JP2004022795A - Device and method for generating multi-bunch laser beam - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a device which is suitable for the generation of a multi-bunch laser beam having uniform pulse intensity. <P>SOLUTION: A laser light source emits a pulsed laser beam. The laser beam emitted from the light source is made incident to a pulse intensity modulator. The modulator modulates the pulse intensity of the incident pulsed laser beam. The pulsed laser beam modulated by means of the modulator is made incident to an optical amplifier. The amplifier amplifies the incident pulsed laser beam. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an apparatus and a method for generating a multi-bunch laser, and more particularly to an apparatus and a method for generating a multi-bunch laser by amplifying a laser beam including a plurality of pulses with an optical amplifier.
[0002]
[Prior art]
As a method of generating a multi-bunch laser beam in which a plurality of laser pulses are grouped, the following two methods are known.
[0003]
The first method is to form a pulse group by optically combining a plurality of laser pulses. The second method is a method of cutting out a part of pulses from a laser pulse train emitted from a mode-locked laser oscillator to form a pulse group, and then amplifying the pulse group.
[0004]
[Problems to be solved by the invention]
The first method is a simple method when the number of pulses in one group (bunch) is small, but as the number of pulses increases, the optical system for synthesizing laser pulses becomes more complicated. In addition, it is difficult to keep the pulse interval in a pulse group constant or to synchronize with another reference frequency.
[0005]
According to the second method, it is possible to easily increase the number of pulses in one group. Further, since the pulse interval is feedback-controlled by the mode-locked laser oscillator, it can be easily synchronized with another reference frequency. However, in the process of optical amplification, a phenomenon occurs in which the forward pulse consumes the gain (gain) of the optical amplifier, and the backward amplification rate decreases. For this reason, the intensity of the rear pulse in the pulse group is smaller than the intensity of the front pulse. In order to make the pulse intensities uniform, any of the following methods may be employed.
[0006]
The first method is a method of reducing an amplification factor per unit volume in a gain medium to realize a substantially uniform amplification factor on a time axis. In this method, the volume of the gain medium must be increased to achieve the required amplification factor. In order to uniformly excite a large gain medium, a high excitation energy is required, which lowers the overall efficiency.
[0007]
A second method is to obtain a highly stable multi-bunch laser beam by controlling a pulse waveform of a flash lamp for exciting a gain medium. However, if this method is adopted, the excitation circuit of the gain medium becomes complicated.
[0008]
An object of the present invention is to provide a multi-bunch laser generating apparatus and a generating method suitable for generating a multi-bunch laser beam having a uniform pulse intensity.
[0009]
[Means for Solving the Problems]
According to one aspect of the present invention, a laser light source that emits a pulsed laser beam, a pulsed laser beam emitted from the laser light source is incident, and a pulse intensity modulator that modulates the pulse intensity of the incident pulsed laser beam, There is provided a multi-bunch laser generation device including a pulse laser beam modulated by a pulse intensity modulator, and an optical amplifier that amplifies the incident pulse laser beam.
[0010]
According to another aspect of the present invention, there is provided a multi-bunch laser generation method including a step of modulating a pulse intensity of a pulse laser beam, and a step of causing the pulse laser beam having the modulated pulse intensity to enter an optical amplifier and amplify the pulse laser beam. Provided.
[0011]
By modulating the pulse intensity by the pulse intensity modulation, it is possible to cancel the effect of the decrease in the amplification factor due to the gain consumption of the optical amplifier, and obtain a multi-bunch laser beam with a uniform pulse intensity.
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 shows a block diagram of a multi-bunch laser generator according to an embodiment of the present invention. The multi-bunch laser generator according to the embodiment includes a mode-locked laser oscillator 1, a pulse slicer 2, a pulse intensity modulator (pulse modulator) 3, and an optical amplifier 4.
[0013]
The mode-locked laser oscillator 1 is composed of, for example, an Nd: YAG laser, an Nd: YLF laser, and emits a linearly polarized pulse laser beam LB1 having a pulse frequency of 119 MHz in synchronization with a reference frequency of 2856 MHz. The pulse laser beam LB1 emitted from the mode-locked laser oscillator 1 enters the pulse slicer 2.
[0014]
The pulse slicer 2 passes only pulses within a predetermined passage period from the laser pulse train of the pulse laser beam LB1 based on a control signal given from the control device 10, and does not pass pulses other than the passage period. Thus, a multi-bunch laser beam LB2 is obtained. The multi-bunch laser beam LB2 that has passed through the pulse slicer 2 enters the pulse intensity modulator 3. The pulse intensity modulator 3 modulates the pulse intensity based on a control signal provided from the control device 10. Thereby, for example, a multi-bunch laser beam LB3 having a higher intensity as the pulse is more rearward is obtained.
