JP5059797B2 - Optical fiber, optical fiber design method, and optical wavelength division multiplexing communication system - Google Patents

Description

  The present invention relates to an optical fiber, an optical fiber design method, and an optical wavelength division multiplexing communication system.

  Currently, high-density wavelength division division multiplexing (DWDM) technology that uses light of several hundreds of different wavelengths for signal transmission is being put into practical use with the aim of applying to long-distance communications such as land and seabed. . At that time, suppression of various nonlinear effects that cause signal degradation due to distortion becomes a problem.

In order to solve this problem, it is effective to increase the effective area A _eff (generally proportional to the square of the mode field diameter), which is one of the optical fiber parameters, and the design of single-mode optical fibers with various refractive index profiles is effective. It has been studied and put into practical use.

However, in the conventional optical fiber (having no holes), there is a problem that when the effective area A _eff is increased, the single mode condition is hardly satisfied, and the bending loss αb is increased. As shown in FIG. 1, the effective area A _eff is about 150 μm ² to obtain a sufficiently small bending loss αb. Further, since the effective area A _eff decreases on the short wavelength side, for example, it decreases to about 100 μm ² near the wavelength of 1 μm.

  On the other hand, a photonic crystal fiber (PCF: Photonic Crystal Fiber) is a fiber having a hole structure arranged in a lattice shape in a cross section as shown in Non-Patent Documents 1 and 2 below. It is made of pure quartz glass and the like, and its loss reduction has rapidly progressed in recent years, and its minimum loss value reaches 0.18 dB / km in the vicinity of a wavelength of 1.55 μm.

  The PCF is drawn, for example, from pure quartz glass, and its cross section has a structure in which holes are regularly arranged in a triangular lattice shape, and several layers of holes surround a central glass core (the following non-patent document). Reference 3). A structure in which a portion corresponding to one central hole is replaced with material glass is hereinafter referred to as a 1-cell structure (see Non-Patent Document 7 below).

With a 1-cell PCF, single mode transmission is possible in the entire wavelength range by setting the ratio d / Λ of the hole diameter d and the hole interval Λ to 0.42 or less (Endlessly Single Mode: ESM) is shown in Non-Patent Document 3 below. Thus, even in the PCF, the expansion of the effective area A _eff has been studied centering on the structure in the vicinity of d / Λ = 0.42 that satisfies the condition of ESM. Thus, the upper limit is about 150 μm ² , and the wavelength used is also limited to the vicinity of the 1.5 μm band. This is because bending loss αb or confinement loss increases on the short wavelength side.

That is, in the PCF close to single-mode optical fiber and ESM conditions of the conventional structure construction, since the bending loss αb or confinement loss can not be long-distance transmission becomes a trade-off, the upper limit of the effective area A _eff is slightly 150 [mu] m ² The operating wavelength range was also limited to around 1.5 μm. This is because the effective area A _eff is about 80 μm ² in a commercially available 1.3 μm-band zero dispersion single-mode fiber (ITU-T G.652). Since the input power limit is proportional to the effective area A _eff , the power can be increased by a factor of about two. However, considering the current situation where the need for higher communication speeds and larger capacities has emerged, It is necessary to further increase the effective area A _eff and the use wavelength range.

  On the other hand, as a technique for actively using a multimode transmission region that transmits two or more modes in an optical fiber having a conventional structure, a mode filter (see Non-Patent Document 4 below), a Restrict-Mode-Excitation method (Non-Patent Non-Patent Document 4). Patent Document 5) has been reported.

  In the former, high-speed transmission of 10 Gbps in the vicinity of a wavelength of 0.8 μm is realized by removing the higher-order mode LP11 that causes signal degradation in front of the light receiver by a mode filter in the transmission in the two-mode wavelength region of SMF. Has been.

  In the latter case, a four-mode optical fiber is connected after the single-mode optical fiber that propagates only LP01, but the connection conditions are controlled so that LP01 is accurately coupled to the center of the core of the corresponding optical fiber. Thus, excitation of other higher-order modes is suppressed, and high-speed transmission of 10 Gbps is realized.

