Patent ID: 12199672

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

The present disclosure is more particularly described in the following examples that are intended as illustrative only since numerous modifications and variations therein will be apparent to those skilled in the art. Like numbers in the drawings indicate like components throughout the views. As used in the description herein and throughout the claims that follow, unless the context clearly dictates otherwise, the meaning of “a”, “an”, and “the” includes plural reference, and the meaning of “in” includes “in” and “on”. Titles or subtitles can be used herein for the convenience of a reader, which shall have no influence on the scope of the present disclosure.

The terms used herein generally have their ordinary meanings in the art. In the case of conflict, the present document, including any definitions given herein, will prevail. The same thing can be expressed in more than one way. Alternative language and synonyms can be used for any term(s) discussed herein, and no special significance is to be placed upon whether a term is elaborated or discussed herein. A recital of one or more synonyms does not exclude the use of other synonyms. The use of examples anywhere in this specification including examples of any terms is illustrative only, and in no way limits the scope and meaning of the present disclosure or of any exemplified term. Likewise, the present disclosure is not limited to various embodiments given herein. Numbering terms such as “first”, “second” or “third” can be used to describe various components, signals or the like, which are for distinguishing one component/signal from another one only, and are not intended to, nor should be construed to impose any substantive limitations on the components, signals or the like.

In the drawings of the present disclosure, the sizes of certain structures or portions may be enlarged relative to other structures or portions for illustrative purposes, and thus are merely used for illustration of the basic structure of the subject matter of the present disclosure.

In addition, spatially relative terms (such as “over,” “above,” “under,” and “below”) in the present disclosure are used to conveniently describe a spatial relationship between one element/feature and another element/feature as shown in the drawings. These spatially relative terms are intended to include different orientations of a device in use or in operation other than the orientations illustrated in the drawings. For example, when the device in the drawing is turned over, elements described as below and/or under other elements or features would then be oriented above the other elements or features. Therefore, the exemplary term “below” encompasses both orientations of “above” and “below”. The device may also be otherwise oriented (rotated 90 degrees or at other orientations), and the spatially relative terms used herein is to be interpreted accordingly. When one element/layer is referred to as being disposed “above” or “connected to” another element/layer, said element/layer can be directly disposed above or connected to another element/layer, or a middle element/layer can be provided therebetween.

As shown inFIG.1, the present disclosure provides a wavelength division multiplexer/demultiplexer based on asymmetric Bragg gratings, which includes a substrate, a bus waveguide10provided on the substrate, and at least two wavelength division multiplexing/demultiplexing units20provided on the bus waveguide10. Reference is made toFIG.2, which is a schematic structural diagram of one of the wavelength division multiplexing/demultiplexing units20. In this diagram, structures such as the substrate and covering layers are omitted, and only the bus waveguide10and structures of other components are shown.FIG.3is a sectional schematic diagram of A-A inFIG.2. Here, the wavelength division multiplexer/demultiplexer is provided on a planar waveguide of a silicon-on-insulator (SOI) structure, and includes a substrate101, a buried oxide layer102, the bus waveguide10, and a cover layer103. The substrate101is a silicon substrate, the buried oxide layer102is silicon dioxide, the bus waveguide10is a strip waveguide made of a silicon material, and the cover layer103is silicon dioxide. In other embodiments, the bus waveguide10can be a ridge waveguide or a waveguide of other materials, such as silicon nitride (SiN), silicon oxynitride (SiON), silicon oxide (SiO2), indium phosphide (InP), gallium nitride (GaN), or lithium niobate (LiNbO3). Alternatively, the wavelength division multiplexer/demultiplexer can be provided on a planar waveguide based on other structures, and the substrate can be a substrate made of other materials, such as a lithium niobate substrate, an indium phosphide substrate, or a gallium nitride substrate.

The wavelength division multiplexing/demultiplexing unit20includes a mode multiplexer21and an asymmetric Bragg grating22. The mode multiplexer21is configured to make an input light incident into the asymmetric Bragg grating22in a TE1 mode or a higher-order mode. The mode multiplexer21includes a first port211, a second port212and a third port213. The first port211and the second port212are respectively configured to input or output optical signals, and the third port213is connected to the asymmetric Bragg grating22. The asymmetric Bragg gratings22in different units of the at least two wavelength division multiplexing/demultiplexing units20have different grating periods, and a light that satisfies a resonance condition with the grating period of the asymmetric Bragg grating22has a resonant wavelength λi. The resonance condition is

2⁢π/Λ=(2⁢πλ⁢i)⁢n_TE0+(2⁢πλ⁢i)⁢n_TE1,
in which Λ is a period of a corresponding asymmetric Bragg grating, λi is a wavelength of a transmitted light, and nTE0and nTE1are respectively the effective refractive indices of a light when being transmitted in the corresponding asymmetric Bragg grating in the TE0 mode and in the TE1 mode.

In the embodiment, as shown inFIG.2, the mode multiplexer21includes a straight-through waveguide214and a cross waveguide215. Two ends of the straight-through waveguide214are respectively the first port211and the third port213, and an end of the cross waveguide215that is adjacent to the first port end211is the second port212. The cross waveguide215and the through waveguide214perform mode coupling. In other embodiments, the mode multiplexer may also adopt other mode multiplexing structures. The asymmetric Bragg grating22has a grating structure that is gradually asymmetrical, an etching depth h of the grating is gradually increased from both ends of the grating toward the middle, and a waveguide width d in the center of the grating gradually narrows from both ends toward the middle, so as to form a certain taper.

