CN115657218A

CN115657218A - All-solid-state light beam scanning system based on lens assistance

Info

Publication number: CN115657218A
Application number: CN202210631784.2A
Authority: CN
Inventors: 吴侃; 陈建平
Original assignee: Shanghai Jiaotong University
Current assignee: Shanghai Jiaotong University
Priority date: 2022-05-25
Filing date: 2022-05-25
Publication date: 2023-01-31

Abstract

A lens-based assisted all-solid-state beam scanning system, comprising: an input coupler, a1 XN optical switch and a transmitting waveguide array integrated on a chip, and an off-chip microlens array, a grating and a cylindrical lens; the transmitting waveguide array consists of N parallel transmitting waveguides vertical to the transmission direction of the laser beams; the micro-lens array is arranged in the light beam output direction of the transmitting waveguide array and is formed by arranging N sub-lenses corresponding to N transmitting waveguides, and the focal plane of each sub-lens is superposed with the transmitting end face of the corresponding transmitting waveguide; the width of the emission end face of the emission waveguide is such that when the emitted light beams irradiate the sub-lens, the width of the light beams in the horizontal direction is equal to the working width of the sub-lens in the horizontal direction, and the width of the light beams in the vertical direction is smaller than the working width of the sub-lens in the vertical direction. The invention adopts the combination of the transmitting waveguide array on the upper end surface and the micro-lens array, thereby achieving the reduction of loss, reducing the far-field scanning blind area to within 1 percent and realizing the rapid light beam scanning.

Description

All-solid-state light beam scanning system based on lens assistance

Technical Field

The invention relates to all-solid-state light beam scanning, in particular to a lens-assisted all-solid-state light beam scanning system.

Background

The laser radar has important application value in the fields of unmanned driving, robots and the like. One of the core components of lidar is beam scanning. In recent years, attention has been paid to an all-solid-state beam scanning technique, i.e., lens-assisted beam scanning (LABS), also known as Focal Plane Switch Array (FPSA). According to the technology, a group of emission unit arrays are arranged on a focal plane of a lens, light beams are switched to be emitted from different emission units, and the light beams emitted by the different emission units are collimated by the lens and then deflected to different pointing angles. Thus, by switching the emitting unit emitting light, beam scanning in the far field can be achieved. The LABS technology is particularly suitable for photonic integration technology, and is expected to realize miniaturized all-solid-state beam scanning and laser radar.

There are currently numerous solutions for the LABS technology, none of which can take into account the overall performance requirements of the beam scanning. These performance requirements include mainly: (1) The integrated photonic platform should accommodate as high waveguide internal 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; (3) The switch switches the beam as fast as possible to achieve fast scanning.

Several comparative techniques, also based on LABS, which are relevant to the present invention, are described and analyzed herein one by one.

The first technology is as follows: silicon-based integrated MEMS switch arrays, grating emitter arrays and off-chip lenses (x.zhang, et al. "a large-scale micro-electromechanical-systems-based silicon semiconductors LiDAR," Nature 603, 253-258 (2022)) are employed. The scheme adopts a silicon-based waveguide, and the waveguide cannot bear higher power due to the existence of a two-photon effect, and generally does not exceed 10mW. The actual emission loss of the emission grating is high, about 3dB. The emission grating is limited by the size of the silicon-based MEMS optical switch and adopts sparse arrangement. However, because the LABS technology is based on the lens imaging principle, the far-field scanning light spots are in a mirror image relationship with the emission light spots at the focal plane of the lens (in the technology, the light spots at the emission grating array), and the far-field scanning blind area is large (> 80%) due to the sparse emission grating array. MEMS optical switches are slow, on the order of microseconds.

The second technology comprises the following steps: silicon-based integrated thermo-optic switch arrays, grating emission arrays, off-chip microlens arrays, and off-chip main lenses (c. Rogers, et al, "a passive 3D imaging sensor on a silicon photonics platform," Nature 590, 256-261 (2021)) are employed. The scheme adopts the silicon-based waveguide, is similar to the technology, and cannot bear higher waveguide internal power. The optical grating loss is high. The thermo-optic switching speed is slow, on the order of microseconds.

