Optical amplification device and optical amplification method

Abstract

An optical amplification device (100) comprises: a polarization splitter (101) that splits signal light into polarization components; a first excitation light combining unit (102) that combines excitation light with each polarization component; a first optical amplification unit (103) that amplifies each polarization component by causing an optical parametric amplification process; a first excitation light separating unit (104) that separates excitation light from each polarization component; an unwanted band separating unit (105) that removes an unwanted frequency component from each polarization component; a second excitation light combining unit (106) that combines the separated excitation light with each polarization component; a second optical amplification unit (107) that amplifies each polarization component by causing an optical parametric amplification process; a second excitation light separating unit (108) that separates excitation light from each polarization component; a polarization combining unit (109) that combines the polarization components; and a band pass filter unit (110) that removes an unwanted frequency component from the combined signal light. The first optical amplification unit (103) is in a phase matching state in which the amplification gain is greater farther away from the central frequency, and the second optical amplification unit (107) is in a phase matching state in which the amplification gain is greater around the central frequency.

Description

[Technical Field]

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

[0002] With the recent launch of the fifth-generation mobile communication system and the widespread use of rich content such as high-resolution video, communication traffic has been increasing exponentially. This has led to a demand for continuous increases in the capacity of optical networks. In optical fiber transmission, the transmission capacity per fiber can be improved by widening the optical transmission bandwidth. Therefore, various studies are being conducted to realize ultra-wideband wavelength division multiplexing transmission.

[0003] In long-distance optical fiber transmission, optical amplifiers are used to compensate for optical losses that occur within the optical fiber. In such transmission systems, the transmission bandwidth of an optical fiber transmission system is limited by the amplification bandwidth of the optical amplifier. Common optical amplifiers include those in which rare-earth elements are doped into the optical fiber. The erbium-doped fiber amplifier (EDFA) is a typical rare-earth doped optical amplifier. The amplification bandwidth of an EDFA is approximately 4 THz within the C-band (approximately 1530–1565 nm), where propagation loss within optical fiber is low. This wavelength band is commonly used in long-distance optical communications. To expand the transmission bandwidth, for example, paralleling heterogeneous amplifiers with different amplification bandwidths, such as by changing the rare-earth elements doped into the optical fiber, is considered. However, in bands outside the C-band, low gain and a high noise figure are issues. Furthermore, considering the cost and operation of the transmission system and the excessive optical loss caused by splitting the optical signal into multiple band components, it is desirable to be able to amplify a wide bandwidth using a single optical amplifier.

[0004] Against this background, optical parametric amplifiers (OPAs) are attracting attention. OPAs are amplifiers that amplify input light by utilizing the nonlinear optical effect in media such as lithium niobate, a second-order nonlinear optical medium, or optical fiber, a third-order nonlinear optical medium. The amplification bandwidth of an OPA depends on the phase matching characteristics in the nonlinear optical medium used as the amplification medium. By realizing a wideband phase matching state through medium design, OPAs can achieve wideband amplification that exceeds the amplification bandwidth of EDFAs. In addition, the center of the phase matching characteristics can be designed to various wavelengths. Therefore, it is possible to amplify various communication wavelength bands other than the conventionally used C-band and L-band.

[0005] In second-order nonlinear optical media using second-harmonic waves as pump light for parametric amplification, the frequency difference between the amplified light and the pump light is large, resulting in a large difference in the effective refractive index for each component. Therefore, designing a medium that satisfies the phase matching condition is challenging. Furthermore, a technique called quasi-phase matching (QPM) can be used to achieve broadband phase matching characteristics using a periodically poled structure that alternates between regions with inverted nonlinear susceptibility signs. To achieve both broadband and gain, a promising configuration is to use periodically poled lithium niobate (PPLN) as an amplifying medium, which is less susceptible to unwanted nonlinear optical effects. An OPA using a PPLN waveguide has demonstrated the feasibility of broadband amplified repeater transmission exceeding 10 THz with an amplification gain of 15 dB (see, for example, Non-Patent Document 1).

[0006] In second-order nonlinear media such as PPLN, the optical parametric amplification process only occurs in specific polarization states. This necessitates a polarization diversity configuration in which the polarization of the input light is split into two orthogonal components before amplification, and then each component is amplified and recombined. Even with third-order nonlinear optical effects, gain is generated according to the polarization state of the pump light. To achieve stable polarization-independent amplification, it is desirable to use a similar polarization diversity configuration.

[0007] When an OPA amplifies an input signal light, an idler light, which is a phase conjugate of the signal light, is generated at a frequency symmetrical to the signal light across the center frequency of the amplification band. For signal transmission, either the idler light or the original signal light can be transmitted. Therefore, optical components not used for transmission are cut using a band-pass filter (BPF) after amplification. If the idler light is extracted as a new optical signal to be transmitted, the OPA can also function as an optical phase conjugate converter (see, for example, Non-Patent Document 2) or a wavelength converter (see, for example, Patent Document 1). One of the features of an OPA is that it can perform various optical signal processing functions, not just as a simple optical amplifier.

