CN104238010B - A kind of front end input waveguide structure of direction coupling optical waveguide detector - Google Patents

Abstract

The invention belongs to the field of optoelectronic technology, and discloses a front-end input waveguide structure of a directionally coupled optical waveguide detector, including a substrate layer, a covering layer, a lower waveguide layer, a gap layer, an upper waveguide layer, and an absorption layer stacked in sequence from bottom to top , the substrate layer, the cover layer, the lower waveguide layer, the gap layer, and the front end of the upper waveguide layer are flush, the absorbing layer is located on the rear upper surface of the upper waveguide layer, the gap layer is the coupling layer, and the front end of the absorbing layer is the same as the front end of the upper waveguide layer The distance is twice the coupling length. The invention overcomes the shortcomings of the waveguide detector and the horizontal coupling waveguide detector, can effectively increase the photocurrent, and also solves the problem of overheating and burning of the front end of the waveguide due to the mode evolution problem.

Description

Front-end input waveguide structure of a directionally coupled optical waveguide detector

technical field

The invention belongs to the field of optoelectronic technology, and relates to a waveguide photodiode, in particular to a front-end input waveguide structure of a directional coupling optical waveguide detector.

Background technique

The high-power high-speed photodetector is a detection device based on the interaction between light and matter, and its function is to convert the incident light signal into a high-power high-frequency signal. High-power high-speed optical detector is an indispensable device in optically controlled phased array radar, ultra-high-speed test system and optical fiber LAN communication, and its performance plays a decisive role in the whole system.

Conventional vertical-incidence photodetectors cannot meet both high-speed and high-power requirements. The main reasons are as follows: one is the saturation effect, which limits the photocurrent; the other is the long transit time, which limits the response frequency; the third is that the light absorption of the intrinsic layer is exponentially attenuated, the volume of the absorption region is thin, and the total photocurrent is small .

In order to overcome the contradiction between high power and high speed, the high-power high-speed photodetector uses a waveguide photodetector (WGPD) to eliminate the influence of electron transit time in the depletion layer on the response speed, thereby overcoming the high-speed photodetector in traditional photodetectors. Contradiction between response performance and quantum efficiency. The structure of the waveguide detector is shown in Figure 1. After the light is incident from the end face of the waveguide 10, it is coupled to the absorbing layer 6 to be absorbed while propagating in the waveguide 10, and is converted into electron-hole pairs, which ensures more uniform absorption in the length direction of the device. The transit time of photogenerated carriers is determined by the thickness of the absorbing layer 6, and the quantum efficiency is determined by the length of the detector, which resolves the contradiction between response efficiency and quantum efficiency. The incident direction of light is perpendicular to the electric field direction of drift, and the absorption layer 6 is much thinner than that of the surface vertical detector, so the drift time of carriers is much faster. Gradually coupled waveguide photodetectors are suitable for monolithic integration, coupled with the continuous improvement of the coupling efficiency of optical fibers and detectors, the quantum efficiency has been further improved.

In 1986, J.E.BOWERS and others obtained the first high-speed side-coupled WGPD with a quantum efficiency of 25% and a 3dB bandwidth of 28GHz. Due to reasons such as fiber diameter and detector optical field diameter, the coupling efficiency of incident light and detector is very low. In 1991, K.Kato and D.Wake et al. proposed a dual-core waveguide detector structure, as shown in Figure 2, adding a layer of doped waveguide 10 around the indium gallium arsenide (InGaAs) absorption layer 6 , so that the coupling efficiency of the optical fiber and the detector reaches 80%, the quantum efficiency reaches 40%, and the bandwidth reaches 50GHz, which is an important breakthrough for the waveguide detector to move towards the practical stage.

According to the classification of the detector structure, there are side-incidence WGPD, dual-waveguide WGPD, gradient coupling WGPD, traveling-wave photodetector (TWPD) and so on.

The existing problems of waveguide detectors are: the distribution of photocurrent along the waveguide is uneven, it is exponentially attenuated, and the coupling loss is large; the photocurrent is very strong at the front end of the waveguide, and gradually weakens in the direction of propagation, and the front end of the waveguide determines the photocurrent. Saturation value, which limits the incident optical power.

