JPH0663869B2 - Optical sampling waveform measuring device - Google Patents

Description

TECHNICAL FIELD The present invention is used in an optical sampling waveform measuring apparatus for measuring an optical pulse waveform used in optical communication or the like by using sum frequency light generation, and particularly, in the background. High SN by removing light
The present invention relates to an optical sampling waveform measuring device capable of measuring an optical pulse waveform with a ratio.

[Conventional technology]

Conventionally, sum frequency light generation (Sum
In optical sampling waveform measurement using Freqency Generation (hereinafter referred to as SFG), an optical pulse to be observed and a sampling optical pulse with a narrower width are guided to a nonlinear optical material, and the cross-correlation signal between them is sum frequency optical ( Below SF
Called light. ) Is used as a retrieval method.

SFG is a phenomenon in which when two light waves with angular frequencies ω ₁ and ω ₂ are incident on a nonlinear optical material, light with a sum angular frequency (ω ₁ + ω ₂ ) is generated. Especially when the angular frequencies of the two light waves are equal,
(Ω _DATA = ω _SAM = ω ₀ ) is the second harmonic generation (Second
Harmonic Generation) (hereinafter referred to as SHG), and light with double the angular frequency 2ω ₀ is generated. SFG achieves phase matching only for incident light of a specific linearly polarized light.

Phase matching means that the phase velocity of the nonlinear polarization wave generated by the incident light and the phase velocity of the SF light generated thereby coincide with each other, at which time the energy conversion occurs most efficiently.

Conventionally, a non-linear optical material that performs "type 1" phase matching has been used for this phase matching (Japanese Patent Application No. 63-35351).
See Japanese Patent Application No. 63-42320). “Type 1” phase matching means that the two incident lights (measured light and sampling light) have the same polarization direction and both are extraordinary light or both ordinary light due to birefringence of the nonlinear optical material. Is what happens.

Nonlinear optical materials generally have optical anisotropy, and when light is incident, two refracted lights appear due to birefringence, and the ray velocity is different except when propagating in the optical axis direction.

KTP, LiNbO ₃ , LiIO ₃ and KNbO as nonlinear optical materials
_{Although 3 and the} like are used, these are uniaxial crystals. In this case, of the two refracted lights, light having a polarization direction in which the light velocity is constant regardless of the propagation direction is called ordinary light, and light having different light velocity depending on the propagation direction is called abnormal light. Therefore,
It is possible to achieve phase matching of the SFG by adjusting the propagation direction of the ray and appropriately determining the ray velocity of the extraordinary ray.

FIG. 5 shows changes in the relative position with time of the pulse to be measured and the sampling optical pulse in the optical sampling waveform measuring method using this SFG, and the low-speed cross-correlation signal waveform obtained thereby. The cross-correlation signal waveform of Δf (Hz) as shown in FIG. 5 is set by making the repetition frequency of the sampling light pulse slightly lower or higher than the repetition frequency f [Hz] of the pulse under measurement, that is, Δf (Hz). To get Therefore, the time axis of the optical waveform to be measured can be enlarged and measured, so that an extremely high-speed optical pulse waveform can be detected.

In the example of FIG. 5, the frequency of the measured light pulse and the frequency of the sampling light pulse are substantially equal to each other, but the sampling frequency itself may be in the vicinity of 1 / f (n / Hz).

FIG. 6 (a) is a block diagram showing an example of a conventional optical sampling waveform measuring apparatus (where f is the repetition frequency of the measured light). In FIG. 6 (a), 1 is a sampling light source that generates a sampling light pulse train at a repetition frequency f−Δf, 2 and 3 are polarization direction controllers that control the polarization directions of the sampling light and the light to be measured, and 4 is the measurement target. A multiplexer for combining light and sampling light, 5 is a "type 1" nonlinear optical material that causes a "type 1" phase-matching SFG, and 6 is an optical filter that removes the measured light and the sampling light transmitted through this material. , 7 is a photodetector for detecting the generated SF light and converting it into an electric signal, 8 is an electric signal processing system for processing the electric signal, and 9 is a display for displaying the measured optical waveform.

Specifically, the polarization direction controllers 2 and 3 are a combination of a quarter-wave plate and a half-wave plate, a Babinet-Soleil compensator, and the multiplexer 4 is a fiber coupler or a beam splitter. Etc. are used.

