CN114236688B - Multimode waveguide chirped Bragg grating delay line chip with the highest order being even-numbered - Google Patents

Abstract

The invention provides a multimode waveguide chirped Bragg grating delay line chip with even-order highest order, which comprises a multimode waveguide and Bragg gratings positioned on two sides of the multimode waveguide; the two groups of Bragg gratings on two sides of the multimode waveguide are of an asymmetric structure at an initial position, the relative displacement is delta L=lambda/2, lambda is a grating period, then the relative displacement gradually decreases, the two groups of Bragg gratings are completely symmetric at a position between 20% and 90% of the multimode waveguide, and the duty ratio of the gratings sequentially increases from the initial position to the terminal. The invention adopts multimode waveguide supporting high-order modes, and adds symmetrical gratings on two sides, thereby constructing high-order multimode gratings; adopting an antisymmetric-symmetrical gradual grating apodization mode to inhibit delay jitter and side lobes; the gradient design of the grating duty ratio is adopted to realize the chirp dispersion delay of the Bragg grating, and two modes of changing the duty ratio gradient interval and changing the tooth length of the externally protruded grating are provided aiming at the chirp dispersion interval regulation and control.

Description

Multimode waveguide chirped Bragg grating delay line chip with the highest order being even-numbered

Technical field

The invention relates to the field of integrated optoelectronics technology, and more specifically to a multi-mode waveguide chirped Bragg grating delay line chip whose highest order is an even-numbered order.

Background technique

Optical delay technology is widely used in the fields of optical communications and radar detection. Phased array antennas based on optical true delay lines can break through the limitations of the aperture effect and achieve large instantaneous bandwidth. The chirped grating delay line chip has the advantages of high integration and continuous delay, so it has received widespread attention. However, it has the disadvantages of large loss and large delay error.

There are two types of chirped grating delay line chips: dual-waveguide reverse-coupled chirped Bragg grating and single-waveguide chirped Bragg grating. The single waveguide chirped Bragg grating delay line can easily achieve a spiral structure. The delay line structure is compact and the chip size is small [APL Photonics, 2020, 5(10):101302].

At present, chirped Bragg grating delay lines are basically based on single-mode waveguide design. In a single-waveguide single-mode chirped Bragg grating delay line, the reflected fundamental mode (TE ₀ ) light and the incident fundamental mode light are mixed together. Special design is required, as shown in Figure 1a, and a 3dB coupler is introduced to separate the incident light and reflected light. Light [OPTICS EXPRESS,20(10):11242]. In this structure, the incident light enters two identical optical delay lines through the 3dB coupler. The two paths of light need to have good amplitude and phase consistency to achieve a good light separation effect. However, due to processing errors, it is difficult for the two delay waveguides to be completely consistent, and the 3dB coupler has a certain power imbalance, which will introduce losses and large crosstalk. The existence of crosstalk will produce delay errors. In addition, the loss of single-mode waveguide is relatively large, and in comparison, the loss of multi-mode wide waveguide is much smaller.

In recent years, multi-mode Bragg grating upload and download filters have been widely studied [Photonics Research, 2021, 9 (5): 759; optics Letters, 2019, 44 (6): 1304], which can realize monochromatic optical signal direction. Upload and download functions of the transmission trunk. As shown in Figure 1b, based on the multi-mode waveguide and antisymmetric Bragg grating design, the fundamental mode of forward transmission (TE ₀ ) and the first-order mode of reverse transmission (TE ₁ ) are coupled, thereby achieving reflection of the incident TE ₀ light The TE ₁ light is converted, and then the reflected TE ₁ light is separated from the main line through the mode demultiplexer. At the same time, the TE ₁ mode is converted to TE ₀ .

Contents of the invention

The technical problem to be solved by the present invention is how to solve the chirped dispersion delay problem of the Bragg grating of a high-order mode multi-mode waveguide.

