtained. This is because it becomes impossible to do so. Therefore, a possible method is to deposit glass fine powder in each layer of glass with different types and concentrations of dopants and make it transparent vitrified to produce a transparent glass rod, and then layer glass containing a specified dopant on top of it. The process of depositing fine powder and turning the fine glass powder into transparent vitrification is repeated. At this time, fluorine doping can be performed by various methods.

First, a fluorine-containing gas is included in the glass raw material.
Second, a gas containing fluorine is used during the dehydration process to remove moisture mixed into the fine glass powder (gases used for this include CF4, SF, etc., and an atmosphere containing these gases is used). The fine glass powder preform is heat-treated in a heat treatment chamber, and the temperature is usually set at a temperature that does not cause the fine glass powder preform to become transparent vitrified). and so on.

Next, regarding the increase in absorption loss due to OH groups, there is no method for manufacturing optical fibers by repeating the process of depositing fine glass powder on a glass rod that has been made transparent many times. , there is a great risk of increased optical fiber loss. In other words, as mentioned above, even if a transparent glass is placed in a fluorine-containing atmosphere, fluorine will not penetrate into the transparent glass in an amount that would substantially affect the refractive index distribution. This is because there is a risk of contamination with OH groups. As for this OH group, even a very small amount tends to cause an increase in loss. In other words, as shown in Figure 16, the amount of fluorine required to lower the refractive index of silica glass by 0.1% is about 0.3 moles, whereas 1 ppm of OH groups in the glass Even if it is mixed, the absorption loss at a wavelength of 1.38 yen increases to about 50 dB/km,
The tail of the absorption loss causes an increase in loss of several dB/km for wavelengths of 1.3 m and 1.55 μm, which are required for optical communications. For example, as a reference example, in order to create a core with the refractive index distribution as shown in Figure 15, the series of steps of depositing glass fine powder on a transparent glass rod and turning it into transparent glass was repeated three times, as shown in Figure 17. The loss wavelength characteristics shown in Figure 1 were obtained. As shown in FIG. 17, wavelength 1.
The loss at 38 pm is as large as approximately 15 dB/km. This is done by depositing fine glass powder on each layer of the core.
Because the transparent vitrification process was repeated three times, it is thought that the contamination of OH groups from the heating source used at that time and the atmosphere around the transparent glass rod accumulated, resulting in the large loss as described above. This increase in loss means that the currently required optical fiber loss in this wavelength range is 0.5 dB/
Considering that it is less than km, it must be said that it is extremely large.

An object of the present invention is to provide a dispersion-shifted optical fiber that achieves minimizing bending loss and increasing the mode field diameter in order to reduce splicing loss without increasing OH group absorption loss. do.

Means for Solving the Problems The dispersion-shifted optical fiber of the present invention has quartz glass as its main component, and its radial refractive index 41 has two or more high refractive index portions in the core portion and the plurality of high refractive index portions. has a low refractive index part with a refractive index not lower than the refractive index of pure silica glass sandwiched between high refractive index parts of the core part, and has a refractive index lower than the refractive index of the pure silica glass surrounding the core part. It is formed to have a cladding part of
The core diameter and refractive index difference are adjusted so that the wavelength is greater than or equal to the wavelength.

In the working core part, the low refractive index part sandwiched between multiple high refractive index parts has a refractive index that is not lower than that of pure silica glass, so there is no need to dope this part with fluorine. , the core part can be easily produced in one glass fine powder deposition process. Not only that, but
The core part is produced in one glass fine powder deposition/transparent glass process, and the fine glass powder forming the cladding part can be deposited on the glass rod of the core part thus produced. Even if OH groups are mixed in during the deposition process, they hardly enter the core, and O
H group absorption loss does not increase.

Embodiment A dispersion-shifted optical fiber according to an embodiment of the present invention is as follows:
The main component is quartz glass, and the refractive index distribution in the radial direction is formed as shown in FIG. That is, the core part 1 has a germanium-doped high refractive index part 2 in the center, a low refractive index part 3 around it, and a germanium-doped high refractive index part 3 around the low refractive index part 3. It has a refractive index portion 4. A cladding portion 5 having a low refractive index is provided around the core portion 1 so as to surround it. Two high refractive index portions 2 in the core portion 1.
The low refractive index portion 3 sandwiched between the two portions 4 is made of pure quartz glass or quartz glass doped with a small amount of germanium, and has a refractive index not lower than that of pure quartz glass. The cladding portion 5 is doped with fluorine so as to have a refractive index lower than that of pure silica glass. Moreover, the core diameter and the refractive index difference are adjusted so that substantially a single mode propagates and the wavelength dispersion becomes a wavelength of 1.4 μm or more. That is,
According to experiments, the refractive index differences Δ1, Δ2, Δ3 shown in FIG.
The parameters of and the parameters of diameters 2a, 2b, and 2c are: Δ3/Δl=0.1~0.6 Δ2/Δl=0.1~0.4 b/a=1.3~2.5 c/a= 1.5 to 4 was effective.

