JP2012129423A - Fiber laser and continuous light oscillation method for fiber laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fiber laser and a continuous light oscillation method for the fiber laser, capable of suppressing generation of a giant pulse light.SOLUTION: A fiber laser includes: a resonator formed by a first mirror 2 and a second mirror 3; a rare-earth doped optical fiber 1 doped with a rare-earth element, provided on an optical path of the resonator; and an excitation light source 5 that excites the rare-earth element. The optical path has a feedback suppression section 7 that suppresses a feedback of spontaneous emission light of the rare-earth element from the first mirror 2 to the rare-earth doped optical fiber 1. The feedback suppression section 7 makes the resonator perform a pulse oscillation by temporarily increasing a feedback amount from the first mirror 2 during the period when there is a risk that parasitic oscillation occurs at ramp-up of the excitation light source 5.

Description

  The present invention relates to a fiber laser and a fiber laser continuous light oscillation method, and more particularly to a fiber laser and a fiber laser continuous light oscillation method capable of suppressing the generation of giant pulsed light due to parasitic oscillation.

  A fiber laser using an optical fiber such as a rare earth-doped optical fiber as a laser medium has been proposed (see, for example, Patent Documents 1 to 5). In a fiber laser using a rare earth doped optical fiber, an optical isolator is inserted in the optical path between the rare earth doped optical fiber and the excitation light source in order to prevent light from the rare earth doped optical fiber from entering the excitation light source.

JP 2001-358388 A JP 2002-530875 A JP 2003-29060 A JP 2008-254006 A JP 2009-101400 A

  When the excitation light source of the fiber laser is started up, there is a phenomenon in which the excitation state of the rare earth-doped optical fiber increases transiently and giant pulsed light with a high peak value is generated. This is called parasitic oscillation, and there is a possibility that the core or end face of the rare earth-doped optical fiber or the excitation light source may be damaged by the giant pulse light.

  Accordingly, an object of the present invention is to provide a fiber laser capable of suppressing the generation of giant pulse light and a continuous light oscillation method of the fiber laser.

  In order to achieve the above object, the fiber laser and the fiber laser continuous light oscillation method of the present invention forcibly induce pulse oscillation during a period when there is a possibility of parasitic oscillation such as when the fiber laser excitation light source is turned on. Thus, the generation of giant pulsed light due to parasitic oscillation is suppressed.

  Specifically, a fiber laser according to the present invention includes a resonator formed by a first reflecting portion and a second reflecting portion, and a rare earth element doped with a rare earth element provided in an optical path of the resonator. A fiber laser comprising an optical fiber and an excitation light source for exciting the rare earth element, wherein the spontaneous emission light of the rare earth element is transmitted from the first reflecting portion to the rare earth doped optical fiber in the optical path. A feedback suppression unit that suppresses feedback, and the feedback suppression unit temporarily increases a feedback amount from the first reflection unit during a period in which parasitic oscillation at the time of startup of the excitation light source may occur. Thus, pulse oscillation is performed by the resonator.

  Parasitic oscillation occurs when spontaneous emission light generated from a rare-earth-doped optical fiber in an excited state is scattered in the rare-earth-doped optical fiber, reciprocates in the rare-earth-doped optical fiber, and is amplified by stimulated emission. Since the feedback suppression unit is provided, steady pulse oscillation can be induced without locally increasing the excited state of the rare earth-doped optical fiber constituting at least a part of the resonator. Thereby, the fiber laser according to the present invention can suppress the generation of giant pulsed light due to parasitic oscillation such as when the excitation light source of the fiber laser is started up.

In the fiber laser according to the present invention, the feedback suppression unit may repeatedly increase the feedback temporarily at a predetermined period until the excitation light source exceeds a power at which the fiber laser can continuously oscillate. .
Giant pulsed light may be generated between the start of startup of the fiber laser excitation light source and the continuous oscillation of the fiber laser. For this reason, the fiber laser according to the present invention can suppress the generation of giant pulsed light due to parasitic oscillation when the excitation light source of the fiber laser is started up.

