CN112711147A - Lithium niobate optical isolator irrelevant to polarization - Google Patents
Lithium niobate optical isolator irrelevant to polarization Download PDFInfo
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- CN112711147A CN112711147A CN202110106888.7A CN202110106888A CN112711147A CN 112711147 A CN112711147 A CN 112711147A CN 202110106888 A CN202110106888 A CN 202110106888A CN 112711147 A CN112711147 A CN 112711147A
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- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour
- G02F1/09—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour based on magneto-optical elements, e.g. exhibiting Faraday effect
- G02F1/093—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour based on magneto-optical elements, e.g. exhibiting Faraday effect used as non-reciprocal devices, e.g. optical isolators, circulators
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/24—Coupling light guides
- G02B6/26—Optical coupling means
- G02B6/27—Optical coupling means with polarisation selective and adjusting means
- G02B6/2746—Optical coupling means with polarisation selective and adjusting means comprising non-reciprocal devices, e.g. isolators, FRM, circulators, quasi-isolators
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/24—Coupling light guides
- G02B6/26—Optical coupling means
- G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
- G02B6/2804—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers
- G02B6/2808—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers using a mixing element which evenly distributes an input signal over a number of outputs
- G02B6/2813—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers using a mixing element which evenly distributes an input signal over a number of outputs based on multimode interference effect, i.e. self-imaging
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- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour
- G02F1/09—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour based on magneto-optical elements, e.g. exhibiting Faraday effect
- G02F1/095—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour based on magneto-optical elements, e.g. exhibiting Faraday effect in an optical waveguide structure
- G02F1/0955—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour based on magneto-optical elements, e.g. exhibiting Faraday effect in an optical waveguide structure used as non-reciprocal devices, e.g. optical isolators, circulators
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Abstract
The invention provides a polarization-independent lithium niobate optical isolator which comprises a first interference arm (1), a second interference arm (2), a first coupler (3) and a second coupler (4) which are integrally formed, wherein the first coupler (3) and the second coupler (4) are respectively formed at two ends of the first interference arm (1) and the second interference arm (2), the first interference arm (1) and the second interference arm (2) at least comprise a reciprocal waveguide and a nonreciprocal waveguide along the length direction, and the polarization-independent lithium niobate optical isolator comprises: the length of the reciprocal waveguide of the first interference arm (1) is different from that of the reciprocal waveguide of the second interference arm (2); the nonreciprocal waveguide comprises a magneto-optical waveguide and a lithium niobate ridge waveguide, the magneto-optical waveguide is positioned above a flat waveguide of the lithium niobate ridge waveguide, and ridges of the magneto-optical waveguide and the lithium niobate ridge waveguide are formed in parallel along the width direction; the first coupler (3) and the second coupler (4) are lithium niobate ridge waveguides, and realize the isolation of reverse input light of a transverse mode and a longitudinal mode at the same time. The invention has the characteristics of no relation with polarization, high waveguide coupling efficiency, easy integration and the like.
Description
Technical Field
The invention relates to the technical field of micro-nano photoelectronics integration, in particular to a lithium niobate optical isolator irrelevant to polarization.
Background
With the development of the information age, the performance requirements for fiber optic communication modules, links, and systems are also gradually increasing in order to better meet the "explosively increasing" information transmission needs thereof. In the optical path, reflected light in the opposite direction to the forward propagating light is generated for various reasons, and for example, when light is coupled into an optical fiber, reflected light in the opposite direction to the original propagating light is generated at the end faces and points due to the presence of connectors and fusion splices. When the photons of the reflected wave return to the device, the photons and the semiconductor material perform secondary action to interfere the normal carrier distribution of the luminescent material, so that self-coupling effect and self-excitation effect are generated between optical path systems, the generation of light with other wavelengths and modes is caused, the transmission stability is damaged, and various adverse effects are brought to the device: for a directly modulated laser, reflected waves can bring chirp to the laser, so that severe fluctuation of a light source signal is caused, the modulation bandwidth is reduced, long-distance transmission of a high-speed signal is not facilitated, and the laser can be even burnt in severe cases; for the optical fiber amplifier, the existence of the reflected wave can increase the noise intensity, thereby reducing the transmission signal-to-noise ratio; for an analog signal transmission system, the anti-electromagnetic wave interference capability of the analog signal transmission system is poor, and reflected waves can seriously affect the communication quality; for a coherent optical communication system, reflected waves increase the spectral width of a carrier signal and cause frequency drift, so that the system cannot meet the conditions of a heterodyne method and cannot work normally.
