JPH07159247A - Light waveform measuring apparatus - Google Patents

Abstract

PURPOSE:To provide an ultrahigh rate light waveform measuring apparatus employing so-called four wave mixing. CONSTITUTION:The light waveform measuring apparatus comprises a nonlinear optical element 1 which receives a light to be measured having a first optical frequency f1, and a sampling optical pulse train having a second optical frequency f1+ or -DELTAf, i.e., a optical frequency difference DELTAf larger than a predetermined band fmax with respect to the first optical frequency f1, and outputs a first light having frequency difference from the first optical frequency f1 equal to the optical frequency difference DELTAf, and a second light having frequency difference from the second optical frequency f1+ or -DELTAf equal to the optical frequency difference DELTAf, a frequency selecting means 2 for receiving outputs from the nonlinear optical element and outputting any one of them selectively, and means 3 for converting the output from the means 2 into an electric signal corresponding to sampled information to be measured.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to the measurement of optical waveforms in the optical field.

[0002]

2. Description of the Related Art In order to observe an optical waveform of signal light amplitude-modulated at a constant repetition frequency, a measurement using a combination of a photodiode and an oscilloscope is general. However, the bandwidth of the photodiode is 30-40 at the highest speed.
It is about GHz, and signal light having a band exceeding this cannot be measured by this measuring method. On the other hand, as a method for observing such an ultra-high-speed optical signal, the document “NTT R
& D, Vol. 40, No. 6 pp825 ”and the literature“ Journal of the Institute of Electronics, Information and Communication Engineers B-1, Vol. J75- ”.
B-1, No. 5 pp372 ”, the optical sampling method has been announced. In this method, a signal light to be observed and a narrow sampling light pulse train are guided to a nonlinear optical crystal, a cross-correlation signal between them is taken out as sum frequency light, and the waveform of the signal light is directly observed.

The optical sampling method will be outlined below with reference to FIG. The frequency band of the included modulation signal is fma
When the signal light that is x and is amplitude-modulated at the repetition frequency f ₀ is the light to be measured and is oscillated by the semiconductor laser 25 or the like,

[0004]

[Equation 1]

A pulse light train having a repetition frequency fs represented by (hereinafter, referred to as equation 1) is defined as a sampling light pulse train. Then, each is made into linearly polarized light whose polarization plane can be controlled through the polarization control device 4. Then, as shown in FIG. 6, the relative position between the measured light and the sampling light pulse train is

[0006]

[Equation 2]

The two lights are shifted by ΔT (hereinafter referred to as the equation 2) by the optical multiplexer 6 and are incident on the nonlinear optical material 21. The nonlinear optical material 21 is made of, for example, a KTP crystal, and is a so-called sum frequency light generation (Sum Frequency Gene) which is an interaction between two incident lights.
ration: SFG) occurs. Sum frequency light generation is the second-order nonlinear susceptibility x when two lights are incident on a certain nonlinear optical material.
^{By (2)} , it refers to a non-linear optical phenomenon that outputs light with a frequency that is the sum of the respective optical frequencies. In general, this phenomenon has a very fast response speed, and the power P _samp of the sump link optical pulse train and the power P _{sig of the} light under measurement between the optical pulse widths thereof.
The sum frequency optical power P is generated by multiplying the product of and the generation efficiency η. The light from the nonlinear optical material 21 is filtered by the optical filter 22.
And the like are transmitted to cut out light of an unnecessary wavelength such as the measured light and sampling light pulse train, and extract only the sum frequency light. This sum frequency light is a low-speed optical signal having the same frequency as the repetition frequency of the sampling light pulse train, and this light is received by the light receiver 23 and converted into an electrical signal. The envelope of the intensity of the sum frequency light is the waveform of the light under measurement magnified along the time axis, and its repetition frequency is nΔ.
f _samp, which is a low frequency. With the above configuration, it is possible to observe an ultrafast optical waveform.

The resolution in this measuring method is the width of the sampling light pulse train, the response speed of the nonlinear optical material 21, the time difference between the measured light and the sampling light pulse train due to the wavelength dispersion of the nonlinear optical material 21, the jitter of the electronic circuit. Etc.

The S / N ratio is determined by the efficiency of the non-linear optical material 21, the efficiency of the light receiver 23, the thermal noise of the electronic circuit, and the like.

[0010]

In the optical sampling method using the above sum frequency light, the KTP crystal has a sum frequency light generation efficiency P as shown in the above document.

[0011]

[Equation 3]

Since it is considerably low, the strength of the generated correlation signal is extremely small. Therefore, it is necessary to use a photomultiplier tube having a multiplication factor higher than that of the photodiode for the photodetector 23, and increase the number of sweeps to improve the S / N ratio by averaging. In such a case, the following problems may occur.

