Patent ID: 12259601

DETAILED DESCRIPTION

Hereinafter, a preferred embodiment of a light shaping device and a light shaping method according to an aspect of the present disclosure will be described in detail with reference to the accompanying drawings.

Schematic Configuration of Light Shaping Device

FIG.1is a view illustrating a schematic configuration of the light shaping device according to an aspect of the present disclosure. As illustrated in the same drawing, a light shaping device1includes a light source2, an intensity modulation unit3that is optically connected to the light source2, and a calculation unit4that calculates modulation conditions in the intensity modulation unit3. For example, the light source2is a laser light source called a solid laser light source, and outputs coherent pulse light as input light L1to the intensity modulation unit3. The intensity modulation unit3modulates a spectrum intensity of an optical pulse that is the input light L1, and outputs the optical pulse of which a temporal width is narrowed as output light L2.

A spectrum waveform of the optical pulse and a temporal waveform are associated with each other in a Fourier transform relationship. For example,FIG.2Ais a view showing an example of a spectrum waveform having a Gaussian shape, andFIG.2Bis a view showing a temporal waveform corresponding to the spectrum waveform. In addition,FIG.3Ais a view showing a waveform obtained by applying cosine-like modulation to the spectrum waveform shown inFIG.2A, andFIG.3Bis a view showing a temporal waveform corresponding to the spectrum waveform. As shown in the drawings, when at least one of a phase and an intensity of the spectrum waveform varies, the temporal waveform also varies in correspondence with the variation.

Light shaping in the intensity modulation unit3uses the Fourier transform relationship. In the light shaping, when a mask (filtering) M is applied to the spectrum waveform of the input light L1, an improvement of a temporal width narrowing rate with respect to an intensity loss of the optical pulse is realized at low cost.FIG.4is a view showing a mask that can be used in the intensity modulation unit. As shown in the same drawing, the mask M is expressed by two parameters including a starting end wavelength λr and a wavelength width W from the starting end wavelength λr. InFIG.4, the mask M is set to be symmetrical with respect to a central wavelength λ0of the input light L1.

Here, the starting end wavelength λr is defined as a wavelength on a side, which is close to the central wavelength λ0, between both ends in a wavelength range expressed by a wavelength width W. In addition, the wavelength width W is defined with the starting end wavelength λr set as a reference. In a mask M on a long wavelength side in comparison to the central wavelength λ0, an end on a short wavelength side in the wavelength range of the mask M becomes the starting end wavelength λr, and a terminating end wavelength becomes λr+W. In a mask M on a short wavelength side in comparison to the central wavelength λ0, an end on a long wavelength side in the wavelength range of the mask M becomes the starting end wavelength λr, and the terminating end wavelength becomes λr−W. InFIG.4, a pair of masks (single masks) symmetrical with respect to the central wavelength λ0is shown, but a setting number of the mask M is not limited thereto, and a plurality of pairs of masks (multi masks) symmetrical with respect to the central wavelength λ0may be set.

Returning toFIG.1, the calculation unit4is constituted by, for example, a computer system including a processor, a memory, and the like. Examples of the computer system include a PC, a microcomputer, a cloud server, and a smart device (smartphone, a tablet terminal, or the like). The calculation unit4may be constituted by a programmable logic controller (PLC), or an integrated circuit such as a field-programmable gate array (FPGA).

The calculation unit4calculates setting values of the starting end wavelength λr and the wavelength width W of the mask M that is used in the intensity modulation unit3on the basis of an allowable value of at least one of an intensity loss rate and a temporal width narrowing rate of the output light L2. For example, the allowable value of the intensity loss rate and the temporal width narrowing rate is acquired by accepting an input from a user of the light shaping device1with an input unit such as a keyboard. A relationship between the starting end wavelength λr and the wavelength width W, and the intensity loss rate and the temporal width narrowing rate will be described later.

In addition, the calculation unit4calculates setting values of the starting end wavelength λr and the wavelength width W of the mask M that is used in the intensity modulation unit3, and a setting number of the mask M on the basis of an allowable value of at least one of a peak intensity and an appearance time region of a side lobe of the output light L2. The side lobe is high order light that appears on both sides of a main pulse in pulse light (refer toFIG.16). A relationship between the setting values of the starting end wavelength λr and the wavelength width W and the setting number of the mask M, and the peak intensity and the appearance time region of the side lobe of the output light L2will be described later.

