Optical spectrum generation device, optical spectrum amplification device, and optical spectrum generation method
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Filing Date
- 2024-09-20
- Publication Date
- 2026-06-02
Abstract
Description
Optical spectrum generating device, optical spectrum amplifying device, and optical spectrum generating method
[0001] The present disclosure relates to an optical spectrum generating device, an optical spectrum amplifying device, and an optical spectrum generating method.
[0002] An apparatus for generating an optical spectrum with a linear peak has been disclosed (see, for example, Patent Document 1) that uses an optical waveguide that generates a peak at a predetermined wavelength by applying a nonlinear effect to pulsed light whose intensity or phase has been modulated.
[0003] International Publication No. 2021 / 251365
[0004] However, with the technology disclosed in Patent Document 1, a relatively large pedestal component (also referred to as a background component or background) remains in the generated optical spectrum, resulting in problems such as a weak effective optical spectrum intensity and a small signal-to-background ratio (SBR).
[0005] The present disclosure has been made in light of these circumstances, and its purpose is to generate a light spectrum with less pedestal components and higher intensity.
[0006] To solve the above problems, an optical spectrum generation device according to one embodiment of the present invention includes a pulsed light source that generates pulsed light, an optical modulator that modulates the pulsed light of a predetermined wavelength from the pulsed light source, and a nonlinear loop mirror configured with an optical branching unit and a loop-shaped optical fiber connected to two ends of the same side of the optical branching unit. The optical modulator is disposed on the loop of the nonlinear loop mirror. The optical fiber of the nonlinear loop mirror generates a peak in the pulsed light modulated by the optical modulator due to a nonlinear effect. The optical path length from the optical branching unit to the optical modulator differs depending on whether the optical path is traced clockwise or counterclockwise around the loop of the nonlinear loop mirror.
[0007] In one embodiment, the modulation of the pulsed light may be intensity modulation.
[0008] In some embodiments, the modulation of the pulsed light may be phase modulation.
[0009] In one embodiment, the optical modulator may generate a modulation at a predetermined wavelength.
[0010] In some embodiments, the optical modulator may generate modulation at a plurality of predetermined wavelengths.
[0011] In some embodiments, the predetermined wavelengths may be discrete and evenly spaced.
[0012] In one embodiment, an optical modulator may modulate pulsed light of a predetermined wavelength to generate a dip, and a nonlinear effect may convert the dip into a peak.
[0013] In one embodiment, the optical fiber that makes up the nonlinear loop mirror may be a highly nonlinear fiber.
[0014] In one embodiment, the optical fiber that makes up the nonlinear loop mirror may be a small-core fiber.
[0015] In one embodiment, the optical modulator may be a gas cell.
[0016] In some embodiments, the gas cell may be a multi-pass cell.
[0017] In an embodiment, the gas in the gas cell may be any of methane, ethane, hydrogen cyanide, carbon monoxide, carbon dioxide, acetylene, nitrogen oxides, or water vapor, or a mixture thereof.
[0018] In one embodiment, the light modulator may be a spectral controller such as an LCOS.
[0019] In some embodiments, the optical spectrum generating device may further comprise a wavelength-shifting fiber for shifting the wavelength of the pulsed light from the pulsed light source.
[0020] In one embodiment, the optical spectrum generation device may further include a first polarization controller for maintaining the polarization state of the pulsed light input to the optical splitter, and a second polarization controller for maintaining the polarization state of the pulsed light output from the optical splitter.
[0021] In some embodiments, the optical fiber of the nonlinear loop mirror may be a polarization-maintaining fiber.
[0022] In one embodiment, the optical fiber of the nonlinear loop mirror may be an anomalous dispersion single mode optical fiber.
[0023] In one embodiment, the ratio of the optical path length from the optical branching device to the optical modulator when tracing the loop of the nonlinear loop mirror clockwise to the optical path length when tracing the loop counterclockwise is greater than 7:3 or less than 3:7.
[0024] In one embodiment, the ratio of the optical path length from the optical branching device to the optical modulator when tracing the loop of the nonlinear loop mirror clockwise to the optical path length when tracing the loop counterclockwise is greater than 9:1 or less than 1:9.
[0025] Another aspect of the present invention is an optical spectrum amplifying device, which includes an optical amplifier at a stage subsequent to the output of an optical branching device of any of the optical spectrum generating devices described above.
[0026] Another aspect of the present invention is a method for generating an optical spectrum. The method includes the steps of generating pulsed light using a pulsed light source, modulating the pulsed light of a predetermined wavelength from the pulsed light source using an optical modulator, and generating a peak in the pulsed light modulated by the optical modulator by a nonlinear effect of a nonlinear loop mirror composed of an optical branching unit and a loop-shaped optical fiber connected to two ends of the same side of the optical branching unit. The optical path length from the optical branching unit to the optical modulator is different when tracing the loop of the nonlinear loop mirror clockwise and when tracing the loop counterclockwise.
[0027] Any combination of the above components, and conversion of the present disclosure into a method, device, system, recording medium, computer program, etc., are also valid aspects of the present disclosure.
[0028] According to the present disclosure, it is possible to generate a light spectrum with a small pedestal component and high intensity.
[0029] 1 is a schematic diagram of an optical spectrum generating device according to a first embodiment. ２1 is a schematic diagram showing the absorption of 1.3 μm to 2.0 μm wavelengths of 1.3 μm to 2.0 μm of a nonlinear optical spectrum generating method using the optical spectrum generating device of FIG. 1; FIG. 2 is a schematic diagram showing the absorption of 1.3 μm to 2.0 μm wavelengths ... 1 is a graph (calculated results) showing the peak intensity and signal-to-background ratio of the optical spectrum output from the nonlinear loop mirror when the input power of the pulsed light input to the nonlinear loop mirror is changed.
