Optical devices and optical amplification devices

Abstract

An optical device according to one embodiment of the present disclosure comprises: a first polarization beam splitter (PBS) and a second PBS, which reflect linearly polarized light of a first polarization component incident thereon and transmit linearly polarized light of a second polarization component orthogonal to the first polarization component, the first PBS and the second PBS being placed such that the first PBS reflects the linearly polarized light of the first polarization component toward a first path between the first PBS and the second PBS out of the first path and a second path between the first PBS and the second PBS, and the second PBS reflects the linearly polarized light of the first polarization component toward the second path; a first polarization rotator and a second polarization rotator, which are placed in the first path; and an optical element, which is placed between the first polarization rotator and the second polarization rotator and transmits only linearly polarized light of a third polarization component.

Description

Technical Field

[0001] The present invention relates to an optical device and an optical amplification device.

Background Art

[0002] Conventionally, in long-distance optical communication, a wavelength band called the C band with wavelengths from 1530 nm to 1565 nm has been widely used. So far, wavelength multiplexing technology and high-speed and multi-valued modulation technology have been researched, developed, and put into practical use, and the optical communication capacity has been increased. Although this increase in capacity continues, since the wavelength multiplexing technology already uses the entire wavelength band of the C band, it has become a concern. Therefore, in recent years, for further increasing the capacity of communication after 5G, expanding the communication wavelength band to the S band (1460 nm to 1530 nm) and the L band (1565 nm to 1625 nm) adjacent to the C band has been considered (see, for example, Non-Patent Document 1).

[0003] For such an expansion of the communication wavelength band, it is necessary to use existing communication equipment, and a wavelength converter is required. A wavelength converter using an optical nonlinear effect has been proposed. As the nonlinear medium, there are those using a highly nonlinear fiber type and a nonlinear crystal (for example, a periodically poled lithium niobate (PPLN) crystal). Unless otherwise stated, this case describes a wavelength converter using a nonlinear optical crystal.

[0004] FIG. 1 is a diagram showing a schematic configuration of a wavelength converter using a nonlinear optical crystal. The wavelength converter 3 shown in FIG. 1 converts the wavelength of the signal light input via the optical fiber 1. The wavelength converter 3 includes an erbium-doped fiber amplifier (EDFA) 4 that amplifies the pump light from the pump light device 2, a SHG module 7 for second harmonic generation (SHG), and an OPA module 6 for performing optical parametric amplification (OPA) on the signal light by the second harmonic. The SHG module 7 includes PPLNs 5 respectively.

[0005] The SHG module 7 has two lenses and a dichroic mirror on its input side to transmit only excitation light of a desired wavelength and couple it to the PPLN5. Only the excitation light of the desired wavelength from the excitation light focused by the dichroic mirror by the first lens is focused by the second lens and coupled to the PPLN5. On the output side of the SHG module 7, there are two lenses and a dichroic mirror to isolate and output only the second harmonic of a desired wavelength. Only the desired second harmonic from the output of the PPLN5, focused by the dichroic mirror by the first lens, is focused by the second lens and coupled to an external optical fiber. The SHG module 7 can also be configured to have only lenses that focus light on one or both of its input and output sides, i.e., a configuration without dichroic mirrors.

[0006] The OPA module 6 has three lenses and a dichroic mirror on its input side for coupling the signal light and the second harmonic from the SHG module 7 to the PPLN5. Only the signal light of the desired wavelength, focused by the first lens and transmitted through the dichroic mirror, and the desired second harmonic, focused by the second lens and reflected by the dichroic mirror, are focused by the third lens and coupled to the PPLN5. On the output side of the OPA module 6, there are two lenses and a dichroic mirror for isolating and outputting only the signal light that has been frequency-converted to the desired frequency. Only the signal light that has been frequency-converted to the desired frequency, focused by the first lens and transmitted through the dichroic mirror, is focused by the second lens and coupled to the external optical fiber.

[0007] Polarization-independent optical amplification is used to increase the capacity of optical communications. The wavelength converter 3 equipped with a PPLN shown in Figure 1 is the basic configuration for amplifying polarization-independent optical amplification. The wavelength converter 3 is also called an optical parametric amplifier. Since the wavelength converter equipped with a PPLN exhibits polarization dependence, polarization-independent optical amplification corresponding to polarization-independent optical amplification has been proposed and demonstrated (see, for example, Non-Patent Document 2). Figures 2 and 3 show the schematic configuration of a polarization-independent optical amplification device.

