JP2017037928A - Fiber laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize a fiber laser in which, even if power of excitation light is increased, it is difficult to raise resin temperature in the vicinity of the end of an optical fiber for amplification.SOLUTION: A fiber laser (1) includes an optical fiber (11) for amplification, a front excitation light source (13a), a rear excitation light source (13b) and a control part (15) for controlling the front excitation light source (13a) and the rear excitation light source (13b) so that power (Pa) of front excitation light and power (Pb) of rear excitation light complementarily and repeatedly increase and decrease.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a bidirectionally pumped fiber laser.

  A fiber laser is widely used as a laser device for processing. A fiber laser is a laser device that uses an optical fiber in which a rare earth element is added to a core (hereinafter referred to as “amplifying optical fiber”) as a laser medium.

  The fiber laser supplies pumping light to an amplification optical fiber, a mirror element connected to one end of the amplification optical fiber, a half mirror element connected to the other end of the amplification optical fiber, and the amplification optical fiber. And an excitation light source. There are pumping light that is supplied to the amplifying optical fiber via a mirror element and that that is supplied to the amplifying optical fiber via a half mirror. The former is forward pumping light, and the latter is backward pumping light. Called. With a bidirectionally pumped fiber laser including both a forward pumping light source and a rear pumping light source, an output power of 1 kW or more can be obtained.

  In the fiber laser, when excitation light enters the amplification optical fiber from the end, the amplification optical fiber absorbs the excitation light and generates heat in the vicinity of the end. At this time, when the temperature of the resin portion of the amplification optical fiber (hereinafter referred to as “resin temperature”) exceeds the heat-resistant upper limit temperature of the material, problems such as deterioration and decomposition may occur. For this reason, the power of the pumping light has to be suppressed so that the temperature of the resin portion of the amplification optical fiber does not exceed the heat-resistant upper limit temperature, which has been a factor in increasing the output of the fiber laser.

  Regarding the heat generation of the amplification optical fiber used in the fiber laser, Patent Literature 1 discloses the following knowledge. That is, the amount of heat generated per unit length of the amplification optical fiber due to the forward pumping light is maximized at the end where the forward pumping light is incident, and decreases exponentially as the distance from the end is increased. In other words, the heat generation of the amplification optical fiber due to the forward pumping light occurs locally in the vicinity of the end where the forward pumping light is incident. Similarly, the amount of heat generated per unit length of the amplification optical fiber due to the backward pumping light is maximized at the end where the backward pumping light is incident, and decreases exponentially as the distance from the end is increased. In other words, the heat generation of the amplification optical fiber due to the backward pumping light locally occurs near the end where the backward pumping light is incident.

  Patent Document 1 discloses a mounting form of an optical fiber for amplification devised based on the above knowledge. That is, there is shown a mounting form in which the amplification optical fiber is wound in a spiral shape with a gap and placed on a heat sink. In the mounting form described in Patent Document 1, (1) the amplification optical fiber is wound in such a way that the vicinity of the end where the heat generation amount is large constitutes the outer periphery of the spiral, and (2) the spacing between the amplification optical fibers. Is equal to or larger than the diameter of the optical fiber for amplification on the outer peripheral side of the spiral, so that the distribution of the amount of heat generated per unit area of the optical fiber for amplification (the amount of heat generated from the portion disposed within the unit area) is uniformly distributed. It is close to. As a result, heat dissipation using the heat dissipation plate is efficiently performed, and as a result, the resin temperature in the vicinity of the end portion of the amplification optical fiber is hardly increased.

JP, 2015-90909, A (publication date: May 11, 2015)

  However, even if the form described in Patent Document 1 is adopted for mounting the amplification optical fiber, the resin temperature in the vicinity of the end portion of the amplification optical fiber rises if the power of the excitation light is increased. That is, even if the form described in Patent Document 1 is adopted for mounting the amplification optical fiber, if the power of the excitation light is further increased, the resin portion deteriorates or decomposes in the vicinity of the end of the amplification optical fiber. obtain.

  The present invention has been made in view of the above-described problems, and the purpose of the present invention is to increase the power of the pumping light (the sum of the power of the front pumping light and the power of the rear pumping light). The object is to realize a fiber laser in which the resin temperature in the vicinity of the end portion hardly rises.

  In order to solve the above problems, a fiber laser according to the present invention includes an amplification optical fiber, a forward pumping light source that supplies forward pumping light to the amplification optical fiber, and backward pumping light to the amplification optical fiber. A backward pumping light source to be supplied, and a controller that controls the forward pumping light source and the backward pumping light source so that the power of the front pumping light and the power of the rear pumping light repeatedly increase and decrease in a complementary manner. It is characterized by that.

  According to the above configuration, since the power of the forward pumping light repeatedly increases and decreases, the resin temperature in the vicinity of the end portion on the side where the forward pumping light of the optical fiber for amplification is incident rises when the power of the forward pumping light increases. Can be reduced when the power of the forward pumping light is reduced. As a result, it is possible to suppress an increase in the resin temperature in the vicinity of the end of the amplification optical fiber on the side where the forward pumping light is incident.

  In addition, according to the above configuration, the power of the backward pumping light repeatedly increases and decreases, so that it increases when the power of the backward pumping light increases, near the end of the amplification optical fiber on the side where the backward pumping light is incident. The resin temperature can be lowered when the power of the backward pumping light is reduced. As a result, it is possible to suppress an increase in the resin temperature in the vicinity of the end of the amplification optical fiber on the side where the backward pumping light is incident.

  Further, according to the above configuration, the increase / decrease in the power of the forward pumping light and the increase / decrease in the power of the rear pumping light are complementary. The sum of the power and the power of the backward pumping light) can be kept constant or substantially constant.

  In summary, it is possible to suppress an increase in the resin temperature in the vicinity of each end of the amplification optical fiber while keeping the power of all pumping light supplied to the amplification optical fiber constant or substantially constant.

  In the fiber laser according to the present invention, the control unit performs ON / OFF control of the front pumping light source and the rear pumping light source so that the front pumping light source and the rear pumping light source blink complementarily. Is preferred.