The third technology: silicon-based integrated thermo-optic switch arrays, long-strip photonic crystal grating arrays, and off-chip lenses (H.Ito, et a1., "Wide beam stepping by slow-light waveguiding gratings and a prism lenses," Optica 7, 47-52 (2020)) are used. The scheme adopts the silicon-based waveguide, is similar to the technology, and cannot bear higher waveguide internal power. The loss of the emitted grating is high, and the far field scanning blind area is also large because the photonic crystal structure cannot be densely arranged. The thermo-optic switching speed is slow, on the order of microseconds.

The fourth technology is as follows: silicon nitride is used to integrate the thermo-optic switch array, the striped grating array and the off-chip lens (C.Li, et al, "Black zone-suppressed beam steering for solid-state Lidar," Photonics Research 9, 1871-1880 (2021)). The scheme adopts the silicon nitride waveguide, can accommodate higher waveguide internal power, but has higher transmission grating loss. The thermo-optic switching speed is slow, on the order of tens of microseconds.

In summary, if a silicon-based integrated waveguide device is used, it cannot withstand higher waveguide internal power due to the two-photon absorption effect of silicon. If a thermo-optic switch or MEMS switch is used, the rate is slower. If grating emission is adopted, the loss is higher. In addition, dense arrangement of emission gratings is also a problem. Therefore, there is a need to develop a new LABS beam scanning technique that can take into account waveguide power tolerance, beam launch loss, far field scan dead zone, and scan speed.

Disclosure of Invention

Aiming at the defects of the prior art, the invention provides a lens-assisted all-solid-state light beam scanning system, which realizes all-solid-state light beam scanning by adopting an on-chip high-speed optical switch and an emission waveguide array and matching with an off-chip micro lens array and a grating, and has waveguide power tolerance, light beam emission loss, dense arrangement of emission units, small far-field scanning blind area and wide application prospect.

The technical solution of the invention is as follows:

a lens-based assisted all-solid-state optical beam scanning system, comprising: an input coupler, a1 XN optical switch and a transmitting waveguide array integrated on a chip, and an off-chip micro-lens array, a grating and a cylindrical lens;

the input coupler is used for receiving the laser beam and is coupled into the chip;

the 1 xN optical switch is connected with the output end of the input coupler and used for receiving the laser beam and transmitting the laser beam to the nth waveguide of the corresponding transmitting waveguide array;

the transmitting waveguide array is composed of N parallel transmitting waveguides vertical to the transmission direction of the laser beam, the input end of each transmitting waveguide is respectively connected with N output ends of the 1 XN optical switch, the transmitting end face of each transmitting waveguide is the same in size, and the transmitting waveguides integrally form the output end face of the chip; the emission waveguides are arranged along the y-axis direction, the chip normal is arranged along the x-axis direction, and the emission waveguide array is arranged along the z-axis direction;

the micro lens array is arranged in the light beam output direction of the transmitting waveguide array and is formed by arranging N sub lenses corresponding to the N transmitting waveguides, and the focal plane of each sub lens is superposed with the transmitting end face of the corresponding transmitting waveguide; the width of the emission end face of the emission waveguide is such that when emitted light beams irradiate the sub-lens, the width of the light beams in the horizontal direction is equal to the working width of the sub-lens in the horizontal direction, the width of the light beams in the vertical direction is smaller than the working width of the sub-lens in the vertical direction, and the working width of each sub-lens in the horizontal direction is not smaller than 99% of the physical width of the sub-lens in the horizontal direction; the micro lens array is used for collimating the light beams emitted from the on-chip transmitting waveguide array;

the grating is arranged in the light beam output direction of the micro lens array, and collimated light beams output by the micro lens array are subjected to far field diffraction deflection in the vertical direction (namely the x-axis direction) through the grating;

the cylindrical lens is arranged in the light beam output direction of the grating, the focal plane of the cylindrical lens is superposed with the output plane of the grating, and the cylindrical lens is used for re-collimating and deflecting the light beam deflected by the far-field diffraction of the grating in the horizontal direction (namely the z-axis direction).

