Spot converter and high power external cavity laser

Abstract

A spot mode converter (SSC) and a high-power external cavity laser chip, the external cavity laser chip including a wide ridge gain chip and a passive external cavity chip, one end of the wide ridge gain chip is coated with a high-reflection film as a reflective end, and the other end is coated with an anti-reflection film to achieve low reflection. By integrating the SSC, efficient coupling of the fundamental mode of the wide ridge gain chip, which is much larger than the size of a single transverse mode waveguide, with a small-sized single-mode waveguide is achieved. The SSC converts the large-scale mode field into the small-scale mode field and effectively reduces the optical coupling loss. By adjusting the longitudinal mode phase shifter and microring filter, mode selection and output wavelength adjustment are achieved. Part of the power is fed back to the wide ridge gain chip to establish external cavity resonance, achieving high-power output. The SSC of the present invention can significantly reduce the device size, is compatible with wide ridge gain chips and inter-chip hybrid integration processes, and further improves the tuning flexibility, integration, and overall stability of the laser while maintaining high-power output.

Description

Technical Field

[0001] The present invention belongs to the technical field of integrated optoelectronic devices, and in particular relates to a spot mode converter and a high-power external cavity laser. Background Art

[0002] With the rapid development of fields such as sensing, lidar, coherent optical communications, and microwave photonics, higher requirements are being placed on the size, weight, and power (SWaP) indicators of related devices. Existing devices based on traditional processes and material systems often struggle to achieve both high performance and miniaturization. Therefore, utilizing chip technologies such as silicon-based optoelectronics to implement various optoelectronic functions has become one of the main approaches to addressing this challenge. By integrating various functional devices such as waveguides, modulators, detectors, and active gain chips on the substrate, the overall size can be significantly reduced while ensuring excellent system performance in terms of high speed, high bandwidth, and high sensitivity.

[0003] However, achieving both miniaturization and high performance requires overcoming the challenges of integrating high-power laser sources into a chip. High power, narrow linewidth, and a wide tuning range are crucial requirements, particularly in coherent optical communications, precision measurement, and high-resolution lidar. Due to the excellent performance of III-V semiconductor materials in terms of optical gain and quantum efficiency, high-power chip-scale lasers often utilize III-V gain chips as the core laser source. To achieve better linewidth control and a wider tunable range, hybrid integration of passive materials such as silicon and III-V gain chips has recently gained favor. These external cavity structures, combined with gain chips, can compress the linewidth to sub-kHz levels while maintaining a tunable range on the order of hundreds of nanometers. However, due to inter-chip coupling losses and losses in the passive structures within the hybrid cavity, the output power of the source is typically limited to tens of milliwatts. While adding a gain chip to the backend can further increase power, it makes it difficult to integrate other components into the overall system, defeating the original purpose of device miniaturization and integration.

[0004] In terms of reducing coupling loss, the spot mode converter (SSC) is one of the key technologies. It can effectively couple the laser spot from the gain zone with the passive waveguide to ensure that the loss is minimized when entering or leaving the waveguide. Although the ordinary conical or inverted conical SSC is simple in concept, it often relies on a long gradient structure to achieve a gradual change in the spot size, which means that the required length is much larger than the change in the waveguide width. According to relevant simulation results, the length of conventional conical and inverted conical SSCs must be more than 20 times the difference in waveguide width before and after the spot mode conversion in order to reduce the insertion loss to less than 0.03dB. Although this form of SSC has a high coupling efficiency, it occupies too much chip area in a compact design, which reduces chip utilization.

[0005] In order to further shorten the SSC length while taking into account excellent mode spot conversion performance, researchers have tried to adjust the sidewall shape from conventional linear or simple curves to nonlinear distribution, so that the mode spot conversion can more reasonably change with the change of waveguide cross-section. However, these more complex sidewall trajectories not only increase the design difficulty, but also easily introduce roughness or irregular scattering on the waveguide sidewalls, thereby inducing multimode phenomena and significantly increasing insertion loss, weakening the bandwidth performance of the entire solution. Another type of improvement idea is to directly etch microlenses at the end of the waveguide or use metamaterial structures to shorten the actual occupied length of the mode spot conversion section. However, this often places extremely high demands on the lithography and etching processes. Once the process deviates slightly, the scattering loss will increase significantly, making it difficult to improve the overall performance to the ideal level.

