Dispersion measurement device and dispersion measurement method

Abstract

To provide a dispersion measuring device and a dispersion measuring method for more accurately measuring the amount of wavelength dispersion of a measurement object.SOLUTION: A dispersion measuring device 1A comprises a pulse laser light source 2, a pulse formation unit 3, a correlator 4, and a computation unit 5b. The pulse formation unit forms, from a light pulse Pa outputted from the pulse laser light source, a light pulse train Pb that includes a plurality of light pulses Pb1, Pb2 mutually having a time difference and mutually differing in center wavelength. The correlator detects the time waveform of correlation light Pc formed from the light pulse train Pb. The computation unit 5b estimates, on the basis of the time waveform of the correlation light Pc, the wavelength dispersion amount of an optical component 7 arranged on the optical path between the pulse laser light source and the correlator. A dispersion medium 8 applies a positive or negative group delay dispersion to the light pulse train Pb so as to heighten the peak intensity of the correlation light Pc, making it greater than or equal to a detection threshold of the correlator. The pulse formation unit applies, to the light pulse Pa, a group delay dispersion which is of the reverse sign to the group delay dispersion applied to the light pulse train Pb.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a dispersion measurement device and a dispersion measurement method. [Background technology]

[0002] Patent Document 1 describes a dispersion measurement device and a dispersion measurement method capable of measuring the amount of chromatic dispersion of a pulsed laser light source. In these devices and methods, an optical pulse train including a plurality of optical pulses having a time difference and different center wavelengths is formed from an optical pulse to be measured output from a pulsed laser light source, the optical pulse train is incident on a correlation optical system, the correlation optical system outputs correlated light including cross-correlation or autocorrelation of the optical pulse train, the time waveform of the correlated light is detected, and the amount of chromatic dispersion of the pulsed laser light source is estimated from features of the detected time waveform of the correlated light. Furthermore, by inserting an object to be measured, such as an optical component, into the optical system, the amount of chromatic dispersion of the object to be measured can also be measured from the time waveform of the correlated light. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2020-169946 Summary of the Invention [Problem to be solved by the invention]

[0004] When measuring the amount of chromatic dispersion of an object to be measured, multiple optical pulses with different center wavelengths and a time difference between them are transmitted through the object to be measured. The amount of chromatic dispersion of the object to be measured can be estimated based on the time waveforms of the multiple optical pulses after passing through the object to be measured. Here, when the peak intensity of the optical pulses incident on the object to be measured is large, the accuracy of measuring the amount of chromatic dispersion may decrease. For example, when the object to be measured is a nonlinear medium such as an optical fiber or an optical waveguide, a nonlinear optical phenomenon occurs in the object to be measured when the peak intensity of the optical pulse exceeds a certain threshold. This phenomenon distorts the time waveform of the optical pulse. Furthermore, for example, an excessively large peak intensity of the optical pulse may damage the object to be measured. For these reasons, when the peak intensity of the optical pulses incident on the object to be measured is large, the amount of chromatic dispersion of the object to be measured may not be accurately measured based on the time waveforms of the optical pulses transmitted through the object to be measured.

[0005] On the other hand, if the peak intensity of the optical pulse incident on the measurement target is reduced, the peak intensity of the optical pulse may not reach the detection threshold of the detector that detects the time waveform of the optical pulse that has passed through the measurement target. In this case, the detection accuracy of the time waveform of the optical pulse decreases, and it becomes impossible to accurately measure the amount of chromatic dispersion of the measurement target based on the time waveform of the optical pulse.

[0006] An object of one aspect of the present invention is to provide a dispersion measurement device and a dispersion measurement method that enable more accurate measurement of the amount of chromatic dispersion of an object to be measured. [Means for solving the problem]

[0007] In order to solve the above-mentioned problems, a dispersion measurement device according to one aspect of the present invention includes a light source, a pulse shaping unit, a dispersive medium, a correlator, and a calculation unit. The light source outputs a first optical pulse. The pulse shaping unit has a spatial light modulator that provides a phase pattern for generating modulated light by imparting a predetermined phase shift for each wavelength to the first optical pulse. The pulse shaping unit forms, from the first optical pulse, an optical pulse train that is modulated light including multiple second optical pulses that have a time difference from one another and different center wavelengths. The dispersive medium has positive or negative group delay dispersion and receives the optical pulse train. The correlator includes a correlation optical system and a detector. The correlation optical system receives the optical pulse train that has passed through the dispersive medium and outputs correlated light including multiple third optical pulses that are a cross-correlation or auto-correlation of the optical pulse train. The detector detects the time waveform of the correlated light. The correlator detects the time waveform of the correlated light formed from the optical pulse train having a peak intensity equal to or greater than a detection threshold. The calculation unit is electrically connected to the correlator. A measurement object is placed on an optical path between the pulse shaping unit and the dispersive medium. The calculation unit estimates the amount of chromatic dispersion of the measurement object based on the time waveform of the correlated light. The dispersive medium imparts group delay dispersion to the multiple second optical pulses included in the optical pulse train, thereby increasing the peak intensity of the second optical pulses to above the detection threshold. The phase pattern includes a pattern for imparting group delay dispersion of an opposite sign to the group delay dispersion of the dispersive medium to the first pulses.

[0008] A dispersion measurement method according to one aspect of the present invention includes an output step, a pulse formation step, a dispersive medium transmission step, a detection step, and a calculation step. In the output step, a first optical pulse is output. In the pulse formation step, a spatial light modulator that provides a phase pattern for generating modulated light by imparting a predetermined phase shift for each wavelength to the first optical pulse is used to form an optical pulse train, which is modulated light including multiple second optical pulses having different center wavelengths and a time difference from each other, from the first optical pulse. In the dispersive medium transmission step, the optical pulse train passes through an object to be measured and then passes through a dispersive medium having positive or negative group delay dispersion. In the detection step, the optical pulse train that has passed through the dispersive medium is received, and correlated light including multiple third optical pulses that are cross-correlation or autocorrelation of the optical pulse train is generated, and the time waveform of the correlated light is detected. In the detection step, the time waveform of the correlated light formed from the optical pulse train having a peak intensity equal to or greater than a detection threshold is detected. In the calculation step, the amount of chromatic dispersion of the object to be measured is estimated based on the time waveform of the correlated light. The phase pattern includes a phase pattern for imparting to the first optical pulse group delay dispersion having an opposite sign to the group delay dispersion possessed by the dispersive medium.

[0009] A dispersion measurement device according to another aspect of the present invention includes a light source, a pulse forming unit, a correlation optical system, a dispersive medium, a detector, and a calculation unit. The light source outputs a first optical pulse. The pulse forming unit has a spatial light modulator that provides a phase pattern for generating modulated light by imparting a predetermined phase shift for each wavelength to the first optical pulse. The pulse forming unit forms, from the first optical pulse, an optical pulse train that is modulated light including multiple second optical pulses that have a time difference from one another and different center wavelengths. The correlation optical system receives the optical pulse train and outputs correlated light including multiple third optical pulses that are cross-correlated or auto-correlated with the optical pulse train. The dispersive medium has positive or negative group delay dispersion and receives the correlated light. The detector receives the correlated light that has passed through the dispersive medium and detects the time waveform of the correlated light that has a peak intensity equal to or greater than a detection threshold. The calculation unit is electrically connected to the detector. A measurement object is disposed on an optical path between the pulse forming unit and the correlation optical system. The calculation unit estimates the amount of chromatic dispersion of the measurement object based on the time waveform of the correlated light. The dispersive medium imparts group delay dispersion to the third optical pulses included in the correlated light, thereby increasing the peak intensity of the third optical pulses to be equal to or greater than the detection threshold. The phase pattern includes a pattern for imparting group delay dispersion of an opposite sign to the group delay dispersion of the dispersive medium to the first pulses.

[0010] A dispersion measurement method according to one aspect of the present invention includes an output step, a pulse formation step, a correlated light output step, a dispersive medium transmission step, a detection step, and a calculation step. In the output step, a first optical pulse is output. In the pulse formation step, a spatial light modulator is used to provide a phase pattern for generating modulated light by imparting a predetermined phase shift for each wavelength to the first optical pulse, thereby forming an optical pulse train from the first optical pulse, which is modulated light including multiple second optical pulses having different center wavelengths and a time difference from each other. In the correlated light output step, after the optical pulse train passes through an object to be measured, correlated light including multiple third optical pulses that are a cross-correlation or autocorrelation of the optical pulse train is output. In the dispersive medium transmission step, the correlated light passes through a dispersive medium having positive or negative group delay dispersion. In the detection step, the time waveform of the correlated light that has passed through the dispersive medium is detected. In the calculation step, the amount of chromatic dispersion of the object to be measured is estimated based on the time waveform of the correlated light. In the detection step, the time waveform of the correlated light having a peak intensity equal to or greater than a detection threshold is detected. In the dispersive medium transmission step, when the correlated light transmits through the dispersive medium, group delay dispersion is imparted to the plurality of third optical pulses included in the correlated light, thereby increasing the peak intensities of the plurality of third optical pulses to be equal to or greater than the detection threshold. The phase pattern includes a pattern for imparting group delay dispersion of an opposite sign to the group delay dispersion of the dispersive medium to the first optical pulses.

[0011] In these devices and methods, a positive or negative group delay dispersion is imparted to the first optical pulse in the pulse forming section (pulse forming step). This suppresses the peak intensity of the second optical pulse, thereby avoiding the influence of a high peak intensity of the second optical pulse on the second optical pulse in the object to be measured. For example, the peak intensity of the second optical pulse propagating through the object to be measured can be made below the threshold of a nonlinear optical phenomenon. After the second pulse passes through the object to be measured, the dispersive medium imparts group delay dispersion of the opposite sign to the group delay dispersion imparted to the first optical pulse to the second or third pulse. Therefore, the peak intensity of the second or third pulse after passing through the object to be measured can be made above the detection threshold of the correlator or detector. This enables the time waveform of the correlated light to be detected with high accuracy. From the above, the amount of chromatic dispersion of the object to be measured can be more accurately measured.

[0012] In the above-described device, the group delay dispersion provided by the phase pattern may be negative, and the group delay dispersion provided by the dispersive medium may be positive. Here, many types of dispersive media provide positive group delay dispersion. Therefore, with this device, more types of dispersive media can be selected as the dispersive medium in the device than when the group delay dispersion provided by the dispersive medium is negative.

