JP2016065948A - Optical pulse waveform shaping device and optical pulse waveform shaping method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical pulse waveform shaping device that can be downsized by a simple configuration and also can get a time waveform of an optical pulse into arbitrary shape, and provide an optical pulse waveform shaping method.SOLUTION: An optical pulse waveform shaping device comprises: expansion and compression parts (16 and 18) that separate an optical pulse into multiple optical pulses having different wavelength components and then spatially expand and compress them; and a spatial modulation part (14) that modulates an amplitude of the optical pulse with a modulation ratio corresponding to an incident position of the entering optical pulse and then outputs it. After using the expansion and compression parts (16 and 18) to expand an optical pulse (L) to be shaped which enters from a pulse light source, the device applies amplitude modulation to the respective optical pulses by moving them to and from the spatial modulation part (14). Then, the device gives different delay time to the optical pulses by using the expansion and compression parts (16 and 18) to compress the amplitude-modulated optical pulses, and outputs them as a shaped optical pulse (L).SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical pulse waveform shaping device and an optical pulse waveform shaping method.

  As a method for shaping the temporal waveform of an optical pulse, a method is known in which light having a wide spectral line width is separated into a plurality of wavelengths using a wavelength dispersion device, and a delay for each of the plurality of wavelengths generated at that time is used. . In principle, this method is the same as the method based on the optical pulse stretching method.

  Non-Patent Document 1 discloses an example of an optical pulse waveform shaping device using the optical pulse expansion method. With reference to FIG. 7, the optical pulse waveform shaping device 100 disclosed in Non-Patent Document 1 will be described. As shown in FIG. 7, the optical pulse waveform shaping device 100 includes a MEMS-MMA (micro electro mechanical system-micro mirror array) 102, a grating (diffraction grating) 104, and a lens 106.

  The input optical pulse (Input Pulse) incident on the optical pulse waveform shaping device 100 is spatially frequency (wavelength) separated by the grating 104, and the frequency separated light is incident on the MEMS-MMA 102 via the lens 106. The MEMS-MMA is a micro mirror array using MEMS, and each of the aluminum mirrors arranged in an array is configured to be driven in the optical axis direction of incident light by MEMS technology.

  The MEMS-MMA 102 changes the optical length for each frequency by reflecting each spatially frequency-separated light by changing the position for each frequency. The light whose optical length is changed for each frequency is output as an output light pulse (Output Pulse) through the lens 106 and the grating 104 again. The output optical pulse has a waveform obtained by synthesizing optical pulses having different frequencies and different delay times, and the waveform of the output optical pulse becomes an envelope of the synthesized optical pulses having different frequencies. The time waveform of the pulse is shaped.

  Further, as another example of the optical pulse waveform shaping device using the optical pulse expansion method, one disclosed in Non-Patent Document 2 is known. The optical pulse waveform shaping device according to Non-Patent Document 2 includes two gratings and an optical member including a lens, a mask, and a lens disposed between the two gratings. The grating, the lens, and the mask are arranged at intervals of the focal length of the lens. A pattern is formed on the mask so that the transmittance varies depending on the position on the mask surface, and the mask acts as a spatial modulator for modulating the amplitude.

  The input light pulse is spatially separated into a plurality of wavelengths by one grating and enters the mask through the lens. The separated wavelengths can be changed in amplitude (amplitude modulated) for each of the plurality of wavelengths by passing through a mask having different transmittance for each of the plurality of wavelengths. After passing through the mask, the light enters the other grating via the lens, and the separated light beams having a plurality of wavelengths are combined and output as an output light pulse. The time waveform of the output light pulse is obtained by shaping the time waveform of the input light pulse for the same reason as the light pulse waveform shaping device 100 of Non-Patent Document 1.

  Furthermore, as another example of the optical pulse waveform shaping device using the optical pulse expansion method, one disclosed in Patent Document 1 is also known. The optical pulse waveform shaping device disclosed in Patent Document 1 includes a pulse generation device and an optical pulse width conversion device, and the optical pulse width conversion device includes a half mirror, a diffraction grating pair, an amplitude adjustment unit, and a reflection mirror. It is configured.

  The input light pulse from the pulse generation device input through the half mirror is given wavelength dispersion by the diffraction grating pair and is spatially separated for each wavelength. The light of each wavelength that is spatially separated and given a different delay time is given a different amplitude change for each wavelength by the amplitude adjusting unit, then reflected by the reflecting mirror, and reversely follows the optical path up to that time. The light of each wavelength synthesized by the diffraction grating pair is reflected by the half mirror and output as an output light pulse. The time waveform of the output light pulse is obtained by shaping the time waveform of the input light pulse for the same reason as the light pulse waveform shaping device 100 of Non-Patent Document 1.

