An on-chip integrated optical frequency comb generation structure and method

Abstract

This invention discloses an on-chip integrated optical frequency comb generation structure and method. The structure is heterogeneously integrated from a laser chip and an integrated optical microcavity chip. The integrated optical microcavity chip includes a polarization beamsplitter, two ring filters, an add-drop microcavity, and two output ports. The polarization beamsplitter splits the laser injected from the laser chip into TE and TM modes with different power ratios, which are then injected into the two ring filters respectively. Each ring filter has a heating electrode to control the wavelength of light transmitted, and the two ring filters have different diameters for transmitting different wavelengths. The add-drop microcavity generates the optical frequency comb, and a heating electrode is positioned above the microcavity to control the resonant wavelength passing through it. This invention solves the problems of large size and complex debugging in existing optical frequency comb debugging systems, and can significantly reduce the size of the microcavity optical frequency comb and lower power consumption.

Description

Technical Field

[0001] This invention belongs to the field of integrated photonics, specifically relating to an on-chip integrated optical frequency comb generation structure and an optical frequency comb generation method. Background Technology

[0002] Microcavity Kerr optical frequency comb technology has shown disruptive potential in many fields due to its advantages of miniaturization, low power consumption and high integration. However, it also faces several challenges, such as manufacturing process, system integration and stability.

[0003] With the continuous advancement of on-chip waveguide technology, integrated optics has been continuously developed, and more and more optical systems can be integrated onto chips using on-chip waveguide devices. This has also made the miniaturization and integration of Kerr optical frequency combs possible. Currently, the optical frequency comb based on the self-injection locking principle is the most integrated Kerr optical frequency comb. Although this method has high integration, it is very difficult to debug, and its reproducibility and stability are poor. The commonly used optical frequency comb debugging method is based on the auxiliary laser method, which uses a reverse-propagating laser beam to compensate for the intracavity power jitter after the pump laser enters the red detuned state, thereby achieving stable intracavity power and enabling it to stably enter the soliton state. The resulting soliton optical frequency comb also has robustness. However, the optical and electrical debugging system of this system is relatively large, and the debugging methods are also relatively complex, which restricts the development of optical frequency combs towards miniaturization. Summary of the Invention

[0004] The purpose of this invention is to provide an on-chip integrated optical frequency comb generation structure and method, which can solve the problems of large size and complex debugging of existing optical frequency comb debugging systems, and can greatly reduce the size of microcavity optical frequency combs and reduce power consumption.

[0005] To achieve the above objectives, one aspect of the present invention provides an on-chip integrated optical frequency comb generating structure, which is heterogeneously integrated with a laser chip and an integrated optical microcavity chip. The integrated optical microcavity chip includes a polarization beam splitter, two ring filters, an add-drop type microcavity, and two output ports. The laser output from the laser chip is injected into the integrated optical microcavity chip, and the integrated optical microcavity chip generates a microcavity optical frequency comb and outputs it from one of the two output ports.

[0006] The polarization beam splitter is used to split the injected laser into TE mode and TM mode with different power ratios, which are injected into two ring filters respectively; the two ring filters are respectively equipped with heating electrodes to control the wavelength passed through, and the two ring filters have different diameters to transmit different wavelengths.

[0007] Add-drop type microcavities are used to generate optical frequency combs. Their Input and Through terminals are used to transmit TE and TM modes after passing through two ring filters, respectively. The Add and Drop terminals are used to output the optical frequency comb, corresponding to the two output ports of the integrated optical microcavity chip. Heating electrodes are provided above the microcavity to control the resonant wavelength passing through the microcavity.

[0008] Another aspect of the present invention provides a method for generating an optical frequency comb, which generates an optical frequency comb using the above-described structure, comprising:

[0009] The laser generated by the laser chip is split into TE and TM modes by a polarization beam splitter and fed into two ring filters respectively. The heating electrodes of the ring filters are finely adjusted to precisely control the wavelength of the transmitted light, so that they have a certain wavelength difference. After being filtered by the ring filters, the light is injected into the add-drop type microcavity in opposite directions. The lower power of the TE mode and TM mode is used as the auxiliary laser, and the other is used as the pump laser. The heating electrodes on the add-drop type microcavity are adjusted to cool the microcavity from a high temperature to a low temperature until an optical frequency comb is generated. The generated optical frequency comb is output through one of the two output ports of the integrated optical microcavity chip.

