A semiconductor nanowire-mediated fiber-chip coupling structure

Abstract

The application discloses a kind of based on semiconductor nanowire mediated optical fiber-chip coupling structure, based on adiabatic coupling method, by standard optical fiber one end is drawn into taper micro-nano optical fiber and the thin end of high refractive index diameter gradient semiconductor nanowire is connected and coupled to form composite structure, gap is filled with a small amount of polymer, and the thick end of diameter gradient semiconductor nanowire in this composite structure and the taper silicon waveguide on chip are adhered and coupled, to realize high coupling efficiency, wide bandwidth, polarization insensitive standard optical fiber and on-chip integrated optical waveguide between bidirectional coupling.Utilize this method can realize high coupling efficiency, wide bandwidth, polarization insensitive standard optical fiber and on-chip integrated optical waveguide between bidirectional coupling, entire coupling structure is stable, the coupling structure is suitable for optical fiber and the coupling of various on-chip devices, conducive to realizing the integration of traditional optical devices and on-chip optical devices.

Description

Technical Field

[0001] This invention belongs to the research fields of nanophotonics and integrated optics, and relates to a technology for coupling optical fiber and on-chip integrated optical waveguide, particularly to an optical fiber-chip coupling structure based on semiconductor nanowires. Background Technology

[0002] The integration of traditional optical devices and on-chip optical devices is crucial for many applications in integrated photonics, such as optical information processing, optical storage, quantum computing, and communication. However, due to the significant mismatch in fundamental mode size and effective refractive index between standard optical fibers and on-chip integrated optical waveguides (at a wavelength of 1550 nm, the fundamental mode diameter of a single-mode standard silica fiber is approximately 10 μm, while the fundamental mode diameter of a typical on-chip integrated silicon waveguide is usually less than 1 μm), achieving efficient coupling between standard optical fibers and on-chip integrated optical waveguides and devices has become a major challenge in integrated photonics. Currently, commonly used coupling methods include end-face coupling, grating coupling, and adiabatic coupling. End-face coupling is the most direct coupling method, where the beam is coupled into and out of the waveguide from the side of the chip. To achieve efficient coupling by matching the mode field, the mode field of the on-chip single-mode waveguide is usually amplified using a mode field converter, a tapered lens fiber, or discrete microlenses at the interface. This method typically has the advantages of high efficiency and wide bandwidth, but usually requires strict mode field matching, resulting in small alignment tolerances. Furthermore, to reduce losses, the coupling interface requires fine polishing, which increases the complexity of fabrication. Also, the coupling waveguide and fiber are horizontally coupled at the end face, meaning the coupling waveguide can only be distributed at the chip edge, which is detrimental to the integration of multiple optical devices on-chip. Grating coupling, based on the Bragg diffraction effect, offers greater flexibility in coupler placement and larger alignment tolerance between the fiber and the grating structure compared to end-face coupling. However, to achieve high coupling efficiency and wide bandwidth, complex grating structures are typically required, such as adding a metal back reflector, depositing a polycrystalline silicon layer, and designing apodized gratings. Grating couplers also often exhibit strong polarization dependence. Adiabatic coupling, based on evanescent field coupling, achieves high-efficiency transmission between the fiber and the chip through evanescent field coupling between a tapered fiber or photonic lead and the waveguide. This method can achieve high coupling efficiency, wide bandwidth, and is insensitive to optical polarization. However, due to the low effective refractive index of tapered optical fibers, to achieve high-efficiency coupling of fiber-to-chip systems, the coupling process is typically performed with the silicon waveguide suspended or the substrate removed in the coupling region to prevent energy leakage to the substrate. This complicates operations and reduces the stability of the coupling structure. Photonic wire coupling, based on 3D printing technology, fabricates polymer waveguides in situ using two-photon lithography. However, this method, being based on polymer waveguides, is generally not heat-resistant, has poor long-term stability, and relatively large surface roughness, making it difficult to implement practically. Therefore, achieving a high-efficiency, wide-bandwidth, polarization-insensitive, and stable coupling structure between a standard optical fiber and an on-chip integrated optical waveguide is a pressing problem in this field. Summary of the Invention

[0003] The purpose of this invention is to address the shortcomings of existing technologies by providing a fiber-to-chip coupling structure mediated by semiconductor nanowires. This invention is based on an adiabatic coupling method, in which a standard optical fiber drawn into a tapered micro / nano fiber at one end is connected and coupled to the thinner end of a high-refractive-index, diameter-gradient semiconductor nanowire to form a composite structure. The gap is filled with a small amount of polymer, and the thicker end of the diameter-gradient semiconductor nanowire in this composite structure is bonded and coupled to a tapered silicon waveguide on the chip, thereby achieving bidirectional coupling between a standard optical fiber and an on-chip integrated optical waveguide with high coupling efficiency, wide bandwidth, and polarization insensitivity.

