Optical Parametric Amplifier

Abstract

Provided is an optical parametric amplifier having two phase-matching wavelengths λ1 and λ2 (λ1 < λ2) and comprising: a PPLN waveguide (71) that performs parametric amplification of a signal light group 20; and an excitation light generation unit (10) that generates excitation light to be input to the PPLN waveguide (71). The phase-matching wavelengths λ1 and λ2 satisfy a relationship λ1 < λsi < λ2 for a wavelength λsi (i = 1, 2, 3...) of the signal light group. The excitation light generation unit (10) generates excitation light having a wavelength in a vicinity of λ1 / 2 and excitation light having a wavelength in a vicinity of λ2 / 2 and inputs the excitation light to the PPLN waveguide (71). The PPLN waveguide (71) comprises a nonlinear optical crystal having a periodic poling structure having two poling periods Λλ1 and Λλ2.

Description

[Technical Field]

[0001] The present invention relates to an optical parametric amplifier used in an optical communication system or a laser device. [Background technology]

[0002] In optical communication systems, erbium-doped fiber amplifiers (EDFAs) are widely used to relay signals that have been attenuated by propagating through optical fibers. EDFAs inject pump light into erbium-doped fiber (EDFs) and amplify the incident light through stimulated emission in the EDF.

[0003] Until EDFAs became practical, the method used was to convert attenuated light into an electrical signal, identify the digital signal, and then convert the electrical signal back into an optical signal. This optical-electrical-optical conversion required many optical and electrical components, which increased the cost of optical communication relays.

[0004] However, with the practical application of EDFAs, it has become possible to amplify optical signals as they are, making it possible to amplify wavelength division multiplexing (WDM) signals, which transmit different information on multiple wavelengths, all at once. This has made it possible to amplify and relay optical signals with a simple configuration, significantly reducing the cost of optical relays. It is no exaggeration to say that the entire system, especially for optical communication networks that transmit optical signals over long distances, is designed with the use of EDFAs in mind.

[0005] The diversification of information and communication technology services in recent years has led to demands for further increases in transmission capacity for optical communication systems that support the backbone of communication networks. According to Shannon's communication theory, spectral efficiency, defined as the ratio of transmission capacity per unit frequency band, is expressed as log2(1+S / N) relative to the signal-to-noise (S / N) ratio. Therefore, the upper limit of the S / N ratio determines the theoretical upper limit of transmission capacity. The S / N ratio in an optical communication receiver is proportional to the power of the optical signal under conditions where so-called shot noise becomes dominant. Therefore, in principle, it makes sense to transmit at high optical power in order to increase spectral efficiency.

[0006] However, in reality, optical fibers, the transmission medium for optical communications, are subject to nonlinear optical effects. It has been pointed out that if the transmission power is increased more than necessary, the S / N ratio of the optical signal will actually deteriorate due to the effects of nonlinear optical effects. This deterioration in the S / N ratio is called the nonlinear Shannon limit, and is being discussed as a phenomenon that could restrict the upper limit of the transmission capacity of optical communication systems.

[0007] As mentioned above, the nonlinear Shannon limit is beginning to reveal a theoretical upper limit to the spectral efficiency of optical communication systems. To further increase communication capacity, it is essential to expand the frequency band used for optical communication.

[0008] However, the following problems still exist in optical communication systems that use the above-mentioned EDFAs. The wavelength bands that can be amplified by the EDFAs widely used in current optical communication systems are limited to the C band (1530-1565 nm) and L band (1565-1625 nm). Therefore, current optical communication systems are built on the assumption that these wavelength bands will be used. However, because the transparent wavelength band of optical fiber itself is extremely wide, if wavelength bands other than the C and L bands could be used, it would be possible to significantly expand the transmission capacity of optical communication.

[0009] Optical amplifiers such as EDFAs that use rare-earth elements as laser media are limited in the wavelength ranges they can amplify because they use transitions between the energy levels of the rare-earth elements. One way to achieve optical amplification that does not have such limitations is to use parametric amplification using second- or third-order nonlinear optical media. A typical example of a third-order nonlinear optical medium is four-wave mixing in optical fiber. However, as mentioned above, the nonlinear optical effect of optical fiber can also cause a degradation in the S / N ratio of optical signals. Therefore, those that use third-order nonlinear optical media are problematic as low-noise optical amplifiers.

[0010] On the other hand, a typical example of a second-order nonlinear optical medium is an optical waveguide made of periodically poled lithium niobate (PPLN). For example, Non-Patent Document 1 shows that wideband optical amplification is possible using difference frequency generation, which is a second-order nonlinear optical effect of PPLN. In the method using PPLN, third-order nonlinear optical effects can be ignored, so it can be considered that there is almost no degradation in signal quality due to nonlinear optical effects.

[0011] FIG. 7 shows the basic configuration of a conventional optical parametric amplifier and wavelength converter using a second-order nonlinear optical medium such as a PPLN waveguide. This configuration is disclosed in Non-Patent Document 2. The conventional configuration uses two second-order nonlinear optical elements 100 and 101 with the same phase-matched wavelength (1550 nm). The second-order nonlinear optical elements 100 and 101 include PPLN waveguides 1000 and 1010, respectively. A laser light source 102 used in optical communications generates fundamental light in the 1550 nm band. An EDFA 103 amplifies the fundamental light to obtain sufficient power to achieve a nonlinear optical effect. The second-order nonlinear optical element 100 is a device for second harmonic generation (SHG) and generates second harmonic light from the amplified fundamental light 200.

