JP3484165B2 - Polarization-maintaining optical fiber - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a core and a clad (a photonic crystal structure clad) of a polarization maintaining optical fiber.
The present invention relates to a polarization-maintaining optical fiber capable of transmitting while maintaining the polarization of an optical signal constant by devising the above. The present invention can be used as a transmission medium used for optical communication networks and optical signal processing.

[0002]

2. Description of the Related Art FIG. 5 shows a conventional optical fiber 10. A photonic crystal structure clad 12 is provided around the core 11 of the optical fiber 10, and a circumference of the photonic crystal structure clad 12 is covered with a jacket 13. In the general literature, the term "photonic crystal structure cladding" is used regardless of the material of the core, while the term "photonic bandgap cladding" is used when the core is hollow or the refractive index of the core is It is used only when it is lower than the refractive index of the clad, but in the following, the term "photonic crystal structure clad" is used in the same sense as the former,
The "photonic crystal structure cladding" also includes the "photonic bandgap cladding".

The photonic crystal structure clad 12 has a diffraction grating (member indicated by ◯ in the figure). The grating is generally formed by holes, but it can also be formed by forming circular cross-section members having different refractive indexes.

Next, the principle of light guiding by the optical fiber 10 having such a structure will be described. In this optical fiber 10, when the material of the core 11 is glass, the equivalent refractive index of the photonic crystal structure cladding 12 is lower than that of the core 11, so that light is confined by total reflection of the refractive index (general single Same as mode fiber confinement)
Is guided in the core 11. On the other hand, when the refractive index of the core 11 is lower than the refractive index of the photonic crystal structure clad 12, or when the core 11 is hollow, the surrounding photonic crystal structure clad 12 and the jacket 1 are provided.
Since No. 3 uses the same silica-based material as the ordinary optical fiber, the refractive index of the material itself is higher than that of the core 11. Therefore, if the clad has the same structure as the clad of the conventional fiber, the refractive index of the core 11 becomes the lowest, and the energy of light cannot be confined in the core 11 as it is.

Therefore, the confinement of light is realized by adopting a structure called a photonic crystal structure in a part of the clad. That is, the light is emitted around the core 11 to the core 1.
A photonic crystal structure clad 12 having a diffraction grating with a grating interval set so as to be confined to 1 is provided.

FIG. 6 is a diagram showing the structure of a photonic crystal structure (photonic crystal structure clad 12). In general, a three-dimensional photonic crystal structure is a diffraction grating that Bragg-reflects light in all directions, and the grating constant (grating spacing) d of the diffraction grating propagates in a medium (core) as shown in FIG. It is realized by setting the wavelength to the same level as the wavelength of light.

As the structure of the crystal lattice which constitutes the photonic crystal structure, several structures other than the square lattice as shown in FIG. 6 are conceivable. FIG. 7 shows various structural examples of the crystal lattice that constitutes the photonic crystal structure.

FIG. 7A shows a square lattice structure (black portion in the figure) having a high refractive index embedded in a medium (white portion in the figure) having a low refractive index, and FIG. FIG. 7C shows a square lattice structure with a low refractive index (white portion in the figure) embedded in a medium with a high refractive index (black portion in the figure).
FIG. 7D shows a medium having a high refractive index (black portion in the drawing), and a triangular lattice structure having a high refractive index (black portion in the drawing) embedded in a medium having a low refractive index (white portion in the drawing). ) Is embedded in a low-refractive-index triangular lattice structure (white portion in the figure), and FIG. 7 (e) shows a low-refractive-index triangular structure embedded in a medium with a low refractive index (white portion in the figure). Each shows a high honeycomb-shaped lattice structure (black portion in the figure).

Reference [JD Joannopoulos et al., Pho
tonic Crystals, Princeton University Press, pp.122
-126, 1995. ”, it is shown that a photonic crystal structure exists in these structures to confine light.

Although a lattice structure of a cylinder or a circular hole is assumed here as the shape of the lattice, this is not limited to the cylinder or the circular hole, and a triangular prism or a triangular hole, a square prism or a square hole, It is possible to realize a photonic crystal structure even in a lattice structure having a shape such as a hexagonal column or a hexagonal hole.

