JP2012156175A - Fiber laser light source device and wavelength conversion laser light source device using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fiber laser light source device that can output stable pulse light from start of pulse light emission.SOLUTION: A fiber laser light source device has: a laser resonator provided with a fiber containing a laser active material and fiber gratings provided at both sides of the fiber; an excitation laser light source for emitting excitation light to one end of the laser resonator; a laser driving unit for supplying current to output excitation light from the excitation laser light source; and a laser controller for controlling the current to be output from the laser driving unit. The laser controller supplies first current larger than threshold current for making the laser resonator output laser light just before pulse light is output from the laser resonator, and after the first current is stopped and then a break period is provided, the laser controller supplies the excitation laser light source with second current larger than the first current to output the pulse light.

Description

  The present invention relates to a fiber laser light source device capable of outputting stable pulsed light from an initial pulse at the start of pulse light emission, and a wavelength conversion laser light source device using the same.

Solid-state lasers such as Nd: YAG laser and Nd: YVO ₄ laser are mainly used as laser light sources that generate light having a wavelength of 1 μm band, and laser processing machines using them and visible light using these lights as fundamental waves. A light source is realized. However, in order to obtain a large laser output, the laser medium needs to be cooled, and the apparatus becomes large. Therefore, there is a demand for a fiber laser light source having a W-class high output that can be reduced in size by simple cooling.

  The basic operation of this fiber laser light source will be described. A fiber grating is provided on both ends of the fiber containing the laser active material to form a resonator that forms a pair of reflecting mirrors, and excitation light from the excitation laser light source is incident from one end of the fiber. When this excitation light is absorbed by the laser active material contained in the fiber, the laser active material generates spontaneous emission. Spontaneous light emission generated inside the fiber is reflected in the resonator many times and goes back and forth, and becomes light with the same phase, and is amplified by the laser active material to increase the light intensity. At the same time, since the wavelength is selected by the fiber grating, laser oscillation with a uniform wavelength is obtained. The fiber and the fiber grating are fusion-bonded at the connection portion, and the excitation laser light source is driven by current.

  If it is a pulse laser light source with a high peak power, the application will further expand such as laser processing such as drilling and marking, and highly efficient wavelength conversion. Therefore, there is a pulsed light generator using a fiber laser using a Q switch. This is because a fiber Bragg grating (hereinafter referred to as “FBG”) is provided at both ends of a fiber to which a rare earth is added as a laser active material, and a laser resonator is formed between two FBGs. Furthermore, pulses are generated from the saturable absorber provided in the resonator by utilizing the saturable absorption characteristics of the laser. The Q-switch operation is enabled by the supersaturated absorption characteristics, and ultrashort pulse laser light is generated from the output end of the fiber laser light source by using mode lock (see, for example, Patent Document 1).

  Further, as a method of emitting light with a light source using a fiber, there is a method of amplifying a modulated seed light source with a fiber amplifier. In such an example, a modulated seed light source is amplified by a fiber amplifier, and a fiber laser oscillator (MO unit) and a fiber laser amplifier (PA unit) are connected in series to generate a high output pulse train with a constant output. In the fiber laser light source device, by devising the timing of the excitation start of the oscillator and the amplifier, a stable and constant output optical pulse train can be obtained from the first stage where an optical pulse is generated (for example, Patent Document 2 and 3). Such a configuration requires an expensive and complicated optical configuration such as a Q switch and an amplifier, resulting in a large and expensive device.

  Also, in a fiber laser light source device composed of a similar fiber and a pair of FBGs sandwiching the same, the input pattern of the pumping laser light source is optimized during continuous light output, and a surge pulse is added at the head of the rectangular optical pulse. There is known a technique for generating a stable rectangular light pulse without accompanying the above (see, for example, Patent Document 4). With such a configuration, it is possible to obtain a stable rectangular light pulse that is optimal for uses such as a display without a surge pulse and various types of analysis. Furthermore, since a pulse using a gain switch can be generated by modulating a laser light source for excitation, a complicated optical configuration is unnecessary, and an inexpensive device can be realized with a simple configuration.

JP 2005-174993 A JP 2007-35696 A JP 2008-181943 A JP 2007-142380 A

  However, in the conventional configuration, stable pulsed light cannot be output from the start of pulsed light emission. The pulsed light is light that changes sharply in a short time, and outputs light having higher power than continuous light output. Therefore, in order to obtain this high optical output, energy is rapidly accumulated in the laser resonator of the fiber laser light source, and the laser resonator is excited when this energy transitions from the supersaturated state to the oscillation (excited) state. Even if light was injected rapidly, stable pulsed light could not be output from the start of pulse emission.

