Microcavity optical frequency comb laser chip structure and preparation method thereof

Abstract

The application discloses a kind of microcavity optical frequency comb laser chip structure and preparation method thereof, the method comprises: obtaining n-type lower cladding layer, n-type lower waveguide layer, light-emitting active region and p-type upper waveguide layer in turn from bottom to top on substrate by epitaxial growth method, then echo wall mode microcavity is obtained by using photoetching etching;Photoetching is carried out on the upper waveguide layer, and a heating layer located above the microcavity is prepared for adjusting the phase in the microcavity;Photoetching is carried out again on the upper waveguide layer, and the contact electrode of the heating layer and the p-type electrode are prepared;Silicon oxide is grown as an insulating layer to protect the waveguide structure and prevent short circuit;Photoetching is carried out again, and the through hole of the contact electrode of the p-type electrode is prepared;The substrate is thinned and peeled off, and then the n-type electrode is prepared to obtain the microcavity laser.The application can realize laser and optical frequency comb two mode emission by a laser chip, can greatly improve the quality factor of microcavity, and can realize Kerr optical frequency comb under lower pump power.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of integrated photonics, and particularly relates to a microcavity optical frequency comb laser chip structure and a preparation method thereof. BACKGROUND

[0002] The Kerr optical frequency comb based on a micro resonant cavity is expected to have important applications in future astronomical observation, integrated microwave photon source, optical communication, miniaturized optical clock and the like due to its miniaturization and ultrahigh repetition frequency. Benefiting from the extremely high field enhancement effect and ultrashort cavity length in a high-quality factor microcavity, when continuous pumping light is coupled into the micro ring resonant cavity, high repetition frequency broadband optical frequency comb output can be realized through four-wave mixing effect at a lower pumping threshold, and the repetition frequency level can be improved by 2 to 3 orders of magnitude compared with other traditional optical frequency comb systems.

[0003] With the continuous progress of on-chip integration technology, miniaturization and integration of the Kerr optical frequency comb are possible. At present, the optical frequency comb realized based on the self-injection locking principle is the most integrated Kerr optical frequency comb. A DFB laser is directly used to pump a high-quality factor on-chip microcavity, and the Rayleigh scattering reflection of the microcavity is reflected back to the laser to realize phase locking through secondary mode selection. Although this method has high integration, there are still two technical problems, one is that due to the small output power of the laser itself, a microcavity with a quality factor of 1E7 is required to realize the formation of the optical frequency comb, which requires a very high preparation process for the microcavity; the other is that the laser and the microcavity are hetero-devices based on different material substrates, which is still not conducive to subsequent large-scale photon integration. These two factors jointly restrict the further development of the integrated microcavity optical frequency comb. SUMMARY

[0004] The purpose of the present application is to provide a microcavity optical frequency comb laser chip structure and a preparation method thereof as well as an optical frequency comb generation method, which can realize two mode emissions of laser and optical frequency comb through one laser chip, greatly improve the quality factor of the microcavity, and realize the Kerr optical frequency comb at a lower pumping power.

[0005] In order to achieve the above purpose, one aspect of the present application provides a preparation method of a microcavity optical frequency comb laser chip structure, comprising:

[0006] Step S1: obtaining, from bottom to top, an n-type lower cladding layer, an n-type lower waveguide layer, a light-emitting active region and a p-type upper waveguide layer on a substrate by epitaxial growth method, and then using photolithography or electron beam exposure combined with ICP etching or wet etching to obtain a whispering gallery mode microcavity;

[0007] Step S2: performing photolithography on the upper waveguide layer, and using a lift-off process to prepare a heating layer above the microcavity for adjusting the phase in the microcavity;

[0008] Step S3: contact electrodes of the heating layer and the p-type electrode are prepared by lift-off process through re-photolithography on the upper waveguide layer;

[0009] Step S4: the lower waveguide layer, the active region, the upper waveguide layer and the p-type electrode are protected and short circuit is prevented by growing silicon oxide as an insulation layer by ICP-CVD;

[0010] Step S5: a via hole of the contact electrode of the p-type electrode is prepared by re-photolithography and etching;

[0011] Step S6: the substrate is thinned and peeled off, and then the n-type electrode is prepared by electron beam evaporation or magnetron sputtering, thereby obtaining a laser with a whispering gallery mode microcavity.

[0012] Preferably, the epitaxial growth method is MOCVD or MBE or ALD.

[0013] Another aspect of the present application provides a microcavity optical frequency comb laser chip structure prepared by the preparation method described above, which can realize laser emission and produce Kerr optical frequency comb by adjusting input conditions.

[0014] Preferably, the whispering gallery mode microcavity includes a micro-ring cavity, a micro-disk cavity and a micro-sphere cavity, which is used to provide a gain structure for laser emission and form Kerr optical frequency comb by nonlinear effect.

[0015] Preferably, the active region satisfies the dual balance of gain and loss, dispersion and Kerr effect, the material of the active region is a Ⅲ-Ⅴ group, a Ⅱ-Ⅵ group material system or a rare earth doped lithium niobate crystal, and the structure of the active region is a quantum well structure, a quantum cascade structure or an interband cascade structure.

