JP2005241284A - Chromatic dispersion measuring apparatus and measuring method - Google Patents

Abstract

Disclosed is a chromatic dispersion measuring apparatus and method capable of measuring a wavelength region longer than 1.7 μm in a short time without being affected by vibration or temperature fluctuation.
An ultrashort optical pulse light source 1 for generating an ultrashort optical pulse, a measured medium 2 for propagating the ultrashort optical pulse generated from the ultrashort optical pulse light source 1, and the measured medium 2 Two-photon absorption detection means 3 for receiving the propagated ultrashort light pulse and outputting a two-photon absorption signal, and from the two-photon absorption signal output from the two-photon absorption detection means, the measured medium 2 A chromatic dispersion measuring device for measuring the chromatic dispersion of a liquid. The measurement principle is different from the conventional optical pulse method, interferometry, and phase shift method, and high-precision measurement is possible without being affected by vibration or temperature fluctuation. In addition, since it is not necessary to adjust the time difference, high-speed measurement is possible, and the propagation distance z of the measured medium may be short, so that a short measured medium can be measured. Furthermore, since the chromatic dispersion is measured from the two-photon absorption signal, the chromatic dispersion in a long wavelength region of ˜1.7 μm or more can be measured.
[Selection] Figure 1

Description

  The refractive index of an optical medium such as an optical fiber changes depending on the wavelength, and every optical medium has a wavelength dispersion that is more or less. Chromatic dispersion is an important parameter that causes chirp and waveform distortion in optical fiber communication and ultrashort optical pulse generation. The present invention relates to an apparatus and method for measuring this chromatic dispersion.

As a chromatic dispersion measuring method, various methods such as an optical pulse method, an interference method, and a phase shift method have been known so far. For example, as a chromatic dispersion measuring device using the interferometry, an optical pulse having a different wavelength is branched into a reference light and a measuring light, and the measuring light affected by the dispersion by the measured medium is interfered with the reference light to cause interference. It is known that the chromatic dispersion is measured by obtaining the group velocity delay time from the optical path difference of the interfering arm that maximizes (see, for example, Patent Document 1). However, since the interference contrast is affected not only by the optical path difference of the interference arm but also by vibrations and temperature fluctuations, it has been difficult to measure with high accuracy. In addition, in order to increase the contrast of interference, it is necessary to adjust the optical path difference between the interference arms, that is, the time difference passing through both interference arms, and it takes a long time to measure chromatic dispersion. The semiconductor photo detector that receives an optical pulse and converts it into an electrical signal has a detectable wavelength band limited by the band gap, and is not sensitive to long wavelength light of ˜1.7 μm or more. It was difficult to measure chromatic dispersion in a long wavelength region of ˜1.7 μm or more. Furthermore, since there is no light source that generates a wavelength-tunable light pulse having a wavelength longer than about 1.7 μm, measurement of wavelength dispersion of about 1.7 μm or more is made more difficult.
Japanese Patent No. 3346595

  As mentioned in the background art above, conventional chromatic dispersion measuring devices are difficult to perform with high precision because they are affected by vibration and temperature fluctuations, and it takes a long time to measure because it is necessary to adjust the time difference. There was a problem that dispersion measurement in a wavelength region longer than 1.7 μm was difficult.

  The present invention has been created to solve such problems. That is, an object of the present invention is to provide a chromatic dispersion measuring apparatus and method capable of measuring a wavelength region longer than 1.7 μm in a short time without being affected by vibration or temperature fluctuation.

  The chromatic dispersion measuring device of the present invention made to solve this problem is an ultrashort optical pulse light source that generates an ultrashort optical pulse, and propagates the ultrashort optical pulse generated from the ultrashort optical pulse light source. Two-photon absorption detection means for receiving the ultrashort light pulse propagated through the measurement medium and outputting a two-photon absorption signal, and outputting two-photon absorption detection means. The wavelength dispersion of the measured medium is measured from the photon absorption signal.

