JPS63208729A - Autocorrelator - Google Patents

Abstract

PURPOSE:To obtain high sensitivity by using a nonlinear optical material with an electrode which detects an overlap part of light as a nonlinear optical material, eliminating the need for phase matching, an obtaining a signal directly as an electric signal. CONSTITUTION:A light beam 1 to be measured is split into two by a beam splitter 2, one is reflected by a mirror 5, and the other is delayed by an optical delay part 3 and then superposed again by a beam mixer 4. This is converged by a lens 6 on the nonlinear material 10 with the electrode. A voltage with depends upon the intensity of the incident light is induced, so this is amplified by an FET preamplifier 7 and a voltage amplifier 8 and its time mean value is calculated by an integrator 9 and inputted to an X-Y recorder 11 as long-axis data. Here, the lateral axis is assigned to an optical delay quantity and a sweep is made to obtain the autocorrelation function of light pulses. Consequently, the need for position matching eliminated and the signal is obtained directly as the electric signal. Thus, the high sensitivity is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a measuring instrument for measuring the time width of a light pulse.

[Conventional technology]

For conventional ultrashort pulse light measuring instruments, see Proceedings of the IE 57.1.
(1969) pp. 14-18 (PROCII!
EDINGS OF IEEE 57.1 (1969
) pp 14-18). but,
These used fluorescence or second harmonic generation.

[Problem that the invention attempts to solve]

When the wavelength of the pulsed light to be measured is variable.

In the method of generating the second harmonic, it is necessary to adjust the angle and temperature of the nonlinear optical crystal in order to satisfy one phase matching, and the applicable wavelength range is also narrow.

In particular, when the wavelength is in the ultraviolet region or its vicinity, the second harmonic is in the vacuum ultraviolet region, making it practically difficult to realize. Furthermore, since the signal is light and a photomultiplier tube is used to convert it into electricity, there is a problem in that there is a need for light shielding, a high-voltage power supply, etc., and the cost is high.

An object of the present invention is to provide a simple device that can measure in a wide wavelength range, particularly in the ultraviolet region, without wavelength dependence in optical system adjustment, and can directly obtain signals electrically.

Furthermore, when measuring the time width of a light pulse, its autocorrelation function is often determined. To do this, the light is divided into two parts, superimposed again, and the degree of overlap is observed using nonlinear effects such as second harmonic generation and two-photon absorption. At this time, there is a problem in that, except when special phase matching is utilized using polarized light in second harmonic generation, the nonlinear process occurring in only one of the lights becomes a background and reduces detection sensitivity. The second harmonic generation method that does not generate background has a narrow wavelength range that satisfies phase matching, and also requires complicated adjustment.

Compared to this, if photon absorption is used, the selection range of nonlinear optical materials will be wider, and higher efficiency can be expected.

A second object of the present invention is to provide a highly sensitive device in which background is removed even when a process that does not involve phase matching is used.

[Means for solving problems]

Conventional devices use second harmonic generation as a method of measuring the degree of overlap between two optical pulses in order to obtain autocorrelation of waveforms.The purpose of the present invention is to solve this problem by using optical rectification. , achieved.

In addition, the second purpose is to divide the light beam into two at frequencies ω1 that are sufficiently lower than the repetition frequency of the light pulses.
．． This is achieved by a device that performs intensity modulation at ω2 and synchronously detects the signal at ω1±ω2.

[Effect]

In optical rectification, a signal proportional to the product of the electric field E of the light to be measured and its complex conjugate quantity E- is obtained, so there is no need for phase matching.Furthermore, it is possible to obtain a signal over a wide wavelength range. When light of 8 frequencies ω, which leads to the formation of
E call. Here x(2
) is the second-order nonlinear susceptibility. Therefore, if this optical rectification is used, DC polarization will always occur regardless of the frequency or wavelength of the light. Furthermore, there is no need for phase matching.

Also, the light beam is divided into two, each with ω1. When the intensity is modulated by ω2, if ωl and ω2 are sufficiently low compared to the repetition frequency of the optical pulse, if they are superimposed again and their autocorrelation is taken using a nonlinear optical process, the signal strength S(τ ) can be written as . In other words, only the overlapping term of the two beams has a component of ω1±ω2. Therefore, ω1
If signals are synchronously detected at a frequency of ±ω2, it becomes possible to eliminate the background, and also eliminate the effects of stray light, etc., making it possible to achieve high sensitivity.

