WO2023218646A1

WO2023218646A1 - Wavelength conversion system

Info

Publication number: WO2023218646A1
Application number: PCT/JP2022/020240
Authority: WO
Inventors: 貴大柏崎; 毅伺梅木
Original assignee: 日本電信電話株式会社
Priority date: 2022-05-13
Filing date: 2022-05-13
Publication date: 2023-11-16

Abstract

Through the present invention, polarization-multiplexed incident light is wavelength converted collectively without an increase in the number of PPLN waveguide-type wavelength conversion means. Provided is a wavelength conversion system that has wavelength conversion means (203X, 203Y) comprising a nonlinear optical material and having a periodic polarization inversion structure, and that performs wavelength conversion collectively of polarization-multiplexed incident light in which a plurality of signal lights are included in each of an upper band and a lower band, the wavelength conversion system comprising a polarization separation means (201) for separating the incident light for each polarization and causing the separated incident light to be incident on the plurality of wavelength conversion means, and a polarization multiplexing means (205) for multiplexing the output from the plurality of wavelength conversion means, signal light and excitation light having orthogonal polarization axes being incident on the wavelength conversion means and turned back by difference frequency generation with a wavelength that is twice that of the excitation light as a center axis, and idler light having different polarization than the signal light before conversion being outputted.

Description

wavelength conversion system

The present invention relates to a wavelength conversion system, and more particularly, to a wavelength conversion system that is applied to an optical communication system or an optical measurement system and includes an optical element using a nonlinear optical effect.

Wavelength conversion technology is used in various application fields such as optical signal wavelength conversion in optical communications, optical processing, medical care, and bioengineering. The wavelength range of light that is subject to wavelength conversion extends from the ultraviolet region to the visible region, the infrared region, and the terahertz region, and even extends to wavelength regions that cannot be directly output by semiconductor lasers. Focusing on the material used for wavelength conversion, lithium niobate (LiNbO ₃ :LN), which is a second-order nonlinear material and has a large nonlinear constant, is used. A wavelength conversion device having an optical waveguide with a periodically poled structure (PPLN) using LN is widely used in commercially available light sources because of its high wavelength conversion efficiency.

The second-order nonlinear optical effect uses three mechanisms in which light with a wavelength λ1 and light with a wavelength λ2 are input to a second-order nonlinear optical material to generate a new wavelength λ3. First, a wavelength conversion operation that satisfies equation (1) is called sum frequency generation (SFG).
1/λ3=1/λ1+1/λ2 Formula (1)
Second, a wavelength conversion operation that satisfies equation (2), which is a modified version of equation (1), with λ1=λ2 is called second harmonic generation (SHG).
λ3=λ1/2 Formula (2)
Thirdly, a wavelength conversion operation that satisfies equation (3) is called difference frequency generation (DFG).
1/λ3=1/λ1-1/λ2 Formula (3)
The wavelength λ1 used when generating the difference frequency according to equation (3) is called excitation light, the wavelength λ2 is called signal light, and the wavelength λ3 is called idler light (converted light). Furthermore, it is also possible to construct an optical parametric oscillator that generates wavelengths λ2 and λ3 that satisfy equation (3) by placing a nonlinear optical material in a resonator and inputting only the wavelength λ1.

For example, in the mid-infrared wavelength range from 2 μm to 5 μm, there are strong absorption lines such as the reference vibration of various environmental gases, so it is desirable to develop a compact mid-infrared light source for environmental gas measurement equipment. ing. As a light source in the mid-infrared region, DFG is considered to be promising because it can use a technically mature excitation light source in the vicinity of 1 μm and a light source for signal light in the communication wavelength band. In addition, since there is a wavelength range of visible light around 0.5 μm that is difficult to realize with semiconductor lasers, visible light such as green light can be produced using an excitation light source around 1 μm using SHG or SFG. A light source that can generate 200 nm is viewed as promising.