[0008] On the other hand, when amplifying with an OPA, the band where idler light is generated must be left open, so signal light cannot be placed in that band. Therefore, to make the most of the OPA's amplification band, it is necessary to divide the signal light into two bands at the center frequency, amplify it using different nonlinear media, and then cut off the unnecessary optical components, i.e., either the original signal light or the idler light, with a wavelength filter, and recombine the optical components of each band. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Publication No. 2020-86031 [Non-patent literature]

[0010] [Non-Patent Document 1] T. Kobayashi et al., "Wide-Band Inline-Amplified WDM Transmission Using PPLN-Based Optical Parametric Amplifier," IEEE Journal of Lightwave Technology, Vol. 39, No. 3, pp. 787-794, Feb. 2021. [Non-patent document 2] T. Umeki et al., "Simultaneous nonlinearity mitigation in 92 × 180-Gbit / s PDM-16QAM transmission over 3840 km using PPLN-based guard-band-less optical phase conjugation," Optics Express, Vol.24, No.15, 16945-16951, 2016. Summary of the Invention [Problem to be solved by the invention]

[0011] In amplifier-repeater transmission, optical amplifiers must compensate for the transmission loss of optical signals within the entire bandwidth used. Even in single-span transmission, they must have sufficient amplification gain within the bandwidth used to ensure the signal-to-noise ratio required for receiving optical signals. Hereinafter, the amplification bandwidth of an optical amplifier with such a desired amplification gain is called the effective amplification bandwidth.

[0012] The phase-matching band of the nonlinear optical medium, which determines the amplification band of an OPA, is determined by the wavelength dependence of the refractive index of the medium and can be adjusted by parameter design during medium manufacturing and temperature conditions. At a temperature that provides optimal phase-matching conditions in a band near the center frequency, the OPA's gain spectrum exhibits flat characteristics near the center frequency. Furthermore, changing the temperature of the medium and changing the phase-matching state decreases the gain near the center frequency and increases the gain at frequencies away from the center. This makes it possible to widen the effective amplification band. Note that changing the temperature in the direction opposite to the direction that widens the effective amplification band changes the gain spectrum so that the flat gain band near the center frequency narrows.

[0013] The expansion of the effective amplification band by temperature control reaches its limit when the gain near the center frequency falls below the desired gain. A possible method for securing the amplification gain is to connect two OPAs in cascade. However, this raises concerns about increased power consumption due to the need for additional pump light, and signal distortion due to gain saturation in the downstream OPA.

[0014] In view of the above circumstances, an object of the present invention is to provide an optical amplifier and an optical amplification method that can widen the amplification band of signal light while reducing an increase in power consumption and signal distortion. [Means for solving the problem]

[0015] an optical amplifier according to one aspect of the present invention, which includes a polarization demultiplexing unit that demultiplexes signal light into first polarization signal light and second polarization signal light, which are two orthogonal polarization components; a first pumping light multiplexing unit that multiplexes pumping light with each of the first polarization signal light and the second polarization signal light; a first optical amplifying unit that generates an optical parametric amplification process to amplify the first polarization signal light and the second polarization signal light that have been multiplexed with the pumping light by the first pumping light multiplexing unit; a first pumping light separating unit that separates the pumping light from each of the first polarization signal light and the second polarization signal light amplified by the first optical amplifying unit; an unwanted band separating unit that removes unwanted frequency components from each of the first polarization signal light and the second polarization signal light from which the pumping light has been separated by the first pumping light separating unit; and a first polarization signal light from which the unwanted frequency components have been removed by the unwanted band separating unit that multiplexes the first polarization signal light from which the unwanted frequency components have been removed by the unwanted band separating unit with the pumping light that has been separated from the first polarization signal light by the first pumping light separating unit. a second optical amplification unit that generates an optical parametric amplification process to amplify the first polarized signal light and the second polarized signal light that have been multiplexed with the pumping light by the second pumping light multiplexer; a second pumping light separation unit that separates the pumping light from each of the first polarized signal light and the second polarized signal light that have been amplified by the second optical amplification unit; a polarization multiplexer that outputs signal light that has been multiplexed with the first polarized signal light and the second polarized signal light that have been separated from the pumping light by the second pumping light separation unit; and a band-pass filter unit that removes unnecessary frequency components from the signal light output by the polarization multiplexer, wherein the first optical amplification unit is in a phase-matched state in which the amplification gain increases as the frequency moves away from a center frequency that is half the frequency of the pumping light, and the second optical amplification unit is in a phase-matched state in which the amplification gain increases near the center frequency.

