Optical filter

Abstract

To provide an optical filter that can change a passing band, and can change a center frequency of the passing band.SOLUTION: An optical filter comprises: generation means that generates continuous light, where the generation means is configured to make a frequency of the continuous light change; modulation means that is configured to enable modulating the continuous light; multiplex means that multiplexes filter object light and pump light the modulation means outputs in a frequency band to thereby output multiplex light including the filter object light and the pump light; a nonlinear medium that allows the multiplex light to pass; and filter means that allows a component of a band width of the filter object light of the multiplex light passing through the nonlinear medium to pass.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present disclosure relates to optical filters. [Background technology]

[0002] In recent years, many reports have been published on optical parametric oscillation in integrated devices of silicon, silicon nitride, etc. Non-Patent Documents 1 and 2 disclose configurations for generating signal light and idler light separated by several hundred THz from pump light by optical parametric oscillation. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] Xiyuan Lu,et.al.,"Milliwatt-threshold visible-telecom optical parametric oscillation using silicon nanophotonics",Optica 6,1535-1541,2019 [Non-Patent Document 2] Renato R.Domeneguetti,et.al.,"Parametric sideband generation in CMOS-compatible oscillators from visible to telecom wavelengths",Optica 8,316-322,2021 Summary of the Invention [Problem to be solved by the invention]

[0004] The present disclosure provides an optical filter that allows the passband to be changed and the center frequency of the passband to be changed. [Means for solving the problem]

[0005] According to one aspect of the present disclosure, an optical filter includes a generating means for generating continuous light, the generating means being configured to be capable of changing the frequency of the continuous light, a modulating means being configured to be capable of modulating the continuous light, a multiplexing means for outputting multiplexed light including the light to be filtered and the pump light output by the modulating means by multiplexing the light to be filtered and the pump light in the frequency domain, a nonlinear medium for passing the multiplexed light, and a filter means for passing a component of the multiplexed light that has passed through the nonlinear medium within the bandwidth of the light to be filtered. Effect of the Invention

[0006] According to the present disclosure, an optical filter is provided that is capable of changing a passband and a center frequency of the passband. [Brief description of the drawings]

[0007] [Figure 1] FIG. 13 is a diagram showing an example of a change in the amount of phase mismatch depending on the frequency. [Diagram 2] FIG. 2 is a diagram showing the relationship between the frequency of pump light and the frequency at which the amount of phase mismatch is zero. [Diagram 3] FIG. 1 is a diagram showing the configuration of an optical filter. [Figure 4] FIG. 1 shows multiple beams before and after a nonlinear medium. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] Hereinafter, the embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the invention according to the claims, and not all combinations of features described in the embodiments are essential to the invention. Two or more features among the multiple features described in the embodiments may be arbitrarily combined. In addition, the same reference numbers are used for the same or similar configurations, and duplicated descriptions are omitted.

[0009] Before describing the embodiments, an overview of optical parametric oscillation will be given.

[0010] Optical parametric oscillation is the generation of signal light with a frequency fs and idler light with a frequency fi by injecting pump light with a frequency fp into a nonlinear medium such as an optical integrated device using silicon (Si) or silicon nitride (Si3N4). Optical parametric oscillation occurs in a frequency (wavelength) region where the phase mismatch amount of the three lights, pump light, signal light, and idler light, is close to zero. The sum of the frequencies of the signal light and idler light (fs+fi) is twice the frequency fp of the pump light. The amount of phase mismatch is determined by the transmission parameters of the nonlinear medium used for optical parametric oscillation, such as the dispersion value. In other words, the frequency fs of the signal light and the frequency fi of the idler light generated by optical parametric oscillation are determined by the nonlinear medium used in optical parametric oscillation and the frequency fp of the pump light. In addition to optical integrated devices, optical parametric oscillation can also be generated by utilizing the nonlinearity of optical fibers.

[0011] For example, as shown in FIG. 1, if pump light of a fixed frequency fp is input to a given nonlinear medium, the phase mismatch amount moves away from 0 as the frequency difference with the frequency fp increases, but then approaches 0. In a first frequency region where the phase mismatch amount on both sides of the frequency fp is far from 0, no optical parametric oscillation occurs, and therefore no signal light / idler light of a frequency within the first frequency region is generated. However, in a second frequency region where the frequency difference with the frequency fp is greater than that of the first frequency region and the phase mismatch amount is closer to 0, optical parametric oscillation occurs, and therefore signal light / idler light of a frequency within the second frequency region can be generated. Note that the phase mismatch amount approaches 0 in the second frequency region due to fourth-order or higher dispersion in the nonlinear medium. Non-Patent Document 1 and Non-Patent Document 2 disclose that optical parametric oscillation due to this fourth-order or higher dispersion generates signal light of a frequency fs and idler light of a frequency fi that are several hundred THz away from the frequency fp of the pump light.

