JPH01302127A - Light pulse measuring apparatus - Google Patents

Abstract

PURPOSE:To eliminate the necessity for making a semiconductor crystal thin and to perform measurement without moving a semiconductor, by splitting a light pulse to be measured into two pulses to give relative light path length difference before synthesizing both pulses to one luminous flux and allowing said luminous flux to be incident to the semiconductor crystal. CONSTITUTION:A light pulse to be measured is split into two pulses by a half mirror 1 while two pulses are reflected by prisms 6, 7 to be again synthesized to one luminous flux which is, in turn, incident to a semiconductor crystal 4 through a lens 8. The electric conductivity change due to two-photon absorption of the crystal 4 is measured by a circuit consisting of a DC power supply 9, a differential amplifier 10 and a load resistor 19 to be recorded on a recorder 5. The prism 7 is reciprocally moved by a moving stand 20 and the relative light path length difference of the split luminous fluxes is changed to perform measurement. The conductivity change of the crystal 4 is proportional to the square of he electric field intensity of light. The continuous time (pulse width) of the light pulse to be measured is calculated from the electric field intensity change of incident light due to light path length difference and the delay time due to said difference. Since only one surface of the crystal 4 is used in measurement, it is unnecessary to make the crystal thin.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention is used to measure the light emission characteristics, transmission characteristics, and other optical characteristics of optical elements. In particular, the present invention relates to a device that measures in detail the waveform and phase of a very short optical pulse whose time width is equal to or less than the response time of existing photodetectors.

[Conventional technology]

FIG. 12 shows the configuration of a conventional optical pulse measuring device.

This device includes a semiconductor crystal 4 having two-photon absorption and conduction characteristics, a semi-transparent mirror 1 that splits a pulsed light beam to be measured into two, and a semiconductor crystal 4 that directs the split light beams in opposite directions.
A reflecting mirror 2.3 for making the light incident on the semiconductor crystal 4, a moving table 20 for moving the semiconductor crystal 4, a DC power supply 9 for supplying DC voltage to the semiconductor crystal 4, an ammeter 24 for measuring the current flowing through the semiconductor crystal 4, and a measuring device. and a recorder 5 for recording the current value.

The semiconductor crystal 4 has an energy forbidden band width equal to 1 of the incident light beam.
It is greater than the energy of a photon, but less than twice this energy. Therefore, in the semiconductor crystal 4, photoconduction due to two-photon absorption occurs in proportion to the square of the combined electric field generated by the correlation of the two reflected light fluxes incident oppositely. This photoconduction is detected as a current change by applying a DC voltage to the semiconductor crystal 4.

Therefore, the semiconductor crystal 4 is moved along the optical axis of the incident pulse, and the current change is recorded by the recorder 5 while being synchronized with this movement.
to be recorded. As a result, an intensity correlation waveform of the incident optical pulse is obtained. The resolution of this intensity correlation waveform depends on the thickness of the semiconductor crystal 4.

Additionally, the applicant has already filed a patent application for a method of measuring optical pulses using a Michelson interferometer.
73547, unpublished at the time of filing, hereinafter referred to as "earlier application")
. In this measurement method, the combined light obtained by an interferometer is incident on a nonlinear crystal that has the ability to generate second harmonic light, and the generated second harmonic light and fundamental wave light are detected by a photodetector and converted into electrical signals. do.

As a result, an intensity correlation waveform is obtained.

[Problem that the invention seeks to solve]

In the conventional device shown in FIG. 12, the electric field strength inside the semiconductor crystal 4 changes by a length approximately equal to the wavelength of the incident light beam due to interference between two opposingly incident light beams.

This change causes a spatial distribution of refractive index inside the semiconductor crystal 4, which has the disadvantage that the incident pulse is affected and the original waveform is distorted.

In addition, in order to avoid this problem and perform high-resolution measurements, that is, to perform measurements that also resolve the above-mentioned interference components, an extremely thin semiconductor crystal whose thickness is about the same as the wavelength of the incident light beam is required. There were drawbacks. Such semiconductor crystals are not only difficult to manufacture, but also difficult to maintain.

Furthermore, in order to converge the incident light flux into the semiconductor crystal 4 in order to improve measurement sensitivity, it is necessary to move the semiconductor crystal 4 during the measurement, so the convergence point of the incident light flux must also be moved accordingly. There was a drawback that it had to be.

Furthermore, the method of the prior application had a drawback in that the generated second harmonic light was weak.

