JPH052252B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a short optical pulse evaluation method, and more specifically, to a method for evaluating short optical pulses. For short optical pulses whose waveforms cannot be measured and evaluated in detail, if the optical pulses form a pulse train that repeats at a constant period, it is possible to analyze both the intensity and phase waveforms of the optical pulses in detail. It relates to a method that can be measured and evaluated.

[Technical background of the invention]

FIG. 3 is a schematic diagram of a conventional short optical pulse measurement and evaluation device. As is clear from this diagram, a part of the optical pulse to be measured is distributed to the spectrum measurement system by the semi-transparent mirror 24, and the remaining part is distributed to the spectrum measurement system. is input to the intensity correlation waveform measurement system. The spectrum measurement system includes a high-resolution spectrometer 26 equipped with a wavelength sending device (scanning mechanism) 27, a focusing lens 25 that focuses the light beam onto the input slit of the spectrometer 26, a photodetector 28, and an amplifier for the output voltage of the detector 28. 29.

In the intensity correlation waveform measurement system, an incident light beam is branched into two light beams by a semi-transparent mirror 1, one of which is reflected by a fixed prism 2, and the other beam is reflected by a movable table 4 on the optical axis. The light beam is reflected by a prism 3 that is movable in the direction, and the above two reflected light beams are combined on a semi-transparent mirror 1 to become one light beam again. This optical system consisting of a semi-transparent mirror and two prisms 2 and 3 has the name Michelson interferometer, and when the optical electric field before incidence is written as E(t) as a function of time t, after recombination The optical electric field is [E(t)+E(t-
τ)]/2. Here, the delay time τ is the amount obtained by dividing the relative optical path wavelength difference l between the optical path passing through the prism 2 and the optical path passing through the prism 3 by the speed of light C, and when the prism 3 is movable by the movable stage 4. For this reason, τ can be changed freely. After this recombination, the light beam is focused by a lens 5 into a crystal 7 having the ability to generate second-order harmonic light, [E(t)+E(t
-τ)] ² , we see the generation of second-order harmonic light whose electric field is proportional. The light emitted from the crystal 7 is focused onto a photodetector 9 by a lens 6. At this time, only the second harmonic light is passed through the optical filter 8,
It reaches the photodetector 9. The response time of the photodetector 9 is
Since the duration of the optical pulse to be measured is extremely long compared to the duration of the second harmonic pulse generated by the crystal 7, the wave height of the output voltage pulse of the photodetector 9 is proportional to the energy of the second harmonic optical pulse. The output voltage pulse of the photodetector 9 is amplified and flattened by the integrating amplifier 30.

The steps of the conventional short optical pulse measurement and evaluation method are shown below.

First, turn the signal changeover switch 31 to the right,
The A/D converter 32 is connected to the spectrum measurement system. In this state, while scanning the center wavelength of the spectrometer 26 at a constant speed using the wavelength sending device 27, the output voltage of the photodetector 28 is read and stored in the computer 18 in time series via the A/D converter 32. let In this way, a data string of the optical pulse spectrum D(λ) is obtained. Next, the signal changeover switch 31
Tilt it to the left and connect the A/D converter to the intensity correlation waveform measurement system. In this state, while moving the moving table 4 at a constant speed, the output voltage of the integrating amplifier 30 is
The information is read and stored in the computer 18 in chronological order via the converter 32. At this time, the integrating amplifier 30
The cutoff frequency is set sufficiently lower than the frequency ₀ = 2V/λ ₀ (1) determined by the center wavelength λ ₀ of the optical pulse to be measured and the moving speed V of the moving table. Therefore, the data string obtained in this way is proportional to 1+2G(τ). Here, G(τ) is the intensity correlation waveform, and using the pulse intensity waveform I(t), G(τ)=∫ ^∞ _-∞ I(t)I(t-τ)dt/∫ ^∞ _-∞ I
It is expressed as (t)I(t)dt/...(2).

This completes the measurement stage and begins the analysis stage.