  Specifically, the fiber laser continuous light oscillation method according to the present invention is a fiber laser continuous light oscillation method using a rare earth-doped optical fiber doped with a rare earth element as a laser medium, wherein the rare earth element is spontaneously emitted. A pumping start process in which the pumping light starts to be incident from the pumping light source while light is prevented from returning from one mirror of the resonator to the rare earth-doped optical fiber, and a shorter time elapses when a predetermined parasitic oscillation occurs. Later, the suppressed feedback is temporarily increased to perform a Q-switch oscillation by the resonator, and the excitation light source has an output larger than the excitation light power exceeding the excitation light power at which the parasitic oscillation can occur. After standing up, a constant state that does not suppress feedback of the spontaneous emission light of the rare earth element to the rare earth-doped optical fiber via one mirror As, has a continuous oscillation step of the continuous light oscillation state, the.

  Parasitic oscillation occurs when spontaneous emission light generated from a rare-earth-doped optical fiber in an excited state is scattered in the rare-earth-doped optical fiber, reciprocates in the rare-earth-doped optical fiber, and is amplified by stimulated emission. For this reason, by having the switching step, it is possible to induce steady pulse oscillation without locally increasing the excited state of the rare earth-doped optical fiber. Thereby, the continuous light oscillation method of the fiber laser according to the present invention can suppress the generation of giant pulsed light due to parasitic oscillation such as the start-up of the excitation light source of the fiber laser.

In the fiber laser continuous light oscillation method of the present invention, in the switching step, Q-switch oscillation may be repeated by temporarily increasing the feedback amount at a predetermined period.
The giant pulsed light may be generated from the start of the start-up of the fiber laser excitation light source until the laser continuously oscillates. For this reason, the continuous light oscillation method of the fiber laser according to the present invention can suppress the generation of giant pulsed light due to parasitic oscillation such as when the fiber laser excitation light source is started up.

  According to the present invention, it is possible to provide a fiber laser and a continuous light oscillation method of a fiber laser that can suppress the generation of giant pulsed light.

The structural example of the fiber laser which concerns on this embodiment is shown. It is an example of the starting state of the fiber laser 10 which concerns on this embodiment, (a) shows laser output power, (b) shows the loss of a feedback suppression part, (c) shows pumping light power. The optical output and inversion distribution rate of a fiber laser not provided with a feedback suppression unit are shown. The optical output and inversion distribution rate of the fiber laser according to the present embodiment are shown. 3 shows a modulation signal of a feedback suppression unit in the first embodiment. The output time change of the fiber laser in Example 1 is shown. 6 shows a modulation signal of a feedback suppression unit in the second embodiment. The output time change of the fiber laser in Example 2 is shown. The output time change of the fiber laser in a comparative example is shown.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, the same reference numerals denote the same components.

  FIG. 1 shows a configuration example of a fiber laser 10 according to this embodiment. The fiber laser 10 according to the present embodiment includes a first mirror 2 that is a first reflecting portion and a second mirror 3 that is a second reflecting portion. The first mirror 2 and the second mirror 3 form a resonator. As at least a part of the optical path of the resonator, a rare earth-doped optical fiber 1 to which a rare earth element is added as a laser amplification medium is used. The fiber laser 10 includes a pumping light source 5 that outputs pumping light for pumping the rare earth element of the rare earth-doped optical fiber 1, and a pumping light combiner that guides pumping light output from the pumping light source 5 to the rare earth-doped optical fiber 1. 6 is provided. Further, the fiber laser 10 has a feedback suppression unit 7 in the optical path of the resonator that suppresses the spontaneous emission light and stimulated emission light from the rare earth element from returning from the first mirror 2 to the rare earth doped optical fiber 1. In addition, a feedback control unit 8 for controlling the feedback amount of the feedback suppression unit 7 is provided.

