JP3743881B2 - Method and apparatus for ultra-high-speed conversion from time signal to two-dimensional spatial signal - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method and an apparatus for converting a time change of a spectrum of an ultrashort light pulse into a two-dimensional spatial distribution of a time axis and a time frequency axis at an extremely high speed.
[0002]
[Prior art]
In general, a system that decomposes a temporal change of a spectrum included in an ultrashort light pulse into two axes of a time axis and a spectrum axis (time frequency axis) and converts it into a spatial distribution uses an ultrafast time gate and a spectral technique. . This method uses an ultrashort optical pulse that is the Fourier limit (an optical pulse in which the phases of the included spectra are aligned) as a time-gated pulse for an ultrashort optical pulse (signal pulse) to be measured. The ultra-fast time gate is enabled by utilizing the high-speed response of the effect. As this nonlinear effect, a second-order or third-order nonlinear effect is used. When a signal pulse and a reference pulse (gate pulse) are incident at the same time in the nonlinear crystal, harmonic light extracted by the gate pulse is emitted from the nonlinear crystal. It becomes possible to know a time frequency distribution included in an arbitrary time of a pulse. Then, by scanning the gate pulse with respect to the time axis, the relationship between the time included in the signal pulse and the time frequency can be obtained. Here, when the signal pulse has a long time width, the time width that the gate pulse must scan also becomes long. As a conventional method of scanning the gate pulse, a method of delaying the gate pulse mechanically by changing the optical path length, a signal pulse and a gate pulse are simultaneously obliquely incident on a diffraction grating or a nonlinear crystal, and a pulse beam A method of scanning a signal pulse with a gate pulse by giving a time delay of incidence in the width direction of the signal has been used.
[0003]
However, in the method using mechanical scanning, it is necessary to repeatedly measure the signal pulse every time the optical path length is changed. Therefore, the method cannot be used when different information is included in each signal pulse. Also, in the method of scanning pulses by oblique incidence on the diffraction grating or nonlinear crystal, the wavefronts of the signal pulse and gate pulse are received by the diffraction grating or nonlinear crystal, so that the size of the diffraction grating or nonlinear crystal results in the gate. This limits the maximum time width that the pulse can scan. In addition, in this method, it is necessary to widen the beam in order to increase the maximum time width. However, since the conversion efficiency by the nonlinear crystal is proportional to the power surface density of the beam, there arises a problem that the conversion efficiency is lowered by expanding the beam.
[0004]
[Problems to be solved by the invention]
The present invention solves these problems of the prior art, does not require repetition of pulse incidence, is not limited by the scanable time width, has a high conversion efficiency, and the time change of the spectrum of an ultrashort light pulse. It is an object of the present invention to provide a method and an apparatus capable of converting the image into a two-dimensional spatial distribution.
[0005]
[Means for Solving the Problems]
In order to achieve the above object, the present invention is a method for converting a time change of a spectrum of an ultrashort optical signal pulse into a two-dimensional spatial distribution of a time axis and a spectrum axis at a high speed, the signal pulse, a spatial The time-delayed gate pulse is incident on the nonlinear crystal through the condensing member, and the second harmonic light emitted by satisfying the phase matching condition in the nonlinear crystal is dispersed by the dispersive element to obtain an image. An ultrafast conversion method from a time signal to a two-dimensional spatial signal is provided.
[0006]
In order to achieve the above object, the present invention is also an apparatus for converting a temporal change in the spectrum of an ultrashort light pulse into a two-dimensional spatial distribution of a time axis and a spectrum axis, comprising: an input part of a signal pulse; A gate pulse generator for generating a gate pulse to which a time delay is given, a signal pulse input unit and a condensing unit for condensing pulses transmitted from the gate pulse generator, and pulsed light that has passed through the condensing unit A non-linear crystal arranged so as to be incident, a dispersive element for dispersing the second harmonic light emitted by satisfying the phase matching condition in the non-linear crystal, and an image forming unit for forming an image of the dispersed harmonic light The present invention also provides an ultrahigh-speed conversion device for converting a time signal into a two-dimensional spatial signal, characterized in that
[0007]
The traveling direction of the gate pulse is parallel to the traveling direction of the signal pulse, and the time delay amount in the gate pulse is different in the spatial direction perpendicular to the traveling direction of the gate pulse and the alignment direction of the signal pulse. ,desirable.
[0008]
The condensing by the condensing member is preferably performed in a spatial direction given a time delay in the gate pulse.
[0009]
It is desirable that the signal pulse is converted into parallel light and the gate pulse is converted into convergent light by passing through a condensing member.
[0010]
Adjusting the pulse width and the time frequency bandwidth of the gate pulse under mutual interlocking so that the time resolution and time frequency resolution of the image by the second harmonic light are in an appropriate state under light weight selection between them. Can do.