In 2008, in order to solve the problem of uneven photocurrent distribution and large coupling loss of waveguide detectors, the University of California, San Diego proposed a directional coupling photodiode solution. As shown in Figure 3, two waveguides placed in parallel plus a coupling layer 4 in the middle, the light is incident from the end face of the waveguide A8 without the coupling layer, while propagating, it is coupled into the waveguide B9 with the absorbing layer 6, and the absorbing layer 6 is located at Waveguide evanescent field location. Initially, the incident optical power is concentrated on the waveguide A8 without the absorbing layer 6, and the optical power in the waveguide B9 with the absorbing layer 6 is very weak, that is, the optical power in the absorbing layer 6 is also very weak. Therefore, this photocurrent is much weaker than the front-end photocurrent of the waveguide type. As the light propagates in the coupler, the optical power coupled to the waveguide with the absorbing layer gradually increases, and the total power will decrease due to the absorption of the waveguide, so At the back end of the directional coupler, the photocurrent will not decay rapidly. Within a certain length, the photocurrent is distributed evenly along the waveguide. Under suitable conditions, the photocurrent can achieve the most uniform distribution. The structural defect is: the air gap at the upper end of the coupling layer 4 has a great influence on the coupling length and the absorption length, and when the width of the air gap changes, the photocurrent distribution will be uneven. The photoetching process of the air gap on the coupling layer 4 is also not easy to manufacture.

In order to improve the vertical direction coupling waveguide diode structure, a vertical direction coupler is also proposed. Directionally coupled waveguide photodetectors must meet a condition called supermode matching to achieve high-power operation. Supermode matching condition: the coupling length of the fundamental supermode and the first-order supermode of the directional coupler is equal to the absorption length of the fundamental supermode and also equal to the absorption length of the first-order supermode. Experiments have proved that the vertically coupled waveguide detector effectively overcomes the shortcomings of the waveguide detector and the horizontally coupled waveguide detector, and can effectively increase the photocurrent. The limiting factor to further increase the photocurrent is the mode evolution at the front end of the waveguide. At the front end of the waveguide, the incident light mainly excites the fundamental supermode and the first-order supermode, and also inevitably excites the higher-order supermode. The excited fundamental supermode and the first-order supermode are necessary for the detector to work, but the high-order supermode will cause the front end of the waveguide to be burned. Local overheating and burning.

Contents of the invention

The purpose of the present invention is to provide a front-end input waveguide structure of a direction-coupled optical waveguide detector, which has a simple structure and solves the above problems well. It also solves the problem of overheating and burning of the front end of the waveguide due to the mode evolution problem.

The technical solution of the present invention is: a front-end input waveguide structure of a directionally coupled optical waveguide detector, including a substrate layer, a cover layer, a lower waveguide layer, a gap layer, an upper waveguide layer, and an absorption layer stacked sequentially from bottom to top. The substrate layer, the cover layer, the lower waveguide layer, the gap layer, and the front end surfaces of the upper waveguide layer are flush, and the absorbing layer is located on the rear upper surface of the upper waveguide layer.

Further, the gap layer is a coupling layer.

Further, the distance between the front end of the absorbing layer and the front end of the upper waveguide layer is twice the coupling length.

Further, the upper surface of the absorption layer is also arranged on the intrinsic layer.

The beneficial effects of the present invention are: the front-end input waveguide structure of the direction-coupled optical waveguide detector used in the present invention effectively overcomes the shortcomings of the waveguide detector and the horizontal direction-coupled waveguide detector, can effectively increase the photocurrent, and also solves the problem of the front-end of the waveguide. The problem of pattern evolution leads to the problem of overheating and burning.

Description of drawings

Figure 1 is a schematic diagram of the structure of an evanescent coupled waveguide fed into a photodiode;

Fig. 2 is a schematic diagram of the structure of a photodiode with a waveguide structure;

Fig. 3 is a schematic structural diagram of a horizontally coupled waveguide photodiode;

Fig. 4 is a structural representation of the present invention;

Fig. 5 is the simplified diagram of the waveguide of the photodetector coupled in vertical direction;

Fig. 6 is a front-end input waveguide cross-sectional refractive index distribution diagram;

Fig. 7 is a kind of simulation result figure of the present invention;

Fig. 8 is another kind of simulation result figure of the present invention;

In the figure: 1. Substrate layer, 2. Covering layer, 3. Lower waveguide layer, 4. Coupling layer, 5. Upper waveguide layer, 51. Front face of upper waveguide layer, 6. Absorbing layer, 61. Front face of absorbing layer, 7. Intrinsic layer, 8. Waveguide A, 9. Waveguide B, 10. Waveguide.

detailed description

In order to make the object, technical solution and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with the accompanying drawings.