[0008] The polarization-independent optical amplifier 20 shown in Figure 2 separates the signal light incident through the optical fiber 1 into polarization components using a polarizing beam splitter (PBS) 21-1. The linear polarization of the first polarization component (S polarization component) is optically parametric amplified by a wavelength converter 3-2, and the linear polarization of the second polarization component (P polarization component), which is orthogonal to the first polarization component, is independently optically parametric amplified by a separate wavelength converter 3-1. The two polarizations after optical parametric amplification are again multiplexed and output by the second PBS 21-2. The excitation light from the excitation light device 2 is split by a 1-input 2-output optical coupler 22 and supplied to the SHG module 7 of the wavelength converter 3-1 and the SHG module 7 of the wavelength converter 3-2, respectively.

[0009] The polarization-independent optical amplifier 20 shown in Figure 2 does not cause interpolar interference as described in Non-Patent Literature 2, but differences in the optical path length of each polarization tend to occur, and especially when using fiber components with optical path lengths of several meters or more, differences in optical path length tend to occur due to thermal expansion and contraction of the fiber and changes in refractive index due to temperature changes.

[0010] The polarization-independent optical amplifier 30 shown in Figure 3 differs from the optical amplifier 20 in Figure 2 in that the OPA module 6 of the wavelength converter 3-1 and the OPA module 6 of the wavelength converter 3-2 are connected via a polarization rotor 31. In the optical amplifier 30, the linear polarization of the first polarization component (S-polarization component) that has passed through the OPA module 6 of the wavelength converter 3-2 is rotated by 90° in the polarization direction angle by the polarization rotor 31, then passes through the OPA module 6 of the wavelength converter 3-1, is reflected by PBS21-1, is multiplexed again, and is output. Similarly, the linear polarization of the second polarization component (P-polarization component) that has passed through the OPA module 6 of the wavelength converter 3-1 is rotated by 90° in the polarization direction angle by the polarization rotor 31, then passes through the OPA module 6 of the wavelength converter 3-2, passes through PBS21-1, is multiplexed again, and is output. [Prior art documents] [Patent Documents]

[0011] [Patent Document 1] Patent No. 6110547 [Non-patent literature]

[0012] [Non-Patent Document 1] Tomoyuki Kato et al., “WDM Transmission in S-Band Using PPLN-Based Wavelength Converters and 400-Gb / s C-band Real-Time Transceivers”, OECC / PSC 2022, July 2022 [Non-Patent Document 2] Takeshi Umeki et al., “PDM Signal Amplification Using PPLN-Based Polarization-Independent Phase-Sensitive Amplifier”, Journal of Lightwave Technology, Vol. 33, No. 7, pp. 1326-1332, April 2015 [Overview of the project]

[0013] The configuration of the optical amplifier 30 shown in Figure 3 has no optical path difference because linearly polarized light with different polarization components propagates in opposite directions along the same optical path. However, this configuration has a problem in that the intense light parametrically amplified by one of the two wavelength converters is reflected by the other wavelength converter and superimposed on orthogonal polarization components, causing signal degradation (see, for example, Patent Document 1 and Non-Patent Document 2).

[0014] This disclosure has been made in view of these problems, and its purpose is to provide an optical device that suppresses the reflection of the orthogonal polarization component in the optical amplification device 30 shown in Figure 3.

[0015] An optical device according to one embodiment of the present disclosure is a first polarization beam splitter (PBS) and a second PBS that reflect the linear polarization of an incident first polarization component and transmit the linear polarization of a second polarization component orthogonal to the first polarization component, wherein the first PBS is arranged such that it reflects the linear polarization of the first polarization component toward the first path of a first and second path between the first PBS and the second PBS, and the second PBS reflects the linear polarization of the first polarization component toward the second path; the optical device comprises a first polarization rotor and a second polarization rotor arranged in the first path; and an optical element arranged between the first polarization rotor and the second polarization rotor that transmits only the linear polarization of a third polarization component.

[0016] As described above, according to one embodiment of the present disclosure, it is possible to provide an optical device that suppresses the reflection of orthogonal polarization components. Furthermore, according to one embodiment of the present disclosure, it is possible to reduce the optical path difference between mutually orthogonal polarization components. [Brief explanation of the drawing]

[0017] [Figure 1] This is a schematic diagram of a wavelength converter equipped with a nonlinear medium having a periodic polarization reversal structure. [Figure 2] It is a schematic configuration diagram of a relay optical amplifier device compatible with polarization multiplexed light. [Figure 3] It is a schematic configuration diagram of a relay optical amplifier device compatible with polarization multiplexed light. [Figure 4] It is a schematic configuration diagram of an optical device according to an embodiment of the present disclosure. [Figure 5] It is a schematic configuration diagram of an optical device according to an embodiment of the present disclosure. [Figure 6] It is a schematic configuration diagram of an optical amplifier device compatible with polarization multiplexed light according to an embodiment of the present disclosure.

