JP2013115147A - Pulse fiber laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To output a light pulse corresponding to an input pulse waveform by suppressing an overshoot of a light pulse due to a delay time and a time constant of at least an exciting laser current control circuit.SOLUTION: A pulse fiber laser which amplifies and outputs a light pulse SD by inputting an exciting laser current SC corresponding to the input pulse waveform to an exciting laser 4 and inputting exciting light to a fiber laser resonator 4 includes: an ACC circuit 3 which supplies the exciting laser current SC to the exciting laser 4; an output light detector 7 which detects the intensity of the light pulse SD; and a control part 2 which performs feed forward control to supply a predetermined exciting laser current to the exciting laser 4 for a predetermined period R1 corresponding to a delay time (t1+t2) from the input of the input pulse waveform until the start of output of the light pulse, and adjusts the exciting laser current SC based upon the intensity of the light pulse SD and performs feedback control the predetermined period later so that the intensity of the light pulse reaches a target value.

Description

  In the present invention, a pump laser current corresponding to an input pulse waveform is input to a pump laser, pump light from the pump laser is input to a fiber laser resonator, and light corresponding to the input pulse waveform is input from the fiber laser resonator. The present invention relates to a pulse fiber laser that amplifies and outputs a pulse.

  Conventionally, pulsed fiber lasers have been used as metal processing such as soldering, marking, display light sources, and light sources for various analyzers. This pulse fiber laser inputs the pump laser current corresponding to the input pulse waveform to the pump laser, inputs the pump light from the pump laser to the fiber laser resonator, and corresponds to the input pulse waveform from the fiber laser resonator. The amplified light pulse is amplified and output.

  Here, in the pulse fiber laser, the inversion distribution and stimulated emission increase sharply due to the steep excitation laser light input, so that the optical pulse output from the fiber laser resonator has an overshoot when it rises. Therefore, in Patent Document 1, the output light from the fiber laser resonator is set so that the output light from the fiber laser resonator rises to a desired output level after rising to a pre-level that does not affect laser processing. Feedback control.

JP 2009-297777 A

  However, the feedback control of the output light from the fiber laser resonator according to Patent Document 1 described above only corresponds to the inversion distribution in the fiber laser resonator with respect to the pump laser current and the amplification control due to stimulated emission, and the input pulse is controlled. Since the delay time and time constant of the pump laser current control circuit are not taken into consideration, the output light may overshoot depending on the control performance of the pump laser current control circuit. In particular, in the case of a pulse fiber laser that outputs an optical pulse having a rise time of about several tens of microseconds, overshoot often occurs.

  The present invention has been made in view of the above, and outputs an optical pulse corresponding to an input pulse waveform while suppressing at least an overshoot of an output pulse due to a delay time and a time constant of an excitation laser current control circuit. An object of the present invention is to provide a pulse fiber laser that can be used.

  In order to solve the above-described problems and achieve the object, a pulse fiber laser according to the present invention inputs an excitation laser current corresponding to an input pulse waveform to the excitation laser, and the excitation light from the excitation laser is resonant with the fiber laser. In a pulse fiber laser that is input to a laser and amplifies and outputs an optical pulse corresponding to the input pulse waveform from the fiber laser resonator, a current supply unit that supplies the pump laser current to the pump laser, and the input pulse waveform is During a predetermined period corresponding to a delay time from the input until the optical pulse starts to be output, feedforward control for supplying a predetermined excitation laser current to the excitation laser is performed, and the predetermined period elapses. After that, the excitation laser current is adjusted based on the intensity of the optical pulse so that the intensity of the optical pulse becomes a target value. Characterized by comprising a control unit for performing feedback control.

  Further, the pulse fiber laser according to the present invention includes, in the above invention, a detection unit that detects the excitation laser current supplied to the excitation laser or the excitation laser beam output from the excitation laser, and the control unit includes: In addition to the feedback control, the excitation laser current or the excitation laser light is fed back in a predetermined period to control the excitation laser current.

