Optical spectrum generating device and optical spectrum generating method

Abstract

[Problem] To provide an optical spectrum generation device for generating an optical spectrum having a narrow line width. [Solution] This optical spectrum generation device has: a short-pulse light source 10; an optical amplifier 11; a wavelength shift fiber 12; a light intensity modulator 13; and an optical waveguide 14. The light intensity modulator 13 is a filter for causing narrow-band absorption in pulse light from the wavelength shift fiber 12, and generating a linear dip of a pulse light spectrum. The optical waveguide 14 propagates the pulse light from the light intensity modulator 13, and generates a nonlinear effect in the pulse light. Through this nonlinear effect, light intensity at the wavelength of the dip is changed, and the dip is converted into a peak.

Description

[Technical field]

[0001] The present disclosure relates to an apparatus and method for generating a linearly peaked optical spectrum. [Background technology]

[0002] An optical spectrum in which spectral lines with very narrow linewidths are arranged discretely and equidistantly like comb teeth is called an optical frequency comb and is used as a frequency ruler. A known method for generating an optical frequency comb is to use a mode-locked laser. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2018 / 181213 Summary of the Invention [Problem to be solved by the invention]

[0004] However, devices that generate optical spectra with linear peaks are expensive, and there is a demand for lower-cost devices.In addition, the wavelengths at which linear peaks can be obtained are limited.

[0005] In view of the above, an object of the present disclosure is to provide an optical spectrum generating device and an optical spectrum generating method that generate an optical spectrum having a linear peak. [Means for solving the problem]

[0006] The inventors have discovered a phenomenon in which, when pulsed light having a linear dip in its spectrum is passed through an optical waveguide, the light intensity at the wavelength of the dip changes periodically according to the propagation distance in the optical waveguide, and dips and peaks appear alternately. The present disclosure is based on this novel phenomenon.

[0007] The present disclosure provides an optical spectrum generating device for generating pulsed light having a spectrum with a peak, the optical spectrum generating device including a pulsed light source for generating pulsed light, an optical modulator for intensity-modulating or phase-modulating a predetermined wavelength of the pulsed light from the pulsed light source, and an optical waveguide for propagating the pulsed light from the optical intensity modulator, causing a nonlinear effect in the pulsed light, and generating a peak at the predetermined wavelength, the spectral width of the pulsed light being at least twice the spectral width of the dip. the optical waveguide is configured so that the pulsed light alternates between peaks and dips at a predetermined wavelength in accordance with the propagation distance of the pulsed light in the optical waveguide, and the length of the optical waveguide is set so that the pulsed light is extracted from the optical waveguide when the predetermined wavelength reaches a peak. The optical spectrum generating device is characterized by the above. Effect of the Invention

[0008] According to the present disclosure, it is possible to inexpensively realize an apparatus for generating an optical spectrum having a linear peak. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram showing the configuration of an optical spectrum generating device according to a first embodiment. [Diagram 2] FIG. 2 is a schematic diagram showing a spectrum shape. [Diagram 3] Graph showing the relationship between the spectrum of pulsed light and the propagation distance. [Figure 4] Graph showing the relationship between light intensity and fiber length. [Diagram 5] Graph showing the relationship between the spectral width of a dip and the spectral width of a peak. [Figure 6] 1 is a graph showing the relationship between the dip absorptance and the peak light intensity. [Figure 7] Graph showing the results of investigating the dependence of the spectral width of the dip on the relationship between light intensity and fiber length. [Figure 8] Graph showing the results of investigating the dependence of peak output of pulsed light on the relationship between light intensity and fiber length. [Figure 9] 1 is a graph showing the relationship between light intensity and fiber length when the spectral shape of the pulsed light is changed to a Gaussian shape. [Figure 10]Graph showing the relationship between light intensity and fiber length when the spectral shape of the pulsed light is changed to a super-Gaussian type. [Figure 11] Graph showing the relationship between light intensity and fiber length when the spectral shape of the dip is changed to a Gaussian type. [Figure 12] Graph showing the spectral shapes and their corresponding time waveforms and phases. [Figure 13] Graph showing the relationship between the spectrum of pulsed light and the propagation distance. [Figure 14] Graphs showing the results of spectrum measurement and graphs showing spectra obtained by numerical calculation. [Figure 15] 1 is a graph showing the measurement results of the spectral shape. [Figure 16] Graph showing an enlarged view of the spectrum shape. [Figure 17] 1 is a graph showing the measurement results of the spectral shape. [Figure 18] FIG. 2 is a diagram showing the configuration of a light intensity modulator 13. [Figure 19] 1 is a graph showing the spectra of input light and output light. [Figure 20] Graph showing the spectrum. [Figure 21] FIG. 13 is a diagram showing the configuration of an optical spectrum generating device according to a second embodiment. [Figure 22] 1 is a graph showing the spectra of input light and output light. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, specific examples of the present disclosure will be described with reference to the drawings, but the present disclosure is not limited to the examples. EXAMPLES

[0011] FIG. 1 is a diagram showing the configuration of an optical spectrum generator according to a first embodiment. As shown in FIG. 1, the optical spectrum generator according to the first embodiment includes a short pulse light source 10, an optical amplifier 11, a wavelength shifting fiber 12, an optical intensity modulator 13, and an optical waveguide 14. The optical spectrum generator according to the first embodiment utilizes a phenomenon discovered by the inventors. The phenomenon is that when pulsed light having a narrow-linewidth dip (a sharp drop in optical intensity) in its spectrum is passed through an optical waveguide 14, the optical intensity at the wavelength of the dip changes periodically according to the propagation distance in the optical waveguide 14, and dips and peaks appear alternately. Hereinafter, this phenomenon may be referred to as spectral peeking. The optical spectrum generator according to the first embodiment utilizes this spectral peeking to convert dips into peaks.