As shown in FIG. 6 (a), the polarization direction controllers 2 and 3
When the polarization directions of the measured light and the sampling light are made to coincide with each other by extraordinary light (ordinary light), and two light waves are made incident on the nonlinear optical crystal 5, "type 1" phase matching is achieved and the SF is a correlation signal of the two light waves. Light is generated.

[Problems to be solved by the invention]

FIG. 6 (b) shows the relationship between the angular frequency and the power of the light wave. The light to be measured and the sampling light incident on the nonlinear optical material 5 are on the left side, and the emitted light generated on the nonlinear optical material 5 is on the right side. Is shown. As is clear from FIG. 6 (b),
In the conventional technique, the angular frequency of the SH light of the measured light itself is 2ω _DATA
And SH light angular frequency 2ω of sampling light pulse itself
_Since the angular frequency (ω _SAM + ω _DATA ) of SF light, which is a cross-correlation signal of _SAM and two lights, is close, it is difficult to extract only SF light.

Further, in the case of "Type 1" phase adjustment, since the angular frequencies ω _DATA and ω _SAM of the measured light and the sampling light are quite close to each other, there is a drawback that the SN ratio of the measured value becomes low for the following reasons. .

FIG. 7 is an explanatory diagram of the background light generation that causes the shot noise that is the reason for this. The upper part of the figure shows the light incident on the non-linear optical material 5 for each combination in which the sum frequency light is generated. If there is a measured light in one combination and there is not "1""0" is shown respectively. The lower part of the figure shows the light emitted from the nonlinear optical material 5 for each combination. The symbols a, b, and c are given to each combination, and the waveform actually obtained as the output of the light receiver 7 is indicated by the symbol d. The circled arrows indicate the direction of polarization, and the thick and thin arrows represent sampling light and measured light, respectively.

That is, in the case of “Type 1” phase matching, as shown in FIG. 7, since the measured light pulse (angular frequency ω _DATA ) and the sampling light pulse (angular frequency ω _SAM ) are incident in the same polarization direction, In addition to SF light (angular frequency ω _SAM + ω _DATA ) [Fig. 7a] which is a cross-correlation signal of two lights, SH light (angular frequency 2ω _DATA ) [Fig. 7b] of the measured light itself and sampling SH light of light pulse itself (angular frequency 2ω _SAM )
[Fig. 7c] is also phase-matched. Therefore, the output light power is the sum of these. Of course, the emitted light also includes light that has not been converted into SF light and has the original frequency.

In optical sampling, since the cross-correlation signal is increased, the sampling light power P (ω _SAM ) is increased as much as possible. Therefore, the cross-correlation signal optical power P (ω _SAM + ω _DATA )
The SH light power P (2ω _SAM ) ∝ {P (ω _SAM )} ^{2 of} the sampling light becomes significantly larger than ∝ {P (ω _SAM ) · P (ω _DATA )}. Therefore, the total output light power is a large background light due to P (2ω _SAM ) ∝ {P (ω _SAM )} ² . On the other hand, the measured light and the sampling light which are not converted into SF light and SH light also pass through the nonlinear optical material 5, but are removed by the optical filter 6. Here, when this output light is detected by the light receiver 7 and converted into an electric signal, the shot noise is proportional to the photocurrent generated at the total output power. Therefore, as shown in FIG. Since a large shot noise component due to P (2ω _SAM ) is superimposed, the SN ratio becomes extremely poor. In the conventional method, since the angular frequencies ω _DATA and ω _SAM of the measured light and the sampling light are quite close to each other, the angular frequency of the SF light (ω _SAM + ω _DATA ) and the autocorrelation SH wave of the measured light and the sampling light are Since the difference between the angular frequencies of (2ω _DATA , 2ω _SAM ) becomes small, it was difficult to separate the two with an optical filter. Therefore, the conventional optical sampling optical waveform measurement uses the background light (signal light and sampling light autocorrelation SH
The S / N ratio is limited by the shot noise due to G).

An object of the present invention is to provide a highly accurate and simple optical sampling waveform measuring device by improving the SN ratio of individual sampling values by eliminating the above drawbacks.

