Wavelength conversion system

Abstract

A wavelength conversion system according to the present disclosure comprises an optical separation unit (110), QPM devices (120a-d) that generate a plurality of wavelength-converted lights by optical parametric amplification, and an optical multiplexing unit (130). The QPM devices (120a-d) are configured such that the spatial distribution of nonlinear constants at a distance z from an incident surface is represented by mathematical expression (2), a quasi-phase-matching condition is represented by mathematical expression (3) using n (where n is an integer), and mathematical expression (4) is satisfied when ZA is the set of all n for which the wavelength λ1(n), which satisfies mathematical expression (3), is a value corresponding to a wavelength band that is intended to be amplified, G(nA) is the parametric gain when the wavelength λ1(n), which satisfies mathematical expression (3), is a value corresponding to a wavelength band that is intended to be amplified, ZB is the set of all n for which the wavelength λ1(n), which satisfies mathematical expression (3), is a value corresponding to a wavelength band that is not intended to be amplified, and G(nB) is the parametric gain when the wavelength λ1(n), which satisfies mathematical expression (3), is a value corresponding to a wavelength band that is not intended to be amplified.

Description

Wavelength Conversion System

[0001] The present disclosure relates to wavelength conversion systems.

[0002] In long-distance optical communications, C-band light (corresponding to wavelengths of 1530-1565 nm) has traditionally been used, and large capacity transmission has been achieved by using wavelength division multiplexed signals. In recent years, to meet the increasing demand for communications, research has been conducted into multi-band simultaneous transmission, which expands the communication wavelength band by using new bands such as S-band (corresponding to wavelengths of 1460-1530 nm), L-band (corresponding to wavelengths of 1565-1625 nm), and U-band (corresponding to wavelengths of 1625-1675 nm) (see, for example, Non-Patent Documents 1, 2, and 3).

[0003] Such multi-band transmission communication systems require optical wavelength filters to separate each wavelength band, and because wavelengths near the filter's cutoff wavelength cannot be used for communication, wavelength regions where no signal light exists (guard bands) are provided.

[0004] Generally, in long-distance optical communication systems, a mechanism for optically amplifying signal light (optical amplifier) ​​is essential. Furthermore, in order to transmit broadband signal light such as in multi-band transmission, the optical amplifier must also have a broadband capability. A known existing technology for amplifying such broadband signal light is to combine multiple types of optical amplifiers. For example, EDFAs, which are currently widely used optical amplifiers, have an amplification band limited to the C-band and L-band, which correspond to the state transitions of erbium atoms. However, by combining multiple types of other optical amplifiers in the transmitting and receiving amplification sections, it becomes possible to optically amplify broadband signal light (see, for example, Non-Patent Document 1).

[0005] However, existing optical amplification technologies for broadband signal light require a large number of amplifiers, which can lead to the expansion of communication systems. Furthermore, depending on the wavelength band of the signal light, amplifiers with poor characteristics must be used, which can degrade communication quality. Therefore, a new optical amplification technology capable of amplifying three or more wavelength bands is needed.

[0006] One candidate for such an optical amplification method is optical parametric amplification. Optical parametric amplification is a second-order nonlinear optical phenomenon in which signal light is amplified in a wavelength band that satisfies a phase matching condition. Quasi-phase matching (hereinafter referred to as QPM), which uses periodic polarization reversal of a nonlinear medium, is widely used as a technique for satisfying such a phase matching condition.

[0007] Because nonlinear optical effects are polarization-dependent, optical amplifiers using optical parametric amplification must be equipped with a mechanism capable of handling signal light of both orthogonal polarizations (X polarization and Y polarization). Optical amplification of both polarizations can be achieved, for example, by a system that separates the signal light into polarizations, amplifies them separately, and then polarization-multiplexes them. Furthermore, because optical parametric amplification requires that a wavelength band corresponding to the wavelength-converted light to be generated be left free, the broadband signal light to be optically amplified is amplified by dividing it into long-wavelength and short-wavelength components centered around the doubled wavelength of the pump light.

[0008] A typical example of a device that realizes QPM (hereinafter referred to as a QPM device) is a periodically poled lithium niobate (hereinafter referred to as PPLN) that uses lithium niobate (LiNbO3) that has a large second-order nonlinear optical coefficient. Furthermore, PPLN used as a QPM device is generally used as an element having an optical waveguide structure (e.g., a ridge waveguide). This is because optical amplifiers that apply the second-order nonlinear optical effect, such as optical parametric amplifiers, require high optical power density to achieve high nonlinear efficiency, and a waveguide structure that confines light to a narrow region is effective.