[0012] The short pulse light source 10 is a light source that emits light with a narrow pulse width (time width). For example, a fiber laser with a ring resonator can be used. The time width (full width at half maximum) of the pulse light is, for example, 10 fs to 100 ps. It is also possible to use an optical frequency comb light source that outputs an optical frequency comb.

[0013] The pulse shape of the pulsed light is arbitrary. For example, 2 Soliton type, Gaussian type, super-Gaussian type, etc. 2 This type is preferred because it allows for a clearer and neater linear peak to be obtained.

[0014] The optical amplifier 11 is a device that amplifies the optical intensity of the pulsed light from the short-pulse light source 10. For example, a rare-earth doped optical fiber amplifier such as an erbium-doped fiber amplifier can be used. Since the nonlinear effect in the downstream optical waveguide 14 occurs above a certain optical intensity, the optical amplifier 11 is used to amplify the optical intensity. In addition, the period of the spectral peeking also depends on the optical intensity. Therefore, the period of the spectral peeking can be adjusted by the amplification factor of the optical amplifier 11.

[0015] The wavelength shift fiber 12 shifts the wavelength of the pulsed light from the optical amplifier 11. This adjusts the position of the absorption peak caused by the optical intensity modulator 13 at the rear stage with respect to the spectrum of the pulsed light. For example, the peak wavelength of the pulsed light is adjusted to be near the wavelength of the absorption peak. Since the amount of wavelength shift in the wavelength shift fiber 12 depends on the optical intensity, the amount of wavelength shift can be controlled by the optical amplifier 11.

[0016] The optical intensity modulator 13 is a filter that causes narrowband absorption of the pulsed light from the wavelength shift fiber 12, and generates a linear dip in the spectrum of the pulsed light. The dip does not have to be one, and multiple dips may be generated. If equally spaced dips are generated, it is possible to generate a spectrum with equally spaced linear peaks by spectral peeking. The wavelength of the dip may be any wavelength within the wavelength band of the pulsed light. However, in order to sufficiently increase the intensity of the peak, it is preferable that the dip is near the central wavelength.

[0017] The spectral shape of the dip by the optical intensity modulator 13 is arbitrary, but it can be Lorentzian, Gaussian, Sech 2 The type is preferable because it can obtain a cleaner and clearer peak.

[0018] The spectral width (full width at half maximum) of the pulsed light is set to be at least twice the spectral width (full width at half maximum) of the dip. By setting the spectral width of the pulsed light or the spectral width of the dip in this manner, it is possible to generate spectral peeking. As long as this is satisfied, the spectral width of the dip can be any value, but is, for example, 10 nm or less, preferably 1 nm or less.

[0019] The absorptance in the dip is not limited, but in order to sufficiently increase the peak light intensity, it is preferably 50% or more, more preferably 70% or more, and even more preferably 90% or more.

[0020] The optical intensity modulator 13 may be any one having a narrow band absorption peak, and may be a gas cell, a diffraction grating, a photonic crystal, or the like. In particular, if a fiber Bragg grating (FBG) is used as the diffraction grating, the optical spectrum generating device of the first embodiment can be configured with all fibers, making it easy to handle. In addition, in the case of a diffraction grating or a photonic crystal, it is also possible to change the wavelength of the absorption peak by controlling the temperature.

[0021] As shown in FIG. 18, a dip may be generated at any wavelength by using a diffraction grating 132 and a spatial light modulator (SLM) 131. That is, the diffraction grating 132 separates the pulsed light into wavelengths, so that the position at which the light reaches the SLM 131 differs depending on the wavelength. The SLM 131 can modulate the light intensity for each position at which the light reaches, so that a dip can be generated by weakening the light intensity of a desired wavelength. The light intensity may be weakened by scattering or by absorption. For example, a method of weakening the light intensity by scattering the light using a liquid crystal diffraction grating or a method of weakening the light intensity by changing the reflection angle using a MEMS mirror may be used. Note that, although both the diffraction grating 132 and the SLM 131 are reflective in FIG. 18, one or both may be transmissive. Also, the wavelength separation may be performed using another wavelength separation element such as a prism instead of the diffraction grating 132.

[0022] In the method using the diffraction grating 132 and the SLM 131, a plurality of dips with equal frequency intervals can be generated, and an optical frequency comb can be generated by converting the dips into peaks by spectral peeking.

