CN113161433B - 100GHz traveling wave vertical direction coupling optical waveguide detector - Google Patents

Abstract

The invention discloses a 100GHz traveling wave vertical direction coupling optical waveguide detector, which comprises a biconical mode converter, a rectangular waveguide structure and a traveling wave vertical direction coupling optical waveguide detector; the biconical mode converter is a left and right side surface Right-angled trapezoid, isosceles trapezoid upper and lower bottom surfaces, and rectangular prism structures with different sizes on the front and rear sides; the rectangular waveguide structure is butt-coupled with the biconical mode converter structure; the traveling wave vertical coupling optical waveguide detector includes a substrate layer, a The waveguide structure and two ground electrodes on the substrate, the waveguide structure is located between the substrate and the signal electrode, the waveguide structure from bottom to top is: lower waveguide layer, coupling layer, upper waveguide layer, depletion layer, The absorption layer, the cladding layer, the lower waveguide layer and the coupling layer are butt-coupled with the rectangular waveguide. The invention integrates a biconical mode converter at the front end of the detector, so that the coupling efficiency is close to 95%; the dual waveguide structure is adopted, so that the photocurrent distribution is uniform and the output power is increased.

Description

100GHz Traveling Wave Vertically Coupled Optical Waveguide Detector

technical field

The invention belongs to the fields of optoelectronic technology and microwave photonics, in particular to a 100GHz traveling wave vertical direction coupling optical waveguide detector.

Background technique

A photodetector is an optoelectronic component that interacts with light and matter. It plays an important role in the fields of optical communication and optoelectronic technology. It converts the optical signal incident on the device into a radio frequency signal to realize the conversion between photoelectric signals. In wired and wireless communication systems, the bandwidth of the photodetector determines the capacity of the communication system, and the high saturation power determines the transmission distance of the signal. Therefore, the photodetector with large bandwidth and high saturation power is indispensable. In photodetectors, these two important parameters are limited by inherent physical factors such as their structures and materials, and it is difficult to achieve the optimum state simultaneously. For example, the bandwidth of conventional normal-incidence photodetectors is limited by the carrier transit time and RC circuit time, and the quantum efficiency is related to the thickness of its intrinsic layer, making the bandwidth and power contradictory; although edge incidence The structure of the waveguide detector improves the contradictory relationship between the bandwidth and power limited by the current-carrying transit time. However, in order to achieve the goal of high power, a longer optical waveguide is required, resulting in a large junction capacitance and a decrease in the overall bandwidth of the detector. . Therefore, in conventional normal-incidence photodetectors and edge-incidence waveguide detectors, large bandwidth and high saturation output power cannot be achieved simultaneously.

For a long time, in order to improve the mutual restriction of large bandwidth and high power of photodetectors, various photodetectors have been reported successively. From traditional PIN photodetectors to single-row carrier photodetector (UTCPD, Uni-Traveling Photodetector) structures, to optical waveguide detectors. In semiconductor physics, it can be known from the conservation of momentum that the mass of electrons is much smaller than that of holes, so the speed of electrons is greater than that of holes. Using this law in a single-row carrier photodetector, a P-type doped absorption layer is introduced, and electrons are used as source carriers to participate in the transition in the depletion layer, so that the bandwidth of the photodetector can be improved, while suppressing the The space charge effect is eliminated, which increases the output power of the detector.

With the rapid development of communications, the requirements for bandwidth and power are getting higher and higher. Waveguide Photodetector (WGPD, Waveguide Photodetector) is proposed, the absorption direction of light and the transit direction of carriers are perpendicular to each other, which overcomes the mutual restriction of bandwidth and quantum efficiency in traditional photodetectors. However, there are the following problems in single-waveguide photodetectors: the photocurrent is exponentially distributed along the waveguide direction, and the optical coupling loss is large. In order to solve the problem of uneven photocurrent distribution and large coupling loss of waveguide detectors, Directional Coupling Waveguide Photodetector (DCPD) is proposed. As shown in Figure 1(a), two waveguides are placed horizontally. Light is incident from the end face of waveguide A without the absorption layer. The incident light signal propagates along the waveguide A and is coupled to the waveguide B with the absorption layer. The absorption layer is located in the waveguide. Evanescent field location. At the beginning, the incident optical power is mainly concentrated in the waveguide A. Due to the coupling effect, the optical power in the waveguide B with the absorption layer is very weak, that is, the optical power in the absorption layer is also very weak, and the generated photocurrent is small. Therefore, the photocurrent of the photodetector with this structure is much weaker than that of the single-waveguide type front-end photocurrent. As the light propagates in the coupler, the optical power in the waveguide B gradually increases, because the waveguide absorbs the optical power. effect, the total optical power will decrease, so at the rear end of the directional coupler, the photocurrent will not decay rapidly, and within a certain length, the photocurrent along the waveguide will be more evenly distributed, and under the condition of mode matching, the photocurrent can be achieve even distribution.