[0009] Several methods are known for adjusting the phase-matching wavelength in a QPM device. One method involves adjusting the polarization reversal period. Another method involves temperature control using a distributed temperature controller attached to the QPM device (see, for example, Patent Documents 1 and 2). This is because the phase-matching condition depends on the refractive index of the nonlinear medium, which in turn depends on temperature. Another method is to adjust the phase-matching wavelength by adjusting the thickness and width of the waveguide, since the effective refractive index of a waveguide-type QPM device depends on the thickness and width of the waveguide.

[0010] An optical parametric amplifier with this configuration can optically amplify a broadband signal light having three or more wavelength bands. Furthermore, this type of optical parametric amplifier allows for flexible control of the phase-matching wavelength band by designing the QPM, and enables broadening of the phase-matching band by shortening the device length.

[0011] However, optical parametric amplifiers have a trade-off problem in that the gain of parametric amplification decreases as the bandwidth of the signal light increases.

[0012] JP 2014-211538 A JP 2014-211539 A

[0013] B. Puttnam et al., “301 Tb / s E, S, C+L-Band Transmission over 212 nm bandwidth with E-band Bismuth-Doped Fiber Amplifier and Gain Equalizer, “The 49th European Conference on Optical Communications (ECOC) 2023, Glasgow, Scotland, Th.C.2.4, 2023.M. Abe et al., “Expansion of Transmission Bandwidth Over 16 THz in S+C+L+U Bands by Combining C-band Transceiver with PPLN-Based Wavelength Converters,” Frontiers in Optics + Laser Science 2023 (FiO, LS), Tacoma, Washington, United States, FM5D.2, 2023.

[0014] The present disclosure has been made in consideration of the above-mentioned problems, and its purpose is to provide a wavelength conversion system that suppresses gain reduction in a desired wavelength band and has a wide band and high gain.

[0015] In response to the above-described problems, the present disclosure provides a wavelength conversion system including an optical separation unit that separates multiband incident light into a plurality of signal lights according to wavelengths and polarization states, a QPM device including a nonlinear medium that generates a plurality of wavelength-converted lights by optical parametric amplification based on the plurality of signal lights and pump light, and an optical multiplexing unit that multiplexes the plurality of wavelength-converted lights, wherein the QPM device is configured such that a spatial distribution of a nonlinear constant at a distance z from an incident surface is expressed by (Equation 2) using a modulation period Λ of the nonlinear constant and a phase modulation function φ(z), and a quasi-phase matching condition among the propagation constant β(λ) of the pump light of wavelength λ, the propagation constant β(λ) of the signal light of wavelength λ, and the propagation constant β(λ) of the wavelength-converted light of wavelength λ is expressed by (Equation 3) using n (n is an integer), and the set of n when the wavelength λ(n) that satisfies (Equation 3) is a value corresponding to the wavelength band to be amplified is Z A , (Equation 3) is satisfied, the parametric gain when the wavelength λ1(n) is a value corresponding to the wavelength band to be amplified is G(n A ), (Equation 3) is satisfied, the set of n when the wavelength λ1(n) is a value corresponding to the wavelength band that is not to be amplified is Z B , (Equation 3) is satisfied, the parametric gain when the wavelength λ1(n) is a value corresponding to the wavelength band that is not to be amplified is G(n B ), there is provided a wavelength conversion system configured to satisfy (Equation 4).

[0016] 1 is a diagram showing a schematic structure of an optical amplification system 100 according to the present disclosure; FIG. 2 is a diagram showing a schematic structure of an optical separation section 110; FIG. 3 is a diagram showing a schematic structure of another form of the optical separation section 110; FIG. 4 is a diagram showing a phase modulation function φ(z) in each of QPM devices 120a-d; FIG. 5 is a diagram showing a poling period in each of QPM devices 120a-d; FIG. 6 is a diagram showing an SHG spectrum in each of QPM devices 120a-d; and FIG. 7 is a diagram showing a parametric gain spectrum of QPM devices 120a-d.

