JP5215699B2 - Photonic crystal fiber - Google Patents

Description

  The present invention relates to a photonic crystal fiber and a mode field diameter (MFD) conversion or expansion method using the optical fiber.

  Currently, optical fibers are widely used as low-loss transmission media for communication. In particular, single mode optical fibers such as SMF (single mode optical fiber) and DSF (dispersion shifted optical fiber) such as the 1.3 micron band zero dispersion optical fiber are capable of high-speed long-distance signal transmission taking advantage of their broadband characteristics. Realized.

  However, since the mode field diameter (MFD) is usually as small as about 10 μm, a conventional single mode optical fiber as shown in FIGS. 1A and 1B is used as a technique for converting and expanding the MFD. A method of extending (SMF) in a tapered shape, or a method of locally expanding MDF by heating the core portion of SMF and diffusing the dopant distribution as shown in FIG. . These methods are described in detail in Non-Patent Document 1, for example.

  Such MFD expansion technology is used in collimators for coupling optical devices to optical fibers. The higher the conversion / expansion performance of MFD, the easier it is to adjust the optical axis when coupling optical devices. It becomes possible to do.

Shojiro Kawakami et al., “Optical fiber and fiber type device”, Baifukan, PP. 201-226, 1996 Hitoshi Fujita et al., “Photonic Crystal Fiber Collimator”, IEICE Technical Report, PN2003-81, pp. 13-17, 2003 Moriyuki Fujita et al., “Photonic Crystal Fiber (1) —Optical Properties—”, Mitsubishi Electric Industrial Time Report No. 99, pp. 1-9 Shojiro Kawakami et al., “Optical fiber and fiber type device”, Baifukan, PP. 41-42, 1996 S. Okude et al., “Fiber Grating Technology and Trends”, IEICE Transactions C, Vol. J83-C, No. 12, PP. 1060-1068, 2000 Katsumi Morishita et al., “Production of Long-Period Gratings in Pure Quartz Photonics Crystal Fiber by Changing Glass Structure”, IEICE Technical Report, OCS 2003-38, pp. 7-12, 2003

  However, with regard to the conventional MFD conversion / enlargement technique as described above, as described in Non-Patent Document 1, with the down taper by the heat drawing in FIG. 1A, the relative refractive index difference between the core and the clad. Since the core diameter is small while Δ is constant, the waveguiding action is small, and therefore there is a problem that loss is likely to occur even with slight bending.

  On the other hand, in the up taper by heating and stretching shown in FIG. 1B, the core diameter increases while the relative refractive index difference Δ between the core and the clad is constant, so that higher-order modes other than the propagation mode are likely to occur. There is a problem that it is difficult to obtain stable characteristics.

  In addition, in the TEC (Thermally-diffused Expanded Core) fiber by distributed heating in FIG. 1C, it is possible to keep the bending loss relatively small while preventing the generation of higher-order modes. However, as described in Non-Patent Document 2, the size of the MFD in the TEC fiber is about 20 μm, that is, the expansion ratio of the MFD (the ratio of the MFD at both ends of the fiber) is limited to about 2 times. It is difficult to achieve an expansion ratio.

  As described above, in the conventional MFD enlargement method, when realizing the MFD enlargement, there are problems of an increase in bending loss and generation of a higher-order mode. In the first place, there has been a major problem to be solved that the enlargement ratio is limited to about twice.

  The present invention has been made in view of the problems of the prior art as described above, and its object is to achieve a mode of a single mode optical fiber connected to the low-mode bending loss and a mode state while maintaining the stability. It is an object of the present invention to provide a photonic crystal fiber that can be greatly enlarged or reduced within a field diameter of twice or more, which has been impossible in the past.

In order to achieve the above object, a photonic crystal fiber according to claim 1, wherein a cladding portion having a periodic hole structure, a first core glass region surrounded by the hole structure, and the first A second core glass region formed by using a refractive index difference caused by a dopant in the core glass region, and a length formed in the whole or a part of the first core glass region or the second core glass region. and an optical fiber Bragg gratings of the period, in a used wavelength, the second single-mode transmission can der with core glass region of the is, the period of the optical fiber Bragg gratings is in the use wavelength, the second The phase matching condition between the lowest order mode propagating in one core glass region and the lowest order mode propagating in the second core glass region is satisfied .

  Here, a ratio d / Λ of a hole diameter d and a hole interval Λ of the hole structure may be 0.4 or less.