The present invention solves the above-mentioned problems and provides an optical pulse measurement device in which there is no interference of incident light flux within a semiconductor crystal, there is no need to use a thin semiconductor crystal, and there is no need to move the semiconductor crystal. With the goal.

[Means for confronting problems]

The optical pulse measurement device of the present invention focuses the optical pulse to be measured, which has been split into two beams, onto a semiconductor crystal by giving a relative optical path length difference to the two beams and then combining them into one beam. It is characterized in that it includes means for.

[For production]

The measured optical pulse beam is split and multiplexed by a Michelson interferometer, and then enters the semiconductor crystal. Therefore, rather than making two opposing light beams enter the semiconductor crystal and cause them to interfere, the interfered light beams are made to enter the semiconductor crystal.

In addition, in order to obtain the correlated waveform of the optical pulse, instead of measuring the spatial distribution of the electric field by moving the semiconductor crystal, by changing the relative optical path length difference of the Michelson interferometer, we can measure the spatial distribution of the electric field by changing the relative optical path length difference of the Michelson interferometer. Reproduced on the surface of a crystal.

Therefore, measurement can be performed without being affected by the thickness of the semiconductor crystal, and highly accurate measurement can be performed without problems in manufacturing or holding thin crystals. Furthermore, it is necessary to move the semiconductor crystal.
Therefore, there is no need to move the convergence point of the incident light beam.

〔Example〕

FIG. 1 is a block diagram of an optical pulse measuring device according to a first embodiment of the present invention.

This device includes a splitting means for splitting a light pulse to be measured into two light beams, that is, a semi-transparent mirror 1, and a focusing means for making these two light beams incident on a common semiconductor crystal 4, that is, a prism 6.7 and a lens 8. , a measuring circuit for measuring changes in electrical conductivity due to two-photon absorption of the semiconductor crystal 4, that is, a DC power supply 9, a differential amplifier 10, a load resistor 19, and a recorder 5.

The condensing means further includes a movable table 20, and the movable table 20, the semi-transparent mirror 1, and the prism 6.7 provide means for giving a relative optical path length difference to two light beams and then combining them into one light beam. configured.

The prism 6 is fixed, and the lens 1.7 is movable in the optical axis direction by a moving table 20. These prisms 6.7
A Michelson interferometer is constituted by this and the semi-transparent mirror 1.

The measured light pulse that entered the Michelson interferometer is split into two beams by a semi-transparent mirror 1, and each beam is split into two beams by a prism 6.
．． After being reflected by 7, it is combined again by the semi-transparent mirror 1,
The light is focused on the semiconductor crystal 4 by the lens 8 .

The semiconductor crystal 4 used has an energy forbidden band width larger than the energy of one photon of the optical pulse to be measured and smaller than the energy of two photons. For example, if the wavelength of the optical pulse to be measured is 6001 m, a zinc selenide crystal, a gallium phosphide crystal, or a semiconductor crystal having a similar energy bandgap is used.

When the combined light beams are incident on the semiconductor crystal 4, two-photon absorption photoconduction occurs in the semiconductor crystal 4 due to the correlation between these light beams. Therefore, when a DC voltage is applied from the DC power supply 9 to the semiconductor crystal 4, a current pulse whose time integral value is proportional to the two-photon energy is generated. The differential amplifier lO and the load resistor 19 perform impedance conversion, and the waveform of the current pulse is recorded on the recorder 5 as a voltage value.

During measurement, the movable table 20 is moved back and forth, and changes in the output voltage of the differential amplifier 10 are recorded on the recorder 5 in synchronization with this movement. As a result, a correlation waveform whose horizontal axis is twice the amount of change in the moving table 20 (change in optical path length) is obtained.

FIG. 2 shows an example of a correlation waveform obtained by this example. In this figure, the horizontal axis is the delay time caused by the difference in optical path length.

FIG. 3 is a block diagram of an optical pulse measuring device according to a second embodiment of the present invention.

In this embodiment, a waveform recording device 1 is connected to the output of a differential amplifier 10 via a coupling circuit 23 instead of a recorder 5.
This embodiment differs from the first embodiment in that it includes a computer 14 and a semitransparent mirror 22, a photodetector 11, and an amplifier 12 for measuring the light intensity output from the Michelson interferometer.

The optical pulse to be measured is incident on the Michelson interferometer as in the first embodiment. The emitted light from the Michelson interferometer enters the semiconductor crystal 4 through the lens 8, is split by the semi-transparent mirror 22, and enters the photodetector 11.

The current pulse generated in the semiconductor crystal 4 is transmitted to the differential amplifier 10.
It is supplied as a voltage waveform to the waveform storage device 13 via the load resistor 19 and the coupling circuit 23. Waveform storage device 1
3 stores the voltage waveform in one of its channels.

