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Patent Issued for Amplification Optical Fiber, and Optical Fiber Amplifier and Resonator Using Same

July 2, 2014



By a News Reporter-Staff News Editor at Journal of Engineering -- From Alexandria, Virginia, VerticalNews journalists report that a patent by the inventor Kashiwagi, Masahiro (Sakura, JP), filed on September 26, 2013, was published online on June 17, 2014.

The patent's assignee for patent number 8755111 is Fujikura Ltd. (Tokyo, JP).

News editors obtained the following quote from the background information supplied by the inventors: "As one of fiber laser devices which are used for a processing machine or medical equipment, an MO-PA (master oscillator-power amplifier) type fiber laser device in which light generated by a seed light source, such as a laser oscillator (MO: Master oscillator), is amplified by an amplifier (PA: power amplifier) to be emitted has been known. As one of amplifiers which are used as the above amplifier, an optical fiber amplifier which amplifies the light using an amplification optical fiber is known.

"In such an amplification optical fiber, a double clad fiber in which an active element such as a rare-earth element is added in a core is generally used. The double clad fiber includes a double clad fiber in which the core propagates only single mode. light and a double clad fiber in which the core propagates multi-mode light. Therefore, in the double clad fiber in which the core propagates only the single mode light, a cross-sectional area of the core is small. Accordingly, in order to obtain a high power laser output, a density of light which may propagate in the core may be increased. In this case, an optical energy is transited to an undesired wavelength by a non-linear optical effect and thus an expected laser output cannot be obtained. Therefore, in accordance with demands for increasing a power of the optical fiber amplifier in recent years, an optical fiber amplifier using a double clad fiber in which a core propagates multi-mode light attracts attentions.

"When the double clad fiber in which the light is propagated in a multi-mode is used as the amplification optical fiber, in the propagated light, an LP01 mode (basic mode) or a higher order mode such as an LP02 mode is pumped. Patent Document 1 discloses an amplification optical fiber in which an active element is added so as to be distributed in accordance with a higher order mode intensity profile. [Patent Document 1] JP 2010-516633 W"

As a supplement to the background information on this patent, VerticalNews correspondents also obtained the inventor's summary information for this patent: "Objects to be Achieved by the Invention

"However, the amplification optical fiber disclosed in Patent Document 1, if the active element is simply added so as to be distributed in accordance with the higher order mode intensity profile, even the LP01 mode other than the higher order mode to be amplified is also amplified at a high amplification factor. In the LP01 mode, a location having a higher intensity is concentrated at a center of the core. Therefore, if the LP01 mode is amplified, a light density becomes too high at the center of the core so that the non-linear optical effect may easily occur, and thus expected emitting light may not be obtained. Further, in general, the amplification optical fiber is used with at least a part which is arranged to be bent. However, in the LP01 mode, in a location where the amplification optical fiber is bent, the light is distributed to an outer peripheral side of the core and a mode field is distorted so that an area is easily decreased. Accordingly, in the LP01 mode, the non-linear optical effect caused by the concentration of light may easily occur in a location where the amplification optical fiber is bent. Therefore, in order to emit high power light, there is a possibility that emitted light having a desired power may not be obtained by the non-linear optical effect.

"Therefore, the present invention has been made in an effort to provide an amplification optical fiber which is capable of emitting light having a desired power, and an optical fiber amplifier and a resonator using the same.

"Means for Achieving the Objects

"In order to solve the above-mentioned problem, the present invention provides an amplification optical fiber including a core and a clad which covers the core. The core propagates light having a predetermined wavelength in at least an LP01 mode, an LP02 mode and an LP03 mode. In the core, when the LP01 mode, the LP02 mode and the LP03 mode are standardized by a power, in at least a part of a region where an intensity of at least one of the LP02 mode and the LP03 mode is stronger than an intensity of the LP01 mode, an active element which stimulates and emits the light having a predetermined wavelength is added with a higher concentration than that in at least a part of a region where the intensity of the LP01 mode is stronger than the intensities of the LP02 mode and the LP03 mode, and at least one of the following Formula (1) and Formula (2) is satisfied. .intg..sub.0.sup.bn(r).times.{I.sub.02(r)-I.sub.01(r)}rdr>0 (1) .intg..sub.0.sup.bn(r).times.{I.sub.03(r)-I.sub.01(r)}rdr>0 (2)