When a defect (= part without a lattice hole = core) is provided in this photonic crystal structure, light is strongly confined in this defect. Therefore, when light is to be guided through a certain structure, as shown in FIG. 6, by making the periphery of the structure (core 11) a photonic crystal structure (photonic crystal structure clad 12), the light is converted into the structure ( It can be confined and propagated in the core 11).

The photonic crystal structure is confined around the core of the optical fiber so that light does not propagate in the radial direction from the center of the core of the optical fiber. That is, as shown in FIG. 5, when the cross section of the optical fiber 10 is seen, it has a lattice-like structure and maintains the same structure in the length direction.
That is, the cross section of the optical fiber 10 has the same structure everywhere (ignoring the fluctuation of the shape due to the manufacturing process of the optical fiber), and there is no structure orthogonal or oblique to the length direction of the optical fiber 10. . By adopting this structure, the light incident on the core 11 can be confined and propagated in the core 11.

[0013]

In the conventional optical fiber 10 described above, it is possible to confine the light in the core 11 and transmit the light into the fiber, but the optical fiber 10 is used for optical communication. In that case, the following problems occur. That is, when the shape of the core 11 of the fiber is circular, there is no mechanism for determining the polarization direction of the light propagating in the core 11, and therefore, a slight fluctuation in the shape of the circular core 11 causes Fluctuation occurs in the polarization direction. Therefore, the polarization of the optical signal after being transmitted through the optical fiber 10 will fluctuate due to temperature fluctuations and vibrations of the optical fiber 10, and the polarization of the optical signal may fluctuate on the receiving side. It was necessary to have a wave-independent configuration.

The polarization-maintaining fibers currently existing are:
There is a PANDA fiber, but this PANDA fiber does not use a photonic crystal structure. This PAND
In the manufacturing process, the A fiber requires a sophisticated technique of forming a hole in the base material of the optical fiber at two positions in the vicinity of the core portion, and further pressing the stress applying material into the hole to generate the fiber. In particular, the process of pushing in the stress-applying material is a major factor that prevents the productivity of PANDA fibers from increasing.
The price of NDA fiber is one of ordinary single mode fiber
It is more than 00 times. Further, the propagation constant difference of the orthogonal polarization modes generated by this PANDA fiber structure cannot be realized so large that it is difficult to set the crosstalk between both modes to -30 dB or more. Therefore, it is difficult to transmit a signal pulse over a long distance while maintaining a single polarization in the PANDA fiber, and the use of the PANDA fiber as a single polarization transmission line has not been realized at present.

In view of the above-mentioned prior art, it is an object of the present invention to provide a polarization maintaining optical fiber capable of transmitting an optical signal while maintaining the polarization of the optical signal constant.

[0016]

In order to solve the above-mentioned problems, the present invention comprises a photonic crystal structure cladding having a diffraction grating with a grating interval Λ so as to confine light in the core. Optical fiber,
The photonic crystal structure clad is divided into four in the circumferential direction, and all the lattices of the first pair of divided sub-portions facing each other along a certain direction have a larger diameter d ₂ , All the gratings of the second pair of divided parts facing each other along the other direction orthogonal to have a smaller diameter d ₁ and the condition for the light intensity distribution near the core to be unimodal is: On the graph where d ₁ / Λ is the vertical axis and d ₂ / Λ is the horizontal axis, the larger diameter d ₂ and the smaller diameter d
₁ of the ratio _{d 1 / d 2, d 1} / Λ = d 2 / Λ d 1 /Λ=0.2(d 2 / Λ) d 1 / Λ = d 2 /Λ-0.20 d 2 / Λ A polarization-maintaining optical fiber having a value such that it is within a region surrounded by four straight lines given by the four equations of 1.0.

