JP2003202445A - Photonic crystal optical fiber and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a PCF whose loss is reduced by reducing loss of the PCF caused by impurities. <P>SOLUTION: The PCF comprises a hollow core 2 whose external diameter is nearly several times as large as the wavelength of light and a clad 4 which is formed around the core 2 and has a plurality of holes 3 formed to constitute a diffraction grating in photonic band-gap structure around the core 2 adjacently, and the hollow core 2 and the plurality of holes 3 of the clad 4 are evacuated. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photonic crystal optical fiber having a photonic bandgap structure, and more particularly, to a photonic crystal having a hollow core and a void in a clad, which has a reduced loss. It relates to a crystal optical fiber.

[0002]

2. Description of the Related Art A conventional optical fiber has a two-layer structure of a core having a high refractive index and a clad having a refractive index slightly lower than that of the core, and its material base is quartz. The core is
In order to make the refractive index slightly higher than that of the clad, quartz has a composition in which an additive such as germanium is added.

In the conventional optical fiber, since the refractive index of the core of the optical fiber is higher than that of the cladding, the light entering the optical fiber is confined in the core by the difference in the refractive indexes and propagates in the optical fiber. can do.
In order to satisfy the single mode condition of the propagating light, the diameter of the core is as small as 5 to 10 μm.

However, in recent years, due to the development of optical amplification technology and wavelength division multiplexing (WDM) technology, the power of light incident on an optical fiber has increased, and various non-linear effect phenomena are likely to occur.

For example, when the self-phase modulation phenomenon, which is one of the nonlinear effect phenomena, occurs, the pulse signal waveform in the optical fiber is distorted and the transmission capacity is limited. In addition, the Brüllian scattering phenomenon, which is also a non-linear phenomenon, is likely to occur.
When this phenomenon occurs, the incident power of the optical fiber diameter is saturated.

When these non-linear effect phenomena occur, the transmission characteristics propagating in the optical fiber are deteriorated. Moreover, even the best transmission loss of the current optical fiber is 0.16 dB /
The main factor is Rayleigh scattering loss due to fluctuations in composition density of the core where light propagates and the clad near the core, but it is much lower in optical fibers used for long-distance transmission between continents. Is desired.

[0007]

By the way, as an optical fiber for solving the problems of the conventional optical fiber, a photonic crystal optical fiber (PCF: Photon i) is used.
c Crystal Fiber) has recently received a lot of attention. PCF is a fiber with a photonic crystal structure provided in the cladding. The photonic crystal structure is a periodic structure of refractive index. Specifically, by providing a honeycomb-like space such as a honeycomb in the clad portion, a photonic band gap (PBG) that is an energy forbidden band of light is obtained. : Photonic Band Ga
p) occurs.

For example, Knight et al., Science
e282, 1476, (1999), reported a PCF using PBG as a guiding principle, and Creg.
an et al., Science 285, 1537, (199
In 9), a hollow core PCF having a PBG structure as a guiding principle is reported. The hollow core PCF shows the possibility of an ultra-low-loss fiber in which Rayleigh scattering, which is the main cause of loss, is extremely small because there is no quartz medium in the core through which light propagates.

Further, Japanese Patent No. 3072842 also discloses a hollow core PCF,
It is described that a loss reduction of about 0.01 dB / km can be expected.

However, in these conventional PCFs, the core in which light propagates and the vicinity thereof form a space in which Rayleigh scattering is small, but the gas existing in the space (in this case, the space is usually filled with air). It has a poor purity. That is, if impurities are present in the space, there is a problem that absorption loss for the impurities occurs.

For example, in the manufacturing process of an optical fiber preform, since molding work by an oxyhydrogen flame is frequently performed, moisture due to the reaction of oxygen and hydrogen remains in the preform, and the air in the hollow portion of the PCF is removed. May be included in. There is a problem that a part of this moisture diffuses into the glass during drawing of the preform, which causes an increase in absorption loss due to molecular vibration of OH groups.

Therefore, an object of the present invention is to provide a PCF in which the loss due to impurities in the PCF is reduced to reduce the loss.