[0016] An optical amplification method according to one aspect of the present invention includes a polarization demultiplexing step of demultiplexing signal light into first polarization signal light and second polarization signal light, which are two orthogonal polarization components; a first pumping light combining step of combining pumping light with each of the first polarization signal light and the second polarization signal light; a first optical amplification step of generating an optical parametric amplification process to amplify the first polarization signal light and the second polarization signal light combined with the pumping light in the first pumping light combining step; a first pumping light separation step of separating the pumping light from each of the first polarization signal light and the second polarization signal light amplified in the first optical amplification step; an unwanted band separation step of removing unwanted frequency components from each of the first polarization signal light and the second polarization signal light from which the pumping light has been separated in the first pumping light separation step; and a first polarization signal light from which the unwanted frequency components have been removed in the unwanted band separation step, wherein the first polarization signal light from which the unwanted frequency components have been removed in the unwanted band separation step is combined with the pumping light separated from the first polarization signal light in the first pumping light separation step. the second optical amplification step generates an optical parametric amplification process to amplify the first polarized signal light and the second polarized signal light multiplexed with the pumping light in the second optical amplification step; the second optical amplification step separates the pumping light from each of the first polarized signal light and the second polarized signal light amplified in the second optical amplification step; the polarization multiplexing step outputs signal light obtained by multiplexing the first polarized signal light and the second polarized signal light from which the pumping light has been separated in the second optical amplification step; and the filtering step removes unnecessary frequency components from the signal light output in the polarization multiplexing step, wherein the first optical amplification step is in a phase matching state in which the amplification gain increases with increasing frequency from a center frequency which is half the frequency of the pumping light, and the second optical amplification step is in a phase matching state in which the amplification gain increases near the center frequency. [Effects of the Invention]

[0017] According to the present invention, it is possible to widen the amplification band of signal light while reducing an increase in power consumption and signal distortion. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a diagram illustrating an example of the configuration of an optical amplifier according to a first embodiment of the present invention. [Figure 2] FIG. 4 is a diagram illustrating an example of an amplification gain of a nonlinear medium in the first embodiment. [Figure 3] FIG. 2 is a diagram showing a comparison between the amplification band spectrum of the optical amplifier in the first embodiment and the amplification band spectrum of the optical amplifier of the prior art. [Figure 4] FIG. 2 is a diagram illustrating a configuration example of an optical amplifier according to the first embodiment. [Figure 5] FIG. 2 is a diagram illustrating a configuration example of an optical amplifier according to the first embodiment. [Figure 6] FIG. 10 is a diagram illustrating an example of the configuration of an optical amplifier according to a second embodiment. [Figure 7] FIG. 10 is a diagram illustrating an example of the configuration of an optical amplifier according to a second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0019] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The present embodiments relate to an optical signal amplification technique using optical parametric amplification.

[0020] The optical amplifier of this embodiment is an OPA configured with cascaded nonlinear optical media with complementary phase matching characteristics (gain spectra). The temperature of the first-stage nonlinear optical media is adjusted to amplify at least a frequency band distant from the center frequency within the target frequency band. Unlike conventional configurations, this adjustment allows for a gain below the desired gain near the center frequency. The second-stage nonlinear optical media complementarily amplifies the band near the center frequency that was not fully amplified by the first-stage media, i.e., the band for which the amplification by the first-stage media did not achieve the desired gain. The temperature of the second-stage media is controlled in the opposite direction to that of the first-stage media to intentionally narrow the amplification band. This effectively reduces the optical power input to the second-stage media, enabling even weak pump light to obtain sufficient gain to complement the gain characteristics of the first-stage media. Therefore, the pump light used in the first-stage nonlinear optical media can be reused, eliminating the need for additional pump light. This means that the effective amplification band can be expanded without increasing power consumption.

[0021] (First embodiment) A first embodiment of the present invention will be described with reference to FIGS.

[0022] 1 is a diagram showing the configuration of an optical amplifier 100 according to the first embodiment. The optical amplifier 100 is an OPA. The optical amplifier 100 includes a polarization demultiplexing unit 101, an excitation light multiplexing unit 102-1, an excitation light multiplexing unit 102-2, a nonlinear medium 103-1, a nonlinear medium 103-2, an excitation light separation unit 104-1, an excitation light separation unit 104-2, an unwanted band separation unit 105-1, an unwanted band separation unit 105-2, an excitation light multiplexing unit 106-1, an excitation light multiplexing unit 106-2, a nonlinear medium 107-1, a nonlinear medium 107-2, an excitation light separation unit 108-1, an excitation light separation unit 108-2, a polarization demultiplexing unit 109, and a bandpass filter 110.

[0023] The polarization demultiplexing unit 101 separates the input signal light into orthogonal polarization components. One of the two orthogonal polarization components is referred to as the first polarization signal light, and the other is referred to as the second polarization signal light. The polarization demultiplexing unit 101 outputs the first polarization signal light to the pump light multiplexing unit 102-1 and inputs the second polarization signal light to the pump light multiplexing unit 102-2.

[0024] The pump light multiplexer 102-i (i = 1, 2) multiplexes the ith polarized signal light input from the polarization demultiplexer 101 with the pump light. The pump light multiplexer 102-i outputs the ith polarized signal light multiplexed with the pump light to the nonlinear medium 103-i. The nonlinear medium 103-i is an optical amplifier that uses a nonlinear optical medium. The nonlinear medium 103-i amplifies the ith polarized signal light input from the pump light multiplexer 102-i by generating an optical parametric process. The nonlinear medium 103-i outputs the amplified ith polarized signal light to the pump light separator 104-i.

[0025] The pump light separating unit 104-i receives the i-th polarized signal from the nonlinear medium 103-i and separates the pump light from the input i-th polarized signal light. The pump light separating unit 104-i outputs the i-th polarized signal light from which the pump light has been separated to the unnecessary band separating unit 105-i and outputs the pump light to the pump light combining unit 106-i. The unnecessary band separating unit 105-i receives the i-th polarized signal light from the pump light separating unit 104-i and removes unnecessary frequency components generated in the optical parametric process from the input i-th polarized signal light, before outputting the signal to the pump light combining unit 106-i.