Further, the present invention provides an optical fiber having a photonic crystal structure clad having a diffraction grating having a grating interval Λ set so as to confine light in the core around the core, in the optical fiber. Is divided into four in the circumferential direction, and only the lattice closest to the core among the lattices of the first divided partial pairs facing each other along a certain direction has a larger diameter d ₂ and the other lattices. Has a smaller diameter d ₁ and all gratings of the second pair of splitting parts facing each other along the other direction orthogonal to the one direction have a smaller diameter d ₁ and As a condition for the intensity distribution to show a single peak, a ratio d _{1 of the} larger diameter d ₂ and the smaller diameter d ₁ on a graph in which d ₁ / Λ is the vertical axis and d ₂ / Λ is the horizontal axis / _{d 2} is, _{d 1} / Λ = _{d 2} Surrounded by four straight lines given by the four equations of _{Λ d 1 /Λ=0.2(d 2 / Λ)} d 1 / Λ = d 2 /Λ-0.20 d 2 /Λ=1.0 Provided is a polarization-maintaining optical fiber, which has a value such that it falls within a range.

[0018]

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[0032]

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[0034]

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[0036]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a sectional view showing a polarization maintaining optical fiber 30 according to the first embodiment of the present invention. The polarization maintaining optical fiber 30 is a single mode optical fiber,
The core 31 is hollow, a photonic crystal structure clad 32 is provided around the core 31, and the periphery of the photonic crystal structure clad 32 is covered with a jacket 33. The photonic crystal structure clad 32 has a diffraction grating (member indicated by ◯ in the figure).

The basic light confining effect in this polarization maintaining optical fiber 30 is the same as in the prior art. A major part of the subject matter of the invention is the shape of the core 31. That is, the cross-sectional shape of the core 31 is rectangular. As described above, since the core has a rectangular cross-sectional shape, modes in the X-axis direction and the Y-axis direction are generated in the electric field of light in the core 31. Regarding the polarization of the light propagating in the core 31, the polarization state parallel to the X axis or the Y axis of the core 31 is maintained, and therefore the polarization state after exiting the polarization maintaining optical fiber 30 is also constant. Becomes

FIG. 2 shows a single-mode polarization maintaining optical fiber 40 according to the second embodiment of the present invention. In FIG. 2, 41 is a core (hollow), 42 is a photonic crystal structure clad, and 43 is a jacket, and the cross-sectional shape of the core 41 is elliptical.

The basic optical confinement effect in the polarization maintaining optical fiber 40 shown in FIG. 2 is the same as that of the prior art. As shown in FIG. 2, when the cross-sectional shape of the core is elliptical, the diameters in the X-axis direction and the Y-axis direction (major axis and minor axis) are different, so only polarization in one direction along the X-axis or Y-axis is possible. Exists, and the single polarization state can be maintained more stably.

Here, as shown in FIG. 1 or 2, when the cores 31 and 41 have a rectangular or elliptical cross-sectional shape, that is, the diameters in the X-axis direction and the Y-axis direction (long axis and short axis).
The principle that polarization can be maintained when the values are different will be described in detail.

Since the propagation constant (that is, the speed of light propagating through the fiber) is different in the mode having the polarization parallel to the X-axis direction or the Y-axis direction, the polarization parallel to the X-axis direction and the Y-axis direction in the fiber. Energy conversion (mode coupling) between polarized waves parallel to is difficult to occur.

Originally, the conventional optical fiber cannot hold a single polarization because the propagation constant difference between two orthogonal modes (HE11x mode and HE11y mode) is δβ (= βx-β).
y), if there is a spatial frequency component close to or equal to δβ in the fluctuation (structure, disturbance, etc.) in the length direction of the optical fiber, mode coupling occurs between both modes, and even if the input is a single polarization. This is because even if it is a wave, other polarized waves are generated by mode coupling in the fiber.

The spatial frequency component corresponding to the fluctuation in the length direction of this optical fiber is usually a low frequency, and δβ is about 0.
When it exceeds 01 cm ^-1 , it decreases sharply. In the present embodiment, since δβ is intentionally increased to suppress this mode coupling, polarized light incident in one direction along the X axis or the Y axis is converted into a component parallel to the other axis. It is possible to maintain a single polarization state because there is a state in which only one direction of polarization exists without being affected.

