JP3404479B2 - Optical sampling waveform observation device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical sampling waveform observing device for observing an optical signal waveform in an ultra-short time region which cannot be directly detected by a photoelectric conversion element.

[0002]

2. Description of the Related Art Conventionally, an optical sampling waveform observing device for observing the waveform of an optical signal is known. This optical sampling waveform observing apparatus uses a sum-frequency generation (Sum-Frequency Generation), which is a second-order nonlinear optical effect.
The optical signal to be measured is observed by converting it to a frequency that can be detected by the photoelectric conversion element by performing optical sampling using the phenomenon of "G"). Specifically, this device is designed to measure an optical signal under measurement (optical angular frequency ωsig)
By summing the sampling pulse light (optical angular frequency ωsamp) having a narrower pulse width than this in the nonlinear optical crystal, the sum frequency optical signal whose optical angular frequency becomes “ωsig + ωsamp” (hereinafter referred to as “SF light”) ) Is taken out, and the SF light is photoelectrically converted to display the waveform of the measured optical signal.

FIG. 6 is a block diagram showing an example of the configuration of the above-mentioned conventional optical sampling waveform observation apparatus. The operation of the conventional optical sampling waveform observing device will be described with reference to FIG. 6. The measured optical signal (optical angular frequency ωsig) S is the sampling pulse light (optical angular frequency ωsamp) P emitted from the sampling pulse light generating unit 1. Together with the optical multiplexer 2
Are combined at, and then input to the nonlinear optical crystal 8. The nonlinear optical crystal 8 generates SF light 3 of the measured optical signal S and the sampling pulse pulse light P based on the second-order nonlinear optical effect.

At this time, the pulse width of the sampling pulsed light P is made sufficiently narrower than the pulse width of the measured optical signal of the measured optical signal, and the repetition frequency of the sampling pulsed light P is an integer of the fundamental frequency of the measured optical signal. 1 to a little higher (or slightly lower), or the repetition frequency of the sampling pulse light P is set to a value close to an integer fraction of the fundamental frequency of the measured optical signal S, and the sampling pulse light P is delayed. The time relative position between the measured optical signal S and the sampling pulse light P is swept by a method of sweeping the delay amount, and the cross-correlation signal between the measured optical signal S and the sampling pulse light P is set as the SF light 3 and is nonlinear. The light can be output from the optical crystal 8 and incident on the light receiving unit 4.

The light receiving section 4 detects (photoelectrically converts) the SF light 3 and outputs it to the electric signal processing section 5. The electric signal processing unit 5 can display the waveform of the measured optical signal S on the waveform display unit 6 by amplifying the output signal of the light receiving unit 4 and then performing analog-digital conversion processing.

As the above-mentioned nonlinear optical crystal 8,
Up to now, generally, KTiO having a relatively high nonlinear optical effect
A PO ₄ (KTP) crystal is used. By using this KTP crystal, the generation efficiency of SF light 3 is 1 × 10 ^−4.
W ^-1 was obtained, and it was 20 at the peak intensity of 2.3 mW of the measured optical signal S as an optical sampling waveform observation device.
A signal to noise (SN) ratio of dB has been obtained. (Ota
Others: "Highly sensitive optica
l sampling system using ti
ming-jitter-reduced gain-
switched optical pulse ", E
LECTRONICS LETTERS, Vol. Three
3, No. 25, pp. 2142-2143, 1997
reference)

[0007]

However, among the optical signals actually observed, there are weak optical signals, and when the waveform is observed after being amplified by an optical amplifier or the like, the nonlinear optical effect in the optical fiber is observed. In some cases, the optical waveform changed, and the original optical signal waveform could not be reproduced because it was buried in the noise of the optical amplifier. Therefore, such a weak optical signal must be directly observed in waveform, and the above SN ratio was insufficient.

