JPH11326065A - Light signal wave-form measuring method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light signal wave-form measuring method effective for a light signal train with any high repetition frequency at a low cost. SOLUTION: A measured light signal train and a light pulse train generated by a short-pulse laser 102 are fed to a mutual correlation unit 107, they are combined and focused in a medium having a light nonlinear effect by the mutual correlation unit 107, the generated nonlinear signal is converted into an electric signal, and the size of the electric signal is measured and recorded in time series by this light signal wave-form measuring method. A resonator length adjusting mechanism 103 adjusting the optical length of a resonator is applied to the short-pulse laser 102, the repetition frequency of the electric signal is converted into a voltage value, and the resonator length adjusting mechanism 103 is driven so that the voltage value becomes a fixed value.

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

[0002]

2. Description of the Related Art In recent years, the application of ultra-high-speed optical signals in the picosecond to femtosecond region has been prosperous in many industrial and academic fields including optical communication and optical information processing. The need to observe the intensity waveform in detail has increased. Here, when measuring the intensity waveform, the first thing that comes to mind as an extension from the method for the slower signal is to convert the optical signal under measurement into a similar electrical signal using a high-speed photodetector, May be observed by existing high-speed electrical signal measuring means.

[0003] However, the photodetector used in this case needs to have a shorter response time than the optical signal to be measured. In the ultra-short time region from the picosecond to the femtosecond region, there is no photodetector that satisfies this condition. Therefore, in the ultra-short time region, it is inevitable to rely on intensity waveform measurement by a cross-correlator described below.

The cross-correlator uses a reference light pulse which is synchronized with the optical signal to be measured and has a shorter time width than the optical signal to be measured. The magnitude of the generated nonlinear signal is measured while interacting with the optical signal while changing the relative delay time between the optical signal and the reference pulse. This is done by gating the optical signal with a short reference pulse and measuring the gated signal output 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 a method similar to the sampling measurement for the electric signal.

Therefore, the measurement using the cross-correlator is referred to as optical sampling, and the reference light pulse with a short time width used in conjunction is sometimes referred to as sampling light pulse. There are various modes and types of optical nonlinear effects to be used, and consequently nonlinear signals.

For example, as the second-order optical nonlinear effect,
Frequency generation effect or two-photon absorption effect is often used
You. In the former, the two lights that are incident, the sum of the respective frequencies
Hit frequency, ie wavelength λ ₁, Λ_TwoΛ₁
λ_Two/ (Λ₁+ Λ_Two) Wavelength (sum frequency light)
You. This is detected by a photodetector sensitive to the sum frequency light wavelength.
To receive and convert the electrical signal proportional to the power of the sum frequency light.
No.

In the latter case, the transmittance of the sampling light pulse decreases with a two-photon transition that occurs when one photon is obtained from each of two incident lights. Therefore, this is converted by a photodetector having sensitivity 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, an electric signal proportional to the two-photon transition probability can be directly obtained by attaching electrodes and collecting carriers generated by two-photon transition.

The third-order optical nonlinear effects include 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. In general, the time resolution related to 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 means for converting an electric signal such as a photodetector. do not do.

This property is the reason why optical sampling measurement using a cross-correlator is performed in an ultra-short time range from the picosecond to the femtosecond region. FIG. 6 shows a conventional optical signal waveform measurement method using a cross-correlator.
It was shown to. First, a general mode of an optical signal waveform measuring method by optical sampling will be described with reference to FIG.

The repetition frequency F shown at the top of FIG.
_{For the} optical signal sequence to be measured repeated with _sig , a sampling pulse sequence having a shorter time width in the middle part of the figure is used to make the two interact in a medium exhibiting an optical nonlinear effect. For example, the cross-correlator, in the case of using the secondary optical non-linear effect in the optical nonlinear effects, as expressions of the cross correlation signal G _c output, the following equation (1) is obtained.

[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 generated by the nonlinear crystal, and the integration is a photodetector having a response time sufficiently slower than the light pulse. Represents the effect of photoelectric conversion. Here, the correlation signal depends on the apparent time difference τ _e between the pulse trains.

If the time width of the sampling pulse is sufficiently shorter than 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, the effect 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 the sampling light pulse waveform I
_{If 0} (t) is known, I ′ ₀ (ω) is also known, and I ′ ₁ (ω) is obtained from equation (2).

By subjecting the obtained I ′ ₁ (ω) to inverse Fourier transform, the intensity waveform of the measured optical signal, I ′ ₁ (ω),
₁ (t) can be obtained. Such an operation is called decorrelation calculation. Now, let us write the repetition frequency F _samp of the sampling pulse train as shown by the following equation. F _samp = F _sig / N−ΔF (3) Here, the natural number N is referred to as a multiple, and ΔF is referred to as a frequency offset. At this time, the apparent time difference τ _e between the sampling optical pulse and the optical signal train to be measured is shifted by Δτ _e = F _samp ^-2 ΔF for each one sampling optical 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. Considering the frequency F _samp , the change frequency of the apparent time difference τ _e by modulating the period of the sampling light pulse is equal to the frequency offset ΔF. Further, since the optical signal under measurement repeats N times during one cycle of the sampling optical pulse, the output waveform of the cross-correlator is eventually observed with the repetition frequency F _obs shown in the following equation.

F _obs = NΔF (4) If a frequency offset ΔF is given between the repetition of the optical signal to be measured and the repetition of the sampling pulse train in accordance with the equation (3), the output of the cross-correlator becomes as follows. The measured optical signal waveform is obtained as if the time axis was _{expanded by Fsamp} / ΔF times. It is sufficient that the electronic circuit for observing and recording the photoelectric conversion in the cross-correlator or its output can respond to this virtually stretched signal, which is exactly the point in the picosecond to femtosecond region by optical sampling measurement. It represents the principle by which an optical signal in an ultra-short time region can be observed.

