Lens-assisted all-solid-state beam scanning system

Abstract

A lens-assisted, all-solid-state beam scanning system comprises an on-chip input coupler, a 1×N optical switch, and an emission waveguide array, as well as an off-chip microlens array, a grating, and a cylindrical lens. The emission waveguide array consists of N parallel emission waveguides perpendicular to the laser beam transmission direction. The microlens array, positioned in the beam output direction of the emission waveguide array, is composed of N sub-lenses arranged corresponding to the N emission waveguides, with the focal plane of each sub-lens coinciding with the emission end facet of the corresponding emission waveguide. The emission end facet width of the emission waveguide is such that when the emitted beam impinges on a sub-lens, the horizontal width of the beam is equal to the horizontal working width of the sub-lens, and the vertical width of the beam is less than the vertical working width of the sub-lens. The present invention combines an on-chip end-face emission waveguide array with a microlens array to reduce losses and reduce the far-field scanning blind zone to less than 1%, enabling rapid beam scanning.

Description

Technical Field

[0001] The present invention relates to all-solid-state light beam scanning, in particular to a lens-assisted all-solid-state light beam scanning system. Background Art

[0002] LiDAR has important application value in the fields of unmanned driving and robotics. One of the core components of LiDAR is beam scanning. In recent years, a fully solid-state beam scanning technology, namely lens-assisted beam steering (LABS), also known as focal plane switch array (FPSA), has received widespread attention. This technology places a set of emitting unit arrays on the focal plane of the lens and switches the light beams emitted from different emitting units. The light beams emitted by different emitting units will be collimated by the lens and deflected to different pointing angles. Therefore, by switching the emitting units, far-field beam scanning can be achieved. LABS technology is particularly suitable for the use of photonic integration technology, and is expected to achieve miniaturized fully solid-state beam scanning and LiDAR.

[0003] Although there are many technical solutions for LABS technology, none of them can meet the comprehensive performance requirements of beam scanning. These performance requirements mainly include: (1) the integrated photonic platform should accommodate as high a waveguide power as possible, and the loss from waveguide input to beam output should be as small as possible to achieve long-distance beam scanning; (2) the far-field scanning blind area should be as small as possible; and (3) the switching of the beam should be as fast as possible to achieve fast scanning.

[0004] Several comparative technologies related to the present invention and also based on LABS are described and analyzed one by one here.

[0005] Technology 1: Using silicon-based integrated MEMS switch array, grating emission array and off-chip lens (X. Zhang, et al.. "A large-scale microelectromechanical-systems-based silicon photonics LiDAR," Nature 603, 253-258 (2022)). This solution uses silicon-based waveguides. Due to the existence of the two-photon effect, the waveguide cannot withstand high power, generally not exceeding 10mW. The actual emission loss of the emission grating is relatively high, about 3dB. The emission grating is limited by the size of the silicon-based MEMS optical switch and is sparsely arranged. However, since the LABS technology is based on the principle of lens imaging, the far-field scanning spot and the emission spot at the focal plane of the lens (in this technology, the spot at the emission grating array) are mirror images, so the sparsely arranged emission grating array results in a large far-field scanning blind area (>80%). The MEMS optical switch is slow, on the order of microseconds.

[0006] Technology 2: Using a silicon-based integrated thermo-optical switch array, a grating emission array, an off-chip microlens array, and an off-chip main lens (C. Rogers, et al., “A universal 3D imaging sensor on a silicon photonics platform,” Nature 590, 256-261 (2021)). This solution uses a silicon-based waveguide, which, like Technology 1, cannot withstand high intra-waveguide power. The emission grating has high losses. The thermo-optical switching speed is slow, on the order of microseconds.

[0007] Technology 3: Using a silicon-based integrated thermo-optical switch array, a long photonic crystal grating array, and an off-chip lens (H.Ito, et al., "Wide beam steering by slow-light waveguide gratings and a prismlens," Optica 7, 47-52 (2020)). This solution uses a silicon-based waveguide, which, like technology 1, cannot withstand high intra-waveguide power. The emission grating has high loss, and due to the photonic crystal structure, it cannot be densely arranged, so the far-field scanning blind area is also large. The thermo-optical switch is slow, on the order of microseconds.