The two-photon absorption signal is proportional to the time integral value of the square intensity of the ultrashort light pulse that has propagated through the measured medium, the ratio (z / L _D ) between the propagation distance z and the dispersion length L _D of the measured medium, and a predetermined value. Therefore, the chromatic dispersion of the measured medium can be obtained from the pulse width To and propagation distance z of the ultrashort light pulse before propagating through the measured medium. Therefore, the measurement principle is different from the conventional optical pulse method, interferometry, and phase shift method, and high-precision measurement is possible without being affected by vibration or temperature fluctuation. In addition, since it is not necessary to adjust the time difference, high-speed measurement is possible, and the propagation distance z of the measured medium may be short, so that a short measured medium can be measured. Furthermore, since the chromatic dispersion is measured from the two-photon absorption signal, the chromatic dispersion in a long wavelength region of ˜1.7 μm or more can be measured.

  The ultrashort optical pulse light source may generate ultrashort optical pulses having different wavelengths.

  Chromatic dispersion can be measured for each different wavelength.

  Further, the ultrashort optical pulse light source may generate an ultrashort optical pulse having a wavelength range of 1.56 μm to 1.9 μm.

  It is possible to measure chromatic dispersion in a long wavelength region of 1.56 μm to 1.9 μm, which has been difficult to measure until now.

  The ultrashort light pulse light source is preferably a fiber laser.

  The chromatic dispersion measuring device can be downsized.

  The ultrashort light pulse is preferably a hyperbolic second type or Gaussian type Fourier limit pulse or a pulse that can be approximated thereto, and more preferably, for example, a femtosecond soliton pulse.

  The two-photon absorption signal becomes strictly proportional to the theoretical value, and the chromatic dispersion measurement accuracy is improved.

  The two-photon absorption detection means may be a semiconductor photodetector.

  The wavelength dispersion measurement wavelength region can be expanded to ˜3.4 μm (= ˜1.7 μm × 2). In addition, high-speed measurement is possible.

  The two-photon detection means may be a combination of an SHG crystal and a power meter.

  Even if the measurement medium has a large absorption loss, two-photon absorption detection can be performed.

  Further, the apparatus may further comprise a light switching means for switching the ultrashort light pulse generated from the ultrashort light pulse light source to a reference light pulse and a measurement light pulse that is incident on the measured medium.

  It is not necessary to separately detect a two-photon absorption signal when there is no medium to be measured, and the chromatic dispersion measurement time can be shortened.

  You may make it further have a spatial coherence regulator which makes the spatial coherence of the measurement light pulse which propagated the said to-be-measured medium and the said reference light pulse the same.

  The measurement accuracy can be improved by removing the influence of the difference in spatial coherence (beam diameter and divergence angle) between the reference light pulse and the measurement light pulse received by the two-photon absorption signal.

  An optical demultiplexer for demultiplexing the reference light pulse and the measurement light pulse propagated through the measured medium; and receiving the reference light pulse and the measurement light pulse demultiplexed by the demultiplexer. And a one-photon absorption detector that outputs a one-photon absorption signal.

  The two-photon absorption signal is affected by the transmission loss and coupling loss of the measured medium, and the influence can be corrected by the output of the one-photon absorption detector.

  Further, the chromatic dispersion measuring method of the present invention for solving the above-mentioned problems is generated in an ultrashort optical pulse generation step for generating an ultrashort optical pulse from an ultrashort optical pulse light source, and in the ultrashort optical pulse generation step. A measured medium propagation step for propagating the ultrashort light pulse to the measured medium, and the two-photon absorption means receiving the ultrashort light pulse propagated through the measured medium in the measured medium propagation step. A two-photon absorption detection step for outputting an absorption signal, and measuring the chromatic dispersion of the measured medium from the two-photon absorption signal detected in the two-photon absorption detection step.

  The ultrashort light pulse light source may generate ultrashort light pulses having different wavelengths.

  Further, the ultrashort optical pulse light source may generate an ultrashort optical pulse having a wavelength range of 1.56 μm to 1.9 μm.

  The ultrashort light pulse light source is preferably a fiber laser.

  The ultrashort light pulse is preferably a hyperbolic second type or Gaussian type Fourier limited pulse.

  The two-photon detection means may be a semiconductor photodetector.

  The two-photon detection means may be a combination of an SHG crystal and a power meter.

  It is preferable to further include a light switching step of switching the ultrashort light pulse generated from the ultrashort light pulse light source to a reference light pulse and a measurement light pulse to be incident on the measured medium by a light switching means.

  Further, it is preferable to further include a spatial coherence adjusting step for making the spatial coherence of the measurement light pulse propagated through the measurement target medium the same as the reference light pulse.