The coupling layer in the horizontally coupled waveguide photodetector is an air gap layer, and the environment has a great influence on the coupling length and absorption length, which will lead to uneven photocurrent distribution. At the same time, the straight air gap between the two waveguides is difficult to etch in the process. To solve these problems, a Vertical Directional Coupling Waveguide Photodetector (VDCPD) is proposed, as shown in Figure 1(b). In the figure, the Cladding Layer is the cladding layer, the Absorber is the absorption layer, the Signal electrode is the signal electrode, the Ground electrode is the ground electrode, the Upper Waveguide is the upper waveguide, the Coupling Layer is the coupling layer, the Lower Waveguide is the lower waveguide, and the InPSubstrate is the InP substrate. The two waveguides of the VDCPD are placed in parallel in the vertical direction, and a low-refractive-index thin film coupling layer is embedded between the upper and lower waveguides. In this structure, when the coupling length of light and the absorption length of light are equal, the photocurrent is uniformly distributed along the photodetector, as shown in Figure 2. In the vertically coupled waveguide detector, the coupling of the incident light in the dual waveguide is the result of the mutual interference of the two supermodes. It is precisely because of the interference of the two supermodes that the optical power in the detector is no longer a purely exponential distribution. In the vertically coupled waveguide photodetector, when the coupling length is equal to the absorption length of the 0th-order supermode and the absorption length of the 1st-order supermode, the photocurrent distribution is the most uniform, which is the supermode matching condition. Compared with DCPD, VDCPD has the following advantages: (1) The absorption length of the zero-order supermode and the first-order supermode can be adjusted by changing the thickness of the upper and lower waveguides; (2) The zero-order supermode can be adjusted by changing the thickness of the gap layer. and the coupling length between the first-order supermode and the first-order supermode; (3) the laser beam output from the fiber and incident on the lower waveguide is more coupled into the zero-order supermode and the first-order supermode, so it has lower coupling loss; ( 4) The output photocurrent is not sensitive to the polarization angle of the incident beam.

SUMMARY OF THE INVENTION

The purpose of the present invention is to overcome the deficiencies of the prior art, and to provide a biconical mode converter integrated at the front end of the detector, so that the coupling efficiency is close to 95%; the single-row carrier structure is adopted, and the waveguide section is designed so that the Its characteristic impedance matches the load impedance, and realizes the 100GHz traveling wave vertical coupling optical waveguide detector in which light waves and microwaves are transmitted in the same waveguide structure.

The object of the present invention is achieved through the following technical solutions: 100GHz traveling wave vertical direction coupling optical waveguide detector, including a biconical mode converter with tapered horizontal and vertical directions, a rectangular waveguide structure and a traveling wave vertical direction coupled optical waveguide detector;

The biconical mode converter is a prismatic structure with right-angled trapezoids on the left and right sides, isosceles trapezoids on the upper and lower bottoms, and two rectangles with different sizes on the front and rear sides. The small rectangular side is used as the waveguide output;

The rectangular waveguide structure is butt-coupled with the waveguide output end of the biconical mode converter structure, and the cross-sectional size of the rectangular waveguide structure is the same as the side surface of the butt-coupled biconical mode converter structure; the rectangular waveguide structure is sequentially from bottom to top Including waveguide layer and cladding;

The traveling wave vertical coupling optical waveguide detector includes a substrate layer, a waveguide structure placed on the substrate, a signal electrode, and two ground electrodes. The waveguide structure is located between the substrate and the signal electrode, and the waveguide structure and the substrate layer are formed. The cross-section of the structure is in a "convex" shape, and the two ground electrodes are symmetrically arranged with respect to the waveguide structure; the waveguide structure from bottom to top is: lower waveguide layer, coupling layer, upper waveguide layer, depletion layer, absorption layer, envelope The cross-sectional dimension of the lower waveguide layer is the same as that of the rectangular waveguide, and the lower waveguide layer and the coupling layer are butt-coupled with the rectangular waveguide.

Further, the input end of the biconical mode converter is connected to the optical fiber, and the circular light spot coupled in from the optical fiber is gradually converted into the rectangular light spot in the rectangular waveguide structure through the gradient waveguide structure, so as to realize the mode conversion, and the biconical light spot is realized. The size of the waveguide input end of the mode converter structure is 6 μm × 6 μm, the size of the waveguide output end is 2.91 μm × 3 μm, and the length is 1 mm.

