JP7645554B2 - Optical spectrum generating device and optical spectrum generating method - Google Patents

Description

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

An optical spectrum in which spectral lines with extremely 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.

International Publication No. 2018/181213

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

Therefore, the present disclosure aims to realize an optical spectrum generating device and an optical spectrum generating method that generate an optical spectrum having a linear peak.

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 depending on the propagation distance in the optical waveguide, causing dips and peaks to appear alternately. This disclosure is based on this novel phenomenon.

The present disclosure relates to an optical spectrum generation device that generates pulsed light having a spectrum having a peak, the optical spectrum generation device comprising: a pulsed light source that generates pulsed light; an optical modulator that intensity-modulates or phase-modulates a predetermined wavelength of the pulsed light from the pulsed light source; and an optical waveguide that propagates the pulsed light from the optical intensity modulator, causes a nonlinear effect in the pulsed light, and generates a peak at the predetermined wavelength, wherein the spectral width of the pulsed light is at least twice the spectral width of the dip, the optical waveguide causes the pulsed light to alternately and periodically repeat peaks and dips at the predetermined wavelength depending on the propagation distance of the pulsed light in the optical waveguide, and the length of the optical waveguide is set such that the pulsed light is extracted from the optical waveguide when the predetermined wavelength reaches its peak .

The present disclosure makes it possible to inexpensively realize an apparatus for generating an optical spectrum having a linear peak.

FIG. 1 is a diagram showing the configuration of an optical spectrum generating device according to a first embodiment. FIG. 2 is a schematic diagram showing a spectrum shape. Graph showing the relationship between the spectrum of pulsed light and the propagation distance. Graph showing the relationship between light intensity and fiber length. Graph showing the relationship between the spectral width of a dip and the spectral width of a peak. 1 is a graph showing the relationship between the dip absorptance and the peak light intensity. Graph showing the results of investigating the dependence of the spectral width of the dip on the relationship between light intensity and fiber length. Graph showing the results of investigating the dependence of peak output of pulsed light on the relationship between light intensity and fiber length. 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. 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. Graph showing the relationship between light intensity and fiber length when the spectral shape of the dip is changed to a Gaussian type. Graph showing the spectral shapes and their corresponding time waveforms and phases. Graph showing the relationship between the spectrum of pulsed light and the propagation distance. Graphs showing the results of spectrum measurement and graphs showing spectra obtained by numerical calculation. 1 is a graph showing the measurement results of the spectral shape. Graph showing an enlarged view of the spectrum shape. 1 is a graph showing the measurement results of the spectral shape. FIG. 2 is a diagram showing the configuration of a light intensity modulator 13. 1 is a graph showing the spectra of input light and output light. Graph showing the spectrum. FIG. 13 is a diagram showing the configuration of an optical spectrum generating device according to a second embodiment. 1 is a graph showing the spectra of input light and output light.

Specific examples of the present disclosure are described below with reference to the figures, but the present disclosure is not limited to the examples.

Figure 1 shows the configuration of the optical spectrum generator of the first embodiment. As shown in Figure 1, the optical spectrum generator of the first embodiment has 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 of the first embodiment utilizes a phenomenon discovered by the inventors. This phenomenon is that when pulsed light having a narrow linewidth dip (a sudden drop in optical intensity) in the spectrum is passed through the optical waveguide 14, the optical intensity at the dip wavelength changes periodically depending on the propagation distance in the optical waveguide 14, and dips and peaks appear alternately. Hereinafter, this phenomenon may be called spectral peeking. The optical spectrum generator of the first embodiment utilizes this spectral peeking to convert dips into peaks.

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.

The pulse shape of the pulsed light is arbitrary, for example, SECH ² type (soliton), Gaussian type, super Gaussian type, etc. SECH ² type is particularly preferable, as it can provide a clearer and more beautiful linear peak.

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 nonlinear effect in the downstream optical waveguide 14 occurs above a certain light intensity, the optical amplifier 11 is used to amplify the light intensity. The period of the spectral peeking also depends on the light intensity. Therefore, the period of the spectral peeking can be adjusted by the amplification factor of 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 caused by the downstream optical intensity modulator 13 relative 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 amount of wavelength shift in the wavelength shift fiber 12 depends on the light intensity, the amount of wavelength shift can be controlled by the optical amplifier 11.

