Ultra-wideband tunable optical attenuator based on dual-nano optical fiber evanescent field coupling

Abstract

The invention belongs to the technical field of optical fiber communication, and particularly relates to an ultra-wideband tunable optical attenuator based on dual-nano optical fiber evanescent field coupling, which comprises a conical nano optical fiber, a uniform nano optical fiber, a first displacement table adjusting bracket and a second displacement table adjusting bracket, the uniform nano optical fiber is fixed on the second displacement table adjusting bracket, and the conical nano optical fiber is coupled with the uniform nano optical fiber. By utilizing the evanescent field coupling principle between the conical nano optical fiber and the uniform nano optical fiber, the transmission spectrum is adjusted by changing the coupling length, and tunable optical attenuation is realized. Related results of simulation, experiments and the like show that optical attenuation with the bandwidth larger than 500 nm in the wavelength range of 1900-1200 nm can be achieved, the maximum extinction ratio is larger than or equal to 15 dB, the length of the core coupling structure is smaller than 221 [mu] m, and good fault tolerance is achieved for transverse assembly imbalance in the range of 500 nm.

Description

Technical Field

[0001] The invention belongs to the technical field of optical fiber communication, and in particular relates to an ultra-wideband tunable optical attenuator based on double-nano optical fiber evanescent field coupling. Background Art

[0002] As one of the core components of optical fiber communication, optical fiber attenuators are widely used in optical fiber communication networks. Their size, attenuation ratio and other characteristics directly affect the integration and power consumption of optical communication systems. In recent years, various types of optical fiber attenuators based on different principles such as acousto-optic effect, light reflection, and equivalent refractive index measurement have been developed and introduced. Among them, attenuators based on micro-nano waveguides such as nano-fibers have attracted much attention due to their highly adjustable dispersion, high integration and good compatibility.

[0003] Traditional optical attenuators based on nanofibers are usually based on methods such as material refractive index modulation or mechanical stretching. For the former, the nanofiber is made of a liquid crystal layer with light-responsive characteristics. When the guided light is transmitted along the optical fiber, automatic light attenuation is achieved based on the light-induced liquid crystal phase change. For the latter, a mechanical clamping device is used to fix and stretch the nanofiber. By changing the stretching length, the nanofiber undergoes different degrees of deformation, which in turn causes the transmittance to change, resulting in tunable light attenuation. The above methods are limited by the range of material refractive index variation or mechanical stretching length, and cannot achieve large optical bandwidth optical attenuation (typical bandwidth <100nm), which restricts the application of such devices in the field of optical communication technology. Summary of the invention

[0004] In view of the technical problem of the demand for broadband miniaturized optical fiber optical attenuators in the development trend of large bandwidth, high integration and high efficiency in the above-mentioned optical communication technology field, the present invention provides an ultra-wideband tunable optical attenuator based on the evanescent field coupling of dual nanofibers, which adopts a dual-gradient nanofiber structure, and adjusts the light transmission spectrum by changing the axial relative position of the two nanofibers to achieve a tunable optical attenuation function within a wide spectrum range of ~500nm, which can solve the problem of insufficient bandwidth of traditional nanofiber attenuators. In addition, the device has good fault tolerance for assembly errors such as lateral dislocation of dual nanofibers, and has good applications in fields such as all-optical communications.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0006] An ultra-wideband tunable optical attenuator based on dual nanofiber evanescent field coupling comprises a tapered nanofiber, a uniform nanofiber, a first displacement stage adjustment bracket, and a second displacement stage adjustment bracket. The tapered nanofiber is fixed on the first displacement stage adjustment bracket, the uniform nanofiber is fixed on the second displacement stage adjustment bracket, and the tapered nanofiber is coupled with the uniform nanofiber.

[0007] The tapered nanofiber and the uniform nanofiber are both prepared by an adiabatic taper method. The front end of the tapered nanofiber is a tapered nanofiber structure with a gradually decreasing diameter, and the front end of the uniform nanofiber is a uniform nanofiber structure with a uniform diameter.

