JP2009129987A - Optical fiber laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical fiber laser capable of easily controlling the absorption characteristics of the exciting light along the longitudinal direction of an optical fiber, planarizing the temperature distribution along the longitudinal direction of the optical fiber, and suppressing the generation of skew rays. <P>SOLUTION: The optical fiber laser has an optical fiber 2 having a clad consisting of two layers around a core doped with a rare-earth element; photocouplers connected to an end of the optical fiber 2; and a plurality of light sources for making an exciting light become incident to the cladding via the photocouplers. The optical fiber 2 has an exciting portion 3, which is wound so as to differentiate the loss in absorption of an exciting light in the longitudinal direction. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a high-power optical fiber laser including a rare earth-doped core and a cladding.

  For the purpose of application to laser processing and medical use, development of a light source with higher output and lower cost is required. In response to these requirements, optical fiber lasers are attracting attention because they can easily extract high-quality and high-quality laser light.

  As an optical fiber used for such a high-power optical fiber laser, a core doped with a rare earth element (Yb, Er, Er / Yb, Tm, Nd, etc.) and a clad composed of a first clad and a second clad are provided. There are double-clad fibers. A coating layer made of an ultraviolet curable resin or the like is provided on the outer periphery of the second cladding.

  Light emitted from an excitation light source such as a multimode LD (semiconductor laser) is incident on one end of the optical fiber as excitation light. The excitation light collected in the first cladding propagates in the optical fiber and excites the rare earth-added element in the core. Then, oscillation light propagates from the excited rare earth element to the core, and high-power laser oscillation light is emitted from the other end of the optical fiber.

  The excitation light propagating in the first cladding is two meridian rays (light rays passing through the core 72) shown in FIG. 7A and skew light rays (light rays not passing through the core 72) shown in FIG. 7B. Kind exists. Since the excitation light is not absorbed unless it passes through the core 72, how to eliminate the skew rays is important for improving the absorption characteristics.

  The prior art document information related to the invention of this application includes the following.

JP-A-5-249328

  However, in the conventional optical fiber, when a Yb-doped (doped) core is used as the core, the light / light conversion efficiency of the laser oscillation light with respect to the excitation light is about 80%, and about 20% of the energy is heat.

  For this reason, in the conventional optical fiber, as the output of the optical fiber laser increases, the temperature rise of the optical fiber, particularly the temperature rise near the incident end of the optical fiber, is large, and the coating layer of the optical fiber may be damaged. There is a problem that the output of the optical fiber laser is limited.

  Also, in the conventional optical fiber laser, in order to suppress the generation of skew rays, the first cladding has a D-shaped or polygonal cross section, or the first cladding and the second cladding as in the case of an air cladding fiber. However, these methods have been insufficient to suppress skew rays.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an optical fiber laser that suppresses an increase in the temperature of an optical fiber in an optical fiber laser and suppresses the generation of skew rays, thereby increasing the output of laser light.

  The present invention has been devised to achieve the above object, and the invention of claim 1 is directed to an optical fiber having a clad composed of two layers around a core doped with a rare earth element, and an end of the optical fiber. An optical fiber laser comprising: an optical coupler connected to a portion; and a plurality of light sources that make excitation light incident on the clad through the optical coupler, wherein the optical fiber absorbs the absorption loss of the excitation light in a longitudinal direction. It is an optical fiber laser having a pumping portion wound so as to be different from each other.

  A second aspect of the present invention is the optical fiber laser according to the first aspect, wherein the optical fiber is wound so that a winding diameter at the pumping portion decreases as the distance from the optical coupler increases in the longitudinal direction. is there.

  According to a third aspect of the present invention, the optical fiber is wound such that the intermediate winding diameter of the excitation portion is minimized and the winding diameter gradually increases as the distance from the intermediate portion of the excitation portion increases. 2. The optical fiber laser according to 2.

  A fourth aspect of the present invention is the optical fiber laser according to any one of the first to third aspects, wherein the optical fiber is wound so that the winding diameter is in a range of 200 to 400 mm.

  According to the present invention, the absorption characteristic of the excitation light along the longitudinal direction of the optical fiber can be easily controlled, the temperature distribution along the longitudinal direction of the optical fiber can be flattened, and the occurrence of skew rays can be suppressed. .

  Preferred embodiments of the present invention will be described below with reference to the accompanying drawings.

  FIG. 1 is a schematic view of the main part of an optical fiber laser showing a preferred first embodiment of the present invention.

