JP3196881B2 - Optical sampling optical waveform measurement method - 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 optical waveform measuring method for measuring an optical pulse waveform used in optical communication or the like by utilizing a second-order nonlinear optical effect.

[0002]

2. Description of the Related Art An optical sampling optical waveform measurement is a method in which an optical pulse to be observed and a sampling optical pulse having a sufficiently narrower pulse width are guided to a nonlinear optical material, and a cross-correlation signal between the two is taken out using a nonlinear optical effect. Since the measured light can be sampled in the optical region, it is expected that the waveform of an ultra-high-speed optical signal can be observed with high time resolution.

Until now, this nonlinear optical effect has mainly been 2
Sum frequency light generation (SFG: Sum)
Frequency Generation) is used (Takara et al .:
"Ultrafast Optical Waveform Measurement Method by Optical Sampling Using Sum-Frequency Light Generation", IEICE Transactions, BI, vol. J75-
BI, No 5, pp. 372-380, 1992).

FIG. 5 shows the relationship between the optical frequency and the light intensity in the case of SFG. In the figure, ν _sam , ν _sig , ν _sfg
Represents a sampling light frequency, a measured light frequency, and a sum frequency light (SF light) frequency, respectively. P _sam , P _sig , P _sfg
Are the sampling light intensity, the measured light intensity, and the sum frequency light intensity, respectively.

[0005] SFG is an optical frequency ν as shown in FIG.
_When two lightwaves of the measured light of _sig and the sampling light of the optical frequency ν _sam are incident on the secondary nonlinear optical material, the optical frequency ν of the sum is obtained.
_sfg = light of ν _sig + ν _sam is a phenomenon that occurs.

FIG. 6 shows a configuration of an optical sampling optical waveform measuring apparatus using these nonlinear optical effects (Takara et al .: "Ultra high-speed optical waveform measuring method by optical sampling using sum frequency light generation"). "IEICE Transactions, BI, vol.J75-BI, No5 , pp.372-380, 1992 years, and forest other:" LiNbO ₃ waveguide and optical sampling using ultrashort optical pulses ", Showa (See the IEICE Spring National Convention Lecture Paper B-671). In the drawing, a thick solid line indicates a flow of an optical signal, and a thin solid line indicates a flow of an electric signal.

Reference numeral 101 denotes a light source to be measured as an external device, 1
02 is a sampling frequency oscillator. As the sampling light source 103, a mode-locked laser, a gain switching semiconductor laser, or the like can be used. Also, an ultrashort optical pulse generation technology combining a mode-locked laser or a gain switching semiconductor laser with a supercontinuum may be used (see Japanese Patent Application No. 6-165566, "Optical Sampling Optical Waveform Measuring Apparatus" Takara et al.).

In general, the conversion efficiency of the nonlinear optical effect of SFG depends on the polarization state of incident light. Accordingly, the measured light having the frequency f ₀ from the measured light source 101 and the sampling light having the frequency f ₀ / N−Ｎf from the sampling light source 102 are guided to the polarization controllers 104 and 105, respectively.
Here, the polarization state of the light pulse is changed to the nonlinear optical material 107.
Adjust to the optimal state for. After that, both lights are coupled to the coupling unit 1.
06 and the sum frequency light or difference frequency light (repetition frequency f
₀ / N- △ f). Conventionally, this nonlinear optical material 10
For example, inorganic crystal KTP, LiNbO ₃ or the like was used. Here, when the level of the second harmonic (optical frequency 2ν _sig ) of the measured light itself or the level of the second harmonic (optical frequency 2ν _sam ) of the sampling light itself cannot be ignored compared to the cross-correlation signal. , Optical bandpass filter 108
Is used to remove these background lights and extract only the cross-correlation signal. Thereafter, the light is received by the photodetector 109 and converted into an electric signal. After that, the signal processing unit 110 performs appropriate signal processing, and then the display unit 111 displays an optical waveform.

That is, in this optical sampling light waveform measuring method, another sampling light pulse having a pulse width narrower than that of the light pulse to be measured is superimposed on the light to be measured, and the light is incident on the nonlinear optical material 107. When the delay of the pulse is swept, the waveform of the cross-correlation signal generated in proportion to the overlapping portion of the two optical pulses is displayed on the display unit 111 to measure the optical waveform to be measured. is there.