[0008] Technology 4: Using a silicon nitride integrated thermo-optical switch array, a strip grating array, and an off-chip lens (C. Li, et al., “Blind zone-suppressed hybrid beam steering for solid-state lidar,” Photonics Research 9, 1871-1880 (2021)). This solution uses a silicon nitride waveguide, which can accommodate higher waveguide power but suffers from higher emission grating losses. Thermo-optical switching is slow, on the order of tens of microseconds.

[0009] In summary, silicon-based integrated waveguide devices cannot handle high intra-waveguide powers due to silicon's two-photon absorption effect. Using thermo-optical or MEMS switches results in slow speeds. Using gratings for transmission results in high losses. Furthermore, densely arranging the transmitting gratings poses challenges. Therefore, a new LABS beam scanning technology is needed that balances waveguide power tolerance, beam transmission losses, far-field scanning blind spots, and scanning speed. Summary of the Invention

[0010] In response to the above-mentioned deficiencies in the existing technology, the present invention provides a lens-assisted all-solid-state beam scanning system. By adopting on-chip high-speed optical switches and emission waveguide arrays, in conjunction with off-chip microlens arrays and gratings, all-solid-state beam scanning is achieved while taking into account waveguide power tolerance, beam emission loss, dense arrangement of emission units and small far-field scanning blind spots, and has broad application prospects.

[0011] The technical solutions of the present invention are as follows:

[0012] A lens-assisted all-solid-state beam scanning system is characterized by comprising: an input coupler, a 1×N optical switch, and a launch waveguide array integrated on a chip, as well as an off-chip microlens array, a grating, and a cylindrical lens;

[0013] The input coupler is used to receive the laser beam and couple it into the chip;

[0014] The 1×N optical switch is connected to the output end of the input coupler, and is used to receive the laser beam and transmit it to the corresponding nth waveguide of the transmitting waveguide array;

[0015] The transmitting waveguide array is composed of N parallel transmitting waveguides perpendicular to the transmission direction of the laser beam. The input end of each transmitting waveguide is respectively connected to the N output ends of the 1×N optical switch. The transmitting end face of each transmitting waveguide is the same size and constitutes the output end face of the chip as a whole. The transmitting waveguide is along the y-axis direction, the chip normal is along the x-axis direction, and the transmitting waveguide array is arranged along the z-axis direction.

[0016] The microlens array is arranged in the light beam output direction of the emitting waveguide array and is composed of N sub-lenses arranged corresponding to the N emitting waveguides, with the focal plane of each sub-lens coinciding with the emitting end face of the corresponding emitting waveguide. The emitting end face width of the emitting waveguide is such that when the emitted light beam impinges on the sub-lens, the horizontal width of the light beam is equal to the horizontal working width of the sub-lens, the vertical width of the light beam is less than the vertical working width of the sub-lens, and the horizontal working width of each sub-lens is no less than 99% of the physical width of the sub-lens in the horizontal direction. The microlens array is used to collimate the light beam emitted from the on-chip emitting waveguide array.

[0017] The grating is arranged in the light beam output direction of the microlens array, and the collimated light beam output by the microlens array is subjected to far-field diffraction deflection in the vertical direction (i.e., the x-axis direction) by the grating;

[0018] The cylindrical lens is arranged in the light beam output direction of the grating, and the focal plane of the cylindrical lens coincides with the output plane of the grating, and is used to re-collimate the light beam after being deflected by the grating far-field diffraction and deflect it in the horizontal direction (i.e., the z-axis direction).

[0019] Preferably, the emitting waveguide widens the waveguide width at the edge of the chip by means of insulating tapering, and the width needs to be greater than the waveguide height, so that the light beam reaching the end face of the waveguide has a larger width in the horizontal direction (i.e., the z-axis direction) and a smaller width in the vertical direction (i.e., the x-axis direction). Therefore, after the light beam is emitted into the free space, a smaller divergence angle in the horizontal direction and a larger divergence angle in the vertical direction are achieved.

[0020] Preferably, the input coupler, the 1×N optical switch and the transmitting waveguide array all operate in a single transverse electric (TE) mode or a single transverse magnetic (TM) mode.

[0021] Furthermore, it also includes a wavelength tunable laser arranged outside the chip, used to output a laser beam to the input coupler; and a controller, used to control the wavelength of the laser beam output by the tunable laser and the output selection of the 1×N optical switch.