The present invention solves the above technical problems through the following technical means:

The multimode waveguide chirped Bragg grating delay line chip with the highest order being an even number includes a multimode waveguide and Bragg gratings located on both sides of the multimode waveguide; the two sets of Bragg gratings on both sides of the multimode waveguide are in non-zero initial positions. Symmetric structure, the relative displacement is ΔL=Λ/2, where Λ is the grating period, and then the relative displacement gradually decreases, and then the two sets of Bragg gratings reach complete symmetry starting from 20% to 90% of the multi-mode waveguide. Displacement ΔL=0; after that, the two sets of Bragg gratings continue to maintain symmetry, and the duty cycle of the gratings increases sequentially from the initial position to the terminal position.

The present invention uses a multi-mode waveguide that supports high-order modes, and adds symmetrical gratings on both sides to construct a high-order multi-mode grating; the anti-symmetric → symmetric gradient grating apodization method is used to suppress delay jitter and side lobes; and the grating duty is used The ratio gradient design realizes the chirped dispersion delay of the Bragg grating, and for the control of the chirped dispersion interval, two methods are proposed, namely changing the duty cycle gradient interval and changing the length of the outer protruding grating teeth.

Furthermore, the grating adopts anti-symmetric to symmetric gradient transverse dislocation apodization.

Further, the grating apodization curve is but is not limited to a Gaussian function.

Further, the grating duty cycle changes linearly.

Further, the parameter selection of the grating period Λ is based on the following formula:

λ＝[n ₁ d(z)+n ₂ (1-d(z))]Λ＝[(n ₁ -n ₂ )d(z)+n ₂ ]Λ (1)

Among them, λ is the wavelength of reflected light, n ₁ and n ₂ represent the effective refractive index of the grating comb area and the non-grating comb area respectively. The effective refractive index is the average of the sum of the effective refractive index of the TE0 mode and the highest order mode. , z is the position of light transmission in the grating delay line, and d(z) represents the grating duty cycle.

Further, the wavelength of the light reflected at the z position of the multi-mode waveguide

λ(z)＝[n ₁ d(z)+n ₂ (1-d(z))]Λ＝[(n ₁ -n ₂ )d(z)+n ₂ ]Λ (2)

Among them, z is the position where the light is transmitted in the grating delay line, and λ(z) is the wavelength of the light reflected at the z position.

Further, the grating duty cycle is 0%≤d(z)≤100%, and the duty cycle gradient interval is ≤(0%-100%).

Further, it also includes a mode demultiplexer; the mode demultiplexer includes an input waveguide and an output waveguide; the incident waveguide is connected to the initial end of the multi-mode waveguide.

Furthermore, there are two ways to control the chirp dispersion interval: changing the duty cycle gradient interval and/or changing the length of the outer protruding grating teeth.

The advantages of the present invention are:

1. Use a multi-mode waveguide that supports high-order modes to reduce the transmission loss of the waveguide, and add symmetrical gratings on both sides to construct a high-order multi-mode grating;

2. Use the antisymmetric → symmetric gradient grating apodization method to suppress delay jitter and side lobes;

3. The gradient design of the grating duty cycle is used to realize the chirped dispersion delay of the Bragg grating, and for the control of the chirped dispersion interval, two methods are proposed: changing the duty cycle gradient interval and changing the length of the outer protruding grating teeth.

Description of drawings

Figure 1a is a comparison of the multi-mode chirped Bragg grating delay line Figure 1b in the prior art and the embodiment of the present invention;

Among them, (a) is a single-mode chirped Bragg grating delay line based on 3dB coupler splitting, (b) is a multi-mode Bragg grating filter based on mode demultiplexing splitting, and Figure 1b is a multi-mode chirped Bragg grating filter in an embodiment of the present invention. Modular chirped Bragg grating delay line;

Figure 2 is a schematic diagram of the design points and structure of a multi-mode chirped Bragg grating delay line in an embodiment of the present invention;

Figure 3 shows a multi-mode waveguide Bragg grating that can achieve TE ₀ / TE ₂ mode conversion;

Figure 4 is a schematic diagram of an antisymmetric → symmetric gradient apodized grating;

Figure 5 is a simulation result diagram using a chirped Bragg grating delay line in an embodiment of the present invention, wherein (a) grating duty cycle: 40%-60%, w _g =180nm; (b) grating duty cycle: 35%-65%, w _g =180nm; (c) Grating duty cycle: 40%-60%, w _g =250nm;

Figure 6 is a multi-mode waveguide chirped Bragg grating with a mode demultiplexer in an embodiment of the present invention.

