JP3378502B2 - Optical signal 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 a technique for observing an optical signal waveform using a short pulse laser light source which generates an optical pulse having an ultra-short time width in the picosecond to femtosecond region. In particular, the present invention relates to a technique for performing observation while suppressing a relative change in the repetition frequency of each of an optical signal train to be measured and a short pulse laser light source that prevents accurate and highly sensitive measurement. The present invention provides a method that can be performed at low cost and that can be performed regardless of the pulse width or repetition frequency of an optical signal.

[0002]

2. Description of the Related Art In recent years, the application of ultrafast optical signals in the picosecond to femtosecond range has become popular in many industrial and academic fields including optical communication and optical information processing. The need to observe the intensity waveform in detail has increased. When measuring the intensity waveform, the first thing that comes to mind as an extension of the method for slower signals is to convert the measured optical signal to a similar electrical signal using a high-speed photodetector, Would be a method of observing with the existing high-speed electrical signal measuring means.

However, the photodetector used in this case is required to have a short response time as compared with the optical signal to be measured. In the picosecond to femtosecond ultra-short time range, no photodetector that satisfies this condition actually exists. Therefore, in the ultra-short time region, there is no choice but to rely on the intensity waveform measurement by the cross-correlator described below.

In the cross-correlator, a reference light pulse that is synchronized with the measured optical signal and has a shorter time width than the measured optical signal is used, and the reference pulse and the measured pulse are measured in a medium exhibiting an optical nonlinear effect. The magnitude of the nonlinear signal generated by interacting with the optical signal is measured while changing the relative delay time between the optical signal and the reference pulse. This is because the optical signal is gated by a short reference pulse and the gated signal output is measured while changing the time position of the gate on the optical signal.
It can be seen that the optical signal waveform is being measured, which is just like a sampling measurement on an electrical signal.

Therefore, the measurement using this cross-correlation meter is called optical sampling, and the reference light pulse with a short time width used accompanying it is sometimes called sampling light pulse. There are various orders and types of optical nonlinear effects used, and consequently the manner in which nonlinear signals are taken.

For example, as the second-order optical nonlinear effect, the sum of
Frequency generation effect or two-photon absorption effect is often used.
It In the former case, the sum of the two incident lights and their respective frequencies
The frequency to hit ₁, Λ₂For the light of₁
λ₂/ (Λ₁+ Λ₂) Wavelength light (sum frequency light) is generated
It This is done by a photodetector sensitive to the sum frequency light wavelength.
Received and converted by the
Get the issue.

In the latter case, the transmittance of the sampling light pulse decreases with the two-photon transition that occurs when one photon is obtained from each of the two incident lights. Therefore, this is converted by a photodetector sensitive to the wavelength of the sampling light pulse to obtain an electric signal proportional to the change in transmittance. Alternatively, in the case of a semiconductor material, it is possible to directly obtain an electric signal proportional to the two-photon transition probability by attaching an electrode and collecting carriers generated by the two-photon transition.

The third-order optical nonlinear effect is saturated absorption,
The optical Kerr effect or the coupling tone generation effect is used, and photoelectric conversion is performed by an appropriate photodetector to obtain an electric signal proportional to the magnitude of each effect. Generally, the time resolution related to the optical sampling measurement is determined by the response time of the optical nonlinear medium or element used and the time width of the sampling light pulse, and is completely dependent on the response time of the conversion means to an electric signal such as a photodetector. do not do.

This property is the reason why the optical sampling measurement using the cross-correlator is performed in the ultra short time range from picosecond to femtosecond. A conventional optical signal waveform measuring method using a cross correlator is shown in FIG.
It was shown to. First, a general aspect of the optical signal waveform measuring method by optical sampling will be described with reference to FIG.

Repetition frequency F shown at the top of FIG.
_{For the} optical signal train to be measured which is repeated with _sig , the sampling pulse train with a shorter time width in the middle part of the figure is used, and the two are made to interact in the medium exhibiting the optical nonlinear effect. For example, in the cross-correlator, when a second-order optical nonlinear effect is used for the optical nonlinear effect, the following expression (1) is obtained as the expression of the output cross-correlation signal G _c .

[0011]

[Equation 1]

Here, I ₁ (t) represents the intensity waveform of the optical signal train to be measured, and I ₀ (t) represents the intensity waveform of the sampling pulse train. The integrand I ₁ (t) I ₀ (t−τ _e ) is the intensity waveform of the second harmonic wave generated in the nonlinear crystal, and the integral is a photodetector having a response time sufficiently slower than that of the optical pulse. Represents the effect of photoelectric conversion by. Here, this correlation signal depends on the apparent time difference τ _e between the pulse trains.

Now, when the time width of the sampling pulse is sufficiently shorter than that of the optical signal to be measured and the sampling pulse can be regarded as a delta function, the expression of the equation (1) can be rewritten as the following equation. , The intensity waveform I ₁ (t) of the measured optical signal can be directly obtained from the cross-correlation signal G _c .

[0014]

[Equation 2]

Even if the finite time width of the sampling pulse cannot be ignored, its influence can be eliminated as follows. Considering the Fourier transform G _c ′ for the delay time τ _e of the cross-correlation signal G _c , it is calculated as in the following equation.

G _c '(ω) = I' ₁ (ω) I ' ^* ₀ (ω)

[0017] _{Here, I '0 (ω),} I' 1 (ω) , respectively, the intensity of the sampling optical pulse waveform I ₀ (t), the Fourier transform of the intensity of the optical signal to be measured waveforms I ₁ (t) is there. If here, the waveform I of the sampling light pulse
_{If 0} (t) is known, I ′ ₀ (ω) is also known, and I ′ ₁ (ω) can be obtained from the equation (2).

By inverse Fourier transforming the I ′ ₁ (ω) thus obtained, the intensity waveform of the measured optical signal, I
₁ (t) can be obtained. Such an operation is called a decorrelation calculation. Now, write the repetition frequency F _samp of the sampling pulse train as shown in the following equation. F _samp = F _sig / N−ΔF (3) Here, the natural number N is called a multiplication number and ΔF is called a frequency offset. At this time, the apparent time difference τ _e between the sampling light pulse and the optical signal train to be measured shifts by Δτ _e = F _samp ⁻² ΔF for each sampling light pulse.

Since the period of the sampling light pulse is equal to F _samp ^-1 , the number of shots required for the time difference τ _e to change by exactly one period is F _samp / ΔF, and the repetition of the sampling light pulse itself Considering the frequency F _samp , the change frequency modulo the period of the sampling optical pulse with the apparent time difference τ _e is equal to the frequency offset ΔF. Further, since the measured optical signal is repeated N times during one cycle of the sampling optical pulse, the output waveform of the cross-correlator is eventually observed at the repetition frequency F _obs shown in the following equation.

F _obs = NΔF (4) Thus, if the frequency offset ΔF is given according to the equation (3) between the repetition of the optical signal to be measured and the repetition of the sampling pulse train, the output of the cross correlator will be It is possible to obtain a measured optical signal waveform in which the time axis is _expanded by F _samp / ΔF times. It suffices that the photoelectric conversion in the cross-correlator, or the electronic circuit for observing and recording its output, be able to respond to this virtually stretched signal, which is exactly what the picosecond to femtosecond range is by optical sampling measurement. This shows the principle by which an optical signal in the ultrashort time region can be observed.

In this case, since the optical signal train to be measured and the sampling pulse train have different repetition frequencies, they can be supplied by independent individual light sources.
It is possible in principle and is rather natural. This is because it is quite difficult to divide the pulse train output from one short pulse light source into two and statically change the repetition frequency of one pulse train to give the frequency offset ΔF. FIG. 6B is a diagram showing a first conventional example of optical signal waveform measurement by optical sampling, in which two solid-state short pulse lasers of the same kind are used and a frequency offset ΔF is provided between the repetitions. There is. For example, Optics Letters
Published in the magazine, Volume 17 (1992), pp. 1286-1288.