As shown in Figure 4, the technical solution of the present invention is: a front-end input waveguide structure of a directionally coupled optical waveguide detector, including a substrate layer 1, a cover layer 2, a lower waveguide layer 3, and a gap layer stacked in sequence from bottom to top. , the upper waveguide layer 5, the absorption layer 6, the substrate layer 1, the cover layer 2, the lower waveguide layer 3, the gap layer, and the front end of the upper waveguide layer 5 are flush, and the absorption layer 6 is located on the rear portion of the upper waveguide layer 5 surface. The gap layer is the coupling layer 4 . The distance between the front end surface 61 of the absorbing layer and the front end surface 51 of the upper waveguide layer is twice the coupling length. The gap layer is a layer of low-refractive-index coupling layer 4 between the lower waveguide layer 3 and the upper waveguide layer 5, which is equivalent to the coupling layer of the above-mentioned directional coupling photodiode. The waveguides 3 and 5 form a coupler in the vertical direction. The incident light is incident from the end face of the lower waveguide layer 3, while being transmitted in the lower waveguide layer 3, it is gradually coupled into the upper waveguide layer 5 through the gap layer 4, and then absorbed by the absorption layer 6. That is, at the input end of the directional coupling optical waveguide detector, a passive vertical directional coupler without an absorbing layer is added. It solves the problem that the photocurrent at the front end of the waveguide is too strong locally under the incidence of high-power laser, which causes the directional coupler to overheat and burn the body. The difference between the input waveguide (front end) and the detection waveguide (back end) is that the input waveguide has no absorption layer, and the detection waveguide has an absorption layer. After adding the input waveguide, the high-power laser is incident from the end face of the input waveguide, and no longer incident from the end face of the detection waveguide. Since the input waveguide has no absorbing layer, the absorption of the high-order modes excited by the waveguide incident laser becomes weaker and slower, and the heat is distributed in a longer space along the input waveguide, so the input waveguide is no longer easy to be burned. When the incident laser reaches the front end of the detection waveguide through twice the coupling length along the input waveguide, the high-order supermode has already been attenuated after passing through the input waveguide, leaving only the fundamental supermode and the first-order supermode, so after entering the waveguide detector, there will be no more The high-order supermode is excited, so that the front end of the detection waveguide will not be caused by excessive absorption and burning.

Embodiment: The following is the working wavelength of 1.55 μm, the material InGaAs (indium gallium arsenide) of the absorption layer 6, the material InGaAsP (indium gallium arsenide phosphide) of the upper waveguide layer 5, the lower waveguide layer 3 and the cover layer 2; the substrate layer 1 An example of a vertically coupled photodetector input front end of the material InP (Indium Phosphide).

First, some basic theoretical parameters of the front-end input waveguide structure of the waveguide coupler are listed:

waveguide material Refractive index InP 3.146 InGaAsP 3.33 InGaAs 3.56-0.1i

Table 1 Refractive index of various materials used in the detector;

Material of each layer Thickness (μm) Upper waveguide layer InGaAsP 3.5 Gap InP 0.09 Lower waveguide layer InGaAsP 3.05 Cladding InGaAsP 0.5 Substrate InP 15 Waveguide Ridge Width 6 waveguide length 1500

Table 2 The structural parameters of the detector.

Theoretical analysis: directionally coupled photodetectors must meet a condition called supermode matching to achieve high-power operation; to meet such a supermode matching condition, vertically coupled photodetectors must be used, and the front-end input waveguide length, the coupling length plays a crucial role in satisfying this supermode matching condition.

The supermode matching condition is: the absorption length of the fundamental supermode = the absorption length of the first-order supermode = twice the coupling length.

Using the effective refractive index method to analyze the photodetectors coupled in the vertical direction, the simplified diagram of the waveguide is shown in Figure 5, and the coordinate axes shown in the figure are established. First, it is equivalent to a slab waveguide in the y direction, and the center waveguide and The effective refractive indices ne and nec of the waveguides on both sides are then equivalent to a slab waveguide in the x direction, and finally the effective refractive index neff of the entire ridge waveguide is obtained. Then substituting neff into formula (1) can calculate the coupling length of the coupler in the vertical direction.

By repeatedly adjusting the thickness of the upper and lower waveguides and the coupling layer, the coupling length can be changed to obtain an ideal coupling length. which is:

In the formula, Lc is the coupling length, β0 is the propagation constant of the fundamental supermode, β1 is the propagation constant of the first-order supermode, λ is the wavelength of light, neff, 0 is the effective refractive index of the fundamental supermode, and neff, 1 is The effective index of the first-order supermode for this mode. Re is a function that takes the real part of the value, and Im is a function that takes the imaginary part of the value.

Table 3 Relationship between changing the thickness of the coupling layer and the thickness of the upper and lower waveguides and the coupling length

In Table 3, t1 is the thickness of the coupling layer, tb1 is the thickness of the lower waveguide layer, tb2 is the thickness of the upper waveguide layer, and Lc is the coupling length. In the case of keeping the total thickness of the upper and lower waveguides unchanged, by changing the thickness of the coupling layer, the coupling length under different thicknesses of the upper and lower waveguides is obtained, and then the front-end input waveguide length is determined to be twice the coupling length. For non-absorbing waveguides, the coupling length calculation is similar to

Working process: light is incident from the lower waveguide layer 3, the fundamental supermode and the first-order supermode are excited in the front-end input waveguide of the photodetector coupled in the vertical direction and propagated forward, and the optical power gradually passes through the coupling layer 4 from the lower waveguide layer 3 Coupled into the upper waveguide layer 5, in the case of no absorption or loss, the propagation constants of the two modes are real numbers, and the interference result shows that the light in the two waveguides changes along the waveguide period, and the optical power will be coupled into the lower waveguide layer 3 , when the optical power in one waveguide is the largest, the optical power in the other waveguide is the smallest, which means that the optical power is periodically coupled from one waveguide to another waveguide, and the total optical power remains unchanged.

