WO2012108248A1

WO2012108248A1 - Q-switched fiber laser

Info

Publication number: WO2012108248A1
Application number: PCT/JP2012/051210
Authority: WO
Inventors: 将邦三室
Original assignee: 株式会社フジクラ
Priority date: 2011-02-08
Filing date: 2012-01-20
Publication date: 2012-08-16
Also published as: JP2012164860A

Abstract

This Q-switched fiber laser is equipped with a resonator-use optical fiber (8), rare earth-doped optical fiber (3) for optical pulse amplification, a Q-switching device (4), resonators (1, 2) for resonating emitted natural light or emitted stimulated light, a partial feedback optical path (5) that has a branching part (5A) and a propagation distance extending part (5B) and is provided between the resonators (1, 2). The branching part (5A) is configured such that light incident into the partial feedback optical path (5) is separated into light to be output into the resonator-use optical fiber (8) and light to be output into the propagation distance extending part (5B) at a pre-determined optical power ratio. The propagation distance extending part (5B) is configured such that by propagating the light incident into the partial feedback optical path (5) within a predetermined optical path length, the propagation distance of the light is extended before the light is emitted into the branching part (5A) again.

Description

Q-switched fiber laser

The present invention relates to a Q-switch type fiber laser that generates an optical pulse having high pulse energy and a wide pulse width (for example, about 100 ns to 500 ns) as an optical pulse used for a light source of a laser processing apparatus.
This application claims priority based on Japanese Patent Application No. 2011-024833 for which it applied on February 8, 2011, and uses the content here.

A Q-switch method is known as a method for obtaining an output light pulse having a high pulse energy from a solid-state laser. In the Q switch method, while pumping is performed in the gain medium, a high gain can be obtained without oscillation by setting the quality factor Q of the resonator to a low value. Also, by returning the quality factor Q of the resonator to a high value, the energy stored in the atoms excited to the upper level is suddenly converted into photons in the resonator, and the output light with high pulse energy. A pulse can be obtained. A Q-switch type fiber laser based on such a Q-switch method is widely used for processing such as welding, cutting and drilling of industrial materials, that is, laser processing. As an optical pulse used as a light source of such a laser processing apparatus, an optical pulse having high pulse energy and a wide pulse width of about 100 ns to 500 ns is desired.

As a method of widening the pulse width of an output optical pulse by using a Q-switch type fiber laser that outputs an optical pulse of high pulse energy, the circulation time of the optical pulse propagating in the resonator is increased by increasing the resonator length. A method for increasing the length is known. For example, in Non-Patent Document 1, “the laser rod is the same, the reflectivity of the mirrors at both ends of the resonator and the power of the pumping light are the same, and two Q-switched lasers with different resonator lengths have the resonator length It is described that a Q-switched laser with a longer length has a longer resonator lifetime and a wider pulse width ”(see FIG. 26.9 of Non-Patent Document 1).

In the method of increasing the circulation time of the optical pulse propagating in the resonator by increasing the cavity length of the Q-switch type fiber laser, the pulse width of the resonator output pulse shape is relatively wide, but it corresponds to the circulation time. The plurality of small signal light pulses are overlapped. For this reason, when the circulation time is long, the sum of small signal light pulses generated at a period corresponding to the circulation time creates a peak and a valley in the entire pulse shape, and relatively large fluctuations in the resonator output pulse shape ( Hereinafter, this will be described as “swell”) (FIG. 11). The generation cycle of this small signal light pulse corresponds to the time for which the light circulates in the resonator.

As described above, the undulation generated in the output light pulse emitted from the Q-switch type fiber laser increases the variation in the characteristics of the output light pulse for each product at the time of product manufacture. As a result, it takes a long time to adjust the pumping light power of the semiconductor laser mounted in the Q-switch type fiber laser, and the pulse width or output power of the output light pulse cannot be adjusted within the product specifications. In some cases.

By the way, as a conventional technique shown in the publicly known literature which aims to stabilize the output light pulse of the fiber laser, a method of obtaining an ultrashort light pulse whose pulse width or repetition frequency does not change by the optical soliton effect (for example, Patent Document 1). See, for example, a method of obtaining a pulse output by changing a fiber length after branching an optical pulse with an optical coupler (see, for example, Patent Document 2), and a fiber laser using an acoustooptic element as a Q switch device. A method of adjusting the timing from the generation of an optical pulse to pulse amplification to stabilize the optical pulse (see, for example, Patent Document 3), and efficiently using the pump combiner to pump light into the power amplifier A method (for example, refer to Non-Patent Document 2) that obtains a 10 W-class output light pulse by introducing it is known.

