JP5014640B2 - Multimode fiber, optical amplifier and fiber laser - Google Patents

Description

  The present invention relates to a multimode fiber provided with a diffraction grating that reflects a fundamental mode of reflected light, an optical amplifier and a fiber laser using the multimode fiber.

  In high power optical fiber amplifiers, fiber lasers, etc., a clad pump structure is generally employed. This is to propagate the pumping light necessary for amplifying the light propagating through the core in the optical fiber through the clad. At the same time that this clad pump structure was widely adopted, a high-power laser diode (hereinafter referred to as LD) with a multimode fiber output was used. In these amplifiers, the gain level may reach 20 dB or more, and the maximum output may reach 1 W to 1 kW.

The optical amplifier and the fiber laser having such a high power amplification medium have the following problems.
-Since the gain of the optical amplifier is extremely large, minute reflected light generated in the vicinity of the emitting portion is greatly amplified inside the optical amplifier and returns to the incident portion.
The signal light has a large power in the optical amplifier, and as a result, a large amount of scattered light is generated backward. This scattered light is further amplified in the optical amplifier and appears as a large reflected light.
-Spontaneous emission light generated in the optical amplifier, leakage light of the signal light emitted from the core, etc. are emitted backward.
These reflected lights may directly destroy the components inside the optical amplifier, or the reflected light may cause the optical amplifier to oscillate, resulting in destruction of the components. Among these components, the LD for the excitation light source, in particular, has a lens inside, and the reflected light forms an image on the chip end surface of the LD, so that it is the most easily broken component.

  The present invention has been made in view of the above circumstances, and provides a multimode fiber, an optical amplifier, and a fiber laser capable of extending the life of the apparatus without being damaged by reflected light returning to the LD side in an optical amplifier or the like. Objective.

In order to achieve the above object, the present invention provides a multimode fiber in which an LD is connected directly or indirectly to one end and an optical oscillator or optical amplifier is connected to the other end directly or via an optical coupler. A first core region doped with Ge in the center, a second core region not containing Ge provided to surround the outside of the first core region, and the second core region A clad made of a material having a refractive index lower than that of the second core region , and generated or amplified by the optical oscillator or the optical amplifier in the first core region , A diffraction grating for reflecting light from the optical oscillator or the optical amplifier toward the LD is provided , and the period of the diffraction grating is determined so as to reflect the fundamental mode at the wavelength of the reflected light from the optical oscillator or the optical amplifier. that was To provide a multi-mode fiber and butterflies.

In the multimode fiber of the present invention, it is preferable that a cross-sectional area occupied by the diffraction grating is in a range of 25% to 75% of a cross-sectional area of the core .

The present invention also provides an optical amplifier comprising the above-described multimode fiber and LD set according to the present invention and an amplification rare earth doped fiber connected to the output side of the multimode fiber .

In the optical amplifier of the present invention, it is preferable that two or more sets of multimode fibers and LDs are provided, and these sets are all connected to one optical coupler .

The present invention also provides a fiber laser comprising the multimode fiber and LD set according to the present invention described above and a rare earth doped fiber for amplification connected to the output side of the multimode fiber .

In the fiber laser of the present invention, it is preferable that there are two or more sets of multimode fibers and LDs, and these sets are all connected to one optical coupler .

Since the multimode fiber of the present invention has a structure in which a diffraction grating that reflects the reflected light returning to the LD side is provided in a part of the core, when the multimode fiber is applied to an optical amplifier or a fiber laser, the LD side Reflected light returning toward the surface is reflected by the diffraction grating and prevents the reflected light from entering the LD, and the reflected light or its amplified light is less likely to cause destruction or performance degradation of the LD, thereby extending the life of the LD. Can be planned.
Since the optical amplifier of the present invention has the above-described set of the multimode fiber and LD according to the present invention and the rare earth-doped fiber for amplification connected to the output side of the multimode fiber, the optical amplifier is directed toward the LD side. Reflected reflected light is reflected by the diffraction grating, prevents this reflected light from entering the LD, and the LD is less likely to break down or degrade performance due to the reflected light or its amplified light, thereby extending the life of the device. it can.
Since the fiber laser of the present invention has the above-described multimode fiber and LD set according to the present invention and the rare earth-doped fiber for amplification connected to the output side of the multimode fiber, the fiber laser is directed toward the LD side. Reflected reflected light is reflected by the diffraction grating, prevents this reflected light from entering the LD, and the LD is less likely to break down or degrade performance due to the reflected light or its amplified light, thereby extending the life of the device. it can.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a view showing an embodiment of a multimode fiber (hereinafter abbreviated as MM fiber) of the present invention, FIG. 1 (a) is a perspective view of the MM fiber, and FIG. 1 (b) is an MM fiber. FIG.

