JP7758968B2 - Wavelength conversion device - Google Patents

Description

The present disclosure relates to a wavelength conversion device that applies a second-order nonlinear optical effect.

Wavelength conversion technologies utilizing second-order nonlinear optical effects are used in a variety of fields, including wavelength conversion of optical signals in optical communications, optical processing, medicine, and bioengineering. The wavelength range of light that can be wavelength converted ranges from the ultraviolet to the visible, infrared, and terahertz regions. Wavelength conversion technologies utilizing second-order nonlinear optical effects are often used to generate light in wavelength ranges that cannot be directly output by semiconductor lasers. On the other hand, wavelength conversion technologies utilizing second-order nonlinear optical effects can also be used in wavelength ranges that can be directly generated by semiconductor lasers but do not provide sufficient power. For example, in optical communication systems, wavelength conversion technologies utilizing second-order nonlinear optical effects are used when wavelength conversion by difference frequency generation (described below) or amplification using the parametric effect is performed. Wavelength conversion devices that perform these wavelength conversion technologies incorporate wavelength conversion elements based on the second-order nonlinear optical effect. A typical material used for wavelength conversion elements is lithium niobate ( _LiNbO3 ), which has a large nonlinear constant. Wavelength conversion devices using _LiNbO3 and having a periodically poled optical waveguide structure are widely used in commercially available light sources due to their high wavelength conversion efficiency.

The principle of wavelength conversion that applies the second-order nonlinear optical effect will be described below. In the second-order nonlinear optical effect, light of wavelength _λ1 and light of wavelength _λ2 are input into a second-order nonlinear medium to generate a new wavelength _λ3 . Among such wavelength conversions, the wavelength conversion expressed by (Equation 1) is called sum frequency generation (hereinafter referred to as SFG).

1/λ ₃ =1/λ ₁ +1/λ ₂ (Formula 1)
In particular, wavelength conversion that satisfies λ ₁ =λ ₂ is called second harmonic generation (hereinafter referred to as SHG).

On the other hand, wavelength conversion that satisfies (Equation 2) is called difference frequency generation (DFR).
: hereinafter referred to as DFG.

1/λ ₃ =1/λ ₁ -1/λ ₂ (Formula 2)
In this DFG, the light with wavelength _λ1 is called pump light, the light with wavelength _λ2 is called signal light, and the light with wavelength _λ3 is called idler light. Furthermore, an optical parametric amplifier can be constructed by inserting a second-order nonlinear medium into the resonator and inputting only _λ1 to generate _λ2 and _λ3 that satisfy Equation 2.

In recent years, wavelength converters used in the field of communications have become capable of optical amplification by second-order nonlinear optical effects due to improvements in wavelength conversion efficiency. Optical amplifiers that perform this optical amplification are called phase-sensitive amplifiers (hereinafter referred to as PSA). Because they can amplify input light without degrading the signal-to-noise ratio, they are expected to replace the erbium-doped fiber amplifiers (hereinafter referred to as EDFAs), which have been widely used until now, as optical amplifiers for long-distance transmission.

Two types of optical amplification operations are known for PSA. One is an operation using degenerate parametric amplification, in which signal light and pump light having a wavelength half that of the signal light are input to a second-order nonlinear medium and the signal light is amplified (see, for example, Non-Patent Document 1). The other is an operation using non-degenerate parametric amplification, in which a pair of signal light and idler light and pump light having a wavelength that is the sum frequency of the signal light and idler light are input and the signal light and idler light are amplified (see, for example, Non-Patent Document 2). The pair of signal light and idler light is generated by the above-mentioned DFG.

In the field of communications, wavelength conversion technologies based on second-order nonlinear optical effects are primarily used, such as DFG and parametric amplification. In DFG and parametric amplification, signal light and idler light exist in the 1.55 μm communications wavelength band, so pump light must be in the 0.78 μm band. Pump light with a wavelength half the communications wavelength band is typically SHG light obtained by wavelength-converting a light source in the communications wavelength band. Such pump light is required to have high power and low noise in order to achieve high gain and low noise in optical amplification devices such as PSA.