[0011]
The present invention can be advantageously applied to the conversion of an ultrashort optical pulse signal whose pulse width (full width at half maximum in time length) is on the order of p (pico) seconds or less, and particularly on the order of f (femto) seconds, which is difficult to convert by electrical means. This is advantageous for a pulse width of.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a diagram illustrating a main part of an apparatus for converting a time signal into a two-dimensional spatial signal according to an embodiment of the present invention. As shown in FIG. 1, let the horizontal axis and the vertical axis be the x-axis and the y-axis, respectively. FIG. 2 shows the state of the optical system and the ultrashort light pulse when the apparatus of FIG. 1 is viewed from the positive y-axis direction. Also, FIG. 3 shows the state of the optical system and the ultrashort light pulse viewed from the x-axis positive direction in FIG.
[0013]
The apparatus shown in the figure includes a signal pulse input unit 1 and a gate pulse generator 2 as a pulse transmitter, and a condensing unit 3 that condenses the pulses transmitted from the signal pulse input unit and the gate pulse generator, A nonlinear crystal 4 is arranged so that the pulsed light that has passed through the condensing part is incident, and an image forming part 5 that forms an image of harmonic light of a signal pulse and a gate pulse transmitted through the nonlinear crystal. The signal pulse input unit 1 and the gate pulse generator 2 may emit pulses that travel in parallel with each other, or may use reflection and diffraction to make the traveling directions of both pulses parallel. Good.
[0014]
The gate pulse generator generates a gate pulse G that is spatially given a time delay so that the delay amount is different in the spatial direction perpendicular to the traveling direction of the gate pulse and the direction in which the gate pulse is aligned. The condensing unit 3 includes three cylindrical lenses 31, 32, and 33, which are arranged so that the axis of the lens cylindrical surface faces the vertical, horizontal, and vertical directions from the side close to the pulse transmission unit. Has been. The image forming unit 5 includes two cylindrical lenses 51 and 52, a diffraction grating (dispersing element) 53, a cylindrical lens 54, and an imaging element 55 from the side close to the nonlinear crystal 4. As for the axis of the lens cylindrical surface, the cylindrical lenses 51 and 54 are in the vertical direction, and the cylindrical lens 52 is in the horizontal direction.
[0015]
The non-linear crystal 4 generates light having a frequency component twice that of incident light when light with strong power is incident. As materials thereof, KTiOPO ₄ , LiB ₃ O ₅ , β-BaB ₂ O _{4 are used.} Etc.
[0016]
In the following, the configuration of the method and apparatus according to the present invention will be described together with the operating principle. Here, for the sake of simplicity, a double pulse having the same distribution in the y-axis direction and having two peaks in time is used as the signal pulse. Assume that pulses having the same distribution in the x-axis direction and different time delays in the y-axis direction are used as the gate pulse. That is, the wavefront of the gate pulse G is inclined with respect to the traveling direction. In FIG. 2, considering the x-axis direction, the signal pulse S and the gate pulse G are superimposed in the nonlinear crystal 4 by the cylindrical lens 33. However, the signal pulse S is reduced by the cylindrical lenses 31 and 32, and after passing through the cylindrical lens 33, enters the nonlinear crystal 4 as parallel light. Further, the gate pulse G is made into focused light by the cylindrical lens 33 and enters the nonlinear crystal 4.
[0017]
In FIG. 3, considering the y-axis direction, the signal pulse S reaches the nonlinear crystal 4 at the same time because it has the same wavefront in the y-axis direction. Conversely, since the gate pulse G has a time delay in the y-axis direction, it reaches the nonlinear crystal in the order from positive to negative in the y-axis direction. Therefore, the wave number vector of the gate pulse G incident on the nonlinear crystal 4 rotates from the y-axis positive direction to the negative direction with time.
[0018]
In the present invention, a second-order nonlinear effect is used. That is, only when the signal pulse S and the gate pulse G reach the nonlinear crystal 4 at the same time, a second harmonic having a sum of the two wave vectors in a new wave vector is generated. In the state of FIG. 3, the second harmonic M1 is generated when the intersection of the signal pulse S1 and the gate pulse G reaches the nonlinear crystal 4, and the intersection of the signal pulse S2 and the gate pulse G is the nonlinear crystal 4. The second harmonic M2 is generated when reaching This makes it possible to convert the time signal of the signal pulse into a wave vector in the y-axis direction of the second harmonic. In FIG. 2, since the phase matching condition is satisfied also in the x-axis direction, the signal pulse, the gate pulse, and the second harmonic can be easily separated. In FIG. 2, the power surface density can be increased by narrowing the signal pulse S using the cylindrical lenses 31 and 33. As a result, it is possible to improve the conversion efficiency of the second harmonic.