[Means for solving problems]

The present invention provides a sampling light generator that generates a sampling light pulse train having a pulse width narrower than that of an optical signal to be measured, a polarization direction controller that controls the polarization directions of the measured light and the sampling light, and the measured light and the sampling light. And a nonlinear optical material that generates a cross-correlation signal of the measured optical pulse and the sampling optical pulse as sum frequency light,
In an optical sampling waveform measuring instrument including an optical filter, a photodetector for detecting the generated sum frequency light and converting it into an electric signal, and an electric processing system for processing the electric signal and displaying an optical pulse waveform to be measured , A sampling light source having a large angular frequency difference with respect to the light to be measured is used as the sampling light generator, a non-linear optical material that causes "type 1" phase matching is used as the non-linear optical material, and sum frequency light is used as the optical filter. It is characterized by using a bandpass filter that transmits only light.

Further, the present invention provides a sampling light generator for generating a sampling light pulse train having a pulse width narrower than that of an optical signal under measurement, a polarization direction controller for controlling the polarization direction of the measurement light and the sampling light, the measurement light and sampling A multiplexer that combines light, a nonlinear optical material that generates the cross-correlation signal of the measured light pulse and the sampling light pulse as sum frequency light, an optical filter, and an electric signal that detects the generated sum frequency light An optical sampling waveform measuring instrument comprising a photodetector for converting into an optical signal and an electrical processing system for processing an electrical signal to display an optical pulse waveform to be measured, wherein the sampling light generator is an angular frequency with respect to the measured light. Is used, and a non-linear optical material that causes "type 2" phase matching is used as the non-linear optical material.

Further, the present invention provides a sampling light generator for generating a sampling light pulse train having a pulse width narrower than that of an optical signal under measurement, a polarization direction controller for controlling the polarization direction of the measurement light and the sampling light, the measurement light and sampling A multiplexer that combines light, a nonlinear optical material that generates the cross-correlation signal of the measured light pulse and the sampling light pulse as sum frequency light, an optical filter, and an electric signal that detects the generated sum frequency light In an optical sampling waveform measuring instrument comprising a photodetector for converting into an optical signal, and an electrical processing system for processing an electric signal to display an optical pulse waveform under measurement A sampling light source with a large difference is used, a non-linear optical material that causes a "type 2" phase matching is used as the non-linear optical material, and a sum frequency is used as the optical filter. Characterized by using a band pass filter that transmits light only.

Here, the “type 1” phase matching means that phase matching occurs when two incident lights have the same polarization direction and both are extraordinary or ordinary with respect to the birefringence of the nonlinear optical crystal. , "Type 2" phase matching refers to a case where phase matching occurs due to extraordinary light and ordinary light that are orthogonal to each other.

[Action]

When a sampling light source that has a large angular frequency difference from the measured light is used, the SF type angular frequency (ω _SAM + ω
_DATA ) and the angular frequency of the SH light of the sampling light itself (2ω
_SAM ) and the angular frequency of the SH light of the measured light itself (2
The difference with ω _DATA ) widens. Therefore, it is possible to remove the background light and detect only the SF light by using a bandpass filter that transmits only the SF light.

In the “type 2” nonlinear optical material, when the sampling light and the measured light whose polarization directions are orthogonal to each other are incident, the “type 2” phase matching is achieved and SFG occurs. However, the SH light of the measured light itself and the SH light of the sampling light itself do not occur because they belong to “type 1” phase matching in the same polarization direction. Therefore, the background light is removed and only the cross-correlation signal of two lights is generated.

That is, a bandpass filter that transmits only a sampling light source and SF light having a large angular frequency difference with the measured light,
By properly combining with a "type 2" nonlinear optical crystal, it is possible to remove background light and improve the SN ratio.

〔Example〕

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 (a) is a block diagram showing the first embodiment of the present invention, and FIG. 1 (b) is an input / output characteristic diagram of the "type 1" nonlinear optical material.