In this case, since the measured optical signal train and the sampling pulse train have different repetition frequencies, they can be supplied by independent light sources respectively.
It is possible in principle, and 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. 6 (b) is a diagram showing a first conventional example of optical signal waveform measurement by optical sampling, using two solid short-pulse lasers of the same type and providing a frequency offset ΔF between the repetitions thereof. I have. For example, Optics Letters
Journal, 17 (1992), pages 1286-1288.

In this example, two similar solid-state laser devices that generate a pulse having a width of 80 fs are used as the signal source laser 630 that generates an optical signal train and the short pulse laser resonator 602 that generates a sampling pulse train. .
Among them, the short-pulse laser resonator 602 includes a resonator length coarse adjustment 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 that has exited 30 is reflected by the reflecting mirror 631 and the reflecting mirror 6.
The light enters the cross-correlator 607 via 16.

The cross-correlator 607 generates a cross-correlation signal G expressed by equation (1) for the two incident pulse trains.
Output _c . This cross-correlation signal _Gc is output from the waveform recorder 612.
Observed and recorded by. At present, among the methods for generating ultrashort light pulses, the light pulse with the shortest time width can be generated stably with the highest repetition frequency 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 generation operation is collectively called passive mode locking or self mode locking.

As a method opposed to this, there is a pulse generation operation performed by positively applying a modulation signal to the laser light source from the outside, and is called active mode locking or forced mode locking. In any pulse generating operation, the repetition frequency of the optical pulse is the reciprocal of the rounding 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 1 / integer or an integral multiple of the repetition frequency determined by the cavity length is externally applied to generate and grow an optical pulse in the cavity. .

Here, it should be recalled that the laser resonator length is inevitably 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 the generated light The time width of the pulse increases. However, the repetition frequency of the optical pulse itself is kept unchanged because the frequency of the modulation signal is determined with priority.

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

In the case of this embodiment, a passive mode-locked laser is used for both the signal source laser 630 and the short pulse laser resonator 602. Further, for the repetition frequency F _sig of the optical signal train from the signal source laser 630, the repetition frequency F _samp of the sampling pulse train is expressed as F _samp = F _sig −ΔF
And the short-pulse laser resonator 602 is set such that the relationship in which the multiplication number N in the equation (3) is equal to 1 holds. To this end, the length of the short-pulse laser resonator 602 is made slightly longer than that of the signal source laser in accordance with the above-described 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 a frequency offset ΔF of 80 Hz is given,
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 small scale easily changes due to drift due to temperature change or fluctuation due to vibration. Therefore, in the configuration in which the resonator length is left to change without any control as in this example, the frequency offset ΔF cannot be kept constant for a long time.

When the frequency offset ΔF changes, the mutual
Repetition frequency F of correlator output waveform _obsOr this
But the apparent time-axis elongation rate F_samp/ ΔF
Changes and is observed and recorded by the waveform recorder 612.
The waveform undergoes expansion and contraction in the time axis direction. As a result,
The time accuracy on the waveform decreases. Or in extreme cases
Are F and F at the left and right ends of the correlator output waveform._samp/ ΔF is different
, Resulting in a distorted observation waveform. These times
The easiest way to avoid this is to use the frequency offset ΔF
The measurement is terminated before the change
You.

In the case of this example, the measurement section is limited to a range of 1 ps around the optical signal pulse, and 1 ps × (80 MHz / 80
(Hz) = 1 μs. In such a case, the measurement has been completed long before a change due to temperature change or vibration occurs, and distortion of the observed waveform due to a change in ΔF can be avoided. As with all measurements in general,
The measurement sensitivity can be higher as the measurement time is longer. this is,
Long measurement time and long-time averaging are equivalent to operating a narrow-band low-pass filter by that much, 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 applied to the measurement, highly sensitive measurement cannot be desired at all. Since this is impractical, control of the frequency offset ΔF
Stabilization methods are 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 external modulation 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 difficult to obtain a short pulse as compared with the passive mode-locking method. As a result, the width of the pulse 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 out by the optical gate 734 at a rate of one out of N pulses, and the pulse width is compressed by the pulse compressor 735. Thereafter, the light enters the cross-correlator 707 via 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. It said cross-correlator 707 outputs a correlation signal G _c of the formula (1) with respect to incident the two pulse trains. 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 of 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 also supplied to the synchronization signal generator 733 at the same time. 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 is applied to the short pulse laser resonator 702 as an external modulation signal.

Accordingly, a pulse train having a repetition frequency of F ₀ -NΔF is generated from the short pulse laser resonator, and the pulse train is thinned out at a rate of one out of N times 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 equation (3) in which the optical signal repetition frequency F _sig is replaced with F ₀ . In this example,
Since the repetition can be multiplied by the signal light converter at the subsequent stage of the signal source laser 730, the repetition frequency F _sig of the optical signal train incident on the cross-correlator 707 is not always equal to the repetition frequency of the signal source laser 730. Therefore, the substitution of the symbol was performed.

Here, F ₀ is an amount to be called the frame repetition frequency of the optical signal train. Thus, in this example,
As a result of constantly fixing the frequency offset ΔF, high-sensitivity optical signal waveform measurement is realized by optical sampling. In the above example, the short pulse laser resonator 702 using the active mode locking method is used, but this is not always necessary. The use of the passive mode-locking method has an advantage that the pulse compressor 735 in the above example is not required because the originally generated pulse width is short.

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

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 the 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-correlator 742 via the reflecting mirror 705 and the reflecting mirror 706. A part of the light is reflected by the split 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-correlator 742, and a part thereof is reflected by the split mirror 714 and received by the high-speed photodetector 738.

The cross-correlator 742 calculates the cross-correlation signal G expressed by the equation (1) for the two incident pulse trains.
Output _c . However, unlike the above-described cross-correlator, the cross-correlator includes a variable optical delay line for changing the relative delay between two pulse trains incident thereon. Control and recording unit 743, while controlling and driving the variable optical delay line, as a function of the relative delay, continue to record the correlation signal G _{c. Assuming} that 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
Sine wave beat signals having a frequency that is an integral multiple of Of these, the repetition frequency F _sig of the optical signal train
Nearest harmonic component NF _samp in becomes the active ingredient stabilization.