  The pulse fiber laser according to the present invention further includes a holding unit that holds a correlation between the intensity of the optical pulse obtained in advance and the excitation laser current in the above invention, and the control unit includes the holding unit. The feedforward control is performed within the predetermined period so that the intensity of the optical pulse does not overshoot based on the correlation held in FIG.

  In the pulse fiber laser according to the present invention as set forth in the invention described above, the control unit decreases the control constant at the time of rising of the current instruction value to the current supply unit as the time constant of the current supply unit decreases. Feed-forward control is performed.

  In the pulse fiber laser according to the present invention, in the above invention, the control unit decreases the control constant until the excitation laser current reaches a target value as the time constant of the current supply unit decreases, and / or Alternatively, the control period is increased.

  In the pulse fiber laser according to the present invention, in the above invention, when the optical pulse output is off, the control unit outputs a minute current less than an oscillation threshold value of the excitation laser beam from the current supply unit. It is characterized by performing.

  In the pulse fiber laser according to the present invention as set forth in the invention described above, the control unit measures the predetermined period with a timer and switches to feedback control of the optical pulse.

  In the pulse fiber laser according to the present invention as set forth in the invention described above, the holding unit holds the correlation in a table or function and can be updated.

  According to the present invention, the control unit feeds a predetermined excitation laser current to the excitation laser within a predetermined period corresponding to a delay time from when the input pulse waveform is input to when the output of the optical pulse is started. In addition to performing forward control, feedback control is performed so that the intensity of the optical pulse becomes a target value by adjusting the excitation laser current based on the intensity of the optical pulse after the predetermined period has elapsed. Therefore, it is possible to output an optical pulse corresponding to the input pulse waveform while suppressing an overshoot of the output pulse due to at least the delay time and time constant of the current supply unit which is a pump laser current control circuit.

FIG. 1 is a schematic diagram showing the configuration of the pulse fiber laser according to the first embodiment of the present invention. FIG. 2 is a time chart showing control processing of the pulse fiber laser shown in FIG. FIG. 3 is a time chart showing the operation when the time constant of the ACC circuit is small in FIG. FIG. 4 is a time chart showing control processing when the time constant of the ACC circuit is small in FIG. FIG. 5 is a schematic diagram showing the configuration of the pulse fiber laser according to the second embodiment of the present invention. FIG. 6 is a time chart showing control processing of the pulse fiber laser shown in FIG. FIG. 7 is a schematic diagram showing a configuration of a pulse fiber laser according to a modification of the second embodiment of the present invention. FIG. 8 is a time chart showing the operation when the time constant of the ACC circuit is small in FIG. FIG. 9 is a time chart showing control processing when the time constant of the ACC circuit is small in FIG. FIG. 10 is a lime chart showing a control process in which the rise of the ACC circuit is made faster in FIG. FIG. 11 is a time chart showing the operation of a conventional pulse fiber laser.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be described with reference to the accompanying drawings.

(Embodiment 1)
FIG. 1 is a schematic diagram showing the configuration of the pulse fiber laser according to the first embodiment of the present invention. The pulse fiber laser includes a holding unit 1, a control unit 2, an ACC (Auto Current Control) circuit 3, an excitation laser 4, a fiber laser resonator 5, and an output light detector 7. The fiber laser resonator 5 has a Yb-doped fiber 5b connected between the total reflection mirror 5a and the output mirror 5c. The fiber laser resonator 5 amplifies and outputs 1 μm band laser light by forward pumping of the pump laser 4.

  When the input pulse SA is input from the input terminal T 1 to the control unit 2, the control unit 2 outputs the instruction value SB of the excitation laser current to the ACC circuit 3. At this time, the ACC circuit 3 has a time constant τ of 10 μs or more. As a result, the excitation laser current SC to which the time constant τ is given is output to the excitation laser 4 corresponding to the instruction value SB. The pump laser 4 outputs pump laser light corresponding to the pump laser current SC to the fiber laser resonator 5. The fiber laser resonator 5 pumps the amplification medium (Yb) by the input pump laser light and outputs a 1 μm band optical pulse SD to the output terminal T2. Further, the output light detector 7 branches a part of the light pulse SD via the coupler 6, detects the intensity of the light pulse SD, and inputs the detection result to the control unit 2. Note that the ACC circuit 3 has a delay of time t1 as shown in FIG. 2 due to the operation threshold of the internal FET and the operational amplifier until the pumping laser current SC is output after the instruction value SB is input. Further, the fiber laser resonator 5 has a delay of time t2 as shown in FIG. 2 until the oscillation threshold is reached after the pump laser light is input from the pump laser 4.