JP 2007-334225 A

Tomoaki Abe, Hiroki Wang, Hiroki Yazawa, “Phase and Amplitude Waveform Shaping of 400nm Femtosecond Laser Pulse Using Two-Dimensional MEMS Spatial Light Modulator”, Keio University Annual Report (2008) A.M.Weiner, J.P.Heritage, and E.M.Kirschner, "High-resolution femtosecond pulse shaping", J.Opt Soc. Am. B, 5, 8 (1988) 1563-1572

  Any of the above-described conventional optical pulse waveform shaping devices can obtain an output optical pulse obtained by shaping the waveform of the input optical pulse by the optical pulse expansion method. However, since the optical pulse waveform shaping device disclosed in Non-Patent Document 1 modulates the amplitude by one reflection by MEMS-MMA, there is a limit to the shaping amount that can be naturally applied.

  On the other hand, since the optical pulse waveform shaping device disclosed in Non-Patent Document 2 has separate gratings for dispersing optical pulses and synthesized gratings, it is necessary to use two expensive gratings, which is disadvantageous in terms of cost. In addition, since the number of optical parts to be configured is large, there are problems in terms of stability and reproducibility. In addition, the entire optical system is large because it passes through each optical element once (does not reciprocate), and the input direction of the input light pulse is different from the output direction of the output light pulse, which is inconvenient in handling. It is.

  Furthermore, since the optical pulse waveform shaping device disclosed in Patent Document 1 also uses two gratings, it is disadvantageous in terms of cost. In addition, since the number of optical parts to be configured is large, there are problems in terms of stability and reproducibility, and it is difficult to downsize the entire optical system.

  The present invention has been made to solve the above-described problems, and can be miniaturized with a simple configuration, and can further shape an optical pulse time waveform into an arbitrary shape. And it aims at providing the optical pulse waveform shaping method.

  In order to achieve the above object, an optical pulse waveform shaping device according to claim 1, wherein an optical pulse waveform shaping device separates an optical pulse into a plurality of optical pulses having different wavelength components and spatially expands or compresses, and an incident A spatial modulation unit that modulates the amplitude of the optical pulse with a modulation factor according to the incident position and emits the optical pulse, and the shaped light pulse incident from the pulse light source is expanded by the expansion / compression unit Thereafter, the spatial modulation unit is reciprocated to perform amplitude modulation for each of the plurality of optical pulses, and the plurality of optical pulses subjected to amplitude modulation are compressed by the expansion / compression unit, so that each of the plurality of optical pulses is different. A delay time is given and emitted as a shaped light pulse.

  According to a second aspect of the present invention, in the first aspect of the invention, the expansion / compression unit includes a diffraction grating that separates an incident light pulse into a plurality of light pulses having different wavelength components, and incident light. A folded portion that reflects the pulse and changes the optical axis and folds back in the incident direction, and diffracts the incident optical pulse by the diffraction grating and separates it into a plurality of optical pulses. The optical pulse is spatially extended by being diffracted by the diffraction grating again after being folded at the folding portion, and after the spatially stretched optical pulse is diffracted by the diffraction grating and folded at the folding portion By differently diffracting and compressing with the diffraction grating, different delay times are given for each of the plurality of optical pulses.

  The invention according to claim 3 is the invention according to claim 2, wherein the folded portion includes a corner cube or a reflecting mirror.

  According to a fourth aspect of the present invention, in the invention according to any one of the first to third aspects of the present invention, the spatial modulation unit has a transmittance according to an incident position of each of the plurality of light pulses. Includes different filter units, and transmits the plurality of incident optical pulses through the filter unit to modulate the amplitude for each of the plurality of optical pulses.

  Further, the invention according to claim 5 is the invention according to any one of claims 1 to 4, wherein the optical axis of the shaped light pulse to be incident and the optical axis of the shaped light pulse to be emitted are Are spatially separated.

  According to a sixth aspect of the present invention, in the fifth aspect of the present invention, the reflecting unit according to the fifth aspect further comprises a reflecting unit that reflects the stretched shaped light pulse emitted from the spatial modulation unit and enters the spatial modulation unit again. In addition.

  According to a seventh aspect of the invention, in the sixth aspect of the invention, the optical axis of the shaped light pulse that is incident and the light of the shaped light pulse that is emitted by adjusting the reflection angle of the reflecting portion. The axis is spatially separated.

  On the other hand, in order to achieve the above object, an optical pulse waveform shaping method according to claim 8 includes: an extension / compression unit that spatially extends or compresses an optical pulse by separating the optical pulse into a plurality of optical pulses having different wavelength components; An optical pulse waveform shaping method in an optical pulse waveform shaping device, comprising: a spatial modulation unit that modulates an amplitude of an optical pulse with a modulation factor according to an incident position and emits the optical pulse; The shaped light pulse incident from the light is expanded by the expansion / compression unit, the expanded shaped light pulse is reciprocated through the spatial modulation unit, and amplitude-modulated for each of the plurality of light pulses. The plurality of optical pulses are compressed by the expansion / compression unit, and a different delay time is given to each of the plurality of optical pulses to be emitted as a shaped light pulse.