When the wavelength division multiplexer/demultiplexer is employed as a demultiplexer, a light containing multiple wavelengths input from one of the first port211and the second port212of the mode multiplexer21is incident into the asymmetric Bragg grating22through a light in a TE(n+1) mode output from the third port213by the mode multiplexer21. The asymmetric Bragg grating22reflects a light with a resonant wavelength λi, its grating period satisfies a resonance condition with the resonant wavelength λi, and converts the reflected light with the resonant wavelength λi into the TE0 mode and transmits the reflected light to the mode multiplexer21, and the reflected light is output from another of the first port211and the second port212. The asymmetric Bragg grating22transmits lights containing wavelengths corresponding to other communication channels (but not the above-mentioned light with the resonant wavelength λi). Here, another one of the first port211and the second port212corresponds to the above-mentioned one of the first port211and the second port212which is configured to input the optical signals. In the TE0 and TE(n+1) modes, n is an integer greater than or equal to zero. For the convenience of description, the explanation below takes n=0 as an example. That is, the transmission in the asymmetric Bragg grating is exemplified as being the TE1 mode. In other embodiments, the transmission in the asymmetric Bragg grating can be carried out in a mode higher than the TE1 mode (e.g., TE2).

Wavelength division multiplexing/demultiplexing of four wavelengths that range from 1261 nm to 1341 nm as required by CWDM4 (which is commonly used in light communications) is taken as an example. Assuming that the four wavelengths are λ1, λ2, λ3, and λ4, at least three wavelength division multiplexing/demultiplexing units are needed for cascading.FIG.4is a schematic structural diagram of the wavelength division multiplexer/demultiplexer according to an embodiment of the present disclosure. The wavelength division multiplexer/demultiplexer includes a first wavelength division multiplexing/demultiplexing unit20a, a second wavelength division multiplexing/demultiplexing unit20b, a third wavelength division multiplexing/demultiplexing unit20c, and a fourth wavelength division multiplexing/demultiplexing unit20dthat are cascaded. The asymmetric Bragg gratings corresponding to the four wavelength division multiplexing/demultiplexing units are a first asymmetric Bragg grating22a, a second asymmetric Bragg grating22b, a third asymmetric Bragg grating22c, and a fourth asymmetric Bragg grating22d, respectively. The demultiplexer is taken as an example. The first port211of the mode multiplexer21is an input port Input, the second port212is an output port Drop1, and the third port213is connected to an incident end of the asymmetric Bragg grating22a. A transmission end of the asymmetric Bragg grating22ais connected to the first port211of the mode multiplexer21at a next stage. Here, the mode multiplexer21is designed in such a manner that the light in the TE0 mode that is input to the mode multiplexer from the first port211will be directly transmitted through the straight-through waveguide and output from the third port213, and the light in the TE1 mode that is input to the mode multiplexer from the third port213will be mode-coupled to the cross waveguide and output from the second port212in the TE0 mode.

For the wavelength division multiplexer/demultiplexer shown inFIG.4, a demultiplexing process is as follows. A composite light containing the four wavelengths λ1, λ2, λ3 and λ4 in the TE0 mode is input to the mode multiplexer21of the first wavelength division multiplexing/demultiplexing unit20afrom the first port211, is transmitted through the straight-through waveguide of the mode multiplexer21and output from the third port213, and is incident into the first asymmetric Bragg grating22aof the first wavelength division multiplexing/demultiplexing unit20a. The first asymmetric Bragg grating22ais designed to transmit the lights containing the wavelengths λ2, Δ3, λ4, and to reflect the light with the wavelength λ1 and perform a mode conversion to convert the mode of the light reflected. The light with the wavelength λ1 is reflected by the first asymmetric Bragg grating22aand converted into the TE1 mode, is transmitted to the third port213of the mode multiplexer21, and then is converted into the TE0 mode and output from the second port212after being mode coupled from the straight-through waveguide to the cross waveguide in the mode multiplexer21. The lights containing the wavelengths λ2, λ3, λ4 are transmitted through the first asymmetric Bragg grating22a, and then are transmitted to the wavelength division multiplexing/demultiplexing unit at a next stage (i.e., the second wavelength division multiplexing/demultiplexing unit20b) along the bus waveguide10in the TE0 mode. That is, the lights containing the wavelengths λ2, λ3, λ4 are input to the first port211of the mode multiplexer21of the second wavelength division multiplexing/demultiplexing unit20b. The second asymmetric Bragg grating22bof the second wavelength division multiplexing/demultiplexing unit20bis designed to transmit the lights containing the wavelengths λ3, λ4, and to reflect the light with the wavelength λ2 and perform the mode conversion to convert the mode of the light reflected. The demultiplexing process of the second wavelength division multiplexing/demultiplexing unit20bis the same as that of a previous stage (i.e., the first wavelength division multiplexing/demultiplexing unit20a). The light with the wavelength λ2 in the TE0 mode is output from the second port212of the mode multiplexer21of the second wavelength division multiplexing/demultiplexing unit20b, and the lights containing the wavelengths λ3, λ4 continue to be transmitted to the wavelength division multiplexing/demultiplexing unit at a next stage (i.e., the third wavelength division multiplexing/demultiplexing unit20c) along the bus waveguide10in the TE0 mode. At this stage, the third asymmetric Bragg grating22cis designed to transmit the light with the wavelength λ4, and to reflect the light with the wavelength λ3 and perform the mode conversion to convert the mode of the light reflected. Afterwards, the light with the wavelength λ3 in the TE0 mode is output from the second port212of the mode multiplexer21of the third wavelength division multiplexing/demultiplexing unit20c, and then the light with the wavelength λ4 is transmitted through the third asymmetric Bragg grating and output in the TE0 mode. The light with the wavelength λ4 in the TE0 mode transmitted through the third asymmetric Bragg grating22ccontinues to be transmitted to the fourth wavelength division multiplexing/demultiplexing unit20dalong the bus waveguide10. The fourth asymmetric Bragg grating22dof the fourth wavelength division multiplexing/demultiplexing unit20dis designed to reflect the light with the wavelength λ4 and perform the mode conversion to convert the mode of the light reflected. Finally, the light with the wavelength λ4 in the TE0 mode is output from the second port212of the mode multiplexer21of the fourth wavelength division multiplexing/demultiplexing unit20d, and demultiplexing of λ1, λ2, λ3, λ4 is completed. When the wavelength division multiplexer/demultiplexer is employed as a multiplexer, a wavelength division multiplexing process is opposite to the above-mentioned demultiplexing process, and the details thereof will not be described herein. The fourth wavelength division multiplexing/demultiplexing unit20dcan filter the light containing other wavelength (e.g., λ1, λ2, λ3) that may be transmitted through previous stages and to the fourth stage. Through filtering, an extinction ratio can be increased, and crosstalk of other wavelengths can be prevented from being introduced into a light channel of the wavelength λ4.