Embodiments for Carrying Out the Invention

[0018] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The same or similar reference numerals indicate the same or similar elements, and repeated descriptions may be omitted. The numerical values in the following description are examples, and the optical device and the optical amplifier device according to the embodiments of the present disclosure can be implemented by substituting other numerical values without departing from the spirit of the present disclosure.

[0019] [First Embodiment] Referring to FIG. 4, an optical device according to the first embodiment of the present disclosure will be described. The optical device according to this embodiment can be used in place of the polarization rotator 31 of the optical amplifier device 30 described with reference to FIG. 3. The optical device 40 shown in FIG. 4 includes a first PBS 41-1 and a second PBS 41-2. A mirror 44-1, and Faraday rotators (FR) 42-1 and 42-2 are arranged on the first path (the path from right to left on the upper side of FIG. 4) between the first PBS 41-1 and the second PBS 41-2. A polarizer 43-1 is arranged between FR 42-1 and FR 42-2. A mirror 44-2, and FR 42-4 and 42-3 are arranged on the second path (the path from left to right on the upper side of FIG. 4) between the first PBS 41-1 and the second PBS 41-2. A polarizer 43-2 is arranged between FR 42-4 and FR 42-3.

[0020] The first PBS41-1 and the second PBS41-2 are configured to reflect the linearly polarized light of the incident first polarization component (S polarization component) and transmit the linearly polarized light of the second polarization component (P polarization component) orthogonal to the first polarization component.

[0021] The mirror 44-1 is arranged to reflect the first polarization component (S polarization component) reflected by the first PBS41-1 toward the FR42-1. The mirror 44-2 is arranged to reflect the first polarization component (S polarization component) reflected by the second PBS41-2 toward the FR42-4.

[0022] The FR42-1 and the FR42-4 are polarization rotators configured to rotate the angle of the polarization direction of the incident linearly polarized light by 45° and then emit it. In FIG. 4, the angle of the polarization direction of the linearly polarized light of the first polarization component incident from the right side of the FR42-1 is 90°, and the angle of the polarization direction of the linearly polarized light emitted from the left side of the FR42-1 is 135°. Also, the angle of the polarization direction of the linearly polarized light of the first polarization component incident from the left side of the FR42-4 is 90°, and the angle of the polarization direction of the linearly polarized light emitted from the right side of the FR42-4 is 135°.

[0023] The polarizers 43-1 and 43-2 are configured to transmit only the linearly polarized light of the polarization component with a preset polarization direction angle. In this embodiment, the preset polarization direction angle is an angle of 135° obtained by adding the polarization direction angle (i.e., 45°) rotated by the FR42-1 to the polarization direction angle of 90° of the linearly polarized light of the first polarization component.

[0024] The Faraday rotators (FR) 42-2 and FR42-3 are polarization rotators configured to rotate the polarization direction angle of incident linearly polarized light by 45° before emission. In Figure 4, the polarization direction angle of linearly polarized light incident from the right side of FR42-2 (i.e., linearly polarized light transmitted through polarizer 43-1) is 135°, and the polarization direction angle of linearly polarized light emitted from the left side of FR42-2 is 180° (i.e., the linearly polarized light of the second polarization component (P-polarized component)). Similarly, the polarization direction angle of linearly polarized light incident from the left side of FR42-3 (i.e., linearly polarized light transmitted through polarizer 43-2) is 135°, and the polarization direction angle of linearly polarized light emitted from the right side of FR42-3 is 180° (i.e., the linearly polarized light of the second polarization component (P-polarized component)).

