WO2021251365A1

WO2021251365A1 - Optical spectrum generation device and optical spectrum generation method

Info

Publication number: WO2021251365A1
Application number: PCT/JP2021/021693
Authority: WO
Inventors: 典彦西澤; 真仁山中
Original assignee: 国立大学法人東海国立大学機構
Priority date: 2020-06-11
Filing date: 2021-06-08
Publication date: 2021-12-16
Also published as: JPWO2021251365A1

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

Optical spectrum generator, optical spectrum generation method

The present disclosure relates to an apparatus for generating an optical spectrum having a linear peak, and a method thereof.

The optical spectrum in which spectral lines with very narrow line widths are discretely arranged in a comb-teeth pattern at equal intervals is called an optical frequency comb and is used as a frequency measure. As a method of generating an optical frequency comb, a method using a mode lock laser is known.

International Publication No. 2018/181213

However, an apparatus that generates an optical spectrum having a linear peak is expensive, and there is a demand for cost reduction of the apparatus. In addition, the wavelength at which a linear peak can be obtained is also limited.

Therefore, an object of the present disclosure is to realize an optical spectrum generator and an optical spectrum generation method for generating an optical spectrum having a linear peak.

When a pulsed light having a linear dip in the spectrum is passed through the optical waveguide, the inventors periodically change the light intensity at the wavelength of the dip according to the propagation distance in the optical waveguide, and the dip and the peak alternate. I found a phenomenon that appears in. The present disclosure is based on this novel phenomenon.

The present disclosure is an optical spectrum generator that generates pulsed light of a spectrum having a peak, and is an optical spectrum light source that generates pulsed light and optical modulation that intensity-modulates or phase-modulates a predetermined wavelength of pulsed light from the pulsed light source. It has a device and an optical waveguide that propagates pulsed light from a light intensity modulator, causes a non-linear effect in the pulsed light, and produces a peak at a predetermined wavelength, and the spectral width of the pulsed light is the spectral width of the dip. It is an optical spectrum generator characterized by being more than twice as large.

According to the present disclosure, it is possible to inexpensively realize an apparatus for generating an optical spectrum having a linear peak.

The figure which showed the structure of the optical spectrum generation apparatus of Example 1. FIG. The figure which showed the spectral shape schematically. A graph showing the relationship between the spectrum of pulsed light and the propagation distance. A graph showing the relationship between light intensity and fiber length. A graph showing the relationship between the spectral width of a dip and the spectral width of a peak. A graph showing the relationship between the absorption rate of dips and the light intensity of peaks. A graph showing the results of investigating the dependence of the spectral width of the dip on the relationship between the light intensity and the fiber length. A graph showing the results of investigating the dependence of the peak output of pulsed light on the relationship between light intensity and fiber length. A graph showing the relationship between the light intensity and the fiber length when the spectral shape of the pulsed light is changed to the Gaussian type. A graph showing the relationship between the light intensity and the fiber length when the spectral shape of the pulsed light is changed to the Super Gaussian type. A graph showing the relationship between the light intensity and the fiber length when the spectral shape of the dip is changed to the Gaussian type. A graph showing the spectral shapes and their corresponding time waveforms and phases. A graph showing the relationship between the spectrum of pulsed light and the propagation distance. A graph showing the measurement results of the spectrum, and a graph showing the spectrum obtained by numerical calculation. A graph showing the measurement results of the spectral shape. A graph showing an enlarged spectrum shape. A graph showing the measurement results of the spectral shape. The figure which showed the structure of the light intensity modulator 13. A graph showing the spectra of input light and output light. A graph showing the spectrum. The figure which showed the structure of the optical spectrum generation apparatus of Example 2. FIG. A graph showing the spectra of input light and output light.

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.

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

The short pulse light source 10 is a light source that emits light having a narrow pulse width (time width). For example, a fiber laser of a ring type resonator can be used. The time width (full width at half maximum) of the pulsed 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.

The pulse shape of the pulsed light is arbitrary, and is, for example, sech ² type (soliton), Gauss type, super Gauss type, or the like. In particular, sech ² type is preferable. A clearer and clearer linear peak can be obtained.

