Patent Grant
10539784
The present disclosure relates to a photonic crystal fiber and a high-power optical transmission system that enable high power optical transmission.
Along with the progress of high-power lasers, applications to industrial processing such as welding using laser light are widely employed. Especially in recent years, high-power fiber lasers having an output of as high as 10 kW have been developed and are expected to be used for medical and industrial applications. In such a high-power fiber laser, for example as illustrated in Non-Patent Literature 1, the core area is enlarged in a short optical fiber of several meters or less, thereby relaxing an output power limit due to nonlinearity. Moreover, in laser processing, the beam quality of emitted light greatly affects the processing efficiency. Since the beam quality strongly depends on a mode state of emitted light, an optical fiber capable of single mode transmission is used in a fiber laser.
Furthermore as illustrated in Non-Patent Literature 2, an optical fiber is coupled to an emitting end of the high-power laser described above, which is applied also to welding processing from a remote place. In this case, the beam quality at the emitting end is affected by an excitation state of a higher order mode in the coupled optical fiber. Therefore, connecting a multimode optical fiber having a large core area as a transmission optical fiber enables transmission of high-power light such as several kilowatts for several tens of meters or more, however, the beam quality at the emitting end becomes low. In order to enhance the beam quality, it is necessary to reduce the number of propagation modes; however, in structure design of the optical fibers in general, reduction of the number of propagation modes and enlargement of a core area are in a trade-off relationship, and thus an attempt to enhance the beam quality results in limiting the power that can be transmitted.
Furthermore as illustrated in Non-Patent Document 3, it is known that, by using a photonic crystal fiber having a hole structure, the trade-off between a single mode operation region and enlargement of a core area can be relaxed as compared with a general optical fiber in which a refractive index distribution is formed by adding a dopant to a core. Therefore, it is known that, in a photonic crystal fiber for a communication application, deterioration of tradition characteristics due to a nonlinear effect can be mitigated in an optical communication system. Furthermore in Patent Literature 1, it is known that the trade-off between the single mode operation region and enlargement of a core area can be further relaxed as compared with a uniform structure by using a photonic crystal fiber in which holes are arranged non-uniformly.
As described above, obtaining a wide core area with a small number of propagation modes in order to obtain high quality and high output power with a long propagation distance is in a trade-off relationship in the conventional optical fibers, and thus there is a problem that high-power light having high output and a high quality cannot be obtained. Furthermore, even in the case of using a photonic crystal fiber, in industrial high-power transmission applications of a kilowatt class far exceeding communication applications, there is a problem that it is unknown how much the trade-off is improved and that structure design suitable for obtaining high quality and high output power is unclear.
Therefore, in order to solve the above problems, an object of the present invention is to provide a photonic crystal fiber in which a wide core area can be obtained with a limited number of propagation modes and a high-power optical transmission system including the photonic crystal fiber and having a high beam quality.
According to the present invention, holes of a photonic crystal fiber are arranged such that, in a cross section, a hole ratio, which is an area of the holes per unit area, is larger in a central side than in an outer side in a portion corresponding to a cladding and that a wide core area can be obtained while the number of modes that can be propagated is limited to several. Moreover, in a high-power optical transmission system according to the present invention, the amount of axis misalignment between the central axis of a laser oscillator and the central axis of the photonic crystal fiber is less than or equal to a certain amount.
Specifically, a first photonic crystal fiber according to the present invention is a photonic crystal fiber having a plurality of holes arranged in the optical fiber along a longitudinal direction,
in which, in a cross section, a hole ratio which is an area of the holes per unit area is larger in a central side than in an outer side in a portion corresponding to a cladding,
an interval among all of the holes is Λ, and a diameter d1 of the holes in the central side is larger than a diameter d of the holes in the outer side, and, when Λ is represented in a horizontal axis and d1/d is represented in a vertical axis, Λ, d1, and d are in a region where respective regions represented by mathematical formulas C1 overlap, and
a bending loss of a basic mode is 1 dB/km with a bending radius of 500 mm or less.