The optical intensity modulator 13 is a filter that causes narrowband absorption of the pulsed light from the wavelength shifting fiber 12, producing a linear dip in the spectrum of the pulsed light. It is not necessary to produce a single dip; multiple dips may be produced. By producing equally spaced dips, 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, to sufficiently increase the intensity of the peak, it is preferable to use a wavelength near the central wavelength.

The spectral shape of the dip by the optical intensity modulator 13 can be any shape, but a Lorentzian type, a Gaussian type, a Sech ² type, etc. are preferable, since a cleaner and clearer peak can be obtained.

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 way, 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.

The absorption rate in the dip can be any value, but in order to sufficiently increase the peak light intensity, it is preferable to have an absorption rate of 50% or more. More preferably, it is 70% or more, and even more preferably, it is 90% or more.

The optical intensity modulator 13 can be any type having a narrowband absorption peak, and can 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 Example 1 can be constructed 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.

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. In other words, 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. In addition, the wavelength may be separated using another wavelength separation element such as a prism instead of the diffraction grating 132.

In the method using the diffraction grating 132 and SLM 131, multiple dips with equal frequency intervals can be generated, and an optical frequency comb can be generated by converting the dips into peaks using spectral peaking.

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

When using a gas cell, it is possible to generate a spectrum with linear peaks spaced at equal intervals, particularly for gases such as methane, ethane, and carbon dioxide, which have numerous equally spaced absorption peaks.

In addition, in Example 1, a dip is generated in the transmitted light of the light intensity modulator 13 by absorbing and reflecting the light using the light intensity modulator 13, but a dip may also be generated in the reflected light instead of the transmitted light.

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 light intensity at the wavelength of the dip, converting the dip into a peak.

The optical waveguide 14 may be any that propagates 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 be a photonic crystal structure. Since the optical spectrum generating device of Example 1 can be constructed entirely of fiber, it is preferable that it be an optical fiber.

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 is 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 are 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.

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

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 peaking. It can also improve the signal-to-noise ratio (the ratio of the peak light intensity to the background light intensity).

Also suitable as the optical waveguide 14 is an optical fiber amplifier. By using an optical fiber amplifier, it is possible to efficiently generate spectral peaking and at the same time increase the peak intensity. For example, an erbium-doped fiber amplifier (EDFA) can be used as the optical fiber amplifier.

The pulsed light may be converted to supercontinuum light before being input to the optical waveguide 14, or may be converted to supercontinuum light after being output from the optical waveguide 14. A highly nonlinear optical fiber may be used for 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 an anomalous dispersion single-mode optical fiber and a highly nonlinear optical fiber connected to the rear of the fiber.

In addition, the optical waveguide 14 may be constructed by connecting multiple types of optical fibers, planar optical waveguides, rectangular optical waveguides, etc.

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

Next, the operation of the optical spectrum generating device of Example 1 will be described.

The pulsed light emitted from the pulsed light source 10 has its light intensity amplified by the optical amplifier 11, its wavelength band adjusted by the wavelength shift fiber 12, and then passed 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)).

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 repeated alternately. 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 with a linear peak at the wavelength that was a dip (see FIG. 2(b)). Even if there are multiple dips, they can be converted to peaks at the same time. 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 the dips equally spaced, the 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.

The period of spectral peeking depends not only on the propagation distance in the optical waveguide 14 but also on the optical intensity of the pulsed light. 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, the greater the nonlinear effect in the optical waveguide 14 and the greater the amount of phase change, resulting in a shorter period of spectral peeking. Of course, both the optical intensity and the propagation distance in the optical waveguide 14 may be controlled.

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

Next, various experimental results regarding the optical spectrum generating device of Example 1 will be explained.

The spectrum of the pulsed light emitted from the optical waveguide 14 of the optical spectrum generating device of the first embodiment was calculated by numerical calculation. Various conditions in this numerical calculation are as follows. The pulsed light input to the optical waveguide 14 was a sech ² type (soliton) with a spectral width of 6 nm and a pulse width of 400 fs, a peak output of 500 W, and a central wavelength of 1650 nm. 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 an absorption rate of 85%. The optical waveguide 14 was an anomalous dispersion single mode fiber, and in the wavelength range of the pulsed light, the second-order dispersion value β ₂ = -33 ps ² /km, the third-order dispersion value β ₃ = 0.18 ps ³ /km, the MFD = 11 μm, and the soliton order N = 0.94. The effect of phase shift due to absorption was ignored.