Preferably, the width of the waveguide at the edge of the chip is widened by the emission waveguide in an insulation tapering manner, and the width is larger than the height of the waveguide, 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), and thus, after the light beam is emitted to a free space, a smaller divergence angle in the horizontal direction and a larger divergence angle in the vertical direction are realized.

Preferably, the input coupler, the 1 xn optical switch, and the transmit waveguide array all operate in a single Transverse Electric (TE) mode or a single Transverse Magnetic (TM) mode.

The laser further comprises a wavelength-adjustable laser arranged outside the chip and used for outputting laser beams to the input coupler; and a controller for controlling the wavelength of the laser beam output by the tunable laser and the output gating of the 1 XN optical switch.

The controller is respectively connected with the wavelength-adjustable laser and the 1 xN optical switch by adopting a wire, bonding or other electrical connection modes.

The end face of the output end of the transmitting waveguide array is polished and coated with an anti-reflection film, so that the loss of light beams when the light beams are transmitted from the waveguides to the free space is reduced.

The chip is sequentially provided with a substrate, a silicon dioxide cladding layer and a waveguide layer from bottom to top, and the input coupler, the 1 xN optical switch and the emission waveguide array are prepared on the waveguide layer along the propagation direction of light beams through etching, deposition and/or stripping processes.

The input coupler is used for realizing low-loss coupling of the laser to the chip and is a tapered waveguide or a cantilever waveguide.

The N waveguide pitches of the transmitting waveguide array are the same as the pitches of all the sub lenses of the micro lens array.

Preferably, the waveguide layer is a thin film lithium niobate material and is x-cut y-pass, i.e., 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.

Preferably, the thin-film lithium niobate waveguide layer is doped with magnesium oxide or other materials capable of inhibiting optical folding effect in lithium niobate, so that the power tolerance of the device is improved.

Preferably, the 1 × N optical switch is based on an electro-optical effect of lithium niobate, and can realize nanosecond-level switching.

Preferably, the thickness of the thin-film lithium niobate waveguide layer is 600nm.

The working principle of the invention is as follows:

for the output wavelength of a certain laser, a light beam is emitted to a free space from a certain emission waveguide array through a1 XN optical switch on a control chip, and is collimated by a micro-lens array corresponding to the emission waveguide array and then is irradiated to a grating, the grating deflects the light beam according to the wavelength of the light beam, the deflected light beam is irradiated to a cylindrical lens, the cylindrical lens deflects the light beam again according to the incident position (determined by the emission waveguide position) of the cylindrical lens, and the deflection direction is vertical to the deflection direction controlled by the wavelength. Therefore, the deflection of the light beam in one direction can be controlled by changing the output wavelength of the laser, and the deflection of the light beam in the other direction can be controlled by changing the transmitting waveguide array of the on-chip light beam, so that two-dimensional light beam scanning is realized.

Compared with the prior art, the invention has the following advantages:

1) The invention does not adopt the on-chip grating emission with higher loss, but adopts the combination of the on-chip end surface emission waveguide array and the off-chip micro lens array, thereby achieving the reduction of loss.

2) The far-field light spots of the invention are not in mirror image relation with the light spots at the emission array like the traditional LABS technology, but in mirror image relation with the light spots on the focal plane of the cylindrical lens (namely the light spots output by the micro-lens array). Therefore, by controlling the divergence angle of the emergent light of the emission waveguide in the horizontal direction, the light spots output by the micro-lens array can be densely arranged, and the light field filling exceeding 99% in the horizontal direction is achieved, so that the scanning blind area of a far field in the horizontal direction can be reduced to be within 1%. In the vertical direction, a method of wavelength tuning and grating deflection is adopted, and the continuous tuning of the wavelength ensures that a far field in the vertical direction has no blind area. Therefore, the total far-field scanning blind area can be reduced to be within 1 percent.