The electric output is amplified by a high frequency amplifier 737 and then applied to a high frequency mixer 740. The electrical output of the high-speed detector 738 that has received 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 sum and difference frequencies of the two applied electrical inputs. Among them, a signal of the frequency NF _samp −F _sig which is a 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 stabilizing the frequency of the oscillator at F _sig / N by feeding back the difference frequency NF _samp −F _sig as an error signal to the oscillator having the free-running frequency F _samp is as follows.
It is commonly known as a phase locked loop (PLL). The coarse resonator tuning mechanism 704 is provided to adjust the free-running frequency of the short-pulse laser resonator to fall within the pull-in frequency range of the phase-locked loop. Thus, in this example, the repetition frequency is always F _samp = F _sig
/ N is obtained as a sampling pulse train stabilized at / N.
This corresponds to the relationship in equation (3) where the frequency offset ΔF is set to zero.

As described above, in this embodiment, 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, the variable optical system is used instead of the cross-correlator 707 in the previous embodiment. It is necessary to use a cross-correlator 742 having a delay line. In the configuration of the present example, between the high-speed photodetector 736 and the high-frequency amplifier 737,
If a frequency shifter that changes the frequency of the sine wave electric signal by + NΔF is inserted, or a frequency shifter of −NΔF is inserted between the high-speed detector 738 and the high-frequency amplifier 739, the equation (3) is satisfied. A sampling pulse train is obtained. In this case, similarly to the previous example, the cross-correlator 7
07 can be used to observe the waveform. Thus, also in this example, the frequency offset ΔF is always fixed to be constant,
By optical sampling, highly sensitive optical signal waveform measurement is realized.

[0043]

However, the above-mentioned conventional optical signal waveform measuring method has the following problems. As described above, the first conventional example is extremely low-sensitivity and impractical, so it is essential to control and stabilize the frequency offset ΔF. Of the second and third conventional examples for performing this, in the second conventional example, a short pulse laser resonator 70 using an active mode locking method is used as a sampling pulse light source.
As a result, the repetition of the measurable optical signal is limited to a range where the pulse generation operation in the short-pulse laser resonator 702 is not impaired. That is, the repetition of the measurable optical signal train is practically fixed.

Furthermore, the width of a pulse that can be generated by the active mode locking method is too wide, and it is difficult to realize the time resolution required for the current optical signal waveform measurement in the current picosecond to femtosecond range. It became necessary to add a container 735. Such pulse compressors contain very powerful optical amplifiers and are therefore very expensive. Thus, in the second prior art, the repetition of the optical signal sequence to be measured is inflexible and the price is high. In the third conventional example, since the sampling light source by the passive mode locking method is used, such a problem can be solved.

That is, the pulse generation operation in the short pulse laser resonator 702 is not affected by the pulse repetition frequency, that is, the length of the resonator. As a result, the pulse width is kept short.
The pulse repetition frequency can be changed. Therefore, the flexibility for the repetition of the optical signal sequence to be measured is smaller than that of the second conventional example.
It is much wider. Furthermore, according to the passive mode-locking method, an optical pulse having a sufficiently short time width can be generated, so that no pulse compressor is required. As a result, the cost can be reduced at a lower cost than in the second conventional example. Such optical sampling methods inherently have the potential to observe even very high repetition rates of optical signal trains, exceeding THz.

It is very easy at present to reduce the time resolution of waveform observation to subpicoseconds or less by using sampling pulses in the femtosecond region. In situations where even the waveform inside the repetition period can be observed,
There is no doubt that the repetition period itself cannot be observed. A subpicosecond period corresponds to a repetition frequency above THz. In order to observe a high repetition optical signal sequence, the repetition frequency itself of the mode-locked laser as the sampling light source does not need to be high at all. At a constant sampling pulse train repetition frequency F _samp , if the multiplication factor N in the equation (3) can be increased, the arbitrarily high optical signal repetition frequency F _{sig is obtained.}
It is because it can respond to.

However, in the third conventional method, high-speed light detection is performed.
Devices 736 and 738, high-frequency amplifiers 737 and 739,
Furthermore, it can be set by the band limitation of the high frequency mixer 740.
The number of multiplications N is limited. In other words, the
F above wavenumber_sig, For example, repetition frequency
Optical signal from mode-locked semiconductor laser with 100 GHz
Sequence cannot be observed using conventional methods.
You. For lower repetition optical signal trains that can be observed
Also generally have a repetition frequency F _sigBecomes higher,
The electronic circuits used, namely the fast photodetectors 736 and 738,
High frequency amplifiers 737 and 739, and high frequency mixer 7
40 becomes expensive.

Moreover, in order to maintain flexibility in the optical signal repetition frequency, not only an electronic circuit operating only in a narrow band but also a wide band circuit is required. In particular, the cost of the microwave band broadband high frequency amplifiers 737 and 739 can easily occupy more than half of the conventional arrangement, resulting in a loss of economics of the method. Up to here, about the third conventional example,
Although the problem caused by the electronic circuit has been described, the second conventional example is not necessarily free from 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 behind it.

As described above, in the conventional optical signal waveform measuring method, (1) the measurement of a high repetition optical signal train is restricted by the band limitation of the electronic circuit, and (2) the economical efficiency is caused 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 any high repetition frequency and can be performed at low cost.

[0050]

According to the optical signal waveform measuring method of the present invention, an optical signal sequence to be measured and a sampling pulse sequence generated from a short pulse laser resonator are made incident on a cross-correlator. , Both the measured optical signal train and the sampling pulse train are multiplexed and focused in a medium having an optical nonlinear effect, the resulting nonlinear signal is converted into an electric signal, and the magnitude is measured in time series. Record. On this occasion,
The short pulse laser is provided with a resonator length adjusting mechanism for adjusting the optical length of the resonator, 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. The adjustment mechanism is controlled and driven.