An optical isolator is a device that allows an optical signal to propagate only in one direction and blocks reflected light, and is called an optical isolator, which is similar to a diode in a circuit and can be used to prevent the reflected light generated in an optical path due to various reasons from adversely affecting forward-transmitted light. Therefore, the optical communication system needs to add isolators at these ports, so that the normal operation of the system can be effectively stabilized, and the transmission quality of signals can be ensured. The indexes for measuring the performance of the optical isolator comprise insertion loss, reverse isolation, return loss, 3dB isolation bandwidth, passband bandwidth, polarization-related loss, temperature characteristics and the like, and in order to enable the optical isolator to play a better effect in a system, the characteristics of high reverse isolation, high working bandwidth, high return loss, high stability and reliability, low insertion loss and the like are the main development directions of the optical isolator.
The lithium niobate has excellent electro-optic coefficient, and when the incident light intensity is small, the electric polarization intensity of the crystal and the incident light field intensity can be described by a linear relation. Under the action of strong light, the electric polarization intensity of the medium and the intensity of incident light have a power series relationship, and the higher-order term of the power series cannot be ignored, which indicates that new frequency radiation is generated when the light is incident into the medium. Based on the excellent optical characteristics of lithium niobate, the optical effects of frequency doubling, sum frequency, difference frequency, parametric oscillation and the like of incident light can be realized. Currently, optical devices made of lithium niobate bulk materials have been commercialized, but these devices have the disadvantages of large device size and low integration level, and an integrated optical platform similar to a Silicon On Insulator (SOI) structure is urgently needed to realize on-chip integration of multifunctional devices.
With the progress of micro-nano optoelectronics in recent years, optoelectronic devices are developing towards miniaturization and integration, people can see the development prospect of an optical communication system in the aspect of on-chip integration, and the goal of realizing photoelectric fusion is to be on the spot. However, the integration of complex active devices on photonic integrated chips is hampered by the lack of an efficient and practical method of integrating optical isolators.
Disclosure of Invention
Technical problem to be solved
Aiming at the problems, the invention provides a lithium niobate optical isolator irrelevant to polarization, which is used for at least partially solving the technical problems that the traditional on-chip device has stronger polarization correlation, and can only realize the effect on a transverse mode or a longitudinal mode generally, and the like.
(II) technical scheme
The invention provides a lithium niobate optical isolator irrelevant to polarization, which comprises a first interference arm 1, a second interference arm 2, a first coupler 3 and a second coupler 4 which are integrally formed, wherein the first coupler 3 and the second coupler 4 are respectively formed at two ends of the first interference arm 1 and the second interference arm 2, the first interference arm 1 and the second interference arm 2 at least comprise a reciprocal waveguide and a non-reciprocal waveguide along the length direction, and the lithium niobate optical isolator comprises: the length of the reciprocal waveguide of the first interference arm 1 is different from that of the reciprocal waveguide of the second interference arm 2; the nonreciprocal waveguide comprises a magneto-optical waveguide and a lithium niobate ridge waveguide, the magneto-optical waveguide is positioned above a flat waveguide of the lithium niobate ridge waveguide, and ridges of the magneto-optical waveguide and the lithium niobate ridge waveguide are formed in parallel along the width direction; the first coupler 3 and the second coupler 4 are lithium niobate ridge waveguides, and realize the isolation of reverse input light of a transverse mode and a longitudinal mode at the same time.
Further, the mode of the light includes a fundamental transverse mode, a fundamental longitudinal mode, or a combination thereof.
Further, the difference in length between the reciprocal waveguides in the first and second interference arms 1 and 2 causes a pi/2 +2m pi phase shift in the two beams, where m is an integer.
Furthermore, the phase shift generated after the forward transmitted light passes through the nonreciprocal waveguide is-pi/2 +2n pi, and the phase shift generated after the backward transmitted light passes through the nonreciprocal waveguide is pi/2 +2n pi, wherein n is an integer.
Further, the magneto-optical material of the magneto-optical waveguide is a magneto-optical active material with a large Faraday coefficient, and comprises doped rare-earth iron garnet.
Further, the magneto-optical material is cerium-doped yttrium iron garnet.