First, the spectrum D(λ) expressed as a function of the wavelength λ is expressed as the spectrum D(λ) as a function of the frequency ω.
Convert to (ω). Optical electric field E(t) of the pulse to be measured,
If the Fourier transform is E~(ω), then the absolute value of E~(ω) can be found from the 1/2 power of D(ω) according to |E~(ω)|=√(ω). Therefore, E~(ω)=|E~(ω)|exp
If the phase φ(ω) of the Fourier component when written as [iφ(ω)] (i: imaginary unit) is determined, the electric field E(t) of the pulse to be measured can be completely reconstructed. Therefore, φ(ω) is expanded around the center wavelength ω ₀ of the spectrum, and φ(ω)=φ ₀ +(ω−ω ₀ )p+(ω−ω ₀ ) ² q+(
ω
−ω ₀ ) ³ r In order to determine the expansion coefficients, q and r (φ ₀ and p are unnecessary), Etr(t)=1/√2π∫ ^∞ _-∞ D(ω)exp[i(ω −ω
₀ ) ² q + i (ω−ω ₀ ) ³ r−iωt〕dω …(3) The least squares method is used so that Gtr(τ) calculated according to the following two equations best matches the actually measured intensity correlation waveform. This allows us to obtain φ(ω) with cubic precision.
can be obtained approximately, so according to equation (3), Etr
(t), the intensity waveform of the pulse to be measured can be found from |Etr(t)| ² , and the phase waveform of the pulse to be measured can be found from arg(Etr(t). In this way, at the analysis stage Since only the third-order coefficients of φ(ω) can be obtained, this method cannot, in principle, reproduce pulse intensity waveforms and phase waveforms of arbitrary shapes in faithful detail. ) can be improved by finding expansion coefficients up to higher orders, but this is difficult because of the increased calculation time.

Furthermore, there are many problems involved in using a spectrometer to measure spectra. The spectrometer used in this case is required to have extremely high resolution. For example, the full width at half maximum of the spectrum of an optical pulse with a width of 1 ps with a center wavelength of 0.6 μm is approximately 5 Å. The resolution required for the instrument is 0.1 Å or less, and as the width of the optical pulse to be measured becomes longer, proportionally better resolution is required.

Spectrometers that satisfy the above specifications include a diffraction grating spectrometer with a focal length of 1 m or more or a swept Fabry-Perot etalon, but the former has a large volume and is difficult to adjust the optical axis, and the latter has a larger capacity than the former. Therefore, careful optical axis adjustment is required, and both methods have the disadvantage of not being able to be easily measured.

As described above, in the conventional short optical pulse measurement evaluation method, in addition to the Michelson interferometer used to measure the intensity correlation waveform of the optical pulse, a high-resolution spectrometer is required for spectrum measurement. The spectrometer accounts for most of the volume and price of measurement and evaluation equipment, which hinders miniaturization, lower cost, and easier adjustment. Furthermore, the accompanying measurement data analysis method remains at a rough approximation, making detailed evaluation of the optical pulse impossible.

[Summary of the invention] The present invention has been made in view of the above points,
It is an object of the present invention to provide a short optical pulse measurement and evaluation method that does not require a spectrometer and allows detailed measurement and evaluation of optical pulses.

Therefore, the short optical pulse measurement and evaluation method according to the present invention is a method for measuring and evaluating repeatedly occurring short optical pulses, in which the light beam to be measured is branched into two light beams using a semi-transparent mirror, and the light beams are divided into two light beams through different paths. After imparting a relative optical path length difference to the two light beams by increasing the light beam, the two light beams are combined again, and the combined light beam is incident and focused into a crystal having a second harmonic light generator,
In the short optical pulse measurement and evaluation method of converting the second harmonic light intensity generated in the crystal into an electrical signal by a photodetector, recording the relative optical path length difference,
Perform Fourier analysis on the recorded data to obtain three spectra distributed with the frequency interval of the interference fringe of the fundamental wave, raise the spectrum near the zero frequency of these spectra to the 1/2 power, and then perform inverse Fourier transform. The intensity waveform of the optical pulse to be measured is reconstructed using This method is characterized by reconstructing the phase of the optical pulse to be measured through calculation.

Further, according to a second short optical pulse measurement and evaluation method according to the present invention, a method of measuring and evaluating repeatedly occurring short optical pulses is provided, in which a light beam to be measured is branched into two light beams by a semi-transparent mirror, and two different light beams are used. After imparting a relative optical path length difference to the two light beams by causing them to pass through a path, the two light beams are combined again, and the combined light beam is incident and focused into a crystal having the ability to generate second harmonic light. In a short optical pulse measurement and evaluation method in which the intensity of second-order harmonic light generated in By forcing the frequency components near DC to be extracted, and by passing the other components through a high frequency pass filter,
After extracting a frequency component near twice the frequency of the interference fringe of the fundamental wave, the amplitude of this component is determined using an AC voltmeter, and the voltage near DC and the value indicated by the AC voltmeter are calculated using the relative optical path. The change in length is recorded, and each pair of recorded data strings is subjected to Fourier analysis to obtain a spectrum, and the above-mentioned
After raising the spectrum obtained from the DC neighborhood component to the 1/2 power, the intensity waveform of the measured optical pulse is reconstructed by inverse Fourier transform, and then the spectrum obtained from the AC voltage instruction value is raised to the 1/2 power. 2
It is characterized in that the phase of the optical pulse to be measured is reconstructed by repeated calculations from the multiplied product and the already determined intensity waveform.

According to the present invention, a method is devised to measure the second harmonic optical electric field correlation waveform instead of the spectrum in the conventional method, and the pulse intensity is measured using a Michelson interferometer and a crystal capable of generating second harmonic light. By simultaneously measuring and recording the correlation waveform and the second harmonic optical electric field correlation waveform and analyzing them, it becomes possible to measure and evaluate the intensity waveform and phase waveform of the optical pulse without using a spectrometer.

[Specific description of the invention]

According to the present invention, the superimposed light flux to which a relative optical path difference is imparted by the Michelson interferometer is focused by the action of a lens in a crystal having the ability to generate second-order harmonic light, and the second-order harmonic light is generated. The intensity of the waves is converted into voltage using a photodetector.

This voltage value is proportional to the following formula, and the present invention is characterized by measuring G(τ) and F ₂ (τ) in the following formula.

1+2G(τ)+4F ₁ (τ)cos(ω ₀ τ+φ ₁ (τ))+F ₂
(τ)cos(2ω ₀ τ+φ ₂ (τ)) …(4) Here, τ is the delay time, ω ₀ = 2πC/λ ₀ is the center angular frequency of the optical pulse to be measured, and F ₂ (τ), _φ2
(τ) is the optical electric field E(t) of the pulse, E(t)=[ε(t)exp(-iω ₀ t)+ε ^* (t)exp
(iω ₀ t)]/2 (* on the shoulder represents a conjugate complex number) is defined by the following formula. This left side is called the second harmonic electric field correlation waveform.

F ₂ (τ) exp (−iφ ₂ (τ)) = ∫ ^∞ _-∞ [ε(t) ² [
ε ^* (t-
τ)〕 ² dt …(5) In the conventional method, a Michelson interferometer is used to
Whereas only G(τ) in equation (4) was measured,
In the present invention, using the same interferometer, G in equation (4) is
(τ) and F ₂ (τ), thereby eliminating the need for spectrum measurement using a spectrometer, which was separately required in the conventional method.

Furthermore, in the analysis stage of determining the intensity waveforms I(t) and φ(t) of the pulse from the two correlated waveforms, in the present invention, the pulse is symmetrical with respect to a certain time, that is, a so-called symmetrical pulse. We proceed with the analysis assuming that. Most of the pulses generated by existing short optical pulse light sources during normal operation are symmetrical, and the pulses remain symmetrical even after being deformed through the optical fiber or optical waveguide, so the above assumption does not apply. It is valid in many application situations of short optical pulse evaluation method. Based on this assumption, in the present invention, from the intensity autocorrelation waveform data G(τ),
The pulse intensity waveform I(t) is reconstructed according to the following equation (6).

I(t)=∫ ^∞ _-∞ ∫ ^∞ _-∞ G(τ)exp(iωτ)dτ1/
2exp(−iωt)dω…(6) With this calculation method, as long as it is a symmetrical pulse,
It has been proven that any complex waveform can be perfectly reproduced. Next, the second-order harmonic electric field correlation waveform data F ₂ (τ) is subjected to Fourier transformation to obtain the Fourier transformation S of the square of the electric field of the pulse to be measured [s(t)] ²
The square of the absolute value of (ω) is obtained. That is, under the assumption of symmetric pulses, ∫ ^∞ _-∞ F ₂ (τ)exp(iωτ)dτ=|S~(ω)| ² …(
7) On the other hand, [ε(t)] ² is determined by the pulse phase φ(t) and intensity waveform I(t) as follows: [ε(t)] ² = I(t)exp[2iφ(t)] By combining this with the definition of S~(ω), we get ∫ ^∞ _-∞ I(t)exp[2iφ(t)+iωt]dt=S~(ω
)…(8), so it was calculated by equation (7) from the already obtained I(t) and data F ₂ (τ) |S~
From (ω)|, iterative calculation exp(2iφ _k+1 ) = arg(F[|S~|arg(F[Iexp(2iφ _k )]))]
...(9) allows the pulse phase φ(t) to be determined with high accuracy.