[0012] The second-order nonlinear optical element 101 is an element for difference frequency generation (DFG), and performs non-degenerate parametric amplification of externally input signal light 203 using second harmonic light 204 output from the second-order nonlinear optical element 100 as pump light. At the same time, wavelength-converted light (idler light) corresponding to the frequency difference between the signal light and the pump light is also generated by the DFG process. The configuration in Figure 7 functions as an optical amplifier if only the amplified signal light is extracted from the output side of the second-order nonlinear optical element 101, and functions as a wavelength converter if only the wavelength-converted light is extracted.

[0013] Figure 8 is a diagram illustrating the conventional optical parametric amplification process and DFG band. Here, we will use the DFG process for explanation, but the principles are similar in the optical parametric amplification process. 200 in Figure 8(b) shows a single-wavelength fundamental wave light output from a single laser light source. 201 in Figure 8(a) shows the phase matching curve for SHG of the PPLN waveguide, 202 shows the phase matching curve for DFG of the PPLN waveguide, 203 shows the signal light, and 205 shows the converted light. The phase matching band for SHG of the PPLN waveguide is narrower than the phase matching band for DFG but is sufficiently wider than the linewidth of the fundamental light.

[0014] Here, we will describe the wavelength conversion bandwidth of a PPLN waveguide when the fundamental wavelength λ0 (frequency ω0) is 1545 nm and the pump wavelength λp (frequency 2ω0) is 772.5 nm. By inputting pump light and signal light into the PPLN waveguide, converted light is generated through the DFG process. For example, if the signal wavelength λs (frequency ωs) is 1540 nm, converted light with a wavelength of 1550 nm is generated through 2ω0-ωs. As shown in Figure 8(a), converted light is generated by folding the signal light on the wavelength axis, centered on the fundamental wavelength λ0.

[0015] In the PPLN waveguide, the quasi-phase matching condition is satisfied among the pump light, signal light, and converted light. In other words, if the effective refractive indices of the pump light, signal light, and converted light in the waveguide are np, ns, and nc, respectively, the PPLN waveguide has a polarization inversion structure with an inversion period Λ that satisfies equation (1). np / λp-ns / λs-nc / λc=1 / Λ ···(1)

[0016] Even if the wavelength of the signal light is changed, the same conversion efficiency can be obtained as long as equation (1) is satisfied between the converted light with a frequency of 2ω0-ωs and the pump light. Specifically, if the wavelength λs (frequency ωs) of the signal light is 1539 nm, for example, converted light with a wavelength of 1551 nm is generated by 2ω0-ωs. At this time, the effective refractive index ns of the signal light and the effective refractive index nc of the converted light also change, but because nc becomes smaller as ns increases due to material dispersion, equation (1) can be satisfied even if the wavelength of the signal light is changed. As a result, a parametric amplifier using a PPLN waveguide can achieve a wide wavelength conversion bandwidth, as shown in Figure 8(a).

[0017] Because material dispersion is not linear, the increase in the effective refractive index ns of the signal light and the decrease in the effective refractive index nc of the converted light are not exactly the same, and the conversion efficiency gradually decreases, limiting the wavelength conversion bandwidth. However, when the wavelength of the fundamental light (1545 nm in this example) is matched to the phase-matching wavelength, a bandwidth of approximately 60 nm centered on the wavelength of the fundamental light can be obtained with a PPLN waveguide length of 45 mm, enabling broader amplification than that of a typical EDFA. Furthermore, as disclosed in Non-Patent Document 1, the shape of the amplification bandwidth can be changed by detuning the phase-matching wavelength from the pump light wavelength, enabling optical amplification over an even wider bandwidth.

[0018] However, although optical parametric amplification using a PPLN waveguide is expected to achieve broadband amplification, it has the following problems. As mentioned above, the parametric amplification process not only amplifies the signal light, but also generates converted light with wavelengths that are folded back from the signal light wavelength around the fundamental light wavelength. Therefore, when the signal light group entering the PPLN waveguide is on both the long-wavelength and short-wavelength sides of the fundamental light wavelength, the converted light for the long-wavelength signal light will be generated in the short-wavelength signal light wavelength band, and the converted light for the short-wavelength signal light will be generated in the long-wavelength signal light wavelength band, so the signal lights must be separated in advance.

[0019] For this reason, when amplifying signal light over the entire parametric amplification band, the input signal light is separated into a short wavelength side and a long wavelength side using a wavelength demultiplexer 300 as shown in Fig. 9. In Fig. 10A, 400 indicates the signal light incident on the wavelength demultiplexer 300, in Fig. 10B, 401 indicates the short wavelength side signal light separated by the wavelength demultiplexer 300, and in Fig. 10C, 402 indicates the long wavelength side signal light separated by the wavelength demultiplexer 300.

[0020] The signal light on the shorter wavelength side is passed through second-order nonlinear optical element 301, and the signal light on the longer wavelength side is passed through second-order nonlinear optical element 302, thereby amplifying and generating converted light. Second-order nonlinear optical elements 301 and 302 include PPLN waveguides 3010 and 3020, respectively. 403 in FIG. 10D denotes the converted light generated by second-order nonlinear optical element 301, and 404 in FIG. 10E denotes the converted light generated by second-order nonlinear optical element 302. When multiplexing the output light of second-order nonlinear optical element 301 and the output light of second-order nonlinear optical element 302, wavelength multiplexer 303 cuts the converted light output from second-order nonlinear optical element 301 and the converted light output from second-order nonlinear optical element 302. In this way, only the original signal light is multiplexed, as shown in FIG. 10F.