Further, the magnetic field on the nonreciprocal waveguide is perpendicular to the direction of light propagation and is tilted by 45 °, and the magnitude of the magnetic field is not less than the saturation magnetization of the magneto-optical material.
Furthermore, the lower cladding of the optical isolator is made of silicon oxide material, the substrate is made of lithium niobate material, and the upper cladding is made of air, silicon oxide material or other semiconductor material with the refractive index smaller than that of the lithium niobate.
Further, the first coupler 3 divides the forward transmitted light into two identical beams of light, and the second coupler 4 combines the two beams of light into one beam of light and outputs the light through the output end of the light.
Further, the first coupler 3 and the second coupler 4 include a 1 × 2 directional coupler, a 1 × 2 multimode interference coupler.
(III) advantageous effects
According to the polarization-independent lithium niobate optical isolator provided by the invention, the cross section size and length of the reciprocal waveguide and the nonreciprocal waveguide are designed, so that the isolation effect of input light of a transverse mode and a longitudinal mode can be realized simultaneously, and the applicability and the usability of the device are improved; lithium niobate material and Ce: YIG material has very similar refractive index, the mode fields of light transmitted in the two waveguides are almost the same under the same size, the mode fields can be butted without loss, and the coupling efficiency is high; the optical isolator is realized on a lithium niobate platform, the diversity of lithium niobate devices is enriched, and a thought is provided for realizing the monolithic integration of various devices on a lithium niobate wafer.
Drawings
FIG. 1 schematically illustrates a structural schematic of a polarization independent lithium niobate optical isolator, in accordance with an embodiment of the present invention;
FIG. 2 is a schematic diagram showing the cross-section of a reciprocal waveguide and the distribution of transverse mode and longitudinal mode fields in a polarization independent lithium niobate optical isolator according to an embodiment of the present invention;
FIG. 3 is a schematic diagram illustrating the cross-section and transverse and longitudinal mode field distributions of a non-reciprocal waveguide in a polarization-independent lithium niobate optical isolator, according to an embodiment of the present invention;
fig. 4 is a graph schematically illustrating simulation results of forward and reverse light transmission characteristics in a polarization-independent lithium niobate optical isolator according to an embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to specific embodiments and the accompanying drawings.
The embodiment of the present disclosure provides a lithium niobate optical isolator unrelated to polarization, including first interference arm 1, second interference arm 2, first coupler 3 and second coupler 4 of integrated into one piece, first coupler 3 and second coupler 4 form respectively at the both ends of first interference arm 1, second interference arm 2, first interference arm 1, second interference arm 2 all include reciprocal waveguide, non-reciprocal waveguide along length direction at least, wherein: the length of the reciprocal waveguide of the first interference arm 1 is different from that of the reciprocal waveguide of the second interference arm 2; the nonreciprocal waveguide comprises a magneto-optical waveguide and a lithium niobate ridge waveguide, the magneto-optical waveguide is positioned above a flat waveguide of the lithium niobate ridge waveguide, and ridges of the magneto-optical waveguide and the lithium niobate ridge waveguide are formed in parallel along the width direction; the first coupler 3 and the second coupler 4 are lithium niobate ridge waveguides, and realize the isolation of reverse input light of a transverse mode and a longitudinal mode at the same time.
The polarization-independent lithium niobate optical isolator comprises a first 3dB coupler 3, a first interference arm 1, a second interference arm 2 and a second 3dB coupler 4, wherein the interference arms comprise reciprocal waveguides 11 and 21 and nonreciprocal waves 12 and 22, and the first interference arm 1 and the second interference arm 2 are isolated from each other. The device body is roughly in a spindle rod shape, the first 3dB coupler 3 and the second 3dB coupler 4 are respectively positioned at two ends of the spindle rod shape, and the first interference arm 1 and the second interference arm 2 are respectively positioned at two sides of the spindle rod shape. The device is integrally disposed on a lower cladding layer formed of silica or other material. The forward transmitted light enters the first 3dB coupler 3 from the input end and is divided into two beams of light with equal power and phase, and the two beams of light enter the first interference arm 1 and the second interference arm 2 respectively, and finally are output through the output end of the second 3dB coupler 4. The reciprocal waveguide 11 and the reciprocal waveguide 21 generate reciprocal phase shift, and the nonreciprocal waveguide 12 and the nonreciprocal waveguide 22 generate nonreciprocal phase shift. The lengths of the two waveguides are designed to ensure that the phase shift of the first interference arm 1 and the second interference arm 2 meets the conditions of constructive light interference of forward transmission and destructive light interference of reverse transmission when the phase shift is combined by the first 3dB coupler 3 or the second 3dB coupler 4, so that the isolation of reverse transmission light is realized.