3) The invention adopts the optical switch based on the lithium niobate high-speed electro-optical effect, the switching speed of the switch can reach nanosecond or even faster, and therefore, the fast light beam scanning can be realized.

4) The invention adopts the thin-film lithium niobate doped with magnesium oxide as the waveguide layer, can inhibit the photorefractive effect in the lithium niobate and improve the power tolerance, and meanwhile, the lithium niobate has no double-photon absorption in a very wide working wave band (500 nm-3 mu m), thereby being capable of supporting higher waveguide internal power.

Drawings

FIG. 1 is a schematic diagram of a lens-assisted all-solid-state beam scanning system in accordance with the present invention.

Fig. 2 is three exemplary implementations of a1 xn optical switch, (a) a binary tree structure, (b) a chain structure, and (c) a multimode interferometer structure.

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

41-1X 2 optical switch input waveguide, 42-1X 2 optical switch, 43-1X 2 optical switch output waveguide, 44-1X N optical switch output waveguide, 45-1X 2 optical switch control wire, i.e. electro-optic phase shifter control wire, 46-1X N optical beam splitter, 47-electro-optic phase shifter, 48-N X N multimode coupler.

Detailed Description

The invention will be further illustrated with reference to the following figures and examples, without thereby limiting the scope of the invention. Embodiments of the present invention include, but are not limited to, the following examples.

Examples

The thickness of the lithium niobate film is 0.6 micron, the thickness of the silicon dioxide cladding is 2 micron, and the thickness of the substrate silicon material is 400 micron. The lithium niobate waveguide is etched to form a strip waveguide or a ridge waveguide. The working wavelength is 1550nm wave band.

Fig. 1 shows a schematic structural diagram of the present invention. Take N =8 as an example.

A thin film lithium niobate based lens-assisted beam scanning device comprising:

the chip comprises a substrate, a silicon dioxide cladding and a lithium niobate film from bottom to top, and integrated waveguides and devices are prepared on the lithium niobate film through etching, deposition, stripping and other processes. On the thin-film lithium niobate chip, along the light beam propagation direction, an input coupler, a1 XN optical switch and an end-face emission waveguide array are arranged in sequence. The micro-lens array, the grating and the cylindrical lens are arranged outside the chip along the light beam propagation direction in sequence. In addition, a laser couples laser light into the chip through an input coupler on the chip. A controller controls the output wavelength of the laser and the output channel of the on-chip 1 xn optical switch.

The lithium niobate thin film is x-cut y-transmission, namely the normal line of the erbium-doped lithium niobate thin film is along the x axis, the propagation direction of optical signals is along the y axis, and the crystal axis is the z axis;

the lithium niobate thin film is doped with magnesium oxide or magnesium or other materials which can inhibit the optical distortion effect in the lithium niobate and improve the power tolerance of the lithium niobate;

the lithium niobate waveguide can be a partially etched ridge waveguide or a fully etched strip waveguide, and silicon oxide or other low-refractive-index cladding layers are covered above the waveguide to reduce waveguide loss;

the input coupler is used for realizing low-loss coupling from the laser to the chip and can be a tapered waveguide, a cantilever waveguide and other structures;

the 1 × N optical switch is an optical switch based on a lithium niobate electro-optic effect, and can be realized by adopting a binary tree structure for the 1 × 2 optical switch, adopting a mach-zehnder interferometer structure and combining a lithium niobate electro-optic phase shifter for the 1 × 2 optical switch, adopting a chain structure for the 1 × 2 optical switch, or connecting a1 × N optical beam splitter to an N × N multimode interferometer after passing through N lithium niobate electro-optic phase shifter waveguides;

