CN110286442B - An optical fiber coupler with adjustable coupling ratio - Google Patents

Abstract

The invention provides an optical fiber coupler with adjustable coupling ratio, which is specifically set as: a tapered optical fiber and a tapered optical fiber probe; the tapered optical fiber includes a core and a cladding wrapped on the outer surface of the core, The radius of the cladding and the fiber core changes according to the shape function of the taper; the tapered fiber probe is placed in the tapered waist region of the tapered fiber, and the shape is the tapered fiber after breaking, and the tapered fiber probe and the tapered fiber are in the same position. The tapered waist region is adsorbed by van der Waals force and electrostatic force. There is no need to fuse the fibers according to a fixed angle. By means of adsorption, a coupling area is formed in the tapered waist area. By adjusting the angle between the probe and the tapered fiber tapered waist area, the coupling ratio between the two fibers can be changed. ; The included angle and the coupling ratio are approximately linear; when the included angle is 30°, the coupling ratio is approximately 50%:50%. The optical fiber coupler using this structure can ideally adjust or control the optical signal and reduce the complexity and cost of the optical fiber coupler.

Description

An optical fiber coupler with adjustable coupling ratio

technical field

The invention relates to the field of lasers, in particular to a fiber coupler capable of realizing adjustable laser output and coupling ratios.

Background technique

Optical fiber coupler (Coupler), also known as splitter (Splitter), connector, adapter, optical fiber flange, is a component used to realize optical signal splitting/combining, or used to extend optical fiber links, belonging to optical passive In the field of components, it is used in telecommunication networks, cable television networks, subscriber loop systems, and local area networks. It is one of the most important optical passive devices with multiple functions, multiple uses. With the in-depth study of optical fiber couplers, there are three main manufacturing methods: polishing method, etching method and fusion taper method. Fused cone drawing method is widely used because of its simple operation method, low fabrication cost and low device loss.

With the rapid development of fiber couplers, polarization-maintaining fiber couplers also began to appear. The polarization-maintaining fiber coupler has the advantages of low polarization crosstalk, low additional loss, and keeping the polarization state of linearly polarized light unchanged during transmission. In recent years, in order to vigorously develop and research polarization-maintaining fiber couplers, the country has invested a lot of money, and a large number of scientific researchers have joined the research team of polarization-maintaining fiber couplers. In the future, polarization-maintaining fiber couplers will develop rapidly and be widely used. At the same time, single-mode polarization fiber coupler, as a polarization-maintaining fiber coupler, also has good performance advantages, which can effectively solve the problems of polarization crosstalk, polarization dependent loss and polarization mode dispersion, and improve the stability of optical communication system.

Besides the above-mentioned polarization-maintaining fiber couplers, one of the most important devices in fiber systems is the directional couplers based on photonic crystal fibers. In recent years, the research on photonic crystal fiber couplers has aroused the interest of many researchers and achieved certain results. According to the structure of photonic crystal fiber couplers, it can be divided into three categories: fused cone type photonic crystal fiber couplers, side polished photonic crystal fiber couplers, and double-core or multi-core photonic crystal fiber couplers. In order to use photonic crystal fibers in communication systems, it is necessary to provide some basic fiber components of photonic crystal fibers. Therefore, the future prospects of photonic crystal fiber couplers must be very impressive.

In recent years, a new type of fiber coupler, the fiber grating coupler, has appeared. Fiber Bragg grating couplers can be easily designed for selective wavelength operation, especially for coarse wavelength division multiplexing systems. Single-mode fibers form long-period fiber grating devices, which allow strong optical coupling from a core mode to a set of cladding modes selected at specific wavelengths, acting as band-stop filters. The early research on grating fiber couplers mainly introduced Bragg gratings in waveguides or fiber couplers to achieve wavelength selectivity. A disadvantage of Bragg grating devices is that they work in a reflective state, and when the reflected signal is recovered, unnecessary optical feedback is generated, resulting in additional losses. On the other hand, long-period fiber grating couplers work without reflection problems during transmission. These advantages greatly promote the wide application of fiber grating couplers, and the potential market is huge.