[0015]
The multi-bunch laser beam LB3 modulated by the pulse intensity modulator 3 enters the optical amplifier 4. The optical amplifier 4 includes a gain medium such as Nd: YAG or Nd: YLF and a light source for exciting the gain medium, and amplifies an incident multi-bunch laser beam. The excitation light source is controlled by the control 10. Thus, an amplified multi-bunch laser beam LB4 is obtained.
[0016]
The configuration and operation of the pulse slicer 2 and the pulse intensity modulator 3 will be described with reference to FIG.
[0017]
FIG. 2A is a schematic diagram of the pulse slicer 2 and the pulse intensity modulator 3. The pulse slicer 2 and the pulse intensity modulator 3 include a half-wave plate 20, a Pockels cell 21, a polarizer 22, and a high-voltage power supply 23. The polarization direction of the pulse laser beam is controlled by the half-wave plate 20, and is incident on the Pockels cell 21. The Pockels cell 21 rotates the polarization direction of the pulse laser beam depending on the magnitude of the voltage applied from the high voltage power supply 23. The pulse laser beam that has passed through the Pockels cell 21 enters the polarizer 22 at an incident angle of 45 °.
[0018]
The polarizer 22 transmits a polarized light component (P component) parallel to the incident surface (corresponding to the paper surface in FIG. 2A) of the incident pulse laser beam, and a polarized light component (S component) perpendicular to the incident surface. Is reflected. When the polarization direction is changed by the Pockels cell 21, the intensity ratio between the P component and the S component changes. Therefore, by controlling the polarization direction by the Pockels cell 21, the intensity of the pulse transmitted through the polarizer 22 can be changed.
[0019]
The optical axis direction of the half-wave plate 20 is adjusted so that the pulse laser beam that has passed through the half-wave plate 20 has only the S component. Therefore, when no voltage is applied to the Pockels cell 21 and the rotation angle of the polarization direction is 0 °, the pulse laser beam does not pass through the polarizer 22. When a voltage is applied to the Pockels cell 21 to rotate the polarization direction by 90 °, the pulse laser beam incident on the polarizer 22 has only the P component, and the intensity of the pulse laser beam transmitted through the polarizer 22 is maximized. .
[0020]
FIG. 2B shows an example of the voltage waveform of the high-voltage power supply 23 when the optical device including the half-wave plate 20, the Pockels cell 21, and the polarizer 22 is operated as the pulse slicer 2. During the period from time t ₁ to t _2, a constant voltage is applied, the other periods, no voltage is applied. Voltage from the time t ₁ is applied to the period t ₂ is sized to the Pockels cell 21 is exactly 90 ° turning the polarization direction. At this time, from time t ₁ period t ₂ only (passing time), the pulse laser beam passes through the polarizer 22.
[0021]
FIG. 2C shows an example of the voltage waveform of the high-voltage power supply 23 when the optical device including the half-wave plate 20, the Pockels cell 21, and the polarizer 22 is operated as the pulse intensity modulator 3. Voltage, than the time t ₀ starts to increase from the previous time point linearly, with triangular shape back to 0V at the time when after time t _1. Voltage at the time point of time t ₁ is controlled to a magnitude that is approximately 90 ° turning the polarization direction.
[0022]
Turning angle of the polarization direction at at time t ₀ is less than 90 ° greater than 0 °. Since the voltage increases with the passage of time, the turning angle gradually increases. Turning angle is 90 ° at time t _1. For this reason, the amount of attenuation by the optical device shown in FIG. That is, the pulse intensity of the pulsed laser beam passing through the polarizer 22 gradually increases with time.
[0023]
Returning to FIG. 1, the description will be continued. The gain medium of the optical amplifier 4 is pumped by the pumping light at a period of, for example, about 200 μs. The time constant of the decrease in the amplification factor is about 200 μs. The interval (bunch width) from the first pulse to the last pulse of the multi-bunch laser beam LB2 cut out by the pulse slicer 2 is about 1 μs. As described above, the time required for the multi-bunch laser beam LB3 to pass through the optical amplifier 4 is sufficiently shorter than the time constant for decreasing the amplification factor.
[0024]
The gain medium of the optical amplifier 4 is excited immediately before the multi-bunch laser beam LB3 modulated by the pulse intensity modulator 3 enters, and is not excited during the period when the multi-bunch laser beam LB3 passes. Therefore, when the pulse of the multi-bunch laser beam LB3 is amplified, the gain of the gain medium is consumed, and the amplification factor of the optical amplifier 4 decreases.