[0016] Still another aspect of the present application provides an optical frequency comb generation method for producing Kerr optical frequency comb by using the structure described above, which comprises:

[0017] injecting current into the laser to reach above the lasing threshold to produce laser close to single mode;

[0018] increasing the current of the laser to reach the parametric oscillation threshold to produce cascaded four-wave mixing;

[0019] fine-tuning the injection current of the microcavity electrode to reach the phase matching condition to produce Kerr optical frequency comb.

[0020] Preferably, a strip waveguide is introduced at a position 50-500 nm away from the microcavity to realize directional coupling output of the optical frequency comb, or the optical frequency comb is output by fiber coupling.

[0021] Preferably, the temperature of the microcavity is adjusted by a heating layer above the microcavity, or a TEC is arranged below the substrate to adjust the temperature of the microcavity.

[0022] The microcavity optical frequency comb laser chip structure, the preparation method thereof and the optical frequency comb generation method according to the above aspect of the present application can realize two mode emissions of laser and optical frequency comb by one laser chip, greatly improve the quality factor of the microcavity, and realize Kerr optical frequency comb at a lower pump power. BRIEF DESCRIPTION OF DRAWINGS

[0023] In order to more clearly illustrate the technical solutions of the present application, the following will briefly introduce the drawings used in the description of the embodiments of the present application. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor based on these drawings:

[0024] Figure 1 is a side view of a microcavity optical frequency comb laser chip structure of an embodiment of the present application;

[0025] Figure 2 is a top view of a microcavity optical frequency comb laser chip structure of an embodiment of the present application. DETAILED DESCRIPTION

[0026] In order to make the objects, technical solutions and advantages of the present application clearer, the technical solutions of the present application will be described clearly and completely in the following with reference to the drawings. Obviously, the described embodiments are only some embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0027] An embodiment of the present application provides a preparation method of a microcavity optical frequency comb laser chip structure, comprising the following steps S1-S6.

[0028] Step S1: After obtaining the lower cladding layer, the lower waveguide layer, the light-emitting active region and the upper waveguide layer on the substrate by an epitaxial growth method such as MOCVD (Metal-Organic Chemical Vapor Deposition), MBE (Molecular Beam Epitaxy) or ALD (Atomic Layer Deposition), the back-and-forth wall mode microcavity is realized by using photolithography or electron beam exposure combined with ICP (Inductively Coupled Plasma) etching or wet etching.

[0029] Step S2: again photoetching on the upper waveguide layer, and preparing the heating electrode above the whispering gallery mode microcavity by using the lift-off process, which is used for precisely regulating the phase in the microcavity;

[0030] Step S3: again photoetching on the upper waveguide layer, and preparing the contact electrode of the heating layer and the p-type electrode of the laser chip by using the lift-off process;

[0031] Step S4: using ICP-CVD to grow silicon oxide as an insulation layer to protect the waveguide and other structures (the lower waveguide layer, the active region, the upper waveguide layer and the p-type electrode) and prevent short circuit;

[0032] Step S5: again photoetching and etching to prepare the through hole of the contact electrode of the p-type electrode;

[0033] Step S6: flip the chip, thin and peel off the substrate, and then prepare the n-type electrode by using electron beam evaporation or magnetron sputtering.

[0034] The embodiment of the present application further provides a microcavity optical frequency comb laser chip structure prepared by the above method. Figure 1 and Figure 2 As shown in the drawings, the microcavity optical frequency comb laser chip structure of the embodiment of the present application comprises an n-type electrode 1, an n-type lower cladding layer 2, an n-type lower waveguide layer 3, a light-emitting active region 4, a p-type upper waveguide layer 5, a p-type electrode 6, a silicon oxide insulation layer 7 and a whispering gallery mode microcavity 9, wherein the through hole 8 is prepared on the p-type electrode 6. The material of the active region 4 can be a light-emitting material of a Ⅲ-Ⅴ group, a Ⅱ-Ⅵ group material system, a rare earth doped lithium niobate crystal, etc. The structure of the active region 4 is a light-emitting structure, which can be a quantum well structure, a quantum cascade structure, an interband cascade structure, etc. The active region 4 has a relatively wide gain bandwidth through the structure, dispersion and gain design, which satisfies the laser emission and the double balance of gain and loss, dispersion and Kerr effect.

[0035] The microcavity optical frequency comb laser chip structure of the embodiment of the present application is essentially a laser with a whispering gallery mode microcavity, which can directly generate an optical frequency comb by adjusting the input conditions of the laser. The microcavity optical frequency comb laser chip structure of the embodiment of the present application has two working modes, which can realize the emission of a single laser and the output of an optical frequency comb by adjusting the input conditions such as current and phase. The directional emission of the laser is realized by an asymmetric structure, or a strip waveguide is introduced at a place 50-500 nm away from the microcavity to realize the directional coupling output of the optical frequency comb.

[0036] The echo wall mode microcavity 9 includes a microcircular cavity, a microdisk cavity, a microsphere cavity, etc.