[0022] The controller is connected to the wavelength tunable laser and the 1×N optical switch respectively by means of wires, bonding or other electrical connection methods.

[0023] The output end face of the emission waveguide array is polished and coated with an anti-reflection film, thereby reducing the loss of the light beam when it is emitted from the waveguide to the free space.

[0024] The chip comprises a substrate, a silicon dioxide cladding layer and a waveguide layer from bottom to top. On the waveguide layer, the input coupler, 1×N optical switch and emission waveguide array are prepared along the light beam propagation direction through etching, deposition and / or stripping processes.

[0025] The input coupler is used to achieve low-loss coupling from the laser to the chip, and is a tapered waveguide or a cantilever beam waveguide.

[0026] The spacing between the N waveguides of the emission waveguide array is the same as the spacing between the sub-lenses of the microlens array.

[0027] Preferably, the waveguide layer is made of thin-film lithium niobate material and is x-cut and y-transmitting, that is, the normal of the thin-film lithium niobate is along the x-axis, the propagation direction of the optical signal is along the y-axis, and the crystal axis is the z-axis.

[0028] Preferably, the thin-film lithium niobate waveguide layer is doped with magnesium oxide or other materials that can suppress the photorefractive effect in lithium niobate, thereby improving the power tolerance of the device.

[0029] Preferably, the 1×N optical switch is based on the electro-optical effect of lithium niobate and can achieve nanosecond switching.

[0030] Preferably, the thickness of the thin film lithium niobate waveguide layer is 600 nm.

[0031] The working principle of the present invention is:

[0032] For a specific laser output wavelength, the on-chip 1×N optical switch is used to control the light beam from a specific emission waveguide array into free space. After being collimated by the microlens array corresponding to the emission waveguide array, it is irradiated onto a grating. The grating deflects the light beam according to its wavelength. The deflected light beam then strikes a cylindrical lens, which deflects it again according to its incident position (determined by the position of the emission waveguide). The deflection direction is perpendicular to the deflection direction controlled by the wavelength. Therefore, changing the laser output wavelength can control the beam deflection in one direction, while changing the on-chip emission waveguide array can control the beam deflection in another direction, thus achieving two-dimensional beam scanning.

[0033] Compared with the prior art, the present invention has the following advantages:

[0034] 1) The present invention does not use on-chip grating emission with high loss, but instead uses a combination of an on-chip end-face emission waveguide array and an off-chip microlens array to achieve loss reduction.

[0035] 2) The far-field light spot of the present invention is no longer in a mirror relationship with the light spot at the emission array as in traditional LABS technology, but is in a mirror relationship with the light spot on the focal plane of the cylindrical lens (that is, the light spot output by the microlens array). In this way, by controlling the divergence angle of the emitted light from the emission waveguide in the horizontal direction, the light spots output by the microlens array can be densely arranged, and more than 99% of the light field can be filled in the horizontal direction, thereby reducing the far-field horizontal scanning blind area to less than 1%. In the vertical direction, the method of wavelength tuning combined with grating deflection is adopted, and the continuous tunability of the wavelength ensures that there is no blind area in the far field in the vertical direction. Therefore, the total far-field scanning blind area can be reduced to less than 1%.

[0036] 3) The present invention uses an optical switch based on the high-speed electro-optical effect of lithium niobate, and the switching speed can reach nanoseconds or even faster, so fast beam scanning can be achieved.

[0037] 4) The present invention uses magnesium oxide-doped thin-film lithium niobate as the waveguide layer, which can suppress the photorefractive effect in the lithium niobate and improve the power tolerance. At the same time, lithium niobate has no two-photon absorption in a very wide operating band (500nm-3μm), so it can support higher power in the waveguide. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1It is a schematic diagram of the lens-assisted all-solid-state beam scanning system of the present invention.

[0039] Figure 2 Three typical implementations of a 1×N optical switch are shown: (a) binary tree structure, (b) chain structure, and (c) multimode interferometer structure.