However, no example has been reported in which these two techniques are applied to an optical fiber in which the effective area A _eff of the basic propagation mode LP01 exceeds 200 μm ² .

Kazunori Takekasa and 4 others, “Comparison of characteristics of solid fiber for broadband transmission and Holey Fiber (HF)”, IEICE General Conference, IEICE, 2008, C-3-5 Takashi Matsui, 4 others, “Examination of effective area expansion of photonic crystal fiber”, IEICE Communication Society, IEICE, 2008, B-13-21 Kunimasa Saitoh, two others, “Endlessly single-mode hole fibers: the influencing of core design”, Optical Society of America, Optics Ex. 13, no. 26, December 26, 2005, p. 10833-10839 Tomoya Shimizu, and four others, "Multi-Band Mode Filter for Shorter Wavelength Region Transmission over Conventional SMF", Optical Fiber Communication Conference and Exhibition National Fiber Optic Engineers Conference, OFC / NFOEC 2008, 2008 years, OMH7. pdf Masanori Imamura, 4 others, "Terminal DCF-free submarine NZ-DSF transmission line by the Restriction Mode Excitation method", IEICE technical report, IEICE, OCS 2008-31, July 2008, p. . 59-62 Kunimasa Saitoh, 1 other, “Empirical relations for simple design of photonic crystal fibers”, Optical Society of America, 1.1. Optis Exp. 1, January 10, 2005, p. 267-274 Masatoshi Tanaka, 4 others, “Characteristics of LMA Photonic Crystal Fiber”, IEICE Technical Report, IEICE, OFT 2008-2, May 2008, p. 5-8

As described above, the conventional method realizes an optical fiber having an effective area A _eff of the basic propagation mode LP01 exceeding 150 μm ² and a WDM system configuration using such an optical fiber and capable of high-speed transmission. I couldn't.

From the above, the present invention realizes an optical fiber having an effective area A _eff exceeding 150 μm ² in a wide wavelength range, which has been impossible in the past, and a WDM system configuration capable of high-speed transmission using the optical fiber, An object of the present invention is to provide an optical fiber, an optical fiber design method, and an optical wavelength division multiplexing communication system that can increase the number of signal wavelengths that can be transmitted in a WDM system and increase the input power by suppressing nonlinear optical effects. To do.

An optical fiber according to a first invention for solving the above-described problems is
A core part and a clad part surrounding the core part, wherein the core part and the clad part are made of a medium having a uniform optical refractive index, and uniform voids along the longitudinal direction are formed in the clad part. In the formed photonic crystal optical fiber,
The ratio d / Λ between the hole diameter d and the hole interval Λ is 0.42 to 0.63, and the signal light has only two basic propagation modes LP01 and first higher-order mode LP11. It has a hole structure capable of propagating a mode, a bending loss αb at a bending radius of 30 mm is 0.005 dB / m or less, and the effective area A _eff is from a wavelength of about 1.0 μm to about 1.7 μm. It is characterized by being 150 μm ² or more in all wavelength regions.

An optical fiber design method according to the second invention for solving the above-mentioned problems is as follows.
A core part and a clad part surrounding the core part, wherein the core part and the clad part are made of a medium having a uniform optical refractive index, and uniform voids along the longitudinal direction are formed in the clad part. In the formed photonic crystal optical fiber,
The ratio d / Λ between the hole diameter d and the hole interval Λ is 0.42 to 0.63, and the signal light has only two basic propagation modes LP01 and first higher-order mode LP11. A method of designing an optical fiber having a hole structure capable of propagating modes,
Determining a lower limit wavelength for use;
The bending loss αb and the effective area A _eff satisfy specified values, and d / Λ at the lower limit wavelength of use is determined;
Determining Λ.

An optical wavelength division multiplexing communication system according to a third invention for solving the above-mentioned problems is
A transmitter having a light source that converts electrical signals and outputs signal lights having different wavelengths;
An optical fiber cable transmission line for propagating the input signal light;
In an optical wavelength division multiplexing communication system comprising a receiver having a light receiving circuit that converts signal light into an electrical signal,
The optical fiber according to claim 1 is used as the optical fiber cable transmission line.

