Directional coupler based on single-mode fiber and arbitrary waveguide coupling and preparation method thereof

Abstract

The invention discloses a directional coupler based on coupling of a single-mode optical fiber and any waveguide and a preparation method of the directional coupler. The directional coupler comprises a substrate, a waveguide cladding arranged on the substrate, and a single-mode optical fiber core and a few-mode waveguide core which are arranged in the waveguide cladding; the single-mode optical fiber core and the few-mode waveguide core are located in the same plane and are arranged in parallel; when the effective refractive index of the fundamental mode LP01 of the single-mode optical fiber core is the same as that of the Emn mode of the few-mode waveguide core, and the interval G between the few-mode waveguide core and the single-mode optical fiber core is matched with the coupling length, the fundamental mode LP01 of the single-mode optical fiber core is coupled and converted into the Emn mode of the few-mode waveguide core; therefore, optical signals of a fundamental mode of the single-mode optical fiber core are coupled and converted into optical signals of an Emn mode of the few-mode waveguide core. By changing the gap G between the few-mode waveguide core and the single-mode optical fiber core in the coupling area and the structure (including width, height and refractive index) of the few-mode waveguide core, optical fiber fundamental mode coupling can be obtained and converted into any waveguide mode.

Description

Technical Field

[0001] The invention relates to the field of integrated photonics, and in particular to a directional coupler based on coupling of a single-mode optical fiber and an arbitrary waveguide and a preparation method thereof. Background Art

[0002] With the rapid development of the Internet and new generation information technology, emerging technologies such as artificial intelligence, the Internet of Things, and cloud computing have increasingly increased their requirements for data transmission, ushering in the challenges of complex transmission requirements such as ultra-high speed, ultra-large capacity, ultra-long distance, and multi-data processing. In this context, the optical fiber communication system, as an important part of the communication system, urgently needs to be upgraded to adapt to the future development trend of communication.

[0003] Traditional optical communication systems have defects such as limited information processing capabilities, high transmission losses, and limited integration. Optical communication systems based on photonic chip integration improve integration and data processing performance by integrating light sources, optical modulation, signal processing, and detection functions on a single chip. In order to meet the complex transmission requirements of ultra-high speed and ultra-long distance, the combination of optical fiber communication systems and photonic chip integrated communication systems has the advantages of ultra-high speed and ultra-long distance, and can meet the requirements of ultra-large capacity and multi-data processing. This is the future development trend of optical communication systems.

[0004] With the rapid development of fiber-optic communication technology and integrated photonics, the coupling technology between optical fiber and waveguide core plays a vital role in optical communication systems. Optical fiber is the key carrier for long-distance optical signal transmission, while waveguide is widely used in integrated photonic devices and chips. In the communication system combining optical fiber communication and optical waveguide communication, it is necessary to efficiently couple the optical signal in the optical fiber to the waveguide core, or to efficiently lead the optical signal in the waveguide to the optical fiber. However, due to the differences in the mode field size and propagation characteristics of optical fiber and waveguide, the coupling efficiency is often affected.

[0005] Existing waveguide and optical fiber coupling technologies include end-face coupling, grating coupling, spatial optical path coupling, etc. Among them, the end-face coupling and spatial optical path coupling methods require the alignment and matching of the optical fiber core and the waveguide core, which have the disadvantages of high alignment accuracy and large insertion loss. The grating coupling method has the disadvantages of complex manufacturing process, strong thermal sensitivity, large insertion loss caused by mode mismatch, and weak stability. Summary of the invention

[0006] In order to overcome the shortcomings of the prior art, the present invention provides a directional coupler based on coupling of a single-mode optical fiber and an arbitrary waveguide.

[0007] The second object of the present invention is to provide a method for preparing a directional coupler based on coupling of a single-mode optical fiber and an arbitrary waveguide.