As shown inFIG.2, the asymmetric Bragg grating22employed in the present disclosure is a Bragg grating that is gradually asymmetrical. Compared with a Bragg grating that is completely asymmetrical, this grating can effectively suppress occurrences of side lobes and has a high extinction ratio. The asymmetric Bragg grating22is designed to simultaneously realize the functions of filtering and mode conversion. When the grating period satisfies the Bragg condition

2⁢π/Λ=(2⁢πλ)⁢n_TE0+(2⁢πλ)⁢n_TE,
the light with a wavelength λ satisfies a resonance relationship with the grating period. When the light with the wavelength λ is incident into the asymmetric Bragg grating in the TE0 mode, the asymmetric Bragg grating22can convert the light into the TE1 mode and reflect the light back. In the above formula, A is the grating period of the asymmetric Bragg grating, λ is the wavelength of a transmitted light, and nTE0 and nTE1 are respectively effective refractive indices of the light transmitted in the TE0 mode and in the TE1 mode in the asymmetric Bragg grating22. The above formula can be simplified into Δ/2Λ=(n_TE0+n_TE1)/2. That is, for one specific wavelength λ, when the grating period A is reasonably selected such that a value of λ/2Λ is equal to a mean value of the effective refractive indices of the TE0 mode and the TE1 mode, the conversion between the two modes can be realized. Referring toFIG.5, relations of parameters respectively vary with the wavelength λ in a typical silicon-on-insulator (SOI) structure are illustrated, and the parameters are the effective refractive indices of the TE0 mode and the TE1 mode in the strip waveguide of the silicon material having a width of 550 nm and a height of 220 nm, the mean value of the effective refractive indices of the TE0 mode and the TE1 mode, and the value of λ/2Λ that is under one specific grating period. In the range of the CWDM4 (1261 nm to 1341 nm), a curve of the λ/2Λ and a curve of the mean value of the effective refractive indices of the TE0 mode and the TE1 mode are intersected at a point A. That is, the above equation is satisfied at the point A. The asymmetric Bragg grating of said grating period will convert the light with the wavelength corresponding to the point A between the TE0 mode and the TE1 mode, and reflect the light. In addition, the curve of λ/2Λ and a curve of the refractive index of the TE0 mode are also intersected at a point B around 1340 nm. That is, the equation λ/2Λ=n_TE0 is satisfied at the point B. When the light transmitted in the waveguide is in the TE0 mode, reflection will also occur at the wavelength corresponding to the point B. In order to prevent the crosstalk caused by the asymmetric Bragg grating reflecting a TE0 mode light with the wavelength that corresponds to the point B, it needs a large difference between the effective refractive indexes of the TE0 mode and the TE1 mode of the waveguide. In this way, the spacing between the wavelengths respectively corresponding to the point A and the point B is large enough to meet requirements of a wavelength range (>80 nm) of the CWDM4.