[0023] For example, for a wavelength range of 50 nm, an SLM131 with 1250 pixels can produce a dip with a spectral resolution of 40 pm, while a SLM with 4000 pixels can produce a dip with a spectral resolution of 12 pm.

[0024] When a gas cell is used, since the absorption of gases such as methane, ethane, and carbon dioxide in particular has multiple absorption peaks at equal intervals, it is possible to generate a spectrum with linear peaks arranged at equal intervals.

[0025] In the first embodiment, a dip is generated in the transmitted light of the light intensity modulator 13 by absorbing and reflecting the light by the light intensity modulator 13. However, a dip may be generated in the reflected light instead of the transmitted light.

[0026] The optical waveguide 14 propagates the pulsed light from the optical intensity modulator 13 and generates a nonlinear effect in the pulsed light. This nonlinear effect changes the optical intensity at the wavelength of the dip, converting the dip into a peak.

[0027] The optical waveguide 14 may be any one that propagates the pulsed light and generates a nonlinear effect in the pulsed light. For example, it may be an optical fiber, a planar optical waveguide, a rectangular optical waveguide, or the like. It may also have a photonic crystal structure. Since the optical spectrum generating device of the first embodiment can be configured entirely from fibers, an optical fiber is preferable.

[0028] The propagation distance in the optical waveguide 14 is set so that the light is output from the optical waveguide 14 at the timing when the dip is converted into a peak. Since the light intensity at the wavelength of the dip fluctuates periodically according to the propagation distance and alternates between dips and peaks, if the propagation distance in the optical waveguide 14 is set appropriately, the light can be output from the optical waveguide 14 at the timing when the peak occurs. In order to sufficiently increase the light intensity of the peak, it is preferable that the propagation distance is set at the timing when the light intensity at the wavelength of the dip becomes near the maximum value. Near the maximum value is, for example, a range that is 0.5 times or more the light intensity of the maximum value. More preferably, it is 0.8 times or more, and even more preferably, it is 0.9 times or more. In particular, it is preferable that the propagation distance is set so that the light is near the first maximum value. In other words, it is preferable that the propagation distance is set to the minimum propagation distance among the propagation distances that become near the maximum value. This is because the light intensity decreases due to Raman scattering and the like as the propagation distance becomes longer. Also, when multiple dips are converted into peaks, the shift in the period of spectral peeking between those dips becomes significant as the propagation distance becomes longer.

[0029] When an optical fiber is used as the optical waveguide 14, it is preferable to use an anomalous dispersion single mode optical fiber. In an anomalous dispersion single mode optical fiber, the waveform can be shaped into a soliton, so that the shape of the generated peak can be made neater and clearer.

[0030] A small-core fiber is also suitable as the optical waveguide 14. A small-core fiber has a large nonlinear effect and can efficiently generate spectral peeking. It can also improve the signal-to-noise ratio (the ratio of the peak light intensity to the background light intensity).

[0031] An optical fiber amplifier is also suitable as the optical waveguide 14. By using an optical fiber amplifier, it is possible to efficiently generate spectral peeking and simultaneously increase the peak intensity. As the optical fiber amplifier, for example, an erbium-doped fiber amplifier (EDFA) can be used.

[0032] Also, the pulsed light may be converted to supercontinuum light before being input to the optical waveguide 14, or the pulsed light may be converted to supercontinuum light after being output from the optical waveguide 14. A highly nonlinear optical fiber can be used for the conversion to supercontinuum light. By converting to supercontinuum light, the signal-to-noise ratio can be improved. In addition, the spectral width becomes very large, so the number of peaks can be increased. For example, the optical waveguide 14 may be formed by a single-mode optical fiber with anomalous dispersion and a highly nonlinear optical fiber connected to the rear of the optical waveguide 14.

[0033] Furthermore, the optical waveguide 14 may be configured by connecting a plurality of types of optical fibers, planar optical waveguides, rectangular optical waveguides, and the like.

[0034] As described above, the optical spectrum generating device of the first embodiment can generate pulsed light having a spectrum with linear peaks. In particular, it can generate an optical spectrum in which linear peaks are arranged at equal intervals in a comb-like pattern. In addition, the wavelength of the dip can be controlled by the optical intensity modulator 13 to set the linear peak to a desired wavelength.

[0035] Next, the operation of the optical spectrum generating device according to the first embodiment will be described.

[0036] The pulsed light emitted from the pulsed light source 10 has its light intensity amplified by the optical amplifier 11, and its wavelength band adjusted by the wavelength shift fiber 12, and then passes through the optical intensity modulator 13. The pulsed light is strongly absorbed at the absorption peak of the optical intensity modulator 13. Therefore, the spectrum of the pulsed light transmitted through the optical intensity modulator 13 has a dip (see FIG. 2(a)).