To achieve the above object, the mode field diameter conversion method according to claim 3, the optical fiber that operates in a single mode in a specific wavelength region, connecting a photonic crystal fiber according to claim 1 or 2 Thus, the mode field diameter is converted.

  With the above configuration, in the photonic crystal fiber of the present invention, even if the core diameter is increased, it is easy to maintain a single mode state, it is possible to suppress bending loss, and a mode field diameter of about 50 μm. realizable.

  For this reason, according to the present invention, it is possible to increase the mode field diameter in the range of twice or more, which was impossible in the past, while maintaining low bending loss and mode stability, and to reduce bending loss. The mode state can be stabilized.

  Hereinafter, the structure of the photonic crystal fiber according to a preferred embodiment of the present invention, and further, the mode field diameter (MFD) conversion method using the structure will be described in detail with reference to the drawings. Describe.

(First embodiment)
As the first embodiment of the present invention, the outer diameter of the fiber changes in a tapered shape, the structure in the cross section is similar in the longitudinal direction, and the ratio d / Λ of the hole diameter d and the hole interval Λ. A structure of a photonic crystal fiber in which is kept constant and a method for producing the fiber will be described.

As a cross-sectional example of the photonic crystal fiber, as shown in FIG. 2, a photonic crystal fiber generally has a structure in which cylindrical holes are periodically arranged in uniform glass. In the photonic crystal fiber having this basic structure, for example, as described in Non-Patent Document 3, the ratio d / Λ of the hole diameter d and the distance Λ between adjacent hole centers is more than about 0.4. It has been theoretically shown that when it is reduced, it propagates only the LP ₀₁ mode (lowest order mode) and operates in a complete single mode, no matter how short the transmission wavelength. Experimentally, Non-Patent Document 2 described above has a structure of d / Λ = 0.14 and Λ = 21 μm, and realizes a propagation mode MFD value of about 50 μm (wavelength 1.55 μm).

  Therefore, a photonic crystal fiber having an appropriate constant d / Λ value and having a structure in the cross-section that is similar in the longitudinal direction while the outer diameter D of the fiber changes in the longitudinal direction is MFD. It can be used as an optical component for conversion and expansion.

  FIG. 3 shows a schematic diagram of the structure of the photonic crystal fiber of the present invention. Generally, when drawing an optical fiber, the structure in the cross section is kept similar. Accordingly, in such a photonic crystal fiber, (1) first, a fiber having an outer diameter D1 is drawn from the base material, and (2) further, the drawing is performed so that a desired outer diameter D2 (where D1> D2) is obtained. It can be obtained by adjusting temperature and tension.

  When the wavelength used is the communication wavelength range of a general-purpose single mode optical fiber, for example, when a wavelength of 1.55 μm is used, the d / Λ condition is further relaxed. For example, even under the condition of d / Λ = 0.5, a single mode state can be maintained in a wide core diameter region as long as the wavelength is 1.55 μm.

  FIG. 4 shows the relationship between the core diameter (horizontal axis) and the ratio of the optical power confined in the core region (vertical axis) for a photonic crystal fiber having a five-layer structure when d / Λ = 0.5 as an example. The calculation results are plotted. Here, the core region is assumed to be a circle having a diameter (2Λ-d). As can be seen from FIG. 4, even if the fiber cross-sectional size and the core diameter change by about 5 times, the ratio of the optical power contained in the core region is almost constant from about 95% to about 98%. From this result, in the photonic crystal fiber as in the present invention, the range of selection of the values of d and Λ is wide, and if an appropriate value is selected for these, the outer diameter and the core diameter are increased in a tapered shape. It can also be seen that the single mode state can be maintained and bending loss can be suppressed.

  The above discussion is based on the premise that quartz glass is used as the material. From the viewpoint of ease of processing, low melting point multi-component glass or plastic polymer is used as the material of the photonic crystal fiber of the present invention. You can also. In that case, the value of d / Λ may be appropriately adjusted according to the refractive index of the material used to maintain the single mode state.

(Second Embodiment)
As a second embodiment of the present invention, the fiber outer diameter changes in a tapered shape, the structure in the cross section of the fiber is similar in the longitudinal direction, and the ratio d / of the hole diameter d to the hole interval Λ An application example of MFD conversion using a photonic crystal fiber in which Λ is kept constant will be described.