  The rare earth doped optical fiber 1 is an optical fiber for optical amplification in which at least one rare earth element such as Yb is added to the core. The current control unit 4 controls the output power of the pumping light by controlling the driving current of the pumping light source 5 composed of a semiconductor laser. The pumping light source 5 outputs pumping light including a wavelength capable of transitioning the rare earth element added to the rare earth-doped optical fiber 1 to the pumping level. The fiber laser 10 according to the present embodiment includes a plurality of pumping light sources 5 in order to sufficiently pump the rare earth element of the rare earth doped optical fiber 1. The excitation light combiner 6 combines the excitation light from the plurality of excitation light sources 5. Excitation light from the excitation light combiner 6 is input to the entire glass portion of the rare earth-doped optical fiber 1. The first mirror 2 and the second mirror 3 are, for example, fiber Bragg gratings, and emit light in the wavelength band of at least a part of spontaneous emission light and stimulated emission light from the rare earth element emitted from the rare earth-doped optical fiber 1. Reflects to the resonator with a predetermined reflectivity. In the fiber laser 10 according to the present embodiment, the pumping light combiner 6 is connected to a fiber on which a fiber Bragg grating (second mirror 3) is formed. The excitation light is incident on the resonator from the fiber on which the second mirror 3 is formed. Spontaneous emission light and stimulated emission light emitted by sufficiently exciting the rare earth element cause laser oscillation in the resonator formed by the first mirror 2 and the second mirror 3. The first mirror 2 is formed so as to transmit at a predetermined transmittance at the laser oscillation wavelength. Laser light generated by laser oscillation is partially transmitted from the first mirror 2 and emitted from the fiber laser 10.

  The feedback suppression unit 7 is inserted in the optical path of the optical resonator between the first mirror 2 and the second mirror 3. In response to a signal from the feedback control unit 8, the feedback suppression unit 7 reflects the spontaneous emission light and the stimulated emission light generated in the rare earth-doped optical fiber 1 by the first mirror 2 and returns to the optical resonator again. Suppress. The feedback suppression unit 7 is, for example, a loss variable optical element such as an acousto-optic element (A / O element) or an electro-optic element (E / O element) that can vary the transmission loss of light in the wavelength band of spontaneous emission light. It is. This acoustooptic device can control the loss of transmitted light by the high-frequency voltage output from the feedback controller 8. By increasing the transmission loss, the spontaneous emission light and the stimulated emission light generated in the rare earth-doped optical fiber 1 can be prevented from returning to the optical resonator again, and by reducing the transmission loss, the rare earth-doped light is reduced. Spontaneous emission light and stimulated emission light generated in the fiber 1 can be reflected by the first mirror 2 and returned to the optical resonator again. The feedback suppression unit 7 is preferably disposed between the rare earth-doped optical fiber 1 and the first mirror 2. Thereby, the fall of the power of the laser output light by the influence of the transmission loss of the feedback suppression part 7 can be prevented.

  The feedback control unit 8 includes an arithmetic circuit, a clock, and a memory, is electrically connected to the current control unit 4, and acquires current control information of the current control unit 4. In the memory, information for instructing the feedback suppression timing of the feedback suppression unit 7 and information on the high-frequency voltage output to the feedback suppression unit 7 are stored in advance.

  When the rare earth element of the rare earth doped optical fiber 1 is in a predetermined inversion distribution state, if the loss of the feedback suppression unit 7 is smaller than the predetermined value and the gain as the resonator is positive, the intended laser oscillation by the resonator Occur. On the other hand, if the loss of the feedback suppression unit 7 is made larger than a predetermined value, even if the rare earth element of the rare earth-doped optical fiber 1 is excited, the gain as a resonator does not become positive. Can be prevented.

  Next, a description will be given of the operation when the conventional fiber laser is started up, that is, when the excitation light source 5 is switched from the OFF state to the ON state. The pumping light source 5 takes a predetermined rising time from the OFF state (output: 0) until the optical output necessary for continuous oscillation of the fiber laser 10 is obtained. Immediately after starting the excitation light source 5 from the OFF state to the ON state (immediately after starting to flow current), the light output of the excitation light source 5 is weak. On the other hand, at this time, since the rare earth element of the rare earth-doped optical fiber 1 is in the ground state (not in the inversion distribution state), large absorption of the excitation light occurs near the position where the excitation light is supplied, and only that portion is in the excited state. It becomes. At this time, in the region where the rare earth element Yb atoms are not excited, the rare earth doped optical fiber 1 acts as an absorber for the excitation light. In the optical fiber region where Yb atoms are excited and stimulated emission occurs (with gain), the spontaneous emission light generated is amplified by stimulated emission. Part of the stimulated emission light amplified by a part of the optical resonator is scattered backward in the optical fiber. The backscattered light is further amplified while returning through the rare earth-doped optical fiber 1 that is excited and has gain. Part of this amplified stimulated emission light is again scattered back in the optical fiber. By repeating this scattering and amplification, a giant pulse having a high peak value due to parasitic oscillation may be generated. Thus, in the rare earth element excitation start state of the rare earth-doped optical fiber 1, the light that has not been scattered propagates through the resonator toward the first mirror 2, but the proportion of the optical fiber region that acts as an absorber is small. Since it is large, the gain of the entire fiber laser resonator is smaller than the loss, and laser oscillation as a fiber laser resonator does not occur.