By using wavelength conversion technology using DFG, it is possible to convert all the light in the 1.55 μm wavelength band, which is mainly used in optical fiber communications, into another wavelength band. Therefore, it can be applied to optical routing in wavelength division multiplexing, wavelength collision avoidance in optical routing, etc. Further, in wavelength conversion using a DFG, signal distortion compensation can be performed by using the fact that the idler light becomes phase conjugate light with respect to the signal light. When the signal light is converted to phase conjugate light at approximately the midpoint of the transmission path, the signal distortion caused by the dispersion generated in the transmission path before conversion to phase conjugate light and the nonlinear optical effect in the fiber is converted into phase conjugate light. They propagate so as to cancel each other out in the subsequent transmission path. Therefore, a wavelength conversion device using the above wavelength conversion technology is considered as one of the key devices for constructing a large-capacity communication optical network.

By using a wavelength conversion element with high wavelength conversion efficiency, it is possible to configure a signal light amplifier called optical parametric amplification by transferring energy from pumping light power to signal light. In particular, phase-sensitive amplifiers, which have amplification characteristics depending on the phase relationship between pump light and signal light, are expected to be a technology that enables low-noise optical amplification. Furthermore, the degenerate optical parametric amplification process can generate photon pairs with quantum correlation, and can generate non-classical states such as squeezed light generation and single photon states with messengers. These optical amplification technologies are expected to be applied to optical quantum computers, sensing technologies using quantum light, and the like.

In order to obtain a highly efficient and broadband nonlinear optical effect in PPLN, an optical waveguide type device is effective. This is because wavelength conversion efficiency is proportional to the power density of light propagating through a nonlinear optical material, and by forming a waveguide structure, light can be confined within a limited range. Until now, studies have been made on diffusion type waveguides called Ti diffusion waveguides and proton exchange waveguides as waveguides using nonlinear optical materials. However, these waveguides have problems from the viewpoint of optical damage resistance and long-term reliability because impurities are diffused into the crystal during the manufacturing process. In diffusive waveguides, when high-intensity light is input into the waveguide, the crystal is damaged due to the photorefractive effect, so there is a limit to the optical power that can be input into the waveguide.

In recent years, research and development has been carried out on ridge-type optical waveguides, which have features such as high optical damage resistance, long-term reliability, and easy device design because the properties of the bulk of the crystal can be used as is. A ridge-type optical waveguide can be formed by bonding two substrates, making one substrate thin, and then performing ridge processing. When bonding two substrates together, the method of bonding them together with an adhesive has a problem in that the thin film cracks when the temperature changes because the adhesive and the substrate have different coefficients of thermal expansion. In addition, since the adhesive is degraded by the second harmonic light generated in the waveguide, waveguide loss increases during operation, and wavelength conversion efficiency deteriorates. Furthermore, due to the non-uniformity of the adhesive layer, the thickness of the single crystal film becomes non-uniform, resulting in a problem that the phase matching wavelength of the wavelength conversion element shifts.

On the other hand, direct bonding technology that does not use adhesive is known. The direct bonding method is a method in which wafers that have been surface-treated using chemicals are stacked on top of each other and bonded using attraction between the surfaces. Bonding is performed at room temperature, but since the bonding strength of the wafers at this time is low, heat treatment at a high temperature is performed to improve the bonding strength. In addition to features such as high optical damage resistance, long-term reliability, and ease of device design, the direct bonding method also has the advantage of avoiding contamination of impurities and absorption of adhesives, etc. when generating light in the mid-infrared region by DFG. It is also viewed as promising. Furthermore, the direct bonding method is expected to be applied not only to nonlinear optical devices but also to high-power optical modulators and the like.

Since the direct bonding method requires heat treatment at a high temperature of about 400° C., the wafers that can be bonded are required not only to have good surface flatness but also to have similar coefficients of thermal expansion. For this reason, direct bonding methods using similar material substrates such as LN, lithium tantalate (LiTaO ₃ :LT), or LN added with additives such as Mg, Zn, Sc, In, and Fe have been investigated.

Oxide-based compound substrates such as LN have large electro-optic constants in addition to second-order nonlinear optical constants, and are widely used as optical modulators using electro-optic effects (EO effects). However, although optical modulators using Ti diffused waveguides have been commercially available, it has been difficult to input high-power light of 100 mW or more. Nonlinear optical devices using the direct bonding method can receive optical inputs on the order of watts, so they can be expected to be applied to the generation of high-intensity optical modulation signals, laser processing technology, etc.