  According to the present invention, it is possible to provide an optical pulse waveform shaping device and an optical pulse waveform shaping method that can be downsized with a simple configuration and that can shape the time waveform of an optical pulse into an arbitrary shape. There is an effect.

It is a top view which shows an example of a structure of the optical pulse waveform shaping apparatus which concerns on 1st Embodiment. It is a side view showing an example of composition of an optical pulse waveform shaping device concerning a 1st embodiment. It is a figure explaining the optical path of each wavelength component wavelength-separated with the diffraction grating of the optical pulse waveform shaping apparatus which concerns on 1st Embodiment. It is a figure which shows an example of the time waveform of the incident light pulse of the optical pulse waveform shaping apparatus which concerns on 1st Embodiment, an expansion | extension light pulse, and an emitted light pulse. It is a figure for demonstrating the optical path length difference of the long wavelength component and short wavelength component of the optical pulse waveform shaping apparatus which concerns on 1st Embodiment. It is a top view which shows an example of a structure of the optical pulse waveform shaping apparatus which concerns on 2nd Embodiment. It is a figure which shows the structure of the optical pulse waveform shaping apparatus which concerns on a prior art.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The application of the optical pulse waveform shaping device according to the present embodiment is not particularly limited, but is used as an example for laser processing using an optical pulse, for example, the waveform of an irradiation light pulse for the purpose of efficient laser processing This is used when shaping

[First Embodiment]
With reference to FIG. 1 thru | or FIG. 5, the structure of the optical pulse waveform shaping apparatus 10 which concerns on this Embodiment is demonstrated. FIG. 1 is a diagram (plan view) showing a configuration of the optical pulse waveform shaping device 10 as viewed from above, and FIG. 2 is a diagram showing a configuration of the optical pulse waveform shaping device 10 shown in FIG. FIG. 3 is a diagram for explaining the optical path of each wavelength component wavelength-separated by the diffraction grating of the optical pulse waveform shaping device 10, and FIG. 4 is the incident light of the optical pulse waveform shaping device 10. FIG. 5 is a diagram illustrating time waveforms of a pulse, an extended light pulse, and an emitted light pulse, and FIG. 5 is a diagram illustrating an optical path length difference between a long wavelength component and a short wavelength component of the optical pulse waveform shaping device 10.

As shown in FIGS. 1 and 2, the optical pulse waveform shaping device 10 includes a reflection mirror 12, a spatial modulator 14, a diffraction grating 16, and a corner cube 18. In the optical pulse waveform shaping device 10, as an example, a broadband ultrashort pulse light (for example, about a pulse width of several ps (picosecond)) generated by an optical pulse generator (not shown) (hereinafter sometimes referred to as “light source”). but is incident as the incident light beam L _in the right direction in the drawing to the left, the light pulse waveform is shaped by the optical pulse waveform shaping apparatus 10 is emitted as an outgoing light beam L _out from the left direction in the drawing to the right . In the present embodiment, the shape of the cross section B _in the incident light beam L _in, as an example, have a substantially circular as shown in FIG. 3 (a).

First, an outline of the operation of the optical pulse waveform shaping device 10 will be described with reference to FIGS. 1 and 2. The incident light beam Lin that is incident in the left direction from the right direction on the paper is wavelength-separated by the diffraction _in the diffraction grating 16 and is incident on the corner cube 18 with the beam width expanded. Reflected by the corner cube 18 and folded, the beam width is further expanded and incident on the diffraction grating 16 again. The light beam diffracted by the diffraction grating 16 is diffracted _in the direction of the incident light beam Lin, passes through the spatial modulator 14, is reflected by the reflection mirror 12, and is transmitted through the spatial modulator 14 again to undergo amplitude modulation. (The amplitude of the light pulse is changed for each separated wavelength).

Amplitude modulated light beam again follows the light path the incident light beam L _in the traced incident on the diffraction grating 16. The light beam diffracted by the diffraction grating 16 and reduced in beam width is reflected and folded by the corner cube 18 to further reduce the beam width, and then is diffracted again by the diffraction grating 16 to output the outgoing light beam L _out. Is emitted as Each wavelength component contained in the output light beam L _out, at the same time the modulation factor in the spatial modulator 14 is given different amplitudes by different, since the optical path length difference is given a different delay time by different, out light pulses contained in Shako beam L _out, to the optical pulses contained in the incident light beam L _in, is shaped in the time axis direction. When the optical pulse waveform shaping device 10 is, for example, an optical pulse waveform shaping device used for laser processing, the emitted light beam L _{out that} is emitted is irradiated onto a workpiece.

As described above, the optical pulse waveform shaping apparatus 10 according to the present embodiment is 2 by reciprocating the incident light beam L _in (diffracts four times the diffraction grating 16) in that the waveform shaping, one of the features There is. Hereinafter, the operation of the optical pulse waveform shaping device 10 according to the present embodiment will be described in more detail.