[0037] Next, the pulsed light from the optical intensity modulator 13 is passed through the optical waveguide 14. Here, the optical intensity at the wavelength of the dip changes periodically according to the propagation distance in the optical waveguide 14, and dips and peaks are alternately repeated. Here, the propagation distance in the optical waveguide 14 is set so that the light is output from the optical waveguide 14 at the timing when the dip is converted to a peak. Therefore, the pulsed light output from the optical waveguide 14 has a spectrum having a linear peak at the wavelength that was the dip (see FIG. 2(b)). Even when there are multiple dips, they can be simultaneously converted to peaks. In addition, when the pulsed light source 10 is an optical frequency comb light source, an optical frequency comb can be extracted, and in particular, by making multiple dips at equal intervals, an optical frequency comb can be extracted at equal intervals. In addition, by controlling the wavelength of the dip with the optical intensity modulator 13, the peak can be set to a desired wavelength.

[0038] The period of the spectral peeking depends on the optical intensity of the pulsed light as well as the propagation distance in the optical waveguide 14. Therefore, instead of controlling the propagation distance in the optical waveguide 14, it is also possible to set the pulsed light to be output from the optical waveguide 14 at the timing when the dip is converted into a peak by controlling the optical intensity of the pulsed light. The stronger the optical intensity of the pulsed light is, the greater the nonlinear effect in the optical waveguide 14 becomes, and the greater the amount of phase change becomes, so that the period of the spectral peeking becomes shorter. Of course, both the optical intensity and the propagation distance in the optical waveguide 14 may be controlled.

[0039] As described above, the optical spectrum generating device of the first embodiment can convert a linear dip in a spectrum into a linear peak, and generate a pulsed light having a spectrum with a linear peak at a desired wavelength. In particular, it can generate an optical spectrum in which a plurality of linear peaks are arranged at equal intervals in a comb-like pattern.

[0040] Next, various experimental results regarding the optical spectrum generating device of the first embodiment will be described.

[0041] The spectrum of the pulsed light radiated from the optical waveguide 14 of the optical spectrum generation device of the first embodiment was calculated by numerical calculation. The various conditions in this numerical calculation are as follows. The pulsed light input to the optical waveguide 14 is a sech light having a spectral width of 6 nm and a pulse width of 400 fs. 2 The dip was a Lorentz type with a central wavelength of 1650 nm and a spectral width (full width at half maximum) of 20 pm, and the absorptance was 85%. The optical waveguide 14 was an anomalous dispersion single mode fiber, and the second-order dispersion value β2 = -33 ps in the wavelength range of the pulse light. 2 / km, third-order dispersion value β3=0.18ps 3 / km, MFD = 11 μm, and soliton order N = 0.94. The effect of phase shift due to absorption was ignored.

[0042] Fig. 3 is a graph showing the results of numerical calculation of the relationship between the spectrum of the pulsed light output from the optical waveguide 14 and the propagation distance in the optical waveguide 14. Fig. 3(a) shows the case where Raman scattering is taken into account, and Fig. 3(b) shows the case where Raman scattering is ignored. Fig. 4 is a graph showing the relationship between the light intensity at a wavelength of 1650 nm and the fiber length (propagation distance) of the optical waveguide 14. In Fig. 3, the solid line shows the case where Raman scattering is taken into account, and the dotted line shows the case where Raman scattering is ignored.

[0043] As shown in Figures 3 and 4, it was found that the light intensity at a wavelength of 1650 nm changes periodically according to the propagation distance, and changes continuously from a dip to a peak and from the peak to a dip. This shows that if the propagation distance of the optical waveguide 14 is appropriately set, an optical pulse with a peaked spectrum can be emitted from the optical waveguide 14. Furthermore, as shown in Figures 3(a) and 4, when Raman scattering is taken into consideration, the spectrum as a whole shifts to the long wavelength side according to the propagation distance, and the optical intensity also decreases overall, but the wavelength at which the dips and peaks change periodically remains at 1650 nm and does not change.

[0044] Fig. 5(a) is a graph showing the relationship between the spectral width of the dip and the spectral width of the peak, and Fig. 5(b) is an enlarged view of the dip and the peak. The pulse width of the pulsed light is 200 fs and 400 fs, and other conditions of the numerical calculation are the same as Figs. 3 and 4.

[0045] As shown in Figure 5, the spectral width of the peak was found to be approximately 0.8 times the spectral width of the dip.

[0046] Figure 6(a) is a graph showing the results of a numerical calculation of the relationship between the dip absorptance and peak light intensity at a wavelength of 1650 nm, and Figure 6(b) is a graph showing the spectrum when the dip absorptance is set to 99%. The propagation distances were set to 0 m and 11.2 m, and the conditions were the same as in Figures 3 and 4, except that Raman scattering was taken into account. The light intensity is a value normalized to the light intensity at a wavelength of 1650 nm when there is no absorption.

[0047] As shown in Figure 6, when the absorptance was between 0 and 25%, the peak light intensity increased approximately linearly and was equivalent to the amount of absorption. When the absorptance was 50% or more, the peak light intensity increased exponentially, and at an absorptance of 99%, the peak light intensity was approximately 240%. As a result, it was found that absorptance of 50% or more is preferable for the dip.