[0034] This graph shows the input power dependence of the peak intensity of the optical spectrum when the fiber length of the nonlinear loop mirror is 30 m. This graph shows the input power required to maximize the peak intensity of the optical spectrum when the fiber length of the nonlinear loop mirror is changed. This graph is an experimental result showing the peak intensity and signal-to-background ratio of the optical spectrum output from the nonlinear loop mirror when the fiber length of the nonlinear loop mirror is changed. In the experimental system of FIG. 12 , OPL1 = 1 m, OPL2 = 9 m, but OPL1 = 3 m, OPL2 = 7 m, this shows the measurement results after changing the position of the CH4 gas cell. This is a schematic diagram showing the experimental system of Experiment 2 using the optical spectrum generating device of FIG. 1. This is a photograph showing the waveform and intensity of the optical spectrum when the second gas cell is not passed through. This is a photograph showing the waveform and intensity of the optical spectrum when the second gas cell is not passed through.34 is a photograph showing the waveform and intensity of the optical spectrum when passed through a second gas cell. 35 is a photograph showing the waveform and intensity of the optical spectrum when passed through a second gas cell. 36 is a diagram showing the waveform (line) when passed through the second gas cell superimposed with data (dots) when not passed through the second gas cell. 37 is an enlarged view of the vicinity of the spectral intensity of 0 in FIG. 28. 38 is a simulation result when a normal nonlinear fiber is used as the optical fiber constituting the nonlinear loop mirror. 39 is a simulation result when a small-core fiber is used as the optical fiber constituting the nonlinear loop mirror. 39 is a simulation result when a highly nonlinear fiber is used as the optical fiber constituting the nonlinear loop mirror. 39 is a diagram showing modulation of each spectrum by a spectral controller. 39 is a diagram showing absorption of pulsed light when passed through a gas cell of a multipass cell. 39 is a diagram showing pulsed light in which the dip in FIG. 34 is converted into a peak by a highly nonlinear fiber. 39 is a diagram showing the spectrum of pulsed light interfered by a nonlinear loop mirror constituted using a highly nonlinear fiber. 39 is a graph showing the result of phase control by a spectral controller. 39 is a graph showing the result of intensity control by a spectral controller. 39 is a graph showing the result of phase control by a spectral controller. 39 is a spectrum of pulsed light before input to an optical branching device. 1 shows the spectrum of pulsed light before it is input to an optical branching device. 2 shows the spectrum of pulsed light before it is input to an optical branching device. 3 shows the spectrum of pulsed light output from an optical branching device. 4 shows the spectrum of pulsed light output from an optical branching device. 5 shows the spectrum of pulsed light output from an optical branching device.
[0030] The present disclosure will be described below based on preferred embodiments with reference to the drawings. In the embodiments and modified examples, identical or equivalent components and parts are designated by the same reference numerals, and redundant description will be omitted where appropriate. The dimensions of the parts in the drawings are enlarged or reduced as appropriate for ease of understanding. Some elements that are not important for explaining the embodiments are omitted from the drawings. Terms including ordinal numbers such as "first" and "second" are used to describe various components, but these terms are used only to distinguish one component from another and do not limit the components.
[0031] Before describing specific embodiments, the present inventors will explain the basic findings. The present inventors discovered a phenomenon in which, when pulsed light having a sudden drop in light intensity (hereinafter referred to as a dip) in its spectrum is passed through an optical waveguide, the light intensity at the wavelength of the dip changes periodically depending on the optical path length of the optical waveguide, and dips and peaks appear alternately. This phenomenon is also called spectral peeking. By passing pulsed light in which a dip has been generated using an optical modulator or the like through an optical waveguide of an appropriate optical path length, the dip can be converted into a peak by the effect of spectral peeking, thereby generating an optical spectrum with a linear peak (see, for example, Patent Document 1).
[0032] However, the optical spectrum generated in this way still has a relatively large pedestal component, which means that the effective optical spectrum intensity is weak and the signal-to-background ratio is small, leaving room for further improvement.
[0033] 1 is a schematic diagram of an optical spectrum generating device 1 according to a first embodiment. The optical spectrum generating device 1 includes a pulsed light source 10, an optical modulator 20, and a nonlinear loop mirror 30. The nonlinear loop mirror 30 is composed of an optical branching device 31 and a loop-shaped optical fiber 32 connected to two terminals T1 and T2 on the same side of the optical branching device 31.
[0034] The optical modulator 20 is disposed on the loop of the nonlinear loop mirror 30. The optical fiber 32 of the nonlinear loop mirror 30 generates a peak in the pulsed light modulated by the optical modulator 20 due to a nonlinear effect.
[0035] The optical modulator 20 may be an optical intensity modulator that modulates the intensity of light, or an optical phase modulator that modulates the phase of light.
[0036] The optical splitter 31 is a device that can split light incident on one optical fiber into two outgoing lights, or conversely, combine light incident on two optical fibers into one outgoing light, and can be an optical coupler, a 2x2 fiber coupler, a cube-type beam splitter, a plate-type beam splitter, or the like.
[0037] The optical path length from the optical branching device 31 to the optical modulator 20 differs depending on whether the loop of the nonlinear loop mirror 30 is traced clockwise or counterclockwise. Specifically, the optical path length OPL1 between the optical branching device 31 and the optical modulator 20 when the pulsed light output from the terminal T1 of the optical branching device 31 traces the loop clockwise differs from the optical path length OPL2 between the optical branching device 31 and the optical modulator 20 when the pulsed light output from the terminal T2 of the optical branching device 31 traces the loop counterclockwise. In the example of FIG. 1 , OPL1<OPL2.
[0038] The optical spectrum generating device 1 further includes a wavelength shifting fiber 40, a first polarization controller 51, a second polarization controller 52, and an optical amplifier 60. However, these components are not essential for implementing the present invention. In particular, the first polarization controller and the second polarization controller are not required when the optical fiber 32 is configured as a polarization-maintaining fiber.
[0039] The pulsed 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 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.
[0040] The pulse shape of the pulsed light is arbitrary. For example, ２ Soliton type, Gaussian type, super Gaussian type, etc. ２ This type is preferred because it allows for a clearer, more linear peak to be obtained.
[0041] The optical amplifier 60 amplifies the optical intensity of the pulsed light from the pulsed light source 10. For example, a rare-earth doped optical fiber amplifier such as an erbium-doped fiber amplifier can be used as the optical amplifier 60. Since the nonlinear effect of the optical fiber 32 of the downstream nonlinear loop mirror 30 occurs above a certain optical intensity, it is advantageous to use the optical amplifier 60 to amplify the optical intensity. Furthermore, the period of spectral peeking also depends on the optical intensity. Therefore, the period of spectral peeking can be adjusted by changing the amplification factor of the optical amplifier 60.