[0013]

The present invention was devised to achieve the above object, and the invention of claim 1 is
A hollow core having an outer diameter of several times the wavelength of light and a plurality of holes formed around the core and forming a photonic bandgap diffraction grating are formed at least around the core. A photonic crystal optical fiber including a clad, wherein the hollow core and a plurality of holes in the clad are evacuated.

According to a second aspect of the present invention, a preform for a photonic crystal optical fiber is provided with a core hole which becomes a hollow core after drawing and a plurality of holes forming a diffraction grating having a photonic bandgap structure after drawing. A plurality of clad holes to be formed, and these core holes and a plurality of clad holes are filled with He gas in advance and hermetically sealed, and then the preform is drawn, and the vacuum of the optical fiber after drawing is drawn. This is a method for producing a photonic crystal optical fiber in which a He gas is extracted by performing a treatment to make a vacuum in a hollow core and a plurality of holes in a clad.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 is a sectional view of a photonic crystal optical fiber (PCF) which is a preferred embodiment of the present invention.

As shown in FIG. 1, a PCF 1 according to the present invention.
Is a hollow core 2 arranged in the center and having an outer diameter φc of 5 to 50 μm, and a plurality of holes 3 formed around the core 2 and having a diameter slightly smaller than the outer diameter φ of the PCF 1. Arranged in a photonic bandgap (PBG)
It is composed of a clad 4 constituting a diffraction grating having a structure and a jacket 5 covering the outer periphery of the clad 4, and the inside of the hollow core 2 and the plurality of holes 3 of the clad 4 are evacuated. The outer diameter φ of PCF1 is 80 to 150 μm.

The hollow core 2 is formed by forming holes having a hexagonal cross section along the longitudinal direction of the PCF 1, and since the inside is kept in a vacuum state, the refractive index is 1. There is. The core 2 which is hollow and kept in a vacuum state is the most desirable form for lowering the loss of the optical fiber, because there is no light scattering factor and light absorption loss due to molecular vibration of the OH group.

The clad 4 has a substantially hexagonal cross section. The clad 4 constitutes a diffraction grating having a PBG structure by arranging a plurality of holes 3 having a circular cross section formed along the longitudinal direction of the PCF 1 around the core 2 in a honeycomb shape. Since the insides of the plurality of holes 3 are also kept in a vacuum state, the refractive index of the cladding 4 is also 1.

In the PCF 1, since the core 2 where the signal light energy is most concentrated is hollow and the vacuum is small, there are few optical loss factors, and it is possible to achieve a much lower loss than the conventional optical fiber loss level of 0.2 dB / km. Expected, 0.01d
There is a possibility of B / km level.

Since the cladding 4 of the PCF 1 has the PBG structure, the light is confined in the core 2 and propagates in the longitudinal direction of the optical fiber without leaking in the radial direction of the fiber due to the Bragg reflection of the diffraction grating.

In the PCF 1, the hollow honeycomb structure clad 4 is also kept in a vacuum state. Therefore, the loss factor when light propagates in the cladding 4 while repeating Bragg reflection is also reduced.

Next, a method of manufacturing the PCF 1 according to the present invention will be described.

In manufacturing the PCF1, first, the PC
As a member to be a preform for F, for example, a pure quartz base material having high purity is used for VAD (Vapor Phase).
It is manufactured by the Axial Deposition method. When an ordinary optical fiber is manufactured by the VAD method, quartz is doped with germanium or the like as an additive in order to increase the refractive index of the core portion.
In No. 1, since light is propagated by the PBG structure, no additive for increasing the refractive index is used. However, PB
Even with the G structure, since the silica glass portion is also present in the region near the core through which light propagates, countermeasures against OH groups remaining in the quartz were taken.