FIG.5is a curve diagram from the simulation of characteristics of the strip waveguide of the silicon material in the typical silicon-on-insulator (SOI) structure which is a reasonable design. The point B is outside the wavelength range of from 1261 nm to 1341 nm as required by the CWDM4, and crosstalk of the wavelength corresponding to the point B will not be introduced. However, in a ridge waveguide structure of silicon material of a silicon-on-insulator, or in waveguide structures of other materials (such as SiN, SiON, SiO2, or lithium niobate) that have a small difference between the effective refractive indexes of the TE0 mode and the TE1 mode, wavelength spacing between the point A and the point B (<80 nm) is smaller than the wavelength spacing as required by the CWDM4, which will cause severe wavelength crosstalk and cannot meet the requirements of the wavelength division multiplexer. Therefore, the wavelength division multiplexer/demultiplexer shown inFIG.4is only suitable for the strip waveguide of silicon material of the silicon-on-insulator. In the strip waveguide of the silicon material, the difference between the effective refractive indexes of the TE0 mode and the TE1 mode is so large as to allow the wavelength spacing between the wavelengths respectively corresponding to the point A and the point B shown inFIG.5to meet the wavelength range (>80 nm) as required by the CWDM4.

FIG.6illustrates another embodiment of the present disclosure. In the wavelength division multiplexer/demultiplexer of the present embodiment, the light is incident into the asymmetric Bragg grating in the TE1 mode, and the light transmitted in the asymmetric Bragg grating and the bus waveguide is mainly in the TE1 mode. As a result, a reflection peak of the TE0 mode at the point B illustrated inFIG.5will not appear. In addition, although there will appear a reflection peak of the TE1 mode at the wavelength corresponding to the intersection point of the curve of λ/2Λ and the curve of the refractive index of the TE1 mode, the wavelength corresponding to the reflection peak of the TE1 mode is outside the wavelength range of the CWDM4 and is at shorter wavelengths, so that the wavelength division multiplexing/demultiplexing within the wavelength range as required by the CWDM4 is not affected. Moreover, modes that may be converted from the TE1 mode (such as TE2/TE3) also appear at shorter wavelengths, similarly, that will not affect the use of the wavelength division multiplexer/demultiplexer.

Specifically, as shown inFIG.6, wavelength division multiplexing/demultiplexing of the four wavelengths in that range from 1261 nm to 1341 nm as required by the CWDM4 (which is commonly used in the light communications) is also taken as an example in the present embodiment. Assuming that the four wavelengths are λ1, λ2, λ3, λ4. The wavelength division multiplexer/demultiplexer includes three wavelength division multiplexing/demultiplexing units30a,30b,30cthat are sequentially cascaded. Each of the wavelength division multiplexing/demultiplexing units30a,30b,30cincludes the mode multiplexer31and the corresponding one of the asymmetric Bragg gratings32a,32b,32c. The three cascaded wavelength division multiplexing/demultiplexing units are respectively the first wavelength division multiplexing/demultiplexing unit30a, the second wavelength division multiplexing/demultiplexing unit30b, and the third wavelength division multiplexing/demultiplexing unit30c. The three wavelength division multiplexing/demultiplexing units respectively correspond to the first asymmetric Bragg grating32a, the second asymmetric Bragg grating32b, and the third asymmetric Bragg grating32c. The asymmetric Bragg gratings of the three wavelength division multiplexing/demultiplexing units have different grating periods, and are used to respectively reflect the lights with the wavelengths λ1, λ2, λ3 but transmit the light with the wavelength λ4. The mode multiplexer31includes the first port311, the second port312, and the third port313. The first port311and the second port312are respectively configured to input or output the optical signals, and the third port313is connected to the asymmetric Bragg gratings32a,32b,32c. The demultiplexer is taken as an example. The first port311of the mode multiplexer31is used as the input port, the second port312is used as the output port, and the third port313is connected to the incident ends of the asymmetric Bragg gratings32a,32b,32c. The transmission ends of the symmetric Bragg gratings32a,32b,32care connected to the first port311of the mode multiplexer31at a next stage. Here, the mode multiplexer31is designed in such a manner that the light in the TE1 mode that is input to the first port311will be directly transmitted through the straight-through waveguide and output from the third port313, the light in the TE0 mode that is input to the third port313will be mode-coupled to the cross waveguide, and output from the second port312in the TE1 mode.

