Size spot converter and high-power external cavity laser

Abstract

The invention discloses a spot-size converter (SSC) and a high-power external cavity laser chip, the external cavity laser chip comprises a wide-ridge gain chip and a passive external cavity chip, one end of the wide-ridge gain chip is plated with a high-reflection film as a reflection end, and the other end of the wide-ridge gain chip is plated with an antireflection film to realize low reflection; and efficient coupling of the fundamental mode of the wide-ridge gain chip with the size far larger than that of the single-transverse-mode waveguide and the small-size single-mode waveguide is realized by integrating the SSC. The SSC converts a large-size mode field and a small-size mode field mutually and effectively reduces optical coupling loss, mode selection and output wavelength adjustability are achieved by adjusting the longitudinal mode phase shifter and the micro-ring filter, part of power is fed back to the wide-ridge gain chip to construct external cavity resonance, and high-power output is achieved. According to the SSC, the device size can be remarkably reduced, a wide-ridge gain chip and an inter-chip hybrid integration process are compatible, and the tuning flexibility, the integration level and the overall stability of the laser are further improved while high-power output is kept.

Description

Technical Field

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

[0002] With the rapid development of sensing, lidar, coherent optical communications, and microwave photonics, people have put forward higher requirements on the size, weight, and power consumption (SWaP) indicators of related devices. Existing devices based on traditional processes and material systems often find it difficult to balance high performance and miniaturization. Therefore, the use of chip technologies such as silicon-based optoelectronics to achieve various optoelectronic functions has become one of the main ways to meet 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 compressed while ensuring the excellent performance of the system in terms of high speed, high bandwidth, and high sensitivity.

[0003] However, to truly achieve both miniaturization and high performance, it is necessary to overcome the chip-based problem of high-power laser sources. Especially in coherent optical communications, precision measurement and high-resolution lidar, high power, narrow linewidth and wide tuning range are crucial indicators. Since III-V semiconductor materials have excellent performance in optical gain, quantum efficiency and other aspects, high-power chip-level lasers often use III-V gain to provide core laser sources. In order to achieve better linewidth control and a wider tunable range, people tend to use hybrid integration solutions of passive materials such as silicon-based and III-V gain in recent years. Its external cavity structure combined with the gain chip can compress the linewidth to the sub-kHz level and at the same time ensure a tunable range of hundreds of nanometers. However, due to the inter-chip coupling loss and the loss of the passive structure in the hybrid cavity, the output power of the light source can usually only be maintained at a level of tens of milliwatts. If a gain chip is added at the back end, although the power can be further increased, it makes it difficult for other devices to be monolithically integrated with the overall system, which goes against the original intention of device miniaturization and integration.

[0004] In terms of reducing coupling losses, 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 idea of ​​ordinary conical or inverted conical SSC is simple, it often relies on a longer 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 make the insertion loss 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 weakens the chip utilization.

[0005] In order to further shorten the SSC length and take 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 multi-mode phenomena and significantly increasing insertion loss, weakening the bandwidth performance of the entire solution. Another type of improvement idea is to directly etch a microlens at the end of the waveguide or use a metamaterial structure 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, how to achieve low-loss, high-power coupling between the laser end and the passive waveguide has become one of the core issues in current optoelectronic chip design. If hybrid integrated external cavity lasers are to be used on a large scale in fields such as perception, communication, and microwave photonics, it is necessary to achieve high-power output and stable narrow linewidth and broadband tuning performance while ensuring low insertion loss. At the same time, it is necessary to continue to study the structure of SSC in depth, balance its coupling efficiency and required area, and improve the repeatability and yield of the manufacturing process. Summary of the invention

[0007] In order to improve the output power of the hybrid integrated external cavity laser chip, the present invention proposes a high-power external cavity laser based on a multi-port waveguide structure and a planar waveguide evanescent wave coupling structure SSC. Compared with conventional hybrid integrated external cavity laser chips, it not only greatly improves the power but also maintains a very small chip area.