[0017] Various embodiments of the present disclosure will be described in detail below with reference to the drawings. The same or similar reference numerals indicate the same or similar elements, and redundant description may be omitted. Materials and numerical values ​​are for illustrative purposes only and are not intended to limit the technical scope of the present disclosure. The following description is an example, and some configurations may be omitted or modified, or additional configurations may be added, as long as they do not deviate from the gist of one embodiment of the present disclosure.

[0018] In the following description, the wavelength conversion system according to the present disclosure will be described as amplifying multi-band signal light including four bands, S-band, C-band, L-band, and U-band, among the wavelength bands of signals used in optical communications. However, this is intended as an example, and it should be noted that the signal light that can be amplified by the optical amplification system according to the present disclosure is not limited to these four bands of communication signals. For example, the signal light that can be amplified by the optical amplification system according to the present disclosure may be multi-band signal light consisting of six bands.

[0019] In the following explanation, the S band is a communication signal wavelength band with a wavelength of 1490.8-1521.4 nm (corresponding to a frequency of 197.05-201.10 THz), the C band is a communication signal wavelength band with a wavelength of 1534.6-1567.1 nm (corresponding to a frequency of 191.30-195.35 THz), the L band is a communication signal wavelength band with a wavelength of 1578.7-1613.1 nm (corresponding to a frequency of 185.85-189.90 THz), and the U band is a communication signal wavelength band with a wavelength of 1628.0-1664.6 nm (corresponding to a frequency of 180.10-184.15 THz).

[0020] Additionally, although wavelength conversion systems according to the present disclosure are hereinafter described in the context of optical amplification systems, this is intended as an example and is not intended to be limiting.

[0021] (System Configuration) Fig. 1 is a diagram showing a schematic structure of an optical amplification system 100 according to the present disclosure. As shown in Fig. 1, the optical amplification system 100 includes an optical separation unit 110 that separates polarization-multiplexed multi-band incident light according to the polarization state and wavelength band, QPM devices 120a-d that optically parametrically amplify each of the signal lights output from the optical separation unit 110, and an optical multiplexing unit 130 that multiplexes each of the wavelength-converted lights output from each of the QPM devices 120a-d.

[0022] Fig. 2 is a diagram showing a schematic structure of the light separating unit 110. As shown in Fig. 2, the light separating unit 110 includes a polarizing beam splitter 111 and wavelength filters 112a and 112b.

[0023] The polarizing beam splitter 111 splits the incident light into X-polarized and Y-polarized waves. As mentioned above, the second-order nonlinear optical effect is polarization-dependent, and therefore splitting is performed to accommodate both X-polarized and Y-polarized waves. Meanwhile, the wavelength filters 112a and 112b split the incident light into short wavelengths (S-band and C-band) and long wavelengths (L-band and U-band). As mentioned above, this is because, in optical parametric amplification, it is necessary to leave open the wavelength band corresponding to the wavelength-converted light to be generated.

[0024] 2, for example, the optical separation unit 110 is depicted as being configured to perform wavelength-dependent separation by wavelength filters 112a and 112b on each of the signal lights separated by the polarization beam splitter 111. However, as shown in Fig. 3, the optical separation unit 110 may be configured to perform polarization-dependent separation by polarization beam splitters 111a and 111b on each of the signal lights of each wavelength band separated by the wavelength filter 112. In other words, separation by polarization and separation by wavelength may be performed in the reverse order.

[0025] Each of the signal lights separated by the optical separation unit 110 is input to a different QPM device 120a-d. For example, an X-polarized light on the short wavelength side (S band and C band) is input to the QPM device 120a, an X-polarized light on the long wavelength side (L band and U band) is input to the QPM device 120b, a Y-polarized light on the short wavelength side (S band and C band) is input to the QPM device 120c, and a Y-polarized light on the long wavelength side (L band and U band) is input to the QPM device 120d. Note that in FIG. 4, the optical amplification system 100 is depicted as including four QPM devices 120a-d, but this is for illustrative purposes only, and the number of QPM devices installed may be set arbitrarily depending on the number of wavelength bands of the signal lights.

[0026] In addition to the signal light separated by the optical separation unit 110, high-power pump light is also incident on the QPM devices 120a-d. The wavelength of the pump light is 786.4 nm, which is due to the fact that 1572.8 nm, which is double the wavelength of the pump light, corresponds to the central wavelength of the short wavelength side (S band and C band) and the long wavelength side (L band and U band), as described above. Pump light with such a wavelength can amplify signal light for all four wavelength bands, from S band to U band.