[0008] The tail ends of the tapered nanofiber and the uniform nanofiber are both standard single-mode optical communication fibers, and the front ends of the tapered nanofiber and the uniform nanofiber are coupled by axial bonding.

[0009] It also includes a light source, the tapered nano-fiber is arranged in the light path direction of the light source, and the supercontinuum output spectrum of the light source is 400nm to 2400nm.

[0010] The initial diameter of the tapered nanofiber is ≤5 μm, the average change angle of the tapered nanofiber is ≥0.05° and ≤5°; and the diameter of the uniform nanofiber is ≤5 μm.

[0011] The coupling length between the tapered nanofiber and the uniform nanofiber is 0-300 μm, and the lateral misalignment width between the tapered nanofiber and the uniform nanofiber is 0-0.5 μm.

[0012] An attenuation method of an ultra-wideband tunable optical attenuator based on dual-nano-fiber evanescent field coupling comprises the following steps:

[0013] S1. The optical signal to be attenuated is transmitted through the tapered nanofiber. Based on the principle of evanescent wave coupling, the optical signal is input into the uniform nanofiber at the front end of the tapered nanofiber and output through the uniform nanofiber.

[0014] The front ends of S2, the tapered nanofiber and the uniform nanofiber are fixed on the first translation stage adjustment bracket and the second translation stage adjustment bracket respectively;

[0015] S3. When the second translation stage adjustment bracket is displaced along the axial direction of the optical fiber relative to the first translation stage adjustment bracket, the front end of the tapered nanofiber is displaced accordingly, causing the length of the coupling region between the tapered nanofiber and the uniform nanofiber to change, resulting in a change in the coupled transmission spectrum, thereby achieving tunable optical attenuation.

[0016] The method for achieving tunable light attenuation in S3 is as follows: by changing the coupling length (L) between the two nanofibers, the transmission spectrum will also change; the coupling region is composed of a tapered portion at the front end of the tapered nanofiber and a uniform portion at the front end of the uniform nanofiber in an axially bonded manner; the tapered portion is arranged in parallel with the uniform portion, and is bonded by adsorption using van der Waals force, and the bracket is adjusted on the second displacement stage to always maintain lateral bonding during the axial displacement process;

[0017] The tapered nanofiber and uniform nanofiber are made of SMF-28 standard optical communication fiber. The refractive index of the fused silica air cladding cylindrical nanofiber is:

[0018]

[0019] Where λ is the wavelength in μm, and n1 is the refractive index of SiO2;

[0020] The cutoff diameter (D) of the fundamental mode at different wavelengths is expressed as

[0021]

[0022] Wherein n2, V = 2.405 is the normalized frequency; the first displacement stage adjustment bracket and the second displacement stage adjustment bracket are micro-nano displacement mechanical structures, providing a controllable displacement with a distance of > 300 μm and a resolution of < 0.1 μm.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] The present invention utilizes the evanescent field coupling principle between tapered nanofibers and uniform nanofibers, and adjusts the transmission spectrum by changing the coupling length to achieve tunable optical attenuation. Simulation and experimental results show that the present invention can achieve optical attenuation with a bandwidth of >500nm in the wavelength range of 1900nm-1200nm, a maximum extinction ratio of ≥15dB, a core coupling structure length of <221μm, and good fault tolerance for lateral assembly misalignment within a range of 500nm (change in optical transmittance <-0.1dB). The results show that the present invention has a broadband tunable optical attenuation function and can be used for optical attenuation in ultra-wideband optical fiber communication systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the implementation methods of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for the implementation methods or the description of the prior art. Obviously, the drawings in the following description are only exemplary, and for ordinary technicians in this field, other implementation drawings can be derived from the provided drawings without creative work.