In the first embodiment, a sampling light generator for generating a sampling light pulse train having a pulse width narrower than that of an optical signal under measurement, polarization direction controllers 2 and 3 for controlling polarization directions of the measured light and the sampling light, The multiplexer 4 that combines the measurement light and the sampling light, the nonlinear optical material that generates the cross-correlation signal of the measured light pulse and the sampling light pulse as the sum frequency light, the optical filter, and the generated sum frequency light An optical sampling waveform measuring instrument including a photodetector 7 as a photodetector for detecting and converting into an electrical signal, an electrical processing system 8 for processing the electrical signal and displaying an optical pulse waveform to be measured, and a display 9. use in, as said sampling light generator, a sampling light source 10 for generating the sampling optical pulse train having an angular frequency which is close to the measured light having an angular frequency (ω _DATA) (ω _SAM) , Using the "Type 1" nonlinear optical material 5 causing phase matching of "type 1" as the nonlinear optical material, in which a band-pass filter 11 which transmits only the sum frequency light as the optical filter.

A feature of the present invention is that the sampling light source 10 and the bandpass filter 11 are used in FIG.

Specifically, KTP is used as a "Type I" nonlinear optical material.
Bulk crystals (made by Ascar), LiNbO ₃ , LiIO ₃ and K
NbO ₃ or the like can be used. The waveguide type may be used instead of the bulk type. Even in the same type of crystal like the KTP crystal of the present embodiment, it is possible to achieve the type 1 or type 2 phase matching by appropriately determining the cut-out angle of the crystal (perpendicular to the light incident direction). It is possible.

The first embodiment is a case where a sampling light source 10 having a large angular frequency difference from the measured light and a bandpass filter 11 that transmits only SF light are used. In this case, Fig. 1 (b)
As shown in, the angular frequency of SF light (ω _SAM + ω _DATA ) of the cross-correlation signal of the two lights, the angular frequency of SH light of the sampling light itself (2ω _SAM ) and the angular frequency of SH light of the measured light itself. The difference with (2ω _DATA ) widens. Therefore, by using the bandpass filter 11 that transmits only SF light,
Only SF light can be detected. For example, if the measured light wavelength is 1.32 μm and the sampling light wavelength is 1.55 μm, SF
The wavelength of the light is 0.707 μm, and the wavelengths of the autocorrelation SH light of the measured light and the sampling light are 0.660 μm and 0.755 μm, respectively. That is, by using the bandpass filter 11 having a half-width of about 0.09 μm centered on 0.707 μm, background light can be removed and only SF light can be extracted.

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

In the second embodiment, a sampling light source 1 having a small angular frequency difference with respect to the light to be measured is used as the sampling light generator, and a "type 2" non-linearity that causes "type 2" phase matching is used as the non-linear optical material. The optical material 12 is used.

The feature of the present invention resides in that the "type 2" nonlinear optical material 12 is used in FIG.

“Type 2” phase matching SFG means that SFG phase matching is achieved by extraordinary light and ordinary light that are orthogonal to each other, as can be seen from FIG. FIG. 3 is an explanatory diagram for explaining an SFG of “Type 2” phase matching. As shown in FIG. 3A, when the sampling light and the light under measurement whose polarization directions are orthogonal to each other are incident, “type 2” phase matching is achieved and SFG occurs. However, as shown in FIGS. 3B and 3C, the SH light of the measured light itself and the SH light of the sampling light itself described in the prior art are generated because they belong to the “type 1” phase matching in the same polarization direction. do not do. Therefore, as shown in FIG. 3d, the background light is removed and only the correlation signal of two lights can be detected, so shot noise is reduced and the SN ratio can be made very high.

FIG. 4 is a block diagram showing the third embodiment of the present invention.

In the third embodiment, a sampling light source 10 having a large angular frequency difference with respect to the light to be measured is used as the sampling light generator, a "type 2" nonlinear optical material 12 is used as the nonlinear optical material, and an SF filter is used as an optical filter. A bandpass filter 11 that transmits only light is used.

The feature of the present invention is that in FIG.
This is because the “type 2” nonlinear optical material 12 and the bandpass filter 11 were used.

That is, the third embodiment has an advantage that the SN ratio is further improved when the first and second embodiments are combined.

〔The invention's effect〕

As described above, according to the present invention, background light can be removed by filtering SF light, or by using a “type 2” nonlinear optical material that causes “type 2” phase matching, or by using both methods together. This has the effect of significantly improving the SN ratio of the value. This makes it possible to reduce the number of times of averaging processing of the optical sampling waveform measuring device, achieve high sensitivity of measurement, downsizing of the device, and the like, and the effect is great.