[0010] According to the on-chip integrated optical frequency comb generation structure and method of the present invention described above, the problems of large size and complex debugging of existing optical frequency comb debugging systems can be solved, and the volume of microcavity optical frequency combs can be greatly reduced and power consumption can be reduced. Attached Figure Description

[0011] To more clearly illustrate the technical solutions of the present invention, the accompanying drawings used in the description of the embodiments of the present invention will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort:

[0012] Figure 1 This is a top view of an on-chip integrated optical frequency comb generation structure according to an embodiment of the present invention. Detailed Implementation

[0013] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0014] One embodiment of the present invention provides an on-chip integrated optical frequency comb generation structure, such as Figure 1As shown, the on-chip integrated optical frequency comb generation structure of this embodiment of the invention is heterogeneously composed of a laser chip 1 and an integrated optical microcavity chip 2. The laser chip 1 and the integrated optical microcavity chip 2 are heterogeneous substrates. A spacer of appropriate height is placed under each of the substrates, and the waveguide end face of the laser waveguide 10 of the laser chip 1 is aligned with the waveguide end face of the microcavity waveguide 20 of the integrated optical microcavity chip 2. After alignment, they are fixed by dispensing adhesive to complete the heterogeneous integration of the two.

[0015] Laser chip 1 is a semiconductor laser chip, capable of outputting single-mode or multi-mode laser light with a typical wavelength of 1550 nm. The integrated optical microcavity chip 2, from left to right, includes a polarization beamsplitter 21, two ring filters (first ring filter 22 and second ring filter 23), an add-drop microcavity 24, and two output ports (first output port 25 and second output port 26). The laser light output from laser chip 1 is injected into the integrated optical microcavity chip 2 through laser waveguide 10. The microcavity optical frequency comb generated by the integrated optical microcavity chip 2 is output from one of the two output ports. The polarization beamsplitter 21 is, for example, a Y-type polarization beamsplitter.

[0016] The polarization beam splitter 21 of the integrated optical microcavity chip 2 splits the injected laser into TE mode and TM mode with different power ratios, which are injected into two ring filters respectively. This prevents the light after passing through the microcavity from being re-injected back into the laser chip 1. In addition, it also facilitates the filtering out of different modes in the output optical comb, thereby reducing phase noise.

[0017] The two ring filters of the integrated optical microcavity chip 2 are each equipped with heating electrodes to precisely control the wavelength passing through the filters. The two ring filters have different diameters to transmit different wavelengths, with their wavelength difference approximating the wavelength difference between the TE and TM modes within N microcavity FSR (free spectral range) periods, where N is an integer ≥ 0. Specifically, the wavelengths transmitted by the two ring filters are the resonant wavelengths of their respective modes, and within the same period... n The wavelength difference, due to the periodicity of the microcavity, means that the resonant wavelengths in the same mode can differ by N times the FSR, thus generating a wavelength difference of N×FSR+ between the transmission wavelengths of the two ring filters of the optical frequency comb. n .

[0018] The add-drop type microcavity of the integrated optical microcavity chip is used to generate an optical frequency comb. Its input and through terminals are used to transmit the TE mode and TM mode after passing through the ring filter, respectively. The add and drop terminals are used to output the optical frequency comb (corresponding to the two output ports of the integrated optical microcavity chip 2, respectively). A heating electrode is also added above the microcavity to control the resonant wavelength passing through the microcavity.

[0019] The integrated optical microcavity chip is fabricated through the following steps:

[0020] Step S1: Grow a layer of lower cladding silicon dioxide on a silicon, SOI, or lithium niobate substrate, typically with a thickness greater than 2 μm;

[0021] Step S2: Grow a silicon nitride or lithium niobate thin film as a waveguide layer under silicon dioxide. The thickness of the waveguide layer needs to be selected within the design spectrum range according to the dispersive structure design.

[0022] Step S3: By combining photolithography or electron beam exposure with ICP etching or wet etching, the waveguide layer is patterned to obtain a polarization beam splitter, two ring filters, and an add-drop microcavity structure.

[0023] Step S4: Use ICP-CVD to grow a layer of silicon dioxide as the top cladding on the patterned waveguide layer;

[0024] Step S5: Photolithography is performed again on the silicon dioxide cladding layer, and a heating layer is prepared above the polarization beam splitter, the ring filter, and the add-drop microcavity using a lift-off process;

[0025] Step S6: Grow another layer of silicon dioxide on the heating layer, etch the position and depth of the contact electrode by photolithography, and fabricate the contact electrode of the heating layer using a lift-off process;

[0026] Step S7: After the contact electrode is completed, another layer of silicon dioxide is grown, and then photolithography is performed to remove the photoresist to obtain the through hole of the contact electrode. The silicon dioxide grown in steps S4, S6 and S7 together serve as the upper cladding layer of the integrated optical microcavity chip. The thickness of the upper cladding layer is generally greater than 2μm.

[0027] The optical frequency comb is generated by splitting the laser generated by the laser into TE and TM modes by a polarization beam splitter and feeding them into two ring filters. The heating electrodes of the ring filters are finely adjusted to precisely control the wavelength of the passing light, so that they have a certain wavelength difference. After being filtered by the ring filters, the light is injected into an add-drop type microcavity in opposite directions. The lower power path is used as an auxiliary laser, and the other path is used as a pump laser. The heating electrodes on the add-drop type microcavity are adjusted to cool down from a high temperature to a low temperature until an optical frequency comb is generated. The generated optical frequency comb is output through one of the two output ports of the integrated optical microcavity chip 2, depending on the path with higher transmission power used as the pump laser.