First, since the thickness of the nonlinear optical material 21 such as KTP is about several millimeters, the wavelength dispersion of this element causes a group delay time difference between the measured light and the sampling light pulse train, which deteriorates the time resolution. Let If the element is thinned in order to avoid this problem, the generation efficiency deteriorates.

Next, it is necessary to achieve phase matching in the generation of the sum frequency light, and the angle condition between the incident light angle on the nonlinear optical material 21 and the crystal axis is severe. Therefore, there are problems in the yield of the nonlinear optical material 21 and the stability of the measuring device.

When a photomultiplier tube is used for the photodetector 23, the photodetector is large and expensive and is not easy to handle.

When a photomultiplier tube is used for the photodetector 23, this photodetector generally has a slower response speed than that of the photodiode. Therefore, n in Expression 1 is increased to increase the repetition frequency fs of the sampling light pulse train. Must be set low. However, if you do so, as can be seen from Equation 2,
When n is large, jitter due to fluctuations in n itself and Δf _samp appears in ΔT, which deteriorates the time resolution. Also,
Generally, when the repetition frequency fs is low in the pulse light source,
It becomes difficult to generate an optical pulse having a narrow pulse width. Therefore, it is difficult to obtain a narrow sampling optical pulse train necessary for achieving high time resolution.

Furthermore, when the measured light has a short wavelength, the wavelength of the sum frequency light generated becomes extremely short, and the usable light receiver is limited.

Finally, if the sum frequency light generation efficiency of the nonlinear optical material 21 is low, it is necessary to increase the number of averaging processes in order to improve the S / N. However, there is also a problem in that the time required for measurement increases as the number of times increases.

[0019]

As described above, the efficiency of the nonlinear optical material greatly influences the performance of the present measurement method. Therefore, in the present invention, the following optical waveform measuring device has been developed by using four-wave mixing which is generally higher in efficiency than the sum frequency light generation as the nonlinear optical effect. This four-wave mixing means the nonlinear optical element 1
When 2 or 3 kinds of light with different optical frequencies are input to
This is a phenomenon in which light of a new optical frequency is generated due to the third-order nonlinear susceptibility x ⁽³⁾ . As will be described later, due to the relative relationship of the optical frequencies used, near degenerate four-wave mixing,
Alternatively, it may be called non-degenerate four-wave mixing, but here it will be generically called four-wave mixing. The outline of the present invention is as follows. For the nonlinear optical element 1, a measured light having a first optical frequency f1 amplitude-modulated with information to be measured having a predetermined band fmax;
Second optical frequency f1 ± Δ having an optical frequency difference Δf larger than the predetermined band fmax with respect to the optical frequency f1 of
and a sampling light pulse train having f. The nonlinear optical element 1 has an optical frequency difference between the first light whose optical frequency difference from the first optical frequency f1 is equal to the optical frequency difference Δf described above due to four-wave mixing and the second optical frequency f1 ± Δf. Output the second light equal to the optical frequency difference Δf described above.
Further, the frequency selecting means 2 which receives the output from the non-linear optical element 1 and selects and outputs either the first light or the second light, and the output from the frequency selecting means 2 are sampled. Conversion means 3 for converting into an electric signal corresponding to the information to be measured.

[0020]

The operation of the present invention will be described below with reference to the case where the semiconductor laser optical amplifier 11 is used as the nonlinear optical element 1. FIG. 3 is a diagram for explaining the principle of generation of four-wave mixing in the semiconductor laser optical amplifier 11, and in particular, this configuration may be called forward four-wave mixing. Semiconductor laser optical amplifier 11
On the other hand, the measured light having the optical frequency f1 and the optical frequency f1 + Δ
and the sampling light pulse train of f. When the light intensity is above a certain level, the carrier density and carrier occupancy probability distribution in the active layer inside the semiconductor laser optical amplifier 11 vibrate with the optical frequency difference Δf of the two light waves, forming a kind of grating. Amplitude modulation and phase modulation are applied to the input light such as the sampling light pulse train by this grating. The modulated light produces modulated sidebands, which are observed as new light f1-? F and f1 + 2? F. Here, in the case of Δf <1 to 2 GHz, the change in carrier density is the main cause of four-wave mixing, and this case is particularly called near-degenerate four-wave mixing. Also, Δf> 1-2
In the case of GHz, the change in the carrier occupancy probability distribution is the main cause, and this case is particularly called nondegenerate four-wave mixing. In the present specification, including them, they are collectively referred to as four-wave mixing.

Comparing the generation efficiency in four-wave mixing with the case of sum frequency light, there is a nonlinear optical element whose efficiency is very high. For example, it is a semiconductor crystal having a quantum well structure. In particular, when the nonlinear optical element 1 is an amplifying medium like a semiconductor laser, the light to be measured and the light generated by the four-wave mixing are in the same wavelength band, so that an amplification effect is obtained and a higher generation efficiency is obtained. .