the lithium niobate electro-optical phase shifter is characterized in that parallel electrodes are arranged on two sides of a lithium niobate waveguide along the y-axis direction, when voltage is applied to the electrodes, an electric field along the z-axis direction is generated, and the electric field can change the refractive index of the waveguide through the electro-optical effect of the lithium niobate waveguide, so that the phase change of light beams in the waveguide is realized;

preferably, the 1 × N optical switch of the present invention employs a binary tree structure based on a1 × 2 optical switch, so that the loss from the input to all the outputs is the same;

the transmitting waveguide array is composed of N waveguides which are arranged in parallel, the waveguide direction is along the y axis, and the array arrangement direction is along the z axis. One end of each waveguide is connected with N outputs of the 1 XN optical switch, the other end of each waveguide extends to the edge of the chip, the roughness of the end face of each waveguide is reduced by adopting an end face polishing process, and an anti-reflection film is plated to reduce the loss of light beams emitted from the waveguides to free space;

the emitting waveguide can change the waveguide size at the chip edge by tapering and the like, so that a light beam emitted to a free space has a large spot size and a small divergence angle in the horizontal direction (namely on a YOZ plane) and has a small spot size and a large divergence angle in the vertical direction (namely on an XOY plane);

the micro-lens array is used for collimating the light beams emitted from the on-chip waveguide array. The microlens array is arranged in a direction parallel to the emission waveguide array, i.e., in the z-axis direction. The center line of the micro lens is along the direction of the y axis, and the distance is equal to the distance of the transmitting waveguide. The end face of the end face emission waveguide array at the edge of the chip is superposed with the focal plane of the micro lens array. Each microlens corresponds to one emission waveguide, and the light beams emitted from the waveguides are collimated after being irradiated on the microlenses. After the light beam emitted in the waveguide is subjected to certain divergence, when the light beam irradiates the micro lens, the width of the light beam in the horizontal direction is just equal to the effective aperture of the micro lens in the horizontal direction, and the width of the light beam in the vertical direction is smaller than or equal to the effective aperture of the micro lens in the vertical direction;

the grating is used for diffracting and deflecting the light beam collimated by the micro lens on an XOY plane according to the wavelength of the light beam. The grating is placed parallel to the microlens array. The grating may be transmissive or reflective;

the cylindrical lens is used for collimating and deflecting the light beam after the diffraction of the grating again. The focal plane of the cylindrical lens coincides with the output plane of the grating, the deflection direction of the output light beam of the grating along the wavelength is aligned to the non-focusing direction of the cylindrical lens, and the position changing direction of the output light beam generated by the on-chip output waveguide switching is aligned to the focusing direction of the cylindrical lens. The cylindrical lens should be large enough to ensure that the light beams output from the grating can all be irradiated into the effective working area of the cylindrical lens at different wavelengths.

And the micro lens array and the cylindrical lens are coated with antireflection films on two surfaces, so that interface reflection is reduced.

The wavelength of the antireflection film corresponds to the working wavelength of the device.

Further, the invention also comprises a wavelength-adjustable laser and a controller. The laser couples the laser into the chip through an on-chip input coupler. The controller is used to control the wavelength of the tunable laser and the output gating of the on-chip 1 xn optical switch. The connection of the controller control signals to the laser and the 1 xn optical switch may be by wire, bonding, or other electrical connection.

Fig. 2 is three exemplary implementations of a1 × N optical switch, respectively a binary tree structure based on a1 × 2 optical switch (fig. 2 a), a chain structure based on a1 × 2 optical switch (fig. 2 b), and a structure based on a multi-mode interferometer (fig. 2 c). For a1 × 2 optical switch, the phase is controlled by an electro-optical phase shifter on one arm of the mach-zehnder interferometer, so that input light can be switched to any one of two output waveguides for output. For the electro-optical phase shifter, metal electrodes are arranged on two sides of a lithium niobate waveguide in parallel, and voltage is applied to the electrodes to change the refractive index of the lithium niobate waveguide, so that the phase of an optical signal in the waveguide is changed. In the figure 41 is a1 × 2 optical switch input waveguide, 42 is a1 × 2 optical switch, 43 is a1 × 2 optical switch output waveguide, 44 is a1 × N optical switch output waveguide, 45 is a1 × 2 optical switch control wire, i.e., an electro-optical phase shifter control line, 46 is a1 × N optical beam splitter, 47 is an electro-optical phase shifter, and 48 is an N × N multimode coupler.