In addition, the calculation unit4may calculate a power magnification of the input light L1for compensating the peak intensity of the output light L2on the basis of a calculation result of the setting values of the starting end wavelength λr and the wavelength width W of the mask M that is used in the intensity modulation unit3. In the aspect ofFIG.1, the calculation unit4and the light source2are connected to each other in a communication possible manner, and a control signal for controlling power of the input light L1on the basis of the calculation result of the power magnification of the input light L1in the calculation unit4is output from the calculation unit4to the light source2.

Configuration Example of Intensity Modulation Unit

FIG.5is a schematic view illustrating an example of a configuration of the intensity modulation unit. As illustrated inFIG.5, the intensity modulation unit3includes a demultiplexing element11that demultiplexes the input light L1for every wavelength, an intensity modulation element12that cuts a predetermined spectrum component in the input light L1that is demultiplexed by the demultiplexing element11, and a multiplexing element13that multiplexes a spectrum component that is output from the intensity modulation element12.

As the demultiplexing element11and the multiplexing element13, for example, a diffraction grating, a prism, a grism, or the like can be used. The diffraction grating may be any of a reflection type diffraction grating and a transmission type diffraction grating, and may be combined with a lens or the like as necessary. In addition, the demultiplexing element11and the multiplexing element13may be configured by combining a fiber bragg grating and a lens. In this case, a fiber input and output type intensity modulation unit3can be configured. As the intensity modulation element12, for example, a spatial light modulator (SLM), a shielding plate, a dielectric multi-layer film mirror, or the like can be used.

In an example illustrated inFIG.5, the demultiplexing element11and the multiplexing element13are constituted by a reflection type diffraction grating21, and the intensity modulation element12is constituted by a spatial light modulator22. The input light L1that is reflected by the reflection type diffraction grating21that is the demultiplexing element11becomes parallel light by a lens23, and is incident to the spatial light modulator22. For example, the spatial light modulator22is a phase modulation type spatial light modulator. The spatial light modulator22cuts a spectrum component corresponding to the mask M by presenting a predetermined diffraction grating pattern to a phase modulation plane.

The diffraction grating pattern presented to the phase modulation plane of the spatial light modulator22is calculated by a modulation pattern calculation device (not illustrated). For example, the modulation pattern calculation device is electrically connected to the calculation unit4, and on the basis of a calculation result of setting values of the starting end wavelength λr and the wavelength width W of the mask M in the calculation unit4, the modulation pattern calculation device calculates a diffraction grating pattern corresponding to the calculation result. Light of which intensity is modulated by the spatial light modulator22is condensed to a lens24, and is multiplexed into the output light L2by the reflection type diffraction grating21that is the multiplexing element13. The output light L2is pulse light of which a temporal width is further narrowed in comparison to the input light L1.

FIG.6is a schematic view illustrating another example of the configuration of the intensity modulation unit. The example illustrated in the same drawing is different from the aspect inFIG.5in that the demultiplexing element11and the multiplexing element13are constituted by a transmission type diffraction grating25. Even in this aspect, a spectrum component corresponding to the mask M is cut by presenting a predetermined diffraction grating pattern to the phase modulation plane of the spatial light modulator22. Accordingly, pulse light of which a temporal width is further narrowed in comparison to the input light L1can be obtained as the output light L2.

FIG.7is a schematic view illustrating still another example of the configuration of the intensity modulation unit. In the example illustrated in the same drawing, the demultiplexing element11and the multiplexing element13are constituted by the reflection type diffraction grating21, and the intensity modulation element12is constituted by a shielding plate26. The shielding plate26is provided with a shielding part26athat shields light, and a passing part26bthrough which light is transmitted in a predetermined pattern. A spectrum component corresponding to the mask M is cut by the shielding part26a, and thus pulse light of which a temporal width is further narrowed in comparison to the input light L1can be obtained as the output light L2. In this aspect, for example, a plurality of shielding plates26in which patterns of the shielding part26aand the passing part26bare different from each other are prepared in advance, and setting values of the starting end wavelength λr and the wavelength width W of the mask M, and a setting number of the mask M can be changed by switching the shielding plates26disposed on an optical path of the input light L1by a switching unit.