"(In this case, r is a distance from the center in the radial direction of the core, I.sub.01(r) is an intensity of the LP01 mode in the distance r from the center in the radial direction of the core, I.sub.02(r) is an intensity of the LP02 mode in the distance r from the center in the radial direction of the core, and I.sub.03(r) is an intensity of the LP03 mode in the distance r from the center in the radial direction of the core, n(r) is a concentration of an added active element. in the distance r from the center in the radial direction of the core, and b is a radius of the core.)

"In the amplification optical fiber of the present invention, in the region where at least one of the LP02 mode and the LP03 mode is stronger than the LP01 mode, the active element is added with a higher concentration than that in at least a part of a region where the intensity of the LP01 mode is stronger than at least a part of the intensities of the LP02 mode and the LP03 mode. In a region where the active element is added with a high concentration the amplification factor of the light is high so that at least one of the LP02 mode and the LP03 mode is amplified at a high amplification factor. In the meantime, at least a part of the region where the intensity of the LP01 mode is higher than the intensities of the LP02 mode and the LP03 mode, the amplification factor of the light is not high so that the amplification of the LP01 mode is suppressed. As described above, in the amplification optical fiber of the present invention, the amplification of the LP01 mode is suppressed, so that at least one of the LP02 mode and the LP03 mode is actively amplified so as to satisfy at least one of the above Formula (1) and Formula (2) in the entire fiber.

"However, a high order mode such as the LP02 mode or the LP03 mode, a location having a high intensity is distributed in the radial direction of the core. Accordingly, even when the high order mode is amplified, the non-linear optical effect is hard to occur as compared with the case when the LP01 mode is amplified. Therefore, even when the high power light is emitted, optical energy is suppressed to be transited to a different wavelength by the non-linear optical effect and desired power light may be emitted.

"According to the knowledge of the inventor, even when the amplification optical fiber is used while being bent in the high order mode, center of light is hard to be deviated to the outer peripheral side of the core, and the area of the mode field is also hard to be small. Therefore, in the amplification optical fiber of the invention, even when the amplification optical fiber is used while being bent, the occurrence of the non-linear optical effect due to the concentration of the light is suppressed and the power of the light may be suppressed to be decreased as compared with the power of the light which is emitted while being used in a linear status.

"Further, in the core, when the LP01 mode, the LP02 mode and the LP03 mode are standardized by a power, in at least a part of a region where the intensity of the LP02 mode is stronger than the intensity of the LP01 mode and at least a part of a region where the intensity of the LP03 mode is stronger than the intensity of the LP01 mode, the active element may be added with a higher concentration than that in at least a part of a region where the intensity of the LP01 mode is stronger than the intensities of the LP02 mode and the LP03 mode, and both Formula (1) and Formula (2) may be satisfied.

"In such an amplification optical fiber, in the entire fiber, the LP02 mode and the LP03 mode are amplified at a higher amplification factor than that of the LP01 mode so as to satisfy the above Formula (1) and Formula (2). Accordingly, higher power light may be emitted.

"Further, in the core, when the LP01 mode, the LP02 mode and the LP03 mode are standardized by a power, in at least a part of a region where both the intensity of the LP02 mode and the intensity of the LP03 mode are stronger than the intensity of the LP01 mode, the active element may be added with a higher concentration than that in at least a part of a region where the intensity of the LP01 mode is stronger than the intensities of the LP02 mode and the LP03 mode.

"According to such an amplification optical fiber, in the entire fiber, the active element may be added in at least a part of the region where both the intensities of the LP02 mode and the LP03 mode are stronger than the intensity of the LP01 mode so as to satisfy the above Formula (1) and Formula (2). Therefore, the active element may be efficiently added so as to amplify the LP02 mode and the LP03 mode at a higher amplification factor than that of the LP01 mode.