In this example, as the signal source laser 630 for generating an optical signal train and the short pulse laser resonator 602 for generating a sampling pulse train, two similar solid-state laser devices for generating a pulse having a width of 80 fs are used. .
The short pulse laser resonator 602 includes a resonator length rough adjusting mechanism 604, and is configured so that the resonator length can be adjusted. The sampling pulse train emitted from the short pulse laser resonator 602 enters the cross correlator 607 via the reflecting mirror 605 and the reflecting mirror 606, while the signal source laser 6
The optical signal train emitted from 30 is reflected by the mirror 631 and the mirror 6.
The light enters the cross-correlator 607 via 16.

The cross-correlator 607 applies the cross-correlation signal G represented by the equation (1) to the above-mentioned two pulse trains incident thereon.
Output _c . This cross-correlation signal G _c is stored in the waveform recorder 612.
Observed and recorded by. At present, among the methods of generating ultrashort light pulses, the reason why the light pulse with the shortest time width can be stably generated with the highest repetition frequency is based on the optical nonlinear effect inside the laser light source itself. This is a case where a pulse generation operation is performed. This kind of pulse generating operation is generically called passive mode locking or self mode locking.

As a method opposed to this, there is a pulse generating operation which is performed by positively applying a modulation signal to the laser light source from the outside, which is called active mode locking or forced mode locking. In any pulse generation operation, the repetition frequency of the optical pulse is the reciprocal of the circulation time of the laser resonator or an integral multiple thereof. In the latter active mode-locking operation, a modulation signal having a frequency that is an integer fraction or an integer multiple of the repetition frequency determined by the cavity length is externally applied to generate and grow an optical pulse in the cavity. .

Recall that the laser cavity length is unavoidably changed due to drift due to temperature change or fluctuation due to vibration. When such a change occurs, in the latter active mode-locking method, a discrepancy occurs between the repetition frequency determined by the resonator length and the frequency of the modulation signal, and as a result, the pulse operation becomes unstable or occurs. The time width of the pulse becomes wider. However, the repetition frequency of the optical pulse itself is kept unchanged because the frequency of the modulated signal is decided with priority.

On the other hand, in the former passive mode-locking method, the pulse forming operation is not affected even if the laser resonator length changes, and therefore the time width of the optical pulse is kept unchanged. However, the repetition of the optical pulse changes following the change in the cavity length. From the opposite point of view, in the passive mode-locked laser, by changing the cavity length, the pulse repetition frequency can be controlled without damaging the generated pulse.

In this example, passive mode-locked lasers are used for both the signal source laser 630 and the short pulse laser resonator 602. Further, the repetition frequency F _samp of the sampling pulse train is F _samp = F _sig −ΔF with respect to the repetition frequency F _sig of the optical signal train from the signal source laser 630.
Then, the short pulse laser resonator 602 is set so that the relationship in which the multiplication number N in Expression (3) corresponds to 1. For this purpose, the length of the short pulse laser resonator 602 is set to be slightly longer than that of the signal source laser, in accordance with the above-mentioned property that the pulse repetition frequency of the passive mode-locked laser can be adjusted by the resonator length. , Resonator length coarse adjustment mechanism 60
Adjust 4.

In this example, for 80 MHz F _sig ,
Since the frequency offset ΔF of 80 Hz is added,
The required cavity length offset is 1 ppm of the cavity length,
That is, it becomes 1.87 μm. The resonator length difference on such a minute scale easily changes due to drift due to temperature change or fluctuation due to vibration. Therefore, the frequency offset ΔF cannot be kept constant over a long period of time in the configuration in which the resonator length is allowed to change without any control as in this example.

When the frequency offset ΔF changes, the mutual
Repetition frequency F of correlator output waveform _obs, Or
It is the same value as the above, but the apparent time axis expansion rate F_samp/ ΔF
Changes and is observed and recorded by the waveform recorder 612.
The waveform undergoes expansion and contraction along the time axis. As a result, observation
The time accuracy on the waveform is reduced. Or if it is extremely
Is F at the left and right ends of the correlator output waveform._samp/ ΔF is different
, Which results in a distorted observed waveform. Times these
The simplest way to avoid is the frequency offset ΔF
The end of the measurement before the change in
It

In the case of this example, the measurement section is limited to the range of 1 ps around the optical signal pulse, and 1 ps × (80 MHz / 80
The measurement is completed within (Hz) = 1 μs. In such a case, the measurement is completed long before the change due to the temperature change or the vibration appears, and the distortion of the observed waveform due to the change of ΔF can be avoided. As with all measurements in general,
The measurement sensitivity can be higher as the measurement time is longer. this is,
This is because the fact that the measurement time is long and the average is taken for a long time is equivalent to operating a narrow band low-pass filter by that amount, and noise is reduced as a result of this band limitation.

From this point of view, in the case of this example in which only 1 μs is dedicated to the measurement, highly sensitive measurement is completely undesired. This is impractical, so control frequency offset ΔF
A method of stabilization is essential. FIG. 7 (a) is a diagram showing a second conventional example of optical signal waveform measurement by optical sampling, in which two active mode-locked lasers are used and an externally modulated signal is applied between them by an electric means. A frequency offset is provided. For example, Electoronics Lette
Published in rs magazine, Volume 32 (1996), pages 2256-2258.

In this example, two similar active mode-locked laser devices are used as the signal source laser 730 for generating the optical signal train and the short pulse laser resonator 702 for generating the sampling pulse train. As described above, in the active mode-locking method, it is more difficult to obtain a short pulse than in the passive mode-locking method, and as a result, the pulse width generated by these lasers in this example is about 4.4 ps. The sampling pulse train emitted from the short-pulse laser resonator 702 is thinned by the optical gate 734 at a rate of 1 for every N pulses, and the pulse width is compressed by the pulse compressor 735. Then, the light enters the cross-correlator 707 through the reflecting mirror 705 and the reflecting mirror 706.

On the other hand, the optical signal train emitted from the signal source laser 730 is modulated or multiplexed by the signal light converter 715, and then enters the cross-correlator 707 via the reflecting mirror 731 and the reflecting mirror 716. The cross-correlator 707 outputs the cross-correlation signal G _c represented by the equation (1) with respect to the two pulse trains that have been incident. The cross-correlation signal G _c is observed and recorded by the waveform recorder 712. The output of a reference oscillator 732 that generates a sine wave having a frequency F ₀ is applied to the signal source laser 730 as an external modulation signal. As a result, the repetition frequency of the optical signal train generated by the signal source laser 730 is always equal to F ₀ .

The output of the reference oscillator 732 is simultaneously supplied to the synchronizing signal generator 733. Synchronous signal generator 733 for sinusoidal input frequency F _0, the sine wave of frequency F ₀ -NΔF a transition only NΔF frequency, and outputs a divide-by-N signal after it binarization. The former of these is applied to the short pulse laser resonator 702 as an external modulation signal.

Therefore, a pulse train having a repetition frequency F ₀ -NΔF is generated from the short pulse laser resonator, and this pulse train is thinned out at a ratio of 1 to N by the optical gate 734 driven by the latter frequency-divided signal. As a result, the repetition frequency F
A sampling pulse train of _samp = F ₀ / N−ΔF is obtained. This corresponds to the relationship in which the optical signal repetition frequency F _sig is replaced by F ₀ in the equation (3). In this example,
The repetition frequency F _sig of the optical signal train incident on the cross-correlator 707 is not necessarily equal to the repetition frequency of the signal source laser 730 because the repetition can be multiplied by the signal light converter after the signal source laser 730. Therefore, the symbols have been intentionally replaced.

F ₀ here is a quantity which should also be called the frame repetition frequency of the optical signal train. Thus, in this example,
As a result of the frequency offset ΔF being constantly fixed, a highly sensitive optical signal waveform measurement is realized by optical sampling. In the above example, the short pulse laser resonator 702 is based on the active mode locking method, but this is not always necessary. Using the passive mode-locking method has the advantage that the pulse compressor 735 in the above example is not required because the width of the pulse originally generated is shorter.

