CN106483600B - A kind of ultrashort vertical waveguide coupler with tolerance of producing extensively - Google Patents

Abstract

The invention discloses an ultra-short vertical waveguide coupler with large fabrication tolerance. The coupler is composed of an upper waveguide, a spacer layer and a lower waveguide in the vertical direction. In the direction of propagation, a vertical coupler is divided into an input area, a coupling area, and an output area. In the coupling region, the width of the upper waveguide gradually decreases, and is divided into front, middle and back sections; the front and back sections are the fast shrinking sections of the upper waveguide width, and the middle section is the mode transfer section. The present invention increases the coupling coefficient by reducing the thickness of the spacer layer between the upper and lower waveguides, thereby shortening the length of the coupler; in the middle section of the coupling area, a suitable deceleration rate and a suitable length of the upper waveguide width are set to make the upper waveguide When the width deviates due to the limitation of actual manufacturing precision, the mode of the upper waveguide can still be efficiently transferred to the lower waveguide, thereby achieving a large manufacturing tolerance.

Description

An Ultrashort Vertical Waveguide Coupler with Large Fabrication Tolerance

technical field

The invention relates to an optical component, in particular to a vertical waveguide coupler.

Background technique

As optical communication systems are upgraded to higher speeds, more and more optical devices are integrated into photonic integrated circuits (PICs) to achieve monolithic integration of various functions. In order to make optical communication devices develop in the direction of smaller size, lower power consumption, and more reliable stability, the integration of active and passive devices will be essential. At present, photonic integration technologies mainly include: Butt-joint regrowth technology, offset quantum well technology, quantum well hybrid technology, etc. The relatively complex manufacturing process of these integrated technologies leads to relatively high manufacturing costs and low yields. In addition, since the structures of active and passive devices cannot be designed separately, the performance of integrated devices is limited. The dual-waveguide vertical coupling technology proposed by Suematsu et al. mainly utilizes the vertical coupling of the upper waveguide from top to bottom to realize the integration of active and passive devices (Y.Suematsu, M.Yamada, and K.Kayashi.Integrated twin -guide AlGaAs laser with multiheterostructure[J].IEEE J.Quantum Electron.,1975,11(7):457-460.). In this technology, different waveguide layers can be grown from bottom to top, which avoids complex re-growth process and greatly simplifies the manufacturing process. At the same time, because the upper and lower waveguides can be designed relatively independently, it has greater flexibility.

The vertical coupling technology has the advantage of simple manufacturing process, and can be well applied to the integration of devices in the integrated optical circuit. According to the supermode coupling theory, the current vertical coupling schemes mainly include resonant couplers (XiankaiSun, Hsi-Chun Liu, and AmnonYariv, Adiabaticity criterion and the shortest adiabaticmode transformer in a coupled-waveguide system, Opt. Lett., 2009, 34(3):280–282.) and adiabatic couplers. In the resonant coupler, the energy is evenly distributed in the symmetric mode and the antisymmetric mode, and the mutual interference of these two eigenmodes is used to realize the upper and lower waveguide coupling (M.Galarza, K.De Mesel, R.Baets, A.Martinez , C. Aramburu, and M. Lopez-Amo, Compactspot-size converters with fiber-matched antiresonant reflecting optical waveguide, Appl. Opt., 2003, 42:4841-4846.). In the initial section, the power is concentrated in the upper waveguide. In the longitudinal propagation direction along the waveguide, the two modes interfere with each other. When the phase difference between the two modes is π, the modes are coherent and cancel in the upper waveguide, and the coherence is strengthened in the lower waveguide, and the power is concentrated in the lower waveguide; and when the two modes When the phase difference of the modes is 2π, the modes are coherent and cancelled in the lower waveguide, and the coherence is strengthened in the upper waveguide, and the power returns to the upper waveguide. In this way, in the propagation direction, the optical power exhibits a periodic behavior, which is coupled from the upper waveguide to the lower waveguide in half a period, and then coupled back to the upper waveguide from the lower waveguide in one period. The coupling length of the resonant coupler depends on the coupling coefficient between the upper and lower waveguides. The larger the coupling coefficient, the shorter the coupling length (A.Wieczorek, B.Roycroft, F.H.Peters, and B.Corbett, Loss analysis and increasing of the fabrication tolerance of resonant coupling by tapering the modebeating section, Opt. Quantum Electron., 2011, 42(8):521–529.). In the resonant coupler, the coupling efficiency is strongly dependent on the length and structure of the coupler due to the periodic coupling caused by the interference of the mode. In actual production, the change of the upper waveguide width due to the manufacturing accuracy and process deviation has a great influence on the coupling efficiency. Large, the manufacturing tolerances of such resonant couplers are usually small. For the adiabatic coupler, the energy is mainly concentrated in the symmetric mode. As the optical confinement factor of the lower waveguide becomes larger, the optical power is slowly coupled from the upper waveguide to the lower waveguide (F.Xia, V.M.Menon, and S.R.Forrest, Photonic integration using asymmetric twin-waveguide (ATG) technology: partIconcepts and theory [J]. IEEE J. Sel. Top. Quantum Electron., 2005, 11(1): 17–29.). Since the width of the upper waveguide of the adiabatic coupler changes very gently, this reduces the dependence of the coupling efficiency on the coupler length, and its manufacturing tolerance is relatively large. However, the adiabatic coupler usually requires a sufficiently long (~200 μm) coupling region to ensure that the upper waveguide mode is finally coupled into the lower waveguide, reducing the loss caused by the mode transition to other higher-order modes. This kind of coupler not only reduces the density of integrated devices, but also increases the insertion loss of the coupler, which greatly limits its practical application. In short, the current vertical coupler has the problem of small manufacturing tolerance or too long length, resulting in low yield of integrated devices and high manufacturing cost, making it unable to be widely used.