  The present invention solves the above-described conventional problems, and an object of the present invention is to provide a fiber laser light source device that can output stable pulsed light from the start of pulsed light emission.

  In order to solve the above-mentioned conventional problems, a fiber laser light source device of the present invention and a wavelength conversion laser light source device using the same include a fiber containing a laser active material, a laser resonator provided with fiber gratings at both ends thereof, An excitation laser light source that makes excitation light incident on one end of the laser resonator, a laser drive unit that supplies a current for outputting excitation light from the excitation laser light source, and a current output from the laser drive unit are controlled. A laser control unit, and the laser control unit gives a first current larger than a threshold current for the laser resonator to output laser light immediately before outputting the pulsed light from the laser resonator, After a stop period is provided after stopping the first current, a second current larger than the first current for outputting pulsed light is applied to the excitation laser light source. It is obtained by, comprising the, a laser control unit to provide.

  According to the fiber laser light source device of the present invention and the wavelength conversion laser light source device using the same, it is possible to output stable pulsed light from the start of pulsed light emission.

Principle of pulsed light generation using a gain switch Diagram showing the relationship between excitation light and pulsed light generation in a fiber laser light source Model diagram explaining generation of pulsed light in a fiber laser light source Configuration diagram of a light source device in an embodiment of the present invention (A) The figure which shows the electric current waveform input into the light source device in the Example of this invention (b) The figure which shows the optical output waveform output from the light source device in the Example of this invention The figure which shows the relationship between the preliminary | backup excitation output and the dispersion | variation in pulse output in the Example of this invention (A) The figure which shows the relationship between light emission stop time and pulse light output (b) The figure which shows the relationship between light emission stop time and optical delay time Diagram showing the relationship of pulsed light output characteristics to current rise time The figure which shows the relationship between excitation light and pulsed light generation in embodiment of this invention

(Problems that arise when using gain switches in fiber laser light sources)
First, the principle of pulse light generation by a gain switch in a laser will be described. FIG. 1A is a schematic diagram of a laser resonator, in which a laser medium 202 is sandwiched between two mirrors 204 and 205 to form a laser resonator. Excitation light 201 is incident on the laser resonator from the outside, and laser-oscillated output light 203 is obtained. In the fiber laser light source, the laser medium 202 corresponds to a doped fiber, and the mirrors 204 and 205 correspond to a fiber grating.

  When pump light 201 whose intensity has been modulated stepwise as shown in FIG. 1 (b) is input to the laser medium 202, the laser medium contains the inside of the laser medium over time as shown in FIG. 1 (c). Energy increases. If the laser medium does not oscillate, the internal energy exceeds the laser oscillation threshold of the laser resonator and becomes supersaturated. When laser oscillation starts in this supersaturated state, the accumulated energy is released at a time, so that a pulsed light output having an output light intensity P1 is generated as shown in FIG. This is the principle of pulsed light emission by the gain switch.

  In the gain switch, an energy storage time t1 is required from the excitation start time to the generation of pulsed light. Hereinafter, this time t1 is referred to as an optical delay time. By using a fiber laser light source using such a gain switch, laser pulse light can be generated with a simple configuration.

  However, when pulse emission is performed using a gain switch as a fiber laser light source, two problems that have not been known in the past have been clarified. The first problem is a phenomenon (first pulse problem) in which an initial pulse at the start of pulse emission does not occur or its output is extremely reduced. Although the output fluctuation of the initial pulse is also observed in the solid-state laser light source, in this case, the output of the initial pulse increases contrary to the fiber laser light source. Therefore, the generation mechanism of the output fluctuation of the initial pulse of the fiber laser light source is different.

  The second problem is a phenomenon in which the optical delay time t1 between the pumping light and the pulsed light output changes depending on the energy incident on the laser resonator. When this optical delay time changes, the output timing of the pulsed light changes. This is a phenomenon that cannot be seen with a solid-state laser light source.

The fiber laser light source is different from the solid laser light source in that the resonator length is very long. The resonator length of the solid-state laser light source is about 1 m at the maximum, and generally about several tens of centimeters is used. On the other hand, since the resonator length of the fiber laser light source is 10 m or more, the influence of the energy accumulated in the resonator on the generation of the laser is different from that of the solid laser light source.
First, the first problem, which is the first pulse, will be described.

  FIG. 2 shows the relationship of the pulsed light output generated when a constant pumping light output is repeatedly incident on a laser resonator (hereinafter referred to as a fiber resonator) of a fiber laser light source. An example will be described in which the pumping light output input to the fiber resonator is controlled at 20 W, a pulse interval of 50 μs, and a duty of 10%.