[0042] The first polarization controller and the second polarization controller change the polarization state by applying external stress to the optical fiber, etc. Such polarization controllers can be configured using elements that can control polarization, such as a quarter-wave plate or a half-wave plate.
[0043] The wavelength shifting fiber 40 shifts the wavelength of the pulsed light from the optical amplifier 60. This adjusts the position of the absorption peak of the downstream optical modulator 20 relative to the spectrum of the pulsed light. For example, the wavelength shifting fiber 40 adjusts the peak wavelength of the pulsed light to be close to the wavelength of the absorption peak. Since the amount of wavelength shift in the wavelength shifting fiber 40 depends on the light intensity, the amount of wavelength shift can be controlled by the optical amplifier 60. For example, a polarization-maintaining fiber (PANDA fiber) can be used as the wavelength shifting fiber 40.
[0044] The optical modulator 20 functions as a filter that generates narrowband absorption for the pulsed light from the wavelength shifting fiber 40 and generates linear dips in the spectrum of the pulsed light. The number of dips generated is not limited to one, and multiple dips may be generated. By generating equally spaced dips, a spectrum with equally spaced linear peaks can be generated by spectral peaking. The wavelength of the dips may be any wavelength within the wavelength band of the pulsed light. However, to sufficiently increase the intensity of the peaks, a wavelength near the center wavelength is preferable.
[0045] As described above, the optical modulator 20 may generate modulation at a predetermined wavelength, such as an absorption peak due to an internal gas. There may be multiple predetermined wavelengths. Furthermore, the multiple predetermined wavelengths may be arranged discretely at equal intervals. In this case, the intervals may not be strictly equal, and there may be an error in the intervals, such as several tens of percent.
[0046] The spectral shape of the dip by the optical modulator 20 is arbitrary, but it can be Lorentzian, Gaussian, or sech type. ２ By adopting these spectral shapes, it is possible to obtain cleaner and clearer peaks.
[0047] The spectral width (full width at half maximum) of the pulsed light is preferably at least twice the spectral width (full width at half maximum) of the dip. By setting the spectral width of the pulsed light or the spectral width of the dip in this manner, spectral peaking can be generated. The spectral width of the dip can be any value as long as it satisfies this requirement, but it is preferably 10 nm or less, for example, and more preferably 1 nm or less.
[0048] The absorptance in the dip is arbitrary, but in order to sufficiently increase the peak light intensity, it is desirable that the absorptance be 50% or more, more desirably 70% or more, and even more desirably 90% or more.
[0049] The optical modulator 20 may be any type that has a narrow band absorption peak, and may be a gas cell filled with gas, a diffraction grating, a photonic crystal, or the like.
[0050] When the optical modulator 20 is configured with a gas cell, the gas in the gas cell may be any gas that generates narrow-band absorption for the pulsed light from the wavelength shifting fiber 40. In particular, methane (CH ４ ), ethane (C ２ H ６ ), carbon monoxide (CO), carbon dioxide (CO ２ ), acetylene (C ２ H ２ ), nitrogen oxides (NO ｘ The absorption of gases such as methane, hydrogen cyanide (HCN), and water vapor (H ２ 1 shows the absorption behavior in the wavelength range of 1.3 μm to 2.0 μm when using fluorine-containing ...
[0051] Furthermore, the gas in the gas cell may be a single gas such as methane, ethane, hydrogen cyanide, carbon monoxide, carbon dioxide, acetylene, nitrogen oxides, or water vapor, or may be a mixture of these gases.
[0052] When the optical modulator 20 is configured with a gas cell, a multi-pass type gas cell (hereinafter referred to as a "multi-pass cell") can be used.
[0053] FIG. 3 is a schematic diagram of a multipass cell 21. The multipass cell 21 includes an entrance 22, an exit 23, a mirror 24, and a mirror 25. The interior of the multipass cell 21 is filled with a gas that has a predetermined absorption peak for light incident through the entrance 22. The light that enters the multipass cell 21 through the entrance 22 is reflected multiple times between the mirrors 24 and 25 before exiting through the exit 23. By using the two mirrors 24 and 25 in this way, the multipass cell 21 can multiplex the optical path, thereby increasing the effective optical path length (in the example of FIG. 3, a 10-fold optical path length can be obtained). This allows for strong absorption even with the same amount of gas.
[0054] FIG. 4 is a photograph of an actual multipass cell.
[0055] The optical splitter 31 of the nonlinear loop mirror 30 has four terminals for optical input and output and is a device that can split light incident from a single fiber into two outgoing beams, or conversely combine two beams into one. In this embodiment, light input to terminal T3 is split within the optical splitter 31 and output from terminals T1 and T2, respectively. On the other hand, the sum of the two beams input to terminals T1 and T2 is output from terminal T3, and the difference of these two beams is output from terminal T4.
[0056] The optical fiber 32 of the nonlinear loop mirror 30 is connected to the terminals T1 and T2 of the optical branching unit 31 to form a loop. The optical fiber 32 generates a peak in the pulsed light modulated by the optical modulator 20 due to a nonlinear effect. This nonlinear effect changes the light intensity at the wavelength of the dip, converting the dip into a peak.
[0057] That is, the optical modulator 20 may be one that modulates pulsed light of a predetermined wavelength to generate a dip, and the nonlinear effect of the optical fiber 32 may be one that converts this dip into a peak.
[0058] 1, the nonlinear loop mirror 30 is configured using an optical fiber 32, but it may be configured using any material that can propagate pulsed light and generate a nonlinear effect in the pulsed light. Instead of an optical fiber, the nonlinear loop mirror 30 may be configured using, for example, a planar optical waveguide or a rectangular optical waveguide. Also, a photonic crystal structure may be used.
[0059] In particular, when the material that generates the nonlinear effect is optical fiber, it is desirable to use anomalous dispersion single-mode optical fiber, because the waveform can be shaped into a soliton in anomalous dispersion single-mode optical fiber, which makes the shape of the generated peaks cleaner and clearer.
[0060] When the material that generates the nonlinear effect is an optical fiber, a polarization-maintaining fiber may be used, in which case there is no need to provide a polarization controller within the device.