  Further, when the output of the pumping light source 5 increases and exceeds a predetermined power, the rare earth element of the rare earth-doped optical fiber 1 can form an inverted distribution state over the entire length of the resonator. The situation of local gain media and absorbers in the length direction is eliminated. In this way, when the rare earth-doped optical fiber 1 is excited over the entire length of the resonator, the gain of the entire resonator is the gain of the parasitic resonance due to scattering that causes parasitic oscillation (resonator). Therefore, parasitic oscillation does not occur and a desired continuous laser oscillation by the resonator can be obtained.

  The conditions for the occurrence of parasitic oscillation before the pumping light source 5 rises from the OFF state to the specified steady state are the absorption coefficient, gain, scattering coefficient, and time constant until the pumping light source of the rare earth-doped optical fiber 1 is started up. Therefore, if the rare earth-doped optical fiber, the excitation light source, and the current control unit to be used are determined, it can be known in advance through experiments, simulations, and the like.

  Therefore, in the fiber laser 10 according to the present embodiment, in order to suppress parasitic oscillation, the Q switch method is used when the fiber laser is started up. That is, in a period in which parasitic oscillation at the time of start-up that can be known in advance can occur, the loss of the feedback suppression unit 7 is changed from a large state to a small state (feedback from a small state to a large state) at a predetermined timing. Q switch pulse oscillation by. In this way, by causing intentional Q switch pulse oscillation, relaxation of the localized excited state in the length direction of the rare earth-doped optical fiber 1 can be promoted. If the output of the excitation light source 5 after the first Q switch pulse oscillation is in a state where parasitic oscillation is possible, the Q switch pulse oscillation is caused again after a predetermined time while further raising the excitation light source 5. By repeating this up to the output of the pumping light source 5 that cannot oscillate parasitically, the loss of the feedback suppression unit 7 is minimized when the output of the pumping light source 5 that cannot oscillate parasitically is reached. It can be performed. Note that the loss controlled by the feedback suppression unit 7 preferably includes an oscillation wavelength (mirror wavelength band) of the resonator.

Next, a continuous light oscillation method of the fiber laser according to this embodiment will be described. FIG. 2 is a diagram illustrating an example of a startup state of the fiber laser 10 according to the present embodiment. 2A shows the laser output power of the fiber laser 10, FIG. 2B shows the loss of spontaneous emission light of the rare earth-doped optical fiber 1 of the feedback suppression unit 7, and FIG. 2C shows the pumping light power. Incidentally, the pumping light power P _S from the rise of the excitation light source 5, a power of the pumping light source 5 is the upper limit that parasitic oscillation can occur, the excitation light source, is determined rare earth doped optical fiber, a first mirror and a second mirror For example, the value is uniquely determined in advance.

  First, in the excitation start step, excitation light starts to enter the rare earth-doped optical fiber 1 from the excitation light source 5. At this time, the feedback suppression unit 7 is set to a state in which the light transmission loss in the wavelength band including the peak of spontaneous emission light of the rare earth-doped optical fiber 1 is a predetermined large value. In this state, since spontaneous emission light and stimulated emission light generated in the rare earth-doped optical fiber 1 are suppressed from being reflected by the first mirror 2 and returning to the optical resonator again, the laser by the fiber laser resonator is suppressed. Oscillation does not occur.