  The MM fiber 10 is provided in a portion that guides light (excitation light) from an LD (not shown), one end side is connected to the LD, and the other end side serving as an output side is connected to the input side of the rare earth-doped fiber. It is connected. As shown in FIG. 1A, the MM fiber 10 includes a core region 12 doped with Ge in the center, and a core region 13 that does not include Ge provided around the outer periphery of the core region 12. A cladding 14 provided around the core region 13 and made of a material having a refractive index lower than that of the core region 13 is provided. A diffraction grating 11 as shown in FIG. (Hereinafter, sometimes referred to as a grating). This MM fiber 10 is provided with a core region 12 doped with Ge in a part of the core, and a grating can be formed only in that region.

  The diffraction grating 11 is formed with a period (lattice constant) so that the excitation light is transmitted but the fundamental mode of the amplified signal light is reflected. In this embodiment, the case where the wavelength of the excitation light from the LD is 980 nm and the wavelength of the amplified signal light is 1064 nm is illustrated, and the formed diffraction grating 11 is fundamental for light having a wavelength of 1064 nm. Only the mode has high reflectivity, and the higher order mode does not show reflection. On the other hand, the light of wavelength 980 nm shows almost no reflection in all modes.

  FIG. 2 is a schematic diagram for explaining the function of the diffraction grating 11 used in this embodiment. FIG. 2A shows a case where signal light having a wavelength of 1064 nm is passed through the diffraction grating 11, and FIG. A case where excitation light having a wavelength of 980 nm is passed is shown. Δβ shown in FIG. 2 represents a propagation constant difference between the propagation mode and the reflected mode. This Δβ is determined by the grating period Λ of the diffraction grating 11. If the grating period is Λ, reflection occurs only when Δβ = 2π / Λ is satisfied.

When signal light having a wavelength of 1064 nm is passed through the diffraction grating 11, as shown in FIG. 2A, the grating has a period coupling from the fundamental mode of signal light to the fundamental mode of reflection. The fundamental mode of signal light is reflected.
On the other hand, when excitation light having a wavelength of 980 nm is passed, coupling between the fundamental mode and the higher order mode or between the higher order mode and the higher order mode occurs as shown in FIG. 2B, but these are very small. The fundamental mode of excitation light having a wavelength of 980 nm is not reflected.

FIG. 3 is a graph illustrating the electric field distribution of the region where the diffraction grating 11 of the MM fiber 10 is formed, the fundamental mode, and the higher-order mode, and FIG. 3A is a graph illustrating the electric field distribution of the fundamental mode. b) is a graph showing the electric field distribution of the higher-order mode.
FIG. 4 is a graph illustrating the transmission spectrum of the MM fiber 10 having the diffraction grating 11. As shown in FIG. 4, in this MM fiber 10, it can be seen that excitation light having a wavelength of 980 nm is transmitted without loss, but signal light having a wavelength of 1064 nm is reflected.

A diffraction grating in an optical fiber is called a fiber grating and is used as a wavelength filter. If this filter is used, it is possible to reflect only the signal light and pass the excitation light. However, in reality, when using a fiber grating, the following restrictions are imposed.
1. Single mode fiber,
2. The signal light wavelength is shorter than the excitation light or sufficiently (preferably 100 nm or more) away.