[0061] The operation of the optical spectrum generating device 1 will be described below with reference to Fig. 1. Pulsed light P1 generated by a pulsed light source 10 has a wavelength WL1. After being amplified by an optical amplifier 60, the pulsed light P1 is input to a wavelength shifting fiber 40. The wavelength of the pulsed light P1 is shifted from WL1 to WL2 by the wavelength shifting fiber 40, resulting in pulsed light P2. The wavelength WL2 of the pulsed light P2 corresponds to the absorption peak of the optical modulator 20 in the subsequent stage.
[0062] The polarization state of the pulsed light P2 is adjusted by the first polarization controller PC1, and then the pulsed light P2 is input to the terminal T1 of the optical splitter 31. The optical splitter 31 splits the pulsed light P2 into two pulsed lights.
[0063] One of the pulsed light beams P3 branched by the optical branching device 31 is output from terminal T1 and input to the optical fiber 32. The pulsed light P3 passes through the optical modulator 20 and the second polarization controller 52 in order, travels clockwise around the loop of the optical fiber 32, and inputs to terminal T2 of the optical branching device 31.
[0064] The optical modulator 20 generates a linear dip in the pulsed light P3. As the pulsed light P3 then passes through the optical fiber 32, the dip is converted into a peak due to a nonlinear effect. At this time, it should be noted that OPL1 (the optical path length between terminal T1 and the optical modulator 20) is less than OPL2 (the optical path length between terminal T2 and the optical modulator 20). That is, after being output from the optical modulator 20, the pulsed light P3 passes through the optical fiber 32 with a sufficiently long optical path length before reaching terminal T2. Therefore, the pulsed light P3 has a sufficiently large peak intensity when it reaches terminal T2. Note that the pulsed light P3 that reaches terminal T2 has a pedestal component.
[0065] The other pulsed light P4 branched by the optical branching device 31 is output from terminal T2 and input to the optical fiber 32. The pulsed light P4 passes through the second polarization controller 52 and the optical modulator 20 in order, traces the loop of the optical fiber 32 counterclockwise, and inputs to terminal T1 of the optical branching device 31.
[0066] A linear dip occurs in the pulsed light P4 due to the optical modulator 20. The pulsed light P3 then passes through the optical fiber 32, but is only subjected to a slight nonlinear effect because OPL1 (the optical path length between the terminal T1 and the optical modulator 20) is sufficiently short. Therefore, when the pulsed light P4 reaches the terminal T1, it has a slight peak or still has a dip. The pulsed light P3 that reaches the terminal T1 has a pedestal component, just like the pulsed light P4 that reaches the terminal T2.
[0067] The pulsed light P3 input to the terminal T2 and the pulsed light P4 input to the terminal T1 are combined by the optical branching device 31. Thereafter, the sum of the pulsed light P3 and the pulsed light P4 is output from the terminal T3, and the difference between the pulsed light P3 and the pulsed light P4 is output from the terminal T4.
[0068] The light output from terminal T4 has an optical spectrum in which the pedestal components of pulsed light P3 and pulsed light P4 are canceled out and have no pedestal component (or in which the pedestal component is reduced). Furthermore, the light output from terminal T4 has an optical spectrum with a strong peak intensity because the dip of pulsed light P4 is subtracted from the peak of pulsed light P3.
[0069] On the other hand, the light output from terminal T3 has an optical spectrum in which the pedestal components of pulsed light P3 and pulsed light P4 are reinforced. Furthermore, the light output from terminal T3 has almost no peak because the peak of pulsed light P3 and the dip of pulsed light P4 cancel each other out.
[0070] FIG. 5 shows a schematic diagram of pulsed light input to terminals T1 and T2 of the optical branching device 31 and pulsed light output from terminals T3 and T4.
[0071] FIG. 6 is a diagram in which the waveform obtained by simulation is superimposed on the schematic waveform of FIG.
[0072] According to this embodiment, it is possible to generate an optical spectrum with a small pedestal component and a high intensity.
[0073] The optical fiber 32 constituting the nonlinear loop mirror 30 may be a highly nonlinear fiber (HNLF) or a small-core fiber (SCF) instead of a normal nonlinear fiber. The highly nonlinear fiber has, for example, a core diameter of 3 μm and a second-order chromatic dispersion β ２ = 6.4 ps ２ / km (wavelength 1.55 μm) normal dispersion highly nonlinear fiber can be used.
[0074] For example, a highly nonlinear fiber has a core diameter of 3 μm and second-order chromatic dispersion β ２ = 6.4 ps ２ / km (wavelength 1.55 μm) normal dispersion highly nonlinear fiber can be used.
[0075] For example, the small core fiber has a core diameter of 6 μm and a second-order chromatic dispersion β ２ =-16 ps ２ / km (wavelength 1.55 μm) small core fiber can be used.
[0076] 7 is a schematic diagram of an optical spectrum generation system 1A configured using a highly nonlinear fiber 32A. The optical spectrum generation system 1A includes a pulsed light source 10, an optical modulator 20, a nonlinear loop mirror 30, a first polarization controller 51, and a second polarization controller 52. The nonlinear loop mirror 30 is configured with an optical branching unit 31 and a loop-shaped optical fiber 32A connected to two terminals T1 and T2 on the same side of the optical branching unit 31. The optical spectrum generation system 1A has a similar configuration to the optical spectrum generation system 1 in FIG. 1, but differs in that the optical fiber 32A constituting the nonlinear loop mirror 30 is a highly nonlinear fiber.
[0077] Furthermore, in the case of the optical spectrum generation device 1A using the highly nonlinear fiber 32A, it should be noted that the optical path length OPL1 between the optical branching device 31 and the optical modulator 20 when the pulsed light output from terminal T1 of the optical branching device 31 traces the loop in a clockwise direction is approximately equal to the optical path length OPL3 between the optical branching device 31 and the second polarization controller 52 when the pulsed light output from terminal T2 of the optical branching device 31 traces the loop in a counterclockwise direction. In the case of the optical spectrum generation device 1A using the highly nonlinear fiber 32A, the position of the optical modulator 20 as seen from the optical branching device 31 and the position of the second polarization controller 52 as seen from the optical branching device 31 are symmetrical in this way, which makes it possible to generate an efficient interference effect.