Fiber optic couplers are becoming more and more important and will become an indispensable part of the field of fiber optic communication and sensing. Various fiber couplers with excellent performance, such as polarization fiber couplers and photonic crystal fiber couplers, are developing rapidly and have a promising prospect.

Single-mode fiber couplers are the most widely used. 2x2 single mode fiber couplers are typical. It has two inputs and two outputs with a coupling section in between. Made by melt stretching process. The stretched portion forms a tapered coupling region. Stretching expands the optical energy in the core region out of the core; at the same time, the two cores are brought closer to each other, both of which enhance the coupling. The input optical signal is distributed through the tapered coupling area and then output from the output end. The optical fiber embedded in the quartz block with a certain curvature is often used to remove part of the cladding, and then two such quartz blocks are closely attached to make the two cores close to each other to form a fiber coupler.

Patent No. 2014108108669 proposes an optical fiber fused device based on a taper method and a manufacturing method thereof. According to a preset diameter of a single-mode fiber, a taper method is used to obtain a single-mode fiber with a preset diameter; The diameter of the single-mode fiber is tapered according to a preset angle to obtain a single-mode fiber cone of a preset angle; the single-mode fiber cone of the preset angle and the few-mode fiber are bonded and fused to obtain an optical fiber. coupler. The obtained single-mode fiber with a preset diameter is taper-drawn according to a preset angle to obtain a single-mode fiber taper with a preset angle.

In the prior art, the optical fiber coupler device is ground, polished, embedded or welded by a preset angle. Once it is shaped, its parameters cannot be changed, its mode field and coupling ratio cannot be adjusted, and its high wavelength dependence cannot be avoided.

SUMMARY OF THE INVENTION

The invention solves the technical problem that the coupling ratio of the existing optical fiber coupler is not adjustable, and provides an optical fiber coupler with adjustable coupling ratio. The optical fiber coupler with this structure can ideally adjust or control the optical signal.

For achieving the above object, the present invention provides the following scheme:

An optical fiber coupler with adjustable coupling ratio, comprising: a tapered fiber and a tapered fiber probe; the tapered fiber includes:

The core and the cladding wrapped on the outer surface of the core; the radii of the cladding and the core change according to the shape function of the taper; the tapered fiber probe is a taper fiber with an outer tapered structure at one end ;

The tapered fiber probe is placed in the tapered waist region of the tapered fiber; the tapered fiber probe and the tapered fiber are adsorbed by van der Waals force and electrostatic force at the tapered waist region.

The tapered fiber probe and the tapered fiber are arranged in parallel at the tapered waist region or the tapered fiber probe surrounds the tapered fiber.

The tapered fiber probe includes a core and a cladding wrapped around the outer surface of the core;

The shape of the tapered fiber probe is a broken tapered fiber.

Both the tapered optical fiber and the tapered optical fiber probe are made by a single-mode optical fiber fusion taper method, and the tapered optical fiber is

The diameter of the cone waist is less than or equal to 5 μm.

Preferably, the tapered optical fiber has an initial radius of 62.5 μm, a core radius of 4.1 μm, a taper length of 14 mm, a cladding radius of the tapered waist region of 2.765 μm, a core radius of 0.1814 μm, and a cladding radius of 0.1814 μm. The refractive index is 1.4629 and the core refractive index is 1.4682.

In the preparation method of the tapered optical fiber probe, the tapered optical fiber after being taper is pulled or truncated.

Optionally, the tapered optical fiber probe is placed in the tapered waist region of the tapered optical fiber, and the overlapping length is 2 mm.

Optionally, the center wavelength of the incident light of the tunable coupling ratio fiber coupler is 1550 nm or 1064 nm. .

The coupling ratio adjustable fiber coupler adjusts the light intensity coupled into the probe fiber by adjusting the angle between the probe and the tapered waist region of the tapered fiber, thereby adjusting the coupling ratio of the fiber coupler.

Optionally, the angle α between the tapered fiber probe and the tapered waist region of the tapered fiber varies from 5° to 90°.