[0040] In the figure: 1-substrate, 2-silica cladding, 3-input coupler, 4-1×N optical switch, 5-transmitting waveguide array, 6-microlens array, 7-grating, 8-cylindrical lens, 9-laser, 10-controller;

[0041] 41-1×2 optical switch input waveguide, 42-1×2 optical switch, 43-1×2 optical switch output waveguide, 44-1×N optical switch output waveguide, 45-1×2 optical switch control wire, i.e., electro-optical phase shifter control wire, 46-1×N optical beam splitter, 47-electro-optical phase shifter, 48-N×N multimode coupler. DETAILED DESCRIPTION

[0042] The present invention will be further described below with reference to the accompanying drawings and examples, but the scope of protection of the present invention shall not be limited thereto. The embodiments of the present invention include but are not limited to the following examples.

[0043] Example

[0044] The lithium niobate film is 0.6 microns thick, the silicon dioxide cladding is 2 microns thick, and the silicon substrate is 400 microns thick. The lithium niobate waveguide is etched into a strip waveguide or a ridge waveguide. The operating wavelength is 1550nm.

[0045] Figure 1 The structural diagram of the present invention is given, taking N=8 as an example.

[0046] A lens-assisted beam scanning device based on thin-film lithium niobate, comprising:

[0047] The chip consists of a substrate, a silicon dioxide cladding, and a lithium niobate thin film from bottom to top. Integrated waveguides and devices are fabricated on the lithium niobate thin film through processes such as etching, deposition, and stripping. On the thin-film lithium niobate chip, along the direction of beam propagation, are an input coupler, a 1×N optical switch, and an end-face emission waveguide array. Outside the chip, along the direction of beam propagation, are a microlens array, a grating, and a cylindrical lens. In addition, a laser couples laser light into the chip via an on-chip input coupler. A controller controls the laser's output wavelength and the output channel of the on-chip 1×N optical switch.

[0048] The lithium niobate film is x-cut and y-transmitted, that is, the normal line of the erbium-doped lithium niobate film is along the x-axis, the optical signal propagation direction is along the y-axis, and the crystal axis is the z-axis;

[0049] The lithium niobate film is doped with magnesium oxide or magnesium or other materials that can suppress the photorefractive effect in lithium niobate and improve its power tolerance;

[0050] The lithium niobate waveguide can be a partially etched ridge waveguide or a fully etched strip waveguide, and the waveguide is covered with silicon oxide or other low refractive index cladding to reduce waveguide loss;

[0051] The input coupler is used to achieve low-loss coupling from the laser to the chip, and can be a tapered waveguide, cantilever beam waveguide or other structures;

[0052] The 1×N optical switch is an optical switch based on the lithium niobate electro-optic effect, and can be implemented by a 1×2 optical switch using a binary tree structure, a 1×2 optical switch using a Mach-Zehnder interferometer structure combined with a lithium niobate electro-optical phase shifter, a 1×2 optical switch using a chain structure, or a 1×N optical beam splitter connected to an N×N multimode interferometer via N lithium niobate electro-optical phase shifter waveguides.

[0053] The lithium niobate electro-optical phase shifter is a device that places parallel electrodes on both sides of a lithium niobate waveguide along the y-axis. When a voltage is applied to the electrodes, an electric field along the z-axis is generated. This electric field changes the refractive index of the waveguide through the electro-optic effect of the lithium niobate waveguide, thereby changing the phase of the light beam in the waveguide.

[0054] Preferably, the 1×N optical switch of the present invention adopts a binary tree structure based on the 1×2 optical switch, so that the loss from input to all outputs is the same;

[0055] The transmitting waveguide array consists of N waveguides arranged in parallel, with the waveguides oriented along the y-axis and the array arranged along the z-axis. One end of each waveguide is connected to the N outputs of a 1×N optical switch, and the other end extends to the edge of the chip. The waveguide end face is polished to reduce roughness and coated with an antireflection coating to minimize light loss when it is emitted from the waveguide into free space.