[0078] In the case of an optical spectrum generating device using a small core fiber, as in the case of a highly nonlinear fiber, the position of the optical modulator 20 as seen from the optical branching device 31 and the position of the second polarization controller 52 as seen from the optical branching device 31 are symmetrical, thereby producing an efficient interference effect.
[0079] The optical modulator may be a spectral controller. Such a spectral controller may be configured using a spatial light modulator (SLM) such as a liquid crystal on silicon (LCOS) or a waveguide device. FIG. 8 is a schematic diagram of an optical spectrum generating device 1B configured using a spectral controller. The optical spectrum generating device 1B includes a beam expander 90, a concave mirror 91, a diffraction grating 92, and a spectral controller 93. In this example, the spectral controller 93 is configured using an LCOS.
[0080] 9 shows modulation for each spectrum by the spectrum controller. In this manner, in this embodiment, optical modulation can be performed for each spectrum.
[0081] Second Embodiment Fig. 10 is a schematic diagram of an optical spectrum amplifying device 2 according to a second embodiment. The optical spectrum amplifying device 2 includes a pulsed light source 10, an optical modulator 20, a nonlinear loop mirror 30, a low-pass filter 71, a third polarization controller 53, a first semiconductor optical amplifier 81, a band-pass filter 72, and a second semiconductor optical amplifier 82. That is, the optical spectrum amplifying device 2 includes the low-pass filter 71, the third polarization controller 53, the first semiconductor optical amplifier 81, the band-pass filter 72, and the second semiconductor optical amplifier 82 in addition to the components of the optical spectrum generating device 1 in Fig. 1. The other components of the optical spectrum amplifying device 2 are the same as those of the optical spectrum generating device 1.
[0082] By configuring the optical spectrum amplifying device 2 according to this embodiment, an optical spectrum having a peak at a specific wavelength can be generated and amplified.
[0083] 11 is a flowchart showing the processing steps of an optical spectrum generation method according to a third embodiment. This method includes step S1 of generating pulsed light using a pulsed light source, step S2 of modulating the pulsed light of a predetermined wavelength from the pulsed light source using an optical modulator, and step S3 of generating a peak in the pulsed light modulated by the optical modulator by a nonlinear effect of a nonlinear loop mirror composed of an optical branching device and looped optical fibers connected to two ends of the same side of the optical branching device. The optical path length from the optical branching device to the optical modulator differs depending on whether the optical path is traced clockwise or counterclockwise around the loop of the nonlinear loop mirror.
[0084] According to this embodiment, it is possible to generate an optical spectrum with a small pedestal component and a high intensity.
[0085] [Verification experiment]
[0086] In order to confirm the effectiveness of the present invention, the inventors conducted the following verification experiment.
[0087] [Experiment 1] Figure 12 shows a schematic diagram of the experimental system for Experiment 1 using the optical spectrum generating device 1 shown in Figure 1. The pulse light source was a ring-type resonator fiber laser using a polarization-maintaining erbium-doped fiber and a single-walled carbon nanotube. This pulse light source produced a sech laser with a repetition rate of 50 MHz, a pulse width of 300 fs, and a center wavelength of 1556 nm. ２ The optical amplifier used was a fully polarization-maintaining erbium-doped fiber amplifier. The wavelength-shifting fiber used was an anomalous dispersion single-mode polarization-maintaining fiber. The output of this optical amplifier had a pulse width of 100 fs and a center wavelength of 1650 nm. A gas cell filled with methane gas was used for the optical modulator 13. The optical fiber of the nonlinear loop mirror was a single-mode fiber with a total length of 10 m. The optical modulator was located 1 m from the optical splitter when tracing the loop clockwise (in other words, 9 m from the optical splitter when tracing the loop counterclockwise). In other words, the ratio of OPL1:OPL2 was 1:9. An optical spectrum analyzer and a power meter were connected to the output of the pulsed light from terminal T4, and the shape of the optical spectrum and the optical intensity were measured.
[0088] 13 to 16 are photographs showing the measurement data obtained in Experiment 1.
[0089] Figure 13 shows the waveform and intensity of the pulsed light before it is input to the nonlinear loop mirror. As shown in the figure, this is a semi-transparent light with a central wavelength of 1650 nm. ２ It has a waveform of this type.
[0090] FIG. 14 shows the waveform and intensity of the pulsed light that has traveled clockwise through the nonlinear loop mirror just before it is input to the terminal T2 of the optical branching device. ２ It can be seen that, for the waveform of the type shown in Fig. 1, evenly spaced dips occur due to absorption in the gas cell, and then the dips are converted into peaks due to nonlinear effects.
[0091] Figures 15 and 16 show the waveform and intensity of the optical spectrum output from terminal T4 of the optical splitter. However, Figure 16 is a logarithmic representation of Figure 15. Comparing Figure 15 with Figure 14, it can be seen that the waveform in Figure 15 has the pedestal component of Figure 14 removed, leaving only the peak. This is because the pulsed light that traveled counterclockwise and the pulsed light that traveled clockwise through the nonlinear loop mirror are combined, and the pedestal component is removed by taking the difference component. As can be seen from Figure 16, the output optical spectrum has a signal intensity of -22.5 dBm and a pedestal component of -47.5 dBm, which indicates a good signal-to-background ratio of 35 dB.
[0092] Figures 17 and 18 are graphs showing the peak intensity and signal-to-background ratio of the optical spectrum output from the nonlinear loop mirror when the input power of the pulsed light input to the nonlinear loop mirror is changed. Figure 17 shows the experimental results, and Figure 18 shows the simulation results. In this experiment, the fiber length of the nonlinear loop mirror was set to 50 m to clearly observe the dependence of the peak intensity and signal-to-background ratio on the input power. In both the experimental and simulation results, the peak intensity and signal-to-noise ratio tend to increase as the input power increases from 0, then decrease after reaching a maximum value. This indicates that the input power can be tuned to an appropriate value to improve the peak intensity or signal-to-background ratio.
[0093] Fig. 19 is a graph showing the input power dependence of the peak intensity of the optical spectrum when the fiber length of the nonlinear loop mirror is 30 m. Comparing Fig. 19 with Fig. 17, the same tendency is observed in both figures.
