An ultra-wideband optical directional coupler

Abstract

The application belongs to the field of optoelectronic technology and particularly relates to an ultra-wideband optical directional coupler, which comprises three single-mode waveguides arranged in parallel at intervals and having the same width, contains straight waveguides and curved waveguides, and one curved waveguide is arranged between two adjacent straight waveguides; straight waveguide ports supporting edge state modes at both ends of a waveguide structure composed of the three single-mode waveguides are respectively used as a coupler input port and an output port; the curved waveguide is a modulation waveguide, and the center distance between the modulation waveguide and one of the straight waveguides adjacent to the modulation waveguide is characterized by a modulation function which changes uniformly and slowly in the propagation direction, the tangent of the function at both ends of the device is parallel to the straight waveguide, and the change relation involves parameters such as the total length of the device, the modulation amplitude of the bending in the direction perpendicular to the propagation direction, and the position of the input port end in the propagation direction. The application greatly improves the response bandwidth of the device.

Description

Technical Field

[0001] This invention belongs to the field of optoelectronic technology, and more specifically, relates to an ultra-wideband optical directional coupler. Background Technology

[0002] Optical directional couplers are one of the important optical components in optical communication systems and a fundamental component in photonic chips. They are used for coupling and interconnecting optical signals between different photonic devices and play a very important role.

[0003] A conventional optical directional coupler consists of two parallel waveguides, and light is coupled between the different waveguides. The length required for light energy to be completely coupled from one waveguide to the other is called the coupling length. Typically, the coupling length is very sensitive to wavelength, which means that conventional couplers can often only achieve high-efficiency energy coupling within a relatively small wavelength range (response bandwidth). This limits the development of broadband on-chip optical transmission and photonic devices.

[0004] Therefore, developing on-chip directional couplers with broadband response has significant research and application value. Summary of the Invention

[0005] In view of the shortcomings of the existing technology and the need for improvement, the present invention provides an ultra-wideband optical directional coupler, the purpose of which is to improve the broadband response of the directional coupler.

[0006] To achieve the above objectives, according to one aspect of the present invention, an ultra-wideband optical directional coupler is provided, comprising: three single-mode waveguides arranged in parallel at intervals and having the same width;

[0007] Among them, the three single-mode waveguides include both straight waveguides and curved waveguides, with a curved waveguide placed between two adjacent straight waveguides;

[0008] The straight waveguide ports supporting boundary state modes at both ends of the waveguide structure composed of the three single-mode waveguides serve as the coupler input and output ports, respectively. The curved waveguide acts as a modulation waveguide, and the relationship between the center-to-center distance d between the modulation waveguide and one of its adjacent straight waveguides and the direction of light propagation is expressed as: d(x)=w+d1+Δd+Δd*f(π*x / L), and the relationship between the center-to-center distance d' between the modulation waveguide and the other straight waveguide and the direction of light propagation is expressed as: d'(x)=w+d2-Δd-Δd*f(π*x / L), where w is the waveguide width, and d1 and d2 are the center-to-center distances of the modulation waveguide at the input port end. The gap width between the two straight waveguides is given by d1 = d2 - 2Δd, where Δd is the modulation amplitude caused by the bending of the modulation waveguide perpendicular to the propagation direction, L is the total length of the coupler, and x is the position of the input port end in the propagation direction; f(·) satisfies the following: the function value between x = 0 and x = L decreases uniformly and monotonically from 1 to -1, and its tangents at x = 0 and x = L are parallel to the straight waveguides; after light enters from the input port, it can excite the boundary state mode of the waveguide structure and evolve into the bulk state mode along the waveguide structure. Under adiabatic evolution, the bulk state mode will be converted into the boundary state mode of the waveguide structure and output from the output port.

[0009] Furthermore, the single-mode waveguide is strip-shaped, cylindrical, planar, or ridge-shaped.

[0010] Furthermore, the values ​​of its structural parameters are determined through simulation based on the actual selected waveguide material. The structural parameters include: the total length of the coupler, d1, d2, Δd, the waveguide width, and the height h of each waveguide. The actual selected waveguide material is determined according to the operating band of the coupler.

[0011] Furthermore, when its operating band is the optical band, the optimal structural parameter values ​​are determined through simulation, and the actual range of structural parameter values ​​is within ±50nm of the optimal structural parameter value.

[0012] Furthermore, the waveguide material is determined based on the following principle when determining the operating band of the coupler: the smaller the material dispersion, the better the broadband response characteristics.

[0013] Furthermore, the relationship between the changes is: d(x)=w+d1+Δd+Δd*cos(π*x / L), d'(x)=w+d2-Δd-Δd*cos(π*x / L).