The optical amplifier 11 is a device that amplifies the light 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 non-linear effect in the optical waveguide 14 in the subsequent stage occurs at a constant light intensity or higher, the optical amplifier 11 is used to amplify the light intensity. The period of spectral peaking also depends on the light intensity. Therefore, the period of spectral peaking can be adjusted by the amplification factor in the optical amplifier 11.

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 by the light intensity modulator 13 in the subsequent stage with respect to the spectrum of the pulsed light. For example, the peak wavelength of the pulsed light is adjusted to be close to the wavelength of the absorption peak. Since the wavelength shift amount in the wavelength shift fiber 12 depends on the light intensity, the wavelength shift amount can be controlled by the optical amplifier 11.

The light intensity modulator 13 is a filter that causes absorption in a narrow band with respect to the pulsed light from the wavelength shift fiber 12 and causes a linear dip in the spectrum of the pulsed light. The number of dips to be generated does not have to be one, and a plurality of dips may be generated. By generating evenly spaced dips, spectral peaking can generate a spectrum of evenly spaced linear peaks. The wavelength of the dip may be any wavelength as long as it is within the wavelength band of the pulsed light. However, in order to sufficiently increase the intensity of the peak, the vicinity of the center wavelength is preferable.

The spectral shape of the dip by the light intensity modulator 13 is arbitrary, but Lorentz type, Gauss type, sech ² type and the like are preferable. You can get a cleaner and clearer peak.

Make sure that the spectral width of the pulsed light (full width at half maximum) is at least twice the spectral width of the dip (full width at half maximum). Spectral peaking can be generated by setting the spectral width of the pulsed light or the spectral width of the dip in this way. The spectral width of the dip is arbitrary as long as it is within the range satisfying this, but is, for example, 10 nm or less, preferably 1 nm or less.

The absorption rate in the dip is arbitrary, but it is preferable to set the absorption rate to 50% or more in order to sufficiently increase the light intensity of the peak. It is more preferably 70% or more, still more preferably 90% or more.

The light intensity modulator 13 is arbitrary as long as it has an absorption peak in a narrow band, and a gas cell, a diffraction grating, a photonic crystal, or the like can be used. In particular, when a fiber Bragg grating (FBG) is used as the diffraction grating, the optical spectrum generator of Example 1 can be configured with all fibers, and handling becomes easy. Further, in the case of a diffraction grating or a photonic crystal, it is possible to change the wavelength of the absorption peak by controlling the temperature.

As shown in FIG. 18, a dip may be generated at an arbitrary wavelength by using a diffraction grating 132 and a spatial light modulator (SLM) 131. That is, the pulsed light is wavelength-separated by the diffraction grating 132 so that the arrival position at the SLM 131 differs depending on the wavelength. Since the SLM 131 can modulate the light intensity for each arrival position of the light, the light intensity of a desired wavelength can be weakened to generate a dip. The light intensity may be weakened by scattering or weakened by absorption. For example, a method of reducing the light intensity by scattering light with a diffraction grating made of a liquid crystal may be used, or a method of weakening the light intensity by changing the reflection angle with a MEMS mirror may be used. In FIG. 18, both the diffraction grating 132 and the SLM 131 are of the reflective type, but one or both of them may be of the transmissive type. Further, the wavelength may be separated by using another wavelength separating element such as a prism instead of the diffraction grating 132.

In the method using the diffraction grating 132 and the SLM 131, a plurality of dips having the same frequency interval can be generated, so that an optical frequency comb can be generated by converting the dips into peaks by spectral peaking.

For example, when the wavelength range is 50 nm, a dip can be generated with a spectral resolution of 40 pm when the number of pixels of the SLM 131 is 1250 and a spectral resolution of 12 pm when the number of pixels is 4000.

When a gas cell is used, in particular, the absorption of gas such as methane, ethane, and carbon dioxide has a large number of absorption peaks at equal intervals, so that it is possible to generate a spectrum in which linear peaks are arranged at equal intervals.