"Further, in the core, when the LP01 mode, the LP02 mode and the LP03 mode are standardized by a power, in the entire region where both the intensity of the LP02 mode and the intensity of the LP03 mode are stronger than the intensity of the LP01 mode, the active element may be added with a higher concentration than that in at least a part of a region where the intensity of the LP01 mode is stronger than the intensities of the LP02 mode and the LP03 mode.

"According to the amplification optical fiber, higher power light may be emitted.

"Further, in the core, when the LP01 mode, the LP02 mode and the LP03 mode are standardized by a power, in an entire region where the intensity of at least one of the LP02 mode and the LP03 mode is stronger than the intensity of the LP01 mode, the active element may be added with a higher concentration than that in at least a part of a region where the intensity of the LP01 mode is stronger than the intensities of the LP02 mode and the LP03 mode, and both Formula (1) and Formula (2) may be satisfied.

"According to such an amplification optical fiber, in a region other than the region where the intensity of at least one of the LP02 mode and the LP03 mode is higher than the intensity of the LP01 mode, the light is amplified at a high amplification factor and the LP02 mode and the LP03 mode are amplified at a higher amplification factor than the LP01 mode in the entire fiber. Accordingly, the non-linear optical effect may be suppressed so that higher power light may be emitted.

"Further, no active element may be added in at least a part of the region where the intensity of the LP01 mode is stronger than the intensities of the LP02 mode and the LP03 mode.

"With this configuration, in at least a part of the region where the intensity of the LP01 mode is strong, the light is not amplified and the amplification of the LP01 mode is suppressed in the entire fiber. Accordingly, the non-linear optical effect may be suppressed.

"In the core, when the LP01 mode, the LP02 mode and the LP03 mode are standardized by a power, in at least a part of a region where the intensity of at least one of the LP02 mode and the LP03 mode is stronger than the intensity of the LP01 mode and the active element which stimulates and emits the light having the predetermined wavelength is added with a higher concentration than that in an entire region where the intensity of the LP01 mode is stronger than the intensities of the LP02 mode and the LP03 mode.

"According to such an amplification optical fiber, in a region where the active element is added with a high concentration, even in a region where the intensity of the LP01 mode is higher than the intensities of the LP02 mode and the LP03 mode, the light is amplified at a high amplification factor. By doing this, in a location where the light is amplified at a high amplification factor, at least one of the LP02 mode and the LP03 mode is amplified at a high amplification factor. Further, in the entire fiber, at least one of the LP02 mode and the LP03 mode is amplified at a higher amplification factor than that of the LP01 mode and higher power light may be emitted.

"Further, in the core, when the LP01 mode, the LP02 mode and the LP03 mode are standardized by a power, in at least a part of a region where the intensity of the LP02 mode is stronger than the intensity of the LP01 mode and at least a part of a region where the intensity of the LP03 mode is stronger than the intensity of the LP01 mode, the active element may be added with a higher concentration than that in an entire region where the intensity of the LP01 mode is stronger than the intensities of the LP02 mode and the LP03 mode, and both Formula (1) and Formula (2) may be satisfied.

"In such an amplification optical fiber, in the entire fiber, the LP02 mode and the LP03 mode are amplified at a higher amplification factor than that of the LP01 mode so that higher power light may be emitted.

"Further, in the core, when the LP01 mode, the LP02 mode and the LP03 mode are standardized by a power, in at least a part of a region where both the intensity of the LP02 mode and the intensity of the LP03 mode are stronger than the intensity of the LP01 mode, the active element may be added with a higher concentration than that in an entire region where the intensity of the LP01 mode is stronger than the intensities of the LP02 mode and the LP03 mode.

"According to such an amplification optical fiber, the active element may be added in at least a part of the region where the intensities of both the LP02 mode and the LP03 mode is higher than the intensity of the LP01 mode. Therefore, in the entire fiber, the active element may be efficiently added so as to amplify the LP02 mode and the LP03 mode at a higher amplification factor than that of the LP01 mode.