[0048] Fig. 7 is a graph showing the results of investigating the dependence of the spectral width of the dip on the relationship between the light intensity at a wavelength of 1650 nm and the fiber length (propagation distance) of the optical waveguide 14. The conditions were the same as those in Figs. 3 and 4, except that the spectral width of the dip was changed.

[0049] As shown in Figure 7, periodic changes in light intensity were observed up to a spectral width of 3 nm, but no periodic changes were observed when the spectral width exceeded 3 nm. This shows that in order to convert a dip into a peak, the spectral width of the pulsed light needs to be at least twice the spectral width of the dip.

[0050] Figure 8 is a graph showing the results of investigating the dependency of the peak output of the pulsed light on the relationship between the light intensity at a wavelength of 1650 nm and the fiber length (propagation distance) of the optical waveguide 14. The conditions were the same as those in Figures 3 and 4, except that the peak output of the pulsed light was changed to 400 W and 500 W.

[0051] As shown in Fig. 8, it was found that the light intensity changes periodically, and the period changes depending on the propagation distance and the peak output. From this, it was found that in order to control the output from the optical waveguide 14 at the timing when the light intensity at the wavelength of 1650 nm becomes maximum, it is necessary to control the propagation distance in the optical waveguide 14 or the peak output of the pulsed light.

[0052] Figure 9(a) shows the spectral shape of the pulsed light. 2 9(b) is a graph showing the relationship between the light intensity at a wavelength of 1650 nm and the fiber length (propagation distance) of the optical waveguide 14 when the spectral shape is changed from a Gaussian type to a Gaussian type. The conditions were the same as those in Figs. 3 and 4, except that the spectral shape was Gaussian and the peak output was changed to 500 W, 700 W, and 1000 W. Also, Fig. 9(b) is a graph showing the spectral shape of pulsed light when the peak output was 500 W and the fiber length was 16 m.

[0053] As shown in Figure 9, even when the spectral shape is Gaussian, the light intensity changes periodically and can be converted from a dip to a peak. 2 Although the peak shape was slightly distorted compared to the normal shape, it was a thin, linear and strong peak.

[0054] Figure 10(a) shows the spectral shape of the pulsed light. 210(b) is a graph showing the relationship between the light intensity at a wavelength of 1650 nm and the fiber length (propagation distance) of the optical waveguide 14 when the type is changed from the Gaussian type to the Super Gaussian type. The conditions were the same as those in Figs. 3 and 4, except that the spectral shape was Super Gaussian and the peak output was changed to 500 W, 750 W, and 1000 W. Also, Fig. 10(b) is a graph showing the spectral shape of the pulsed light when the peak output was 500 W and the fiber lengths were 0 m and 26 m.

[0055] As shown in Figure 10, even when the spectral shape is a super-Gaussian type, the light intensity changes periodically and can be converted from a dip to a peak. 2 Although the peak shape was slightly distorted compared to the standard, it was still a thin, linear, and intense peak. It was also found that the overall spectrum shape became narrower.

[0056] Comparing Figures 3, 4, 9, and 10, the spectral shape of the pulsed light is sech 2 The sech type was found to be the most preferred. 2 In the steady state, the soliton pulse of this type undergoes a uniform phase shift over the time waveform, and the pulse waveform is also maintained stable, so it is thought that the peak shape is cleaner than that of the Gaussian or super-Gaussian type.

[0057] Fig. 11(a) is a graph showing the relationship between the light intensity at a wavelength of 1650 nm and the fiber length (propagation distance) of the optical waveguide 14 when the spectral shape of the dip is changed from Lorentzian to Gaussian. The conditions were the same as those in Figs. 3 and 4, except that the spectral shape of the dip was changed to Gaussian and the peak output was set to 500 W. Fig. 11(b) is a graph showing the spectral shapes of the pulsed light when the fiber lengths were 0 m and 26 m.

[0058] As shown in FIG. 11, even when the spectral shape of the dip was Gaussian, a thin, linear, and strong peak was obtained, similar to the case of the Lorentzian type.

[0059] 12(a) to (c) show the spectral shapes when the fiber lengths are 0 m, 6 m, and 12 m, and FIG. 12(d) to (f) are graphs showing the corresponding time waveforms and phases.

[0060] From FIG. 12, the reason why dips and peaks appear periodically can be considered as follows. As shown in FIG. 12(d) to (f), the time waveform is expressed by superposition of narrow pulses and wide pulses. The wide pulses correspond to the dips and peaks in the spectral shape, and the narrow pulses correspond to the parts other than the dips and peaks. When the fiber length is 0 m, as shown in FIG. 12(d), the narrow pulses and the wide pulses have a phase difference of π and cancel each other out, resulting in a dip in the spectral shape as shown in FIG. 12(a). When the pulsed light propagates through the optical waveguide 14, the narrow pulses are strong in intensity and therefore continuously undergo a phase shift due to the nonlinear effect. On the other hand, the wide pulses are weak in intensity and therefore the phase shift is small enough to be ignored. Therefore, the phase difference between the narrow pulse and the wide pulse changes periodically according to the fiber length, and when the phase difference is 0 or an even multiple of π as shown in Fig. 12(f), the narrow pulse and the wide pulse reinforce each other, resulting in a strong linear peak in the spectrum as shown in Fig. 12(c). When the phase difference is π or an odd multiple of π as shown in Fig. 12(d), the spectrum shows a linear dip as shown in Fig. 12(a). In this way, the light intensity changes periodically according to the fiber length, and dips and peaks appear alternately and periodically.