  In FIG. 2, on the vertical axis, FIG. 2A shows the pumping light output incident on the fiber resonator, and FIG. 2B shows the pulsed light output generated from the fiber resonator with respect to the incident pumping light. And the horizontal axis represents time. The excitation light is generated for a certain period of time and then stopped for a certain period of time. The pumping light that performs this series of operations enters the fiber resonator at a pulse interval T. By repeatedly making this excitation light incident on the fiber resonator, pulse light is generated from the fiber laser light source. The pump light and pulsed light output correspond to the nth incident pump light En and the pulsed light output Pn generated at that time, respectively.

  As shown in FIG. 2, when the pump light is incident on the fiber resonator, the pulse lights P1 and P2 are not generated in the pump light E1 and E2 at the start of pulse emission, and the minute pulse light P3 and P4 are generated from the pump lights E3 and E4. It was found that P4 was generated, and thereafter, pulsed lights P5 and P6 with stable outputs were generated from the excitation lights E5 and E6.

  This mechanism will be described with reference to FIGS. FIG. 3 shows the distribution of accumulated energy inside the fiber resonator. The horizontal axis is distance, and indicates the position from the entrance to the exit of the fiber resonator. The vertical axis represents the energy of inversion distribution accumulated in the fiber resonator. The arrow of the pumping light indicates a state in which constant pumping light is input from the incident part side into the fiber resonator, and the arrow of the pulsed light indicates a state in which a pulse laser is generated.

  Now, as shown in FIG. 2A, the pulsed light output generated when the constant excitation light is incident on the fiber resonator at the pulse interval T is generated as follows. When the excitation light E1 is incident, energy is accumulated inside the fiber resonator as shown in FIG. Thereafter, when the pumping light E1 is stopped, the pumping light absorption coefficient of the fiber resonator is high, so that the pumping light is attenuated inside the fiber, and the stored energy is also decreased from the incident side toward the emitting side (FIG. 3B). . That is, the excitation light E1 is not attenuated inside the fiber and does not reach the laser oscillation state, and as a result, the pulsed light P1 is not generated.

  Next, when the excitation light E2 is incident on the fiber resonator (after 50 μs from the incidence of E1), the energy immediately before remaining in the excitation light E1 is added, and energy is accumulated in the fiber. With the stop of the excitation light E2, this energy is attenuated and does not reach the laser oscillation state, and the pulsed light P2 is not generated. The residual energy at this time is larger in the fiber resonator than when the pumping light E1 is stopped. Next, when the pumping light E3 is incident on the fiber resonator (after 100 μs from the incidence of E1), the energy just before remaining in the pumping light E2 is added and the accumulated energy exceeds the threshold value of the fiber resonator. P3 is generated (FIG. 3C). This pulsed light P3 is generated because the fiber resonator is in a laser oscillation state. At this time, since the pumping light and the output pulsed light propagate inside the fiber resonator, the energy distribution spreads over the entire fiber resonator. In other words, the energy distribution accumulated inside the fiber resonator is changed by laser oscillation, and thereafter attenuates as the pumping light stops (FIG. 3D).

  Further, when the excitation light E4 is incident (after 150 μs from the incidence of E1), the residual energy of the immediately preceding E3 is added to enter a laser oscillation state, so the output of the pulsed light P4 is a pulse larger than that of the immediately preceding pulsed light P3. Light is output. At this time, the residual energy of the stop of the pumping light E4 is larger in the fiber resonator than when the pumping light E3 is stopped. When the excitation light is repeatedly incident in this way, the excitation light E5 200 μs after the incidence of E1 generates a pulsed light P5 larger than P4. The state inside the resonator at this time is as shown in FIG.

  Then, the pumping light E5 is stopped, and the residual energy inside the fiber resonator immediately before the next pumping light E6 is incident is substantially the same as the residual energy just before the pumping light E5 is incident (FIG. 3 (f )), Pulsed light P6 equivalent to pulsed light P5 is generated. Thereafter, when the excitation light is repeatedly incident, the residual energy immediately before the excitation light is incident is saturated, and the laser is oscillated by adding the saturated energy, so that stable pulsed light is repeatedly generated (En, Pn).