[0008]

In order to solve the above problems, the present invention has the following respective configurations. That is, the optical sampling waveform observing apparatus according to the present invention is a sampling pulse light generating means for generating sampling pulse light having a pulse width narrower than that of the measured optical signal and a low repetition frequency, the measured optical signal and the sampling pulse light. Optical combining means for combining the sampling pulse light output from the generating means, and the output light of the optical combining means are cross-correlated in the nonlinear optical crystal, and the optical frequency of the measured optical signal and the sampling pulse light generation Optical cross-correlation means for generating sum frequency light having a sum of optical frequencies of sampling pulsed light output from the means, photoelectric conversion means for converting the light generated by the optical cross-correlation means into an electrical signal, An optical sampler having waveform display means for displaying the time waveform of the measured optical signal from the electric signal output from the photoelectric conversion means. A waveform observing device, wherein the nonlinear optical crystal is configured to periodically invert the polarization of the nonlinear optical crystal so that the phase of the sum frequency light generated from the inside of the nonlinear optical crystal is matched. Is characterized by. According to the above structure, when the polarization inversion period of the nonlinear optical crystal is used as the quasi phase matching condition, the SF light generation efficiency is maximized. As a result, a large SF light is generated from the nonlinear optical crystal, so that the signal component is improved,
The SN ratio is improved.

Further, the optical sampling waveform observing device according to the present invention is the optical sampling waveform observing device according to the above-mentioned invention, wherein the period in which the polarization of the nonlinear optical crystal is inverted is set in the inside of the nonlinear optical crystal. It is characterized in that the phase of the second harmonic light having a frequency twice as high as the optical frequency of the sampling pulse light generated from each position and outputted from the sampling pulse light generating means is not matched. In this case, by setting the polarization inversion period of the nonlinear optical crystal so as to satisfy the quasi-phase matching condition, and setting the above relationship, the sampling pulsed light is sh
Since the SH light as a noise component due to the generation of light is easily separated, the S light from the measured optical signal is
Only F light is used for measurement.

Further, the optical sampling waveform observing device according to the present invention is the optical sampling waveform observing device according to any one of the preceding inventions, wherein the sum frequency light is selectively extracted from the output light from the nonlinear optical crystal. It is characterized in that it has means for leading to photoelectric conversion means. By selectively sending the sum frequency light to the photoelectric conversion means in this way, it becomes possible to display a waveform in the electric signal processing section.

Further, the optical sampling waveform observing device according to the present invention is the optical sampling waveform observing device according to any one of the preceding claims, wherein a periodically poled lithium niobate crystal is used as the nonlinear optical crystal. It is characterized by

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS (First Embodiment) FIG. 1 is a block diagram showing an example of the configuration of an optical sampling waveform observing apparatus according to the first embodiment of the present invention. Elements common to the elements shown in FIG. 6 according to the technique are individually denoted by the same reference numerals, and description of those elements will be omitted as appropriate. The optical sampling waveform observing device H of the configuration example shown in FIG. 1 includes a sampling pulse light generating section (sampling pulse light generating means) 1, an optical combining section (optical combining means) 2, an optical cross-correlation means 30, a light receiving section 4, and an electric signal. Periodically poled lithium niobate crystal (hereinafter abbreviated as PPLN crystal) is used as the nonlinear optical crystal used as the optical cross-correlation means 30 having the processing section 5 and the waveform display section 6. 6 are equivalent to those of the conventional apparatus shown in FIG.

That is, the measured optical signal (optical angular frequency ωsig)
For S, an optical pulse (optical angular frequency ωsamp) P for sampling emitted from the sampling pulse light generation unit 1 is combined by the optical combining unit 2, and the measured optical signal S and the sum frequency light by the sampling pulse light P are combined. Optical cross-correlation means with quasi-phase matching condition (periodically poled lithium niobate crystal:
PPLN crystal) 30.

The optical cross-correlation means 30 of the PPLN crystal is 2
Based on the following non-linear optical effect, the sum frequency light of the measured optical signal S and the sampling pulse light P is generated. At this time,
The pulse width of the sampling pulsed light P is made sufficiently narrower than the pulse width of the measured optical signal of the measured optical signal S, and the repetition frequency of the sampling pulsed light P is set to an integer fraction of the fundamental frequency of the measured optical signal S. A method of making it slightly higher (or lower), or setting the repetition frequency of the sampling pulse light P near one integer of the fundamental frequency of the measured optical signal S, delaying the sampling pulse light P, and sweeping the delay amount The relative position in time between the measured optical signal S and the sampling pulse light P can be swept by a method or the like, and the cross-correlation signal between the measured optical signal S and the sampling pulse light P is used as the sum frequency light and the optical mutual of the PPLN crystal is It can be output from the correlation means 30.