An optical demultiplexing step for demultiplexing the reference light pulse and the measurement light pulse propagated through the measured medium, and receiving the reference light pulse and the measurement light pulse demultiplexed in the demultiplexing step. And a one-photon absorption detection step for outputting a one-photon absorption signal.
(Operation of the invention)
When an optical pulse of light intensity I (t, 0) propagates through a dispersion medium with chromatic dispersion β ₂ , the pulse width increases according to the propagation distance, and when propagating a distance z, I (t, z) is obtained (see FIG. 4). ). Here, t represents the time measured in the coordinate system traveling with the pulse at the county velocity. Therefore, it can be seen that I (t, z) has information on chromatic dispersion.

Assuming that the wavelength of the optical pulse is λ, the angular frequency is ω (= λ / 2π), and the pulse width is T ₀ , the electric field amplitude U (t, 0) of the optical pulse before propagation, and the electric field of the optical pulse after propagation The amplitude U (t, z) is
U (t, 0) = (1 / 2π) ∫U (ω, 0) exp (−iωt) dω (1)
U (t, z) = (1 / 2π) ∫U (ω, z) exp (-iωt) dω (2)
It is expressed. Where U (ω, 0) is the Fourier transform of U (t, 0), U (ω, z) is the Fourier transform of U (t, z),
U (ω, 0) = ∫U (t, 0) exp (iωt) dt (3)
U (ω, z) = ∫U (t, z) exp (iωt) dt = U (ω, 0) exp (iβ ₂ ω ² z / 2) (4)
It is expressed. If the optical pulse is Gaussian,
U (t, 0) = exp (-t ² / 2T ₀ ) (5)
For hyperbolic second type (sech type)
U (t, 0) = sech (t / T ₀ ) (6)
It is expressed. Further, the extent to which the light pulse propagates through the dispersion medium is determined by the dispersion distance L _D defined by the following equation (translated by Takashi Odaki and Koichi Yamada “Nonlinear Fiber Optics”, Yoshioka Shoten Publishing, May 25, 1997, p67-78).

L _D = T ² ₀ / | β ₂ | (7)
Although the integration range in equations (1) to (4) is omitted, it is −∞ to ∞.

Since the pulse intensity is proportional to the square of the electric field amplitude, the light intensity I (t, 0) before propagation and the light intensity I (t, z) after propagation are calculated from the equations (1) and (2) as I ( t, 0) ∝ | (1 / 2π) ∫U (ω, 0) exp (-iωt) dω | ² (8)
I (t, z) ∝ | (1 / 2π) ∫U (ω, z) exp (-iωt) dω | ² (9)
It is expressed.

On the other hand, since the two-photon absorption signal is proportional to the integral value of the squared intensity of the pulse, the two-photon absorption signal D ₂ (z) when the light pulse propagated through the measured medium is received and the light not propagated through the measured medium. The two-photon absorption signal D ₂ (0) when receiving a pulse (equivalent to the light pulse before propagating through the measured medium, ie, the reference light pulse) is D ₂ (z) ∝∫I ² (t, z ) dt (10)
D ₂ (0) ∝∫I ² (t, 0) dt (11)
It is expressed.

When the optical pulse is a sech type, the relationship between D ₂ (z) / D ₂ (0) and z / L _D using the above equations (1) to (4) and (6) to (11) Is simulated, the relationship shown in FIG. 5 is obtained. Since z / L _{D on the} horizontal axis is a dimensionless parameter and equal to | β ₂ | z / T ² ₀ , z / L _D is obtained by measuring the two-photon absorption signal, and the propagation distance z and the light before propagation are obtained. The dispersion value | β ₂ | is obtained from the pulse width T ₀ of the pulse.

  As described above, the chromatic dispersion measuring apparatus of the present invention measures the chromatic dispersion from the two-photon absorption signal of the optical pulse propagated through the measured medium, so that the conventional optical pulse method, interference method, phase shift method and measurement principle are used. However, it is possible to measure with high accuracy without being affected by vibration and temperature fluctuation, and to measure a long wavelength region of up to 1.7 μm or more. In addition, since it is not necessary to adjust the time difference, high-speed measurement is possible, and the propagation distance z of the measured medium may be short, so that a short measured medium can be measured.