  That is, when pulse emission starts, excitation light enters the fiber resonator, but the loss of the fiber resonator is large and the laser does not enter an oscillation state. Therefore, no pulse light is generated from the initial pulse at the start of pulse emission. Thereafter, the excitation light is repeatedly incident, and the immediately preceding residual energy is added to enter a laser oscillation state, and pulse light is generated by the gain switch. Further, when the excitation light is repeatedly incident, the immediately preceding energy distribution is saturated, and this saturated energy is added to cause laser oscillation. Therefore, stable output pulse light can be repeatedly generated.

  Next, a mechanism that changes the pulsed light output timing according to energy incident on the fiber resonator, which is a second problem, will be described. The change in the output timing of the pulsed light is also caused by the energy inside the fiber resonator as in the first problem. Since the excitation light absorption coefficient of the fiber resonator is high, the excitation light is attenuated inside the fiber, so that the internal energy accumulated in the fiber resonator is reduced. When the internal energy decreases, it takes time to reach the laser oscillation state, and the pulse generation, that is, the optical delay time becomes longer. This optical delay time varies depending on the intensity of pumping light and the energy distribution inside the fiber resonator.

The above is the mechanism of occurrence of problems that occur when pulse emission is performed using a gain switch as a fiber laser light source.
Hereinafter, a light source device according to an embodiment of the present invention will be described with reference to the drawings. In addition, what attached | subjected the same code | symbol in drawing may abbreviate | omit description.

(Embodiment 1)
FIG. 4 is a configuration diagram of the light source device according to Embodiment 1 of the present invention. As shown in FIG. 4, the fiber laser light source 101 includes a fiber resonator using a pumping laser light source 102 and a Yb-doped double clad fiber, and FBGs 104 and 105 fused to both ends of the fiber. The configuration is as follows. The core diameter of the fiber 103 was about 6 μm and the length was about 16 m. The excitation laser light source 102 is a semiconductor laser having a wavelength of 915 nm and is arranged so that excitation light can be input to one FBG 104 of the fiber resonator. By inputting excitation light, laser output light having a wavelength of 1000 nm to 1100 nm is generated from the FBG 105 by resonating in the fiber resonator. The resonance wavelength may be selected by the reflection wavelength of the FBG.

  In this embodiment, an FBG reflection wavelength capable of generating laser output light having a wavelength of 1064 nm is selected. When this laser output light is guided to the SHG module 106, the wavelength-converted short wavelength light is output from the SHG module 106. Laser output light generated from the fiber resonator is generated by inputting a current output from the laser driving unit 107 to the excitation laser light source 102. In this laser drive unit, the output current is controlled by the laser control unit 108.

A feature of the present invention resides in how to supply a current for driving the excitation laser light source 102. That is, the laser control unit 108 controls the laser driving unit 107. FIG. 5 shows a schematic diagram of a current waveform (FIG. 5A) input to the excitation laser light source 102 and a light output waveform (FIG. 5B) output from the fiber laser light source 101 at this time. Yes.
In FIG. 5, first, the laser control unit 108 gives a preliminary light emission current that is a first current to the excitation laser light source 102 via the laser driving unit 107, and generates a preliminary output of continuous light from the fiber laser light source 101. Next, after stopping the first current, a pause period is provided after the first current is stopped (light emission stop step). Subsequently, a pulse light emission current as a second current is applied to generate pulsed light from the gain switch. There is a feature of outputting (pulse emission process).

  In FIG. 5, the first current in the preliminary light emission process is a current for outputting continuous light at a low output from the fiber laser light source 101, and a current equal to or higher than a threshold current required for the fiber resonator to oscillate. It is necessary to provide the laser light source 102 for excitation. The light emission stop process is a process in which laser oscillation of the fiber resonator is stopped and energy is accumulated inside the fiber resonator in order to perform gain switching. In this light emission stop process, the current is set to zero or less than the threshold current at which the excitation laser light source 102 outputs excitation light. After that, by supplying the second current to the excitation laser light source 102 in the pulse emission process, the fiber resonator can emit pulses by the gain switch by adding the energy stored immediately before, so that the pulse emission is stabilized from the start of the pulse emission. Output pulsed light is obtained.

As described with reference to FIGS. 2 and 3, the fiber laser light source has a long resonator length and a high pumping light absorption coefficient, so that the pumping light is attenuated inside the fiber. For this reason, in the initial pulse, energy necessary for generating a pulse by the fiber resonator does not propagate through the fiber, so that the output pulse light has a long optical delay time and a low output. The preliminary light emission process and the light emission stop process solve this problem. In these steps, energy can be accumulated inside the fiber resonator, so that energy attenuation of the initial pulse in the pulse emission process can be suppressed, and stable pulsed light output can be generated from the start of pulse emission.
Below, the current drive method in the pulse light source of this invention is demonstrated in detail divided into each process.