Next, in the switching step, the transmission loss of the feedback suppression unit 7 is temporarily reduced for a time (t ₁ ) that is shorter than the time at which parasitic oscillation may occur from the start of excitation in the fiber laser 10. That is, feedback of spontaneous emission light and stimulated emission light to the rare earth-doped optical fiber 1 through the first mirror 2 is increased. By this time (t ₁ ), the rare earth-doped optical fiber 1 is in a predetermined excitation state by the incidence of excitation light. As a result, the loss of the resonator is reduced transiently and Q-switch oscillation occurs. State of lowering the transmission loss of the feedback suppression unit 7 will _be maintained until _{t 2 = t 1 + t a} . The output of the pumping light source 5 at t ₂ in the first round of Q after the switch pulse oscillation, if the parasitic oscillation and may state (P _S hereinafter), while further promoting the launch of the excitation light source 5, the feedback suppression unit 7 The amount of the transmission loss is such that spontaneous emission light and stimulated emission light generated in the rare earth-doped optical fiber 1 are reflected by the first mirror 2 and are fed back to the optical resonator to cause no laser oscillation by the resonator. Increase again (t ₂ ). Thereafter, after a predetermined time (t _b ) has elapsed (t ₃ = _t 2 + t _b ), the transmission loss of the feedback suppression unit 7 is temporarily reduced again to cause Q switch pulse oscillation. Such repeated operation, the excitation light source 5, the output power to stabilize the output to a predetermined output power after exceeding the P _S (t _S). In the continuous light oscillation method of the fiber laser according to the present embodiment, four Q-switch oscillations are caused in the switching process.

FIG. 2B shows an example in which the change in the loss amount of the feedback suppression unit 7 is increased or decreased in a rectangular shape. However, if the change in the feedback amount can cause the Q switch oscillation at a predetermined timing, the change mode Is not limited to a rectangular shape, and may be a pulse shape. At this time, the time t ₁ from the start of excitation until the loss of the feedback suppression unit 7 is reduced is set to be shorter than the time at which parasitic oscillation may occur. Parasitic oscillation does not occur in the interval from the start of excitation to the first Q switch oscillation (t ₁ ) and in the interval between the first Q switch oscillation and the second and subsequent Q switch oscillations (t _b ). Set to interval. These intervals t ₁ and t _b can be determined in advance by experiment or simulation if the pumping light source 5, the rare earth-doped optical fiber 1, the first mirror 2, and the second mirror 3 are determined.

  Next, in the continuous oscillation step, after the output power of the excitation light source 5 exceeds Ps, the loss of the feedback suppression unit 7 is minimized and stable continuous laser oscillation is performed. With the above-described startup method, the fiber laser 10 can continuously oscillate without causing parasitic oscillation from the OFF state. Thereby, damage to the excitation light source 5, the fiber core, and the end face can be prevented by the giant pulse generated by the parasitic oscillation, and the continuous oscillation can be safely performed.

  Next, the fact that parasitic oscillation can be suppressed when the fiber laser 10 according to the present embodiment is started up will be described from the relationship between the optical output of the fiber laser and the inversion distribution rate. According to the inventors, it has been found that the giant pulse due to parasitic oscillation depends on the inversion distribution rate when the rare earth-doped optical fiber is in an excited state. FIG. 3 shows the light output and inversion distribution rate of a fiber laser that does not include the feedback suppression unit 7. The excitation light power (steady time) was 5 W, and the rise time of the excitation light itself was 0. 3A shows the time change of the laser output power, and FIG. 3B shows the time change of the inversion distribution ratio (%) in the fiber length (m) from the second mirror 3. As for the time, the incident start time of excitation light is set to 0 from an unexcited state (ground state). The excitation light is incident from the side of the fiber length: 0 (second mirror 3). As shown in FIG. 3B, the inversion distribution rate increases with time. For example, the inversion distribution rate of the incident end of the excitation light increases from 0.02 ms after the start of the fiber laser (0 ms), and the inversion distribution rate in the fiber length direction 0.05 ms after the start of the fiber laser. In 0.057 ms after the start of the fiber laser, a part of the fiber region in the fiber length direction reaches an inversion distribution rate of 100%. Thereafter, as shown in FIG. 3A, a giant pulse due to parasitic oscillation is generated. When the giant pulse is generated, the inversion distribution rate is as high as 100%, and the excited electrons emit energy at one time, resulting in a pulse having a very high peak value.