[0025] The operation of the optical device 40 in this embodiment will now be explained. Linearly polarized light (S-polarized component with a polarization angle of 90°) polarized perpendicular to the plane of the paper from the right side of the optical device 40 is reflected by PBS 41-1, then by mirror 44-1 located in the first path, and rotated by FR 44-1 by an angle of 45° in the polarization direction. Subsequently, linearly polarized light with a polarization angle of 135° emitted from FR 44-1 passes through polarizer 43-1, which has a preset polarization angle of 135°, and is further rotated by FR 42-2 by an angle of 45° in the polarization direction. Linearly polarized light with a polarization angle of 180° emitted from FR42-2 (i.e., linearly polarized light polarized in the horizontal direction of the paper, perpendicular to the polarization (S-polarization component) when incident on the optical device 40 (P-polarization component with a polarization angle of 180°)) passes through PBS41-2 and is output from the left side of the optical device 40. Similarly, linearly polarized light polarized in the vertical direction of the paper from the left side of the optical device 40 (S-polarization component with a polarization angle of 90°) is reflected by PBS41-2, travels from left to right along a second path, passes through PBS41-1, and is output from the left side of the optical device 40.

[0026] Linearly polarized P-polarized light output from the left side of the optical device 40 is reflected by some optical surface and re-incident to the optical device 40 as reflected light. At this time, the reflected light, which is linearly polarized with a polarization angle of 180° (P-polarized component), passes through PBS 41-2 and then through FR 42-2, which is positioned in the first path, and its polarization angle is rotated by 45°. Linearly polarized light with a polarization angle of 225° emitted from FR 42-2 cannot pass through polarizer 43-1, which has a preset polarization angle of 135°, and is blocked. However, in reality, it is not possible to completely block it by increasing the polarization extinction ratio in PBS and polarizer. The polarization component with a polarization angle of 135° in the reflected light passes through polarizer 43-1 (approximately 30 dB of reflected light incident on polarizer 43-1). When the reflected light that has passed through polarizer 43-1 passes through FR42-1, its polarization angle is rotated by 45°. The reflected light emitted from FR42-1 (linearly polarized with a polarization angle of 180°) passes through PBS41-1 and does not return to the right side of the optical device 40 (the original incident position). However, even here, it is not possible to completely block the light by increasing the polarization extinction ratio of PBS41-1. Approximately 30 dB of the reflected light incident on PBS41-1 is reflected by PBS41-1 and reaches the original incident position. Assuming that 1% (20 dB) of the emitted light output from the left side of the optical device 40 is reflected light, the reflected light reaching the original incident position will be approximately 20 dB + 30 dB + 30 dB = 80 dB relative to the emitted light. Even if the reflected light is amplified by about 20 dB, there is a difference of 60 dB relative to the signal, and the effect on crosstalk to the signal light incident from the left side of the optical device 40 is almost negligible.

[0027] In this embodiment, two FRs are placed in each of the first and second paths. However, the number of FRs placed in each path can be one or three or more, provided there are no issues with the optical arrangement or the amount of light required for the system. Furthermore, although the explanation has been based on a spatial optical system, the functionality may also be realized by integrated devices using process integration technologies such as silicon photonics.

[0028] As described above, the optical device 40 of this embodiment can suppress the reflection of orthogonal polarization components. Although an inter-polarization optical path difference may occur in the optical device 40, this optical path length difference can be made smaller.

[0029] [Second Embodiment] Next, an optical device according to a second embodiment of this disclosure will be described with reference to Figure 5. The optical device according to this embodiment can be used in place of the polarization rotor 31 of the optical amplifier 30 described with reference to Figure 3. The optical device 50 shown in Figure 5 differs from the optical device 40 shown in Figure 4 in that FR52-1 and FR52-1 are arranged in the first and second paths instead of FR42-1 and FR42-4. The optical device 50 shown in Figure 5 also differs from the optical device 40 shown in Figure 4 in that a third PBS51-1 and a fourth PBS51-2 are arranged in the first and second paths instead of polarizers 43-1 and 43-2. ​​Furthermore, the optical device 50 shown in Figure 5 differs from the optical device 40 shown in Figure 4 in that photodetectors 53-1 and 53-2 are provided near the first and second paths.

[0030] FR52-1 and FR52-2 are polarization rotors configured to rotate the polarization direction angle of incident linearly polarized light by 43° before emission. In Figure 5, the polarization direction angle of the first polarization component of linearly polarized light incident from the right side of FR52-1 is 90°, and the polarization direction angle of the linearly polarized light emitted from the left side of FR52-1 is 133°. Similarly, the polarization direction angle of the first polarization component of linearly polarized light incident from the left side of FR52-2 is 90°, and the polarization direction angle of the linearly polarized light emitted from the right side of FR52-2 is 133°.

[0031] The third PBS51-1 and the fourth PBS51-2 are configured to transmit linearly polarized light with a polarization angle of 135° and reflect polarization components with a polarization angle other than 135°.