The magneto-optical waveguide is arranged above the slab waveguide of the lithium niobate ridge waveguide along the light propagation direction, is arranged side by side with the ridge of the lithium niobate ridge waveguide, and forms a nonreciprocal waveguide together with the lithium niobate waveguide for generating nonreciprocal phase shift. The first coupler 3 and the second coupler 4 are lithium niobate ridge waveguides, and are suitable for splitting or combining light in both the transverse mode and the longitudinal mode.
It should be noted that, the length difference between the reciprocal waveguides in the first interference arm 1 and the second interference arm 2 may be implemented in a manner that the reciprocal waveguide 11 in the first interference arm 1 is curved outward, the reciprocal waveguide 21 in the second interference arm 2 is linear, the reciprocal waveguide is curved outward in a direction away from the second interference arm 2, and the reciprocal waveguide is curved outward without being curved inward, which is beneficial to reducing crosstalk between the two interference arms and simultaneously is convenient to load a reverse magnetic field to the two interference arms. Of course, the reciprocal waveguide in the second interference arm 2 may be curved outwardly, and the reciprocal waveguide in the first interference arm 1 may be linear.
It should be noted that, the mutual inductance waveguide lengths in the first interference arm 1 and the second interference arm 2 may be different in a manner that the first interference arm 1 and the second interference arm 2 are bent in a transverse direction perpendicular to the original waveguide direction, and then the mutual inductance waveguide is designed to have different lengths in the transverse direction and then bent back to the original waveguide direction. Thus, the reciprocal waveguides can be distributed on both sides of the non-reciprocal waveguide, and the effect produced by the device is the same as the effect produced by the reciprocal waveguides distributed on the same side of the non-reciprocal waveguide, with only slight differences in size and spatial arrangement.
Here, the non-reciprocal phase shift refers to the characteristic that light in two opposite directions in a certain object will exhibit different phase shifts, and similarly, the reciprocal phase shift refers to the characteristic that light in two opposite directions in a certain object will exhibit the same phase shift. According to the lithium niobate waveguide magneto-optical isolator, transmission light in different directions can generate irreversible nonreciprocal phase shift through the magneto-optical waveguide, reversible reciprocal phase shift can be generated through lithium niobate interference arms with different lengths, forward light is overlapped and enhanced through the reciprocal and nonreciprocal phase shift of the two lights by utilizing the interference principle of coherent light, and reverse light is counteracted and weakened.
On the basis of the above embodiment, the mode of light includes a fundamental transverse mode, a fundamental longitudinal mode, or a combination of both.
The on-chip device often has stronger polarization dependence, namely the effect of a transverse mode or a longitudinal mode can be realized usually, the optical isolator can realize the isolation effect of input light of the transverse mode and the longitudinal mode simultaneously through the design of the section size and the length of reciprocal and nonreciprocal waveguides, and the applicability and the usability of the device are improved.
On the basis of the above embodiment, the difference in length between the reciprocal waveguides in the first 1 and second 2 interference arms causes a pi/2 +2m pi phase shift in the two beams, where m is an integer.
The specific phase change process of forward and backward propagating light through the reciprocal and non-reciprocal waveguides is described in detail herein.
The forward transmitted light is equally divided into two beams of light 1, 2 with equal phase and intensity through the first 3dB coupler 3, and the two beams enter the first interference arm 1 and the second interference arm 2 respectively (assuming that the initial phases are both 0):
after passing through the unequal-length reciprocal waveguides of the interference arms, the phase of light in the first interference arm 1 becomes pi/2, and the phase of light in the second interference arm 2 is also 0;
after passing through the equal-length nonreciprocal waveguides of the interference arms, the phase of light in the first interference arm 1 becomes pi/4, and the phase of light in the second interference arm 2 becomes pi/4;
the phase difference of the forward light in the first interference arm 1 and the second interference arm 2 is 0, the condition of optical interference phase lengthening is achieved, and output is output from the second 3dB coupler 4 with high transmittance.