[0056] The emission waveguide can change its waveguide size at the chip edge by means of tapering, etc., so that the light beam emitted into free space has a large spot size and a small divergence angle in the horizontal direction (i.e., on the YOZ plane), and a small spot size and a large divergence angle in the vertical direction (i.e., on the XOY plane);

[0057] The microlens array is used to collimate the light beam emitted from the on-chip waveguide array. The arrangement direction of the microlens array is parallel to the emitting waveguide array, that is, along the z-axis direction. The center line of the microlens is along the y-axis direction, and the spacing is equal to the spacing of the emitting waveguide. The end face of the end-face emitting waveguide array at the edge of the chip coincides with the focal plane of the microlens array. Each microlens corresponds to an emitting waveguide, and the light beam emitted from the waveguide is collimated after being irradiated onto the microlens. And after the light beam emitted from the waveguide is diverged to a certain extent, when it is irradiated onto the microlens, the width of the light beam in the horizontal direction is exactly equal to the effective aperture of the microlens in the horizontal direction, and the width of the light beam in the vertical direction should be less than or equal to the effective aperture of the microlens in the vertical direction;

[0058] The grating is used to diffract and deflect the light beam collimated by the microlens according to its wavelength in the XOY plane. The grating is placed parallel to the microlens array. The grating can be either transmissive or reflective.

[0059] The cylindrical lens is used to re-collimate and deflect the beam diffracted by the grating. The focal plane of the cylindrical lens coincides with the grating output plane. The wavelength-dependent deflection of the grating output beam should align with the non-focusing direction of the cylindrical lens. The positional shift of the output beam, resulting from on-chip output waveguide switching, should align with the focusing direction of the cylindrical lens. The cylindrical lens should be sufficiently large to ensure that the grating output beam, regardless of wavelength, falls within its effective operating area.

[0060] The microlens array and cylindrical lens are both coated with anti-reflection films on both sides to reduce interface reflection.

[0061] The wavelength of the antireflection film corresponds to the operating wavelength of the device.

[0062] Furthermore, the present invention includes a wavelength-tunable laser and a controller. The laser is coupled into the chip via an on-chip input coupler. The controller controls the wavelength of the tunable laser and the output gating of the on-chip 1×N optical switch. The controller's control signals can be connected to the laser and the 1×N optical switch via wires, bonding, or other electrical connections.

[0063] Figure 2 There are three typical implementation methods of 1×N optical switch, namely, binary tree structure based on 1×2 optical switch ( Figure 2 a) Chain structure based on 1×2 optical switches ( Figure 2 b), and structures based on multimode interferometers ( Figure 2c) For a 1×2 optical switch, the phase is controlled by an electro-optical phase shifter on one arm of the Mach-Zehnder interferometer, switching the input light to either of the two output waveguides. The electro-optical phase shifter is designed by placing metal electrodes parallel to each other on either side of the lithium niobate waveguide and applying a voltage to the electrodes to change the refractive index of the lithium niobate waveguide, thereby altering the phase of the optical signal in the waveguide. In the figure, 41 represents the input waveguide of the 1×2 optical switch, 42 represents the 1×2 optical switch, 43 represents the output waveguide of the 1×2 optical switch, 44 represents the output waveguide of the 1×N optical switch, 45 represents the control line of the 1×2 optical switch, or the control line of the electro-optical phase shifter, 46 represents the 1×N optical beam splitter, 47 represents the electro-optical phase shifter, and 48 represents the N×N multimode coupler.

[0064] Across the entire device, on-chip losses are dominated by the 1×N optical switch. By optimizing the losses of the 1×2 optical switch, these losses can be controlled to within 0.5dB. The light beam emitted from the waveguide and passing through the microlens, grating, and cylindrical lens incurs virtually no losses, resulting in extremely low insertion loss. Lithium niobate, without the double-photon absorption effect, can withstand hundreds of milliwatts of optical power within the waveguide. Combined with the system's extremely low losses, the present invention can achieve very high transmission power. The scanning blind zone is determined by the light spot arrangement on the microlens exit plane. As mentioned above, a blind zone of less than 1% can be achieved. Regarding beam scanning time, the horizontal scanning time is determined by the switching time of the 1×N optical switch. Due to the high-speed electro-optical effect of lithium niobate, this switching time can be controlled to less than 1 nanosecond. The vertical scanning time is determined by the laser wavelength tuning speed and can also be controlled to less than 10 nanoseconds, thus enabling high-speed beam scanning. In summary, the present invention can simultaneously achieve extremely low insertion loss, high transmission power, extremely small blind area and high-speed beam scanning, while various solutions of the comparative technology cannot achieve these performances at the same time.

[0065] The above is a detailed description of a preferred embodiment of the present invention, but it is not intended to limit the scope of the present invention. Any modifications, equivalent changes and improvements made within the scope of the present invention should still be included in the scope of protection of the present invention.

