SD-OCT system based on photonic chip

Abstract

An integrated spectral domain optical coherence tomography (OCT) imaging system based on a photonic chip includes a light source chip, an interferometer chip, and a spectral detector. The light source chip outputs a broad spectrum of light to the interferometer chip. After being split by the interferometer chip, the light is output to a sample under test through a sample arm and a reference arm, respectively, and to a reflector built into the interferometer chip. The reflected light from the sample arm scans the sample under test, generating an interference signal with the light from the reference arm on the interferometer chip. The interference signal is then output to the spectral detector, which processes the signal to obtain an image of the sample under test. The present invention utilizes an integrated photonic chip to replace the discrete components in traditional systems, proposing an integrated solution for the entire imaging system, including the light source, interferometer, and spectrometer. This system is suitable for operating in the commonly used OCT imaging bands with central wavelengths of 850nm, 1060nm, 1300nm, and 1700nm, significantly reducing size and cost.

Description

Technical Field

[0001] The present invention relates to a technology in the field of optical imaging, specifically an integrated spectral domain optical coherence tomography (SD-OCT) system based on a photonic chip. Background Art

[0002] Optical coherence tomography (OCT) is a non-invasive imaging modality that provides deep-resolution, high-resolution images of tissue microstructure. However, most components rely on spatial optics, which are discrete and require active alignment and are prone to misalignment. The cost, size, and misalignment of these components significantly limit the application of traditional OCT.

[0003] Silicon photonic chips can be used to manufacture optical devices in large quantities at low cost through standard complementary metal oxide semiconductor manufacturing processes. However, existing silicon photonic chips are mainly designed for SD-OCT systems with a central wavelength of around 1300 nm. Since silicon materials themselves cannot transmit light below 1.1 microns, these chips are not suitable for applications with wavelengths of 850 nm or 1060 nm, such as ophthalmic systems. In addition, these efforts are limited to the miniaturization of individual discrete components and do not propose system-level integrated solutions. Summary of the Invention

[0004] In response to the above-mentioned shortcomings of the existing technology, the present invention proposes an integrated spectral domain optical coherence tomography system based on a photonic chip. The integrated photonic chip replaces the discrete components in the traditional system, and an integrated solution is proposed for the entire imaging system, including components such as the light source, interferometer, and spectrometer. The system can be applied to the commonly used OCT imaging bands with working center wavelengths of 850 nm, 1060 nm, 1300 nm, and 1700 nm, significantly reducing the size and cost.

[0005] The present invention is achieved through the following technical solutions:

[0006] The present invention relates to an integrated spectral domain optical coherence tomography system based on a photonic chip, comprising: a light source chip, an interferometer chip and a spectral detector, wherein: the light source chip outputs a wide spectrum of light to the interferometer chip, and after being split by the interferometer chip, the light is output to a sample to be measured through a sample arm and a reference arm and a reflector built into the interferometer chip respectively; the reflected light of the sample arm scanning the sample to be measured and the light of the reference arm generate an interference signal on the interferometer chip, which is then output to the spectral detector; the spectral detector obtains an image of the sample to be measured through signal processing.

[0007] The light source chip is a silicon nitride resonator or a co-packaged superluminescent diode chip and a silicon nitride photonic chip.

[0008] The interferometer chip includes: an input end, a beam splitter, a reference arm, a sample arm, an output end and a reflector, wherein: the input end is directly coupled or indirectly coupled and packaged with the light source chip, the output end scans the sample by cooperating with a MEMS scanner or scanning the sample through an optical phased array, and the optical path of the reference arm matches the sum of the on-chip and off-chip optical paths of the sample arm.

[0009] All devices on the interferometer chip are composed of a high-refractive-index waveguide layer and a low-refractive-index cladding layer, wherein the high-refractive-index waveguide layer is made of silicon nitride material and the low-refractive-index cladding is made of silicon dioxide material. The high-refractive-index waveguide layer confines light and achieves low-loss propagation, and can be integrated with the light source chip and detector on the same substrate material.

[0010] The direct coupling is achieved by, but not limited to, a spot converter, and the indirect coupling and packaging are achieved by, but not limited to, optical fibers, micro lenses, etc.

[0011] The beam splitter preferably has a splitting ratio of 50:50.

[0012] The matching is achieved by finely adjusting the optical path length by thermally adjusting the length of the reference arm. Preferably, the thermal adjustment is achieved by integrating a microheater that achieves a nanometer-level adjustment resolution at an applied voltage of 1 mV.

[0013] The spectrum detector is a wide spectrum, high spatial resolution detector, and its detection spectrum width matches the spectrum bandwidth of the light source chip. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 Schematic diagram of the waveguide structure (a) and simulation diagram of the waveguide light mode (b);

[0015] Figure 2 It is a schematic diagram of the present invention;

[0016] Figure 3 Schematic diagram of the interferometer chip structure in the embodiment;

[0017] Figure 4 This is a schematic diagram of the structure of the light source chip embedded after pre-etching the groove in the embodiment;

[0018] Figure 5 Schematic diagram of the structure of the spectrometer detector in the embodiment;

[0019] Figure 6 The dispersion calculation simulation diagram of silicon nitride waveguide with fixed width and variable height;

[0020] Figure 7 A simulation diagram of the dispersion calculation of a silicon nitride waveguide with fixed height and varying width.

[0021] Figure 8 This is the normalized loss test result of the processed silicon nitride waveguide near the wavelength of 1060 nm.

[0022] Figure 9 The figure shows the normalized loss test results of the processed silicon nitride waveguide near the wavelength of 1310 nm.

[0023] Figure 10 An optical frequency comb generated by pumping the processed silicon nitride chip near 1060 nm (measured through a filter). DETAILED DESCRIPTION

[0024] like Figure 2 As shown, this embodiment relates to an integrated spectral domain optical coherence tomography (SD-OCT) system based on a photonic chip, including: a light source chip, an interferometer chip and a spectral detector, wherein: the light source chip outputs a wide spectrum of light to the interferometer chip, which is then split by the interferometer chip and output to the sample to be tested and the reflector built into the interferometer chip through the sample arm and the reference arm respectively. The light of the sample arm scans the sample to be tested, and the reflected light and the light of the reference arm generate an interference signal on the interferometer chip and then are output to the spectral detector. The spectral detector obtains an image of the sample to be tested through signal processing.

[0025] The light source chip of this embodiment is implemented in any of the following ways:

[0026] a) Using a silicon nitride resonator to generate a frequency comb as a light source, a broadband spectrum is generated through self-injection locking and nonlinear processes. To generate a frequency comb with a central wavelength of 1060 nm, the dispersion calculation results for different silicon nitride waveguide sizes are shown in the figure below. Figure 6 and Figure 7 shown.

[0027] The bandwidth and frequency interval of the broadband spectrum are determined by the geometry of the resonant cavity; preferably, the thickness of the silicon nitride is 700 nm, and the width range of the silicon nitride waveguide is simulated as follows: Figure 7 As shown in FIG. 1 , the dispersion near 1060 nm can be guaranteed to be the desired anomalous dispersion in the range of 700-1500 nm, and the width of silicon nitride is 1300 nm.

[0028] b) Using a co-packaged superluminescent diode chip and a silicon nitride photonic chip, specifically: first pre-etching grooves on a silicon substrate wafer and then embedding the superluminescent diode.

[0029] The pre-etched grooves are defined by photolithography, and the silicon substrate wafer is etched using a high aspect ratio plasma etching process or KOH wet etching, such as Figure 4As shown in Figure 2, for optical coupling, the silicon nitride device's edge coupler uses a flared structure to match the output mode spot of the superluminescent diode, ensuring optimal coupling. Finally, the superluminescent diode chip and the silicon nitride photonic chip are packaged on the same carrier using adhesive and UV curing.

[0030] The interferometer chip is integrated based on a silicon nitride photonic platform. Specifically, the reference arm will consist of a long, low-loss silicon nitride waveguide and an adjustable integrated microheater, which can provide greater flexibility to match the path length required by the sample arm.

[0031] The sample arm of the interferometer chip is integrated using a co-packaging process with a MEMS scanner.

[0032] The spectral detector is implemented in any of the following ways:

[0033] ① Such as Figure 5 As shown, an arrayed waveguide grating is combined with a CCD sensor, wherein: the arrayed waveguide grating includes: an input waveguide, an input star coupler, an array waveguide, an output star coupler and an output waveguide connected in sequence, wherein: when the wavelength is When the broadband light is coupled to the input waveguide, the beam diverges in the input star coupler as a free propagation area, and the divergent beam is coupled to the array waveguide; the length of each waveguide in the array waveguide increases linearly, resulting in different phase delays caused by a single waveguide, so that only plane waves with the same phase delay constructively interfere on the focal line on the image plane of the output star coupler; finally, each output waveguide transmits A single wavelength in .

[0034] This embodiment intends to design and prepare a silicon nitride arrayed waveguide grating with a working center wavelength of 1060 nm that can be used for OCT imaging. In addition, this embodiment will also integrate a CCD sensor by integrating a lens on the end face of the silicon nitride photonic chip, such as Figure 5 shown.

[0035] ② A co-propagating fixed Fourier transform spectrometer (FTS) is fabricated using silicon nitride. Specifically, the optical signal is first split into two parallel silicon nitride ridge waveguides with different ridge widths to obtain the desired overlap of the evanescent fields of the two stimulated waveguide modes. Since the propagation constants of the stimulated modes are slightly different, an interference pattern is generated between the two waveguides. By placing a grating between the waveguides, the interference pattern can be diffracted upward to the photodiode array.

[0036] After specific actual experiments, the normalized transmission loss of the fabricated silicon nitride waveguide measured at the central wavelengths of 1060 nm and 1310 nm is as follows: Figure 8 and Figure 9As shown, it can be seen that the transmission loss of the silicon nitride waveguide can remain basically unchanged in the range of 1030-1070nm and 1270-1330nm, indicating that the loss is wavelength-insensitive, proving that the waveguide can be applied in a wide spectrum range.

[0037] Through specific practical experiments, it was found that at a pump wavelength of around 1060 nm, with a pump power of approximately 700 mW, a silicon nitride height of approximately 700 nm, and a width of approximately 1300 nm, the silicon nitride chip can generate a broad-spectrum optical frequency comb suitable for SD-OCT due to dispersion control.

[0038] The light source in this embodiment, that is, the coupling and co-packaging of the gain chip or laser chip and the integrated photonic chip, can also be achieved by flip-chip bonding or direct growth of the gain chip; supercontinuum technology can also be used to replace the light source in this embodiment; the spectrometer can be realized by connecting a spectrometer to the chip.

[0039] Compared with the existing technology, the present invention proposes an integrated solution for the entire imaging system. Through the photonic chip, components including light sources, interferometers, spectrometers, etc. can be miniaturized and integrated. The new integrated system will be several orders of magnitude smaller than the current system in size, reduced from several cubic meters to several cubic centimeters, and lighter in weight. The operating wavelength of this device has a broadband selectability feature. The same photonic chip can cover the commonly used OCT imaging bands such as 850 nm, 1060 nm, 1300 nm, and 1700 nm, and is used for the diagnosis of chronic ophthalmic diseases, dermatology, dentistry, oncology and other fields. Compared with the existing system with separate components, the present invention uses integrated waveguides with polarization stability, is insensitive to factors such as vibration, is not prone to misalignment, and has better overall system stability.

[0040] The above-mentioned specific implementation can be partially adjusted in different ways by those skilled in the art without departing from the principles and purpose of the present invention. The scope of protection of the present invention shall be based on the claims and shall not be limited by the above-mentioned specific implementation. All implementation schemes within its scope shall be subject to the constraints of the present invention.

Claims

1. An integrated spectral domain optical coherence tomography system based on a photonic chip, characterized in that: include: A light source chip, an interferometer chip, and a spectrum detector, wherein: the light source chip outputs a broad spectrum of light to the interferometer chip, which is then split by the interferometer chip and output to the sample to be measured through the sample arm and the reference arm, respectively, and to the reflector built into the interferometer chip; the light from the sample arm scans the sample to be measured, and the reflected light from the light from the reference arm generates an interference signal on the interferometer chip, which is then output to the spectrum detector; the spectrum detector obtains an image of the sample to be measured through signal processing; The interferometer chip includes: an input end, a beam splitter, a reference arm, a sample arm, an output end, and a reflector, wherein: the input end is directly coupled or indirectly coupled to the light source chip and packaged, the output end scans the sample by cooperating with a MEMS scanner or scanning the sample using an optical phased array, and the optical path of the reference arm matches the sum of the on-chip and off-chip optical paths of the sample arm; The light source chip is implemented in any of the following ways: a) Using a silicon nitride resonator as a light source, a broadband spectrum is generated through self-injection locking and nonlinear processes, producing a frequency comb with a central wavelength of 1050 nm. At a pump wavelength of 1050 nm, a pump power of 700 mW, a silicon nitride height of 700 nm, and a width of 1300 nm, the silicon nitride chip produces a broad-spectrum optical frequency comb suitable for SD-OCT due to dispersion control. b) using a co-packaged superluminescent diode chip and a silicon nitride photonic chip, specifically: pre-etching a groove on a silicon substrate wafer and then embedding the superluminescent diode; The spectral detector is realized in the following manner: A co-propagating fixed Fourier transform spectrometer is fabricated using silicon nitride. Specifically, the optical signal is first split into two parallel silicon nitride ridge waveguides with different ridge widths to achieve the desired overlap of the evanescent fields of the two stimulated waveguide modes. Because the propagation constants of the stimulated modes are slightly different, an interference pattern is generated between the two waveguides. By placing a grating between the waveguides, the interference pattern is diffracted upward to a photodiode array. All devices on the interferometer chip are composed of a high-refractive-index waveguide layer and a low-refractive-index cladding layer. The high-refractive-index waveguide layer binds light and achieves low-loss propagation. The light source chip and detector are integrated on the same substrate material. The interferometer chip is integrated based on a silicon nitride photonic platform. Specifically, the reference arm consists of a long, low-loss silicon nitride waveguide and an adjustable integrated microheater to match the path length required by the sample arm. The light source chip is a silicon nitride resonator or a co-packaged superluminescent diode chip and a silicon nitride photonic chip; The matching is achieved by thermally adjusting the length of the reference arm through an integrated microheater to finely adjust the optical path length. The microheater achieves a nanometer-level adjustment resolution at an applied voltage of 1 mV. The spectrum detector is a wide spectrum, high spatial resolution detector, and its detection spectrum width matches the spectrum bandwidth of the light source chip; The pre-etched grooves are defined by photolithography, and the silicon substrate wafer is etched using a high-aspect-ratio plasma etching process or KOH wet etching. The edge coupler of the silicon nitride device adopts a trumpet-mouth structure to match the output mode spot of the radiant light-emitting diode. After ensuring optimal coupling, the superluminescent diode chip and the silicon nitride photonic chip are packaged on the same carrier using adhesive and ultraviolet light curing.

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