[0026] The structures, proportions, sizes, etc. illustrated in this specification are only used to match the contents disclosed in the specification so as to facilitate understanding and reading by persons familiar with the technology. They are not used to limit the conditions under which the present invention can be implemented, and therefore have no substantial technical significance. Any structural modification, change in proportion or adjustment of size shall still fall within the scope of the technical contents disclosed in the present invention without affecting the effects and purposes that can be achieved by the present invention.

[0027] Figure 1It is a schematic diagram of the structure of the present invention.

[0028] Figure 2 It is a schematic diagram of the coupling structure of the present invention.

[0029] Figure 3 The single-mode waveguide condition diagram of uniform nano-optical fibers with different diameters at different wavelengths of the present invention is shown in FIG.

[0030] Figure 4 The diagram is a schematic diagram of the relationship between the cutoff wavelength of the tapered region at the front end of the tapered nanofiber of the present invention and the position. The minimum diameter required to support the fundamental mode increases with the increase of the wavelength.

[0031] Figure 5 The figure is a relationship diagram of the coupling transmission spectrum of the present invention with the coupling length. When the coupling length changes, the minimum coupling initial diameter required for the tapered part at the front end of the tapered nano-fiber changes accordingly.

[0032] Figure 6 The cross-sectional distribution diagram of the transmission light field in the tapered region at the front end of the tapered nanofiber of the present invention. The input wavelengths from top to bottom are (a) 1.2 μm (b) 1.55 μm (c) 1.9 μm.

[0033] Figure 7 It is a schematic diagram showing that the transmission efficiency of a single optical fiber decreases with its diameter at different transmission wavelengths according to the present invention.

[0034] Figure 8 It is a schematic diagram of the lateral dislocation of the present invention.

[0035] Fig. 9 This is a diagram showing the effect of lateral misalignment on transmission characteristics of the present invention. DETAILED DESCRIPTION

[0036] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. These descriptions are only to further illustrate the features and advantages of the present invention, rather than to limit the claims of the present invention. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0037] The specific implementation of the present invention is further described in detail below in conjunction with the accompanying drawings and examples. The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0038] The terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of this application, unless otherwise specified, "plurality" means two or more.

[0039] In the description of this application, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0040] An ultra-wideband tunable optical attenuator based on dual nanofiber evanescent field coupling comprises a tapered nanofiber 1-1, a uniform nanofiber 1-2, a first displacement stage adjustment bracket 2-1, and a second displacement stage adjustment bracket 2-2. The tapered nanofiber 1-1 is fixed on the first displacement stage adjustment bracket 2-1, the uniform nanofiber 1-2 is fixed on the second displacement stage adjustment bracket 2-2, and the tapered nanofiber 1-1 is coupled with the uniform nanofiber 1-2. The optical signal to be attenuated is transmitted through the tapered nanofiber 1-1, and is transmitted to the uniform nanofiber 1-2 at the front end of the tapered nanofiber 1-1 based on the evanescent wave coupling principle, and is output along the tapered nanofiber; the front ends of the tapered nanofiber 1-1 and the uniform nanofiber 1-2 are respectively fixed on the first displacement stage adjustment bracket 2-1 and the second displacement stage adjustment bracket 2-2; when the second displacement stage adjustment bracket 2-2 is displaced along the axial direction of the optical fiber relative to the first displacement stage adjustment bracket 2-1, the front end of the tapered nanofiber 1-1 is displaced accordingly, so that the length of the coupling region of the tapered nanofiber 1-1 and the uniform nanofiber 1-2 changes, resulting in a change in the coupled transmission spectrum, thereby realizing tunable optical attenuation.

[0041] Further, if Figure 2 As shown, the tapered nanofiber 1-1 and the uniform nanofiber 1-2 are both prepared by the adiabatic taper method. The front end of the tapered nanofiber 1-1 is a tapered nanofiber structure with a gradually decreasing diameter, and the front end of the uniform nanofiber 1-2 is a uniform nanofiber structure with a uniform diameter.