An optical wavelength division multiplexing communication system according to a fourth invention for solving the above-mentioned problems is as follows.
A transmitter having two or more light sources that convert electrical signals and output signal lights having different wavelengths, and an optical multiplexer that combines the signal lights output from the light sources;
An optical fiber cable transmission line for propagating the input signal light;
An optical wavelength division multiplexing communication system comprising: an optical demultiplexer that demultiplexes signal light; and a receiver that includes a plurality of light receiving circuits that convert signal light having different wavelengths output from the optical demultiplexer into electrical signals. In
The optical fiber according to claim 1 is used as the optical fiber cable transmission line.

An optical wavelength division multiplexing communication system according to a fifth invention for solving the above-mentioned problems is the optical wavelength division multiplexing communication system according to the third invention or the fourth invention,
In the transmitter at the front stage of the optical fiber cable transmission path, at the receiver at the rear stage of the optical fiber cable transmission path, or at least one of the middle portions of the optical fiber cable transmission path, the signal light A mode filter that propagates only the basic propagation mode LP01 is provided.

An optical wavelength division multiplexing communication system according to a sixth invention for solving the above-mentioned problems is an optical wavelength division multiplexing communication system according to any one of the third to fifth inventions,
A mode filter using a photonic crystal optical fiber that propagates only the basic propagation mode LP01 of the signal light of the light source in the transmitter is provided.

  According to the present invention, it is possible to provide an optical fiber, an optical fiber design method, and an optical wavelength division multiplexing communication system that enable an increase in the number of signal wavelengths that can be transmitted and an increase in input power in a WDM system.

It is the figure which showed the example of the cross section of PCF of 1 cell structure concerning this invention. It is the figure which showed the example of the V value of PCF, and the relationship of (LAMBDA) / (lambda). It is the figure which showed the example of the relationship between V value and (LAMBDA) / (lambda) in SMF. It is the figure which showed the result of having calculated | required the relationship between d / Λ of PCF and bending loss αb by theoretical calculation. It is the figure which showed the result of having calculated | required the relationship of d / Λ of PCF and bending loss αb when the wavelength λ is 1.06 μm by theoretical calculation. It is the figure which showed the result of having calculated | required the relationship of d / Λ of PCF and bending loss αb when the wavelength λ is 1.55 μm by theoretical calculation. It is the flowchart which showed the procedure of the design method of the structure which expands the effective area _{Aeff based} on 1st Example of this invention. It is the figure which showed the wavelength dependence in 1.0 micrometer-1.7 _micrometers of effective cross-sectional area _Aeff of PCF in the structure which expands effective cross-sectional area _{Aeff based} on 1st Example of this invention. It is the figure which showed the structural example which installs the mode filter in the transmission-path side ahead of an optical multiplexer in the WDM communication system which concerns on 2nd Example of this invention. It is the figure which showed the structural example which installs the mode filter in the transmission line side behind an optical demultiplexer in the WDM communication system which concerns on 2nd Example of this invention. It is the figure which showed the structural example which installs the mode filter in either of the connection points in the middle of a PCF transmission line in the WDM communication system which concerns on 2nd Example of this invention. It is the figure which showed the measurement result of the near field pattern (near-field image) of PCF which satisfy | fills ESM conditions with d / (LAMBDA) which concerns on 3rd Example of this invention.

  Embodiments of an optical fiber, an optical fiber design method, and an optical wavelength division multiplexing communication system according to the present invention will be described below with reference to the drawings.

In view of the above-described problems, the inventors of the present application have studied as described in detail below. As a result, the two-mode PCF in which the basic propagation mode LP01 and the first higher-order mode LP11 can exist is close to the two-mode limit. In the PCF of the structure, the effective area A _eff of LP01 is expanded to about 150 μm ² or more, and the bending loss αb of LP01 is suppressed to be equal to or less than that of ordinary SMF (ITU-T G.652). Further, the present inventors have found that an optical wavelength division multiplexing communication system capable of high-speed transmission can be constructed by using the above-described mode filter and the restrict-mode-exclusion method in combination.