The repetition frequency of the passive mode-locked laser is
A method called a phase locked loop is already known as a technique for synchronizing with a reference oscillator. For example, IEEE Journ
al of Quantum Electronics, Volume 28 (1992), 289
-296 pages, or Optics Letters, 19 volumes (1994)
-Published on pages 481-483. If this phase-locked loop is applied to optical signal waveform measurement by optical sampling,
For example, the configuration shown in FIG. 7B as a third conventional example of optical signal waveform measurement by optical sampling can be conceived.

In this example, a passive mode-locked laser device is used as the short pulse laser resonator 702 for generating a sampling pulse train, and the short pulse laser resonator 702 has a resonator for adjusting its repetition frequency. A coarse adjustment mechanism 704 and a resonator length fine adjustment mechanism 703 are provided. The sampling pulse train emitted from the short pulse laser resonator 702 enters the cross-correlation meter 742 via the reflecting mirror 705 and the reflecting mirror 706. A part of the light is reflected by the branch mirror 713 and received by the high-speed photodetector 736. On the other hand, the optical signal train incident on the signal light incident end 701 is incident on the cross-correlation meter 742, and a part thereof is reflected by the branch mirror 714 and received by the high-speed photodetector 738.

The cross-correlation meter 742 receives the cross-correlation signal G expressed by the equation (1) for the above-mentioned two pulse trains incident thereon.
Output _c . However, unlike the above-mentioned cross-correlator, this cross-correlation meter has a variable optical delay line for changing the relative delay of the two pulse trains incident therein. The controller / recorder 743 records and records the cross-correlation signal G _c as a function of the relative delay while controlling and driving the variable optical delay line. When the repetition frequency of the sampling pulse train generated by the short pulse laser resonator is F _{sa mp} , the electric output of the high-speed photodetector 736 is F _samp.
It is a superposition of sinusoidal beat signals having a frequency that is an integral multiple of. Of these, the repetition frequency F _sig of the optical signal sequence
The harmonic component NF _samp closest to is the effective component for stabilization.

The electric output is amplified by the high frequency amplifier 737 and then applied to the high frequency mixer 740. The electrical output of the high-speed detector 738 that receives the optical signal train is
After being amplified by the high frequency amplifier 739, it is also applied to the high frequency mixer 740. The high frequency mixer 74
0 outputs the frequency of the sum and difference of two applied electric inputs. Among them, the signal of the frequency component NF _samp −F _sig , which is the difference component, is extracted by the low-pass filter 741 and supplied to the drive circuit 711 of the resonator length fine movement mechanism 703 via the integration circuit 710.

As described above, a method of feeding back the difference frequency NF _samp -F _sig as an error signal to the oscillator having the free-running frequency F _samp and stabilizing the oscillator frequency to F _sig / N is as follows.
Generally, it is known as a phase locked loop (PLL). The resonator coarse adjustment mechanism 704 is provided to adjust the free-running frequency of the short pulse laser resonator so as to fall within the pull-in frequency range of this phase-locked loop. Thus, in this example, the repetition frequency is always F _samp = F _sig
A sampling pulse train stabilized at / N is obtained.
This corresponds to the relationship in which the frequency offset ΔF is set to zero in the equation (3).

As described above, in this example, the frequency offset ΔF is always fixed to zero, and the relative delay between the optical signal train and the sampling pulse train is fixed. As a result, instead of the cross-correlator 707 of the previous example, a variable optical system is used. It is necessary to use a cross-correlator 742 that contains a delay line. In the configuration of this example, between the high-speed photodetector 736 and the high-frequency amplifier 737,
If a frequency shifter that shifts the frequency of the sine wave electric signal by + NΔF is inserted, or if a frequency shifter of −NΔF is also inserted between the high-speed detector 738 and the high-frequency amplifier 739, it follows the formula (3). A sampling pulse train is obtained. In this case, as in the previous examples, the cross correlator 7
The waveform can be observed using 07. Thus, also in this example, the frequency offset ΔF is always fixed to be constant,
Optical sampling enables highly sensitive optical signal waveform measurement.

[0043]

However, the above-mentioned conventional optical signal waveform measuring method has the following problems. As described above, since the first conventional example has extremely low sensitivity and is impractical, it is indispensable to control / stabilize the frequency offset ΔF. In the second conventional example of the second and third conventional examples that do this, the short pulse laser resonator 70 by the active mode locking method is used as the sampling pulse light source.
As a result of using 2, the measurable optical signal repetition is limited to a range in which the pulse generation operation in the short pulse laser resonator 702 is not impaired. That is, the repeatability of the measurable optical signal train is effectively fixed.

Furthermore, since the pulse width that can be generated by the active mode locking method is too wide, it is difficult to realize the time resolution required for the current optical signal waveform measurement in the picosecond to femtosecond region, resulting in pulse compression. It became necessary to add a container 735. Such a pulse compressor contains a very powerful optical amplifier and is therefore very expensive. Thus, the second conventional example is inevitably inflexible in repetition of the optical signal train to be measured and has a high price, which is inevitable in the marketability. In the third conventional example, such a problem can be solved because a sampling light source based on the passive mode-locking method is used.

That is, the pulse generation operation in the short pulse laser resonator 702 is not affected by the pulse repetition frequency, that is, the resonator length, so that the pulse width is kept short,
The pulse repetition frequency can be changed. Therefore, the flexibility of repeating the optical signal train to be measured is greater than that of the second conventional example.
Remarkably wide. In addition, the passive mode-locking method can generate light pulses of sufficiently short duration so that no pulse compressor is required. As a result, the cost can be reduced as compared with the second conventional example. Such an optical sampling method inherently has a potential of observing a very high repetitive optical signal train exceeding THz.

It is very easy at present that the time resolution of waveform observation is set to sub-picosecond or less by using the sampling pulse in the femtosecond region. In the situation where even the waveform inside the repetition period can be observed,
There is no doubt that the repetition period itself cannot be observed. The sub-picosecond period thus corresponds to a repetition frequency above THz. In order to observe a highly repetitive optical signal train, the repetition frequency itself of the mode-locked laser as a sampling light source need not be high at all. Even at a constant sampling pulse train repetition frequency F _samp , if the multiplication factor N in the equation (3) can be large, the optical signal repetition frequency F _sig can be arbitrarily high.
Because it can correspond to.

However, in the third conventional method, high-speed light detection is performed.
736 and 738, high frequency amplifiers 737 and 739,
Furthermore, it can be set by limiting the band of the high frequency mixer 740.
There is a limit to the multiplication number N. That is, the week they can respond
F above the wave number_sigCannot be applied to, for example, repetition frequency
Optical signal by mode-locked semiconductor laser with number of 100 GHz
Sequences, etc. cannot be observed by conventional methods.
It For lower repeatable optical signal trains that can be observed
Also, in general, the repetition frequency F _sigThe higher the
The electronic circuits used, i.e. the high speed photodetectors 736 and 738,
High frequency amplifiers 737 and 739, and further high frequency mixer 7
40 becomes expensive.

Moreover, in order to maintain the flexibility of the optical signal repetition frequency, an electronic circuit which operates only in a narrow band and a wide band is required. In particular, the prices of the high frequency amplifiers 737 and 739 having a wide band in the microwave band easily occupy a majority of the conventional configurations, resulting in a loss of economical efficiency of the method. Up to this point, regarding the third conventional example,
Although the problem caused by the electronic circuit has been described, the second conventional example is not immune to this problem. This is because, in the configuration of the second conventional example, the synchronization signal generator 733 includes a high frequency amplifier and a high frequency mixer in the shadow.

As described above, in the conventional optical signal waveform measuring method, (1) the measurement of a high repetition optical signal train is limited by the band limitation of the electronic circuit, and (2) the economical efficiency is obtained by the price of the high frequency electronic circuit. There was a problem that was damaged. An object of the present invention is to solve these difficulties and to provide an optical signal waveform measuring method which is effective for an optical signal train having a high repetition frequency and can be inexpensive.