The demultiplexing process of the wavelength division multiplexer/demultiplexer shown inFIG.6is as follows. A composite light containing the four wavelengths λ1, λ2, λ3, λ4 in the TE1 mode is input to the first port311of the mode multiplexer31of the first wavelength division multiplexing/demultiplexing unit30a, is transmitted through the straight-through waveguide of the mode multiplexer31and output from the third port313, and is incident into the first asymmetric Bragg grating32aof the first wavelength division multiplexing/demultiplexing unit30ain the TE1 mode. The first asymmetric Bragg grating32ais designed to transmit the light containing the wavelengths λ2, λ3, λ4, and to reflect the light with the wavelength λ1 and perform the mode conversion to convert the mode of the light reflected. The light with the wavelength λ1 is reflected by the first asymmetric Bragg grating32aand converted into the TE0 mode, is transmitted to the third port313of the mode multiplexer31, and then is converted into the TE1 mode and output from the second port312after being mode coupled from the straight-through waveguide to the cross waveguide in the mode multiplexer31. After being transmitted through the first asymmetric Bragg grating32a, the light containing the wavelengths λ2, λ3, λ4 continues to be transmitted to the second wavelength division multiplexing/demultiplexing unit30bat a next stage along the bus waveguide10in the TE1 mode, and is input to the first port311of the mode multiplexer31of the second multiplexing/demultiplexing unit30b. The second asymmetric Bragg grating32bof the second wavelength division multiplexing/demultiplexing unit30bis designed to transmit the light containing the wavelengths λ3, λ4, and to reflect the light with the wavelength λ2 and perform the mode conversion to convert the mode of the light reflected. The demultiplexing process of the second wavelength division multiplexing/demultiplexing unit30bis the same as that of a previous stage. The light with the wavelength λ2 will be output from the second port312of the mode multiplexer31of the second wavelength division multiplexing/demultiplexing unit30bin the TE1 mode, and the light containing the wavelengths λ3, λ4 will continue to be transmitted to the third wavelength division multiplexing/demultiplexing unit30cat a next stage along the bus waveguide10in the TE1 mode. At this stage, the third asymmetric Bragg grating32cis designed to transmit the light with the wavelength λ4, and to reflect the light with the wavelength λ3 and perform the mode conversion to convert the mode of the light reflected. Finally, the light with the wavelength λ3 in the TE1 mode is output from the second port312of the mode multiplexer31of the third wavelength division multiplexing/demultiplexing unit30c, and the light with the wavelength λ4 in the TE1 mode is transmitted through the third asymmetric Bragg grating32cand output. In this way, demultiplexing of λ1, λ2, λ3, λ4 is completed. When the wavelength division multiplexer/demultiplexer is employed as a multiplexer, the wavelength division multiplexing process is opposite to the above-mentioned demultiplexing process, and the details thereof will not be described herein.

In the present embodiment, the light incident into the asymmetric Bragg grating is in the TE1 mode, so that the light transmitted in the asymmetric Bragg grating is in the TE1 mode. Accordingly, the crosstalk caused by reflection of the asymmetric Bragg grating for the TE0 mode can be prevented. In addition, the crosstalk caused by reflection peaks of the TE1 mode or higher-order modes that may exist at short wavelengths can be effectively prevented by performing reflecting and filtering light with short wavelengths first and sequentially increasing the filtered wavelengths in a cascade manner. For example, in the demultiplexing process, the order of the four wavelengths is λ1<λ2<λ3<λ4, so that the crosstalk caused by the reflection peaks of the TE1 mode or a higher-order mode at short wavelengths can be prevented. Moreover, there is no need to additionally dispose filter devices on the bus waveguides between the wavelength division multiplexing/demultiplexing units at each stage, which allows the manufacturing process to be simplified.

The wavelength division multiplexer/demultiplexer shown inFIG.7, further improved on the basis of the embodiment shown inFIG.6, a fourth wavelength division multiplexing/demultiplexing unit30dis cascaded after the third wavelength division multiplexing/demultiplexing unit30c. A fourth asymmetric Bragg grating32dof the fourth wavelength division multiplexing/demultiplexing unit30dis designed to reflect the light with the wavelength λ4 and perform the mode conversion to convert the mode of the light reflected. Finally, the light with the wavelength λ4 in the TE1 mode is output from the second port of the mode multiplexer31of the fourth multiplexing/demultiplexing unit30d. The fourth wavelength division multiplexing/demultiplexing unit30dis used to filter the light containing other wavelength (e.g., those having the wavelengths λ1, λ2, λ3) that may be transmitted through previous stages and to the fourth stage. Through filtering, the extinction ratio can be increased, and the crosstalk of other wavelengths can be prevented from being introduced into the light channel of the wavelength λ4.

The wavelength division multiplexer/demultiplexer shown inFIG.8, further improved on the basis of the embodiment shown inFIG.6orFIG.7, a first mode converter33is provided to the output port of each wavelength division multiplexing/demultiplexing unit. That is, the wavelength division multiplexing/demultiplexing units30a,30b,30c,30dfurther include the first mode converter33. The first mode converter33is connected to the second port312of the mode multiplexer31. During the wavelength division multiplexing process, the first mode converters33of the wavelength division multiplexing/demultiplexing units30a,30b,30c,30dat each stage are respectively used to convert the light in the TE1 mode that is output from the second port312of the corresponding mode multiplexer31into the TE0 mode. In the embodiment, the structure of the first mode converter33is the same as that of the mode multiplexer31, and includes a straight-through waveguide and a cross waveguide. Two ends of the straight-through waveguide are respectively the first port and the third port, and one end of the cross waveguide that is adjacent to the first port is the second port. But, the first mode converter33only makes use of the second port and the third port to perform the mode conversion. In other embodiments, the first mode converter33may also adopt other mode conversion structures.