  According to the above configuration, since the forward pumping light source blinks, the resin temperature in the vicinity of the end of the amplification optical fiber on the side where the forward pumping light is incident, which has risen during the lighting period of the front pumping light source, is calculated. Can be reduced during the extinguishing period. As a result, it is possible to suppress an increase in the resin temperature in the vicinity of the end of the amplification optical fiber on the side where the forward pumping light is incident.

  Further, according to the above configuration, since the backward pumping light source blinks, the resin temperature in the vicinity of the end of the amplification optical fiber on the side where the backward pumping light is incident, which has risen during the lighting period of the backward pumping light source, is pumped backward. It can be lowered during the light-off period of the light source. As a result, it is possible to suppress an increase in the resin temperature in the vicinity of the end of the amplification optical fiber on the side where the backward pumping light is incident.

  Further, according to the above configuration, since the blinking of the forward pumping light source and the blinking of the backward pumping light source are complementary, the power of all the pumping light supplied to the amplification optical fiber can be kept constant or substantially constant. it can.

  In summary, it is possible to suppress an increase in the resin temperature in the vicinity of each end of the amplification optical fiber while keeping the power of all pumping light supplied to the amplification optical fiber constant or substantially constant.

  In the fiber laser according to the present invention, the control unit turns off the backward pumping light source, turns on the front pumping light source for a predetermined lighting time, turns off the front pumping light source, It is preferable to repeat the control to turn on the backward excitation light source for a predetermined lighting time.

  According to the above configuration, by setting the lighting time of the front pumping light source and the lighting time of the rear pumping light source to be sufficiently short, the resin temperature at each end of the amplification optical fiber is changed to the resin portion of the amplification optical fiber. The heat resistance upper limit temperature can be kept lower.

  The fiber laser according to the present invention includes a first thermometer for measuring a resin temperature in the vicinity of the front pumping light source side end of the amplification optical fiber, and an end of the amplification optical fiber on the rear pumping light source side. A second thermometer for measuring a nearby resin temperature, and the control unit turns off the rear excitation light source, and the resin temperature measured by the first thermometer is predetermined. The front excitation light source is turned on until a predetermined threshold temperature is reached, then the front excitation light source is turned off, and the rear excitation light source is measured until the resin temperature measured by the second thermometer reaches the threshold temperature. It is preferable to repeat the control to turn on.

  According to the above configuration, the resin temperature at each end of the amplification optical fiber is kept lower than the heat resistant upper limit temperature by setting the threshold temperature lower than the heat resistant upper limit temperature of the resin portion of the amplification optical fiber. Can do.

  In the fiber laser according to the present invention, the amplification optical fiber is wound so as to form one spiral, and the fiber laser is a heat dissipation block that is in thermal contact with the amplification optical fiber, The heat sink per unit area in the area | region which contacts the inner peripheral part of the said optical fiber for amplification is further provided with the heat dissipation block larger than the heat absorption amount per unit area in the area | region which contacts the outer peripheral part of the said optical fiber for amplification Is preferable.

  According to the above configuration, since the amplification optical fiber is wound so as to form one spiral, the area required for mounting the amplification optical fiber can be reduced, and as a result, the fiber laser can be downsized. can do.

  Also, when the distribution of heat generation per unit length of the amplification optical fiber is symmetric, the heat generation per unit area in the inner periphery of the amplification optical fiber (the heat generation from the portion arranged within the unit area) ) Is larger than the amount of heat generated per unit area at the outer periphery of the amplification optical fiber. Therefore, when the distribution of the heat absorption amount per unit area of the heat dissipation block is uniform, the resin temperature in the inner periphery of the amplification optical fiber is higher than the resin temperature in the outer periphery of the amplification optical fiber.

  On the other hand, if the distribution per unit area of the heat dissipation block is made non-uniform as described above, the difference in resin temperature between the inner and outer peripheral portions of the amplification optical fiber is reduced (amplification optical fiber). The resin temperature at the inner periphery of the resin can be lowered).

  In the fiber laser according to the present invention, the amplification optical fiber is wound so as to form one spiral, and of the front pumping light source and the rear pumping light source, the inner side of the amplification optical fiber is arranged. The lighting time of the pumping light source connected to the end is shorter than the lighting time of the pumping light source connected to the outer end of the amplification optical fiber among the front pumping light source and the rear pumping light source. Is preferred.

  According to the above configuration, since the amplification optical fiber is wound so as to form one spiral, the area required for mounting the amplification optical fiber can be reduced, and as a result, the fiber laser can be downsized. can do.

  Also, when the distribution of heat generation per unit length of the amplification optical fiber is symmetric, the heat generation per unit area in the inner periphery of the amplification optical fiber (the heat generation from the portion arranged within the unit area) ) Is larger than the amount of heat generated per unit area at the outer periphery of the amplification optical fiber. Therefore, when the distribution of the heat absorption amount per unit area of the heat dissipation block is uniform, the resin temperature in the inner periphery of the amplification optical fiber is higher than the resin temperature in the outer periphery of the amplification optical fiber.

  On the other hand, if the lighting time of the front pumping light source and the rear pumping light source can be made different as described above, that is, if the distribution of heat generation per unit length of the amplification optical fiber is asymmetric, the amplification light The difference in resin temperature between the inner and outer peripheral portions of the fiber can be reduced (the resin temperature at the inner peripheral portion of the amplification optical fiber can be lowered).

  In the fiber laser according to the present invention, the amplification optical fiber is wound so that a first section from one end to an intermediate point forms a first spiral having an outer periphery in the vicinity of the one end. It is preferable that the second section from the middle point to the other end is wound so as to form a second spiral having an outer periphery in the vicinity of the other end.

  According to the above configuration, since the amplification optical fiber is wound so as to form two spirals, the area required for mounting the amplification optical fiber can be reduced, and as a result, the fiber laser can be downsized. can do.

  According to the above configuration, since the section from each end of the amplification optical fiber to the intermediate point is wound so as to form a spiral having the vicinity of the end as the outer periphery, The amount of heat generated per unit area in the vicinity of the end of the amplification optical fiber can be reduced compared to the case where the spiral is wound so as to form a spiral. Accordingly, it is possible to further suppress an increase in the resin temperature in the vicinity of the end portion of the amplification optical fiber.