In the present invention, in order to solve the above problems, a two-mode PCF in which the increase in the number of modes (V value) is very gradual with respect to the core diameter and MFD is used. In the two-mode PCF, the PCF structure having an effective area A _eff of 150 μm ² or more is determined by determining the structure parameters Λ and d of the PCF in consideration of the condition that the bending loss αb is equal to or less than that of the SMF. Can be determined.

  In addition, in a WDM communication system using the PCF as a transmission line, a high-speed transmission of about 10 Gbps per wavelength is realized by suppressing transmission deterioration caused by the higher-order mode LP11 by using a mode filter and a restrict-mode-excitation method together. can do.

In the two-mode PCF used in the present invention, the effective area A _eff of 250 μm ² or more in the region near the wavelength of 1.55 μm, which is most commonly used for current communication, and the SMF is equal to or less than that Bend loss αb can be realized, and the wavelength dependence of the effective area A _eff can also be suppressed to a very small value.

In addition, when assuming the use of a region from a wavelength of 1.0 μm or more to a wavelength of about 1.7 μm, which is expected as a new high-speed transmission wavelength region, the effective area A _eff of 150 μm ² or more is equivalent to that of SMF. The following bending loss αb can be realized.

  Further, in the WDM communication system using the PCF as a transmission path, for example, if a PCF that satisfies ESM conditions that operate in a single mode at all wavelengths is separately used as a mode filter, the generation of the LP11 higher order mode is suppressed, and the LP11 higher order is provided. Since the mode can be effectively removed regardless of the wavelength, DWDM transmission with a large wavelength and a large input can be performed using a wide wavelength range, and the transmission capacity and distance can be greatly realized.

Next, a first embodiment of the present invention will be described.
As shown in FIG. 1, the optical fiber according to the present embodiment has a core part and a clad part surrounding the core part, and the core part and the clad part are made of a medium having a uniform optical refractive index. PCF1 in which a plurality of uniform holes 2 are formed in the portion along the longitudinal direction. In this example, in the PCF 1 having the 1-cell structure having the air holes 2 shown in FIG. 1, the structural parameters (diameter of the air holes 2) for realizing the increase in the effective area A _eff and the bending loss αb below the general-purpose SMF are obtained. The present invention relates to a method for selecting the distance Λ) between d and the hole 2.

In FIG. 2, the horizontal axis represents the ratio Λ / λ between Λ and wavelength λ, and the relationship between the vertical axis and the normalized frequency V _eff is shown. On the other hand, FIG. 3 shows the relationship between the abscissa axis ratio a / λ between the core portion radius a and the wavelength λ, and the ordinate axis normalized frequency V _eff for an optical fiber having a conventional structure.

The normalized frequency V _eff is given by the following formula (1).
n ₁ and n ₂ are the refractive indexes of the core portion and the cladding portion. In the PCF1 having the 1-cell structure, n ₂ is replaced with the effective refractive index n _eff of the hole cladding portion, and the radius a of the core portion is approximately Λ / √3.

As an optical fiber having a conventional structure, an optical fiber having a step-type refractive index distribution in which the relative refractive index difference between the core and the cladding is Δ, and n ₁ and Δ are those of the quartz core SMF doped with GeO ₂ is used. Typical values are used. Here, the effective refractive index n _eff of the PCF 1 is calculated using an empirical formula described in Non-Patent Document 6 above, and a scalar wave of the finite element method using an equivalent refractive index model for a step index type optical fiber. Calculation is performed by obtaining the electric field distribution E by approximate analysis.

The empirical formula described in Non-Patent Document 6 is shown in the following formula (2), and the coefficients used are shown in the following Table 1.

The effective area A _eff is obtained using the following formula (3).

The number of modes propagating in the optical fiber changes depending on the value of the normalized frequency V _eff . In other words, if the normalized frequency V _eff is 2.405 or less, the optical fiber is single-mode operation, the normalized frequency V _eff is equal to or less than a value of between 3.832 from 2.405, the These optical fibers operate in two modes.