However, in any of the conventional techniques shown in the above-mentioned documents, it has not been considered to prevent the occurrence of undulations appearing in the output light pulse generated by the Q-switched fiber laser.

JP-A-5-102582 Japanese Patent Laid-Open No. 10-125983 JP 2007-234943 A

As described above, in the conventional Q-switched fiber laser, the optical pulse propagating in the resonator is obtained by increasing the resonator length of the Q-switched fiber laser while keeping the pulse energy of the output light pulse at a high level. When the circulation time is increased and the pulse width is increased to about 100 ns to 500 ns, there is a problem that the output light pulse is swelled.

The present invention has been made against the background described above, and an object thereof is to eliminate undulations appearing in the pulse shape of an optical pulse having a high pulse power and a wide pulse width (for example, 100 ns to 500 ns). The present invention also provides a Q-switched fiber laser that outputs such an optical pulse.

The inventors of the present invention have solved the above-mentioned problem by providing a partial feedback optical path in the resonator of the Q-switch type fiber laser that can prevent the output light pulse from being waved.

Next, the Q-switch type fiber laser according to the first to eighth aspects of the present invention will be described.

A Q-switched fiber laser according to a first aspect of the present invention includes a resonator optical fiber, a rare-earth doped optical fiber for optical pulse amplification, a Q-switch device, and a resonance for resonating spontaneous emission or stimulated emission light. And a partial feedback optical path having a branching portion and a propagation distance extending portion and provided in the resonator. Here, the branching unit separates the light incident on the partial feedback optical path into light emitted to the resonator optical fiber and light emitted to the propagation distance extension unit at a constant optical power ratio. Is configured to do. The propagation distance extension unit is configured to extend the propagation distance of the light by propagating the light incident on the partial feedback optical path within a certain optical path length and to enter the branching unit again. ing.

In the Q-switched fiber laser of the first aspect described above, a partial feedback optical path having a branch portion and a propagation distance extension portion is provided inside the resonator. With this structure, the propagation distance of the light branched in the direction of the propagation distance extension at the branching portion is extended by the propagation distance extension provided in the partial feedback optical path. This structure is substantially equivalent to a configuration in which multiple resonators having different optical path lengths are arranged. As a result, a plurality of small signal light pulses generated with a period corresponding to the circulation time in the resonator are superimposed on the output light pulse, so that the undulation valley portion of the output light pulse disappears, and the Q-switch type fiber laser It is possible to obtain a pulse-shaped light pulse without undulation.
In the present invention, “eliminating waviness” includes reducing waviness.

In the Q-switched fiber laser according to the second aspect of the present invention, an optical fiber coupler having an incident waveguide and an output waveguide is provided as a branching portion of the partial feedback optical path, and an optical fiber is used as a propagation distance extending portion. It is preferable that both ends of the optical fiber are connected to the incident waveguide and the output waveguide.

In the Q-switch type fiber laser of the second aspect, by providing an optical fiber coupler and an optical fiber as a branching portion and a propagation distance extending portion of a partial feedback optical path, the configuration is simple and the resonator is configured at a relatively low cost. can do.

The Q-switched fiber laser according to the third aspect of the present invention is the Q-switched fiber laser according to the second aspect, wherein the power ratio of light branched from the optical fiber coupler to the optical fiber is 60% ± 20. % Is preferable.

In the Q-switched fiber laser of the third aspect, the optical power ratio of the optical pulse branched from the optical fiber coupler of the partial feedback optical path to the optical fiber for the propagation distance extension is within the range of 60% ± 20%. By doing so, the shape of the optical pulse output from the Q-switch type fiber laser can be made into a smooth pulse shape without undulation.

A Q-switched fiber laser according to a fourth aspect of the present invention is the Q-switched fiber laser according to the first aspect, wherein a fiber Bragg grating is provided as a branch part of the partial feedback optical path, and is reflected by the fiber Bragg grating. In addition, it is preferable that a light propagation path that propagates through the resonator and enters the fiber Bragg grating again functions as a propagation distance extension.

In the Q-switch type fiber laser of the fourth aspect, a fiber Bragg grating is provided as a branch part of the partial feedback optical path, reflected by the fiber Bragg grating, propagated through the resonator, and again incident on the fiber Bragg grating. The light propagation path up to this point functions as a propagation distance extension. For this reason, the space required for installing the partial feedback optical path is reduced, and the entire resonator of the Q-switch type fiber laser can be downsized.

The Q-switched fiber laser according to the fifth aspect of the present invention is the Q-switched fiber laser according to the fourth aspect, wherein the reflectance of the fiber Bragg grating is preferably in the range of 1% to 10%. .