According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects: the present invention provides an optical fiber coupler with adjustable coupling ratio. There is no need for traditional grinding and splicing, and there is no need to splicing and splicing the optical fiber according to a fixed angle. It only needs to adsorb the tapered fiber probe to the tapered waist region of the tapered fiber with van der Waals force, and then the tapered fiber can be fused at the secondary tapered waist. The coupling ratio between the two fibers can be changed by adjusting the included angle between the probe and the tapered waist region of the tapered fiber; the included angle and the coupling ratio are approximately linearly related; when the included angle is 30°, the coupling ratio is approximately 50%:50%. The optical fiber coupler using this structure can ideally adjust or control the optical signal and reduce the complexity and cost of the optical fiber coupler.

Description of drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings required in the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some of the present invention. In the embodiments, for those of ordinary skill in the art, other drawings can also be obtained according to these drawings without creative labor.

1 is a structural schematic diagram of a coupling ratio adjustable fiber coupler provided by the present invention;

2 is a schematic diagram of a tapered fiber probe surrounding the tapered fiber;

Figure 3 is the mode field distribution diagram of different included angles, in which: (a) is the mode field distribution diagram of the included angle of 5°, (b) is the mode field distribution diagram of the included angle of 30°, and (c) is the mode field of the included angle of 70°. Distribution map, (d) is the 90° mode field distribution map;

Fig. 4 is the optical field intensity distribution diagram of the optical fiber coupler provided by the present invention with an included angle of 30°;

FIG. 5 is a relationship curve between the included angle and the coupling ratio provided by the present invention.

Detailed ways

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, but not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

The purpose of the present invention is to provide an optical fiber coupler with adjustable coupling ratio. The optical fiber coupler with this structure can ideally adjust or control optical signals and reduce the complexity and cost of the optical fiber coupler.

In order to make the above objects, features and advantages of the present invention more clearly understood, the present invention will be described in further detail below with reference to the accompanying drawings and specific embodiments.

FIG. 1 is a structural schematic diagram of a coupling ratio adjustable fiber coupler provided by the present invention. As shown in FIG. 1, a coupling ratio adjustable fiber coupler includes: a tapered fiber 1 and a tapered fiber probe 2; The tapered optical fiber 1 includes a core 1-1 and a cladding 1-2 wrapped around the outer surface of the core 1-1; the radii of the cladding 1-2 and the core 1-1 are in a tapered shape function changes; the tapered fiber probe 2 includes a core 2-1 and a cladding 2-2 wrapped on the outer surface of the core 2-1; the tapered fiber probe 2 is placed on the tapered fiber The tapered waist region of 1 is in the shape of a broken tapered fiber; the tapered fiber probe is placed in the tapered waist region of the tapered fiber; the tapered fiber probe and the tapered fiber are at the tapered waist region adsorption by van der Waals forces.

The working principle of the above-mentioned coupling ratio adjustable fiber coupler is: as shown in Figure 1, it is realized by using the transmission characteristics and coupling characteristics unique to the tapered micro-nano fiber. The ordinary single-mode fiber is tapered into a tapered fiber by the fusion taper method, where r ₀ is the radius of the single-mode fiber, l is the pre-tapered length of the single-mode fiber, L ₀ is the initial length of the single-mode fiber before tapering, ε is the retraction factor, r _W is the diameter of the taper waist region, assuming that the taper waist region diameter L _W of the tapered fiber is uniform.

A tapered fiber probe was prepared by tapering by the fusion tapering method again, and the probe was placed on the beam waist region of the tapered fiber for spectral coupling. The relationship between the drawn length of the tapered fiber made by the fusion taper method and the fiber radius r is:

Tunable fiber couplers mainly use the coupling between two tapered fibers to achieve optical signal attenuation. Therefore, the attenuator model is theoretically analyzed by the coupled wave theory. Considering that both the bi-tapered fiber and the probe-type tapered fiber are single-mode step-weak guide fibers, which meet the conditions of local mode coupling, the theory of local mode coupling can be used for theoretical analysis. Under the approximation of weak conduction and weak coupling, ignoring the self-coupling effect, assuming that the fiber has no absorption loss, the coupling equation is:

是光纤在鼓励状态下的纵向模传播常数，

是耦合系数，实际情况下可认为

和

的值是相等的，求解得：where A(z) and B(z) are the mode field amplitudes of the two fibers,

is the longitudinal mode propagation constant of the fiber in the encouraged state,

is the coupling coefficient, which can be considered in practice

and

are equal, and solve for:

其中，

in,

The coupling coefficient is:

where ρ is the fiber radius; d is the distance between the centers of the two fibers; U is the transverse propagation constant of the core; W is the transverse attenuation constant _of the cladding; V is the normalized frequency of the isolated fiber _; Order-modified Bessel functions of the second kind. The power distribution of the coupler is given by:

F2 is the maximum coupling power between the ^two fibers. According to the above equations (6) and (7), it can be found that the power of the coupling region is periodically exchanged. This shows that by choosing an appropriate interaction length, arbitrary power distributions between the two interacting waveguides can be achieved.

When the transmittance of the coupling optical system is not considered, the coupling efficiency mainly depends on the overlapping area of the incident light field distribution in the base film field distribution in the single-mode fiber, and the integral area is the entire coupling surface. The coupling ratio of an optical attenuator is defined as the ratio of the power left in the tapered fiber to the transmitted power coupled into the fiber probe:

E _if (r, θ) is the incident field strength, E _ff (r, θ) is the mode field distribution coupled to the biconical fiber, E _jf (r, θ) is the mode field distribution coupled to the tapered probe, r is the tapered fiber radius and θ is the angle of incidence. It can be seen from the above analysis that the coupling ratio has a great relationship with the tapered fiber radius r and the incident angle θ.

There are three main preparation methods for tapered optical fibers: polishing method, etching method and fusion taper drawing method. The characteristics of the optical fiber taper made by the fusion drawing method are that the diameter of the cladding and the core of the optical fiber are gradually tapered along the axial direction of the optical fiber. It can generally be considered that the ratio of the diameters of the cladding to the core remains constant throughout the tapered waist region. The tapered fiber probe quickly pulls or truncates the tapered fiber, and the section naturally forms a smooth plane. This method of making optical fiber tapers is easy to control, has good repeatability, and has a smooth surface after tapering, so it is an ideal manufacturing method.

The invention selects the SME-28e single-mode fiber produced by Corning as the tapered fiber, the cladding radius R ₁ =62.5μm, the core radius R ₂ =4.1μm, and the tapered length l=14mm. Through the calculation of the taper shape function, the cladding radius R ₁₁ =2.765μm, the core radius R ₂₁ =0.1814μm, the cladding refractive index n ₁ =1.4629, the core refractive index n ₂ = 1.4682, the probe is the same size as the tapered fiber. Using Mode solution as the simulation software, the entire coupling length was calculated using the EME resolver, the EME method is a fully vector and bidirectional technique to solve Maxwell's equations. The method relies on the modal decomposition of the electromagnetic field into a fundamental set of eigenmodes, which is computed by dividing the geometry into elements and then solving for the modes at the interfaces between adjacent elements. Select the incident light with a wavelength of λ=1550nm from port 1 (as shown in Figure 1) into the core of the tapered optical fiber, and set up a monitor to observe the field distribution of the coupler.

In order to increase the contact surface, the tapered fiber probe surrounds the tapered fiber several times, as shown in Figure 2, to obtain a better adsorption effect.

FIG. 3(a) to FIG. 3(d) are the mode field distribution diagrams provided by the present invention with included angles of 5°, 30°, 70°, and 90°, respectively. It can be seen from Figure 3 that as the included angle increases, the light field coupled into the probe gradually weakens. When the included angle α increases to 70°, as shown in Figure 3(c), it can be seen that in addition to the coupled mode field distribution in the part where the probe is close to the tapered waist region of the tapered fiber, the part of the fiber where the probe is raised There is no light field distribution, i.e. there is no light output from the fiber tip. It can be seen from Figure 3(d) that when the included angle α is greater than 70°, there is no light field distribution in the raised part of the probe. The critical angle of the light field output can be determined to be approximately 60° through the mode field distribution simulation.