[0008] The technical solution of the present invention is 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 inverted tapered waveguides arranged at equal intervals, wherein the front end width of the inverted tapered waveguide is smaller than the rear end width thereof, and is used to convert the output mode spot of the wide ridge gain chip to meet the input requirements of the planar waveguide evanescent wave coupling structure;

[0011] The planar waveguide evanescent wave coupling structure is composed of a triangular planar waveguide and a narrow straight waveguide. There is a certain distance between the hypotenuse of the triangular planar 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 section 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 planar 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 planar 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 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.

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

[0015] A wide ridge gain chip, with a high-reflection film on one end and an anti-reflection film 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 pattern spot 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 pattern spot converter is the above-mentioned pattern spot converter, which is used to convert the fundamental mode pattern spot output by the wide ridge gain chip, which is much larger than the size of the single-mode waveguide, into a smaller pattern spot matching 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 micro-ring filter and the free spectrum range is expanded by using the vernier effect, and 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 plated with a high reflection film with a reflectivity greater than 90%, and the other end is plated 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 planar waveguide evanescent wave coupling structure SSC. It then enters the longitudinal mode phase shifter, and then enters the multimode interferometer, and then enters the first microring resonator and the second microring resonator input ends respectively from the two channels output from the multimode interferometer. The light at the output ends of the first microring filter and the second microring filter returns along the direction of the output end of the second microring filter and the output end of the first microring filter, respectively, so that part of the power is transmitted in the opposite direction and fed back to the wide ridge gain chip, and the other part of the power and the two lights that are not coupled into the two microring resonators together constitute four output lights 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 and is used to match the large-size mode spot of the wide-ridge gain chip; the rear-end waveguide width is relatively wide and is used to be connected in series with the planar waveguide evanescent wave coupling structure with low loss.

[0023] The planar waveguide evanescent wave coupling structure is composed of a triangular planar waveguide and a narrow straight waveguide. The spacing between the hypotenuse of the triangular structure planar waveguide and the straight waveguide changes linearly, and the closer to the output end of the straight waveguide, the larger the spacing. Through the evanescent field interaction, the large-scale mode field in the lateral direction is coupled to the straight waveguide with low loss.

[0024] The radii of the first microring filter and the second microring filter are slightly different, and the vernier effect is used to increase the free spectral range.

[0025] The first micro-ring filter and the second micro-ring filter adopt a multi-mode waveguide structure to reduce waveguide transmission loss, while reducing optical power density and improving 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 III-V group materials 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 may adopt a multi-transverse mode straight waveguide structure, a tapered waveguide structure, or the like.

[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 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 the conventional III-V hybrid integrated external cavity laser, a solution to increase the power by using a wide ridge gain chip is proposed. Since the waveguide of the wide ridge gain chip is tens of microns or even hundreds of microns wide, it can provide watt-level output power.

[0034] 2. In order to convert the large-size mode field of the wide-ridge gain chip into the single-mode waveguide of the passive chip with low loss and high integration, a multi-port structure and a planar waveguide evanescent wave coupling structure SSC are designed. Compared with conventional tapered SSC, the required chip area can be reduced by more than 10 times while maintaining consistent low insertion loss.

[0035] 3. Conventional tapered SSC cannot achieve significant longitudinal changes in the mode spot under the condition that the waveguide thickness remains unchanged, while the multi-port structure can achieve changes in the longitudinal size of the mode spot, 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 This is a schematic diagram of the SSC structure of the multi-port structure and the 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 mode field distribution diagram of the multi-port structure

[0040] Figure 5 This is the output end 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] Fig. 9This is the electric field distribution diagram of the mode spot conversion process of the planar waveguide evanescent wave coupling structure

[0045] In the 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 in conjunction with the accompanying drawings and embodiments, but the protection scope of the present invention should not be limited thereto.

[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 includes a multi-port structure and a slab waveguide evanescent wave coupling structure SSC, a longitudinal mode phase shifter, a multi-mode interferometer, a first microring filter and a second microring filter, which together constitute an external cavity structure.

[0048] The wide ridge gain chip provides high power for the laser. The wide ridge gain chip active area adopts a high gain quantum well structure design. 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 of ≤0.1%. Conventional gain chips use a single transverse mode ridge waveguide with a width of 3 to 5 μm, while the wide ridge gain chip in this embodiment uses a 90 μm wide multi-transverse mode ridge straight waveguide, so it has a higher saturation power. Under the input current condition of 12A, the maximum output power can achieve 5W.