[Mathematical Formulas C1]
d1/d≤0.633Λ−5.467 (Λ≤11.8 μm)
d1/d≤−0.0429Λ+2.486 (11.8 μm≤Λ≤15.4 μm)
d1/d≤0.0454Λ+1.13 (Λ≥15.4 μm)
d1/d≥1 (Λ≤16.8 μm)
d1/d≥0.117Λ−0.96 (Λ≥16.8 μm) (C1)
It is preferable that the number of propagation modes of the first photonic crystal fiber according to the present invention is three or less in order not to deteriorate the beam quality even in a case where incident light from a laser oscillator and a mode field diameter of the photonic crystal fiber are not matched.
Moreover, the first photonic crystal fiber according to the present invention enables high power transmission of light and thus does not generate output saturation due to stimulated Raman scattering upon propagation of light of 90 kW·m.
A second photonic crystal fiber according to the present invention is a photonic crystal fiber having a plurality of holes arranged in the optical fiber along a longitudinal direction,
in which, in a cross section, a hole ratio which is an area of the holes per unit area is larger in a central side than in an outer side in a portion corresponding to a cladding,
a diameter of all of the holes is d, and an interval Λ1 of the holes in the central side is smaller than an interval Λ of the holes in the outer side, and, when Λ is represented in a horizontal axis and d/Λ is represented in a vertical axis, Λ and d are in a region where respective regions represented by mathematical formulas C2 overlap, and
a bending loss of a basic mode is 1 dB/km with a bending radius of 500 mm or less.
[Mathematical Formulas C2]
d/Λ≤0.24Λ−2.22 (Λ≤10.8 μm)
d/Λ≤0.00667Λ+0.293 (10.8 μm≤Λ≤19.5 μm)
d/Λ≤0.01Λ+0.23 (Λ≥19.5 μm)
d/Λ≥0.3 (Λ≤19.2 μm)
d/Λ≥0.0195Λ−0.075 (Λ≥19.2 μm) (C2)
It is preferable that the number of propagation modes of the second photonic crystal fiber according to the present invention is three or less in order not to deteriorate the beam quality even in a case where incident light from a laser oscillator and a mode field diameter of the photonic crystal fiber are not matched.
Moreover, the second photonic crystal fiber according to the present invention enables high power transmission of light and thus does not generate output saturation due to stimulated Raman scattering upon propagation of light of 90 kW·m.
A third photonic crystal fiber according to the present invention is a photonic crystal fiber having a plurality of holes arranged in the optical fiber along a longitudinal direction,
in which, in a cross section, a hole ratio which is an area of the holes per unit area is larger in a central side than in an outer side in a portion corresponding to a cladding,
three or more layers having different hole ratios from each other are arranged concentrically with a layer closer to the center having a larger hole ratio,
a diameter d of all the holes are the same, and, when an interval Λ between a hole in a central layer closest to the center and a hole in an adjacent layer adjacent to the central layer is represented in a horizontal axis and d/Λ is represented in a vertical axis, Λ and d are in a region where respective regions represented by mathematical formulas C3 overlap, and
a bending loss of a basic mode is 1 dB/km with a bending radius of 500 mm or less.
[Mathematical Formulas C3]
d/Λ≤0.22Λ−2.01 (Λ≤10.9 μm)
d/Λ≤−0.000769Λ+0.398 (10.9 μm≤Λ≤16.1 μm)
d/Λ≤0.004Λ+0.32 (Λ≥16.1 μm)
d/Λ≥0.00172Λ+0.322 (Λ≤15.2 μm)
d/Λ≥0.0064Λ+0.250 (Λ≥15.2 μm) (C3)
It is preferable that the number of propagation modes of the third photonic crystal fiber according to the present invention is three or less in order not to deteriorate the beam quality even in a case where incident light from a laser oscillator and a mode field diameter of the photonic crystal fiber are not matched.
Moreover, the third photonic crystal fiber according to the present invention enables high power transmission of light and thus does not generate output saturation due to stimulated Raman scattering upon propagation of light of 90 kW·m.