[0013] In the above-described device, the phase pattern may be a phase pattern for imparting group delay dispersion to the first optical pulses such that the peak intensities of the plurality of second optical pulses are below a threshold of a nonlinear optical phenomenon in the measurement target. With this device, the occurrence of a nonlinear optical phenomenon is suppressed, and therefore distortion of the time waveform of the plurality of second optical pulses can be suppressed, and ultimately distortion of the time waveform of the correlated light can be suppressed. [Effects of the Invention]

[0014] According to a dispersion measurement device and a dispersion measurement method according to one aspect of the present invention, it is possible to more accurately measure the amount of chromatic dispersion of an object to be measured. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a diagram illustrating a schematic configuration of a dispersion measurement device according to an embodiment of the present invention. [Figure 2] FIG. 2 is a diagram illustrating a configuration example of a pulse forming section. [Figure 3] FIG. 2 shows the modulation surface of an SLM. [Figure 4] FIG. 1 shows an example of a phase pattern presented to an SLM. [Figure 5] FIG. 1 shows an example of a phase pattern presented to an SLM. [Figure 6] 1A, 1B, and 1C are diagrams showing an example of the relationship between group delay dispersion imparted to an optical pulse and the peak intensity of the optical pulse. [Figure 7] 10 is a diagram conceptually showing changes in peak intensity in the time waveform of an optical pulse. FIG. [Figure 8] 10A, 10B, and 10C are diagrams showing examples of band-controlled multi-pulses. [Figure 9] 10A, 10B, and 10C are diagrams showing examples of multi-pulses that are not band-controlled, as comparative examples. [Figure 10] FIG. 1 is a diagram schematically illustrating a correlation optical system for generating correlation light including autocorrelation of an optical pulse train, as an example of the configuration of the correlation optical system. [Figure 11] FIG. 10 is a diagram schematically illustrating a correlation optical system for generating correlation light including the cross-correlation of optical pulse trains, as another configuration example of the correlation optical system. [Figure 12] FIG. 10 is a diagram schematically illustrating a correlation optical system for generating correlation light including the cross-correlation of optical pulse trains, as yet another configuration example of the correlation optical system. [Figure 13] 1A and 1B are diagrams for conceptually explaining the feature quantities of correlated light, showing an example of the time waveform of correlated light when the chromatic dispersion of an optical component is zero, and an example of the time waveform of correlated light when the chromatic dispersion of an optical component is not zero. [Figure 14] FIG. 2 is a diagram illustrating an example of the hardware configuration of a control device. [Figure 15]1 is a flowchart showing a dispersion measurement method using the dispersion measurement device. [Figure 16] (a) The spectral waveform of a single optical pulse. (b) The temporal intensity waveform of an optical pulse. [Figure 17] (a) shows the spectral waveform of the output light from the pulse shaping section when a rectangular wave-shaped phase spectrum modulation is applied in the SLM. (b) shows the temporal intensity waveform of the output light from the pulse shaping section. [Figure 18] FIG. 1 is a diagram showing the configuration of a device that calculates a modulation pattern of an SLM. [Figure 19] FIG. 2 is a block diagram showing the internal configuration of a phase spectrum design unit and an intensity spectrum design unit. [Figure 20] FIG. 10 is a diagram illustrating a procedure for calculating a phase spectrum by an iterative Fourier transform method. [Figure 21] FIG. 10 is a diagram illustrating a procedure for calculating a phase spectrum function in a phase spectrum design unit. [Figure 22] FIG. 10 is a diagram showing a procedure for calculating the spectral intensity in the intensity spectrum design unit. [Figure 23] FIG. 10 is a diagram showing an example of a procedure for generating a target spectrogram in a target generation unit. [Figure 24] FIG. 10 is a diagram illustrating an example of a procedure for calculating an intensity spectrum function. [Figure 25] 1A and 1B are diagrams showing a spectrogram and a target spectrogram obtained by changing the spectrogram, respectively; [Figure 26] 1 is a diagram showing an example of intensity changes in the time waveform of an optical pulse in a dispersion measurement device according to an embodiment of the present invention. [Figure 27] 1 is a diagram showing an example of intensity changes in the time waveform of an optical pulse in a dispersion measurement device according to an embodiment of the present invention. [Figure 28] FIG. 10 is a diagram showing another configuration of a dispersion measurement device as a first modified example. [Figure 29] 10 is a flowchart showing a dispersion measurement method using the dispersion measurement device in the first modified example. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, embodiments of a dispersion measuring device and a dispersion measuring method according to the present invention will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are given the same reference numerals, and duplicated explanations will be omitted.

[0017] FIG. 1 is a diagram illustrating a schematic configuration of a dispersion measurement apparatus 1A according to one embodiment of the present invention. The dispersion measurement apparatus 1A is an apparatus for measuring the amount of chromatic dispersion of an optical component 7 to be measured, and includes a pulse laser light source 2 (light source), a pulse shaping unit 3, a dispersive medium 8, a correlator 4, and a control device 5. The optical input end 3a of the pulse shaping unit 3 is optically coupled to the pulse laser light source 2, either spatially or via an optical waveguide such as an optical fiber. The optical input end 7a of the optical component 7 is optically coupled to the optical output end 3b of the pulse shaping unit 3, either spatially or via an optical waveguide such as an optical fiber. The optical input end 8a of the dispersive medium 8 is optically coupled to the optical output end 7b of the optical component 7, either spatially or via an optical waveguide such as an optical fiber. The optical input end 4a of the correlator 4 is optically coupled to the optical output end 8b of the dispersive medium 8, either spatially or via an optical waveguide such as an optical fiber. The control device 5 is electrically connected to the pulse shaping unit 3 and the correlator 4. The correlator 4 includes a correlation optical system 40 and a detector 400. An optical output end 40b of the correlation optical system 40 is optically coupled to the detector 400 either spatially or via an optical waveguide such as an optical fiber. The control device 5 includes a control unit 5a, a calculation unit 5b, and an output unit 5c.

[0018] The pulsed laser light source 2 outputs a coherent optical pulse Pa (first optical pulse). The pulsed laser light source 2 is, for example, a femtosecond laser, and in one embodiment, is a solid-state laser light source such as a LD directly pumped Yb:YAG pulsed laser. The time waveform of the optical pulse Pa is, for example, a Gaussian function. The full width at half maximum (FWHM) of the optical pulse Pa is, for example, in the range of 10 fs to 10,000 fs, and in one example, is 100 fs. This optical pulse Pa is an optical pulse having a certain bandwidth and includes multiple continuous wavelength components. In one embodiment, the bandwidth of the optical pulse Pa is 10 nm, and the center wavelength of the optical pulse Pa is 1030 nm.

[0019] The pulse shaping unit 3 is a part that shapes, from an optical pulse Pa, an optical pulse train Pb that includes multiple optical pulses Pb1, Pb2 (multiple second optical pulses) that have a time difference from each other and different center wavelengths. The optical pulse train Pb is a group of single pulses generated using each wavelength band after dividing the spectrum that makes up the optical pulse Pa into multiple wavelength bands. Note that there may be overlapping portions at the boundaries between the multiple wavelength bands. In the following description, the optical pulse train Pb may also be referred to as a "band-controlled multi-pulse."

[0020] FIG. 2 is a diagram showing an example of the configuration of the pulse shaping unit 3. This pulse shaping unit 3 includes a diffraction grating 12, a lens 13, a spatial light modulator (SLM) 14, a lens 15, and a diffraction grating 16. The diffraction grating 12 is a dispersing element in this embodiment and is optically coupled to the pulse laser light source 2. The SLM 14 is optically coupled to the diffraction grating 12 via the lens 13. The diffraction grating 12 spatially separates the multiple wavelength components contained in the optical pulse Pa into individual wavelengths. Note that other optical components such as a prism may be used as the dispersing element instead of the diffraction grating 12. The optical pulse Pa is incident obliquely on the diffraction grating 12 and is dispersed into multiple wavelength components. The light P1 containing these multiple wavelength components is condensed by the lens 13 into each wavelength component and formed into an image on the modulation surface of the SLM 14. The lens 13 may be a convex lens made of a light-transmitting member or a concave mirror having a concave light-reflecting surface.

[0021] The SLM 14 applies a predetermined phase shift for each wavelength to the optical pulse Pa to convert the optical pulse Pa into an optical pulse train Pb (modulated light). Specifically, the SLM 14 receives a control signal from the control unit 5a (see FIG. 1) to apply a phase shift to the optical pulse Pa to generate the optical pulse train Pb. A phase pattern is presented to the SLM 14 upon receiving the control signal output from the control unit 5a. The SLM 14 simultaneously performs phase modulation and intensity modulation of the light P1 using the presented phase pattern. In this manner, the SLM 14 mutually shifts the phases of the multiple wavelength components output from the diffraction grating 12. Note that the SLM 14 may perform only phase modulation or only intensity modulation. The SLM 14 is, for example, a phase modulation type. In one embodiment, the SLM 14 is an LCOS (Liquid Crystal on Silicon) type. Note that although a transmissive SLM 14 is shown in the drawings, the SLM 14 may also be a reflective type.

[0022] FIG. 3 is a diagram showing the modulation surface 17 of the SLM 14. As shown in FIG. 3, on the modulation surface 17, multiple modulation regions 17a are arranged along a certain direction A, and each modulation region 17a extends in a direction B intersecting with direction A. Direction A is the direction of light dispersion by the diffraction grating 12. This modulation surface 17 functions as a Fourier transform plane, and each of the multiple modulation regions 17a is incident on a corresponding wavelength component after dispersion. The SLM 14 modulates the phase and intensity of each incident wavelength component in each modulation region 17a independently from the other wavelength components. Note that since the SLM 14 of this embodiment is a phase modulation type, intensity modulation is achieved by a phase pattern (phase image) presented on the modulation surface 17.

[0023] Each wavelength component of modulated light P2 modulated by SLM 14 is focused to a single point on diffraction grating 16 by lens 15. In this case, lens 15 functions as a focusing optical system that focuses modulated light P2. Lens 15 may be a convex lens made of a light-transmitting member, or a concave mirror with a concave light-reflecting surface. In addition, diffraction grating 16 functions as a combining optical system that combines each wavelength component after modulation. In other words, lens 15 and diffraction grating 16 focus and combine the multiple wavelength components of modulated light P2 together to form a bandwidth-controlled multi-pulse (optical pulse train Pb).

[0024] The phase pattern presented on modulation surface 17 is a phase pattern for generating optical pulse train Pb, superimposed with a phase pattern for imparting positive or negative group delay dispersion (GDD) to optical pulses Pa (i.e., making optical pulse train Pb have positive or negative group delay dispersion). FIGS. 4 and 5 show examples of the spectral waveform (spectral phase G11 and spectral intensity G12) of output light from pulse shaping section 3. In FIGS. 4 and 5, the horizontal axis represents wavelength (nm), the left vertical axis represents the intensity value (arbitrary unit) of the intensity spectrum, and the right vertical axis represents the phase value (rad) of the phase spectrum. The spectral waveform shown in FIG. 4 represents the case where negative group delay dispersion is imparted. The spectral waveform shown in FIG. 5 represents the case where positive group delay dispersion is imparted.