Detailed ways

In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are part of the present invention. Examples, not all examples. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts fall within the scope of protection of the present invention.

As shown in Figure 2, the design points of the multi-mode chirped Bragg grating delay line include three aspects: 1. Use a multi-mode waveguide 1 that supports TE2 mode, and add symmetrical gratings 2 on both sides to construct a high-order multi-mode grating. ; 2. Use the antisymmetric → symmetric gradient grating apodization method to suppress delay jitter and side lobes; 3. Use the gradient design of the grating duty cycle to achieve the chirped dispersion delay of the Bragg grating, and target the chirped dispersion interval For control, two methods are proposed: changing the duty cycle gradient interval and changing the length of the protruding grating teeth.

The following is a step-by-step introduction based on the design logic:

1. High-order multi-mode grating design

The loss of optical waveguides mainly comes from the scattering loss caused by the roughness of the waveguide sidewalls. The wider the waveguide width, the lower the sidewall mode field intensity, and the lower the loss. Compared with the TE ₁ multi-mode waveguide (waveguide width 1.1um) used in the upload and download filters, the loss of the TE ₂ multi-mode waveguide (waveguide width 1.3um) will undoubtedly be lower. The present invention adopts TE ₂ multi-mode Bragg grating, whose structure is shown in Figure 3. First, determine the waveguide width based on the high-order mode order. For TE ₂ mode, through simulation calculation, the appropriate waveguide width is 1.3um. Then, periodic structures are introduced on both sides of the multi-mode waveguide to form a Bragg grating. In view of the symmetrical field distribution characteristics of TE ₂ , the Bragg grating also needs to adopt a symmetric design, that is, there is no misalignment of the upper and lower gratings to achieve TE ₀ / TE ₂ mode reverse coupling. . The parameter selection of the grating period Λ is based on the following formula:

λ＝[n ₁ d+n ₂ (1-d)]Λ＝[(n ₁ -n ₂ )d+n ₂ ]Λ (1)

Among them, λ is the wavelength of reflected light, n ₁ and n ₂ represent the effective refractive index of the grating comb area and the non-grating comb area respectively. The effective refractive index is the effective refractive index of the forward propagating TE ₀ mode and the reverse propagating TE ₂ mode. The average value of the sum of refractive indices, that is, (n_TE ₀ +n_TE ₂ )/2, the values of n_TE ₀ and n_TE ₂ are related to the waveguide width w and the grating tooth length w _g , which can be obtained through simulation calculations.

After the symmetrical grating is introduced, the TE ₀ mode transmitted in the forward direction and the TE ₂ mode transmitted in the reverse direction are coupled in the multi-mode waveguide. This coupling causes the TE ₀ mode incident light to be converted into TE ₂ mode reflected light and returned from the original path. This reflection is for a specific wavelength, and the wavelength can be changed by reasonably designing the grating period Λ, duty cycle d and other parameters.

