JP2005106751A - Phase characteristic measuring device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a phase characteristic measuring device which can accurately and easily measure phase characteristics of a light pulse to be measured even in the case of low wavelength resolution of a spectroscopic measuring part. <P>SOLUTION: A double light pulse outputted from a double light pulse generating part 10 and a chirp light pulse outputted from a chirp imparting part 20 are inputted to a sum frequency light generating part 30 so that a sum frequency light is generated. The double light pulse outputted from the double light pulse generating part 10 is inputted to a higher harmonic light generating part 40 so that a higher harmonic light is generated. Either of the sum frequency light or higher harmonic light is selected by a selection part 60, and the selected light is inputted to a spectroscopic measuring part 50 to be spectroscopically measured thereby. Based on the result of spectroscopic measurement of the higher harmonic light by the spectroscopic measuring part 50, the result of spectroscopic measurement of the sum frequency light by the spectroscopic measuring part 50 is corrected by an analysis part 70, and the result of spectroscopic measurement of the sum frequency light corrected is analyzed to measure the phase characteristics of the light pulse to be measured. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a phase characteristic measuring apparatus for measuring a phase characteristic of an optical pulse.

  A streak camera is known as an apparatus for measuring an optical pulse waveform in the time domain. A streak camera inputs a light pulse to be measured to a photoelectric conversion surface to generate photoelectrons, passes the photoelectrons between a pair of deflecting plates and enters a fluorescent screen, and sweeps voltage synchronized with the generation timing of the light pulse. Is applied between a pair of deflection plates. Thereby, a temporal change in the intensity of the light pulse is observed as a spatial change in the fluorescence intensity on the phosphor screen. However, when trying to measure the waveform of an optical pulse with a short pulse width, the streak camera cannot measure the optical pulse waveform in the time domain because the time resolution is insufficient.

Therefore, a phase characteristic measurement technique is known in which the amplitude and phase of the optical pulse to be measured are measured in the spectral domain, and the optical pulse waveform in the time domain is reconstructed based on the measurement result. SPIDER (spectral phase interferometry for direct electric-field reconstruction) technology, which is one of such technologies, uses a spectroscopic measurement unit to calculate the sum frequency light spectrum of a double light pulse and a chirped light pulse generated from a measured light pulse. The phase characteristic of the light pulse to be measured is measured based on the spectroscopic measurement result (see, for example, Non-Patent Document 1).
Chris Iaconis, et al., "Self-Referencing Spectral Interferometry for Measuring Ultrashort Optical Pulses", IEEE Journal of Quantum Electronics, Vol.35, No.4, pp.501-509 (1999)

  By the way, in order to measure fine interference fringes in the sum frequency light spectrum with high accuracy, the spectroscopic measurement unit requires high wavelength resolution. Further, when the pulse width of the light pulse to be measured is about 10 fs, the sum frequency light spectrum of the double light pulse and the chirp light pulse has a spread of about 100 nm, so the spectroscopic measurement unit has a wide spectrum measurement range. Is also required. That is, the spectroscopic measurement unit requires both high wavelength resolution and a wide spectrum measurement range in order to measure the sum frequency light spectrum with high accuracy.

  However, a spectroscope that can be used as a spectroscopic measurement unit capable of both high wavelength resolution and a wide spectrum measurement range is expensive. Further, when the pulse width of the light pulse to be measured is narrower, the required specification for the spectroscope becomes more severe. Thus, even when the above-mentioned SPIDER technique is used, it is difficult to measure the phase characteristics of the measured light pulse with high accuracy when the measured light pulse has an ultrashort pulse width.

  FIG. 8 is a diagram for explaining the problems of the conventional phase characteristic measuring apparatus. Here, the calculation result by simulation is shown by setting the pulse width of the light pulse to be measured to 5 fs. FIG. 8A shows a sum frequency light spectrum S1 measured by a spectroscopic measurement unit having a sufficiently high wavelength resolution and a sum frequency light spectrum S2 measured by a spectroscopic measurement unit having a low wavelength resolution. FIG. 8B shows a phase characteristic P1 obtained by analyzing the sum frequency light spectrum S1 measured by a spectroscopic measurement unit having a sufficiently high wavelength resolution, and a sum frequency measured by a spectroscopic measurement unit having a low wavelength resolution. The phase characteristic P2 obtained by the analysis of the optical spectrum S2 is shown. FIG. 8B also shows the intensity spectrum I of the light pulse to be measured for reference, but this is not obtained by the SPIDER technique. As can be seen from this figure, when the wavelength resolution of the spectroscopic measurement unit is insufficient, the sum frequency light spectrum S2 measured by the spectroscopic measurement unit does not have a sufficient degree of modulation of the interference fringe waveform. The phase characteristic P2 obtained by the analysis of the sum frequency light spectrum S2 has an error with respect to the true phase characteristic P1.

  The present invention has been made in order to solve the above-described problems, and is capable of easily and accurately measuring the phase characteristics of the optical pulse to be measured even when the wavelength resolution of the spectroscopic measurement unit is low. An object is to provide a characteristic measuring apparatus.

  The phase characteristic measuring apparatus according to the present invention includes (1) inputting an optical pulse to be measured, converting the optical pulse to be measured into a double optical pulse having a pulse interval τ, and outputting the double optical pulse. Double optical pulse generator that can adjust the interval τ; (2) Input the probe light pulse, give a linear chirp to the probe light pulse, convert it to a chirp light pulse, and output this chirp light pulse The chirping unit, (3) the double optical pulse output from the double optical pulse generation unit, and the chirped optical pulse output from the chirping unit are input, and the optical frequency of each of the double optical pulse and the chirped optical pulse. A sum frequency light generation unit that generates and outputs a sum frequency light having an optical frequency equal to the sum of (4), and inputs a double light pulse output from the double light pulse generation unit, A harmonic light generator that generates and outputs harmonic light having an optical frequency that is twice the optical frequency of the double optical pulse, (5) a spectroscopic measurement unit that spectroscopically measures the input light, and (6) sum frequency light generation A selection unit that selectively inputs either the sum frequency light output from the unit or the harmonic light output from the harmonic light generation unit to the spectroscopic measurement unit; and (7) output from the sum frequency light generation unit. The optical pulse to be measured is corrected and analyzed based on the result of the spectroscopic measurement of the harmonic light output from the harmonic light generation unit. And an analysis unit for measuring the phase characteristic of the above.