  As shown in FIG. 1, in the optical fiber laser 1 according to the first embodiment, an optical coupler (not shown) is connected to both ends of the optical fiber 2, and the optical fiber 2 is passed through both optical couplers. The excitation light from a light source (not shown) is incident on the clad of the substrate, the rare earth element of the core is excited, and a high-output laser oscillation light is output from one optical coupler side. The optical fiber 2 is wound so that the absorption loss of the pumping light varies in the longitudinal direction to form the pumping section 3.

  More specifically, the optical fiber laser 1 includes an absorption characteristic adjusting member 4 for winding the optical fiber 2. The absorption characteristic adjusting member 4 has a circular cross-sectional shape, and its diameter is formed so as to continuously change along the longitudinal direction of the absorption characteristic adjusting member 4 (vertical direction in FIG. 1). Is formed with a predetermined curvature so as to be vertically symmetrical so that the diameter of the center (the middle between both ends of the absorption characteristic adjusting member 4) is small and the diameter increases toward both ends.

  In the first embodiment, the absorption characteristic adjusting member 4 has a longitudinal center diameter of 200 mm and both end diameters of 500 mm.

  The excitation portion 3 is formed by winding the optical fiber 2 around the absorption characteristic adjusting member 4. At this time, the optical fiber 2 may be wound so that the winding diameter decreases as the distance from the optical couplers connected to both ends of the optical fiber 2 increases in the longitudinal direction. That is, the intermediate winding diameter of the optical fiber 2 between the two optical couplers is minimized, and the winding is preferably performed symmetrically so that the winding diameter increases as the distance from the center increases.

  In 1st Embodiment, the winding diameter of the excitation part 3 was changed in the range of 200-400 mm. This is because if the winding diameter is less than 200 mm, the absorption loss increases, and if it exceeds 400 mm, the temperature at both ends of the optical fiber 2 becomes high.

  The excitation method is not particularly limited, but either side excitation or end surface excitation can be employed.

  As shown in FIG. 2, the optical fiber 2 used in the optical fiber laser 1 according to the first embodiment includes a core 21 to which a rare earth element is added, and a clad 22 formed on the outer periphery of the core 21. The clad 22 includes an inner first clad 23 and an outer second clad 24.

  The core 21 is obtained by adding a small amount of rare earth elements such as Yb, Er, Er / Yb, Tm, and Nd (doped) to pure quartz. In the first embodiment, the excitation light Le has a wavelength λe (915 nm or 975 to 980 nm), and Yb is used as a rare earth element in order to emit the laser light L having a wavelength λ (1030 to 1100 nm). Yb is a rare earth element suitable for absorption of the excitation light L having the wavelength λe and amplification (stimulated emission) of the light having the wavelength λ.

  In the first embodiment, a plurality of holes penetrating along the longitudinal direction of the optical fiber 2 are arranged in a honeycomb shape so that the core 21 and the clad 22 constitute a PCF (photonic crystal fiber). A photonic crystal structure is formed on the first cladding 23.

  As an example of the optical fiber laser 1 according to the first embodiment, the optical fiber laser 1 of FIG. 3 will be described.

  As shown in FIG. 3, the optical fiber laser 1 includes an optical unit 32 that includes a light source and outputs laser oscillation light L, and drives an LD driver (not shown) that is connected to the optical unit 32 and drives the light source. Mainly composed of devices.

  The optical unit 32 includes the optical fiber 2 and light source units 33A and 33B provided near both ends of the optical fiber 2 (outside both optical couplers 36a and 36b described later).

  The light source unit 33A includes a plurality of excitation light sources 34 for emitting high-output excitation light, a plurality of excitation light paths 35 connected to the excitation light sources 34, and the excitation light paths 35, respectively. The optical couplers 36 a and 36 b are optically connected to optically couple the light emitted from each excitation light source 34 to the optical fiber 2.

  As each excitation light source 34, a multi-mode LD suitable for inexpensive optical transmission is used. In the first embodiment, as an example, a multi-mode LD that emits excitation light Le having a wavelength λe (915 nm or 975 to 980 nm) is used.

  Each excitation light source 34 is connected in series for each of the light source units 33A and 33B, and these are connected to the driving device described above. As each excitation optical path 35, a multimode optical fiber or an optical waveguide is used. As the optical couplers 36a and 36b, a multi-coupler or an excitation combiner is used.