[0026] The pump light multiplexer 106-i multiplexes the pump light separated by the pump light separator 104-i with the ith polarized signal light from which unnecessary frequency components have been removed by the unnecessary band separator 105-i. The pump light multiplexer 106-i outputs the ith polarized signal light multiplexed with the pump light to the nonlinear medium 107-i. The nonlinear medium 107-i is an optical amplifier using a nonlinear optical medium. The nonlinear medium 107-i amplifies the ith polarized signal light input from the pump light multiplexer 106-i by generating an optical parametric process. The nonlinear medium 107-i outputs the amplified ith polarized signal light to the pump light separator 108-i. The pump light separator 108-i inputs the ith polarized signal light from the nonlinear medium 107-i and separates the pump light from the input ith polarized signal light. The pump light separating unit 108-i outputs the i-th polarized signal light from which the pump light has been separated to the polarization multiplexing unit 109.

[0027] The polarization multiplexer 109 multiplexes the first polarized signal light input from the pump light separator 108-1 and the second polarized signal light input from the pump light separator 108-2, and outputs the multiplexed signal light to the band-pass filter 110. The band-pass filter 110 receives the signal light from the polarization multiplexer 109 and removes, from the input signal light, unnecessary frequency components generated in the optical parametric processes of the nonlinear medium 107-1 and the nonlinear medium 107-2. The band-pass filter 110 outputs the signal light from which the unnecessary frequency components have been removed.

[0028] The signal light input to the optical amplifier 100 is first split into two bands separated by the center frequency of the amplification band. This is because, during parametric amplification, idler light, which is a phase conjugate of the signal light, is generated at a frequency symmetrical to the center frequency. The two bands of signal light are then split into orthogonal polarization components by the polarization splitter 101. This is because nonlinear phenomena in nonlinear media are polarization-dependent. The split signal light is then multiplexed with pump light by pump light multiplexers 102-1 and 102-2 and parametrically amplified by nonlinear media 103-1, 103-2, 107-1, and 107-2. After amplification, the pump light is separated from the signal light of each polarization component by the pump light splitters 108-1 and 108-2. After the pump light is separated, the signal light of each polarization component is again polarization-combined by the polarization multiplexer 109.

[0029] The multiplexing of the pumping lights by the pumping light multiplexing units 102-1 and 102-2 and the demultiplexing of the pumping lights by the pumping light demultiplexing units 108-1 and 108-2 are performed using wavelength multiplexing filters, dichroic mirrors, etc. Components of the signal light and idler light that are not used for transmission are cut by the bandpass filter 110. Typically, the bandpass filter 110 extracts signal light components in the same band as the signal light input to the optical amplifier 100 and outputs the extracted signal light components to a subsequent stage. However, the bandpass filter 110 may also extract idler light and output the extracted idler light to a subsequent stage. This allows the optical amplifier 100 to be used as a phase conjugate converter or a wavelength converter.

[0030] In this embodiment, two nonlinear media, 103-i and 107-i, are arranged in the paths of each polarization component. The temperatures of these two nonlinear media, 103-i and 107-i, are adjusted by Peltier elements, heaters, or the like attached to the media so that they have complementary gain spectra. The pump light used for amplification by the first-stage nonlinear medium, 103-i, is separated by the pump light separation unit, 104-i, and then combined with the signal light by the subsequent pump light combination unit, 106-i, and used for amplification by the nonlinear medium, 107-i. Note that the demultiplexing of the pump light by the pump light separation unit, 104-i, and the combination of the pump light by the pump light combination unit, 106-i, are performed using wavelength multiplexing filters, dichroic mirrors, and the like. The pump light separation unit, 104-i, between the nonlinear media, 103-i and 107-i, passes either the same band as the signal light input to the optical amplifier 100, or the idler light band.

[0031] Typically, the power of pump light once used decreases due to slight attenuation associated with nonlinear processes within the medium, losses in the pump light splitter, and losses in the pump light multiplexer. Furthermore, to linearly amplify signal light that has been amplified once and has high optical power, even higher pump light optical power is required. For these reasons, reuse of pump light usually immediately causes gain saturation due to pump depletion, resulting in nonlinear distortion of the signal light. However, as in this embodiment, by narrowing the phase matching characteristics of the subsequent nonlinear medium 107-i, the pump light is not consumed in the band outside the signal light. In other words, the signal light power input to the subsequent nonlinear medium 107-i can be effectively considered lower than the true input power, thereby suppressing gain saturation. Another factor that makes this configuration possible is that only the central band needs to be amplified with a small gain.

[0032] FIG. 2 shows an example of the amplification gain of the first-stage nonlinear medium 103-i and the second-stage nonlinear medium 107-i. In the first-stage nonlinear medium 103-i, the amplification gain around the center wavelength of the phase matching characteristic is low, and the amplification gain is accordingly large at wavelengths away from the center wavelength. The center wavelength of the phase matching characteristic is a wavelength corresponding to the center frequency, which is half the frequency of the pump light. In addition, the second-stage nonlinear medium 107-i has an amplification gain spectrum that complementarily amplifies the wavelength around the center wavelength of the first-stage nonlinear medium 103-i.