  In this phase characteristic measuring apparatus, the optical pulse to be measured is input to the double optical pulse generator, converted into a double optical pulse with a pulse interval τ, and the double optical pulse is output from the double optical pulse generator. At this time, the output pulse interval τ can be adjusted. On the other hand, the probe light pulse is input to the chirp applying unit to be given a linear chirp and converted into a chirped light pulse. The chirped light pulse is converted into the chirped applying unit. Is output from. The double light pulse output from the double light pulse generation unit and the chirp light pulse output from the chirping unit are input to the sum frequency light generation unit, and the optical frequency of each of the double light pulse and the chirp light pulse is input. A sum frequency light having an optical frequency equal to the sum is generated and output by the sum frequency light generator. The double light pulse output from the double light pulse generator is input to the harmonic light generator, and the harmonic light having an optical frequency twice as high as the optical frequency of the double light pulse is generated by the harmonic light generator. Is output. One of the sum frequency light output from the sum frequency light generation unit and the harmonic light output from the harmonic light generation unit is selected by the selection unit and input to the spectroscopic measurement unit, and the spectroscopic measurement unit performs spectroscopic measurement. Is done. The result of the spectral measurement of the sum frequency light output from the sum frequency light generation unit by the spectroscopic measurement unit is the result of the spectral measurement of the harmonic light output from the harmonic light generation unit. The phase characteristics of the light pulse to be measured are measured by correcting and analyzing based on the above.

  The phase characteristic measuring apparatus according to the present invention splits an optical pulse to be measured into two parts, inputs one of the two splits into a double optical pulse generator, and inputs the other into a chirp applying part as a probe optical pulse. It is preferable to further include The probe light pulse input to the chirping unit may be a short pulse of light output from another light source synchronized with the light pulse to be measured, but when a light branching unit as described above is provided, A part of the optical pulse to be measured input to the double optical pulse generator is branched and extracted by the optical branching unit and used as the probe optical pulse.

  The phase characteristic measuring apparatus according to the present invention preferably further includes a storage unit that stores correction information at the time of correction in the analysis unit. As long as the spectroscopic measurement unit is the same, the device function (response function) of the spectroscopic measurement unit is considered to be constant. Therefore, when the above storage unit is provided, each spectroscopic measurement unit The correction information obtained in step 1 may be stored in the storage unit.

  The phase characteristic measuring apparatus according to the present invention further includes a light detection unit that detects the intensity of the harmonic light output from the harmonic light generation unit, and the analysis unit performs fringe decomposition SHG autocorrelation based on the detection result by the light detection unit. It is preferable to obtain the waveform. In this case, the intensity of the harmonic light output from the harmonic light generation unit is detected by the light detection unit, and the fringe decomposition SHG autocorrelation waveform is obtained by the analysis unit based on the detection result by the light detection unit. Then, based on the fringe decomposition SHG autocorrelation waveform, information on the pulse width and chirp of the optical pulse to be measured is easily obtained. Further, the pulse interval in the double light pulse when the peak of the fringe decomposition SHG autocorrelation waveform is obtained is calibrated as 0 point.

  In the phase characteristic measuring apparatus according to the present invention, the sum frequency light generation unit and the harmonic light generation unit may each include a separate nonlinear optical crystal, or the sum frequency light generation unit and the harmonic light generation unit share a non-linearity. An optical crystal may be included.

  According to the present invention, even when the wavelength resolution of the spectroscopic measurement unit is low, the phase characteristics of the light pulse to be measured can be easily measured with high accuracy.

  The best mode for carrying out the present invention will be described below in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted.

  First, a schematic configuration of the phase characteristic measuring apparatus according to the present embodiment will be described. FIG. 1 is a schematic configuration diagram of a phase characteristic measuring apparatus 1 according to the present embodiment. The phase characteristic measuring apparatus 1 includes a double light pulse generation unit 10, a chirping unit 20, a sum frequency light generation unit 30, a harmonic light generation unit 40, a spectroscopic measurement unit 50, a selection unit 60, an analysis unit 70, and a storage unit 80. .

  The double light pulse generator 10 receives the light pulse to be measured, converts the light pulse to be measured into a double light pulse having a pulse interval τ, and outputs the double light pulse. Further, the pulse interval τ can be adjusted. The double optical pulse generator 10 is, for example, a Michelson interferometer or a Mach-Zehnder interferometer as a specific optical system. Further, the adjustment of the pulse interval τ in the double light pulse can be performed, for example, by adjusting the position of one reflecting mirror in the Michelson interferometer, or by adjusting one optical path length in the Mach-Zehnder interferometer. Is possible.

  The chirp applying unit 20 receives the probe light pulse, gives a linear chirp to the probe light pulse, converts it to a chirp light pulse, and outputs the chirp light pulse. The chirp imparting unit 20 may be, for example, a material having wavelength dispersion characteristics (for example, optical glass such as BK7 or SF10) itself, or may be configured by a diffraction grating. The chirp applying unit 20 gives a chirp light pulse in which the time and the instantaneous frequency at the time have a linear relationship within the pulse width by giving a distribution to the propagation speed for each electric field frequency component included in the input light. Can be output. Here, the instantaneous frequency is an amount defined by time differentiation of the phase. The probe light pulse input to the chirp applying unit 20 may be a part of the measured optical pulse input to the double optical pulse generation unit 10 that is branched off by the optical branching unit, or may be extracted. It may be a short pulse of light output from another light source synchronized with the measurement light pulse.