Figure 3 is a graph showing the results of a 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. Figure 3(a) shows the case where Raman scattering is taken into account, and Figure 3(b) shows the case where Raman scattering is ignored. Figure 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 Figure 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.

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, it is possible to emit an optical pulse with a spectrum having a peak 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 is 1650 nm and does not change.

Figure 5(a) is a graph showing the relationship between the spectral width of the dip and the spectral width of the peak, and Figure 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 numerical calculation conditions are the same as those in Figures 3 and 4.

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.

Figure 6(a) is a graph showing the results of a numerical calculation of the relationship between the dip absorption rate and peak light intensity at a wavelength of 1650 nm, and Figure 6(b) is a graph showing the spectrum when the dip absorption rate 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.

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

Figure 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 Figures 3 and 4, except that the spectral width of the dip was changed.

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

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.

As shown in Figure 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 a wavelength of 1650 nm is maximized, it is sufficient to control the propagation distance in the optical waveguide 14 or the peak output of the pulsed light.

Fig. 9(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 pulsed light is changed from Sech ² type to 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. Fig. 9(b) is a graph showing the spectral shape of the pulsed light when the peak output was 500 W and the fiber length was 16 m.

As shown in Figure 9, even when the spectrum shape is Gaussian, the light intensity changes periodically and it is possible to convert dips into peaks. In addition, although the shape of the peak is slightly distorted compared to the SECH ² type, it is still a thin, linear, and strong peak.

Fig. 10(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 pulsed light is changed from Sech ² type to Super Gaussian type. The conditions were the same as those in Figs. 3 and 4, except that the spectral shape was Super Gaussian type and the peak output was changed to 500 W, 750 W, and 1000 W. Fig. 10(b) is a graph showing the spectral shapes of the pulsed light when the peak output was 500 W and the fiber lengths were 0 m and 26 m.

As shown in Figure 10, even when the spectral shape is a super-Gaussian type, the light intensity changes periodically and it is possible to convert dips into peaks. In addition, although the shape of the peak is slightly distorted compared to the SECH ² type, it is a thin, linear, and strong peak. It was also found that the overall spectral shape becomes narrower.

3, 4, 9, and 10, it was found that the spectral shape of the pulsed light is most favorable for the sech ² type. 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 stably, so it is thought that the peak shape is cleaner than that of the Gaussian or super-Gaussian types.

Figure 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 Figures 3 and 4, except that the spectral shape of the dip was Gaussian and the peak output was 500 W. Figure 11(b) is a graph showing the spectral shape of the pulsed light when the fiber lengths were 0 m and 26 m.

As shown in Figure 11, even when the spectral shape of the dip was Gaussian, a thin, linear, and strong peak was obtained, just as in the case of the Lorentzian shape.

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

From Figure 12, the reason why dips and peaks appear periodically can be considered as follows. As shown in Figures 12(d) to (f), the time waveform is expressed by the superposition of narrow 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 Figure 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 Figure 12(a). When pulsed light propagates through the optical waveguide 14, the narrow pulses are strong and therefore continuously subjected to a phase shift due to nonlinear effects. On the other hand, the wide pulses are weak and 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 shape as shown in Fig. 12(c). Also, when the phase difference is π or an odd multiple of π as shown in Fig. 12(d), the spectrum shape 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.

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 near 1650 nm. The pulsed light was a sech ² type with a pulse width of 400 fs, a peak output of 2 kW, and a central wavelength of 1650 nm. Other conditions were the same as those in FIGS. 3 and 4.

As shown in Figure 13, even in the case of multiple dips, the light intensity at the wavelengths of those dips changes periodically depending on the propagation distance, and it was found that it is possible to simultaneously convert into multiple linear peaks. Furthermore, 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 the further 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. Furthermore, Figure 13 shows that it is possible to generate a pulse train with an ultra-high repetition rate of 290 GHz.

Next, the optical spectrum generating device of Example 1 was actually manufactured, and its output was measured by 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 a polarization-maintaining Er-doped fiber and a single-walled carbon nanotube, and was used that output pulse 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 ² type soliton pulse with a pulse width of 200 fs. 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 shifting 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 transmitted through the gas cell was passed through the single mode fiber.