[0042] Furthermore, the tail ends of the tapered nanofiber 1-1 and the uniform nanofiber 1-2 are both standard single-mode optical communication fibers, and the front ends of the tapered nanofiber 1-1 and the uniform nanofiber 1-2 are coupled by axial bonding.

[0043] Furthermore, a light source is included. The tapered nano-optical fiber 1-1 is arranged in the light path direction of the light source. Preferably, the supercontinuum output spectrum of the light source is 400nm to 2400nm.

[0044] Further, preferably, the initial diameter of the tapered nano-fiber 1-1 is 1 μm, the average variation angle of the tapered nano-fiber 1-1 is ≥0.05° and ≤5°; and the diameter of the uniform nano-fiber 1-2 is 0.6 μm.

[0045] Further, preferably, the coupling length between the tapered nano-fiber 1-1 and the uniform nano-fiber 1-2 is 0-300 μm, and the lateral misalignment width between the tapered nano-fiber 1-1 and the uniform nano-fiber 1-2 is 0-0.5 μm.

[0046] The specific analysis is as follows:

[0047] The signal to be attenuated is input into the optical fiber 1-1 by components such as an optical fiber adapter.

[0048] Among them, Figure 2 As shown, the tapered nanofiber 1-1 and the uniform nanofiber 1-2 are prepared by the adiabatic taper method, and the tail ends of both are standard single-mode optical communication fibers, wherein: the front end of the tapered nanofiber 1-1 is a tapered nanofiber structure with a gradually decreasing diameter, and the front end of the uniform nanofiber 1-2 is a uniform nanofiber structure with a uniform diameter. The uniform fiber adopts a braided size decreasing structure to avoid the influence of end face reflection. By changing the coupling length (L) between the two nanofibers, the transmission spectrum will also change.

[0049] The coupling region is composed of the tapered portion at the front end of the tapered nanofiber 1-1 and the uniform portion at the front end of the uniform nanofiber 1-2 in an axially bonded manner. The tapered portion is arranged in parallel with the uniform portion, and is bonded by adsorption using van der Waals force, and the bracket 2-2 is adjusted on the second displacement stage to always maintain lateral bonding during the axial displacement process.

[0050] The nanofiber is made of SMF-28 standard optical communication fiber. The refractive index of the fused silica (SiO2) air cladding cylindrical nanofiber can be given by (1)

[0051]

[0052] Where λ is the wavelength in μm, and n1 is the refractive index of SiO2.

[0053] Among them, using formula (1), the cutoff diameter (D) of the fundamental mode at different wavelengths can be expressed as

[0054] Among them, using formula (1), the cutoff diameter (D) of the fundamental mode at different wavelengths can be expressed as

[0055]

[0056] Where n2, V = 2.405 is the normalized frequency.

[0057] The first displacement stage adjustment bracket 2 - 1 and the second displacement stage adjustment bracket 2 - 2 are micro-nano displacement mechanical structures, which can provide controllable displacement with a distance of > 300 μm and a resolution of < 0.1 μm.

[0058] Among them, the nano-optical fiber works in a single-mode state to ensure high-efficiency coupling.

[0059] in, Figure 4 The schematic diagram of the relationship between the cutoff wavelength of the front end tapered region of optical fiber 1 and the position is shown. For the tapered nanofiber, the minimum diameter required to support the fundamental mode increases with the increase of wavelength.

[0060] Among them, Figure 5 As shown, since the initial diameter of the coupling process changes with the coupling length, tunable transmission can be obtained by changing the coupling length.