  The fiber laser according to the present invention includes a first heat dissipation block that is in thermal contact with the first section of the amplification optical fiber, and a second heat contact that is in thermal contact with the second section of the amplification optical fiber. It is preferable that the second heat dissipating block further includes a second heat dissipating block spaced apart from the first heat dissipating block.

  According to said structure, since the 1st thermal radiation block and the 2nd thermal radiation block are spaced apart, the heat | fever generate | occur | produced in the 1st area of the optical fiber for amplification is 2nd area of the optical fiber for amplification. It is possible to prevent conduction through the heat dissipation block. Similarly, heat generated in the second section of the amplification optical fiber can be prevented from being conducted to the first section of the amplification optical fiber through the heat dissipation block. Therefore, the increase in the resin temperature in the vicinity of the end portion of the amplification optical fiber can be suppressed more effectively than in the case where the first heat dissipation block and the second heat dissipation block are not separated from each other.

  According to the present invention, it is possible to realize a fiber laser in which the resin temperature in the vicinity of the end of the optical fiber for amplification is hardly increased even if the power of the pumping light (the sum of the power of the forward pumping light and the power of the rear pumping light) is increased can do.

It is a block diagram which shows the structure of the fiber laser which concerns on the 1st Embodiment of this invention. It is a figure which shows the specific example of the control method of the front excitation light source and back excitation light source in the fiber laser shown in FIG. (A) is a graph showing the time change of the power of the forward pumping light output from a front pumping light source, (b) is a graph showing the time change of the power of the back pumping light output from a back pumping light source. And (c) is a graph showing the time change of the resin temperature at the front pumping light source side end of the amplification optical fiber, and (d) is the resin at the rear pumping light source side end of the amplification optical fiber. It is a graph showing the time change of temperature. It is a top view which shows the 1st mounting form of the optical fiber for amplification with which the fiber laser shown in FIG. 1 is provided. It is a figure which shows the structural example of the thermal radiation block adapted to the mounting form of the optical fiber for amplification shown in FIG. (A) is a top view which shows the 1st structural example of a thermal radiation block, (b) is a top view which shows the 2nd structural example of a thermal radiation block. It is a figure which shows the specific example of the control method of the front excitation light source and back excitation light source which were adapted to the mounting form of the optical fiber for amplification shown in FIG. (A) is a graph showing the time change of the power of the forward pumping light output from a front pumping light source, (b) is a graph showing the time change of the power of the back pumping light output from a back pumping light source. is there. It is a top view which shows the 2nd mounting form of the optical fiber for amplification with which the fiber laser shown in FIG. 1 is provided. It is a block diagram which shows the structure of the fiber laser which concerns on the 2nd Embodiment of this invention.

<< First Embodiment >>
[Configuration of fiber laser]
The configuration of the fiber laser 1 according to the first embodiment of the present invention will be described with reference to FIG. FIG. 1 is a block diagram showing a configuration of a fiber laser 1 according to this embodiment.

  As shown in FIG. 1, the fiber laser 1 includes an amplification optical fiber 11, a mirror element 12a, a half mirror element 12b, a front pumping light source 13a, a rear pumping light source 13b, an output optical fiber 14, a control unit 15, and a heat dissipation block. 16 is provided.

  The amplification optical fiber 11 is an optical fiber whose core functions as a laser medium. For example, the amplification optical fiber 11 is an optical fiber in which a rare earth element is added to the core. Examples of the rare earth element that can be used include Er (erbium), Yb (ytterbium), Nd (neodymium), and the like. The rare earth element added to the core of the amplification optical fiber 11 transitions to the inverted distribution state by the pumping light supplied from the front pumping light source 13a and the rear pumping light source 13b. Then, the rare earth element transitioned to the inversion distribution state generates laser light by chained stimulated emission using spontaneous emission light as seed light.

  In the present embodiment, a double clad fiber is used as the amplification optical fiber 11. Here, the double-clad fiber is sometimes referred to as (1) a cylindrical core and (2) a cylindrical first clad that covers the side surface of the core and has a lower refractive index than the core (“inner clad”). ), (3) a cylindrical second clad that covers the outer surface of the first clad and has a lower refractive index than the first clad (sometimes referred to as “outer clad”), and (4) the second clad And a cylindrical coating covering the outer surface of the optical fiber. Usually, the base material of the core and the first cladding is silica glass, and the base material of the second cladding and the coating is resin. By using the double clad fiber, it becomes possible to confine the excitation light supplied from the front excitation light source 13a and the rear excitation light source 13b in the first cladding having a cross-sectional area wider than that of the core. This is advantageous for increasing the output of the fiber laser 1.

  The mirror element 12a is configured to reflect the laser beam generated in the amplification optical fiber 11, and is fused to one end A of the amplification optical fiber 11 (hereinafter referred to as "fusion point A"). It is worn. The half mirror element 12 b is configured to reflect a part of the laser light generated in the amplification optical fiber 11 and transmit the remaining part, and the other end B of the amplification optical fiber 11. (Hereinafter referred to as “fusion point B”). The laser light generated in the amplification optical fiber 11 is alternately reflected by the mirror element 12a and the half mirror element 12b, and is recursively amplified in the process of repeatedly reciprocating the amplification optical fiber 11. Of the laser light recursively amplified by the amplification optical fiber 11, the laser light transmitted through the half mirror element 12 b is output to the outside via the output optical fiber 14 (for example, irradiating the object to be processed) )

  In the present embodiment, fiber Bragg gratings are used as the mirror element 12a and the half mirror element 12b. Here, the fiber Bragg grating refers to an optical fiber in which the refractive index of the core periodically changes along the longitudinal direction (a set of high refractive index cross sections constitutes the grating). The fiber Bragg grating selectively reflects light in a wavelength band whose central wavelength is a Bragg wavelength corresponding to the period of the grating.

  In the fiber Bragg grating used as the mirror element 12a, the reflectance of the grating at a wavelength λ (for example, 1050 nm) predetermined as the wavelength of the laser beam to be oscillated is 99% or more, and It is set to transmit forward excitation light having a wavelength λ ′ (for example, 900 nm) supplied from a forward excitation light source 13a described later. In the fiber Bragg grating used as the half mirror element 12b, the period of the grating is supplied from a rear excitation light source 13b described later so that the reflectance at the wavelength λ is 10% or more and 20% or less. It is set so as to transmit backward pumping light having a wavelength λ ′.