  The above limitations are a major problem when considering high power amplifiers. This is because the LD using MM fiber is usually used in the cladding excitation system. In addition, when comparing the excitation light and the signal light, the excitation light necessarily has a short wavelength and is often 100 nm or less (for example, 1550 nm and 1480 nm, 1064 nm and 980 nm, etc.). Therefore, the present invention proposes a filter that appropriately blocks adjacent signal light and allows excitation light to pass through while using the MM fiber.

In order to achieve this object, the point of interest in the present invention is to form the diffraction grating 11 only in a part of the MM fiber 10.
Specifically, a part doped with Ge is formed in a part of the core, and the diffraction grating 11 is formed in this part. At this time, if the portion doped with Ge includes the lowest order mode among the many guided modes, that is, the portion where the electric field of the fundamental mode is maximized, the diffraction grating 11 is included in this portion. Only formed.

  By the way, among many propagation modes existing in the MM fiber 10, which mode is reflected by the grating with a constant wavelength is determined by the period of the grating. In general, when the grating period is Λ, reflection is performed only when Δβ = 2π / Λ (Δβ = propagation constant difference between the propagation mode and the reflected mode) is satisfied. This is a phase matching condition and a necessary condition for reflection.

  In order for light to be actually reflected, it is necessary that the electric field overlap be greater. In particular, according to the present invention, since the grating is formed only in a part of the core when viewed in cross section, the overlap of the two electric fields is important only in the part where the grating exists.

In the present invention, the period is determined so that the fundamental modes are reflected by the grating at the wavelength to be reflected.
A grating exists only in the vicinity where the electric field distribution in the fundamental mode is maximized. As a result, a large reflectance can be obtained in the fundamental mode at the wavelength to be reflected, but only a small amount of reflection occurs at other wavelengths. This is because there are no propagation modes that satisfy the coupling condition depending on the grating period at wavelengths longer than the wavelength to be reflected, and there are propagation modes that satisfy the phase matching condition at short wavelengths. This is because the magnitude of reflection by the grating is small because is significantly different from the fundamental mode.

Moreover, in the case of a grating, unlike absorption and scattering, since it reflects to the waveguide mode, the slightly generated reflected light returns to the LD again and contributes to the oscillation of the LD, so that it hardly causes a decrease in the output of the LD. . A good example of this is an LD with a fiber grating, which forms a single resonator with the grating and the LD, so that the output is hardly reduced by reflection.
On the other hand, the fundamental mode has the highest power ratio among the many propagation powers existing in the fiber, and sufficient reflectivity with respect to the total power can be obtained by reflecting only the fundamental mode. Is possible.
Furthermore, the light condensed at the smallest spot by the LD lens is the fundamental mode, and the effect of not returning this mode to the LD is very large. Therefore, it is possible to sufficiently satisfy the object of the present invention only by blocking the fundamental mode.

  Since the MM fiber 10 of the present embodiment has a structure in which a diffraction grating 11 that reflects reflected light returning to the LD side is provided in a part of the core, when the MM fiber 10 is applied to an optical amplifier or a fiber laser, Reflected light returning toward the LD side is reflected by the diffraction grating 11, and this reflected light is prevented from entering the LD, and the reflected light or its amplified light is less likely to cause destruction or performance degradation of the LD. Life can be extended.

  Next, embodiments of the optical amplifier and the fiber laser of the present invention will be described. An optical amplifier and a fiber laser (hereinafter abbreviated as an optical amplifier) of the present invention are connected to the above-described set of the MM fiber 10 and LD with a diffraction grating according to the present invention and the output side of the MM fiber 10. And a rare earth-doped fiber for amplification.

  FIG. 5 is a diagram showing a first embodiment of an optical amplifier according to the present invention. An optical amplifier 20A according to this embodiment includes an MM fiber 10, an LD 22 connected to one end of the MM fiber 10, and the MM. An amplification rare earth-doped fiber 21 connected to the other end side (output side) of the fiber 10 is provided. The LD 22 and an optical fiber for guiding signal light are connected to one end side of the MM fiber 10. It is desirable that the connecting portion on one end side has a structure in which excitation light and signal light from the LD 22 are put into the MM fiber 10 using an appropriate optical coupler.

  The rare earth-doped fiber 21 can be appropriately selected from quartz glass based optical fibers in which the core is doped with one or more rare earth elements such as Yb, Er, and Tm.