The embodiment shown inFIG.9, the same as the embodiment shown inFIG.6, both are the wavelength division multiplexer/demultiplexer used for four wavelengths. The wavelength division multiplexer/demultiplexer includes three wavelength division multiplexing/demultiplexing units that are sequentially cascaded. Wherein, each of the wavelength division multiplexing/demultiplexing units40a,40b,40cincludes a mode multiplexer41and the corresponding one of the asymmetric Bragg gratings42a,42b,42c. The three cascaded wavelength division multiplexing/demultiplexing units are respectively the first wavelength division multiplexing/demultiplexing unit40a, the second wavelength division multiplexing/demultiplexing unit40b, and the third wavelength division multiplexing/demultiplexing unit40c. The three wavelength division multiplexing/demultiplexing units respectively correspond to the first asymmetric Bragg grating42a, the second asymmetric Bragg grating42b, and the third asymmetric Bragg grating42c. The asymmetric Bragg gratings of the three wavelength division multiplexing/demultiplexing units have different grating periods, and are used to respectively reflect the light with the wavelengths λ1, λ2, λ3 but transmit the light with the wavelength λ4. The difference between the present embodiment and the embodiment shown inFIG.6is as follows. In the present embodiment, the mode multiplexer41is designed in such a manner that the light input to the second port412is in the TE0 mode and converted into the TE1 mode through being mode-coupled from the cross waveguide to the straight-through waveguide and output from the third port413. Then, the light in the TE0 mode that is input to the third port413will be directly transmitted through the straight-through waveguide and output from the first port411, and the light in the TE1 mode that is input to the third port413will be converted into TE0 mode through being mode-coupled from the straight-through waveguide to the cross waveguide and output from the second port412. Moreover, there is a second mode converter50provided between every two adjacent ones of the wavelength division multiplexing/demultiplexing units40a,40b,40c. The second mode converter50includes two ports, and the two ports are respectively connected to the asymmetric Bragg grating42aor42bat a previous stage on one side of the second mode converter50and the second port412of the mode multiplexer41at a next stage on another side of the second mode converter50. When the wavelength division multiplexer/demultiplexer is employed as a demultiplexer, the second port412of the mode multiplexer41of the first wavelength division multiplexer/demultiplexer unit40ais used as an input port for receiving the composite light in the TE0 mode, the third port413is connected to an incident end of the first asymmetric Bragg grating42a, a transmission end of the first asymmetric Bragg grating42ais connected to one port of the second mode converter50, another port of the second mode converter50is connected to the second port of the mode multiplexer41of the second wavelength division multiplexing/demultiplexing unit40bat a next stage, and so on. The second mode converter50is used to convert the light transmitted through the asymmetric Bragg grating from the TE1 mode to the TE0 mode, and transmit the light to the second port412of the mode multiplexer41of the wavelength division multiplexer/demultiplexer unit at a next stage. The first port411of each mode multiplexer41and the third asymmetric Bragg grating42cof the third wavelength division multiplexing/demultiplexing unit40care respectively used to output the light of the channels after demultiplexing. The structure of the second mode converter50is the same as that of the mode multiplexer41, and includes a straight-through waveguide and a cross waveguide. Two ends of the straight-through waveguide are respectively the first port and the third port, and one end of the cross waveguide that is adjacent to the first port is the second port. But, the second mode converter50only makes use of the second port and the third port to perform the mode conversion. In other embodiments, the second mode converter50may also adopt other mode conversion structures.

The demultiplexing process is as follows. A composite light containing the four wavelengths λ1, λ2, λ3, λ4 in the TE0 mode is input to the second port412of the mode multiplexer41of the first wavelength division multiplexing/demultiplexing unit40a, and is mode coupled from the cross wavelength to the straight-through waveguide of the mode multiplexer41and output from the third port413in the TE1 mode. The light in the TE1 mode is incident into the first asymmetrical waveguide42aof the first wavelength division multiplexing/demultiplexing unit40a. The first asymmetric Bragg grating42ais designed to transmit the light containing the wavelengths λ2, λ3, λ4, and to reflect the light with the wavelength λ1 and perform the mode conversion to convert the mode of the light reflected. The light with the wavelength λ1 is reflected by the first asymmetric Bragg grating42aand converted into the TE0 mode, is transmitted to the third port413of the mode multiplexer41, and then is output from the first port411after being transmitted through the straight-through waveguide of the mode multiplexer41. After being transmitted through the first asymmetric Bragg grating42a, the light containing the wavelengths λ2, λ3, λ4 continues to be transmitted to the second mode converter50in the TE1 mode, then is converted into the TE0 mode by the second mode converter50and transmitted in the bus waveguide10, the light in the TE0 mode is transmitted through the bus waveguide10and into the second wavelength division multiplexing/demultiplexing unit40bat a next stage, and is input from the second port412of the mode multiplexer41of the second wavelength division multiplexing/demultiplexing unit40b. The second asymmetric Bragg grating42bof the second wavelength division multiplexing/demultiplexing unit40bis designed to transmit the light containing the wavelengths λ3, λ4, and to reflect the light with the wavelength λ2 and perform the mode conversion to convert the mode of the light reflected. The demultiplexing process of the second wavelength division multiplexing/demultiplexing unit40bis the same as that of a previous stage. The light with the wavelength λ2 in the TE0 mode is output from the first port411of the mode multiplexer41of the second wavelength division multiplexing/demultiplexing unit40b, and the light containing the wavelengths λ3, λ4 continues to be transmitted to the second mode converter50in the TE1 mode, then is converted into the TE0 mode by the second mode converter50and transmitted in the bus waveguide10, the light in the TE0 mode is transmitted through the bus waveguide10and into the third wavelength division multiplexing/demultiplexing unit40cat a next stage, and is input from the second port412of the mode multiplexer41of the third wavelength division multiplexing/demultiplexing unit40c. At this stage, the third asymmetric Bragg grating42cis designed to transmit the light with the wavelength λ4, and to reflect the light with the wavelength λ3 and perform the mode conversion to convert the mode of the light reflected. Finally, the light with the wavelength λ3 in the TE0 mode is output from the first port411of the mode multiplexer41of the third wavelength division multiplexing/demultiplexing unit40c, and the light with the wavelength λ4 in the TE1 mode is transmitted through the third asymmetric Bragg grating42cand output. In this way, demultiplexing of the four wavelengths λ1, λ2, λ3, λ4 is completed. When the wavelength division multiplexer/demultiplexer is used as a multiplexer, the wavelength division multiplexing process is opposite to the above-mentioned demultiplexing process, and the details thereof will not be described herein. In the structure of the present embodiment, the light transmitted in the bus waveguide10is still in the TE0 mode, transmission loss is small, and requirements on the size of the waveguide are few.