In Example 1, the light intensity modulator 13 absorbs and reflects the light to cause a dip in the transmitted light of the light intensity modulator 13, but the reflected light is not a transmitted light but a dip is generated. There may be.

The optical waveguide 14 propagates the pulsed light from the light intensity modulator 13 and causes a non-linear effect in the pulsed light. Due to this non-linear effect, the light intensity at the wavelength of the dip is changed and the dip is converted into a peak.

The optical waveguide 14 may be arbitrary as long as it propagates pulsed light and causes a non-linear effect in the pulsed light. For example, it may be an optical fiber, a planar optical waveguide, a rectangular optical waveguide, or the like. Further, a photonic crystal structure may be used. An optical fiber is preferable from the viewpoint that the optical spectrum generator of the first embodiment can be configured by all fibers.

The propagation distance in the optical waveguide 14 is set so as to be output from the optical waveguide 14 at the timing when the dip is converted to a peak. Since the light intensity at the wavelength of the dip fluctuates periodically according to the propagation distance and the dip and the peak are repeated alternately, if the propagation distance in the optical waveguide 14 is set appropriately, the light is optical at the timing of the peak. It can be output from the waveguide 14. 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 is near the maximum value. The vicinity of the maximum value is, for example, a range in which the light intensity of the maximum value is 0.5 times or more. It is more preferably 0.8 times or more, still more preferably 0.9 times or more. In particular, it is preferable that the propagation distance is set so as to be near the initial maximum value. That is, it is preferable that the propagation distance is set to the minimum of the propagation distances near the maximum value. This is because the longer the propagation distance, the lower the light intensity due to Raman scattering and the like. Further, when converting a plurality of dips into peaks, the deviation of the period of spectral peaking between those dips becomes remarkable as the propagation distance becomes long.

When the optical waveguide 14 is an optical fiber, it is preferable to use a single-mode optical fiber with anomalous dispersion. In the anomalous dispersion single-mode optical fiber, the waveform can be shaped into soliton, so the shape of the generated peak can be made clearer and clearer.

Further, a small diameter core fiber is also suitable as the optical waveguide 14. The small-diameter core fiber has a large non-linear effect and can efficiently generate spectral peaking. In addition, the signal-to-noise ratio (ratio of peak light intensity to background light intensity) can be improved.

An optical fiber amplifier is also suitable as the optical waveguide 14. If an optical fiber amplifier is used, spectral peaking can be efficiently generated, and at the same time, the peak intensity can be increased. As the optical fiber amplifier, for example, an erbium-doped fiber amplifier (EDFA) can be used.

Further, the pulsed light may be converted into supercontinuum light before the input to the optical waveguide 14, or the pulsed light may be converted into supercontinuum light after output from the optical waveguide 14. A highly non-linear optical fiber can be used for conversion to supercontinuum light. By converting to supercontinuum light, the SN ratio can be improved. Moreover, since the spectral width becomes very large, the number of peaks can be increased. For example, the optical waveguide 14 may be configured by an anomalous dispersion single-mode optical fiber and a highly non-linear optical fiber connected to a subsequent stage.

Further, the optical waveguide 14 may be configured by connecting a plurality of types of optical fibers, a planar optical waveguide, a rectangular optical waveguide, or the like.

As described above, according to the optical spectrum generator of the first embodiment, it is possible to generate pulsed light having a spectrum having a linear peak. In particular, it is possible to generate an optical spectrum in which linear peaks are arranged in a comb-teeth pattern at equal intervals. Further, by controlling the wavelength of the dip with the light intensity modulator 13, the linear peak can be set to a desired wavelength.

Next, the operation of the optical spectrum generator of the first embodiment will be described.