[0014] Furthermore, the initial center-to-center distance between the modulation waveguide and the two straight waveguides at x=0 is determined based on the following principle: at x=0, the center-to-center distance between the modulation waveguide and one of the straight waveguides is d=w+d2, and the center-to-center distance with the other straight waveguide is d'=w+d1. The greater the difference between d and d', the more localized the boundary state mode optical field energy of the input port and the output port is in a single waveguide, and the better the directional coupling transmission effect.

[0015] In summary, the above-described technical solutions conceived in this invention can achieve the following beneficial effects:

[0016] (1) In view of the problem that the response bandwidth of current directional optical couplers is narrow, this invention proposes an ultra-wideband optical directional coupler, which is composed of three single-mode waveguides with the same width and arranged in parallel at intervals. Specifically, the three single-mode waveguides include both straight waveguides and curved waveguides. A curved waveguide is set between two adjacent straight waveguides as a modulation waveguide. The relationship between the center distance d between the modulation waveguide and one of the adjacent straight waveguides and the change with the direction of light propagation is specifically expressed to characterize the curved structure. The entire coupler structure is relatively simple, applicable to different materials and bands, and has strong scalability. After theoretical analysis and experimental verification, the coupler proposed in this invention has a large response bandwidth.

[0017] (2) When designing the coupler, if the working band of the coupler to be designed is the optical band, the optimal structural parameter values ​​are determined by simulation. The actual structural parameter values ​​of the coupler to be designed can be within ±50nm. Through simulation verification, the structural parameters determined within this range can broaden the bandwidth. Therefore, the structural redundancy of the coupler of this invention is high. Attached Figure Description

[0018] Figure 1 A schematic diagram of an ultra-wideband optical directional coupler structure provided in an embodiment of the present invention;

[0019] Figure 2 A schematic diagram of an exemplary ultra-wideband optical directional coupler structure provided for an embodiment of the present invention;

[0020] Figure 3 A comparison of the coupling efficiency spectra of an ultrawideband directional coupler and a conventional dual-waveguide directional coupler provided in an embodiment of the present invention;

[0021] Figure 4 A comparison diagram of typical wavelength optical field distributions of an ultra-wideband directional coupler and a conventional dual-waveguide directional coupler provided in an embodiment of the present invention;

[0022] Figure 5 Crosstalk spectrum of an ultra-wideband directional coupler provided in an embodiment of the present invention;

[0023] Figure 6 A schematic diagram illustrating the redundancy of the coupling efficiency spectrum of the ultra-wideband directional coupler provided in this embodiment of the invention relative to the waveguide width;

[0024] Figure 7 This is a schematic diagram illustrating the redundancy of the coupling efficiency spectrum of the ultra-wideband directional coupler provided in an embodiment of the present invention relative to the waveguide spacing. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0026] Example 1

[0027] An ultra-wideband optical directional coupler includes: three single-mode waveguides arranged in parallel at intervals and with the same width; wherein the three waveguides include both straight waveguides and curved waveguides, and a curved waveguide is arranged between two adjacent straight waveguides;

[0028] The straight waveguide ports supporting boundary state modes at both ends of the waveguide structure composed of three single-mode waveguides serve as the input and output ports of the coupler, respectively. The curved waveguide acts as a modulation waveguide, and the relationship between the center-to-center distance d between the modulation waveguide and one of its adjacent straight waveguides and the direction of light propagation is expressed as: d(x)=w+d1+Δd+Δd*f(π*x / L), and the relationship between the center-to-center distance d' between the modulation waveguide and the other straight waveguide and the direction of light propagation is expressed as: d'(x)=w+d2-Δd-Δd*f(π*x / L), where w is the waveguide width, and d1 and d2 are the center-to-center distances of the modulation waveguide at the input port. The gap width between the two straight waveguides is given by d1 = d2 - 2Δd, where Δd is the modulation amplitude caused by the bending of the modulation waveguide perpendicular to the propagation direction, L is the total length of the coupler, and x is the position of the input port end in the propagation direction. f(·) satisfies the following: the function value between x = 0 and x = L decreases uniformly and monotonically from 1 to -1, and its tangents at x = 0 and x = L are parallel to the straight waveguides. After light enters from the input port, it can excite the boundary state mode of the waveguide structure and evolve into the bulk mode along the waveguide structure. Under the condition of adiabatic evolution, the bulk mode will be converted into the boundary state mode of the waveguide structure and output from the output port.

[0029] It should be noted that the three single-mode waveguides are arranged in parallel with spacing, and the bending direction of the curved waveguide between two adjacent straight waveguides is the length direction of the straight waveguide, which is the direction of light propagation.