[0061] FIG. 13 is a graph showing the results of numerical calculations of the relationship between the spectrum of the pulsed light output from the optical waveguide 14 and the propagation distance in the optical waveguide 14, in the case where the dips are multiple absorption lines of methane around 1650 nm. The pulsed light is a sech light with a pulse width of 400 fs. 2 The peak output was 2 kW and the central wavelength was 1650 nm. Other conditions were the same as in Figures 3 and 4.

[0062] As shown in Figure 13, even if there are multiple dips, the optical intensity at the wavelengths of those dips changes periodically depending on the propagation distance, and it is possible to simultaneously convert them into multiple linear peaks. In addition, since the absorption lines of methane are arranged at equal intervals, the converted multiple linear peaks are also at equal intervals. It was also found that the intensity of each peak is strong near the central wavelength of the pulsed light and becomes weaker as it moves away from the central wavelength. It was also found that the periodicity of each peak shifts as the propagation distance increases. This is thought to be because the amount of phase shift is wavelength dependent. Figure 13 also shows that it is possible to generate a pulse train with an ultra-high repetition rate of 290 GHz.

[0063] Next, the optical spectrum generating device of Example 1 was actually fabricated, its output was measured using an optical spectrum analyzer and optical power, and the spectral shape at each average output was measured. The specific device configuration was as follows. The short pulse light source 10 was a fiber laser with a ring-type resonator using polarization-maintaining Er-doped fiber and single-walled carbon nanotubes, and outputted pulsed light with a repetition rate of 28 MHz, a pulse width of 300 fs, and a central wavelength of 1556 nm. The optical amplifier 11 was a fully polarization-maintaining Er-doped fiber amplifier. The wavelength shifting fiber 12 was an anomalous dispersion single-mode polarization-maintaining fiber, and its output was a sech with a pulse width of 200 fs. 2 The pulsed light was a soliton pulse of the 1000-nm type. The output was adjusted in the optical amplifier 11 so that the central wavelength was 1650 nm. The optical intensity modulator 13 was a gas cell filled with methane gas, and the output from the wavelength shift fiber 12 was passed through a long-pass filter and then through the gas cell. The optical waveguide 14 was a 20 m single mode fiber, and the pulsed light that had passed through the gas cell was passed through the single mode fiber.

[0064] Figure 14(a) is a graph showing the measured spectrum, and Figure 14(b) is a graph showing the spectrum obtained by numerical calculation. As shown in Figure 14(a), at an average output of 1.0 mW, multiple equally spaced absorption lines due to methane gas were observed. It was found that as the average output increased, the spectral width was compressed due to the soliton effect, the light intensity at the wavelength of the absorption line changed, and linear dips could be converted into linear peaks. Furthermore, comparing Figures 14(a) and 14(b), the measured results roughly matched the results of numerical calculation.

[0065] Next, the optical waveguide 14 was replaced with a small-core fiber with a fiber length of 5 m and MFD = 5.5 μm, and the spectral shape was measured in the same manner. Figure 15 is a graph showing the measurement results of the spectral shape. As shown in Figure 15, it was found that the light intensity at the wavelength of the absorption line changed, and linear dips could be converted into linear peaks. In addition, since the small-core fiber has a higher soliton order than the single-mode fiber used in Figure 14, the spectral width became wider as the average output power increased. As a result of the wider spectral width, the number of dips and peaks also increased. In addition, the larger the average output power, the more the spectral shape was distorted due to self-phase modulation and Raman scattering.

[0066] Figure 16 is a graph showing an enlarged view of the spectral shape near the wavelength of 1650 nm. As shown in Figure 16, the dip with a spectral width of 20 pm was converted into a peak with a spectral width of 18 pm, which roughly matched the results of the numerical calculation. In addition, the background output level was low due to the collapse of the pulse shape, and a high signal-to-noise ratio was obtained.

[0067] Next, the optical waveguide 14 was replaced with a single mode fiber and a highly nonlinear optical fiber with normal dispersion connected in sequence, and the spectrum shape was measured in the same manner. The single mode fiber in the front stage was 10 cm long, and the highly nonlinear optical fiber in the rear stage was 5 m long. The second-order dispersion value β2 of the highly nonlinear optical fiber was 6.4 ps 2 / km, third-order dispersion value β3=0.0057ps 3 / km, and the nonlinear coefficient is 23 W at a wavelength of 1.56 μm. -1km -1 It was decided.

[0068] Figure 17 is a graph showing the measurement results of the spectral shape. As shown in Figure 17, it was found that the optical intensity at the wavelength of the absorption line changes, and that the linear dips and linear peaks change periodically as the average output increases. In addition, it was found that the spectral width is greatly broadened, the number of peaks increases, and the signal-to-noise ratio also increases due to the strong self-phase modulation caused by the highly nonlinear optical fiber.