For the explanation of this mode coupling, for example,
Okamoto: "Fundamentals of optical waveguides" (Corona Publishing) pp, 106-
122.

The above-mentioned principle of polarization maintaining is the same in other embodiments described below.

FIG. 3 shows a single-mode polarization maintaining optical fiber 50 according to the third embodiment of the present invention. In FIG. 3, 51 is a core (hollow), 52 is a photonic crystal structure clad, and 53 is a jacket. Further, the basic light confinement effect is the same as that of the conventional technique.

In the third embodiment, the core 51 has a circular cross section. In addition, the lattice spacing of the photonic crystal structure clad 52 is different in two axial directions orthogonal to each other in a plane perpendicular to the optical axis, specifically, the X-axis direction and the Y-axis direction. That is, the lattice spacing in the X-axis direction is longer than the lattice spacing in the Y-axis direction. For this reason, there is a state in which there is only polarized light in one direction along the X axis or the Y axis, and it is possible to more stably maintain the single polarized state.

FIG. 4 shows a single-mode polarization maintaining optical fiber 60 according to a fourth embodiment of the present invention. In FIG. 4, 61 is a core (hollow), 62 is a photonic crystal structure clad, and 63 is a jacket. Further, the basic light confinement effect is the same as that of the conventional technique.

In the fourth embodiment, the core 61 has a circular cross section. The photonic crystal structure clad 62 has a square-shaped diffraction grating (member indicated by a circle in the figure) and has four divided portions 62 in the circumferential direction.
It is divided into a, 62b, 62c and 62d. And in each divided part 62a, 62b, 62c, 62d,
Although the lattice spacing between the lattice holes is the same, the diameter of each lattice in the divided portions 62a and 62c facing each other along the first Y axis is large, and the divided portion 6 facing each other along the second X axis is large.
The diameter of each lattice in 2b and 62d is small.
Note that the lattice is generally composed of holes,
It can also be constructed by forming circular members having different refractive indexes in cross section.

In this way, the first opposing divided portion 6
Since the diameters of the respective gratings in 2a and 62c are large and the diameters of the respective gratings in the second opposing divided portions 62b and 62d are small, only polarized light in one direction along the X axis or the Y axis exists. There is a state in which a single polarization state can be maintained more stably.

In the above-mentioned embodiment, the core is hollow, but the core is solid and the material of the core having a refractive index lower than that of the material of the photonic crystal structure clad is adopted. Also has the same effect.

FIG. 8 is a sectional view showing a polarization maintaining optical fiber 70 according to the fifth embodiment of the present invention. The polarization maintaining optical fiber 70 is a single mode optical fiber,
A photonic crystal structure clad 72 is provided around the core 71.
The periphery of 2 is covered with a jacket 73. The basic light confinement effect in this polarization maintaining optical fiber 70 is the same as that of the conventional technique.

In the fifth embodiment, the core 71 has a circular cross section. Further, the photonic crystal structure clad 72 has a triangular diffraction grating (member indicated by a circle in the figure), and has four divided portions 72 in the circumferential direction.
It is divided into a, 72b, 72c and 72d. And in each divided part 72a, 72b, 72c, 72d,
The lattice spacing Λ between the lattice holes is the same, but the diameter d ₂ of each lattice in the divided portions 72a and 72c facing each other along the first Y axis is the same as the diameter of the divided portions facing each other along the second X axis. Part 7
It is larger than the diameter d ₁ of each lattice in 2b and 72d (d ₂ > d ₁ ). By doing so, a propagation constant difference is generated in the X direction and the Y direction, and the polarization maintaining function can be realized.

The polarization maintaining optical fiber according to the fifth embodiment can be realized in the following three types depending on the form of the core 71.

(1) Type in which the core 71 is hollow: In this case, the propagation constant difference of the fiber is given by the ratio of the diameters of the lattice holes, so the shape of the core 71 is not particularly limited, and it may be square, rectangular, or elliptical. Although good, it is generally convenient to make a fiber in the shape of a circle.