[0030] Furthermore, the relationship between the center-to-center distance between the modulation waveguide and one of its adjacent straight waveguides varies with the direction of light propagation, characterized by a modulation function that varies uniformly and slowly in the direction of propagation.

[0031] To more clearly illustrate the directional coupler described in this embodiment, a structural schematic diagram with relevant parameters is now provided, as follows: Figure 1 As shown.

[0032] The coupler structure described in this embodiment can be implemented through the following design process:

[0033] Step 1: Introduce a modulation waveguide that bends along the propagation direction between two parallel waveguides, calculate the relationship between the coupling strength between the waveguides and the waveguide spacing, and design, optimize and determine the material and structural parameters of the directional coupler based on the coupled wave equation theory, including waveguide width, waveguide spacing and waveguide length, etc.

[0034] Step 2: Set the directional coupler structure and corresponding parameters in the simulation software, simulate and calculate the transmittance (coupling efficiency) and the optical field evolution of different wavelengths, calculate the response bandwidth, and further optimize the structural parameters of the coupler through simulation calculation;

[0035] Step 3: Select the optimal structural parameters and use simulation software to simulate and calculate the performance indicators of the on-chip directional coupling device, including response bandwidth, coupling efficiency, crosstalk, etc.

[0036] The coupler structure design in this embodiment not only broadens the response width but also features a small size.

[0037] As a preferred embodiment, the structural parameters of the directional coupler proposed in this embodiment can be determined through simulation based on the actual selected waveguide material. These structural parameters include: the total length of the coupler, the bending structure (i.e., the two aforementioned variations), the waveguide width, and the waveguide height. The actual selected waveguide material is determined according to the coupler's operating wavelength. Therefore, different materials have different response wavelengths, and thus, the directional coupler can be made from different materials. Materials that can be selected for optical directional couplers include dielectric materials such as silicon, lithium niobate, and silicon nitride, as well as metallic materials that can be used to fabricate waveguides.

[0038] As a further preferred embodiment, the waveguide material can be determined based on the following principle when determining the operating wavelength of the coupler: the smaller the material dispersion, the better the broadband response characteristics. For the innovative structure of the directional coupler proposed in this embodiment, experiments and theoretical studies have shown that the smaller the material dispersion, the better the broadband response characteristics. Therefore, while the directional coupler can be made of different materials, the wavelength range and response bandwidth of the device can be further expanded based on the material selection principles proposed in this embodiment.

[0039] Based on directional couplers, broadband directional couplers with different response bands can be designed and fabricated. The coupler structure proposed in this embodiment is not limited by materials. Appropriate waveguide materials and structures can be selected according to the actual required operating band. It has a wide range of applications and is of great significance for realizing broadband directional couplers with different bands.

[0040] The response wavelength range (i.e., operating band) of the directional coupler can include microwave band and optical band, depending on the waveguide material and structure. Microwave band directional couplers use microwave waveguides, while optical band directional couplers use dielectric waveguides with optical band response. This can be considered a preferred implementation. When the operating band of the coupler is the optical band, the optimal structural parameter values ​​are determined through simulation. The actual range of structural parameter values ​​can be within ±50nm of the optimal structural parameter value.

[0041] In other words, based on the directional coupler structure proposed in this embodiment, the directional coupler has good redundancy in structural dimensions. When the device structural parameters (waveguide width, waveguide spacing, etc.) vary within ±50nm relative to the designed standard dimensions (i.e., the optimal structural parameter values), it can still maintain a broadband spectral response, that is, it can still obtain ultra-wideband directional coupling characteristics.

[0042] In a preferred embodiment, the relationship between the center-to-center spacing d of the modulation waveguide and one of its adjacent straight waveguides as a function of the light propagation direction is expressed as: d(x) = w + d1 + Δd + Δd*cos(π*x / L), and the relationship between the center-to-center spacing d' of the modulation waveguide and the other straight waveguide as a function of the light propagation direction is expressed as: d'(x) = w + d2 - Δd - Δd*cos(π*x / L). That is, preferably, the modulation function can be a cosine function. It should be noted that the above description of the coupler structure, combined with... Figure 1 As can be seen, in the coupler structure proposed in this embodiment, the bending tangent of the curved waveguide is parallel to the straight waveguide at both the input and output ends of the waveguide array structure. The reason why the modulation function is preferably a cosine function is that the modulation waveguide has central symmetry in the intervals x=0 and x=L, and its tangents at both ends are exactly parallel to the straight waveguide, which is an ideal modulation method.