  FIG. 5 shows an example in which the size of the MFD is converted by inserting the photonic crystal fiber of the present invention between the SMF and an optical device (such as an optical wavelength filter or a light source). In FIG. 5, 51 is an SMF (single mode optical fiber), 52 is a photonic crystal fiber (PCF) according to the present invention, and 53 is an optical device. On the connection side with the SMF 51, it is preferable that the MFD of the photonic crystal fiber 52 and the MFD of the SMF 51 substantially coincide with each other in order to reduce connection loss. For the connection of both 51 and 52, various optical connectors may be used, or fusion splicing may be used.

  As in the configuration shown in FIG. 5, for example, when the optical device 53 is an optical wavelength filter, the wavelength selectivity of the filter 53 is increased by enlarging the MFD by the photonic crystal fiber 52 in front of the optical wavelength filter 53. It becomes possible to improve.

  When a light source (not shown) having a large light spot size is coupled to the SMF 51, the light source is arranged at the position of the optical device 53, and the MFD size of the SMF 51 is determined from the spot size of the light source by the photonic crystal fiber 52. High-efficiency optical input becomes possible by converting to.

  FIG. 6 shows an example in which the size of the MFD is converted by inserting the photonic crystal fiber according to the present invention between a single mode optical fiber (SMF) and a multimode optical fiber (MMF). In FIG. 6, 61 is an SMF, 62 is a photonic crystal fiber (PCF), and 63 is an MMF. By converting the MFD size from the MFD size of the MMF 63 to the MFD size of the SMF 61 by the photonic crystal fiber 62, it is possible to input light with high efficiency to the SMF 61.

  With recent development of optical LAN (local area network) technology, MMF may be used for optical wiring such as Ethernet (registered trademark). In the case of MMF, when the transmission distance is short, the low-order mode occupies the main mode component, but the core diameter of the MMF is about 50 μm and is about five times larger than the core diameter of the SMF. Therefore, the low-order mode component is dominant. Even under such conditions, when an optical signal is input as it is from the MMF 63 to the SMF 61, a connection loss of about 15 dB or more occurs.

  However, as shown in the configuration diagram of FIG. 6, the photonic crystal fiber 62 is inserted between the MMF 63 and the SMF 61, and the MFD of the photonic crystal fiber 62 and the MFD of the SMF 61 are substantially matched on the connection side with the SMF 61. Also, on the connection side with the MMF 63, the optical signal input from the MMF 63 to the SMF 61 can be realized with a relatively low connection loss by setting the MFD of the photonic crystal fiber to about 50 μm, which is substantially the same as the MFD of the MMF 63. Here, various optical connectors may be used for connection between the optical fibers, or fusion splicing may be used.

(Third embodiment)
As a third embodiment of the present invention, a basic structure of a photonic crystal fiber that realizes MFD conversion by providing long-period optical fiber grating will be described.

  As already described in the first embodiment of the present invention, a normal photonic crystal fiber has a structure in which cylindrical holes are periodically arranged in a uniform glass material, as shown in FIG.

  On the other hand, as shown in the cross-sectional view of FIG. 7, the photonic crystal fiber of the present invention includes the first core glass region 72 surrounded by the clad portion 71 composed of holes and glass, in addition to the first core glass region 72. A second core glass region 73 is formed inside the core glass region 72 using a refractive index difference caused by the dopant. The first core region 72 acts as a clad for the second core glass region 73.

  Here, when the hole diameter is d and the interval between adjacent hole centers is Λ, the first core glass region 72 has a core diameter 2Λ-d adjacent to the innermost plurality of holes. The second core glass region 73 is a circular region having a core diameter 2 a that is concentric with the first core glass region 72.

Further, as shown in the schematic diagram of the refractive index distribution in FIG. 7, the ratio C ₂ / C ₁ between the _first core diameter C ₁ = 2Λ−d and the second core diameter C ₂ = 2a must be sufficiently small. For example, the influence of the first core region 72 and the second core region 73 on each other is so small that it can be ignored. That is, at this time, the mode 74 that propagates using the first core region 72 as a core portion and the mode 75 that propagates using the second core region 73 as a core portion can exist independently.

For a certain wavelength λ, in order for the first core region 72 to act as a single mode core, as already mentioned in the first embodiment of the present invention, d / Λ is small (typically 0.4 or less) is preferable. On the other hand, in order for the second core region 73 to function as a single mode core, it is a standard that the normalized frequency ν (λ) at the use wavelength λ of the following equation (1) is 2.4 or less. Here, n _2c is the refractive index of the second core region 73, n _1c is the refractive index of the first core region 72, and a is the core radius of the second core region 73. W ₂ (λ) is ½ of the MDF of the second core region 73.