Claims

1. A lens-assisted all-solid-state beam scanning system, characterized in that: include: The input coupler, 1×N optical switch, and launch waveguide array integrated on the chip, as well as the off-chip microlens array, grating, and cylindrical lens; The input coupler is used to receive the laser beam and couple it into the chip; The 1×N optical switch is connected to the output end of the input coupler, and is used to receive the laser beam and transmit it to the corresponding nth waveguide of the transmitting waveguide array; The transmitting waveguide array is composed of N parallel transmitting waveguides perpendicular to the transmission direction of the laser beam. The input end of each transmitting waveguide is respectively connected to the N output ends of the 1×N optical switch. The transmitting end face of each transmitting waveguide is the same size and constitutes the output end face of the chip as a whole. The transmitting waveguide is along the y-axis direction, the chip normal is along the x-axis direction, and the transmitting waveguide array is arranged along the z-axis direction. The microlens array is arranged in the light beam output direction of the emitting waveguide array and is composed of N sub-lenses arranged corresponding to the N emitting waveguides, with the focal plane of each sub-lens coinciding with the emitting end face of the corresponding emitting waveguide. The emitting end face width of the emitting waveguide is such that when the emitted light beam impinges on the sub-lens, the horizontal width of the light beam is equal to the horizontal working width of the sub-lens, the vertical width of the light beam is less than the vertical working width of the sub-lens, and the horizontal working width of each sub-lens is no less than 99% of the physical width of the sub-lens in the horizontal direction. The microlens array is used to collimate the light beam emitted from the on-chip emitting waveguide array. The grating is arranged in the light beam output direction of the microlens array, and the collimated light beam output by the microlens array is subjected to far-field diffraction deflection in the vertical direction (i.e., the x-axis direction) by the grating; The cylindrical lens is arranged in the light beam output direction of the grating, and the focal plane of the cylindrical lens coincides with the output plane of the grating, and is used to re-collimate the light beam after being deflected by the grating far-field diffraction and deflect it in the horizontal direction (i.e., the z-axis direction).

2. The lens-assisted all-solid-state beam scanning system according to claim 1, characterized in that: The emitting waveguide is widened at the edge of the chip by insulating tapering. The width must be greater than the waveguide height, so that the light beam reaching the end face of the waveguide has a larger width in the horizontal direction (i.e., the z-axis direction) and a smaller width in the vertical direction (i.e., the x-axis direction). Therefore, after the light beam is emitted into free space, a smaller divergence angle in the horizontal direction and a larger divergence angle in the vertical direction are achieved.

3. The lens-assisted all-solid-state beam scanning system according to claim 1, characterized in that: The input coupler, 1×N optical switch and launch waveguide array all operate in a single transverse electric (TE) mode or a single transverse magnetic (TM) mode.

4. The lens-assisted all-solid-state beam scanning system according to claim 1, characterized in that: The spacing between the N waveguides of the emission waveguide array is the same as the spacing between the sub-lenses of the microlens array.

5. The lens-assisted all-solid-state beam scanning system according to any one of claims 1 to 4, characterized in that: Also includes the a wavelength tunable laser, configured to output a laser beam to the input coupler; and The controller is used to control the wavelength of the laser beam output by the tunable laser and the output gating of the 1×N optical switch.

6. The lens-assisted all-solid-state beam scanning system according to claim 5, characterized in that: The controller sampling wire, bonding or other electrical connection methods are respectively connected to the wavelength tunable laser and the 1×N optical switch.

7. The lens-assisted all-solid-state beam scanning system according to any one of claims 1 to 6, characterized in that: The emitting end face of the emitting waveguide array is polished and coated with an anti-reflection film, thereby reducing the loss of the light beam when it is emitted from the waveguide to the free space.

8. The lens-assisted all-solid-state beam scanning system according to any one of claims 1 to 6, characterized in that: The chip comprises a substrate, a silicon dioxide cladding layer and a waveguide layer from bottom to top. On the waveguide layer, the input coupler, 1×N optical switch and emission waveguide array are prepared along the light beam propagation direction through etching, deposition and / or stripping processes.

9. The lens-assisted all-solid-state beam scanning system according to any one of claims 1 to 6, characterized in that: The input coupler is a tapered waveguide or a cantilever beam waveguide, which is used to achieve low-loss coupling from the laser to the chip.

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