[0006] Therefore, achieving low-loss, high-power coupling between the laser end and the passive waveguide has become a core issue in current optoelectronic chip design. If hybrid integrated external cavity lasers are to be widely used in fields such as sensing, communications, and microwave photonics, high-power output and stable narrow linewidth and broadband tuning performance must be achieved while ensuring low insertion loss. Furthermore, further research is needed into the structure of SSCs to balance their coupling efficiency with the required area and to improve the repeatability and yield of the manufacturing process. Summary of the Invention

[0007] To increase the output power of hybrid integrated external cavity laser chips, this paper proposes a high-power external cavity laser based on a multi-port waveguide structure and a slab waveguide evanescent wave coupling (SSC) structure. Compared to conventional hybrid integrated external cavity laser chips, this significantly increases power while maintaining a very small chip area.

[0008] The technical solutions of the present invention are as follows:

[0009] On the one hand, the present invention provides a spot converter, which is characterized in that it includes a multi-port waveguide structure and a planar waveguide evanescent wave coupling structure connected in series;

[0010] The multi-port waveguide structure is composed of a row of equally spaced inverted tapered waveguides, wherein the front end width of the inverted tapered waveguide is smaller than the rear end width, and is used to convert the output mode spot of the wide-ridge gain chip to meet the input requirements of the slab waveguide evanescent wave coupling structure;

[0011] The slab waveguide evanescent wave coupling structure consists of a triangular slab waveguide and a narrow straight waveguide. There is a certain distance between the hypotenuse of the triangular slab waveguide and the straight waveguide, and the distance changes linearly along the transmission direction. The mode spot is transformed in lateral size through the evanescent wave coupling effect.

[0012] Furthermore, the inverted tapered waveguide is composed of a waveguide segment that is narrow at the front end, gradually widens in the middle, and is wider at the rear end, and matches the evanescent wave coupling structure of the slab waveguide. When light is transmitted from the front end to the rear end in the multi-port waveguide, the longitudinal mode spot size gradually converges as the waveguide width gradually changes, thereby matching the fundamental mode of the evanescent wave coupling structure of the slab waveguide.

[0013] Furthermore, the mode spot is expanded or compressed in the longitudinal direction by adjusting the front end width, number and spacing distance of the inverted tapered waveguide; and by adjusting the rear end width and spacing distance of the inverted tapered waveguide, it is connected in series with the input end of the planar waveguide evanescent wave coupling structure with low loss.

[0014] On the other hand, the present invention also provides a high-power external cavity laser chip, which is characterized by comprising:

[0015] Wide-ridge gain chip, with a high-reflection coating on one end and an anti-reflection coating on the other end; the waveguide of the wide-ridge gain chip can adopt a multi-transverse mode straight waveguide structure or a tapered waveguide structure, and a quantum well, quantum wire or quantum dot epitaxial structure is used in the active area to provide high gain;

[0016] A passive external cavity chip, integrating a spot mode converter, a longitudinal mode phase shifter, a multimode interferometer, a first microring filter, a second microring filter, and a waveguide structure connecting them; wherein the spot mode converter is the spot mode converter described above, and is used to convert the fundamental mode spot mode output by the wide-ridge gain chip, which is much larger than the size of the single-mode waveguide, into a smaller spot mode that matches the single-mode waveguide of the passive chip;

[0017] The passive external cavity chip and the wide ridge gain chip together form a resonant cavity. The longitudinal mode is selected by the microring filter and the free spectrum range is expanded by using the vernier effect. At the same time, part of the light is fed back to the wide ridge gain chip to realize laser oscillation.