2. Antisymmetry→symmetric gradient grating apodization

The sudden introduction of a grating structure into the waveguide will cause strong interface reflection at the initial position of the grating, which will in turn cause reflection spectrum side lobes and delay jitter. In order to suppress side lobes and delay jitter, the grating needs to be apodized. In the present invention, the method of lateral dislocation apodization is adopted, and the coupling intensity is changed through the relative displacement of the gratings on both sides. For the TE ₁ mode with an antisymmetric mode field, the lateral dislocation apodization of the grating is antisymmetric → symmetric gradient grating apodization. In the present invention, since the TE ₂ mold has a symmetrical mode field, a symmetric → anti-symmetric gradient grating apodization is designed. The starting end of the apodization area is the initial position of the grating, and the ending end is any position between 20% and 90% of the grating. From this position onwards, there is a symmetrical structure of the grating on both sides. This example starts with symmetry at 50%. As shown in Figure 4, at the beginning of the apodization region, the grating teeth have an asymmetric structure, and the relative displacement is half of the period, that is, when ΔL = Λ/2, the two sets of gratings are completely misaligned and the coupling coefficient is 0; at the apodization At the end of the region, when the displacement ΔL=0, the two sets of gratings are completely aligned, and the coupling coefficient reaches the maximum. To make the coupling coefficient from small to large, ΔL should be changed from large to small, and it needs to change according to a certain function, such as Gaussian function, cosine function, Blackman function, hyperbolic tangent function, Sinc function and Cauchy function, etc., in Figure 4 , the grating apodization curve is a Gaussian function.

Compared with the common apodization method in which the grating tooth length w _g gradually changes, the transverse offset apodization method can ensure that the grating comb teeth have a larger w _g at the initial position, effectively reducing the processing difficulty of the grating comb teeth, and can be compared with the existing Match the minimum line width of the chip processing technology.

3. Linear gradient of duty cycle to achieve chirped dispersion

In order to achieve chirped dispersion delay, that is, light of different wavelengths is reflected at different positions of the grating delay line, a gradient refractive index needs to be introduced. The grating duty cycle is the ratio of the length of the grating to the period within one cycle. Gradient grating duty cycle is an important means to realize chirped Bragg grating [Scientific Reports, 2016, 6:30235]. The present invention adopts a scheme of gradient grating duty cycle, but due to its special protruding grating tooth structure, there are two ways to control the chirp dispersion interval: a. Duty cycle gradient interval, b. Protruding grating tooth length .

Assume that the width of the multimode waveguide is w, the grating width is w _g , the grating period is Λ, and the duty cycle is d(z). In the chirped Bragg grating, formula (1) becomes:

λ(z)＝[n ₁ d(z)+n ₂ (1-d(z))]Λ＝[(n ₁ -n ₂ )d(z)+n ₂ ]Λ (2)

Among them, z is the position where the light is transmitted in the grating delay line, and λ(z) is the wavelength of the light reflected at the z position. It can be seen from Equation 1 that if the duty cycle d(z) changes linearly, then λ(z) will also change linearly. When the duty cycle linearly increases, a linear delay with a positive dispersion coefficient can be achieved. On the contrary, when the duty cycle linearly decreases, a linear delay with a negative dispersion coefficient can be achieved. It can also be seen from Equation 2 that when the period Λ is fixed, the variation interval of λ (chirped dispersion interval) depends on the gradient interval of the duty cycle and the values of n ₁ and n ₂ , where n ₁ and n ₂ vary with the grating teeth. Therefore, there are two ways to control the chirp dispersion interval: a. Duty cycle gradient interval, b. Outward protruding grating tooth length.

Figure 5 shows the simulation results of the chirped Bragg grating delay line. The multimode waveguide width w=1.3μm, the waveguide thickness is 220nm, the grating width _wg =180nm, and the grating period Λ=300nm. In Figure 5a, the grating width w _g =180nm, the grating duty cycle changes from 40% to 60%, and the chirp dispersion delay interval is 1549~1559nm (bandwidth 10nm). In Figure 5b, the grating width w _g =180nm, the grating duty cycle changes from 30% to 70%, and the chirp dispersion delay interval is 1548~1561nm (bandwidth 12nm). In Figure 5c, the grating width w _g =250nm, the grating duty cycle changes from 40% to 60%, and the chirp dispersion delay range is 1551~1569nm (bandwidth 18nm). (a) Grating duty cycle: 40%-60%, w _g =180nm; (b) Grating duty cycle: 35%-65%, w _g =180nm; (c) Grating duty cycle: 40%-60 %, w _g =250nm.