[0025] By imparting positive or negative group delay dispersion to the optical pulse Pa incident on the modulation plane 17, the peak intensities of the time waveforms of the optical pulses Pb1 and Pb2 change. FIG. 6 is a diagram showing an example of the relationship between the group delay dispersion imparted to the optical pulse Pa by the modulation plane 17 and the peak intensities of the optical pulses Pb1 and Pb2. In FIG. 6, the vertical axis represents the peak intensities of the optical pulses Pb1 and Pb2, and the horizontal axis represents the group delay dispersion imparted to the optical pulse Pa by the modulation plane 17. As shown in FIG. 6, the peak intensities of the optical pulses Pb1 and Pb2 depend on the group delay dispersion. That is, when the group delay dispersion is zero, the peak intensities of the optical pulses Pb1 and Pb2 are maximum, and the larger the absolute value of the group delay dispersion, the smaller the peak intensities of the optical pulses Pb1 and Pb2 become. Therefore, by imparting positive or negative group delay dispersion to the optical pulse Pa on the modulation plane 17, the peak intensities of the optical pulses Pb1 and Pb2 can be reduced compared to when no group delay dispersion is imparted. In the description of this embodiment, the "peak intensity of an optical pulse" means the peak intensity of an optical pulse in the time domain unless otherwise specified. When group delay dispersion is imparted to an optical pulse Pa at modulation plane 17, the peak intensities of the optical pulses Pb1 and Pb2 in the time domain are reduced while the intensities of the optical pulses Pb1 and Pb2 in the spectral domain are maintained.

[0026] The phase pattern presented on the modulation surface 17 is a phase pattern for imparting group delay dispersion to the optical pulse Pa in the optical component 7 such that the maximum intensities of the optical pulses Pb1 and Pb2 are below the threshold of the nonlinear optical phenomenon. As described above, when the phase pattern imparts positive or negative group delay dispersion to the optical pulse Pa, the peak intensity of the optical pulse train Pb output from the pulse shaping unit 3 is reduced. Furthermore, by sufficiently increasing the absolute value of the group delay dispersion imparted to the optical pulse Pa, it becomes possible for the peak intensities of the optical pulses Pb1 and Pb2 propagating through the optical component 7 to be below the threshold of the nonlinear optical phenomenon. Note that the sign of the group delay dispersion matches the sign of the group velocity dispersion, which is the group delay dispersion per unit length.

[0027] 7 is a diagram conceptually illustrating changes in the peak intensities of optical pulses Pb1 and Pb2. As shown in FIG. 7, by imparting positive or negative group delay dispersion to optical pulse Pa in pulse shaping section 3, the peak intensities of optical pulses Pb1 and Pb2 fall below threshold Q1 of the nonlinear optical phenomenon of optical component 7. Note that if optical component 7 is a long-distance optical waveguide such as an optical fiber, the peak intensities of optical pulses Pb1 and Pb2 will further change due to group delay dispersion of optical component 7. In such a case, it is desirable that the peak intensities of optical pulses Pb1 and Pb2 fall below threshold Q1 of the nonlinear optical phenomenon at both optical input end 7a and optical output end 7b of optical component 7.

[0028] The SLM 14 generates an optical pulse train Pb including optical pulses Pb1 and Pb2 from the optical pulse Pa. FIG. 8 shows an example of a bandwidth-controlled multi-pulse. In this example, an optical pulse train Pb consisting of optical pulses Pb1 and Pb2 is shown. FIG. 8(a) is a spectrogram showing time on the horizontal axis and wavelength on the vertical axis, with light intensity represented by color shading. FIG. 8(b) shows the time waveform of the optical pulse train Pb. The time waveform of each optical pulse Pb1, Pb2 is, for example, a Gaussian function. As shown in FIGS. 8(a) and 8(b), the peaks of the optical pulses Pb1, Pb2 are separated from each other in time, and the propagation timings of the optical pulses Pb1, Pb2 are shifted from each other. In other words, one optical pulse Pb1 is delayed in time with respect to another optical pulse Pb2, and the optical pulses Pb1, Pb2 have a time difference from each other. The optical pulses Pb1 and Pb2 have different center wavelengths. The central wavelength of the optical pulse Pb1 is, for example, 805 nm, and the central wavelength of the optical pulse Pb2 is, for example, 795 nm. The time interval (peak interval) between the optical pulses Pb1 and Pb2 is, for example, within a range of 10 fs to 10,000 fs, and is, for example, 2,000 fs. The FWHM of the optical pulses Pb1 and Pb2 is, for example, within a range of 10 fs to 5,000 fs, and is, for example, 300 fs.

[0029] FIG. 8(c) shows the spectrum obtained by combining two optical pulses Pb1 and Pb2. As shown in FIG. 8(c), the spectrum obtained by combining two optical pulses Pb1 and Pb2 has a single peak. However, as shown in FIG. 8(a), the center wavelengths of the two optical pulses Pb1 and Pb2 are offset from each other. The single peak shown in FIG. 8(c) roughly corresponds to the spectrum of optical pulse Pa. The peak wavelength interval between adjacent optical pulses Pb1 and Pb2 is determined by the spectral bandwidth of optical pulse Pa and is generally within a range of twice the full width at half maximum. For example, when the full width at half maximum (FWHM), which is the spectral bandwidth of optical pulse Pa, is 10 nm, the peak wavelength interval is 10 nm. As a specific example, when the central wavelength of optical pulse Pa is 1550 nm, the peak wavelengths of optical pulse Pb1 and optical pulse Pb2 can be 1555 nm and 1545 nm, respectively.

[0030] FIG. 9 shows an example of a multipulse that is not bandwidth-controlled as a comparative example. In this example, an optical pulse train Pd consisting of two optical pulses Pd1 and Pd2 is shown. Similar to FIG. 8(a), FIG. 9(a) is a spectrogram in which the horizontal axis represents time and the vertical axis represents wavelength, and light intensity is represented by color shading. FIG. 9(b) shows the time waveform of the optical pulse train Pd. FIG. 9(c) shows the spectrum obtained by combining the two optical pulses Pd1 and Pd2. As shown in FIGS. 9(a) to 9(c), the peaks of the two optical pulses Pd1 and Pd2 are separated from each other in time, but the center wavelengths of the two optical pulses Pd1 and Pd2 are the same. The pulse shaping unit 3 of this embodiment does not generate such an optical pulse train Pd, but generates an optical pulse train Pb having different center wavelengths, as shown in FIG. 8.

[0031] Referring again to FIG. 1, the dispersive medium 8 receives the optical pulse train Pb output from the pulse shaping unit 3. The dispersive medium 8 has positive or negative group delay dispersion. The sign of the group delay dispersion of the dispersive medium 8 is opposite to the sign of the group delay dispersion imparted to the optical pulse Pa in the pulse shaping unit 3. That is, if the pulse shaping unit 3 imparts positive group delay dispersion to the optical pulse Pa, the dispersive medium 8 has negative group delay dispersion. Conversely, if the pulse shaping unit 3 imparts negative group delay dispersion to the optical pulse Pa, the dispersive medium 8 has positive group delay dispersion. By imparting this group delay dispersion to the optical pulses Pb1 and Pb2 included in the optical pulse train Pb, the dispersive medium 8 increases the peak intensities of the optical pulses Pb1 and Pb2, unlike the pulse shaping unit 3, and makes the peak intensities of the optical pulses Pb1 and Pb2 above the detection threshold of the correlator 4. The optical pulse train Pb that has passed through the dispersive medium 8 is output from the optical output end 8b.

[0032] Referring again to Fig. 7, as described above, the dispersive medium 8 imparts to the optical pulse train Pb group delay dispersion with an opposite sign to the group delay dispersion imparted to the optical pulse Pa in the pulse shaping section 3. This increases the peak intensity of the optical pulse train Pb in the dispersive medium 8. This makes it possible to make the intensity of the optical pulse train Pb equal to or greater than the detection threshold Q2 of the correlator 4.

[0033] The above-mentioned action will be explained with reference to Fig. 6. For example, if negative group delay dispersion is imparted to optical pulse Pa in pulse shaping section 3, the peak intensities of optical pulses Pb1 and Pb2 are reduced, as indicated by arrow B1 in the figure. If positive group delay dispersion is then imparted to optical pulses Pb1 and Pb2 in dispersive medium 8, the peak intensities of optical pulses Pb1 and Pb2 are increased, as indicated by arrow B2 in the figure. Furthermore, if positive group delay dispersion is imparted to optical pulse Pa in pulse shaping section 3, the peak intensities of optical pulses Pb1 and Pb2 are reduced, as indicated by arrow B3 in the figure. If negative group delay dispersion is then imparted to optical pulses Pb1 and Pb2 in dispersive medium 8, the peak intensities of optical pulses Pb1 and Pb2 are increased, as indicated by arrow B4 in the figure.

[0034] The dispersion medium 8 may be any medium having a non-zero dispersion. The dispersion medium 8 may be, for example, an optical fiber, a light-guiding member such as an optical waveguide, or a semiconductor or dielectric optical crystal. Examples of optical fibers include single-mode fibers, multimode fibers, rare-earth-doped fibers, photonic crystal fibers, dispersion-shifted fibers, and double-clad fibers. Examples of optical waveguides include semiconductor microwaveguides such as SiN or InP. Examples of semiconductor or dielectric optical crystals include diamond, SiO2, LiNbO3, LiTaO3, lanthanum-doped lead zirconate titanate (PLZT), Si, Ge, fullerene, graphite, graphene, carbon nanotubes, GaN, GaAs, magnetic materials, organic materials, or polymer materials. The dispersion medium 8 may also be, for example, a grating pair or a prism pair. Furthermore, the dispersion medium 8 may also be, for example, glass such as BK7. If the dispersion of the optical component 7 is large, the dispersion medium 8 should be, for example, SF11 or the like, which has large dispersion. The dispersion medium 8 is preferably one that does not cause nonlinear optical phenomena, but any medium that is less likely to cause nonlinear optical phenomena will suffice. The dispersion medium 8 may be, for example, one that has a sufficiently small nonlinear optical effect.

[0035] The correlation optical system 40 receives the optical pulse train Pb output from the dispersive medium 8 and outputs correlated light Pc including multiple optical pulses (multiple third optical pulses) that are cross-correlated or auto-correlated with the optical pulse train Pb. In this embodiment, the correlation optical system 40 includes a lens 41, an optical element 42, and a lens 43. The lens 41 is provided on the optical path between the pulse shaping unit 3 and the optical element 42 and focuses the optical pulse train Pb output from the pulse shaping unit 3 onto the optical element 42. The optical element 42 is a light emitter that includes at least one of a nonlinear optical crystal that generates second harmonic generation (SHG) and a phosphor. Examples of nonlinear optical crystals include KTP (KTiOPO4) crystal, LBO (LiB3O5) crystal, and BBO (β-BaB2O4) crystal. Examples of phosphors include coumarin, stilbene, and rhodamine. The optical element 42 receives the optical pulse train Pb and generates correlated light Pc including cross-correlated or auto-correlated light of the optical pulse train Pb. The lens 43 collimates or focuses the correlated light Pc output from the optical element 42. The correlated light Pc is generated to more accurately calculate the feature amount of the time waveform of the optical pulse train Pb.