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

At this time, a resonator length adjusting mechanism for adjusting the optical length of the resonator is provided to the short pulse laser, and both the measured optical signal train and the sampling pulse train are connected to the first cross-correlator. Before being incident on the optical path, the duplicates of the measured optical signal train and sampling pulse train obtained are incident on a second cross-correlator. In the second cross-correlator, the duplicates are converted into optical signals. Combined and 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, so that this voltage value becomes a constant value, The resonator length adjustment mechanism is controlled and driven. At the same time, an origin of time-series measurement and 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 conventional optical signal waveform measurement method, in order to control and stabilize the frequency offset ΔF in the equation (3), two optical signals of a frequency F _sig and a frequency F _samp are used. And a method of establishing a relative frequency relationship using an electronic nonlinear element called a high-frequency mixer. For this reason, first, it is necessary to associate the sine wave electric signal with the optical pulse train.

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

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

To this end, attention is paid to the fact that the repetition frequency F _obs of the output waveform of the cross-correlator is given by equation (4). That is, F _obs is the product of the multiplier N and the frequency offset ΔF, and the multiplier can be regarded as constant. Because, before the multiplier itself changes, it is necessary to undergo a very large change in the frequency offset before that. In the feedback control operation in which the frequency offset ΔF is stabilized, the change in the multiplier 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 given by the following equation (1).
Of nothing but the cross-correlation signal G _c, the time resolution is determined by the duration of the sampling pulse, as described above, high enough to observe the waveform itself repeating the subject optical signal train. Here, since it is a repetitive waveform, the time width during which the waveform itself changes is naturally smaller than the pulse repetition period. Therefore, it is quite obvious that the present method for sufficiently detecting even a change in 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 a narrow pulse width of the sampling pulse train and can handle a signal train of any high repetition. On the other hand, in the present invention, a mechanism for finely adjusting the optical length of the resonator is provided to the short pulse laser that generates the sampling pulse train. As described above, when the resonator length is changed, the repetition frequency F _samp of the sampling pulse train changes accordingly.

Further, according to equation (3), the frequency F _samp
As a result, feedback is applied to the frequency offset ΔF. This corresponds to the operation element of the automatic control system of the present invention. As a result that it is desirable that the pulse width of the generated sampling pulse is not impaired by such an operation of changing the resonator length, 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. Therefore, if a laminated piezoelectric actuator having 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 factors of the frequency offset ΔF are:
A change in the resonator length of about 1 μm due to vibration is obtained, and the stroke of 15 μm of the laminated piezoelectric actuator above is sufficient to offset this change. On the other hand, in the short pulse fiber laser, the resonator length is about 10 m, and the main change factor of the frequency offset ΔF is a temperature drift of the resonator length on the order of 5 ppm / ° C.

Here, in the process of the above piezoelectric actuator, a change in the resonator length of only 1.5 ppm is given, and even a temperature drift of 1 ° C. is insufficient. However, a method of increasing the control range by winding the fiber constituting the resonator around a bobbin made of a piezoelectric material is known, and a sufficient control range can be realized by using this. As described above, the operation elements of the automatic control system according to the present invention can be realized very easily and inexpensively, and
It is a versatile technique that can be implemented on various types of known passively mode-locked short pulse lasers.

In general, in the short pulse laser based on the passive mode locking method, even if the resonator length is changed greatly beyond the width of the fine adjustment mechanism, the pulse width of the generated sampling pulse is not damaged. 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, the range of the optical signal sequence repetition frequency F _{sig that} can be supported can be expanded. Now, the length of the short-pulse laser resonator is L, and the stroke of the coarse adjustment mechanism is ΔL.
Let's write At this time, the repetition frequency of the sampling pulse train has a variation width around F _samp by rF _samp .

Here, r = ΔL / L is a relative coarse adjustment stroke. As a result, when the number of multiplication is N, as the optical signal train repetition frequency F _sig, around the NF _samp with variation of only RNF _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 multiplier 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 _larger than F _c = N _c F _samp , at least once within the stroke of the coarse adjustment mechanism, the equation (3)
F _samp that satisfies the relationship That is, in other words, the optical signal train having the above repetition frequency critical frequency F _c is always measurable. 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 moving table having a stroke of 25 mm is used for the coarse adjustment mechanism, a critical frequency of 6 GHz can be obtained, and it can cope with any repetition currently or assumed in optical communication or optical information processing. in this way,
The optical signal waveform measuring method of the present invention can provide a general-purpose method having extremely high flexibility in repetition of an optical signal train. It should be noted that, for the same purpose of adjusting the resonator length, the two types of fine adjustment mechanism and coarse adjustment mechanism are provided.
At first glance it seems like a wasteful duplication, but there is a reason.

That is, the adjusting mechanism having a large movable range is expensive, regardless of whether it is mechanical or electro-optical, and it is generally difficult to avoid deterioration of responsiveness. That is, it is difficult to balance the stroke and the responsiveness of the adjusting mechanism. In the stabilization of the frequency offset ΔF by the feedback control operation, it suffices that the change in the resonator length due to vibration or temperature drift can be offset, and a large stroke is not required as described above.
Regarding responsiveness, an order of 10 kHz is required for measures against vibration, and an order of 10 Hz is required for measures against temperature drift. On the other hand, it is desirable that the stroke is large for the purpose of expanding the compatibility with the repetition of the optical signal train.

If the stroke is small, the critical frequency of the equation (6) becomes too high, and practically only a discrete repetition frequency can be handled. On the other hand, a static adjustment is sufficient for this purpose and no responsiveness is required. It is extremely impractical to find an adjustment mechanism that satisfies the above two different requirements at the same time, and therefore they are divided between the fine adjustment mechanism and the grand adjustment mechanism. Based on the repetition frequency F _obs of the cross-correlation signal G _{c as} a detection element, in order to obtain an error signal for controlling the resonator length as an 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 an 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, so that by applying feedback in the direction of shortening the resonator length, F _samp increases, 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 has f
It is also called a -V converter (fV converter), and its operating principle can be roughly divided into two types: an analog type and a digital type. In the analog system, an input waveform is stepped by a comparator, a monostable multi is triggered to generate a constant area pulse, which is further smoothed by a low-pass filter and output. This method can be performed at low cost with a small number of components, but it is difficult to expect long-term accuracy of the absolute frequency.