In the Q-switched fiber laser of the fifth aspect, the reflectance of the fiber Bragg grating in the partial feedback optical path is in the range of 1% to 10%. For this reason, the shape of the optical pulse output from the Q switch type fiber laser can be made into a smooth pulse shape without undulation.

In the Q-switched fiber laser according to the sixth aspect of the present invention, in the Q-switched fiber laser according to any one of the first to fifth aspects, an acousto-optic element is preferably provided as a Q-switch device.

In the Q-switch type fiber laser of the sixth aspect, an acousto-optic element is provided as a Q-switch device. For this reason, the loss of the Q switch can be precisely controlled, and the driving time can be shortened, so that the response of the output light pulse emitted from the Q switch type fiber laser can be made high speed.

The Q-switched fiber laser according to the seventh aspect of the present invention is preferably the same as the Q-switched fiber laser according to any one of the first to sixth aspects, wherein a Fabry-Perot resonator is provided as a resonator. .

In the Q-switched fiber laser of the seventh aspect, the fiber length of the resonator can be shortened and the number of components can be reduced by providing the Fabry-Perot resonator in the Q-switched fiber laser.

In the Q-switched fiber laser according to the eighth aspect of the present invention, the Q-switched fiber laser according to any one of the first to third aspects is preferably provided with a ring resonator as a resonator.

In the Q-switched fiber laser of the eighth aspect, the loss in the resonator can be reduced by providing the ring-type resonator in the Q-switched fiber laser.

According to the Q-switched fiber laser of the present invention, it is possible to generate an optical pulse without undulation in the pulse shape, and even when an optical pulse having a high pulse energy and a wide pulse width is obtained, an optical pulse without undulation is obtained. Can be generated. Therefore, the problem that it takes time to adjust each product when a Q-switch type fiber laser product is actually manufactured, or the problem that the product cannot be adjusted is solved.

1 is a schematic diagram showing the overall configuration of a Q-switched fiber laser according to a first embodiment of the present invention. It is the schematic which shows the whole structure of the Q switch type fiber laser by the 2nd Embodiment of this invention. In the Q-switched fiber laser according to the third embodiment of the present invention, an optical fiber coupler is provided as a branch part of the partial feedback optical path, and an optical fiber is provided as a propagation distance extension part of the partial feedback optical path. It is the schematic which shows the whole structure of a Q switch type fiber laser. A Q-switched fiber laser according to a third embodiment of the present invention, wherein a fiber Bragg grating is provided as a branch part of a partial feedback optical path, and is reflected by the fiber Bragg grating as a propagation distance extension part of the partial feedback optical path. FIG. 2 is a schematic view showing the overall configuration of a Q-switched fiber laser that functions as a light propagation path from the inside of the resonator until it enters the fiber Bragg grating again. In the Q-switch type fiber laser according to the fourth embodiment of the present invention, a Q switch is provided with an optical fiber coupler as a branch part of the partial feedback optical path and an optical fiber as a propagation distance extension part of the partial feedback optical path It is the schematic which shows the whole structure of an optical fiber laser. 1 is a schematic diagram illustrating an overall configuration of a Q-switched fiber laser used in Example 1. FIG. 3 is a graph showing a relationship of optical power with respect to time of an output optical pulse emitted from a Q-switch type fiber laser in Example 1. FIG. 6 is a graph showing the relationship of the optical power with respect to the time of the output optical pulse in the Q-switched fiber laser in Example 2, and the power of the incident optical pulse incident on the optical delay adding unit with respect to the incident optical pulse incident on the partial feedback optical path. The case where the branching amount is 30% is shown. FIG. 6 is a graph showing the relationship of the optical power with respect to the time of the output optical pulse in the Q-switched fiber laser in Example 2, and the power of the incident optical pulse incident on the optical delay adding unit with respect to the incident optical pulse incident on the partial feedback optical path. The case where the branching amount is 70% is shown. FIG. 6 is a graph showing the relationship of the optical power with respect to the time of the output optical pulse in the Q-switched fiber laser in Example 2, and the power of the incident optical pulse incident on the optical delay adding unit with respect to the incident optical pulse incident on the partial feedback optical path. The case where the branching amount is 90% is shown. It is the graph which showed the relationship of the optical power with respect to the time of the output optical pulse in the Q switch type fiber laser in Example 3, and has shown the case where the feedback length (loop length) in a partial feedback optical path is 1000 mm. It is the graph which showed the relationship of the optical power with respect to the time of the output optical pulse in the Q switch type fiber laser in Example 3, and has shown the case where the feedback length in a partial feedback optical path is 2000 mm. It is the graph which showed the relationship of the optical power with respect to the time of the output optical pulse in the Q switch type fiber laser in Example 3, and has shown the case where the feedback length in a partial feedback optical path is 3000 mm. 6 is a graph showing a relationship of optical power with respect to time of an output light pulse in a Q-switch type fiber laser in Example 4. In a conventional Q-switched fiber laser, a graph showing the relationship of the optical power with respect to the time of the output optical pulse when the circulation time of the optical pulse propagating in the resonator is increased by increasing the resonator length. is there.