ν (λ) = πa (n _2c (λ) ² −n _1c (λ) ² ) ^0.5 / λ (1)
W ₂ (λ) = a (0.65 + 1.619 · ν (λ) ^−1.5 + 2.879 · ν (λ) ⁻⁶ ) (2)

If the values of d, Λ, and a are set so as to satisfy the above conditions, the lowest-order propagation mode LP ₀₁ (first core) inherent to the first core region 72 and inherent to the second core region 73 There may be two propagation modes of the lowest order propagation mode LP ₀₁ (second core). Such photonic crystal fiber, selects the source of the MFD of the light incident from the (not shown), if matching the MFD = 2W ₁ of LP ₀₁ (first core), propagation mode LP ₀₁ (first core) Can be excited. On the other hand, if the MFD of light incident from the light source is matched with MFD = 2W ₂ of LP ₀₁ (second core) in the photonic crystal fiber, LP ₀₁ (second core) is selectively excited. Can do.

For example, as described in Non-Patent Document 4, it is known that the value of W ₂ is often given by equation (2) in a fiber having a step-type core as shown in FIG. . Therefore, if the structural parameters of the equations (1) and (2) are known, the value of W ₂ can be estimated, and it becomes easier to selectively excite LP ₀₁ (second core).

(Fourth embodiment)
As a fourth embodiment of the present invention, a method for imparting long-period optical fiber grating to the photonic crystal fiber of FIG. 7 whose basic structure is described in the third embodiment and its MFD conversion operation will be described.

Table 1 below shows an example of the material structure of the photonic crystal fiber of the present invention. For the clad portion 71, the first core glass region 72, and the second core glass region 73 shown in FIG. 7, for example, glass materials having a combination of (i) (ii) (iii) in the table are used. Can be used. Here, GeO ₂ (germanium oxide) and F (fluorine) are the most representative dopants used in silica-based optical fibers. However, in (iii), the GeO ₂ doping amount of the _second core glass region 73 needs to be maximized.

Of the material configurations shown in Table 1, the GeO ₂ -doped quartz glass portion is irradiated with ultraviolet rays to change the refractive index of the irradiated portion, whereby long-period light as shown in FIG. 8 or FIG. It is possible to provide fiber gratinging. An example of a method that can be applied to a GeO ₂ -doped quartz glass portion is a step exposure method, which is described in Non-Patent Document 5, for example. As a method applicable not only to GeO ₂ doped quartz but also to F doped quartz and pure quartz glass, a method of performing femtosecond laser irradiation, a method of applying periodic stress, a periodic refractive index by heating and quenching There are ways to give variation. In particular, there is a report example that the method using heating and quenching is applied to a photonic crystal fiber made of pure quartz as described in Non-Patent Document 6, for example.

  Therefore, the first core glass region or the second core glass can be applied to the photonic crystal fiber of the present invention by using an appropriate method according to the material configuration of the fiber from among these methods. Long period optical fiber gratings can be formed in all or part of the region.

Next, in the following, as described in the third embodiment of the present invention, the lowest-order propagation mode LP ₀₁ (first core) inherent to the first core region 72 of FIG. In contrast to a photonic crystal fiber having a structure in which two propagation modes of the lowest order propagation mode LP ₀₁ (second core) unique to the optical fiber can exist, as shown in FIG. 8, a long-period optical fiber having a period Q ₁ An example of operation when the grating is formed will be described.

  In long period optical fiber gratings, coupling between modes occurs between the two modes when the phase matching condition is met. That is, coupling occurs when the propagation constant difference Δβ between modes is expressed by the following equation (3), and mode conversion occurs. Q is the period of the optical fiber grating.

    Δβ = 2π / Q (3)

Accordingly, at the wavelength λ ₁ , the propagation constant of the propagation mode LP ₀₁ (second core) (75) inherent to the second core region 73 is β _A , and the propagation mode LP ₀₁ (first property inherent to the first core region 72 is (1 core) (74), if the propagation constant is β _B , by selecting the period Q = Q ₁ in equation (3) so that Δβ = β _A -β _B , as shown in FIG. It is possible to convert the mode and convert MFD = 2W ₂ corresponding to the second core to MFD = 2W ₁ (W ₂ <W ₁ ) corresponding to the first core.

In general, since the propagation constant has wavelength dependence, in order to perform the same operation at the wavelength λ ₂ different from the wavelength λ ₁ , (Δβ = β _A −β _B at the wavelength λ ₂ ( 3) may be selected period _Q = Q ₂ of the equation.