[0019]
Next, the second harmonics emitted from the nonlinear crystal 4 are Fourier-transformed using the lenses 51 and 52 and are incident on the diffraction grating 53. A frequency distribution is obtained in the x-axis direction by Fourier transforming the light dispersed by the diffraction grating 53 using the lens 54. In the example shown in the figure, the incident on the diffraction grating 53 is in the vertical direction, but the oblique incidence on the diffraction grating 53 makes it possible to widen the time window and facilitate imaging with a normal device. The advantage of becoming.
[0020]
As described above, a frequency distribution in the x-axis direction and a time distribution in the y-axis direction are obtained, and the signal pulse can be converted into a two-dimensional spatial distribution corresponding to time and frequency.
[0021]
In the above description, the signal pulse having two peaks has been described as an example. However, in practice, the signal pulse takes the form of a pulse train having an arbitrary pulse width and pulse interval, and the gate pulse has the same pulse width. Often takes the form of a pulse train with pulse intervals. Also in these cases, the signal pulse and the gate pulse overlap each other on the nonlinear crystal so that any pulse in the signal pulse train intersects any pulse in the gate pulse train.
[0022]
Note that gate pulses having different time delay amounts in the spatial direction perpendicular to the traveling direction of the gate pulses and the direction in which the signal pulses are arranged can be obtained, for example, as shown in FIG. 4 or FIG. In the example of FIG. 4, a phase object A having an optical path length different in the pulse wavefront direction is placed in the optical path of the pulse G0. The phase object A has a refractive index different from that of the pulse propagation space. In the example of FIG. 4, there is a larger time delay when a longer part is passed with respect to a shorter part of the optical path length.
[0023]
In the example of FIG. 5, diffraction gratings P1 and P2 having different diffraction angles in the pulse wavefront direction are placed in the optical path of the pulse G0. In the diffraction grating P2, the position of the diffraction angle is almost opposite to that of the diffraction grating P1. Accordingly, the light having a large diffraction angle reaches the diffraction grating P1 through an optical path longer than that of the light having a small diffraction angle, and is diffracted again to become a pulse G1 ′ that travels in substantially the same direction as the first traveling direction. The wavefront tilts with a long time delay.
[0024]
For each distribution in the two-dimensional spatial distribution obtained here, the principle that the product of the width in the time direction (time resolution) and the width in the time frequency direction (time frequency resolution) does not fall below a certain value holds. This is the case when the above-described conversion is performed using, for example, a pulse having a Fourier limit or a phase close to that, such as a laser pulse, as a gate pulse. In the present invention, it is possible to arbitrarily set the respective resolutions while satisfying the Fourier limit. Each resolution is determined by a time-frequency component included in the gate pulse. Here, the time frequency distribution obtained in the two-dimensional distribution is obtained in the form of a total combination of the time frequency distribution of the signal pulse and the time frequency distribution of the gate pulse. Therefore, when the width of the time frequency band included in the gate pulse is wide, the time frequency resolution is lowered, but the pulse width of the gate pulse is shortened, so that the time resolution is improved. Conversely, when the pulse width of the gate pulse is wide, the time resolution decreases, but the time frequency bandwidth included in the gate pulse is narrowed, so the frequency band of the gate pulse that interacts with the signal pulse is narrowed, As a result, the time frequency resolution is improved. Therefore, by adjusting the pulse width and the time frequency bandwidth of the gate pulse under mutual interlocking, the time resolution and the time frequency resolution can be brought into an appropriate state under light weight selection between them. For example, when handling a pulse with coarse information in the time direction and fine information in the time frequency direction, such as a signal used in optical communication, as a signal pulse, a gate pulse with a relatively wide pulse width and a narrow time frequency bandwidth May be used. In addition, when a pulse having fine information in the time direction and rough information in the time frequency direction is handled as a signal pulse as in optical measurement, a gate pulse having a relatively narrow pulse width and a wide time frequency bandwidth may be used. Become. By selecting the gate pulse in this way, it is possible to provide a conversion system corresponding to a wide range of applications.
[0025]
【The invention's effect】
As described above, according to the present invention, it is possible to provide a method for converting a time signal into a two-dimensional spatial signal and an apparatus for carrying out the method, which have the following effects. That is, in the present invention, gate pulses having different time delay amounts in the spatial direction perpendicular to the traveling direction of the gate pulse and the alignment direction of the signal pulse are used, and the two pulses are crossed after passing through the condensing member. Incident on the nonlinear crystal. Since the second harmonic light is generated by intersecting the signal pulse and the gate pulse having a time delay on the nonlinear crystal and the image is obtained, the harmonic wave number vector becomes the time delay of the gate pulse. Accordingly, the direction is changed, and the image forming position changes. The image is formed through spectroscopy by a dispersion element such as a diffraction grating. Therefore, it is possible to convert a time signal included in an ultrashort light pulse into a two-dimensional spatial distribution corresponding to time and time frequency at a very high speed by rotating the harmonic wave vector and forming an image by spectroscopy. .