[0036] An example of the configuration of the correlation optical system 40 will now be described in detail. FIG. 10 is a schematic diagram illustrating a correlation optical system 40A, as an example of the configuration of the correlation optical system 40, for generating correlation light Pc including the autocorrelation of the optical pulse train Pb. This correlation optical system 40A has a beam splitter 44 as an optical branching component that branches the optical pulse train Pb into two. The beam splitter 44 is optically coupled to the pulse forming unit 3 shown in FIG. 1 and transmits a portion of the optical pulse train Pb input from the pulse forming unit 3 and reflects the remainder. The branching ratio of the beam splitter 44 is, for example, 1:1. One of the optical pulse trains Pba branched by the beam splitter 44 passes through an optical path 40c including multiple mirrors 45 and reaches the lens 41. The other optical pulse train Pbb branched by the beam splitter 44 passes through an optical path 40d including multiple mirrors 46 and reaches the lens 41. The optical lengths of the optical paths 40c and 40d are different from each other. Therefore, the multiple mirrors 45 and the multiple mirrors 46 constitute a delay optical system that imparts a time difference between one optical pulse train Pba and the other optical pulse train Pbb split by the beam splitter 44. Furthermore, at least a portion of the multiple mirrors 46 are mounted on a moving stage 47, making the optical length of the optical path 40d variable. Therefore, with this configuration, the time difference between the optical pulse trains Pba and Pbb can be varied.

[0037] In this example, the optical element 42 includes a nonlinear optical crystal. The lens 41 focuses each of the optical pulse trains Pba and Pbb toward the optical element 42, and also causes the optical axes of the optical pulse trains Pba and Pbb to intersect with each other at a predetermined angle in the optical element 42. As a result, in the optical element 42, which is a nonlinear optical crystal, a second harmonic is generated starting from the intersection of the optical pulse trains Pba and Pbb. This second harmonic is correlated light Pc, which includes the autocorrelation of the optical pulse train Pb. The correlated light Pc is collimated or focused by the lens 43 and then input to the detector 400.

[0038] FIG. 11 is a schematic diagram illustrating a correlation optical system 40B, as another example of the correlation optical system 40, for generating correlation light Pc including the cross-correlation of the optical pulse train Pb. In this correlation optical system 40B, the optical pulse train Pb reaches the lens 41 through an optical path 40e, while the reference optical pulse Pr, which is a single pulse, reaches the lens 41 through an optical path 40f. The optical path 40f includes multiple mirrors 48 and is bent in a U-shape. Furthermore, at least some of the multiple mirrors 48 are mounted on a moving stage 49, making the optical length of the optical path 40f variable. Therefore, with this configuration, the time difference between the optical pulse train Pb and the reference optical pulse Pr (the difference in timing at which they reach the lens 41) can be varied.

[0039] In this example, the optical element 42 also includes a nonlinear optical crystal. The lens 41 focuses the optical pulse train Pb and the reference optical pulse Pr toward the optical element 42, and causes the optical axis of the optical pulse train Pb and the optical axis of the reference optical pulse Pr to intersect each other at a predetermined angle in the optical element 42. As a result, in the optical element 42, which is a nonlinear optical crystal, a second harmonic is generated starting from the intersection of the optical pulse train Pb and the reference optical pulse Pr. This second harmonic is correlated light Pc, which includes the cross-correlation of the optical pulse train Pb. The correlated light Pc is collimated or focused by the lens 43 and then input to the detector 400.

[0040] FIG. 12 is a schematic diagram illustrating a correlation optical system 40C, as yet another example of the correlation optical system 40, for generating correlation light Pc including the cross-correlation of the optical pulse train Pb. In this example, the SLM 14 in the pulse shaping unit 3 is a polarization-dependent spatial light modulator that modulates in a first polarization direction. In contrast, the polarization plane of the optical pulse Pa input to the pulse shaping unit 3 is tilted with respect to the polarization direction modulated by the SLM 14, and the optical pulse Pa includes a polarization component in the first polarization direction (arrow Dp1 in the figure) and a polarization component in a second polarization direction (symbol Dp2 in the figure) that is orthogonal to the first polarization direction. Furthermore, the polarization of the optical pulse Pa is not limited to the above-described polarization (tilted linear polarization) but may also be elliptically polarized.

[0041] The polarization component of the optical pulse Pa in the first polarization direction is modulated by the SLM 14 and output from the pulse forming unit 3 as an optical pulse train Pb. On the other hand, the polarization component of the optical pulse Pa in the second polarization direction is not modulated by the SLM 14 and is output as is from the pulse forming unit 3. This unmodulated polarization component is provided to the correlation optical system 40 as a reference optical pulse Pr, which is a single pulse, on the same axis as the optical pulse train Pb. The correlation optical system 40 generates correlation light Pc, which includes the cross-correlation of the optical pulse train Pb, from the optical pulse train Pb and the reference optical pulse Pr. In this configuration example, by delaying the optical pulse train Pb in the SLM 14 and making the delay time variable (arrow E in the figure), the time difference between the optical pulse train Pb and the reference optical pulse Pr (the difference in timing at which they arrive at the lens 41) can be made variable, and the correlation optical system 40 can preferably generate correlation light Pc, which includes the cross-correlation of the optical pulse train Pb.

[0042] 13(a) shows the time waveform of the correlated light Pc when the amount of chromatic dispersion of the optical component 7 is zero. FIG. 13(b) shows the time waveform of the correlated light Pc when the amount of chromatic dispersion of the optical component 7 is not zero. In this example, the peak intensities PE1 to PE3 of the optical pulses Pc1 to Pc3 contained in the correlated light Pc are significantly reduced compared to FIG. 13(a), and the full widths at half maximum W1 to W3 of the optical pulses Pc1 to Pc3 are significantly enlarged compared to FIG. 13(a). Furthermore, the peak time interval G 1,2 is significantly longer than in Figure 13(a).

[0043] In this way, when the chromatic dispersion of the optical component 7 is not zero, the feature quantities of the time waveform of the correlation light Pc (peak intensities PE1 to PE3, full widths at half maximum W1 to W3, peak time interval G 1,2 ,G 2,3) changes significantly compared to when the chromatic dispersion of the optical component 7 is zero. The amount of change depends on the chromatic dispersion of the optical component 7. Therefore, by observing changes in the feature quantities of the time waveform of the correlated light Pc, the chromatic dispersion of the optical component 7 can be determined accurately and easily. However, in the above observation, the chromatic dispersion of the optical component 7 may be corrected using the known chromatic dispersion of the pulsed laser light source 2.

[0044] Referring again to FIG. 1, the detector 400 receives the correlated light Pc output from the correlation optical system 40. The detector 400 detects the time waveform of the correlated light Pc formed from the optical pulse train Pb having a peak intensity equal to or greater than the detection threshold of the correlator 4. The detector 400 includes a photodetector such as a photodiode. The detector 400 detects the time waveform of the correlated light Pc by converting the intensity of the correlated light Pc into an electrical signal. The electrical signal, which is the detection result, is provided to the calculation unit 5b. In this embodiment, the detection threshold is a value determined based on the characteristics of the correlation optical system 40 and the detector 400. When the optical pulse train Pb having a peak intensity equal to or greater than the detection threshold enters the correlation optical system 40, the detector 400 can accurately detect the time waveform of the optical pulse train Pb.

[0045] The calculation unit 5b is electrically connected to the detector 400. The calculation unit 5b estimates the amount of chromatic dispersion of the optical component 7 based on the feature quantities of the time waveform provided by the detector 400. As described above, according to the knowledge of the present inventors, when correlated light Pc including cross-correlation or autocorrelation of the optical pulse train Pb is generated, various feature quantities (e.g., pulse interval, peak intensity, pulse width, etc.) in the time waveform of the correlated light Pc have a significant correlation with the amount of chromatic dispersion of the measurement target. Therefore, the calculation unit 5b can accurately estimate the amount of chromatic dispersion of the measurement target optical component 7 by evaluating the feature quantities of the time waveform of the correlated light Pc.

[0046] The output unit 5c outputs the result of the estimation of the amount of chromatic dispersion in the calculation unit 5b. The output unit 5c is, for example, a display device that displays the result of the estimation of the amount of chromatic dispersion.

[0047] The optical component 7 is disposed on the optical path between the pulse shaping unit 3 and the dispersive medium 8. The optical component 7 is, for example, a light-guiding member such as an optical fiber or an optical waveguide. Examples of optical fibers include single-mode fibers, multimode fibers, rare-earth-doped fibers, photonic crystal fibers, dispersion-shifted fibers, and double-clad fibers. Examples of optical waveguides include semiconductor microwaveguides such as SiN and InP. Alternatively, the optical component 7 may be, for example, a semiconductor or dielectric optical crystal. In this case, the optical component 7 may be diamond, SiO2, LiNbO3, LiTaO3, PLZT, Si, Ge, fullerene, graphite, graphene, carbon nanotubes, GaN, GaAs, a magnetic material, an organic material, a polymer material, or the like.

[0048] Here, we will explain the combination of the sign of the group delay dispersion imparted by the phase pattern presented to the SLM 14, the sign of the group delay dispersion of the optical component 7, and the sign of the group delay dispersion of the dispersive medium 8. In this embodiment, the group delay dispersion imparted to the optical pulse Pa by the phase pattern presented to the SLM 14 reduces the peak intensities of the optical pulses Pb1 and Pb2, and the group delay dispersion of the dispersive medium 8 increases the peak intensities of the optical pulses Pb1 and Pb2. Therefore, the sign of the group delay dispersion imparted by the phase pattern presented to the SLM 14 and the sign of the group delay dispersion of the dispersive medium 8 may be opposite to each other. The sign of the group delay dispersion imparted by the optical component 7 may be either positive or negative. However, it is more preferable that the group delay dispersion imparted by the phase pattern presented to the SLM 14 be negative and the group delay dispersion imparted by the dispersive medium 8 be positive. Here, many types of dispersive media impart positive group delay dispersion. Therefore, according to this device, more types of dispersive media can be selected as the dispersive medium 8 in this embodiment than when the group delay dispersion given by the dispersive medium 8 is negative group delay dispersion.

[0049] Fig. 14 is a diagram schematically illustrating an example of the hardware configuration of the control device 5. As shown in Fig. 14, the control device 5 can be physically configured as a normal computer including a processor (CPU) 51, main storage devices such as a ROM 52 and a RAM 53, input devices 54 such as a keyboard, mouse, and touch screen, output devices 55 such as a display (including a touch screen), a communication module 56 such as a network card for transmitting and receiving data to and from other devices, and an auxiliary storage device 57 such as a hard disk.