[0049] The enlarged image of the multi-port structure and the planar waveguide evanescent wave coupling structure SSC is shown in Figure 2 , which 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 ridge width of 90μm is 2.8μm, and the slow-axis mode field size is 84μm (in the present 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 result shows 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 overlap integral value of the mode field and the fundamental mode field of the wide ridge gain chip is the highest. The mode field distribution is shown in Figure 4The input end of the planar waveguide evanescent wave coupling structure is a 400nm thick, 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 distance 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 planar 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 planar 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. Figure 6 As shown, the longitudinal dimension of the mode field is compressed from 2.8 μm to 1 μm.

[0050] The other part is the planar waveguide evanescent wave coupling structure, which consists of a triangular planar waveguide and a narrow straight waveguide, with a certain spacing 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 planar waveguide and the straight waveguide. The spacing between the two waveguides determines the coupling strength. The closer to the output end of the planar 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 planar waveguide with a Gaussian distribution enter the straight waveguide through evanescent wave coupling with low loss. The acute angle of the triangle is related to the propagation constant β of the planar waveguide and the straight waveguide, The angle is calculated to be about 31 degrees. 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 lower, 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 and increase the strength of the evanescent wave coupling. The waveguide width and the waveguide spacing at the output end are simulated. When the waveguide width is 0.4μm and the waveguide spacing at the output end 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 similar to the 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. Fig. 9 shown.

[0051] The longitudinal mode phase shifter adjusts the phase so that the longitudinal mode of the laser is aligned with the center of the filter passband. The longitudinal mode phase shifter adopts a thermo-optical phase shifter.

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

[0053] The first microring filter and the second microring filter are used to select the longitudinal mode of the laser. The radius of the second microring filter is slightly different from that of the first microring filter, and the vernier effect is used to increase the free spectral range. The microring filters all use a multimode waveguide structure to reduce the waveguide transmission loss, reduce the optical power density of the waveguide, and improve the stability of the laser. In order to maintain the fundamental mode transmission in the multimode waveguide during the bending process, Euler bending is used to reduce the excitation of high-order modes while maintaining a small radius.

[0054] The multi-mode interferometer, the first micro-ring filter, the second micro-ring 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 pattern 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 inverted tapered waveguides arranged at equal intervals, wherein the front end width of the inverted tapered waveguide is smaller than the rear end width thereof, and is used to convert the output mode spot of the wide ridge gain chip to meet the input requirements of the planar waveguide evanescent wave coupling structure; The planar waveguide evanescent wave coupling structure is composed of a triangular planar waveguide and a narrow straight waveguide. There is a certain distance between the hypotenuse of the triangular planar 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.

2. The pattern spot converter according to claim 1, characterized in that: The inverted tapered waveguide is composed of a waveguide section that is narrow at the front end, gradually widens in the middle, and is wide at the rear end, and matches the evanescent wave coupling structure of the planar 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 planar waveguide.

3. The pattern spot converter according to claim 2, 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.

4. A high-power external cavity laser chip, characterized in that: include: A wide ridge gain chip, with a high-reflection film on one end and an anti-reflection film 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 mode spot 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 mode spot converter is the mode spot converter according to any one of claims 1 to 3, and is used to convert the fundamental mode mode spot output by the wide ridge gain chip, which is much larger than the size of the single-mode waveguide, into a smaller mode spot 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 micro-ring filter and the free spectrum range is expanded by using the vernier effect, and part of the light is fed back to the wide ridge gain chip to realize laser oscillation.

5. The high power external cavity laser chip according to claim 4, 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 divide 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 the waveguide transmission loss and improve the laser stability; the multimode interferometer, the first microring filter, the second microring filter and the waveguide connected therebetween together constitute a Sagnac annular reflector, which serves as the rear cavity surface of the laser and has the function of optical feedback.

6. The high power external cavity laser chip according to claim 4, 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.

7. The high power external cavity laser chip according to claim 4, 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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