[0043] In a preferred embodiment, the initial center-to-center spacing between the modulation waveguide and the two straight waveguides at x=0 is determined based on the following principle: the center-to-center spacing between the modulation waveguide and one of the straight waveguides at x=0 is d=w+d2, and the center-to-center spacing with the other straight waveguide is d'=w+d1. The greater the difference between d and d', the more localized the boundary state mode optical field energy of the input port and the output port is within a single waveguide, resulting in better directional coupling transmission. This preferred method is used to provide direction for the values ​​of d1, d2, and Δd.

[0044] This invention proposes an ultra-wideband directional coupler by introducing a waveguide that bends and modulates along the propagation direction between two parallel straight waveguides to extend the response bandwidth of the directional coupler. The design and fabrication of the on-chip ultra-wideband directional coupler can be divided into three steps. To more specifically illustrate the characteristics and performance of the coupler proposed in this invention, the design process described above will be discussed in detail below:

[0045] (1) Design of material and structural parameters for directional couplers

[0046] First, select appropriate materials for designing the waveguide and directional coupler based on the required operating band. Then, introduce a waveguide with the same waveguide width (w) and a curved modulation along the propagation direction between two parallel waveguides. The relationship between the center-to-center distance of the modulation waveguide and one of its adjacent straight waveguides varies with the light propagation direction, for example: d(x) = w + d1 + Δd + Δd*cos(π*x / L), d'(x) = w + d2 - Δd - Δd*cos(π*x / L), where w is the waveguide width, d1 and d2 are the gap widths between the modulation waveguide and the two adjacent straight waveguides at the input port, and d1 = d2 - 2Δd, where Δd is the modulation amplitude of the waveguide bending perpendicular to the propagation direction, and L is the total length of the directional coupler; or, the relationship can be a composite function, etc.

[0047] (2) Simulation Design and Optimization of Directional Couplers

[0048] In the simulation software, the structure and parameters of the directional coupler are set, the transmittance (i.e. coupling efficiency) of different wavelengths and the optical field evolution of the corresponding wavelengths are simulated and calculated, the response bandwidth is calculated, and the structural parameters are adjusted through simulation calculation to further optimize the response bandwidth of the directional coupler.

[0049] (3) Performance characterization of directional couplers

[0050] Based on the simulation optimization results, the optimal structural parameters are selected, and the performance indicators of the directional coupling device are calculated using simulation software, including response bandwidth, coupling efficiency, crosstalk, etc.

[0051] To calculate the performance parameters of the directional coupling device, this embodiment utilizes optical simulation software for simulation calculations. In the simulation, the waveguide material is designed as thin-film lithium niobate, and the waveguide structure is surrounded by silicon oxide material, such as... Figure 2 As shown in the figure, the transverse electric (TE) mode optical field of a single waveguide is selected as the input, with the input port shown in the figure. Its wavelength range is 1300-1700 nm. The transmittance as a function of wavelength is monitored at the end of another straight waveguide. Figure 3As shown, the coupling efficiency spectra of the designed on-chip ultra-wideband directional coupler and the conventional dual-waveguide directional coupler are compared. In the figure, the -0.2dB (~95%) bandwidth of the ultra-wideband directional coupler reaches 210nm, while the -0.2dB (~95%) bandwidth of the conventional dual-waveguide directional coupler of the same length is only 70nm, representing a three-fold increase. Figure 4 As shown, the typical wavelength optical field distributions of the designed on-chip ultra-wideband directional coupler and the conventional dual-waveguide directional coupler are compared. The figure shows that the output of the ultra-wideband directional coupler at different wavelengths is mainly concentrated in the straight waveguide corresponding to the output port, while the conventional dual-waveguide directional coupler has outputs in several waveguides. This indicates that the directional coupling effect of the ultra-wideband directional coupler is significantly better than that of the conventional dual-waveguide directional coupler. Furthermore, the ratio of the light intensity emitted from the opposite end of the incident waveguide to the light intensity emitted from the output port is defined as the crosstalk of the directional coupler. Figure 5 As shown, the crosstalk of this ultra-wideband directional coupler is less than -10dB in the 1360-1700nm range. The redundancy of the structural parameters was tested by varying the waveguide width and waveguide spacing of the ultra-wideband directional coupler, as shown in the figure. Figure 6 and Figure 7 As shown, the on-chip directional couplers designed within a waveguide width and spacing range of ±50nm all exhibit broadband response characteristics, indicating that the designed on-chip ultra-wideband directional couplers have good redundancy in structural parameters. In other words, the structural design of this invention reduces the sensitivity of device response to structural dimensions.