On the whole device, the on-chip loss is dominated by the 1 multiplied by N optical switch, and the loss of the 1 multiplied by 2 optical switch can be controlled within 0.5dB by optimizing. The light beam is emitted from the waveguide, and almost no loss is generated by the micro lens, the grating and the cylindrical lens, so that the invention has extremely low insertion loss. Lithium niobate has no double-photon absorption effect and can bear the optical power in the waveguide of hundreds of milliwatts, so that the invention can realize very high transmitting power by combining the extremely low loss of the system. The scanning blind area is determined by the arrangement of the spots on the exit plane of the microlens, and as already explained above, it is possible to achieve a blind area of less than 1%. In the light beam scanning time, the scanning time in the horizontal direction is determined by the switching time of the 1 XN optical switch, the switching time can be controlled below 1 nanosecond due to the high-speed electro-optical effect of the lithium niobate, and the scanning time in the vertical direction is determined by the wavelength tuning speed of the laser and can also be controlled below 10 nanoseconds, so that the invention can realize high-speed light beam scanning. In summary, the present invention can simultaneously achieve extremely low insertion loss, high emission power, extremely small blind area, and high-speed beam scanning, and various schemes of the comparison technique cannot simultaneously achieve these performances.

The above description is provided to illustrate a preferred embodiment of the present invention, but not to limit the scope of the invention. All changes, equivalents, and improvements that come within the scope of the invention are intended to be embraced therein.

Claims

1. A lens-based assisted all-solid-state beam scanning system, comprising: an input coupler, a1 XN optical switch and a transmitting waveguide array integrated on a chip, and an off-chip micro-lens array, a grating and a cylindrical lens;

2. The lens-based assisted all-solid-state light beam scanning system according to claim 1, wherein the emission waveguide widens the waveguide width at the edge of the chip in an insulation tapering manner, and the width is larger 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. z-axis direction) and a smaller width in the vertical direction (i.e. x-axis direction), thereby realizing a smaller divergence angle in the horizontal direction and a larger divergence angle in the vertical direction after the light beam is emitted to the free space.

3. The lens-assisted all-solid-state light beam scanning system according to claim 1, wherein the input coupler, the 1 x N optical switch, and the transmit 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, wherein the N waveguide pitches of the launching waveguide array are the same as the respective sub-lens pitches of the micro-lens array.

5. The lens-based assisted all-solid-state beam scanning system of any one of claims 1 to 4, further comprising an off-chip disposed

The wavelength-adjustable laser is used for outputting a laser beam to the input coupler; and

and the controller is used for controlling the wavelength of the laser beam output by the adjustable laser and the output gating of the 1 XN optical switch.

6. The lens-based assisted all-solid-state beam scanning system of claim 5, wherein the controller is connected to the wavelength tunable laser and the 1 xn optical switch by wire bonding or other electrical connection.

7. The lens-assisted all-solid-state beam scanning system according to any one of claims 1 to 6, wherein the emitting end faces of the emitting waveguide array are polished and coated with an antireflection film so as to reduce loss of the light beam emitted from the waveguides to free space.

8. The lens-based assisted all-solid-state optical beam scanning system according to any one of claims 1 to 6, wherein the chip comprises, from bottom to top, a substrate, a silica cladding layer and a waveguide layer, and the input coupler, the 1 XN optical switch and the emission waveguide array are formed on the waveguide layer along the light beam propagation direction by 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, wherein the input coupler is a tapered waveguide or a cantilever waveguide for low-loss coupling of the laser to the chip.