Comparing FIG. 2 and FIG. 3, in the 1-cell structure PCF 1 of FIG. 2, if d / Λ is 0.42 or less, single-mode transmission is possible in the entire wavelength range, that is, the ESM condition is satisfied. If d / Λ is 0.63 or less, transmission in 2 modes or less is possible in the entire wavelength range. This is because the effective refractive index n _eff of the hole cladding portion has a relatively strong wavelength dependence, and as a result, the relative refractive index difference Δ increases as the wavelength λ increases. On the other hand, in the conventional SMF, the normalized frequency V _eff increases monotonously with _respect to a / λ as shown in FIG. 3, and it is extremely difficult to obtain conditions of two modes or less in a structure where a / λ is 5 or more. . This is because in SMF, the relative refractive index difference Δ is substantially constant regardless of the wavelength.

From this result, in the PCF1 having the 1-cell structure, in the structure in which d / Λ is 0.42 to 0.63, by increasing the hole interval Λ, the radius a (= Λ / √3) of the core portion and The effective area A _eff can be enlarged.

Next, FIG. 4 shows the result of theoretically calculating the relationship between Λ of PCF1 and bending loss αb in order to investigate the structural region where the bending loss αb is equal to or less than SMF.
The following formula (4) was used for calculating the bending loss αb. In the following formula (4), ρ is an effective core radius, Δ is a relative refractive index difference, R is a bending radius, W is a laterally normalized propagation constant of the cladding, and the calculation result is that the wavelength λ is It is a calculation result at 1.55 μm.

  5 and 6, regions where the SMF bending loss αb is less than the standard value (bending radius R = 30 mm, 0.005 dB / m) are indicated by hatching. 5 shows the calculation result when the wavelength λ is 1.06 μm, and FIG. 6 shows the calculation result when the wavelength λ is 1.55 μm.

Next, a method for designing a structure for increasing the effective area A _{eff according} to the first embodiment of the present invention will be described.
FIG. 7 is a flowchart showing a procedure of a method for designing a structure for increasing the effective area A _{eff according} to the first embodiment of the present invention.
As shown in FIG. 7, first, in step P1, a use lower limit wavelength is determined.
Next, in step P2, d / Λ is set to 0.42, Λ is determined in step P3, and in step P4, the normalized frequency V _eff at that time is calculated using the above equation (2). A _eff is calculated.

Next, in step P5, when the effective area A _eff is less than 150 μm ² , the process returns to step P3, the value of Λ is enlarged, and the effective area A _eff calculation procedure is repeated.
When the effective area A _eff is 150 μm ² or more in step P5, the bending loss αb is calculated using the above equation (4) in step P6.

Next, in step P7, when the bending loss αb is larger than 0.005 dB / m, the process returns to step P2 to increase d / Λ and repeat the effective area A _eff calculation procedure.
In Step P7, when the effective area A _eff is 150 μm ² and the bending loss αb is 0.005 dB / m, d / Λ and Λ are extracted in Step P8 to determine the structural parameters.

If the lower limit wavelength is 1.06 μm, and Λ is in the range of 8 μm to 16 μm, the d / Λ is adjusted in the range of 0.63 or less, and the effective area is less than the specified value of the bending loss αb of SMF. A _eff can be enlarged. The maximum effective area A _eff obtained when the lower limit wavelength is 1.06 μm is 216 μm ² . FIG. 8 shows the wavelength dependence of the effective area A _eff of the PCF 1 having this structure from 1.0 μm to 1.7 μm. FIG. 8 shows that the effective area A _eff is 200 μm ² or more in all wavelength regions.

Further, if the lower limit wavelength is 1.45 μm, the condition of the bending loss αb is more relaxed than when 1.06 μm, so if Λ is in the range of 8 μm to 18 μm, d / Λ is in the range of 0.63 or less. If adjusted, the effective area A _eff can be increased below the specified value of the bending loss αb of the SMF. The maximum effective area A _eff obtained when the lower limit wavelength is 1.45 μm is 298 μm ² .

When the lower limit wavelength is 1.45 μm, the effective area is less than the specified value of the bending loss αb of SMF by adjusting d / Λ in the range of 0.63 or less even when Λ is in the range of 18 μm or more. A _eff can be enlarged. The maximum effective area A _eff obtained under the above conditions is 330 μm ² .