Further, the rectangular waveguide structure is used to stabilize the rectangular light spot output from the mode converter, and its cross-sectional size is 2.91 μm×3 μm and the length is 0.5 mm.

Further, the lower waveguide layer and the coupling layer of the traveling wave vertical coupling optical waveguide detector are coupled to the rectangular waveguide, and the incident light is coupled to the upward waveguide while propagating in the traveling wave vertical coupling optical waveguide detector, and is in the absorption layer. It is absorbed in the medium to generate photogenerated carriers, and under the action of an external reverse bias (the signal electrode is connected to the negative electrode of the DC power supply, and the ground electrode is connected to the positive electrode of the DC power supply), an optical current is formed, and is transmitted along the waveguide to the load; the traveling wave is perpendicular to the direction. The length of the coupled optical waveguide detector is 0.5 mm, and both the ground electrode and the signal electrode are metal electrodes with a thickness of 0.1 μm.

Further, under the bottom surface of the biconical mode converter, there is a substrate layer with the bottom surface having the same size as the bottom surface of the biconical mode converter and the same height as the substrate layer of the optical waveguide detector for coupling in the vertical direction of the traveling wave. A substrate layer is arranged under the waveguide layer of the rectangular waveguide structure. The bottom surface of the substrate layer has the same size as the bottom surface of the rectangular waveguide layer, the height is the same as the substrate layer of the traveling wave vertical coupling optical waveguide detector, and the length is the same as that of the rectangular waveguide.

Compared with the traditional photodetector, the 100GHz traveling wave vertically coupled optical waveguide detector of the present invention has the following advantages: (1) a biconical mode converter is integrated in the front end of the detector, so that the coupling efficiency is close to 95%; (2) the use of The double-waveguide structure, under the supermode matching condition, makes the photocurrent distribution uniform and increases the output power; (3) Compared with the traditional photodetector, the present invention adopts a single-row carrier structure, and the waveguide cross section is designed to make its characteristics The impedance matches the load impedance, which realizes the transmission of light waves and microwaves in the same waveguide structure, and the response bandwidth reaches about 100GHz. In theory, the detector can be infinitely long, and it is easier to integrate with external loads (antennas, etc.).

Description of drawings

Figure 1 is a dual waveguide photodetector, wherein Figure 1 is (a) a horizontally coupled waveguide photodetector, and (b) is an asymmetric vertically coupled waveguide photodetector;

Fig. 2 is a photocurrent distribution diagram of the two detectors shown in Fig. 1 under different conditions;

FIG. 3 is the overall structure of the 100GHz traveling wave vertically coupled optical waveguide detector proposed by the present invention;

Figure 4 is the equivalent circuit model of the transmission line, wherein (a) is the epitaxial layer structure of the optical waveguide detector coupled in the vertical direction of the traveling wave, and Figure 4 (b) is the equivalent circuit model;

Figure 5 is the impedance diagram, the propagation length and the electrical phase velocity diagram calculated by the equivalent circuit model;

6 is the structure of the biconical mode converter of the first part of the 100 GHz traveling wave vertically coupled optical waveguide detector proposed by the present invention, 6(a) is the refractive index distribution diagram of the front and rear ends, 6(b) is the input light spot and The output spot diagram, 6(c) is the normalized optical power distribution diagram in the biconical mode converter;

Fig. 7 is the refractive index distribution diagram of the second part of the rectangular waveguide structure of the 100 GHz traveling wave vertically coupled optical waveguide detector proposed by the present invention;

FIG. 8 is the result of simulation analysis of the epitaxial layer structure of the traveling wave vertical coupling optical waveguide detector; wherein, (a) is the third part of the rectangular waveguide structure of the 100 GHz traveling wave vertical coupling optical waveguide detector proposed by the present invention The refractive index distribution diagram of , 8(b) is the total optical power distribution diagram; 8(c) is the simulation diagram of the optical waveguide detector coupled in the vertical direction of the traveling wave;

Fig. 9 is a 3dB bandwidth diagram limited by the carrier transit time for different intrinsic layer thicknesses;

Figure 10 is a 3dB bandwidth diagram of the optical group velocity and electrical phase velocity limitations in the traveling wave vertically coupled optical waveguide detector.