A ridge-type waveguide has a core formed on a base substrate according to a waveguide pattern, and has a step-type refractive index distribution. The three sides of the core that are not in contact with the base substrate are in contact with an air layer (with a refractive index of 1). However, as a practical problem, if the core layer is exposed, there is a concern that the characteristics may change over time due to adhesion of airborne dirt or dust. Furthermore, in consideration of the mechanical strength necessary for forming a film such as an AR coat on the end face of the optical waveguide, an over cladding layer that also serves as a protective film may be provided.

On the other hand, the periodic poled structure is a structure for performing quasi-phase matching, and the crystal orientation is reversed for each coherence length of the fundamental wave and the wavelength-converted wave, and the sign of the nonlinear constant is reversed. It is a structure that compensates for the amount. It has high practical value in that it can perform wavelength conversion over a wide range from the mid-infrared region to the visible region without using special nonlinear optical crystals. Generally, the refractive index of a nonlinear optical material has temperature dependence, and in order to strictly satisfy the quasi-phase matching condition in a second-order nonlinear optical element, it is necessary to keep the temperature of the optical element constant. Usually, a temperature measuring element such as a thermistor or thermocouple is provided at or near the secondary nonlinear optical element, and its resistance value is monitored. Then, depending on the monitoring results, a mechanism is provided to keep the optical element at a constant temperature using a temperature controller such as a heater or a Peltier element.

When using a PPLN optical waveguide, in order to achieve high conversion efficiency, the polarization of the three lights, pumping light, signal light, and idler light, is oriented parallel to the polarization direction of the crystal. It was designed with a polarization inversion period that would allow for matching. This is to use the maximum d ₃₃ among the second-order nonlinear optical constants d _ij (3x6 matrix elements). Phase matching in which three lights with the same polarization interact efficiently is called Type 0 phase matching. Optical parametric amplification of 30 dB or more has been reported using a PPLN waveguide using Type 0 phase matching. Further, by using a plurality of PPLN waveguide type optical parametric amplifiers in parallel, phase conjugate conversion in an optical communication system has been successfully achieved (see, for example, Non-Patent Document 1).

However, in the method using Type 0 phase matching, the optical parametric amplifier becomes a device that acts only on one polarized wave. Therefore, when applied to an optical communication system that handles polarization-multiplexed signal light, it is necessary to prepare a plurality of optical parametric amplifiers to support XY polarization. Furthermore, in wavelength conversion, the conversion destination band and polarization resources must be opened in advance, so by using a wavelength demultiplexer, it is necessary to use four PPLN optical parametric amplifiers. Further, in optical parametric amplification, it is necessary to prepare optical fiber amplifiers in order to prepare strong pumping light, so it is necessary to prepare the number of optical fiber amplifiers according to the number of PPLN optical parametric amplifiers. Therefore, the system size tends to be large and the power consumption also tends to be large. A conventional PPLN waveguide type optical parametric amplifier using Type 0 phase matching has a problem in that the system configuration becomes large because wavelength resources cannot be utilized to the maximum.

An object of the present invention is to provide a wavelength conversion system that can wavelength-convert polarization-multiplexed incident light all at once without increasing the number of PPLN waveguide type wavelength conversion means.

In order to achieve such an object, one embodiment of the present invention has a wavelength conversion means made of a nonlinear optical material and having a periodic polarization inversion structure, and has a plurality of wavelength conversion means for each of upper sideband waves and lower sideband waves. A wavelength conversion system that collectively performs wavelength conversion on polarization-multiplexed incident light that includes signal light of The wavelength converting means includes a wave separating means and a polarization combining means for combining the outputs from the plurality of wavelength converting means, and the wavelength converting means receives a signal light and a pumping light whose polarization axes are perpendicular to each other, It is characterized in that it is folded back around a central axis having a wavelength twice that of the excitation light by frequency generation, and outputs idler light having a polarization different from that of the signal light before conversion.