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

Next, the pulsed light from the light intensity modulator 13 is passed through the optical waveguide 14. Here, the light intensity at the wavelength of the dip changes periodically according to the propagation distance in the optical waveguide 14, and the dip and the peak are alternately repeated. Here, the propagation distance in the optical waveguide 14 is set so as to be 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 a dip (see FIG. 2B). If you have multiple dips, you can convert them to peaks at the same time. Further, when the pulse light source 10 is used as an optical frequency comb light source, the optical frequency combs can be cut out, and in particular, by making a plurality of dips at equal intervals, the optical frequency combs can be cut out at equal intervals. Further, by controlling the wavelength of the dip with the light intensity modulator 13, the peak can be set to a desired wavelength.

The period of spectral peaking depends not only on the propagation distance in the optical waveguide 14 but also on the light intensity of the pulsed light. Therefore, instead of controlling the propagation distance in the optical waveguide 14, it is possible to set the output from the optical waveguide 14 at the timing when the dip is converted to the peak by controlling the light intensity of the pulsed light. The stronger the light intensity of the pulsed light, the larger the non-linear effect in the optical waveguide 14, and the larger the amount of phase change, so that the period of spectral peaking becomes shorter. Of course, both the light intensity and the propagation distance in the optical waveguide 14 may be controlled.

As described above, according to the optical spectrum generator of Example 1, a linear dip in the spectrum can be converted into a linear peak, and pulsed light of a spectrum having a linear peak at a desired wavelength is generated. be able to. In particular, it is possible to generate an optical spectrum in which a plurality of linear peaks are arranged in a comb-teeth shape at equal intervals.

Next, various experimental results regarding the optical spectrum generator of Example 1 will be described.

The spectrum of the pulsed light emitted from the optical waveguide 14 of the optical spectrum generator of Example 1 was obtained by numerical calculation. Various conditions in this numerical calculation are as follows. The pulsed light input to the optical waveguide 14 had a spectrum width of 6 nm, a pulse width of 400 fs, a sech ² type (soliton), a peak output of 500 W, and a center wavelength of 1650 nm. The dip was a Lorentz type with a center wavelength of 1650 nm and a spectrum width (full width at half maximum) of 20 pm, and had an absorption rate of 85%. The optical waveguide 14 is an anomalous dispersion single mode fiber, and the secondary dispersion value β ₂ = −33 ps ² / km, the tertiary dispersion value β ₃ = 0.18 ps ³ / km, and MFD = 11 μm in the wavelength range of the pulsed light. The soliton order N = 0.94. Moreover, the influence of the phase shift due to absorption was ignored.

FIG. 3 is a graph showing the result obtained by 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. 3A shows a case where Raman scattering is taken into consideration, and FIG. 3B shows a case where Raman scattering is ignored. Further, 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 is the case where Raman scattering is considered, and the dotted line is the case where Raman scattering is ignored.

As shown in FIGS. 3 and 4, it was found that the light intensity at a wavelength of 1650 nm changes periodically according to the propagation distance, and continuously changes from dip to peak and from peak to dip. From this, it was found that if the propagation distance of the optical waveguide 14 is appropriately set, an optical pulse having a spectrum having a peak can be radiated from the optical waveguide 14. Further, as shown in FIGS. 3A 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 light intensity also decreases as a whole, but with a dip. The wavelength at which the peak changed periodically was 1650 nm and did not change.

FIG. 5A is a graph showing the relationship between the spectral width of the dip and the spectral width of the peak, and FIG. 5B is an enlarged view of the dip and the peak. The pulse widths of the pulsed light are 200 fs and 400 fs, and the other numerical calculation conditions are the same as in FIGS. 3 and 4.

As shown in FIG. 5, it was found that the spectral width of the peak was about 0.8 times the spectral width of the dip.

FIG. 6A is a graph showing the result of numerically calculating the relationship between the dip absorption rate at a wavelength of 1650 nm and the light intensity of the peak, and FIG. 6B is a graph showing the dip absorption rate of 99%. It is a graph which showed the spectrum at the time of. The propagation distance was set to 0 m and 11.2 m, and the conditions were the same as in FIGS. 3 and 4 except that Raman scattering was taken into consideration. The light intensity is a value specified by the light intensity at a wavelength of 1650 nm when there is no absorption.