  The forward pumping light source 13a is configured to supply the forward pumping light to the amplification optical fiber 11, and is connected to one end of the amplification optical fiber 11 via the mirror element 12a. The backward pumping light source 13b is configured to supply backward pumping light to the amplification optical fiber 11, and is connected to the other end of the amplification optical fiber 11 via the half mirror element 12b. Most of the forward pumping light supplied from the forward pumping light source 13 a and the backward pumping light supplied from the rear pumping light source 13 b are coupled to the cladding mode of the amplification optical fiber 11 and are connected to the core of the amplification optical fiber 11. The added rare earth element is changed to an inversion distribution state. As described above, the laser light is generated by the chain stimulated emission that occurs in the rare earth element transitioned to the inversion distribution state.

  In the present embodiment, a high-output light source including N LDs (Laser Diodes) 13a1 and one combiner 13a2 is used as the forward excitation light source 13a. Here, N is an arbitrary natural number of 2 or more. FIG. 1 shows a case where N = 6. Each LD 13a1 is a light source that outputs laser light. The combiner 13a2 is a combiner having N + 1 input ports (one of which is unused) and one output port, and combines the laser beams output from each of the N LDs 13a. The laser light output from the combiner 13a2 is supplied to the amplification optical fiber 11 as forward pumping light.

  On the current path 13a4 of the drive current Ia supplied to each LD 13a1, a constant current source 13a5 having a variable current amount is provided. The amount of current of the constant current source 13a5, that is, the magnitude of the drive current Ia supplied to each LD 13a1 is controlled by the control unit 15 described later.

  Similarly, in the present embodiment, a high-output light source composed of N LDs (Laser Diodes) 13b1 and one combiner 13b2 is used as the backward pumping light source 13b. Here, N is an arbitrary natural number of 2 or more. FIG. 1 shows a case where N = 6. Each LD 13b1 is a light source that outputs laser light. The combiner 13b2 is a combiner having N + 1 input ports (one of which is connected to the output optical fiber 14) and one output port, and combines the laser light output from each of the N LDs 13b. Combine. The laser light output from the combiner 13b2 is supplied to the amplification optical fiber 11 as backward pumping light.

  On the current path 13b4 of the drive current Ib supplied to each LD 13b1, a constant current source 13b5 having a variable current amount is provided. The amount of current of the constant current source 13b5, that is, the magnitude of the drive current Ib supplied to each LD 13b1 is controlled by the control unit 15 described later.

  The forward pumping light supplied from the forward pumping light source 13a is absorbed by the amplification optical fiber 11 mainly in the vicinity of the fusion point A, and heats the part. For this reason, the temperature (hereinafter referred to as “resin temperature”) Ta of the resin portion of the amplification optical fiber 11 in the vicinity of the fusion point A increases. Similarly, the backward pumping light output from the backward pumping light source 13b is absorbed by the amplification optical fiber 11 mainly in the vicinity of the fusion point B and causes the part to generate heat. For this reason, the resin temperature Tb of the amplification optical fiber 11 in the vicinity of the fusion point B increases.

  However, heat generation in the resin portion of the amplification optical fiber 11 due to the forward pumping light locally occurs in the vicinity of the fusion point A. That is, the influence of the forward excitation light on the increase in the resin temperature Tb in the vicinity of the fusion point B is small and can be ignored. Similarly, heat generation in the resin portion of the amplification optical fiber 11 due to the backward pumping light locally occurs in the vicinity of the fusion point B. That is, the influence of the backward excitation light on the increase in the resin temperature Ta in the vicinity of the fusion point A is small and can be ignored.

  In order to prevent the temperature Ta, Tb of the resin part of the amplification optical fiber 11 in the vicinity of the fusion points A, B from exceeding the heat resistant upper limit temperature T0 of the resin material constituting the resin part due to such heat generation, the fiber In the laser 1, the following measures are taken.

  Countermeasure 1: The power Pa of the forward pumping light and the power Pb of the rear pumping light repeatedly increase and decrease (for example, the power Pa of the front pumping light periodically changes and the power Pb of the rear pumping light changes to the front pumping. A control unit 15 is provided to control the front pumping light source 13a and the rear pumping light source 13b so that the sum Pa + Pb with the light power Pa changes periodically).

  Countermeasure 2: A heat dissipation block 16 that is in thermal contact with the amplification optical fiber 11 is provided.

  By countermeasure 1, the power Pa of the forward pumping light repeatedly increases and decreases, so that the resin temperature Ta that rises when the power Pa of the forward pumping light increases can be lowered when the power Pa of the front pumping light decreases. As a result, an increase in the resin temperature Ta can be suppressed. Further, since the power Pb of the back pumping light repeatedly increases and decreases, the resin temperature Tb that has risen when the power Pb of the back pumping light increases can be lowered when the power Pb of the back pumping light decreases, As a result, an increase in the resin temperature Tb can be suppressed. Further, since the increase / decrease in the power Pa of the forward pumping light and the increase / decrease in the power Pb of the rear pumping light are complementary, the power of the total pumping light supplied to the amplification optical fiber 11 (the power Pa of the front pumping light and the rear pumping light The sum Pa + Pb) of the light power Pb can be kept constant or substantially constant. Accordingly, it is possible to suppress an increase in the resin temperatures Ta and Tb while keeping the power of the total pumping light supplied to the amplification optical fiber 11 constant or substantially constant.

  By the measure 2, the heat generated in the resin portion of the amplification optical fiber 11 can be released to the heat dissipation block 16. As a result, the increase in the resin temperatures Ta and Tb can be further suppressed.

  A specific example of a method for controlling the front excitation light source 13a and the rear excitation light source 13b will be described later with reference to another drawing.

[Specific example of control method of front excitation light source and rear excitation light source]
Next, a specific example of a method for controlling the front excitation light source 13a and the rear excitation light source 13b will be described with reference to FIG. In FIG. 2, (a) is a graph showing the time change of the power Pa of the forward pumping light output from the forward pumping light source 13a, and (b) is the power of the rear pumping light output from the rear pumping light source 13b. It is a graph showing the time change of Pb, (c) is a graph showing the time change of the resin temperature Ta of the optical fiber 11 for amplification in the vicinity of the fusion point A, (d) is the graph of the fusion point B. It is a graph showing the time change of the resin temperature Tb of the optical fiber 11 for amplification in the vicinity.