The incident light beam L _in is incident on the diffraction grating 16 of the reflection type is diffracted and separated into a plurality of wavelengths (the variance). At that time, the diffraction angles (see FIG. 3B) are different for each of a plurality of wavelengths, and the shorter the wavelength, the smaller the diffraction angle defined in FIG. 3B, so that each wavelength diffracted by the diffraction grating 16 is reduced. The light is spatially separated.

With reference to FIG. 3, the diffraction in the diffraction grating 16 is demonstrated in detail. In FIG. 3, the incident light beam _{L in} is incident at point _{P 1} on the surface of the diffraction grating 16. Since the grooves S of the diffraction grating 16 according to the present embodiment are formed in the X direction as shown in FIG. 2, each spatially separated wavelength is expressed in the Z direction (Y− In the Z plane). In the following description, among the wavelength components diffracted by the diffraction grating 16, the longest wavelength component λ _{R is} the longest wavelength component and the short wavelength component λ _B is the shortest wavelength component.

As shown in FIG. 3B, each wavelength component included _in the incident light beam Lin is wavelength-separated according to the formation state of the groove S in the diffraction grating 16, and the short wavelength component λ _{B is} converted from the long wavelength component λ _R. Line up in the Z direction. A plurality of wavelength components having different wavelengths exist between the long wavelength component λ _R and the short wavelength component λ _B, and the number of wavelength components depends on the way in which the grooves S are formed in the diffraction grating 16.

Here, the diffraction angle of the incident light beam L _in incident to the point P _1, the normal h at a point P ₁ of the diffraction grating 16 is defined by an angle measured to the optical axis of each wavelength component which is diffracted. FIG. 3B shows the diffraction angle θ _D of the center wavelength λ of the light source, the diffraction angle θ _DB of the short wavelength component λ _B , and the diffraction angle θ _DR of the long wavelength component λ _R according to this definition. . Due to the characteristics of the diffraction grating 16, the diffraction angle θ _DB of the short wavelength component λ _B is smaller than the diffraction angle θ _DR of the long wavelength component λ _R. As described above, each wavelength component included _in the incident light beam Lin is spatially separated by the diffraction in the diffraction grating 16.

As shown in FIGS. 1 and 2, each wavelength component spatially separated by the diffraction grating 16 is reflected by the corner cube 18 and folded. At this time, the light beam, which is the sum of the wavelength components, is further spatially expanded and incident on the diffraction grating 16, and is diffracted by the diffraction grating 16 _in the direction of the incident light beam Lin. Since the optical path traced to this point differs depending on each wavelength component, each wavelength component diffracted by the diffraction grating has a different delay time. For this reason, the light beam diffracted by the diffraction grating 16 is spatially expanded, and at the same time, each wavelength component included in the light beam is also expanded in the time axis direction.

Referring now to FIG. 3, is diffracted by the incident light beam L _in the diffraction grating 16, it is folded back at corner cube 18, to be diffracted by the diffraction grating 16 again, long wavelength components lambda _R and short wavelength The optical path of the component λ _B will be described.

The long wavelength component λ _R diffracted at the point P ₁ of the diffraction grating 16 is reflected at the point P ₂ of the corner cube 18 and proceeds to the point P ₃ . The long wavelength component λ _R reflected at the point P ₃ of the corner cube 18 is diffracted at the point P ₅ of the diffraction grating 16 and proceeds to the point P ₇ of the spatial modulator 14.

On the other hand, the short wavelength component λ _B diffracted at the point P ₁ of the diffraction grating 16 is reflected at the point P ₃ of the corner cube 18 and proceeds to the point P ₂ . The short wavelength component λ _B reflected at the point P ₂ of the corner cube 18 is diffracted at the point P ₄ of the diffraction grating 16 and proceeds to the point P ₆ of the spatial modulator 14.

As described above, the light beam _{in which} the beam diameter of the incident light beam Lin is spatially expanded to a predetermined length W _p (length from the point P ₆ to the point P ₇ ) is transmitted in the spatial modulator 14. Amplitude modulation is applied to change the amplitude of the optical pulse for each wavelength component. The cross section _Bout of the light beam after passing through the spatial modulator 14 has an elliptical shape having a major axis in the Z direction, as shown in FIG.
Therefore, the _{W p} represents the length of the major axis of the _{B out} of the elliptical shape.

The spatial modulator 14 according to the present embodiment is a kind of a spatial filter (optical filter) having a different transmittance depending on the position on the incident surface. For example, on the transparent glass substrate, the thickness of the spatial modulator 14 depends on the position. A thin film (for example, a metal thin film) whose transmittance is changed by varying the thickness is formed.
A position corresponding to each transmittance of the spatial modulator 14 corresponds to a spatial position of each wavelength component separated by the diffraction grating 16.