[0050]

According to the optical signal waveform measuring method of the present invention, an optical signal train to be measured and a sampling pulse train generated from a short pulse laser resonator are incident on a cross correlator, and the cross correlator is used. , Both the optical signal train to be measured and the sampling pulse train are combined / focused in a medium having an optical nonlinear effect, the generated nonlinear signal is converted into an electrical signal, and the magnitude is measured in time series. Record. On this occasion,
A resonator length adjusting mechanism for adjusting the optical length of the resonator is added to the short pulse laser, the repetition frequency of the electric signal is converted into a voltage value, and the resonator length is adjusted so that the voltage value becomes a constant value. It is characterized by controlling and driving the adjusting mechanism.

In the above description, one cross-correlator is used for both waveform measurement and resonator length control, but a separate cross-correlator can be provided for each. That is, another configuration of the optical signal waveform measuring method of the present invention is such that the measured optical signal train and the sampling pulse train generated from the short pulse laser resonator are incident on the first cross-correlator, In the correlator, both the optical signal train to be measured and the sampling pulse train are combined and focused in a medium having an optical nonlinear effect, the generated nonlinear signal is converted into an electrical signal, and its magnitude is time-series. Measure and record.

At this time, the short pulse laser is provided with a resonator length adjusting mechanism for adjusting the optical length of the resonator, and both the measured optical signal train and the sampling pulse train are subjected to the first cross-correlation meter. Before being incident on the second cross-correlator, a duplicate of each of the measured optical signal train and the sampling pulse train, which is obtained, is incident on a second cross-correlator, and the two replicas are separated by a light beam. Combined / focused in a medium having a non-linear effect, the generated non-linear signal is converted into an electric signal, the repetition frequency of the electric signal is converted into a voltage value, and the voltage value becomes a constant value. The resonator length adjusting mechanism is controlled and driven. At the same time, the origin of time-series measurement / recording of the magnitude of the electric signal originating from the first cross-correlator is determined based on the electric signal originating from the second cross-correlator.

[Operation] In the optical signal waveform measuring method of the conventional example, in order to control / stabilize the frequency offset ΔF in the equation (3), two sinusoidal electric signals of frequency F _sig and frequency F _samp are applied. , A high-frequency mixer was used to establish a relative frequency relationship using an electronic nonlinear element. For this reason, first, an association between the sinusoidal electrical signal and the optical pulse train is required.

Therefore, the second conventional example requires an active mode-locked light source for generating an optical pulse train synchronized with these sinusoidal electric signals, while the third conventional example requires an optical signal train to be measured. A fast photodetector was required to convert each of the sampling pulse trains into these sinusoidal electrical signals. On the other hand, in order to obtain the signal level necessary for the operation of this high frequency mixer, a high frequency amplifier has been indispensable. Based on this consideration, it can be seen that the problem of the conventional example is exactly rooted in the use of the high frequency mixer.

On the other hand, the cross-correlator conventionally used for optical sampling is essentially a non-linear element of light. In addition, the signal level necessary for the operation of this non-linear element is naturally obtained due to the fact that optical sampling measurements are possible. Thus, the cross-correlator provided in the configuration of the optical signal waveform measuring method from the beginning is replaced with the high-frequency mixer of the conventional example, and the repetition frequencies F _sig and F _{samp are changed.}
The basic idea of the present invention is to perform the automatic control of the frequency offset ΔF by using this comparison.

For this reason, attention is paid to the fact that the repetition frequency F _obs of the output waveform of the cross-correlator is given by the equation (4). That is, F _obs is the product of the multiplication number N and the frequency offset ΔF, of which the multiplication number can be regarded as constant. This is because, before the multiplication number itself changes, it is necessary to undergo a very large change in the frequency offset, and during the feedback control operation in which the frequency offset ΔF is stabilized, the change in the multiplication number cannot occur. Because. The above corresponds to the detection element of the automatic control system of the present invention.

The output waveform of the cross-correlator is as follows:
The time resolution of the cross-correlation signal G _c is determined by the time width of the sampling pulse as described above, and is high enough to observe the repeating waveform itself of the optical signal train to be measured. Here, since it is called a repetitive waveform, the time width in which the waveform itself changes is naturally smaller than the pulse repetition period. Therefore, it is quite self-evident that the present method for sufficiently detecting even the change of the waveform itself is not limited by the repetition period.

As described above, the detection element of the automatic control system according to the present invention can advantageously use the narrow pulse width of the sampling pulse train, and can cope with a high repeating signal train. On the other hand, in the present invention, the short pulse laser that generates the sampling pulse train is provided with a mechanism for finely adjusting the optical length of the resonator. As described above, if the resonator length is changed, the repetition frequency F _samp of the sampling pulse train changes accordingly.

Further, according to the equation (3), the frequency F _samp
The frequency offset ΔF is fed back in accordance with the change of. This corresponds to the operating element of the automatic control system of the present invention. It is desirable that the pulse width of the sampling pulse generated is not impaired by such an operation of changing the cavity length. As a result, in the present invention, the short pulse laser that generates the sampling pulse train is based on the passive mode locking method described above. It is better to use. Among the passive mode-locked lasers, the current solid-state short pulse laser has a cavity length of 0.75 to 2 m, so if a stacked piezoelectric actuator with a stroke of 15 μm is used, the repetition frequency F _{samp is} 7.5 to 20 ppm. Can be changed.

This corresponds to a change width of the frequency offset ΔF of 0.56 to 4 kHz. In the case of such a laser, the main change factor of the frequency offset ΔF is
It is a resonator length change of about 1 μm due to vibration, and the stroke of 15 μm of the stacked piezoelectric actuator above is sufficient to offset this change. On the other hand, in the short pulse fiber laser, the resonator length reaches about 10 m, and the main cause of change in the frequency offset ΔF is temperature drift of the resonator length of the order of 5 ppm / ° C.

In the above process of the piezoelectric actuator, only a resonator length change of 1.5 ppm is given, and even a temperature drift of 1 ° C. is insufficient as it is. However, there is known a method of increasing the control range by winding a fiber forming a resonator around a bobbin made of a piezoelectric material, and a sufficient control range can be realized by using this method. Thus, the operating element of the automatic control system of the present invention can be realized very easily and inexpensively, and
It is a versatile technique that can be implemented for various known types of passively mode-locked short-pulse lasers.

In general, in the short pulse laser by the passive mode-locking method, the pulse width of the sampling pulse generated is not impaired even if the cavity length is changed greatly exceeding the width by the above fine adjustment mechanism. Therefore, a mechanism for roughly adjusting the optical length of the resonator can be provided separately from the fine adjustment mechanism. With such a coarse adjustment mechanism, it is possible to widen the range of the optical signal train repetition frequency F _sig that can be _handled . Now, let the length of the short pulse laser resonator be L and the process of the coarse adjustment mechanism be ΔL.
Let's write At this time, the repetition frequency of the sampling pulse train has a variation width of rF _samp around F _samp .

Here, r = ΔL / L is a relative rough adjustment process. As a result, when the multiplication number is N, the optical signal train repetition frequency F _sig has a change width of rNF _samp around NF _samp . This compatible F _sig is discrete when the multiplication number N is small. However, as the multiplication number N increases, the change width around the multiplication number overlaps with the change width for the adjacent multiplication number. The multiplication number N _{c at} which this occurs for the first time is calculated by the following equation.

[0064]

[Equation 3]

Therefore, for F _{sig equal to} or greater than F _c = N _c F _samp , at least once in the course of the coarse adjustment mechanism, equation (3)
You will see the appearance of F _samp that satisfies the relationship That is, in other words, an optical signal train having a repetition frequency equal to or higher than the critical frequency F _c can be measured without fail. This critical frequency F
_c is a simple calculation, it can be shown that represented by as shown in the following equation, only stroke ΔL and the light velocity c of the coarse mechanism. F _c = c / (2ΔL) (6)

For example, if a parallel displacement table having a stroke of 25 mm is used for the coarse adjustment mechanism, 6 GHz can be obtained as a critical frequency, and it is possible to cope with all repetitions currently used or assumed in optical communication or optical information processing. in this way,
INDUSTRIAL APPLICABILITY The optical signal waveform measuring method of the present invention can provide a general-purpose method having extremely high flexibility in repeating an optical signal train. In addition, for the same purpose of adjusting the resonator length, two types of the fine adjustment mechanism and the coarse adjustment mechanism are provided.
Seemingly wasteful duplication, there's a reason for this.