  In the control method according to this specific example, the front excitation light source 13a and the rear excitation light source 13b blink in a complementary manner (they repeat turning on and off while satisfying the relationship that when one is turned on, the other is turned off). Thus, this is a control method for ON / OFF control of the front excitation light source 13a and the rear excitation light source 13b.

  That is, from time t1, Δt seconds (predetermined time), the control unit 15 (a) sets the constant current source 13a5 so that the drive current Ia supplied to the LD 13a1 becomes I0 (predetermined current value). The front pumping light source 13a is turned on by control, and (b) the rear pumping light source 13b is turned off by controlling the constant current source 13b5 so that the drive current Ib supplied to the LD 13b1 is equal to or lower than the threshold current. Thereby, from time t1 to Δt seconds, (a) the power Pa of the forward pumping light becomes P0 (predetermined power) as shown in (a) of FIG. 2, and (b) the power Pb of the rear pumping light. Becomes 0 [W] as shown in FIG.

  Next, from time t2 = t1 + Δt to Δt seconds, the control unit 15 (a) controls the constant current source 13a5 so that the drive current Ia supplied to the LD 13a1 is equal to or lower than the threshold current, and turns off the front excitation light source 13a. At the same time, (b) the rear excitation light source 13b is turned on by controlling the constant current source 13b5 so that the drive current Ib supplied to the LD 13b1 becomes I0. As a result, the power Pa of the forward pumping light becomes 0 [W] as shown in (a) of FIG. 2 for Δt seconds from the time t2, and (b) the power Pb of the backward pumping light is shown in FIG. As shown in (b) of FIG.

  Thereafter, the control unit 15 repeats the above ON / OFF control. Thus, the forward pumping light source 13a and the backward pumping light are supplied while the power of the total pumping light supplied to the amplification optical fiber 11 (the sum Pa + Pb of the forward pumping light power Pa and the backward pumping light power Pb) is maintained at P0. The lighting time of the light source 13b can be suppressed to Δt.

  By controlling the forward pumping light source 13a as described above, the resin temperature Ta of the amplification optical fiber 11 in the vicinity of the fusion point A changes as shown in FIG. That is, for each period (lighting period + lighting period), the resin temperature Ta is the period in the lighting period (t1 to t2, t3 to t4, t5 to t6, t7 to t8, t9 to t10) of the front excitation light source 13a. After the temperature rises from the initial temperature to the maximum temperature of the cycle, during the extinguishing period (t2 to t3, t4 to t5, t6 to t7, t8 to t9) of the front excitation light source 13a, the maximum temperature of the cycle Decrease to final temperature. At this time, as shown by the dotted line in FIG. 2C, the maximum temperature in each cycle gradually rises as the cycles are repeated, and the way of the rise is the resin temperature when the front excitation light source 13a is continuously lit. It is slower than how Ta increases.

  Here, the temperature increase amount (maximum temperature-starting temperature) in the lighting period gradually decreases as the period is repeated, and the temperature decrease amount (maximum temperature-final temperature) in the extinguishing period gradually increases as the period increases. This is because as the resin temperature Ta increases, the temperature difference between the resin temperature Ta and the environmental temperature Te (for example, the temperature of the heat dissipation block 16) increases, and as a result, the heat dissipation efficiency increases. For this reason, as shown by the dotted line in FIG. 2C, the maximum temperature of each cycle gradually increases until the temperature increase amount in the lighting period and the temperature decrease amount in the extinguishing period are balanced, After the temperature increase amount balances with the temperature decrease amount during the extinguishing period, the steady temperature T1 is maintained.

  When the front excitation light source 13a is continuously lit, the resin temperature Ta reaches a steady temperature T2 higher than the steady temperature T1 when the front excitation light source 13a is blinked, as indicated by a chain line in FIG. . For this reason, as shown in FIG. 2C, the upper limit heat-resistant temperature T0 is higher than the steady temperature T1 when the front excitation light source 13a blinks, and the steady temperature T2 when the front excitation light source 13a is continuously lit. If the forward pumping light source 13a is continuously turned on, the resin portion of the amplification optical fiber 11 deteriorates or decomposes. If the front pumping light source 13a is blinked, the amplification optical fiber 11 The resin part does not deteriorate or decompose.

  Further, by controlling the backward pumping light source 13b as described above, the resin temperature Tb of the amplification optical fiber 11 in the vicinity of the fusion point B changes as shown in FIG. That is, for each period, the resin temperature Tb is from the start temperature of the period to the maximum temperature of the period in the lighting period (t2 to t3, t4 to t5, t6 to t7, t8 to t9) of the rear excitation light source 13b. After rising, in the extinguishing period (t3 to t4, t5 to t6, t7 to t8, t9 to t10) of the rear excitation light source 13b, the temperature decreases from the highest temperature in the cycle to the final temperature in the cycle. At this time, as shown by a dotted line in FIG. 2D, the maximum temperature in each cycle gradually increases as the cycles are repeated. The method of increasing is the resin temperature when the front excitation light source 13b is continuously lit. It is slower than how Ta increases.

  Here, the temperature increase amount (maximum temperature-starting temperature) in the lighting period gradually decreases as the period is repeated, and the temperature decrease amount (maximum temperature-final temperature) in the extinguishing period gradually increases as the period increases. This is because as the resin temperature Tb increases, the temperature difference between the resin temperature Tb and the environmental temperature Te (for example, the temperature of the heat dissipation block 16) increases, and as a result, the heat dissipation efficiency increases. For this reason, the maximum temperature of each cycle gradually increases until the temperature increase amount in the lighting period and the temperature decrease amount in the extinguishing period are balanced as shown by a dotted line in FIG. After the temperature increase amount balances with the temperature decrease amount during the extinguishing period, the steady temperature T1 is maintained.