[0033] FIG. 3 compares the amplification gain spectrum of the optical amplifier 100 of this embodiment shown in FIG. 1 with that of a conventional optical amplifier. The conventional optical amplifier has an OPA configuration consisting of a single nonlinear medium. Measurements of the amplification gain when continuous light is input using a PPLN waveguide with a phase-matching characteristic center wavelength of 1545.32 nm (half the pump light frequency) were performed by sweeping the wavelength of the input light. For both the optical amplifier 100 of this embodiment and the conventional optical amplifier, temperature control was performed to maximize the bandwidth over which a 15 dB gain was obtained. The input power of the pump light was the same for both the present embodiment and the conventional optical amplifier. As shown in FIG. 3, the optical amplifier 100 of this embodiment achieves an effective amplification bandwidth approximately 8 nm wider than that of the conventional optical amplifier. This embodiment achieves such bandwidth expansion with high power efficiency without adding pump light or increasing output power.

[0034] 4 is a diagram showing the configuration of an optical amplifier 200. The optical amplifier 200 is configured to use the entire band extending from the center frequency of the phase matching characteristics of the OPA to the low-frequency side and the high-frequency side. In FIG. 4, parts that are the same as those in the optical amplifier 100 shown in FIG. 1 are given the same reference numerals, and their description will be omitted. The optical amplifier 200 includes a band demultiplexing unit 201, an OPA 202-1, an OPA 202-2, and a band combining unit 203.

[0035] The band demultiplexing unit 201 demultiplexes the signal light input to the optical amplifier 200 into two bands with the center frequency as the boundary. The band demultiplexing unit 201 outputs one of the demultiplexed signal lights to the OPA 202-1 and outputs the other demultiplexed signal light to the OPA 202-2.

[0036] 1 except that the bandpass filter 110 is removed. The polarization demultiplexing unit 101 of the OPA 202-1 and the polarization demultiplexing unit 101 of the OPA 202-2 receive the signal light from the band demultiplexing unit 201. The polarization multiplexing unit 109 of the OPA 202-1 and the polarization multiplexing unit 109 of the OPA 202-2 output the polarization-combined signal light to the band multiplexing unit 203.

[0037] The band multiplexer 203 multiplexes the signal light input from the polarization multiplexer 109 of OPA 202-1 and the signal light input from the polarization multiplexer 109 of OPA 202-2. The band multiplexer 203 uses a filter to cut out components not used for transmission from the combined signal light, and extracts and outputs the signal light in the band used for transmission. As a result, the band multiplexer 203 outputs signal light from which idler light has been cut, or idler light from which signal light has been cut.

[0038] In an OPA, it is necessary to leave the band where idler light is generated free. Therefore, a configuration is required in which the signal light is split into two bands at the center frequency, and after processing each band, the two bands are combined again. Therefore, optical amplifier 200 has another configuration similar to the optical amplifier 100 described above, and is connected in parallel via a band multiplexing / demultiplexing filter consisting of band demultiplexing unit 201 and band multiplexing unit 203. The configuration of optical amplifier 100 corresponds to OPA 202-1 and OPA 202-2.

[0039] In order to suppress the reflected light from the nonlinear medium, an isolator may be provided before and after the nonlinear medium. An example of providing an isolator is shown in Figure 5.

[0040] Fig. 5 is a configuration diagram of OPA 300. In Fig. 5, parts that are the same as those in optical amplifier 100 shown in Fig. 1 are given the same reference numerals, and their description will be omitted. OPA 300 shown in Fig. 5 differs from optical amplifier 100 shown in Fig. 1 in that it includes isolators 301-1, 301-2, 302-1, 302-2, 303-1, 303-2, 304-1, 304-2, 305-1, and 305-2.

[0041] The isolator 301-i (i = 1, 2) is provided between the polarization demultiplexing unit 101 and the pumping light multiplexing unit 102-i. The isolator 301-i passes the ith polarized signal light in the direction from the polarization demultiplexing unit 101 to the pumping light multiplexing unit 102-i and blocks light in the direction from the pumping light multiplexing unit 102-i to the polarization demultiplexing unit 101. The isolator 302-i is provided between the pumping light separation unit 104-i and the unwanted band separation unit 105-i. The isolator 302-i passes the ith polarized signal light in the direction from the pumping light separation unit 104-i to the unwanted band separation unit 105-i and blocks light in the direction from the unwanted band separation unit 105-i to the pumping light separation unit 104-i. The isolator 303-i is provided between the unwanted band separation unit 105-i and the pumping light multiplexing unit 106-i. The isolator 303-i passes the i-th polarized signal light in the direction from the unwanted band separator 105-i to the pumping light multiplexer 106-i, and blocks light in the direction from the pumping light multiplexer 106-i to the unwanted band separator 105-i.