<Preliminary light emission process>
In the preliminary light emission step, energy is accumulated in the laser active medium by generating continuous light as described above. In FIG. 5, the preliminary light emission current is input to the excitation laser light source 102, and continuous light is output from the fiber laser light source 101. What is necessary for the preliminary light emission step is a stable continuous light output, and the fiber resonator needs to be in a laser oscillation state. If the pumping light intensity input to the fiber resonator for this purpose (the pumping light intensity corresponds to the preliminary light emission current in FIG. 5) is a certain value or more, the pulse in the pulse emission process is independent of the pumping light intensity. The light output does not change. However, when the excitation light intensity is low, the pulse light output in the pulse emission process is affected. This will be described with reference to FIG.

  FIG. 6 shows the relationship between the output of the excitation laser light source 102 in the preliminary light emission process and the initial pulsed light output variation at the start of pulsed light emission in the pulse light emission process. The vertical axis represents the relative pulse output variation in the pulse emission process, the numeral 0 indicates a state in which no pulsed light is emitted, and the numeral 100 indicates the target pulsed light output. The length of the arrow indicates that any pulsed light output is generated during that time.

  As shown in FIG. 6, when a preliminary light emission current having an excitation light intensity equal to or lower than the threshold value at which the fiber resonator performs laser oscillation, the pulse output in the pulse light emission process varies greatly. When the preliminary light emission intensity exceeds this threshold, the variation in pulsed light output gradually decreases. When the preliminary emission intensity exceeded about 2 W, no variation was observed, and thereafter a constant pulse output was obtained regardless of the value of the preliminary emission current. This is presumably because the laser oscillation is unstable in the vicinity of the threshold, and the energy accumulation becomes insufficient.

  For this reason, in the preliminary light emission process, a preliminary light emission current and a preliminary light emission time for obtaining stable laser oscillation are required. On the other hand, even when the excitation light intensity was varied at a value exceeding 2 W, the pulsed light output did not change. From this result, it is considered that the energy stored by laser oscillation is saturated at an excitation light intensity of about 2 W. As a result, the preliminary excitation power is preferably about 2 W or more.

In the preliminary light emission step, the time until the continuous light is generated depends on the value of the preliminary light emission current. When the current value is small, the time until continuous light is output becomes longer. In order to obtain a stable pulsed light output, it is necessary to generate stable continuous light in the preliminary light emission process. For this reason, it is desirable that the time of the preliminary light emission process is longer than the time when stable continuous light emission starts from the start of current application. Specifically, the preliminary light emission current that is the first current is a value that the excitation laser light source 102 outputs 2 W or more in FIG. 5A, and the preliminary light emission time is the continuous light emitted from the fiber resonator by laser oscillation. Is preferably set to a time (eg, 1 ms or more) during which the signal is output stably.
Further, for example, in FIG. 4, the laser light output from the fiber laser light source 101 is separated by a sampling mirror, and a part of the separated light is monitored by a photodetector, and laser control is performed with desired continuous light. It is good also as a structure (not shown).

<Light emission stop process>
The light emission stop process is a process in which the laser oscillation in the immediately preceding preliminary light emission process is stopped and energy is accumulated in the fiber resonator in order to generate an initial pulse. Immediately before the next pulse emission step, a state in which there is no laser oscillation or relaxation oscillation inside the fiber resonator, that is, a state in which there is no internal energy fluctuation, is required for a certain time or more. For this reason, a light emission stop process is required. Here, it is necessary to store energy for generating an initial pulse. Since this energy is absorbed by the fiber resonator, when the light emission stop time is changed in FIG. 5, the energy also changes and affects the pulsed light output.

<Pulse emission process>
The pulse light emission step is a step of emitting pulsed light as shown in FIG. The energy remaining in the fiber resonator through the preliminary light emission process through the light emission stop process is added to give a pulsed light emission current as a second current, and the fiber resonator is excited to generate pulsed light by the gain switch. In this step, as described with reference to FIG. 1, the internal energy is brought into a supersaturated state, and pulse light is generated by releasing the accumulated energy all at once. The optical delay time from the start of input of the pulsed light emission current to the generation of the pulsed light is the time indicated by the optical delay time t1 in FIG. 1 (c), and the time until the energy of the fiber resonator reaches the threshold value for generating the pulsed light. It is. For this reason, it is necessary to make the current rise time for generating pumping light from the pumping laser light source faster than the optical delay time. The generated pulse light output, pulse width, and delay time depend on the pulse light emission current.