Hereinafter, embodiments of the present invention will be described below with reference to FIGS.

FIG. 1 shows a Q-switched fiber laser according to a first embodiment of the present invention. This Q-switch type fiber laser is a Fabry-Perot type resonance comprising a high reflectivity structure 1 and a low reflectivity structure 2 arranged in a resonator optical fiber 8 as a resonator for resonating spontaneous emission light or stimulated emission light. Has a vessel. Between the high reflectivity structure 1 and the low reflectivity structure 2, a rare earth doped optical fiber 3, a partial feedback optical path 5, and a Q switch device 4 are provided. The Q switch device 4 instantaneously emits an optical pulse after sufficiently storing energy in the resonator. The partial feedback optical path 5 includes a branching part 5A and a propagation distance extension part 5B. An optical fiber coupler 5a is disposed as the branching portion 5A, and an optical fiber 5b is disposed as the propagation distance extending portion 5B. The optical fiber coupler 5a has an incident waveguide and an output waveguide. The optical fiber for resonator 5 and one end (first end) of the optical fiber 5b having the function of the propagation distance extension 5B are connected to the incident waveguide of the optical fiber coupler 5a. The output waveguide of the optical fiber coupler 5a is connected to the resonator optical fiber 8 and the other end (second end) of the optical fiber 5b having the function of the propagation distance extension 5B. The end of the optical fiber 5b connected to the output waveguide has a structure that leads to the incident waveguide of the optical fiber coupler 5a.
In an actual Q-switch type fiber laser, a driver (drive unit) (not shown) is provided in the pumping light source or the Q-switch device, but the configuration of the driver itself may be the same as that in the past.
The positions where the optical fiber coupler 5a and the optical fiber 5b are inserted into the Q switch type fiber laser are between the rare earth doped optical fiber 3 and the Q switch device 4 or between the Q switch device 4 and the low reflectivity structure 2. Any one is acceptable. Further, the optical fiber coupler 5a and the optical fiber 5b can be inserted between the high reflectivity structure 1 and the rare earth-doped optical fiber 3, and in this case, the transmittance of the pumping light at the branching portion is close to 100%. Is desirable.

Next, the operation and function of the Q-switch type fiber laser of the first embodiment shown in FIG. 1 will be described. The excitation light incident on the Fabry-Perot resonator of the Q-switched fiber laser propagates through the resonator optical fiber 8, passes through the high reflectivity structure 1, and the Q value of the resonator is set low by the Q switch device 4. In this state, the light is incident on the rare earth doped optical fiber 3 to excite the rare earth element of the rare earth doped optical fiber 3. In the state in which the inversion distribution rate of the rare earth-doped optical fiber is high, the Q value of the resonator is increased by the Q switch device 4 so that the spontaneous emission light generated in the excited rare earth-doped optical fiber 3 has a high reflectance. Reflected by the structure 1 or the low reflectivity structure 2 into the resonator. When the reflected light enters the excited rare-earth doped optical fiber 3, stimulated emission is generated, optically amplified, and incident on the partial feedback optical path 5. The optical pulse incident on the partial feedback optical path 5 is split into light A (first light) and light B (second light) at a constant optical power ratio by the optical fiber coupler 5a. The light A passes through the Q switch device 4 via the resonator optical fiber 8 and is reflected by the low reflectivity structure 2. On the other hand, the light B is incident on the optical fiber 5b of the partial feedback optical path 5 and propagates to extend the propagation distance of one round of the optical fiber 5b, and returns to the optical fiber coupler 5a again. The light B returned to the optical fiber coupler 5 a is branched into light C (third light) and light D (fourth light) at a constant power ratio, and the light C propagates to the Q switch device 4. The other light D enters the optical fiber 5b again, the propagation distance for one round of the optical fiber 5b is extended, and returns to the optical fiber coupler 5a again. This partial feedback operation is repeated, and the light propagation distance is extended according to the number of times of feedback. Then, as a result of the superposition of a plurality of small signal light pulses generated at a period according to the circulation time in the resonator on the output light pulse, the low reflectance structure 2 is changed into a pulse shape via the resonator optical fiber 8. An unpulsed light pulse is output.