FIG.8Ais a schematic view illustrating still another example of the configuration of the intensity modulation unit. In the example illustrated inFIG.8A, the demultiplexing element11and the multiplexing element13are constituted by the transmission type diffraction grating25, and the intensity modulation element12is constituted by one or a plurality of (here, two) dielectric multi-layer film mirrors27. The dielectric multi-layer film mirrors27function as a so-called band-stop filter. As shown inFIG.8B, the dielectric multi-layer film mirror27on one side has high reflectance in a band corresponding to the starting end wavelength λr and the wavelength width W on a longer wavelength side in comparison to the central wavelength a of input light. In addition, as shown inFIG.8C, the dielectric multi-layer film mirror27on the other side has a high reflectance in a band corresponding to the starting end wavelength λr and the wavelength width W on a shorter wavelength side in comparison to the central wavelength λ0of the input light.

A spectrum component corresponding to the mask M is cut by the dielectric multi-layer film mirrors27, and thus pulse light of which a temporal width is further narrowed in comparison to the input light L1can be obtained as the output light L2. In this aspect, for example, a plurality of the dielectric multi-layer film mirrors27in which reflection bands are different are prepared in advance, and the setting values of the starting end wavelength λr and the wavelength width W of the mask M, and the setting number of the mask M can be changed by switching the dielectric multi-layer film mirrors27disposed on an optical path of the input light L1by a switching unit.

FIG.9is a schematic view illustrating still another example of the configuration of the intensity modulation unit. The example illustrated in the same drawing is different form the aspect illustrated inFIG.5toFIG.7, andFIG.8Ain that the intensity modulation unit3is constituted by only one or a plurality of (here, two) dielectric multi-layer film mirrors27. Even in this aspect, a spectrum component corresponding to the mask M is cut by the dielectric multi-layer film mirrors27, and thus pulse light of which a temporal width is further narrowed in comparison to the input light L1can be obtained as the output light L2. Note that, for example, reflection bands of the two dielectric multi-layer film mirrors27are the same as shown inFIG.8BandFIG.8C. The configuration in which the dielectric multi-layer film mirrors27disposed on the optical path of the input light L1are switched by the switching unit is also the same as in the case ofFIG.8A.

Operation of Light Shaping Device

FIG.10is a flowchart illustrating an example of an operation of the light shaping device described above. As illustrated in the same drawing, in the light shaping device1, first, an input of allowable values of the intensity loss rate and the temporal width narrowing rate of the output light L2are received (step S01). In addition, an input of allowable values of a peak intensity and an appearance time region of a side lobe of the output light L2is received (step S02). Next, parameters of the mask M are set on the basis of the input allowable values. Specifically, in the calculation unit4, setting values of the starting end wavelength λr and the wavelength width W of the mask M are calculated (step S03). In addition, a setting number of the mask M is calculated (step S04). In addition, a power magnification of the input light L1is calculated on the basis of the setting values of the starting end wavelength λr and the wavelength width W of the mask M (step S05).

After calculating the parameters of the mask M, setting of the intensity modulation unit3is performed so that a mask based on the calculation result is formed (step S06). The setting content of the intensity modulation unit3is different depending on an aspect of the intensity modulation unit3. In the case of using the spatial light modulator22as the intensity modulation unit3, a phase pattern is presented to the spatial light modulator22, and in the case of using the shielding plate26or the dielectric multi-layer film mirror27as the intensity modulation unit3, switching of the shielding plate26or the dielectric multi-layer film mirror27disposed on an optical path of the input light L1is performed.

After setting the intensity modulation unit3, setting of the light source2based on the calculation result in step S05(setting of the power of the input light L1) is performed (step S07), and output of an optical pulse that becomes the input light L1is initiated from the light source2on the basis of the setting (step S08). The input light L1output from the light source2is input to the intensity modulation unit3, and in the intensity modulation unit3, a spectrum intensity is modulated by using the mask M that is set (step S09). Due to the modulation of the spectrum intensity by using the mask M, an optical pulse of which a temporal width is narrowed is output from the intensity modulation unit3as the output light L2(step S10).

Reception of the input of the allowable values in step S01and step S02may be executed in any order or simultaneously. Reception of the input of the allowable value in step S02may be omitted. The calculation in step S03and step S04may be executed in any order or simultaneously. In the case of omitting the reception of the input of the allowable value in step S02, the calculation in step S04may be omitted. The calculation of the power magnification of the input light L1in step S05may be omitted. In the case of omitting the calculation of the power magnification of the input light L1in step S05, setting of the light source2in step S07is also omitted.