[0004] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows:

[0005] This invention discloses a fiber-to-chip coupling structure mediated by semiconductor nanowires, comprising a tapered micro / nanofiber drawn from one end of a standard optical fiber, a semiconductor nanowire with a gradually tapered diameter mediated in the middle, a polymer bonding the tapered micro / nanofiber and the gradually tapered semiconductor nanowire, and a tapered silicon waveguide on the chip. The thinner end of the gradually tapered semiconductor nanowire is connected and coupled to the tapered micro / nanofiber drawn from one end of the standard optical fiber to form a composite structure. The gap at the connection is filled with a small amount of polymer. The thicker end of the gradually tapered semiconductor nanowire is bonded to the tapered silicon waveguide on the chip. This achieves bidirectional coupling between a standard optical fiber and an on-chip integrated optical waveguide, characterized by high coupling efficiency, wide bandwidth, and polarization insensitivity.

[0006] As a further improvement, the present invention draws one end of a standard optical fiber into a tapered micro / nano fiber with a slow taper change to meet the thermal insulation requirements. The taper angle at the end is within 10 degrees, and it is prepared by a hot drawing method.

[0007] As a further improvement, the semiconductor nanowires with gradually varying diameters described in this invention are made of cadmium sulfide or zinc oxide, with a refractive index greater than 2 in the communication band, and are prepared by chemical vapor deposition.

[0008] As a further improvement, the diameter-gradient semiconductor nanowires of the present invention have a length ranging from 50 μm to 200 μm, and a diameter that is gradually varied, with the thin end diameter ranging from 350 nm to 550 nm and the thick end diameter ranging from 600 nm to 1 μm.

[0009] As a further improvement, the polymer described in this invention is polystyrene (PS) or polymethyl methacrylate (PMMA).

[0010] As a further improvement, the tapered silicon waveguide on the chip described in this invention has a length of 10-20 μm and a width that varies from 100 nm to 400 nm. The tapering changes slowly to meet the thermal insulation requirements. It is manufactured using electron beam exposure and inductively coupled plasma dry etching processes.

[0011] Compared with the prior art, the beneficial effects of the present invention are:

[0012] A composite structure is formed by drawing standard optical fiber into a tapered micro / nano fiber and coupling it with high-refractive-index, tapered-diameter semiconductor nanowires. This composite structure is then coupled to a tapered silicon waveguide on a chip. This method enables bidirectional coupling between standard optical fiber and on-chip integrated optical waveguides with high coupling efficiency, wide bandwidth, and polarization insensitivity. Furthermore, due to the high refractive index of the semiconductor nanowires (greater than 2 for communication band materials), the thicker end of the semiconductor nanowires in the composite structure can be attached to the substrate and coupled to the silicon waveguide during coupling without needing to suspend the silicon waveguide to prevent mode leakage to the substrate, thus making the entire coupling structure more stable. This coupling structure is suitable for coupling optical fibers and various on-chip devices, facilitating the integration of traditional optical devices and on-chip optical devices. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the fiber-chip coupling structure mediated by semiconductor nanowires of the present invention;

[0014] In the figure, 1-a tapered micro / nano fiber drawn from one end of a standard optical fiber, 2-polymer, 3-semiconductor nanowire with a gradually changing diameter, 4-tapered silicon waveguide, 5-silicon dioxide layer, and 6-silicon substrate.

[0015] Figure 2 These are electron microscope images of the tapered micro / nano optical fiber and semiconductor nanowire composite structure used in the examples of this invention.

[0016] Figure 3 These are CCD images taken under an optical microscope of the tapered micro / nano fiber and semiconductor nanowire composite structure used in this invention example; (top) and (bottom) images.

[0017] Figure 4 These are electron microscope images of the silicon-based structure on the chip of this invention.

[0018] Figure 5 This is a diagram of a device for testing the coupling efficiency of a standard optical fiber and an on-chip integrated optical waveguide, as an example of the present invention.

[0019] In the figure, 7-tunable laser, 8-fiber polarization controller, 1-tapered micro / nano fiber drawn from one end of a standard fiber, 2-polymer, 3-diameter-gradient semiconductor nanowire, 4-tapered silicon waveguide, 9-grating, 10-standard fiber, 11-optical power meter.

[0020] Figure 6 These are optical microscope images of standard optical fibers and on-chip integrated optical waveguides used in the testing of examples of this invention.

[0021] Figure 7This is a graph showing the bidirectional coupling efficiency between fiber to chip and chip to fiber in the TM mode of this invention.

[0022] Figure 8 This is a graph showing the bidirectional coupling efficiency curves between fiber to chip and chip to fiber in the TE mode of this invention. Detailed Implementation

[0023] This invention discloses a fiber-to-chip coupling structure mediated by semiconductor nanowires 3. One end of a standard optical fiber is drawn into a tapered micro / nano fiber, which is then coupled to the thinner end of a high-refractive-index, tapered-diameter semiconductor nanowire 3 to form a composite structure. The thicker end of the high-refractive-index, tapered-diameter semiconductor nanowire 3 in this composite structure is then coupled to a tapered silicon waveguide 4 on the chip. This method enables bidirectional coupling between a standard optical fiber and an on-chip integrated optical waveguide, achieving high coupling efficiency, wide bandwidth, and polarization insensitivity. This method is applicable to the coupling of optical fibers and various on-chip devices, facilitating the integration of traditional optical devices and on-chip optical devices.

[0024] Specifically, a composite structure is formed by drawing one end of a standard optical fiber into a tapered micro / nano fiber and connecting it to the thinner end of a high-refractive-index diameter graded-semiconductor nanowire 3. The gap is filled with a small amount of polymer 2, and the thicker end of the high-refractive-index diameter graded-semiconductor nanowire 3 in this composite structure is bonded and coupled to a tapered silicon waveguide 4 on the chip. The tapered micro / nano fiber in the composite structure is made from one end of a standard optical fiber by a thermal drawing method. By controlling parameters such as drawing speed, flame size, and airflow speed, the shape and diameter of the tapered micro / nano fiber can be controlled, ultimately obtaining a tapered micro / nano fiber with a slow taper change (the taper angle at the end of the micro / nano fiber is usually within 10 degrees) and meeting the thermal insulation requirements. The high-refractive-index diameter graded-semiconductor nanowire 3 in the composite structure is grown by vapor deposition, and its material is cadmium sulfide and zinc oxide. The refractive index of the material is high (greater than 2) in the communication band. By controlling parameters such as growth temperature and gas pressure, semiconductor nanowires 3 with suitable diameter and length and smooth surface morphology can be obtained. The semiconductor nanowires 3 used in the experiment ranged in length from 50 μm to 200 μm, with a gradually changing diameter. The diameter of the thinner end ranged from 350 nm to 550 nm, and the diameter of the thicker end ranged from 600 nm to 1 μm. Using micro-nano manipulation methods, tapered micro-nano fibers and semiconductor nanowires 3 of suitable diameter were assembled to form a composite structure. The thinner end of the semiconductor nanowires 3 with a gradually changing diameter was connected and coupled to the tapered micro-nano fiber, and the coupling length was adjusted to obtain the optimal coupling conditions. To ensure that the optimal coupling conditions remained unchanged during subsequent coupling processes, a small amount of polymer 2 was filled into the connection gap between the tapered micro-nano fiber and the semiconductor nanowires 3 to form a stable composite structure. The polymer 2 material was polystyrene (PS) and polymethyl methacrylate (PMMA). The on-chip silicon-based structure was fabricated from a silicon wafer on an insulating substrate (SOI) using electron beam lithography and inductively coupled plasma dry etching processes. It included a tapered silicon waveguide 4 for coupling with the fiber composite cascade structure, a standard silicon strip waveguide for transmitting optical signals, and gratings for inputting and outputting on-chip optical signals. The light source is provided by a tunable laser. Under a microscope, the thicker end (approximately 1 μm) of the gradually tapered semiconductor nanowire 3 in the composite structure is brought into contact and coupled with the on-chip tapered waveguide. The output optical signal is then fed into a power meter. The polarization of the optical signal is controlled by a fiber polarization controller and grating design in the optical path. Ultimately, the bidirectional coupling efficiency between the silicon-based waveguide and the standard optical fiber under different polarizations is obtained from the input and output optical signals and the grating efficiency at different wavelengths.