4. Mode demultiplexer splitting

Since the input mode and the reflection (output) mode are of different orders, a mode demultiplexer can be used to separate the input and output light. The schematic diagram of the mode demultiplexer is shown in Figure 6. By rationally designing the geometric parameters of the mode demultiplexer , so that the refractive index of the fundamental mode and the higher-order mode transmitted in the two waveguides is similar. Through the waveguide tapering, the loss coefficient of the higher-order mode reflected in the multi-mode waveguide is gradually increased, which can force most of the energy of the higher-order mode to be transferred to the other waveguide. The fundamental mode excited in . The simulation results show that the energy transmittance of the mode demultiplexer is above 95%, and the crosstalk between different ports is <-10dB.

The above embodiments are only used to illustrate the technical solutions of the present invention, but not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions of the foregoing embodiments. The recorded technical solutions may be modified, or some of the technical features thereof may be equivalently replaced; however, these modifications or substitutions shall not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of each embodiment of the present invention.

Claims (9)

1. The multimode waveguide chirped Bragg grating delay line chip with even-order highest order is characterized by comprising a multimode waveguide and Bragg gratings positioned on two sides of the multimode waveguide; the two groups of Bragg gratings on two sides of the multimode waveguide are in an asymmetric structure at an initial position, the relative displacement is delta L=Λ/2, wherein Λ is a grating period, then the relative displacement gradually decreases, and then the two groups of Bragg gratings are completely symmetric at a position between 20% and 90% of the multimode waveguide, and the relative displacement delta L=0; and then the two groups of Bragg gratings continue to be symmetrical, and the duty ratio of the gratings increases from the initial position to the terminal.

2. The multimode waveguide chirped bragg grating delay line chip of even-order highest order according to claim 1, wherein the grating employs antisymmetric→symmetric graded lateral dislocation apodization.

3. The multimode waveguide chirped bragg grating delay line chip of even-order highest order of claim 2 wherein the grating apodization curve is a gaussian function.

4. A multimode waveguide chirped bragg grating delay line chip having an even-order highest order according to any one of claims 1 to 3, wherein the grating duty cycle is linearly graded.

5. A multimode waveguide chirped bragg grating delay line chip with an even-order highest order according to any one of claims 1 to 3, wherein the parameter selection of the grating period Λ is according to the following formula:

λ＝[n ₁ d(z)+n ₂ (1-d(z))]Λ＝[(n ₁ -n ₂ )d(z)+n ₂ ]Λ (1)

wherein λ is the wavelength of reflected light, n ₁ 、n ₂ The effective refractive indexes of the grating comb tooth region and the non-grating comb tooth region are respectively represented, the effective refractive index is the average value of the sum of the effective refractive indexes of the TE0 mode and the highest order mode, z is the transmission position of light in a grating delay line, and d (z) represents the grating duty ratio.

6. The even-order highest order multimode waveguide chirped bragg grating delay line chip of claim 5 wherein the wavelength of light reflected at the multimode waveguide z-position

λ(z)＝[n ₁ d(z)+n ₂ (1-d(z))]Λ＝[(n ₁ -n ₂ )d(z)+n ₂ ]Λ (2)

Where z is the position where light is transmitted in the grating delay line and λ (z) is the wavelength of light reflected at the z position.

7. The multimode waveguide chirped bragg grating delay line chip of even-order highest order of claim 6 wherein the grating duty cycle is 0% to 100%.

8. A multimode waveguide chirped bragg grating delay line chip having an even-order highest order according to any one of claims 1 to 3, further comprising a mode de-multiplexer; the mode demultiplexer includes an incident waveguide and an output waveguide; the incident waveguide communicates with an initial end of the multimode waveguide.

9. The multimode waveguide chirped bragg grating delay line chip with even-order highest order as claimed in claim 6, wherein the chirped dispersion interval is controlled by two ways: changing the duty cycle transition interval and/or changing the external protrusion grating tooth length.