[0028] In summary, the on-chip integrated optical frequency comb generation structure and method of this invention, combined with on-chip waveguide technology, enables a robust optical system to be realized through on-chip devices and on-chip debugging methods. The optical system debugging steps are simple, and a robust optical frequency comb can be generated simply by temperature control. This solves the problems of large size and complex debugging of existing optical frequency comb debugging systems, and can greatly reduce the size of miniaturized microcavity optical frequency combs and reduce power consumption.

[0029] The foregoing has only described certain exemplary embodiments of the present invention by way of illustration. Undoubtedly, those skilled in the art can modify the described embodiments in various ways without departing from the spirit and scope of the present invention. Therefore, the foregoing drawings and descriptions are illustrative in nature and should not be construed as limiting the scope of protection of the claims of the present invention.

Claims

1. An on-chip integrated optical frequency comb generation structure, characterized by, The laser chip and the integrated optical microcavity chip are heterogeneously integrated, the integrated optical microcavity chip comprises a polarization beam splitter, two ring filters, an Add-drop type microcavity and two output ports, laser output by the laser chip is injected into the integrated optical microcavity chip, and the integrated optical microcavity chip generates a microcavity optical frequency comb and outputs from one of the two output ports; The polarization beam splitter is used for splitting the injected laser into TE mode and TM mode, and has different power ratios, and is injected into the two ring filters respectively; the two ring filters are respectively provided with heating electrodes for controlling the wavelength size passed through, and the two ring filters have different diameters for transmitting different wavelengths; The Add-drop type microcavity is used for generating an optical frequency comb, the Input and Through ends thereof are used for transmitting the TE mode and TM mode passed through the two ring filters respectively, and the Add and Drop ends are used for outputting the optical frequency comb, corresponding to the two output ports of the integrated optical microcavity chip, and a heating electrode is arranged above the microcavity for controlling the resonant wavelength passed through the microcavity; The integrated optical microcavity chip is prepared by the following steps: Step S1: growing a lower cladding layer of silicon dioxide on a silicon or SOI or lithium niobate substrate; Step S2: growing a silicon nitride or lithium niobate film as a waveguide layer on the lower cladding layer of silicon dioxide; Step S3: realizing pattern transfer of the waveguide layer by photolithography or electron beam exposure combined with ICP etching or wet etching to obtain a polarization beam splitter, two ring filters and an Add-drop type microcavity; Step S4: growing a layer of silicon dioxide as an upper cladding layer on the waveguide layer with the pattern prepared by ICP-CVD; Step S5: performing photolithography on the upper cladding layer of silicon dioxide, and preparing a heating layer above the polarization beam splitter, the two ring filters and the Add-drop type microcavity by using a lift-off process; Step S6: growing a layer of silicon dioxide again on the heating layer, and etching the position and depth of the contact electrode by photolithography to prepare the contact electrode of the heating layer by using a lift-off process; Step S7: growing a layer of silicon dioxide again, and then performing photolithography and etching to obtain a through hole of the contact electrode.

2. The structure of claim 1, wherein The wavelength difference of the two ring filters transmitting wavelengths is N x FSR n where FSR is the free spectral range of the microcavity, N is the number of FSR periods, n is the wavelength difference of the TE and TM modes in the same period.

3. The structure of claim 1 or 2, wherein The laser chip is a semiconductor laser chip, and outputs single-mode or multi-mode laser, and the polarization beam splitter is a Y-type polarization beam splitter.

4. The structure of claim 1 or 2, wherein The silicon dioxide grown in steps S4, S6 and S7 is used as the upper cladding layer of the integrated optical microcavity chip together, and the thickness of the upper cladding layer is greater than 2 μm.

5. The structure of claim 1 or 2, wherein The waveguide end face of the laser chip is aligned with the waveguide end face of the integrated optical microcavity chip, and after alignment, the laser chip and the integrated optical microcavity chip are fixed by dispensing to complete the heterogenous integration.

6. The structure of claim 1 or 2, wherein The thickness of the lower cladding layer of silicon dioxide is greater than 2 μm.

7. A method of optical frequency comb generation, comprising: The structure of any one of claims 1-6 is used to generate an optical frequency comb, comprising: The laser generated by the laser chip is divided into TE mode and TM mode by a polarization beam splitter and sent into two ring filters respectively, the heating electrodes of the ring filters are finely adjusted, the through wavelengths are accurately controlled, the ring filters have a certain wavelength difference, and after filtering, the ring filters are oppositely injected into an Add-drop type microcavity, one of the TE mode and the TM mode with lower power is used as auxiliary laser, and the other is used as pump laser, the heating electrodes on the Add-drop type microcavity are adjusted, the microcavity is reduced from high temperature to low temperature, until the optical frequency comb is generated, and the generated optical frequency comb is output through one of the two output ports of the integrated optical microcavity chip.

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