  As specific values, FIG. 7 shows the relationship between the light emission stop process and the pulse light output in the pulse light emission process in this embodiment. FIG. 7 shows the relationship between the initial pulse output (FIG. 7A) generated when the light emission stop time is used as a parameter and the optical delay time at this time (FIG. 7B). The fiber used is a Yb-doped fiber with a core diameter of 6 μm, the resonator length is 16 m, and the pumping light is a semiconductor laser with a wavelength of 915 nm. Here, in the preliminary light emission step, the fiber laser light source is in a state of lasing. In FIG. 7A, when the light emission stop time is less than 15 μsec, the pulsed light output decreases. This is because the relaxation oscillation inside the fiber resonator is not sufficiently settled and the supersaturated state is hindered. The generation of pulsed light by the gain switch in the present embodiment requires a state in which the laser oscillation in the fiber resonator is completely contained, and it is considered that a time of about 15 μsec is necessary for this.

  On the other hand, when the light emission stop time exceeds 60 μsec, the pulsed light output decreases again. Furthermore, if it exceeds 230 microseconds, a pulsed light output will fall rapidly. This indicates that the upper limit of the holding time of the energy accumulated in the preliminary light emission process is about 230 μsec (B in the figure). When this upper limit is exceeded, the loss of stored energy increases and the pulsed light output decreases. On the other hand, it was also found that a very stable state in which the pulsed light output does not fluctuate exists between 15 to 60 μs. Therefore, in the case of the fiber resonator according to the present embodiment, it is more preferable that the light emission stop time is set to 15 μsec or more and 60 μsec or less which is the range of A in FIG. Within this range, the pulsed light output hardly changes depending on the light emission stop time, and stable and high-powered pulsed light output characteristics can be obtained from the initial pulse at the start of pulsed light emission.

  The light emission stop state is such that the loss of accumulated energy does not affect the generation of pulsed light, and a length for creating a stationary state capable of generating pulsed light is required. When 230 μsec is exceeded, the stored energy loss increases, and when it is shorter than 15 μsec, the supersaturated state of the fiber resonator is inhibited.

  In FIG. 7B, the light emission stop time increases and the light delay time increases. This is because if the light emission stop time becomes longer, the energy stored in the fiber is absorbed by the fiber, and the energy is attenuated with time. Therefore, as the residual energy decreases, it takes time to reach the threshold value for generating pulsed light, and thus the optical delay time becomes longer. If the light emission stop time is less than 15 μsec, the optical delay time decreases rapidly, because the relaxation oscillation of the fiber resonator is not sufficiently settled and the supersaturated state is inhibited. That is, the continuous light in the preliminary light emission process is not completely stopped. The generation of pulsed light by the gain switch in the present embodiment requires a state in which the laser oscillation in the fiber resonator is completely contained, and it is considered that a time of about 15 μsec is necessary for this.

  Further, FIG. 8 shows a pulse output characteristic generated when the current rise time of the second current in FIG. 5 is used as a parameter. In FIG. 8, the left side of the vertical axis shows the optical delay time, and the right side shows the pulsed light output. The laser drive conditions of the laser controller 108 at this time are a preliminary light emission current 2A, a preliminary light emission time 1 ms, a light emission stop time 20 us, and a pulse light emission current 10A, respectively. From FIG. 8, when the current rise time is shortened, the optical delay time is shortened and the pulsed light output is increased. Although the optical delay time changes depending on the current rise time, it was found that a stable state that does not fluctuate in the pulsed light output has a current rise time of 1.3 μs or less. This is considered that the pulsed light output generated by the supersaturated state of the fiber resonator of the present embodiment is saturated with respect to the current rise time.

  Further, when the current rise time becomes longer than the optical delay time, the pulsed light output is significantly reduced. Therefore, it is desirable that the current rise time be shorter than the optical delay time.

  When a current is step-applied to the excitation laser light source 102, as shown in FIG. 1, the excitation light intensity is input to the fiber resonator in a step shape, and the fiber resonator becomes oversaturated and generates pulsed light. However, if the current rise time becomes long, the rise time of the excitation light intensity also becomes long, which affects the generated pulsed light output. That is, as the current rise time becomes longer, it takes time for the fiber resonator to become supersaturated, and the pulse light generated from the fiber resonator increases or decreases the output delay time. For this reason, in order to obtain a stable and high-power pulsed light output, the current rise time must be at least shorter than the optical delay time.