In the embodiment of FIG. 1, in the optical fiber coupler 5a, the power ratio of the light branched to the optical fiber 5b (the power ratio of the branched light to the power of the light incident on the optical fiber coupler 5a) is 60% ± 20%. It is desirable to be within the range. When the power ratio is in the range of 60% ± 20%, the shape of the light pulse emitted from the Q-switch type fiber laser becomes extremely smooth. If the power ratio is less than 40%, the undulation of the pulse shape is not improved, and even if it exceeds 80%, the undulation of the pulse shape is not improved.
Further, by increasing the population inversion ratio in the rare earth-doped optical fiber 3, the amplification of the light pulse is promoted, and the pulse energy of the light pulse is increased. Considering the feasibility, it is desirable that the addition density of the rare earth element is in the range of 2.0 × 1025 to 4.0 × 1025 [/ m ³ ]. When the addition density is lowered, the pulse power of the output light pulse is lowered.

FIG. 2 shows a Q-switched fiber laser according to a second embodiment of the present invention. 2, the same members as those of the Q-switch type fiber laser shown in FIG. 1 are denoted by the same reference numerals, and the description thereof is omitted or simplified. Also, in each drawing described below, similarly, the same members as those in the Q-switch type fiber laser shown in FIG. 1 are denoted by the same reference numerals, and the description thereof is omitted or simplified.
In the Q-switched fiber laser of the second embodiment shown in FIG. 2, a fiber Bragg grating 5c is provided as a branching part 5A of the partial feedback optical path 5. The Q-switch type fiber laser of the second embodiment is reflected by the fiber Bragg grating 5c, propagates through the resonator, is reflected by the high reflectivity structure 1, and enters the fiber Bragg grating 5c again. The light propagation path has a structure that functions as the propagation distance extension 5B. The position where the optical fiber Bragg grating 5c is inserted into the Q-switch type fiber laser must be between the Q-switch device 4 and the low reflectivity structure 2.

Next, the operation and function of the partial feedback optical path in the Q-switch type fiber laser of the second embodiment shown in FIG. 2 will be described. The light incident on the partial feedback optical path 5 is branched into light E (fifth light) and light F (sixth light) according to the reflectance of the fiber Bragg grating 5c, and the light E passes through the fiber Bragg grating 5c. It is transmitted and reflected by the low reflectivity structure 2. The light F is reflected by the fiber Bragg grating 5c, propagates through the resonator optical fiber 8, is reflected by the high reflectivity structure 1 through the Q switch device 4 and the rare earth-doped optical fiber 3, and again is added to the rare earth-doped optical fiber 3. Is incident on. Then, the light F generates stimulated emission light in the rare earth-doped optical fiber 3, passes through the Q switch device 4, and enters the fiber Bragg grating 5 c again. Accordingly, the propagation distance extension 5B in the partial feedback optical path 5 of the embodiment of FIG. 2 passes through the rare-earth-doped optical fiber 3 and the Q switch device from the exit end face of the high reflectivity structure 1 via the resonator optical fiber 8. It becomes a light propagation path to a position reflected by the fiber Bragg grating 5c. The extended propagation distance is the reciprocal distance of the propagation path. The above-described operation in the partial feedback optical path 5 extends the light propagation distance according to the number of feedbacks. Then, as a result of the superposition of a plurality of small signal light pulses generated at a period according to the circulation time in the resonator on the output light pulse, the low reflectance structure 2 is changed into a pulse shape via the resonator optical fiber 8. An unpulsed light pulse is output.

In the embodiment of FIG. 2, the reflectance of the fiber Bragg grating 5c (the ratio of the power of the light reflected by the fiber Bragg grating 5c to the power of the light incident on the fiber Bragg grating 5c) is 1% to 10%. It is desirable to be within the range. If the reflectance is in the range of 1% to 10%, the undulation of the shape of the light pulse emitted from the Q-switch type fiber laser is eliminated. If the reflectance is less than 1%, the number of feedbacks is small, and thus the pulse shape undulation is not improved. On the other hand, if the reflectivity exceeds 10%, the resonator output power may be lowered, resulting in a decrease in pulse energy.

FIG. 3 shows a Q-switched fiber laser according to a third embodiment of the present invention.
The Q-switch type fiber laser of the third embodiment shown in FIG. 3 has a configuration in which an acoustooptic device 4 a is disposed as the Q-switch device 4.