The backward transmitted light is equally divided into two beams of light 1, 2 with equal phase and intensity through the second 3dB coupler 4, and the two beams enter the first interference arm 1 and the second interference arm 2 respectively (assuming that the initial phases are both 0):
after the equal-length nonreciprocal waveguides of the interference arms, the phase of light in the first interference arm 1 is changed into pi/4, and the phase of light in the second interference arm 2 is changed into-pi/4;
after passing through the non-equal length reciprocal waveguides of the interference arms, the phase of the light in the first interference arm 1 becomes 3 pi/4, and the phase of the light in the second interference arm 2 becomes-pi/4;
the phase difference of the reverse light in the first interference arm 1 and the second interference arm 2 is pi, so that the condition of destructive light interference is achieved, and the output is output from the first 3dB coupler 3 with low transmittance.
Of course, it is not limited herein that the forward propagating light and the backward propagating light can only generate pi/2 phase shift after passing through the reciprocal waveguide, and pi/2 +2m pi phase shift can be applied, where m is an integer. The forward transmitted light generates different phase shifts through the reciprocal waveguide and generates opposite phase shifts through the non-reciprocal waveguide; the backward transmitted light generates opposite phase shift through the nonreciprocal waveguide, different phase shifts are generated through the reciprocal waveguide, and the forward transmitted light is overlapped and enhanced, the backward transmitted light is mutually offset and weakened through the design of the phase shift of the two sections of waveguides, so that the optical isolation effect is achieved.
On the basis of the above embodiment, the phase shift generated after the forward transmitted light passes through the nonreciprocal waveguide is-pi/2 +2n pi, and the phase shift generated after the backward transmitted light passes through the nonreciprocal waveguide is pi/2 +2n pi, wherein n is an integer.
Here, the phase shift generated after the forward propagating light passes through the non-reciprocal waveguide 12 of the first interference arm 1 and the phase difference generated after the forward propagating light passes through the non-reciprocal waveguide 22 of the second interference arm 2 are-pi/2 +2n pi, for example, in the above-mentioned embodiment regarding the specific phase change process, the non-reciprocal waveguide 12 of the first interference arm 1 obtains-pi/4, and the non-reciprocal waveguide 22 of the second interference arm 2 obtains pi/4; here, the phase shift is a phase difference, that is, a phase shift of the first nonreciprocal waveguide minus the second nonreciprocal waveguide is-pi/2 +2n pi (n is 0).
The phase shift generated by the backward propagating light after passing through the non-reciprocal waveguide 12 of the first interference arm 1 and the phase difference generated by the forward propagating light after passing through the non-reciprocal waveguide 22 of the second interference arm 2 are pi/2 +2n pi, for example, in the embodiment regarding the specific phase change process, the non-reciprocal waveguide 12 of the first interference arm 1 obtains pi/4 and the non-reciprocal waveguide 22 of the second interference arm 2 obtains-pi/4, that is, the phase shift of the first non-reciprocal waveguide minus the second non-reciprocal waveguide is pi/2 +2n pi (n is 0).
On the basis of the above embodiments, the magneto-optical material of the magneto-optical waveguide is a magneto-optically active material with a large faraday coefficient, including doped rare-earth iron garnet.
The Faraday coefficient represents the optical rotation performance of the magneto-optical material, and the larger the Faraday coefficient is, the optical rotation performance is represented, and the better the nonreciprocal property is. The doped rare earth iron garnet material has the characteristics of low dielectric loss, high density and high Faraday coefficient.
On the basis of the above embodiment, the magneto-optical material is cerium-doped yttrium iron garnet.
The lithium niobate material has very similar refractive index with the Ce-YIG material, the mode fields of light transmission in the two waveguides are almost the same under the same size, the mode fields can be butted without loss, and the coupling efficiency is high.
On the basis of the above embodiment, the magnetic field on the non-reciprocal waveguide is perpendicular to the direction of light propagation and is tilted by 45 °, with the magnitude of the magnetic field being not less than the saturation magnetization of the magneto-optical material.