[0094] Figure 20 is a graph showing the input power required to maximize the peak intensity of the optical spectrum when the fiber length of the nonlinear loop mirror is changed. It can be seen from Figure 20 that the shorter the fiber length, the greater the input power required. This is thought to be because the nonlinear effect on pulsed light is roughly expressed as the product of the fiber length and the input power.
[0095] Figure 21 shows experimental results of the peak intensity and signal-to-background ratio of the optical spectrum output from the nonlinear loop mirror when the fiber length of the nonlinear loop mirror is changed. As the fiber length increases, both the peak intensity and the signal-to-noise ratio increase, then decrease after reaching a maximum. This indicates that the fiber length can be tuned to an appropriate value to improve the peak intensity or signal-to-background ratio.
[0096] In the experimental system of FIG. 7, OPL1=1 m, OPL2=9 m is changed to OPL1=3 m, OPL2=7 m. ４ The results are obtained by changing the position of the gas cell. The effect of reducing the pedestal component is lower than when OPL1 is 1 m and OPL2 is 9 m, but it can be seen that a certain degree of effect is obtained.
[0097] This shows that the desired effect can be achieved when the ratio of the optical path length from the optical branching device to the optical modulator between the optical path length when tracing the loop of the nonlinear loop mirror clockwise and the optical path length when tracing the loop counterclockwise is greater than 7:3 or less than 3:7. Furthermore, the above experiment shows that the ratio of the optical path length from the optical branching device to the optical modulator between the optical path length when tracing the loop of the nonlinear loop mirror clockwise and the optical path length when tracing the loop counterclockwise is more preferably greater than 9:1 or less than 1:9.
[0098] [Experiment 2] Fig. 23 shows a schematic diagram of an experimental system for Experiment 2 using the optical spectrum generation device 1 shown in Fig. 1. This experimental system differs from the experimental system shown in Fig. 7 in that a second CH is added to the output terminal T4 of the optical splitter. ４ The optical spectrum after passing through this second gas cell is observed using a power meter and an optical spectrum analyzer.
[0099] Figures 24 and 25 are photographs showing the waveform and intensity of the optical spectrum when the light was not passed through the second gas cell. However, Figure 25 is a logarithmic representation of Figure 24. Figures 26 and 27 are photographs showing the waveform and intensity of the optical spectrum when the light was passed through the second gas cell. However, Figure 27 is a logarithmic representation of Figure 26.
[0100] Comparing Figures 26 and 27 with Figures 24 and 25, it can be seen that when passing through the second gas cell, the intensity of the peak decreases, but the pedestal component remains almost unchanged.
[0101] Figure 28 shows the waveform (line) when the signal was passed through the second gas cell, superimposed with the data (dots) when the signal was not passed through the second gas cell. Figure 29 is an enlarged view of the area around the zero spectral intensity in Figure 28. These figures also show that when the signal was passed through the second gas cell, the peak intensity decreased while maintaining its shape, and the pedestal component remained almost unchanged.
[0102] A system with such a second gas cell is expected to be applicable to, for example, a spectrometer. In conventional spectroscopy, the spectral waveform contains a pedestal component, so it is necessary to observe the entire spectral waveform. In contrast, in the system shown in Figure 23, the pedestal component is removed, so the presence or absence of a target gas can be determined by simply measuring the intensity without observing the entire spectral waveform.
[0103] [Experiment 3] Experiment 3 is an experiment to verify the effect of using a small-core fiber and a highly nonlinear fiber as the optical fiber that constitutes the nonlinear loop mirror.
[0104] Simulation results are shown in Figures 30 to 33. Figure 30 shows the simulation results when a conventional nonlinear fiber (30 m) is used as the optical fiber constituting the nonlinear loop mirror. The horizontal axis represents the input power of the pulsed light input to the nonlinear loop mirror, and the vertical axis represents the peak intensity and signal-to-background ratio of the optical spectrum output from the nonlinear loop mirror. The solid line in the graph represents the peak intensity, and the dotted line represents the signal-to-background ratio (same below). Here, the pulsed light source outputs SECH2 pulsed light with a repetition rate of 160 MHz, a pulse width of 20 fs, and a center wavelength of 1556 nm. Because the repetition rate is high at 160 MHz, as shown in the figure, an input power of approximately 50 mW is required to obtain the initial peak output intensity. Furthermore, an input power of approximately 40 mW is required to obtain the initial peak signal-to-background ratio.
[0105] Figure 31 shows the simulation results when a small-core fiber is used as the optical fiber constituting the nonlinear loop mirror. It can be seen that the input power required to obtain the first peak in output intensity drops to about 40 mW. It can also be seen that the input power required to obtain the first peak in signal-to-background ratio drops to about 30 mW.
[0106] Figure 32 shows the simulation results when a highly nonlinear fiber is used as the optical fiber constituting the nonlinear loop mirror. It can be seen that the input power required to obtain the first peak power further drops to about 10 mW to 20 mW for both the output intensity and the signal-to-background ratio.
[0107] Figure 33 shows the output spectrum from an optical splitter when a highly nonlinear fiber is used for the optical fiber that makes up the nonlinear loop mirror. It can be seen that the pedestal component is effectively removed in all peaks.
[0108] The experimental results are shown in Figures 34 to 36. Figure 34 shows the absorption of pulsed light when it passes through the gas cell of the multipass cell.
[0109] FIG. 35 shows pulsed light in which the dip in FIG. 34 is converted into a peak by a highly nonlinear fiber.
[0110] Figure 36 shows the spectrum of pulsed light interfered by a nonlinear loop mirror constructed using a highly nonlinear fiber. As shown in the figure, it is clear that the pedestal component has been effectively removed from all peaks.
[0111] [Simulation 4] Simulation 4 verifies the effect of spectrum control by the spectrum controller. The results are shown in Figs. 37 to 45. Fig. 37 is a graph showing the results of phase control by the spectrum controller. Fig. 38 is a graph showing the results of intensity control by the spectrum controller.
[0112] As shown in FIGS. 37 and 38, when spectrum control is performed by a spectrum controller, phase modulation produces a peak that is 5 to 6 times larger than that produced by intensity modulation.