  FIG. 4 shows the light output and inversion distribution rate of the fiber laser 10 according to the present embodiment. 4A shows the time change of the laser output power, and FIG. 4B shows the time change of the inversion distribution ratio (%) in the fiber length (m) from the second mirror 3. When the feedback suppression unit 7 repeats the feedback suppression state and the feedback state of spontaneous emission light 100 times between time 0 and 1 ms, steady pulse oscillation corresponding to this period occurs, and the inversion distribution rate is 100 in the fiber length direction. It rises gradually without making a localization that becomes%. At 1 ms from the start of the fiber laser, the pumping light source 5 outputs pumping light of pumping light power that continuously oscillates between the first mirror 2 and the second mirror 3. In this case, after 1 ms from the start of the fiber laser, the transmission loss of the feedback suppression unit 7 is minimized (maximum feedback rate) so that spontaneous emission light is fed back to the rare earth-doped optical fiber 1. By doing so, the output power of the fiber laser oscillates over the entire length of the rare-earth-doped optical fiber 1 and becomes higher than a predetermined inversion distribution rate, and finally shifts to continuous oscillation without causing parasitic oscillation. Note that the peak value of the vibration of the optical output at the start of continuous oscillation is sufficiently smaller than the peak value of the optical output at the time of parasitic oscillation, and does not damage the components constituting the fiber laser.

  The feedback suppression unit 7 is not limited to a loss variable optical element as long as it can perform Q switching by increasing or decreasing the feedback amount of spontaneous emission light to the rare earth-doped optical fiber 1 at high speed (for example, 1 μs or less). For example, the feedback suppression unit 7 may be a reflectance variable unit that varies the reflectance of the first mirror 2, an attenuation factor varying unit that varies the attenuation factor of light, or the first The reflection direction variable means for changing the reflection direction of the mirror 2 may be used.

  The rare earth element-doped optical fiber 1 may have a double clad structure coated with a low refractive index resin so that high-intensity excitation light can enter. Even if the excitation light source 5 is operated by the current control unit 4 so that the high-intensity excitation light is incident on the rare earth element-doped optical fiber 1, the light from the plurality of excitation light sources 5 is optically coupled by the excitation light combiner 6. Good. In order to make high-intensity excitation light incident on the rare earth element-doped optical fiber 1, it is preferable to use high-power semiconductor laser light as the excitation light source 5. Furthermore, in the fiber laser 10 according to the present embodiment, the pumping light is incident through the second mirror 3. However, if the pumping light is introduced into the cladding of the rare earth-doped optical fiber 1, the first A configuration in which a coupler is provided between the resonators of the mirror 2 and the second mirror 3 may be employed. The first mirror 2 on the emission side is a fiber black mirror, but may be a multilayer mirror as long as it exhibits a predetermined reflectance with respect to a desired oscillation wavelength. You may use the Fresnel reflection of the end surface of the acousto-optic element used in the feedback suppression part 7, or an electro-optic element (E / O element).

  In the fiber laser 10 according to the present embodiment, the backward pumping method in which the excitation light is incident on the emission end face of the rare earth-doped optical fiber 1 is used, but the forward pumping method or the front / back pumping method may be used. In this case, the feedback suppression unit 7 is preferably disposed at a position where the gain of the rare earth-doped optical fiber 1 is highest when pump light is incident. Thereby, the power of the laser oscillation light can be increased.

The fiber laser output was measured using the fiber laser according to the present embodiment.
The conditions evaluated in Example 1 are described below. The rare earth-doped optical fiber 1 used had an absorption amount of 620 dB / m per unit length at a wavelength of 976 nm, a fiber transmission loss of 25 dB / km, a core / cladding diameter of 10 μm / 130 μm, and a fiber length of 11 m. The reflection band of the second mirror 3 is 1064.187 nm to 1064.525 nm and the excitation light loss is 0.1 dB. The first mirror 2 has a central reflection wavelength of 1064.38 nm and a reflectance of 10%, and the excitation light source 5 includes two high-power semiconductor lasers with a wavelength of 915 nm and an output power of 10 W per unit. used. An optical power meter was used as a photodetector for measuring the fiber laser output. As shown in FIG. 5, the voltage applied to the feedback suppression unit 7 was a rectangular wave having a repetition frequency of 100 kHz, a duty ratio of 10%, and a voltage of 0V-5V.