[0018] Furthermore, the wide-ridge gain chip has a ridge waveguide structure that is tens of microns or even hundreds of microns wide, providing high power for the laser.

[0019] The passive external cavity chip increases the cavity length of the laser, improves the optical delay, and provides a filtering function with a wide range of adjustable wavelengths.

[0020] Furthermore, one end of the wide-ridge gain chip is coated with a high-reflection film with a reflectivity greater than 90%, and the other end is coated with an anti-reflection film with a reflectivity ≤ 0.1%.

[0021] Furthermore, the light generated by the wide-ridge gain chip undergoes low-loss mode spot conversion through a multi-port structure located at the input end face of the passive external cavity chip and a slab waveguide evanescent wave coupling structure (SSC). It then enters a longitudinal mode phase shifter and then a multimode interferometer. The two output channels of the multimode interferometer then enter the input ends of the first and second microring resonators, respectively. The light from the output ends of the first and second microring filters returns toward the output ends of the second and first microring filters, respectively, allowing part of the power to be transmitted in opposite directions and fed back to the wide-ridge gain chip. The remaining power, combined with the two light paths that were not coupled into the two microring resonators, forms four output lights, which serve as the output of the laser.

[0022] The multi-port structure is formed by a row of inverted cone structures arranged at equal intervals. The front-end waveguide width is relatively small, which is used to match the large-size mode spot of the wide-ridge gain chip; the rear-end waveguide width is relatively wide, which is used to be connected in series with the flat waveguide evanescent wave coupling structure with low loss.

[0023] The slab waveguide evanescent wave coupling structure consists of a triangular slab waveguide and a narrow straight waveguide. The spacing between the hypotenuse of the triangular slab waveguide and the straight waveguide varies linearly, increasing toward the output end of the straight waveguide. Through evanescent field interaction, large-scale transverse mode fields are coupled into the straight waveguide with low loss.

[0024] The radii of the first microring filter and the second microring filter are slightly different, and the free spectral range is increased by using the vernier effect.

[0025] The first micro-ring filter and the second micro-ring filter adopt a multi-mode waveguide structure to reduce waveguide transmission loss, reduce optical power density, and improve the stability of the laser during high-power operation.

[0026] Preferably, the wide-ridge gain chip can use materials with high luminous efficiency, such as those of the III-V family, as the material system of the epitaxial structure.

[0027] Preferably, the active region of the wide-ridge gain chip adopts a quantum well structure, a quantum wire structure or a quantum dot structure.

[0028] Preferably, the waveguide of the wide-ridge gain chip can adopt a multi-transverse-mode straight waveguide structure, a tapered waveguide structure, etc.

[0029] Preferably, the evanescent wave coupling structure SSC based on the multi-port structure and the planar waveguide can use silicon nitride, silicon, lithium niobate or other materials with suitable refractive index difference as waveguide materials.

[0030] Preferably, the phase shifter may be a phase shifter using a thermo-optical, electro-optical or other phase modulation mechanism.

[0031] Preferably, the multimode waveguide bending parts of the first microring filter and the second microring filter can adopt special bending methods such as Euler bending or Bessel bending, so as to improve the integration while ensuring fundamental mode transmission.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] 1. Based on conventional III-V hybrid integrated external cavity lasers, a solution is proposed to increase power by using wide-ridge gain chips. Because the waveguide of wide-ridge gain chips is tens or even hundreds of microns wide, they can provide watt-level output power.

[0034] 2. To achieve low-loss, high-integration conversion of the wide-ridge gain chip's large mode field into the passive chip's single-mode waveguide, a multi-port structure and a slab waveguide evanescent wave coupling (SSC) structure were designed. Compared to conventional tapered SSCs, the required chip area can be reduced by more than 10 times while maintaining consistent low insertion loss.

[0035] 3. Conventional tapered SSCs cannot achieve significant longitudinal changes in the mode spot size while maintaining a constant waveguide thickness. However, the multi-port structure can achieve longitudinal changes in the mode spot size, thereby improving the coupling efficiency between the wide-ridge gain chip and the external cavity chip. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is a schematic diagram of the structure of the SSC high-power external cavity laser chip based on the multi-port waveguide structure and the slab waveguide evanescent wave coupling structure.