FIG. 1 is a diagram showing a wavelength conversion system using conventional Type 0 phase matching, FIG. 2 is a diagram showing a wavelength conversion system according to the first embodiment of the present invention, FIG. 3 is a diagram showing a wavelength conversion system according to a second embodiment of the present invention, FIG. 4 is a diagram showing a PPLN waveguide included in the wavelength conversion means, FIG. 5 is a diagram showing an example of the wavelength conversion system of the first embodiment.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows a wavelength conversion system using conventional Type 0 phase matching. This figure shows the configuration of a conventional wavelength conversion system that can collectively perform wavelength conversion on a plurality of polarization-multiplexed signal lights. The structure of the incident light includes a plurality of signal lights in each of the upper and lower band waves, and the upper and lower band waves are arranged around a wavelength twice that of the excitation light. In FIG. 1, a lower sideband wave is schematically shown on the left and an upper sideband wave is shown on the right side, centering on a wavelength twice that of the excitation light.

The incident light is separated into X polarization and Y polarization by the polarization separation means 101, and further separated into four bands, an upper side band and a lower side band, by the wavelength separation means 102X and 102Y. Each sideband signal light is input to wavelength conversion means 103XU, 103XL, 103YU, and 103YL including PPLN waveguides using Type 0 phase matching, and by difference frequency generation, the central axis is twice the wavelength of the excitation light. are converted to their respective folded wavelengths. In FIG. 1, the excitation light source in the wavelength conversion means is not shown. The wavelength-converted light has the same polarization as the input signal light. Pumping light and signal light are removed from the output of the wavelength conversion means, and idler light is multiplexed by wavelength multiplexing means 104X, 104Y and polarization multiplexing means 105 and output.

As described above, the conventional wavelength conversion system requires a plurality of optical parametric amplifiers corresponding to XY polarization, and requires a total of four wavelength conversion means. Therefore, in this embodiment, wavelength conversion is performed using half the number of conventional optical parametric amplifiers by using wavelength conversion means including a PPLN waveguide that utilizes Type 2 phase matching.

[First embodiment]
FIG. 2 shows a wavelength conversion system according to a first embodiment of the present invention. A plurality of signal lights are included in each of the upper side band wave and the lower side band wave, and wavelength conversion can be performed on the polarization multiplexed incident light all at once. The configuration of the incident light is the same as that of the conventional example, and is separated into X polarized light and Y polarized light by polarization separation means 201, and is transmitted to wavelength conversion means 203X and 203Y including PPLN waveguides using Type 2 phase matching. It is incident. At this time, as will be described later with reference to FIG. 4, the polarization axis of the incident light is aligned with the eigenaxis of the PPLN waveguide of the wavelength conversion means 203X, 203Y. The characteristic axis of a PPLN waveguide is the direction of polarization, that is, the vertical direction of the ridge-shaped optical waveguide (Y-axis direction in Figure 4) or the direction perpendicular to it (X-axis direction in Figure 4), and the polarization of the incident light Make the axis parallel to one of the eigenaxes. Therefore, it is desirable that the polarization axis of an optical system such as an optical fiber connecting between the polarization separation means 201 and the wavelength conversion means 203X, 203Y is maintained. Specifically, the polarization axes can be aligned by using

polarization maintaining fibers

211X and 211Y. Further, a planar optical circuit that maintains polarization may be used, or a polarization controller may be inserted between the polarization separation means 201 and the wavelength conversion means 203X, 203Y.

In the case of Type 2 phase matching, light with a wavelength that is folded back around a center axis that is twice the wavelength of the excitation light due to difference frequency generation is generated as idler light with a polarization different from that of the signal light before conversion. In FIG. 2, the excitation light source in the wavelength conversion means is not shown. In the wavelength conversion means 203X, the lower sideband light of the X polarization is converted into the upper sideband wave of the Y polarization. Similarly, X-polarized upper sideband light is converted to Y-polarized lower sideband light. Similarly, in the wavelength converting means 203Y, the signal light is converted into idler light having a polarization orthogonal to the polarization of the signal light.

