JPH09133585A - Optical pulse train measuring method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for acquiring the phase information of autocorrelation with relatively high sensitivity even at the time of low intensity of light without substantially limiting the measurable wavelength range. SOLUTION: A light of frequency f0 emitted from a light source 1 is introduced to an interferometer 2 having two optical paths, e.g. a Mach-Zehnder(MZ) interferometer. The two optical path interferometer 2 has a roll of providing an optical path length difference and a roll for enabling heterodyne detection by imparting a frequency difference to the light between two optical paths. A light transmitted through the two optical path interferometer 2 is converted through a light receiver 3 into an electric signal which is then processed for each spectrum through an intensity analyzer 4 thus obtaining an autocorrelation. According to the method, not only a normal frequency difference component is taken out but the components, produced by adding the frequency difference to integer times of modulation frequency (repetitive frequency) of light source or subtracting the frequency difference therefrom, are measured simultaneously thus completing the information.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an autocorrelation measuring method for optical pulses or modulated light.

[0002]

2. Description of the Related Art A method using the second harmonic of a non-linear material is well known for measuring the autocorrelation of light, and has become a standard method for evaluating the pulse width (new edition of Laser Handbook, Asakura Shoten, p. 298 (1989)). However, in this method, the pulse width can be determined fairly accurately, but phase information, that is, information such as chirping is difficult to obtain. In addition, there is a wavelength region that cannot be measured because a non-linear material is required, and since the non-linear effect is used, the sensitivity is low at low light intensity.

On the other hand, evaluation of the pulse waveform by the self-heterodyne method is considered. This method, in an interferometer consisting of two optical paths, attempts to evaluate the pulse width by giving a frequency difference to the light in the two optical paths by a frequency converter and measuring the difference frequency component in the interference signal. It is not possible to obtain an accurate pulse width, but it is only a guide.

[0004]

To summarize the above problems, it is difficult to obtain phase information by the method using the second harmonic, and the accurate pulse width cannot be obtained by the self-heterodyne method. That is.

Therefore, it is an object of the present invention to provide a method for extracting information on the phase of the autocorrelation of an optical pulse and a method for accurately determining the pulse width in the self-heterodyne method.

[0006]

Since the purpose is to extract phase information, it is characterized by measuring the autocorrelation of the electric field. (The method using the second harmonic measures the autocorrelation of the light intensity.) The measurement system consists of a light source that emits light pulses or modulated light, an interferometer consisting of two optical paths,
A system in which a light receiver and a device for detecting a signal for each frequency are connected in series, and an optical delay device is provided in at least one optical path of an interferometer consisting of two optical paths, and a frequency converter is provided in at least one optical path of the interferometer. And the frequency converter gives a frequency difference to the light in the two optical paths. The difference frequency component exists in the detected signal component, and the amplitude of the component gives the autocorrelation of the electric field (self-heterodyne method). However, it is not possible to extract all the information of the autocorrelation only with the difference frequency component. Therefore, in the present invention, the autocorrelation information is made sufficient by taking out the frequency component obtained by adding the difference frequency to an integer multiple of the modulation frequency of the light source or the frequency component subtracted therefrom.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a basic measurement system. Light having a frequency f ₀ emitted from a light source 1 is guided to an interferometer 2 having two optical paths such as a Mach-Zehnder (MZ) interferometer. The two optical path interferometer 2 has a role to give an optical path difference and a frequency difference to the light in the two optical paths to enable heterodyne detection. The light that has passed through the two-optical path interferometer 2 is converted into an electric signal by the photodetector 3 and is subjected to signal processing for each spectrum by the frequency-specific intensity analyzer 4 to obtain an autocorrelation.

The light source 1 generates an optical pulse or modulated light, and is a mode-synchronized laser, an amplitude-modulated laser, a phase-modulated laser, or the like. The light source 1 is also achieved by a combination of a continuous wave laser and an external modulator for amplitude or phase modulation.

The basic structure of the interferometer 2 is shown within a dotted line.
The optical path is divided into two by the Harm mirror 26. The frequency converter 21 gives a frequency difference f ₁ to the light in the two optical paths. The frequency converter 21 is realized by an acousto-optic element, but can also be realized by a non-linear optical element, an element by a half-wave plate rotation system, a wavelength conversion semiconductor laser element, or the like. 2
Reference numeral 2 is an optical delay device for adjusting the difference in optical path length and gives a delay time τ. The light that has passed through the two optical paths 23 and 24 is recombined by the half mirror 27 and guided to the light receiver 3.