  FIG. 9 is obtained by adding a preliminary light emission process and a light emission stop process according to the present invention to the input / output light characteristics of FIG. In the preliminary light emission step, the excitation light E0 is input to the fiber laser light source and the continuous light P0 is output. After the fiber resonator enters the oscillation state, the laser oscillation is stopped at a predetermined emission stop time (Ts) in the emission stop process. By inputting the excitation light E1 in the pulse emission process, high-power pulse light is generated from the initial pulse P1. Here, as described above, the optical delay time of the generated pulsed light varies depending on the light emission stop time Ts.

  This change in optical delay time depends on the residual energy inside the fiber resonator. Also in the pulse emission process, if the pulse interval T is changed, the residual energy also changes, so the output of the pulse light and the optical delay time change, and the initial pulse light and the output of the second and subsequent pulse lights vary. Therefore, it is desirable to set the light emission stop time Ts to be the same as the time (excitation stop time) Tp from when the pulse light is stopped until the next excitation light is input in the pulse light emission process. When continuous pulse oscillation is performed with Ts and Tp being the same time, the residual energy immediately before the pulse emission becomes equivalent, and the pulse oscillation is performed by adding that energy. It was possible to improve.

  This emission stop time also depends on the structure of the Yb-doped double clad fiber. In this embodiment, the fiber has a core diameter of 6 μm and a length of 16 m. Further, the pumping light having a wavelength of about 915 nm and the output light having a wavelength of about 1064 nm were used. When the core diameter of the fiber laser is increased to about 10 μm and the design of the resonator such as the reflectance of the FBG is adjusted, the minimum value of the emission stop time can be shortened to about 5 μs. By optimizing the resonator structure, the stable light emission stop time during which the pulsed light output does not change can be set to 5 μs to 80 μs. By shortening the light emission stop time, the maximum value of the pulse repetition frequency can be increased.

  As described above, the preliminary light emission process and the light emission stop process are provided, and the light emission stop time of the light emission stop process and the excitation stop time of the pulse light emission process are set to the same time, so The output characteristics of the pulsed light after the first are the same, and stable pulsed light output can be obtained from the start of pulsed light emission.

  In the present embodiment, the fiber 103 has been described as a Yb-doped fiber, but any other fiber containing a laser active substance may be used. That is, rare earth such as Er, Pr, Nd, Tm, and Ho doped as a laser active material, or a fiber in which these are mixed may be used. The oscillation wavelength can be selected by changing the rare earth to be doped. In addition, if the fiber is divided into a plurality of portions in the traveling direction, and fibers having different doping materials or doping concentrations are connected to each other, high-speed pulse modulation becomes possible.

  A plurality of excitation laser light sources may be prepared and separately distributed to the excitation laser light source in the pulse emission process and the excitation laser light source in the preliminary emission process. For example, the pumping laser light source is composed of four semiconductor lasers (LDs), and one of the LDs is used for the preliminary light emission process that requires only a low output, and the pulsed light is excited by four LDs. Is also possible. In the preliminary excitation that requires a low output operation, there is an advantage that a low output can be stably operated when the number of LDs is small.

  In the configuration of this embodiment, there is a possibility that the output pulsed light returns to the excitation laser light source. When the return light returns to the output end face of the excitation laser light source, the end face of the LD is broken. In order to prevent this, the excitation laser light source is preferably provided with a wavelength selection filter for preventing return light. By providing a wavelength selection filter near the output of the excitation laser light source, the return light from the fiber laser light source can be suppressed.

  Next, in this embodiment, a wavelength-converted laser light source that generates short-wavelength light by wavelength-converting the output from the fiber laser light source with a wavelength conversion element will be described. In FIG. 5, if the pulsed light output from the fiber laser light source 101 is guided to the SHG module 106, the wavelength-converted short wavelength pulsed light is output from the SHG module. In this embodiment, the reflection wavelength of the FBG is selected, and laser light having a wavelength of 1064 nm is generated from the fiber. The light having a wavelength of 1064 nm can be converted into green light having a wavelength of 532 nm by the SHG module 106.

  Here, the SHG module 106 includes a wavelength conversion element having nonlinear characteristics, the conversion efficiency changes depending on the wavelength of the fundamental wave output from the fiber laser light source 101, and the harmonic output is proportional to the square of the fundamental wave. And increase. By utilizing this characteristic, the output intensity of continuous light generated by the preliminary light emission process on the wavelength conversion element output side can be significantly suppressed. For example, assuming that the pulse output in the pulse light emission process is 100 W on the output side of the fiber laser light source and the preliminary output of continuous light in the preliminary light emission process is 1 W, the wavelength conversion element output side has 50 W and 0.005 W, respectively. That is, on the output side of the fiber laser light source, the preliminary output of continuous light is 1/100 of the output of the pulsed light, but on the wavelength conversion element output side, this ratio is greatly reduced to 1/10000. . That is, the output of the preliminary light emission process can be significantly suppressed by passing the wavelength conversion element. As a result, laser processing similar to that of a normal pulse light source without the influence of the preliminary output becomes possible. Further, for example, in FIG. 4, the harmonic output light output from the SHG module 106 is separated by a sampling mirror, and a part of the separated light is monitored by a photodetector, and the preliminary light emission process is performed in a desired continuous state. It is good also as a structure which carries out laser control with light (not shown).