[0008] The technical solution of the present invention to solve the above technical problems is:

[0009] A directional coupler based on coupling of a single-mode optical fiber with an arbitrary waveguide comprises a substrate, a waveguide cladding arranged on the substrate, and a single-mode optical fiber core and a few-mode waveguide core arranged in the waveguide cladding, wherein:

[0010] The single-mode optical fiber core and the few-mode waveguide core are located in the same plane, and the two are arranged in parallel;

[0011] When the single-mode optical fiber core and the few-mode waveguide core meet the phase matching condition, that is, the fundamental mode LP of the single-mode optical fiber core 01 With the few-mode waveguide core E mn When the effective refractive index of the modes is the same and the interval G between the few-mode waveguide core and the single-mode optical fiber core matches the coupling length, the fundamental mode LP of the single-mode optical fiber core 01 coupled and transformed into the E of the few-mode waveguide core mn The mode is output from the few-mode waveguide core, realizing the coupling of the fundamental mode optical signal of the single-mode fiber core and converting it into the E mn The optical signal of the mode.

[0012] Preferably, the coupling length L between the few-mode waveguide core and the single-mode optical fiber core is c for:

[0013]

[0014] Where k is the coupling coefficient.

[0015] Preferably, the waveguide cladding includes a waveguide upper cladding and a waveguide lower cladding.

[0016] Preferably, the substrate is a silicon-based substrate.

[0017] A method for preparing a directional coupler based on coupling of a single-mode optical fiber and an arbitrary waveguide comprises the following steps:

[0018] Step S1: performing etching, taper, and photolithography processing on the single-mode optical fiber;

[0019] Step S2: Processing the surface of the silicon-based substrate to ensure that it is smooth and flat, and applying a cladding material onto the silicon-based substrate as a waveguide lower cladding;

[0020] Step S3: coating the surface of the waveguide lower cladding layer with a film, and coating a layer of few-mode waveguide photoresist on the surface of the film, spreading the few-mode waveguide photoresist on the surface of the silicon-based substrate at a low speed, and then volatilizing the few-mode waveguide photoresist at a high speed to reach a predetermined thickness;

[0021] Step S4: placing the silicon-based substrate on which the few-mode waveguide photoresist has been spin-coated into an equipment cabin of a mask photolithography device, and performing mask photolithography on the few-mode waveguide photoresist according to the designed few-mode waveguide graphic file;

[0022] Step S5: placing the obtained silicon-based substrate into a developer to remove the portion of the few-mode waveguide photoresist that is not exposed to the ultraviolet light, and performing development and fixing;

[0023] Step S6: placing the single-mode optical fiber core in step S1 on the same plane as the few-mode waveguide core in the silicon-based substrate and fixing them, and the interval between the few-mode waveguide core and the single-mode optical fiber core is controlled to be G;

[0024] Step S7: preparing a waveguide upper cladding layer on the surface of the few-mode waveguide core and the waveguide lower cladding layer.

[0025] Preferably, in step S1, the steps of processing the single-mode optical fiber are:

[0026] Step S101: using alcohol to clean the single-mode optical fiber, immersing the single-mode optical fiber in an etchant, observing the corrosion condition of the single-mode optical fiber cladding and controlling the corrosion time to ensure that the single-mode optical fiber core is not excessively corroded and the single-mode optical fiber cladding is completely corroded;

[0027] Step S102: neutralizing the corrosive agent with a neutralizer, washing the corroded single-mode optical fiber with clean water, and then drying the single-mode optical fiber core with dry nitrogen;

[0028] Step S103: placing the corroded single-mode optical fiber core on an optical fiber taper machine, formulating taper parameters according to the single-mode optical fiber design requirements, and observing the taper process of the single-mode optical fiber core using a microscope to ensure the uniformity and accuracy of the taper;

[0029] Step S104: fix the tapered single-mode optical fiber on the operating table, use a heater to heat the single-mode optical fiber core to soften it; use a bending mold to bend the single-mode optical fiber core into a predetermined shape, stop heating and cool the single-mode optical fiber;

[0030] Step S105: using a microscope to inspect the single-mode optical fiber core at the bent portion to ensure that the single-mode optical fiber core has no cracks, excessive stretching or deformation.