  When the rear excitation light source 13b is continuously turned on, the resin temperature Tb reaches a steady temperature T2 higher than the steady temperature T1 when the rear excitation light source 13b is blinked, as indicated by a chain line in FIG. . For this reason, as shown in FIG. 2C, the upper limit heat resistant temperature T0 is higher than the steady temperature T1 when the backward pumping light source 13b is blinked, and the steady temperature T2 when the backward pumping light source 13b is continuously lit. If the rear pumping light source 13b is continuously turned on, the resin portion of the amplification optical fiber 11 deteriorates or decomposes. If the rear pumping light source 13b is blinked, the amplification optical fiber 11 The resin part does not deteriorate or decompose.

[First implementation of amplification optical fiber]
Next, a first mounting form of the amplification optical fiber 11 will be described with reference to FIG. FIG. 3 is a top view showing a first mounting form of the amplification optical fiber 11.

  In the present embodiment, the amplification optical fiber 11 is wound on the heat radiation block 16 so as to form one spiral, as shown in FIG. By adopting such a mounting form, the area required for mounting the amplification optical fiber 11 can be kept small.

  Even if the amplification optical fiber 11 is wound so that the vicinity of the fusion point A constitutes the outer periphery of the spiral (the vicinity of the fusion point B constitutes the inner periphery of the spiral), the fusion point A May be wound so as to constitute the inner circumference of the spiral (the vicinity of the fusion point B constitutes the outer circumference of the spiral), but the former is assumed in FIG. 3 and the following description.

  When the mounting form shown in FIG. 3 is employed, when the distribution of the heat generation amount per unit length of the amplification optical fiber 11 is symmetric, the heat generation amount per unit area in the inner periphery of the amplification optical fiber 11 is It becomes larger than the calorific value per unit area in the outer periphery of the optical fiber 11. Therefore, when the heat absorption amount per unit area of the heat dissipation block 16 is uniform, the resin temperature Tb in the inner peripheral portion of the amplification optical fiber 11 is higher than the resin temperature Ta in the outer peripheral portion of the amplification optical fiber 11. Become. It should be noted that the distribution of the heat generation amount per unit length of the amplification optical fiber 11 is symmetrical from the distribution of the heat generation amount per unit length as a function of the distance from the fusion point A and the fusion point B. It means that the distribution of the calorific value per unit length as a function of the distance is identical or substantially identical.

  When the resin temperatures Ta and Tb of the amplification optical fiber 11 are different between the inner peripheral portion and the outer peripheral portion as described above, for example, the following problem occurs. That is, when the lighting time Δt of the front pumping light source 13a and the rear pumping light source 13b is set so that the resin temperature Ta in the outer peripheral part of the amplification optical fiber 11 does not exceed the heat resistant upper limit temperature T0, the inner peripheral part of the amplification optical fiber 11 This causes a problem that the resin temperature Tb exceeds the heat-resistant upper limit temperature T0.

  As a method of avoiding such a problem, (1) a method of making the heat absorption amount per unit area of the heat dissipation block 16 non-uniform, and (2) a heat generation amount per unit length of the amplification optical fiber 11 A method of making the distribution asymmetric is conceivable.

(1) Method of making the heat absorption amount per unit area of the heat dissipation block 16 non-uniform per unit area of the heat dissipation block 16 in the region in contact with the inner periphery (near the fusion point B) of the amplification optical fiber 11 The amount of heat absorption is made larger than the amount of heat absorption per unit area of the heat dissipation block 16 in the region in contact with the outer peripheral portion of the amplification optical fiber 11 (near the fusion point A). As a result, the resin temperature Tb in the inner peripheral portion of the amplification optical fiber 11 is unlikely to exceed the upper limit heat resistant temperature T0.

  A configuration example of the heat dissipation block 16 in which the distribution of the heat absorption amount per unit area is made non-uniform as described above is shown in FIGS. 4 (a) and 4 (b). 4A is a plan view showing a first configuration example of the heat dissipation block 16, and FIG. 4B is a plan view showing a second configuration example of the heat dissipation block 16.

  A spiral flow path 16a is provided on the back surface of the heat dissipation block 16 shown in FIG. 4A, and the spiral flow path 16a is cooled so that the inner peripheral side is upstream (the outer peripheral side is downstream). Liquid is flowing. The amplification optical fiber 11 is disposed such that its inner peripheral portion overlaps with the inner peripheral side of the spiral flow channel 16a and its outer peripheral portion overlaps with the outer peripheral side of the spiral flow channel 16a.

  According to the above configuration, the amount of heat absorbed per unit area of the heat dissipation block 16 in the region in contact with the inner peripheral portion of the amplification optical fiber 11 is changed to that of the heat dissipation block 16 in the region in contact with the outer peripheral portion of the amplification optical fiber 11. The amount of heat absorbed per unit area can be made larger. As a result, the resin temperature Tb in the inner peripheral portion of the amplification optical fiber 11 is unlikely to exceed the upper limit heat resistant temperature T0.

  Two annular channels 16a1 and 16a2 arranged concentrically are provided on the back surface of the heat dissipation block 16 shown in FIG. 4B, and the temperature Tlow of the coolant flowing through the small-diameter annular channel 16a1 is The temperature of the coolant flowing through the large-diameter annular channel 16a2 is lower than High. The amplification optical fiber 11 is arranged on the surface of the heat radiation block 16 so that the inner peripheral side thereof overlaps with the small-diameter annular channel 16a1 and the outer peripheral side thereof overlaps with the large-diameter annular channel 16a2.

  According to the above configuration, the amount of heat absorbed per unit area of the heat dissipation block 16 in the region in contact with the inner peripheral portion of the amplification optical fiber 11 is changed to that of the heat dissipation block 16 in the region in contact with the outer peripheral portion of the amplification optical fiber 11. The amount of heat absorbed per unit area can be made larger. As a result, the resin temperature Tb in the inner peripheral portion of the amplification optical fiber 11 is unlikely to exceed the upper limit heat resistant temperature T0.

(2) Method of making the distribution of the heat generation amount per unit length of the amplification optical fiber 11 asymmetrical The heat generation amount per unit length in the inner periphery (near the fusion point B) of the amplification optical fiber 11 is The heat generation amount per unit length in the outer peripheral portion (near the fusion point A) of the amplification optical fiber 11 is made smaller. As a result, the resin temperature Tb in the inner peripheral portion of the amplification optical fiber 11 is unlikely to exceed the upper limit heat resistant temperature T0.