That is, the adjusting mechanism having a large movable range is expensive, whether it is mechanical or electro-optical, and generally, it is inevitable that the response is deteriorated. That is, the stroke and response of the adjusting mechanism are difficult to achieve at the same time. In stabilizing the frequency offset ΔF by the feedback control operation, it is sufficient to cancel the change in the resonator length due to vibration or temperature drift, and as described above, a large stroke is not required.
As for responsiveness, 10 kHz order is required for vibration countermeasures and 10 Hz order for temperature drift countermeasures. On the other hand, for the purpose of widening the adaptability to the repetition of the optical signal train, it is desirable that the process is long.

This is because, if the stroke is short, the critical frequency of the equation (6) becomes too high, and only the discrete repetition frequency can be dealt with in practice. On the other hand, static adjustments are sufficient for this purpose and no responsiveness is required. It is extremely unrealistic to find an adjusting mechanism that satisfies the above two different requirements at the same time, and therefore divides them between the fine adjusting mechanism and the rough adjusting mechanism. Based on the repetition frequency F _obs of the cross-correlation signal G _{c which is} the detection element, in order to obtain an error signal for controlling the resonator length which is the operation element, the repetition frequency is converted into a voltage value, and this voltage value and a constant reference voltage. The difference from the value is used as the error signal. When this error signal is positive, the frequency offset ΔF is too large, that is, the repetition frequency F _samp of the sampling pulse train is too low, and therefore F _samp is increased by applying feedback in the direction of shortening the resonator length, Negative feedback is applied to the frequency offset ΔF.

It is known that the function of converting the repetition frequency into a voltage value can be realized by an electronic circuit called a frequency-voltage converter. Such an electronic circuit is
It is also called a -V converter (f-V converter), and its operation principle is roughly divided into two types: an analog system and a digital system. In the analog system, an input waveform is stepped by a comparator, a monostable multi is triggered to generate a constant area pulse, and further it is smoothed by a low pass filter to be an output. This method has a small number of parts and can be performed at low cost, but it is difficult to expect long-term accuracy of the absolute frequency.

In the present invention, only the change of the repetition frequency is important, and absolute frequency accuracy is not required, so that the present analog system is sufficiently practical. As another analog system, the FM demodulator can be generally regarded as a frequency-voltage converter in a broad sense. However, the FM demodulator has an operating frequency range that is too narrow, and it is difficult to directly apply the present invention to the present invention. In the digital method, the input waveform is pulsed by the comparator, and after counting for a certain period, the count value is held and D
What is converted to an analog voltage with the / A converter and output is
Widely available on the market.

However, in such a product, the accuracy of the absolute frequency is emphasized, and for the purpose of the present invention, the update cycle of the output is too slow, in other words, the responsiveness is insufficient.
As a digital system, it is better to count a period of a pulsed waveform with a built-in clock pulse, hold the count value, convert it into an analog voltage with a D / A converter, and output a new one. , Suitable for the purposes of the invention. The principle and constituent elements of the present invention have been described above. Finally, a time axis calibration method performed on time series data in which the magnitude of correlator output is measured and recorded will be described. If the repetition frequency F _sig of the optical signal train to be measured is known in advance, the repetition cycle T _obs on the measured / recorded time-series data is made to correspond to the true repetition cycle 1 / F _sig , so that the time axis Can be calibrated.

Even if the repetition frequency F _sig is not known with sufficient accuracy, the repetition frequency F of the sampling pulse train is
_{If the} repetition frequency F _sig is known to the extent that _samp can be measured and the multiplication number N≈F _sig / F _samp can be specified, the frequency offset ΔF can be calculated from the repetition frequency F _obs = NΔF of the correlator output.
And the time axis expansion rate F _samp / ΔF is obtained, the time axis on the time series data is reduced accordingly,
The time axis for the optical signal train to be measured is obtained. Even if the repetition frequency F _sig of the optical signal train to be measured is not known at all prior to the waveform measurement according to the present invention, the time axis can be calibrated by the following procedure. In such a case, the repetition frequency F _sig would not be detected at all with the other existing methods, so that an extremely high repetition rate can be assumed.

Then, the repetition frequency F _sig is well above the above-mentioned critical frequency F _c , and it is found that the frequency offset ΔF becomes equal to zero at least twice during the stroke ΔL of the coarse adjustment mechanism. The amount of movement of the coarse adjustment mechanism during that period is Δ
When written as L ′, the approximate value of the repetition frequency F _sig can be obtained by the following equation. F _sig = c / (2ΔL ′) (7) The calculation by the above equation (7) is approximate to the repetition frequency F _samp of the sampling pulse train in that the frequency offset ΔF is ignored, but many If you give enough precision. Further, when high accuracy is required, the repetition frequency F _{sa mp} of the sampling pulse train is measured, and the multiplication number _{N≈F sig} / F _samp is determined together with the repetition according to the equation (7). The time axis may be converted via the time axis expansion rate.

[0074]

DETAILED DESCRIPTION OF THE INVENTION The structure of the present invention will be described below with reference to the drawings. [First Configuration of the Present Invention] FIG. 1 is a diagram showing a first configuration of the optical signal waveform measuring method of the present invention. As described above, one cross-correlator is used for waveform measurement and resonator length control. Shows the configuration shared by both. In this configuration, a passive mode-locked laser device is used as the short pulse laser resonator 102 that generates a sampling pulse train. The short pulse laser resonator 102 includes a resonator length coarse adjustment mechanism 104,
The resonator length is configured to be statically controllable.

Further, the short pulse laser resonator 102 is
The resonator length fine adjustment mechanism 103 is provided, and the resonator length can be dynamically controlled. In a laser resonator having an independent end face mirror, as the resonator length fine adjustment mechanism 103,
A so-called displacement mirror device is used in which an end face mirror is attached to one end of a (multilayer type) piezoelectric actuator and the position of the end face mirror is changed by the voltage applied to the piezoelectric actuator. In the case of an optical fiber laser resonator, the resonator length fine adjustment mechanism 103 may have a structure in which a part of the fiber is expanded and contracted by a piezoelectric actuator.

The sampling pulse train emitted from the short pulse laser resonator 102 has a reflecting mirror 105 and a reflecting mirror 106.
Then, the light enters the cross-correlator 107 via. On the other hand, the optical signal train to be measured incident on the signal light incident end 101 also receives the cross-correlator 1
It is incident on 07. The cross-correlator 107 outputs the cross-correlation signal G _c represented by the equation (1) with respect to the two pulse trains that have been incident. This cross-correlation signal G _c is the waveform recorder 1
Observed and recorded by 12. At the same time, the cross-correlation signal G _c is supplied to the frequency-voltage converter 108 and converted into a voltage value proportional to the repetition frequency F _obs . The voltage value is compared with the reference voltage 109, and the difference from the reference voltage 109 is input to the integrating circuit 110.

The output of the integrator circuit 110 is supplied to the drive circuit 111 corresponding to the above-described resonator length fine adjustment mechanism 103, and adjusts the resonator length of the short pulse laser resonator 102. As the operation of this configuration, when the optical signal train to be measured is given, first, the resonator length coarse movement mechanism 104 is adjusted to make the cross-correlation signal G
The repetition frequency F _{obs of c} is brought close to the range obtained by multiplying the repetition variable range of the sampling pulse train determined by the stroke of the resonator fine adjustment mechanism 103 by the multiplication number N. In this case, the output voltage value of the frequency-voltage converter 108 is the reference voltage 10
The resonator length may be changed by the resonator length coarse movement mechanism 104 in a direction approaching the value of 9.

When the repetition frequency F _obs reaches the range,
The resonator length control by the automatic control system is automatically started.
After that, the resonator length coarse movement mechanism 104 is fixed, and while continuing the resonator length adjustment operation by the automatic control, the waveform recorder 1
The magnitude of the cross-correlation signal G _c output by the cross-correlator 107 is observed and recorded by the method 12. Thus, in the first configuration of the optical signal waveform measuring method of the present invention,
High-sensitivity measurement of waveforms of optical signal trains with ultra-short widths in the picosecond to femtosecond range is realized.