That is, the optical fiber according to the present invention has a core part and a cladding part surrounding the core part, the core part and the cladding part are made of a medium having a uniform optical refractive index, and the cladding part extends along the longitudinal direction. In the PCF 1 in which a plurality of uniform holes are formed, the ratio d / Λ between the diameter d of the holes 2 and the interval Λ of the holes 2 is 0.42 to 0.63, and the fundamental of the signal light It has a hole structure capable of propagating only two modes, the propagation mode LP01 and the first higher-order mode LP11, the bending loss αb at a bending radius of 30 mm is 0.005 dB / m or less, and the effective area A _eff is The size is 150 μm ² or more in all wavelength regions from a wavelength of about 1.0 μm to about 1.7 μm.

The optical fiber design method according to the present invention includes a core part and a clad part surrounding the core part, and the core part and the clad part are made of a medium having a uniform optical refractive index, and the clad part is elongated in the longitudinal direction. In the PCF 1 in which a plurality of uniform holes are formed along the direction, the ratio d / Λ between the diameter d of the holes 2 and the interval Λ of the holes 2 is 0.42 to 0.63, and the signal A method for designing an optical fiber having a hole structure capable of propagating only two modes of a fundamental propagation mode LP01 and a first higher-order mode LP11, the step of determining a lower limit wavelength, a bending loss αb, The effective cross-sectional area A _eff satisfies a specified value, and includes a step of determining d / Λ at a lower limit wavelength for use and a step of determining Λ.

Next, a second embodiment of the present invention will be described.
The present embodiment relates to a configuration example of a WDM communication system using the PCF 1 shown in the first embodiment as a transmission path.
Hereinafter, an example of the entire system configuration when using a mode filter and when using a restrict-mode-excitation method will be described. The restrict-mode-excitation method described in Non-Patent Document 5 is a target. A single mode optical fiber that propagates only the LP01 mode is accurately connected to the center of the core of the multimode optical fiber in front of the multimode fiber (on the side closer to the light source) for effective single mode transmission. It is a technique to realize.

  This is synonymous with the fact that the mode filter that cuts off the higher-order mode is disposed in the front stage (between the light source and the transmission line) of the transmission line optical fiber. Therefore, a configuration example of the WDM communication system corresponding to the usage position of the mode filter is shown below.

  The transmitter 11 in the WDM communication system shown in FIG. 9 includes N light sources 17 that convert electrical signals and output signal lights having different wavelengths, and an optical multiplexer 12 that multiplexes n signal lights. Has been. The receiver 15 receives an optical demultiplexer 14 that demultiplexes the combined signal light into N signal lights, and n signal lights that are demultiplexed by the optical demultiplexer 14. And a light receiving circuit 18 for converting the signal.

  The optical multiplexer 12 of the transmitter 11 and the optical demultiplexer 14 of the receiver 15 are connected by a PCF transmission path 13. The signal light output from the light source 17 of the transmitter 11 is combined by the optical multiplexer 12, and this combined signal light is propagated by the PCF transmission path 13 and received by the light receiving circuit 18 of the receiver 15 for electrical use. Converted to a signal. In the WDM communication system shown in FIG. 9, the mode filter 16 is installed in the transmitter 11 in the previous stage of the PCF transmission path 13.

  On the other hand, in the configuration shown in FIG. 9, a higher-order mode LP11 may occur in the middle of the PCF transmission path 513 depending on conditions such as transmission distance, bending, and connection point, and signal degradation due to multimode dispersion may occur. . In that case, a mode filter 19 for removing the higher-order mode LP11 is installed in the receiver 15 at the rear stage of the PCF transmission line 13 as shown in FIG. 10, or in the middle of the PCF transmission line 13 as shown in FIG. It is possible to prevent transmission degradation by installing it at the connection point. Of course, it is also possible to install a plurality of mode filters 16 and 19 at different locations depending on the situation.

For use in WDM, it is desirable that the mode filters 16 and 19 to be used have a wider band (wavelength region for blocking higher-order modes), and the WDM wavelength region capable of high-speed and long-distance transmission is the mode filter 16. , 19 is limited by the band range. However, since the two-mode PCF 1 with an enlarged effective area A _{eff according} to the present invention realizes two or less modes in all wavelength regions, for example, as shown in FIG. By using 19, it is possible to select a wide range of operating wavelengths.