[0032] The photodetectors 53-1 and 53-2 are positioned near the first and second paths and are configured to detect light reflected by PBS 51-1 and PBS 51-2.

[0033] The operation of the optical device 50 in this embodiment will now be explained. Linearly polarized light (S-polarized component with a polarization angle of 90°) polarized perpendicular to the plane of the paper from the right side of the optical device 50 is reflected by PBS41-1, then by mirror 44-1 located in the first path, and rotated by FR52-1 by 43°. Subsequently, the linearly polarized component with a polarization angle of 135° (99.9%) included in the linearly polarized light with a polarization angle of 133° emitted from FR52-1 is transmitted through the third PBS51-1, and the polarization components with polarization angles other than 135° (0.1%) are reflected by the third PBS51-1. Furthermore, the linearly polarized component with a polarization angle of 135° that has been transmitted through the third PBS51-1 has its polarization angle rotated by 45° by FR42-2. Linearly polarized light with a polarization angle of 180° emitted from FR42-2 (i.e., linearly polarized light polarized in the horizontal direction of the paper, perpendicular to the polarization (S-polarization component) when incident on the optical device 50 (P-polarization component with a polarization angle of 180°)) is transmitted through PBS41-2 and output from the left side of the optical device 50. Polarization components with polarization angles other than 135° reflected by the third PBS51-1 are incident on the photodetector 53-1. Similarly, linearly polarized light polarized in the vertical direction of the paper from the left side of the optical device 50 (S-polarization component with a polarization angle of 90°) is reflected by PBS41-2, travels from left to right along the second path, passes through PBS41-1, and is output from the left side of the optical device 50.

[0034] Polarization components with polarization angles other than 135° reflected by the third PBS51-1 and fourth PBS51-2 are incident on photodetectors 53-1 and 53-2. This allows monitoring of signal light (e.g., parametric amplification components) incident on the optical device 50. For example, it can be used to detect error signals when an optical parametric amplifier is configured with polarization diversity. It can also be used to monitor gain, and therefore can be used for gain adjustment monitoring. In this embodiment, the rotation angle of the polarization direction by FR52-1 and FR52-2 is set to 43.0°, but the amount of monitored light can be increased or decreased by adjusting the rotation angle according to the required amount of light for the system.

[0035] The linearly polarized P-polarized component output from the left side of the optical device 50 is reflected by some optical surface and re-enters the optical device 50 as reflected light. At this time, the reflected light, similar to the optical device 40 in Figure 4, passes through PBS41-2 and then through FR42-2, which is positioned in the first path, and its polarization direction angle is rotated by 45°. The linearly polarized light with a polarization direction angle of 225° emitted from FR42-2 cannot pass through the third PBS51-1 and is reflected. In some cases, a portion of the reflected light may pass through the third PBS51-1. When the reflected light that has passed through the third PBS51-1 passes through FR42-1, its polarization direction angle is rotated by 43°. The reflected light emitted from FR42-1 (polarized component with a polarization direction angle of 178°) passes through PBS41-1 and does not return to the right side of the optical device 50 (the original incident position). Although it is not possible to completely block the signal by increasing the polarization extinction ratio in PBS41-1, as with the case of optical device 40 in Figure 4, there is almost no effect on crosstalk to the signal light incident from the left side of optical device 50.

[0036] As described above, the optical device 50 of this embodiment can suppress the reflection of orthogonal polarization components. Although an inter-polarization optical path difference may occur in the optical device 50, this optical path length difference can be made smaller.

[0037] [Third Embodiment] Next, with reference to Figure 6, an optical amplification device according to a third embodiment of this disclosure will be described. The optical amplification device 60 shown in Figure 6 is an optical parametric amplifier arranged in a polarization diversity configuration. The optical amplification device 60 differs from the optical amplification device 30 shown in Figure 3 in that the optical device 40 shown in Figure 4 and the optical device 50 shown in Figure 5 are arranged in place of the polarization rotor 31. When the optical device 50 shown in Figure 5 is used in the optical amplification device 60, as described above, the parametric amplification component (signal light) output from the optical parametric amplification (OPA) modules of the wavelength converter 3-1 and the wavelength converter 3-2 can be monitored.