〔Example〕

An embodiment of the present invention will be described below with reference to FIG. The light beam 1 to be measured is divided into two by a beam splitter 2,
After guiding the light by a mirror 5 and delaying one side by an optical delay unit 3, the beam mixer 4 overlaps the two beams again. This is narrowed down to an electrode-attached nonlinear optical material 10 using a lens 6. Here, as a nonlinear optical material with electrodes, K D P
(K HIP Oa) was used as an electrode by vapor-depositing silver. Since a voltage dependent on the intensity of the incident light is induced, this signal is amplified by the FET preamplifier 7 and the voltage amplifier, and then the time average is taken by the integrator 9, and the X-Y
Input on the vertical axis of the recorder 11. Here, by taking the optical delay amount on the horizontal axis and sweeping it, it is possible to obtain the autocorrelation function of the optical pulse. Mode-locked Ndδ+: The results of measurements for YAG laser are shown in FIG. From this, the time width of the optical pulse can be determined.

Next, another embodiment of the present invention will be described with reference to FIG.

A beam 31 formed by an optical pulse train to be measured is divided into two by a beam splitter 32, each guided by a mirror 33, and intensity modulated by choppers 34 and 35 at frequencies fx and fz. At this time, the repetition frequency of the optical pulse is 4
For MHz, jt=10kHz, fz=12kHz
I made it z. Then, the two beams are transferred to the nonlinear optical material 3
Overlap in the center of 6. A fundamental wave of Nd8+:YAG was used as the laser, and an ethanol solution of the organic dye Rhodamine 6G was used as the nonlinear optical material. Rhodamine is 2
Fluorescence is emitted by photon absorption, which is detected by the photodiode array 7. With the difference frequency converter 39, ft-fz
=2kHz and input it as a synchronization signal to the lock-in amplifier 40. The photodiode array performs channel sweeping by a driver 38, performs synchronous detection for each channel, and inputs the output to the computer 41.

After one channel has been measured in this manner, the driver 38 measures the next channel, and this can be repeated to obtain the spatial distribution of rhodamine fluorescence intensity. This corresponds to the autocorrelation function of the optical pulse, and from this the time width of the pulse can be determined. FIG. 4 shows the self-phase r! of the optical pulse determined using this method. Showing the JX function,
In contrast, FIG. 5 shows a signal obtained by a conventional method that does not use synchronous detection and in which no background occurs.

〔Effect of the invention〕

According to the present invention, there is no need for one-phase matching and the signal is directly obtained as an electrical signal, so the configuration of the device and its adjustment can be extremely simplified.

In addition, regardless of the wavelength of the measurement light, from infrared light to ultraviolet light,
Measurement is possible without changing the optical system.

Furthermore, the background can be removed using any nonlinear optical effect. Furthermore, the influence of stray light from external electric lights etc. can also be removed. Therefore, higher sensitivity is possible.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an autocorrelator according to an embodiment of the present invention, and FIG. 2 is obtained thereby. FIG. 3 is a diagram showing the autocorrelation function of light, FIG. 3 is a diagram showing the configuration of an autocorrelator according to another embodiment of the present invention, FIG. 4 is a diagram showing the autocorrelation function of light obtained thereby, and FIG. FIG. 3 is a diagram showing a signal obtained by a conventional method. 1...Measuring light, 2...Beam splitter, 3...
Optical delay unit, 4... Beam mixer, 7.8... Amplifier, 10... Nonlinear optical material with electrode, 11... X-Y
recorder. 31...Measuring light, 32...Beam splitter, 33
...Mirror, 34.35...Chopper, 36...
Nonlinear optical material, 37... Photodiode array (PDA), 38... PDA driver, 39... Difference frequency generator, 40... Lock-in amplifier, 41... Computer. No 1 Figure 4 Hi-4 Mi no Ifl (been 3L volume) 3rd Figure jl /6 stone door L 41 Tsuyu and Ichita cold 4 Figure 1 Be Pi 11 + Sefunori 1 Tsukasa (!PF) Acinej

Claims (1)

[Claims] 1. In an autocorrelator consisting of a beam splitter that separates light into two, a nonlinear optical material, an electric signal amplification system, and an optical system, the nonlinear optical system uses optical rectification as a nonlinear optical effect. An autocorrelator characterized in that the material is a nonlinear optical material with electrodes that detects overlapping parts of light. 2. In an autocorrelator consisting of a beam splitter that splits light into two, a nonlinear optical material, and a signal processing system, each of the two split beams is divided into two beams with an intensity at a different frequency that is sufficiently lower than the repetition frequency of the optical pulse. An autocorrelator characterized by having a device for modulating, and a device for synchronously detecting a signal having an intensity proportional to an autocorrelation function at a frequency corresponding to the difference or sum thereof.