The magnetic fields in the first interference arm 1 and the second interference arm 2 are perpendicular to the light propagation direction, the first nonreciprocal waveguide 12 in the first interference arm 1 is inclined 45 ° to the upper right, the second nonreciprocal waveguide 22 in the second interference arm 2 is inclined 45 ° to the lower left, of course, the second nonreciprocal waveguide 22 may be inclined 45 ° to the upper right, the first nonreciprocal waveguide 12 may be inclined 45 ° to the lower left, and in short, the inclination directions of the two are opposite. The inclination angle of 45 degrees is to ensure that the input light of the transverse mode can obtain nonreciprocal displacement, and the input light of the longitudinal mode can obtain nonreciprocal phase shift, so that the characteristic that the polarization of the device is irrelevant is finally realized.
On the basis of the above embodiment, the lower cladding of the optical isolator is made of silicon oxide material, the substrate is made of lithium niobate material, and the upper cladding is made of air, silicon oxide material or other semiconductor material with refractive index smaller than that of lithium niobate.
The device can be directly filled with air, and can also be covered with a layer of semiconductor material with the refractive index smaller than that of lithium niobate, so that the device is protected, leakage of partial power caused by direct scattering of light into the air is prevented, and the transmission efficiency is improved.
On the basis of the above embodiment, the first coupler 3 splits the forward transmitted light into two identical beams, and the second coupler 4 combines the two beams into one beam and outputs the beam through the output end.
The lithium niobate material has a high electro-optic coefficient, can realize low driving voltage and large-range tuning under a short waveguide length, and has good tuning performance. The first 3dB coupler 3 divides the forward transmitted light into two same beams of light, and the second 3dB coupler 4 combines the two beams of light into one beam of light and outputs the light through the output end of the light; the second 3dB coupler 4 divides the reversely transmitted light into two same beams, and the first 3dB coupler 3 combines the two beams into one beam and outputs the beam through the output end of the beam.
On the basis of the above embodiments, the first coupler 3 and the second coupler 4 include a 1 × 2 directional coupler and a 1 × 2 multimode interference coupler.
The coupler plays the roles of beam combination and beam splitting simultaneously, the 1 x 2 directional coupler has extremely low insertion loss, but has higher sensitivity to wavelength and polarization, and is commonly used for having higher requirements on low loss of devices, determining the polarization state of transmitted light and stabilizing the wavelength transmission; the performance of the 1 multiplied by 2 multimode interference coupler is slightly influenced by structural parameters, the process tolerance is large, the preparation is easy, meanwhile, the polarization correlation is small, the polarization insensitive optical power splitter can be realized through proper design, and the loss is slightly high, so that the 1 multiplied by 2 multimode interference coupler is usually used for the situation of multi-polarization common transmission or polarization uncertainty.
The present invention is further illustrated by the following specific embodiments.
Fig. 1 is a schematic structural diagram of an embodiment, in which a lower cladding of a device is a silicon oxide material, a substrate is a lithium niobate material, and an upper cladding is air. The first 3dB coupler 3 and the second 3dB coupler 4 both adopt a 1 multiplied by 2 multimode interference coupler structure, and the length is 30 um. Magneto-optical material Ce: the Faraday rotation coefficient of YIG is-4500 degree/cm, and magnetic fields perpendicular to the propagation direction, 45 degrees outside the oblique sheet and 45 degrees inside the oblique sheet, which are antiparallel and have the size of 1400Gs, are respectively loaded on the non-reciprocal waveguides in the first interference arm 1 and the second interference arm 2, so that the two interference arms can generate different non-reciprocal phase shifts on the transverse mode and the longitudinal mode.
The example operates with a center wavelength of 1550nm and modes of transverse and longitudinal modes.
The cross-sectional structure of the reciprocal waveguide and the mode field distribution of the transverse mode and the longitudinal mode thereof are shown in fig. 2, the reciprocal waveguide is a ridge-shaped waveguide made of pure lithium niobate material, the total height is 600nm, the height of the flat plate is 300nm, the height of the ridge is 300nm, and the width is 800 nm.
The nonreciprocal waveguide interface structure and the mode field distribution of the transverse mode and the longitudinal mode in the nonreciprocal waveguide interface structure are shown in fig. 3, and the nonreciprocal waveguide interface structure is formed by a lithium niobate ridge waveguide and a magneto-optical material Ce: YIG strip waveguides. Ce: YIG had a Faraday rotation coefficient of 4500 DEG/cm and a length of 1785 μm. The total height of the lithium niobate ridge waveguide is 600nm, the height of the flat plate is 300nm, the width is 400nm, and the ratio of Ce: the YIG waveguide is arranged above a flat plate of the lithium niobate ridge waveguide and beside the ridge waveguide in parallel, and has the width of 400nm and the height of 300 nm.