Contents of the invention

The technical problem to be solved by the present invention is to propose a new ultra-short vertical coupler with a large manufacturing tolerance, which is used to reduce the size of the optical integrated device, improve the manufacturing tolerance of the optical integrated device, and then improve the optical integrated device. Yield.

In order to solve the above technical problems, the present invention proposes a new ultra-short vertical waveguide coupler with a large manufacturing tolerance. On the vertical section of the ultra-short vertical waveguide coupler, from top to bottom are the upper waveguide, the space layer and lower waveguide.

In the direction of propagation, the ultrashort vertical coupler is divided into an input area, a coupling area and an output area; in the input area, the mode propagation constant β ₁ of the upper waveguide is greater than the mode propagation constant β 2 of the lower waveguide, and the mode propagation constant β ₂ of the upper and lower waveguides Half of the mode propagation constant difference, that is, δβ, δβ is greater than the coupling coefficient κ between the two waveguides, that is, δβ>κ, δβ=(β ₁ -β ₂ )/2, the upper waveguide mode is not affected by the existence of the lower waveguide; The coupling region, as the width of the upper waveguide gradually decreases, is divided into three sequentially connected sections: the front section, the middle section and the back section; the front section and the back section are the fast shrinking sections of the upper waveguide width, and the middle section is the mode transfer section;

By reducing the thickness of the spacer layer between the upper and lower waveguides, the coupling coefficient κ between the two waveguides is increased, thereby reducing the coupling length. The upper limit of the coupling coefficient increase is half of the maximum propagation constant difference δβ that can be realized between the upper and lower waveguides in the input area.

Further, in the middle of the coupling region, the width of the upper waveguide gradually decreases, so that the mode propagation constant of the upper waveguide decreases from slightly larger than that of the lower waveguide to slightly smaller than the mode propagation constant of the lower waveguide; the width of the upper waveguide gradually decreases to satisfy the following formula:

β ₁ (A) = β ₂ + Δβ

β ₁ (β)=β ₂ -Δβ

In the above formula, β ₁ (A) is the mode propagation constant corresponding to the initial width of the upper waveguide in the middle section of the coupling region, β ₁ (B) is the mode propagation constant corresponding to the end width of the upper waveguide, and Δβ is the maximum The offset of the upper waveguide mode propagation constant brought by the manufacturing tolerance; the middle section of the coupling region, that is, the length of the mode transfer section is 1.5 to 4 times of L _c , and L _c is defined as the straight waveguide coupling length.