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

  FIG. 1 is a schematic configuration diagram of a chromatic dispersion measuring apparatus showing an embodiment of the present invention. As shown in this figure, the chromatic dispersion measuring apparatus includes an ultrashort light pulse light source 1 and a measurement light pulse that causes the ultrashort light pulse generated from the ultrashort light pulse light source 1 to enter the reference light pulse and the measured medium 2. The optical switching means 4 for switching to the reference light pulse, the spatial coherence adjuster 5 for making the spatial light coherence of the measurement light pulse propagated through the measured medium 2 the same as the reference light pulse, and the measurement light pulse propagated through the reference light pulse and the measured medium 2 , A one-photon absorption detector 7 that receives the reference light pulse and the measurement light pulse that are demultiplexed by the optical demultiplexer 6 and outputs a one-photon absorption signal, Two-photon absorption detection means 3 that receives the reference light pulse and measurement light pulse that have passed through the duplexer 6 and outputs a two-photon absorption signal; and processing means that inputs and processes the two-photon absorption signal and the one-photon absorption signal 8. Here, the basic components are an ultrashort optical pulse light source 1, a measured medium 2, and a two-photon absorption detection means 3. For example, even if the light switching means 4 is not provided, the measured medium 2 is inserted between the ultrashort optical pulse light source 1 and the two-photon absorption detecting means 3 to detect the two-photon absorption, and then the measured medium 2 is removed to remove the two-photon absorption. By detecting photon absorption, chromatic dispersion can be measured.

  The ultrashort optical pulse light source 1 desirably generates ultrashort optical pulses having different wavelengths. If an ultrashort optical pulse having a single wavelength is generated, a plurality of ultrashort optical pulse light sources are required. As an ultrashort optical pulse light source that generates ultrashort optical pulses with different wavelengths, for example, an ultrashort optical pulse from a femtosecond fiber laser is used to modulate the intensity of the ultrashort optical pulses and enter the fiber to generate soliton pulses with different wavelengths. be able to. This tunable ultrashort optical pulse light source is disclosed in Japanese Patent No. 3390755, and the amount of Raman scattering shift is changed by changing the intensity of the ultrashort optical pulse incident on the fiber, which is longer than the incident ultrashort optical pulse. The optical pulse shifted to the wavelength side becomes an ultrashort pulse by the soliton effect by the self-phase modulation and chromatic dispersion of the fiber.

  The medium to be measured 2 may be any medium that transmits the ultrashort light pulse generated from the ultrashort light pulse light source 1, and the length is not particularly limited.

  As the two-photon absorption detection means 3, an avalanche photodiode (APD), a PIN photodiode, or the like can be used. A combination of an SHG crystal and a power meter may be used. Even when the measured medium 2 has a large attenuation, two-photon absorption can be detected.

  The light switching means 4 is composed of 45 ° rotating mirrors 41 and 41 ′, and the mirrors 41 and 41 ′ are both set to the positions of the solid lines, so that the ultrashort light pulse does not propagate through the measured medium 2 and the reference light pulse. It becomes. By setting both the mirrors 41 and 41 ′ to the dotted line position, the ultrashort light pulse becomes a measurement light pulse that propagates through the measured medium 2. The mirrors 41 and 41 'may be configured with a beam splitter. In this case, the reference light pulse and the measurement light pulse are divided by the time difference due to the optical path difference and detected by the two-photon absorption detection means 3. If the optical path difference is small, an optical delay line may be inserted in the optical path of the measurement light pulse. By using a beam splitter, there are no mechanically moving parts, and the reliability of the dispersion measuring apparatus is increased.

  The spatial coherence adjuster 5 makes the spatial coherence (beam diameter, divergence angle) of the reference light pulse and the measurement light pulse the same so that the two-photon absorption signal is influenced only by the dispersion of the measured medium 2. The optical fiber 51 has a length of about 5 cm, a condensing lens 52, and a collimator lens 52 ′. When wavelength dispersion or absorption loss of the optical fiber 51 becomes a problem, a spatial filter such as a pinhole may be used instead of the optical fiber 51.