[0037] The microcavity optical frequency comb laser chip structure of the embodiment of the present application has two ways to adjust the temperature of the microcavity, one is a heating layer above the microcavity, and the other is a TEC (ThermoElectric Cooler, semiconductor cooler) under the substrate of the laser chip.

[0038] The microcavity optical frequency comb laser chip structure of the embodiment of the present application can output the optical frequency comb through waveguide-to-space coupling or fiber coupling, and the packaging structure is a butterfly packaging.

[0039] The embodiment of the present application also provides a method for generating an optical frequency comb, which generates the optical frequency comb through the microcavity optical frequency comb laser chip structure of the above embodiment.

[0040] In summary, the microcavity optical frequency comb laser chip structure and the preparation method thereof of the embodiment of the present application make the laser into a microcavity structure, so as to have double matching conditions of gain and loss and dispersion and nonlinear effect for the generation of the optical frequency comb.

[0041] According to the microcavity optical frequency comb laser chip structure, the preparation method thereof and the optical frequency comb generation method, one chip structure is both a laser light source and a microcavity for generating an optical frequency comb, compared with a traditional technical solution, the application greatly simplifies the generation device and the debugging process of the optical frequency comb, and discards the traditional scheme of accurately regulating and controlling discrete devices, and two mode emissions of laser and optical frequency comb can be realized through one optical laser chip, and the problem that the existing integrated microcavity optical frequency comb is of a heterogeneous substrate and is not conducive to photon integration is solved. In addition, in the process of generating the microcavity optical frequency comb by using the microcavity optical frequency comb laser chip structure, there is no coupling loss between discrete devices, the quality factor of the microcavity is greatly improved, and Kerr optical frequency comb can be realized at a lower pump power even without an optical amplifier, and the power consumption of the device is greatly reduced.

[0042] It is to be understood that the above description is merely a description of certain exemplary embodiments of the application, and modifications can be made to the described embodiments in various ways by those of ordinary skill in the art without departing from the spirit and scope of the application. Therefore, the above drawings and descriptions are illustrative in nature, and should not be construed as limiting the scope of protection of the claims of the application.

Claims

1. A method for fabricating a microcavity optical frequency comb laser chip structure, characterized in that, include: Step S1: An n-type lower cladding layer, an n-type lower waveguide layer, a light-emitting active region, and a p-type upper waveguide layer are sequentially obtained from bottom to top on the substrate using epitaxial growth methods. Then, a whispering-gallery mode microcavity is obtained by combining photolithography or electron beam exposure with ICP etching or wet etching. The active region satisfies the dual balance between gain and loss, and dispersion and Kerr effect. The whispering-gallery mode microcavity is used to provide a gain structure for the lasing of the laser and forms a Kerr optical frequency comb using nonlinear effects. Step S2: Perform photolithography on the upper waveguide layer and use a lift-off process to fabricate a heating layer located above the microcavity to adjust the phase in the microcavity; Step S3: Photolithography is performed again on the upper waveguide layer, and the contact electrode and p-type electrode of the heating layer are fabricated by lift-off process; Step S4: Use ICP-CVD grown silicon oxide as an insulating layer to protect the lower waveguide layer, active region, upper waveguide layer and p-type electrode, and prevent short circuits; Step S5: Perform photolithography etching again to prepare the through-hole of the contact electrode of the p-type electrode; Step S6: Thin and peel off the substrate, and then use electron beam evaporation or magnetron sputtering to prepare an n-type electrode to obtain a laser chip structure with a whispering-gallery mode microcavity. The structure can both emit laser light and generate a Kerr frequency comb by adjusting the input conditions.

2. The preparation method according to claim 1, characterized in that, The epitaxial growth method is MOCVD, MBE, or ALD.

3. A microcavity optical frequency comb laser chip structure, prepared using the preparation method described in claim 1 or 2.

4. The structure as described in claim 3, characterized in that, The whispering-gallery mode microcavities include micro-ring cavities, micro-disc cavities, and micro-sphere cavities.

5. The structure as described in claim 3 or 4, characterized in that, The active region is made of group III-V or group II-VI materials or rare earth-doped lithium niobate crystals, and the active region is structured as a quantum well structure, a quantum cascade structure, or an interband cascade structure.

6. A method for generating an optical frequency comb, characterized in that, Generating a Kerr optical frequency comb using the structure described in any one of claims 3-5 includes: By injecting current into the laser above the lasing threshold, a near-single-mode laser can be generated. Increase the laser current to reach the parametric oscillation threshold and generate cascaded four-wave mixing; By finely adjusting the injection current of the microcavity electrode to achieve phase matching conditions, a Kerr optical frequency comb is generated.

7. The optical frequency comb generation method as described in claim 6, characterized in that, By introducing a strip waveguide at a distance of 50~500nm from the microcavity, the directional coupling output of the optical frequency comb can be achieved, or the optical frequency comb can be output through optical fiber coupling.

8. The method for generating an optical frequency comb as described in claim 7, characterized in that, The temperature of the microcavity can be regulated by using a heating layer above the microcavity, or by setting a TEC under the substrate.

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