(2) Type in which the material of the core 71 has a refractive index lower than that of the material of the photonic crystal structure clad: In this case as well, the propagation constant difference of the fiber is given by the ratio of the diameters of the lattice holes. Therefore, the shape of the core 71 is not particularly limited, and may be a square, a rectangle, or an ellipse, but generally a circle is convenient for producing the fiber.

(3) Type in which the material of the core 71 is the same as the material of the photonic crystal structure clad: In this case, the shape of the core 71 is the inner part surrounded by the nearest lattice. Regarding this, PCT application publication number WO00
/ 49436 has already disclosed a structure in which a diameter of a lattice of a photonic crystal structure in a solid glass core is made different in two orthogonal directions to generate a two-fold symmetry axis to generate a polarization maintaining function. However, in this document, there is no specific description about the practical range of the ratio of the diameter of the lattice. The ratio of the diameters of the grating not only directly affects the polarization maintaining function, but also affects the light intensity distribution in the core. That is, if the ratio exceeds a certain value, the light confinement on the side where the grating diameter is small becomes weak and the light intensity distribution is greatly distorted.

FIG. 9 shows the light intensity distribution around the core 71 obtained by using the finite element method, which is one of the numerical analysis methods. One contour line is drawn each time the light intensity changes by 10%. ing. In the calculation, it is assumed that the lattice spacing Λ is approximately equal to twice the wavelength of light propagating in the fiber, more accurately, Λ = 2 μm and the wavelength of light is 0.85 μm. FIG. 9A shows a case where the light intensity distribution near the core 71 shows a single peak (d ₁ /Λ=0.6, d ₂ / Λ = 0.
9), and FIG. 9B shows a case where the light intensity distribution near the core 71 exhibits bimodal characteristics (d ₁ /Λ=0.1, d ₂ /
Λ = 0.9) is shown. In the case of FIG. 9A, the light intensity distribution near the core has almost the same shape as that of the conventional optical fiber, and it is possible to connect the conventional optical fiber with low loss. In the case of the light intensity distribution as shown in (b), it is not only difficult to connect the conventional optical fiber with low loss, but also the light is not sufficiently confined in the portion where the diameter of the grating is small. There is also the danger that light will escape to the outside of the fiber, for example when the fiber is bent.

In this embodiment, the condition of the lattice diameter for the light intensity distribution near the core to show a single peak is specified as follows.

FIG. 10 shows the range of d ₁ / Λ and d ₂ / Λ so that the light intensity distribution near the core exhibits a single peak. Figure 10
In the figure, the horizontal axis represents d ₂ / Λ and the vertical axis represents d ₁ / Λ, and the range in which the light intensity distribution near the core exhibits a single peak is shown as a shaded area. Since d ₂ > d ₁ is defined, the significant region in FIG. 10 is only the lower part of the straight line d _{2 =} d ₁ . As is apparent from FIG. 10, the light intensity distribution exhibits a single peak only in a limited range, and d ₂ and d are unnecessarily large.
_When the ratio of ₁ is increased, the light intensity distribution deviates from the unimodal property and sufficient optical confinement cannot be performed in the optical fiber.

In the case of Λ = 2 μm shown in FIG. 10, the range exhibiting the unimodal characteristic can be expressed as a region surrounded by four straight lines given by the following equation. d ₁ / Λ = d ₂ / Λ (1) d ₁ /Λ=0.2 (d ₂ / Λ) (2) d ₁ / Λ = d ₂ /Λ−0.20 (3) d ₂ / Λ = 1.0 (4)

When the lattice spacing Λ changes, the range showing the unimodal property changes a little, but generally, as shown in FIG. 10, d ₁ / Λ
= D ₂ / Λ is a part of the lower region.

FIG. 11 shows the light intensity distribution obtained by using the values at the points A to F shown in FIG. As is clear from FIG. 11, the light intensity distributions at points A to D (FIGS. 11A to 11D) show a clean unimodal characteristic, but the light intensity distribution at point E (FIG. 11E )), The central region of the light spread in the left-right direction is almost equal to the core region, and the central region of the light spread in the left-right direction is larger than the core region in the light intensity distribution at point F (FIG. 11 (f)). It is bimodal.