[0012] The frequency range of the second frequency region is extremely narrow, and therefore the signal light (idler light) has an extremely narrow band, for example, on the order of several hundred MHz. When calculating the relationship between the frequency fp of the pump light and the frequency at which the phase mismatch amount becomes zero due to fourth-order or higher dispersion, the slope is greater than 1, as shown in Figure 2. This means that when the frequency of the pump light is changed by Δf, the frequency of the signal light (idler light) changes by αΔf (α corresponds to the slope in Figure 2 and is greater than 1). In other words, when the frequency of the pump light is changed by Δf, the change in the frequency of the signal light (idler light) is greater than Δf.

[0013] The present inventors have completed the present invention based on the above findings. The present embodiment will be described in detail below.

[0014] FIG. 3 is a configuration diagram of an optical filter according to this embodiment. The variable light source 10 generates continuous light with a frequency fp and outputs it to the modulation unit 50. The variable light source 10 can change the frequency fp of the continuous light it generates. In other words, the frequency fp is not a fixed value but a changeable value. The modulation unit 50 generates modulated light by modulating the continuous light with a frequency fp with any data, for example, data that repeats 0 and 1, and outputs the modulated light to the multiplexing unit 20. The modulation method in the modulation unit 50 can be any modulation method such as intensity modulation or angle modulation. The bandwidth of the modulated light depends on the modulation speed, modulation method, etc. Note that the modulation unit 50 can be provided with a filter that limits the bandwidth of the modulated light, and the band-limited modulated light can be output to the multiplexing unit 20. Furthermore, the modulation unit 50 can be configured to output the continuous light as it is to the multiplexing unit 20. For example, by modulating the continuous light with successive 0s or 1s, the output of the modulation unit 50 becomes substantially unmodulated. In the following, the light output from the modulation unit 50 will be referred to as "modulated light" even if it is not modulated. The light to be filtered (hereinafter, "filtered light") is input to the multiplexing unit 20 via the polarization control unit 60. The polarization control unit 60 is provided to make the polarization plane of the filtered light coincide with the polarization plane of the modulated light. The polarization control unit 60 may be disposed between the variable light source 10 and the modulation unit 50 or between the modulation unit 50 and the multiplexing unit 20, and the polarization plane of the continuous light or the modulated light may be adjusted to make the polarization plane of the modulated light coincide with the polarization plane of the filtered light. Alternatively, the polarization control unit 60 may be configured to adjust the arrangement of the variable light source 10 to make the polarization plane of the modulated light coincide with the polarization plane of the observation target light, in which case the polarization control unit 60 may be omitted.

[0015] The multiplexing section 20 multiplexes the modulated light and the light to be filtered, and outputs the first multiplexed light to the nonlinear medium 30. FIG. 4(A) shows the first multiplexed light input to the nonlinear medium 30 when the modulation section 50 does not perform modulation and the continuous light of frequency fp generated by the tunable light source is output as it is to the multiplexing section 20. In this embodiment, the modulated light output by the modulation section 50 is used as pump light. Therefore, as shown in FIG. 4(A), the first multiplexed light includes pump light 80 and light to be filtered 90. In FIG. 4(A), reference numeral 81 indicates signal light generated by optical parametric oscillation in the nonlinear medium 30 using the pump light 80 of frequency fp.

[0016] Due to the optical parametric oscillation in the nonlinear medium 30, components of the filter target light 90 other than the band where the signal light 81 is generated are attenuated, so that only the components corresponding to the band of the signal light 81 are output from the nonlinear medium 30. Therefore, the second multiple light output from the nonlinear medium 30 is as shown in FIG. 4(B). In FIG. 4(B), reference numeral 82 denotes filtered light obtained by cutting out the band part where the signal light 81 is generated from the filter target light 90. As described above, the bandwidth of the signal light 81 is on the order of several hundreds of MHz, and therefore the bandwidth of the filtered light 82 is also on the order of several hundreds of MHz. Note that in FIG. 4, the power of the pump light 80 of the first multiple light (FIG. 4(A)) and the power of the pump light 80 of the second multiple light (FIG. 4(B)) are shown in the same way, but the power of the pump light 80 may also vary in the nonlinear medium 30.

[0017] The separation unit 40 is configured to pass components of the second multiple light in a predetermined band that includes at least the bandwidth of the light to be filtered 90. The predetermined band is set so as not to include the pump light 80 and the idler light (not shown in FIG. 4). In other words, the separation unit 40 performs filtering to attenuate (remove) the pump light 80 and the idler light (not shown in FIG. 4) included in the second multiple light, and outputs the filtered second multiple light. The second multiple light includes only the filtered light 82. In this way, narrowband filtering can be performed in the modulation unit 50 by passing continuous light without modulation.