Detailed ways

In order to solve the problems of narrow response bandwidth and low output power of the waveguide detector, the invention first integrates a biconical mode converter and a rectangular optical waveguide at the front end of the detector to improve the optical coupling efficiency of the optical fiber to the waveguide detector; Then, the waveguide width of the cross section of the waveguide detector and the thickness of the intrinsic layer are designed by using the traveling wave theory, so that the characteristic impedance of the detector is 50Ω, which matches the load and realizes the electric traveling wave transmission. Finally, according to the supermode According to the matching theory, the thickness of the coupling layer of the optical waveguide is reasonably designed, so that the coupling length of the mode and the absorption length are equal, the photocurrent is uniformly distributed along the waveguide, and at the same time, it can suppress the space charge effect and improve the external quantum efficiency.

In order to improve the coupling efficiency of light from the fiber to the detector, the present invention integrates a biconical mode converter at the front end of the VDCPD, so that the line mode in the fiber matches the mixing in the waveguide; finally, in order to improve the bandwidth of the VDCPD, While maintaining its high power, a 100GHz traveling wave photodetector is designed, as shown in Figure 3. In the figure, Light is the incident light, L _covert is the length of the biconical mode converter, L _WG is the length of the rectangular waveguide, and L _TWPD is the length of the traveling wave vertical coupling optical waveguide detector, Cladding Layer is the cladding layer, Absorber is the absorption layer, Depletion Layer is the depletion layer, Upper Waveguide is the upper waveguide, Coupling Layer is the coupling layer, Lower Waveguide is the lower waveguide, Signalelectrode is Signal electrode, Ground electrode is ground electrode, InP Substrate is InP substrate. The difference from the lumped photodetector is that the traveling wave photodetector is a distributed structure, and the bandwidth of the detector is determined by the carrier transit time, optical group velocity and electrical phase velocity mismatch, optical absorption coefficient, microwave loss, etc. By designing the cross-section of the VDCPD, its characteristic impedance matches the load impedance of the external circuit to support traveling wave transmission. Incident light propagates in the detector, and due to the evanescent field, photogenerated carriers are generated in the absorber layer, forming a photocurrent. The photocurrent is continuously enhanced by the new photocurrent generated by the optical signal during the propagation process. When the group velocity of the optical signal in the detector matches the phase velocity of the resulting electrical signal, the bandwidth of the detector will only be related to the carrier transit time. To realize the traveling wave detector, the key is to design the width of the detector, the thickness of the intrinsic layer, and the structure of the transmission line, so that the characteristic impedance of the detector is 50Ω, and at the same time, the group velocity of light in the waveguide is synchronized with that generated by the detector The electrical signals have the same phase velocity.

The technical solutions of the present invention are further described below with reference to the accompanying drawings.

As shown in FIG. 3 , the 100 GHz traveling wave vertically coupled optical waveguide detector of the present invention includes three parts, which are a biconical mode converter with tapered horizontal and vertical directions, a rectangular waveguide structure and a traveling wave. Vertically coupled optical waveguide detector;

The biconical mode converter is a prismatic structure with right-angled trapezoids on the left and right sides, isosceles trapezoids on the upper and lower bottoms, and two rectangles with different sizes on the front and rear sides. The small rectangular side is used as the waveguide output;

The rectangular waveguide structure is butt-coupled with the waveguide output end of the biconical mode converter structure, and the cross-sectional size of the rectangular waveguide structure is the same as the side surface of the butt-coupled biconical mode converter structure; the rectangular waveguide structure is sequentially from bottom to top Including waveguide layer and cladding layer (coupling layer);

The traveling wave vertical coupling optical waveguide detector includes a substrate layer, a waveguide structure placed on the substrate, a signal electrode, and two ground electrodes. The waveguide structure is located between the substrate and the signal electrode, and the waveguide structure and the substrate layer are formed. The cross-section of the structure is in a "convex" shape, and the two ground electrodes are symmetrically arranged with respect to the waveguide structure; the waveguide structure from bottom to top is: lower waveguide layer, coupling layer, upper waveguide layer, depletion layer, absorption layer, envelope The cross-sectional dimension of the lower waveguide layer is the same as that of the rectangular waveguide, and the lower waveguide layer and the coupling layer are butt-coupled with the rectangular waveguide.

Further, the input end of the biconical mode converter is connected to the optical fiber, and the circular light spot coupled in from the optical fiber is gradually converted into the rectangular light spot in the rectangular waveguide structure through the gradient waveguide structure, so as to realize the mode conversion, and the biconical light spot is realized. The size of the waveguide input end of the mode converter structure is 6 μm × 6 μm, the size of the waveguide output end is 2.91 μm × 3 μm, and the length is 1 mm.

Further, the rectangular waveguide structure is used to stabilize the rectangular light spot output from the mode converter, and its cross-sectional size is 2.91 μm×3 μm and the length is 0.5 mm.