[0021] As described above, although an optical parametric amplifier using a PPLN waveguide is capable of wideband amplification, a configuration such as that shown in FIG. 9 is required when using the entire amplification band. The configuration in Figure 9 has two problems. The first problem is that the long-wavelength signal light and the short-wavelength signal light must be amplified separately, which requires two PPLN waveguides for amplification, increasing the number of components and making the configuration more complex. The second problem is that the signal light must be separated by a wavelength demultiplexer before amplification, which increases the noise figure of the amplifier by the amount of the transmission loss of the wavelength demultiplexer. If the noise figure increases, it will no longer be possible to transmit signals over long distances while maintaining signal quality, no matter how wide the bandwidth. Therefore, excess noise in the amplifier must be suppressed as much as possible. [Prior art documents] [Non-patent literature]

[0022] [Non-Patent Document 1] MHChou, I. Brener, KRParameswaran and MMFejer, “Stability and bandwidth enhancement of difference frequency generation (DFG)-based wavelength conversion by pump detuning”, ELECTRONICS LETTERS, 10th June 1999, Vol. 35, No. 12, pp. 978-990 [Non-patent document 2] T. Umeki, O. Tadanaga, A. Takada, and M. Asobe, “Phase sensitive degenerate parametric amplification using directly-bonded PPLN ridge waveguides”, Optics Express, Vol. 19, No. 7, pp. 6326-6332, 2011 Summary of the Invention [Problem to be solved by the invention]

[0023] The present invention has been made to solve the above problems, and has as its object to provide an optical parametric amplifier that has a simple configuration and is capable of wideband, low-noise optical amplification. [Means for solving the problem]

[0024] The optical parametric amplifier of the present invention comprises a first optical waveguide having two phase-matched wavelengths λ1 and λ2 (λ1<λ2) and configured to parametrically amplify a signal light group, and a pump light generator configured to generate pump light to be input to the first optical waveguide, wherein the phase-matched wavelengths λ1 and λ2 are set to satisfy the relationship λ1<λsi<λ2 with respect to the wavelengths λsi (i=1, 2, 3, . . . ) of the signal light group, and the pump light generator generates a phase-matched wavelength λ1 near λ1 / 2. Waves of Long excitation light and near λ2 / 2 Waves of and a long excitation light beam, which is input to the first optical waveguide, and the first optical waveguide has two polarization inversion periods Λ λ1 ,Λ λ2 The nonlinear optical crystal has a periodically poled structure having a refractive index of light having a wavelength of λ1 / 2 in the first optical waveguide. λ1 / 2 , the refractive index of light with wavelength λ2 / 2 is n λ2 / 2 , the refractive index of light of wavelength λ1 is n λ1 , the refractive index of light of wavelength λ2 is n λ2 When the polarization inversion period Λ λ1 ,Λ λ2 is n λ1 / 2 / (λ1 / 2)-2n λ1 / λ1=1 / Λ λ1 , n λ2 / 2 / (λ2 / 2)-2n λ2 / λ2=1 / Λ λ2 The present invention is characterized in that the following relationship is satisfied. [Effects of the Invention]

[0025] According to the present invention, an optical parametric amplifier is constructed using a first optical waveguide having two phase-matched wavelengths λ1 and λ2 at both ends of the signal light band. This eliminates the need for a wavelength demultiplexer compared to conventional optical parametric amplifiers, thereby reducing the number of components and suppressing excessive loss, thereby enabling amplification with the noise figure inherent to the optical parametric amplifier and enabling wideband, low-noise optical amplification. [Brief explanation of the drawings]

[0026] [Figure 1] FIG. 1 is a block diagram showing the configuration of an optical parametric amplifier according to a first embodiment of the present invention. [Figure 2] FIG. 2 is a diagram illustrating the operation of parametric amplification in the PPLN waveguide according to the first embodiment of the present invention. [Figure 3] FIG. 3 is a block diagram showing another configuration of the optical parametric amplifier according to the first embodiment of the present invention. [Figure 4] FIG. 4 is a block diagram showing another configuration of the optical parametric amplifier according to the first embodiment of the present invention. [Figure 5] FIG. 5 is a block diagram showing the configuration of an optical parametric amplifier according to a second embodiment of the present invention. [Figure 6] FIG. 6 is a diagram showing the amplification band of an optical parametric amplifier according to a second embodiment of the present invention. [Figure 7] FIG. 7 is a block diagram showing the basic configuration of a conventional optical parametric amplifier and wavelength converter. [Figure 8] FIG. 8 is a diagram illustrating a conventional optical parametric amplification process and a DFG band. [Figure 9] FIG. 9 is a block diagram showing another configuration of a conventional optical parametric amplifier. [Figures 10A-10F] 10A to 10F are diagrams showing the spectra of the signal light, the converted light, and the amplified signal light in the optical parametric amplifier of FIG. DETAILED DESCRIPTION OF THE INVENTION

[0027] [First Example] An embodiment of the present invention will be described below with reference to the drawings. This embodiment proposes a configuration that uses a second-order nonlinear optical medium, such as PPLN, having two phase-matching wavelengths λ1 and λ2 (λ1<λ2). By using two second-harmonic pump lights with wavelengths equal to the two phase-matching wavelengths λ1 and λ2, it is possible to achieve broadband optical amplification without excessive optical loss, without the need to split the signal light into two, shorter and longer wavelengths. Multiple phase-matching wavelengths can be achieved, for example, by providing multiple periodic structures in multiple stages in a periodically poled structure within a PPLN waveguide. Alternatively, a chirped periodic structure, in which the structure gradually changes from one characteristic period to another within the element, may be provided. Alternatively, a multi-QPM (Quasi-Phase-Matching) element may be used, which can be achieved by applying long-period spatial periodic phase modulation to a basic periodic structure.