  Further, in this embodiment, an example of using a wavelength of 1.0 μm or more is given. However, if the mode filters 16 and 19 operating at 1.0 μm or less are used, for example, 0.8 μm used for a LAN or the like. High-speed transmission using near-wavelength light is also possible. Naturally, not only WDM transmission but also transmission using only one wavelength is possible. In this case, the optical multiplexer 12 and the optical demultiplexer 14 are unnecessary. is there.

  As the mode filters 16 and 19, an optical fiber that propagates in a single mode at a desired wavelength, such as the Restrict-Mode-Excitation method described in Non-Patent Document 5, can be used. In addition, the coupler type used in Non-Patent Document 4 above, the type that bends the optical fiber to remove higher-order modes, the type that uses a tapered optical fiber, and the fiber grating are used. There are types, and these may be used as appropriate. Further, from the viewpoint of the connectivity (connection loss) between the mode filters 16 and 19 and the PCF transmission line 13, the mode field diameters (MFD) of the mode filters 16 and 19 and the PCF transmission line 13 should be matched as much as possible. desirable.

Next, a third embodiment of the present invention will be described.
In this embodiment, in each WDM communication system configuration shown in the second embodiment, the ESM-structured PCF 1 is used as the mode filters 16 and 19 in the transmitter 11, in the middle of the PCF transmission line 13, or in the receiver 15. It relates to a system configuration example provided in

  As described in the second embodiment, it is desirable that the mode filters 16 and 19 to be used have a wider band (wavelength range for blocking higher-order modes), and the MFD matches the PCF transmission line 13. It is desirable. A PCF 1 having an ESM structure is suitable as the mode filters 16 and 19 that satisfy both conditions.

  As described in Non-Patent Document 3, in the PCF1 having the 1-cell structure, when the structural parameter d / Λ is smaller than about 0.42, only the LP01 mode is propagated no matter how short the transmission wavelength is. It has been theoretically clarified that it operates in a single mode (ESM operation).

  As an example, FIG. 12 shows the measurement result of the near field pattern (near-field image) of PCF1 where d / Λ is 0.3 and satisfies the ESM condition. 12A shows the measurement result when the transmission wavelength is 0.85 μm, FIG. 12B shows the measurement result when the transmission wavelength is 1.30 μm, and FIG. 12C shows the transmission wavelength when the transmission wavelength is 1.55 μm.

  As shown in FIG. 12, PCF1 satisfying the ESM condition with d / Λ of 0.3 has a wavelength of 0.85 μm, 1.30 μm, and 1.55 μm, regardless of the measurement result in a very short state. It can be seen that a centrally symmetric pattern showing the propagation of LP01 only is obtained. Accordingly, such PCF 1 may be appropriately used as the mode filters 16 and 19.

  When the difference between the MFD values of the two-mode PCF transmission line 13 and the mode filters 16 and 19 is large, connection loss may occur. However, if d / Λ is 0.42 or less in a 1-cell structure, the ESM condition can be maintained as shown in FIG. 2, and therefore MFD can be obtained by using PCF1 in which the value of Λ is adjusted to an appropriate value under this condition. Therefore, the connection loss can be made sufficiently small.

  For the connection of the mode filters 16 and 19 by the PCF transmission line 13 and the PCF 1, the fusion connection can be used after the conditions such as the discharge time are optimized. Further, for example, as shown in Non-Patent Document 7, the end face of each PCF 1 can be processed and connected to a connector.

  In the above-mentioned PCF mode filter, the effect has been confirmed for an optical fiber having a normal structure (A) application of bending, (B) application of refractive index matching agent, (C) increase of coating film thickness, etc. By using this method, propagation of clad mode light can be prevented. Therefore, if these methods are used, the mode filters 16 and 19 can be shortened and miniaturized.