[0025]
In the above embodiment, the intensity of the pulse after the multi-bunch laser beam LB3 incident on the optical amplifier 4 is higher. That is, the amplification factor of a pulse having a high intensity is smaller than the amplification factor of a pulse having a low intensity. By adjusting the degree of increase in the pulse intensity of the multi-bunch laser beam LB3, the pulse intensity of the multi-bunch laser beam LB4 amplified by the optical amplifier 4 can be made uniform.
[0026]
Amplification under conditions that the number of pulses is 100, the pulse width is 3 to 10 ps, the pulse frequency is 119 MHz, the pulse energy is 1 nJ before pulse intensity modulation, the pulse energy is 750 μJ after amplification, and the pulse intensity fluctuation width of the amplified multi-bunch laser beam is 10% or less. In the method according to the above embodiment, a calculation result was obtained that the volume of the gain medium of the optical amplifier 4 should be 0.3 cm ³ and the intensity of the pump light should be 0.6 J.
[0027]
FIG. 3A shows a waveform of the multi-bunch laser beam before amplification, and FIG. 3B shows a waveform of the multi-bunch laser beam after amplification. The pulse intensity of the multi-bunch laser beam before amplification ideally increases exponentially as the number of pulses increases (with time).
[0028]
On the other hand, when trying to amplify under the same conditions using the conventional method, a calculation result was obtained that the volume of the gain medium had to be 3.6 cm ³ and the intensity of the pump light had to be 7.2 J. As described above, by performing amplification by the method according to the above embodiment, the gain medium can be reduced and the intensity of the pump light can be reduced.
[0029]
In the above-described embodiment, the voltage applied to the Pockels cell 21 constituting the pulse intensity modulator 3 is linearly increased. However, the waveform may be other monotonically increasing waveform. A suitable voltage waveform is selected depending on the rate of decrease in gain due to the passage of the laser pulse through the optical amplifier 4. For example, the difference between the maximum value and the minimum value of the pulse intensity of the pulse laser beam LB4 amplified by the optical amplifier 4 is determined when the pulse laser beam not modulated by the pulse intensity modulator 3 enters the optical amplifier 4. The pulse intensity may be modulated so as to be smaller than the difference between the maximum value and the minimum value of the pulse intensity of the subsequent pulsed laser beam. In order to perform such modulation, an exponentially increasing waveform may be preferred.
[0030]
In the above embodiment, one pulse slicer 2 and one pulse intensity modulator 3 are arranged as shown in FIG. 1, but the voltage waveform applied to the Pockels cell 21 shown in FIG. 2A is controlled. Accordingly, the functions of the pulse slicer 2 and the pulse intensity modulator 3 can be realized by an optical device including one Pockels cell.
[0031]
FIG. 2D shows an example of a voltage waveform when the functions of the pulse slicer 2 and the pulse intensity modulator 3 are realized by one Pockels cell. At time t ₀ , the voltage rises sharply, and between time t ₀ and t ₁ , the voltage increases monotonically, for example, linearly. At time t _1, the voltage falls rapidly. The rise and fall times of the voltage at times t ₀ and t ₁ are shorter than the period of the pulse of the pulse laser beam LB 1 emitted from the mode-locked laser oscillator 1.
[0032]
Thus, the voltage applied to the periods other than between time t ₀ of t ₁ and 0V, the period from time t ₀ to t _1, the waveform obtained by superimposing a DC component in the waveform of the voltage increases monotonously Thereby, two functions of the pulse slicer 2 and the pulse intensity modulator 3 can be realized by one optical device including the Pockels cell 21 shown in FIG.
[0033]
FIG. 4 shows a block diagram of an X-ray generator using the multi-bunch laser beam generator according to the above embodiment. The multi-bunch laser beam emitted from the multi-bunch laser generator 30 according to the above embodiment enters the wavelength conversion element 31. The wavelength conversion element 31 generates a fourth harmonic of the incident multi-bunch laser beam. The fourth harmonic enters the photocathode of the electron gun 32. Photoelectrons emitted from the photocathode are accelerated by the accelerating electric field in the electron gun 32, and a pulsed electron beam is emitted.
[0034]
The pulsed electron beam emitted from the electron gun 32 is accelerated by the linac 33. The pulsed laser beam emitted from the pulsed laser oscillator collides with the pulsed electron beam accelerated by the linac 33. This collision generates X-rays due to the inverse Compton scattering process. The multi-bunch laser generator 30, the electron gun 32, the linac 33, and the pulse laser oscillator 34 are synchronized by a controller 35.