  The sum frequency light generation unit 30 inputs the double light pulse output from the double light pulse generation unit 10 and the chirp light pulse output from the chirping unit 20, and each of the double light pulse and the chirp light pulse is input. Sum frequency light having an optical frequency equal to the sum of the optical frequencies is generated and output. The sum frequency light generation unit 30 includes a nonlinear optical crystal such as BBO, for example, and generates and outputs a sum frequency light by inputting a double light pulse and a chirp light pulse collinearly or non-collinearly to the nonlinear optical crystal. .

  The harmonic light generation unit 40 receives the double light pulse output from the double light pulse generation unit 10, and generates and outputs harmonic light having an optical frequency twice as high as the optical frequency of the double light pulse. The harmonic light generation unit 40 includes a nonlinear optical crystal such as BBO, for example, and generates and outputs harmonic light by inputting a double light pulse to the nonlinear optical crystal. The nonlinear optical crystal included in the sum frequency light generation unit 30 and the nonlinear optical crystal included in the harmonic light generation unit 40 may be common or may be separate from each other.

  The spectroscopic measurement unit 50 is a spectroscope that receives the light output from the selection unit 60 and spectroscopically measures the light. The selection unit 60 selectively inputs either the sum frequency light output from the sum frequency light generation unit 30 or the harmonic light output from the harmonic light generation unit 40 to the spectroscopic measurement unit 50. The selection unit 60 is not necessarily arranged between the sum frequency light generation unit 30, the harmonic light generation unit 40, and the spectroscopic measurement unit 50.

  The analysis unit 70 uses the spectroscopic measurement unit 50 to measure the harmonic light output from the harmonic light generation unit 40 based on the spectral measurement result of the sum frequency light output from the sum frequency light generation unit 30. Based on the result, correction and analysis are performed to measure the phase characteristic of the light pulse to be measured. At this time, the analysis unit 70 may correct the result of the spectral measurement of the sum frequency light and then analyze the corrected result of the spectral measurement to measure the phase characteristic of the light pulse to be measured. The analysis unit 70 may measure the phase characteristic of the light pulse to be measured by correcting the result of analyzing the result of the spectral measurement of the sum frequency light. For example, a computer is suitably used as the analysis unit 70.

  The storage unit 80 stores correction information at the time of correction in the analysis unit 70. Since the device function (response function) of the spectroscopic measurement unit 50 is considered to be constant as long as the spectroscopic measurement unit 50 is the same, the storage unit 80 stores correction information obtained for each spectroscopic measurement unit. Should be remembered. For example, when a computer is used as the analysis unit 70, a storage device (for example, a hard disk device) attached to the computer is used as the storage unit 80, and the spectrum used in the analysis operation by the analysis unit 70 is used. Correction information corresponding to the measurement unit is read from the storage device and used.

  The sum frequency light generated by the sum frequency light generation unit 30 may propagate through the space and enter the spectroscopic measurement unit 50, or may be guided to the spectroscopic measurement unit 50 by an optical fiber. Further, the harmonic light generated by the harmonic light generation unit 40 may propagate through the space and enter the spectroscopic measurement unit 50, or may be guided to the spectroscopic measurement unit 50 or the light detection unit 90 by an optical fiber.

  Next, the operation of the phase characteristic measuring apparatus 1 will be described. The optical pulse to be measured is input to the double optical pulse generation unit 10 and converted into a double optical pulse having a pulse interval τ, and the double optical pulse is output from the double optical pulse generation unit 10. At this time, the output pulse interval τ can be adjusted. On the other hand, the probe light pulse is input to the chirp applying unit 20 to be given a linear chirp and converted into a chirped light pulse. Output from the unit 20. The double light pulse output from the double light pulse generation unit 10 and the chirp light pulse output from the chirp applying unit 20 are input to the sum frequency light generation unit 30, and each of the double light pulse and the chirp light pulse is output. Sum frequency light having an optical frequency equal to the sum of the optical frequencies is generated and output by the sum frequency light generation unit 30.

  The double light pulse output from the double light pulse generation unit 10 is input to the harmonic light generation unit 40, and the harmonic light having an optical frequency twice as high as the optical frequency of the double light pulse is generated by the harmonic light generation unit 40. Generated and output. One of the sum frequency light output from the sum frequency light generation unit 30 and the harmonic light output from the harmonic light generation unit 40 is selected by the selection unit 60 and input to the spectroscopic measurement unit 50 to perform spectroscopic measurement. Spectroscopic measurement is performed by the unit 50. Then, based on the result of the spectral measurement of the harmonic light output from the harmonic light generation unit 40 by the analysis unit 70, the sum frequency light output from the sum frequency light generation unit 30 is converted into the spectral measurement unit. The result of spectroscopic measurement by 50 is corrected, and the result of spectroscopic measurement of the corrected sum frequency light is analyzed to measure the phase characteristic of the light pulse to be measured.

  The phase characteristic measuring apparatus 1 corrects the result of the spectral measurement of the sum frequency light based on the result of the spectral measurement of the harmonic light as described above, so that even if the wavelength resolution of the spectroscopic measurement unit 50 is low. The phase characteristics of the light pulse to be measured can be easily measured with high accuracy.

  Next, an analysis operation in the analysis unit 70 of the phase characteristic measuring apparatus 1 according to the present embodiment will be described. FIG. 2 is a flowchart for explaining an analysis operation in the analysis unit 70 of the phase characteristic measuring apparatus 1 according to this embodiment. In the flowchart shown in this figure, the result (namely, correction information) obtained by spectral measurement of the harmonic light output from the harmonic light generation unit 40 via the selection unit 60 by the spectroscopic measurement unit 50 has already been obtained and is stored in the storage unit 80. It is assumed that it is remembered.