[0069] 18 was used as the optical intensity modulator 13. The diffraction grating 132 had a resolution of 900 lines / mm, and the SLM 131 had a pitch of 20 μm and 800 pixels.

[0070] Fig. 19 is a graph showing the spectrum of the input light and the output light to the optical intensity modulator 13 of Fig. 18. As shown in Fig. 19, a plurality of equally spaced dips could be generated at a desired wavelength.

[0071] Figure 20 is a graph showing the spectrum after the light from the optical intensity modulator 13 in Figure 18 is passed through an optical fiber. As shown in Figure 20, the dip could be converted into a peak. As a result, it was found that the use of the optical intensity modulator 13 in Figure 18 makes it possible to generate a peak at a desired wavelength. EXAMPLES

[0072] Fig. 21 shows the configuration of an optical spectrum generating device of Example 2. The optical spectrum generating device of Example 2 is similar in other respects to the optical spectrum generating device of Example 1, except that the optical intensity modulator 13 is replaced with an optical phase modulator 23.

[0073] The optical phase modulator 23 is a device that modulates the phase of an arbitrary wavelength. Its configuration can be a combination of a diffraction grating 132 and an SLM 131, as in FIG. 18. However, it is different in that the SLM 131 modulates the phase instead of the intensity. When the pulsed light having a predetermined wavelength phase-modulated by the optical phase modulator 23 is passed through the optical waveguide 14, spectrum peaking occurs at the phase-modulated wavelength as in the first embodiment. That is, dips and peaks appear alternately and repeatedly at the phase-modulated wavelength according to the transmission distance of the optical waveguide 14. Therefore, by appropriately setting the transmission distance of the optical waveguide 14, it is possible to generate pulsed light having a spectrum with a linear peak at a predetermined wavelength. The phase modulation amount may be any amount other than 0, but the closer it is to π, the greater the peak intensity can be. For example, the phase modulation amount is set to 0.1 to π (rad) or -π to -0.1 (rad).

[0074] The reason why spectral peaking occurs not only in intensity modulation but also in phase modulation is as suggested in Fig. 12. That is, a difference occurs in the amount of phase shift in the optical waveguide 14 between the phase-modulated region and the other regions due to nonlinear effects. Therefore, the phase difference between the phase-modulated region and the other regions changes periodically according to the transmission distance, and they reinforce each other when the phase difference is 0 or an even multiple of π, and they weaken each other when the phase difference is π or an odd multiple of π. As a result, the optical intensity changes periodically according to the transmission distance in the optical waveguide 14, and dips and peaks appear alternately and periodically.

[0075] It is to be noted that both intensity modulation and phase modulation may be performed, which can similarly result in spectral peaking and generate peaks.

[0076] Fig. 22 shows the results of numerically calculating the spectrum of pulsed light emitted from the optical waveguide 14 of the optical spectrum generation device of Example 2. Fig. 22(a) is a graph showing the power spectrum and phase spectrum of the pulsed light input to the optical waveguide 14. Fig. 22(b) is a graph showing the power spectrum of the pulsed light output from the optical waveguide 14. As shown in Fig. 22(a), the input pulsed light is phase modulated at a predetermined frequency interval. It was found that a peak is generated at the phase-modulated wavelength, as shown in Fig. 22(b).

[0077] (Various variations) The optical spectrum generating device of the present disclosure is suitable for generating a spectrum having multiple linear peaks. Light with such a spectrum can be used for optical multiplexing communications, etc.

[0078] The optical spectrum generating device of the present disclosure is also suitable for extracting optical frequency combs at equal intervals. Conventional optical frequency comb generation methods have narrow comb intervals, and in practice, it has been necessary to widen the comb interval or thin out the comb lines appropriately, but such control has been difficult. However, according to the present disclosure, the optical frequency comb can be extracted at desired intervals due to the characteristics of the optical intensity modulator 13, thereby improving practicality. For example, while the comb interval in conventional optical frequency combs is several tens of MHz, the present disclosure can extract it at intervals of several hundreds of GHz.

[0079] Furthermore, the optical spectrum generating device of the present disclosure can generate pulsed light with a high repetition rate, making it suitable for optical sampling.

[0080] The wavelength of the peak generated by the present disclosure is not limited, and a peak of any wavelength can be generated. For example, a peak can be generated in the mid-infrared band or the far-infrared band. [Industrial Applicability]

[0081] The present disclosure can be used for generating optical frequency combs, optical wavelength division multiplexing communications, optical sampling, and the like. [Explanation of symbols]

[0082] 10: Pulsed light source 11: Optical amplifier 12: Wavelength shifting fiber 13: Optical intensity modulator 14: Optical waveguide 23: Optical phase modulator