[0061] The specific implementation parameters are as follows:

[0062] The light source is a supercontinuum output spectrum of 400 to 2400nm;

[0063] Optical bandwidth 0.7μm (from 1.2μm to 1.9μm);

[0064] The average change angle of the tapered nanofiber 1-1 was 0.19 degrees;

[0065] The initial diameter of the tapered nanofiber 1-1 was 1 μm;

[0066] The diameter of the uniform nanofiber 1-2 is 0.6 μm;

[0067] The test coupling length is 0-300μm;

[0068] The tested lateral misalignment width is 0-0.5μm;

[0069] The specific analysis is as follows:

[0070] like Figure 3 As shown, the area between the two curves represents the single-mode condition. In order to ensure single-mode waveguiding through wavelengths from 1 μm to 2.5 μm, the diameter of the nanofiber should be less than 1.2 μm. Figure 4 As shown, the average change angle of the tapered nanofiber is 0.19 degrees, and the black arrows indicate the attenuation regions at different wavelengths, where the waveguide mode transforms into spatial light modulation, resulting in a gradual decrease in transmittance.

[0071] The transmission process was analyzed by the finite difference time domain (FDTD) method. The initial diameter of the tapered nanofiber used in the simulation was 1 μm and the change angle was 0.19 degrees. Figure 5 As shown in Figure 1, within the diameter range of 0.5 to 0.19 μm and within the wavelength range of 1.9 μm to 1.2 μm, the transmittance at different wavelengths decreases. Figure 6 As shown in the figure. As the diameter gradually decreases, the energy of the waveguide mode with a longer wavelength no longer meets the single-mode waveguide condition, resulting in a significant increase in loss. For example, light with a wavelength of 1.9μm begins to experience significant loss near the 30μm position. For a wavelength of 1.2μm, the loss occurs near the 150μm position.

[0072] Among them, the transmission characteristics of tapered nanofibers with different transmission wavelengths as the diameter changes are as follows: Figure 7 As shown. In region I, as the diameter decreases, the long-wave transmittance decreases, while the short-wave transmittance remains unchanged. In this case, the coupler can be used as a tunable short-pass filter. In region II, as the diameter decreases, the transmittance of different wavelengths decreases at almost the same rate (40 dB / μm), and the coupler can be used as an optical attenuator.

[0073] In addition, if Figure 8 , Fig. 9 As shown in the figure, when the initial diameter of the tapered nanofiber and the diameter of the uniform nanofiber are 0.9μm and 0.6μm respectively and they fit closely to each other, the input light can be coupled to the uniform nanofiber with extremely low loss (e.g., loss <-0.15dB) when the coupling length is 260μm. Assuming H is the axial misalignment distance between the two nanofibers, when H changes from 0 to 0.5μm, the transmittance changes by <0.1dB. The results show that the attenuator has good tolerance for small lateral misalignment assembly.

[0074] From the above, it can be seen that the present invention innovatively uses optical evanescent field coupling to achieve light attenuation between nanofibers. By using the evanescent coupling between tapered nanofibers and uniform nanofibers, the fiber coupling characteristics can be effectively modulated by changing the coupling length. Both simulation and experimental results show that within a wide optical bandwidth of more than 500nm, the maximum extinction ratio of the device is >15dB.

[0075] Only the preferred embodiments of the present invention are described in detail above, but the present invention is not limited to the above embodiments. Various changes can be made within the knowledge scope of ordinary technicians in this field without departing from the purpose of the present invention, and various changes should be included in the protection scope of the present invention.

Claims

1. An ultra-wideband tunable optical attenuator based on dual nanofiber evanescent field coupling, characterized in that: The invention comprises a tapered nanofiber (1-1), a uniform nanofiber (1-2), a first displacement stage adjustment bracket (2-1), and a second displacement stage adjustment bracket (2-2); the tapered nanofiber (1-1) is fixed on the first displacement stage adjustment bracket (2-1), the uniform nanofiber (1-2) is fixed on the second displacement stage adjustment bracket (2-2), and the tapered nanofiber (1-1) is coupled with the uniform nanofiber (1-2).

2. The ultra-wideband tunable optical attenuator based on dual nanofiber evanescent field coupling according to claim 1, characterized in that: The tapered nanofiber (1-1) and the uniform nanofiber (1-2) are both prepared by an adiabatic taper method; the front end of the tapered nanofiber (1-1) is a tapered nanofiber structure with a gradually decreasing diameter, and the front end of the uniform nanofiber (1-2) is a uniform nanofiber structure with a uniform diameter.