Preferably, in the front section of the coupling area, the width of the upper waveguide decreases rapidly, and its end width is the same as the initial width of the middle section of the coupling area. The width of the upper waveguide in the front section of the coupling area is rapidly reduced, which can shorten the length of the coupler.

Also preferably, in the rear section of the coupling region, the width of the upper waveguide decreases rapidly, the initial width of the rear section is the same as the end width of the middle section, the end width of the rear section is the minimum width allowed by the production, and the end of the rear section is the termination end , Anti-reflective coating is coated on the terminal end face to reduce reflection. Likewise, the width of the upper waveguide in the rear section of the coupling region decreases rapidly, which can shorten the length of the coupler.

In the middle of the coupling region, the upper waveguide modes are rapidly transferred down the waveguide by means of resonance transfer.

In the middle section of the coupling region, when the width of the upper waveguide deviates due to the limitation of manufacturing precision, the upper waveguide mode can still be efficiently transferred to the lower waveguide at different positions in the middle section.

In the output region there is only the lower waveguide.

The present invention increases the coupling coefficient by reducing the thickness of the spacer layer between the upper and lower waveguides, thereby shortening the length of the coupler; at the same time, by adopting a suitable width deceleration rate and a suitable length in the middle of the coupling region, the following effects are achieved: namely When the width of the upper waveguide in the middle section of the coupling region changes within a certain range, the mode of the upper waveguide can be efficiently transferred to the lower waveguide at different positions in the middle section, so that when the width of the waveguide deviates due to the limitation of the actual manufacturing accuracy, the mode of the upper waveguide remains the same. Efficient transfer down the waveguide enables large fabrication tolerances.

Description of drawings

The technical solutions of the present invention will be further specifically described below in conjunction with the accompanying drawings and specific embodiments.

FIG. 1 is a schematic structural diagram of an ultra-short vertical waveguide coupler with a large manufacturing tolerance according to the present invention.

Fig. 2 is a schematic diagram of a cross-sectional refractive index distribution of an example of the present invention.

FIG. 3 is a top structural view of the upper waveguide of the mode transfer section 7 using a straight waveguide.

FIG. 4 is a schematic diagram of the power conversion simulation of the upper and lower waveguides when the upper waveguide of the mode transfer section 7 adopts a straight waveguide.

FIG. 5 is a graph showing the variation of the coupling efficiency of the lower waveguide with the propagation distance L when the upper waveguide of the mode transfer section 7 adopts width-decreasing waveguides with different slopes.

FIG. 6 is a top view structure diagram of the coupler of the present invention when the upper waveguide of the mode transfer section 7 adopts a width-decreasing waveguide with a certain slope.

FIG. 7 is a schematic diagram showing the tolerance of the coupling efficiency of the coupler of the present invention as the upper waveguide varies with the upper waveguide width when the upper waveguide of the mode transfer section 7 adopts a straight waveguide and the present invention (a width-decreasing waveguide with a certain slope).

FIG. 8 is a schematic diagram of a simulation result of an example of the present invention when the coupling efficiency is at an extreme value. At this time, the upper waveguide width of the mode transfer section 7 changes by Δw ₁ .

FIG. 9 is a schematic diagram of simulation results of another example of the present invention when the coupling efficiency is at an extreme value. At this time, the upper waveguide width of the mode transfer section 7 changes by Δw ₂ .

Detailed ways

As shown in Figure 1, the side view of the optical waveguide structure of the ultra-short vertical waveguide coupler with a large manufacturing tolerance of the present invention, the lateral direction consists of a cover layer 1, an upper waveguide layer 2, a spacer layer 3, and a lower waveguide layer from top to bottom. 4 and substrate 5, the spacer layer 3 serves as a low-refractive index layer between the upper waveguide layer 2 and the lower waveguide layer 4, and the upper and lower waveguides constitute a vertical coupler to gradually couple light from the upper waveguide layer to the lower waveguide layer. waveguide layer. In the direction of propagation, it is divided into an input region 9 , a coupling region ( 6 , 7 , 8 ), and an output region 10 .