FIG. 1 shows the basic configuration of a wavelength conversion device 100 that generates second harmonic waves (SHG light) of input light by SHG. The wavelength conversion device 100 includes a wavelength conversion element 102 that performs wavelength conversion on pump light 101, an optical filter 104 that transmits only wavelength-converted light 103 output from the wavelength conversion element 102, and a temperature regulator 105 that controls the temperature of the wavelength conversion element 102. When input light 101 is input to the wavelength conversion element 102, wavelength-converted light 103 that is most efficiently converted at a wavelength that satisfies the phase matching condition is output. At this time, residual pump light 106, which is a residual component of the pump light 101, may also be output from the wavelength conversion element 102. This residual pump light 106 is removed by the optical filter 104 because it adversely affects the optical amplification characteristics when input as pump light to an optical amplifier such as a PSA. In addition, in the wavelength conversion in the wavelength conversion element 102, the wavelength conversion characteristics depend on the temperature of the wavelength conversion element 102, and therefore the temperature is controlled by the temperature regulator 105 so as to maintain the phase matching condition. The temperature regulator 105 may be, for example, a Peltier element or a heater.

In optical amplification in an optical amplifier such as a PSA, wavelength-converted light (e.g., wavelength-converted light 103 shown in FIG. 1 ) generated by a wavelength conversion device (e.g., wavelength conversion device 100 shown in FIG. 1 ) can be used as pump light. In this case, to ensure the stability of the amplified light generated by the optical amplifier, the input pump light (wavelength-converted light) is required to have stable power. However, even if a temperature regulator (e.g., temperature regulator 105 shown in FIG. 1 ) is operating, fluctuations in the ambient temperature can cause temperature fluctuations in the wavelength conversion element and optical losses, which can disrupt the satisfied phase matching condition and cause the power of the wavelength-converted light to become unstable. Therefore, it is necessary to compensate for these instabilities and fluctuations.

Fig. 2 is a diagram showing the configuration of a wavelength conversion device 200 for stabilizing wavelength-converted light according to the prior art. In addition to the configuration of the wavelength conversion device 100 shown in Fig. 1, the wavelength conversion device 200 further includes a demultiplexer 201 that demultiplexes a portion of the output wavelength-converted light 103, a detector 202 that detects a portion of the wavelength-converted light 103 demultiplexed by the demultiplexer 201, an averaging device 203 that averages the output of the detector 202 to stabilize fluctuations in phase noise of the wavelength-converted light 103, and a computing device 204 that generates a feedback signal for temperature control of the temperature regulator 105 by computation based on the output of the averaging device 203 and transmits the feedback signal to the temperature regulator 105. In the wavelength conversion device 200 configured in this manner, the temperature of the wavelength conversion element 102 is detuned in the positive and negative directions at regular intervals, and the temperature regulator 105 is feedback-controlled based on power fluctuations of the wavelength-converted light 103 that occur due to the temperature detuning.

FIG. 3 shows the behavior of wavelength-converted light when the temperature of the wavelength conversion element is detuned. FIG. 3(a) shows the temperature dependence of the phase control curve of the wavelength conversion device, FIG. 3(b) shows an enlarged view of the vicinity of the peak in FIG. 3(a), and FIG. 3(c) shows the relationship between the detuning temperature and the output of the wavelength-converted light. In the figure, the temperature that satisfies the phase matching condition is represented as T ₀ , the temperature of the wavelength conversion element that has shifted from T ₀ to the higher temperature side is represented as T ₀ +ΔT, and the temperature of the wavelength conversion element that has shifted from T ₀ to the lower temperature side is represented as T ₀ -ΔT. In addition, in FIG. 3(b), the phase matching wavelength is shown with a dashed line. As shown in FIG. 3(a), the phase matching curve of the wavelength conversion element shifts linearly with the temperature of the wavelength conversion element. It can be said that the amount of wavelength shift per unit temperature is generally determined by the physical parameters of the second-order nonlinear medium applied to the wavelength conversion element. In the wavelength conversion device 200 according to the prior art, the temperature of the wavelength conversion element 102 is periodically detuned in the positive and negative directions based on such characteristics, and the temperature fluctuation of the wavelength conversion element 102 is calculated from the power fluctuation (amount of change) as shown in Fig. 3(c). Then, a feedback signal is generated based on the calculation result, and the temperature regulator 105 is controlled by the feedback signal, thereby further stabilizing the power of the output wavelength-converted light 103.