Therefore, in order to practically realize the polarization-maintaining optical fiber of the present embodiment, it is necessary to set the size of each grating diameter within the range exhibiting the unimodal characteristic as shown in FIG. .

FIG. 12 is a sectional view showing a polarization maintaining optical fiber 80 according to the sixth embodiment of the present invention. The polarization-maintaining optical fiber 80 is a single-mode optical fiber, a photonic crystal structure cladding 82 is provided around a core 81 thereof, and a circumference of the photonic crystal structure cladding 82 is covered with a jacket 83. There is. The basic light confinement effect in this polarization maintaining optical fiber 80 is the same as that of the conventional technique.

In the fifth embodiment, the core 81 has a circular cross section. The photonic crystal structure clad 82 has diffraction gratings (members indicated by ◯ in the drawing) arranged in an equilateral triangle, and is divided into four divided portions 82a, 82b, 82c, 82d in the circumferential direction. . Then, the divided parts 82a, 82b, 82c, 82
In d, the lattice spacing Λ between the lattice holes is the same, but the diameter d ₂ of at least some of the lattices in the divided portions 82a and 82c facing each other along the first Y-axis is the same. The diameter is larger than the diameter d ₁ of each lattice in the divided portions 82b, 82d facing each other along the second X axis with the remaining lattices in the portions 82a, 82c (d ₂ >).
d ₁ ).

More specifically, in this embodiment, only the four lattices closest to the inner core 81 of the divided portions 82a and 82c facing each other along the first Y axis have the diameters d ₂ of other lattices. It is larger than the diameter d ₁ . In this case, the number of gratings having a larger diameter d ₂ can be suppressed to the minimum, which is convenient in fiber production as compared with the fifth embodiment described above.

The polarization-maintaining optical fiber according to the sixth embodiment is also (1) described in the fifth embodiment,
It can be realized in three types of forms (2) and (3). (3)
In the case of the above type, the range in which the light intensity distribution near the core exhibits a single peak is similar to that of the fifth embodiment. That is, FIG.
As shown in (a), when the condition of the lattice diameter is satisfied in order that the light intensity distribution near the core exhibits a single peak, most of the light intensity exists in the core region, and thus the large lattice diameter is large. The region where is used may be limited only to the immediate vicinity of the core region as shown in FIG.

[0070]

As described above, in the present invention, light is confined in the core by using the photonic crystal structure for the cladding, and the core has a rectangular or elliptical cross section. Therefore, the optical signal can be transmitted while maintaining the polarization of the optical signal in the optical fiber. As a result, transmission of high-speed and high-power light can be realized while maintaining polarization, and the optical signal processing circuit can be configured with a simple configuration.

Further, in the present invention, in the polarization-maintaining optical fiber, while making the cross-sectional shape of the core circular, the lattice spacing of the photonic crystal structure clad is different in two axial directions orthogonal to each other, or the photonic crystal structure clad is formed. The structure is such that the lattice is divided into four in the circumferential direction and the diameter of the lattice of the first opposing division is large, and the diameter of the lattice of the second opposing division adjacent to the first division is small. did.
With such a configuration, the optical signal can be transmitted while maintaining the polarization of the optical signal in the optical fiber. As a result, transmission of high-speed and high-power light can be realized while maintaining polarization, and the optical signal processing circuit can be configured with a simple configuration.

[Brief description of drawings]

FIG. 1 is a sectional view showing a single-mode polarization maintaining optical fiber according to a first embodiment of the present invention.

FIG. 2 is a sectional view showing a single-mode polarization maintaining optical fiber according to a second embodiment of the present invention.

FIG. 3 is a sectional view showing a single-mode polarization maintaining optical fiber according to a third embodiment of the present invention.

FIG. 4 is a sectional view showing a single-mode polarization maintaining optical fiber according to a fourth embodiment of the present invention.

FIG. 5 is a sectional view showing a conventional optical fiber.

FIG. 6 is an explanatory diagram showing a configuration of a photonic crystal structure.