The light beam branched by the semi-transparent mirror 22 enters the photodetector 11 and is converted into an electrical signal proportional to the light intensity. This electrical signal is amplified by an amplifier 12, and the waveform storage device 1
stored in 3 separate channels.

At the time of measurement, the movable table 20 is moved at a speed V, and while synchronized with this, changes in the output voltage of the differential amplifier 10 and changes in the output voltage of the amplifier 12 are stored in the waveform storage device 13. As a result, two correlated waveforms for the position of the moving table 20 are obtained.

At this time, the sampling interval Δt1, the repetition interval P of the measured optical pulse, and the moving speed V of the moving table 20 are set to satisfy the following relationship: P(Δt(λ./(2V)).However, λ.

is the center wavelength of the optical pulse to be measured.

Next, the obtained correlation waveform is analyzed by the computer 14. First, the data of the output voltage of the differential amplifier 10 stored in the waveform storage device 13 is read. The output voltage S2(τ) of the differential amplifier 10 is Sa(τ) oe 1
+2 c2(r)+4Re (F2(r) exp
(i a+(It))+Re CFa(r) exp(
2i a+0r)). Here, r is the delay time, ω. is the center angular frequency of the optical pulse to be measured, and ω, = 2πC/λ.

There is a relationship between However, C is a luminous flux.

Here, the electric field E (t) of the optical pulse to be measured is expressed as E(t)=
Let mσexp(iφ(t) −i ω.B +c.

However, φ(1) is an arbitrary phase modulation, and C is a constant.

At this time, C, (τ), Fl (τ), F2 (τ) are
x exp (2i (φ(1)−φ(t+r))
, ldt. Therefore, of the spectrum obtained by Fourier transforming the output voltage S2(τ) of the differential amplifier 10, the component near zero frequency is the Fourier transform of 02(τ), and the component near 2fo is the Fourier transform of F,(τ). Each corresponds to a Fourier transform.

Next, the computer 14 reads data on the output voltage of the amplifier 12 stored in the waveform storage device 13.

The output voltage S+(τ) of the amplifier 12 is S,(r)oc 1 +Re(G,(r)exp(i
(too r)]. Here, G1(τ) is x exp Ci (φ(1)−φ(t+r))]
Defined by dt. Of the spectrum obtained by Fourier analysis of the output voltage S1(τ), the component near fo is G,
This corresponds to the Fourier transform of (τ).

The two output voltages S2(τ) and 5l are shown in Figures 4 and 5.
The waveforms of (r) are shown, and FIGS. 6 and 7 show data obtained by Fourier analysis of the respective data.

The computer 14 further processes the data according to the following formula.

First, from G2(r), is found. Similarly, F2(τ), G, (τ) can be found. Here, IU=F, T, (I(t)) uU=F, T, (u(t)) ε−=F, T, (ε(t)) u (t) = ε(1)ε (1) 1- (t) = JTKexp [i < 6 (t)
It is E. The symbols F, T indicate that a Fourier transform is performed. Therefore, t=F,T, (l t l exp (φ,))
U= ε ε ■=εεI (G9 is the complex conjugate of ε) I=F, T,
[l I l arg (F, T, (1) )] u
=F, T, Clu arg (F, T, (
u) ) ) jul=1 exp(2iφ, .+)=arg (u) By repeatedly calculating the following, the intensity I and phase φ of the optical pulse to be measured can be determined with high accuracy. Here, arg indicates that the phase of a complex number is to be determined.

FIG. 8 shows the pulse intensity waveform obtained by the above calculation, and FIG. 9 shows the phase waveform.

FIG. 1O is a block diagram of an optical pulse measuring device according to a third embodiment of the present invention.

This embodiment device has a coupling circuit 23 instead of the recorder 5;
This embodiment differs from the first embodiment in that it includes an oscilloscope 15, a low-pass filter 16, a high-pass filter 17, and a detector 18, and that the movable table 20 outputs a trigger signal to the oscilloscope 15.

The optical pulse to be measured is incident on a Michelson interferometer. The light beam emitted from the interferometer is passed through the semiconductor crystal 4 by the lens 8.
and generates a correlation signal.

A current pulse generated by two-photon absorption conduction in the semiconductor crystal 4 is supplied to a differential amplifier 10 and a load resistor 19. The output voltage of the differential amplifier 10 is distributed and supplied to a low-pass filter 16 and a high-pass filter 17 via a coupling circuit 23 . The output of low pass filter 16 is supplied to oscilloscope 15.