[0028] 1 is a block diagram showing the configuration of an optical parametric amplifier according to this embodiment. The optical parametric amplifier includes a laser light source 1 that generates fundamental light with a wavelength near a phase-matched wavelength λ1, a laser light source 2 that generates fundamental light with a wavelength near a phase-matched wavelength λ2, EDFAs 3 and 4 that amplify the fundamental light from the laser light sources 1 and 2, a multiplexer 5 that multiplexes the fundamental light amplified by the EDFAs 3 and 4, a second-order nonlinear optical element 6 that has phase-matched wavelengths λ1 and λ2 and includes a PPLN waveguide 61 (second optical waveguide) that generates second-harmonic light from the output light of the multiplexer 5, a second-order nonlinear optical element 7 that has phase-matched wavelengths λ1 and λ2 and includes a PPLN waveguide 71 (first optical waveguide) that parametrically amplifies a signal light group 20, and a bandpass filter 8 whose passband is set to pass the signal light group after parametric amplification and remove wavelength-converted light. The laser light sources 1 and 2, the EDFAs 3 and 4, the multiplexer 5, and the second-order nonlinear optical element 6 constitute a pump light generation unit 10.

[0029] The second-order nonlinear optical element 6 includes a spatial optical system 60, a PPLN waveguide 61, and a spatial optical system 62. The second-order nonlinear optical element 7 includes a spatial optical system 70, a PPLN waveguide 71, and a spatial optical system 72.

[0030] The spatial optical system 60 guides the light incident from the multiplexer 5 to the PPLN waveguide 61. The spatial optical system 62 guides the light output from the PPLN waveguide 61 to the output port of the second-order nonlinear optical element 6. The spatial optical system 70 multiplexes the signal light group and the pump light, and guides the combined light to the PPLN waveguide 71. The spatial optical system 72 guides the light output from the PPLN waveguide 71 to the output port of the second-order nonlinear optical element 7.

[0031] This embodiment uses two PPLN waveguides 61 and 71 having two phase-matching wavelengths λ1 and λ2 (λ1<λ2). The phase-matching wavelengths λ1 and λ2 satisfy the relationship λ1<λsi<λ2 with respect to the wavelengths λsi (i=1, 2, 3, . . .) of the signal light group 20. In other words, the phase-matching wavelengths λ1 and λ2 are the wavelengths at both ends of the signal light band.

[0032] The PPLN waveguide 61 has two polarization periods Λ λ1 ,Λ λ2 The PPLN waveguide 71 is made of a nonlinear optical crystal (LiNbO3 in this embodiment) having a periodically poled structure having a poling period Λ. λ1 ,Λ λ2 satisfies the following relationship: n λ1 / 2 / (λ1 / 2)-2n λ1 / λ1=1 / Λ λ1 ···(2) n λ2 / 2 / (λ2 / 2)-2n λ2 / λ2=1 / Λ λ2 ···(3)

[0033] n λ1 / 2 is the refractive index of light with wavelength λ1 / 2 in the PPLN waveguides 61 and 71, and n λ2 / 2 is the refractive index of light with wavelength λ2 / 2 in the PPLN waveguides 61 and 71, and n λ1 is the refractive index of light of wavelength λ1 in the PPLN waveguides 61 and 71, and nλ2 is the refractive index of light of wavelength λ2 in the PPLN waveguides 61 and 71.

[0034] Laser light sources 1 and 2 generate fundamental light waves with wavelengths λ1 and λ2, respectively. EDFAs 3 and 4 amplify the fundamental light waves from laser light sources 1 and 2 to obtain sufficient power to achieve a nonlinear optical effect. A combiner 5 combines the fundamental light waves 21 and 22 amplified by EDFAs 3 and 4.

[0035] The PPLN waveguide 61 having phase-matched wavelengths λ1 and λ2 generates second harmonic light from the fundamental light waves 21 and 22. As a result, in this embodiment, second harmonic light waves 23 and 24 with two wavelengths are generated.

[0036] The PPLN waveguide 71 having phase-matched wavelengths λ1 and λ2 performs parametric amplification of the signal light group 20 using the second harmonic lights 23 and 24 output from the PPLN waveguide 61 as pumping lights.

[0037] The operation of parametric amplification in the PPLN waveguide 71 will be explained below with reference to Fig. 2. In this embodiment, the phase-matching wavelength λ1 of the PPLN waveguides 61 and 71 is 1530 nm, the phase-matching wavelength λ2 is 1602 nm, and the waveguide length is 40 mm. As shown in Fig. 2(a), the wavelengths λsi (i = 1, 2, 3...) of the signal light group 20 are closely spaced in the region from 1530 nm to 1602 nm.

[0038] Second harmonic light 23, 24 generated from fundamental light 21, 22 with wavelengths λ1, λ2 are incident as pump light into PPLN waveguide 71. If the amplification bands of PPLN waveguide 71 are a 3 dB band centered on wavelength λ1 and a 3 dB band centered on wavelength λ2, each 3 dB band is a band of approximately 70 nm.

[0039] The short-wave component of the signal light group 20 is amplified using second-harmonic light 23 with a wavelength λ1 / 2=765 nm as pump light. The long-wave component of the signal light group 20 is amplified using second-harmonic light 24 with a wavelength λ2 / 2=801 nm as pump light. Phase-matching wavelengths λ1 and λ2 are set in a synthesis band such that the gain of the entire amplified signal light group is equal. In Figure 2(c), 500 indicates the gain of the amplified short-wave signal light, and 501 indicates the gain of the amplified long-wave signal light.