FIG. 7 shows a temporal change in the relative position of the optical pulse to be measured and the sampling optical pulse in the optical sampling optical waveform measuring method using the SFG, and a low-speed cross-correlation signal optical waveform obtained thereby. The repetition frequency of the sampling light pulse is set to Δf more than an integer fraction (f ₀ / N [Hz]) of the repetition frequency of the measured light pulse.
By lowering (or increasing) by (Hz), a correlation signal waveform as shown in FIG. 7C is obtained. Here, N =
1 is shown. The envelope of the correlation signal waveform is obtained by enlarging the measured light waveform on the time axis, and the repetition frequency is Δf (Hz).

Therefore, this optical sampling optical waveform measuring method uses the nonlinear optical effect having a very fast response speed of about fsec to sample the optical waveform to be measured and expand the time axis of the optical waveform to be measured. Therefore, it is possible to detect a high-speed optical pulse waveform with extremely high time resolution.

The signal-to-noise ratio (SN ratio) of the optical waveform measured by the SFG optical sampling optical waveform measuring method will be described below (Takara et al .: "Ultra-high-speed optical waveform measuring method by optical sampling using sum frequency light generation"). , IEICE Transactions, BI, vol. J75-BI, No. 5, pp. 372-380, 1992).

SFG has [Type 1] and [Type 2] phase matching, and the S / N ratio of SFG optical sampling light wave measurement is different from each other. The phase matching means that the phase velocity of the nonlinear polarization wave generated by the incident light coincides with the phase velocity of the sum frequency light generated thereby, and at this time, the energy conversion occurs most efficiently. "Type 1"
8A is a condition for phase matching when two lights having the same deflection direction are incident, as shown in FIG. 8A. As described above, the mutual _relationship between the signal light (ν _sig ) and the sampling light (ν _sam ) is obtained. In addition to the SF light (ν _sig + ν _sam ) which is a correlation signal, the second harmonic (2ν _sig , 2ν _sam ) which is an autocorrelation signal of the signal light and the sampling light almost satisfies the phase matching condition.
For this reason, background light is generated and shot noise increases.
On the other hand, in "Type 2", as shown in FIG. 8 (b), the phases are matched when the deflection directions of the two incident lights are orthogonal, so that the phase matching is achieved only for the generation of the sum frequency light, and the background light can be removed.

"Type 1", "Type 2" phase matching SF
When a nonlinear optical material that performs G is used, the SN ratio (SNR) of each sampling value can be approximated by the following equation. However, it was assumed that the shot noise was dominant in the noise component, and the thermal noise and circuit noise were negligible.

(Equation 1)

(Equation 2)

(Equation 3) Here, P _sfg is the sum frequency light (SF light) power, P _shg (s
ig) is the second harmonic light power of the measured light, P _shg (sam)
Is the second harmonic power of the sampling light, and η is SFG
Conversion efficiency, P _sig is the optical power to be measured, P _sam is the sampling optical power, η _q is the quantum efficiency of the light receiver, λ is the wavelength of SF light, c is the light speed, B is the light receiving system band, and τ is the pulse width. .
In the case of “Type 1” phase matching, the normal use condition P _sig <
For <P _sam , SNR = A · ηP _sig 2 (2), which is determined by the SFG conversion efficiency and the value of the measured light power. On the other hand, in the case of “type 1” phase matching, the SN ratio also depends on the sampling light power.

Therefore, the optical sampling optical waveform measuring apparatus using "type 2" phase matching SFG as a cross-correlation signal generating means can remove the background light and the S / N ratio is proportional to the measured light and the sampling light. By increasing the output of light, a high SN ratio can be measured.

[0016]

To actually observe an optical waveform, an SN ratio of 30 dB or more is required (Takara et al .:
"Ultrafast Optical Waveform Measurement Method by Optical Sampling Using Sum-Frequency Light Generation", IEICE Transactions, BI, vol. J75-
BI, No 5, pp. 372-380, 1992). However, in the conventional nonlinear optical material, the SF of the “type 2” phase matching in the 1.3 μm band and the 1.5 μm band used in optical communication is used.
G was possible and none had sufficiently high SFG efficiency. KTP used mainly until now
Conversion efficiency of the crystal and LiNbO ₃ crystal is 10 ^−5.
It is about W ⁻¹ . In the case where the waveguide structure is made of KTP or LiNbO ₃ , the effective working length and the optical power density are increased, so that the efficiency inside the waveguide is improved by about two orders of magnitude. Since the coupling efficiency is 10% or less, the actual SFG conversion efficiency of the device is not so different from that of the crystal (Yamabayashi et al .: “Li
Optical Sampling Using NbO ₃ Waveguides and Ultrashort Optical Pulses ”, Proceedings of the 1988 IEICE Spring National Convention B-671).