The AC voltage output from the high-pass filter 17 is detected by the detector 18.
supplied to The detector 18 detects this AC voltage and outputs a voltage value corresponding to the AC voltage value to the oscilloscope 15.

During measurement, the movable table 21 is reciprocated at high speed at a speed V, and a trigger signal is supplied to the oscilloscope 15 in synchronization with this. The oscilloscope 15 has a low pass filter 16
and the output of the detector 18 are displayed simultaneously. At this time, the output waveform of the low-pass filter 16 is )G2(τ)
1, and the output of the detector 18 corresponds to IF2(τ)1. The phase characteristics of the pulse can be measured using these two waveforms, and the duration of the optical pulse to be measured can be measured from the output of the low-pass filter 16.

FIG. 11 shows an example of the waveform obtained in this example.

In this embodiment, by changing the movement amplitude of the moving stage 20 depending on the length of the optical pulse to be measured, it is also possible to measure relatively long optical pulses. On the other hand, the low-pass filter 1
The cutoff frequency fLP of the filter 6 and the cutoff frequency fHP of the high pass filter 17 are rLP<fo<fHp<2
f.

be determined to satisfy. Here, f is the interference fringe frequency of the fundamental wave, and fo=2V/λ.

Defined by

In this example, since the frequency components near zero and the frequency components near 2fo are separated by two filters, when the pulse width is extremely short and the correlation waveform changes rapidly, the frequency component due to the frequency characteristics of the filter An error will occur. Therefore, the lower limit of the pulse width to which this embodiment can be applied is about a multiple of the oscillation period of the optical electric field of the optical pulse to be measured.

〔Effect of the invention〕

As explained above, the optical pulse measuring device of the present invention can measure the spatial distribution of an electric field using only a single surface of a semiconductor crystal, so there is no need to consider the thickness of the semiconductor crystal, and it is easy to hold it. be. Therefore, the manufacturing cost of the semiconductor crystal and its support is reduced. Furthermore, since there is no need to move the semiconductor crystal, the optical system of the condensing means can also be manufactured at low cost.

According to the present invention, the intensity waveform and phase waveform of the optical pulse to be measured can be determined instantly with high precision using a small and inexpensive device. Also, the phase of the optical pulse is determined at high speed. It can be used to adjust the light source and has great effects.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an optical pulse measuring device according to a first embodiment of the present invention. FIG. 2 is a diagram showing an example of a correlation waveform. FIG. 3 is a block diagram of an optical pulse measuring device according to a second embodiment of the present invention. FIG. 4 is a diagram showing the output voltage waveform of the differential amplifier 10. FIG. 5 is a diagram showing the output voltage waveform of the amplifier 12. FIG. 6 is a diagram showing Fourier transform of the waveform of FIG. 4. FIG. 7 is a diagram showing Fourier transform of the waveform of FIG. 5. FIG. 8 is a diagram showing the intensity waveform of the optical pulse to be measured obtained by calculation. FIG. 9 is a diagram showing a phase waveform of a measured optical pulse obtained by calculation. FIG. 10 is a block diagram of an optical pulse measuring device according to a third embodiment of the present invention. FIG. 11 is a diagram showing an example of a waveform obtained by the third embodiment. FIG. 12 is a block diagram of a conventional optical pulse measuring device. 1.22...Semi-transparent mirror, 2.3...Reflecting mirror, 4...
Semiconductor crystal, 5... Recorder, 6.7... Prism,
8... Lens, 9... DC power supply, 10... Differential amplifier, 11... Photodetector, 12... Amplifier, 13...
...Waveform recording device, 14...Computer, 15...
・Oscilloscope, 16...Low pass filter, 17
...High-pass filter, 18...Detector, 19...
・Load resistance, 20...Moving table, 23...Coupling circuit,
24...Ammeter. Patent Applicant Nippon Telegraph and Telephone Corporation Agent Patent Attorney Ide Nao Takayoshi 1 Mouth Shoulder 2 Times 2 Noyahi, Tsukasa OgL Horse and Shi; , Kagaku 60 Lu 70 Fan 11 times

Claims (1)

[Claims] 1. Branching means for branching the optical pulse to be measured into two light beams, condensing means for making these two light beams incident on a common semiconductor crystal, and electrical conduction by two-photon absorption of the semiconductor crystal. In the optical pulse measuring device equipped with a measurement circuit for measuring a change in intensity, the light focusing means includes means for giving a relative optical path length difference to the two light beams and then combining them into one light beam. An optical pulse measurement device featuring: