Integrated optical gyroscope and method for manufacturing the same

Abstract

The application discloses an integrated optical gyroscope and a preparation method thereof. An optical signal enters a chip through an end face coupler, passes through a beam splitter, enters a lithium niobate modulation area through a vertical adiabatic coupler to modulate the optical signal, and enters a waveguide ring through a mode conversion structure after the optical signal is modulated. The mode conversion structure is arranged at an inlet and an outlet of the waveguide ring and is used for converting modes so that the optical signal is transmitted in the waveguide ring for multiple times. The hetero-integrated structure is adopted, so that the modulation speed and the loss of the optical gyroscope are reduced. Meanwhile, the waveguide ring adopts the mode conversion to utilize the waveguide ring for multiple times. Through different modes, the effective length of the waveguide ring on the gyroscope chip is increased, and the sensitivity of the gyroscope is improved.

Description

Technical Field

[0001] This invention belongs to the field of optical gyroscope technology, and relates to an integrated optical gyroscope and its manufacturing method. Background Technology

[0002] Based on different sensing principles and structural designs, optical gyroscopes can be mainly classified into the following categories:

[0003] 1) Fiber Optic Gyroscope (FOG): Based on the Sagnac effect, it uses the phase difference of light waves propagating in opposite directions within a long fiber optic coil to detect rotational angular velocity. Its advantages lie in high measurement accuracy and good stability, and it has been widely used in high-end fields such as aerospace and marine navigation. However, the need for precisely wound fiber optic coils and high-quality optical components results in a large system size and high cost. Furthermore, optical fibers are sensitive to environmental vibrations and temperature changes, requiring additional vibration damping and temperature control devices.

[0004] 2) Integrated Optical Gyroscope (IOG): This type of gyroscope uses silicon-based or lithium niobate-based waveguides instead of traditional optical fibers, achieving chip-level integration of the optical system through semiconductor processes. This design significantly reduces size (down to the centimeter or even millimeter level) and weight, while also lowering manufacturing costs. However, due to the much higher transmission loss of waveguides compared to optical fibers, and the unavoidable backscattering noise, its measurement accuracy is currently significantly lower than that of FOG. In recent years, the performance of IOG has been continuously improving by employing high refractive index difference waveguides such as silicon nitride and coherent detection technology.

[0005] 3) Resonant Optical Gyroscope (ROG): ROG detects rotation based on the resonant frequency splitting effect in an optical loop resonator. Its core is a high-quality micro-ring resonator. Compared to FOG, ROG has a more compact structure and faster response speed. However, it demands extremely high manufacturing precision; even minute dimensional errors or surface roughness can significantly reduce the Q value. Furthermore, the resonant frequency is extremely sensitive to temperature changes, requiring complex temperature control and frequency stabilization systems. Currently, laboratory-level ROG accuracy can reach 0.1-1° / h, but overcoming technological and stability bottlenecks is still necessary for its engineering application.

[0006] An integrated optical gyroscope is a novel angular velocity sensor based on silicon photonics technology, utilizing the Sagnac effect to measure rotational motion. Compared to traditional fiber optic gyroscopes (FOGs) and microelectromechanical systems (MEMS) gyroscopes, integrated optical gyroscopes combine high precision and miniaturization, achieving a compact design by integrating lasers, modulators, waveguides, and detectors onto a silicon photonic chip. Its working principle involves calculating angular velocity by detecting the phase difference between clockwise and counterclockwise propagating light, exhibiting characteristics such as high shock resistance and low power consumption. However, due to high on-chip losses, poor thermal stability, and the integration of on-chip light sources, the production of integrated optical gyroscopes remains a challenge. Currently, this technology is optimizing performance through heterogeneous integration and calibration algorithms, leading to this invention. Summary of the Invention

[0007] The purpose of this invention is to provide an integrated optical gyroscope and its fabrication method. A new direction-sensing tensor kernel norm is proposed, which increases the effective length of the waveguide loop on the gyroscope chip through different modes, thereby improving the sensitivity of the gyroscope.

[0008] The technical solution to achieve the purpose of this invention is as follows:

[0009] An integrated optical gyroscope is disclosed in which an optical signal enters the chip through an end-face coupler, passes through a beam splitter, and enters the lithium niobate modulation region through a vertical thermal coupler for optical signal modulation. After optical modulation, the signal enters the waveguide ring through a mode conversion structure located at the entrance and exit of the waveguide ring to switch modes, enabling the light to be transmitted multiple times within the waveguide ring.

[0010] In a preferred embodiment, the mode conversion structure includes a first tapered waveguide with a gradually widening width at the entrance of the waveguide ring, a second tapered waveguide with a gradually narrowing width at the exit of the waveguide ring, the second tapered waveguide connecting to a loop waveguide, the loop waveguide having a width smaller than the width of the waveguide ring, the loop waveguide extending above the first tapered waveguide, the loop waveguide and the first tapered waveguide having a certain distance between them, an extension waveguide above the second tapered waveguide, the extension waveguide having a width smaller than the width of the waveguide ring, and the extension waveguide connecting to a vertical adiabatic coupler.

[0011] In a preferred embodiment, the adjacent waveguides of the waveguide ring are alternately wide and narrow waveguides, and a gradually changing waveguide with a gradually changing width is provided at the junction of the wide and narrow waveguides.

[0012] In the preferred embodiment, the wide waveguide width is 3μm, the narrow waveguide width is 2.5μm, the waveguide spacing is 0.4μm, and the length of the gradient waveguide at the junction of the wide and narrow waveguides is 10μm.

[0013] In the preferred technical solution, the optical signal is coupled from the light source into the coupler. After being processed by the chip, the optical signal with angular velocity is transmitted to the coupler. The optical signal enters the detector, and the output current is converted from analog to digital. The digital signal is demodulated, and the demodulated signal is input to the controller. The controller feeds back a drive signal, which controls the driver to apply the modulation voltage to the lithium niobate modulation region electrode.

[0014] In the preferred technical solution, the chip adopts a heterogeneous integration structure of silicon nitride and lithium niobate, the passive device adopts silicon nitride material, and the modulator adopts lithium niobate.

[0015] In a preferred embodiment, the chip is arranged from bottom to top as follows: a silicon substrate, a buried oxide layer, a silicon nitride waveguide, a silicon nitride cladding layer, a benzocyclobutene layer, a lithium niobate ridge waveguide layer, a metal electrode, and a cladding layer. The silicon nitride waveguide includes a mode conversion structure and a waveguide ring.

[0016] This invention also discloses a method for fabricating an integrated optical gyroscope, comprising the following steps:

[0017] Step 1: Fabricate silicon nitride waveguides using damascus and LPCVD processes. The silicon nitride waveguide includes a mode conversion structure and a waveguide ring. The mode conversion structure is located at the entrance and exit of the waveguide ring and is used to convert modes, allowing light to propagate multiple times within the waveguide ring.

[0018] Step 2: Grow silica and perform chemical mechanical polishing;

[0019] Step 3: Bond lithium niobate wafers onto silicon nitride wafers;

[0020] Step 4: Thin and etch the wafer. The first etching depth is 250nm, and the second etching depth is 250nm to obtain the lithium niobate ridge waveguide layer.

[0021] Step 5: Obtain the lithium niobate waveguide by photolithography and inductively coupled plasma etching, and use the lithium niobate waveguide as the modulation region;

[0022] Step 6: Deposit silicon oxide and etch the silicon oxide in the electrode area;

[0023] Step 7: Prepare metal electrodes using a metal lift-off process;

[0024] Step 8: Deposit silicon oxide and create windows in the gasket.

[0025] Compared with the prior art, the significant advantages of this invention are:

[0026] By employing a heterogeneous integrated structure, the modulation speed and loss of the optical gyroscope are reduced. Simultaneously, mode switching is used in the waveguide ring to reuse it multiple times, increasing the effective length of the on-chip waveguide ring through different modes and improving the gyroscope's sensitivity. A novel waveguide ring structure is also proposed, which saves waveguide ring area and increases waveguide ring length. Attached Figure Description

[0027] Figure 1 This is a system structure diagram of the integrated optical gyroscope in this embodiment;

[0028] Figure 2 This is a schematic diagram of the integrated optical gyroscope chip in this embodiment;

[0029] Figure 3 This is a schematic diagram of the waveguide ring structure of the integrated optical gyroscope in this embodiment;

[0030] Figure 4 This is a side view of the integrated optical gyroscope chip in this embodiment;

[0031] Figure 5 A diagram showing the coupling efficiency of mode switching;

[0032] Figure 6 The diagram shows the coupling efficiency of the vertical thermal coupler.

[0033] Figure 7 This is a schematic diagram of a waveguide ring structure according to another embodiment;

[0034] Figure 8 (a)- Figure 8 (h) is a flowchart of the chip fabrication process. Detailed Implementation

[0035] The principle of this invention is as follows: the waveguide ring of this invention uses mode switching to utilize the waveguide ring multiple times. By using different modes, the effective length of the waveguide ring on the gyroscope chip is increased, thereby improving the sensitivity of the gyroscope.