In the present invention, only the change in the repetition frequency is important, and the absolute frequency accuracy is not required. As another analog system, generally, an FM demodulator can be regarded as a frequency-voltage converter in a broad sense. However, the operating frequency range of the FM demodulator is too narrow, and it is difficult to apply it to the present invention as it is. In the digital system, the input waveform is pulsed by a comparator, counted for a certain period, and then the count value is held.
A / A converter converts it to analog voltage and outputs it.
It is widely available on the market.

However, in such products, the absolute frequency accuracy is emphasized, and for the purpose of the present invention, the output updating cycle is too slow, in other words, the response is insufficient.
As a digital method, it is better to newly create a method that counts the period of the pulsed waveform using the built-in clock pulse, holds the count value, converts it to an analog voltage with a D / A converter, and outputs it. Suitable for the purposes of the present invention. The principle and components of the present invention have been described above. Finally, a time axis calibration method performed on time-series data obtained by measuring and recording the magnitude of correlator output will be described. If the repetition frequency F _sig of the optical signal sequence to be measured is known in advance, the repetition period T _obs on the measured and recorded time-series data is made to correspond to the true repetition period 1 / F _sig , and the time axis is reduced. Can be calibrated.

Even when the repetition frequency F _sig is not known with sufficient accuracy, the repetition frequency F
_{If the} repetition frequency F _sig is known to the extent that the _samp can be measured and the multiplication factor N ≒ F _sig / F _samp can be specified, the frequency offset ΔF is obtained from the repetition frequency F _obs = NΔF of the correlator output.
Is determined, and the time-axis elongation rate F _samp / ΔF is obtained. Therefore, the time axis on the time-series data is reduced according to this, and
The time axis for the optical signal sequence to be measured is obtained. Even if the repetition frequency F _sig of the optical signal sequence to be measured is not known at all before the waveform measurement according to the present invention, the time axis can be calibrated by the following procedure. In such a case, extremely high repetition can be assumed, since the repetition frequency F _sig cannot be detected at all using other current methods.

If so, the repetition frequency F _sig is well above the critical frequency F _c , and a point where the frequency offset ΔF is equal to zero is found at least twice in the stroke ΔL of the coarse adjustment mechanism. The amount of movement of the coarse adjustment mechanism during that time is Δ
When L ′ is written, an approximate value of the repetition frequency F _sig is obtained as in the following equation. F _sig = c / (2ΔL ′) (7) The calculation by the above equation (7) is approximate in that the repetition frequency F _samp of the sampling pulse train ignores the frequency offset ΔF. If give enough accuracy. Furthermore, if requiring high precision, measures the repetition frequency F _{sa mp} sampling pulse train, the multiplication factor N ≒ F _sig / F _samp determined in conjunction with repeated according to formula (7), below, in accordance with the above-described subsequent The conversion of the time axis via the time axis expansion rate may be performed.

[0074]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The configuration 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 an optical signal waveform measuring method according to the present invention. As described above, one cross-correlator is used for waveform measurement and resonator length control. Shows a 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 controlled.

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

The sampling pulse train emitted from the short-pulse laser resonator 102 is reflected by a reflecting mirror 105 and a reflecting mirror 106.
And enters the cross-correlator 107. On the other hand, the optical signal sequence to be measured incident on the signal light incident end 101 also
07. The cross-correlator 107 outputs a correlation signal G _c of the formula (1) with respect to incident the two pulse trains. This cross-correlation signal G _c is
12 to be observed and recorded. At the same time, the cross-correlation signal _Gc is supplied to the frequency-voltage converter 108, where it is converted to a voltage value proportional to the repetition frequency _Fobs . The voltage value is compared with a reference voltage 109, and a difference from the reference voltage 109 is input to an integration circuit 110.

The output of the integration circuit 110 is supplied to a 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. The operation of this configuration is as follows. When an optical signal sequence to be measured is given, first, the resonator length coarse adjustment mechanism 104 is adjusted, and the cross-correlation signal G
The repetition frequency F _{obs of c} is made closer 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 multiplier N. To this end, the output voltage value of the frequency-voltage converter 108 corresponds to 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,
Resonator length control by the automatic control system is automatically started.
Thereafter, the resonator length coarse movement mechanism 104 is fixed, and while the resonator length adjusting operation by automatic control is continued, the waveform recorder 1
12 going to observe and record the magnitude of the cross correlation signal G _c to the output of the cross correlator 107 by. Thus, in the first configuration of the optical signal waveform measuring method of the present invention,
High-sensitivity measurement of the waveform of an optical signal train having an ultra-short duration 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 according to the present invention, wherein separate cross-correlators are used for waveform measurement and resonator length control, respectively. Is shown. In this configuration, the sampling pulse train emitted from the short-pulse laser resonator 202 is
After passing through 5, the beam is split into two by a splitting 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 waveform measurement via a reflecting mirror 206.

On the other hand, the optical signal sequence to be measured incident on the signal light incident end 201 is split into two by a split mirror 214, one of which is incident on the cross-correlator 207, and the other is a signal light conversion optionally inserted. After passing through the correlator 215, the light enters the cross-correlator 217 via the reflecting mirror 216. The cross-correlator 2
07 is the equation (1) for the two incident pulse trains.
In output a cross correlation signal G _c represented, 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
Since the repetition can be multiplied, it enters the cross-correlator 217.
Frequency F of the optical signal train_sigIs the signal light incident end 20
The repetition frequency F of the optical signal train at 1₀Is not necessarily equal to
Does not go away. That is, the resonator length system by the cross-correlator 207
The operation is performed at the frame repetition frequency F ₀Made against
On the other hand, the waveform measurement is modified by the signal
This is performed on the optical signal train. Such a configuration is significant
What is important is that, in general, the operating characteristics and characteristics of the signal-to-optical converter 215
Optical signal train output from the converter for evaluation and verification of performance
In this case, the intensity waveform is measured.