  The optical demultiplexer 6 may be a combination of a wave plate and a polarizing beam splitter, but a half mirror that reflects part of incident light and transmits the remaining part is preferable. The reflectance of the half mirror is set so that the one-photon absorption detector 7 detects one-photon absorption. That is, when an avalanche photodiode (APD), PIN photodiode, or the like is used as the one-photon absorption detector 7, the reflectance of the half mirror is set so that two-photon absorption does not occur. If two-photon absorption cannot be prevented only by adjusting the reflectance, an attenuation filter or the like may be inserted between the optical demultiplexer 6 and the one-photon absorption detector 7.

The processing means 8 is preferably a computer. By inputting the relationship between the two-photon absorption signal ratio D ₂ (z) / D ₂ (0) and z / L _D shown in FIG. 5 in advance, the two-photon absorption signal ratio is corrected by the one-photon absorption signal ratio. The dispersion value | β ₂ | can be obtained simply by inputting the propagation distance z and the pulse width T ₀ before propagation. Furthermore, by making the measured medium 2 a medium whose dispersion value is known, not only the absolute value of the dispersion value but also the sign can be obtained. When an oscilloscope is used as the processing means 8, the relative wavelength dependence of the dispersion value can be observed by scanning the wavelength of the ultrashort light pulse generated from the ultrashort light pulse light source 1. Then, the two-photon absorption signal ratio with an ultrashort optical pulse having a different wavelength may be corrected with the one-photon absorption signal ratio, and the relative value of the dispersion value may be calibrated using the relationship shown in FIG.

The two-photon absorption signal ratio D ₂ (z) / D ₂ (0) is defined as D ₁ (z) for the one-photon absorption signal of the measurement light pulse and D ₁ (0) for the one-photon absorption signal of the reference light pulse. Then
(D ₂ (z) / D ₂ (0)) / (D ₁ (z) / D ₁ (0)) ² (12)
It is corrected by.

  FIG. 1 is a schematic configuration diagram of a chromatic dispersion measuring apparatus according to the first embodiment of the present invention. FIG. 2 is a configuration diagram of the ultrashort optical pulse light source 1 of FIG.

  The ultrashort optical pulse light source 1 includes a laser 11 that generates an ultrashort optical pulse having a single wavelength, an intensity modulator 12 that modulates the intensity of the ultrashort optical pulse, and a constant polarization fiber 13 having a length of 100 m. Have. The intensity modulator 12 includes an acousto-optic modulation element 121 and a drive control microcomputer 122, and can continuously change the intensity of the passing ultrashort light pulse with a period of 1 ms. A half-wave plate 14 for adjusting the polarization axis of the constant polarization fiber 13 is inserted after the intensity modulator 12.

  The laser 11 is a commercially available fiber laser that generates an ultrashort optical pulse having a wavelength of 1.55 μm, a pulse width of 100 fs, a pulse repetition period of 50 MHz, and an average output of 60 mW. The ultrashort light pulse generated from the laser 11 is intensity-modulated by the intensity modulator 12, the polarization axis is adjusted by the half-wave plate 14, condensed by the lens 15, and incident on the constant polarization fiber 13. When an ultrashort light pulse with a wavelength of 1.55 μm is incident, a stimulated Raman scattered light pulse shifted to a longer wavelength side than 1.55 μm is induced according to the intensity of the pulse, and a soliton by self-phase modulation and group velocity dispersion The induced optical pulse is converted into an ultrashort optical pulse by the effect, and the sech2 type ultrashort optical pulse (soliton pulse) having a pulse width of 210 fs whose wavelength is changed from 1.62 μm to 1.90 μm from the polarization fiber 13. Is emitted. The wavelength of the emitted ultrashort light pulse is continuously scanned in the range of 1.62 to 1.90 μm with a sawtooth wave having a period of 1 ms. The emitted ultrashort light pulse is collimated by the lens 15 ′ and the incident ultrashort light pulse of 1.55 μm is cut by the optical filter 16 and then incident on the light switching means 4.

  The two-photon absorption detection means 3 is an avalanche photodiode, and the one-photon absorption detector 7 is a PIN photodiode. The processing means 8 is an oscilloscope.

  FIG. 3 shows a case where the medium 2 to be measured is an acousto-optic modulator (AMTIR (GeAsSe Glass)) having a length of 1 cm, a constant polarization fiber (PMF) having a length of 50 cm, and a dispersion shifted fiber (DSF) having a length of 50 cm. It is a chromatic dispersion measurement result. It was confirmed that the results measured by the conventional interferometry agreed well.