  At both ends of the optical fiber 2, light reflecting portions 37a and 37b for reflecting and exciting the excitation light Le incident on the optical fiber 2 are provided inside the optical couplers 36a and 36b. In the first embodiment, two FBGs (fiber Bragg gratings) that transmit the pumping light wavelength and have a high reflectance with respect to the oscillation light wavelength are formed in the optical fiber 2, and the light reflecting unit 37a and 37b.

  The FBG serving as the light reflecting portion 37b (the emission side of the laser oscillation light L of the optical fiber 2) is formed with a different lattice spacing from the FBG serving as the light reflecting portion 77a so as to partially reflect the laser oscillation light. Is done.

  The main part of the optical fiber laser 1 shown in FIG. 1 is installed between the optical couplers 36a and 36b. That is, the absorption characteristic adjusting member 4 is installed in the middle of the optical fiber 2 between the optical couplers 36a and 36b, and the optical fiber 2 is wound around the absorption characteristic adjusting member 4 to form the excitation unit 3.

  Next, an example of the optical fiber laser 1 according to the first embodiment will be described.

  First, using the optical fiber 2 in which the core 21 has a diameter of 50 μm, the Yb concentration in the core 21 is 1000 ppm, the first cladding 23 has an outer diameter of 600 μm, and the second cladding 24 has an outer diameter of 700 μm, The excitation portion 3 was formed by winding the optical fiber 2 around the absorption characteristic adjusting member 4 so that the winding diameter was 200 mm and the winding diameter at the beginning (end of winding) was 400 mm, and the optical fiber laser 1 was manufactured.

  In the optical fiber laser 1 according to the first embodiment, the output is 5 kW or more, preferably 10 kW or more, and the fiber length is 30 m.

  Here, the absorption loss (@ 975 nm) with respect to the bending diameter (winding diameter) is shown in FIG.

  As shown in FIG. 4, when the bending diameter (winding diameter) is changed from 200 mm to 400 mm, the absorption loss (@ 975 nm) linearly changes from about 0.75 dB / m to about 0.3 dB / m. That is, when the winding diameter is reduced, the absorption loss increases, and when the winding diameter is increased, the absorption loss decreases.

  Thermal analysis during use (operation) of the manufactured optical fiber laser 1 was performed. The result is shown in FIG.

  For comparison, in FIG. 5, as a conventional example, the thermal analysis result of an optical fiber laser in which the optical fiber 2 of FIG.

  As shown in FIG. 5, in the conventional example, the fiber temperature in the vicinity of the optical coupler (both ends of the optical fiber 2) where the excitation light enters is high, and the temperature is as high as 277.degree. In addition, the temperature is low between the two optical couplers, and the difference between the maximum temperature and the minimum temperature is large.

  On the other hand, in the present invention, the temperature distribution at the time of use is about 170 ° C. or less over the entire length of the fiber, and the flat temperature distribution has a small difference between the maximum temperature and the minimum temperature. Compared to the conventional example, the maximum temperature differs by about 107 ° C.

  That is, using the optical fiber 2 in which the Yb concentration of the core 21 is about 1000 ppm, the diameter of the core 21 is about 50 μm, and the outer diameter of the first cladding 23 is about 600 μm, the winding diameter of the excitation unit 3 is in the range of 200 mm to 400 mm. When pump light having a wavelength of 975 to 980 nm is incident on the optical fiber 2, the optical fiber laser 1 has an output of 10 kW or more and a fiber temperature in use of about 170 ° C. or less over the entire length of the optical fiber 2. .

  Next, the operation of the first embodiment will be described together with the operation of the optical fiber laser 1 in FIG.

  When each excitation light source 34 is driven by the driving device, excitation light is emitted from each excitation light source 34, and the excitation light from all the excitation light sources 34 in the light source sections 33A and 33B is emitted by the optical couplers 36a and 36b. The pumping light Le enters the optical fiber 2 from both sides.

  The incident excitation light Le passes through the excitation unit 3 formed by winding the optical fiber 2 between the optical couplers 36 a and 36 b around the absorption characteristic adjusting member 4. In the first embodiment, the excitation unit 3 minimizes the winding diameter at the central portion of the absorption characteristic adjusting member 4 so that the winding diameter increases as the distance from the central portion of the absorption characteristic adjusting member 4 increases. Is formed.

  The excitation light Le is coupled to the core 21 to excite the rare earth added element. The excited oscillation light from the rare earth element is excited between the light reflecting portions 37 a and 37 b to generate a high-power laser oscillation light L, and the laser oscillation light L is output from the emission end of the optical fiber 2.