A fourth photonic crystal fiber according to the present invention is a photonic crystal fiber having a plurality of holes arranged in the optical fiber along a longitudinal direction,
in which, in a cross section, a hole ratio which is an area of the holes per unit area is larger in a central side than in an outer side in a portion corresponding to a cladding,
three or more layers having different hole ratios from each other are arranged concentrically with a layer closer to the center having a larger hole ratio,
a diameter d of all the holes are the same, and, when an interval Λ between a hole in a central layer closest to the center and a hole in an adjacent layer adjacent to the central layer is represented in a horizontal axis and d/Λ is represented in a vertical axis, Λ and d are in a region where respective regions represented by mathematical formulas C4 overlap, and
a bending loss of a basic mode is 1 dB/km with a bending radius of 500 mm or less.
[Mathematical Formulas C4]
d/Λ≤0.22Λ−2.01 (Λ≤11 μm)
d/Λ≤0.407 (11 μm≤Λ≤18.7 μm)
d/Λ≤0.00333Λ+0.345 (Λ≥18.7 μm)
d/Λ≥0.00167Λ+0.323 (Λ≥14.5 μm)
d/Λ≥0.00625Λ+0.255 (Λ≥14.5 μm) (C4)
It is preferable that the number of propagation modes of the fourth photonic crystal fiber according to the present invention is four or less in order not to deteriorate the beam quality even in a case where incident light from a laser oscillator and a mode field diameter of the photonic crystal fiber are not matched.
Moreover, the fourth photonic crystal fiber according to the present invention enables high power transmission of light and thus does not generate output saturation due to stimulated Raman scattering upon propagation of light of 90 kW·m.
By allowing a hole structure of a photonic crystal fiber nonuniform in the range of the mathematical formulas C1 to C4, it is possible to obtain a wide core area while the number of modes that can be propagated is limited to several. Therefore, the present invention enables provision of a photonic crystal fiber in which a wide core area can be obtained with a limited number of propagation modes.
Furthermore, a high-power optical transmission system according to the present invention includes a laser oscillator, the photonic crystal fiber, and a coupling part for emitting light from the laser oscillator to the photonic crystal fiber,
in which, in the coupling part, an amount of misalignment between a central axis of the light emitted from the laser oscillator and a central axis of the photonic crystal fiber is 0.95 or less as a relative value relative to a mode field radius of the photonic crystal fiber, and a beam radius of the light from the laser oscillator relative to a mode field radius of the photonic crystal fiber is 0.5 or more.
Since an LP01 mode and LP21 have component peaks at positions shifted from the center of the fiber, the coupling efficiency increases when there is axis misalignment at a connecting part of the optical fiber. Therefore, by allowing the amount of misalignment between the central axis of the light emitted from the laser oscillator and the central axis of the photonic crystal fiber to be 0.95 or less as a relative value with respect to the mode field radius of the photonic crystal fiber, the coupling efficiency between the LP01 mode and LP21 from a laser emitting part to the photonic crystal fiber can be reduced. Therefore, even when an effective cross-sectional area of the photonic crystal fiber is enlarged, the power of a propagation mode other than the basic mode can be reduced, and the beam quality can be enhanced. Therefore, the present invention can provide a high-power optical transmission system that includes a photonic crystal fiber in which a wide core area can be obtained with a limited number of propagation modes and has a high beam quality.
The present invention can provide a photonic crystal fiber in which a wide core area can be obtained with a limited number of propagation modes and a high-power optical transmission system including the photonic crystal fiber and having a high beam quality.
Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. Note that components having the same symbol in the present description and the drawings represent items identical to each other.
A photonic crystal fiber of the present embodiment has a plurality of holes arranged in the optical fiber along a longitudinal direction, in which, in a cross section, a hole ratio which is an area of the holes per unit area is larger in a central side than in an outer side in a portion corresponding to a cladding, and the number of propagation modes in which propagation is possible is 3 or less.