On the other hand, the response speed of the four-wave mixing occurs inevitably as the first optical frequency f1 and the second optical frequency f1 + Δf.
And is proportional to the reciprocal of the optical frequency difference Δf. Therefore, in order to observe an optical waveform having a constant measurement band fmax, the nonlinear optical element 1 having high generation efficiency is required under the condition that the optical frequency difference Δf is larger than the measurement band fmax. In this regard, for example, the document "Terahertz four-wave"
mixing spectroscopy for study ultrafast dynamics i
na semiconductor optical amplifier; Appl. Phys. L
ett. 63 (9), pp. 1179 ”, Δf =
Even at 80 GHz, the generation efficiency is about -20 dB, which is much higher than the case of sum frequency light generation by a general nonlinear optical element. Due to these facts, the requirement for the sensitivity of the light receiver is relaxed, and it becomes possible to use a photodiode, which is fast and easy to handle, as the light-to-electricity converting means 3. In the above description, the optical frequency of the sampling optical pulse train is f1 + Δf, but this is f1-Δf.
, And then f1-2Δ as four-wave mixing.
Light of f and f1 + Δf is generated.

[0023]

EXAMPLES Examples of the present invention will be described below. The first embodiment will be described below with reference to FIG.

First, a measured light (optical frequency f1) having a predetermined frequency band fmax and amplitude-modulated at a repetition frequency f ₀ is polarized through a polarization control device 4 including a polarizer and a half-wave plate. Linearly polarized light with controllable wavefront.

The sampling light source 5 converts a trigger signal (f ₀ or an integer fraction of f ₀ ) from the outside into a repeating frequency fs according to the equation 1 to generate a sampling light pulse train with a narrow pulse width (light frequency f1 + Δf). To do. The optical section of the sampling light source 5 is composed of, for example, a mode-locked semiconductor laser, a gain-switched semiconductor laser, a mode-locked fiber laser, or the like. The polarization plane is adjusted by passing the sampling light pulse train through the polarization controller 4. The polarization planes of the measured light and the sampling light pulse train are adjusted so that the respective polarization directions are the same in the nonlinear optical element 1 and the four-wave mixing is efficiently generated. If the polarization planes of the light to be measured and the sampling light pulse train are adjusted and fixed in advance, these polarization control devices 4 are unnecessary and are not an essential mechanism in this measurement device.

The polarized light to be measured and the sampling light pulse train are multiplexed by the optical multiplexer 6. Here, as the optical multiplexer 6, an optical coupler including an optical fiber coupler or a dielectric multilayer film can be used. However, even if the optical multiplexer 6 is not provided, it is possible to directly input two lights to the nonlinear optical element 1 by using the optical duplexer as in the second embodiment.

The lights multiplexed by the optical multiplexer 6 are input to the non-linear optical element 1 which generates four-wave mixing. Particularly, when the semiconductor laser optical amplifier 11 is used for the nonlinear optical element 1, four-wave mixing is efficiently generated. Another advantage is that the semiconductor laser optical amplifier 11 has a very short element length of about several hundreds of μm, so that the dispersion due to the wavelength when transmitting light is small and the deterioration of resolution is very small. . In this embodiment, the semiconductor laser optical amplifier 11 is used for the nonlinear optical element 1, but four-wave mixing can also be generated by a semiconductor crystal having a quantum well structure, an optical fiber, or the like. When the optical frequency f1 of the measured light and the optical frequency f1 + Δf of the sampling light pulse train are superposed inside the nonlinear optical element 1, the measured light f1 and the sampling light pulse train f1 + are generated by four-wave mixing.
In addition to Δf, light having frequencies f1−Δf and f1 + 2Δf is generated and output. The light generated at this time is generated only when the measured light and the sampling light pulse train temporally overlap with each other, and is generated at the same repetition frequency fs as the sampling light pulse train.

Of the four light waves generated by the non-linear optical element 1, one of the four light waves is taken out by the frequency selecting means 2 and the measured light and the sampling light pulse train are suppressed. In this embodiment, an interference filter using a dielectric multilayer film is used as the frequency selecting means 2. As the frequency selecting means 2, a diffraction grating, a Fabry-Perot etalon, or the like may be used.

The output from the frequency selecting means 2 is converted from an optical signal into an electric signal by the converting means 3. Conversion means 3
Is a photodiode or a photomultiplier tube.
This electric signal is processed by the following electric signal processing system 12. The electric signal processing system 12 includes an A / D converter, a CPU,
It is composed of a memory for storing processing programs and data. The electric signal that has entered the electric signal processing system 12 from the converting means 3 is converted into a digital signal by an A / D converter and then averaged as necessary to improve the S / N. Since this signal is a signal that exists discretely depending on the sampling frequency, it is envelope-processed and converted into a waveform having a continuous connection. By these processes, the ultrahigh-speed optical waveform of the measured light is converted into a low frequency of the repetition frequency nΔf and displayed on the display means 13 such as a cathode ray tube or liquid crystal.