  When the excitation light Le passes through the excitation unit 3, absorption loss is reduced at both ends of the optical fiber 2 near the optical couplers 36a and 36b where the excitation light Le is incident, because the winding diameter of the optical fiber 2 is large. The optical fiber 2 wound around the center of the characteristic adjusting member 4 has a large absorption loss due to its small winding diameter.

  For this reason, in the optical fiber laser 1, the temperature distribution along the longitudinal direction of the optical fiber 2 is flattened, and the heat generated when the pumping light Le is introduced can be kept lower than the conventional optical fiber over the entire length of the optical fiber 2. it can. Thereby, the fall of the optical output by heat_generation | fever can be suppressed.

  Therefore, according to the optical fiber laser 1, by changing the winding diameter of the optical fiber 2 of the excitation unit 3, the absorption characteristic and temperature distribution of the excitation light Le along the longitudinal direction of the optical fiber 2 can be easily controlled. By controlling the heat generation during oscillation, a higher-power optical fiber laser can be realized.

  Further, since the presence or absence of the occurrence of skew rays has a dependency on the winding diameter and shape, the excitation light propagating through the first cladding 23 by varying the winding diameter (bending shape) along the longitudinal direction of the optical fiber 2. Le becomes easy to couple to the core 21, generation of skew rays can be suppressed, and the excitation light Le can be efficiently coupled to the core 21.

  When the optical fiber laser 1 as in the example of FIG. 3 is configured, the optical couplers 36 a and 36 b multiplex pumping light having a wavelength of 975 nm from a large number of pumping light sources 34. It can be incident from both sides and there is little heat loss in the fiber (especially at the end).

  As the second embodiment, in the case of single-side excitation (when an optical coupler is connected only to one end of the optical fiber 2), an optical fiber laser 61 shown in FIG. 6 may be used. The optical fiber laser 61 includes a conical absorption characteristic adjusting member 62, the optical fiber 2 is wound around the absorption characteristic adjusting member 62 to form a pumping unit 63, and the pumping unit 63 has an optical coupler side. The winding diameter (on the right side in FIG. 6) is large, and the winding diameter decreases as the distance from the optical coupler increases. Thereby, at one end portion of the optical fiber 2 near the optical coupler, the absorption loss is reduced because the winding diameter of the optical fiber 2 is large, and as the distance from the optical coupler is increased, the winding diameter is reduced and the absorption loss is increased. . Thereby, the effect similar to 1st Embodiment is acquired.

  In the above embodiment, the excitation part 3 (or 63) is formed by winding the optical fiber 2 around the absorption characteristic adjusting member 4 (or 62). However, the present invention is not limited to this, and the longitudinal direction of the optical fiber 2 is taken along. The excitation diameter 3 (or 63) may be formed by winding the optical fiber 2 in a spiral shape as long as the winding diameter (bending shape) can be changed.

It is a principal part schematic diagram of the optical fiber laser which shows the suitable 1st Embodiment of this invention. It is the schematic of the optical fiber used for the optical fiber laser which concerns on 1st Embodiment. It is the schematic which shows an example of the optical fiber laser which concerns on 1st Embodiment. It is a figure which shows the relationship between the bending diameter and absorption loss of the optical fiber shown in FIG. It is a figure which shows the result of the thermal analysis of the optical fiber laser which concerns on 1st Embodiment, and the conventional optical fiber laser. It is a principal part schematic of the optical fiber laser which concerns on 2nd Embodiment. FIG. 7A is a diagram for explaining meridian rays, and FIG. 7B is a diagram for explaining skew rays.

Explanation of symbols

1 Optical fiber laser 2 Optical fiber 3 Excitation section

Claims (4)

  An optical fiber having a clad composed of two layers around a core to which a rare earth element is added, an optical coupler connected to an end of the optical fiber, and excitation light incident on the clad through the optical coupler An optical fiber laser comprising a plurality of light sources, wherein the optical fiber has a pumping portion wound so that the absorption loss of the pumping light varies in the longitudinal direction.   2. The optical fiber laser according to claim 1, wherein the optical fiber is wound so that a winding diameter at the pumping portion decreases as the distance from the optical coupler increases in the longitudinal direction.   3. The optical fiber laser according to claim 1, wherein the optical fiber is wound so that a winding diameter in the middle of the excitation unit is minimized and the winding diameter gradually increases as the distance from the middle of the excitation unit increases.   The optical fiber laser according to claim 1, wherein the optical fiber is wound so that the winding diameter is in a range of 200 to 400 mm.

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