[0031] Preferably, in step S1, the taper angle of the single-mode optical fiber core is 0.5 to 3 degrees; the waist diameter of the tapered optical fiber is 5 to 20 microns, and the bending radius of the single-mode optical fiber core is 30 to 50 millimeters.

[0032] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0033] 1. The directional coupler based on coupling of single-mode optical fiber and arbitrary waveguide of the present invention forms a directional coupler by placing a single-mode optical fiber core and a few-mode waveguide core that have been corroded and tapered in the same plane, thereby realizing the function of coupling the optical signal of the single-mode optical fiber and converting it into the optical signal of the few-mode waveguide; in addition, the directional coupler based on coupling of single-mode optical fiber and arbitrary waveguide of the present invention can couple the fundamental mode of the single-mode optical fiber and convert it into an arbitrary mode of the few-mode waveguide, and the directional coupler based on coupling of single-mode optical fiber and arbitrary waveguide of the present invention has the characteristics of flexible design, strong stability and low insertion loss.

[0034] 2. The directional coupler based on coupling of single-mode optical fiber and arbitrary waveguide of the present invention has the characteristics of strong structural stability, flexible design, strong scalability, etc. By controlling the structure (width, height), material and spacing between the few-mode waveguide core and the single-mode optical fiber core of the few-mode waveguide, the fundamental mode LP of the single-mode optical fiber core can be satisfied. 01 Any E of the few-mode waveguide core mn Mode phase matching conditions are used to achieve the coupling of the fundamental mode LP0 of the single-mode fiber and convert it into any E mn Function of mode.

[0035] 3. The directional coupler based on coupling of single-mode optical fiber and arbitrary waveguide of the present invention also has the characteristics of high coupling efficiency and low precision requirement, and can be widely used in the application field of combining integrated photonics and optical fiber optics. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is a schematic diagram of a directional coupler based on coupling of a single-mode optical fiber and an arbitrary waveguide according to the present invention;

[0037] Figure 2 A cross-sectional view of the coupling region of the directional coupler based on coupling of a single-mode optical fiber and an arbitrary waveguide according to the present invention;

[0038] Figure 3 Schematic diagram of the processing process of single-mode optical fiber;

[0039] Figure 4 The present invention is a flow chart of the preparation of a directional coupler based on coupling of a single-mode optical fiber to an arbitrary waveguide.

[0040] In the figure: 1-1 - single-mode optical fiber cladding; 1-2 - single-mode optical fiber core; 1-3 - few-mode waveguide core; 1-4 - waveguide upper cladding; 1-5 - waveguide lower cladding; 1-6 - substrate. DETAILED DESCRIPTION

[0041] The present invention is further described in detail below in conjunction with embodiments and drawings, but the embodiments of the present invention are not limited thereto.

[0042] See also Figure 1-Figure 4The directional coupler based on coupling of a single-mode optical fiber with an arbitrary waveguide of the present invention comprises a substrate, a waveguide cladding arranged on the substrate, and a single-mode optical fiber core and a few-mode waveguide core arranged in the waveguide cladding, wherein:

[0043] The substrate is a silicon-based substrate;

[0044] The waveguide cladding includes a waveguide upper cladding and a waveguide lower cladding;

[0045] The single-mode optical fiber core and the few-mode waveguide core are located in the same plane, and the two are arranged in parallel;

[0046] When the single-mode optical fiber core and the few-mode waveguide core meet the phase matching condition, that is, the fundamental mode LP of the single-mode optical fiber core 01 With the few-mode waveguide core E mn When the effective refractive index of the modes is the same and the interval G between the few-mode waveguide core and the single-mode optical fiber core matches the coupling length, the fundamental mode LP of the single-mode optical fiber core 01 coupled and transformed into the E of the few-mode waveguide core mn The mode is output from the few-mode waveguide core, realizing the coupling of the fundamental mode optical signal of the single-mode fiber core and converting it into the E mn The optical signal of the mode.