FIG. 6 shows changes in the output time of the fiber laser in the first embodiment. Since the transmission loss of the feedback suppression unit 7 is temporally changed when the fiber laser is started up, a steady pulse corresponding to the transmission loss fluctuation period is output. As described above, when the fiber laser according to the first embodiment is started, generation of a giant pulse due to parasitic oscillation is avoided. After the pumping light power exceeds the continuous oscillation threshold P _S, the feedback amount of spontaneous emission and stimulated emission light to the rare-earth doped optical fiber 1 of the feedback suppression unit 7 by the maximum (transmission loss is low) It becomes possible to shift to continuous oscillation safely.

  In the second embodiment, the conditions evaluated in the first embodiment and the feedback suppression unit 7 are different. In Example 2, a variable optical attenuator is used for the feedback suppression unit 7, and the voltage applied to the feedback suppression unit 7 is a rectangular wave having a repetition frequency of 100 kHz, a duty ratio of 10%, and a voltage of 0V-5V as shown in FIG. .

FIG. 8 shows changes in the output time of the fiber laser in the second embodiment. Since the transmission loss of the feedback suppression unit 7 is temporally changed when the fiber laser is started up, a steady pulse corresponding to the transmission loss fluctuation period is output. As described above, when the fiber laser according to the second embodiment is started, generation of a giant pulse due to parasitic oscillation is avoided. The safety that the pumping light power is maximum (low transmission loss state) and the feedback amount of spontaneous emission and stimulated emission light to the rare-earth doped optical fiber 1 of the feedback suppression unit 7 after exceeding the continuous oscillation threshold P _S It is possible to shift to continuous oscillation.

Comparative example

  In the fiber laser having the same configuration as that of Example 1, the feedback suppression unit 7 was kept constant with the maximum amount of spontaneously emitted light fed back to the rare earth-doped optical fiber 1 (low transmission loss), and the fiber laser output was measured. This state is substantially equivalent to not arranging the feedback suppression unit 7 in the resonator between the first mirror 2 and the second mirror 3. FIG. 9 shows changes in the output time of the fiber laser in the comparative example. About 15 μs after the start of the fiber laser, a giant pulse due to parasitic oscillation was generated. The giant pulse has such a power that the second mirror 3 may be damaged.

  Since the fiber laser of the present invention is a high-power fiber laser, it can be applied to a wide range of industries such as laser processing in addition to the information communication industry.

1: rare earth doped optical fiber 2: first mirror 3: second mirror 4: current control unit 5: excitation light source 6: excitation light combiner 7: feedback suppression unit 8: feedback control unit 10: fiber laser

Claims (4)

A resonator formed by the first reflecting portion and the second reflecting portion;
A rare earth-doped optical fiber provided in the optical path of the resonator and doped with a rare earth element;
An excitation light source for exciting the rare earth element;
A fiber laser comprising:
The optical path includes a feedback suppression unit that suppresses spontaneous emission of the rare earth element from the first reflection unit to the rare earth-doped optical fiber,
The feedback suppression unit temporarily increases a feedback amount from the first reflection unit to cause pulse oscillation by the resonator during a period in which parasitic oscillation at the time of startup of the excitation light source may occur. A fiber laser.   The said feedback suppression part repeats increasing the said feedback amount temporarily with a predetermined period until the said excitation light source exceeds the power which fiber laser can oscillate continuously. Fiber laser. A fiber laser continuous light oscillation method using a rare earth-doped optical fiber doped with a rare earth element as a laser medium,
In the state in which the spontaneous emission light of the rare earth element suppresses the feedback from one mirror of the resonator to the rare earth-doped optical fiber, the excitation start step for starting the incidence of the excitation light from the excitation light source,
A switching step of temporarily increasing the suppressed feedback after a shorter period of time than a predetermined parasitic oscillation occurs, and causing a Q-switch oscillation by the resonator;
After the pumping light source rises to an output larger than the pumping light power exceeding the pumping light power at which the parasitic oscillation can occur, the rare earth element spontaneous emission light is returned to the rare earth-doped optical fiber via one mirror. As a constant state that does not suppress, a continuous oscillation process to make a continuous light oscillation state,
A continuous light oscillation method of a fiber laser, comprising:   4. The fiber laser continuous light oscillation method according to claim 3, wherein in the switching step, the feedback amount is temporarily increased at a predetermined period to repeat Q-switch oscillation.

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