  Each wavelength component included in the light beam is incident on the reflection mirror 12 and totally reflected after the spatial modulator 14 has changed the amplitude of the optical pulse to a predetermined amplitude (amplitude modulated). In the present embodiment, an example in which the light is totally reflected by the reflection mirror 12 will be described as an example. However, the present invention is not limited to this, and if it is desired to give a loss to the light beam, partial reflection may be used. Good.

Next, the optical path of each wavelength component after being reflected by the reflection mirror 12 will be described. The light of each wavelength component reflected by the reflection mirror 12 again passes through the range from the point P ₆ to the point P ₇ of the spatial modulator 14 and proceeds toward the diffraction grating 16. At this time, the light of each wavelength component is again modulated by the spatial modulator 14. As described above, in the present embodiment, since the spatial modulator 14 receives the amplitude modulation twice, the modulation rate (attenuation rate) for each wavelength component is higher than that of the prior art.

The long wavelength component λ _R emitted from the spatial modulator 14 is diffracted at the point P ₅ of the diffraction grating 16 and travels toward the point P ₃ of the corner cube 18. The long wavelength component λ _R reflected at the point P ₃ is further reflected at the point P ₂ and turned back, and goes toward the point P ₁ of the diffraction grating 16.

On the other hand, the short wavelength component λ _B emitted from the spatial modulator 14 is diffracted at the point P ₄ of the diffraction grating 16 and travels toward the point P ₂ of the corner cube 18. The short wavelength component λ _B reflected at the point P ₂ is further reflected at the point P ₃ and turned back, and is directed to the point P ₁ of the diffraction grating 16 in the same manner as the long wavelength component λ _R.

The incident light beam L _{in which} has been spatially extended to the length W _p is spatially compressed again through the above path, and is emitted as the outgoing light beam L _out . Outgoing light beam L _out is thus to spatially compressed, becomes an incident light beam L _in the same substantially circular beam shape, the optical pulse of each wavelength component are different each delay time, the time axis When viewed above, it is stretched.

Referring now to FIG. 2, the incident light beam L _in and the outgoing beam L _out, the positional relationship with respect to the optical system will be described. In FIG. 2, the spatial modulator 14 is not shown in order to avoid complexity. In the present embodiment, _in order to avoid interference between the incident light beam Lin and the outgoing light beam _Lout , the angle of the reflection mirror 12 is slightly inclined with respect to the vertical direction.

As shown in FIG. 2, the incident light beam _{L in} is incident at point _{P 1} of the direct diffraction grating 16. Diffracted by the diffraction grating 16 and spatially compressed by the above-described path of the corner cube 18 → the diffraction grating 16 → the spatial modulator 14 → the reflection mirror 12 → the spatial modulator 14 → the diffraction grating 16 → the corner cube 18 → the diffraction grating 16. The light beam thus focused is again focused on the point P ₁ ′ on the surface of the diffraction grating 16. In the above description with reference to FIG. 3, for ease of understanding, been described as focusing the light beam is also the point P ₁ which is spatially compressed, interference between the incident light beam L _in the outgoing light beam L _out to avoid, in practice by adjusting the angle with respect to the vertical direction of the reflecting mirror 12, and is focused at different points P _{1 'and} P _1.

The light beam focused on the point P ₁ ′ is reflected by the diffraction grating 16 and is emitted as the outgoing light beam L _out . Thus, the optical pulse waveform shaping apparatus 10 according to the present embodiment, by adjusting the angle of the reflecting mirror 12, spatial cross as the incident light beam L _in the outgoing light beam L _out does not interfere The position (position in the vertical plane) is shifted. In this embodiment, the lower side in the vertical direction of the incident light beam L _in, is exemplified a mode of placing the outgoing light beam L _out on the upper side in the vertical direction, the arrangement relationship may be reversed .

Next, the operation of the optical pulse waveform shaping device 10 according to the present embodiment configured as described above will be described with reference to FIG. 4 (a) is the time waveform of the incident light beam L _in the incident light pulse P _in an optical pulses contained, FIG. 4 (b), spatial, after being temporally extended, the spatial modulator 14 time waveform before the light beam is a light pulse included in the elongated light pulse P _S incident on, Fig. 4 (c), the time waveform of the output optical pulse P _out is the optical pulses contained in the output light beam L _out Respectively.

Incident light pulse P _in the present embodiment shown in FIG. 4 (a) is a broadband ultrashort pulses as previously described, several ps, band one example as an example the time width t _in the incident light pulse P _in Is about several nm (nanometer). Here, in this embodiment, the time width of the optical pulse is defined by FWHM (Full Width at Half Maximum).

As shown in FIG. 4 (b), the elongated light pulse P _S incident light pulse P _in is extended in the time axis direction, a light pulse with a pulse width time width t _w, is separated by the diffraction grating 16 In addition, the optical pulses of the respective wavelength components are arranged from the short wavelength component λ _B to the long wavelength component λ _R with different delay times (different times). Time waveform of the elongated light pulse P _S is the envelope of the optical pulses of respective wavelength components (envelope). Time width _{t w} of the elongated light pulse _{P S} is the number 10ps as an example. In the optical pulse waveform shaping device 10 according to the present embodiment, the short wavelength component λ _B is arranged at a position earlier in time than the long wavelength component λ _{R in} terms of the method.