[0026]
Thus, since the gate pulse performs scanning based on the time delay with respect to one signal pulse, conversion can be performed without requiring repetition of pulse incidence. Further, since the signal pulse and the gate pulse are condensed through the condensing member and enter the nonlinear crystal, the signal pulse and the gate pulse are collected without being limited by the time width that can be scanned due to the size of the nonlinear crystal. Since the pulse is incident on the nonlinear crystal, high conversion efficiency can be obtained.
[0027]
Here, when the two-dimensional spatial distribution is regarded as an image, the present system can be handled as an ultra-high-speed image display system. In addition, in this system, since the relationship between time and time frequency included in one signal pulse can be measured in real time, it is possible to apply an extremely short optical pulse to an optical measurement system.
[Brief description of the drawings]
FIG. 1 is a perspective view illustrating a main part of an ultra-high speed conversion device from a time signal to a two-dimensional spatial signal according to an embodiment of the present invention.
2 is a diagram showing an optical system and an ultrashort light pulse when the apparatus of FIG. 1 is viewed from the positive y-axis direction.
FIG. 3 is a diagram showing an optical system and an ultrashort light pulse when the apparatus of FIG. 1 is viewed from the positive direction of the x-axis.
FIG. 4 is an explanatory diagram showing an example of a technique for generating a gate pulse to which a spatial delay is given spatially.
FIG. 5 is an explanatory diagram showing another example of a method for generating a gate pulse to which a spatial delay is given spatially.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Signal pulse input part 1, 2 ... Gate pulse generator,
3 ... light condensing part, 4 ... nonlinear crystal,
5 ... image forming unit, 53 ... diffraction grating (dispersing element),
G ... Gate pulse, S ... Signal pulse

Claims (9)

This is a method for converting the time change of the spectrum of an ultrashort optical signal pulse into a two-dimensional spatial distribution of the time axis and the spectrum axis at an extremely high speed, which comprises a signal pulse and a gate pulse with a spatial time delay. The second harmonic light is incident on the nonlinear crystal through the condensing member, and the second harmonic light emitted by satisfying the phase matching condition in the nonlinear crystal is dispersed by a dispersive element to obtain an image from a two-dimensional space. Ultra-high speed conversion method to signal. The traveling direction of the gate pulse is parallel to the traveling direction of the signal pulse, and the time delay amount in the gate pulse is different in a spatial direction perpendicular to the traveling direction of the gate pulse and the alignment direction of the signal pulse. The ultrafast conversion method according to claim 1, wherein: 3. The ultrafast conversion method according to claim 2, wherein the condensing by the condensing member is performed in a spatial direction given a time delay in the gate pulse. The ultrafast conversion method according to any one of claims 1 to 3, wherein the signal pulse is converted into parallel light and the gate pulse is converted into convergent light by passing through the light collecting member. Adjusting the pulse width and the time frequency bandwidth of the gate pulse under mutual interlocking so that the time resolution and time frequency resolution of the image by the second harmonic light are in an appropriate state under light weight selection between them. The ultrafast conversion method according to any one of claims 1 to 4, wherein: A device that converts the temporal change of the spectrum of an ultrashort light pulse into a two-dimensional spatial distribution of the time axis and the spectrum axis, and generates a signal pulse input section and a gate pulse that is spatially given a time delay. A gate pulse generator, a condensing part for condensing pulses sent from the signal pulse input part and the gate pulse generator, and a nonlinear crystal arranged so as to make incident the pulsed light that has passed through the condensing part, From a time signal, comprising: a dispersive element for dispersing second harmonic light emitted by satisfying phase matching conditions in the nonlinear crystal; and an image forming unit for forming an image of the dispersed harmonic light. Ultra-high-speed converter for two-dimensional spatial signals. The traveling direction of the gate pulse is parallel to the traveling direction of the signal pulse, and the time delay amount in the gate pulse is different in a spatial direction perpendicular to the traveling direction of the gate pulse and the alignment direction of the signal pulse. The ultrahigh-speed conversion device according to claim 6, wherein 8. The ultra high-speed conversion device according to claim 7, wherein the condensing by the condensing member is performed in a spatial direction given a time delay in the gate pulse. 9. The signal conversion apparatus according to claim 6, wherein the signal pulse is converted into parallel light and the gate pulse is converted into convergent light by passing through the light collecting member.

Images