[Second Configuration of the Present Invention] FIG. 2 is a diagram showing a second configuration of the optical signal measuring method of the present invention. In order to measure the waveform and control the cavity length, individual cross-correlators are provided. Shows the configuration addressed to. In this configuration, the sampling pulse train emitted from the short pulse laser resonator 202 is reflected by the reflecting mirror 20.
After passing through 5, the light is split into two by a branching mirror 213, one of which is incident on a cross-correlator 207 for controlling the resonator length, and the other is incident on a cross-correlator 217 for measuring a waveform through a reflecting mirror 206.

On the other hand, the optical signal train to be measured incident on the signal light incident end 201 is divided into two by the branching mirror 214, one of which is incident on the cross-correlator 207 and the other is signal-optical conversion that is arbitrarily inserted. After passing through the analyzer 215, it enters the cross-correlator 217 via the reflecting mirror 216. Cross-correlator 2
07 is the equation (1) for the above two pulse trains that are incident.
The cross-correlation signal G _c represented by is output, and the cross-correlation signal G _c is observed and recorded by the waveform recorder 212. At this time, the origin of the time-series observation / recording of the waveform recorder 212 is determined with reference to the output of the cross-correlator 207 for controlling the resonator length.

In this configuration, the signal light converter 215 allows
Since the repetition can be multiplied, it is incident on the cross-correlator 217.
Repetition frequency F of optical signal train_sigIs the signal light incident end 20
Repetition frequency F of optical signal train in 1₀Not necessarily equal to
It doesn't get bad. That is, the resonator length control by the cross-correlator 207 is performed.
The operation is the frame repetition frequency F ₀Was made against
On the other hand, the waveform measurement is not modified by the signal light converter 215.
It is performed on the beam signal sequence. Such a configuration is significant
What is meant is generally the operating characteristics and characteristics of the signal / light converter 215.
Optical signal train output by the converter to evaluate and verify performance
This is a case where the intensity waveform of is measured.

For example, in the case where the signal light converter 215 is an optical multiplexer, its operation performance can be examined by observing the uniformity of the output pulse train, and after the optical multiplexing, an optical switch is operated. In that case, the extinction ratio of the optical switch can be evaluated, and so on. Of course, as the signal light converter 215,
It is also possible to insert simpler / basic optical elements / materials. Generally speaking, the signal-to-optical converter 215 in this case is
Not only an optically linear response but also a non-linear response may be used, and the output wavelength may be different from the incident light wavelength.

This is because, as mentioned above, the wavelengths of the two lights involved in the cross-correlation measurement may be different from each other. Further, the light output from the signal light converter 215 need not be coherent (has a deterministic phase) with respect to the incident light. This is because the cross-correlation measurement here detects only the intensity waveform of the output optical signal sequence completely separated from its phase. On the other hand, for more classical (linear) cross-correlation measurement using an interferometer, the output of the signal light converter 215 should be coherent with the incident light and the input and output wavelengths should at least overlap. is necessary. In comparison with this, the optical signal waveform measuring method using non-linear cross-correlation like this example investigates the characteristics of the signal optical converter 215, that is, the optical element / material, in a remarkably wide category. It can provide the means.

As described above, various objects can be regarded as the signal light converter 215, and the purpose of the present invention is to suppress the fluctuation of the frequency offset ΔF by using the cross-correlator 207 for controlling the resonator length. Needless to say, there are various examples of this configuration without departing from the scope. Also, in general, a cross-correlation signal G _c containing a very large amount of noise
Therefore, it is difficult to successfully perform the resonator length control operation. Therefore, in the cross-correlator 207 for controlling the resonator,
It is necessary to ensure that a good signal-to-noise ratio is ensured for the cross-correlation signal G _c that is generated by incidence with an optical signal sequence that is not too weak as a reference. For this purpose, if necessary, the branch mirror 21
4 and an optical amplifier can be inserted between the cross-correlator 207.

The optical amplifier generally has a finite amplification (wavelength) bandwidth and also has chromatic dispersion, and as a result, distorts the waveform of the optical signal train more or less. Therefore, when the waveform is inserted on the side of the cross-correlator 217 for measuring the waveform, the measured waveform is merely an output waveform of the optical amplifier, and is generally different from the original waveform input to the optical amplifier. However, on the side of the cross-correlator 207 for controlling the resonator, any waveform distortion is allowed as long as it does not hinder the detection of the repetition frequency of the optical signal train, and there is no influence on the waveform measurement itself.

Under the same circumstances, as the cross-correlator 207 for controlling the resonator, it is possible to apply a resonator having a low time resolution within a range capable of responding to the repetition frequency of the optical signal train, with sensitivity as the first meaning. This is because even in the cross-correlator, the high-speed measuring device is always leaked, and it is difficult to achieve both sensitivity and resolution. As described above, this configuration in which the individual cross-correlators are addressed for the waveform measurement and the cavity length control is also meaningful for the measurement of a weak optical signal train.

[Structure of Cross Correlator of the Present Invention] FIG.
The configuration example of the cross-correlator used in the present invention will be described with reference to FIG. FIG. 3A is a diagram showing a configuration of a cross-correlator utilizing a sum frequency generation effect belonging to the second-order optical nonlinear effect. This type of cross-correlator, or a cross-correlator constructed by adding a variable optical delay line for changing the relative delay to it, is the most commonly used standard measuring device at present. In this case, the cross-correlator 307 is composed of, for example, reflecting mirrors 320 and 321, a right-angle reflecting prism 322, a focusing lens 323, a nonlinear crystal 324, a diaphragm 325, a condenser lens 326, and a photodetector 327. here,
The reflecting mirrors 320 and 321 and the right-angled reflecting prism 322 narrow the interval between the two incident beams and focus lens 323.
It is installed for incident on top.

The two pulse trains that have passed through the focusing lens 323 intersect with each other in the nonlinear crystal 324 placed at the focus of the focusing lens 323, and at the same time, the respective beams are focused. As a result, the two pulse trains have a nonlinear crystal 324.
Among them, they interact in a state of high light intensity. The sum frequency light generated by the sum frequency generation effect in this example is emitted in the direction sandwiched by the two incident lights. Aperture 32
5 is arranged so as to have an opening in the emitting direction of this sum frequency light, and as a result, only the sum frequency light generated by the interaction of the two pulse trains passes through this diaphragm 325 and reaches the condenser lens 326. be able to.

This sum frequency light is photoelectrically converted by the photodetector 327, and an electric signal proportional to the magnitude of the cross-correlation signal G _c is obtained. FIG. 3B shows a configuration of a cross-correlator applying the two-photon transition in the semiconductor photodetector which also belongs to the second-order optical nonlinear effect. The cross-correlator of this type, or a cross-correlation meter formed by adding a variable optical delay line for changing the relative delay thereto, has a feature that the number of parts is small and the cost is low. Such a cross-correlator is composed of, for example, reflecting mirrors 320 and 321, a right-angle reflecting prism 322, a focusing lens 323, and a photodetector 328. Here, the applications of the reflecting mirrors 320 and 321 and the right-angle reflecting prism 322 are the same as in the case of FIG.

The two pulse trains that have passed through the focusing lens 323 have the photodetector 3 placed at the focal point of the focusing lens 323.
28 is narrowed down. In this example, the photodetector 32
Reference numeral 8 is a semiconductor photodetector, and the wavelength λ _g corresponding to the bandgap energy satisfies the following formulas with respect to the wavelengths λ ₁ and λ ₂ of the _two pulse trains incident. λ _g <λ ₁ , λ _g <λ ₂ , λ ₁ λ ₂ / (λ ₁ + λ ₂ ) <λ _g (8) Under this condition, this photodetector 328 is sensitive to the incident pulse train in the linear region. Do not have.

However, in the non-linear region in which two photons are involved, carriers are generated and a photocurrent proportional to G ₀ + G ₁ + G _c is generated. Here, G _c is the above-mentioned cross-correlation signal. Further, G ₀ and G ₁ represent non-linear signals generated by each pulse train alone, and are signals that do not depend on the relative delay τ _e of the two pulse trains. However, depending on the relationship between the incident light wavelength and the bandgap wavelength, G ₀ ,
In some cases, _one of G ₁ does not occur.