In
In
Here, the maximum value of output light power can be increased by enlarging an effective cross-sectional area of the basic mode, and the effective cross-sectional area can be enlarged by increasing the inter-hole interval Λ of the PCF. Therefore, at an intersection of the solid line and the broken line, propagation in the LP02 mode and increase in the bending loss can be simultaneously prevented, and the effective cross-sectional area can be maximized at a bending radius corresponding to the broken line.
Here, the relationship between the maximum value of output light power (maximum output power) and the effective cross-sectional area of the basic mode will be described. The maximum output power Pth is determined by an output power limit (stimulated Raman threshold) due to stimulated Raman scattering out of the nonlinearity and is expressed by the following mathematical formula.
[Mathematical Formula C5]
Note that the above mathematical formula is widely known as a mathematical formula for deriving a saturation threshold of output power by the stimulated Raman including Non-Patent Literature 4, where gR represents a Raman gain coefficient, Aeff represents an effective cross-sectional area, and L represents a transmission distance.
Leff can be derived from a transmission loss α and the transmission distance L by Leff=(1−exp(αL))α, however in a case where the transmission distance is relatively short such as about 1 km or less, Leff and L can be regarded as equivalent. Therefore, as a high-power transmission performance, a product Pth·L of the induced Raman threshold Pth and the transmission distance L which is a parameter proportional to Aeff can be used. Note that, according to Non-Patent Literature 4, in the case of pure quartz, gR is about 1.0 e−13 m/W at a wavelength of 1.06 μm. For example, where Aeff is 500 μm2, the high-power transmission performance Pth·L is about 90 kW·m.
Furthermore, structural conditions under which the effective cross-sectional area is a predetermined value is illustrated by a dotted line in
In
[Mathematical Formulas C1]
d1/d≤0.633Λ−5.467 (Λ≤11.8 μm)
d1/d≤−0.0429Λ+2.486 (11.8 μm≤Λ≤15.4 μm)
d1/d≤0.0454Λ+1.13 (Λ≥15.4 μm)
d1/d≥1 (Λ≤16.8 μm)
d1/d≥0.117Λ−0.96 (Λ≥16.8 μm) (C1)
Note that, in general, it is known that the bending loss of a PCF increases more as the wavelength becomes shorter, and it is obvious that the bending loss becomes further smaller as the wavelength becomes longer than 1.07 μm.
In
In
[Mathematical Formulas C2]
d/Λ≤0.24Λ−2.22 (Λ≤10.8 μm)
d/Λ≤0.00667Λ+0.293 (10.8 μm≤Λ≤19.5 μm)
d/Λ≤0.01Λ+0.23 (Λ≤19.5 μm)
d/Λ≥0.3 (Λ≤19.2 μm)
d/Λ≥0.0195Λ−0.075 (Λ≥19.2 μm) (C2)
There is a certain correlation between the effective cross-sectional area Aeff and the allowable bending radius R, and assuming Mathematical Formula 1 using proportional coefficients a and b results in a high correlation with the result of
Aeff≤aRb (1)
For example in
For example, by using a PCF obtained by the structure design described above, in the case of transmission of about 50 m as illustrated in
In a PCF of the present embodiment, three or more layers having different ratios of holes 11 from each other are arranged concentrically with a layer closer to the center has a larger ratio of the holes 11, and the number of propagation modes that can propagate is 4 or less.