However, the configuration of the wavelength conversion device 200 according to such conventional technology requires demultiplexing a portion of the wavelength-converted light 103, which poses a problem of power attenuation of the wavelength-converted light 103. To compensate for this power attenuation, conventionally, measures have been taken such as amplifying the pump light 101 using an optical amplifier such as an EDFA. However, this method is known to pose a problem in that spontaneous emission light is mixed into the pump light 101 during the amplification process of the pump light 101, and this spontaneous emission light undergoes a parametric process, causing noise in the wavelength-converted light.

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 T. Umeki, O. Tadanaga, M. Asobe, Y. Miyamoto, and H. Takenouchi, “First demonstration of high-order QAM signal amplification in PPLN-based phase sensitive amplifier,” Optics Express Vol.22, No.3, pp.2473-2482, 2014

The present disclosure has been made in consideration of the above-mentioned problems, and its purpose is to provide a wavelength conversion device that performs wavelength conversion using SFG and that is capable of suppressing power attenuation of output wavelength-converted light more than conventional wavelength conversion devices.

In response to the above-described problems, the present disclosure provides a wavelength conversion device that receives pump light as input and outputs wavelength-converted light by sum frequency generation, the wavelength conversion device comprising: a wavelength conversion element that performs wavelength conversion based on a second-order nonlinear optical effect; a temperature regulator that controls the temperature of the wavelength conversion element; a detector that detects the power of residual pump light that passes through the wavelength conversion element; and a calculation device that generates a control signal for the temperature regulator based on the output of the detector, wherein the calculation device is configured to generate a control signal for causing the temperature regulator to detune the temperature of the wavelength conversion element, calculate temperature fluctuations of the wavelength conversion element based on power fluctuations of the residual pump light that occur in response to the detuning of the temperature of the wavelength conversion element, and generate a control signal for controlling the temperature regulator based on the temperature fluctuations so that the temperature of the wavelength conversion element becomes a temperature that corrects fluctuations in the phase matching conditions of the wavelength conversion element.

1 is a diagram showing the basic configuration of a wavelength conversion device that generates second harmonic waves (SHG light) of input light by SHG. FIG. 1 is a diagram showing the configuration of a wavelength conversion device for stabilizing wavelength-converted light according to the prior art. 3A and 3B are diagrams showing the behavior of wavelength-converted light when the temperature of a wavelength conversion element is detuned, in which FIG. 3A shows the temperature dependence of the phase control curve of the wavelength conversion device, FIG. 3B shows an enlarged view of the vicinity of the peak in FIG. 3A, and FIG. 3C shows the relationship between the detuning temperature and the output of wavelength-converted light. FIG. 10 is a diagram showing the power spectra of wavelength-converted light and residual pump light in the vicinity of a phase-matching wavelength. 1 is a diagram illustrating a configuration of a wavelength conversion device according to an embodiment of the present disclosure. 1 is a diagram illustrating a configuration of a wavelength conversion device according to an embodiment of the present disclosure. 1 is a diagram illustrating a configuration of a wavelength conversion device according to an embodiment of the present disclosure.

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. 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.

The wavelength conversion device according to the present disclosure is similar to the prior art in that it periodically detunes the temperature of a wavelength conversion element and feedback-controls a temperature regulator based on the power fluctuations caused by the detuning. However, the wavelength conversion device according to the present disclosure differs from the prior art in that it detects residual pump light, which has conventionally been removed by an optical filter or the like, and performs feedback control based on the power fluctuations of the residual pump light caused by the periodic detuning.

In the description of this specification, the wavelength conversion device is described as generating wavelength-converted light by SHG, but this is not limited to this, and any wavelength conversion device based on SFG will achieve the same effect.

In wavelength conversion based on SHG, as described above, a portion of the pump light passes through the wavelength conversion element as residual pump light and can be output simultaneously with the wavelength-converted light (in this case, SHG light). However, because a portion of the energy of the residual pump light is transferred to the wavelength-converted light, the power of the residual pump light that passes through the wavelength conversion element is attenuated near the wavelength that satisfies the phase matching condition (this phenomenon is called pump depression). The inventors have focused on this power attenuation due to pump depression and found that, based on this phenomenon, it is possible to feedback-control the temperature of the wavelength conversion element in the same way as in conventional technology.

FIG. 4 is a diagram showing the power spectra of wavelength-converted light and residual pump light near the phase-matching wavelength. Note that each spectrum in the diagram is a spectrum obtained when using a wavelength conversion device 500, which will be described later. Also, in the diagram, as in FIG. 3, the temperature that satisfies the phase-matching condition is represented as T ₀ , the temperature of the wavelength conversion element that has shifted from T ₀ to a higher temperature is represented as T ₀ +ΔT, and the temperature of the wavelength conversion element that has shifted from T ₀ to a lower temperature is represented as T ₀ -ΔT. As shown in the diagram, the power spectrum of the residual pump light has a distribution that exhibits a dip (minimum value) at the phase-matching wavelength when the temperature of the wavelength conversion element is T ₀ , and, like the wavelength-converted light, shifts linearly with temperature fluctuations. It can be seen that the amount of shift coincides with the amount of shift of the wavelength-converted light. In other words, it can be said that the degree to which the phase-matching condition of the wavelength conversion element is deviated can be determined by monitoring not only the wavelength-converted light but also the residual pump light. Based on this principle, the wavelength conversion device according to the present disclosure is configured to control the temperature of the wavelength conversion element by monitoring the residual pump light.

(First embodiment)
A first embodiment of the present disclosure will be described in detail below with reference to the drawings. The wavelength conversion device in this embodiment has the same basic configuration as the wavelength conversion device 200 described above, but is configured to separate residual excitation light, which has conventionally been removed by an optical filter, and use it for detection.

5 is a diagram illustrating a configuration of a wavelength conversion device 500 according to an embodiment of the present disclosure. The wavelength conversion device 500 according to this embodiment includes a wavelength conversion element 102 that performs wavelength conversion on input light 101 including pump light 101, a dichroic mirror 501 that transmits wavelength-converted light 103 and reflects residual pump light 106 from the output light output from the wavelength conversion element 102, a detector 502 that detects the residual pump light 106 separated by the dichroic mirror 501, and a calculation device 204 that generates a feedback signal for controlling the temperature of a temperature regulator 105 by calculation based on the output of the detector 502 and transmits the feedback signal to the temperature regulator 105. As can be seen from the diagram, the basic configuration of the wavelength conversion device 500 is similar to that of a conventional wavelength conversion device (e.g., the wavelength conversion device 200). However, it differs from conventional wavelength conversion devices in that it includes a dichroic mirror 501 instead of the optical filter 104 and does not require an averaging device 203 for stabilizing fluctuations in the phase noise of the detected light.

The wavelength conversion element 102 included in the wavelength conversion device 500 may be, for example, a ridge-type waveguide with LiNbO ₃ having a periodically poled structure as a second-order nonlinear medium. However, without being limited thereto, the second-order nonlinear medium of the wavelength conversion element 102 may be either LiTaO ₃ or LiNb(x)Ta(1-x)O ₃ (where 0≦x≦1), or any of these containing at least one element selected from the group consisting of Mg, Zn, Sc, and In as an additive.

Wavelength conversion is performed using the wavelength conversion device 500 having such a configuration, and SHG light is generated as the wavelength-converted light 103. At this time, the temperature of the wavelength conversion element 102 is periodically detuned by the temperature regulator 105, causing a fluctuation in the spectrum of the residual pump light 106 near the phase-matched wavelength. Then, the fluctuation behavior is monitored by the detector 502, and the arithmetic unit 204 generates a feedback signal based on the output of the detector 502 to control the temperature regulator 105. The wavelength of the pump light 101 may be any wavelength in the range from the O band to the L band among optical communication wavelengths.

In this way, the wavelength-converted light 103 output from the wavelength converter 500 is stabilized by suppressing fluctuations in power due to environmental temperature and optical loss. Furthermore, as described above, the wavelength converter 500 does not monitor the output wavelength-converted light 103, so the power attenuation of the wavelength-converted light 103, which occurred in conventional technology, is also suppressed. Therefore, it is possible to supply stable wavelength-converted light 103 to an external device such as a PSA more efficiently than before.