  As described above, the bending loss αb of the propagation mode LP01 is larger in the PCF1 that satisfies such an ESM condition than in the two-mode PCF transmission line 13. However, in using the parts such as the mode filters 16 and 19, there is no need to use a small bending diameter such as R = 30 mm, and unlike the transmission line, bending is not applied by laying or the like. Therefore, the range of the allowable bending diameter is greatly relaxed as compared with the transmission line. Therefore, by mounting and fixing within the range of the conditions, it is possible to obtain characteristics that have no practical problem.

  As described above, according to the optical fiber, the optical fiber design method, and the optical wavelength division multiplexing communication system according to the present invention, it is possible to suppress the nonlinear optical effect in a wide wavelength range that could not be realized by the prior art, and to achieve the WDM. Through an increase in the number of signal wavelengths that can be transmitted in the system and an increase in input power, the distance for high-speed transmission of 10 Gbps or more can be greatly increased.

  The present invention provides, for example, an optical fiber for utilizing an optical wavelength division multiplexing (WDM) technique in which a plurality of signal lights having different wavelengths are used and light having a relatively large power is input to the optical fiber, an optical fiber design method, and It can be used in an optical wavelength division multiplexing communication system.

1 PCF (photonic crystal fiber)
2 Hole 11 Transmitter 12 Optical multiplexer 13 PCF transmission line 14 Optical demultiplexer 15 Receiver 16 Mode filter 17 Light source 18 Light receiving circuit 19 Mode filter

Claims (6)

A core part and a clad part surrounding the core part, wherein the core part and the clad part are made of a medium having a uniform optical refractive index, and uniform voids along the longitudinal direction are formed in the clad part. In the formed photonic crystal optical fiber,
The ratio d / Λ between the hole diameter d and the hole interval Λ is 0.42 to 0.63, and the signal light has only two basic propagation modes LP01 and first higher-order mode LP11. It has a hole structure capable of propagating a mode, a bending loss αb at a bending radius of 30 mm is 0.005 dB / m or less, and the effective area A _eff is from a wavelength of about 1.0 μm to about 1.7 μm. An optical fiber characterized by being 150 μm ² or more in all wavelength regions. A core part and a clad part surrounding the core part, wherein the core part and the clad part are made of a medium having a uniform optical refractive index, and uniform voids along the longitudinal direction are formed in the clad part. In the formed photonic crystal optical fiber,
The ratio d / Λ between the hole diameter d and the hole interval Λ is 0.42 to 0.63, and the signal light has only two basic propagation modes LP01 and first higher-order mode LP11. A method of designing an optical fiber having a hole structure capable of propagating modes,
Determining a lower limit wavelength for use;
The bending loss αb and the effective area A _eff satisfy specified values, and d / Λ at the lower limit wavelength of use is determined;
A method of designing an optical fiber, comprising: determining Λ. A transmitter having a light source that converts electrical signals and outputs signal lights having different wavelengths;
An optical fiber cable transmission line for propagating the input signal light;
In an optical wavelength division multiplexing communication system comprising a receiver having a light receiving circuit that converts signal light into an electrical signal,
An optical wavelength division multiplexing communication system using the optical fiber according to claim 1 as the optical fiber cable transmission line. A transmitter having two or more light sources that convert electrical signals and output signal lights having different wavelengths, and an optical multiplexer that combines the signal lights output from the light sources;
An optical fiber cable transmission line for propagating the input signal light;
An optical wavelength division multiplexing communication system comprising: an optical demultiplexer that demultiplexes signal light; and a receiver that includes a plurality of light receiving circuits that convert signal light having different wavelengths output from the optical demultiplexer into electrical signals. In
An optical wavelength division multiplexing communication system using the optical fiber according to claim 1 as the optical fiber cable transmission line. In the transmitter at the front stage of the optical fiber cable transmission path, at the receiver at the rear stage of the optical fiber cable transmission path, or at least one of the middle portions of the optical fiber cable transmission path, the signal light The optical wavelength division multiplexing communication system according to claim 3 or 4, further comprising a mode filter that propagates only the basic propagation mode LP01. The mode filter using the photonic crystal optical fiber which propagates only the fundamental propagation mode LP01 of the signal light of the light source in the transmitter is provided. Optical wavelength division multiplexing communication system.