  The optical amplifier 20 according to the present embodiment drives the LD 22 so that excitation light (for example, light having a wavelength of 980 nm) enters the rare earth-doped fiber 21 through the MM fiber 10 and excites rare earth ions doped in the core. Then, the signal light (for example, wavelength 1064 nm) is incident on the rare earth doped fiber 21 through the MM fiber 10, whereby the signal light is amplified and the amplified signal light is output from the output side of the rare earth doped fiber 21.

  As shown in FIG. 1B, the signal light output for some reason is reflected, and when the reflected light enters the MM fiber 10, it is reflected by the diffraction grating 11 provided in the MM fiber 10. As described above, the optical amplifier 20 of the present embodiment prevents the reflected light from entering the LD 22 by connecting the MM fiber 10 with a diffraction grating to the incident side of the rare earth-doped fiber 21, and reflects the reflected light or its amplified light. As a result, the LD 22 is less likely to be broken or degraded in performance, and the life of the apparatus can be extended.

  FIG. 6 is a diagram showing a second embodiment of the optical amplifier of the present invention. The optical amplifier 20B of the present embodiment uses a plurality of sets of MM fibers 10 to which LDs 22 are connected. The output side of the fiber 10 and an optical fiber for guiding signal light are connected to the incident side of the rare earth-doped fiber 21 via the optical coupler 23, respectively.

In the present embodiment, a clad pump structure is employed, and a double clad fiber in which two or more layers of clads are provided around the core is used as the rare earth-doped fiber 21, and signal light is incident on the core and excitation light is transmitted to the core. It is configured to be incident on the inner cladding.
Similar to the optical amplifier 20A of the first embodiment described above, the optical amplifier 20B of the present embodiment prevents the reflected light from entering the LD 22, and the LD 22 may break down or degrade performance due to the reflected light or its amplified light. As a result, the life of the apparatus can be extended.

FIG. 7 is a diagram showing a third embodiment of the optical amplifier of the present invention. The optical amplifier 20C of the present embodiment includes an MM fiber 10 in which an LD 22 is connected to a rare earth doped fiber 21 via an optical coupler 24. Are connected and an isolator 25 is interposed.
Similar to the optical amplifier 20A of the first embodiment described above, the optical amplifier 20B of the present embodiment prevents the reflected light from entering the LD 22, and the LD 22 may break down or degrade performance due to the reflected light or its amplified light. As a result, the life of the apparatus can be extended.

The above-described embodiments are merely examples, and the present invention is not limited to these embodiments, and various changes and modifications can be made.
For example, the fiber used in the optical amplifier of the present invention need not be limited to a Yb-doped fiber, but can be applied to a rare earth-doped fiber that can be used as an optical amplification medium, such as a Yb-Er-doped fiber, an Er-doped fiber, or a Tm-doped fiber.
The optical amplifier of the present invention can be used on the backward pumping side when performing bidirectional pumping.
Further, the present invention can be used not only for an optical amplifier but also for an excitation light source of an optical oscillator.
The fiber grating can also be a long period type (see FIG. 8). In this case, since the fundamental mode component of the propagating light component becomes relatively small, the effect of condensing on the LD end face by the LD imaging system is diminished, and damage to the LD can be effectively prevented. .
Further, in the present invention, it is of course possible to form a grating on a single mode fiber instead of an MM fiber.
In some cases, a wavelength filter using a dielectric multilayer film instead of a fiber grating may be used.

The apparatus shown in FIGS. In these three examples, all use Yb-doped fiber as the excitation medium. 5 and 7 show a core pump type optical amplifier, and FIG. 6 shows a clad pump type optical amplifier.
In any case, the pumping LD is connected to the Yb-doped fiber through a coupler, and the light reflected and returned from the coupler is reflected by a multimode grating disposed in the immediate vicinity of the LD.

  FIG. 2 shows the result of looking at this example in terms of wave number. Only the fundamental modes satisfy the magnitude of Δβ determined by the grating period Λ at the wavelength of 1064 nm (signal wavelength), whereas at the wavelength of 980 nm (excitation wavelength), the fundamental mode—higher order mode or higher order mode— It can be seen that this is a higher order mode.