As shown in FIG. 4 (c), the time waveform of the output optical pulse P _out, shaping the time waveform of the elongated light pulse P _S, a waveform having different pulse shapes. The spatial modulator 14 according to the present embodiment increases the transmittance of a short-wavelength optical pulse arranged at the head portion on the time axis, and decreases the transmittance of a long-wave long light pulse that follows on the time axis. ing. The shape of the emitted light pulse P _out, since the envelope of the optical pulse of each wavelength component, the pulse waveform of the output optical pulse P _out has a portion having a high peak value, which is placed at the head region, high the peak value It has a shape that is combined with a portion that maintains a low peak value following the portion.

The shape of the outgoing light pulse _Pout shown in FIG. 4C is an example, and by adjusting the distribution condition of transmittance in the spatial modulator 14, the condition of wavelength dispersion by the groove forming method in the diffraction grating 16, and the like, It can be shaped into any shape.

Next, referring to FIG. 5, a description will be given of a method of calculating the time width t _w of the elongated light pulse P _S. FIG. 5A is a plan view of the optical pulse waveform shaping device 10 according to the same embodiment as FIG. 1, and FIG. 5B is a diagram illustrating a portion of the diffraction grating 16 in FIG. the optical axis of the incident light beam L _in, is a diagram showing along with the optical axis of the long wavelength components lambda _R contained in the elongated light pulse P _S. FIG. 5 (b), the incident light beam L _in diffracted by the point P ₁ of the diffraction grating 16 after being folded back is reflected by the corner cube 18, it is diffracted by the diffraction grating 16 again beam diameter spatially The state until it is expanded is shown.

With reference to FIG. 5B, a method of calculating the optical path length difference of the long wavelength component λ _R among the wavelength components separated by the diffraction grating 16 will be described. δL is the difference in optical path length of the central wavelength lambda and a long wavelength component lambda _R of the spectrum of the light source relative to the point P _1. δL is one difference in optical path length due to the passage of, so two passes in this embodiment, the difference of the optical path length to the emitted light beams L _out is emitted from the incident light beam L _in the incident 2δL. Further, since the same manner the difference of the optical path length of 2δL also shorter wavelength component lambda _B occurs, the optical path length difference which is a difference in optical path length of the entire separated wavelength components becomes 4DerutaL. Time width _{t w} of the elongated light pulse _{P S,} since obtained by dividing the optical path length difference at the speed of _light, and _t w = _4δL / c.

And notations are defined as follows, with reference to FIG. 5, described in more detail the method of calculating the time width t _w of the elongated light pulse P _S.

Linear _{L 1:} the incident light beam _{L in} the optical path linearly _{L 2:} optical path length lines wavelength component lambda _R folded back by corner cube 18 _{L 3:} linear _{L 2} long wavelength components followed the lambda _R of the light diffraction grating 16 in the reflected light path straight h after: normal linear h 'to the surface of the diffraction grating 16 at point P _1: normal theta _[delta] relative to the surface of the diffraction grating 16 at point P _5: the incident light beam L _in the diffraction grating 16 An angle indicating the width of the diffraction angle of the diffracted light diffracted and spread by (assuming that θ _δ << 1 in this embodiment)
θ _D : diffraction angle of the center wavelength λ of the light source (optical pulse generator) spectrum (angle formed by the optical axis of the diffracted light of the center wavelength λ and the normal h)
theta _I: incident angle to the diffraction grating 16 of the incident light beam _{L in} (angle formed normal h and the straight line _{L 1)}
x _1: length between the points _{P 1} between the intersection and the point _{P 1} of the perpendicular and the line _{L 2} drawn to a straight line _{L 2} R: each wavelength in distance (corner cube 18 and the diffraction grating 16 and the corner cube 18 length between the turning point and the point P ₁ component)
W: A length of ½ of the beam size on the surface of the diffraction grating 16 of the light beam reflected and folded by the corner cube 18 (= W _p / 2, distance between points P ₁ and P ₅ )
c: speed of light m: diffraction order λ: center wavelength of spectrum of light source a: difference in optical path length on line L ₂ b: difference in optical path length on line L ₃ d: number density of grooves S in diffraction grating 16 (number) / Mm) (See Fig. 2)

From the definition of FIG. 5B, δL is given by δL = a + b. a is a difference in optical path length between the center wavelength λ and the long wavelength component λ _R that is generated before the light beam diffracted by the diffraction grating 16 reaches the diffraction grating 16 after being reflected by the corner cube 18. Further, b is a difference in optical path length when the long wavelength component λ _R is diffracted again _in the direction of the incident light beam Lin after entering the diffraction grating 16 again.

First, the following formula (1) is established from the diffraction conditions in the diffraction grating 16.