Next, the light receiving section (photoelectric conversion means) 4 detects (photoelectrically converts) this sum frequency light and outputs it to the electric signal processing section 5. The electric signal processing section 5 amplifies the output signal of the light receiving section 4 and then performs analog-to-digital conversion processing to display the waveform of the measured optical signal S on the waveform display section (waveform display means such as a display) 6. The waveform of the measurement optical signal S can be visually recognized.

Next, the optical cross-correlation means 30 of the PPLN crystal is used.
The generation efficiency of the SF light 3 due to can be expressed as in Equation 1 described later. However, in Equation 1, η _sf is S
Generation efficiency of F light, P _sf is light intensity of SF light, P _sig is light intensity of optical signal to be measured, P _samp is light intensity of sampling pulse light P, μ ₀ is magnetic permeability in vacuum, and ε ₀ is vacuum. _Where ω _sig is the optical angular frequency of the measured optical signal, ω _samp is the optical angular frequency of the sampling pulse light P, and d ₃₃ is the PPL.
Non-linear optical constant of N crystal, l is the length of PPLN crystal,
k _sf is the wave number vector of SF light, k _sig is the wave number vector of the measured optical signal, k _samp is the wave number vector of the sampling pulse light P, Λ is the polarization inversion period of the PPLN crystal, n
_sig is the refractive index of the PPLN crystal at the wavelength of the measured optical signal, and n _samp is P at the wavelength of the sampling pulse light.
The refractive index of the PLN crystal, n _sf, is P at the wavelength of SF light.
The refractive index of the PLN crystal, A represents the beam cross-sectional area within the PPLN crystal.

[0017]

[Equation 1]

FIG. 2 is a characteristic diagram showing an example of the relationship between the SF light generation efficiency and the polarization inversion period Λ based on Expression 1.
From the characteristics shown in FIG. 2, when the polarization inversion period Λ of the PPLN crystal is set to 2π / (Ksf-Ksig-Ksamp) (this can be called a quasi phase matching condition), the SF light generation efficiency η is obtained. It will be the maximum. Therefore, it is clear that a large amount of SF light is generated from the optical cross-correlation means 30 of the PPLN crystal, the signal components in the light receiving unit 4 and the electric signal processing unit 5 are improved, and the SN ratio of the optical sampling waveform observation device is improved. Became.

(Second Embodiment) The optical sampling waveform observing apparatus according to the second embodiment of the present invention has the same general structure as that of the first embodiment. However, in the second embodiment, the polarization inversion period Λ of the PPLN crystal and the wavelength of the sampling pulse light P are set so as to satisfy the quasi-phase matching condition of SF light generation, and at the same time, the polarization inversion period Λ of the PPLN crystal is set. , The sampling pulse light is set so as to deviate from the pseudo phase matching condition of the second harmonic (SH) light.

Generation of SH light 3 of sampling pulse light P
The efficiency can be expressed as shown in Equation 2 below. However
However, in Formula 2, the same symbols as in Formula 1 are the same as in Formula 1.
Represents the meter, η _shIs the SH light of the sampling pulse light P
Generation efficiency, P_shIs the light intensity of SH light, k_shIs the wave of SH light
A number vector, n_shIs a PPLN crystal at the wavelength of SH light
Represents the refractive index of.