Due to the presence of this constant part G ₀ + G ₁ , when using this type of cross-correlator, the cross-correlation signal G _c
Even if is zero, the output signal of the cross-correlator is not zero. As a result, this type of cross-correlator has
Although it is cheaper than the format (a), it has a drawback that it is difficult to measure a wide dynamic range. Such a drawback is also found in a cross-correlator that applies absorption saturation, which belongs to a higher-order third-order optical nonlinear effect.

[0093]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The first embodiment of the present invention will be described below with reference to FIGS.
An example of the configuration will be described in detail. In this embodiment, at a wavelength of 1.53 μm, the repetition frequency is 179 MHz,
The C _r doped YAG laser for generating a pulse train having a width 80fs was short pulse laser resonator 102. In this case, as the resonator length fine adjustment mechanism 103, a so-called displacement mirror in which a total reflection end face mirror is attached to one end of the piezoelectric actuator and the position of the end face mirror is changed by the voltage applied to the piezoelectric actuator is used.

Here, as the piezoelectric actuator, since a laminated piezoelectric actuator having a stroke of 15 μm (voltage of 100 V) is used, it is possible to sufficiently cancel the resonator length change of the order of 1 μm due to vibration. On the other hand, as the resonator coarse adjustment mechanism 104, a parallel displacement table with a micrometer having a stroke of 25 mm was used. Since this laser has a cavity length of 0.84 m, the ratio of this step to the cavity length, that is, the relative coarse adjustment step r is 3%.

FIG. 4A shows the range of the optical signal train repetition frequency F _{sig that} can be applied to the multiplication factor N in the short pulse laser resonator 102 of this embodiment. In the figure, the respective ranges when the number of multiplications is increased / decreased by 1 are shown in an overlapping manner. As can be seen from this, as the number of multiplications increases, the range for the next number of multiplications approaches, and when the number of multiplications reaches 34, the ranges of both sides are completely merged.

The multiplication number at this time is positively equal to the multiplication number N _c represented by the equation (5). Here, when the repetition frequency F _sig corresponding to this multiplication number is read from the figure, it becomes 6 GHz,
This is nothing but the critical frequency F _c of the equation (6). Thus, according to the short pulse laser resonator 102 of this embodiment,
Any repetitive optical signal train of 6 GHz or higher can be measured. Of the optical pulse train with an average output of 100 mW generated by the short pulse laser resonator 102, 2 mW was used as a sampling pulse train for optical signal waveform measurement. This sampling pulse train was made incident on the optical fiber by a coupling lens, and 0.8 mW of which was coupled into the optical fiber.

In this example, the sampling pulse train is guided to the cross-correlator 107 through the short length (1 m) optical fiber in this manner in order to increase the degree of freedom regarding the arrangement of the devices. The cross-correlator 107 used in this example has the configuration shown in FIG. That is, as the nonlinear effect, a sum frequency generation effect in a 2 mm long lithium niobate (LiNbO ₃ ) crystal is used, and the generated sum frequency light is photoelectrically converted by a photomultiplier tube, and the amplification sensitivity is 5 × 10 5.
It was converted to a voltage value by a ⁴ V / A current amplifier.

FIG. 4B shows such a cross-correlator 10
7 shows the result of optical sampling measurement by itself of the sampling pulse train. Such a signal is particularly called an autocorrelation signal. For such measurement,
Immediately before entering the cross-correlator 107, the sampling pulse train was branched outside, and one of them was connected to the signal light incident end 101 instead of the optical signal train via the variable optical delay line. Such measurement of the autocorrelation signal is necessary at least once after the waveform measuring device is assembled in order to estimate the time resolution according to the time width of the sampling pulse.

When the sampling pulse is of the optical fiber incident type as in the present embodiment, the necessity of measuring the autocorrelation signal is particularly increased. What is important is that the pulse width of the sampling pulse is generally shorter than that of the measured optical signal train, and even when measuring the measured optical signal train in the picosecond region, a sampling pulse with a width of the sub-picosecond to femtosecond region is used. It is customary to be done. Such an optical pulse having an ultra-short time width is susceptible to deformation due to wavelength dispersion of the optical fiber, and therefore, even if the pulse width immediately after the short pulse laser resonator 102 that generated the optical pulse is known. , There is no guarantee that the original pulse width is maintained when propagating and exiting the optical fiber.

Therefore, the autocorrelation signal measurement of the sampling pulse actually emitted from the optical fiber is performed,
It is necessary to estimate the time resolution according to the width.
In the case of this example, from the autocorrelation signal measurement result of FIG.
Intensity waveform I ₀ of the sampling pulse after emission from the optical fiber
It has been found that (t) can be sufficiently approximated by a Gaussian function having a full width at half maximum of 166fs. In FIG. 4B, the autocorrelation waveform with respect to this approximate waveform is shown with a white circle superimposed on the measured value.

Here, it should be noted that the width of the sampling pulse after the emission from the optical fiber is actually more than twice as wide as that immediately after the emission from the short pulse laser resonator 102. This is nothing but the manifestation of the effect of chromatic dispersion of the optical fiber. Thus, the waveform I ₀ (t) of the sampling pulse and thus its Fourier transform I ′
_{If 0} (t) is known, the decorrelation calculation according to the equation (2) can be performed for the subsequent waveform measurement for the optical signal sequence.

Note that the vertical axis of FIG. 4 (b) is scaled on a logarithmic scale. From this, it can be seen that the short pulse laser resonator 102 used in this example can generate a sampling pulse in which the tail of the pulse is sharply attenuated and which is very suitable for optical sampling measurement. At the same time, it is also shown that the cross-correlator 107 used in this example can easily secure a dynamic range close to three digits. This is non-coaxially incident on the nonlinear crystal, FIG.
This is a feature of the cross-correlator having the configuration shown in (a).

In the case of this example, the optical path from the optical signal train entrance end 101 to the cross-correlator 107 is also composed of an optical fiber, and a fiber connector is attached to the optical signal train entrance end 101. Thereby, the optical signal train to be measured can be introduced into the configuration of this example by a very simple means of coupling the fiber connectors. In this example, the cross-correlator 107
A digital oscilloscope having a waveform storage / arithmetic function was used as the waveform recorder 112 for observing and recording the output of 1., and the above-mentioned analog circuit system was used as the frequency-voltage converter 108.

In FIG. 5A, the repetition frequency F _sig is 1
The measurement result by this Example of the optical signal train of 9.9 GHz is shown. This optical signal train was generated by a monolithic semiconductor laser having an electroabsorption modulator on a chip. It is known that this semiconductor laser, by active mode-locking operation, produces a pulse train equal to the repetition of a sine wave applied to the modulator. Therefore, the above repetition frequency was known prior to measurement from the reading of the frequency setting of the applied sine wave source.

As a result, the time base calibration for the time series data on the waveform recorder 112 is performed by the above-mentioned first method, that is,
The repetition period of the optical signal train on the oscilloscope (in this case,
About 0.45 ms) could be achieved by making the true repetition period 1 / F _sig (in this case, 50.25 ps) correspond. The average incident power at the optical signal train entrance end 101 was 1.8 mW, and the measurement was performed under the conditions that the multiplication factor N was 111 and the frequency offset ΔF was about 20 Hz. The lower part of FIG. 5A shows the recording of the waveform by one time sweep of the oscilloscope, and the upper part shows the recording of the waveform obtained by averaging over 100 sweeps.

The measurement time required for the averaging measurement in the upper row is
It was 30 seconds. It can be clearly seen that the lower single sweep waveform contains a lot of noise, but the upper averaging measurement has reduced it and greatly improved the signal-to-noise ratio. Thus, this example demonstrates that the fluctuation of the frequency offset ΔF is successfully suppressed and the measurement sensitivity is improved by taking a long measurement time.