For example, if the measured light peak power P _sig is 10
The S / N ratio will be obtained for the case where the peak power P _sam is 10 mW and the sampling light peak power is 10 mW. APD as receiver
Η _q = 0.7, λ = 800 nm, B = 20M
If Hz and τ = 1 ps, then A = 5 × 10 ⁶ , and the S / N ratio of each sampled value is only -20 dB or less. Therefore, conventionally, the S / N ratio has been improved by averaging a large number of sampling measured values or amplifying the optical power to be measured and the sampling optical power by an optical amplifier (Japanese Patent Application No. 1-175538, “Optical sampling waveform”). Measuring device ”Takara et al.).

However, when averaging is performed, only an optical signal that periodically repeats the same waveform can be measured.
In the case of an optical signal whose intensity is modulated as shown in the waveform (A) of
As a cross-correlation signal between the measured light pulse and the sampling light pulse shown in FIG. 9B, a sum frequency light pulse as shown in FIG. 9C is extracted. In this case,
Since the light modulated to “0” and “1” is averaged,
The display unit displays an averaged light waveform as shown in FIG. Therefore, there is a drawback that an eye pattern representing a mark space waveform required for evaluating a transmission waveform cannot be measured.

FIG. 10 is a block diagram showing a case where the measured light power and the sampling light power are amplified by one-stage optical amplifiers 112 and 113, respectively. This optical amplifier 112, 1
As 13, a rare earth-doped optical fiber amplifier, a semiconductor laser amplifier, or a Raman amplifier can be used. Until now, rare earth-doped optical fiber amplifiers have been mainly used because high gain and high output are possible. The gain and average output of a rare earth-doped optical fiber amplifier depend on the average power of the incident light.
B, the saturation output is about 100 mW. For example, a repetition frequency of 10 MHz (a period of 0.1 μs) and a pulse width of 1 p
When an optical pulse train having a peak power of 10 mW is amplified, the peak power is 1 when only the duty ratio is considered.
00 mW × (0.1 μs / 1 ps) = 10 kW, but the amplification of the peak power is actually 1 due to the limitation by the gain.
0W is the limit. On the other hand, if the measured light is signal light used in optical communication, the peak power is determined by the average output, the duty ratio, and the mark ratio. For example, a repetition frequency of 10
Assuming 0 GHz (period 10 ps), pulse width 5 ps, and mark ratio 0.5, the peak power after amplification is 100 mW ×
(10 ps / 5 ps) /0.5=400 mW. When the SN ratio is calculated from the amplified sampling light power and the measured light power, the S / N ratio is about 23 dB, which does not reach the required S / N ratio of 30 dB. Accordingly, since a sufficient SN ratio cannot be obtained with a single-stage optical amplifier configuration, conventionally, two or more optical amplifiers are cascaded as in the optical amplifiers 113a and 113b indicated by broken lines in FIG. Needed to be improved. However, in this case, there is a disadvantage that the configuration becomes complicated and the cost increases.

In order to solve the above-mentioned problems, the present invention relates to an optical sampling optical waveform measuring apparatus using a nonlinear optical effect.
Adamantylamino-5-nitropyridine (2-adamanty
lamino-5-nitropyridine: AANP) by using, to improve the SNR of each sampling value, an optical sampling light configured to be able to measure the eye pattern
It is to provide a waveform measurement method .

[0021]

SUMMARY OF THE INVENTION Optical sampling of the present invention
The optical waveform measurement method includes a sampling light generating unit that generates a sampling light pulse train (optical frequency νsam) having a narrower pulse width than a light to be measured (optical frequency νsig) having repetition, and a coupling that combines the measured light and the sampling light. Unit, a delay sweeping means for sweeping the delay of the sampling light with respect to the measured light, and a sum frequency light (optical frequency vsfg = vsig + vsam) as a cross-correlation signal between the measured light signal and the sampling light
Organic nonlinear optical crystal 2-A
Damantylamino-5-nitropyridine (2-adamantyla
mino-5-nitropyridine: AANP) , a photodetector that detects the generated sum frequency light and converts it into an electrical signal, and an electrical processing system that processes the electrical signal and displays the measured optical pulse waveform. Optical sampler using sampling optical waveform measurement device
In ring optical waveform measurement method, the wavelength 1535~1580n
m is used as the light to be measured .