[0021]

[Equation 2]

Second harmonic of sampling pulse light P (S
H) light is easily generated because the light intensity of the sampling pulse light P is much higher than that of the optical signal S to be measured, and the SH light of the sampling pulse light P is separated near the wavelength of the SF light 3. Hateful. If the SH light of the sampling pulse light P is input to the light receiving unit 4 at the same time as the SF light 3,
The SH light of the sampling pulse light P becomes noise and deteriorates the SN ratio of the optical sampling waveform observation device. Big S
In order to obtain SH light as a noise component of the sampling pulse light P as much as possible while obtaining the F light 3,
The polarization inversion period Λ of the PLN crystal and the wavelength of the sampling pulse light are appropriately set, that is, the polarization inversion period Λ of the PPLN crystal is set so as to deviate from the quasi phase matching condition of the second harmonic (SH) light of the sampling pulse light. There is a need.

In FIG. 3, the wavelength of the measured optical signal S is 1559 nm.
In the case of, a characteristic diagram showing an example of the relationship between the generation efficiency of the SH light of the SF light 3 and the sampling pulse light P and the wavelength of the sampling pulse light P based on Expressions 1 and 2. In FIG. 3, a solid line represents the relationship between the generation efficiency of the SF light 3 and the wavelength of the sampling pulse light P, and a broken line represents the relationship between the generation efficiency of the SH light of the sampling pulse light P and the wavelength of the sampling pulse light P. There is.

For example, referring to FIG. 3, when the polarization inversion period Λ of the PPLN crystal is set to 18.92 μm and the wavelength of the sampling pulse light P is set to 1533 nm, the quasi phase matching condition of the SF light 3 is satisfied as shown by the solid line, The maximum SF light 3 generation efficiency is obtained. When the wavelength of the sampling pulse light P is 1533 nm as indicated by the wavy line, the SH light generation efficiency of the sampling pulse light P is very small. Therefore, if the wavelength of the measured optical signal is 1559 nm, the PPL
If the polarization inversion period Λ of the N crystal is set to 18.92 μm and the wavelength of the sampling pulse light P is set to 1533 nm, the large SF light 3
And the SH light of the sampling pulse light P that becomes noise can be suppressed. As a result, it is possible to obtain the effect of improving the SN ratio of the optical sampling waveform observation device.

(Third Embodiment) FIG. 4 is a block diagram showing a configuration example of an optical sampling waveform observing apparatus according to a third embodiment of the present invention, which is common to the elements shown in FIG. The same reference numerals are given to the elements, and the description thereof will be omitted as appropriate. The configuration of the device according to the third embodiment shown in FIG. 4 is different from the configuration shown in FIG. 1 in that the SF light 3 output from the optical cross-correlation means 30 of the PPLN crystal is transmitted and the SH light of the sampling pulse light P is transmitted. It is characterized in that an optical bandpass filter 7 that suppresses and outputs to the light receiving unit 4 is provided. By providing this optical bandpass filter 7, a large amount of SF light 3 can be obtained, and at the same time, the SH light of the sampling pulse light P that becomes noise can be suppressed, so that the SN ratio of the optical sampling waveform observation apparatus can be improved. You can

FIG. 5 is a characteristic diagram showing an example of the relationship between the measured optical signal peak intensity and the SN ratio of the optical sampling waveform observing apparatus according to the third embodiment of the present invention. In the figure ●
The mark indicates the experimental result of the present invention using PPLN crystal, the solid line indicates the theoretical calculation result, and the mark indicates the conventional KTP (KTiOPO ₄ ).
The experimental results of an optical sampling waveform observation device using a crystal as a nonlinear optical crystal are shown.

From the results shown in FIG. 5, the optical sampling waveform observing apparatus using the PPLN crystal according to the present invention as compared to the conventional optical sampling waveform observing apparatus using the KTP crystal as the nonlinear optical crystal is better. The S / N ratio is improved by about 15 dB. Moreover, the PPLN according to the present invention
It was revealed that the optical sampling waveform observation device using the crystal as a nonlinear optical crystal exhibits an extremely excellent SN ratio close to the theoretical calculated value.

[0028]

As described above, according to the present invention, a nonlinear optical crystal in which polarization is periodically inverted is used as the nonlinear optical crystal of the optical cross-correlation means of the optical sampling waveform observation apparatus. It is possible to generate light, and it is possible to improve the SN ratio of the optical sampling waveform observation device. Therefore, according to the device of the present invention, it is possible to directly measure the waveform of the weak optical signal, and it is possible to perform the optical sampling waveform observation that is not affected by the deterioration of the waveform due to the optical amplifier or the noise.