[0037] Figure 2 Schematic diagram of the SSC structure with multi-port structure and planar waveguide evanescent wave coupling structure

[0038] Figure 3 This is the fundamental mode field distribution diagram of the wide ridge gain chip

[0039] Figure 4 This is the input end face mode field distribution diagram of the multi-port structure

[0040] Figure 5 This is the output end face mode field distribution diagram of the multi-port structure

[0041] Figure 6 This is the electric field distribution diagram of the multi-port structure mode spot conversion process

[0042] Figure 7 This is the input end mode field distribution diagram of the planar waveguide evanescent wave coupling structure

[0043] Figure 8 This is the output end mode field distribution diagram of the planar waveguide evanescent wave coupling structure

[0044] Figure 9This is the electric field distribution diagram of the mode spot conversion process of the planar waveguide evanescent wave coupling structure

[0045] Figure: 1- wide ridge gain chip, 2- silicon nitride chip, 21- SSC, 22- longitudinal mode phase shifter, 23- multimode interferometer, 24- first microring filter, 25- second microring filter DETAILED DESCRIPTION

[0046] The present invention is further defined below with reference to the accompanying drawings and embodiments, but the scope of protection of the present invention should not be limited thereby.

[0047] Figure 1 This is a schematic diagram of the structure of a high-power external cavity laser chip based on a multi-port waveguide structure and a slab waveguide evanescent wave coupling structure (SSC). As shown in the figure, the high-power external cavity laser chip includes a III-V broad-ridge gain chip and a 400nm thick silicon nitride chip. The silicon nitride chip contains the multi-port structure and slab waveguide evanescent wave coupling structure (SSC), a longitudinal mode phase shifter, a multimode interferometer, a first microring filter, and a second microring filter, which together form the external cavity structure.

[0048] Wide-ridge gain chips provide high power for lasers. The active region of the wide-ridge gain chip utilizes a high-gain quantum well structure. One end of the wide-ridge gain chip is coated with a high-reflection film, resulting in a reflectivity greater than 90%, while the other end is coated with an anti-reflection film, resulting in a reflectivity ≤0.1%. Conventional gain chips utilize a single-transverse-mode ridge waveguide with a width of 3 to 5 μm. The wide-ridge gain chip in this embodiment utilizes a 90 μm-wide multi-transverse-mode ridge straight waveguide, resulting in higher saturation power. At an input current of 12 A, the maximum output power can reach 5 W.

[0049] The enlarged image of the multi-port structure and the planar waveguide evanescent wave coupling structure SSC is shown in the figure below. Figure 2 , consists of two parts. The first part is a multi-port structure, which consists of a row of equally spaced inverted tapered waveguides. The multi-port structure is used to perform low-loss mode spot conversion between the output mode spot of the wide-ridge gain chip and the input mode spot of the planar waveguide evanescent wave coupling structure. According to the simulation results, the fast-axis mode field size of the fundamental mode of the wide-ridge gain chip with a 90μm ridge width is 2.8μm, and the slow-axis mode field size is 84μm (in this invention, the mode field sizes are value), such as Figure 3 As shown. The front end width of the inverted tapered waveguide in the multi-port structure is very narrow, which is used to amplify the mode field outside the waveguide, thereby increasing the longitudinal size of the mode spot. The front end width, waveguide spacing, and number of waveguides of the inverted tapered waveguide are simulated. The results show that when the front end width of the inverted tapered waveguide is 0.2μm, the inverted tapered waveguide spacing is 0.61μm, and the number of inverted tapered waveguides is 111, the mode field has the highest overlap integral value with the fundamental mode field of the wide ridge gain chip. The mode field distribution is shown in the figure below. Figure 4The input end of the planar waveguide evanescent wave coupling structure is a 400nm thick and 90μm wide silicon nitride waveguide, and the longitudinal size of its fundamental mode spot is 1μm, as shown in Figure 7 Then, the rear end width of the inverted tapered waveguide and the waveguide spacing are simulated to improve the overlap integral of the mode field at the multi-port output end and the mode field at the input end of the slab waveguide evanescent wave coupling structure. After parameter scanning, when the rear end width of the inverted tapered waveguide is 0.46μm and the waveguide spacing is 0.35μm, the mode field overlap integral with the mode field at the input end of the slab waveguide evanescent wave coupling structure is optimal. At this time, the mode field distribution is as follows: Figure 5 Finally, the length of the multi-port structure is simulated to determine the final structure. The results show that when the length of the multi-port structure is 75μm, the insertion loss is as low as 0.03dB. The multi-port structure mode spot conversion process is shown in Figure 6 As shown, the longitudinal size of the mode field is compressed from 2.8 μm to 1 μm.