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Optical sampling optical waveform measurement of the present invention
The usual method is to use an organic nonlinear optical crystal 2-
Adamantylamino-5-nitropyridine (2-adamanty
Optical sampling with lamino-5-nitropyridine (AANP)
Wavelength 1535 to 1580 n using an optical waveform measuring device
m is used as the light to be measured. Figure 1 is a book
Optical sampling used in the optical sampling optical waveform measuring method of the invention
FIG. 2 is a configuration diagram illustrating an example of a pulling light waveform measurement device. This
The optical sampling optical waveform measuring apparatus of the example of Example 1 is an optical sampling optical waveform measuring apparatus using a type 2 phase matching SFG, and uses an organic nonlinear optical crystal 2-adamantylamino-5-nitropyridine (2-adamantyla) as a nonlinear optical material.
mino-5-nitropyridine: AANP)
The components other than the non-linear optical material are conventional technologies (FIG. 10).
Is the same as In the figure, reference numeral 114 denotes an AANP crystal.

FIG. 2 is a diagram showing the molecular structure and crystal structure of AANP (S. Tomaru et. Al., "Nonlinear optic."
al properties of 2-adamantylamino-5-nitropyridine
crystals ", Appl.Phys.Lett., 58 (23), 10pp.2583-2585,19
91, Kaino, et al .: "Preparation of Organic Nonlinear Optical Crystal-Application to Wavelength Conversion-", NTT R & D, vol.40, No5, 199
1). This AANP has a high second-order nonlinear susceptibility of d ₃₁ = 80 pm / V. Figure 3 is AANP
This figure shows the relationship between the crystal orientation of the crystal and the phase matching wavelength of the second harmonic generation, and the angle phase matching with the 1.3 μm and 1.55 μm band basic light generally used in optical communication. Thus, SFG of type 2 phase matching is possible.
At present, it is possible to produce crystals with a length of 20 mm or more by using the indirect heating laser pedestal method (Yokoo).
Others: "Phase-matched orientation single crystal growth of highly efficient organic nonlinear optical material by modified LHPG method," Applied Physics Society Spring Meeting, 1994, 29p-M-2).

FIG. 4 shows the dependence of the SFG conversion efficiency of the type 2 phase matching of the AANP crystal having a length of 2 mm on the light wavelength to be measured. At this time, the wavelength of the sampling light is 15
It was fixed at 36 nm. As you can see from the figure, AA
The NP crystal is capable of high-efficiency SFG with a conversion efficiency of 10 ⁻⁴ W ⁻¹ or more in a wide wavelength region of 1535 to 1580 nm.
This value is one digit higher than the conventional KTP crystal. This wavelength region can be sufficiently applied to the current 1.55 μm band optical communication system. Therefore, by using this AANP, it is possible to perform high-efficiency type 2 phase-matching SFG in the 1.55 μm band. Even in the configuration in which the sampling light is amplified by only one optical amplifier shown in FIG. Is 33 dB or more, and a sufficiently high SN ratio is obtained for optical waveform measurement.

Further, by optimizing the incident angle using an AANP crystal having a length of 10 mm or more, 2 × 10 ^-3
SFG conversion efficiency of W ⁻¹ or more is possible. This AANP
By using a crystal, the peak power of the light to be measured is 1
If the power is 0 mW or more, an SN ratio of 30 dB or more can be obtained even in a configuration in which the measured light is not amplified (sampling light power 10 W, measured light power 400 mW) as shown in FIG. it can.

The AANP crystal has a wavelength of 1.3 μm.
In the band, a conversion efficiency higher by about one digit than that in the 1.55 μm band is possible, and an optical waveform measurement with a higher SN ratio is possible. That is, by using an AANP crystal as a nonlinear optical material, a high-efficiency type 2 phase-matched SFG in 1.3 μm band and 1.55 μm band used in optical communication is used.
Is possible, and the S / N ratio of the sampling value can be significantly improved compared to the conventional case. As a result, it is possible to reduce the number of optical amplifiers for sampling light required to be plural in the conventional eye pattern measurement to one. Further, the number of optical amplifiers for the light to be measured can be reduced.