Each time component included in the light source separated by the diffraction grating 16 is a time difference that is the time difference of the entire wavelength components generated by giving different delay times in the optical system of the optical pulse waveform shaping device 10 according to the present embodiment. The width _tw is finally given by the following formula (2).
Hereinafter, derivation of Expression (2) will be described.

As described above, in the optical system of the optical pulse waveform shaping device 10 according to the present embodiment, the diffraction grating 16 is reciprocated twice, and the center wavelength λ and the short wavelength component λ _B are the same as the long wavelength component λ _R. A difference in optical path length is generated. Therefore, all wavelengths (spectra) time width t _w is the delay time of the light pulses (expansion amount) resulting from the optical path length difference of the components from the long wavelength components lambda _R to the short wavelength component lambda _B is formula (3 below ).

Here, the following series of expressions is established from the above definition.

From the above series of equations, δL can be described as the following equation (4).

From equation (1)
Therefore, this is substituted into the equation (4) to obtain the above equation (2).

The optical pulse waveform shaping device 10 according to the present embodiment configured as described above has the following characteristics.
(1) Since one diffraction grating is used, the number of parts can be reduced, which is advantageous in terms of cost.
(2) Since the diffraction grating is diffracted four times, when the same pulse expansion amount is obtained, the size of the entire optical system can be about ½. (If the optical system has the same size, the pulse expansion amount can be doubled.)
(3) Since the spatial modulator is transmitted twice, a large shaping amount can be obtained.
Therefore, the waveform shaping of the optical pulse can be performed efficiently.
(4) The shape (light intensity distribution) of the outgoing light beam, which is a light beam extended in time, is substantially the same as the shape of the incident light beam. Therefore, it is not necessary to consider the direction of the cross section of the light beam, and handling is easier.

  As described above in detail, according to the optical pulse waveform shaping device according to the present embodiment, it is possible to reduce the size with a simple configuration and to shape the optical pulse time waveform into an arbitrary shape. A pulse waveform shaping device and an optical pulse waveform shaping method are obtained.

[Second Embodiment]
The optical pulse waveform shaping device 10a according to the present embodiment will be described with reference to FIG.
In the present embodiment, the reflection type diffraction grating 16 in the optical pulse waveform shaping device 10 according to the first embodiment is replaced with a transmission type diffraction grating 16a. Therefore, the same components as those in FIGS. 1 to 3 are denoted by the same reference numerals, and the description thereof is omitted.

Points P ₁₁ to P ₁₇ in FIG. 6 correspond to points P ₁ to P ₇ in FIG. 3, respectively. The incident light beam L _in the diffraction enters the P ₁₁ point grid 16a, distributed to each wavelength component included in the light source, after being diffracted, and enters again the diffraction grating 16a is folded back at corner cube 18. The light beam is diffracted again by the diffraction grating 16 a, and the light beam is expanded spatially and temporally and is incident on the spatial modulator 14. After being subjected to amplitude modulation for each wavelength component by the spatial modulator 14, it is totally reflected by the reflection mirror 12, subjected to amplitude modulation again by the spatial modulator 14, and is incident on the diffraction grating 16 a.

Light beam diffracted by the diffraction grating 16a is folded back and reflected by the corner cube 18, and is incident again to the point P ₁₁ of the diffraction grating 16a while being spatially compressed. The beam diffracted by the diffraction grating 16a is emitted from the optical pulse waveform shaping device 10a as an emitted light beam _Lout .

The optical path of the incident light beam L _in the long wavelength components lambda _R and lambda _B after diffraction at point P ₁₁ can also be considered similarly to the optical pulse waveform shaping apparatus 10. That is, when defined by an angle measured from the normal to the incident surface of the diffraction grating 16a at point P ₁₁ of the diffraction angle theta _D in the diffraction grating 16a, the diffraction angle theta _DR of long wavelength components lambda _R is the diffraction angle of the short wavelength component θ _DB smaller. Accordingly, as shown in FIG. 6, the arrangement relationship between the long wavelength component λ _R and the short wavelength component λ _B is λ _R on the left side and λ _B on the right side as viewed from the front of the paper.

The optical path of the long wavelength component λ _R will be described.
Long wavelength components lambda _R diffracted at point _{P 11} is a point _{P 12} of the corner cube 18, is reflected at point _{P 13,} is incident on the point _{P 15} of the diffraction grating 16a. The long wavelength component λ _R reflected by the point P ₁₅ enters the point P ₁₇ of the spatial modulator 14, undergoes amplitude modulation, is totally reflected by the reflection mirror 12, and is subjected to amplitude modulation by the spatial modulator 14 again. Te is incident at point _{P 15} of the diffraction grating 16a.

The long wavelength component λ _R diffracted at the point P ₁₅ is reflected at the points P ₁₃ and P ₁₂ of the corner cube 18 and is incident again on the point P ₁₁ of the diffraction grating 16a, and is diffracted as an outgoing light beam L _out. Exit.
The optical path of the short wavelength component λ _B can be similarly traced.