  Here, a correlation between the intensity of the optical pulse detected by the output light detector 7 and the excitation laser current SC is obtained in advance, and this correlation is held in the holding unit 1. When obtaining this correlation, the LD current monitor 10 is used, and the LD current monitor 10 detects the excitation laser current SC input to the excitation laser 4 and inputs it to the control unit 2.

  As shown in FIG. 2 (d), this correlation indicates that the instruction value SB and the excitation laser current SC are such that the intensity of the optical pulse SD reaches the target value Pm for each input pulse SA without overshooting. (FIGS. 2B and 2C) are held in the holding unit 1.

  The correlation between the intensity of the optical pulse detected by the output light detector 7 and the excitation laser current SC varies depending on the aging of the excitation laser and the output light detector. Therefore, for each input pulse SA, the instruction value SB and the excitation current SC when the optical pulse reaches the target value Pm are recorded, and the relationship between the instruction value SB held in the holding unit 1 and the excitation laser current SC. Is updated by the control unit 2.

  Here, the optical pulse control by the control unit 2 will be described based on the timing chart shown in FIG. First, when the input pulse SA is input at time T1 (FIG. 2A), the control unit 2 at the time T1 based on the waveform of the instruction value SB of the holding unit 1 corresponding to the input pulse SA. Then, a constant value less than the upper limit value SBmax of the instruction value SB is output to the ACC circuit 3 (FIG. 2 (b)).

  When the instruction value SB is input at time T1, the ACC circuit 3 outputs the excitation laser current SC at time T2 after the delay time t1 (FIG. 2 (c)). However, the ACC circuit 3 has a circuit time constant τ, and rises to the target value Im of the excitation laser current SC with this time constant τ.

  The excitation laser 4 immediately outputs the excitation laser light to the fiber laser resonator 5 in response to the input of the excitation laser current SC, but the optical pulse SD rises after a delay time t2 until the oscillation threshold value of the fiber laser resonator 5 is reached (FIG. 2 (d)). Here, as described above, the excitation laser current SC rises such that no overshoot occurs in the rise of the optical pulse SD.

  The feedforward control described above is performed until time T4 when the light pulse SD rises to the target value Pm. After this time T4, the control unit 2 uses the intensity of the light pulse monitored by the output light detector 7. In addition, feedback control is performed so that the intensity of the light pulse becomes the target value Pm. That is, in the region R1 shown in FIG. 2, feedforward control is performed, and in the region R2, power feedback control is performed by feeding back an optical pulse. Note that the period corresponding to the region R1 is determined in accordance with the delay time (t1 + t2) of the ACC circuit 3, and is a predetermined period in which the optical pulse output overshoot does not occur. Within the period of Then, feedback control is performed from time T4 when the predetermined period ends.

  Thereby, the optical pulse SD becomes a pulse corresponding to the input pulse SA without overshooting. As a result, it is possible to prevent quality degradation and workability degradation of the workpiece. In addition, when applied to a display light source, stable luminance can be obtained. In addition, since the control is performed in advance with the appropriate waveform of the instruction value SB and the excitation laser current SC, the delay time (t1 + t2) from the input of the input pulse SA to the start of rising of the optical pulse SD can be shortened, and The rise time t3 can also be shortened. As a result, an optical pulse maintaining a rise time of about several tens of microseconds can be obtained.

  FIG. 11 shows the case where the power feedback control is performed from the time when the input pulse SA is input. In this case, since the optical pulse SD is not fed back at least until the time T3, the instruction value SB is the upper limit. The value SBmax is reached, and after the delay time t1, the excitation laser current SC rises rapidly, and as a result, the optical pulse SD has overshooted.