[0047] In this embodiment, a directional coupler structure is used to achieve optical fiber fundamental mode coupling to a few-mode waveguide, and the coupling region spacing G is generally 5 microns to 10 microns. Such a spacing requirement makes it easy to manipulate the spacing between the single-mode optical fiber and the few-mode waveguide in the experiment, while also shortening the coupling region length and reducing the device size.

[0048] The specific design of coupling arbitrary waveguides with single-mode optical fibers adopts the basic principle of directional couplers, which are based on the mode coupling theory, that is, based on the mutual coupling and interference effects of light waves in two adjacent waveguides or optical fibers; when the distance between adjacent waveguides and optical fibers is close enough, the light mode field distributions of the waveguide core and the optical fiber core will overlap, resulting in the exchange of light energy between the waveguide core and the optical fiber core; since a mode of the waveguide core has the same effective refractive index as a mode of the optical fiber core, when the two cores are close enough, the overlap of the mode field distributions in the modes with the same effective refractive index will be close to 100%, and the light energy of the two core modes will affect and convert each other; therefore, designing a certain core spacing and coupling length can achieve complete conversion of optical fiber mode signals into waveguide optical signals.

[0049] According to the mode coupling theory, in a directional coupler, the mode fields of two adjacent waveguides or optical fibers can couple with each other; the coupling wave equations of the single-mode fiber core and the few-mode waveguide core are:

[0050]

[0051] Where: E1 and E2 are the fundamental mode LP of the single-mode optical fiber core. 01 And few-mode waveguide core E mn The mode electric field of the mode; z is the position along the waveguide propagation direction; k is the coupling coefficient, which is related to the waveguide spacing, waveguide refractive index difference and light wavelength; the coupling coefficient k is a key parameter of the coupling efficiency and can be calculated by the mode overlap integral, where the coupling coefficient k is:

[0052]

[0053] Where: n1 and n2 are the fundamental mode LP of the single-mode fiber core. 01 and E of the few-mode waveguide core mn The effective refractive index of the mode; E1 and E2 are the fundamental mode LP of the single-mode fiber core 01 And few-mode waveguide core E mn mode; θ1 and θ2 are the fundamental mode LP of the single-mode fiber core 01 and E of the few-mode waveguide core mn The phase angle of the mode;

[0054] The output power is calculated by the square of the electric field amplitude:

[0055] P1(z)=|E1(z)| 2 =P0cos 2 (k);

[0056] P2(z)=|E2(z)| 2 =P0sin 2 (k);

[0057] Where: P0 is the total power input to the single-mode optical fiber core;

[0058] When sin(k) = 1, energy is fully transferred. Therefore, the minimum length required to achieve full power transmission is the coupling length:

[0059]

[0060] When the coupling region length of the directional coupler meets the above conditions, the optical energy of the single-mode fiber core will be completely coupled and exchanged into the few-mode waveguide.

[0061] See also Figure 1-Figure 4 The method for preparing a directional coupler based on coupling of a single-mode optical fiber and an arbitrary waveguide of the present invention comprises the following steps:

[0062] Step S1: performing etching, tapering, and photolithography on the single-mode optical fiber; specifically:

[0063] Use alcohol to clean the single-mode optical fiber, immerse the single-mode optical fiber in the corrosive agent, observe the corrosion condition of the single-mode optical fiber cladding and control the corrosion time to ensure that the single-mode optical fiber core will not be over-corroded and the single-mode optical fiber cladding will be completely corroded; use a neutralizer to neutralize the corrosive agent, and use clean water to clean the corroded single-mode optical fiber, and then blow dry the single-mode optical fiber core with dry nitrogen; place the corroded single-mode optical fiber core on the optical fiber taper machine, formulate the taper parameters according to the single-mode optical fiber design requirements, and use a microscope to observe the taper process of the single-mode optical fiber core to ensure the uniformity and accuracy of the taper (the uniformity and accuracy of the single-mode optical fiber taper can be compared by comparing the realized optical fiber taper diameter displayed by the taper machine with the set taper optical fiber diameter, and the taper machine will display the curve of the change of diameter relative to length); Fix the tapered single-mode optical fiber on the operating table, use a heater to heat the single-mode optical fiber core to soften it; use a bending mold to bend the single-mode optical fiber core into a predetermined shape, then stop heating and cool the single-mode optical fiber; use a microscope to check the single-mode optical fiber core at the bent part to ensure that the single-mode optical fiber core has no cracks, excessive stretching or deformation (whether the optical fiber core of the single-mode optical fiber has cracks, excessive stretching or deformation after tapering is to ensure that the single-mode optical fiber used in subsequent experiments does not have the above problems, to ensure that subsequent experiments can proceed normally, and to ensure that there are no problems with the optical fiber made in this step. If you need to accurately confirm that the optical properties of the single-mode optical fiber after tapering are good and the structure is intact, you need to build a test bench to test the optical properties of the single-mode optical fiber, including loss, output mode field state, etc.).