[0040] In this embodiment, two second-harmonic lights generated from two independent laser light sources 1 and 2 are used as pump lights, and in parametric amplification, the signal light is amplified while preserving its phase information. Therefore, the gain bandwidth of the signal light is obtained as a superposition of the amplification bandwidths of the two second-harmonic lights. In Figure 2(c), 502 indicates the gain of the entire amplified signal light group.

[0041] What is important here is that the wavelength-converted light generated by the parametric amplification in the PPLN waveguide 71 is generated outside the band of the signal light. In Fig. 2(b), 25 denotes the wavelength-converted light generated for the signal light on the short wavelength side, and 26 denotes the wavelength-converted light generated for the signal light on the long wavelength side.

[0042] When using a second-order nonlinear optical element with a phase-matched wavelength within the band of the signal light, as in the conventional configuration, wavelength-converted light is generated within the band of the signal light, so it was necessary to separate the signal light into short-wavelength and long-wavelength sides in advance to avoid interference.

[0043] On the other hand, in this embodiment, there is no need to separate the signal light. Therefore, only one second-order nonlinear optical element is required for parametric amplification, reducing the number of components. Furthermore, in this embodiment, a wavelength demultiplexer that separates the signal light before amplification is not required, so excessive loss due to the wavelength demultiplexer does not occur, and degradation of the noise figure of the optical parametric amplifier can be suppressed.

[0044] The optical parametric amplifier of this embodiment can obtain a flat gain for a signal light group in a wavelength range of approximately 70 nm, and unnecessary wavelength-converted light components after parametric amplification can be removed by using a bandpass filter 8. As a result, this embodiment can achieve wideband amplification with a bandwidth approximately twice that of a conventional EDFA with a simple configuration.

[0045] As described above, in this embodiment, an optical parametric amplifier is configured using PPLN waveguides 61 and 71 having two phase-matched wavelengths λ1 and λ2 at both ends of the signal light band. Compared to conventional optical parametric amplifiers, this embodiment does not require a wavelength demultiplexer, which not only reduces the number of components but also suppresses excessive loss, enabling amplification with the noise figure inherent to the optical parametric amplifier and enabling wideband, low-noise optical amplification.

[0046] The present invention aims to realize parametric amplification of signal light with a simple configuration and low noise, and does not target the use of wavelength-converted light.

[0047] In this embodiment, the phase matching wavelengths λ1 and λ2 are set to 1530 nm and 1602 nm, respectively, but the present invention is not limited to these wavelengths and any phase matching wavelengths λ1 and λ2 can be set. Furthermore, in this embodiment, fundamental light beams having the same wavelengths as the phase matching wavelengths λ1 and λ2 are generated by the laser light sources 1 and 2, but the wavelengths of the fundamental light beams do not have to match the phase matching wavelengths λ1 and λ2, as long as they are close to the phase matching wavelengths λ1 and λ2. In other words, the wavelengths of the second harmonic pump light do not have to match λ1 / 2 and λ2 / 2, as long as they are close to λ1 / 2 and λ2 / 2.

[0048] In addition, in this embodiment, after the two fundamental wave lights 21 and 22 are multiplexed, the two second harmonic lights 23 and 24 are generated using the PPLN waveguide 61 having two phase-matched wavelengths λ1 and λ2, but the present invention is not limited to such a configuration.

[0049] 3, two second-order nonlinear optical elements 11 and 12 having different phase-matching wavelengths may be prepared, and second-harmonic light 23 and 24 output from the second-order nonlinear optical elements 11 and 12 may be multiplexed by a multiplexer 5. The second-order nonlinear optical elements 11 and 12 have PPLN waveguides 110 and 120, respectively. The phase-matching wavelength of the PPLN waveguide 110 (second optical waveguide) is λ1, and the phase-matching wavelength of the PPLN waveguide 120 (third optical waveguide) is λ2. In this case, the laser light sources 1 and 2, the EDFAs 3 and 4, the second-order nonlinear optical elements 11 and 12, and the multiplexer 5 constitute a pumping light generating unit 10a.

[0050] 4, a laser light source 1b that generates second harmonic light 23 with a wavelength near λ1 / 2 and a laser light source 2b that generates second harmonic light 24 with a wavelength near λ2 / 2 may be used without using the PPLN waveguides 61, 110, and 120. In this case, the laser light sources 1b and 2b, the EDFAs 3 and 4, and the multiplexer 5 constitute the pumping light generating unit 10b.

[0051] [Second Example] Next, a second embodiment of the present invention will be described. In the first embodiment, two second harmonic lights generated from fundamental light waves of wavelengths λ1 and λ2 are input as pump light into a PPLN waveguide having two phase-matched wavelengths λ1 and λ2 outside the wavelength range of the signal light, thereby realizing broadband optical parametric amplification. In contrast, this embodiment is configured to achieve an even broader bandwidth.

[0052] 5 is a block diagram showing the configuration of an optical parametric amplifier according to this embodiment. The optical parametric amplifier according to this embodiment includes laser light sources 1c_1 and 1c_2 that generate fundamental light with a wavelength near the phase-matching wavelength λ1, laser light sources 2c_1 and 2c_2 that generate fundamental light with a wavelength near the phase-matching wavelength λ2, EDFAs 3c_1, 3c_2, 4c_1, and 4c_2 that amplify the fundamental light from the laser light sources 1c_1, 1c_2, 2c_1, and 2c_2, a multiplexer 5c that multiplexes the fundamental light amplified by the EDFAs 3c_1, 3c_2, 4c_1, and 4c_2, second-order nonlinear optical elements 6 and 7, and a bandpass filter 8. The laser light sources 1 and 2, the EDFAs 3 and 4, the multiplexer 5, and the second-order nonlinear optical element 6 constitute a pump light generation unit 10c.

[0053] In this embodiment, two PPLN waveguides 61 and 71 having two phase-matched wavelengths λ1 and λ2 are used, as in the first embodiment. The phase-matched wavelengths λ1 and λ2 are wavelengths at both ends of the band of the signal light to be amplified. The PPLN waveguides 61 and 71 have two polarization inversion periods Λ λ1 ,Λ λ2 The multi-stage periodically poled structure has a poled period Λ λ1 ,Λ λ2 satisfies the relationships of equations (2) and (3).

[0054] In this embodiment, four second harmonic lights are used as pump lights, and therefore, four laser light sources 1c_1, 1c_2, 2c_1, and 2c_2 are used to generate fundamental light. EDFAs 3c_1, 3c_2, 4c_1, and 4c_2 amplify the fundamental light from the laser light sources 1c_1, 1c_2, 2c_1, and 2c_2 to obtain power sufficient to achieve a nonlinear optical effect. A multiplexer 5c multiplexes the fundamental light 30 to 33 amplified by the EDFAs 3c_1, 3c_2, 4c_1, and 4c_2.

[0055] The PPLN waveguide 61 having the phase-matching wavelengths λ1 and λ2 generates second-harmonic light from the fundamental light beams 30 to 33, respectively. As a result, in this embodiment, second-harmonic light beams 34 to 37 with four wavelengths are generated. In this embodiment, the laser light source 1c_1 generates fundamental light beam with a wavelength λ1_1 equal to the phase-matching wavelength λ1, and the laser light source 1c_2 generates fundamental light beam with a wavelength λ1_2 slightly detuned to the shorter wavelength side from the phase-matching wavelength λ1. Furthermore, the laser light source 2c_1 generates fundamental light beam with a wavelength λ2_1 equal to the phase-matching wavelength λ2, and the laser light source 2c_2 generates fundamental light beam with a wavelength λ2_2 slightly detuned to the shorter wavelength side from the phase-matching wavelength λ2.

[0056] The PPLN waveguide 71 having phase-matched wavelengths λ1 and λ2 performs parametric amplification of the signal light group 20 using the second harmonic lights 34 to 37 output from the PPLN waveguide 61 as pump light.

[0057] The amplification band in the configuration of this example is shown in Figure 6. In this example, the phase-matching wavelength λ1 of the PPLN waveguides 61 and 71 is 1520 nm, and the phase-matching wavelength λ2 is 1625 nm. The wavelength λ1_1 (= λ1) of the fundamental light generated by the laser light source 1c_1 is 1520 nm, and the wavelength λ1_2 of the fundamental light generated by the laser light source 1c_2 is 1519.78 nm. The wavelength λ2_1 (= λ2) of the fundamental light generated by the laser light source 2c_1 is 1625 nm, and the wavelength λ2_2 of the fundamental light generated by the laser light source 2c_2 is 1624.78 nm. A 20 mm-long PPLN waveguide having a bandwidth for second harmonics with wavelengths of 0.4 nm or more was used as the PPLN waveguide 61, and the four fundamental light waves were converted collectively into second harmonic light.

[0058] The shorter wavelength component of the signal light group 20 is amplified using second harmonic light of wavelength λ1_1 / 2 generated from fundamental light of wavelength λ1_1 (=λ1) as pump light. In this case, the amplification band of the PPLN waveguide 71 is a 3 dB band centered on the phase-matched wavelength λ1, with a width of about 60 nm, as shown by 600 in Figure 6, and is a flat band centered on the wavelength λ1.

[0059] Furthermore, the shorter wavelength component of the signal light group 20 is amplified using second harmonic light of wavelength λ1_2 / 2 generated from the fundamental wave light of wavelength λ1_2 as pump light. In this case, although the gain of the amplification band of the PPLN waveguide 71 decreases near the phase-matched wavelength λ1 as shown by 601 in Fig. 6, gain can be obtained over a wider band than when second harmonic light of wavelength λ1_1 / 2 is used as pump light.

[0060] The longer wavelength component of the signal light group 20 is amplified using second harmonic light of wavelength λ2_1 / 2 generated from fundamental light of wavelength λ2_1 (=λ2) as pump light. In this case, the amplification band of the PPLN waveguide 71 is a 3 dB band centered on the phase-matched wavelength λ2, with a width of about 60 nm, as shown by 602 in Figure 6, and is a flat band centered on the wavelength λ2.

[0061] Furthermore, the longer wavelength component of the signal light group 20 is amplified using second harmonic light of wavelength λ2_2 / 2 generated from the fundamental light of wavelength λ2_2 as pump light. In this case, although the gain of the amplification band of the PPLN waveguide 71 decreases near the phase-matched wavelength λ2 as shown by 603 in Fig. 6, gain can be obtained over a wider band than when second harmonic light of wavelength λ2_1 / 2 is used as pump light.

[0062] This embodiment aims to utilize the characteristics of the amplification band described above to achieve a wide and flat characteristic for the combined band of four second-harmonic pump lights. In Figure 6, 604 indicates the gain of the entire amplified signal light group. As shown in Figure 6, this embodiment achieves a wide amplification band with a flatness of less than 1 dB in a band exceeding 100 nm in width from wavelength 1520 nm to 1625 nm.

[0063] When parametric amplification is performed using multiple second-harmonic pump lights, signal light is generally wavelength-converted using different wavelengths of second-harmonic pump lights, so even when the same signal light is converted, the wavelength of the converted light differs for each second-harmonic pump light. As a result, converted light with shifted wavelengths overlaps, causing interference that corrupts the original signal information. However, since this embodiment is intended only for the parametric amplification of signal light, the above-mentioned problem does not occur even when multiple different second-harmonic pump lights are used, and optical amplification is possible while maintaining the phase information of the signal.

[0064] As described above, in this embodiment, by using PPLN waveguides 61 and 71 having two phase-matched wavelengths λ1 and λ2 at both ends of the signal light band and multiple second-harmonic pump lights, it is possible to achieve optical amplification with a wider bandwidth than conventional EDFAs. According to this embodiment, compared to conventional optical parametric amplifiers, a wavelength demultiplexer is not required, which not only reduces the number of components but also suppresses excessive loss, making it possible to amplify with the noise figure inherent to the optical parametric amplifier, thereby enabling wideband and low-noise optical amplification.

[0065] In this embodiment, four fundamental light beams are used by providing a plurality of laser light sources 1c_1 and 1c_2 that generate fundamental light beams with wavelengths near λ1 and a plurality of laser light sources 2c_1 and 2c_2 that generate fundamental light beams with wavelengths near λ2, but more light sources may be used, and it is not necessary to match the number of light sources on the short wavelength side and the long wavelength side. Also, in this embodiment, independent light sources are used, but multiple fundamental light beams may be generated using an optical modulator or the like.

[0066] 3 may be applied to this embodiment. In this case, the following may be provided: a plurality of laser light sources 1 generating fundamental light having a wavelength near λ1, a plurality of laser light sources 2 generating fundamental light having a wavelength near λ2, a plurality of EDFAs 3 amplifying the fundamental light from the laser light sources 1, a plurality of EDFAs 4 amplifying the pumping light from the laser light sources 2, a plurality of second-order nonlinear optical elements 11 having a phase-matched wavelength λ1 and generating second harmonic light from the fundamental light amplified by the EDFAs 3, a plurality of second-order nonlinear optical elements 12 having a phase-matched wavelength λ2 and generating second harmonic light from the fundamental light amplified by the EDFAs 4, and a multiplexer 5 multiplexing the second harmonic light generated by the second-order nonlinear optical elements 11 and the second harmonic light generated by the second-order nonlinear optical elements 12.

[0067] 4 may also be applied to this embodiment. In this case, a plurality of laser light sources 1b that generate pumping light with a wavelength near λ1 / 2, a plurality of laser light sources 2b that generate pumping light with a wavelength near λ2 / 2, a plurality of EDFAs 3 that amplify the pumping light from laser light sources 1b, and a plurality of EDFAs 4 that amplify the pumping light from laser light sources 2b may be provided, and the pumping light amplified by EDFAs 3 and 4 may be multiplexed by multiplexer 5.

[0068] Furthermore, in this embodiment, the phase matching wavelengths λ1 and λ2 are set to 1520 nm and 1625 nm, but the present invention is not limited to these wavelengths, and any phase matching wavelengths λ1 and λ2 can be set.

[0069] In the first and second embodiments, LiNbO3 is used as the nonlinear optical crystal that constitutes the optical waveguide of the second-order nonlinear optical element. However, the present invention is not limited to this, and LiTaO3 or LiNb (x) Ta (1-x) O3 (0≦x≦1) may also be used. LiNbO3, LiTaO3 or LiNb (x) Ta (1-x) A nonlinear optical crystal in which O3 is doped with at least one element selected from Mg, Zn, Sc, and In may be used.

[0070] Some or all of the above embodiments can be described as, but are not limited to, the following supplementary notes.

[0071] (Supplementary Note 1) A first optical waveguide having two phase-matched wavelengths λ1 and λ2 (λ1<λ2) and configured to perform parametric amplification of a signal light group, and a pumping light generating unit configured to generate pumping light to be input to the first optical waveguide, wherein the phase-matched wavelengths λ1 and λ2 are set to satisfy the relationship λ1<λsi<λ2 with respect to the wavelengths λsi (i=1, 2, 3, . . . ) of the signal light group, and the pumping light generating unit generates pumping light of one or more wavelengths near λ1 / 2 and pumping light of one or more wavelengths near λ2 / 2 and inputs them to the first optical waveguide, and the first optical waveguide has two polarization inversion periods Λ λ1 ,Λ λ2 The nonlinear optical crystal has a periodically poled structure having a refractive index of light having a wavelength of λ1 / 2 in the first optical waveguide. λ1 / 2 , the refractive index of light with wavelength λ2 / 2 is n λ2 / 2 , the refractive index of light of wavelength λ1 is n λ1 , the refractive index of light of wavelength λ2 is n λ2 When the polarization inversion period Λ λ1 ,Λ λ2 is n λ1 / 2 / (λ1 / 2)-2n λ1 / λ1=1 / Λ λ1 , n λ2 / 2 / (λ2 / 2)-2n λ2 / λ2=1 / Λ λ2 Satisfy the relationship.

[0072] (Supplementary Note 2) In the optical parametric amplifier according to Supplementary Note 1, the pump light generating section comprises one or more first light sources configured to generate fundamental light of one or more wavelengths near the phase matching wavelength λ1, one or more second light sources configured to generate fundamental light of one or more wavelengths near the phase matching wavelength λ2, a multiplexer configured to multiplex the fundamental light generated by the first light source and the fundamental light generated by the second light source, and a second optical waveguide having the phase matching wavelengths λ1 and λ2 and configured to generate second harmonic light from the fundamental light multiplexed by the multiplexer, λ1 ,Λ λ2 The second harmonic light generated by the second optical waveguide is input to the first optical waveguide as pump light.