Further, the lower waveguide layer and the coupling layer of the traveling wave vertical coupling optical waveguide detector are coupled to the rectangular waveguide, and the incident light is coupled to the upward waveguide while propagating in the traveling wave vertical coupling optical waveguide detector, and is in the absorption layer. It is absorbed in the medium to generate photogenerated carriers, and under the action of an external reverse bias (the signal electrode is connected to the negative electrode of the DC power supply, and the ground electrode is connected to the positive electrode of the DC power supply), an optical current is formed, and is transmitted along the waveguide to the load; the traveling wave is perpendicular to the direction. The length of the coupled optical waveguide detector is 0.5 mm, and both the ground electrode and the signal electrode are metal electrodes with a thickness of 0.1 μm.

Further, under the bottom surface of the biconical mode converter, there is a substrate layer with the bottom surface having the same size as the bottom surface of the biconical mode converter and the same height as the substrate layer of the optical waveguide detector for coupling in the vertical direction of the traveling wave. A substrate layer is arranged under the waveguide layer of the rectangular waveguide structure. The bottom surface of the substrate layer has the same size as the bottom surface of the rectangular waveguide layer, the height is the same as the substrate layer of the traveling wave vertical coupling optical waveguide detector, and the length is the same as that of the rectangular waveguide. All the substrate layers of the present invention use InP substrates, and the thickness of the substrate layers is several hundreds of microns.

The invention is limited to design the cross section of the third part of the structure. By adjusting the waveguide width W ₂ and the thickness t _d of the intrinsic layer, the characteristic impedance of the optical waveguide detector coupled in the vertical direction of the traveling wave is changed, so that the impedance is the same as that of the external The impedance of the load is matched to achieve the purpose of high-speed and high-power transmission; then, in order to improve the optical coupling efficiency of the fiber to the detector, a biconical mode converter and a rectangular waveguide are integrated at the front end of the detector. The circular light spot in the fiber matches the quasi-rectangular light spot in the detector, so as to improve the coupling efficiency and the internal quantum efficiency.

Manufacturing process: On the InP substrate, grow the InP stress buffer layer to the InGaAsP waveguide layer (undoped bulk material) in turn, and use ion implantation technology for doping to form the vertical vertical in the first part of the biconical mode converter structure. The vertical trapezoidal refractive index distribution in the direction, followed by etching the horizontal isosceles trapezoid structure and the subsequent core layer of the rectangular waveguide structure using photolithography technology, so that the length of the biconical mode converter L _convert = 1mm, and then etched The InP coupling layer is grown on the good rectangular waveguide. As for the P-InGaAs contact layer and electrodes, photolithography and reactive etching techniques are used to form the second part of the rectangular waveguide structure with L _WG = 0.5mm and the third part of L _TWPD = 0.5mm The epitaxial layer structure of the traveling wave coupled optical waveguide detector in the vertical direction. Since the metal electrode is too thick, it will cause the metal electrode to fall off during the production process. If the metal electrode is too thin, the ohmic contact will be poor, and the Schottky barrier may be traveled, which will affect the performance. After a lot of experiments, the thickness of the metal electrode is determined to be 0.1 in the present invention. μm. So far, the entire ridge waveguide structure as shown in FIG. 3 is completed.

The working principle of the present invention is:

The light exits from the fiber and is coupled into the front end of the biconical mode converter. The mode is diffracted in the mode converter, and the circular light spot emitted from the fiber is diffracted into a quasi-rectangular light spot. The incident light passing through the mode converter is in the rectangular shape. Mode stabilization is performed in the optical waveguide, so as to better couple with the lower waveguide in the optical waveguide detector in the vertical direction of the traveling wave coupling. The light coupled from the rectangular waveguide into the lower waveguide is coupled in the upper and lower waveguides of the optical waveguide detector coupled in the vertical direction of the traveling wave. Due to the presence of a P-type doped absorption layer in the evanescent field of the upper waveguide, the light will be absorbed, resulting in The photogenerated carriers form a photocurrent under the action of an external bias voltage.

In the process of light propagation, the fundamental supermode and the first-order supermode are excited, and the two modes interfere in the directional coupler, so that the optical power is distributed more uniformly. In the case of no absorption or loss, the propagation constants of the two modes are real numbers, and the interference results show that the light in the waveguide changes periodically along the waveguide, and the optical power is periodically coupled back and forth from the coupling layer to the upper or lower waveguide, and the total optical power does not Change. In the case of an absorption layer, the propagation constants of the two modes are complex numbers, and the interference results show that the optical power in the waveguide periodically changes along the longitudinal direction of the waveguide, and the total optical power is affected by absorption attenuation and interference at the same time. Therefore, the incident light is absorbed in the absorption layer, and an electrical signal is continuously generated, and the electrical signal is continuously strengthened by a new electrical signal generated by the optical signal during the propagation process. When the characteristic impedance of the detector matches the load impedance, that is, there is no reflection at the load end, the detector obtains the maximum output power.