It is a schematic block diagram of the wavelength dispersion measuring apparatus which shows one Embodiment of this invention. 1 is a configuration diagram of an ultrashort optical pulse light source of a wavelength dispersion measuring apparatus according to Embodiment 1. FIG. 3 is a graph showing chromatic dispersion measured by the chromatic dispersion measuring apparatus of Example 1. It is a bird's-eye view explaining the phenomenon which an optical pulse spreads by propagating through a dispersion medium. It is a graph which shows the propagation dependence of two-photon absorption signal ratio.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Ultrashort light pulse light source 2 Medium to be measured 3 Two-photon absorption detection means 4 Light switching means 5 Spatial coherence adjuster 6 Optical demultiplexer 7 One-photon absorption detector 8 Processing means

Claims (14)

An ultrashort optical pulse light source that generates an ultrashort optical pulse;
A medium to be measured for propagating the ultrashort light pulse generated from the ultrashort light pulse light source;
Two-photon absorption detection means for receiving the ultrashort light pulse propagated through the measured medium and outputting a two-photon absorption signal;
And measuring the chromatic dispersion of the medium to be measured from the two-photon absorption signal output from the two-photon absorption detection means.   2. The chromatic dispersion measuring apparatus according to claim 1, wherein the ultrashort optical pulse light source generates ultrashort optical pulses having different wavelengths.   The wavelength dispersion measuring apparatus according to claim 1, wherein the ultrashort optical pulse light source generates an ultrashort optical pulse having a wavelength range of 1.56 μm to 1.9 μm.   4. The chromatic dispersion measuring apparatus according to claim 1, wherein the ultrashort light pulse is a hyperbolic second type or a Gaussian type.   5. The optical switching unit according to claim 1, further comprising: a light switching unit that switches the ultrashort light pulse generated from the ultrashort light pulse light source to a reference light pulse and a measurement light pulse that is incident on a measured medium. Chromatic dispersion measurement device.   6. The chromatic dispersion measuring apparatus according to claim 5, further comprising a spatial coherence adjuster that makes the spatial coherence of the measurement light pulse propagated through the measurement target medium the same as the reference light pulse. An optical demultiplexer for demultiplexing the reference light pulse and the measurement light pulse propagated through the measurement medium;
A one-photon absorption detector that receives the reference light pulse demultiplexed by the demultiplexer and the measurement light pulse and outputs a one-photon absorption signal;
The chromatic dispersion measuring device according to claim 5, further comprising: An ultrashort optical pulse generating step for generating an ultrashort optical pulse from an ultrashort optical pulse light source;
A measured medium propagation step for propagating the ultrashort light pulse generated in the ultrashort light pulse generation step to the measured medium;
A two-photon absorption detection step of receiving the ultrashort light pulse propagated through the measured medium in the measured medium propagation step by a two-photon absorption means and outputting a two-photon absorption signal;
And measuring the chromatic dispersion of the measured medium from the two-photon absorption signal detected in the two-photon absorption detection step.   9. The chromatic dispersion measuring method according to claim 8, wherein the ultrashort optical pulse light source generates ultrashort optical pulses having different wavelengths.   9. The chromatic dispersion measuring method according to claim 8, wherein the ultrashort optical pulse light source generates an ultrashort optical pulse having a wavelength range of 1.56 [mu] m to 1.9 [mu] m.   11. The chromatic dispersion measuring method according to claim 8, wherein the ultrashort light pulse is a hyperbolic second type or a Gauss type.   9. A light switching step of switching the ultrashort light pulse generated from the ultrashort light pulse light source to a reference light pulse and a measurement light pulse to be incident on a measured medium by an optical switching means. 11. The method for measuring chromatic dispersion according to 11.   The chromatic dispersion measuring method according to claim 12, further comprising a spatial coherence adjusting step for making the spatial coherence of the reference light pulse and the measurement light pulse propagated through the measured medium the same. An optical demultiplexing step for demultiplexing the reference light pulse and the measurement light pulse propagated through the measured medium;
A one-photon absorption detection step for receiving the reference light pulse and the measurement light pulse demultiplexed in the demultiplexing step and outputting a one-photon absorption signal;
The chromatic dispersion measuring method according to claim 12, further comprising:

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