[0052] In summary, under the same simulation conditions, the -0.2dB (~95%) bandwidth of the ultra-wideband directional coupler is three times that of the conventional dual-waveguide directional coupler, and the bandwidth of other coupling efficiencies is also significantly improved, as shown in Table 1. This is of great significance for realizing broadband on-chip optical devices and optical interconnects.

[0053] Table 1

[0054] Ultra-wideband directional coupler Conventional directional coupler -0.2dB bandwidth 1440-1650nm (210nm) 1490-1560nm (70nm) -0.5dB bandwidth 1330-1700nm (>370nm) 1460-1600nm (140nm) -1dB bandwidth 1300-1700nm (>400nm) 1420-1630nm (210nm)

[0055] In summary, this invention addresses the problem of narrow response bandwidth in current directional optical couplers by proposing an ultra-wideband optical directional coupler with a simple structure, large response bandwidth, high structural redundancy, and strong scalability (n can be determined according to actual needs). Simulation calculations show that, for the same total coupler length L, the -0.2dB (~95%) bandwidth of the ultra-wideband directional coupler is three times that of a conventional dual-waveguide directional coupler, and the crosstalk in the 1360-1700nm range is less than -10dB. It also exhibits good redundancy in the device's structural parameters, which is of great significance for realizing broadband on-chip optical devices and optical interconnects.

[0056] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. An ultra-wideband optical directional coupler, characterized in that, include: Three single-mode waveguides spaced parallel to each other and of equal width; Among them, the three single-mode waveguides include both straight waveguides and curved waveguides, with a curved waveguide placed between two adjacent straight waveguides; The straight waveguide ports supporting boundary state modes at both ends of the waveguide structure composed of the three single-mode waveguides serve as the coupler input and output ports, respectively. The curved waveguide acts as a modulation waveguide, and the relationship between the center-to-center distance d between the modulation waveguide and one of its adjacent straight waveguides and the direction of light propagation is expressed as: d(x)=w+d1+Δd+Δd*f(π*x / L), and the relationship between the center-to-center distance d' between the modulation waveguide and the other straight waveguide and the direction of light propagation is expressed as: d'(x)=w+d2-Δd-Δd*f(π*x / L), where w is the waveguide width, and d1 and d2 are the center-to-center distances of the modulation waveguide at the input port end. The gap width between the two straight waveguides is given by d1 = d2 - 2Δd, where Δd is the modulation amplitude caused by the bending of the modulation waveguide perpendicular to the propagation direction, L is the total length of the coupler, and x is the position of the input port end in the propagation direction; f(·) satisfies the following: the function value between x = 0 and x = L decreases uniformly and monotonically from 1 to -1, and its tangents at x = 0 and x = L are parallel to the straight waveguides; after light enters from the input port, it can excite the boundary state mode of the waveguide structure and evolve into the bulk state mode along the waveguide structure. Under adiabatic evolution, the bulk state mode will be converted into the boundary state mode of the waveguide structure and output from the output port.

2. The ultra-wideband optical directional coupler according to claim 1, characterized in that, The single-mode waveguide can be strip-shaped, cylindrical, flat, or ridge-shaped.

3. The ultra-wideband optical directional coupler according to claim 1, characterized in that, The values ​​of its structural parameters are determined through simulation based on the actual selected waveguide material. The structural parameters include: the total length of the coupler, d1, d2, Δd, the waveguide width, and the height h of each waveguide. The actual selected waveguide material is determined according to the operating band of the coupler.

4. The ultra-wideband optical directional coupler according to claim 3, characterized in that, When its operating band is the optical band, the optimal structural parameter values ​​are determined through simulation. The actual range of structural parameter values ​​is within ±50nm of the optimal structural parameter value.

5. The ultra-wideband optical directional coupler according to claim 3, characterized in that, The waveguide material is determined based on the following principle when determining the operating band of the coupler: the smaller the material dispersion, the better the broadband response characteristics.

6. The ultra-wideband optical directional coupler according to claim 1, characterized in that, The relationship is as follows: d(x)=w+d1+Δd+Δd*cos(π*x / L), d'(x)=w+d2-Δd-Δd*cos(π*x / L).

7. The ultra-wideband optical directional coupler according to claim 1, characterized in that, The initial center-to-center spacing between the modulation waveguide and the two straight waveguides at x = 0 is determined based on the following principle: at x = 0, the center-to-center spacing between the modulation waveguide and one of the straight waveguides is d = w + d2, and the center-to-center spacing with the other straight waveguide is d' = w + d1. The greater the difference between d and d', the more localized the boundary state mode optical field energy of the input port and the output port is in a single waveguide, and the better the directional coupling transmission effect.

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