A second embodiment will be described below with reference to FIG. Similar to the first embodiment, the semiconductor laser optical amplifier 11 is used for the nonlinear optical element 1 in the second embodiment as well, but in this embodiment, the configuration called backward four-wave mixing shown in FIG. 4 is applied. ing. Also in this configuration, the principle of generation of four-wave mixing is the same as described above. However, it is effective when the pulse width of the sampling light pulse train is sufficiently long with respect to the time required for the measured light to propagate through the semiconductor laser optical amplifier 11. In the second embodiment, the light to be measured is input from one end of the semiconductor laser optical amplifier 11, and the optical duplexer 8
The sampling optical pulse train is input from the port A of the optical amplifier and input to the semiconductor laser optical amplifier 11 via the port B of the optical duplexer 8. Then, four-wave mixing is generated, and the generated light is extracted from the port C of the optical duplexer 8 via the port B. The processes after the frequency selecting means 2 are the same as those in the first embodiment.

In the case of the second embodiment, as shown in FIG. 4, in the output from the optical duplexer, the direction of this sampling light pulse train is opposite to the direction of the extraction direction of the four-wave mixing wave. Therefore, the sampling light intensity appearing at the output end of the duplexer is smaller than that in the first embodiment. Therefore, when the frequency selecting means 2 cuts out the sampling light pulse and the light to be measured to extract the four-wave mixing wave, the intensity of the sampling light pulse can be reduced more than that in the first embodiment, and S / It contributes to the improvement of N ratio.
It should be noted that the same operation can be performed by exchanging the sampling light pulse train and the terminal on which the measured light enters.

[0032]

The effects of the present invention can be summarized as follows. First, in four-wave mixing, the measured light and the optical frequency of the sampling light pulse train are close to each other, so that they are hardly affected by the chromatic dispersion of the nonlinear optical element 1. Particularly, when the semiconductor laser optical amplifier 11 is used for the nonlinear optical element 1, the element length is extremely short, which is extremely advantageous.

Secondly, in the four-wave mixing, the condition for phase matching is loose, and therefore the angle condition between the incident light angle on the nonlinear optical element 1 and the crystal axis is relaxed. Therefore, the yield of the nonlinear optical element 1 and the stability of the measuring device can be improved.

Thirdly, a photodiode which is small in size, inexpensive and easy to handle can be used for the light receiver.

Fourthly, when a high-speed photo diode is used for the light receiver, it is not necessary to increase n in the equation 1, so that the jitter generated in the equation 2 can be easily suppressed. Moreover, since the repetition frequency fs of the sampling light source 5 is high, it is easy to generate an optical pulse having a narrow pulse width. From these things, it is easy to improve the time resolution.

Fifth, since the wavelengths of the four-wave mixing are close to those of the light to be measured, there are many types of light receivers that can be used even when the measurement light has a short wavelength.

Sixth, when the photomultiplier tube is used for the photodetector, the number of averaging processes can be reduced as compared with the conventional one because the generation efficiency of the nonlinear optical element 1 is increased, so that the time required for measurement is reduced. Can be shortened.

[Brief description of drawings]

FIG. 1 is a block diagram showing a first embodiment of the present invention.

FIG. 2 is a block diagram showing a second embodiment of the present invention.

FIG. 3 is a principle diagram of forward four-wave mixing.

FIG. 4 is a principle diagram of backward four-wave mixing.

FIG. 5 is a block diagram showing a conventional technique.

FIG. 6 is a chart of optical sampling measurements.

[Explanation of symbols]

 1 Non-linear optical element. 2 Frequency selection means. 3 Conversion means. 4 Polarization control device. 5 Sampling light source. 6 Optical multiplexer. 8 optical duplexer. 11 Semiconductor laser optical amplifier. 12 Electric signal processing system. 13 Display means.

Claims (1)

[Claims] 1. A light to be measured having a first optical frequency f1 amplitude-modulated with information to be measured having a predetermined band fmax, and the predetermined band f with respect to the first optical frequency f1.
Upon receiving the sampling optical pulse train having the second optical frequency f1 ± Δf having the optical frequency difference Δf larger than max, the optical frequency difference between the measured light and the first optical frequency f1 becomes the optical frequency difference Δf. A non-linear optical element (1) for outputting equal first light and second light in which the optical frequency difference between the second optical frequency f1 ± Δf is equal to the optical frequency difference Δf; Frequency selection means (2) for receiving and selecting one of the outputs and outputting it, and conversion means (3) for converting the output from the frequency selection means into an electrical signal corresponding to the sampled information to be measured. Optical waveform measuring device having a.