[0036] Example 1:

[0037] like Figure 1 As shown, the optical signal is coupled from the light source into the coupler. After being processed by the chip, the optical signal with angular velocity is transmitted to the coupler. The optical signal enters the detector, and the output current is converted from analog to digital. The digital signal is demodulated and the demodulated signal is input to the controller. The controller feeds back the drive signal and controls the driver to apply the modulation voltage to the lithium niobate modulation region electrode.

[0038] like Figure 2As shown, an integrated optical gyroscope has an optical signal entering the chip through an end-face coupler 1. After passing through a beam splitter 2 on the chip, the light is split in two and enters a vertical thermally adiabatic coupler 3. The optical signal is then modulated in a lithium niobate modulation region 4 via the vertical thermally adiabatic coupler 3. After modulation, the light enters a waveguide ring 52 through a mode conversion structure 51. The mode conversion structure 51 is located at the entrance and exit of the waveguide ring 52 to switch modes, allowing the light to be transmitted multiple times within the waveguide ring 52. This increases the effective length of the waveguide ring in the optical gyroscope.

[0039] Specifically, such as Figure 3 As shown, the mode conversion structure 51 includes a first tapered waveguide 5111 with a gradually widening width at the entrance of the waveguide ring 52, and a second tapered waveguide 5121 with a gradually narrowing width at the exit of the waveguide ring 52. The second tapered waveguide 5121 is connected to a loop waveguide 53. The width of the loop waveguide 53 is smaller than the width of the waveguide ring 52. The loop waveguide 53 extends above the first tapered waveguide 5111 and has a certain distance between it and the first tapered waveguide 5111. An extension waveguide 54 is provided above the second tapered waveguide 5121. The width of the extension waveguide 54 is smaller than the width of the waveguide ring 52. The extension waveguide 54 is connected to a vertical thermally adiabatic coupler (VAC) 3.

[0040] The first tapered waveguide 5111 and the upper loop waveguide 53 extending to the first tapered waveguide 5111 constitute the first mode conversion structure 511, and the second tapered waveguide 5121 and the extended waveguide 54 above the second tapered waveguide 5121 constitute the second mode conversion structure 512.

[0041] The first graded waveguide 5111 and the second graded waveguide 5121 are trapezoidal in shape.

[0042] In waveguide ring 52, the optical signal first enters waveguide ring 52 in TE0 mode. It first passes through a first tapered waveguide 5111, whose width gradually increases. At this point, the width of the optical waveguide widens, and the TE0 mode optical signal passes through the first mode conversion structure 511. Due to the refractive index mismatch, the TE0 mode light in the lower wide waveguide (first tapered waveguide 5111) of the first mode conversion structure 511 will not couple from the wide waveguide to the upper narrow waveguide of the mode conversion structure 511. Then, the TE0 mode light enters waveguide ring 52 and passes through the second mode conversion structure 512. Again, due to the different waveguide widths, the TE0 mode light does not couple because the refractive index matching condition is not met. It then passes through the second tapered waveguide 5121, where the waveguide width narrows, and then passes through the loop waveguide 53. When the light passes through the first mode conversion structure 511 for the second time, the TE0 mode light of the narrow waveguide and the TE1 mode light of the wide waveguide satisfy the refractive index matching condition. The TE0 mode light of the narrow waveguide in the upper part of the first mode conversion structure 511 will be coupled into the TE1 mode light and enter the wide waveguide in the lower part of the first mode conversion structure 511 for the second time. The TE1 mode light enters the waveguide ring 52. In the second mode conversion structure 512, the TE1 mode of the wide waveguide will be coupled into the TE0 mode and enter the narrow waveguide (extended waveguide 54). The optical signal is transmitted out of the waveguide ring 52 through VAC.

[0043] Optical signals are transmitted between different materials through a vertical thermally adiabatic coupler (VAC), which improves the integration of the optical gyroscope.

[0044] like Figure 1 As shown, the optical signal finally passes through the modulator and beam splitter, and then through the end-face coupler, transmitting the optical signal with angular velocity to the photodetector. After signal processing, the corresponding angular velocity is obtained. At the same time, the phase of the entire system can be calibrated using the signal, and the calibration signal is input to the electrodes of the chip's modulation area.

[0045] It should be noted that mode conversion is performed by adding mode conversion structures 51 at the entrance and exit of waveguide ring 52, so that the light of TE0 mode and TE1 mode can propagate multiple times in waveguide ring 52. However, it is not limited to adding only one pair of mode conversion structures. Multiple pairs of mode conversion structures can be added so that the light of higher order modes such as TE2 and TE3 can be transmitted multiple times in waveguide ring 52.

[0046] This invention presents a novel integrated optical gyroscope employing a heterogeneous integration structure of silicon nitride and lithium niobate. Its passive components, such as beam splitters and waveguide rings, are made of silicon nitride. Utilizing the low loss and high stability of silicon nitride, the accuracy of the gyroscope can be improved. Lithium niobate constitutes the modulation region, employing electronic modulation. The large electro-optic coefficient of lithium niobate enables fast, low-drive modulation. The silicon nitride and lithium niobate materials transmit optical signals via a VAC converter.

[0047] like Figure 4 As shown, the chip is arranged from bottom to top as follows: silicon substrate 12, buried oxide layer 13, silicon nitride waveguide 14, silicon nitride top cladding 13, benzocyclobutene (BCB) 15, lithium niobate ridge waveguide layer 16, metal layer 17, and cladding layer 13.

[0048] Specifically, the dimensions of each layer are described below using a specific embodiment as an example, but are not limited to these dimensions:

[0049] The silicon substrate 12 has a thickness of 700 μm, and the buried oxide layer 13 is silicon dioxide with a thickness between 3 μm and 4.7 μm. The silicon nitride waveguide 14 has a thickness of 400 nm.

[0050] The silicon nitride cladding layer 13 is 200 nm thick, made of silicon dioxide, and has a refractive index of approximately 1.456. The BCB15 layer is 300 nm to 500 nm thick, made of BCB, and has a refractive index of approximately 1.6.

[0051] The lithium niobate thin film layer 16 has a thickness of 0.5 μm, with a first etching depth of 250 nm and a second etching depth of 250 nm. Its electro-optic coefficient r 33 It is 30.9 pm / V, and the relative permittivity at room temperature is 27.9–44 (depending on the direction of the applied electric field).

[0052] Metal layer 17 uses gold as the electrode, but is not limited to this material, and includes a central metal signal electrode and two lateral metal ground electrodes. The central metal signal electrode has a width of 20 μm and an electrode spacing of 5 μm.

[0053] The coating layer 13 is made of silicon dioxide, with a thickness of more than 3 μm and a refractive index of approximately 1.46.

[0054] Simulation results are as follows Figure 5 As shown, in the TE0 and TE1 mode conversion structure, over 97% of the light achieves mode conversion. The loss is less than 0.13dB.

[0055] like Figure 6 As shown, the coupling efficiency of VAC at a wavelength of 1.31 μm is 92.8%, with a loss of 0.32 dB. This indicates that the loss caused by heterogeneous integration is acceptable. Compared to thermal tuning, lithium niobate exhibits significantly faster adjustment speed and more stable performance through electro-optic coefficient adjustment. The width of the central electrode in the lithium niobate modulation region is 20 μm, but not limited to this size; the width of the electrodes on both sides is greater than 200 μm, but not limited to this size; gold electrodes are grown through lift-off, with a depth of 1 μm, but not limited to this size; and the electrode spacing is 5 μm, but not limited to this size.

[0056] In another embodiment, such as Figure 7As shown, in order to further reduce the area, the width of each waveguide in the waveguide ring 52 can be changed to form an alternating pattern of wide waveguide 18 and narrow waveguide 19. That is, the adjacent waveguides of the waveguide ring 52 are alternating between wide and narrow waveguides, and a gradually changing waveguide with a gradually changing width is set at the junction 20 of the wide and narrow waveguides.

[0057] Because of the refractive index mismatch between the wide and narrow waveguides, no coupling occurs between adjacent waveguides in the waveguide ring. This reduces crosstalk between waveguides, decreases the spacing between waveguides, and further reduces the area of ​​the waveguide ring.

[0058] Specifically, the wide waveguide 18 has a width of 3μm, the narrow waveguide 19 has a width of 2.5μm, the waveguide spacing is 0.4μm, and the junction 20 between the wide and narrow waveguides is implemented using a 10μm long taper.

[0059] In another embodiment, such as Figure 8 As shown, a method for fabricating an integrated optical gyroscope includes the following steps:

[0060] Step 1: Fabricate silicon nitride waveguide 14 using damascus and LPCVD processes, such as... Figure 8 As shown in (a), the silicon nitride waveguide includes a mode conversion structure and a waveguide ring. The mode conversion structure is disposed at the entrance and exit of the waveguide ring and is used to convert the mode so that the light can be transmitted multiple times in the waveguide ring.

[0061] Step 2: Grow silica 13 and perform chemical mechanical polishing treatment, such as... Figure 8 As shown in (b);

[0062] Step 3: Bond lithium niobate wafers onto the silicon nitride wafer, such as... Figure 8 As shown in (c);

[0063] Step 4: Thin and etch the wafer. The first etching depth is 250 nm, and the second etching depth is 250 nm, to obtain the lithium niobate ridge waveguide layer 16, as shown. Figure 8 As shown in (d);

[0064] Step 5: Obtain the lithium niobate waveguide through photolithography and inductively coupled plasma etching. The lithium niobate waveguide serves as the modulation region, such as... Figure 8 As shown in (e);

[0065] Step 6: Deposit silicon oxide and etch the silicon oxide in the electrode area, such as... Figure 8 As shown in (f);

[0066] Step 7: Fabricate metal electrodes using a metal lift-off process, such as... Figure 8 (g) is shown;

[0067] Step 8: Deposit silicon oxide and create windows in the gasket, such as... Figure 8 As shown in (h).

[0068] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. An integrated optical gyroscope, characterized in that, Optical signals enter the chip through an end-face coupler, pass through a beam splitter, and then enter the lithium niobate modulation region through a vertical thermally adiabatic coupler for modulation. After modulation, the light enters the waveguide ring through a mode conversion structure. The mode conversion structure is located at the entrance and exit of the waveguide ring and is used to switch modes, allowing the light to be transmitted multiple times within the waveguide ring. The mode conversion structure includes a first tapered waveguide with a gradually widening width at the entrance of the waveguide ring, and a second tapered waveguide with a gradually narrowing width at the exit of the waveguide ring. The second tapered waveguide connects to a loop waveguide, the width of which is smaller than the width of the waveguide ring. The loop waveguide extends above the first tapered waveguide, and the loop waveguide and the first tapered waveguide are spaced apart. An extension waveguide is located above the second tapered waveguide, the width of which is smaller than the width of the waveguide ring, and the extension waveguide is connected to the vertical thermally adiabatic coupler.

2. The integrated optical gyroscope according to claim 1, characterized in that, The adjacent waveguides of the waveguide ring are alternately wide and narrow waveguides, and a gradually changing waveguide with a gradually changing width is set at the junction of the wide and narrow waveguides.

3. The integrated optical gyroscope according to claim 2, characterized in that, The wide waveguide has a width of 3μm, the narrow waveguide has a width of 2.5μm, the waveguide spacing is 0.4μm, and the length of the gradient waveguide at the junction of the wide and narrow waveguides is 10μm.

4. The integrated optical gyroscope according to claim 1, characterized in that, The optical signal is coupled from the light source into the coupler. After being processed by the chip, the optical signal with angular velocity is transmitted to the coupler. The optical signal enters the detector. The output current is converted from analog to digital, and the digital signal is demodulated. The demodulated signal is input to the controller. The controller feeds back the drive signal, which controls the driver to apply the modulation voltage to the lithium niobate modulation region electrode.

5. The integrated optical gyroscope according to claim 1, characterized in that, The chip adopts a heterogeneous integration structure of silicon nitride and lithium niobate, with the passive device made of silicon nitride and the modulator made of lithium niobate.

6. The integrated optical gyroscope according to claim 5, characterized in that, The chip is arranged from bottom to top as follows: a silicon substrate, a buried oxide layer, a silicon nitride waveguide, a silicon nitride cladding layer, a benzocyclobutene layer, a lithium niobate ridge waveguide layer, a metal electrode, and a cladding layer. The silicon nitride waveguide includes a mode conversion structure and a waveguide ring.

7. A method for fabricating an integrated optical gyroscope, used to fabricate the integrated optical gyroscope according to any one of claims 1-6, characterized in that, Includes the following steps: Step 1: Fabricate silicon nitride waveguides using damascus and LPCVD processes. The silicon nitride waveguide includes a mode conversion structure and a waveguide ring. The mode conversion structure is located at the entrance and exit of the waveguide ring and is used to convert modes, allowing light to propagate multiple times within the waveguide ring. Step 2: Grow silica and perform chemical mechanical polishing; Step 3: Bond lithium niobate wafers onto silicon nitride wafers; Step 4: Thin and etch the wafer. The first etching depth is 250 nm, and the second etching depth is 250 nm to obtain the lithium niobate ridge waveguide layer. Step 5: Obtain the lithium niobate waveguide by photolithography and inductively coupled plasma etching, and use the lithium niobate waveguide as the modulation region; Step 6: Deposit silicon oxide and etch the silicon oxide in the electrode area; Step 7: Prepare metal electrodes using a metal lift-off process; Step 8: Deposit silicon oxide and create windows in the gasket.

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