[0064] In the above process, in order to ensure a smooth transition of the tapered optical fiber, ensure high coupling efficiency and reduce mode mismatch loss, the taper angle of the single-mode optical fiber core is 0.5 to 3 degrees; the waist diameter of the tapered optical fiber is 5 to 20 microns, and the bending radius of the single-mode optical fiber core is 30 to 50 mm.

[0065] In this embodiment, judging by the diameter of the single-mode optical fiber core after corrosion, when the optical fiber is excessively corroded, it is observed through a microscope that the diameter of the single-mode optical fiber core is significantly smaller than the design value; or, judging by the surface quality of the optical fiber core, deterioration and excessive corrosion lead to an uneven surface of the optical fiber, and obvious etching marks or pits appear on the optical fiber core through microscopic observation; or, judging by the optical properties of the optical fiber core after corrosion, an excessively corroded core or cladding will lead to increased optical loss; or, by building a test device to measure the optical fiber transmission loss, if the loss is significantly higher than the design value, it may be a manifestation of excessive corrosion.

[0066] Step S2: Processing the surface of the silicon-based substrate to ensure that it is smooth and flat; applying a cladding material onto the silicon-based substrate as a waveguide lower cladding;

[0067] Step S3: coating the surface of the waveguide lower cladding layer with a film, and coating a layer of few-mode waveguide photoresist on the surface of the film, spreading the few-mode waveguide photoresist on the surface of the silicon-based substrate at a low speed, and then volatilizing the few-mode waveguide photoresist at a high speed to reach a predetermined thickness;

[0068] Step S4: placing the silicon-based substrate on which the few-mode waveguide photoresist has been spin-coated into an equipment cabin of a mask photolithography device, and performing mask photolithography on the few-mode waveguide photoresist according to the designed few-mode waveguide graphic file;

[0069] Step S5: placing the obtained silicon-based substrate into a developer to remove the portion of the few-mode waveguide photoresist that is not exposed to the ultraviolet light, and performing development and fixing;

[0070] Step S6: placing the single-mode optical fiber in the silicon-based substrate on the same plane as the few-mode waveguide and fixing it, and the interval between the few-mode waveguide core and the single-mode optical fiber core is controlled to be G;

[0071] Step S7: preparing a waveguide upper cladding layer on the surface of the few-mode waveguide and the waveguide lower cladding layer.

[0072] The above is a preferred embodiment of the present invention, but the embodiment of the present invention is not limited to the above content. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. A directional coupler based on coupling of a single-mode optical fiber and an arbitrary waveguide, characterized in that: The invention comprises a substrate, a waveguide cladding arranged on the substrate, and a single-mode optical fiber core and a few-mode waveguide core arranged in the waveguide cladding, wherein: The single-mode optical fiber core and the few-mode waveguide core are located in the same plane, and the two are arranged in parallel; When the single-mode optical fiber core and the few-mode waveguide core meet the phase matching condition, that is, the fundamental mode LP of the single-mode optical fiber core 01 With the few-mode waveguide core E mn When the effective refractive index of the modes is the same and the interval G between the few-mode waveguide core and the single-mode optical fiber core matches the coupling length, the fundamental mode LP of the single-mode optical fiber core 01 coupled and transformed into the E of the few-mode waveguide core mn The mode is output from the few-mode waveguide core, realizing the coupling of the fundamental mode optical signal of the single-mode fiber core and converting it into the E mn The optical signal of the mode.