[0050] The other part is the slab waveguide evanescent wave coupling structure, which consists of a triangular slab waveguide and a narrow straight waveguide, with a certain distance between the two waveguides. The guided mode is coupled due to the interaction of the evanescent field, causing the mutual transfer of mode power between the slab waveguide and the straight waveguide. The spacing between the two waveguides determines the coupling strength. The closer to the output end of the slab waveguide evanescent wave coupling structure, the larger the waveguide spacing is, and it increases linearly. Such a spacing design can make the fundamental mode field of the triangular slab waveguide with a Gaussian distribution enter the straight waveguide through the evanescent wave coupling with low loss. The acute angle of the triangle is related to the propagation constant β of the slab waveguide and the straight waveguide. The angle is calculated to be about 31 degrees, and after simulation optimization, the coupling efficiency is highest when the angle is 30.6 degrees. In order to achieve rapid coupling of the mode field within a limited length, the initial spacing of the straight waveguide needs to be set low, set to 0.35μm. The width of the narrow straight waveguide is smaller than the conventional width of the single-mode waveguide, in order to diffuse the mode spot of the straight waveguide into the silica to a certain extent, thereby increasing the strength of the evanescent wave coupling. The waveguide width and the output end waveguide spacing are simulated. When the waveguide width is 0.4μm and the output end waveguide spacing is 1.12μm, the insertion loss of the flat waveguide evanescent wave coupling structure is only 0.02dB. The flat waveguide evanescent wave coupling structure is used to efficiently convert the lateral size of the mode spot from 84μm to 2.5μm, which is consistent with the results of the simulation. Figure 8 The mode spot size of the 0.4μm wide and 400nm thick silicon nitride narrow straight waveguide shown in the figure is matched. The waveguide width is then widened from 0.4μm to 1μm through a short inverted tapered waveguide. The mode spot conversion process of the planar waveguide evanescent wave coupling structure is shown in the figure. Figure 9 shown.

[0051] The longitudinal mode phase shifter adjusts the phase to align the laser longitudinal mode with the center of the filter passband. The longitudinal mode phase shifter uses a thermo-optical phase shifter.

[0052] The multimode interferometer is used to split the incident light after SSC into two paths.

[0053] The first and second microring filters are used to select the longitudinal mode of the laser. The second microring filter has a slightly different radius from the first, utilizing the vernier effect to increase the free spectral range. Both microring filters utilize a multimode waveguide structure to reduce waveguide transmission losses and optical power density, thereby improving laser stability. To maintain fundamental mode transmission in the multimode waveguide during bending, Euler bending is employed to minimize excitation of higher-order modes while maintaining a small radius.

[0054] The multimode interferometer, the first microring filter, the second microring filter and the waveguide connected therebetween together form a Sagnac ring reflector, which serves as the rear cavity surface of the laser and has the function of optical feedback.

[0055] By adjusting the longitudinal mode phase shifter, the first microring filter and the second microring filter, the resonant wavelength of the Fabry-Perot cavity is aligned with the resonant wavelength of the microring vernier effect to achieve mode selection, and the laser output wavelength can therefore be adjusted.