The following is an example of a traveling wave optical waveguide detector with a working wavelength of 1.55μm, the absorption layer material InGaAs, the waveguide layer material InGaAsP, the substrate layer and the coupling layer material InP, and the metal electrode is the gold electrode, as shown in Figure 3.

The basic theoretical parameters of the 100GHz traveling wave vertically coupled optical waveguide detector, the refractive index and conductivity of the materials used are shown in Table 1, and the thickness, width and length of the structural parameters of each part are shown in Table 2.

Table 1

Material Thickness(μm) Index(n) Conductivity(s/m) Type In_0.53Ga_0.47As 0.05 3.56 848583 P-Contact InP 0.8 3.21 59624 P-Block In_0.53Ga_0.47As 0.11 3.56 72015 P-Absorber In_0.47Ga_0.29As_0.64P_0.36 0.25 3.33 0 Spacer In_0.47Ga_0.29As_0.40P_0.60 0.2 3.33 1374 N-Depletion In_0.47Ga_0.29As_0.40P_0.60 2.39 3.33 9366 Upper-Waveguide InP 0.09 3.21 9366 Coupling Layer In_0.47Ga_0.29As_0.40P_0.60 2.82 3.33 9366 Lower-Waveguide In_0.53Ga_0.47As 0.3 3.56 848583 N-Contact InP 0.3 3.21 0 Buffer InP - 3.21 113230 Substrate

Table 2

W ₃ is the width of the ridge waveguide and the ground electrode of the coupled waveguide detector in the vertical direction of the traveling wave, and W ₄ is the width of the ground electrode of the coupled waveguide detector in the vertical direction of the traveling wave.

The realization principle of the present invention is further elaborated below:

The bandwidth of the 100 GHz traveling wave vertically coupled optical waveguide detector is mainly determined by two factors, namely, the transit time of electrons, and the mismatch between the optical group velocity and the electrical phase velocity. Therefore, the microwave characteristics of the detector are very important. In the third part of the present invention, the equivalent circuit model of the transmission line is used to analyze the microwave characteristics of the optical waveguide detector coupled in the vertical direction of the traveling wave per unit length. Sophisticated analysis, the study found that the electrical phase velocity and characteristic impedance are related to the detector inductance and capacitance.

The invention models the equivalent circuit of the optical waveguide detector coupled in the vertical direction of the traveling wave, and the multi-layer transmission line telegraph equation can be derived from Maxwell's equations and the equivalent structure of the multi-layer transmission line as:

i表示探测器波导各层，探测器的特性阻抗

R为串联电阻，L为单位长度电感，C为单位长度电容，G为单位长度电导，L＝μ₀D/w，C_i＝ε_iw/d_i，G_i＝σ_iw/d_i。where: V(z) and I(z) are the voltage and current at different positions z along the transmission line, respectively,

i represents each layer of the detector waveguide, the characteristic impedance of the detector

R is series resistance, L is inductance per unit length, C is capacitance per unit length, G is conductance per unit length, L=μ ₀ D/w, C _i =ε _i w/d _i , G _i =σ _i w/d _i .

From the theoretical derivation, it can be seen that the characteristics of the traveling wave optical waveguide detector can be explained by applying resistance and capacitance in the equivalent circuit. In the circuit model shown in Figure 4, as shown in Figure 4(a), the barrier layer is R ₂ C ₂ In parallel, the absorption layer is represented by R ₃ C ₃ in parallel, the space charge region is represented by R ₄ C ₄ in parallel and R ₅ C ₅ in parallel, the collection layer is represented by R ₆ C ₆ in parallel, and the waveguide layer and buffer layer are represented by R ₇ respectively. C ₇ and R ₈ C ₈ in parallel, the series resistance R ₁ represents the resistance effect of the doping material and the ohmic contact, L is the detector inductance, and the equivalent circuit model is shown in Figure 4(b).

From the equivalent circuit model, it can be known that the microwave propagation constant is:

The electrical phase velocity is:

where α is the electrical attenuation constant, β is the electrical propagation constant, and ω is the angular frequency of the microwave signal to which the detector responds.