Operation and Effect

As described above, in the light shaping device1, the spectrum intensity of the input light L1is modulated with the mask M expressed by the starting end wavelength λr and the wavelength width W from the starting end wavelength λr. According to this, narrowing of a temporal width exceeding a theoretical limit in the case of modulating a spectrum phase becomes possible, and a temporal width narrowing rate with respect to an intensity loss of the optical pulse can be improved. In addition, parameters of the mask M are relatively simple, and continuous or high-accuracy intensity modulation with respect to each spectrum component is not necessary. Accordingly, it is possible to construct the intensity modulation unit3without introducing an expensive intensity modulator. As a result, a reduction in cost of the light shaping device1can be realized.

In addition, the mask M is set to be symmetrical with respect to the central wavelength λ0of the input light L1. When using such a mask M, the temporal width narrowing rate with respect to the intensity loss of the optical pulse can be further improved.

In addition, the light shaping device includes the calculation unit4that calculates setting values of the starting end wavelength λr and the wavelength width W of the mask M that is used in the intensity modulation unit3on the basis of an allowable value of at least one of the intensity loss rate and the temporal width narrowing rate of the output light L2. According to this, the calculation unit4performs optimal setting of the mask M in consideration of the intensity loss rate and the temporal width narrowing rate. Accordingly, convenience of the light shaping device1can be improved.

In this embodiment, the calculation unit4calculates the setting values of the starting end wavelength λr and the wavelength width W of the mask M that is used in the intensity modulation unit3on the basis of an allowable value of at least one of a peak intensity and an appearance time region of a side lobe of the output light L2. In addition, the calculation unit4calculates a setting number of the mask M that is used in the intensity modulation unit3on the basis of an allowable value of at least one of the peak intensity and the appearance time region of the side lobe of the output light L2. According to this, the calculation unit4performs optimal setting of the mask M in consideration of the side lobe. Accordingly, convenience of the light shaping device1can be further improved.

In addition, in this embodiment, the calculation unit4calculates a power magnification of the input light L1for compensating a peak intensity of the output light L2on the basis of setting values of the starting end wavelength λr and the wavelength width W of the mask M that is used in the intensity modulation unit3. According to this, the peak intensity of the output light L2can be compensated in a level that is equivalent to a case where intensity modulation using the mask M is not performed.

In the example illustrated inFIG.5toFIG.7, andFIG.8A, the intensity modulation unit3includes the demultiplexing element11that demultiplexes the input light L1for every wavelength, the intensity modulation element12that cuts a predetermined spectrum component in the input light L1that is demultiplexed by the demultiplexing element11, and the multiplexing element13that multiplexes spectrum components output from the intensity modulation element12. According to this, the intensity modulation unit3that modulates the spectrum intensity of the input light L1can be realized in a simple configuration.

As illustrated inFIG.5andFIG.6, in a case where the intensity modulation element12is constituted by the spatial light modulator22, the mask M can be formed dynamically and in a high degree of freedom due to a phase pattern presented to the spatial light modulator22. In a case where the intensity modulation element12is constituted by the shielding plate26as illustrated inFIG.7, and in a case where the intensity modulation element12is constituted by one or a plurality of the dielectric multi-layer film mirrors27as illustrated inFIG.8A, the mask M can be formed in a simple configuration, and a further reduction in cost of the light shaping device1can be realized. In the example illustrated inFIG.9, the intensity modulation unit3is constituted by one or a plurality of the dielectric multi-layer film mirrors27. In this case, the mask M can be formed in a simpler configuration in proportion to omission of the demultiplexing element11and the multiplexing element13, and a further reduction in cost of the light shaping device1can be realized.

In light shaping in the related art, a method of narrowing the temporal width of the pulse light by broadening a spectrum band is typically used. On the other hand, in the light shaping of the present disclosure, further narrowing of the temporal width of the pulse light is realized by cutting an arbitrary spectrum component with the mask M. The narrowing of the temporal width of the pulse light represents that a time differential value of the pulse light increases. That is, the narrowing of the temporal width of the pulse light effectively operates on an increase in an amplitude of terahertz waves or pulse shaping when generating terahertz wave pulses which vary in response to a differential value. In addition, the narrowing of the temporal width of the pulse also contributes to an improvement of time resolution of pulse measurement represented by time of flight (TOF) measurement.