[0042] The isolator 304-i is provided between the pumping light separation unit 104-i and the pumping light multiplexing unit 106-i. The isolator 304-i passes pumping light in the direction from the pumping light separation unit 104-i to the pumping light multiplexing unit 106-i and blocks light in the direction from the pumping light multiplexing unit 106-i to the pumping light separation unit 104-i. The isolator 305-i is provided between the pumping light separation unit 108-i and the polarization multiplexing unit 109. The isolator 305-i passes the ith polarized signal light in the direction from the pumping light separation unit 108-i to the polarization multiplexing unit 109 and blocks light in the direction from the polarization multiplexing unit 109 to the pumping light separation unit 108-i.

[0043] As shown in Figure 5, by placing an isolator immediately before or after each of the nonlinear media 103-1, 103-2, 107-1, and 107-2, or both, reflected light can be effectively suppressed. Note that isolators may be provided immediately before or after either the first-stage nonlinear media 103-1 and 103-2 or the second-stage nonlinear media 107-1 and 107-2, or both. It is also desirable to place an isolator in the process of splitting and recombining the pump light to prevent multiple reflections of the pump light between the two media.

[0044] (Second embodiment) In the second embodiment, the optical power of the pump light input to the subsequent nonlinear medium is adjusted. The second embodiment of the present invention will be described with reference to FIGS.

[0045] Fig. 6 is a diagram showing an example of the configuration of an optical amplifier 400 according to the second embodiment. In Fig. 6, the same components as those in the optical amplifier 100 according to the first embodiment shown in Fig. 1 are assigned the same reference numerals, and their description will be omitted. The optical amplifier 400 shown in Fig. 6 differs from the optical amplifier 100 shown in Fig. 1 in that it includes an optical attenuator 401-1 between the pumping light separation unit 104-1 and the pumping light multiplexing unit 106-1, and an optical attenuator 401-2 between the pumping light separation unit 104-2 and the pumping light multiplexing unit 106-2. The optical attenuators 401-i (i = 1, 2) attenuate the pumping light separated by the pumping light separation unit 104-i and output the pumping light to the pumping light multiplexing unit 106-i.

[0046] If the optical power of the pump light input to the subsequent-stage nonlinear medium 107-i is too strong, the gain increases, but the gain saturation tolerance to the input signal light power generally decreases. Therefore, it is desirable to attenuate the optical power of the pump light input to the subsequent-stage nonlinear medium 107-i to an extent that the minimum necessary gain is obtained. Therefore, in the second embodiment, as shown in Figure 6, an optical attenuator 401-i is placed in the pump light path between the initial-stage nonlinear medium 103-i and the subsequent-stage nonlinear medium 107-i.

[0047] Furthermore, optical amplifiers are usually equipped with an automatic gain control (AGC) mechanism. FIG. 7 is a diagram showing an example of the configuration of an optical amplifier 500 equipped with an automatic gain control mechanism. In FIG. 7, the same components as those in the optical amplifier 100 of the first embodiment shown in FIG. 1 are given the same reference numerals, and their description will be omitted. The optical amplifier 500 shown in FIG. 7 differs from the optical amplifier 100 shown in FIG. 1 in that it has a variable optical attenuator (VOA) 501. , 502-1, 502-2, and 503. The variable optical attenuator 501 is provided in the upstream of the polarization demultiplexing unit 101. The variable optical attenuator 502-1 is provided between the pumping light separation unit 104-1 and the pumping light multiplexing unit 106-1. The variable optical attenuator 502-2 is provided between the pumping light separation unit 104-2 and the pumping light multiplexing unit 106-2. The variable optical attenuator 503 is provided in the downstream of the bandpass filter 110. Also, FIG. 7 shows a light source 504 for the pumping light input to the pumping light multiplexing unit 102-1 and the pumping light input to the pumping light multiplexing unit 102-2. The optical tap 510 is connected in the upstream of the optical amplifier 500, and the optical tap 520 is connected in the downstream of the optical amplifier 500. Furthermore, the optical amplifier 500, the optical tap 510, and the optical tap 520 are connected to a gain control device 530. The gain control device 530 is an AGC.

[0048] The optical tap 510 branches a portion of the signal light input to the optical amplifier 500. The optical tap 510 outputs the branched signal light to the gain control device 530, and outputs the remaining branched signal light to the variable optical attenuator 501 of the optical amplifier 500. The variable optical attenuator 501 attenuates the signal light input from the optical tap 510 and outputs it to the polarization demultiplexing unit 101. The variable optical attenuator 502-i (i = 1, 2) attenuates the pump light separated by the pump light separation unit 104-2 and outputs it to the pump light multiplexing unit 106-i. The variable optical attenuator 503 attenuates the signal light output by the bandpass filter 110 and outputs it. The optical tap 520 branches a portion of the signal light attenuated by the variable optical attenuator 503 of the optical amplifier 500. The optical tap 520 outputs the branched signal light to the gain control device 530, and outputs the remaining branched signal light.

[0049] The gain control device 530 includes a monitor unit 531 and a control unit 532. The monitor unit 531 monitors the signal light branched by the optical tap 510 and the signal light branched by the optical tap 520. Based on the monitoring results of the monitor unit 531, the control unit 532 controls the gain when attenuating the signal light in each of the variable optical attenuators 501, 502-1, 502-2, and 503, and controls the pump light power by changing the injection current to the light source 504. With the configuration shown in Fig. 7, AGC can be performed not only by controlling the injection power to the pump light, but also by controlling the temperature of the nonlinear medium and the VOA for the pump light described above.