FIG. 7 is an explanatory diagram showing a configuration example of a crystal lattice that constitutes a photonic crystal structure.

FIG. 8 is a sectional view showing a single-mode polarization maintaining optical fiber according to a fifth embodiment of the present invention.

9 is a diagram showing the light intensity distribution near the core in the polarization-maintaining optical fiber of FIG. 8 in the case of showing a single peak and in the case of showing a double peak.

10 is a diagram showing an example of a graph showing a range of a lattice diameter for showing a single-peaked light intensity distribution near the core in the polarization-maintaining optical fiber of FIG.

11 is a diagram showing the light intensity distribution near the core in the polarization maintaining optical fiber of FIG. 8 for the values of the six grating diameters shown in FIG.

FIG. 12 is a sectional view showing a single-mode polarization maintaining optical fiber according to a sixth embodiment of the present invention.

[Explanation of symbols]

10, 30, 40, 50, 60, 70, 80 Polarization-maintaining optical fiber 11, 31, 41, 51, 61, 71, 81 Core 12, 32, 42, 52, 62, 72, 82 Photonic crystal structure cladding 13, 33, 43, 53, 63, 73, 83 Jackets 62a, 62b, 62c, 62d, 72a, 72b, 7
2c, 72d, 82a, 82b, 82c, 82d Divided part

Front Page Continuation (56) References Special Table 2002-537575 (JP, A) International Publication 99/064903 (WO, A1) International Publication 99/064904 (WO, A1) International Publication 00/49436 (WO, A1) A ． Ortigosa-Blanch, et al. , Highly birefringent photonic crystal fibers, OPTICS LETTERS, USA, September 15, 2000, Vol. 25, No. 18, P.I. 1325-1327 (58) Fields surveyed (Int.Cl. ⁷ , DB name) G02B 6/00 G02B 6/10 G02B 6/16-6/22

Claims (2)

(57) [Claims] 1. An optical fiber comprising a photonic crystal structure clad around a core, the photonic crystal structure clad having a diffraction grating in which a lattice spacing Λ is set so as to confine light in the core. Are divided into four parts with respect to each other, and all the gratings of the gratings of the first divided sub-pair which are opposed to each other along a certain direction have a larger diameter d ₂ and are relative to each other along the other direction orthogonal to the one direction. All the lattices of the facing second split sub-pair have a smaller diameter d ₁ , and the condition for the light intensity distribution near the core to be unimodal is to have d ₁ / Λ as the vertical axis and d ₂ The larger diameter d ₂ and the smaller diameter d on the graph with / Λ being the horizontal axis.
₁ of the ratio _{d 1 / d 2, d 1} / Λ = d 2 / Λ d 1 /Λ=0.2(d 2 / Λ) d 1 / Λ = d 2 /Λ-0.20 d 2 / Λ A polarization-maintaining optical fiber having a value such that it is within a region surrounded by four straight lines given by four equations of 1.0. 2. An optical fiber comprising a photonic crystal structure clad around a core, the diffraction grating having a grating interval Λ set so as to confine light in the core, wherein the photonic crystal structure clad has a circumferential direction. Of the lattices of the first split sub-pair that are opposed to each other along a certain direction, only the lattice closest to the core has a larger diameter d ₂ , and the other lattices have a smaller diameter. has d _1, all grid of the second divided portion to be opposed along the other direction perpendicular to said certain direction has more smaller diameter d _1, the light intensity distribution in the vicinity of the core is a single As a condition for showing the peak characteristic, a ratio d ₁ / d ₂ between the larger diameter d ₂ and the smaller diameter d ₁ is plotted on a graph with d ₁ / Λ being the vertical axis and d ₂ / Λ being the horizontal axis. , D ₁ / Λ = d ₂ / Λ d ₁ / Λ = 0 .2 (d ₂ / Λ) d ₁ / Λ = d ₂ /Λ−0.20 d ₂ /Λ=1.0, which is within the area surrounded by the four straight lines given by the four equations. A polarization-maintaining optical fiber characterized by a value.