  FIG. 3 shows the electric field distribution in the fundamental mode and the higher order mode. There are an infinite number of higher-order modes, but in any case the coupling with the fundamental mode is very small. The fiber used at this time has a core diameter of 100 μm and a cladding diameter of 125 μm.

  FIG. 4 shows a transmission spectrum in this example. It can be seen that a large transmission loss occurs due to reflection in the band of signal light, but only a negligible loss occurs at the wavelength of the excitation light.

  In this embodiment, the cross-sectional area ratio of the diffraction grating portion occupying the core is 50%. However, if this cross-sectional area ratio exceeds 75%, the higher-order mode reflectivity increases. Loss at the wavelength may increase and become 2 dB or more. This is not desirable because a clear drop in the LD output is observed. On the other hand, if the cross-sectional area ratio is less than 25%, the reflectance of the fundamental mode decreases, and for example, the loss at the signal wavelength in FIG. 4 may change from −16 dB to about −4 dB. In this case, the reflected light cannot be sufficiently attenuated from the viewpoint of LD protection, which is not desirable.

An embodiment of MM fiber of the present invention is shown, (a) is a perspective view of MM fiber, and (b) is a sectional view of MM fiber. It is the schematic explaining the function of the diffraction grating provided in the MM fiber of this invention, (a) is the case where the signal light of wavelength 1064nm passes through this diffraction grating, (b) is the case of passing the excitation light of wavelength 980nm Indicates. FIG. 4 is a graph illustrating an electric field distribution of a fundamental mode and a higher-order mode, a region where a diffraction grating of the MM fiber is formed, (a) a graph showing an electric field distribution of a fundamental mode, and (b) an electric field of a higher-order mode It is a graph which shows distribution. It is a graph which illustrates the transmission spectrum by MM fiber of the present invention. 1 is a configuration diagram illustrating a first embodiment of an optical amplifier according to the present invention. FIG. It is a block diagram which shows 2nd Embodiment of the optical amplifier of this invention. It is a block diagram which shows 3rd Embodiment of the optical amplifier of this invention. It is the schematic which shows the effect at the time of using a long period grating.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... MM fiber, 11 ... Diffraction grating, 12 ... Core area | region (Core area | region doped with Ge), 13 ... Core area | region (core area | region which does not contain Ge), 14 ... Cladding, 20A, 20B, 20C ... Optical amplifier, 21 ... rare earth doped fiber, 22 ... LD, 23, 24 ... optical coupler, 25 ... isolator.

Claims (6)

A multimode fiber having a laser diode connected directly or indirectly to one end and an optical oscillator or optical amplifier connected to the other end directly or via an optical coupler,
A first core region doped with Ge in the center; a second core region not containing Ge provided surrounding the outside of the first core region; and surrounding the outside of the second core region. And a clad made of a material having a refractive index lower than that of the second core region,
The first core region is provided with a diffraction grating that is generated or amplified by the optical oscillator or the optical amplifier and reflects light directed from the optical oscillator or the optical amplifier toward the laser diode ,
A multimode fiber characterized in that the period of the diffraction grating is determined so as to reflect the fundamental mode at the wavelength of the reflected light from the optical oscillator or optical amplifier . The multimode fiber according to claim 1, wherein a cross-sectional area occupied by the diffraction grating is in a range of 25% to 75% of a cross-sectional area of the core. Claim 1 or 2 and a set of multimode fiber and the laser diode according to an optical amplifier, characterized in that it comprises a amplifying rare-earth-doped fiber connected to the output side of the multimode fiber. 4. The optical amplifier according to claim 3 , wherein there are two or more sets of the multimode fiber and laser diode, and these sets are all connected to one optical coupler. A set of multimode fiber and the laser diode according to claim 1 or 2, fiber laser and having a amplifying rare-earth-doped fiber connected to the output side of the multimode fiber. 6. The fiber laser according to claim 5 , wherein there are two or more sets of the multimode fiber and the laser diode, and these sets are all connected to one optical coupler.

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