[0113] In the following simulations, phase modulation is applied only to the center of the spectrum. Figure 40 shows the spectrum of the pulsed light in Figure 39 (1) before it is input to the optical branching device. The solid line shows the optical signal that has traveled clockwise through the nonlinear loop mirror in Figure 1, for example, and the dotted line shows the optical signal that has traveled counterclockwise (same below). Figure 41 shows the spectrum of the pulsed light in Figure 39 (2) before it is input to the optical branching device. Figure 42 shows the spectrum of the pulsed light in Figure 39 (3) before it is input to the optical branching device.
[0114] Fig. 43 shows the spectrum of pulsed light output from the optical branching device in Fig. 39(1). Fig. 44 shows the spectrum of pulsed light output from the optical branching device in Fig. 39(2). Fig. 45 shows the spectrum of pulsed light output from the optical branching device in Fig. 39(3). It can be seen that the pedestal component is effectively removed from the spectrum of pulsed light output from the optical branching device at any input power in Fig. 39(1), (2), and (3).
[0115] [Each Aspect of the Present Disclosure] Each aspect of the present disclosure will be summarized below. An optical spectrum generating device according to one aspect of the present disclosure includes a pulsed light source that generates pulsed light, an optical modulator that modulates the pulsed light of a predetermined wavelength from the pulsed light source, and a nonlinear loop mirror that is composed of an optical branching unit and a loop-shaped optical fiber connected to two ends of the same side of the optical branching unit. The optical modulator is disposed on the loop of the nonlinear loop mirror. The optical fiber of the nonlinear loop mirror generates a peak in the pulsed light modulated by the optical modulator due to a nonlinear effect. The optical path length from the optical branching unit to the optical modulator differs depending on whether the optical path is traced clockwise or counterclockwise around the loop of the nonlinear loop mirror.
[0116] According to this aspect, it is possible to generate a light spectrum with less pedestal components and with high intensity.
[0117] In one embodiment, the modulation of the pulsed light is intensity modulation.
[0118] In one embodiment, the modulation of the pulsed light is phase modulation.
[0119] In one aspect, the optical modulator generates a modulation at a predetermined wavelength.
[0120] In one aspect, the optical modulator generates modulation at a plurality of predetermined wavelengths.
[0121] In one embodiment, the predetermined plurality of wavelengths are arranged discretely and at equal intervals.
[0122] According to this aspect, a plurality of dips arranged discretely at equal intervals can be generated using an optical modulator.
[0123] In one embodiment, an optical modulator modulates pulsed light of a predetermined wavelength to generate a dip, and a nonlinear effect converts the dip into a peak.
[0124] According to this aspect, a peak can be generated from a dip generated using an optical modulator.
[0125] In one embodiment, the optical fiber that constitutes the nonlinear loop mirror is a highly nonlinear fiber.
[0126] According to this embodiment, a peak can be generated at a lower intensity.
[0127] In one embodiment, the optical fiber that constitutes the nonlinear loop mirror is a small-core fiber.
[0128] According to this embodiment, a peak can be generated at a lower intensity.
[0129] In one aspect, the optical modulator is a gas cell.
[0130] According to this aspect, the dip can be generated by utilizing gas absorption.
[0131] In one embodiment, the gas cell is a multipass cell.
[0132] According to this embodiment, strong absorption can be obtained with the same amount of gas.
[0133] In some embodiments, the gas in the gas cell is any one of methane, ethane, hydrogen cyanide, carbon monoxide, carbon dioxide, acetylene, nitrogen oxides, or water vapor, or a mixture thereof.
[0134] According to this aspect, the gas cell can be configured with a specific type of gas.
[0135] In one aspect, the optical modulator is a spectral controller such as an LCOS.
[0136] According to this aspect, modulation can be performed for each spectrum.
[0137] In one embodiment, the optical spectrum generating device further comprises a wavelength-shifting fiber that shifts the wavelength of the pulsed light from the pulsed light source.
[0138] According to this aspect, the peak wavelength of the pulsed light can be adjusted to be close to the wavelength of the absorption peak of the optical modulator.
[0139] In one aspect, the optical spectrum generating device further includes a first polarization controller for maintaining the polarization state of the pulsed light input to the optical splitter, and a second polarization controller for maintaining the polarization state of the pulsed light output from the optical splitter.
[0140] According to this embodiment, an optical fiber that is not a polarization-maintaining fiber can be used.
[0141] In one embodiment, the optical fiber of the nonlinear loop mirror is a polarization-maintaining fiber.
[0142] According to this embodiment, a polarization controller is not required.
[0143] In one embodiment, the optical fiber of the linear loop mirror is an anomalous dispersion single mode optical fiber.
[0144] According to this embodiment, the shape of the generated peak can be made clearer and more distinct.
[0145] In one aspect, the ratio of the optical path length from the optical splitter to the optical modulator when tracing the loop of the nonlinear loop mirror clockwise to the optical path length when tracing the loop counterclockwise is greater than 7:3 or less than 3:7.
[0146] According to this embodiment, it is possible to generate an optical spectrum with a strong spectral intensity and a large signal-to-background ratio.
[0147] In one aspect, the ratio of the optical path length from the optical splitter to the optical modulator when tracing the loop of the nonlinear loop mirror clockwise to the optical path length when tracing the loop counterclockwise is greater than 9:1 or less than 1:9.
[0148] According to this embodiment, it is possible to generate an optical spectrum with stronger spectral intensity and a larger signal-to-background ratio.
[0149] Another aspect of the present invention is an optical spectrum amplifying device, which includes an optical amplifier at a stage subsequent to the output of an optical branching device of any of the optical spectrum generating devices described above.
[0150] According to this aspect, an optical spectrum having a peak at a specific wavelength can be generated and amplified.
[0151] Another aspect of the present invention is a method for generating an optical spectrum. The method includes the steps of generating pulsed light using a pulsed light source, modulating the pulsed light of a predetermined wavelength from the pulsed light source using an optical modulator, and generating a peak in the pulsed light modulated by the optical modulator by a nonlinear effect of a nonlinear loop mirror composed of an optical branching unit and a loop-shaped optical fiber connected to two ends of the same side of the optical branching unit. The optical path length from the optical branching unit to the optical modulator is different when tracing the loop of the nonlinear loop mirror clockwise and when tracing the loop counterclockwise.
[0152] According to this aspect, it is possible to generate a light spectrum with less pedestal components and with high intensity.