As shown in FIG. 6, when the absorption rate was 0 to 25%, the light intensity of the peak increased approximately linearly and was equivalent to the absorption amount. When the absorption rate was 50% or more, the peak light intensity increased exponentially, and when the absorption rate was 99%, the peak light intensity was about 240%. As a result, it was found that the absorption rate of the dip is preferably 50% or more.

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 in FIGS. 3 and 4 except that the spectral width of the dip was changed.

As shown in FIG. 7, a periodic change in light intensity was observed up to a dip spectrum width of 3 nm, but no periodic change was observed when the dip spectrum width exceeded 3 nm. From this, it was found that the spectral width of the pulsed light needs to be at least twice the spectral width of the dip in order to convert the dip into a peak.

FIG. 8 is a graph showing the results of investigating the dependence of the peak output of 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 in FIGS. 3 and 4 except that the peak output of the pulsed light was changed to 400 W and 500 W.

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 is maximized, the propagation distance in the optical waveguide 14 or the peak output of the pulsed light should be controlled. rice field.

FIG. 9A 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 pulsed light ^{is changed from sech 2 type to Gauss type.} .. The spectral shape was Gaussian, and the conditions were the same as in FIGS. 3 and 4 except that the peak outputs were changed to 500 W, 700 W, and 1000 W. Further, FIG. 9B is a graph showing the spectral shape of the pulsed light when the peak output is 500 W and the fiber length is 16 m.

As shown in FIG. 9, it was found that even when the spectral shape is Gaussian, the light intensity changes periodically and can be converted from a dip to a peak. Moreover, although ^{the shape of the peak was slightly distorted as compared with the sec 2} type, it was a thin, linear and strong peak.

FIG. 10A 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 pulsed light ^{is changed from sech 2 type to Super Gauss type.} be. The spectral shape was a Super Gaussian type, and the conditions were the same as in FIGS. 3 and 4 except that the peak outputs were changed to 500 W, 750 W, and 1000 W. Further, FIG. 10B is a graph showing the spectral shape of the pulsed light when the peak output is 500 W, the fiber length is 0 m, and the fiber length is 26 m.

As shown in FIG. 10, it was found that 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. Moreover, although ^{the shape of the peak was slightly distorted as compared with the sec 2} type, it was a thin, linear and strong peak. It was also found that the overall spectral shape became narrower.

Comparing FIGS. 3, 4, 9 and 10, ^{it was found that the sech 2} type is the most preferable for the spectral shape of the pulsed light. The sech ² type soliton pulse undergoes a uniform phase shift over the time waveform in the steady state, and the pulse waveform is also maintained stable, so it is thought that the peak shape appears more clearly than the Gauss type and Super Gauss type. ..

FIG. 11A 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 the Lorentz type to the Gauss type. The conditions were the same as in FIGS. 3 and 4 except that the spectral shape of the dip was Gaussian and the peak output was 500 W. Further, FIG. 11B is a graph showing the spectral shape of the pulsed light when the fiber length is 0 m and 26 m.

As shown in FIG. 11, even when the spectral shape of the dip was Gaussian, it was possible to obtain a thin, linear and strong peak as in the case of the Lorentz type.

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

From Fig. 12, the reason why dips and peaks appear periodically is considered as follows. As shown in FIGS. 12 (d) to 12 (f), the time waveform is represented by a superposition of a narrow pulse and a wide pulse. Wide pulses correspond to dips and peaks in the spectral shape, and narrow pulses correspond to parts other than dips and peaks. When the fiber length is 0 m, as shown in FIG. 12 (d), the phases of the narrow pulse and the wide pulse are different by π and are canceled out. Therefore, as shown in FIG. 12 (a), the spectral shape is dip. Become. When the pulsed light propagates through the optical waveguide 14, the narrow pulse has a high intensity and is continuously subjected to a phase shift due to a non-linear effect. On the other hand, a wide pulse has a weak intensity, so the phase shift is negligibly small. Therefore, when the phase difference between the narrow pulse and the wide pulse changes periodically according to the fiber length and the phase difference becomes 0 or even multiples of π as shown in FIG. 12 (f). In addition, the narrow pulse and the wide pulse strengthen each other, resulting in a strong linear peak in the spectral shape as shown in FIG. 12 (c). Further, when the phase difference becomes π or an odd multiple of π as shown in FIG. 12 (d), a linear dip is obtained in the spectral shape 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.