Specifically, when the soot base material produced by the VAD method is sintered and vitrified into a transparent atmosphere in a chlorine gas atmosphere, a pure quartz base material having an OH group content of 0.1 ppm or less is obtained. As shown in Fig. 2, this pure quartz base material was
After stretching so that 21 becomes 80 mm, the length is 200
Preform 21 for PCF cut to mm
And

When the preform 21 is drawn into a fiber to form a fiber, a core hole 22 having an outer diameter φ22 of 5 mm is formed in the center of the cross section of the preform 21.
It is formed by punching with an ultrasonic drill.
Similarly, when the preform 21 is drawn around the core hole 22 to form a fiber, a plurality of cladding holes 23 having an outer diameter φ23 of 2 mm are formed so as to form a diffraction grating having a PBG structure. An ultrasonic drill is used to perform a hollowing process to form a honeycomb shape. After this, in order to facilitate the drawing process, the preform 21 is
Stretching is performed so as to be about mm.

Next, the core hole 22 and the plurality of clad holes 23 of the preform 21 after the drawing process are processed to fill with He gas.

Specifically, as shown in FIG. 3A, one end 21a of the preform 21 is sealed, and the other end 21b is connected to a quartz tube 30 having the same outer diameter as the preform 21, A vacuum pump 31 connected to the quartz tube 30 sucks air existing in the core hole and the plurality of clad holes of the preform 21 to bring the inside of the preform 21 into a vacuum state.

As shown in FIG. 3B, the same quartz tube 30 is used for the core hole 22 and the plurality of clad holes 23 in a vacuum state, and He gas is used for the core hole and the plurality of clad holes. And then, a part of the quartz tube 30 is sealed and cut.

At this time, the He gas pressure in the preform 21 is preferably about atmospheric pressure. The reason is that if the inside of the preform 21 is in a vacuum state from the beginning, the core hole and the plurality of cladding holes inside the preform 21 are crushed during drawing in the next step, and the PBG structure cannot be realized. On the other hand, if He gas is left in the preform 21 in a high pressure state, He gas will not be drawn during drawing.
The jacket layer of the fiber cannot withstand the gas pressure due to the expansion of the gas and the outer diameter fluctuates.
This is because the e-gas may break through and damage the fiber.

Here, the reason why He gas is selected as the gas to be filled in the preform 21 is that its diffusion rate is overwhelmingly higher than that of other gases and that it can easily pass through the quartz glass layer. Because it is natural gas. Similarly, hydrogen exists as a gas with a high diffusion rate, but when hydrogen diffuses into quartz glass, it may combine with defects in the glass to generate new loss factors, and it is also flammable and explosive. Since it is a strong gas and difficult to handle, it was not selected as the gas to be used.

The preform 21 filled with He gas is
Fiber is formed by a normal optical fiber drawing operation. Specifically, a PC after drawing with an outer diameter φ of 125 μm
Although it is set to F, the wire can be drawn to have an outer diameter φ of about 80 to 150 μm, if necessary.

Next, as shown in FIG. 4, the PC after drawing
F41 was put in a treatment tank 40 under high vacuum with a pressure of 1.33 Pa (0.01 Torr) or less, and vacuum treatment was performed for 73 hours to remove He gas from the drawn PCF41, thereby removing the He gas from the hollow core and the clad. When the inside of the holes is evacuated, the PCF 1 shown in FIG. 1 is completed. This PCF
In Fig. 1, the residual gas in the hollow core and in the plurality of holes in the clad was analyzed by Raman spectroscopy.
It was confirmed that the e-gas component was below the detection limit and did not remain.

The P according to the present invention produced as described above
The characteristics of CF1 and the conventional PCF in which air remained in the core and the clad were compared. The loss characteristics of the PCF1 according to the present invention have wavelengths of 0.8 μm, 1.3 μm, and 1.55.
3.98 dB / km and 3.57 dB / k in μm, respectively
m, 3.53 dB / km. In addition, conventional PCF
Loss characteristics of 0.8μm, 1.3μm, 1.55
4.01 dB / km and 4.68 dB / k in μm, respectively
m, 3.55 dB / km.