[0025] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments:

[0026] Figure 1 This is a schematic diagram of the fiber-to-chip coupling structure mediated by semiconductor nanowire 3 according to the present invention; the fiber-to-chip coupling structure mediated by semiconductor nanowire 3 is as follows: Figure 1As shown, a tapered micro / nano fiber 1 drawn from one end of a standard optical fiber is coupled to the thinner end of a semiconductor nanowire 3 with a gradually varying diameter to form a composite structure. After adjusting the coupling length to achieve optimal coupling, a small amount of polymer 2 is filled into the gap at the connection point to increase the stability of the composite structure. The composite structure is then coupled to an on-chip tapered silicon waveguide 4. Below the on-chip tapered silicon waveguide 4 is a 2-micrometer-thick silicon dioxide layer 5, and below the silicon dioxide layer 5 is a silicon substrate layer 6. Due to the high refractive index of the semiconductor nanowire 3, during coupling, the thicker end of the semiconductor nanowire 3 in the composite structure can adhere to the substrate and couple with the silicon waveguide without being completely suspended, while the tapered fiber end needs to be suspended to prevent optical power leakage to the substrate.

[0027] Electron microscope images of the tapered micro / nano fiber and semiconductor nanowire composite structure used in this invention example are shown below. Figure 2 As shown, a hot-drawing method is used to draw one end of a standard optical fiber into a tapered micro / nano fiber. Specifically, the standard optical fiber, with its coating stripped, is placed over a flame until the heated area melts (for approximately tens of seconds). One end of the fiber is then quickly pulled away from the flame, thus producing a high-quality single-ended tapered micro / nano fiber. By controlling parameters such as drawing speed, flame size, and airflow velocity, the shape and diameter of the tapered micro / nano fiber can be controlled, ultimately obtaining tapered micro / nano fibers with a tip taper angle of less than 10 degrees that meet the thermal insulation requirements and experimental specifications. High-refractive-index diameter-gradient semiconductor nanowires 3 (cadmium sulfide, zinc oxide) are grown using vapor deposition. By controlling parameters such as growth temperature and gas pressure, the diameter and length of the semiconductor nanowires 3 can be controlled. The diameter-gradient semiconductor nanowires 3 used in the example are shown below. Figure 2 As shown on the right, the surface is smooth, the length is 97 μm, and the diameter gradually changes from 523 nm to 912 nm, exhibiting a well-defined hexagonal structure. Using micro-nano manipulation methods, tapered micro / nano fibers and semiconductor nanowires 3 are assembled to form a composite structure. Signal light is passed through one end of the tapered micro / nano fiber from a standard fiber, and the intensity of the scattered light at the final output end of the nanowire is monitored in real time using an optical microscope and infrared CCD imaging (e.g., ...). Figure 3 (As shown below), the length of the coupling region was adjusted and optimized until the output intensity at the nanowire end reached its maximum. To ensure that the optimal coupling conditions remained unchanged during subsequent coupling processes, a small amount of polymer 2 was filled into the gap between the tapered micro / nano fiber and the semiconductor nanowire 3 to form a stable composite structure.

[0028] The on-chip silicon structure used in this invention is fabricated using an insulator-on-silicon wafer with a 340nm thick top silicon layer and a 2μm thick silicon dioxide layer 5, via electron beam lithography and inductively coupled plasma dry etching. Electron microscope images of the silicon structure used to test the coupling efficiency of optical fibers and chips are shown below. Figure 4As shown, the structure includes a tapered waveguide for coupling with the composite structure of micro / nano optical fiber and semiconductor nanowire 3, a curved and a straight strip waveguide, and an end connected to a standard grating. The tapered silicon waveguide 4 used in this example has a length of 10 μm and a width varying from 100 nm to 400 nm. The standard grating is used for inputting or outputting optical signals at one end, and the polarization of the light in the waveguide can be controlled by controlling the characteristic parameters of the grating.