In the case of applying the pulse light to an imaging technology such as a streak camera and a sequentially timed all-optical mapping photography (STAMP), the narrowing of the temporal width of the pulse light contributes to an improvement of an imaging speed and time resolution. Due to the improvement of the time resolution, a reduction in a time interference noise is expected, and an influence of a motion blur is reduced, and thus an improvement of an S/N ratio is expected. In a laser processing field, due to the narrowing of the temporal width of the pulse light, time for which laser light interacts with a substance can be reduced. According to this, it is possible to enhance a non-thermal processing effect while reducing an influence of heat on an object to be processed. In a laser microscope field, an improvement of induction efficiency of non-linear optical effect or an improvement of the S/N ratio is expected.

Verification of Relationship Between Parameters of Mask and Narrowing of Temporal Width of Pulse Light

Hereinafter, a relationship between parameters of a mask and narrowing of a temporal width of pulse light will be described. First, description will be given of a result of verification on narrowing of the temporal width of the pulse light in the case of performing intensity modulation by using the mask. As the mask, a pair of single masks symmetrical with respect to the central wavelength λ0is assumed (refer toFIG.4). As the input light, a single pulse of which a wavelength width is 5 nm and the central wavelength λ0is 800 nm is assumed. In a case where the input light is set as a Fourier transform limit pulse of which a phase spectrum is a flat, the full width at half maximum becomes approximately 135 fs.FIG.11Ais a view showing a variation of a temporal width of output light in a case where the parameters (the starting end wavelength λr and the wavelength width W) of the mask are changed. In the same drawing, the horizontal axis represents the starting end wavelength λr, the vertical axis represents the temporal width (full width at half maximum) of the output light, and simulation results in a case where W is 0.73 nm and W is 1.22 nm are plotted. In addition, a result of a case where intensity modulation with the mask is not performed (W is 0 nm: Fourier transform limit pulse) is plotted in combination so as to show a relative variation of the temporal width.

From the result inFIG.11A, it can be seen that the larger the wavelength width W of the mask is, the further the temporal width of the output light is narrowed. In addition, it can be seen that the further the starting end wavelength λr is close to the central wavelength λ0of the input light, the further the temporal width of the output light is narrowed. When W is 1.22 nm, in a case where λr is near the central wavelength λ0, the temporal width of the output light is less than 110 fs, and narrowing of the temporal width by approximately 22% is expected. On the other hand,FIG.11Bis a view showing a result near the central wavelength inFIG.11Ain an enlarged manner. As shown inFIG.11B, it can be seen that the further the starting end wavelength λr is close to the central wavelength λ0of the input light, the further the temporal width of the output light gradually decreases, but under a condition where the starting end wavelength λr and the central wavelength λ0are the same as each other, the temporal width of the output light slightly increases. From the results, it can be concluded that equalizing of the starting end wavelength λr and the central wavelength λ0is not necessarily a condition capable of narrowing the temporal width of the output light most.

FIG.12is a view showing a variation of the intensity loss rate of the output light in a case where the parameters of the mask are changed. In the same drawing, the horizontal axis represents the starting end wavelength λr, the vertical axis represents intensity loss rate, and simulation results in a case where W is 0.24 nm, W is 0.49 nm, W is 0.73 nm, W is 0.98 nm, and W is 1.22 nm are plotted. From the results inFIG.12, it can be seen that the further the starting end wavelength λr is close to the central wavelength λ0, the higher the intensity loss rate of the output light becomes. In addition, it can be seen that the larger the wavelength width W of the mask is, the higher the intensity loss rate of the output light becomes.

When considering this result in combination with the result inFIG.11A, it can be seen that the temporal width narrowing rate and the intensity loss rate are in a trade-off relationship. Accordingly, it is considered that the parameters of the mask are necessary to be set within an allowable range of the intensity loss in practical use when narrowing the temporal width of the output light. In this regard, in Japanese Unexamined Patent Publication No. 2016-218141 as citation list, narrowing of the temporal width by approximately 15% is shown by allowing approximately 60% of intensity loss. In the method of the present disclosure, from the results inFIG.11AandFIG.12, narrowing of the temporal width by approximately 22% can be realized by allowing approximately 40% of intensity loss. In addition, in narrowing of the temporal width by approximately 15%, approximately 25% of intensity loss may be allowed. Accordingly, it can be seen that a high temporal width narrowing effect is obtained at a smaller intensity loss.