  FIG. 5 shows a control example of the front pumping light source 13a and the rear pumping light source 13b for making the distribution of the heat generation amount per unit length of the amplification optical fiber 11 non-uniform. In FIG. 5, (a) is a graph showing the time change of the power Pa of the forward pumping light output from the forward pumping light source 13a, and (b) is the power of the rear pumping light output from the rear pumping light source 13b. It is a graph showing the time change of Pb.

  In the control example shown in FIG. 5, the lighting time Δtb of the backward pumping light source 13 b connected to the inner peripheral end point (fusion point B) of the amplification optical fiber 11 is used as the outer end face of the amplification optical fiber 11. It is longer than the lighting time Δta of the front excitation light source 13a connected to (fusion point A).

  According to the above configuration, the heat generation amount per unit length in the inner peripheral portion of the amplification optical fiber 11 can be made smaller than the heat generation amount per unit length in the outer peripheral portion of the amplification optical fiber 11. As a result, the resin temperature Tb in the inner peripheral portion of the amplification optical fiber 11 is unlikely to exceed the upper limit heat resistant temperature T0.

[Second Implementation of Amplifying Optical Fiber]
Next, a second mounting form of the amplification optical fiber 11 will be described with reference to FIG. FIG. 6 is a top view showing a second mounting form of the amplification optical fiber 11.

  In the present embodiment, the amplification optical fiber 11 is wound on the heat radiation blocks 161 and 162 so as to form two spirals as shown in FIG. Also by adopting such a mounting form, the area required for mounting the amplification optical fiber 11 can be reduced.

  Note that the amplification optical fiber 11 is divided into two equal parts if the point is between the fusion point A of the amplification optical fiber 11 and the intermediate point M (the point between the fusion point A and the fusion point B). The first spiral formed by the section up to (not required) is wound so that the vicinity of the fusion point A constitutes the outer periphery (the vicinity of the intermediate point M constitutes the inner periphery of the spiral) Is preferred. Further, in the second spiral formed by the second section from the intermediate point M to the fusion point B of the amplification optical fiber 11, the vicinity of the fusion point B constitutes the outer periphery of the spiral (of the intermediate point M). It is preferably wound so that the vicinity constitutes the inner periphery of the spiral. This is because the amount of heat generated per unit area in the vicinity of the fusion points A and B can be reduced compared to the case where the vicinity of the fusion points A and B is wound so as to form the inner periphery.

  When the mounting form shown in FIG. 6 is adopted, when the distribution of heat generation amount per unit length of the amplification optical fiber 11 is symmetric, the heat generation amount per unit area of the amplification optical fiber 11 in the vicinity of the fusion point A The amount of heat generated per unit area of the amplification optical fiber 11 in the vicinity of the fusion point B becomes equal. Therefore, as in the case where the mounting form shown in FIG. 3 is adopted, the heat absorption amount per unit area of the heat radiation block 16 is made non-uniform, or the heat generation amount distribution per unit length of the amplification optical fiber 11 is asymmetrical. You do n’t have to.

  In the mounting form shown in FIG. 6, the first heat radiation block 161 that is in thermal contact with the first section of the amplification optical fiber 11 is used to generate the first section of the amplification optical fiber 11. A configuration is adopted in which heat is released and heat generated in the second section of the amplification optical fiber 11 is released using the second heat radiation block 162 that is in thermal contact with the second section of the amplification optical fiber 11. ing. Here, the first heat radiation block 161 and the second heat radiation block 162 are separated from each other and are not in thermal contact with each other.

  By adopting such a configuration, heat generated in the first section of the amplification optical fiber 11 is conducted to the second section of the amplification optical fiber 11 through the heat dissipation block (that is, amplification light). It is possible to prevent the resin temperature Tb of the fiber 11 from being raised). Similarly, heat generated in the second section of the amplification optical fiber 11 is conducted to the first section of the amplification optical fiber 11 through the heat dissipation block (that is, the resin temperature Ta of the amplification optical fiber 11 is increased). Can be prevented.

〔Example〕
The inventors of the fiber laser 1 having an output power of 2 kW configured as follows, when the lighting time Δt of the front pumping light source 13a and the rear pumping light source 13b is 1 msec, the resin temperature Ta of the amplification optical fiber 11 , Tb was confirmed to be kept lower than the heat resistant upper limit temperature T0.

  Front pumping light source 13a and rear pumping light source 13b: each composed of 18 LDs and one pump combiner that combines the laser beams output from the LDs. The output of each LD was 140W. The pump combiner includes 18 excitation ports (core diameter 105 μm, clad diameter 125 μm, NA 0.22) and one output port (core diameter 25 μm, clad diameter 400 μm, core NA 0.06, clad NA 0.46). An 18-input combiner was used.

  Amplifying optical fiber 11: A double clad fiber having a core diameter of 25 μm, a clad diameter of 400 μm, a core NA of 0.06, and a clad NA of 0.46 was used.

  Mirror element 12a: A fiber Bragg grating having a reflectance of 99% was used.

  Half mirror element 12b: A fiber Bragg grating having a reflectance of 10% was used.

<< Second Embodiment >>
The configuration of the fiber laser 1 ′ according to the second embodiment of the present invention will be described with reference to FIG. FIG. 7 is a block diagram showing the configuration of the fiber laser 1 ′ according to this embodiment.

  As shown in FIG. 7, the fiber laser 1 ′ includes an amplification optical fiber 11, a mirror element 12a, a half mirror element 13b, a front pumping light source 13a, a rear pumping light source 13b, an output optical fiber 14, a control unit 15, and heat dissipation. A block 16 is provided. Since these blocks have the same functions as the corresponding blocks (blocks with the same reference numerals) of the fiber laser 1 except for the control unit 15, the description thereof is omitted here.

  The fiber laser 1 'further includes two thermometers 17a and 17b. The first thermometer 17a is a means for measuring the resin temperature Ta of the amplification optical fiber 11 in the vicinity of the fusion point A, and the second thermometer 17b is for amplification in the vicinity of the fusion point B. This is for measuring the resin temperature Tb of the optical fiber 11.