When the relationship between the bending loss characteristics and the mode field diameter of the dispersion-shifted optical fiber according to an embodiment of the present invention was investigated, the results shown in FIG. 2 were obtained. As can be seen from Fig. 2, according to the dispersion-shifted optical fiber with the refractive index distribution shown in Fig. 1, the zero dispersion wavelength is 1.55 μm.
When the mode field diameter is 8 μm, the bending loss is improved to about 115 compared to the step type. This improvement in bending loss characteristics is thought to be due to the provision of multiple peaks in the refractive index distribution and the effect of widening the mode field diameter while maintaining a state in which the propagation constant of the propagating mode does not decrease significantly. Considering that, as mentioned above, in a single-modal refractive index distribution, the mode field diameter is not affected even if the distribution is changed considerably, the effect of this multi-flavored distribution is significant. Further, since the amount of high refractive index dopant can be reduced in this refractive index distribution as a whole, it is expected that Ley scattering loss will be reduced compared to the refractive index distribution shown in FIG.

Although the distribution pattern is basically as shown in Figure 1,
As a slightly modified version of this, the one shown in FIGS. 3A to 3H can be considered. That is, the refractive index at the center may be concave as shown in FIG. 3A, or as shown in FIGS. 3B, C,
As shown in D, the refractive index at the center may be entirely or partially graded, such as triangular, Gaussian, or trapezoidal. Furthermore, the boundaries between each layer may not be clear and there may be some sag. Alternatively, there may be three or more layers of high refractive index portions (FIG. 3F), or the distribution as shown in FIGS. 3G and H may be used.

In order to manufacture a dispersion-shifted optical fiber with such a refractive index distribution, in addition to the so-called MCVD method and the external method, VAD
Law is considered. In the case of the MCVD method, since the refractive index distribution in the radial direction of the optical fiber is controlled in the radial direction, the above-mentioned Soshi type distribution can be easily produced without special consideration. In particular, in the case of the external method that conventionally uses a mandrel to deposit fine glass powder, a pure quartz glass rod is used instead of the mandrel, and this mandrel is not pulled out after the glass fine powder deposition process. If you keep it,
This is advantageous because it is possible to easily create a distribution having a concave shape at the center as shown in FIG. 3A.

Next, a dispersion-shifted optical fiber having the refractive index distribution shown in FIG. 1 was actually fabricated using the VAD method. In order to produce an optical fiber having such a refractive index distribution with low loss, it is advantageous to produce the core portion in one step of depositing fine glass powder. Therefore, a general VAD single mode fiber base material manufacturing apparatus equipped with multiple burners as shown in FIG. 4 was used. In FIG. 4, three burners 41 to 43 generate fine glass powder, which is deposited in the axial direction to form a fine glass powder sintered body 44. Unnecessary gas and the like are exhausted through the exhaust pipe 45. A raw material gas containing germanium as a dopant gas is passed through the burner 41 disposed at the bottom, and a main raw material gas such as silicon tetrachloride that generates fine powder of silica glass through oxidation or hydrolysis is supplied.

Only the main raw material gas is supplied to the burner 42 without the dopant gas, and the raw material gas containing the dopant is supplied to the burner 43. Specifically, at the flow rates shown in the first table,
Oxygen/hydrogen gas, which is a raw material gas and a combustion/heating gas, was supplied to each of the burners No. 43 to 43 together with Ar gas (carrier gas) (flow rate unit: cc/min).

Burner Ar 5iCI 4 GeC14 Oxygen
Hydrogen The thus produced glass fine powder sintered body 44 is heated at approximately 800°C in a helium gas atmosphere containing chlorine gas.
Dehydration treatment was performed at a temperature of 1,640° C., and transparent vitrification was performed in a helium atmosphere at a temperature of 1,640° C. to produce a core preform.

The refractive index distribution of this core preform is as shown in Figure 5.This core preform is stretched to a diameter of about 10 mm, and a fine glass powder made of pure quartz containing no dopants is placed on top of it. was deposited. The deposition conditions were as follows.

Carrier gas (Ar) 250 secmSil
l 4 230 secm Seal gas (A r) 400 sccm Oxygen
40ooscc11 hydrogen 40
00 secThe thus obtained composite preform was CF
By heating in a helium gas atmosphere containing 4% by volume of 4, the 01 group in the deposited layer of fine glass powder was removed and fluorine was doped into the glass fine powder, which was becoming transparent. In this way, the entire glass powder became transparent. A second-stage preform was obtained, and its refractive index distribution was measured, as shown in Figure 6.The ratio of the outer diameter to the core diameter of the preform obtained here was approximately 4, This preform is drawn in a heating furnace and has a standard optical fiber outer diameter of 12
5 pm, the zero dispersion wavelength could not be made greater than the predetermined value of 1.44 m, so the preform was stretched once more and pure silica glass fine powder was deposited on it.
The process of turning this into transparent glass in a fluorine-containing atmosphere was repeated. As a result, the ratio of the outer diameter of the preform to the core diameter was within an appropriate range. The thus obtained preform was drawn into a fiber in a furnace at about 2000° C. to obtain an optical fiber.