The wavelength division multiplexer/demultiplexer shown inFIG.10, further improved on the basis of the embodiment shown inFIG.9, a second mode converter50and a fourth wavelength division multiplexing/demultiplexing unit40dare cascaded after the third wavelength division multiplexing/demultiplexing unit40c. The light with the wavelength λ4 in the TE1 mode is transmitted and output from the third asymmetric Bragg grating42cof the third wavelength division multiplexing/demultiplexing unit40c, is converted into the TE0 mode by the second mode converter50and transmitted along the bus waveguide10, and then is transmitted to the fourth wavelength division multiplexing/demultiplexing unit40dthrough the bus waveguide10. The fourth asymmetric Bragg grating42dof the fourth wavelength division multiplexing/demultiplexing unit40dis designed to reflect the light with the wavelength λ4 and perform the mode conversion. Finally, the light with the wavelength λ4 is output from the first port411of the mode multiplexer41of the fourth wavelength division multiplexing/demultiplexing unit40din the TE0 mode. The fourth wavelength division multiplexing/demultiplexing unit40dis used to filter the light containing other wavelengths (e.g., those having the wavelengths λ1, λ2, λ3) that may be transmitted through previous stages and to the fourth stage. Through filtering, the extinction ratio can be increased, and the crosstalk of other wavelengths can be prevented from being introduced into the light channel of the wavelength λ4.

The embodiment shown inFIG.11, the same as the embodiment shown inFIG.6, both are the wavelength division multiplexer/demultiplexer used for four wavelengths. The wavelength division multiplexer/demultiplexer includes three wavelength division multiplexing/demultiplexing units that are sequentially cascaded. Wherein, each of the wavelength division multiplexing/demultiplexing units60a,60b,60cincludes a mode multiplexer61, and the corresponding one of the asymmetric Bragg gratings62a,62b,62c. The three cascaded wavelength division multiplexing/demultiplexing units are respectively the first wavelength division multiplexing/demultiplexing unit60a, the second wavelength division multiplexing/demultiplexing unit60b, and the third wavelength division multiplexing/demultiplexing unit60c. The three wavelength division multiplexing/demultiplexing units respectively correspond to the first asymmetric Bragg grating62a, the second asymmetric Bragg grating62b, and the third asymmetric Bragg grating62c. The asymmetric Bragg gratings of the three wavelength division multiplexing/demultiplexing units have different grating periods, and are used to respectively reflect the light with the wavelengths λ1, λ2, λ3 but transmit the light with the wavelength λ4. The difference between the present embodiment and the embodiment shown inFIG.6is as follows. In the present embodiment, the first port611of the mode multiplexer61of the wavelength division multiplexing/demultiplexing unit is designed to transmit the light in the TE1 mode, and the second port612is used to transmit the light in TE0 mode. That is, the light that is transmitted from the first port611to the third port613of the mode multiplexer61is in the TE1 mode, the mode of the light is unchanged from input to output, and the light that is transmitted from the third port613to the second port612is in the TE0 mode, the mode of the light is also unchanged from input to output. For example, the light in the TE1 mode that is input to the third port613will be output from the first port611in the TE1 mode, and the light in the TE0 mode that is input to the third port613will be output from the second port612in the TE0 mode. The asymmetric Bragg gratings62a,62bcorresponding to the wavelength division multiplexing/demultiplexing units60a,60bat each stage cascaded are respectively connected to the first port611of the first mode multiplexer61of the wavelength division multiplexing/demultiplexing units60b,60cat a next stage. When the wavelength division multiplexer/demultiplexer is employed as a demultiplexer, the first port611of the mode multiplexer61of the first wavelength division multiplexer/demultiplexer unit60ais used as an input port for receiving the composite light in the TE1 mode. The second port612of the mode multiplexer61at each stage and the third asymmetric Bragg grating62cof the third wavelength division multiplexing/demultiplexing unit60care respectively used to output the light of the channels in the TE0 mode after demultiplexing.