Here, even the optical pulse waveform shaping apparatus 10a according to the present embodiment, similar to the optical pulse waveform shaping apparatus 10, slightly inclined angle of the reflecting mirror 12 with respect to the vertical direction, the emitted light beam and the incident light beam L _in it may be configured so as to avoid interference with the L _out.

The optical pulse waveform shaping device 10a according to the present embodiment that operates as described above also exhibits the optical pulse waveform shaping action shown in FIG. 4 as with the optical pulse waveform shaping device 10 according to the first embodiment. be able to. However, in the optical pulse waveform shaping device 10a, unlike the optical pulse waveform shaping device 10, in the time waveform of the outgoing light pulse _Pout shown in FIG. 4C, the long wavelength component λ _R is temporally different from the short wavelength component λ _B. are arranged in an early position (the position of the long wavelength components lambda _R and short wavelength components lambda _B are interchanged). As apparent from FIG. 6, towards the optical path length of the long wavelength components lambda _R is because shorter than the optical path length of the short wavelength component lambda _B.

In each of the above-described embodiments, the embodiment using the corner cube as the means for turning back the diffracted light diffracted by the diffraction grating has been described as an example. However, the present invention is not limited to this. For example, a reflector may be used instead. Good. In this case, in the time waveform of the outgoing light pulse _Pout shown in FIG. 4C, the long wavelength component λ _R is arranged at a position earlier in time than the short wavelength component λ _B (the long wavelength component λ _R and the short wavelength The position of the component λ _B is switched).

10, 10a Optical pulse waveform shaping device 12 Reflection mirror 14 Spatial modulator 16, 16a Diffraction grating 18 Corner cube 100 Optical pulse waveform shaping device 102 MEMS-MMA
104 grating 106 lens L _in incident light beam L _out outgoing light beam P _in incident light pulse P _S extension light pulse P _out outgoing light pulse S groove

Claims (8)

An extension / compression unit that separates the optical pulse into a plurality of optical pulses having different wavelength components and spatially expands or compresses the optical pulse;
A spatial modulation unit that modulates the amplitude of the light pulse with a modulation rate according to the incident position and emits the light pulse; and
The shaped optical pulse incident from the pulse light source is expanded by the expansion / compression unit, and then the spatial modulation unit is reciprocated to amplitude-modulate the plurality of optical pulses, and the amplitude-modulated optical pulses An optical pulse waveform shaping device that emits a shaped optical pulse by giving a different delay time to each of the plurality of optical pulses by being compressed by the expansion / compression unit. The extension / compression unit includes a diffraction grating that separates an incident optical pulse into a plurality of optical pulses having different wavelength components, and a folding unit that reflects the incident optical pulse and changes the optical axis and folds it back in the incident direction.
The incident optical pulse is diffracted by the diffraction grating and separated into a plurality of optical pulses, and the separated plurality of optical pulses are folded back by the folding section and then diffracted by the diffraction grating again to make the optical pulse spatial. Each of the plurality of light pulses is diffracted by the diffraction grating, diffracted by the diffraction grating, diffracted again by the diffraction grating, and compressed. The optical pulse waveform shaping device according to claim 1, wherein different delay times are provided. The optical pulse waveform shaping device according to claim 2, wherein the folding portion includes a corner cube or a reflecting mirror. The spatial modulation unit includes a filter unit having different transmittance according to the incident position of each of the plurality of light pulses, and transmits the plurality of incident light pulses through the filter unit for each of the plurality of light pulses. The optical pulse waveform shaping device according to any one of claims 1 to 3, wherein the amplitude is modulated. The optical pulse waveform shaping device according to any one of claims 1 to 4, wherein an optical axis of the shaped optical pulse to be incident and an optical axis of the shaped optical pulse to be emitted are spatially separated. The optical pulse waveform shaping device according to claim 5, further comprising a reflection unit that reflects the stretched shaped optical pulse emitted from the spatial modulation unit and makes it incident on the spatial modulation unit again. The optical pulse waveform shaping device according to claim 6, wherein the optical axis of the shaped optical pulse to be incident and the optical axis of the shaped optical pulse to be emitted are spatially separated by adjusting a reflection angle of the reflection unit. The optical pulse is separated into a plurality of optical pulses of different wavelength components and expanded / compressed spatially, and the incident optical pulse is modulated by the modulation rate according to the incident position. An optical pulse waveform shaping method in an optical pulse waveform shaping device comprising:
The to-be-shaped light pulse incident from the pulse light source is expanded by the expansion / compression unit,
Amplifying the stretched shaped optical pulse for each of the plurality of optical pulses by reciprocating the spatial modulation unit,
The plurality of amplitude-modulated optical pulses are compressed by the expansion / compression unit,
An optical pulse waveform shaping method in which a different delay time is given to each of the plurality of optical pulses and emitted as a shaped optical pulse.