Claims

1. A spot converter, characterized in that: It includes a serially connected multi-port waveguide structure and a planar waveguide evanescent wave coupling structure; The multi-port waveguide structure is composed of a row of equally spaced inverted tapered waveguides, wherein the front end width of the inverted tapered waveguide is smaller than the rear end width, and is used to convert the output mode spot of the wide-ridge gain chip to meet the input requirements of the slab waveguide evanescent wave coupling structure; The slab waveguide evanescent wave coupling structure is composed of a triangular slab waveguide and a narrow straight waveguide. There is a certain distance between the hypotenuse of the triangular slab waveguide and the straight waveguide, and the distance changes linearly along the transmission direction. The mode spot is transformed in the lateral size through the evanescent wave coupling effect. The inverted tapered waveguide consists of a waveguide segment that is narrow at the front end, gradually widens in the middle, and is wider at the rear end, and matches the evanescent wave coupling structure of the slab waveguide. When light is transmitted from the front end to the rear end in the multi-port waveguide, the longitudinal mode spot size gradually converges as the waveguide width gradually changes, thereby matching the fundamental mode of the evanescent wave coupling structure of the slab waveguide.

2. The pattern converter according to claim 1, characterized in that: The mode spot is expanded or compressed in the longitudinal direction by adjusting the front end width, number and spacing distance of the inverted tapered waveguide; and the input end of the planar waveguide evanescent wave coupling structure is connected in series with low loss by adjusting the rear end width and spacing distance of the inverted tapered waveguide.

3. A high-power external cavity laser chip, characterized in that: include: Wide-ridge gain chip, with a high-reflection coating on one end and an anti-reflection coating on the other end; the waveguide of the wide-ridge gain chip can adopt a multi-transverse mode straight waveguide structure or a tapered waveguide structure, and a quantum well, quantum wire or quantum dot epitaxial structure is used in the active area to provide high gain; A passive external cavity chip, integrating a spot mode converter, a longitudinal mode phase shifter, a multimode interferometer, a first microring filter, a second microring filter, and a waveguide structure connecting them; wherein the spot mode converter is the spot mode converter according to any one of claims 1-2, and is used to convert the fundamental mode spot mode output by the wide-ridge gain chip, which is much larger than the size of the single-mode waveguide, into a smaller spot mode that matches the single-mode waveguide of the passive chip; The passive external cavity chip and the wide ridge gain chip together form a resonant cavity. The longitudinal mode is selected by the microring filter and the free spectrum range is expanded by using the vernier effect. At the same time, part of the light is fed back to the wide ridge gain chip to realize laser oscillation.

4. The high-power external cavity laser chip according to claim 3, characterized in that: The longitudinal mode phase shifter is used to adjust the phase of the resonant longitudinal mode so as to align it with the resonant frequency of the microring filter; the multimode interferometer is used to split the incident light after passing through the SSC into two paths; the first microring filter and the second microring filter are used to select the laser longitudinal mode, and the radii of the two are slightly different. The vernier effect is used to increase the free spectrum range, and both use a multimode waveguide structure to reduce waveguide transmission loss and improve laser stability; the multimode interferometer, the first microring filter, the second microring filter and the waveguide connecting them together constitute a Sagnac annular reflector, which serves as the rear cavity surface of the laser and has the function of optical feedback.

5. The high-power external cavity laser chip according to claim 3, characterized in that: The waveguide width of the wide-ridge gain chip is in the range of tens of microns to hundreds of microns, and the saturation power is significantly higher than that of a conventional single-transverse-mode waveguide gain chip, and can achieve watt-level or higher output.

6. The high-power external cavity laser chip according to claim 3, characterized in that: By adjusting the longitudinal mode phase shifter, the first microring filter and the second microring filter, the resonant wavelength of the Fabry-Perot cavity is aligned with the resonant wavelength of the microring vernier effect, thereby achieving mode selection and output wavelength adjustment of the laser.

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