In the above formula, the symbol F represents performing a Fourier transform, and arg represents calculating the phase of a complex number.

The above analysis method is different from the conventional approximation method up to a finite dimension, in principle, it incorporates up to an infinite order, and is used to obtain the intensity, waveform, and phase of the pulse to be measured with high accuracy.

Example 1 FIG. 1 shows the configuration of an example of an apparatus for implementing the short optical pulse evaluation method according to the present invention.

The illustrated device is equipped with a Michelson interferometer consisting of a semi-transparent mirror 1, a fixed prism 2, and a prism 3 movable by a movable stage 4, and a helium ion laser used to calibrate the relative optical path difference length of this interferometer. The light from 11 and the pulsed beam to be measured enter the Michelson interferometer in parallel. The helium neon laser beam emitted from the interferometer is mirror 13 and lens 1.
4 to the Si photodetector 15;
outputs an electrical signal proportional to the light intensity. Its output is amplified by amplifier 16 and applied to one of the channels of waveform storage 17. On the other hand, the pulsed beam to be measured emitted from the interferometer is transmitted through the lens 5.
focuses in a crystal 7, for example a lithium iodate crystal 7, which has the ability to generate second harmonic light by
Look at the generation of second harmonics. The light emitted from the crystal 7 is focused on the Si detector 9 by the lens 6, but at this time, only the second harmonic light reaches the photodetector 9 due to the filter 8 interposed in the middle. outputs a voltage pulse with a wave height proportional to the second harmonic optical energy. This voltage pulse is amplified and flattened by an integrating amplifier 10 and applied to another one of the channels of the waveform storage device. A computer 18 is connected to the waveform storage device 17 for reading out and analyzing the stored data after measurement.

The short optical pulse evaluation device described above operates as follows.

While moving the moving table 4 at a constant moving speed V,
A waveform storage device 17 measures and records the output voltages of the amplifier 16 and the integrating amplifier 10, respectively, at regular sampling intervals. At this time, the sampling interval Δt, the time constant T of the integrating amplifier 10, the repetition interval P of the pulse to be measured, and the moving speed V of the moving table are set to satisfy the relationship P<<T<Δt<<λ ₀ /(2V)). do. Here, λ ₀ is the center wavelength of the pulsed light to be measured.

In the following analysis stage, the computer 1 first analyzes the output voltage changes of the integrating amplifier 10 and the output voltage changes of the amplifier 16 stored in the waveform storage device 17.
8, and by referring to the latter, distortion of the former waveform due to unevenness in the moving speed of the moving table 4 is corrected. In this way, correlation data as shown in FIG. 4 is obtained.

FIG. 5 shows the spectrum obtained by Fourier analysis of this data. In the figure, ₀ is the interference fringe frequency of the fundamental wave defined by equation (1). In this spectrum, the peak near the zero frequency corresponds to the Fourier transform ∫ ^∞ _-∞ G(τ)exp(iωτ)dτ of the intensity correlation waveform G(τ).

Therefore, by using this and calculating according to equation (6), the pulse intensity waveform I(t)
is required. Furthermore, in the spectrum, the peak near frequency ₂₀ corresponds to |S~(ω)| ² , so
Using this to the 1/2 power and the already determined I(t), iterative calculations are performed according to equation (9) to determine the pulse phase.

In order to verify the effectiveness of the above analysis method, we used a computer to generate the expected measurement data according to equation (4) for the pulse having the intensity waveform and phase shown in Figure 6, and then used it as described above. The phase of the intensity waveform obtained by analysis according to the analysis method is shown in FIG. This revealed that the intensity waveform was completely reproduced, and the phase was also reproduced with high accuracy except for the tails of weak pulses.

The measurement method shown in this example uses an extremely short optical pulse whose pulse width is several times to several tens of times the period of the vibration of the optical electric field, for example, from 10 fs to 60 fs with a center wavelength of 0.6 μm. This is effective for measuring and evaluating optical pulses with a certain width. When applied to optical pulses having a width larger than that, the storage capacity of the waveform storage device 17 and the computer 18 will be large, so it is recommended to apply Example 2 below to the measurement of the above-mentioned long pulses. desirable. On the other hand, since the second embodiment is subject to errors for extremely short optical pulses as shown above, for this reason, the first and second embodiments have a complementary relationship.

Example 2 FIG. 2 shows the configuration of an example of an apparatus for implementing the short optical pulse evaluation method according to the present invention.