FIG. 4 is a light field intensity distribution diagram of a fiber coupler with an included angle of 30° provided by the present invention. Taking the angle α=30° as shown in Figure 3(b) as an example to calculate the coupling ratio, the coupling mode field distribution curve of the fiber coupler is shown in Figure 4. The left area is the mode field of the tapered fiber, and the right area is the mode field of the tapered probe. They are respectively integrated and compared to obtain the coupling ratio of the fiber coupler. When the included angle α=30°, the coupling ratio is 1.05, indicating that the optical field in the probe is the same as that of the tapered fiber (ie, the coupling ratio is approximately 50%:50%).

FIG. 5 is a relationship curve between the angle α between the probe provided by the present invention and the tapered waist region of the tapered optical fiber and the coupling ratio η. It can be seen that as the angle α increases, the coupling ratio η also increases, and the curve is fitted linearly. It can be seen that the relationship between the angle α and the coupling ratio η is approximately the same as the straight line y=0.48843+0.02038x, Then it can be concluded that the new fiber coupler with this structure can perform linear modulation of light. Therefore, the tunability of the fiber coupler can be achieved by changing the angle to achieve the intensity of light coupled into the probe fiber.

The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments, and the same and similar parts between the various embodiments can be referred to each other.

In this paper, specific examples are used to illustrate the principles and implementations of the present invention. The descriptions of the above embodiments are only used to help understand the methods and core ideas of the present invention; meanwhile, for those skilled in the art, according to the present invention There will be changes in the specific implementation and application scope. In conclusion, the contents of this specification should not be construed as limiting the present invention.

Claims (9)

1. A tunable coupling ratio fiber optic coupler, comprising: a tapered optical fiber and a tapered fiber probe;

the tapered optical fiber comprises a fiber core and a cladding wrapped on the outer surface of the fiber core; the radius of the cladding and the fiber core of the fiber-reinforced composite material is changed according to a tapering shape function;

the tapered optical fiber probe is a tapered optical fiber with an outer tapered structure at one end;

the tapered optical fiber probe is placed in the taper waist area of the tapered optical fiber, the tapered optical fiber probe and the tapered optical fiber are adsorbed at the taper waist area through van der Waals force and electrostatic force, and the included angle α between the tapered optical fiber probe and the taper waist area of the tapered optical fiber is changed from 5 degrees to 90 degrees;

the coupling ratio is the ratio of the power remaining in the tapered fiber to the transmitted power coupled into the tapered fiber probe.

2. The adjustable coupling ratio fiber coupler of claim 1, wherein the tapered fiber probe and the tapered fiber are configured to be disposed in parallel at the waist region.

3. The adjustable coupling ratio fiber coupler of claim 1, wherein the tapered fiber probe and the tapered fiber are configured at the waist region such that the tapered fiber probe surrounds the tapered fiber.

4. The adjustable-coupling-ratio fiber coupler of claim 1, wherein the tapered optical fiber and the tapered fiber probe are both made by a single-mode fiber fused biconical taper method, and the taper waist diameter of the tapered optical fiber is less than or equal to 5 μm.

5. The adjustable-coupling-ratio fiber coupler of claim 4, wherein the initial radius is 62.5 μm, the core radius is 4.1 μm, the taper length is 14mm, the cladding radius in the taper waist region is 2.765 μm, the core radius is 0.1814 μm, the cladding index is 1.4629, and the core index is 1.4682.

6. The adjustable coupling ratio fiber coupler of claim 2, wherein the tapered fiber probe is prepared by: and (4) breaking or cutting off the tapered optical fiber after tapering.

7. The adjustable coupling ratio fiber coupler of claim 2, wherein the tapered fiber probe is disposed at the waist region of the tapered fiber, and the overlapping length is 2 mm.

8. The adjustable-coupling-ratio optical fiber coupler according to claim 2 or 3, wherein the central wavelength of the incident light is 1550nm or 1064 nm.

9. The adjustable coupling ratio fiber coupler of claim 2 or 3, wherein the coupling ratio of the fiber coupler is adjusted by adjusting the angle between the tapered fiber probe and the tapered fiber waist region to adjust the intensity of light coupled into the tapered fiber probe.

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