FIG. 13 shows the results obtained by 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 when the dip is a plurality of absorption lines in the vicinity of 1650 nm of methane. It is a graph. The pulsed light was a sech ² type with a pulse width of 400 fs, a peak output of 2 kW, and a center wavelength of 1650 nm. Other conditions were the same as in FIGS. 3 and 4.

As shown in FIG. 13, even when having a plurality of dips, the light intensity at the wavelengths of those dips changes periodically according to the propagation distance, and can be simultaneously converted into a plurality of linear peaks. I understood. Moreover, since the absorption lines of methane are arranged at equal intervals, the converted plurality of linear peaks are also at equal intervals. It was also found that the intensity of each peak was strong near the center wavelength of the pulsed light and weakened as the distance from the center wavelength increased. It was also found that the periodicity of each peak shifts as the propagation distance increases. It is considered that this is because the phase shift amount has a wavelength dependence. Further, from FIG. 13, it was found that a pulse train having an ultra-high repetition rate of 290 GHz can be generated.

Next, the optical spectrum generator of Example 1 was actually manufactured, its output was measured by an optical spectrum analyzer and optical power, and the spectrum shape at each average output was measured. The specific device configuration is as follows. The short pulse light source 10 is a fiber laser of a ring-type resonator using a polarization-retaining Er-doped fiber and a single-layer carbon nanotube, and is used to output pulsed light having a repetition rate of 28 MHz, a pulse width of 300 fs, and a center wavelength of 1556 nm. board. As the optical amplifier 11, a fully polarized wave holding type Er-doped fiber amplifier was used. As the wavelength shift fiber 12, an anomalous dispersion single-mode polarization holding fiber was used, and the output thereof was a sech ² type soliton pulse having a pulse width of 200 fs. Further, the output was adjusted in the optical amplifier 11 so that the center wavelength was 1650 nm. The light 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 passed through the gas cell. The optical waveguide 14 was a 20 m single-mode fiber, and pulsed light transmitted through the gas cell was passed through the single-mode fiber.

FIG. 14 (a) is a graph showing the measurement results of the spectrum, and FIG. 14 (b) is a graph showing the spectrum obtained by numerical calculation. As shown in FIG. 14A, at an average output of 1.0 mW, a plurality of equally spaced absorption lines due to methane gas were observed. It was found that when the average output increased, the spectral width was compressed by the soliton effect, the light intensity at the wavelength of the absorption line changed, and the linear dip could be converted into a linear peak. Further, when FIG. 14 (a) and FIG. 14 (b) were compared, the measurement results were substantially in agreement with the results of the numerical calculation.

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

FIG. 16 is an enlarged graph showing the spectral shape near the wavelength of 1650 nm. As shown in FIG. 16, a dip having a spectrum width of 20 pm was converted into a peak having a spectrum width of 18 pm, which was in good agreement with the result of the numerical calculation. In addition, the background output level was low due to the collapse of the pulse shape, and a high SN ratio was obtained.

Next, the optical waveguide 14 was replaced with a single-mode fiber and a normally dispersed high-non-linear optical fiber connected in order, and the spectral shape was measured in the same manner. The single-mode fiber in the first stage was set to 10 cm, and the high-non-linear optical fiber in the rear stage was set to 5 m. _{The secondary dispersion value β 2} = 6.4 ps ² / km and the tertiary dispersion value β ₃ = 0.0057 ps ³ / km of the highly nonlinear optical fiber were set, and the nonlinear coefficient was set to 23 W ^-1 km ^-1 at a wavelength of 1.56 μm.