[0153] The present disclosure has been described above based on the embodiments. These embodiments are merely examples, and it will be understood by those skilled in the art that various modifications are possible in the combination of the respective components and the respective treatment processes, and that such modifications are also within the scope of the present invention.
[0154] The present disclosure relates to an optical spectrum generating device, an optical spectrum amplifying device, and an optical spectrum generating method.
[0155] REFERENCE SIGNS LIST 1 optical spectrum generating device, 2 optical spectrum amplifying device, 10 pulse light source, 20 optical modulator, 21 multipass cell, 22 input port, 23 output port, 24 mirror, 25 mirror, 30 nonlinear loop mirror, 31 optical branching device, 32 optical fiber, 40 wavelength shifting fiber, 51 first polarization controller, 52 second polarization controller, 60 optical amplifier, 71 low-pass filter, 72 band-pass filter, 81 first semiconductor optical amplifier, 82 second semiconductor optical amplifier, 90 beam expander, 91 concave mirror, 92 diffraction grating, 93 spectrum controller, OPL1 optical path length, OPL2 optical path length, P1 pulse light, P2: pulsed light, P3: pulsed light, P4: pulsed light, S1: step of generating pulsed light, S2: step of modulating pulsed light of a predetermined wavelength, S3: step of generating a peak in the modulated pulsed light, T1: terminal, T2: terminal, T3: terminal, T4: terminal, WL1: wavelength, WL2: wavelength.
Claims
1. A pulse light source that generates pulsed light, An optical modulator for modulating pulsed light of a predetermined wavelength from the pulsed light source, comprising an optical modulator for generating dips, A nonlinear loop mirror consisting of an optical splitter and loop-shaped optical fibers connected to the two ends on the same side of the optical splitter, Equipped with, The optical modulator is positioned on the loop of the nonlinear loop mirror, The optical fiber of the nonlinear loop mirror generates a peak in the pulsed light modulated by the optical modulator due to the spectral peaking effect, due to the nonlinear effect. The optical path length from the optical splitter to the optical modulator differs depending on whether the loop of the nonlinear loop mirror is traced clockwise or counterclockwise. The optical splitter is an optical spectrum generating device characterized by generating an optical spectrum with a low base component in the wavelength direction and high intensity by taking the difference between pulsed light input by tracing the loop of the nonlinear loop mirror clockwise and pulsed light input by tracing it counterclockwise.
2. The optical spectrum generating apparatus according to claim 1, characterized in that the modulation of the pulsed light is intensity modulation.
3. The optical spectrum generating apparatus according to claim 1, characterized in that the modulation of the pulsed light is phase modulation.
4. The optical spectrum generating apparatus according to claim 1, characterized in that the optical modulator generates modulation to a predetermined wavelength.
5. The optical spectrum generating apparatus according to claim 4, characterized in that the optical modulator generates modulation to a predetermined number of wavelengths.
6. The optical spectrum generating apparatus according to claim 5, characterized in that the predetermined plurality of wavelengths are arranged discretely at equal intervals.
7. The optical modulator modulates the pulsed light of the predetermined wavelength to generate a dip, The optical spectrum generating apparatus according to claim 1, characterized in that the nonlinear effect converts the dip into a peak.
8. The optical spectrum generating apparatus according to claim 1, characterized in that the optical fiber constituting the nonlinear loop mirror is a highly nonlinear fiber.
9. The optical spectrum generating apparatus according to claim 1, characterized in that the optical fiber constituting the nonlinear loop mirror is a small core fiber.
10. The optical spectrum generating apparatus according to claim 1, characterized in that the optical modulator is a gas cell.
11. The optical spectrum generating apparatus according to claim 10, characterized in that the gas cell is a multi-pass cell.
12. The optical spectrum generating apparatus according to claim 10, characterized in that the gas of the gas cell is one of methane, ethane, hydrogen cyanide, carbon monoxide, carbon dioxide, acetylene, nitrogen oxides, or water vapor, or a mixture thereof.
13. The optical spectrum generating apparatus according to claim 1, characterized in that the optical modulator is a spectral controller.
14. The optical spectrum generating apparatus according to claim 1, further comprising a wavelength shifting fiber for shifting the wavelength of pulsed light from the pulsed light source.
15. A first polarization controller for maintaining the polarization state of pulsed light input to the optical splitter, A second polarization controller for maintaining the polarization state of pulsed light output from the optical splitter, The optical spectrum generating apparatus according to claim 1, further comprising the features described above.
16. The optical spectrum generating apparatus according to claim 1, characterized in that the optical fiber of the nonlinear loop mirror is a polarization-maintaining fiber.
17. The optical spectrum generating apparatus according to claim 1, characterized in that the optical fiber of the nonlinear loop mirror is an anomalous dispersion single-mode optical fiber.
18. The optical spectrum generating apparatus according to claim 1, characterized in that, with respect to the optical path length from the optical splitter to the optical modulator, the ratio of the optical path length when tracing the loop of the nonlinear loop mirror clockwise to the optical path length when tracing it counterclockwise is greater than 7:3 or less than 3:
7.
19. The optical spectrum generating apparatus according to claim 1, characterized in that, with respect to the optical path length from the optical splitter to the optical modulator, the ratio of the optical path length when tracing the loop of the nonlinear loop mirror clockwise to the optical path length when tracing it counterclockwise is greater than 9:1 or less than 1:
9.
20. An optical spectrum amplification device characterized by having an optical amplifier downstream of the output of an optical splitter of an optical spectrum generation device according to any one of claims 1 to 18.
21. A step of generating pulsed light using a pulsed light source, A step of modulating pulsed light of a predetermined wavelength from the pulsed light source using an optical modulator that generates dips, The steps include generating a peak in the pulsed light modulated by the optical modulator due to the spectral peaking effect by the nonlinear effect of a nonlinear loop mirror, which is composed of an optical splitter and loop-shaped optical fibers connected to the two ends of the optical splitter on the same side, Includes, The optical path length from the optical splitter to the optical modulator differs depending on whether the loop of the nonlinear loop mirror is traced clockwise or counterclockwise. The optical splitter is characterized by generating an optical spectrum with a low base component in the wavelength direction and high intensity by taking the difference between pulsed light input by tracing the loop of the nonlinear loop mirror clockwise and pulsed light input by tracing it counterclockwise.