[0027] Each of the QPM devices 120a-d performs parametric amplification based on the signal light and pump light and emits wavelength-converted light. Therefore, quasi-phase matching is satisfied among the signal light, wavelength-converted light, and pump light. The QPM devices 120a-d may be, for example, optical waveguide wavelength conversion elements that use PPLN as a nonlinear medium. The QPM devices 120a-d are also designed to achieve high gain (these design aspects will be described later).

[0028] The optical multiplexing section 130 multiplexes the wavelength-converted light beams emitted from the devices 120a-d and emits the multiplexed light beams. The optical multiplexing section 130 has a configuration in which the configuration of the optical demultiplexing section 110 shown in FIGS.

[0029] The optical amplification system 100 having such a configuration is capable of optically amplifying each wavelength band of a broadband signal light having three or more wavelength bands (four wavelength bands in this case) without enlarging the system.

[0030] (QPM Device Design) Next, a detailed description of the design for achieving high gain for the QPM devices 120a-d will be given below.

[0031] Let us now assume that the wavelength of the signal light (each light separated by the optical separation unit 110) incident on each of the QPM devices 120a-d is λ1, the wavelength of the pump light is λ3, and the wavelength of the emitted wavelength-converted light is λ2. As described above, in the QPM devices 120a-d, optical parametric amplification based on the second-order nonlinear optical effect is performed, and therefore the wavelength-converted light corresponds to the difference frequency light between the pump light and the incident light, and its wavelength satisfies the relationship of (Equation 1).

[0032]

[0033] In the QPM devices 120a-d that perform such optical parametric amplification, the spatial distribution of the nonlinear constant at a distance z from the incident surface is expressed by (Equation 2) using the modulation period Λ0 and the phase modulation function φ(z) at the distance z.

[0034]

[0035] Furthermore, the phase modulation function φ(z) is ph Then, the quasi-phase matching condition among the propagation constant β(λ3) at the wavelength λ3 of the pump light, the propagation constant β(λ1) at the wavelength λ1 of the incident light, and the propagation constant β(λ2) at the wavelength λ2 of the difference frequency light is expressed by (Equation 3).

[0036]

[0037] Here, n is an integer.

[0038] Now, let G(n) be the parametric gain at the wavelength λ of the incident light that satisfies (Equation 3), and let Z(n) be the set of desired n (here, corresponding to one of the four wavelength bands to be communicated). A , a set of unwanted wavelength bands (here, corresponding to wavelength bands not involved in communication, for example, guard bands) n is denoted as Z B In such a case, in order to obtain a high gain in the QPM devices 120a-d, the parametric gain G(n) at this time should satisfy (Equation 4).

[0039]

[0040] This (Equation 4) means that the parametric gain G(n) has a high value when n = -1 and when n = 1, and the parametric gain G(n) has a low value (close to 0 dB) when n is other than this. For example, in the QPM device 120a, the wavelength bands of the S band, the C band, the L band, and the U band are G(n A ) and G(n B ) corresponds to the guard band. A is a value selected to achieve such a setting, and may be any value depending on the design. For example, in the QPM device 120a, n=-1 corresponds to the wavelength at which the parametric gain in the S band and U band reaches its peak, and n=1 corresponds to the wavelength at which the parametric gain in the C band and L band reaches its peak. In this way, n A The value of may be arbitrarily set to a desired value (here, corresponding to the wavelength band of the signal light to be communicated).

[0041] QPM device evaluations often refer to the spectrum of second harmonic generation (SHG), which is the inverse of parametric amplification. SHG is a nonlinear optical effect in which a fundamental wave is incident on a QPM device, generating light with doubled frequency. The SHG power spectrum when the wavelength of the fundamental wave is varied has the following qualitative correspondence with the parametric amplification spectrum: When the wavelength of the pump light for parametric amplification is λ3, the SHG spectrum is extracted from the wavelength side longer than 2λ3, folded back toward the shorter wavelength side at λ3, and further stretched in the wavelength direction around 2λ3, resulting in the parametric amplification spectrum. In a QPM device that achieves the parametric gain spectrum shape described above, the SHG spectrum has two peaks on the wavelength side longer than 2λ3, corresponding to the peaks for n = 1 and n = -1, starting from the shorter wavelength side.