Next, in the following, as shown in FIG. 9, the propagation mode LP ₀₁ (first core) (74) inherent to the first core region 72 is changed to the higher-order mode LP _0n (76) (n is 2 or more). An operation in the case of conversion to the propagation mode LP ₀₁ (second core) (75) inherent to the second core region 73 via an integer) will be described. The high-order mode LP _0n (76) is a high-order mode with respect to the first core glass region 71 and has a centrally symmetric light intensity.

In this case, if the propagation constant of the propagation mode LP ₀₁ (first core) inherent to the first core region 72 is β _B and the propagation constant of the higher-order mode LP _0n is β _n , then Δβ = β _B −β _n. Thus, by selecting the period Q = Q ₃ in the equation (3), both modes are converted. Here, by appropriately setting the core diameter a of the second core region 73, as shown in FIG. 9, a portion with high optical power near the center of LP _0n (the mode shape is a ring shape) by binding to the core region 73, it is possible to convert the MFD = 2W ₁ corresponding to the first core region 72 to the _MFD = 2W ₂ corresponding to the second core _(W 2 <W _1).

In general, since the propagation constant has wavelength dependence, in order to perform the same operation at a wavelength λ ₂ different from the wavelength λ ₁ , (3 is set so that Δβ = β _B −β _n at λ _2. It is sufficient to select the period Q = Q ₄ in the formula (1).

(Other embodiments)
In the above, the preferred embodiment of the present invention has been described by way of example. However, the embodiment of the present invention is not limited to the above-described example, and the constituent members thereof are within the scope of the claims. Various modifications such as replacement, change, addition, increase / decrease in number, change in shape design, etc. are all included in the embodiments of the present invention. In addition, the present invention may be applied to a system composed of a plurality of devices, or may be applied to an apparatus composed of one device.

  INDUSTRIAL APPLICABILITY The present invention can be used as a method for converting or expanding a mode field diameter when connecting an optical fiber and an optical device, and optical fibers.

The conventional technique for converting and expanding the mode field diameter (MFD) of a single mode optical fiber is shown, (A) shows the DOWN taper by the heat drawing process, (B) shows the UP taper by the heat drawing process, (C) is a conceptual diagram showing a TFC (Thermally-diffused Expanded Core) fiber by distributed heating treatment. It is sectional drawing which shows an example of the cross section of the photonic crystal fiber of this invention. It is a schematic diagram which shows an example of the structure of the photonic crystal fiber of this invention. It is a characteristic view which shows an example of the relationship between the core diameter of the photonic crystal fiber of this invention, and the ratio (confinement coefficient) of the optical power confined in a core area | region. It is a schematic diagram which shows an example of application of MFD conversion by the photonic crystal fiber of this invention. It is a schematic diagram which shows another example of application of MFD conversion by the photonic crystal fiber of this invention. It is explanatory drawing which shows an example of the relationship between the structure of the photonic crystal fiber of this invention, and refractive index distribution. It is a conceptual diagram which shows an example of MFD conversion operation | movement of the photonic crystal fiber of this invention. It is a conceptual diagram which shows another example of the MFD conversion operation | movement of the photonic crystal fiber of this invention.

Explanation of symbols

51 SMF (single mode optical fiber)
52 PCF (Photonic Crystal Fiber)
53 Optical device 61 SMF
62 PCF
63 MMF (multimode optical fiber)
71 Cladding portion made of holes and glass 72 First core region 73 Second core region 74 Propagation mode (lowest mode) LP ₀₁ unique to first core region
75 Propagation mode (lowest order mode) LP ₀₁ unique to the second core region
76 Higher order mode LP _0n for the first core glass region

Claims (3)

A cladding having a periodic hole structure;
A first core glass region surrounded by the pore structure;
A second core glass region formed using a refractive index difference caused by a dopant in the first core glass region;
A long-period optical fiber grating formed in the whole or a part of the first core glass region or the second core glass region, and the second core glass region is used at a used wavelength. Single mode transmission is possible, and the period of the optical fiber grating is the lowest order mode that propagates through the first core glass region and the lowest order mode that propagates through the second core glass region at the wavelength used. A photonic crystal fiber characterized by satisfying a phase matching condition between 2. The photonic crystal fiber according to claim 1 , wherein a ratio d / Λ of a hole diameter d and a hole interval Λ of the hole structure is 0.4 or less. A mode field characterized in that the mode field diameter is converted by connecting the photonic crystal fiber according to claim 1 or 2 to an optical fiber that operates in a single mode in a specific wavelength region. Diameter conversion method.

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