Pumping light and signal light are removed from the outputs of the wavelength conversion means 203X and 203Y, and idler light is multiplexed by the polarization multiplexing means 205 and output. Note that the optical system between the wavelength conversion means 203X, 203Y and the polarization multiplexing means 205 also desirably maintains its polarization axis.

The polarization separation means 201 and the polarization multiplexing means 205 may use an optical fiber type directional coupler, or may be separated by a dielectric multilayer coating. Furthermore, a waveguide type directional coupler may be used using a planar optical circuit.

[Second embodiment]
FIG. 3 shows a wavelength conversion system according to a second embodiment of the invention. A plurality of signal lights are included in each of the upper side band wave and the lower side band wave, and wavelength conversion can be performed on the polarization multiplexed incident light all at once. The structure of the incident light is the same as that of the conventional example, and is separated into an upper sideband wave and a lower sideband wave by the wavelength separation means 302, and is incident on wavelength conversion means 303U and 303L including PPLN waveguides using Type 2 phase matching. be done. Unlike the first embodiment, the polarization of the light incident on the PPLN waveguides of the wavelength conversion means 303U and 303L may be of any type.

In the case of Type 2 phase matching, light with a wavelength that is folded back around a center axis that is twice the wavelength of the excitation light due to difference frequency generation is generated as idler light with a polarization different from that of the signal light before conversion. In FIG. 3, the excitation light source in the wavelength conversion means is not shown. In the wavelength conversion means 303U, the X-polarized upper-sideband light is converted into the Y-polarized lower-sideband light. Similarly, Y-polarized upper-sideband light is converted to X-polarized lower-sideband light. Similarly, in the wavelength converting means 303L, the signal light is converted into idler light having a polarization orthogonal to the polarization of the signal light.

Pumping light and signal light are removed from the output of the wavelength conversion means, and idler light is multiplexed by the wavelength multiplexing means 304 and output.

The wavelength separation means 302 and wavelength multiplexing means 304 may be of any type as long as they can separate or combine the upper and lower sideband waves based on the center frequency of wavelength conversion. For example, an optical fiber type directional coupler, a grating mirror, or an arrayed waveguide grating (AWG) made of a planar optical circuit can be used.

[PPLN waveguide]
FIG. 4 shows a PPLN waveguide included in the wavelength conversion means. FIG. 4(a) shows a PPLN waveguide using conventional Type 0 phase matching, and FIG. 4(b) shows a PPLN waveguide using Type 2 phase matching included in the wavelength conversion means 203, 303 described above. shows. In each case, a ZnO-doped LN substrate is directly bonded to a base substrate 401 made of LT, and a ridge-shaped PPLN waveguide 402 is formed by processing the LN substrate.

In the PLN waveguide, a quasi-phase matching condition is satisfied between the three waves of excitation light, signal light, and converted light. The wavelengths of the excitation light, signal light, and converted light are λp, λs, and λc, respectively, and the effective refractive indices in the waveguide are np, ns, and nc, respectively.
np/λp-ns/λs-nc/λc=1/Λ (Equation 4)
It has a polarization inversion structure with an inversion period Λ that satisfies the following.

In the case of Type 0 phase matching, phase matching is performed in such a way that the polarizations of the three lights, the excitation light, the signal light, and the idler light, are parallel to the polarization direction (Y-axis). That is, the signal light and the pumping light are input with the same polarization, and the output idler light is also output with the same polarization. On the other hand, in the case of Type 2 phase matching, the signal light and pump light are input with orthogonal polarization, and the output idler light is output with orthogonal polarization to the signal light, that is, the same polarization as the pump light. be done.

At this time, as shown in FIG. 4(b), it is preferable to perform phase matching so that the polarization axis of the incident excitation light is in the same direction as the polarization direction (Y-axis), as the wavelength conversion efficiency is good. However, if the period of polarization inversion is modified, wavelength conversion can be performed even if the polarization axis of the excitation light is orthogonal to the polarization direction (X-axis). It should be parallel to one of the axes.