[0050] The computer processor 51 can realize the function of the calculation unit 5b by means of a chromatic dispersion amount calculation program. In other words, the chromatic dispersion amount calculation program causes the computer processor 51 to operate as the calculation unit 5b. The chromatic dispersion amount calculation program is stored in a storage device (storage medium) inside or outside the computer, such as an auxiliary storage device 57. The storage device may be a non-transitory recording medium. Examples of the recording medium include recording media such as flexible disks, CDs, and DVDs, recording media such as ROMs, semiconductor memories, cloud servers, etc. An output device 55 such as a display (including a touch screen) operates as the output unit 5c.

[0051] Auxiliary storage device 57 stores feature quantities of the time waveform of correlated light Pc that are theoretically calculated in advance (or measured in advance) assuming that the amount of chromatic dispersion of optical component 7 is zero. By comparing these feature quantities with the feature quantities of the time waveform of correlated light Pc detected by detector 400, it is possible to determine the extent to which the feature quantities of correlated light Pc have changed due to the amount of chromatic dispersion of optical component 7. Therefore, calculation unit 5b can estimate the amount of chromatic dispersion of optical component 7 by comparing the feature quantities stored in auxiliary storage device 57 with the feature quantities of the time waveform of correlated light Pc detected by detector 400.

[0052] 15 is a flowchart showing a dispersion measuring method using the dispersion measuring apparatus 1A having the above configuration. First, in an output step S101, the pulse laser light source 2 outputs an optical pulse Pa.

[0053] Next, in the pulse forming step S102, the pulse forming unit 3 receives the optical pulse Pa and generates an optical pulse train Pb. Specifically, the pulse forming unit 3 forms the optical pulse train Pb, which is modulated light including multiple optical pulses Pb1 and Pb2 with different center wavelengths and a time difference from each other, from the optical pulse Pa output from the pulse laser light source 2. For example, the multiple wavelength components included in the optical pulse Pa are spatially separated by wavelength, and the SLM 14 is used to shift the phases of the multiple wavelength components relative to each other, and then the multiple wavelength components are focused. This makes it easy to generate the optical pulse train Pb. Additionally, in the pulse forming step S102, the phase pattern provided to the SLM 14 imparts positive or negative group delay dispersion to the optical pulse Pa. As described in the description of the pulse forming unit 3, this makes it possible to reduce the peak intensity in the time domain while maintaining the intensity in the spectral domain in the formed optical pulses Pb1 and Pb2.

[0054] Next, in a dispersive medium transmission step S103, the optical pulse train Pb passes through a dispersive medium 8. Specifically, after the optical pulse train Pb output from the pulse shaping unit 3 passes through the optical component 7, the optical pulse train Pb passes through a dispersive medium 8 having a group delay dispersion with an opposite sign to the group delay dispersion imparted to the optical pulse Pa. When the optical pulse train Pb passes through, the dispersive medium 8 imparts group delay dispersion to the optical pulses Pb1 and Pb2 included in the optical pulse train Pb. As a result, when the optical pulse train Pb passes through the dispersive medium 8, the dispersive medium 8 increases the peak intensities in the time domain of the optical pulses Pb1 and Pb2, making it possible to make the peak intensities of the optical pulses Pb1 and Pb2 equal to or greater than the detection threshold of the correlator 4.

[0055] Next, in detection step S104, the time waveform of the correlated light Pc is detected. Specifically, correlation optical system 40 receives the optical pulse train Pb output from dispersive medium 8, and outputs correlated light Pc including a plurality of optical pulses Pc1 to Pc3 which are cross-correlations or auto-correlations of the optical pulse train Pb. Then, detector 400 detects the time waveform of the correlated light Pc. As an example, after the optical pulse train Pb passes through the dispersive medium 8, correlation optical system 40 uses optical element 42 which includes at least one of a nonlinear optical crystal and a phosphor to generate correlated light Pc including cross-correlations or auto-correlations of the optical pulse train Pb.

[0056] For example, as shown in Fig. 10, an optical pulse train Pb is branched into two, one of the branched optical pulse trains Pbb is time-delayed with respect to the other optical pulse train Pba, and correlated light Pc containing the autocorrelation of the optical pulse train Pb is generated from the time-delayed one optical pulse train Pbb and the other optical pulse train Pba. Also, for example, as shown in Fig. 11, an optical pulse train Pb and a reference optical pulse Pr are input, and the reference optical pulse Pr is time-delayed with respect to the optical pulse train Pb, and correlated light Pc containing the cross-correlation of the optical pulse train Pb is generated from the time-delayed reference optical pulse Pr and the optical pulse train Pb. Also, for example, as shown in FIG. 12, an optical pulse train Pb is generated by modulating only the polarization component of an optical pulse Pa in a first polarization direction in the SLM 14, and the polarization component in a second polarization direction is used as a reference optical pulse Pr. The optical pulse train Pb is time-delayed relative to the reference optical pulse Pr in the SLM 14, and correlation light Pc including the cross-correlation of the optical pulse train Pb is generated from the time-delayed optical pulse train Pb and the reference optical pulse Pr.

[0057] Subsequently, in calculation step S105, calculation unit 5b estimates the amount of chromatic dispersion of optical component 7 based on the feature quantities of the time waveform of correlated light Pc. Specifically, first, calculation unit 5b acquires the feature quantities of the time waveform of correlated light Pc that have been theoretically calculated in advance (or measured in advance) on the assumption that the chromatic dispersion of optical component 7 is zero. Next, calculation unit 5b acquires the feature quantities of the time waveform of correlated light Pc detected in detection step S104. Here, the feature quantities are, for example, peak intensities E1 to E3, full widths at half maximum W1 to W3, and peak time intervals G shown in FIG. 1,2 ,G 2,3 Next, the calculation unit 5b compares the feature amounts of the two acquired time waveforms with each other to estimate the amount of chromatic dispersion of the optical component .

[0058] Here, we will explain in detail the phase modulation for generating bandwidth-controlled multi-pulses in the SLM 14 of the pulse shaping unit 3 shown in FIG. 2. The region before the lens 15 (spectral region) and the region after the diffraction grating 16 (time domain) have a Fourier transform relationship with each other, and phase modulation in the spectral domain affects the time-domain intensity waveform. Therefore, the output light from the pulse shaping unit 3 can have various time-domain intensity waveforms different from that of the optical pulse Pa according to the modulation pattern of the SLM 14. FIG. 16(a) shows, as an example, the spectral waveform (spectral phase G21 and spectral intensity G22) of a single-pulse optical pulse Pa, and FIG. 16(b) shows the time-domain intensity waveform of the optical pulse Pa. Furthermore, FIG. 17(a) shows, as an example, the spectral waveform (spectral phase G31 and spectral intensity G32) of the output light from the pulse shaping unit 3 when a rectangular-wave phase spectral modulation is applied in the SLM 14, and FIG. 17(b) shows the time-domain intensity waveform of the output light. In Figures 16(a) and 17(a), the horizontal axis represents wavelength (nm), the left vertical axis represents intensity values ​​(arbitrary units) of the intensity spectrum, and the right vertical axis represents phase values ​​(rad) of the phase spectrum. In Figures 16(b) and 17(b), the horizontal axis represents time (femtoseconds), and the vertical axis represents light intensity (arbitrary units). In this example, by applying a rectangular phase spectrum waveform to the output light, the single pulse of the optical pulse Pa is converted into a double pulse accompanied by higher-order light. Note that the spectrum and waveform shown in Figure 17 are just one example, and the time-intensity waveform of the output light from the pulse shaping unit 3 can be shaped into various shapes by combining various phase spectra and intensity spectra.

[0059] FIG. 18 shows the configuration of a modulation pattern calculation device 20 that calculates the modulation pattern of SLM 14. The modulation pattern calculation device 20 is, for example, a computer with a processor, such as a personal computer; a smart device such as a smartphone or tablet; or a cloud server. The calculation unit 5b shown in FIG. 1 may also function as the modulation pattern calculation device 20. The modulation pattern calculation device 20 is electrically connected to SLM 14, calculates a phase modulation pattern for approximating the time intensity waveform of the output light from the pulse forming unit 3 to a desired waveform, and provides a control signal including the phase modulation pattern to SLM 14. The modulation pattern is data for controlling SLM 14 and includes a table of the intensity of the complex amplitude distribution or the intensity of the phase distribution. The modulation pattern is, for example, a computer-generated hologram (CGH).

[0060] The modulation pattern calculation device 20 of this modification stores in the control unit 5a phase patterns including a phase pattern for phase modulation that imparts a phase spectrum to output light to obtain a desired waveform, and a phase pattern for intensity modulation that imparts an intensity spectrum to output light to obtain a desired waveform. To this end, as shown in FIG. 18 , the modulation pattern calculation device 20 has an arbitrary waveform input unit 21, a phase spectrum design unit 22, an intensity spectrum design unit 23, and a modulation pattern generation unit 24. That is, the processor of the computer provided in the modulation pattern calculation device 20 realizes the functions of the arbitrary waveform input unit 21, the phase spectrum design unit 22, the intensity spectrum design unit 23, and the modulation pattern generation unit 24. Each function may be realized by the same processor or by different processors.

[0061] The computer processor can realize each of the above functions by means of a modulation pattern calculation program. Therefore, the modulation pattern calculation program causes the computer processor to operate as an arbitrary waveform input unit 21, a phase spectrum design unit 22, an intensity spectrum design unit 23, and a modulation pattern generation unit 24 in the modulation pattern calculation device 20. The modulation pattern calculation program is stored in a storage device (storage medium) inside or outside the computer. The storage device may be a non-transitory recording medium. Examples of the recording medium include recording media such as a flexible disk, CD, DVD, etc., recording media such as ROM, semiconductor memory, cloud server, etc.

[0062] The arbitrary waveform input unit 21 accepts input of a desired time-intensity waveform from an operator. The operator inputs information about the desired time-intensity waveform (e.g., pulse interval, pulse width, number of pulses, etc.) to the arbitrary waveform input unit 21. The information about the desired time-intensity waveform is provided from the arbitrary waveform input unit 21 to the phase spectrum design unit 22 and the intensity spectrum design unit 23. The phase spectrum design unit 22 calculates a phase spectrum of the output light of the pulse shaping unit 3 suitable for realizing the given desired time-intensity waveform. The intensity spectrum design unit 23 calculates an intensity spectrum of the output light of the pulse shaping unit 3 suitable for realizing the given desired time-intensity waveform. The modulation pattern generation unit 24 calculates a phase modulation pattern (e.g., a computer-generated hologram) for applying the phase spectrum calculated by the phase spectrum design unit 22 and the intensity spectrum calculated by the intensity spectrum design unit 23 to the output light of the pulse shaping unit 3. A control signal SC including the calculated phase modulation pattern is then provided to the SLM 14. The SLM 14 is controlled based on the control signal SC.

[0063] Fig. 19 is a block diagram showing the internal configuration of the phase spectrum design unit 22 and the intensity spectrum design unit 23. As shown in Fig. 19, the phase spectrum design unit 22 and the intensity spectrum design unit 23 each have a Fourier transform unit 25, a function substitution unit 26, a waveform function correction unit 27, an inverse Fourier transform unit 28, and a target generation unit 29. The target generation unit 29 includes a Fourier transform unit 29a and a spectrogram correction unit 29b. The functions of each of these components will be described in detail later.