The two-path interferometer 2 can be modified in various ways, one of which is shown in FIG. In FIG. 2, two frequency converters 21 are used so that the effect of dispersion generated in the frequency converter 21 is equalized in the two optical paths. Similarly in FIG. 3, the effect of dispersion generated in the frequency converter 21B is equalized in the two optical paths, and the direction of the optical beam emitted from the frequency converter 21B is different depending on the frequency of light. By utilizing the role of the Haugh Mira-26 and the frequency converter 2
It has one role at the same time.

The structure of the intensity analyzer for each frequency 4 is shown in FIG. Bandpass filter-41A, B, C, D,
An integral multiple nf ₂ of the modulation (repetition) frequency f ₂ of the light source 1 is added to the f ₁ component of the frequency difference of the light on the two optical paths, f ₁ (f ₁ +
nf ₂ ) component is taken out independently, and detectors 42A, B,
The strength of each component is extracted by C and D. Here, n represents an arbitrary integer. The information of the autocorrelation to be obtained is the detector 42
It is obtained by processing the outputs of A, B, C and D by the arithmetic unit 43.

In the above-mentioned embodiment, the optical path is in the air and the splitting / combining of light is performed by the Hafmirers 26 and 27. However, it is also possible to use an optical fiber in the optical path. -26 and 27 are replaced with optical couplers.

(Embodiment 1) As a light source 1, a mode synchronization laser is used.
Optical path 23 when using the- (repetition f ₂ = 1 / T)
Optical pulse trains 101 and 102 at and 24 are shown in FIG. Here, T represents the period of the optical pulse train. The optical pulse train on the optical path 25 is a combination of 101 and 102. The delay time τ is adjusted by the optical delay device 22.

FIG. 6 shows the output waveform of the intensity analyzer for each frequency 4. When the delay time τ is changed by the optical delay device 22,
The degree of overlap between the two optical pulse trains changes, and an autocorrelation waveform that reflects this is obtained. Autocorrelation waveform is detected by detector 4
2A, B, C, D output. From the outputs of the detectors 42A, B, C and D, the signal f ₁ · (f ₁ +
The f ₂ ) (f ₁ + 2f ₂ ) (f ₁ + nf ₂ ) component is obtained.
Here, n represents an arbitrary integer. If the mode synchronization is perfect, the normalized waveforms of any of the above components are in agreement. However, when the mode synchronization is incomplete and chirping is present, the peak position is different for each component (Δτ ≠ 0), and the chirp amount can be evaluated from this difference.

The pulse width of the mode-synchronized laser light can be obtained from a waveform obtained by adding the autocorrelation waveforms of the above components with weights according to the power spectrum intensity. If the mode synchronization is perfect, the pulse width can be determined only by measuring the f ₁ component.

(Embodiment 2) FIG. 7 shows the autocorrelation waveform of the f ₁ component when the light source 1 is a single mode laser to which phase modulation is applied. The vibration waveform in FIG. 7 is described by the Bessel function of order 0, and the phase modulation degree, that is, the amount of chirp is obtained from the vibration period. Similarly, the autocorrelation waveform of the (f ₁ + nf ₂ ) component is described by the n-th order Bessel function. FIG.
Shows the case where the modulation frequency is 1.5 GHz and the chirp amount is 22 GHz.

When phase modulation is applied to the laser, amplitude modulation generally occurs at the same time. If the amplitude modulation degree is measured at this time, the α parameter in the semiconductor laser can be determined by combining it with the phase modulation degree.

When the amplitude modulation degree is large, the laser light becomes a pulse, but the pulse width is measured by measuring the autocorrelation waveforms of all (f ₁ + nf ₂ ) components and adding them with weights according to the spectrum intensity. It can be obtained from the width of the combined waveform.

(Embodiment 3) In Embodiments 1 and 2, the measurement concerning the property of the light source itself was shown, but the same measurement can be carried out for elements other than the light source. FIG. 8 shows a measurement system to which the dispersion medium 51 is added. When a mode synchronous laser is used as a light source, the light pulse is chased due to the dispersion of the medium 51.
Ping occurs, and the (f ₁ + nf ₂ ) -order autocorrelation waveform is n
Causes a peak shift depending on. This phenomenon provides a very accurate dispersion measurement method.

(Embodiment 4) FIG. 9 shows a measurement system in which the dispersion medium 51 is set in the interferometer 2. This system provides a very accurate dispersion measurement method. Mode synchronization laser as a light source
When the laser is used, chirping occurs in the optical pulse due to the dispersion of the medium 51, and a peak shift depending on n occurs in the (f ₁ + nf ₂ ) -order autocorrelation waveform, and the dispersion can be measured. .