  By the way, when the time constant τ of the ACC circuit 3 is small, the excitation laser current SC suddenly rises as shown in FIG. As a result, the pump laser light corresponding to the pump laser SC is rapidly input to the fiber laser resonator 5, and the inversion distribution and stimulated emission increase rapidly in the fiber laser resonator 5 to cause a large overshoot. It becomes a pulse.

  Therefore, in the first embodiment, as shown in FIG. 4, when the time constant τ of the ACC circuit 3 is small, the instruction value SB is controlled to gradually increase. As a result, the excitation laser current SC rises rapidly after time T2, but since the instruction value SB is small and then rises slowly, the optical pulse SD reaches the target value Pm without overshooting, as in FIG. To do. The control unit 2 performs PI control or PID control, and the instruction value SB described above can be gradually increased by reducing these control constants. That is, control constants such as proportional gain, integral constant, and differential constant are reduced.

(Embodiment 2)
In Embodiment 1 described above, feedforward control is performed until time T4. However, in Embodiment 2, excitation laser current SC or excitation laser light feedback control is further added to this feedforward control. Yes.

  FIG. 5 is a schematic diagram showing the configuration of the pulse fiber laser according to the second embodiment of the present invention. In this pulse fiber laser, the LD current monitor 10 is provided in the pulse fiber laser of the first embodiment, and the control unit 2 performs feedback control of the current value of the excitation laser current SC monitored by the LD current monitor 10 until time T4. . The holding unit 1 holds a correlation between the intensity of the optical pulse SD and the excitation laser current SC. That is, the target value Im of the excitation laser current SC is held.

  FIG. 6 is a timing chart showing optical pulse control of the pulse fiber laser shown in FIG. In FIG. 6, after the time T1 when the input pulse SA is input, the control unit 2 monitors the excitation laser current SC, but since it is not detected after the time T1, the instruction value SB rises to the upper limit value SBmax ( FIG. 6 (b)). However, since the control unit 2 detects the excitation laser current SC from the time point T2, the excitation laser current SC approaches the target value Im in the vicinity of the time point T3, so that the instruction value SB becomes small after the time point T3. As a result, the excitation laser current SC converges to the target value Im without causing a large overshoot. Thereby, the optical pulse SD reaches the target value Pm at time T4 without overshooting.

  In the second embodiment, the feedback control by the excitation laser current SC is further performed in the region R1, and the feedback control by the optical pulse SD is performed in the region R2, so that an optical pulse without overshoot can be output. .

  In FIG. 5, the feedback control by the excitation laser current SC is applied in the region R1, but the present invention is not limited to this, and the feedback control by the excitation laser light may be added in the region R1. That is, as shown in FIG. 7, instead of the LD current monitor 10, an LD light monitor 11 for monitoring the excitation laser light is provided via the coupler 11a, and the excitation laser light detected by the LD light monitor 11 is controlled by the control unit 2. To give feedback. Also in this case, the same control as the feedback control of the excitation laser current SC is performed. The holding unit 1 holds the correlation between the intensity of the optical pulse SD and the excitation laser beam. That is, the target value of the excitation laser beam is held.

  The correlation between the intensity of the optical pulse SD and the pumping laser light changes depending on the aging of the fiber laser resonator and the output photodetector. Therefore, for each input pulse SA, the intensity of the excitation laser beam and the optical pulse SD when the optical pulse reaches the target value Pm is recorded, and the correlation between the intensity of the held optical pulse SD and the excitation laser current SC is recorded. The relationship is updated by the control unit 2.

  By the way, when the time constant τ of the ACC circuit 3 is small, the excitation laser current SC suddenly rises as shown in FIG. As a result, the pump laser light corresponding to the pump laser SC is rapidly input to the fiber laser resonator 5, and the inversion distribution and stimulated emission increase rapidly in the fiber laser resonator 5 to cause a large overshoot. It becomes a pulse.

  Therefore, in the second embodiment, as shown in FIG. 9, when the time constant τ of the ACC circuit 3 is small, the instruction value SB is controlled to gradually increase. As a result, the excitation laser current SC rises rapidly after time T2, but since the instruction value SB is small and then rises slowly, the optical pulse SD reaches the target value Pm without overshooting, as in FIG. To do. The control unit 2 performs PI control or PID control, and the instruction value SB described above can be gradually increased by reducing these control constants. That is, control constants such as proportional gain, integral constant, and differential constant are reduced. Or you may make it the control part 2 lengthen the control period of feedback control.