Furthermore, unlike conventional wavelength conversion devices (e.g., the wavelength conversion device 200), the wavelength conversion device 500 does not require an averaging device (e.g., the averaging device 203), and can achieve stability of the wavelength-converted light 103 equivalent to that of conventional devices. This is because, as shown in FIG. 4, the power of the residual pump light 106 is greater than that of the wavelength-converted light 103. Since the power of this residual pump light 106 has a value large enough that noise can be ignored, it is not necessary to stabilize fluctuations in phase shift due to noise. Therefore, the wavelength conversion device 500 does not require an averaging device, and therefore has the advantage of simplifying the system configuration.

Second Embodiment
A second embodiment of the present disclosure will be described in detail below with reference to the drawings. The wavelength conversion device in this embodiment further improves the stability of wavelength-converted light by hermetically sealing a wavelength conversion mechanism and a branching mechanism for branching output light from a wavelength conversion element inside a metal housing.

6 is a diagram illustrating a configuration of a wavelength conversion device 600 according to an embodiment of the present disclosure. As shown in the figure, the wavelength conversion device 600 according to this embodiment includes, in addition to the configuration of the wavelength conversion device 500, an optical fiber 602 that introduces input light 101 into the interior of a metal housing 601, a collimating lens 603 for excitation light that collimates excitation light output from the optical fiber 602, a focus lens 604 arranged to focus the collimated excitation light on the wavelength conversion element 102, a collimating lens 605 that collimates output light from the wavelength conversion element 102 (including the wavelength-converted light 103 and the residual excitation light 106), and a focus lens 606 that focuses the wavelength-converted light 103 of the output light collimated by the collimating lens 605 on the output end. The metal housing 601 further includes a focus lens 606 arranged to reflect the residual pump light 106, an optical fiber 607 for guiding the wavelength-converted light 103 focused by the focus lens 606 to the outside, a mirror 608 for reflecting the residual pump light 106 out of the output light collimated by the collimator lens 605, a focus lens 609 arranged to focus the residual pump light 106 reflected by the mirror 608 at the output end, a fiber 610 for guiding the residual pump light 106 focused by the focus lens 609 to the detector 502, and a plurality of optical windows 611 for inputting and outputting light from the outside to the inside of the metal housing 601 and from the inside to the outside. Note that since the pump light 101 and the residual pump light 106 have the same wavelength, it is preferable to use the same optical fiber for the optical fiber 602 and the optical fiber 610 in terms of optical design.

In the wavelength conversion device 600 of this embodiment, the dichroic mirror 501 is installed inside the metal housing 601 and between the collimator lens 605 and the focus lens 606. In addition, in the wavelength conversion device 600 of this embodiment, the detector 502 and the arithmetic unit 204 are installed outside the metal housing 601, and the detector 502 is optically connected to an optical fiber 610.

Like the wavelength conversion device 200 and the wavelength conversion device 500, the calculation device 204 is communicably connected to the temperature regulator 105, and the temperature regulator 105 is configured to be able to receive a feedback signal generated by the calculation device 204. The feedback signal may be an electrical signal or an optical signal, but in either case, a mechanism is required to prevent the feedback signal from being blocked by the metal casing 601 (for example, if the feedback signal is an electrical signal, it is necessary that the electrical signal be configured to be transmitted to the temperature regulator 105 via a terminal installed in the metal casing 601, and that the metal casing 601 and the terminal be electrically insulated).

Even when the wavelength conversion device 600 configured in this manner is used, by performing wavelength conversion to generate SHG light as in the first embodiment, stable wavelength-converted light 103 can be generated, similar to wavelength conversion devices according to conventional technology (e.g., wavelength conversion device 200).

Furthermore, like the wavelength conversion device 500, the wavelength conversion device 600 does not monitor the output wavelength-converted light 103, and therefore is able to perform wavelength conversion while suppressing power attenuation of the wavelength-converted light 103. In addition, like the wavelength conversion device 500, the wavelength conversion device 600 does not require an averaging device, and therefore has the advantage of being able to simplify the system configuration.

Furthermore, as described above, the wavelength conversion device 600 has a structure in which the mechanism for performing wavelength conversion and the mechanism for splitting the output light from the wavelength conversion element are hermetically sealed inside the metal housing 601. Therefore, it is less susceptible to the effects of external temperature fluctuations and the like, and is able to generate wavelength-converted light 103 that is more stable than that of the wavelength conversion device 500.