[0050] As described above, according to this embodiment, the OPA configuration in which two nonlinear media with complementary gain characteristics are connected enables the amplification band to be broadened with high power efficiency without the need to add pump light or increase output power.

[0051] According to the above-described embodiment, the optical amplifier device includes a polarization demultiplexing unit, a first pumping light multiplexing unit, a first optical amplifying unit, a first pumping light separating unit, an unwanted band separating unit, a second pumping light multiplexing unit, a second optical amplifying unit, a second pumping light separating unit, a polarization demultiplexing unit, and a bandpass filter unit. The polarization demultiplexing unit corresponds to, for example, the polarization demultiplexing unit 101 in the embodiments. The first pumping light multiplexing unit corresponds to, for example, the pumping light multiplexing units 102-1 and 102-2 in the embodiments. The first optical amplifying unit corresponds to, for example, the nonlinear media 103-1 and 103-2 in the embodiments. The first pumping light separating unit corresponds to, for example, the pumping light separating units 104-1 and 104-2 in the embodiments. The unwanted band separating unit corresponds to, for example, the unwanted band separating units 105-1 and 105-2 in the embodiments. The second pumping light combining unit corresponds to, for example, the pumping light combining units 106-1 and 106-2 in the embodiments. The second optical amplification unit corresponds to, for example, the nonlinear media 107-1 and 107-2 in the embodiments. The second pumping light separation unit corresponds to, for example, the pumping light separation units 108-1 and 108-2 in the embodiments. The polarization multiplexing unit corresponds to, for example, the polarization multiplexing unit 109 in the embodiments. The band-pass filter unit corresponds to, for example, the band-pass filter 110 in the embodiments.

[0052] The polarization demultiplexing unit demultiplexes the signal light into first polarization signal light and second polarization signal light, which are two orthogonal polarization components. The first pump light multiplexing unit multiplexes pump light with each of the first polarization signal light and the second polarization signal light. The first optical amplifying unit generates an optical parametric amplification process to amplify the first polarization signal light and the second polarization signal light multiplexed with the pump light by the first pump light multiplexing unit. The first pump light separating unit separates the pump light from each of the first polarization signal light and the second polarization signal light amplified by the first optical amplifying unit. The unnecessary band separating unit removes unnecessary frequency components from each of the first polarization signal light and the second polarization signal light from which the pump light has been separated by the first pump light separating unit. The second pump light multiplexer multiplexes the first polarized signal light from which unnecessary frequency components have been removed by the unnecessary band separator with the pump light separated from the first polarized signal light by the first pump light separator, and multiplexes the second polarized signal light from which unnecessary frequency components have been removed by the unnecessary band separator with the pump light separated from the second polarized signal light by the first pump light separator. The second optical amplifier generates an optical parametric amplification process to amplify the first polarized signal light and the second polarized signal light multiplexed with the pump light by the second pump light multiplexer. The second pump light separator separates the pump light from each of the first polarized signal light and the second polarized signal light amplified by the second optical amplifier. The polarization multiplexer outputs signal light obtained by multiplexing the first polarized signal light and the second polarized signal light from which the pump light has been separated by the second pump light separator. The bandpass filter removes unnecessary frequency components from the signal light output by the polarization multiplexer. The first optical amplifier is in a phase-matched state in which the amplification gain increases with increasing frequency from a center frequency that is half the frequency of the pump light. The second optical amplifier is in a phase-matched state in which the amplification gain increases near the center frequency. The second optical amplifier complementarily amplifies the first polarization signal light and the second polarization signal light that do not meet the predetermined amplification gain in the first optical amplifier within the target frequency band for amplification.

[0053] The optical amplifying device may further include an adjustment unit. The adjustment unit corresponds to, for example, the optical attenuators 401-1 and 401-2 in the embodiments. The adjustment unit adjusts the optical power of the pumping light output from the first pumping light separation unit to the second pumping light multiplexing unit. The attenuation amount of the pumping light in the adjustment unit may be fixed or variable.

[0054] The optical amplifying device may further include a control unit. The control unit corresponds to, for example, the gain control device 530 of the embodiment. The control unit controls the gain of the adjustment unit according to the monitoring results of the signal light before being separated by the polarization demultiplexing unit and the signal light output by the band-pass filter unit.

[0055] The optical amplification device may further include an isolator at one or both of the upstream and downstream stages of one or both of the first optical amplification unit and the second optical amplification unit, which allows light to pass in the direction from the first optical amplification unit to the second optical amplification unit and blocks light in the direction from the second optical amplification unit to the first optical amplification unit.

[0056] The bandpass filter section passes the amplified signal light or idler light.