The ideal length of the front-end input waveguide is twice the coupling length. The light enters the front-end input waveguide from the lower waveguide, propagates in the front-end input waveguide, and converts between the upper and lower waveguides. In the case of 2 times the coupling length, when reaching the detection waveguide, the light returns to the lower waveguide, enters the waveguide detector from the lower waveguide of the detection waveguide, and the light begins to be absorbed by the absorbing layer. After about 2 times the coupling length, the light is completely absorbed and the photoelectric conversion is completed.

The specific process of determining the length of the front-end input waveguide is as follows: use BeamProp software to simulate the vertical coupler with the front-end input waveguide, change the length of the input waveguide, analyze the simulation results, and then use Matlab to draw the photocurrent distribution diagram for comparison Determine the length value of the front-end input waveguide when the distribution fluctuation of the initial end of the photocurrent reaches the minimum, to overcome the easy saturation of the optical power at the front end of the waveguide, the nonlinearity, thermal effect, and even burnout of the device caused by excessive photocurrent. The problem of uneven distribution of directional photocurrent.

Combining simulation data or drawings to give conclusive opinions: use BeamPROP software to carry out numerical simulation of the above structural parameters, the input light adopts Gaussian beam, and the input light is irradiated on the end surface of the waveguide layer under the input end. The cross-sectional refractive index distribution of the front-end input waveguide is shown in Figure 6, and the simulation results are shown in Figure 7.

In the simulation result diagram of Figure 7, the left block diagram represents the change of the optical field distribution inside the square front-end input waveguide along the z direction (the length direction of the waveguide). In the case of no absorption or loss, the optical power is cycled in the upper and lower waveguide layers. When the optical power in one waveguide is the largest, the optical power in the other waveguide is the smallest, which means that the optical power is periodically coupled from one waveguide to another waveguide, and the total optical power remains unchanged. The box diagram on the right is the change diagram of optical power. When the waveguide length is 0, the first curve on the left is the change of the total power in the waveguide, the second curve is the change of power in the lower waveguide layer, and the third curve is the change of the upper waveguide. Changes in power in layers. It can be seen that, at the position of about 280 μm, the optical power of the upper waveguide layer reaches the maximum, and the optical power of the lower waveguide layer reaches the minimum, so the coupling length is at the position of about 280 μm. From the simulation results in Figure 8, it can be seen that after the input waveguide is added, the light couples and propagates in the input waveguide. After the light couples and propagates in the input waveguide for about 2 cycles, it is connected to the vertical coupler, the absorbing layer absorbs, and generates light current.

The light is coupled and propagated in the front-end input waveguide. When the light is coupled and propagated in the input waveguide for about 2 cycles, that is, when the length of the input waveguide is 2 times the coupling length, it is connected to the vertical coupler, and the absorption layer absorbs, resulting in the overall distribution of photocurrent relatively uniform. Compared with no front-end waveguide, the photocurrent distribution waveguide at the initial end with the input waveguide is greatly reduced, and the stability of the photocurrent is significantly improved, which is very important for the design of the vertical coupler.

Those skilled in the art will appreciate that the embodiments described herein are to help readers understand the principles of the present invention, and it should be understood that the protection scope of the invention is not limited to such specific statements and embodiments. All possible equivalent replacements or changes made according to the above descriptions are deemed to belong to the protection scope of the claims of the present invention.

Claims (3)

1. A kind of 1. front end input waveguide structure of direction coupling optical waveguide detector, it is characterised in that：Including from the bottom up successively Substrate layer, coating, lower waveguide layer, clearance layer, upper ducting layer, the absorbed layer of stacking, the substrate layer, coating, lower waveguide Layer, clearance layer, upper ducting layer front end face are concordant, and the absorbed layer is located at upper ducting layer rear upper surface；The absorbed layer front end Face and the distance of upper ducting layer front end face are 2 times of coupling lengths.
2. 2. the front end input waveguide structure of coupling optical waveguide detector in direction according to claim 1, it is characterised in that：Institute It is coupling layer to state clearance layer.
3. 3. the front end input waveguide structure of coupling optical waveguide detector in direction according to claim 1, it is characterised in that：Institute State absorbed layer upper surface and be also provided at intrinsic layer.