In step S <b> 11, the sum frequency light output from the sum frequency light generation unit 30 is spectrally measured by the spectroscopic measurement unit 50 via the selection unit 60, and the spectrum of the sum frequency light is acquired by the analysis unit 70. The sum frequency light spectrum acquired here includes the contribution of the device function of the spectroscopic measurement unit 50. When the wavelength resolution of the spectroscopic measurement unit 50 is insufficient, the shortage of wavelength resolution is caused. It has deteriorated. Assuming that the true sum frequency light spectrum is I (λ) and the device function of the spectroscopic measurement unit 50 is H (λ), the acquired sum frequency light spectrum O (λ) is expressed by the following equation (1). It is represented by a convolution of (λ) and H (λ). Here, λ represents the wavelength of light. F represents a Fourier transform operation, and F ⁻¹ represents an inverse Fourier transform operation.

続くステップＳ１２では、ステップＳ１１で取得された和周波光スペクトルＯ(λ)は、記憶部８０により記憶されている補正情報に基づいて補正されて、分光計測部５０の装置関数Ｈ(λ)の寄与が除去されて、これにより、真の和周波光スペクトルＩ(λ)が得られる。すなわち、下記(2)式で表されるように、デコンボリューション演算により、真の和周波光スペクトルＩ(λ)が得られる。

In subsequent step S12, the sum frequency light spectrum O (λ) acquired in step S11 is corrected based on the correction information stored in the storage unit 80, and the device function H (λ) of the spectroscopic measurement unit 50 is corrected. The contribution is removed, thereby obtaining a true sum frequency light spectrum I (λ). That is, as expressed by the following equation (2), a true sum frequency light spectrum I (λ) is obtained by deconvolution calculation.

ステップＳ１２で求められた真の和周波光スペクトルは、これまでの波長λの関数から光周波数ｆの関数に変換される（ステップＳ１３）。そして、光周波数ｆの関数としての和周波光スペクトルは、フーリエ変換され（ステップＳ１４）、そのフーリエ変換結果がフィルタリングされて−ＡＣ成分およびＤＣ成分が除去され（ステップＳ１５）、そのフィルタリングされたものが逆フーリエ変換される（ステップＳ１６）。この逆フーリエ変換により得られた位相波形からωτが減算され（ステップＳ１７）、この減算結果が積分されて被測定光パルスの位相特性が測定される（ステップＳ１８）。ここで、ωは、光の角周波数であり、真空中の光の速度ｃを用いて「ω＝２πｃ／λ」なる式で表される。

The true sum frequency light spectrum obtained in step S12 is converted from the function of wavelength λ so far into a function of optical frequency f (step S13). Then, the sum frequency optical spectrum as a function of the optical frequency f is Fourier transformed (step S14), the Fourier transformation result is filtered to remove the -AC component and DC component (step S15), and the filtered one. Are subjected to inverse Fourier transform (step S16). Ωτ is subtracted from the phase waveform obtained by the inverse Fourier transform (step S17), and the subtraction result is integrated to measure the phase characteristic of the optical pulse to be measured (step S18). Here, ω is an angular frequency of light, and is represented by an expression “ω = 2πc / λ” using the speed of light c in vacuum.

  By the way, as the simplest case, when the response function is constant in all wavelength regions of the spectroscopic measurement unit 50 (or when it can be regarded as constant), it is used in the deconvolution calculation of the above equation (2). The device function H (λ) to be obtained can be easily determined. That is, light having a sufficiently narrow spectrum width (impulse input) is input to the spectroscopic measurement unit 50, and the spectrum (impulse response function) measured by the spectroscopic measurement unit 50 at this time is obtained. A device function is determined.

  However, in general, the spectroscopic measurement unit 50 cannot obtain an apparatus function that is constant at all wavelengths due to the influence of aberration of the optical system for forming an image on the output surface. Hereinafter, for convenience of explanation, a method for obtaining the device function of the spectroscopic measurement unit 50 will be described assuming that the space of the optical frequency axis or the wavelength axis is called real space and the space obtained by Fourier transform is called Fourier space. .

  The impulse response function represents the degree of deterioration of the degree of modulation at all optical frequencies or wavelengths when considered in Fourier space. In the deconvolution operation (equation (2) above), the Fourier transform result of the acquired sum frequency light spectrum I (λ) is divided by the Fourier transform result of the device function O (λ). However, as shown in FIG. 3, the range of the sum frequency light spectrum I (λ) to be subjected to the deconvolution calculation is limited to a narrow portion.

  FIG. 3 is a diagram illustrating a sum frequency light spectrum and a device function spectrum in Fourier space. An enlarged view of the vicinity of the -AC component of the sum frequency light spectrum in FIG. 9A is shown in FIG. As shown in this figure, in Fourier space, the sum frequency light spectrum is divided into three parts, -AC component, DC component, and + AC component, and -AC component and DC component are filtered out of them (in FIG. 2). In step S15), only the + AC component is left as a necessary component.

  Therefore, the device function O (λ) of the spectroscopic measurement unit 50 only needs to be obtained within a limited necessary range including the + AC component of the sum frequency light spectrum I (λ). Further, the + AC component of the sum frequency light spectrum I (λ) appears in the vicinity of a position corresponding to the pulse interval τ in the double light pulse generated by the double light pulse generator 10. From the above, in the phase characteristic measuring apparatus 1 according to the present embodiment, the apparatus function of the spectroscopic measurement unit 50 is obtained as follows.

FIG. 4 is a flowchart for explaining an apparatus function acquisition operation in the analysis unit 70 of the phase characteristic measuring apparatus 1 according to this embodiment. Here, the harmonic light output from the harmonic light generation unit 40 is spectrally measured by the spectroscopic measurement unit 50 via the selection unit 60. Further, the pulse interval in the double light pulse output from the double light pulse generator 10 and input to the harmonic light generator 40 is τ ₁ within a limited necessary range including the + AC component of the sum frequency light spectrum in FIG. ˜τ _n are sequentially set (n is an integer of 2 or more).