For the SHG module, for example, MgO: LiNbO ₃ and a material in which a polarization reversal structure is formed can be used. Other titanyl potassium phosphate (KTiOPO ₄ : KTP), Mg: LiTaO ₃ , and the like, and You may use what formed the polarization inversion structure in these materials. Further, it is particularly effective for nonlinear optical crystals having a domain-inverted structure, such as Mg: LiNbO ₃ (congruent composition / stoichiometric composition), Mg: LiTaO ₃ (congruent composition / stoichiometric composition), and KTP. .

  In the embodiment of the present invention, as an example of wavelength conversion, conversion from infrared light (1064 nm) to visible light (532 nm) is taken as an example, but in addition to second harmonic generation, sum frequency generation, difference Any frequency generation and parametric oscillation can be used as long as they utilize a structure that matches the phase of light by utilizing the period of the domain-inverted structure.

  Thus, the present invention can also be applied as a wavelength-converted laser light source in which pulse light output from a fiber laser light source is converted into a harmonic by a wavelength conversion element. When converted into the second harmonic as wavelength conversion, it becomes possible to generate short wavelength light, and the range of the workpiece is widened. Since the wavelength conversion uses a nonlinear phenomenon, the intensity distribution of the pulse can be modulated. The wavelength conversion has the advantage that the pulse width can be compressed to about ½.

  Furthermore, using a nonlinear optical crystal having a periodic domain-inverted structure as the wavelength conversion element is effective for shaping the pulse waveform. Since the wavelength conversion element having a periodic domain-inverted structure has no walk-off, a long action length can be used. As the action length becomes longer, the wavelength tolerance for wavelength conversion becomes narrower. Since the pulse waveform has a tail when it falls, the fall time is delayed, but the wavelength of this tail portion and the wavelength of the pulse peak are slightly different. For this reason, when wavelength conversion is performed by a wavelength conversion element having a periodic domain-inverted structure, there is an advantage that the conversion efficiency of the tail portion is reduced and the pulse waveform is shaped. This is effective in that the pulse waveform can be shaped in addition to the compression of the pulse width by the nonlinear optical effect.

  According to the fiber laser light source and the wavelength conversion laser device short wavelength light source using the fiber laser light source according to the present invention, a stable pulsed light output can always be obtained from the initial pulse at the start of pulse emission. With such a high-power pulse fiber laser light source, a short wavelength light source suitable for industrial applications such as laser processing and laser annealing can be realized.

DESCRIPTION OF SYMBOLS 101 Fiber laser light source 102 Laser light source for excitation 103 Fiber 104 FBG (Faber Bragg grating)
105 FBG (Faber Bragg Grating)
106 SHG module 107 Laser drive unit 108 Laser control unit

Claims (5)

A fiber containing a laser active substance and a laser resonator provided with fiber gratings at both ends thereof;
An excitation laser light source that makes excitation light incident on one end of the laser resonator;
A laser driver for supplying a current for outputting excitation light from the excitation laser light source;
A laser control unit for controlling the current output from the laser drive unit,
The laser control unit gives a first current larger than a threshold current for the laser resonator to output laser light immediately before outputting the pulse light from the laser resonator,
After providing a rest period after stopping the first current,
A fiber laser light source device comprising: a laser control unit that supplies a second current larger than the first current for outputting pulsed light to the excitation laser light source. 2. The fiber laser light source device according to claim 1, wherein a time for applying the first current is longer than a time from when the laser resonator supplies the first current to when the laser beam starts to output continuous light. The fiber laser light source device according to claim 1, wherein the pause period is longer than a time period during which relaxation oscillations in the laser resonator disappear. 2. The fiber laser light source device according to claim 1, wherein the pause period is substantially equal to a time during which the laser control unit stops the laser driving unit when the periodic continuous pulsed light is output by the second current. A fiber laser light source device according to claim 1;
A wavelength conversion laser light source device comprising: a wavelength conversion unit that converts the wavelength of laser light output from the fiber laser light source device and outputs a harmonic.