The demultiplexing process is as follows. A composite light containing the four wavelengths λ1, λ2, λ3, λ4 in the TE1 mode is input to the first port611of the mode multiplexer61of the first wave demultiplexing/demultiplexing unit60a, and is transmitted through the mode multiplexer61and output from the third port613in the TE1 mode, then the light in the TE1 mode is incident into the first asymmetric Bragg grating62aof the first wavelength division multiplexing/demultiplexing unit60a. The first asymmetric Bragg grating62ais designed to transmit the light containing the wavelengths λ2, λ3, λ4, and to reflect the light with the wavelength λ1 and perform the mode conversion to convert the mode of the light reflected. The light with the wavelength λ1 is reflected by the first asymmetric Bragg grating62aand converted into the TE0 mode, and then is transmitted to the third port613of the mode multiplexer61. After passing through the mode multiplexer61, the light with the wavelength λ1 is output from the second port612in the TE0 mode. After being transmitted through the first asymmetric Bragg grating62a, the light containing the wavelengths λ2, λ3, λ4 continues to be transmitted in the bus waveguide10in the TE1 mode, then is transmitted to the second wavelength division multiplexing/demultiplexing unit60bat a next stage through the bus waveguide10, and is input to the first port611of the mode multiplexer61of the second wavelength division multiplexing/demultiplexing unit60b. The second asymmetric Bragg grating62bof the second wavelength division multiplexing/demultiplexing unit60bis designed to transmit the light containing the wavelengths λ3, λ4, and to reflect the light with the wavelength λ2 and perform the mode conversion. The demultiplexing process of the second wavelength division multiplexing/demultiplexing unit60bis the same as that of the previous stage. While the light with the wavelength λ2 is output from the second port612of the mode multiplexer61of the second wavelength division multiplexing/demultiplexing unit60bin the TE0 mode, the light containing the wavelengths λ3, λ4 continues to be transmitted in the bus waveguide10in the TE1 mode, then is transmitted to the third wavelength division multiplexing/demultiplexing unit60cat a next stage through the bus waveguide10, and is input to the first port611of the mode multiplexer61of the wavelength division multiplexing/demultiplexing unit60c. At this stage, the third asymmetric Bragg grating62cis designed to transmit the light with the wavelength λ4, and to reflect the light with the wavelength λ3 and perform the mode conversion. Finally, the light with the wavelength λ3 is output from the second port612of the mode multiplexer61of the third wavelength division multiplexing/demultiplexing unit60cin the TE0 mode, and the light with the wavelength λ4 in the TE1 mode is transmitted through the third asymmetric Bragg grating62cand output. In this way, demultiplexing of the four wavelengths λ1, λ2, λ3, λ4 is completed. When the wavelength division multiplexer/demultiplexer is used as a multiplexer, the wavelength division multiplexing process is opposite to the above-mentioned demultiplexing process, and the details thereof will not be described herein.

The wavelength division multiplexer/demultiplexer shown inFIG.12, further improved on the basis of the embodiment shown inFIG.11, a fourth wavelength division multiplexing/demultiplexing unit60dis cascaded after the third wavelength division multiplexing/demultiplexing unit60c. A fourth asymmetric Bragg grating62dof the fourth wavelength division multiplexing/demultiplexing unit60dis designed to reflect the light with the wavelength λ4 and perform the mode conversion to convert the mode of the light reflected. Finally, the light with the wavelength λ4 in the TE0 mode is output from the second port612of the mode multiplexer61of the fourth wavelength division multiplexing/demultiplexing unit60d. The fourth wavelength division multiplexing/demultiplexing unit60dis used to filter the light containing other wavelengths (e.g., those having the wavelengths λ1, λ2, λ3) that may be transmitted through previous stages and to the fourth stage. Through filtering, the extinction ratio can be increased, and the crosstalk of other wavelengths can be prevented from being introduced into the light channel of the wavelength λ4.

The above-mentioned embodiments are described by taking a wavelength division multiplexer/demultiplexer of four-wavelengths as an example. Certainly, different numbers of wavelength division multiplexing/demultiplexing units can be cascaded to realize wavelength division multiplexing/demultiplexing of different numbers of channels. Various cascading modifications are within the protection scope of the present disclosure. For example, an eight-channel wavelength division multiplexer/demultiplexer may include at least seven wavelength division multiplexing/demultiplexing units for cascading or a combination of cascading and parallel connection, so as to perform wavelength division multiplexing or demultiplexing on lights of eight different wavelengths. Naturally, the eight-channel wavelength division multiplexing/demultiplexing unit may include at least eight wavelength division multiplexing/demultiplexing units for cascading or a combination of cascading and parallel connection.

The present disclosure further provides an optical module that includes a photonic integrated chip and the wavelength division multiplexer/demultiplexer in any one of the above embodiments is disposed in the photonic integrated chip. The wavelength division multiplexer/demultiplexer is used as a wavelength division multiplexer at a light transmitting end, and is used as a wavelength division demultiplexer at a light receiving end. Naturally, optical active devices (such as signal modulators and/or photodetectors) or optical passive devices (such as couplers) can also be disposed in the photonic integrated chip.

The foregoing description of the exemplary embodiments of the disclosure has been presented only for the purposes of illustration and description and is not intended to be exhaustive or to limit the disclosure to the precise forms disclosed. Many modifications and variations are possible in light of the above teaching.

The embodiments were chosen and described in order to explain the principles of the disclosure and their practical application so as to enable others skilled in the art to utilize the disclosure and various embodiments and with various modifications as are suited to the particular use contemplated. Alternative embodiments will become apparent to those skilled in the art to which the present disclosure pertains without departing from its spirit and scope.