[0042] As an example, each of the QPM devices 120a-d is a waveguide-type element that uses PPLN as a nonlinear medium, and each of the QPM devices 120a-d has a core size of 8 × 8 μm and a length of 45 mm. Furthermore, each of the QPM devices 120a-d is configured so that the polarizations of the pump light, signal light, and wavelength-converted light are all the same in TM (Transverse-Magnetic) mode, and so that the type 0 quasi-phase matching condition is satisfied.

[0043] Furthermore, each of the QPM devices 120a-d is configured to have a phase modulation function φ(z) as shown in FIG. 4 and a poling period as shown in FIG.

[0044] 4 is a diagram showing the phase modulation function φ(z) in each of the QPM devices 120a-d. As shown in FIG. 4, each of the QPM devices 120a-d has a phase modulation function φ(z) with a modulation period Λ equivalent to a length of 45 mm. ph The device is configured to have:

[0045] 5 is a diagram showing the poling period in each of the QPM devices 120a-d. As shown in FIG. 5, the center of the poling period in each of the QPM devices 120a-d is configured to be 17.9 μm. This center of the poling period corresponds to the modulation period Λ of the nonlinear constant spatial distribution in Equation 2 above.

[0046] 6 is a diagram showing the SHG spectrum in each of the QPM devices 120a-d. As shown in FIG. 6, the QPM devices 120a-d are configured so that the SHG has two peaks and the SHG power between these two peaks is approximately 0 W.

[0047] The QPM devices 120a-d are set so that the shorter wavelength peak of the two peaks is at double the wavelength of the pump light (1572.8 nm). The parametric gain spectrum is obtained by extracting the wavelength longer than 1572.8 nm of the SHG spectrum, folding it back to the shorter wavelength side, and then stretching it in the wavelength direction.

[0048] The peak on the short wavelength side of the SHG corresponds to the peak wavelengths of the C and L bands in parametric gain, and corresponds to the case where n = 1 in (Equation 3). The peak on the long wavelength side of the SHG corresponds to the peak wavelengths of the S and U bands in parametric gain, and corresponds to the case where n = -1 in (Equation 3). The part between the peaks of the SHG spectrum where the power is 0 W corresponds to the wavelengths of the guard bands between the S and C bands and between the L and U bands in parametric gain, and corresponds to the case where n = 0 in (Equation 3).

[0049] Furthermore, the QPM devices 120a-d are configured so that the peak on the shorter wavelength side (peak near a wavelength of 1572.8 nm) is narrower than the peak on the longer wavelength side (peak near a wavelength of 1573.5 nm).

[0050] This is because the stretching in the wavelength direction is not uniform in the corresponding relationship between the SHG spectrum and the parametric gain spectrum. With parametric gain, the further away from the center wavelength is, the greater the mismatch in quasi-phase matching becomes (because the relative refractive index fluctuations become greater), and the narrower the peak width becomes. Therefore, by designing the peak on the long-wavelength side of the SHG spectrum to be wide, it is possible to make the peak width uniform in the parametric gain spectrum.

[0051] 7 is a diagram showing the spectrum of the parametric gain of the QPM devices 120a-d. As an example, FIG. 7 shows the spectrum of the Patrick gain obtained when the peak wavelengths in the C and L bands satisfy the quasi-phase matching condition when n=1 in (Equation 3), and when the peak wavelengths in the S and U bands satisfy the quasi-phase matching condition when n=-1 in (Equation 3). When n is not 1 or -1, the set Z B (i.e., an undesired n), the parametric gain is significantly smaller than the parametric gain in the four communication wavelength bands, and exhibits a value close to 0 dB.

[0052] The optical amplification system 100 including the QPM devices 120a-d having such a setting is capable of optically amplifying signal light having three or more wavelength bands, and can obtain even higher gain.

[0053] In the above description, an optical amplifier for transmitting signal light of multiple bands including S, C, L, and U bands is used, but other optical communication wavelength bands may be selected for the signal light, and the number of wavelength bands is not limited to four. In such a case, the parameters (Λ, Λ) in the quasi-phase matching condition of (Equation 3) are ph , and n) may be set appropriately.

[0054] The above description is based on an optical amplifier and is directed to the case where output light with the same wavelength as the input signal light is used, but a configuration in which wavelength-converted light is used is also possible.