In fig. 2 and 3, the mode field distributions of the same light on the cross sections of the reciprocal waveguide and the non-reciprocal waveguide are very similar, and the mode overlap integral is more than 99%, so that the waveguide coupling efficiency is high, and the nearly lossless butt coupling between the waveguides is realized.
The length difference of the reciprocal waveguides in the first interference arm 1 and the second interference arm 2 is selected to be 212nm, and reciprocal phase shift of 0.5 pi is generated. Fig. 4 shows transmission characteristics of forward and reverse light transmission in this embodiment. It can be seen that > 30dB isolation can be achieved at a center wavelength of the device of 6nm around 1550 nm.
The above-mentioned embodiments are intended to illustrate the objects, technical solutions and advantages of the present invention in further detail, and it should be understood that the above-mentioned embodiments are only exemplary embodiments of the present invention, and are not intended to limit the present invention, and any modifications, equivalents, improvements and the like made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (10)
1. The utility model provides a lithium niobate optical isolator irrelevant with polarization, includes integrated into one piece's first interference arm (1), second interference arm (2), first coupler (3) and second coupler (4) form respectively the both ends of first interference arm (1), second interference arm (2), first interference arm (1), second interference arm (2) all include reciprocal waveguide, nonreciprocal waveguide along length direction at least, wherein:
the reciprocal waveguide of the first interference arm (1) and the reciprocal waveguide of the second interference arm (2) are different in length;
the non-reciprocal waveguide comprises a magneto-optical waveguide and a lithium niobate ridge waveguide, the magneto-optical waveguide is positioned above a slab waveguide of the lithium niobate ridge waveguide, and ridges of the magneto-optical waveguide and the lithium niobate ridge waveguide are formed in parallel along the width direction;
the first coupler (3) and the second coupler (4) are lithium niobate ridge waveguides, and isolation of reverse input light of a transverse mode and a longitudinal mode is achieved simultaneously.
2. The polarization-independent lithium niobate optical isolator of claim 1, wherein the mode of light comprises a fundamental transverse mode, a fundamental longitudinal mode, or a combination thereof.
3. The polarization-independent lithium niobate optical isolator according to claim 1, wherein a difference in length of the reciprocal waveguides in the first interference arm (1) and the second interference arm (2) causes a pi/2 +2m pi phase shift in the two beams, where m is an integer.
4. The polarization independent lithium niobate optical isolator of claim 3, wherein the phase shift imparted by the forward propagating light after passing through the non-reciprocal waveguide is-pi/2 +2n pi, and the phase shift imparted by the backward propagating light after passing through the non-reciprocal waveguide is pi/2 +2n pi, where n is an integer.
5. The polarization-independent lithium niobate optical isolator of claim 1, wherein the magneto-optical material of the magneto-optical waveguide is a magneto-optically active material with a large faraday coefficient, comprising a doped rare earth iron garnet.
6. The polarization-independent lithium niobate optical isolator of claim 5, wherein the magneto-optical material is cerium-doped yttrium iron garnet.
7. The polarization independent lithium niobate optical isolator of claim 6, wherein a magnetic field on the non-reciprocal waveguide is perpendicular to a direction of light propagation and tilted by 45 °, the magnitude of the magnetic field being not less than a saturation magnetization of the magneto-optical material.
8. The polarization independent lithium niobate optical isolator of claim 1, wherein the lower cladding of the optical isolator is a silicon oxide material, the substrate is a lithium niobate material, and the upper cladding is air, a silicon oxide material or other semiconductor material having a refractive index less than that of lithium niobate.
9. The polarization-independent lithium niobate optical isolator according to claim 1, wherein the first coupler (3) splits the forward transmitted light into two identical beams, and the second coupler (4) combines the two beams into one beam and outputs the beam through its output.
10. The polarization-independent lithium niobate optical isolator according to claim 9, wherein the first coupler (3) and the second coupler (4) comprise a 1 x 2 directional coupler, a 1 x 2 multimode interference coupler.
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| US20220382084A1 (en) * | 2021-05-27 | 2022-12-01 | Kyocera Corporation | Nonreciprocal waveguide, isolator, optical switch, optical transceiver, data center, and manufacturing method |
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