[Example]
FIG. 5 shows an example of the wavelength conversion system of the first embodiment. The structure of the incident light is the same as that of the conventional example, and is separated into X polarization and Y polarization by polarization separation means 501, and is sent to wavelength conversion means 503X and 503Y including PPLN waveguides using Type 2 phase matching. It is incident. At this time, the polarization axis of the incident light is aligned with the eigenaxis of the PPLN waveguide of the wavelength conversion means 503X, 503Y. Specifically, the polarization separating means 501 and the wavelength converting means 503X, 503Y are connected using polarization maintaining fibers 511X, 511Y, and the polarization axis of the signal light and the eigenaxis of the PPLN waveguide are connected. Align with the eigenaxis of polarization direction.

The wavelength conversion means 503X, 503Y include

PPLN waveguides

531X, 531Y using Type 2 phase matching, into which the excitation light from the

excitation light sources

532X, 532Y and the signal light from the polarization separation means 501 are input. . The signal light and the excitation light are combined by a combining mirror installed immediately before the PPLN waveguide of the wavelength conversion means 503X, 503Y. As shown in FIG. 4(b), the core size of the PPLN waveguide 402 is 8×8 μm, the polarization inversion period of PPLN is approximately 9 μm, and the TE mode of the pumping light, the TM mode of the signal light, and the TE mode of the idler light. It was confirmed that the three types of light interact.

In the case of Type 2 phase matching, light with a wavelength that is twice the wavelength of the pumping light as the central axis is generated as idler light with a polarization different from that of the signal light before conversion. In the wavelength conversion means 503X, the lower sideband light of the X polarization is converted into the upper sideband wave of the Y polarization. Similarly, X-polarized upper sideband light is converted to Y-polarized lower sideband light. Similarly, in the wavelength conversion means 503Y, the signal light is converted into idler light having a polarization orthogonal to the polarization of the signal light.

Pumping light and signal light are removed from the outputs of the wavelength conversion means 503X and 503Y, and the idler light is multiplexed by the polarization multiplexing means 505 and output. It was confirmed that the desired wavelength conversion was performed by generating a difference frequency in the wavelength conversion means 503X and 503Y. Note that

polarization maintaining fibers

512X, 512Y are also used between the wavelength converting means 503X, 503Y and the polarization multiplexing means 505 to align the polarization axis and the eigenaxis.

Although PPLN is used in this embodiment, a nonlinear optical material that does not have a periodically poled structure may be used, and a material having a second-order nonlinear optical constant may be used in addition to LN. .

Claims

It is made of a nonlinear optical material and has a wavelength conversion means with a periodic polarization inversion structure. The upper sideband wave and the lower sideband wave each contain a plurality of signal lights, and the polarization-multiplexed incident light is collectively converted into wavelengths. A wavelength conversion system that performs conversion,
polarization separation means that separates the incident light into polarized waves and causes the separated light to enter a plurality of wavelength conversion means;
and polarization multiplexing means for multiplexing outputs from the plurality of wavelength conversion means,
The wavelength conversion means receives a signal light and a pump light whose polarization axes are perpendicular to each other, and is folded back around a central axis having a wavelength twice that of the pump light by difference frequency generation, which is different from the signal light before conversion. A wavelength conversion system characterized by outputting polarized idler light.
Claim 1, wherein the polarization separating means and the plurality of wavelength converting means and between the plurality of wavelength converting means and the polarization multiplexing means are connected by polarization maintaining fibers. The wavelength conversion system described in .
The wavelength conversion system according to claim 2, wherein the polarization direction of the wavelength conversion means and the polarization axis of the excitation light are in the same direction.
It is made of a nonlinear optical material and has a wavelength conversion means with a periodic polarization inversion structure. The upper sideband wave and the lower sideband wave each contain a plurality of signal lights, and the polarization-multiplexed incident light is collectively converted into wavelengths. A wavelength conversion system that performs conversion,
wavelength separation means that separates an upper band wave and a lower side band wave of the incident light and causes them to enter a plurality of wavelength conversion means;
and wavelength multiplexing means for multiplexing outputs from the plurality of wavelength conversion means,
The wavelength conversion means receives a signal light and a pump light whose polarization axes are perpendicular to each other, and is folded back around a central axis having a wavelength twice that of the pump light by difference frequency generation, which is different from the signal light before conversion. A wavelength conversion system characterized by outputting polarized idler light.