[0055] In the above description, the design of the QPM device is realized by adjusting the polarization inversion pitch as shown in FIG. 5, but it may also be achieved by controlling the temperature distribution or adjusting the effective refractive index by adjusting the waveguide width and thickness.

[0056] The nonlinear medium is not limited to PPLN as long as it exhibits a second-order nonlinear optical effect. For example, the QPM device may use LiTaO3, LiNb (x) Ta (1-x) The nonlinear medium may include O3 (0≦x≦1) or a material containing O3 and at least one element selected from Mg, Zn, Sc, and In as an additive.

[0057] As described above, the wavelength conversion system according to the present disclosure enables wavelength conversion of broadband signal light having three or more wavelength bands and can achieve high gain for each wavelength band. Such a wavelength conversion system is expected to be applied to communication systems requiring long-distance optical transmission.

Claims

1. A wavelength conversion system comprising: an optical separation unit that separates multi-band incident light into a plurality of signal lights according to wavelength and polarization state; a QPM device including a nonlinear medium that generates a plurality of wavelength-converted lights by optical parametric amplification based on the plurality of signal lights and pump light; and an optical multiplexing unit that multiplexes the plurality of wavelength-converted lights, wherein the spatial distribution of the nonlinear constant at a distance z from the incident surface is expressed by (Equation 1) using the modulation period Λ0 of the nonlinear constant and the phase modulation function φ(z), The quasi-phase matching condition among the propagation constant β(λ3) of the pump light of wavelength λ3, the propagation constant β(λ1) of the signal light of wavelength λ1, and the propagation constant β(λ2) of the wavelength-converted light of wavelength λ2 is expressed by (Equation 2) using n (n is an integer), The set of n when the wavelength λ(n) that satisfies the above (Equation 2) corresponds to the wavelength band to be amplified is Z A When the wavelength λ1(n) that satisfies the above (Equation 2) is a value corresponding to the wavelength band to be amplified, the parametric gain is G(n A ), the set of n when the wavelength λ1(n) that satisfies the above (Equation 2) corresponds to the wavelength band that is not to be amplified is Z B When the wavelength λ1(n) that satisfies the above (Equation 2) is a value corresponding to the wavelength band that is not to be amplified, the parametric gain is G(n B ), a wavelength conversion system configured to satisfy (Equation 3) when 2. A wavelength conversion system comprising: an optical separation unit that separates multiband incident light into a plurality of signal lights according to wavelengths and polarization states; a QPM device including a nonlinear medium that generates a plurality of wavelength-converted lights by optical parametric amplification based on the plurality of signal lights and pump light; and an optical multiplexing unit that multiplexes the plurality of wavelength-converted lights, wherein the spatial distribution of the nonlinear constant at a distance z from the incident surface is expressed by (Equation 4) using the modulation period Λ0 of the nonlinear constant and the phase modulation function φ(z), The quasi-phase matching condition among the propagation constant β(λ3) of the pump light of wavelength λ3, the propagation constant β(λ1) of the signal light of wavelength λ1, and the propagation constant β(λ2) of the wavelength-converted light of wavelength λ2 is expressed by (Equation 5) using n (n is an integer), The set of n when the wavelength λ2(n) that satisfies the above (Equation 5) corresponds to the wavelength band to be converted is Z A The parametric gain when the wavelength λ2(n) that satisfies the above (Equation 5) is a value corresponding to the wavelength band to be converted is G(n A ), the set of n when the wavelength λ2(n) that satisfies the above (Equation 5) is a value corresponding to the wavelength band that is not to be wavelength converted is Z B The parametric gain when the wavelength λ2(n) that satisfies the above (Equation 5) is a value corresponding to the wavelength band that is not to be wavelength converted is G(n B ), a wavelength conversion system configured to satisfy (Equation 6) when 3. The wavelength conversion system according to claim 1 or 2, wherein the QPM device has an optical waveguide structure.

4. The nonlinear medium is LiTaO3, LiNb (x) Ta (1-x) 3. The wavelength conversion system according to claim 1, wherein the material is selected from O3 (0≦x≦1) or a material containing at least one element selected from Mg, Zn, Sc, and In as an additive.

5. A wavelength conversion system according to claim 1 or 2, wherein the quasi-phase matching condition is adjusted by polarization inversion, effective refractive index modulation by temperature distribution, or width and thickness of the nonlinear medium of the QPM device.

Images