3. The ultra-wideband tunable optical attenuator based on dual nanofiber evanescent field coupling according to claim 2, characterized in that: The tail ends of the tapered nanofiber (1-1) and the uniform nanofiber (1-2) are both standard single-mode optical communication fibers, and the front ends of the tapered nanofiber (1-1) and the uniform nanofiber (1-2) are coupled by axial bonding.

4. The ultra-wideband tunable optical attenuator based on dual nanofiber evanescent field coupling according to claim 1, characterized in that: It also comprises a light source, wherein the tapered nano-optical fiber (1-1) is arranged in the light path direction of the light source, and the supercontinuum output spectrum of the light source is 400nm to 2400nm.

5. The ultra-wideband tunable optical attenuator based on dual nanofiber evanescent field coupling according to claim 1, characterized in that: The initial diameter of the tapered nanofiber (1-1) is ≤5 μm, the average change angle of the tapered nanofiber (1-1) is ≥0.05° and ≤5°; and the diameter of the uniform nanofiber (1-2) is ≤5 μm.

6. The ultra-wideband tunable optical attenuator based on dual nanofiber evanescent field coupling according to claim 1, characterized in that: The coupling length between the tapered nanofiber (1-1) and the uniform nanofiber (1-2) is 0-300 μm, and the lateral misalignment width between the tapered nanofiber (1-1) and the uniform nanofiber (1-2) is 0-0.5 μm.

7. The attenuation method of an ultra-wideband tunable optical attenuator based on dual-nano-fiber evanescent field coupling according to any one of claims 1 to 6, characterized in that: The following steps are involved: S1, the optical signal to be attenuated is transmitted via the tapered nano-optical fiber (1-1), is transmitted to the uniform nano-optical fiber (1-2) at the front end of the tapered nano-optical fiber (1-1) based on the evanescent wave coupling principle, and is outputted along the tapered nano-optical fiber; The front ends of S2, the tapered nanofiber (1-1) and the uniform nanofiber (1-2) are respectively fixed on the first displacement stage adjustment bracket (2-1) and the second displacement stage adjustment bracket (2-2); S3. When the second translation stage adjustment bracket (2-2) is displaced along the optical fiber axis relative to the first translation stage adjustment bracket (2-1), the front end of the tapered nanofiber (1-1) is displaced accordingly, causing the length of the coupling region between the tapered nanofiber (1-1) and the uniform nanofiber (1-2) to change, resulting in a change in the coupled transmission spectrum, thereby achieving tunable optical attenuation.

8. The attenuation method of an ultra-wideband tunable optical attenuator based on dual-nano-fiber evanescent field coupling according to any one of claim 7, characterized in that: The method for achieving tunable light attenuation in S3 is as follows: by changing the coupling length (L) between the two nanofibers, the transmission spectrum will also change; the coupling region is composed of a tapered portion at the front end of the tapered nanofiber (1-1) and a uniform portion at the front end of the uniform nanofiber (1-2) in an axially bonded manner; the tapered portion is arranged in parallel with the uniform portion, and is bonded by adsorption using van der Waals force, and the bracket (2-2) is adjusted on the second displacement stage to always maintain lateral bonding during the axial displacement process; The tapered nanofiber (1-1) and the uniform nanofiber (1-2) are made of SMF-28 standard optical communication fiber. The refractive index of the fused silica air cladding cylindrical nanofiber is: Where λ is the wavelength in μm, and n1 is the refractive index of SiO2; The cutoff diameter (D) of the fundamental mode at different wavelengths is expressed as Wherein n2, V=2.405 is the normalized frequency; the first displacement stage adjustment bracket (2-1) and the second displacement stage adjustment bracket (2-2) are micro-nano displacement mechanical structures, providing controllable displacement with a distance of >300 μm and a resolution of <0.1 μm.

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