[0064] Here, the desired time-intensity waveform is expressed as a function in the time domain, and the phase spectrum is expressed as a function in the frequency domain. Therefore, the phase spectrum corresponding to the desired time-intensity waveform can be obtained, for example, by an iterative Fourier transform based on the desired time-intensity waveform. Figure 20 shows the procedure for calculating the phase spectrum using the iterative Fourier transform method. First, an initial intensity spectrum function A0(ω) and a phase spectrum function Ψ0(ω), which are functions of frequency ω, are prepared (process number (1) in the figure). In one example, these intensity spectrum function A0(ω) and phase spectrum function Ψ0(ω) represent the spectral intensity and spectral phase of the input light, respectively. Next, the intensity spectrum function A0(ω) and the phase spectrum function Ψ n A waveform function (a) in the frequency domain including (ω) is prepared (processing number (2) in the figure).

number

[0065] Next, the above function (a) is subjected to a Fourier transform from the frequency domain to the time domain (arrow A1 in the figure). This results in a time-intensity waveform function b n (t) and time phase waveform function Θ n The waveform function (b) in the frequency domain including (t) is obtained (processing number (3) in the figure).

number

number

number

number

[0066] Next, the intensity spectrum function B included in the above function (e) n In order to constrain (ω), it is replaced with the initial intensity spectrum function A0(ω) (process number (7) in the figure).

number

[0067] However, while the iterative Fourier method described above can control the time-intensity waveform, it has the problem of being unable to control the frequency components (band wavelengths) that make up the time-intensity waveform. Therefore, the modulation pattern calculation device 20 of this embodiment calculates the phase spectral function and intensity spectral function that form the basis of the modulation pattern using the calculation method described below. FIG. 21 is a diagram showing the calculation procedure for the phase spectral function in the phase spectrum design unit 22. First, an initial intensity spectral function A(ω) and a phase spectral function Φ(ω), which are functions of frequency ω, are prepared (process number (1) in the figure). In one example, these intensity spectral function A(ω) and phase spectral function Φ(ω) represent the spectral intensity and spectral phase of the input light, respectively. Next, a first waveform function (g) in the frequency domain that includes the intensity spectral function A(ω) and the phase spectral function Φ(ω) is prepared (process number (2-a)). Here, i is an imaginary number.

number

[0068] Next, the Fourier transform unit 25 of the phase spectrum design unit 22 performs a Fourier transform from the frequency domain to the time domain on the function (g) (arrow A3 in the figure), thereby obtaining a second waveform function (h) in the time domain that includes the time intensity waveform function a0(t) and the time phase waveform function φ0(t) (Fourier transform step, process number (3)).

number

[0069] Next, the function substitution unit 26 of the phase spectrum design unit 22 substitutes the time-intensity waveform function Target0(t) based on the desired waveform input at the arbitrary waveform input unit 21 into the time-intensity waveform function b0(t) as shown in the following formula (i) (processing number (4-a)).

number

[0070] Next, the function substitution unit 26 of the phase spectrum design unit 22 substitutes the time-intensity waveform function a0(t) with the time-intensity waveform function b0(t) as shown in the following formula (j): That is, the time-intensity waveform function a0(t) included in the above function (h) is replaced with the time-intensity waveform function Target0(t) based on the desired waveform (function substitution step, process number (5)).

number

[0071] Next, the waveform function correction unit 27 of the phase spectrum design unit 22 corrects the second waveform function so that the spectrogram of the second waveform function (j) after the replacement approaches a target spectrogram generated in advance according to a desired wavelength band. First, the second waveform function (j) after the replacement is subjected to a time-frequency transform to convert the second waveform function (j) into a spectrogram SG 0,k (ω, t) (processing number (5-a) in the figure). The subscript k indicates the kth conversion process.

[0072] Here, time-frequency transformation refers to the process of converting a composite signal such as a time waveform into three-dimensional information consisting of time, frequency, and the strength of the signal component (spectral intensity) by performing frequency filtering or numerical calculation (multiplying by a window function while shifting it to derive a spectrum for each time) on the signal. In this embodiment, the result of this transformation (time, frequency, spectral intensity) is defined as a "spectrogram."

[0073] Examples of time-frequency transforms include the Short-Time Fourier Transform (STFT) and wavelet transforms (Haar wavelet transform, Gabor wavelet transform, Mexican Hat wavelet transform, Morlet wavelet transform).

[0074] Furthermore, a target spectrogram TargetSG0(ω,t) that has been generated in advance according to a desired wavelength band is read from the target generation unit 29. This target spectrogram TargetSG0(ω,t) is approximately the same value as the target time waveform (time intensity waveform and its constituent frequency components), and is generated by the target spectrogram function of processing number (5-b).

[0075] Next, the waveform function correction unit 27 of the phase spectrum design unit 22 calculates the spectrogram SG 0,k Pattern matching is performed between the second spectrogram TargetSG0(ω,t) and the target spectrogram TargetSG0(ω,t) to check the similarity (degree of agreement). In this embodiment, an evaluation value is calculated as an index representing the similarity. Then, in the following process number (5-c), it is determined whether the obtained evaluation value satisfies a predetermined termination condition. If the condition is satisfied, the process proceeds to process number (6); if not, the process proceeds to process number (5-d). In process number (5-d), the time phase waveform function φ0(t) included in the second waveform function is replaced with an arbitrary time phase waveform function φ 0,k (t). The second waveform function after the time-phase waveform function has been changed is converted back into a spectrogram by a time-frequency transform such as STFT. After that, the above-mentioned processes (5-a) to (5-d) are repeated. In this way, the spectrogram SG 0,k The second waveform function is corrected so that (ω, t) gradually approaches the target spectrogram TargetSG0(ω, t) (waveform function correction step).

[0076] Thereafter, the inverse Fourier transform unit 28 of the phase spectrum design unit 22 performs an inverse Fourier transform on the modified second waveform function (arrow A4 in the figure) to generate a third waveform function (k) in the frequency domain (inverse Fourier transform step, process number (6)).

number

[0077] 22 is a diagram showing the calculation procedure of the spectral intensity in the intensity spectrum design unit 23. Note that, since the process numbers (1) to (5-c) are the same as the calculation procedure of the spectral phase in the phase spectrum design unit 22 described above, the explanation will be omitted. The waveform function correction unit 27 of the intensity spectrum design unit 23 calculates the spectrogram SG 0,k If the evaluation value indicating the similarity between (ω, t) and the target spectrogram TargetSG0(ω, t) does not satisfy a predetermined termination condition, the time-phase waveform function φ0(t) included in the second waveform function is constrained to an initial value, and the time-intensity waveform function b0(t) is set to an arbitrary time-intensity waveform function b 0,k (t) (Processing No. (5-e)). The second waveform function after the time intensity waveform function has been changed is converted back into a spectrogram by a time-frequency transform such as STFT. After that, processing Nos. (5-a) to (5-c) are repeated. In this way, the spectrogram SG 0,k The second waveform function is corrected so that (ω, t) gradually approaches the target spectrogram TargetSG0(ω, t) (waveform function correction step).

[0078] Then, the inverse Fourier transform unit 28 of the intensity spectrum design unit 23 performs an inverse Fourier transform on the modified second waveform function (arrow A4 in the figure) to generate a third waveform function (m) in the frequency domain (inverse Fourier transform step, process number (6)).

number

[0079] Next, in process number (7-b), the filter processing unit of the intensity spectrum design unit 23 calculates the intensity spectrum function B 0,k (ω) is subjected to filtering based on the intensity spectrum of the input light (filtering step). Specifically, the intensity spectrum function B 0,kThe intensity spectrum obtained by multiplying (ω) by the coefficient α is cut off in the portion that exceeds the cutoff intensity for each wavelength determined based on the intensity spectrum of the input light. 0,k This is to prevent the spectral intensity of the input light from exceeding that of the input light. In one example, the cutoff intensity for each wavelength is set to match the intensity spectrum of the input light (in this embodiment, the initial intensity spectral function A0(ω)). In that case, as shown in the following formula (n), the intensity spectral function αB 0,k At frequencies where (ω) is greater than the intensity spectral function A0(ω), the intensity spectral function A TWC-TFD The value of the intensity spectrum function A0(ω) is taken as the value of (ω). Also, the value of the intensity spectrum function αB 0,k At frequencies where (ω) is less than or equal to the intensity spectral function A0(ω), the intensity spectral function A TWC-TFD (ω) as the value of the intensity spectrum function αB 0,k The value of (ω) is taken in (process number (7-b) in the figure).

number

[0080] The modulation pattern generator 24 generates the phase spectrum function Φ TWC-TFD The spectral phase indicated by (ω) and the intensity spectrum function A calculated by the intensity spectrum design unit 23 TWC-TFD A phase modulation pattern (for example, a computer-generated hologram) for imparting the spectral intensity indicated by (ω) to the output light is calculated (data generation step).

[0081] FIG. 23 shows an example of the procedure for generating a target spectrogram TargetSG0(ω,t) in the target generator 29. The target spectrogram TargetSG0(ω,t) represents a target time waveform (a time-intensity waveform and its constituent frequency components (wavelength band components)). Therefore, creating the target spectrogram is an extremely important process for controlling the frequency components (wavelength band components). As shown in FIG. 23, the target generator 29 first inputs a spectral waveform (initial intensity spectral function A0(ω) and initial phase spectral function Φ0(ω)) and a desired time-intensity waveform function Target0(t). It also inputs a time function p0(t) containing desired frequency (wavelength) band information (processing number (1)).

[0082] Next, the target generation unit 29 generates a phase spectrum function Φ for realizing the time intensity waveform function Target0(t) using, for example, the iterative Fourier transform method shown in FIG. IFTA (ω) is calculated (Processing No. (2)).

[0083] Next, the target generator 29 generates the phase spectrum function Φ IFTA The intensity spectrum function A to realize the time intensity waveform function Target0(t) by the iterative Fourier transform method using (ω) IFTA (ω) is calculated (processing number (3)). Here, FIG. 24 shows the intensity spectrum function A IFTA FIG. 10 is a diagram illustrating an example of a procedure for calculating (ω).