"In the core, when the LP01 mode, the LP02 mode and the LP03 mode are standardized by a power, in the entire region where both the intensity of the LP02 mode and the intensity of the LP03 mode are stronger than the intensity of the LP01 mode, the active element may be added with a higher concentration than that in the entire region where the intensity of the LP01 mode is stronger than the intensities of the LP02 mode and the LP03 mode.

"According to such an amplification optical fiber, in the entire region where the intensities of the LP02 mode and the LP03 mode are stronger than the intensity of the LP01 mode, the light is amplified at a higher amplification than that in the entire region where the intensity of the LP01 mode is higher than the intensities of the LP02 mode and the LP03 mode. Accordingly, the LP02 mode and the LP03 mode are amplified at a higher amplification factor than that of the LP01 mode and higher power light may be emitted.

"In the core, when the LP01 mode, the LP02 mode and the LP03 mode are standardized by a power, in the entire region where the intensity of at least one of the LP02 mode and the LP03 mode is stronger than the intensity of the LP01 mode, the active element may be added with a higher concentration than that in the entire region where the intensity of the LP01 mode is stronger than the intensities of the LP02 mode and the LP03 mode, and both Formula (1) and Formula (2) are satisfied.

"According to such an amplification optical fiber, in a region other than the region where the intensity of the LP01 mode is higher than that of at least one of the LP02 mode and the LP03 mode, the light is amplified at a high amplification factor. Accordingly, in the entire fiber, the LP02 mode and the LP03 mode may be amplified at a high amplification factor and higher power light may be emitted.

"No active element may be added in the region where the intensity of the LP01 mode is stronger than the intensities of the LP02 mode and the LP03 mode.

"According to the amplification optical fiber, the light is not amplified in the region where the intensity of the LP01 mode is stronger than the intensities of the LP02 mode and the LP03 mode, and the amplification of the LP01 mode may be suppressed in the entire fiber and the occurrence of the non-linear optical effect may be further suppressed.

"The amplification optical fiber may further include a pair of stress applying units which pinches the core in the clad. By providing the stress applying units, single polarized light may be propagated.

"An optical fiber amplifier of the present invention includes the amplification optical fiber, a seed light source which allows seed light including an LP02 mode and an LP03 mode to enter the amplification optical fiber; and a pumping light source which emits pumping light which pumps the active element of the amplification optical fiber.

"According to the optical fiber amplifier, even when the LP01 mode is pumped, the LP02 mode or the LP03 mode are amplified at a higher amplification factor than that of the LP01 mode so that even when the high power light is emitted, desired power light may be emitted.

"Further, in the optical fiber amplifier, the seed light which enters the amplification optical fiber may pumps only an axial symmetrical mode of the amplification optical fiber.

"According to the optical fiber amplifier, the asymmetrical high order mode does not propagate in the amplification optical fiber, so that the asymmetrical high order mode is not amplified and emitted, which may allow light to be emitted with a good beam quality.

"A resonator according to the present invention includes the amplification optical fiber, a pumping light source which emits pumping light which pumps the active element of the amplification optical fiber; a first FBG (Fiber Bragg Grating) which is provided at one side of the amplification optical fiber to reflect at least part of light having a wavelength of light which is emitted by the active element which is pumped by the pumping light; and a second FBG which is provided at the other side of the amplification optical fiber to reflect light having the same wavelength as the light reflected by the first FBG at a reflectance which is lower than that of the first FBG.

"According to the resonator, during the resonance of the light, when the light propagates in the core of the amplification optical fiber, the LP02 mode or the LP03 mode is amplified to be stronger than the LP01 mode. Therefore, the non-linear optical effect is suppressed so that even when the high power light is emitted, desired power light may be emitted.

"Effect of the Invention

"As described above, the present invention provides an amplification optical fiber which is capable of emitting light having a predetermined power, and an optical fiber amplifier and a resonator using the same."

For additional information on this patent, see: Kashiwagi, Masahiro. Amplification Optical Fiber, and Optical Fiber Amplifier and Resonator Using Same. U.S. Patent Number 8755111, filed September 26, 2013, and published online on June 17, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=8755111.PN.&OS=PN/8755111RS=PN/8755111

Keywords for this news article include: Fujikura Ltd.

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Source: Journal of Engineering


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