In FIG. 3, by changing the amplitude of the high-frequency signal corresponding to the material of the acoustooptic element 4a, the diffraction efficiency of the acoustooptic element 4a changes and the loss of the module including the acoustooptic element 4a changes. The power of the light pulse incident on the acousto-optic element 4a changes according to the loss control state of the module and is emitted. When the resonator loss is high by the acousto-optic element 4a, the rare-earth-doped optical fiber 3 is excited, and when the inversion distribution rate is higher than a predetermined level, the resonator loss is caused by the acousto-optic element 4a. By lowering, an optical pulse with high pulse power is emitted.

Next, a modification of the Q-switch type fiber laser of the third embodiment will be described with reference to FIG.
In the modification of the Q-switched fiber laser shown in FIG. 4, a fiber Bragg grating 5c is provided as a branching part 5A of the partial feedback optical path 5 in the Q-switched fiber laser of the third embodiment shown in FIG. The propagation path of the light reflected by the Bragg grating 5c, propagating through the resonator, reflected by the high reflectivity structure 1, and entering the fiber Bragg grating again functions as the propagation distance extension 5B. It should be noted that the position where the fiber Bragg grating 5c is inserted into the Q-switched fiber laser must be between the acoustooptic device 4a and the low reflectivity structure 2 as shown in FIG.

FIG. 5 shows a Q-switch type fiber laser according to a fourth embodiment of the present invention. This Q-switch type fiber laser has a ring resonator composed of a pump coupler 1a and an output coupler 2a disposed in the resonator optical fiber 8 as a resonator for resonating spontaneous emission light or stimulated emission light. This Q-switch type fiber laser has a rare-earth doped optical fiber 3, a band-pass filter 6, a Q-switch device 4, an optical fiber coupler 5a as a branching section 5A and a propagation distance from the side (position) close to the pumping coupler 1a. The partial feedback optical path 5 having the optical fiber 5b as the extension 5B is arranged.

In addition, the position where this partial feedback optical path 5 is inserted into the Q-switch type fiber laser is the position between the rare earth-doped optical fiber 3 and the band-pass filter 6, and between the band-pass filter 6 and the Q-switch device 4. It is one of the position, the position between the Q switch device 4 and the output coupler 2a, and the position between the output coupler 2a and the excitation coupler 1a.
In this embodiment, the effect of the present invention can be obtained even if the arrangement of the bandpass filter 6 and the Q switch device 4 is exchanged, and the effect of the present invention can be obtained even if the arrangement of the Q switch device 4 and the output coupler 2a is exchanged. can get.

Next, the operation and function of the Q-switch type fiber laser of the fourth embodiment shown in FIG. 5 will be described. The pumping light incident on this ring resonator passes through the pumping coupler 1a and enters the rare earth-doped optical fiber 3 to excite the rare earth element. Light generated in the excited rare earth-doped optical fiber 3 passes through the bandpass filter 6 and the Q switch device 4 and enters the partial feedback optical path 5. The incident light is branched into light A and light B at a constant optical power ratio by the optical fiber coupler 5a which is the branching part 5A of the partial feedback optical path 5, and the light A propagates to the output coupler 2a. On the other hand, the light B is incident on the optical fiber 5b which is the propagation distance extending portion 5B and propagates through a certain optical path length, thereby extending the propagation distance and returning to the optical fiber coupler 5a again. The light B returned to the optical fiber coupler 5a is branched into light C and light D at a constant optical power ratio, the light C propagates to the output coupler 2a, and the other optical pulse D is again transmitted to the optical fiber 5b. Incident light is propagated through a certain optical path length, thereby extending the propagation distance and returning to the optical fiber coupler 5a again. The optical pulse incident on the output coupler 2a is branched at a constant optical power ratio, and one optical pulse (first optical pulse) is emitted as an output optical pulse emitted from the resonator, while the other light is emitted. The pulse (second optical pulse) again passes through the excitation coupler 1 a and enters the rare earth-doped optical fiber 3. At this time, stimulated emission light is generated in the rare earth-doped optical fiber 3, and the stimulated emission light excites the ring-shaped resonator optical fiber 8. The light propagating through the resonator optical fiber 8 is again incident on the rare-earth doped optical fiber 3 in the excited state, whereby an optical pulse is generated, and a plurality of small signal lights generated at a period corresponding to the circulation time in the resonator. As a result of superimposing the pulse on the output optical pulse, an optical pulse having no undulation in the pulse shape is output.