The refractive index distribution of the optical fiber thus obtained was as shown in FIG. Furthermore, the loss wavelength characteristics and wavelength dispersion characteristics of the optical fiber were as shown in FIGS. 8 and 9, respectively. The loss at a wavelength of 1.55 μm is as low as 0.21 dB/km, which is a value sufficiently suitable for long-distance non-repeater transmission of look or more. Also, the loss at a wavelength of 1.38 μm is approximately 3
dB/km, which is a value at which the influence of absorption loss of OH groups does not appear in transmission in the 1.5 pLm band. In contrast, in order to form the conventional refractive index distribution as shown in Figure 15, fine glass powder was deposited and transparent vitrified for each layer of the core three times.
In the reference example that was repeated twice, the loss at a wavelength of 1.38° was approximately 15 dB/km, as shown in Figure 17, as described above.
This is a large number, and when compared with this reference example,
It can be seen that the dispersion shifted optical fiber of this invention has very low loss. Of course, this dispersion-shifted optical fiber is
．． Other wavelengths such as 3 μm can also be used. moreover,
The wavelength dispersion value at a wavelength of 1.55 pm is approximately 1 psec
/km/nm, small, 200km, 400M
It has characteristics that can withstand bit/sea transmission. Further, when the bending loss was measured, it was found to be relatively low at 10 dB/m at a wavelength of 1.55 m. Despite the low bending loss, the mode field diameter was as large as 8.5 m as calculated.

In addition, in the case of the VAD method, instead of using a large number of burners as described above, a single reciprocating burner 51 is used, as shown in FIG. The fine glass powder preform 52 may be made while forming the refractive index distribution of the core part by using the same method. After stretching to a desired outer diameter, fine glass powder made of pure quartz glass is deposited thereon.Next, the composite preform of the transparent glass rod and the deposited layer of fine glass powder is treated in a heating furnace. Then, the layer of fine glass powder 8a is made transparent and vitrified, and fluorine is doped into the cladding. If the cladding/core diameter ratio is insufficient, repeat the above steps of stretching, glass fine powder deposition, and heat treatment, as shown in Figure 3H.
When forming a refractive index distribution like A thick-walled glass tube may also be coated.

Effects of the Invention In the dispersion-shifted optical fiber according to the present invention, the bending loss is suppressed to a low level, and the mode field diameter is increased, so that the splice loss is decreased. Moreover, in actual production, there is little chance of OH groups being mixed in, and an increase in absorption loss due to OH groups does not occur.

[Brief explanation of drawings]

FIG. 1 is a graph showing the refractive index distribution of a dispersion-shifted optical fiber according to an embodiment of the present invention, FIG. 2 is a graph showing the relationship between mode field diameter and bending loss, and FIG. 3 is a graph showing the relationship between mode field diameter and bending loss.
, B, C, D, E, F, G, and H are graphs representing the refractive index distributions of modified examples, FIG. 4 is a schematic diagram showing the manufacturing method by the VAD method of the example, and FIG. A graph showing the refractive index distribution of the actually obtained core preform. Figure 6 is a graph showing the refractive index distribution of the preform obtained by depositing glass fine powder on the core preform and then converting it into transparent glass. , FIG. 7 is a graph showing the refractive index distribution of the finally obtained dispersion-shifted optical fiber, and FIG.
9 and 9 are graphs showing the loss wavelength characteristics and wavelength dispersion characteristics of the optical fiber obtained in this example, respectively.
Figure 0 is a schematic diagram showing an example of another manufacturing method, Figure 11 is a refractive index distribution diagram of a typical conventional single mode fiber,
Figure 12 is a graph showing conventional examples of transmission loss characteristics and wavelength dispersion characteristics. Figure 13 A, B, C, and D are graphs representing refractive index distribution of conventional dispersion-shifted optical fibers. Figure 14 is waveguide dispersion. Fig. 15 is a graph showing the refractive index distribution of another conventional dispersion-shifted optical fiber, and Fig. 16 shows the relationship between the fluorine concentration in silica glass and the refractive index. FIG. 17 is a graph showing the loss wavelength characteristics of a dispersion-shifted optical fiber of a reference example. 1... Core part? , 4...high refractive index section 3
...Low refractive index part 5...Clad part 41-4
3.51...Burner 44.52...Glass fine powder sintered body 45...Exhaust pipe

Claims (2)

[Claims] (1) Mainly composed of silica glass, with two or more high refractive index parts in the core part, and a low refractive index that is not lower than the refractive index of pure silica glass sandwiched between the multiple high refractive index parts. The refractive index distribution in the radial direction is formed so as to have a refractive index portion and a cladding portion having a refractive index lower than the refractive index of pure silica glass surrounding the core portion. A dispersion-shifted optical fiber characterized in that a core diameter and a refractive index difference are adjusted so that a single mode propagates and the wavelength dispersion becomes a wavelength of 1.4 μm or more. (2) Germanium as the main dopant in the core,
2. The dispersion-shifted optical fiber according to claim 1, wherein fluorine is used as a dopant in the cladding portion.