For example, when the signal light converter 215 is an optical multiplexer, its operation performance can be examined by checking the uniformity of the output pulse train. After optical multiplexing, the optical switch is operated. In this case, the extinction ratio of the optical switch can be evaluated, and so on. Of course, as the signal light converter 215,
Simpler and basic optical elements and materials can be inserted. Generally speaking, the signal light converter 215 in this case is
Not only those exhibiting an optically linear response but also those having a non-linear response may be used, and the output wavelength may be different from the wavelength of the incident light.

This is because, as described 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 does not need to be coherent (having 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 train completely separated from its phase. On the other hand, in a more classical (linear) cross-correlation measurement using an interferometer, the output of the signal light converter 215 must be coherent with the incident light and the input and output wavelengths at least overlap. is necessary. In comparison with this, the optical signal waveform measurement method using the nonlinear cross-correlation as in this example examines the characteristics of the signal-to-optical converter 215, ie, optical elements and materials in a much wider category. It can provide a means.

As described above, there are various objects that can be regarded as the signal light converter 215, and the purpose of the present invention to suppress the fluctuation of the frequency offset ΔF by using the cross-correlator 207 for controlling the resonator length is described. It goes without saying that there are various cases in this configuration without departing from the scope. Also, in general, the cross-correlation signal G _c containing a very large amount of noise
Therefore, it is difficult to perform the cavity length control operation successfully. Therefore, the cross-correlator 207 for resonator control includes:
Incident unduly not weak optical signal train as a reference, it is necessary to ensure good signal-to-noise ratio correlation signal G _c in which they occur. For this purpose, if necessary, the split mirror 21
4 and the cross-correlator 207, an optical amplifier can be inserted.

An optical amplifier generally has a finite amplification (wavelength) bandwidth and also has chromatic dispersion, resulting in more or less distortion of the waveform of an optical signal train. Therefore, when the waveform is inserted on the side of the cross-correlator 217 for waveform measurement, the measured waveform is merely an output waveform of the optical amplifier and generally differs 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 the detection of the repetition frequency of the optical signal train is not hindered, and there is no influence on the waveform measurement itself.

In the same manner, the cross-correlator 207 for controlling the resonator can be applied with a low time resolution within a range that can respond to the repetition frequency of the optical signal train, with the sensitivity being the primary definition. This is because the cross-correlator does not always leak out of the high-speed measuring device, and it is difficult to achieve both sensitivity and resolution. As described above, this configuration in which an individual cross-correlator is addressed for waveform measurement and resonator length control is also meaningful for measuring a weak optical signal train.

[Configuration of Cross Correlator of the Present Invention] FIG.
FIG. 1 shows a configuration example of a cross-correlator used in the present invention. FIG. 3A is a diagram showing a configuration of a cross-correlator using a sum frequency generation effect belonging to a second-order optical nonlinear effect. This type of cross-correlator or a cross-correlator configured by adding a variable optical delay line for changing the relative delay to this type is currently the most frequently used and a standard measuring device. In this case, the cross-correlator 307 includes, for example, reflecting mirrors 320 and 321, a right-angle reflecting prism 322, a focusing lens 323, a non-linear crystal 324, a stop 325, a condenser lens 326, and a photodetector 327. here,
The reflecting mirrors 320 and 321 and the right-angle reflecting prism 322 reduce the distance between the two incident beams to form a focusing lens 323.
It is installed to enter above.

The two pulse trains that have passed through the focusing lens 323 cross each other in the nonlinear crystal 324 placed at the focal point of the focusing lens 323, and at the same time, each beam is narrowed down. As a result, the two pulse trains form the nonlinear crystal 324.
It interacts in a state where the light intensity is high. Sum frequency light generated by the sum frequency generation effect in this example is emitted in a direction sandwiched between two incident lights. Aperture 32
5 is arranged so as to have an opening in the emission direction of the sum frequency light. As a result, only the sum frequency light generated by the interaction of the two pulse trains passes through the stop 325 and reaches the condenser lens 326. be able to.

[0089] The sum frequency light is photoelectrically converted by the photodetector 327, an electric signal proportional to the magnitude of the cross correlation signal G _c is obtained. FIG. 3 (b) shows a configuration of a cross-correlator applying two-photon transition in a semiconductor photodetector which also belongs to the second-order optical nonlinear effect. This type of cross-correlator, or a cross-correlator configured by adding a variable optical delay line for changing the relative delay to the cross-correlator, has a feature that the number of parts is small and the cost is low. Such a cross-correlator includes, 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 passing through the focusing lens 323 are applied to the light detector 3 placed at the focal point of the focusing lens 323.
28. In this example, the photodetector 32
8 is a semiconductor photodetector, the wavelength lambda _g corresponding to the band gap energy, the wavelength lambda ₁ of the two pulse trains to be incident, with respect to lambda _2, satisfies the following equation. λ _g <λ ₁ , λ _g <λ ₂ , λ ₁ λ ₂ / (λ ₁ + λ ₂ ) <λ _g (8) Under this condition, the photodetector 328 has sensitivity to a pulse train incident in the linear region. Do not have.

However, in the non-linear region involving two photons, carriers are generated, and a photocurrent proportional to G ₀ + G ₁ + G _c is generated. Here, G _c is the cross-correlation signal described above. G ₀ and G ₁ represent non-linear signals generated by the respective pulse trains alone, and are signals independent of the relative delay τ _e of the two pulse trains. However, depending on the relationship between the incident light wavelength and the band gap wavelength, G ₀ ,
In some cases, _one of G1 does not occur.