The microwave characteristics of the optical waveguide detector coupled in the vertical direction of the wave are obtained from the equivalent circuit model, as shown in Figure 5. 5(a) is the frequency-dependent characteristic impedance of the traveling wave detector, the abscissa Frequency is the frequency, and the ordinate Characteristic Impedance is the characteristic impedance; at the frequency of 1GHz, the real part of the characteristic impedance is very large; at 1-200GHz , the real part is about 50Ω, and the imaginary part is close to 0; above 200GHz, both the real part and the imaginary part of the characteristic impedance increase. Therefore, at 100GHz, the vertical coupling of the optical waveguide detector and the external load can achieve a good match. 5(b) Frequency on the abscissa is the frequency, and Propagation Length on the ordinate is the propagation length, indicating that at 100 GHz, the attenuation length of the microwave is 100 μm, so the length of the electric waveguide is not considered in the present invention. 5(c) Frequency on the abscissa is the frequency, and Electrical Phase Velocity on the ordinate is the electrical phase velocity; it shows that at 100 GHz, the optical group velocity is about 4 times the electrical phase velocity, so the velocity mismatch will have an impact on the bandwidth of the traveling wave detector . The equivalent circuit model is compared and verified by HFSS simulation, and the two are consistent within the allowable error range.

In the present invention, the front-end cross-sectional area of the biconical mode converter using Beampro software is larger, and the circular light spot emitted by the ball lens fiber can better coincide with the front-end of the mode converter, which is beneficial to the coupling of light. The circular light spot emitted from the fiber is gradually transformed into a rectangular-like light spot in the mode converter, and the mode conversion efficiency is as high as 90%. Fig. 6(b) is the transverse field distribution graph at z=0 and z=1000; Fig. 6(c) is the normalized value of Monitor Value, where Pathway Monitor is the monitoring path, Power is the optical power. In the above figures, X is the waveguide width, Y is the waveguide thickness, and Z is the propagation length.

Figure 7 is the transverse field distribution diagram of the rectangular waveguide at z=1000; X is the width of the waveguide, and Y is the thickness of the waveguide; the light spot emitted by the mode converter propagates for a certain distance in the rectangular waveguide, and has achieved the effect of stabilizing the mode, making the conversion The latter mode is consistent with the mode in the waveguide under the waveguide detector, improving the coupling efficiency, as shown in Fig. 6(b).

The epitaxial layer structure of the optical waveguide detector coupled in the vertical direction of the traveling wave is simulated and analyzed, as shown in Figure 8. Figure 8(a) is the lateral refractive index distribution at z=1500; Figure 8(b) is the optical power distribution in the traveling wave detector; the attenuation length of the optical power in the detector is about 300 μm and the coupling length of the mode It is about 150 μm, which approximately meets the supermode matching condition, and the photocurrent in the traveling wave photodetector is relatively uniformly distributed on the waveguide with a length of 300 μm. Figure 8(c) shows the optical power distribution of the entire structure of the present invention. From the curve in the figure, the coupling efficiency of light is about 90% and the absorption efficiency is 100%. Therefore, high output power can be theoretically achieved. In the figure, Monitor Value is the normalized value of the monitor, Pathway Monitor is the monitoring path, Power is the optical power, X in the above figures is the waveguide width, Y is the waveguide thickness, and Z is the propagation length.

In the present invention, the 3dB bandwidth determined by the carrier transition is mainly determined by the drift time of the carriers in the intrinsic layer. Therefore, the Silvaco software is used to simulate and analyze the optical waveguide detector coupled in the vertical direction of the wave. Transit velocity in the intrinsic layer. The bandwidth determined by the carrier drift time is obtained from t=t _d /ve and f _3dB = 1/ _2πt , as shown in Figure 9. In the figure, the abscissa Thickness of Space Charge Region is the space charge region, and the ordinate is 3dB Bandwidth is the 3dB bandwidth, and Carrier Drift is the carrier drift. When the intrinsic layer thickness is 0.25μm, the 3dB determined by the carrier drift time is 100GHz. The overall bandwidth is mainly determined by the carrier transit time and velocity mismatch. Thus, the bandwidth limited by the speed mismatch is further analyzed. In a traveling wave photodetector, the photocurrent is:

where Q is the total charge, ω _f is the characteristic frequency of the forward wave, ω _r is the characteristic frequency of the backward wave, γ is the reflection coefficient of microwave at the input of the traveling wave detector, l is the length of the detector, υ _o and υ _e are the optical group velocity and the electrical phase velocity, respectively. The 3dB bandwidth limited by the above available speed mismatch is shown in Figure 10. In the figure, Frequency is the frequency, and FractionalPhotocurrent is the photocurrent ratio. When the input end of the traveling wave detector is mismatched, that is, when γ=1, the bandwidth is about 150 GHz; when the input end of the traveling wave detector is matched, that is, when γ=0, the bandwidth is about 500 GHz. Although, when the input ends are mismatched, the bandwidth is much smaller than when the input is matched, but the output power is twice as large as the mismatch. Therefore, high output power can be achieved with input mismatch, provided the bandwidth is sufficient.