[0029] This invention demonstrates the coupling efficiency between a standard optical fiber and an on-chip silicon waveguide using a semiconductor nanowire-mediated fiber-to-chip coupling structure. The test setup is shown in the figure below. Figure 5 As shown. When testing the coupling efficiency from the standard fiber to the on-chip silicon waveguide, the light output from the tunable laser 7 is input through the standard fiber, polarized by a fiber polarization controller 8, and then input into the composite structure of the tapered micro / nano fiber and semiconductor nanowire 3. This structure is formed by coupling the tapered micro / nano fiber 1 (drawn from one end of the standard fiber) to the thinner end of the semiconductor nanowire 3, with a small amount of polymer 2 filling the gap at the connection point to increase the stability of the composite structure. Next, under a microscope, the thicker end of the semiconductor nanowire 3 is coupled to the on-chip tapered waveguide. Then, the standard fiber 10 is used to receive the optical signal output from the grating 9 and input it into the optical power meter 11. An optical microscope image of the coupling process is shown below. Figure 6 As shown. When testing the coupling efficiency from the silicon-based waveguide on the chip to the standard optical fiber, the input and output ends are reversed. Light enters the silicon waveguide on the chip through the grating from the standard optical fiber. By adjusting the coupling between the tapered micro / nano fiber semiconductor nanowire 3 composite structure and the tapered silicon waveguide 4, the light is coupled out from the standard optical fiber connected to the micro / nano fiber and input into the power meter.

[0030] The polarization of light in the system is controlled by a fiber polarization controller and the design of grating characteristic parameters in the on-chip silicon-based structure. In this example, a composite coupling structure is formed using cadmium sulfide nanowires with diameters gradually increasing from 515 nm to 860 nm and tapered micro / nano fibers. Polystyrene is filled into the gaps, and the composite structure is coupled to an on-chip tapered waveguide. The bidirectional coupling efficiency curves between fiber-to-chip and chip-to-fiber in TM and TE modes, measured in the 1520-1640 nm range, are shown below. Figure 7-8 As shown. In TM mode, as Figure 7 As shown, the bidirectional coupling efficiency from fiber to silicon waveguide and from silicon waveguide to fiber is similar, with the highest coupling efficiency exceeding 85% at certain wavelengths (1570nm), and both exhibiting a high 3dB bandwidth exceeding 100nm. In TM mode, such as... Figure 8As shown, the bidirectional coupling efficiency from fiber to silicon waveguide and from silicon waveguide to fiber both reach over 90% at certain wavelengths (1560nm), and both also exhibit a high 3dB bandwidth exceeding 100nm. This demonstrates that the coupling structure possesses high coupling efficiency and wide coupling bandwidth. Furthermore, the test results in TE and TM modes are similar, indicating that the coupling structure exhibits good polarization independence.

[0031] It will be understood by those skilled in the art that the above are merely preferred examples of the invention and are not intended to limit the invention. Although the invention has been described in detail with reference to the foregoing examples, those skilled in the art can still modify the technical solutions described in the foregoing examples or make equivalent substitutions for some of the technical features. All modifications and equivalent substitutions made within the spirit and principles of the invention should be included within the scope of protection of the invention.

Claims

1. A fiber-chip coupling structure mediated by semiconductor nanowires, characterized in that, The system comprises a tapered micro / nanofiber drawn from one end of a standard optical fiber, a diameter-gradient semiconductor nanowire mediated in the middle, a polymer bonding the tapered micro / nanofiber and the semiconductor nanowire, and a tapered silicon waveguide on a chip. The thinner end of the diameter-gradient semiconductor nanowire is bonded to the thinner end of the tapered micro / nanofiber drawn from one end of the standard optical fiber, with a small amount of polymer filling the gap at the connection. The thicker end of the diameter-gradient semiconductor nanowire is bonded to the thinner end of the tapered silicon waveguide on the chip. The tapered micro / nanofiber drawn from one end of the standard optical fiber has a slow taper change to meet thermal insulation requirements, and its taper angle at the end is within 10 degrees. It is fabricated using a hot-drawing method. The diameter-gradient semiconductor nanowire... The conductor nanowires are made of cadmium sulfide or zinc oxide, with a refractive index greater than 2 in the communication band, and are prepared by chemical vapor deposition. The length of the diameter-gradient semiconductor nanowires ranges from 50 μm to 200 μm, with a diameter gradient of 350 nm to 550 nm at the thin end and 600 nm to 1 μm at the thick end. The polymer is polystyrene (PS) or polymethyl methacrylate (PMMA). The tapered silicon waveguides on the chip have a length of 10-20 μm and a width that varies from 100 nm to 400 nm, with a slow taper to meet thermal insulation requirements. They are manufactured using electron beam lithography and inductively coupled plasma dry etching processes.

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