FIG.13is a view showing a variation of the peak intensity of the output light in a case where the parameters of the mask are changed. In the same drawing, the horizontal axis represents the starting end wavelength λr and the vertical axis represents a peak intensity ratio. InFIG.13, simulation results in a case where W is 0.24 nm, W is 0.49 nm, W is 0.73 nm, W is 0.98 nm, and W is 1.22 nm are plotted as inFIG.12. The peak intensity ratio of the output light is a peak intensity of a temporal waveform with respect to a peak intensity of a temporal waveform in a case where intensity modulation with the mask is not performed (Fourier transform limit pulse). In a case where a peak intensity ratio exceeds 1, this case represents that the peak intensity further increases in comparison to the Fourier transform limit pulse by the intensity modulation using the mask. In a case where the peak intensity ratio is less than 1, this case represents that the peak intensity further decreases in comparison to the Fourier transform limit pulse by the intensity modulation using the mask.

From the result inFIG.13, in a case where W is 0.24 nm, the peak intensity ratio is approximately 0.9, whereas the further W increases, the further the peak intensity ratio decreases. In a case where W is 1.22 nm, the peak intensity ratio is less than 0.6. From this, it can be seen that the larger the wavelength width W of the mask is (that is, the higher the intensity loss rate and the temporal width narrowing rate are), the further the peak intensity of the output light decreases.

In a case where the peak intensity of the output light becomes a problem in application of the light shaping device1, the problem can be compensated by raising power of the input light.FIG.14is a view showing power of the input light which is necessary for compensation of the peak intensity. In the same drawing, the horizontal axis represents the starting end wavelength λr and the vertical axis represents the power magnification of the input light. InFIG.14, simulation results in a case where W is 0.24 nm, W is 0.49 nm, W is 0.73 nm, W is 0.98 nm, and W is 1.22 nm are plotted as inFIG.13. The power magnification of the input light is a ratio between power of the input light in a case where intensity modulation with the mask is not performed and power of the input light which is necessary for obtaining a peak intensity that is the same as a peak intensity of a temporal waveform of the output light in a case where intensity modulation with the mask is not performed. From results inFIG.14, for example, in a case where W is 1.22 nm, when increasing the power of the input light by approximately 1.85 times, the peak intensity of the output light can be compensated in the same level as in a case where intensity modulation with a mask is not performed.

FIG.15is a view showing a relationship between accuracy of the mask and the temporal width of the output light. In the same drawing, the horizontal axis represents the starting end wavelength λr and the vertical axis represents the temporal width (full width at half maximum) of the output light. In the above-described simulation, an ideal model (refer toFIG.4) in which a spectrum intensity of the input light of a wavelength region of which an intensity is modulated with the mask is set to 0 is used. However, in an actual device, it is also assumed that the spectrum intensity of the input light may not be completely 0 due to the configuration of the intensity modulation unit3, or the like. InFIG.15, under a condition that W is 1.22 nm, an intensity (transmittance) of light that is not subjected to intensity modulation with the mask and is transmitted is changed, and a simulation result of a temporal width of the output light with respect to each transmittance is plotted. Conditions of the transmittance are set to three conditions of 0%, 10%, and 20%. In addition, for comparison, a result in the case of Fourier transform limit pulse (TL pulse) is plotted in combination.

As shown inFIG.15, for example, at λr of 800.02 nm, the temporal width of the output light in a case where the transmittance is 0% is 109 fs, whereas the temporal width of the output light in a case where the transmittance is 10% is 118 fs, and the temporal width of the output light in a case where the transmittance is 20% is 121 fs. From the results, it can be seen that the transmittance of the input light at a wavelength region of which an intensity is modulated with the mask has an influence on the temporal width narrowing rate of the output light. In addition, even when a transmittance difference is 10%, it can be seen that in a case where the transmittance increases from 0% to 10%, a decrease in the temporal width narrowing rate of the output light is as large as two or more times in comparison to a case where the transmittance increases from 10% to 20%. Accordingly, from the viewpoint of improving the temporal width narrowing rate of the output light, it can be concluded that it is preferable that the transmittance of the input light at a wavelength region of which an intensity is modulated with the mask is suppressed to 10% or less, and more preferably near 0%.

Next, description will be given of a result of verification on a side lobe of the output light in a case where intensity modulation is performed with the mask. As described above, the starting end wavelength λr and the wavelength width W of the mask are parameters having an influence on the temporal width of the output light, and are also parameters having an influence on a shape of the side lobe of the output light. It is assumed that the peak intensity and the appearance time region of the side lobe become a problem in application of the light shaping device1.