[0027]

As described above, the optical sump of the present invention is
The ring light waveform measurement method uses a light to be measured (light
Sampling optical pulse with narrower pulse width than frequency νsig)
Generation of sampling light to generate a light train (optical frequency vsam)
Unit, a coupling unit that combines the measured light and the sampling light,
Delay that sweeps the delay of the sampling light with respect to the measured light
Sweep means and cross-correlation between optical signal under measurement and sampling light
Sum frequency light (optical frequency vsfg = vsig + vsa) as a signal
m) An organic nonlinear optical crystal 2 of a nonlinear optical material that generates
-Adamantylamino-5-nitropyridine (2-adaman
tylamino-5-nitropyridine: AANP) and the generated sum frequency
A photodetector that detects several light beams and converts them into an electrical signal;
Processing system that processes the signal and displays the measured optical pulse waveform
Optical sampling using an optical sampling optical waveform measuring device equipped with
In the pulling light waveform measurement method, the wavelength is 1535 to 158.
By using 0 nm light as the light to be measured , a highly efficient type 2 phase matching S
Since FG becomes possible and the S / N ratio of the sampling value can be greatly improved compared to the conventional one, an eye representing a mark space waveform necessary for evaluating a transmission waveform only by amplifying the sampling light in one stage of the optical amplifier is provided. Pattern measurement becomes possible. Therefore, there are great effects such as simplification of the configuration and significant cost reduction.

[Brief description of the drawings]

FIG. 1 is used in the optical sampling optical waveform measurement method of the present invention.
1 is a configuration diagram of an optical sampling optical waveform measurement device to be used.

FIG. 2 is a diagram showing a molecular structure and a crystal structure of a nonlinear optical crystal AANP.

FIG. 3 is a diagram showing a relationship between a crystal orientation of a nonlinear optical crystal AANP and a phase matching wavelength.

FIG. 4 is a diagram showing the relationship between the sum frequency light generation efficiency and the wavelength of the light to be measured.

FIG. 5 is a diagram showing a relationship between an optical frequency of sum frequency light generation and light intensity.

FIG. 6 is a configuration diagram of a conventional optical sampling optical waveform measuring device.

FIG. 7 is a time chart of an optical waveform to be measured, a sampling optical waveform, and a cross-correlation signal optical waveform.

FIG. 8 is a diagram illustrating SFG of type 1 and type 2 phase matching.

FIG. 9 is a time chart of a measured optical waveform, a sampling optical waveform, a cross-correlation signal optical waveform, and an averaged optical waveform.

FIG. 10 is a configuration diagram of a conventional optical sampling optical waveform measurement device including an optical amplifier.

[Explanation of symbols]

101: light source to be measured, 102: sampling frequency oscillator, 103: sampling light generator, 104, 10
5: polarization controller, 106: coupler, 107: nonlinear optical material, 108: optical filter, 109: photodetector, 110: signal processing unit, 111: Display, 112 ...
.Optical amplifiers for the light to be measured, 113, 113a, 113b
..Optical amplifier for sampling light, 114 ... Organic nonlinear optical crystal AANP

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Atsushi Yokoo 3-19-2 Nishishinjuku, Shinjuku-ku, Tokyo Nippon Telegraph and Telephone Corporation (72) Inventor Masatoshi Saruwatari 3-19, Nishishinjuku, Shinjuku-ku, Tokyo No. 2 Nippon Telegraph and Telephone Corporation (56) References JP-A-3-41329 (JP, A) JP-A-5-100257 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB Name) G01J 11/00 G02F 1/361 H04B 3/46 H04B 10/08

Claims (1)

(57) [Claims] 1. A light under measurement (optical frequency ν) having repetition
sig) A sampling light generation unit that generates a sampling light pulse train (optical frequency νsam) with a narrower pulse width, a coupling unit that combines the measured light and the sampling light, and sweeps the delay of the sampling light with respect to the measured light Delay sweeping means, and an organic nonlinear optical crystal 2 -adaman of a nonlinear optical material that generates sum frequency light (optical frequency vsfg = vsig + vsam) as a cross-correlation signal between the optical signal to be measured and the sampling light.
Tylamino-5-nitropyridine (2-adamantylamino-5
-nitropyridine: AANP) , an optical detector that detects the generated sum frequency light and converts it into an electrical signal, and an electrical sampling optical waveform that has an electrical processing system that processes the electrical signal and displays the optical pulse waveform to be measured. Optical sampling light using a measuring device
In the waveform measurement method , light having a wavelength of 1535 to 1580 nm is used as the light to be measured.
An optical sampling optical waveform measurement method characterized by using: