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

Optical Parametric Amplifier

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

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

[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, and to collectively amplify wavelength division multiplexing (WDM) signals, which transmit different information on multiple wavelengths. This has made it possible to amplify and relay optical signals with a simple configuration, significantly reducing the cost of optical relays. In particular, it is no exaggeration to say that the entire system of optical communication networks that transmit optical signals over long distances is designed with the use of EDFAs in mind.

[0005] With the recent diversification of information and communication technology services, optical communication systems that support the backbone of communication networks are required to further increase their transmission capacity. According to Shannon's communication theory, the spectral efficiency, which is defined as the ratio of transmission capacity per unit frequency band, is expressed as logarithm of the signal-to-noise (S / N) ratio. ２The S / N ratio is (1 + S / N). Therefore, the upper limit of the S / N ratio determines the theoretical upper limit of the 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 the frequency utilization efficiency.

[0006] However, in reality, optical fibers, which are the transmission medium for optical communications, have 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 deteriorates due to the influence 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 limit the upper limit of the transmission capacity of optical communication systems.

[0007] As mentioned above, the nonlinear Shannon limit is beginning to bring into view the theoretical upper limit of the spectral efficiency in 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 EDFAs, which are widely used in current optical communication systems, are limited to the C band (1530-1565 nm) and the L band (1565-1625 nm). Therefore, current optical communication systems are built on the premise of using these wavelength bands. 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 utilize transitions between the energy levels of the rare-earth elements. A method for achieving optical amplification that does not suffer from 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 the use of four-wave mixing in an optical fiber. However, as mentioned above, the nonlinear optical effect of an optical fiber can also cause a degradation in the S / N ratio of an optical signal. 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 by 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 second harmonic generation (SHG) element, generating 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 through 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] FIG. 8 is a diagram illustrating a conventional optical parametric amplification process and DFG band. Here, the DFG process is used for explanation, but the principles are similar in the optical parametric amplification process. Reference numeral 200 in FIG. 8(b) indicates a single-wavelength fundamental wave light output from a single laser light source. Reference numeral 201 in FIG. 8(a) indicates a phase matching curve for SHG of a PPLN waveguide, reference numeral 202 indicates a phase matching curve for DFG of a PPLN waveguide, reference numeral 203 indicates signal light, and reference numeral 205 indicates converted light. The phase matching band for SHG of a PPLN waveguide is narrower than the phase matching band for DFG, but is sufficiently wider than the linewidth of the fundamental wave light.

[0014] Here, we will describe the wavelength conversion band of a PPLN waveguide when the wavelength λ0 (frequency ω0) of the fundamental wave is 1545 nm and the wavelength λp (frequency 2ω0) of the pump light 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 wavelength λs (frequency ωs) of the signal light is 1540 nm, converted light with a wavelength of 1550 nm is generated by 2ω0 - ωs. As shown in Figure 8(a), converted light is generated by folding the signal light on the wavelength axis centered on the wavelength λ0 of the fundamental wave.

[0015] In the PPLN waveguide, the quasi-phase matching condition is satisfied among the three waves of 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 between the converted light with a frequency of 2ω0-ωs and the pump light as long as formula (1) is satisfied. Specifically, for example, if the wavelength λs (frequency ωs) of the signal light is 1539 nm, 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 since nc decreases by the amount that ns increases due to material dispersion, formula (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 obtain a wide wavelength conversion band 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 completely equal, 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 the bandwidth 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 and 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 described above, in the parametric amplification process, not only is the signal light amplified, but converted light is also generated at wavelengths that are obtained by folding the signal light wavelength around the fundamental light wavelength. Therefore, when a group of signal lights incident on a PPLN waveguide is on both the long-wavelength and short-wavelength sides of the fundamental light wavelength, converted light for the long-wavelength signal light is generated in the short-wavelength signal light wavelength band, and converted light for the short-wavelength signal light is generated in the long-wavelength signal light wavelength band, so it is necessary to separate the signal lights 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. Reference numeral 400 in Fig. 10A denotes the signal light incident on the wavelength demultiplexer 300, 401 in Fig. 10B denotes the short wavelength side signal light separated by the wavelength demultiplexer 300, and 402 in Fig. 10C denotes the long wavelength side signal light separated by the wavelength demultiplexer 300.

[0020] The short-wavelength signal light is passed through second-order nonlinear optical element 301, and the long-wavelength signal light 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. Reference numeral 403 in FIG. 10D denotes the converted light generated by second-order nonlinear optical element 301, and reference numeral 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, optical parametric amplifiers using PPLN waveguides are capable of wideband amplification, but when the entire amplification band is used, a configuration like that shown in Figure 9 is required. The configuration shown 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 complicating the configuration. 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 transmission loss of the wavelength demultiplexer. If the noise figure increases, it becomes impossible 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.

[0022] 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 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

[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.

[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 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 The present invention is characterized in that the following relationship is satisfied.

[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.

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

[0027] [First Example] An example of the present invention will now be described with reference to the drawings. In this example, a second-order nonlinear optical medium such as PPLN having two phase-matching wavelengths λ1 and λ2 (λ1<λ2) is used. By using two second-harmonic pump lights with wavelengths equal to the two phase-matching wavelengths λ1 and λ2, a configuration is proposed that can achieve broadband optical amplification without excessive optical loss without the need to split the signal light into two, short and long 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 may be provided, in which the structure gradually changes from one characteristic period to another within the element. 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 combiner 5, and the second-order nonlinear optical element 6 constitute a pump light generating section 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] In this embodiment, two PPLN waveguides 61 and 71 having two phase-matched wavelengths λ1 and λ2 (λ1<λ2) are used. The phase-matched 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-matched wavelengths λ1 and λ2 are wavelengths at both ends of the signal light band.

[0032] The PPLN waveguide 61 has two polarization inversion periods Λ λ１ , Λ λ２A nonlinear optical crystal (in this example, LiNbO ３ The PPLN waveguide 71 has the same structure as the PPLN waveguide 61. λ１ , Λ λ２ satisfies the following relationship: 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 n λ２ 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 power sufficient 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 described 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 of 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 of 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 of 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 light beams generated by two independent laser light sources 1 and 2 are used as pump light, 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 light beams. 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 deterioration 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 the 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 this 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 pump 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 a pumping light generating unit 10b.

[0051] Second Embodiment Next, a second embodiment of the present invention will be described. In the first embodiment, broadband optical parametric amplification was achieved by inputting, as pump light, two second-harmonic light waves generated from fundamental light waves of wavelengths λ1 and λ2 into a PPLN waveguide having two phase-matched wavelengths λ1 and λ2 outside the wavelength range of the signal light. In contrast, this embodiment is configured to achieve an even wider 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 generating 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 formulas (2) and (3).

[0054] In this embodiment, four second harmonic lights are used as pump lights, so fundamental light is generated using four laser light sources 1c_1, 1c_2, 2c_1, and 2c_2. 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 sufficient power 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 embodiment is shown in Figure 6. In this embodiment, the phase-matched wavelength λ1 of the PPLN waveguides 61 and 71 is 1520 nm, and the phase-matched 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 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 long 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 Fig. 6, 604 indicates the gain of the entire amplified signal light group. As shown in Fig. 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 a plurality of 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 a larger number of 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 a plurality of fundamental light beams may be generated using an optical modulator or the like.

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

[0068] In addition, 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, the nonlinear optical crystal constituting the optical waveguide of the second-order nonlinear optical element is LiNbO ３ However, the present invention is not limited to this, and LiTaO ３ or LiNb （ｘ） Ta （１－ｘ） O ３ (0≦x≦1) may also be used. ３ , LiTaO ３ or LiNb （ｘ） Ta （１－ｘ） O ３Alternatively, a nonlinear optical crystal may be used in which at least one element selected from Mg, Zn, Sc, and In is added to the nonlinear optical crystal.

[0070] Some or all of the above embodiments may 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 λ1 / 2, one or more second light sources configured to generate pumping light of one or more wavelengths near 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.

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

[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 generating unit, 60, 62, 70, 72...spatial optical system, 61, 71, 110, 120...PPLN waveguide.

Claims

1. A first optical waveguide having two phase-matching wavelengths λ1 and λ2 (λ1 < λ2) and configured to perform parametric amplification of a signal optical group; An excitation light generation unit configured to generate excitation light input to the first optical waveguide; The phase-matching wavelengths λ1 and λ2 are set so as to satisfy the relationship λ1 < λsi < λ2 with respect to the wavelengths λsi (i = 1, 2, 3,...) of the signal optical group; The excitation light generation unit generates excitation light having a wavelength near λ1 / 2 and excitation light having a wavelength near λ2 / 2 and inputs them to the first optical waveguide; The first optical waveguide is made of a nonlinear optical crystal having a periodically poled inversion structure with two poling inversion periods Λ λ１ , Λ λ２ , and is characterized by being an optical parametric amplifier.

2. In the optical parametric amplifier according to Claim 1, The excitation light generation 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 multiplexer configured to multiplex the fundamental light generated by the first light source and the fundamental light generated by the second light source; It is composed of 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, The second optical waveguide is composed of a nonlinear optical crystal having a periodically poled inversion structure with the poling inversion period Λ λ１ , Λ λ２ and has a periodically poled inversion structure with a period Λ An optical parametric amplifier, wherein the second harmonic light generated by the second optical waveguide is input to the first optical waveguide as excitation light.

3. In the optical parametric amplifier according to Claim 1, The excitation light generation unit A first light source configured to generate excitation light having a wavelength near λ1 / 2; A second light source configured to generate excitation light having a wavelength near λ2 / 2; It is composed of 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 excitation light multiplexed by the multiplexer is input to the first optical waveguide.

4. In the optical parametric amplifier according to Claim 1, The excitation light generation 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 light generated by the first light source; A third optical waveguide having the phase-matching wavelength λ2 and configured to generate a second harmonic wave from the fundamental optical wave generated by the second light source; A multiplexer configured to multiplex the second harmonic wave generated by the second optical waveguide and the second harmonic wave generated by the third optical waveguide; The second optical waveguide is made of a nonlinear optical crystal having a periodically poled structure with the poling inversion period Λ λ１ and is composed of a nonlinear optical crystal having a periodically poled structure with the poling inversion period Λ The third optical waveguide is made of a nonlinear optical crystal having a periodically poled structure with the poling inversion period Λ λ２ and has a periodically poled structure with the poling inversion period Λ An optical parametric amplifier, wherein the second harmonic wave multiplexed by the multiplexer is input into the first optical waveguide as excitation light. **Claim 5** In the optical parametric amplifier according to any one of claims 1 to 4, The non-linear optical crystal is LiNbO ３ , LiTaO ３ or LiNb （ｘ） Ta （１－ｘ） O ３ (0 ≤ x ≤ 1), or is composed of a material obtained by adding at least one selected from the group consisting of Mg, Zn, Sc, and In as an additive to any of these materials. A parametric optical amplifier characterized by this. **Claim 6** In the optical parametric amplifier according to any one of claims 1 to 4, Let the refractive index of light with wavelength λ1 / 2 in the first optical waveguide be n λ１／２ and the refractive index of light with wavelength λ2 / 2 be n λ２／２ , the refractive index of light with wavelength λ1 be n λ１ , and the refractive index of light with wavelength λ2 be n λ２ . When this is the case, the polarization inversion period Λ λ１ , Λ λ２ satisfies the relationship n λ１／２ / (λ1 / 2) - 2n λ１ / λ1 = 1 / Λ λ１ , n λ２／２ / (λ2 / 2) - 2n λ２ / λ2 = 1 / Λ λ２ . A parametric optical amplifier characterized by satisfying the above relationships.