For example, the side lobe is high order light that appears on both sides of a main pulse in pulse light as shown inFIG.16. An example inFIG.16is a temporal waveform of the output light in a case where intensity modulation is performed by using masks (refer toFIG.4) symmetrical with respect to the central wavelength λ0, and in a case where the masks are symmetrical with respect to the central wavelength λ0, side lobes of the output light also appear to be symmetrical with respect to the main pulse. Here, a first peak that appears in a time region on both sides of the main pulse is referred to as a first side lobe, and the subsequent peak that appears in a time region on an outer side of the above-described time region is referred to as a second side lobe.

FIG.17is a view showing a variation of a peak intensity of the side lobe in a case where the parameters of the mask are changed. In the same drawing, the horizontal axis represents the starting end wavelength λr, the vertical axis represents a ratio of the peak intensity of the side lobe (first side lobe) to the peak intensity of the main pulse, and simulation results in a case where W is 0.73 nm and W is 1.22 nm are plotted. From the results inFIG.17, it can be seen that the larger the wavelength width W of the mask is, the further the peak intensity of the side lobe tends to increase. In addition, it can be seen that the peak intensity of the side lobe varies in accordance with a value of the starting end wavelength λr. In the example inFIG.17, it can be seen that in a region where λr is 800 to 801 nm, the larger λr is, the further peak intensity of the side lobe decreases. This tendency is also true of a region where λr is 801 to 802 nm and a region where λr is 802 to 805 nm.

FIG.18is a view showing a variation of the appearance time of the side lobe in a case where the parameters of the mask are changed. In the same drawing, the horizontal axis represents the starting end wavelength λr, the vertical axis represents the appearance time of the side lobe (first side lobe), and simulation results in a case where W is 0.73 nm and W is 1.22 nm are plotted. The appearance time of the side lobe corresponds to a time interval between the peak position of the main pulse and the peak position of the side lobe. From the results inFIG.18, it can be seen that the larger the wavelength width W of the mask is, the further the appearance time of the side lobe tends to deviate from the appearance time of the main pulse. In addition, it can be seen that the appearance time of the side lobe varies in accordance with a value of the starting end wavelength λr. In the example inFIG.18, in a region where λr is 800 to 801 nm, it can be seen that the larger λr is, the further the appearance time of the side lobe is close to the appearance time of the main pulse. This tendency is also true of a region where λr is 801 to 802 nm and a region where λr is 802 to 805 nm.

From results inFIG.17andFIG.18, it can be seen that the peak intensity and the appearance time region of the side lobe can be controlled through selection of the starting end wavelength λr and the wavelength width W of the mask. Note that, in the results inFIG.17andFIG.18, a discontinuity is shown in the plot. The discontinuity disappears when the first side lobe is closer to the main pulse to a certain extent in the case of changing the starting end wavelength λr, and occurs because the second side lobe replaces the first side lobe.

Next, description will be given of a relationship between the setting number of the mask and the temporal waveform of the output light. In the above-described simulations, a pair of single masks symmetrical with respect to the central wavelength λ0is assumed (refer toFIG.4). However, here, as shown inFIG.19, consideration will be given on a case where a plurality of pairs of masks (multi masks) are set to be symmetrical with respect to the central wavelength λ0. InFIG.19, two pairs of masks (double masks) symmetrical with respect to the central wavelength λ0are shown. Masks (primary masks M1) on the central wavelength λ0side are expressed by a starting end wavelength λr1and a wavelength width W1. Masks (secondary masks M2) on an outer side of the primary masks M1are expressed by a starting end wavelength λr2and a wavelength width W2.

FIG.20is a view showing a variation of the temporal width of the output light in a case where the parameters of the mask are changed. In the input light, a single pulse in which the wavelength width is 5 nm and the central wavelength λ0is 800 nm is assumed. In a case where the input light is set as a Fourier transform limit pulse of which a phase spectrum is flat, the full width at half maximum becomes approximately 135 fs. For simplification of simulation, the starting end wavelength λr1of the primary masks is set to a constant of 800.49 nm, and the wavelength width W1is set to a constant of 0.49 nm. InFIG.20, the horizontal axis represents the starting end wavelength λr2, the vertical axis represents the temporal width (full width at half maximum) of the output light, and simulation results in a case where W2is 0.24 nm, W2is 0.73 nm, and W2is 1.22 nm are plotted. In addition, a result in a case where intensity modulation with a mask is not performed (W1=W2=0 nm: the case of a Fourier transform limit (TL) pulse) is plotted in combination in order to show a relative variation of the temporal width.