[0057] Although the embodiments of the present invention have been described in detail above with reference to the drawings, the specific configurations are not limited to these embodiments, and include designs within the scope of the present invention that do not deviate from the gist of the present invention. [Explanation of symbols]

[0058] 100 Optical Amplifier 101 Polarization splitter 102-1, 102-2 Pumping light multiplexing section 103-1, 103-2 Nonlinear media 104-1, 104-2 Pump light separation unit 105-1, 105-2 Unnecessary band separation section 106-1, 106-2 Pumping light multiplexing section 107-1, 107-2 Nonlinear media 108-1, 108-2 Pump light separation unit 109 Polarization multiplexing section 110 Bandpass Filter 200 Optical Amplifier 201 Band splitter 203 band multiplexing section 301-1, 301-2, 302-1, 302-2, 303-1, 303-2, 304-1, 304-2, 305-1, 305-2 Isolators 400 Optical Amplifier 401-1, 401-2 Optical attenuator 500 Optical Amplifier 501, 502-1, 502-2, 503 Variable Optical Attenuators 504 Light source 510, 520 optical tap 530 Gain control device 531 Monitor section 532 Control Unit

Claims

1. a polarization demultiplexing unit that demultiplexes the signal light into first polarization signal light and second polarization signal light, which are two orthogonal polarization components; a first pumping light multiplexing unit that multiplexes pumping light with each of the first polarization signal light and the second polarization signal light; a first optical amplifying unit that generates an optical parametric amplification process to amplify the first polarization signal light and the second polarization signal light that are multiplexed with the pump light by the first pump light multiplexing unit; a first pumping light separating unit that separates the pumping light from each of the first polarization signal light and the second polarization signal light amplified by the first optical amplifier unit; an unnecessary band separating unit that removes unnecessary frequency components from each of the first polarization signal light and the second polarization signal light into which the pump light is separated by the first pump light separating unit; a second pumping light multiplexing unit that multiplexes the first polarized signal light from which unnecessary frequency components have been removed by the unnecessary band separating unit and the pumping light separated from the first polarized signal light by the first pumping light separating unit, and multiplexes the second polarized signal light from which unnecessary frequency components have been removed by the unnecessary band separating unit and the pumping light separated from the second polarized signal light by the first pumping light separating unit; a second optical amplifying section that generates an optical parametric amplification process to amplify the first polarization signal light and the second polarization signal light that are multiplexed with the pump light by the second pump light multiplexing section; a second pumping light separating unit that separates the pumping light from each of the first polarization signal light and the second polarization signal light amplified by the second optical amplifier unit; a polarization multiplexing unit that outputs signal light obtained by multiplexing the first polarized signal light and the second polarized signal light obtained by separating the pumping light by the second pumping light separating unit; a band-pass filter unit that removes unnecessary frequency components from the signal light output from the polarization multiplexing unit, the first optical amplification unit is in a phase matching state in which an amplification gain increases as the frequency moves away from a center frequency that is half the frequency of the pumping light, the second optical amplification unit is in a phase-matched state in which the amplification gain is large near the center frequency; Optical amplifier.

2. further comprising an adjustment unit that adjusts the optical power of the pumping light output from the first pumping light separation unit to the second pumping light multiplexing unit.

2. The optical amplifier according to claim 1.

3. a control unit that controls a gain of the adjustment unit in accordance with a monitoring result of the signal light before being separated by the polarization demultiplexing unit and the signal light output by the band-pass filter unit.

3. The optical amplifier according to claim 2.

4. further comprising an isolator at one or both of a front stage and a rear stage of one or both of the first optical amplification unit and the second optical amplification unit, the isolator passing light in a direction from the first optical amplification unit to the second optical amplification unit and blocking light in a direction from the second optical amplification unit to the first optical amplification unit; 2. The optical amplifier according to claim 1.

5. the bandpass filter section passes the amplified signal light or the idler light; 5. The optical amplifier according to claim 1.

6. a polarization demultiplexing step of demultiplexing the signal light into two orthogonal polarization components, a first polarization signal light and a second polarization signal light; a first pumping light multiplexing step of multiplexing pumping light with each of the first polarization signal light and the second polarization signal light; a first optical amplification step of generating an optical parametric amplification process to amplify the first polarization signal light and the second polarization signal light multiplexed with the pump light in the first pump light multiplexing step; a first pumping light separation step of separating the pumping light from each of the first polarization signal light and the second polarization signal light amplified in the first optical amplification step; an unnecessary band separation step of removing unnecessary frequency components from each of the first polarization signal light and the second polarization signal light obtained by separating the pump light in the first pump light separation step; a second pumping light combining step of combining the first polarized signal light from which unnecessary frequency components have been removed by the unnecessary band separating step and the pumping light separated from the first polarized signal light by the first pumping light separating step, and combining the second polarized signal light from which unnecessary frequency components have been removed by the unnecessary band separating step and the pumping light separated from the second polarized signal light by the first pumping light separating step; a second optical amplification step of amplifying the first polarization signal light and the second polarization signal light multiplexed with the pump light in the second pump light multiplexing step by generating an optical parametric amplification process; a second pumping light separation step of separating the pumping light from each of the first polarization signal light and the second polarization signal light amplified in the second optical amplification step; a polarization multiplexing step of outputting signal light obtained by multiplexing the first polarized signal light and the second polarized signal light from which the pump light has been separated in the second pump light separation step; a filtering step of removing unnecessary frequency components from the signal light outputted by the polarization multiplexing step, In the first optical amplification step, a phase matching state is established in which the amplification gain increases as the frequency moves away from a center frequency that is half the frequency of the pump light, In the second optical amplification step, a phase matching state is established in which an amplification gain is large near the center frequency. Optical amplification method.

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