With respect to the arithmetic mean waveform in the upper part of FIG.
As a result of performing decorrelation calculation using the waveform of the sampling pulse obtained above, no significant difference was brought about in the waveform. This is because the width of the optical signal train waveform in this measurement is
It is 7.5 ps, which is the result that the width of the sampling pulse is sufficiently narrow. Therefore, it can be regarded that the arithmetic mean waveform in the upper part of FIG. 5A directly gives the intensity waveform I ₁ (t) of the optical signal train to be measured. Figure 5
Further, (b) shows the result of comparing the auto-correlation signal obtained by calculating the auto-correlation signal and directly measuring the auto-correlation signal for the arithmetic mean waveform obtained in this example.

The measurement of such an autocorrelation signal was performed by the same procedure as that performed for the sampling pulse above. The calculated value and the measured value agree fairly well. In particular, note that the tails of the correlation signals do not completely recover to zero, which is a perfect match. This is a result demonstrating that the true value is reflected up to the intensities slightly visible before and after the pulse on the arithmetic mean waveform. That is, in the arithmetic mean waveform, the measurement is realized with sufficient accuracy even if the intensity is only 1.5% with respect to the peak value.

Thus, this example achieved a measurement with extremely low noise (signal-to-noise ratio> 100). From a different point of view, since the peak value of the present optical signal is about 12 mW, it means that the present measurement could accurately measure up to an instantaneous power level of about 0.2 mW. This is a level at which the optical signal train generated by the semiconductor laser can be measured with sufficient margin without using an optical amplifier. That is, the present embodiment can easily measure the output light of a semiconductor light source, which is considered to be the most practical in the field of optical communication / information processing, without using an optical amplifier.

Therefore, the cost of the optical amplifier is reduced, and there is no fear of waveform distortion due to the optical amplifier. As a result, high measurement accuracy is guaranteed. Thus, detailed measurement of the output waveform signal waveform contributes greatly to the evaluation of the characteristics of such a semiconductor light source. In the comparison between the calculated value of the autocorrelation signal and the actually measured value, a sharp spike is found at the origin of the delay time in the actually measured value, but it is not found in the calculated value. This does not mean that the time resolution of this embodiment is insufficient.

This spike is called a coherence spike, and is a manifestation of the fact that the optical signal train has a fine structure that statistically fluctuates from pulse to pulse. Such a microstructure is
In waveform measurement by optical sampling as in this embodiment,
Essentially not detected. Because optical sampling is
This is because it is an averaging measurement as such, and only an envelope curve obtained by smoothing the oscillating microstructure is detected. As described above, the apparatus of this embodiment can stabilize the frequency offset ΔF and realize the optical signal waveform measuring method with high sensitivity and high accuracy.

[0112]

According to the optical signal measuring method of the present invention, it is possible to perform the measurement without suffering the waveform distortion due to the repetition frequency fluctuation of the optical signal train to be measured or the optical length fluctuation of the resonator of the short pulse laser. Further, the optical signal measuring method of the present invention,
There is no limitation due to the band of the high-frequency electronic circuit, and therefore, the upper limit of the applicable repetition frequency is determined only by the time resolution of the signal waveform measurement, and practically any high repetition frequency can be supported. Further, the optical signal measuring method of the present invention can be performed universally without depending on the repetition frequency of the optical signal train to be measured,
Moreover, since it can be carried out at low cost, a great industrial effect can be obtained.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a first configuration of an optical signal measuring method of the present invention.

FIG. 2 is an explanatory diagram showing a second configuration of the optical signal measuring method of the present invention.

FIG. 3 is a diagram showing a configuration example of a cross-correlator used in the configuration of the present invention, FIG. 3 (a) is an explanatory diagram showing a configuration using a sum frequency generation effect, and FIG. 3 (b) is a semiconductor photodetector. It is explanatory drawing which shows the structure which uses the two-photon transition in the inside.

FIG. 4 is a diagram showing characteristics of a short pulse light source used in an embodiment of the present invention, FIG. 4 (a) is a graph showing a signal repetition frequency with respect to a multiplication number, and FIG. 4 (b) is a sampling pulse. 5 is a graph showing an autocorrelation signal of

FIG. 5 is a diagram showing characteristics obtained according to an embodiment of the present invention, and FIG. 5 (a) is a diagram illustrating sweeping of a sampled correlator output signal.
FIG. 5B is a graph showing the case of 00 times and a single sweep, and FIG. 5B is a graph showing comparison with the actual measurement value of the autocorrelation signal calculated from the measurement signal.

FIG. 6 is an explanatory view showing the principle of the optical signal waveform measuring method and the configuration of the first conventional example, FIG. 6 (a) is a general explanatory view of the optical signal waveform measuring method by optical sampling, and FIG. 8) is an explanatory view showing a configuration of a first conventional method.

FIG. 7A is an explanatory diagram showing a configuration of a second conventional method, and FIG. 7B is an explanatory diagram showing a configuration of a third conventional method.

[Explanation of symbols]

101 Signal light incident end 102 Short pulse laser resonator 103 Resonator length fine adjustment mechanism 104 Resonator length coarse adjustment mechanism 105,106 Reflector 107 cross-correlator 108 Frequency-voltage converter 109 Reference voltage 110 Integrator circuit 111 drive circuit 112 waveform recorder 201 Signal light incident end 202 Short pulse laser resonator 203 Resonator length fine adjustment mechanism 204 Resonator length coarse adjustment mechanism 205, 206 Reflector 207 Cross-correlator 208 Frequency-voltage converter 209 Reference voltage 210 Integrator circuit 211 drive circuit 212 waveform recorder 213, 214 Branch mirror 215 Signal Optical Converter 216 Reflector 217 Cross-correlator 307 Cross-correlator 320, 321 reflector 322 Right angle reflection prism 323 Focusing lens 324 Non-linear crystal 325 aperture 326 Condensing lens 327 Photodetector 328 Photodetector 602 Short pulse laser resonator 604 Resonator length coarse adjustment mechanism 605,606 Reflector 607 Cross-correlator 612 Waveform recorder 616 reflector 630 Signal source laser 631 reflector 701 Signal light incident end 702 Short pulse laser resonator 703 Resonator length fine adjustment mechanism 704 Resonator length coarse adjustment mechanism 705,706 Reflector 707 Cross-correlator 710 Integrator circuit 711 drive circuit 712 Waveform recorder 713,714 Branching mirror 716 Reflector 736 High-speed photodetector 737 high frequency amplifier 738 High-speed photodetector 739 high frequency amplifier 740 High frequency mixer 741 low pass filter 742 Cross Correlator 743 Control / Recorder

Claims (2)

(57) [Claims] 1. An optical signal train to be measured and an optical pulse train generated by a short pulse laser are made incident on a cross-correlator, and are combined / focused in a medium having an optical nonlinear effect in the cross-correlator. In the optical signal waveform measuring method for converting a generated non-linear signal into an electric signal and measuring and recording the magnitude of the electric signal in time series, a resonance for adjusting the optical length of the resonator to the short pulse laser. An optical signal waveform measuring method, characterized in that a cavity length adjusting mechanism is provided, the repetition frequency of the electric signal is converted into a voltage value, and the cavity length adjusting mechanism is driven so that the voltage value becomes a constant value. . 2. An optical signal train to be measured and an optical pulse train generated by a short pulse laser are incident on a cross-correlator, which is combined and focused in a medium having an optical nonlinear effect to generate. In the optical signal waveform measuring method for converting a non-linear signal into an electric signal and measuring and recording the magnitude of the electric signal in time series, a resonator for adjusting the optical length of the resonator to the short pulse laser. A length adjustment mechanism is added to divide each of the two pulse trains into two before reaching the cross-correlator,
Each of the obtained two pulse trains is incident on a separate cross-correlator, and at the separate cross-correlator, the two copies are combined and focused in a medium having an optical nonlinear effect, and generated. A non-linear signal is converted into an electric signal, the repetition frequency of the electric signal is converted into a voltage value, and the resonator length adjusting mechanism is driven so that the voltage value becomes a constant value. Then, the origin of the time-series measurement / recording of the magnitude of the electric signal is determined, and the optical signal waveform measuring method.