From the results inFIG.20, it can be seen that the larger the wavelength width W2of the mask is, the further the temporal width of the output light is narrowed. In addition, it can be seen that the further the starting end wavelength λr2of the secondary masks is close to a terminating end wavelength (=λr1+W1) of the primary masks, the further the temporal width of the output light is narrowed. That is, it can be said that a temporal width narrowing condition of the output light in the case of the multi masks is a case where the wavelength width W of the masks is large, and the starting end wavelength λr is closer to the central wavelength λ0as in the single mask.

FIG.21is a view showing a variation of the peak intensity of the side lobe in a case where the parameters of the mask are changed. InFIG.21, the horizontal axis represents the starting end wavelength λr2, the vertical axis represents a ratio of the peak intensity of the side lobe (first side lobe) to the peak intensity of the main pulse, and simulation results in a case where W2is 0.24 nm, W2is 0.73 nm, and W2is 1.22 nm are plotted. The parameters of the input light are the same as the case inFIG.20. From the results inFIG.21, it can be seen that the larger the wavelength width W2of the mask is, the further the peak intensity of the first side lobe increases. In addition, it can be seen that the peak intensity of the first side lobe varies in accordance with a value of the starting end wavelength λr2.

In the light shaping of the present disclosure, as described above, the temporal width narrowing rate and the intensity loss rate are in a trade-off relationship. In practical use of the light shaping, the temporal width narrowing rate is set within an allowable range of the intensity loss rate. Here, in the following simulation, verification is made on which temporal waveform of the output light is obtained in the single mask and the double masks in a case where the intensity loss rate is set to a constant value.

FIG.22BandFIG.24show temporal waveforms of the output light which are obtained with respect to models of the single mask and the double masks, respectively by setting the intensity loss rate to 30%. InFIG.22A, a model of two single masks in which parameters are different from each other is shown. In Case 1, λr is set to 800.56 nm, and W is set to 0.98 nm. In Case 2, λr is set to 801.15 nm, and W is set to 1.22 nm.

FIG.22Bis a view showing a temporal waveform of the output light in a case where intensity modulation is performed by using the mask. As shown in the same drawing, in any of Case 1 and Case 2, the temporal width (full width at half maximum) of the output light is 116.1 fs. On the other hand, in Case 1 and Case 2, a great difference occurs in a shape of the first side lobe. A ratio of the peak intensity of the first side lobe to the peak intensity of the main pulse is 1.09% in Case 1 and 5.89% in Case 2. In addition, an appearance time of the first side lobe is approximately 300 fs in Case 1 and approximately 600 fs in Case 2.

FIG.23AtoFIG.23Cshow models of three double masks in which parameters are different from each other. In Case 1, λr1is set to 800.49 nm, W1is set to 0.49 nm, λr2is set to 801.12 nm, and W2is set to 0.49 nm. In Case 2, λr1is set to 800.49 nm, W1is set to 0.49 nm, λ2is set to 802.00 nm, and W2is set to 0.73 nm. In Case 3, λr1is set to 800.49 nm, W1is set to 0.49 nm, λr2is set to 802.73 nm, and W2is set to 1.22 nm.

FIG.24is a view showing a temporal waveform of the output light in a case where intensity modulation is performed by using the masks. As shown in the same drawing, in Case 1 to Case 3, the temporal width (full width at half maximum) of the output light is approximately in a range of 116.2 fs to 118.2 fs. The ratio of the peak intensity of the first side lobe to the peak intensity of the main pulse is 1.05% in Case 1, 0.65% in Case 2, and 1.80% in Case 3. In addition, the appearance time of the first side lobe is approximately 300 fs in Case 1, approximately 470 fs in Case 2, and approximately 280 fs in Case 3.

From the results inFIG.22BandFIG.24, it can be seen that narrowing of a temporal width that is the same as in the single mask can be realized even in the case of using the multi masks. In addition, at least in a case where the time region is within a range of −1000 fs to 1000 fs, it can be seen that parameters capable of suppressing the peak intensity of the first side lobe exists in the case of using the double masks in comparison to the case of using the single mask. Since the number of parameters is large in the case of using the double masks, the degree of freedom of adjustment of the peak intensity and the appearance time region of the first side lobe is higher in comparison to the case of using the single mask.

1: light shaping device,3: intensity modulation unit,4: calculation unit,11: demultiplexing element,12: intensity modulation element,13: multiplexing element,22: spatial light modulator,26: shielding plate,27: dielectric multi-layer film mirror, L1: input light, L2: output light, M: mask.