  The controller 15 controls the magnitudes of the drive currents Ia and Ib supplied to the excitation light sources 13a and 13b based on the resin temperatures Ta and Tb measured by the two thermometers 17a and 17b. Specifically, the following control is performed using a predetermined threshold temperature Tth lower than the heat-resistant upper limit temperature T0 of the resin material constituting the resin portion of the amplification optical fiber 11.

  That is, the control unit 15 controls the driving current Ia supplied to the front excitation light source 13a to I0 (predetermined current value) until the resin temperature Ta reaches the threshold temperature Tth, thereby controlling the front excitation light source 13a. Light. During this time, the rear excitation light source 13b is kept in the off state. Then, when the resin temperature Ta reaches the threshold temperature Th, the control unit 15 controls the drive current Ia supplied to the forward excitation light source 13a to be equal to or less than the threshold current and turns off the forward excitation light source 13a.

  Next, until the resin temperature Tb reaches the threshold temperature Tth, the control unit 15 controls the drive current Ib supplied to the rear pumping light source 13b to I0 and lights the rear pumping light source 13b. During this time, the front excitation light source 13a is kept off. Then, when the resin temperature Tb reaches the threshold temperature Th, the control unit 15 controls the drive current Ib supplied to the rear excitation light source 13b to be equal to or lower than the threshold current and turns off the rear excitation light source 13b.

  Thereafter, the control unit 15 repeats the above ON / OFF control. Thus, the resin temperatures Ta and Tb are respectively maintained while maintaining the power of all pumping light supplied to the amplification optical fiber 11 (the sum Pa + Pb of the power Pa of the front pumping light and the power Pb of the rear pumping light) at P0. It can be suppressed to a threshold temperature Tth lower than the heat-resistant upper limit temperature T0.

≪Additional notes≫
Note that the present invention is not limited to the above-described embodiments and examples, and various modifications can be made within the scope shown in the claims, and technical means disclosed in the embodiments or examples are appropriately used. Embodiments obtained in combination are also included in the technical scope of the present invention.

  For example, in each of the embodiments described above, ON / OFF control of the drive currents Ia and Ib supplied to the excitation light sources 13a and 13b is performed so that the front excitation light source 13a and the rear excitation light source 13b are alternately lit. However, the present invention is not limited to this. That is, if the power Pa of the front pumping light and the power Pa of the rear pumping light repeatedly increase and decrease, control of the drive currents Ia and Ib supplied to the pumping light sources 13a and 13b is not necessarily ON / OFF. There is no need for OFF control. For example, when the power Pa of the forward pumping light rises / falls in a curve, a configuration in which the power Pb of the rear pumping light falls / rises so as to satisfy Pa + Pb = P0 (constant) is also within the scope of the present invention. included. Further, the minimum value of the power Pa of the forward pumping light is larger than 0 [W] (all the LDs 13a1 are lit with a power smaller than the full power Pa / N, or some of the LDs 13a1 are full power Pa / N. Is turned on and the remaining LD 13a1 is turned off) or the minimum value of the power Pb of the backward pumping light is larger than 0 [W] (all the LDs 13b1 are turned on with a power smaller than the full power Pb / N, or A configuration in which a part of the LD 13b1 is turned on at full power Pa / N and the remaining LD 13b1 is turned off is also included in the scope of the present invention.

DESCRIPTION OF SYMBOLS 1,1 'Fiber laser 11 Optical fiber for amplification 12a Mirror element 12b Half mirror element 13a Front pumping light source 13b Back pumping light source 14 Output optical fiber 15 Control part 16 Heat radiation block

Claims (8)

An optical fiber for amplification;
A forward pumping light source for supplying forward pumping light to the amplification optical fiber;
A backward pumping light source for supplying backward pumping light to the amplification optical fiber;
A controller that controls the front pumping light source and the rear pumping light source so that the power of the front pumping light and the power of the rear pumping light repeat complementary increase and decrease;
A fiber laser characterized by that. The control unit performs ON / OFF control of the front excitation light source and the rear excitation light source so that the front excitation light source and the rear excitation light source blink complementarily.
The fiber laser according to claim 1. The control unit turns off the rear excitation light source, turns on the front excitation light source for a predetermined lighting time, turns off the front excitation light source, and determines the rear excitation light source in advance. Repeat the control to turn on over the lighting time,
The fiber laser according to claim 2. A first thermometer for measuring a resin temperature in the vicinity of the front pumping light source side end of the amplification optical fiber; and a first thermometer for measuring a resin temperature in the vicinity of the rear pumping light source side end of the amplification optical fiber. 2 thermometers, and
The control unit turns off the backward excitation light source, turns on the forward excitation light source until the resin temperature measured by the first thermometer reaches a predetermined threshold temperature, and then turns on the forward excitation light source. The light source is turned off, and the control to turn on the back excitation light source is repeated until the resin temperature measured by the second thermometer reaches the threshold temperature.
The fiber laser according to claim 2. The amplification optical fiber is wound so as to form one spiral,
The fiber laser is a heat dissipation block that is in thermal contact with the amplification optical fiber, and the amount of heat absorbed per unit area in the region in contact with the inner periphery of the amplification optical fiber is the outer circumference of the amplification optical fiber. A heat dissipating block larger than the amount of heat absorbed per unit area in the region in contact with the part,
The fiber laser according to any one of claims 1 to 4, wherein: The amplification optical fiber is wound so as to form one spiral,
Of the front pumping light source and the rear pumping light source, the lighting time of the pumping light source connected to the inner peripheral end of the amplification optical fiber is the amplification power of the front pumping light source and the rear pumping light source. Shorter than the lighting time of the excitation light source connected to the outer peripheral end of the optical fiber,
The fiber laser according to claim 3. The amplification optical fiber is wound so that a first section from one end to an intermediate point forms a first spiral having an outer periphery in the vicinity of the one end. The second section to the end of the second end is wound so as to form a second spiral having an outer periphery in the vicinity of the other end.
The fiber laser according to any one of claims 1 to 4, wherein: A first heat dissipation block in thermal contact with the first section of the amplification optical fiber;
A second heat dissipation block that is in thermal contact with the second section of the amplification optical fiber, the second heat dissipation block being spaced apart from the first heat dissipation block;
The fiber laser according to claim 7.

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