In step S21, the double optical pulses output from the double pulse generator 10 is set to a pulse interval tau _1, in step S22, the harmonic light is spectroscopically measured by spectroscopic measurement section 50 output from the harmonic light generator 40 The spectrum of the harmonic light is acquired by the analysis unit 70. If the resolution of the spectroscopic measurement unit 50 is sufficiently high, interference fringes with a frequency interval corresponding to 1 / τ ₁ with a modulation factor of approximately 100% are recognized in the acquired harmonic light spectrum. However, when the resolution of the spectroscopic measurement unit 50 is low, the acquired harmonic light spectrum is modulated into interference fringes having a modulation degree corresponding to the resolution. Therefore, based on the obtained fringe information of the harmonic light spectrum, the response to the frequency of 1 / τ _i in the impulse response function can be obtained in subsequent steps S23 to S27.

The harmonic light spectrum obtained in step S22 is converted from the function of wavelength λ so far into a function of optical frequency f (step S23). The harmonic optical spectrum as a function of the optical frequency f is Fourier transformed (step S24), the Fourier transformation result is filtered to remove the -AC component and DC component (step S25), and the filtered one. Are subjected to inverse Fourier transform (step S26). This inverse Fourier transform result, the amplitude A ₁ and phase phi ₁ the device functions of the spectroscopic measurement unit 50 corresponding to the pulse interval tau ₁ is determined (step S27).

Subsequently, in step S29, double optical pulses output from the double pulse generator 10 is set to a pulse interval tau _2, in step S22, the harmonic light by spectroscopic measurement section 50 output from the harmonic light generator 40 is spectroscopy and the spectrum of the harmonic light is obtained by the analysis unit 70, similarly, the amplitude a ₂ and phase phi ₂ of the device functions of the spectroscopic measurement unit 50 corresponding to the pulse interval tau ₂ are obtained. Further similarly, the amplitude _{A i} and phase phi _i of the device function of the spectral measurement unit 50 corresponding to the pulse interval tau _i is calculated (i = 3~n).

As described above, the device function of the spectroscopic measurement unit 50 is obtained discretely as a series of amplitudes A _i and phases φ _i (i = 1 to n). Thus, since the device function of the spectroscopic measurement unit 50 is obtained from the equally spaced interference fringes on the optical frequency axis instead of the wavelength axis, the sum frequency light spectrum contributes to the device function as shown in the flowchart of FIG. Although it may be converted into a function of the optical frequency f after being removed, the contribution of the device function is removed after the sum frequency light spectrum is converted into a function of the optical frequency f as shown in the flowchart of FIG. Is preferred.

  FIG. 5 is a flowchart for explaining another example of the analysis operation in the analysis unit 70 of the phase characteristic measuring apparatus 1 according to this embodiment. Also in the flowchart shown in this figure, the result (that is, correction information) obtained by spectral measurement of the harmonic light output from the harmonic light generation unit 40 through the selection unit 60 by the spectroscopic measurement unit 50 has already been obtained and is stored in the storage unit 80. It is assumed that it is remembered.

  In step S <b> 31, the sum frequency light output from the sum frequency light generation unit 30 is spectrally measured by the spectroscopic measurement unit 50 via the selection unit 60, and the spectrum of the sum frequency light is acquired by the analysis unit 70. In the subsequent step S32, the acquired sum frequency light spectrum is converted from a function of the wavelength λ so far to a function of the optical frequency f. Thereafter, in step S33, the sum frequency light spectrum as a function of the optical frequency f is corrected based on the correction information stored in the storage unit 80, and the contribution of the device function of the spectroscopic measurement unit 50 is removed. Thereby, a true sum frequency light spectrum is obtained.

  The true sum frequency light spectrum is Fourier transformed (step S34), the result of the Fourier transformation is filtered to remove the -AC component and the DC component (step S35), and the filtered one is the inverse Fourier transform. (Step S36). Ωτ is subtracted from the phase waveform obtained by the inverse Fourier transform (step S37), and the subtraction result is integrated to measure the phase characteristic of the optical pulse to be measured (step S38).

Note that the device function of the spectroscopic measurement unit 50 may be approximated by a polynomial based on n sets of amplitudes A _i and phases φ _i obtained discretely. As the simplest method, n = 2, and the apparatus function of the spectroscopic measurement unit 50 may be linearly approximated based on two sets of amplitude A _i and phase φ _i . Further, in the deconvolution calculation (the above equation (2)) performed in step S12 in FIG. 2 or step S33 in FIG. 5, it is only necessary to approximate the true sum frequency light spectrum by successive approximation calculation.

  Next, a specific configuration example of the phase characteristic measuring apparatus according to the present invention will be described. FIG. 6 is a diagram illustrating a first configuration example of the phase characteristic measuring apparatus according to the present embodiment. The phase characteristic measuring apparatus 2 shown in this figure includes a double optical pulse generation unit 10A, a chirping unit 20, a spectroscopic measurement unit 50, an analysis unit 70, a storage unit 80, a beam splitter BS1, a beam splitter BS2, a mirror M1, and a lens L. The nonlinear optical crystal NC and the shutter S are provided.

  Among these, each of the double light pulse generation unit 10A, the chirping unit 20, the spectroscopic measurement unit 50, the analysis unit 70, and the storage unit 80 is the same as the component having the same name already described with reference to FIG. is there. However, the double optical pulse generation unit 10A in the phase characteristic measuring apparatus 2 includes a beam splitter BS3, a mirror M2, and a mirror M3, and constitutes a Michelson interferometer.

  That is, the beam splitter BS3 receives the optical pulse to be measured that has arrived from the beam splitter BS1, splits it into two, outputs one optical pulse obtained by the two branches to the mirror M2, and outputs the other optical pulse to the mirror M3. Output to. Further, the beam splitter BS3 inputs the optical pulse reflected by the mirror M2, and also inputs the optical pulse reflected by the mirror M3, and outputs these as a double optical pulse to the beam splitter BS2. Either the mirror M2 or the mirror M3 can be moved in a direction parallel to the normal line of the reflecting surface, and this movement can adjust the pulse interval τ in the double light pulse.