In the input region, the mode propagation constant β ₁ of the upper waveguide is greater than the mode propagation constant β ₂ of the lower waveguide, half of the difference between the mode propagation constants of the upper and lower waveguides δβ=(R ₁ -β ₂ )/2 is greater than the coupling coefficient κ between the two waveguides , that is, δβ>κ, the upper waveguide mode is not affected by the existence of the lower waveguide; in the coupling region, as the width of the upper waveguide gradually decreases, it can be divided into three regions, namely, the front section of the rapid contraction section 6: the rapid reduction of the upper waveguide The width of the waveguide reduces the mode propagation constant to make it close to the mode propagation constant of the lower waveguide; the middle mode transfer section 7: gradually reduces the width of the upper waveguide, and the mode propagation constant of the upper waveguide decreases from slightly larger than that of the lower waveguide to slightly smaller than that of the lower waveguide The mode propagation constant of the upper waveguide mode is quickly transferred to the lower waveguide through resonance transfer; the rear fast shrinkage section 8: quickly reduce the width of the upper waveguide again to the cut-off of the upper waveguide mode, and suppress the light in the lower waveguide from coupling back to the upper waveguide. The output area has only the lower waveguide. By reducing the thickness of the spacer layer between the upper and lower waveguides, the coupling coefficient κ between the two waveguides is increased, thereby reducing the coupling length. The upper limit of the coupling coefficient increase is half of the maximum propagation constant difference δβ that can be realized between the upper and lower waveguides in the input area.

The width of the middle section 7 of the upper waveguide gradually decreases to satisfy the following formula:

β ₁ (A) = β ₂ + Δβ

β ₁ (B)=β ₂ -Δβ

In the above formula, β ₁ (A) is the mode propagation constant corresponding to the initial width of the upper waveguide in the middle section of the coupling region, β ₁ (B) is the mode propagation constant corresponding to the end width of the upper waveguide, and Δβ is the maximum The offset of the upper waveguide mode propagation constant brought by the manufacturing tolerance; the middle section of the coupling region, that is, the length of the mode transfer section is 1.5 to 4 times of L _c , and L _c is defined as the straight waveguide coupling length.

The ultra-short vertical waveguide coupler with large manufacturing tolerance in this embodiment has an operating wavelength of 1.30 μm, and the refractive index and structural parameters of different waveguide layers are given in the following table.

Table 1 Refractive index and structural parameters of each waveguide layer of the vertical coupler

Corresponding waveguide layer Refractive index thickness Overlay 1 3.2044 2μm Upper waveguide layer 2 3.3448 0.498μm Spacer 3 3.2044 d Lower waveguide layer 4 3.3048 0.4μm Substrate layer 5 3.2044 2μm

Fig. 2 is a graph showing the refractive index profile of the cross-section of the example of the present invention. For a vertical coupler with a constant upper waveguide width (that is, a straight waveguide), its top view structure is shown in Figure 3. According to the coupled wave theory, in the propagation direction, the normalized power distribution of the upper and lower waveguides is:

Where 2θ=arctan(κ/δβ), κ is the coupling coefficient, and δβ is half of the difference between the propagation constants β ₁ and β ₂ of the upper and lower waveguide modes during propagation (β ₁ -β ₂ )/2. It can be seen that the smaller the δβ, the higher the power transfer efficiency of the upper and lower waveguides; if the efficiency of power transfer to the lower waveguide mode is required to be greater than 90%, then: δβ/κ<1/3, that is, half of the difference between the propagation constants of the upper and lower waveguides must be Satisfy δβ<κ/3. When δβ=0, that is, when the phases are completely matched, the upper and lower waveguides realize the complete transfer of power. At this time, the width of the upper waveguide, that is, the matching width w _m , as shown in the figure, the coupling length L _c is:

It can be seen that the coupling length depends on the coupling coefficient between the upper and lower waveguides, the greater the coupling coefficient, the shorter the coupling length. The coupling coefficient depends on the degree of overlap between the upper and lower waveguide modes. The thinner the spacer layer, the greater the overlap between the two waveguide modes and the greater the coupling coefficient. Therefore, to obtain a small coupling length, the spacer layer between the two waveguides needs to be thin. Here, in order to increase the coupling coefficient and reduce the length of the vertical coupler, the thickness d of the spacer layer between the upper and lower waveguides is taken as 0.3 μm. Fig. 4 is a schematic diagram of the simulation results of power conversion between the upper and lower waveguides when the upper waveguide has a matching width. It can be seen that the optical power exhibits a periodic back-and-forth coupling behavior in the propagation direction.