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 increases, the soliton effect compresses the spectral width, changing the light intensity at the wavelength of the absorption line and converting a linear dip into a linear peak. Furthermore, when Figures 14(a) and 14(b) are compared, the measured results roughly matched the results of the numerical calculation.

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 changes, and linear dips can 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 widened as the average output power increased. As a result of the broadening of the spectral width, the number of dips and peaks also increased. In addition, the larger the average output power, the greater the distortion of the spectral shape due to self-phase modulation and Raman scattering.

Figure 16 is a graph showing an enlarged view of the spectral shape around a 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 collapse of the pulse shape resulted in a low background output level, and a high signal-to-noise ratio was obtained.

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

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

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

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

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.

Figure 21 is a diagram showing the configuration of an optical spectrum generating device of Example 2. In the optical spectrum generating device of Example 2, the optical intensity modulator 13 of the optical spectrum generating device of Example 1 is replaced with an optical phase modulator 23, and the other configurations are the same.

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 whose predetermined wavelength is phase-modulated by the optical phase modulator 23 is passed through the optical waveguide 14, spectrum peaking occurs in the phase-modulated wavelength as in the first embodiment. That is, dips and peaks appear alternately and repeatedly in 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 whose spectrum has a linear peak at a predetermined wavelength. The phase modulation amount can 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).

The reason why spectral peaking occurs not only in intensity modulation but also in phase modulation is as suggested in Figure 12. In other words, a difference in the amount of phase shift occurs in the optical waveguide 14 between the phase-modulated area and the area other than the phase-modulated area due to nonlinear effects. Therefore, the phase difference between the phase-modulated area and the area other than the phase-modulated area changes periodically according to the transmission distance, and they strengthen each other when the phase difference is 0 or an even multiple of π, and weaken each other when the phase difference is 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.

It is also possible to perform both intensity modulation and phase modulation. This can result in spectral peaking and generate peaks.

Figure 22 shows the results of numerical calculation of the spectrum of pulsed light emitted from the optical waveguide 14 of the optical spectrum generating device of Example 2. Figure 22(a) is a graph showing the power spectrum and phase spectrum of the pulsed light input to the optical waveguide 14. Figure 22(b) is a graph showing the power spectrum of the pulsed light output from the optical waveguide 14. As shown in Figure 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 Figure 22(b).

(Various modified examples)
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.

The optical spectrum generating device disclosed herein 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 tens of MHz, this can be extracted at intervals of hundreds of GHz according to the present disclosure.

In addition, the optical spectrum generating device disclosed herein can produce pulsed light with a high repetition rate, making it suitable for optical sampling.

The wavelength of the peak generated by this 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.

This disclosure can be used for generating optical frequency combs, optical wavelength division multiplexing communications, optical sampling, etc.

10: Pulse light source 11: Optical amplifier 12: Wavelength shifting fiber 13: Optical intensity modulator 14: Optical waveguide 23: Optical phase modulator

Claims (19)

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: 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: 2. The optical spectrum generation device of claim 1, wherein the length of the optical waveguide is set to a minimum distance at which the predetermined wavelength reaches a peak. 4. The optical spectrum generating device according to claim 1 , wherein the pulse shape of the pulsed light is a Sech ² type. 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. 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. 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. The optical spectrum generating device according to claim 7 , wherein the spectral shape of the dip is Lorentzian or Gaussian. 9. The optical spectrum generating device according to claim 7 , wherein the optical intensity modulator is a gas. 10. The optical spectrum generating device of claim 9 , wherein the gas is methane. 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: 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. 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. 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. Furthermore, the device has 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 from the optical modulator into the optical waveguide or after outputting the pulsed light from the optical waveguide. 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 . 15. The optical spectrum generating device according to claim 1, wherein the optical waveguide is an optical fiber amplifier. 1. A method for generating pulsed light having a peaked spectrum, comprising:
After subjecting the pulsed light to intensity modulation or phase modulation at a predetermined wavelength, 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: 1. A method for generating a pulsed light having a spectrum with a peak, comprising:
After subjecting the pulsed light to intensity modulation or phase modulation at a predetermined wavelength, 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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