[Brief description of drawings]

FIG. 1 (a) is a block diagram showing the first embodiment of the present invention. Figure 1 (b) shows the input / output characteristics of the "type 1" nonlinear optical material. FIG. 2 is a block diagram showing the second embodiment of the present invention. FIG. 3 is an explanatory diagram of background light removal by the “type 2” phase matching SFG. FIG. 4 is a block diagram showing the third embodiment of the present invention. FIG. 5 is an explanatory diagram of generation of a cross-correlation signal between the measured light pulse and the sampling light pulse. FIG. 6 (a) is a block diagram showing a conventional example. Figure 6 (b) shows the input / output characteristics of the "type 1" nonlinear optical crystal. FIG. 7 is an explanatory diagram of background light generation in the conventional example. 1, 10 ... Sampling light source, 2, 3 ... Polarization direction controller, 4 ... Multiplexer, 5 ... "Type 1" nonlinear optical material, 6 ... Optical filter, 7 ... Photoreceiver, 8 ... Electric signal processing system, 9 ... Display, 11 ... Band pass filter, 12
…… “Type 2” nonlinear optical material, ω _DATA …… Angular frequency of measured light, ω _SAM …… Angular frequency of sampling light pulse.

Claims (3)

[Claims] 1. A sampling light generator for generating a sampling light pulse train having a pulse width narrower than that of an optical signal under measurement, a polarization direction controller for controlling the polarization direction of the measurement light and the sampling light, and the measurement light and the sampling light. , A nonlinear optical material that generates a cross-correlation signal of the measured optical pulse and the sampling optical pulse as sum frequency light, an optical filter, and the generated sum frequency light is detected and converted into an electrical signal. An optical sampling waveform measuring instrument comprising a photodetector for converting and an electrical processing system for processing an electric signal to display an optical pulse waveform to be measured, wherein the sampling light generator is an angular frequency difference with respect to the measured light. When a sampling light source with a large size is used and the polarization directions of the two incident lights are the same as the nonlinear optical material and both are extraordinary rays or both ordinary rays with respect to birefringence An optical sampling waveform measuring apparatus characterized by using a non-linear optical material that causes phase matching and using a band pass filter that transmits only sum frequency light as the optical filter. 2. A sampling light generator for generating a sampling light pulse train having a pulse width narrower than that of the measured light signal, a polarization direction controller for controlling the polarization direction of the measured light and the sampling light, and the measured light and the sampling light. , A nonlinear optical material that generates a cross-correlation signal of the measured optical pulse and the sampling optical pulse as sum frequency light, an optical filter, and the generated sum frequency light is detected and converted into an electrical signal. An optical sampling waveform measuring instrument comprising a photodetector for converting and an electrical processing system for processing an electric signal to display an optical pulse waveform to be measured, wherein the sampling light generator has an angular frequency with respect to the measured light. As the nonlinear optical material, the polarization directions of two incident light beams are orthogonal to each other, and when one of them is an extraordinary light beam and the other is an ordinary light beam, the phase is close to each other. An optical sampling waveform measuring device characterized by using a non-linear optical material that produces matching. 3. A sampling light generator for generating a sampling light pulse train having a pulse width narrower than that of the measured light signal, a polarization direction controller for controlling the polarization direction of the measurement light and the sampling light, and the measurement light and the sampling light. , A nonlinear optical material that generates a cross-correlation signal of the measured optical pulse and the sampling optical pulse as sum frequency light, an optical filter, and the generated sum frequency light is detected and converted into an electrical signal. An optical sampling waveform measuring instrument comprising a photodetector for converting and an electrical processing system for processing an electrical signal to display an optical pulse waveform to be measured, wherein the sampling light generator is an angular frequency difference with respect to the optical signal to be measured. As the nonlinear optical crystal, the polarization directions of the two incident light beams are orthogonal to each other and the phase adjustment is performed when birefringence occurs when one is extraordinary and the other is ordinary. An optical sampling waveform measuring device characterized by using a non-linear optical material that causes a mismatch and using a bandpass filter that transmits only sum frequency light as the optical filter.