Considering that in the actual manufacturing process, due to the influence of manufacturing accuracy and process repeatability, the upper waveguide width often deviates from the ideal matching width, so the mode transfer section usually adopts a waveguide structure with decreasing width. Figure 5 shows the relationship between the coupling efficiency of the upper waveguide power and the lower waveguide in the mode transfer section 7 as a function of the propagation length. The figure shows the coupling efficiency curves when the upper waveguide is a waveguide with different slope widths and a straight waveguide. It can be seen that the use of an upper waveguide structure with decreasing width can destroy the periodic back-and-forth coupling of power in the case of a straight waveguide, making the peak value of the back coupling smaller, thereby reducing the sensitivity of the coupler to structural changes; under a certain slope, The complete suppression of back coupling can be achieved, and the matching width w _m can be achieved when the upper waveguide width changes in a relatively large range under a certain slope, so that multiple power efficient transfer points are included, thereby increasing the fabrication Tolerance; when the slope increases, usually a longer coupling length is required to increase the tolerance of the coupler, which will also increase the transmission loss of the coupler. Therefore, while increasing the tolerance of the coupler, a relatively short coupler length should also be considered in order to improve integration and reduce transmission loss.

FIG. 6 is a top view structure diagram of the coupler of the present invention when the mode transfer section 7 adopts an upper waveguide with a certain slope and decreasing width. The figure includes the fast shrinking section of the waveguide width on the front and rear sections of the coupler. Here, the length of the middle section of the coupler, that is, the mode matching section, is 2 to 4 times the coupling length L _c of the straight waveguide. In this way, when the width of the waveguide on the mode transfer section increases or decreases in the same direction, the width of the waveguide can reach the matching width w _m . Specifically, in the case of width changes Δw ₁ and Δw ₂ , the mode matching width w _m can exist in two positions before and after, and the corresponding optical power can reach the optimal coupling condition, so that the coupling extreme value can occur in the two different width cases Next, this increases the tolerance of the waveguide width on the mode shifting section. Under the condition of existing lithographic precision, this vertical waveguide coupler with large manufacturing tolerance will significantly improve the yield of integrated devices.

In order to verify the manufacturing tolerance of the present invention, the ultra-short vertical waveguide coupler of the present invention is simulated by using BeamPROP software. In the simulation, the lengths of the two upper waveguide width rapid contraction sections of the front section and the rear section of the coupler are respectively set 6μm and 5μm. By analyzing the coupling efficiency when the upper waveguide of the mode transfer section 7 adopts a straight waveguide and a tapered waveguide structure, it is found that: for the straight waveguide structure shown in Fig. The coupling length is about 20 μm. The simulated coupling efficiency varies with the width of the upper waveguide as shown in the dotted line in Figure 7. It can be seen that the coupling efficiency curve has only one maximum value, and the upper waveguide width tolerance is about -0.06 μm when the coupling efficiency is 90%. <Δw<0.075μm; for the graded waveguide structure shown in Figure 4, when the length of the middle mode transfer section is about 40μm, the simulated coupling efficiency varies with the width of the upper waveguide as shown in the solid line in Figure 7, which is different from the straight waveguide and the graded waveguide There are two maxima in the coupling efficiency, and the upper waveguide width tolerance is about -0.2μm<Δw<0.19μm when the coupling efficiency is 90%, which is about 3 times the lower tolerance of the straight waveguide.