The polarization inversion period of the nonlinear optical crystal is
By setting the length that does not match the phase of the second harmonic light having a frequency twice that of the sampling pulsed light,
Since the SH light as the noise component caused by the sampling pulsed light generating the SH light can be easily separated, only the SF light from the measured optical signal can be effectively used for the measurement, and the noise component It is possible to perform an optical sampling waveform observation that is not affected by.

Furthermore, if the sum frequency light of the light emitted from the nonlinear optical crystal is selectively guided to the photoelectric conversion means, the waveform of the weak optical signal can be directly and surely measured, and the waveform of the waveform by the optical amplifier can be measured. Optical sampling waveform observation that is not affected by deterioration or noise can be performed more reliably. Also,
By using a periodically inverted lithium niobate crystal as the non-linear optical crystal, it is possible to perform optical sampling waveform observation with an S / N ratio close to the ideal theoretical calculation result without being affected by waveform deterioration or noise. it can.

[Brief description of drawings]

FIG. 1 is a block diagram showing the main configuration of an optical sampling waveform observation apparatus according to a first embodiment and a second embodiment of the present invention.

FIG. 2 is a characteristic diagram showing a result of measuring a relationship between a generation efficiency of SF light and a polarization inversion period Λ by the device of the present invention.

FIG. 3 is a characteristic diagram showing measurement results of the relationship between the generation efficiency of SF light and the generation efficiency of the second harmonic of sampling pulse light and the wavelength of sampling pulse light by the device of the present invention.

FIG. 4 is a block diagram showing a configuration of an optical sampling waveform observation device according to a third embodiment of the present invention.

FIG. 5 is a characteristic diagram showing a measurement result of the relationship between the measured optical signal peak intensity and the SN ratio of the optical sampling waveform observation apparatus according to the third embodiment of the present invention.

FIG. 6 is a block diagram showing a configuration example of an optical sampling waveform observation apparatus according to a conventional technique.

[Explanation of symbols]

H ... Optical sampling waveform observing device, P ... Sampling light pulse, S ... Optical signal to be measured, 1 ... Sampling pulse light generating unit (light pulse generating means), 2 ... Optical combining unit (Photosynthetic means), 3 ... SF light, 30 ... Optical cross-correlation means (P
PLN crystal), 4 ... Light receiving part (photoelectric conversion means), 5 ... Electric signal processing part, 6 ... Waveform display part (waveform display means), 7 ...
..Optical band filters

Claims (3)

(57) [Claims] 1. A sampling pulse light generating means for generating sampling pulse light having a pulse width narrower than that of an optical signal to be measured and a low repetition frequency, and an optical pulse output from the optical signal to be measured and the sampling pulse light generating means. Of the sampling pulse light output from the sampling pulse light generating means and the optical frequency of the optical signal under measurement by cross-correlating the output light of the optical multiplexing means with each other in the nonlinear optical crystal. An optical cross-correlation means for generating sum frequency light having a sum frequency with an optical frequency, a photoelectric conversion means for converting the light generated by the optical cross-correlation means into an electric signal, and an electric power output from the photoelectric conversion means. An optical sampling waveform observation apparatus having waveform display means for displaying a time waveform of the measured optical signal from a signal, wherein the nonlinear optical crystal is Of the as the phase of the sum frequency light are aligned to occur from the interior of the nonlinear optical crystal, and the polarization of the nonlinear optical crystal one which periodically inverted, the counter
A switching period is generated from the inside of the nonlinear optical crystal.
The length that the phase of the second harmonic light of the pulling pulse light does not match
Optical sampling waveform measuring apparatus characterized by being configured to. 2. The output light from the nonlinear optical crystal
A means for selectively guiding the sum frequency light to the photoelectric conversion means
The optical sampling waveform observation device according to claim 1, further comprising a step . 3. The periodic polarization as the nonlinear optical crystal
The optical sampling waveform observation device according to claim 1 or 2, wherein an inverted lithium niobate crystal is used .