To sum up, the 3dB bandwidth of the optical waveguide detector coupled in the vertical direction of the traveling wave is 100 GHz, which shows that the invention is feasible.

Those of ordinary skill in the art will appreciate that the embodiments described herein are intended to assist readers in understanding the principles of the present invention, and it should be understood that the scope of protection of the present invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific modifications and combinations without departing from the essence of the present invention according to the technical teaching disclosed in the present invention, and these modifications and combinations still fall within the protection scope of the present invention.

Claims (6)

1. A 100GHz traveling wave vertical coupling optical waveguide detector, characterized in that it includes a biconical mode converter with tapered horizontal and vertical directions, a rectangular waveguide structure and a traveling wave vertical coupling optical waveguide detector; The biconical mode converter is a prismatic structure with right-angled trapezoids on the left and right sides, isosceles trapezoids on the upper and lower bottoms, and two rectangles with different sizes on the front and rear sides. Small rectangular side as the waveguide output; The rectangular waveguide structure is butt-coupled with the waveguide output end of the biconical mode converter structure, and the cross-sectional size of the rectangular waveguide structure is the same as the side surface of the butt-coupled biconical mode converter structure; the rectangular waveguide structure is sequentially from bottom to top Including waveguide layer and cladding; The traveling wave vertical coupling optical waveguide detector includes a substrate layer, a waveguide structure placed on the substrate, a signal electrode, and two ground electrodes. The waveguide structure is located between the substrate and the signal electrode, and the waveguide structure and the substrate layer are formed. The cross-section of the structure is in a "convex" shape, and the two ground electrodes are symmetrically arranged with respect to the waveguide structure; the waveguide structure from bottom to top is: lower waveguide layer, coupling layer, upper waveguide layer, depletion layer, absorption layer, envelope The cross-sectional dimension of the lower waveguide layer is the same as that of the rectangular waveguide, and the lower waveguide layer and the coupling layer are butt-coupled with the rectangular waveguide. 2 . The 100 GHz traveling wave vertical coupling optical waveguide detector according to claim 1 , wherein the input end of the biconical mode converter is connected to the optical fiber, and the circular optical fiber coupled in from the optical fiber is formed by the tapered waveguide structure. 3 . The shape light spot is gradually converted into a rectangular light spot in a rectangular waveguide structure to realize mode conversion. The size of the input end of the waveguide of the biconical mode converter structure is 6 μm × 6 μm, the size of the output end of the waveguide is 2.91 μm × 3 μm, and the length is 1 mm. 3 . The 100 GHz traveling wave vertically coupled optical waveguide detector according to claim 1 , wherein the rectangular waveguide structure is used to stabilize the rectangular light spot output from the mode converter, and its cross-sectional size is 2.91 μm× 3μm, length is 0.5mm. 4 . The 100 GHz traveling wave vertical coupling optical waveguide detector according to claim 1 , wherein the lower waveguide layer and the coupling layer of the traveling wave vertical coupling optical waveguide detector are coupled and docked with the rectangular waveguide, and the incident light In the optical waveguide detector coupled in the vertical direction of the traveling wave, the traveling wave is coupled to the waveguide while propagating, and is absorbed in the absorption layer to generate photo-generated carriers, which form an optical current under the action of an external reverse bias, and are transmitted along the waveguide to the load; The length of the optical waveguide detector coupled in the vertical direction of the traveling wave is 0.5 mm, and both the ground electrode and the signal electrode are metal electrodes with a thickness of 0.1 μm. 5 . The 100 GHz traveling wave vertically coupled optical waveguide detector according to claim 1 , wherein the bottom surface of the biconical mode converter is provided with a bottom surface having the same size as the bottom surface of the biconical mode converter. 6 . , and the height is the same substrate layer as the substrate layer of the optical waveguide detector coupled in the vertical direction of the traveling wave. 6 . The 100 GHz traveling wave vertically coupled optical waveguide detector according to claim 1 , wherein a substrate layer is provided below the waveguide layer of the rectangular waveguide structure, and the bottom surface of the substrate layer has the same size as the bottom surface of the rectangular waveguide layer, 7 . The height is the same as the substrate layer of the optical waveguide detector coupled in the vertical direction of the traveling wave, and the length is the same as that of the rectangular waveguide.

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