Figures 8 and 9 show the simulation results of two extreme coupling efficiencies of the coupler when the upper waveguide width changes in the mode transfer section are Δw ₁ and Δw ₂ , respectively, where the left figure represents the vertical coupler internal light The change of the field distribution along the propagation direction, and the right figure is the change diagram of the normalized optical power. On the right, the black and gray curves represent the upper and lower waveguide power as a function of propagation length, respectively. It can be seen from Fig. 8 that at the position of z=30 μm or so, the power of the lower waveguide increases significantly, and the optical power transfers rapidly. At the end of the vertical coupler, the down-waveguide power reaches its maximum and the coupling efficiency is about 97%. Similarly, in FIG. 9 , the optical power coupling mainly occurs at a position around z=42 μm, and the coupling efficiency is about 96%, corresponding to the maximum coupling positions in FIG. 8 one after the other. .

Finally, it should be noted that the above specific embodiments are only used to illustrate the technical solutions of the present invention and not limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that the present invention can be Modifications or equivalent replacements of the technical solutions without departing from the spirit and scope of the technical solutions of the present invention shall fall within the scope of the claims of the present invention.

Claims (6)

1. An ultrashort vertical waveguide coupler with a large manufacturing tolerance, characterized in that, on the vertical section from top to bottom of the ultrashort vertical waveguide coupler, there are respectively an upper waveguide, a spacer layer and a lower waveguide; In the direction of propagation, the ultrashort vertical coupler is divided into an input area, a coupling area and an output area; in the input area, the mode propagation constant β ₁ of the upper waveguide is greater than the mode propagation constant β 2 of the lower waveguide, and the mode propagation constant β ₂ of the upper and lower waveguides Half of the mode propagation constant difference, that is, δβ, δβ is greater than the coupling coefficient κ between the two waveguides, that is, δβ>κ, δβ=(β ₁ -β ₂ )/2, the upper waveguide mode is not affected by the existence of the lower waveguide; The coupling region, as the width of the upper waveguide gradually decreases, is divided into three sequentially connected sections: the front section, the middle section and the back section; the front section and the back section are the fast shrinking sections of the upper waveguide width, and the middle section is the mode transfer section; Increase the coupling coefficient κ between the two waveguides by reducing the thickness of the spacer layer between the upper and lower waveguides, thereby reducing the coupling length, and the upper limit of the increase in the coupling coefficient is half of the maximum propagation constant difference δβ that can be realized between the upper and lower waveguides in the input area; In the middle of the coupling region, the width of the upper waveguide gradually decreases, and the width of the upper waveguide gradually decreases to satisfy the following formula: β ₁ (A) = β ₂ + Δβ β ₁ (B)=β ₂ -Δβ In the above formula, β ₁ (A) is the mode propagation constant corresponding to the initial width of the upper waveguide in the middle section of the coupling region, β ₁ (B) is the mode propagation constant corresponding to the end width of the upper waveguide, Δβ is the upper waveguide The offset of the upper waveguide mode propagation constant brought by the maximum manufacturing tolerance; the length of the middle section of the coupling region is 1.5 to 4 times of L _c , and L _c is defined as the straight waveguide coupling length, L _c = π/(2κ) . 2. The ultra-short vertical waveguide coupler with large manufacturing tolerance according to claim 1, characterized in that, in the front section of the coupling region, the width of the upper waveguide decreases rapidly, and its end width is the same as the beginning of the middle section of the coupling region Same width. 3. The ultra-short vertical waveguide coupler with large manufacturing tolerance according to claim 1 or 2, characterized in that, in the rear section of the coupling region, the width of the upper waveguide decreases rapidly, and the initial width of the rear section is the same as that of the middle section The width of the end of the end is the same, the end width of the rear section is the minimum width allowed by the production, the end of the rear section is the termination end, and the end surface of the termination end is coated with an anti-reflection film to reduce reflection. 4. The ultra-short vertical waveguide coupler with large manufacturing tolerance according to claim 1, characterized in that, in the middle section of the coupling region, the upper waveguide mode is quickly transferred to the lower waveguide by means of resonance transfer. 5. The ultra-short vertical waveguide coupler with large manufacturing tolerance according to claim 1, characterized in that, in the middle section of the coupling region, when the upper waveguide width deviates due to the limitation of manufacturing accuracy, the upper waveguide mode can still Efficient down waveguide transfer at various locations in the midsection. 6. The ultrashort vertical waveguide coupler with large manufacturing tolerances according to claim 1, characterized in that there is only the lower waveguide in the output region.