[0038] The polarization-independent optical amplifier 60 shown in Figure 5 separates the signal light incident via the optical fiber 1 into polarization components by a fifth PBS 21-1. The linear polarization of the first polarization component (S-polarization component) is optically parametrically amplified by the wavelength converter 3-2, and the linear polarization of the second polarization component (P-polarization component), which is orthogonal to the first polarization component, is independently optically parametrically amplified by a separate wavelength converter 3-1. The excitation light from the excitation light device 2 is split by a 1-input 2-output optical coupler 22 and supplied to the SHG module 7 of the wavelength converter 3-1 and the SHG module 7 of the wavelength converter 3-2, respectively. The linear polarization of the first polarization component (S-polarization component) that has passed through the OPA module 6 of the wavelength converter 3-2 is rotated by 90° in the polarization direction by the optical device 40 shown in Figure 4 or the optical device 50 shown in Figure 5, then passes through the OPA module 6 of the wavelength converter 3-1, is reflected by the PBS 21-1, re-multiplexed, and output. Similarly, the linear polarization of the second polarization component (P-polarization component) that has passed through the OPA module 6 of the wavelength converter 3-1 is rotated by 90° in the polarization direction angle by the optical device 40 or optical device 50, then passes through the OPA module 6 of the wavelength converter 3-2, passes through PBS21-1, is multiplexed again, and output. [Industrial applicability]

[0039] This disclosure provides optical devices and optical amplifiers usable in optical communications. [Explanation of Symbols]

[0040] 1 Optical fiber 2. Excitation light device 3, 3-1, 3-2 Wavelength Converters 4. Erbium-doped fiber amplifier (EDFA) 5. Periodic reversal of polarization lithium niobate (PPLN) 6. Optical Parametric Amplification (OPA) Module 7. Second Harmonic Generator (SHG) Module 20, 30, 60 Optical Amplifier 21-1, 21-2, 41-1, 41-2, 51-1, 51-2 Polarization Beam Splitter (PBS) 22 1-input 2-output optical coupler 31 Polarization rotor 40, 50 Optical Devices 42-1, 42-2, 42-3, 42-4, 52-1, 52-4 Faraday Rotator (FR) 43-1,43-2 Polarizer (Pol.) 44-1, 44-2 Mirror (M) 53-1, 53-2 Photodetector 62 Polarization-maintaining fiber

Claims

1. A first polarization beam splitter (PBS) and a second PBS that reflect the linearly polarized light of an incident first polarization component and transmit the linearly polarized light of a second polarization component perpendicular to the first polarization component, wherein the first PBS is arranged such that it reflects the linearly polarized light of the first polarization component toward the first path of the first and second paths between the first PBS and the second PBS, and the second PBS is arranged such that it reflects the linearly polarized light of the first polarization component toward the second path, A first polarization rotor and a second polarization rotor are arranged in the first path described above, An optical element that transmits only linearly polarized light of the third polarization component, positioned between the first polarization rotor and the second polarization rotor, An optical device comprising, The first polarization rotator is a Faraday rotator configured to rotate the polarization direction by an angle less than 45°, The second polarization rotator is a Faraday rotator configured to rotate the polarization direction by 45°, The optical element is a third PBS configured to transmit only polarization components with a polarization angle of 135° that have passed through the first polarization rotor, and to reflect polarization components with a polarization angle other than 135° that have passed through the first polarization rotor. An optical device further comprising a photodetector into which the polarization component reflected by the third PBS is incident.

2. The optical device according to claim 1, wherein, of the light incident on the first PBS and emitted from the second PBS, the light that is incident on the second PBS again does not incident on the first PBS.

3. A third polarization rotor and a fourth polarization rotor are arranged in the second path described above, An optical element that transmits only the linearly polarized light of the third polarization component, disposed between the third polarization rotor and the fourth polarization rotor, The optical device according to claim 1, further comprising the above.

4. An optical device according to any one of claims 1 to 3, A first wavelength converter equipped with a first optical parametric amplification (OPA) module, A second wavelength converter equipped with a second OPA module, A polarization-maintaining fiber connecting the first PBS and the first wavelength converter of the optical device, and the second PBS and the second wavelength converter of the optical device, An optical amplification device equipped with this device.

5. The first wavelength converter is an optical parametric amplifier further comprising a first second harmonic generator (SHG) module, The optical amplification apparatus according to claim 4, wherein the second wavelength converter is an optical parametric amplifier further comprising a second SHG module.

6. The system further comprises a fifth PBS that reflects the first polarization component of the signal light and transmits the second polarization component, The first polarization component is incident on the first wavelength converter, and the second polarization component is incident on the second wavelength converter. The optical amplification apparatus according to claim 4, wherein the fifth PBS transmits the optical signal output from the first wavelength converter and reflects the optical signal output from the second wavelength converter.

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