[0035]
Since the pulse intensity of the multi-bunch laser emitted from the multi-bunch laser generator 30 is uniform, the electron density of each pulse of the pulsed electron beam emitted from the electron gun 32 can be made uniform.
[0036]
Further, the multi-bunch laser generator 30, the wavelength conversion element 31, and the electron gun 32 shown in FIG. 4 can also be used as a pulsed electron beam source of the free electron laser generator.
[0037]
Further, the multi-bunch laser generator according to the above embodiment can be used as a laser light source for a laser drill. By changing the number of pulses in one group or the shape of the envelope connecting the peaks of the pulse intensities, the shape of the hole to be machined can be controlled. Furthermore, high temporal resolution can be obtained by using the multi-bunch laser generator according to the above-described embodiment as a light source for laser measurement by the pump probe method.
[0038]
Although the present invention has been described with reference to the embodiments, the present invention is not limited thereto. For example, it will be apparent to those skilled in the art that various modifications, improvements, combinations, and the like can be made.
[0039]
【The invention's effect】
As described above, according to the present invention, the pulse intensity of the pulse laser beam after amplification is modulated by modulating the pulse intensity of the pulse laser beam incident on the optical amplifier in response to the decrease in the gain of the optical amplifier. The strength can be as large as desired.
[Brief description of the drawings]
FIG. 1 is a block diagram of a multi-bunch laser generator according to an embodiment.
FIG. 2 is a schematic diagram of a pulse slicer and a pulse intensity modulator, and a graph showing an example of a voltage waveform applied to a Pockels cell.
FIG. 3 is a graph showing a multi-bunch laser beam waveform before amplification and a multi-bunch laser beam waveform after amplification in a multi-bunch laser generator according to an embodiment.
FIG. 4 is a block diagram of an X-ray generator using the multi-bunch laser generator according to the embodiment.
[Explanation of symbols]
REFERENCE SIGNS LIST 1 mode-locked laser oscillator 2 pulse slicer 3 pulse intensity modulator 4 optical amplifier 10 controller 20 波長 wavelength plate 21 Pockels cell 22 polarizer 23 high-voltage power supply 30 multi-bunch laser generator 31 wavelength conversion element 32 electron gun 33 linac 34 Pulse laser oscillator 35 Controller

Claims (8)

A laser light source for emitting a pulsed laser beam,
A pulsed laser beam emitted from the laser light source is incident, a pulse intensity modulator that modulates the pulse intensity of the incident pulsed laser beam,
A multi-bunch laser generator comprising: an optical amplifier that receives a pulse laser beam modulated by the pulse intensity modulator and amplifies the incident pulse laser beam. 2. The multi-bunch laser generator according to claim 1, wherein the pulse intensity modulator reduces an attenuation amount of the pulse intensity over time. 3. Further, the pulsed laser beam emitted from the laser light source is disposed between the laser light source and the pulse intensity modulator. The multi-bunch laser generator according to claim 1 or 2, further comprising a pulse slicer for entering the modulator. The difference between the maximum value and the minimum value of the pulse intensity of the pulsed laser beam amplified by the optical amplifier, after amplification when the pulsed laser beam not modulated by the pulse intensity modulator is incident on the optical amplifier. The multi-bunch laser generator according to claim 1, wherein the pulse intensity modulator modulates the pulse intensity so that the pulse intensity becomes smaller than a difference between a maximum value and a minimum value of the pulse intensity of the pulse laser beam. Modulating the pulse intensity of the pulsed laser beam;
Amplifying a pulse laser beam having a modulated pulse intensity by inputting the pulse laser beam into an optical amplifier. 6. The multi-bunch laser generation method according to claim 5, wherein in the step of modulating the pulse intensity, the amount of attenuation of the pulse intensity is reduced with time. Before the step of modulating the pulse intensity of the pulsed laser beam, further, from a plurality of laser pulses of the pulsed laser beam, has a step of cutting out a plurality of pulses only for a certain passing period, in the step of modulating the pulse intensity 7. The multi-bunch laser generation method according to claim 5, wherein the intensity of the plurality of cut out pulses is modulated. The difference between the maximum value and the minimum value of the pulse intensity of the pulse laser beam amplified by the optical amplifier is the maximum of the pulse intensity of the amplified pulse laser beam when a pulse laser beam whose pulse intensity is not modulated is incident. The method according to claim 5, wherein the pulse intensity is modulated so as to be smaller than a difference between the value and the minimum value.

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