Due to the existence of this constant part G ₀ + G ₁ , when using this kind of cross-correlator, the cross-correlation signal G _c
Is zero, the output signal of the cross-correlator will not be zero. As a result, the cross-correlator of this type is similar to that of FIG.
Compared to the method (a), it is inexpensive, but has a drawback that it is difficult to perform a measurement with a wide dynamic range. Such a defect is also found in a cross-correlator that applies absorption saturation belonging to a higher-order third-order optical nonlinear effect.

[0093]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS.
An example of the configuration will be described in detail. In this embodiment, at a wavelength of 1.53 μm, a repetition frequency of 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 was used, in which a total reflection end mirror was attached to one end of a piezoelectric actuator, and the position of the end mirror was changed by a voltage applied to the piezoelectric actuator.

Here, since a laminated piezoelectric actuator having a stroke of 15 μm (voltage 100 V) is used as the piezoelectric actuator, a change in the resonator length on the order of 1 μm due to vibration can be sufficiently canceled. On the other hand, as the resonator coarse adjustment mechanism 104, a parallel moving table with a micrometer having a stroke of 25 mm was used. Since the present laser has a resonator length of 0.84 m, the ratio of this step to the resonator 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 used for the multiple N in the short pulse laser resonator 102 of this embodiment. In the drawing, respective ranges when the multiplication factor is increased or decreased by 1 are shown in an overlapping manner. As can be seen, as the multiplication factor increases, the range for the next multiplication factor approaches, and when the multiplication factor reaches 34, it completely merges with the adjacent ranges.

The multiplication factor at this time is exactly equal to the multiplication factor _Nc 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 formula (6). Thus, according to the short pulse laser resonator 102 of the present embodiment,
Optical signal trains of any repetition above 6 GHz 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 measuring the optical signal waveform. This sampling pulse train was incident on the optical fiber by a coupling lens, of which 0.8 mW was coupled into the optical fiber.

In this example, the sampling pulse train is guided to the cross-correlator 107 via a short (1 m) optical fiber in order to increase the degree of freedom in the arrangement of devices. The cross-correlator 107 used in this example has the configuration shown in FIG. That is, as a non-linear 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 an amplification sensitivity of 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 of the sampling pulse train by itself. Such a signal is particularly called an autocorrelation signal. For such measurements,
Immediately before the incidence on the cross-correlator 107, the sampling pulse train was branched off outside, and one of them was connected to the signal light incidence end 101 instead of the optical signal train via a variable optical delay line. Such an autocorrelation signal measurement is required at least once after assembling the waveform measuring device in order to estimate the time resolution based on the time width of the sampling pulse.

As in the present embodiment, when the sampling pulse is of an optical fiber incidence type, the necessity of measuring the autocorrelation signal particularly increases. The pulse width of the sampling pulse is generally shorter than that of the optical signal sequence under measurement.Even when dealing with the optical signal sequence under measurement in the picosecond region, a sampling pulse with a width in the subpicosecond to femtosecond region is used. It is customary. Such an ultrashort-duration light pulse is liable to undergo deformation due to chromatic dispersion of the optical fiber, and therefore, even if the pulse width immediately after the short-pulse laser resonator 102 that generated the light pulse is known. However, there is no guarantee that the original pulse width is maintained when the light propagates and exits the optical fiber.

Therefore, an autocorrelation signal measurement is performed on the sampling pulse actually emitted from the optical fiber, and
It is necessary to estimate the time resolution based on the width.
In the case of this example, from the autocorrelation signal measurement result of FIG.
The intensity waveform I ₀ of the sampling pulse after exiting 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 166 fs. FIG. 4B shows an autocorrelation waveform corresponding to the approximated waveform with a white circle superimposed on the measured value.

Here, it should be noted that the width of the sampling pulse after emission from the optical fiber is actually twice as large as that immediately after emission from the short-pulse laser resonator 102. This is nothing but the effect of the chromatic dispersion of the optical fiber. In this way, the waveform I ₀ (t) of the sampling pulse and, consequently, its Fourier transform I ′
Knowing ₀ (t), the decorrelation calculation in accordance with equation (2) can be performed for the subsequent waveform measurement of the optical signal train.

Note that the vertical axis in FIG. 4B is scaled on a logarithmic scale. This indicates that the short-pulse laser resonator 102 used in this example can generate a sampling pulse that has a sharply attenuated pulse tail and is very suitable for optical sampling measurement. At the same time, it is shown that the cross-correlator 107 used in this example can easily secure a dynamic range close to three digits. This is due to the non-coaxial incidence on the nonlinear crystal, FIG.
This is a feature of the cross-correlator having the configuration shown in FIG.

In the case of this example, the optical path from the input end 101 of the optical signal train to the cross-correlator 107 is also constituted by an optical fiber, and a fiber connector is attached to the input end 101 of the optical signal train. As a result, the optical signal sequence 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 the above, and the above-described analog circuit type was used as the frequency-voltage converter 108.

FIG. 5A shows that 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 electro-absorption modulator on a chip. It is known that this semiconductor laser generates a pulse train equal to the repetition of a sine wave applied to the modulator by an active mode locking operation. Therefore, the repetition frequency was known prior to the measurement from the reading of the frequency setting of the applied sine wave source.

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

The measurement time required for the averaging measurement in the upper stage is
30 s. Although the lower single-sweep waveform contains a lot of noise, it can be clearly seen in the upper-average measurement that it is reduced and the signal-to-noise ratio is greatly improved. Thus, according to the present embodiment, it was demonstrated that the fluctuation of the frequency offset ΔF was successfully suppressed, and the measurement sensitivity was improved by taking a long measurement time.

With respect to the averaging waveform in the upper part of FIG.
As a result of executing the decorrelation operation using the waveform of the sampling pulse obtained above, no significant difference was brought to the waveform. This is because the width of the optical signal train waveform in this measurement is
7.5 ps, which is a result of the sufficiently narrow sampling pulse width. Therefore, it can be considered that the averaging waveform in the upper part of FIG. 5A directly gives the intensity waveform I ₁ (t) of the measured optical signal sequence. FIG.
(B) shows the result of calculating an autocorrelation signal for the averaged waveform obtained according to the present embodiment and comparing it with an autocorrelation signal obtained by directly measuring the autocorrelation signal.