[0073] (Supplementary Note 3) In the optical parametric amplifier described in Supplementary Note 1, the pumping light generating unit comprises one or more first light sources configured to generate pumping light of one or more wavelengths near the λ1 / 2, one or more second light sources configured to generate pumping light of one or more wavelengths near the λ2 / 2, and a multiplexer configured to multiplex the pumping light generated by the first light source and the pumping light generated by the second light source, and the pumping light multiplexed by the multiplexer is input to the first optical waveguide.

[0074] (Supplementary Note 4) In the optical parametric amplifier according to Supplementary Note 1, the pump light generating section comprises one or more first light sources configured to generate fundamental light of one or more wavelengths near the phase matching wavelength λ1, one or more second light sources configured to generate fundamental light of one or more wavelengths near the phase matching wavelength λ2, one or more second optical waveguides having the phase matching wavelength λ1 and configured to generate second harmonic light from the fundamental light generated by the one or more first light sources, one or more third optical waveguides having the phase matching wavelength λ2 and configured to generate second harmonic light from the fundamental light generated by the one or more second light sources, and a multiplexer configured to multiplex the second harmonic light generated by the second optical waveguide and the second harmonic light generated by the third optical waveguide, λ1 and the third optical waveguide is made of a nonlinear optical crystal having a periodically poled structure having the poled period Λ λ2 The second harmonic light multiplexed by the multiplexer is input as pump light to the first optical waveguide. [Industrial Applicability]

[0075] The present invention can be applied to techniques for amplifying optical signals. [Explanation of symbols]

[0076] 1, 1b, 1c_1, 1c_2, 2, 2b, 2c_1, 2c_2...laser light source, 3, 3c_1, 3c_2, 4, 4c_1, 4c_2...EDFA, 5, 5c...wave combiner, 6, 7, 11, 12...second-order nonlinear optical element, 8...bandpass filter, 10, 10a to 10c...pumping light generation unit, 60, 62, 70, 72...spatial optical system, 61, 71, 110, 120...PPLN waveguide.

Claims

1. a first optical waveguide having two phase-matched wavelengths λ1 and λ2 (λ1<λ2) and configured to perform parametric amplification of a signal light group; an excitation light generating unit configured to generate excitation light to be input to the first optical waveguide; the phase matching wavelengths λ1 and λ2 are set to satisfy the relationship λ1<λsi<λ2 with respect to the wavelengths λsi (i=1, 2, 3 . . . ) of the signal light group, the pumping light generating unit generates pumping light having a wavelength near λ1 / 2 and pumping light having a wavelength near λ2 / 2 and inputs the generated pumping light into the first optical waveguide; The first optical waveguide has two polarization inversion periods Λ λ１ , Λ λ２ and wherein the polarization inversion periods Λ λ1 and Λ λ2 satisfy the relationships n λ1 / 2 / (λ1 / 2)-2n λ1 / λ1=1 / Λ λ1 and n λ2 / 2 / (λ2 / 2)-2n λ2 / λ2=1 / Λ λ2, where n λ1 / 2 is the refractive index of light with wavelength λ1 / 2 in the first optical waveguide, n λ2 / 2 is the refractive index of light with wavelength λ2 / 2, n λ1 is the refractive index of light with wavelength λ1, and n λ2 is the refractive index of light with wavelength λ2.

2. 2. The optical parametric amplifier according to claim 1, The excitation light generating unit a first light source configured to generate fundamental light having a wavelength near the phase matching wavelength λ1; a second light source configured to generate fundamental light having a wavelength near the phase matching wavelength λ2; a combiner configured to combine the fundamental wave light generated by the first light source and the fundamental wave light generated by the second light source; a second optical waveguide having the phase-matched wavelengths λ1 and λ2 and configured to generate second harmonic light from the fundamental wave light multiplexed by the multiplexer; The second optical waveguide has the polarization inversion period Λ λ１ , Λ λ２ The nonlinear optical crystal has a periodically poled structure, An optical parametric amplifier, wherein second harmonic light generated by the second optical waveguide is input as pump light to the first optical waveguide.

3. 2. The optical parametric amplifier according to claim 1, The excitation light generating unit a first light source configured to generate excitation light having a wavelength near λ 1 / 2; a second light source configured to generate excitation light having a wavelength near λ / 2; a multiplexer configured to multiplex the excitation light generated by the first light source and the excitation light generated by the second light source, an optical parametric amplifier, wherein the pumping light multiplexed by the multiplexer is input to the first optical waveguide;

4. 2. The optical parametric amplifier according to claim 1, The excitation light generating unit a first light source configured to generate fundamental light having a wavelength near the phase matching wavelength λ1; a second light source configured to generate fundamental light having a wavelength near the phase matching wavelength λ2; a second optical waveguide having the phase-matching wavelength λ1 and configured to generate second-harmonic light from the fundamental wave light generated by the first light source; a third optical waveguide having the phase matching wavelength λ2 and configured to generate second harmonic light from the fundamental wave light generated by the second light source; a multiplexer configured to multiplex the second harmonic light generated by the second optical waveguide and the second harmonic light generated by the third optical waveguide, The second optical waveguide has the polarization inversion period Λ λ１ The nonlinear optical crystal has a periodically poled structure, The third optical waveguide has the polarization inversion period Λ λ２ The nonlinear optical crystal has a periodically poled structure, an optical parametric amplifier, wherein the second harmonic light multiplexed by the multiplexer is input as pump light to the first optical waveguide;

5. 5. The optical parametric amplifier according to claim 1, The nonlinear optical crystal is LiNbO ３ , LiTaO ３ or LiNb （ｘ） Ta （１－ｘ） O ３ (0≦x≦1) or any of these materials to which at least one selected from the group consisting of Mg, Zn, Sc, and In is added as an additive.

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