  The beam splitter BS1 acts as an optical branching unit that splits the optical pulse to be measured into two parts, inputs one of the two splits to the double optical pulse generation unit 10A, and inputs the other to the chirping unit 20 as a probe optical pulse. Is. That is, the beam splitter BS1 inputs the optical pulse to be measured and branches it into two, outputs one optical pulse obtained by the two branches to the double optical pulse generator 10A, and outputs the other optical pulse to the mirror M1. To do. The mirror M1 reflects the optical pulse that has arrived from the beam splitter BS1 to the chirping unit 20.

  The beam splitter BS2 receives the double light pulse output from the double light pulse generation unit 10A and also receives the chirp light pulse output from the chirping unit 20, and outputs the double light pulse and the chirp light pulse. Combine and output to the lens L. The lens L receives the double light pulse and the chirped light pulse that are combined and output by the beam splitter BS2, and condenses the light pulses on the nonlinear optical crystal NC. The nonlinear optical crystal NC generates and outputs sum frequency light having an optical frequency equal to the sum of the optical frequencies of the double light pulse and the chirped light pulse that are collected by the lens L and incident.

  A shutter S is provided in the middle of the optical path from the beam splitter BS1 through the mirror M1 to the beam splitter BS2. This shutter S can be freely placed and retracted on this optical path, and permits and prohibits the input of chirped light pulses to the nonlinear optical crystal NC. When the shutter S is on the optical path, the input of the chirped light pulse to the nonlinear optical crystal NC is prohibited. Therefore, the nonlinear optical crystal NC inputs only the double light pulse, and the optical frequency of the double light pulse is reduced. Harmonic light having twice the optical frequency is generated and output. On the other hand, when the shutter S is retracted from the optical path, the input of the chirped light pulse to the nonlinear optical crystal NC is permitted. Therefore, the nonlinear optical crystal NC inputs both the double light pulse and the chirped light pulse. , Generate and output sum frequency light. That is, the nonlinear optical crystal NC serves as both a sum frequency light generation unit and a harmonic light generation unit. In addition, the shutter S functions as a selection unit that selectively inputs either the sum frequency light or the harmonic light to the spectroscopic measurement unit 50.

  In the phase characteristic measuring apparatus 2 configured as described above, the shutter S is placed on the optical path, so that the input of the chirped light pulse to the nonlinear optical crystal NC is prohibited and is output from the double light pulse generating unit 10A. The pulse interval in the double light pulse is sequentially set to several appropriate values. A double light pulse having a pulse interval of each value is input to the nonlinear optical crystal NC, and the spectrum of the harmonic light generated by the nonlinear optical crystal NC is measured by the spectroscopic measurement unit 50. Then, according to the flowchart shown in FIG. 4, the measured harmonic light spectrum is analyzed by the analysis unit 70, the device function of the spectroscopic measurement unit 50 is obtained, and the correction information is stored in the storage unit 80.

  Subsequently, when the shutter S is retracted from the optical path, the input of the chirped light pulse to the nonlinear optical crystal NC is permitted, and the pulse interval in the double light pulse output from the double light pulse generator 10A is an appropriate value. Set to The double light pulse and the chirped light pulse are input to the nonlinear optical crystal NC, and the spectrum of the sum frequency light generated by the nonlinear optical crystal NC is measured by the spectroscopic measurement unit 50. Then, according to the flowchart shown in FIG. 2 or FIG. 5, the analysis unit 70 corrects the sum frequency light spectrum based on the correction information stored in the storage unit 80, and based on the corrected sum frequency light spectrum. Thus, the phase characteristic of the light pulse to be measured is measured.

  FIG. 7 is a diagram illustrating a second configuration example of the phase characteristic measuring apparatus according to the present embodiment. The phase characteristic measuring apparatus 3 shown in this figure includes a double light pulse generation unit 10B, a chirping unit 20, a sum frequency light generation unit 30, a harmonic light generation unit 40, a spectroscopic measurement unit 50, an analysis unit 70, a storage unit 80, A light detection unit 90, a beam splitter BS1, mirrors M1 to M5, a lens L, and a concave mirror CM are provided.

  Among these, the double light pulse generation unit 10B, the chirp applying unit 20, the sum frequency light generation unit 30, the harmonic light generation unit 40, the spectroscopic measurement unit 50, the analysis unit 70, and the storage unit 80 are respectively shown in FIG. This is the same as the component having the same name already described. However, the double light pulse generation unit 10B in the phase characteristic measuring apparatus 3 includes a beam splitter BS2, a beam splitter BS3, a corner reflector CR1, and a corner reflector CR2, and constitutes a Michelson interferometer.

  In other words, the beam splitter BS2 inputs the optical pulse to be measured that has arrived from the beam splitter BS1, splits it into two, outputs one optical pulse obtained by the two branches to the corner reflector CR1, and outputs the other optical pulse to the corner. Output to the reflector CR2. The beam splitter BS3 inputs the light pulse reflected by the corner reflector CR1 and also inputs the light pulse reflected by the corner reflector CR2, and outputs these as double light pulses to the mirror M2 and the lens L, respectively. To do. Either the corner reflector CR1 or the corner reflector CR2 can be moved in the direction of entering and exiting the optical pulse, and this movement can adjust the pulse interval τ in the double optical pulse.

  The beam splitter BS1 acts as an optical branching unit that splits the optical pulse to be measured into two parts, inputs one of the two splits to the double optical pulse generation unit 10B, and inputs the other to the chirping unit 20 as a probe optical pulse. Is. That is, the beam splitter BS1 inputs the optical pulse to be measured and branches it into two, outputs one optical pulse obtained by the two branches to the double optical pulse generator 10B, and outputs the other optical pulse to the mirror M1. To do. The mirror M1 reflects the optical pulse that has arrived from the beam splitter BS1 to the chirping unit 20.