The measurement of the autocorrelation signal was performed by the same procedure as that performed on the sampling pulse. The calculated and measured values agree fairly well. In particular, it should be noted that the point that the tail of the correlation signal does not completely recover to zero completely matches. This is a result of proving that the true value is reflected up to the intensity slightly remaining before and after the pulse on the averaging waveform. That is, in the averaging waveform, even with an intensity of only 1.5% of the peak value, the measurement is realized with sufficient accuracy.

Thus, according to the present embodiment, measurement with much lower noise (signal-to-noise ratio> 100) was achieved. From a different point of view, the peak value of the present optical signal is about 12 mW, which means that the actual measurement can be performed with high accuracy 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 a sufficient margin without using an optical amplifier. That is, the present embodiment can easily measure the output light of a semiconductor light source that is currently regarded as the most practical in the field of optical communication and information processing without using an optical amplifier.

Therefore, the cost of the optical amplifier is reduced, and there is no concern about waveform distortion due to the optical amplifier, so that high measurement accuracy is guaranteed. As described above, the detailed measurement of the output waveform signal waveform greatly contributes 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 not in the calculated value. This does not indicate the lack of time resolution according to the present embodiment.

This spike is called a coherence spike, which indicates that the optical signal train has a fine structure that fluctuates statistically for each pulse. Such a microstructure is
In the waveform measurement by optical sampling as in the present embodiment,
Essentially undetectable. Because optical sampling is
This is because the measurement itself is an averaging measurement, and detects only an envelope in which the oscillating fine structure is leveled. As described above, the apparatus of the present embodiment stabilizes the frequency offset ΔF and realizes a highly sensitive and accurate optical signal waveform measuring method.

[0112]

According to the optical signal measuring method of the present invention, it is possible to perform measurement without suffering from waveform distortion due to the repetition frequency fluctuation of the optical signal sequence 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 can correspond to any repetition frequency that is practically high. In addition, the optical signal measurement method of the present invention can be performed universally without depending on the repetition frequency of the optical signal sequence to be measured,
Moreover, since it can be implemented at low cost, a great industrial effect can be obtained.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram illustrating a first configuration of an optical signal measurement method according to the present invention.

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

3A and 3B are diagrams illustrating a configuration example of a cross-correlator used in the configuration of the present invention, wherein FIG. 3A is an explanatory diagram illustrating a configuration using a sum frequency generation effect, and FIG. 3B is a semiconductor photodetector; It is explanatory drawing which shows the structure using two-photon transition in a middle.

4A and 4B are graphs showing characteristics of a short pulse light source used in an embodiment of the present invention. FIG. 4A is a graph showing a signal repetition frequency with respect to a multiple, and FIG. 4B is a graph showing a sampling pulse. 5 is a graph showing an autocorrelation signal of FIG.

FIG. 5 is a graph showing characteristics obtained according to the embodiment of the present invention, and FIG.
FIG. 5B is a graph showing a comparison between the measured value and the actual value of the autocorrelation signal calculated from the measured signal.

6A and 6B are explanatory diagrams showing the principle of the optical signal waveform measuring method and the configuration of the first conventional example. FIG. 6A is a general explanatory diagram of the optical signal waveform measuring method by optical sampling, and FIG. () Is an explanatory diagram showing the configuration of the 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]

 Reference Signs List 101 signal light incident end 102 short pulse laser resonator 103 resonator length fine adjustment mechanism 104 resonator length coarse adjustment mechanism 105, 106 reflecting mirror 107 cross-correlator 108 frequency-voltage converter 109 reference voltage 110 integration 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 mirror 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 light converter 216 Reflector mirror 217 Cross correlator 307 Cross correlator 320, 321 Reflector mirror 322 Right angle reflection prism 323 Converging lens 324 Nonlinear crystal 325 Aperture 326 Condenser lens 327 Light detection 328 Light detection 602 Short pulse laser resonator 604 Resonator length coarse adjustment mechanism 605, 606 Reflector mirror 607 Cross correlator 612 Waveform recorder 616 Reflector mirror 630 Signal source laser 631 Reflector mirror 701 Signal light incident end 702 Short pulse laser resonator 703 Resonator Long fine adjustment mechanism 704 Resonator length coarse adjustment mechanism 705,706 Reflector mirror 707 Cross-correlator 710 Integrator 711 Drive circuit 712 Waveform recorder 713,714 Branch mirror 716 Reflector mirror 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)

[Claims] An optical signal train to be measured and an optical pulse train generated by a short-pulse laser are incident on a cross-correlator, and multiplexed and focused in a medium having an optical nonlinear effect in the cross-correlator. In the optical signal waveform measuring method for converting a generated nonlinear signal into an electric signal and measuring and recording the magnitude of the electric signal in a time-series manner, a method for adjusting the optical length of a resonator to the short-pulse laser. An optical signal waveform measuring method, comprising: providing a device length adjusting mechanism, converting the repetition frequency of the electric signal into a voltage value, and driving the resonator length adjusting mechanism so that the voltage value becomes a constant value. . 2. An optical signal sequence to be measured and an optical pulse sequence generated by a short-pulse laser are made incident on a cross-correlator, where they are multiplexed and focused in a medium having an optical nonlinear effect. A non-linear signal to be converted into an electric signal, and the magnitude of the electric signal is measured and recorded in a time-series manner. Providing a length adjustment mechanism, bisecting each of the two pulse trains before reaching the cross-correlator,
Copies of each of the two obtained pulse trains are incident on a separate cross-correlator, where the two copies are combined and focused in a medium having an optical nonlinear effect. The 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, based on the electric signal. And determining an origin of time-series measurement and recording of the magnitude of the electric signal.