The illustrated device includes a Michelson interferometer similar to that in Example 1, a crystal and filter capable of generating second harmonic light, and a photodetector. The voltage pulse generated by the photodetector is amplified and flattened by an integrating amplifier 10, and then passed through a low frequency pass filter 19.
and distributed to the harmonic pass filter 20.
The output voltage of the low frequency pass filter is determined by the A/D converter 22.
The data is read into the computer 18 via the computer 18 and stored. On the other hand, the output AC voltage of the high frequency pass filter is supplied to the AC voltmeter 21, detected, and converted into a voltage value corresponding to the AC voltage value, and then read into the computer 18 via the A/D converter 23. be remembered.

The operation of the short optical pulse evaluation apparatus described above will be explained next.

While moving the movable table 4 at a constant speed V, the computer reads and stores the outputs of each A/D converter at constant time intervals. At this time, the time constant T of the integrating amplifier 10, the repetition interval P of the pulse to be measured, and the moving speed V of the moving table are set so as to satisfy the relationship P<<T<<λ ₀ /(2V)). The data loading time interval can be set arbitrarily long. The cutoff frequency LP of the low frequency pass filter and the cutoff frequency HP of the high frequency pass filter are determined to satisfy the relationship LP < ₀ < HP ( ₀ is the interference fringe frequency of the fundamental wave).

As described above, in this embodiment, since the data reading time interval can be set arbitrarily long, it is possible to measure pulses of any width with a certain storage capacity. However, in this embodiment, since the changing components of the voltage generated by the photodetector 9 (Equation (4)) near frequencies 0, ₀ , and ₂₀ are separated using a filter,
The pulse width is extremely short, G (τ), F ₁ (τ), F ₂
When (τ) changes rapidly, errors caused by deformation of G(τ) and F ₂ (τ) due to the frequency characteristics of the filter cannot be ignored. Therefore, the lower limit of the applicable pulse width in this embodiment is about ten times the oscillation period of the optical electric field of the pulse.

As described above, two types of correlation data as shown in FIG. 7 are obtained. In the figure, the solid line is the intensity correlation waveform G(τ) obtained by the low frequency pass filter, and the dotted line is the second harmonic optical electric field correlation waveform obtained by the high frequency pass filter and AC voltmeter.
F ₂ (τ). However, what is actually measured as the indicated value of the AC meter here is |F ₂ (τ)|, so the point where the indicated value becomes zero is detected and the waveform measured at that point is plotted on the horizontal axis. F ₂ (τ) was obtained by folding around the center.

In the analysis stage below, first, the pulse intensity waveform I(t) is obtained by calculating according to equation (6) using data G(τ), and then from data F ₂ (τ) ( 7) Calculated by formula |S~(ω)
| Using ₂ to the 1/2 power and the determined I(t), perform iterative calculations according to equation (9) to find the pulse phase.

As a verification of the effectiveness of the above analysis method, for the pulse with the intensity waveform and phase shown in Figure 6,
G(τ) in the measurements assumed according to equations (2) and (5),
F ₂ (τ) was calculated and analyzed according to the above analysis method. Since the original pulse is the same as that used in the similar verification of the first example, the obtained intensity waveform and phase are also exactly the same as those obtained in the previous example, and are shown in Fig. 8. It became as shown. From this, it was confirmed that the analysis method according to this example also reproduced the intensity waveform perfectly and had high accuracy in reproducing the phase except at the tail of the pulse.

FIG. 9 shows the intensity waveform and phase obtained by measuring and evaluating the optical pulses generated by the semiconductor laser using the short optical pulse evaluation method according to this embodiment. The semiconductor laser used was an InGaAsP laser with a wavelength of 1.3 μm.
Pulses were generated by superimposing and modulating the injected current with a 200MHz sine wave. The moving speed V of the moving table was 1.55 mm/min, and the fringe frequency of the fundamental wave was 40 Hz, and the cut-off frequency of the low-frequency pass filter was set to 10 Hz, and the cut-off frequency of the high-frequency pass filter was set to 70 Hz, and measurements were performed. .

From the analysis results in Figure 9, the pulse has an intensity waveform with a gentle tail, and the intensity is 1/5 of the peak.
It can be seen that it has a parabolic phase over a wide range up to a certain degree.