[0084] First, the initial intensity spectrum function A k=0 (ω) and the phase spectrum function Ψ0(ω) are prepared (processing number (1) in the figure). Next, the intensity spectrum function A k A frequency domain waveform function (o) including the phase spectrum function Ψ(ω) is prepared (process number (2) in the figure).

number

[0085] Next, the function (o) is subjected to a Fourier transform from the frequency domain to the time domain (arrow A5 in the figure). This results in a time-intensity waveform function b k The frequency domain waveform function (p) containing (t) is obtained (processing number (3) in the figure).

number

[0086] Next, the time-intensity waveform function b included in the above function (p) k (t) is replaced with a time-intensity waveform function Target0(t) based on the desired waveform (process numbers (4) and (5) in the figure).

number

number

[0087] Next, the function (r) is subjected to an inverse Fourier transform from the time domain to the frequency domain (arrow A6 in the figure). This results in the intensity spectrum function C k (ω) and the phase spectrum function Ψ k The waveform function (s) in the frequency domain including (ω) is obtained (processing number (6) in the figure).

number

number

[0088] In addition, the intensity spectrum function C in the frequency domain after inverse Fourier transform k (ω) is filtered based on the intensity spectrum of the input light. Specifically, the intensity spectrum function C k In the intensity spectrum represented by (ω), a portion exceeding the cutoff intensity for each wavelength determined based on the intensity spectrum of the input light is cut off. In one example, the cutoff intensity for each wavelength is determined based on the intensity spectrum of the input light (for example, the initial intensity spectrum function A k=0 (ω)). In this case, the intensity spectrum function C is set to match the k (ω) is the intensity spectrum function A k=0 At frequencies greater than (ω), the intensity spectral function A k (ω) as the value of the intensity spectrum function A k=0 (ω) is taken into account. Also, the intensity spectrum function C k (ω) is the intensity spectrum function A k=0 At frequencies below (ω), the intensity spectrum function A k (ω) as the value of the intensity spectrum function C k The value of (ω) is taken in (process number (7-b) in the figure).

number

[0089] Thereafter, by repeating the above steps (2) to (7-b), the intensity spectrum function A k The intensity spectrum shape represented by (ω) can be made closer to the intensity spectrum shape corresponding to the desired time intensity waveform. IFTA (ω) is obtained.

[0090] Referring again to Figure 23, the phase spectrum function Φ IFTA (ω) and the intensity spectrum function A IFTA By calculating (ω), a third waveform function (v) in the frequency domain including these functions is obtained (process number (4)).

number

number

[0091] The spectrogram correction unit 29b of the target generation unit 29 converts the fourth waveform function (w) into a spectrogram SG IFTA (ω, t) (Processing No. (6)). Then, in Processing No. (7), a spectrogram SG is generated based on the time function p0(t) containing the desired frequency (wavelength) band information. IFTA By modifying (ω, t), the target spectrogram TargetSG0(ω, t) is generated. For example, the spectrogram SG IFTA The characteristic pattern appearing in (ω, t) is partially extracted, and the frequency components of that portion are manipulated based on the time function p0(t). A specific example of this will be explained in detail below.

[0092] For example, consider the case where a triple pulse with a time interval of 2 picoseconds is set as the desired time intensity waveform function Target0(t). In this case, the spectrogram SG IFTA (ω, t) results as shown in Figure 25(a). In Figure 25(a), the horizontal axis indicates time (unit: femtoseconds) and the vertical axis indicates wavelength (unit: nm). The spectrogram values ​​are indicated by the brightness of the figure, with the brighter the image, the larger the spectrogram value. This spectrogram SGIFTA At (ω, t), the triple pulse appears as domains D1, D2, and D3 separated on the time axis by 2 picosecond intervals. The central (peak) wavelength of domains D1, D2, and D3 is 800 nm.

[0093] If one wishes to control only the time intensity waveform of the output light (simply obtain a triple pulse), there is no need to manipulate these domains D1, D2, and D3. However, if one wishes to control the frequency (wavelength) band of each pulse, one must manipulate these domains D1, D2, and D3. That is, as shown in FIG. 25(b), moving each domain D1, D2, and D3 independently in the direction along the wavelength axis (vertical axis) means changing the constituent frequencies (wavelength bands) of each pulse. Such a change in the constituent frequencies (wavelength bands) of each pulse is performed based on the time function p0(t).

[0094] For example, when the peak wavelength of domain D2 is fixed at 800 nm and the peak wavelengths of domains D1 and D3 are translated by -2 nm and +2 nm, respectively, the spectrogram SG IFTA (ω, t) changes to the target spectrogram TargetSG0(ω, t) shown in Figure 25(b). For example, by performing such processing on the spectrogram, it is possible to create a target spectrogram in which the constituent frequencies (wavelength bands) of each pulse are arbitrarily controlled without changing the shape of the time-intensity waveform.

[0095] The effects obtained by the dispersion measurement apparatus 1A and dispersion measurement method of the present embodiment described above will be described.

[0096] When estimating the amount of chromatic dispersion of the optical component 7, multiple optical pulses Pb1 and Pb2 having different center wavelengths and a time difference between them are transmitted through the optical component 7. The amount of chromatic dispersion of the optical component 7 can then be estimated based on the time waveform of the correlated light Pc obtained from the multiple optical pulses Pb1 and Pb2. Here, when the peak intensities of the optical pulses Pb1 and Pb2 are high, the measurement accuracy of the amount of chromatic dispersion may decrease. For example, when the peak intensities of the optical pulses Pb1 and Pb2 exceed a certain threshold in the optical component 7, a nonlinear optical phenomenon occurs. FIG. 26 conceptually illustrates the time waveforms of the optical pulses Pb1 and Pb2, which have peak intensities equal to or greater than the threshold Q1 of the nonlinear optical phenomenon, before and after they pass through the optical component 7. As shown in FIG. 26, when a nonlinear optical phenomenon occurs, the time waveforms of the optical pulses Pb1 and Pb2 are distorted. Alternatively, the high peak intensities of the optical pulses Pb1 and Pb2 may, for example, damage the optical component 7. In this case, too, the time waveforms of the optical pulses Pb1 and Pb2 are distorted. When the time waveforms of the optical pulses Pb1 and Pb2 are distorted, the time waveform of the correlated light Pc is also distorted. Therefore, when the peak intensities of the optical pulses Pb1 and Pb2 are large, it may not be possible to accurately measure the amount of chromatic dispersion of the optical component 7 based on the time waveform of the correlated light Pc.

[0097] To address this issue, it is conceivable to reduce the peak intensities of the optical pulses Pb1 and Pb2 when generating the optical pulse train Pb in the pulse shaping unit 3, for example. However, simply reducing the peak intensities of the optical pulses Pb1 and Pb2 in the pulse shaping unit 3 reduces the detection accuracy of the time waveform, which may result in an inaccurate measurement of the amount of chromatic dispersion of the optical component 7 based on the time waveform of the correlated light Pc. FIG. 27 conceptually illustrates the time waveforms of the optical pulses Pb1 and Pb2, whose peak intensities have been reduced, before and after they pass through the optical component 7. As shown in FIG. 27, when the peak intensities of the optical pulses Pb1 and Pb2 are reduced in the pulse shaping unit 3, the peak intensities of the optical pulses Pb1 and Pb2 after passing through the optical component 7 may fall below the detection threshold of the correlator 4. In this case, it may be impossible to accurately measure the amount of chromatic dispersion of the optical component 7.

[0098] In the dispersion measurement apparatus 1A and method of this embodiment, positive or negative group delay dispersion is imparted to the optical pulse Pa in the pulse shaping unit 3 (pulse shaping step S102). This suppresses the peak intensities of the optical pulses Pb1 and Pb2, making it possible, for example, to make the peak intensities of the optical pulses Pb1 and Pb2 propagating through the optical component 7 below the threshold of a nonlinear optical phenomenon. Furthermore, for example, it is possible to prevent optical pulses Pb1 and Pb2 with peak intensities large enough to degrade the optical component 7 from entering the optical component 7, thereby preventing damage to the optical component 7. Furthermore, when the optical pulses Pb1 and Pb2 pass through the dispersive medium 8 after passing through the optical component 7, they are given group delay dispersion with an opposite sign to the group delay dispersion imparted to the optical pulse Pa. This makes it possible to make the peak intensities of the optical pulses Pb1 and Pb2 above the detection threshold of the correlator 4. This makes it possible to accurately detect the time waveform of the correlated light Pc. From the above, this embodiment makes it possible to accurately measure the amount of chromatic dispersion of the optical component 7.

[0099] As in this embodiment, the phase pattern presented by SLM 14 may be a phase pattern for imparting to optical pulse Pa such group delay dispersion that the peak intensities of optical pulses Pb1, Pb2 fall below the threshold of a nonlinear optical phenomenon in optical component 7. In this case, the occurrence of a nonlinear optical phenomenon is suppressed, thereby suppressing distortion of the time waveforms of optical pulses Pb1, Pb2, and ultimately making it possible to suppress distortion of the time waveform of correlated light Pc.

[0100] In this embodiment, a correlator 4 including a correlation optical system 40 and a detector 400 is employed to detect the optical pulse train Pb transmitted through the optical component 7 being measured. As shown in FIGS. 10 to 12 , the correlation optical system 40 is an optical system that spatially and temporally superimposes the optical pulse train Pb with itself or another pulse train. Specifically, by sweeping one pulse train over time, a correlation waveform conforming to the time waveform shape of the optical pulse train Pb is detected. Generally, pulse sweeping is performed by spatially varying the optical path length using a drive stage or the like, so the amount of movement of the stage corresponds to the amount of time delay in the correlation waveform. In this case, the amount of time delay relative to the amount of stage movement is very small. Therefore, by employing the correlator 4, the pulse shape can be observed in the detector 400 with a high time resolution scale reaching the order of femtoseconds, thereby enabling more accurate measurement of the chromatic dispersion of the optical component 7 being measured.

[0101] (First Modification) Fig. 28 is a diagram showing a dispersion measurement apparatus 1B in the first modified example. As shown in Fig. 28, in the first modified example, a dispersive medium 8 is arranged not on the optical path between the pulse forming section 3 and the correlation optical system 40, but on the optical path between the correlation optical system 40 and a detector 400. Note that an optical component 7 is arranged on the optical path between the pulse forming section 3 and the correlation optical system 40. The correlation optical system 40 receives an optical pulse train Pb that has passed through the optical component 7, and outputs correlation light Pc that includes optical pulses Pc1 to Pc3 that are cross-correlations or auto-correlations of the optical pulse train Pb. Note that a control device 5 is electrically connected to the pulse forming section 3 and the detector 400.

[0102] A dispersive medium 8 having positive or negative group delay dispersion receives the correlated light Pc output from the correlation optical system 40. At this time, the dispersive medium 8 imparts group delay dispersion to the optical pulses Pc1 to Pc3 contained in the correlated light Pc, thereby making the intensities of the optical pulses Pc1 to Pc3 equal to or greater than a threshold value (described later) of the detector 400.

[0103] Detector 400 receives correlated light Pc that has passed through dispersive medium 8 and detects the time waveform of correlated light Pc that has an intensity equal to or greater than a detection threshold. Note that the detection threshold in the first modified example is a value that is determined based on the characteristics of detector 400, and by having correlated light Pc that has an intensity equal to or greater than the detection threshold incident on detector 400, detector 400 can accurately detect the time waveform of correlated light Pc.