  The concave mirror CM receives the double light pulse output from the double light pulse generation unit 10B and reflected by the mirror M2, and also receives the chirp light pulse output from the chirping unit 20, and receives these light pulses. Is condensed and made incident on the sum frequency light generation unit 30. The sum frequency light generation unit 30 includes a nonlinear optical crystal having an optical axis set in an appropriate orientation, and is equal to the sum of the optical frequencies of the double light pulse and the chirped light pulse incident after being collected by the concave mirror CM. Sum frequency light having a frequency is generated and output by the nonlinear optical crystal.

  The lens L receives the double light pulse output from the double light pulse generation unit 10 </ b> B, and focuses the double light pulse on the harmonic light generation unit 40 to make it incident. The harmonic light generation unit 40 includes a nonlinear optical crystal having an optical axis set in an appropriate direction, and outputs harmonic light having an optical frequency twice as high as the optical frequency of the incident double light pulse collected by the lens L. Generate and output with nonlinear optical crystal.

  The mirror M3 is provided in the middle of the optical path from the mirror M5 to the spectroscopic measurement unit 50, and can be freely placed and retracted on this optical path. When the mirror M3 is arranged on this optical path, the mirror M3 can reflect the sum frequency light output from the sum frequency light generation unit 30 and make the sum frequency light incident on the spectroscopic measurement unit 50. On the other hand, when the mirror M3 is retracted from the optical path, the harmonic light output from the harmonic light generation unit 40 and sequentially reflected by the mirror M4 and the mirror M5 can enter the spectroscopic measurement unit 50. That is, the mirror M3 acts as a selection unit that selectively inputs either the sum frequency light or the harmonic light to the spectroscopic measurement unit 50.

  The mirror M4 is provided in the middle of the optical path from the harmonic light generation unit 40 to the light detection unit 90, and can be arranged and retracted on the optical path. When the mirror M4 is arranged on this optical path, the harmonic light output from the harmonic light generation unit 40 can be reflected to the mirror M5, and the mirror M5 can measure the harmonic light reaching from the mirror M4. Reflect toward 50. On the other hand, when the mirror M4 is retracted from the optical path, the harmonic light output from the harmonic light generation unit 40 can be input to the light detection unit 90. The light detection unit 90 measures the intensity of the harmonic light output from the harmonic light generation unit 40. For example, a photomultiplier tube is preferably used.

  In the phase characteristic measuring apparatus 3 configured as described above, the mirror M3 is retracted from the optical path, the mirror M4 is disposed on the optical path, and the number of pulse intervals in the double optical pulse output from the double optical pulse generator 10B is several. Are set to appropriate values in sequence. A double light pulse having a pulse interval of each value is input to the harmonic light generation unit 40, and the harmonic light generated by the harmonic light generation unit 40 is input to the spectroscopic measurement unit 50 through the mirror M4 and the mirror M5, and is subjected to spectroscopy. The harmonic light spectrum is measured by the measurement unit 50. Then, according to the flowchart shown in FIG. 4, the measured harmonic light spectrum is analyzed by the analysis unit 70, the device function of the spectroscopic measurement unit 50 is obtained, and the correction information is stored in the storage unit 80.

  Subsequently, the mirror M3 is disposed on the optical path, and the pulse interval in the double light pulse output from the double light pulse generator 10B is set to an appropriate value. The double light pulse and the chirped light pulse are input to the sum frequency light generation unit 30, and the sum frequency light generated by the sum frequency light generation unit 30 is input to the spectroscopic measurement unit 50 via the mirror M3. To measure the sum frequency light spectrum. Then, according to the flowchart shown in FIG. 2 or FIG. 5, the analysis unit 70 corrects the sum frequency light spectrum based on the correction information stored in the storage unit 80, and based on the corrected sum frequency light spectrum. Thus, the phase characteristic of the light pulse to be measured is measured.

Further, in the phase characteristic measuring apparatus 3, when the mirror M4 is retracted from the optical path, the harmonic light generated by the harmonic light generating unit 40 enters the light detecting unit 90, and the intensity of the harmonic light is detected by the light detecting unit 90. Is detected. The intensity G _FR (τ) of the harmonic light detected by the light detector 90 depends on the amplitude characteristic A (t) and phase characteristic φ (t) of the light pulse to be measured and the pulse interval τ of the double light pulse.

Then, the analysis unit 70 obtains a fringe decomposition SHG autocorrelation waveform representing the relationship between the pulse interval τ in the double light pulse and the harmonic light intensity G _FR (τ) based on the detection result by the light detection unit 90. In this fringe decomposition SHG autocorrelation waveform, when the pulse interval τ is 0, the harmonic light intensity G _FR (τ) peaks and the contrast ratio (G _FR (0) / G _FR (∞)) is 8.

  When the fringe-resolved SHG autocorrelation waveform is measured, the harmonic light intensity is calculated assuming the amplitude characteristic A (t) and the phase characteristic φ (t) of the optical pulse to be measured. The degree of coincidence with the measurement result is evaluated, and the amplitude characteristic A (t) and the phase characteristic φ (t) that maximize the degree of coincidence are obtained. In this way, information on the pulse width and chirp of the light pulse to be measured can be easily obtained. Further, the pulse interval in the double light pulse when the peak of the fringe decomposition SHG autocorrelation waveform is obtained is calibrated as 0 point.

  In this phase characteristic measuring apparatus 3, since the corner reflectors CR1 and CR2 are used in the double light pulse generation unit 10B, the light pulses reflected by the corner reflectors CR1 and CR2 do not return to the beam splitter BS1.

  The light pulses reflected by the corner reflectors CR1 and CR2 included in the double light pulse generation unit 10B are combined and branched by the beam splitter BS3, and output to the mirror M2 and the lens L as double light pulses. . The double light pulse output from the double light pulse generator 10B to the lens L is input to the harmonic light generator 40. Then, the spectrum of the harmonic light generated by the harmonic light generation unit 40 is measured by the spectroscopic measurement unit 50 and analyzed by the analysis unit 70, whereby the device function of the spectroscopic measurement unit 50 is obtained. Further, the intensity of the harmonic light generated by the harmonic light generation unit 40 is measured by the light detection unit 90 and analyzed by the analysis unit 70, thereby obtaining a fringe-decomposed SHG autocorrelation waveform. Information relating to the pulse width and chirp of the optical pulse is simply obtained, and the pulse interval in the double optical pulse is calibrated.