2. The directional coupler based on coupling of single-mode optical fiber and arbitrary waveguide according to claim 1, characterized in that: The coupling length L between the few-mode waveguide core and the single-mode optical fiber core c for: Where k is the coupling coefficient.

3. The directional coupler based on coupling of single-mode optical fiber and arbitrary waveguide according to claim 2, characterized in that: The waveguide cladding includes a waveguide upper cladding and a waveguide lower cladding.

4. The directional coupler based on coupling of single-mode optical fiber and arbitrary waveguide according to claim 3, characterized in that: The substrate is a silicon-based substrate.

5. A method for preparing a directional coupler based on coupling of a single-mode optical fiber and an arbitrary waveguide as claimed in claim 4, characterized in that: The following steps are involved: Step S1: performing etching, taper, and photolithography processing on the single-mode optical fiber; Step S2: Processing the surface of the silicon-based substrate to ensure that it is smooth and flat, and applying a cladding material onto the silicon-based substrate as a waveguide lower cladding; Step S3: coating the surface of the waveguide lower cladding layer with a film, and coating a layer of few-mode waveguide photoresist on the surface of the film, spreading the few-mode waveguide photoresist on the surface of the silicon-based substrate at a low speed, and then volatilizing the few-mode waveguide photoresist at a high speed to reach a predetermined thickness; Step S4: placing the silicon-based substrate on which the few-mode waveguide photoresist has been spin-coated into an equipment cabin of a mask photolithography device, and performing mask photolithography on the few-mode waveguide photoresist according to the designed few-mode waveguide graphic file; Step S5: placing the obtained silicon-based substrate into a developer to remove the portion of the few-mode waveguide photoresist that is not exposed to the ultraviolet light, and performing development and fixing; Step S6: placing the single-mode optical fiber core in step S1 on the same plane as the few-mode waveguide core in the silicon-based substrate and fixing them, and the interval between the few-mode waveguide core and the single-mode optical fiber core is controlled to be G; Step S7: preparing a waveguide upper cladding layer on the surface of the few-mode waveguide core and the waveguide lower cladding layer.

6. The method for preparing a directional coupler based on coupling of a single-mode optical fiber and an arbitrary waveguide according to claim 5, characterized in that: In step S1, the steps of processing the single-mode optical fiber are: Step S101: using alcohol to clean the single-mode optical fiber, immersing the single-mode optical fiber in an etchant, observing the corrosion condition of the single-mode optical fiber cladding and controlling the corrosion time to ensure that the single-mode optical fiber core is not excessively corroded and the single-mode optical fiber cladding is completely corroded; Step S102: neutralizing the corrosive agent with a neutralizer, washing the corroded single-mode optical fiber with clean water, and then drying the single-mode optical fiber core with dry nitrogen; Step S103: placing the corroded single-mode optical fiber core on an optical fiber taper machine, formulating taper parameters according to the single-mode optical fiber design requirements, and observing the taper process of the single-mode optical fiber core using a microscope to ensure the uniformity and accuracy of the taper; Step S104: fix the tapered single-mode optical fiber on the operating table, use a heater to heat the single-mode optical fiber core to soften it; use a bending mold to bend the single-mode optical fiber core into a predetermined shape, stop heating and cool the single-mode optical fiber; Step S105: using a microscope to inspect the single-mode optical fiber core at the bent portion to ensure that the single-mode optical fiber core has no cracks, excessive stretching or deformation.

7. The method for preparing a directional coupler based on coupling of a single-mode optical fiber and an arbitrary waveguide according to claim 6, characterized in that: In step S1 , the taper angle of the single-mode optical fiber core is 0.5 to 3 degrees; the waist diameter of the tapered optical fiber is 5 to 20 microns, and the bending radius of the single-mode optical fiber core is 30 to 50 millimeters.

Images