[0018] As described above, the frequency fs of the signal light 81 also changes by changing the frequency of the pump light 80. Therefore, the center frequency of the passband of the optical filter can be changed by changing the frequency of the continuous light generated by the tunable light source 10. Note that, as described above, when the frequency of the pump light 80 is changed by Δf, the frequency of the signal light 81, i.e., the frequency of the filtered light 82, changes by αΔf (α is greater than 1). Therefore, the variable width of the center frequency of the passband of the filter is larger than the range in which the frequency can be changed by the tunable light source 10. In other words, the variable width of the frequency of the continuous light generated by the tunable light source 10 may be narrower than the bandwidth of the light 90 to be filtered.

[0019] Here, when modulation is performed in the modulation section 50 and modulated light is output, the bandwidth of the pump light 80 in Figures 4(A) and 4(B) widens. Therefore, the bandwidth of the signal light 81 also widens accordingly. Note that when the bandwidth of the pump light 80 is increased by X times, the bandwidth of the signal light 81 widens by αX times, where α corresponds to the slope in Figure 2. As the bandwidth of the signal light 81 widens, the bandwidth of the filtered light 82 also widens. In other words, the passband width of the filter can be controlled by controlling the bandwidth of the pump light 80 by controlling the modulation speed, modulation method, whether or not to modulate in the modulation section 50, etc.

[0020] In this embodiment, filtering is performed using the signal light 81, but filtering can also be performed using idler light.

[0021] As described above, an optical filter capable of changing the passband and changing the center frequency of the passband is realized by utilizing the optical parametric oscillation in the nonlinear medium 30. The nonlinear medium 30 is configured so that optical parametric oscillation based on the pump light 80 included in the first multiple light occurs. Preferably, the nonlinear medium 30 is configured so that optical parametric oscillation based on fourth-order or higher dispersion occurs. The tunable light source 10 is configured so that the frequency of the continuous light can be changed in a range from a first frequency to a second frequency. Here, when the tunable light source 10 changes the frequency of the continuous light in a range from the first frequency to the second frequency, the nonlinear medium 30 is configured so that the range in which the frequency of the signal light or the idler light generated by the optical parametric oscillation based on the pump light 80 changes includes the band of the filter target light 90. Alternatively, the tunable range of the frequency in the tunable light source 10 is set so that the frequency of the signal light or the idler light generated by the optical parametric oscillation based on the pump light 80 can be changed in a range including the band of the filter target light 90.

[0022] The above configuration makes it possible to provide an optical filter that can change the passband and the center frequency of the passband, which can contribute to Goal 9 of the United Nations Sustainable Development Goals (SDGs) "Build resilient infrastructure, promote sustainable industrialization and foster innovation."

[0023] The invention is not limited to the above-described embodiment, and various modifications and variations are possible within the scope of the gist of the invention. [Explanation of symbols]

[0024] 10: variable light source, 50: modulation section, 20: multiplexing section, 30: nonlinear medium, 40: separation section

Claims

1. Generating means for generating continuous light, the generating means being configured to be able to change the frequency of the continuous light; Modulating means configured to be able to modulate the continuous light; Multiplying means for outputting multiplexed light including the light to be filtered and the pump light by multiplexing the light to be filtered and the pump light output from the modulating means in the frequency domain; A non-linear medium through which the multiplexed light passes; Filtering means for passing a component of the bandwidth of the light to be filtered among the multiplexed light that has passed through the non-linear medium; An optical filter comprising the above.

2. The modulating means outputs, as the pump light, modulated light generated by modulating the continuous light, and is configured to be able to change the bandwidth of the pump light by changing the modulation method or modulation speed of the continuous light. The optical filter according to claim 1.

3. The modulating means outputs the pump light by band-limiting the modulated light generated by modulating the continuous light. The optical filter according to claim 2.

4. The modulating means is configured to be able to output the continuous light as the pump light. The optical filter according to claim 2.

5. The non-linear medium is configured such that optical parametric oscillation based on the pump light included in the multiplexed light occurs. The optical filter according to claim 1.

6. The non-linear medium is configured such that the optical parametric oscillation based on the pump light occurs based on fourth-order or higher dispersion. The optical filter according to claim 5.

7. The generating means is able to change the frequency of the continuous light in a range from a first frequency to a second frequency, When the frequency of the continuous light is changed in the range from the first frequency to the second frequency, the non-linear medium is configured such that the range in which the frequency of the signal light or idler light generated by the optical parametric oscillation based on the pump light changes includes the band of the light to be filtered. The optical filter according to claim 5.

8. The non-linear medium is an optical integrated device or an optical fiber. The optical filter according to claim 1.

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