  Here, at the initial stage of the period t1 shown in FIG. 9, since the value of the control value SB is small, the rise of the excitation laser current SC is delayed, and the period t1 becomes long. This is because the ON threshold value of the FET or operational amplifier in the ACC circuit 3 has not been reached. For this reason, as shown in FIG. 10, when the optical pulse output is off, control is performed to output a minute current less than the oscillation threshold value of the excitation laser beam from the ACC circuit 3 to the excitation laser 4. Make rise faster. Thereby, the period t1 is shortened, and the time from the rise of the input pulse SA to the rise of the optical pulse SD to the target value can be shortened.

  In the first and second embodiments described above, the switching from the region R1 to the region R2 has been described as the time when the intensity of the optical pulse has reached the target value. Alternatively, a predetermined time from the time T1 when the input pulse SA is input to the time T4 when the intensity of the optical pulse reaches the target value Pm may be switched by a timer.

  In the first and second embodiments described above, at time T4, the feedforward control or the feedforward control including the excitation laser current or the feedback control of the excitation laser light is switched to the feedback control of the optical pulse output. This switching time is not limited to time T4, and may be before or after time T4. That is, the predetermined period corresponding to the region R1 may be short or long. In short, feedforward control may be performed within a predetermined period so as not to cause overshoot of the optical pulse output.

DESCRIPTION OF SYMBOLS 1 Holding part 2 Control part 3 ACC circuit 4 Excitation laser 5 Fiber laser resonator 5a Total reflection mirror 5b Output mirror 6, 11a Coupler 7 Output photodetector 10 LD current monitor 11 LD light monitor T1 input terminal T2 output terminal SA input pulse SB indicated value SC excitation laser current SD optical pulse R1 region (predetermined period)

Claims (8)

The pump laser current corresponding to the input pulse waveform is input to the pump laser, the pump light from the pump laser is input to the fiber laser resonator, and the optical pulse corresponding to the input pulse waveform is amplified and output from the fiber laser resonator. In the pulse fiber laser
A current supply for supplying the excitation laser current to the excitation laser;
During a predetermined period corresponding to a delay time from when the input pulse waveform is input to when the output of the optical pulse is started, feedforward control for supplying a predetermined excitation laser current to the excitation laser is performed, and After a predetermined period of time, a control unit that performs feedback control so that the intensity of the optical pulse becomes a target value by adjusting the excitation laser current based on the intensity of the optical pulse;
A pulse fiber laser comprising: A detector that detects an excitation laser current supplied to the excitation laser or an excitation laser beam output from the excitation laser;
2. The pulse fiber according to claim 1, wherein the controller controls the excitation laser current by feeding back the excitation laser current or the excitation laser light in addition to the feedback control within the predetermined period. laser. A holding unit for holding a correlation between the intensity of the optical pulse determined in advance and the excitation laser current;
The control unit performs the feedforward control within the predetermined period so that the intensity of the optical pulse does not overshoot based on the correlation held in the holding unit. Or the pulse fiber laser of 2.   The said control part performs feedforward control which makes small the control constant at the time of the rising of the electric current instruction value to the said current supply part as the time constant of the said current supply part becomes small. The pulse fiber laser according to any one of the above.   The said control part makes a control constant small and / or lengthens a control period until it becomes the target value of the said excitation laser current as the time constant of the said current supply part becomes small. The pulse fiber laser according to any one of the above.   6. The control unit according to claim 1, wherein when the optical pulse output is off, the control unit performs control to output a minute current less than an oscillation threshold value of the excitation laser beam from the current supply unit. Fiber laser described in 1.   The pulse fiber laser according to claim 1, wherein the control unit measures the predetermined period with a timer and switches to feedback control of the optical pulse.   The pulse fiber laser according to claim 1, wherein the holding unit holds the correlation as a table or a function and can be updated.

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