These loss levels are still larger than those of ordinary optical fibers, but when the loss factors were investigated, PCF
It was found that the structural imperfection loss due to the manufacturing precision of the preform for automobiles is largely dominant. However, the Rayleigh scattering coefficient obtained from this result is the PCF1 according to the present invention.
Further, the conventional PCF was about 0.2, which were significantly lower than the smallest Rayleigh scattering coefficient of about 0.6 in the conventional optical fiber.

Further, the PCF1 according to the present invention has a loss at a wavelength of 1.3 μm lower than that of the conventional PCF. This is OH at wavelength 1.39 μm
This is because the influence of the absorption peak due to the molecular vibration of the group is suppressed to a lower level. It can be seen that the PCF1 according to the present invention can achieve both reduction of scattering loss and reduction of absorption loss.

As described above, in the PCF of the present invention, since the inside of the hollow core and the inside of the plurality of holes in the clad are evacuated, there is no light absorption loss due to light scattering factors and impurities, and thus the conventional The Rayleigh scattering loss, which cannot be achieved with the optical fiber, can be reduced, and the absorption loss due to impurities can be reduced, resulting in a very low loss.

In the PCF manufacturing method of the present invention, P
When manufacturing a CF preform, the core hole and the plurality of clad holes are filled with He gas, which has a large diffusion coefficient, at about atmospheric pressure, hermetically sealed, and drawn in that state. By making the pressure in the reform substantially equal to the atmospheric pressure, it is possible to prevent the core hole and the plurality of cladding holes from being crushed, and to realize a photonic bandgap structure in the fiber.

Further, since the PCF obtained after drawing is vacuum-treated to extract the He gas existing inside the fiber by utilizing its large diffusion coefficient, the inside of the hollow core of the PCF and the clad are clad. It is possible to realize a low-loss PCF by reducing the scattering loss of light propagating through the fiber and the absorption loss due to impurities by making a vacuum inside the plurality of holes.

[0040]

As described above, the PCF of the present invention
Is a very low loss because it is possible to reduce the Rayleigh scattering loss that could not be achieved by the conventional optical fiber, and the absorption loss due to impurities can be reduced.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a preferred embodiment of the present invention.

FIG. 2 is a cross-sectional view of a PCF preform.

FIG. 3 (a) is a schematic view illustrating a method of vacuum-treating a PCF preform. Figure 3 (b) shows
It is a schematic diagram explaining the method of filling the He gas into the preform for PCF.

FIG. 4 is a schematic diagram illustrating a method of extracting He gas from PCF after drawing.

[Explanation of symbols]

1 Photonic crystal optical fiber (PCF) 2 hollow core 3 holes 4 clad 5 jacket φc Hollow core outer diameter φ PCF outer diameter

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Kazumasa Ozono             1-6-1, Otemachi, Chiyoda-ku, Tokyo             Standing Wire Co., Ltd. (72) Inventor             1-6-1, Otemachi, Chiyoda-ku, Tokyo             Standing Wire Co., Ltd. (72) Inventor Kazushi Osuga             1-6-1, Otemachi, Chiyoda-ku, Tokyo             Standing Wire Co., Ltd. F-term (reference) 2H050 AB04Z AC01 AC64                 4G021 BA00 HA00

Claims (3)

[Claims] 1. A hollow core having an outer diameter of about several times the wavelength of light, and a plurality of cavities formed around the core and forming a photonic bandgap structure diffraction grating at least around the core. And a clad in which holes are formed,
A photonic crystal optical fiber, characterized in that the inside of the hollow core and the inside of a plurality of holes in the clad are evacuated. 2. A preform for a photonic crystal optical fiber, a core hole which becomes a hollow core after drawing, and a plurality of clads which become holes forming a diffraction grating having a photonic bandgap structure after drawing. Photonic crystal characterized by forming holes for use in the core and a plurality of holes for the cladding, filling He gas in advance and hermetically sealing, and then drawing a preform to form a fiber. Optical fiber manufacturing method. 3. The photonic crystal light according to claim 2, wherein the drawn photonic crystal optical fiber is subjected to a vacuum treatment to remove the He gas so as to form a vacuum in the hollow core and the plurality of holes in the clad. Fiber manufacturing method.