Claims

1. An optical spectrum generating device for generating pulsed light having a spectrum with a peak, comprising: a pulsed light source that generates the pulsed light; an optical modulator that intensity-modulates or phase-modulates a predetermined wavelength of the pulsed light from the pulsed light source; an optical waveguide that propagates the pulsed light from the optical modulator and causes a nonlinear effect on the pulsed light, a spectral width of the pulsed light is equal to or greater than twice the spectral width of the predetermined wavelength, the optical waveguide is configured so that the pulsed light alternates between peaks and dips at the predetermined wavelength in accordance with a propagation distance of the pulsed light in the optical waveguide; the length of the optical waveguide is set so that the pulsed light is extracted from the optical waveguide when the predetermined wavelength reaches a peak. An optical spectrum generating device comprising:

2. An optical spectrum generating device for generating pulsed light having a spectrum with a peak, comprising: a pulsed light source that generates the pulsed light; an optical modulator that intensity-modulates or phase-modulates a predetermined wavelength of the pulsed light from the pulsed light source; an optical waveguide that propagates the pulsed light from the optical modulator and causes a nonlinear effect on the pulsed light, a spectral width of the pulsed light is equal to or greater than twice the spectral width of the predetermined wavelength, the optical waveguide is configured so that the pulsed light alternates between peaks and dips at the predetermined wavelength in accordance with a propagation distance of the pulsed light in the optical waveguide; the light intensity of the pulsed light is set so that the pulsed light is extracted from the optical waveguide when the predetermined wavelength reaches a peak. An optical spectrum generating device comprising:

3. An optical spectrum generating device as described in claim 1, wherein the length of the optical waveguide is set to the minimum distance at which the specified wavelength peaks.

4. The pulse shape of the pulsed light is ２ 4. The optical spectrum generating device according to claim 1, characterized in that it is of the type.

5. 5. The optical spectrum generating device according to claim 1, wherein the optical modulator generates intensity modulation or phase modulation for a plurality of wavelengths.

6. 6. The optical spectrum generating device according to claim 5, wherein the optical modulator generates intensity modulation or phase modulation for a plurality of wavelengths that are discretely arranged at equal intervals.

7. 7. The optical spectrum generating device according to claim 1, wherein the optical modulator is an optical intensity modulator that generates a dip by intensity-modulating the predetermined wavelength.

8. 8. The optical spectrum generating device according to claim 7, wherein the spectral shape of the dip is Lorentzian or Gaussian.

9. 9. The optical spectrum generating device according to claim 7, wherein the optical intensity modulator is a gas.

10. 10. The optical spectrum generating device of claim 9, wherein the gas is methane.

11. The optical intensity modulator includes: a wavelength separation element that separates the wavelengths of the pulsed light from the pulsed light source; a spatial light intensity modulator that causes light from the wavelength separation element to enter a different position for each wavelength and intensity-modulates the light for each position; 9. The optical spectrum generating device according to claim 7 or 8, further comprising:

12. 7. The optical spectrum generating device according to claim 1, wherein the optical modulator is an optical phase modulator that phase-modulates the predetermined wavelength.

13. The optical phase modulator comprises: a wavelength separation element that separates the wavelengths of the pulsed light from the pulsed light source; a spatial light phase modulator that causes light from the wavelength separation element to enter at different positions for each wavelength and phase-modulates the light for each position; 13. The optical spectrum generating device according to claim 12, further comprising:

14. 14. The optical spectrum generating device according to claim 1, wherein the pulsed light source is an optical frequency comb light source that outputs an optical frequency comb.

15. Further comprising a highly nonlinear optical fiber, 15. The optical spectrum generating device according to claim 1, wherein a highly nonlinear optical fiber converts the pulsed light from the optical modulator into supercontinuum light before inputting the pulsed light into the optical waveguide or after outputting the pulsed light from the optical waveguide.

16. the optical waveguide is composed of an anomalous dispersion single mode fiber and a highly nonlinear optical fiber connected to a rear stage of the anomalous dispersion single mode fiber, 15. The optical spectrum generating device according to claim 1, wherein the highly nonlinear optical fiber converts the pulsed light into supercontinuum light.

17. 15. The optical spectrum generating device according to claim 1, wherein the optical waveguide is an optical fiber amplifier.

18. 1. A method for generating pulsed light having a peaked spectrum, comprising: After intensity modulation or phase modulation is caused in the predetermined wavelength of the pulsed light, the pulsed light is propagated through an optical waveguide to cause a nonlinear effect in the pulsed light; the optical waveguide is configured so that the pulsed light alternates between peaks and dips at the predetermined wavelength in accordance with a propagation distance of the pulsed light in the optical waveguide; the length of the optical waveguide is set so that the pulsed light is extracted from the optical waveguide when the predetermined wavelength reaches a peak.

2. A method for generating an optical spectrum comprising:

19. 1. A method for generating pulsed light having a peaked spectrum, comprising: After intensity modulation or phase modulation is caused in the predetermined wavelength of the pulsed light, the pulsed light is propagated through an optical waveguide to cause a nonlinear effect in the pulsed light; the optical waveguide is configured so that the pulsed light alternates between peaks and dips at the predetermined wavelength in accordance with a propagation distance of the pulsed light in the optical waveguide; the light intensity of the optical waveguide is set so that the pulsed light is extracted from the optical waveguide when the predetermined wavelength reaches a peak.

2. A method for generating an optical spectrum comprising:

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