[0104] 29 is a flowchart showing a dispersion measuring method using the dispersion measuring apparatus 1B having the above configuration. First, in an output step S201, the pulse laser light source 2 outputs a light pulse Pa.

[0105] Next, in pulse forming step S202, the pulse forming unit 3 receives the optical pulse Pa and generates an optical pulse train Pb. Specifically, the pulse forming unit 3 forms, from the optical pulse Pa output from the pulse laser light source 2, the optical pulse train Pb, which is modulated light including a plurality of optical pulses Pb1, Pb2 that have a time difference from each other and different center wavelengths. At this time, the pulse forming unit 3 imparts positive or negative group delay dispersion to the optical pulse Pa. That is, the phase pattern presented to the SLM 14 imparts positive or negative group delay dispersion to the optical pulse Pa. This makes it possible to reduce the intensity in the time domain of the formed optical pulses Pb1 and Pb2 while maintaining the intensity in the spectral domain.

[0106] Subsequently, in correlation light output step S203, correlation optical system 40 outputs correlation light Pc. Specifically, after the optical pulse train Pb passes through optical component 7, correlation optical system 40 outputs correlation light Pc including optical pulses Pc1 to Pc3 which are cross-correlations or auto-correlations of the optical pulse train Pb. As an example, after the optical pulse train Pb passes through optical component 7, correlation optical system 40 uses optical element 42 which includes at least one of a nonlinear optical crystal and a phosphor to generate correlation light Pc including cross-correlations or auto-correlations of the optical pulse train Pb.

[0107] Next, in dispersive medium transmission step S204, the correlated light Pc passes through the dispersive medium 8. Specifically, the dispersive medium 8 receives the correlated light Pc output from the correlation optical system 40. The correlated light Pc passes through the dispersive medium 8, which has a group delay dispersion with an opposite sign to the group delay dispersion imparted to the optical pulse Pa. This increases the intensity in the time domain of the optical pulses Pc1 to Pb3 when the correlated light Pc passes through the dispersive medium 8, making it possible to raise the intensity above the detection threshold of the detector 400.

[0108] Subsequently, in detection step S205, detector 400 detects the time waveform of correlated light Pc that has passed through dispersive medium 8. Specifically, detector 400 detects the time waveform of correlated light Pc that has an intensity equal to or greater than the detection threshold of detector 400.

[0109] Subsequently, in calculation step S206, the calculation unit 5b estimates the amount of chromatic dispersion of the optical component 7 based on the feature amount of the time waveform of the correlated light Pc.

[0110] In the dispersion measurement apparatus 1B and method of the first modification, positive or negative group delay dispersion is imparted to the optical pulse Pa in the pulse shaping unit 3 (pulse shaping step S202). This suppresses the intensities of the optical pulses Pb1 and Pb2 included in the optical pulse train Pb, making it possible, for example, to make the maximum intensities of the optical pulses Pb1 and Pb2 propagating through the optical component 7 below the threshold of a nonlinear optical phenomenon. Then, after the optical pulses Pb1 and Pb2 pass through the optical component 7, the optical pulses Pc1 to Pb3 are given group delay dispersion with an opposite sign to the group delay dispersion imparted to the optical pulse Pa when they pass through the dispersive medium 8. This makes it possible to make the intensities of the optical pulses Pc1 to Pb3 above the detection threshold of the detector 400. This makes it possible to accurately detect the time waveform of the correlated light Pc. From the above, it becomes possible to accurately measure the amount of chromatic dispersion of the optical component 7. [Explanation of symbols]

[0111] 1A, 1B... Dispersion measuring device, 2... Pulse laser light source, 3... Pulse forming section, 3a... Optical input end, 3b... Optical output end, 4... Correlator, 40, 40A, 40B, 40C... Correlation optical system, 4a... Optical input end, 40b... Optical output end, 40c to 40f... Optical path, 400... Detector, 5... Control device, 5a... Control section, 5b... Calculation section, 5c... Output section, 7... Optical component (measurement object), 8... Dispersion medium, 8a... Optical input end, 8b... Optical output end, 12... Diffraction grating, 13, 15... Lens, 14... Spatial light modulator (SLM), 16... Diffraction grating, 17... Modulation surface, 17a... Modulation region, 20... Modulation pattern calculation device, 21... Arbitrary waveform input section, 22... Phase spectrum design section, 23... Intensity spectrum design section, 24... Modulation pattern generation section, 25... Fourier transform unit, 26...function substitution unit, 27...waveform function correction unit, 28...inverse Fourier transform unit, 29...target generation unit, 29a...Fourier transform unit, 29b...spectrogram correction unit, 41, 43...lens, 42...optical element, 44...beam splitter, 45, 46, 48...mirror, 47, 49...moving stage, 51...processor, 54...input device, 55...output device, 56...communication module, 57...auxiliary storage device, Pa...optical pulse (first optical pulse), Pb, Pd...optical pulse train, Pb1, Pb2...optical pulse (second optical pulse), Pd1, Pd2...optical pulse, Pba, Pbb...optical pulse train, Pc...correlated light, Pc1, Pc2, Pc3...optical pulse, Pr...reference optical pulse, SC...control signal, G11, G21...spectral phase.

Claims

1. a light source that outputs a first light pulse; a pulse forming unit that has a spatial light modulator that presents a phase pattern for generating modulated light by giving the first light pulse a predetermined phase shift for each wavelength, and that forms, from the first light pulse, an optical pulse train that is the modulated light including a plurality of second light pulses that have a time difference from each other and different center wavelengths; a dispersive medium having positive or negative group delay dispersion and receiving the optical pulse train; a correlation optical system that receives the optical pulse train that has passed through the dispersive medium and outputs correlated light that includes a plurality of third optical pulses that are cross-correlated or auto-correlated with the optical pulse train; and a correlator that includes a detector that detects a time waveform of the correlated light, and detects the time waveform of the correlated light formed from the optical pulse train that has an intensity equal to or greater than a detection threshold; a calculation unit electrically connected to the correlator, a measurement object is disposed on an optical path between the pulse forming unit and the dispersive medium; the calculation unit estimates the amount of chromatic dispersion of the object to be measured based on the time waveform of the correlated light; the dispersive medium imparts group delay dispersion to the second optical pulses included in the optical pulse train, thereby increasing peak intensities of the second optical pulses to be equal to or greater than the detection threshold; the phase pattern includes a pattern for imparting to the first optical pulses group delay dispersion of an opposite sign to the group delay dispersion possessed by the dispersive medium so as to reduce peak intensities of the plurality of second optical pulses.

2. a light source that outputs a first light pulse; a pulse forming unit that has a spatial light modulator that presents a phase pattern for generating modulated light by giving the first light pulse a predetermined phase shift for each wavelength, and that forms, from the first light pulse, an optical pulse train that is the modulated light including a plurality of second light pulses that have a time difference from each other and different center wavelengths; a correlation optical system that receives the optical pulse train and outputs correlated light including a plurality of third optical pulses that are cross-correlations or auto-correlations of the optical pulse train; a dispersive medium having positive or negative group delay dispersion and receiving the correlated light; a detector that receives the correlated light that has passed through the dispersive medium and detects a time waveform of the correlated light that has a peak intensity equal to or greater than a detection threshold; a calculation unit electrically connected to the detector, a measurement object is disposed on an optical path between the pulse forming unit and the correlation optical system; the calculation unit estimates the amount of chromatic dispersion of the object to be measured based on the time waveform of the correlated light; the dispersion medium imparts group delay dispersion to the third optical pulses included in the correlated light, thereby increasing peak intensities of the third optical pulses to be equal to or greater than the detection threshold; the phase pattern includes a pattern for imparting to the first optical pulses group delay dispersion of an opposite sign to the group delay dispersion possessed by the dispersive medium so as to reduce peak intensities of the plurality of second optical pulses.

3. the group delay dispersion given by the phase pattern is negative group delay dispersion; 3. The dispersion measuring device according to claim 1, wherein the group delay dispersion provided by the dispersive medium is positive group delay dispersion.

4. The plurality of second optical pulses have peak intensities equal to or greater than a threshold of a nonlinear optical phenomenon when no group delay dispersion is imparted to the first optical pulses; 4. The dispersion measurement device according to claim 1, wherein the phase pattern is a phase pattern for imparting group delay dispersion to the first optical pulse such that peak intensities of the plurality of second optical pulses are below a threshold of a nonlinear optical phenomenon in the measurement object.

5. an output step of outputting a first optical pulse; a pulse forming step of forming, from the first light pulse, an optical pulse train that is the modulated light including a plurality of second light pulses that have a time difference from one another and different center wavelengths, using a spatial light modulator that presents a phase pattern for generating modulated light by giving the first light pulse a predetermined phase shift for each wavelength; a dispersive medium transmission step in which the optical pulse train transmits through a dispersive medium having positive or negative group delay dispersion after transmitting through a measurement object; a detection step of receiving the optical pulse train that has passed through the dispersive medium, generating correlated light including a plurality of third optical pulses that are cross-correlated or auto-correlated with the optical pulse train, and detecting a time waveform of the correlated light formed from the optical pulse train that has a peak intensity equal to or greater than a detection threshold; a calculation step of estimating the amount of chromatic dispersion of the object to be measured based on the time waveform of the correlated light, in the dispersive medium transmission step, when the optical pulse train transmits through the dispersive medium, group delay dispersion is imparted to the plurality of second optical pulses included in the optical pulse train, thereby increasing peak intensities of the plurality of second optical pulses to be equal to or greater than the detection threshold; the phase pattern includes a pattern for imparting to the first optical pulses group delay dispersion of an opposite sign to the group delay dispersion possessed by the dispersive medium so as to reduce peak intensities of the plurality of second optical pulses.

6. an output step of outputting a first optical pulse; a pulse forming step of forming, from the first light pulse, an optical pulse train that is the modulated light including a plurality of second light pulses that have a time difference from one another and different center wavelengths, using a spatial light modulator that presents a phase pattern for generating modulated light by giving the first light pulse a predetermined phase shift for each wavelength; a correlation light output step of outputting correlation light including a plurality of third optical pulses that are cross-correlations or auto-correlations of the optical pulse train after the optical pulse train has passed through a measurement target; a dispersive medium transmission step in which the correlated light is transmitted through a dispersive medium having positive or negative group delay dispersion; a detecting step of detecting a time waveform of the correlated light transmitted through the dispersive medium; a calculation step of estimating the amount of chromatic dispersion of the object to be measured based on the time waveform of the correlated light, In the detecting step, a time waveform of the correlated light having an intensity equal to or greater than a detection threshold is detected; in the dispersive medium transmission step, when the correlated light transmits through the dispersive medium, group delay dispersion is imparted to the third optical pulses included in the correlated light, thereby increasing the intensities of the third optical pulses to be equal to or greater than the detection threshold; the phase pattern includes a pattern for imparting to the first optical pulses group delay dispersion of an opposite sign to the group delay dispersion possessed by the dispersive medium so as to reduce peak intensities of the plurality of second optical pulses.

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