FIG. 17 is a graph showing the measurement results of the spectral shape. As shown in FIG. 17, it was found that the light intensity at the wavelength of the absorption line changed, and the linear dip and the linear peak changed periodically as the average output increased. It was also found that the spectral width was greatly widened due to the strong self-phase modulation by the highly nonlinear optical fiber, the number of peaks increased, and the SN ratio also increased.

Next, as the light intensity modulator 13, a combination of the diffraction grating 132 and SLM131 shown in FIG. 18 was used. The diffraction grating 132 used was 900 lines / mm, and the SLM 131 was 800 pixels at a pitch of 20 μm.

FIG. 19 is a graph showing spectra of input light and output light to the light intensity modulator 13 of FIG. As shown in FIG. 19, a plurality of evenly spaced dips could be generated at a desired wavelength.

FIG. 20 is a graph showing a spectrum after passing light from the light intensity modulator 13 of FIG. 18 through an optical fiber. As shown in FIG. 20, the dip could be converted into a peak. As a result, it was found that the peak can be generated at a desired wavelength by using the light intensity modulator 13 of FIG.

FIG. 21 is a diagram showing the configuration of the optical spectrum generator of the second embodiment. The optical spectrum generator of the second embodiment replaces the optical intensity modulator 13 of the optical spectrum generator of the first embodiment with an optical phase modulator 23, and has the same other configurations.

The optical phase modulator 23 is a device that modulates the phase of an arbitrary wavelength. The configuration may be a combination of the diffraction grating 132 and the SLM 131, as in FIG. However, it differs in that it is phase-modulated rather than intensity-modulated by SLM131. When pulsed light whose phase has been phase-modulated by the optical phase modulator 23 is passed through the optical waveguide 14, spectral 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, pulsed light having a spectrum having a linear peak at a predetermined wavelength can be generated. The phase modulation amount may be arbitrary as long as it is not 0, but the closer to π, the larger the peak intensity can be. For example, the phase modulation amount is 0.1 to π (rad) or −π to −0.1 (rad).

As described above, the reason why spectral peaking occurs even in phase modulation instead of intensity modulation is as suggested in FIG. That is, a difference in the amount of phase shift in the optical waveguide 14 occurs due to the non-linear effect between the region subjected to the phase modulation and the portion other than the region subjected to the phase modulation. Therefore, when the phase difference between the region subjected to phase modulation and the portion other than the region subjected to phase modulation changes periodically according to the transmission distance and the phase difference becomes 0 or an even multiple of π. They strengthen each other and weaken each other when the phase difference becomes π or an odd multiple of π. As a result, the light intensity changes periodically according to the transmission distance in the optical waveguide 14, and dips and peaks appear alternately and periodically.

Note that both intensity modulation and phase modulation may be performed. Similarly, spectral peaking can occur and peaks can be generated.

FIG. 22 is a result obtained by numerically calculating the spectrum of the pulsed light radiated from the optical waveguide 14 of the optical spectrum generator of the second embodiment. FIG. 22A is a graph showing the power spectrum and phase spectrum of the pulsed light input to the optical waveguide 14. FIG. 22B is a graph showing the power spectrum of the pulsed light output from the optical waveguide 14. As shown in FIG. 22A, the pulsed light to be input is phase-modulated at predetermined frequency intervals. Then, as shown in FIG. 22 (b), it was found that a peak was generated at the wavelength subjected to the phase modulation.

(Various deformation examples)
The optical spectrum generator of the present disclosure is suitable for generating a spectrum having a plurality of linear peaks. Light having such a spectrum can be used for optical multiplex communication and the like.

Further, the optical spectrum generator of the present disclosure is suitable for cutting out optical frequency combs at equal intervals. In the conventional optical frequency comb generation method, the comb interval is narrow, and practically, it is required to widen the comb interval or appropriately thin the comb lines, but such control is difficult. However, according to the present disclosure, the optical frequency combs can be cut out at desired intervals due to the characteristics of the optical intensity modulator 13, so that practicality can be improved. For example, in the conventional optical frequency comb, the comb interval is several tens of MHz, but according to the present disclosure, this can be cut out at an interval of several hundred GHz.