〔Effect of the invention〕

As is clear from the above explanation, according to the short optical pulse evaluation method according to the present invention, the intensity waveform and phase of the optical pulse to be measured can be determined with higher precision than the conventional method using a small and inexpensive measuring device. becomes possible. According to the present invention, since the pulse phase can be determined in extremely detailed manner, the present invention is particularly effective in optimizing and adjusting a pulse compressor using an optical fiber. Furthermore, the measuring device of the present invention can also be used for general purposes such as adjusting and evaluating short optical pulse light sources.
Since it is smaller and cheaper than conventional methods, it can be widely used.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of the configuration for implementing the first embodiment of the short optical pulse evaluation method according to the present invention, FIG. 2 is a schematic diagram of the configuration for implementing the second embodiment, and FIG. The figure is a schematic diagram of the configuration of a conventional short optical pulse evaluation device, FIG. 4 is a diagram showing the measurement results of the device of the first embodiment according to the present invention, and FIG. 5 is the result of Fourier analysis of the measurement results of FIG. 4. FIG. 6 is a diagram showing the intensity waveform and phase of the original pulse used to verify the validity of the analysis methods of the first and second embodiments of the present invention, and FIG. A diagram showing the measurement results of the device of the second embodiment. FIG. 8 shows the assumed measurement results calculated from the original pulse of FIG. 6, and the analysis method of the first and second embodiments of the present invention. FIG. 9 shows the intensity waveform and phase of the pulse to be measured obtained by analyzing the measured data in the second embodiment of the present invention. FIG. 1... Semi-transparent mirror, 2, 3... Prism, 4... Moving table,
5, 6... Lens, 7... Crystal having second harmonic generation ability, 8... Optical filter, 9... Photodetector, 10...
Integrating amplifier, 11... Helium neon laser, 1
2, 13... Mirror, 14... Lens, 15... Photodetector,
16...Amplifier, 17...Waveform storage device, 18...Computer, 19...Low frequency pass filter, 20...High frequency pass filter, 21...AC voltmeter, 22, 23...
A/D converter, 24...semi-transparent mirror, 25...lens, 2
6... Spectrometer, 27... Wavelength sending device, 28... Photodetector, 29... Amplifier, 30... Integrating amplifier, 31... Changeover switch, 32... A/D converter.

Claims (1)

[Scope of Claims] 1. A method for measuring and evaluating repeatedly occurring short light pulses, which splits the light beam to be measured into two light beams using a semi-transparent mirror, and makes the two light beams pass through different paths. After giving a desired optical path length difference, the two beams are combined again, and the combined beam is focused into a crystal that has the ability to generate second-order harmonic light, and the intensity of the second-order harmonic light generated within the crystal is In a short optical pulse measurement evaluation method in which a detector converts into an electrical signal, changes in the electrical signal with respect to the relative optical path length difference are recorded, and the recorded data is subjected to Fourier analysis to determine the frequency interval of the interference fringe of the fundamental wave. After obtaining three spectra distributed with , and raising the spectrum near the zero frequency among these spectra to the 1/2 power, the intensity waveform of the optical pulse to be measured is reconstructed by inverse Fourier transform. after,
The feature is that the phase of the optical pulse to be measured is reconstructed by iterative calculation from the spectrum near the frequency twice the frequency of the interference fringe of the fundamental wave raised to the 1/2 power and the determined intensity waveform. Short optical pulse measurement evaluation method. 2. A method for measuring and evaluating repeatedly occurring short light pulses, in which the light beam to be measured is branched into two light beams using a semi-transparent mirror, and the two light beams are given a relative optical path length difference by passing through different paths. After that, the two beams are combined again, the combined beam is focused into a crystal that has the ability to generate second harmonic light, and the intensity of the second harmonic light generated within the crystal is detected as an electrical signal by a photodetector. In the method for measuring and evaluating short optical pulses, the electrical signal is branched into two, and one of them is passed through a low frequency pass filter.
After extracting a frequency component in the vicinity of twice the frequency of the interference fringe of the fundamental wave by extracting a nearby frequency component and passing the other one through a high frequency pass filter, the frequency component of the component is measured with an AC voltmeter. The amplitude is determined, the voltage near DC and the indicated value of the AC voltmeter are recorded as changes with respect to the relative optical path length difference, and the pair of recorded data strings is subjected to Fourier analysis to obtain a spectrum. After raising the spectrum obtained from the components to the 1/2 power, the intensity waveform of the optical pulse to be measured is reconstructed by inverse Fourier transform, and then the spectrum obtained from the AC voltage indication value is raised to the 1/2 power. and
A short optical pulse measurement and evaluation method characterized by reconstructing the phase of an optical pulse to be measured by iterative calculation from a previously determined intensity waveform.