Two lasers (center wavelengths λ ₁ and λ ₂ )
There is also a method using. These lasers are assumed to be modulated at the frequency f ₂ and may or may not be mode-synchronized. First center wavelength lambda ₁ of les as the light source 1 - The - measuring the autocorrelation waveform using. Next light source 1
Is replaced with a laser having a center wavelength λ _{2 and} a similar autocorrelation waveform is measured. The peak position of the autocorrelation waveform differs when the wavelength is λ ₁ and λ ₂ , and the dispersion can be measured from this difference.

(Embodiment 5) Medium 5 in FIGS. 8 and 9
If a non-linear medium is selected as 1, the amount of self-phase modulation caused by the non-linear effect, that is, the higher-order refractive index can be measured.

(Embodiment 6) FIG. 10 shows a measurement system including a modulator 52. Dispersion generated in modulator 52 by this system
Chirping can be measured.

[0024]

As described above, according to the present invention, the phase information of the light autocorrelation, that is, the light source itself, the modulator, the dispersion medium,
It enables highly accurate measurement of chirping, dispersion, phase shift, high-order refractive index, etc. of nonlinear media.

Further, according to the present invention, the width of the optical pulse can be accurately measured even in the heterodyne method. Since the method according to the invention does not use non-linear effects, the measurable wavelength range is determined only by the sensitivity range of the detector, giving a very wide measuring range. Also, unlike the method using the second harmonic, the sensitivity is relatively high even at low light intensity.

[Brief description of the drawings]

FIG. 1 shows a basic measuring system of an autocorrelation measuring method according to the present invention.

FIG. 2 is a two-path interferometer using two frequency converters.

FIG. 3 is a two-path interferometer in which a frequency converter also has a half mirror function.

FIG. 4 is a configuration diagram of a frequency-specific intensity analyzer 4.

FIG. 5 is a pulse train of two optical paths in a two-path interferometer.

FIG. 6 is an autocorrelation waveform of the f ₁ component and the (f ₁ + nf ₂ ) component of the received light signal of the mode synchronization pulse train.

FIG. 7 is an autocorrelation waveform when phase modulation is applied to a single mode laser.

FIG. 8: Measurement system for evaluating a dispersion medium.

FIG. 9 shows a measurement system in which a dispersion medium is incorporated in a two-path interferometer.

FIG. 10: Measurement system for evaluating a modulator.

[Explanation of symbols]

1: Light source 2: 2 Optical path interferometer 3: Light receiver 4: Frequency intensity analyzer 21: Frequency converter 21B: Frequency converter 2
2: Optical delay device 23, 24, 25: Optical path 26, 27: Harm mirror 41A, B, C, D: Band pass filter 42
A, B, C, D: Detector 43: Operator 51: Dispersion medium 52: Modulator 53: Transmitter 101: Pulse train of optical path 23 102: Pulse train of optical path 24

Claims (6)

[Claims] 1. A system in which a light source for generating an optical pulse or modulated light, an interferometer consisting of two optical paths, a light receiver, and a device for detecting a signal for each frequency are connected in series, and interference consisting of two optical paths. An optical delay device for changing the optical path length is provided in at least one optical path of the interferometer, and a frequency converter is provided in at least one optical path of the interferometer, and a frequency difference is generated between the light in the two optical paths by the frequency converter. An optical pulse train measuring method for obtaining an autocorrelation of light by measuring the difference frequency component of the received light signal and the frequency component of the sum or difference of the integer multiple of the modulation frequency and the difference frequency. 2. A method for measuring an optical pulse train, wherein the light source according to claim 1 is composed of a single light source or an external modulator. 3. A method of measuring an optical pulse train, wherein the light modulation according to claim 1 is performed by the light source itself or the external modulator according to claim 2. 4. The optical pulse train measuring method according to claim 1, wherein the pulse width of the optical pulse is obtained by arithmetic operation from the obtained autocorrelation. 5. The optical pulse train measurement method according to any one of claims 1 to 3, wherein chirping, dispersion, and phase shift generated in the light source are obtained from the obtained autocorrelation. Method. 6. The optical pulse train measuring method according to claim 1, wherein the optical element or the optical sample is set between the light source and the interferometer consisting of two optical paths or inside the interferometer consisting of two optical paths. Chirping, dispersion,
A method for measuring an optical pulse train, which comprises measuring a phase shift.