Further, according to the optical spectrum generator of the present disclosure, pulsed light having a high repetition rate can be obtained, which is suitable for optical sampling.

The wavelength of the peak generated by the present disclosure is not limited, and a peak of any wavelength can be generated. For example, peaks can be generated even in the mid-infrared band and the far-infrared band.

This disclosure can be used for generation of optical frequency combs, optical wavelength division multiplexing, optical sampling, and the like.

10: Pulse light source 11: Optical amplifier 12: Wavelength shift fiber 13: Optical intensity modulator 14: Optical wave guide 23: Optical phase modulator

Claims

An optical spectrum generator that generates pulsed light in a spectrum with a peak.
A pulse light source that generates pulsed light and
An optical modulator that intensity-modulates or phase-modulates a predetermined wavelength of pulsed light from the pulsed light source,
An optical waveguide that propagates pulsed light from the light modulator, causes the pulsed light to have a non-linear effect, and produces the peak at the predetermined wavelength.
Have,
The spectral width of the pulsed light is at least twice the spectral width of the predetermined wavelength.
An optical spectrum generator characterized by this.
The optical spectrum generator according to claim 1, wherein the pulse shape of the pulsed light is sech 2 type.
The optical spectrum generator according to claim 1 or 2, wherein the light modulator causes intensity modulation or phase modulation at a plurality of wavelengths.
The optical spectrum generator according to claim 3, wherein the light modulator causes intensity modulation or phase modulation at a plurality of wavelengths discretely arranged at equal intervals.
The optical spectrum generator according to any one of claims 1 to 4, wherein the light modulator is an optical intensity modulator that intensity-modulates the predetermined wavelength to generate a dip. ..
The optical spectrum generator according to claim 5, wherein the spectral shape of the dip is a Lorentz type or a Gauss type.
The optical spectrum generator according to claim 5 or 6, wherein the light intensity modulator is a gas.
The optical spectrum generator according to claim 7, wherein the gas is methane.
The optical spectrum generator according to claim 5 or 6, wherein the optical intensity modulator is a fiber Bragg grating.
The light intensity modulator is
A wavelength separation element that separates the pulsed light from the pulse light source by wavelength,
A spatial light intensity modulator in which light from the wavelength separation element is incident at different positions for each wavelength and the intensity of the light is modulated for each position.
The optical spectrum generator according to claim 5 or 6, wherein the optical spectrum generator comprises the above.
The optical spectrum generator according to any one of claims 1 to 4, wherein the light modulator is an optical phase modulator that phase-modulates the predetermined wavelength.
The optical phase modulator is
A wavelength separation element that separates the pulsed light from the pulse light source by wavelength,
A spatial optical phase modulator in which light from the wavelength separation element is incident at different positions for each wavelength and phase-modulates the light for each position.
11. The optical spectrum generator according to claim 11.
The optical spectrum generator according to any one of claims 1 to 12, wherein the pulse light source is an optical frequency comb light source that outputs an optical frequency comb.
The optical spectrum generator according to any one of claims 1 to 13, wherein the optical waveguide is an anomalous dispersion single-mode optical fiber.
The optical spectrum generator according to any one of claims 1 to 13, wherein the optical waveguide is a small-diameter core fiber.
Claims 1 to 13 are characterized in that the optical waveguide is composed of an anomalous dispersion single-mode fiber and a highly nonlinear optical fiber connected to a subsequent stage of the anomalous dispersion single-mode fiber. The optical spectrum generator according to any one of the above items.
The optical spectrum generator according to any one of claims 1 to 13, wherein the optical waveguide is an optical fiber amplifier.
The optical spectrum generator according to any one of claims 1 to 17, wherein the pulse width of the pulsed light is 10 fs to 100 ps.
It is an optical spectrum generation method that generates pulsed light of a spectrum having a peak.
After causing intensity modulation or phase modulation at a predetermined wavelength of the pulsed light, the pulsed light is propagated to the optical waveguide to cause a non-linear effect on the pulsed light, and the peak is generated at the predetermined wavelength.
A method for generating an optical spectrum.