Example 1
The overall configuration of the Q-switched fiber laser used in Example 1 is shown in FIG. In this Q switch type fiber laser, based on the configuration of the Q switch type fiber laser of the first embodiment, a high reflection fiber Bragg grating 1b as a high reflectivity structure 1 and a low reflection fiber as a low reflectivity structure 2 are used. Bragg grating 2b, Yb-doped optical fiber 3a as rare earth-doped optical fiber 3, acoustooptic element 4a as Q-switch device 4, optical fiber coupler 5a as branching portion 5A of partial feedback optical path 5, and partial An optical fiber 5b as a propagation distance extension 5B of the feedback optical path 5 is disposed on the same resonator optical fiber 8, and a semiconductor laser 7 is provided as a pumping light source. The semiconductor laser 7 and the acoustooptic device 4a are equipped with a driver (not shown).

In the Q-switched fiber laser shown in FIG. 6, the pumping light emitted from the semiconductor laser 7 passes through the highly reflective fiber Bragg grating 1b, and the acoustooptic element 4a sets the Q value to be low. It enters 3a and excites the Yb element. As a result, the inversion distribution of the Yb-doped optical fiber 3a is increased. When the Q value of the resonator is increased by the acoustooptic device 4a in this state, the spontaneous emission light generated in the Yb-doped optical fiber 3a is converted into a highly reflective fiber Bragg grating. The light is reflected into the resonator by 1b and the low reflection fiber Bragg grating 2b. The reflected light is incident on the excited Yb-doped optical fiber 3a to generate stimulated emission light. The stimulated emission light is reflected into the resonator by the high reflection fiber Bragg grating 1b and the low reflection fiber Bragg grating 2b. The The reflected light is incident again on the Yb-doped optical fiber 3a in the excited state, so that an optical pulse is generated. Light incident on the partial feedback optical path 5 during the generation of the optical pulse is branched into light A and light B at a constant optical power ratio by the optical fiber coupler 5a of the partial feedback optical path 5, and the light A is acousto-optic. Propagate to element 4a. On the other hand, the light B propagates to the optical fiber 5b of the partial feedback optical path 5 and propagates a certain optical path length, thereby extending the propagation distance and returning to the optical fiber coupler 5a again. The light B returned to the optical fiber coupler 5a is branched into light C and light D at a constant optical power ratio, and the light C propagates to the acoustooptic device 4a. The other light D is again propagated to the optical fiber 5b and propagates a certain optical path length, thereby extending the propagation distance and returning to the optical fiber coupler 5a again. This partial feedback operation is repeated, and the light propagation distance is extended according to the number of times of feedback. As a result of the operation of the above-described Q-switched fiber laser, a plurality of small signal light pulses generated at a period corresponding to the circulation time in the resonator are superimposed on the output light pulse. As a result, the low reflection fiber Bragg grating 2b An optical pulse with no undulations is output.

FIG. 7 shows output light pulses obtained when the optical components having the optical characteristics shown in Table 1 are arranged in the Q-switched fiber laser shown in FIG.

As described above, according to the Q switch type fiber laser provided with the partial feedback optical path of the first embodiment, the pulse power is high, the pulse width is 100 ns to 500 ns, and the smooth pulse without the small signal pulse or the undulation It was confirmed that a light pulse having a shape was generated.

(Example 2)
A Q-switched fiber laser having the same configuration as in Example 1 was used. In the optical fiber coupler 5a of the partial feedback optical path 5, the optical power branched to the optical fiber 5b which is the propagation distance extension 5B is 30%, 70% and 90% of the optical pulse incident on the partial feedback optical path 5. The output light pulse waveforms when changed are shown in FIGS. 8A to 8C. Table 2 shows measurement results of fluctuations in the pulse width and average power of the output light pulse when a lateral pressure is applied to the resonator optical fiber 8.

As shown in Example 2 above, by setting the power branching amount of the light incident on the propagation distance extension portion with respect to the light incident on the partial feedback optical path in the partial feedback optical path of the Q-switched fiber laser to 70%, It was confirmed that the fluctuation of the pulse width of the output light pulse and the fluctuation of the average power with respect to the side pressure can be remarkably reduced.

(Example 3)
A Q-switched fiber laser having the same configuration as in Example 1 was used. In the optical fiber coupler 5a of the partial feedback optical path 5, the optical power branched to the optical fiber 5b which is the propagation distance extension 5B is fixed to 70% of the optical pulse incident on the partial feedback optical path 5, and the optical fiber 5b. 9A to 9C show output light pulse waveforms when the length (feedback length) is changed to 1000 mm, 2000 mm, and 3000 mm. In any output optical pulse waveform when the length of the optical fiber 5b of the partial feedback optical path of the Q switch type fiber laser is changed to 1000 mm, 2000 mm, and 3000 mm, the peak of the center power does not collapse, and a large swell Neither has occurred.
Further, Table 3 shows the measurement results of the fluctuation of the pulse width of the output light pulse and the fluctuation of the average power when the lateral pressure is applied to the resonator optical fiber 8. The fluctuation of the average power of the output optical pulse is remarkably low when the length of the optical fiber 5b in the partial feedback optical path is 1000 mm, compared to the resonator length of 2000 mm of the Q-switch type fiber laser.