In
In
[Mathematical Formulas C3]
d/Λ≤0.22Λ−2.01 (Λ≤10.9 μm)
d/Λ≤−0.000769Λ+0.398 (10.9 μm≤Λ≤16.1 μm)
d/Λ≥0.004Λ+0.32 (Λ≥16.1 μm)
d/Λ≥0.00172Λ+0.322 (Λ≥15.2 μm)
d/Λ≥0.0064Λ+0.250 (Λ≥15.2 μm) (C3)
In
[Mathematical Formulas C4]
d/Λ≤0.22Λ−2.01 (Λ≤11 μm)
d/Λ≤0.407 (11 μm≤Λ≤18.7 μm)
d/Λ≤0.00333Λ+0.345 (Λ≥18.7 μm)
d/Λ≥0.00167Λ+0.323 (Λ≥14.5 μm)
d/Λ≥0.00625Λ+0.255 (Λ≥14.5 μm) (C4)
a specification value determining step of determining a wavelength of light propagated in the photonic crystal fiber, power Pth of the light propagated in the photonic crystal fiber, and a propagation distance L through which the light is propagated in the photonic crystal fiber;
an effective cross-sectional area calculating step of calculating a required effective cross-sectional area Aeff of the photonic crystal fiber by utilizing a mathematical formula C5 on the basis of the power Pth and the propagation distance L having been determined in the specification value determining step and a Raman gain coefficient gR;
a hole structure detecting step of calculating an effective cross-sectional area Aeff from a diameter d and an interval Λ of the holes of the photonic crystal fiber, and detecting the diameter d and the interval Λ of the holes of the Aeff satisfying or exceeding the required Aeff having been calculated in the effective cross-sectional area calculating step on the basis of a plotted graph having a horizontal axis of d/Λ and a vertical axis of Λ;
a bending radius determining step of determining a region of allowable bending radius with which a basic mode can be propagated in the photonic crystal fiber;
a region detecting step of detecting an overlapping region in which a non-propagating region in which the LP02 mode or the LP31 mode is not propagated, a region of the allowable bending radius having been determined in the bending radius determining step, and a region based on a product of power of light from a laser oscillator and the propagation distance L overlap in a graph representing the interval Λ of the holes on a horizontal axis and a ratio (d1/d) of a diameter d1 of the holes on the central side and the diameter d of holes adjacent to the holes in the central side from the outer side thereof on a vertical axis or in a graph representing the interval Λ of the holes adjacent to the holes in the central side from the outer side thereof on a horizontal axis and a ratio (d/Λ) of the diameter d and the interval Λ of the holes on a vertical axis; and
a structure determining step of determining the interval Λ and the ratio (d1/d) or the interval Λ and the ratio (d/Λ) in the overlapping region as a structure of the photonic crystal fiber.
In a specification value determining step S01, the wavelength, the transmission distance, and the output power Pth are set as parameters. In an effective cross-sectional area calculating step S02, an effective cross-sectional area is calculated by the mathematical formula (C5) on the basis of the specifications having been set. Note that theoretically, L in the mathematical formula (C5) can be replaced by the interaction length defined by Leff=(1−exp(αL))/α; however, since it is assumed that the optical fiber of the present invention has a relatively short transmission distance such as 1 km or less, and Leff and L are equivalent values, the transmission distance L is used. Note that the transmission distance is not limited to 1 km or less and can be similarly applied as long as Leff and L can be regarded as equivalent.
In a hole structure detecting step S03, a hole structure giving the calculated effective cross-sectional area is detected. Specifically, a structure such as that of
In a region detecting step, steps from S05 to S07 are performed.
In step S05, the bending loss in the LP01 mode at the allowable bending radius having been set in the bending radius determining step S04 in the PCF having the hole structure detected in the hole structure detecting step S03 is calculated (for example, the bending loss in the LP01 mode is in the range of 1 dB/km or less, and a left side region of a broken curve in
If the hole structure is included in the region in which the condition of the number of propagation modes is 3 or less, that is, the region under the solid line of the LP02 mode non-propagation condition in
Noted that the three propagation modes refer to the LP01 mode, the LP11 mode, and the LP21 mode, and the four propagation modes refer to the LP01 mode, the LP11 mode, the LP21 mode, and the LP02 mode.
The PCF according to the present invention can be applied to industrial processing using high-power light.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2015-240984 | Dec 2015 | JP | national |
The present application is a divisional of U.S. patent application Ser. No. 15/772,263, filed Apr. 30, 2018, which is a U.S. National Stage Entry of International Patent Application No. PCT/JP2016/084024, filed Nov. 17, 2016, which claims priority to Japanese Patent Application No. 2015-240984, filed Dec. 10, 2015, and the entire content of these applications is herein incorporated by reference.
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| Number | Date | Country | |
|---|---|---|---|
| 20190339513 A1 | Nov 2019 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 15772263 | US | |
| Child | 16507570 | US |