  In the phase characteristic measuring apparatus 3, separate nonlinear optical crystals are used in the sum frequency light generation unit 30 and the harmonic light generation unit 40. In the nonlinear optical crystal as the sum frequency light generation unit 30, double light pulses and chirp light pulses are incident non-linearly from the concave mirror CM, so that sum frequency light is generated with high efficiency corresponding to the incident direction of these light pulses. The azimuth of the optical axis is set so that the phase matching condition for satisfying the condition is satisfied. On the other hand, since the nonlinear optical crystal as the harmonic light generation unit 40 receives only the double light pulse, the phase matching condition for generating the harmonic light with high efficiency corresponding to the incident direction of the light pulse. The azimuth of the optical axis is set so that As described above, the orientation of the optical axis of each of the nonlinear optical crystal as the sum frequency light generation unit 30 and the nonlinear optical crystal as the harmonic light generation unit 40 can be appropriately set in accordance with each purpose.

  Further, in this phase characteristic measuring apparatus 3, since the concave mirror CM is used as a condensing optical system for condensing and entering the double light pulse and the chirped light pulse to the sum frequency light generation unit 30, this condensing optical system is used. There is no chromatic dispersion in the system, and highly accurate measurement is possible. Further, a concave mirror may be used as a condensing optical system for condensing and entering the double light pulse to the harmonic light generation unit 40.

  When optical components that include dispersion are used in condensing optical systems, beam splitters, etc., the amount of dispersion of optical components used in advance is evaluated, and if the amount of dispersion of the optical components is subtracted from the analysis results, quantitative performance is obtained. Will not be damaged.

  In the above embodiment, as an example of the method for calibrating the pulse interval τ of the double light pulse, an example using the analysis result of the correlation waveform by the fringe decomposition SHG autocorrelator is shown, but the present invention is not limited to this. For example, apart from this apparatus, a spectrometer having sufficient resolution is prepared for calibration, and a double light pulse is input to the spectrometer. The spectrum obtained as a result of the observation is a spectrum similar to the spectrum S1 shown in FIG. For this spectrum, up to step S36 is executed in the algorithm shown in FIG. Since the phase waveform obtained at this time has a phase gradient of ωτ, the value of the pulse interval τ can be determined from the calculation result. The double light pulse used at this time may use a fundamental wave or a second harmonic.

It is a schematic block diagram of the phase characteristic measuring apparatus 1 which concerns on this embodiment. It is a flowchart explaining the analysis operation | movement in the analysis part 70 of the phase characteristic measuring apparatus 1 which concerns on this embodiment. It is a figure which shows the sum frequency light spectrum in Fourier space, and an apparatus function spectrum. It is a flowchart explaining the apparatus function acquisition operation | movement in the analysis part 70 of the phase characteristic measuring apparatus 1 which concerns on this embodiment. It is a flowchart explaining the analysis operation | movement in the analysis part 70 of the phase characteristic measuring apparatus 1 which concerns on this embodiment. It is a figure which shows the 1st structural example of the phase characteristic measuring apparatus which concerns on this embodiment. It is a figure which shows the 2nd structural example of the phase characteristic measuring apparatus which concerns on this embodiment. It is a figure explaining the problem of the conventional phase characteristic measuring apparatus.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1-3 ... Phase characteristic measuring apparatus, 10 ... Double light pulse generation part, 20 ... Chirp provision part, 30 ... Sum frequency light generation part, 40 ... Harmonic light generation part, 50 ... Spectral measurement part, 60 ... Selection part, 70 ... analysis part, 80 ... storage part, 90 ... light detection part.

Claims (5)

A double optical pulse generator that inputs a measured optical pulse, converts the measured optical pulse into a double optical pulse with a pulse interval τ, outputs the double optical pulse, and can adjust the pulse interval τ. When,
A chirp applying unit that inputs a probe light pulse, gives a linear chirp to the probe light pulse, converts the probe light pulse into a chirp light pulse, and outputs the chirp light pulse;
An optical frequency equal to the sum of the optical frequencies of the double optical pulse and the chirped optical pulse is inputted by inputting the double optical pulse output from the double optical pulse generating unit and the chirped optical pulse output from the chirp applying unit. A sum frequency light generator that generates and outputs a sum frequency light having
A double light pulse output from the double light pulse generator, a harmonic light generator that generates and outputs harmonic light having an optical frequency twice as high as the optical frequency of the double light pulse;
A spectroscopic measurement unit that spectroscopically measures the input light;
A selection unit that selectively inputs either the sum frequency light output from the sum frequency light generation unit and the harmonic light output from the harmonic light generation unit to the spectroscopic measurement unit;
Based on the result of spectroscopic measurement of the sum frequency light output from the sum frequency light generation unit, based on the result of spectroscopic measurement of the harmonic light output from the harmonic light generation unit. Correcting and analyzing to measure the phase characteristic of the light pulse to be measured,
A phase characteristic measuring apparatus comprising:   An optical branching unit is further provided that splits the optical pulse to be measured into two parts, inputs one of the two branches into the double optical pulse generation unit, and inputs the other into the chirping unit as the probe optical pulse. The phase characteristic measuring apparatus according to claim 1.   The phase characteristic measuring apparatus according to claim 1, further comprising a storage unit that stores correction information at the time of correction in the analysis unit. A light detection unit for detecting the intensity of the harmonic light output from the harmonic light generation unit;
The analysis unit obtains a fringe decomposition SHG autocorrelation waveform based on a detection result by the light detection unit,
The phase characteristic measuring apparatus according to claim 1. The phase characteristic measuring apparatus according to claim 1, wherein the sum frequency light generation unit and the harmonic light generation unit include a common nonlinear optical crystal.

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