As shown in Example 3 above, in the partial feedback optical path of the Q-switched fiber laser, the optical power branched into the optical fiber 5b that is the propagation distance extension 5B is incident on the partial feedback optical path 5. In the case of 70%, an output light pulse without undulation was obtained regardless of the feedback length. In particular, it has been confirmed that by setting the feedback length of the partial feedback optical path to 1000 mm with respect to the resonator length of 2000 mm, the output optical pulse shape can be made extremely smooth and the fluctuation of the average power of the output optical pulse with respect to the side pressure can be significantly reduced. It was done. It has been confirmed that the feedback length of the partial feedback optical path is preferably in the range of 500 mm to 1500 mm with respect to the resonator length of 2000 mm. However, when the resonator length is different, the feedback length is It is necessary to select appropriately according to the length.

Example 4
The configuration of the Q-switched fiber laser used in Example 4 is based on the configuration of the Q-switched fiber laser of the second embodiment. Specifically, a fiber Bragg grating having a reflectance of 4% is provided as a branch part of the partial feedback optical path, reflected by the fiber Bragg grating 5c, propagated through the resonator, reflected by the high reflectance structure 1, and again. The light propagation path until it enters the fiber Bragg grating 5c functions as the propagation distance extension 5B. The output light pulse waveform obtained from this Q-switch type fiber laser is shown in FIG.

As shown in FIG. 10, it was confirmed that the undulation of the output light pulse shape was eliminated also in the case of Example 4.

DESCRIPTION OF SYMBOLS 1 High reflectivity structure 1a Excitation coupler 1b High reflection fiber Bragg grating 2 Low reflectivity structure 2a Output coupler 2b Low reflection fiber Bragg grating 3 Rare earth addition optical fiber 3a Yb addition optical fiber 4 Q switch apparatus 4a Acoustooptic element 5 Partial feedback Optical path 5A Branch part 5B Propagation distance extension part 5a Optical fiber coupler 5b Optical fiber 5c Fiber Bragg grating 6 Bandpass filter 7 Semiconductor laser 8 Optical fiber for resonator

Claims

A Q-switched fiber laser,
An optical fiber for a resonator;
A rare earth doped optical fiber for optical pulse amplification;
A Q switch device;
A resonator for resonating spontaneous emission or stimulated emission; and
A partial feedback optical path having a bifurcation and a propagation distance extension and provided in the resonator;
With
The branching unit is configured to separate light incident on the partial feedback optical path into light emitted to the resonator optical fiber and light emitted to the propagation distance extension unit at a constant optical power ratio. Configured,
The propagation distance extension unit is configured to extend the propagation distance of the light by propagating light incident on the partial feedback optical path within a certain optical path length, and to enter the branching unit again. Q-switch type fiber laser characterized by the above.
The Q-switched fiber laser according to claim 1,
As a branch of the partial feedback optical path, an optical fiber coupler having an incident waveguide and an output waveguide is provided,
An optical fiber is provided as a propagation distance extension,
Both ends of the optical fiber are connected to the incident waveguide and the output waveguide. A Q-switched fiber laser, wherein:
The Q-switched fiber laser according to claim 2,
The Q-switch type fiber laser, wherein a power ratio of light branched from the optical fiber coupler to the optical fiber is in a range of 60% ± 20%.
The Q-switched fiber laser according to claim 1,
As a branch of the partial feedback optical path, a fiber Bragg grating is provided,
An optical Q-switch type fiber laser characterized in that a light propagation path from a light reflected by a fiber Bragg grating, propagated through the resonator, and again incident on the fiber Bragg grating functions as a propagation distance extension.
The Q-switched fiber laser according to claim 4,
The Q-switch type fiber laser, wherein the reflectance of the fiber Bragg grating is in the range of 1% to 10%.
In the Q-switched fiber laser according to any one of claims 1 to 5,
A Q-switch type fiber laser comprising an acousto-optic device as the Q-switch device.
In the Q-switched fiber laser according to any one of claims 1 to 6,
A Q-switch type fiber laser comprising a Fabry-Perot resonator as the resonator.
In the Q-switched fiber laser according to any one of claims 1 to 3,
A Q-switch type fiber laser comprising a ring-type resonator as the resonator.