(Third embodiment)
A third embodiment of the present disclosure will be described in detail below with reference to the drawings. The wavelength conversion device according to this embodiment is the wavelength conversion device 600 described in the second embodiment, except that the detector 502 for detecting the residual pump light 106 is installed inside a metal housing 601.

7 is a diagram illustrating a configuration of a wavelength conversion device 700 according to an embodiment of the present disclosure. As shown in the figure, the wavelength conversion device 700 according to this embodiment has a structure in which the detector 502 for detecting the residual pump light 106 in the wavelength conversion device 600 described in the second embodiment is installed inside a metal housing 601. Here, the detector 502 included in the wavelength conversion device 700 is a large-diameter detector. Furthermore, with this structure, the mirror 608, the focus lens 609, the optical window 611 installed between the mirror 608 and the focus lens 609, and the optical fiber 610 included in the wavelength conversion device 600 are not required.

The wavelength converter 700 having such a configuration has the advantage that the optical alignment is simplified compared to the wavelength converter 600 described in the second embodiment. That is, the wavelength converter 600 requires alignment to introduce the residual pump light 106 demultiplexed from the output light into the optical fiber 610, but the detector 502 installed in the wavelength converter 700 is a large-diameter detector that can receive collimated light, so it can be optically coupled by passive alignment. As a result, the lead time for module assembly can be shortened.

Furthermore, if such a wavelength conversion device 700 is used to perform wavelength conversion and generate SHG light, as in the first and second embodiments, it will be possible to generate stable wavelength-converted light 103, as in a wavelength conversion device according to conventional technology (e.g., the wavelength conversion device 200).

The wavelength conversion device according to the present disclosure, unlike the prior art, is capable of stabilizing the power of wavelength-converted light while suppressing power attenuation of the output wavelength-converted light. Such a wavelength conversion device can efficiently supply input light to an optical amplifier such as a PSA, and is therefore expected to be applied to the light source of the optical amplifier.

Claims (6)

A wavelength conversion device that receives pump light as input and outputs wavelength-converted light by sum frequency generation,
a wavelength conversion element that performs wavelength conversion based on a second-order nonlinear optical effect;
a temperature controller for controlling the temperature of the wavelength conversion element;
a detector for detecting the power of residual excitation light transmitted through the wavelength conversion element;
a calculation unit that generates a control signal for the temperature regulator based on the fact that the power of the residual excitation light output by the detector has a linear relationship with the temperature of the wavelength conversion element ;
Equipped with
The computing device
generating a control signal for causing the temperature regulator to detune the temperature of the wavelength conversion element;
calculating a temperature fluctuation of the wavelength conversion element based on a power fluctuation of the residual pump light that occurs in response to a temperature detuning of the wavelength conversion element;
generating a control signal for controlling the temperature regulator based on the temperature fluctuation so that the temperature of the wavelength conversion element becomes a temperature that corrects the fluctuation of the phase matching condition of the wavelength conversion element;
The wavelength conversion device is configured as follows. a metal housing in which the wavelength conversion element and the temperature regulator are hermetically sealed;
an optical window disposed in the metal housing, for inputting the excitation light, the wavelength-converted light, or the residual excitation light into the metal housing and outputting it to the outside;
The wavelength conversion device according to claim 1 , further comprising: The wavelength conversion device described in claim 2, wherein the detector is a large-diameter detector and is installed inside the metal housing. A wavelength conversion device according to any one of claims 1 to 3, wherein the wavelength of the pump light is in the range of O-band to L-band among communication wavelengths. A wavelength conversion device according to any one of claims 1 to 4, wherein the wavelength conversion element is a ridge-type waveguide equipped with a second-order nonlinear medium, and the second-order nonlinear medium has a periodically poled structure. The wavelength conversion device of claim 5, wherein the material applied to the second-order nonlinear medium is either LiNbO ₃ , LiTaO ₃ , or LiNb(x)Ta(1-x)O ₃ (where 0≦x≦1), or any of these materials containing at least one element selected from the group consisting of Mg, Zn, Sc, and In as an additive.