KR100585016B1 - Single mode optical fiber structure having high-order mode extinction filtering function - Google Patents

Abstract

BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an optical fiber for transmitting light and an optical communication field using the same, wherein the incident light is inputted by a plurality of modes from an input optical fiber, and the cross-sectional size or refractive index of the core gradually decreases when viewed sequentially in the longitudinal axis of the waveguide It has a structure consisting of a transition region, a single-mode region supporting only a single mode even in a short wavelength, and another transition region in which the cross-sectional size or refractive index of the core is restored. The optical fiber high-order mode rejection filter is characterized in that the light of the light exits the cladding in the single mode region and does not return to the core, thereby eliminating the optical power of the higher order mode and outputting the light of the unremoved base mode back to the optical fiber. to provide. This is particularly easily achieved by giving the optical fiber a tapered waist-like structure in which the diameters of the core and the cladding are reduced and then recovered, and the cladding sheath with the same or higher refractive index as the cladding outside. According to the present invention, a single mode transmission characteristic is given to an optical fiber in a wavelength band instead of a single mode, thereby providing a gain in optical signal transmission, and in particular, a short wavelength optical signal is transmitted with high speed modulation without modal dispersion. Can be.

Optical communication, optical fiber, single-mode fiber, mono-mode fiber, inter-modal dispersion, modal dispersion, mode filter, fundamental mode mode, optical waveguide, optical taper

Description

SINGLE MODE OPTICAL FIBER STRUCTURE HAVING HIGH-ORDER MODE EXTINCTION FILTERING FUNCTION}

1 is a perspective view of a single mode optical fiber with a higher order mode cancellation filtering function according to a preferred embodiment of the present invention,

2A is a graph of an impulse response curve in a single mode optical fiber to which an optical fiber high order mode cancellation filter is applied according to the present invention;

Figure 2b is a graph of the impulse response curve in a typical single mode optical fiber.

The present invention relates to the structural features of optical fiber used as a transmission medium in the field of optical communication. In particular, the present invention relates to a single mode optical fiber structure having a high-order mode elimination filtering function capable of obtaining single mode transmission characteristics in a short wavelength region using a standardized single mode optical fiber. It is about.

Optical fiber used as a transmission medium for optical communication is composed of a dielectric core and a fiber cladding having a lower refractive index than the core. The optical fiber has a cylindrical axis due to a higher refractive index of the core than the cladding. Light can be transmitted in the direction.

Optical fibers used in optical communication may be divided into a single mode optical fiber having one fundamental mode and a multimode optical fiber having a plurality of modes in an optical axis length direction or an axis perpendicular to the propagation direction of light. In the case of single-mode fiber, the optical signals transmitted are all made by one base mode, so there is no modal dispersion due to time delay between modes, thus having a wide bandwidth. Single-mode fiber is currently the main transmission medium for long-distance optical communications, and its utilization is increasing in short-distance optical communications in recent years.

Fibers are characterized by normalized frequency or V-number. For an optical fiber having a step-index profile, the refractive index of the core is n ₁ , the refractive index of the cladding is n ₂ , the radius of the core is a and the wavelength of the light source is λ. It can be expressed as.

If V is less than 2.405, there is only one transmission mode in the core. In other words, if the wavelength is longer than this in a given optical fiber condition, there is a specific cutoff wavelength λ _c which becomes a single mode condition.

Therefore, single-mode optical fibers usually use longer wavelengths than the cut-off wavelength, and even short-wavelength single-mode optical fibers become multi-mode optical fibers that exhibit the characteristics of mode dispersion (Ref. K. Okamoto, "Fundamentals of Optical Waveguide ", Academic Press, San Diego, 2000).

Conventional single mode optical fiber mainly uses a light source having a cutoff wavelength of about 1.1 to 1.2 m and a wavelength of 1.3 m and 1.5 m. Hereinafter, such an optical fiber will be referred to as a "standard single mode optical fiber". If the radius of the core is smaller than that of the standard single-mode fiber, the cutoff wavelength is shortened, and thus the single-wavelength single-mode fiber is not widely used. The short wavelength refers to a cutoff wavelength of a standardized single mode optical fiber, a wavelength shorter than 1.1 to 1.2 mu m, especially a wavelength between 600 and 1000 nm for which a semiconductor laser light source can be easily obtained.

When a standard wavelength of single-mode fiber is shorter than the cutoff wavelength, for example, 850 nm, the standard single-mode fiber has multiple modes. If the V-number is between 2.4 and 3.3, there are two modes, LP ₀₁ and LP ₁₁ , in terms of linearly-polarized mode. If V is greater than 3.3 and between 5.0, LP ₀₁ , LP ₁₁ , LP ₀₂ , LP ₂₁ , and 4 modes are available.

Since the wavelength and the V-number are inversely related, a single-mode fiber with a blocking wavelength of 1.3 μm will have a V of 4.8 at 650 nm. Thus, a standardized single mode fiber will have a V less than 4.8 at 650 nm wavelength and be approximately between 4.5 and 4.8. Four LP modes are possible in this area. At 850nm, V-numbers range from 3.4 to 3.7, enabling three LP modes.

As such, a standard single-mode fiber will have less than four modes at short wavelengths.

On the other hand, if the light propagating through the optical fiber in a short wavelength is always implemented in a single mode, the following advantages can be obtained.

Waveguides of light in a single mode, especially in ground mode, through fiber optics allow long-distance transmission without compromising the quality of the beam. In particular, the base mode of the optical fiber having a cylindrical structure has a vertical electric field distribution similar to that of a Gaussian beam, which can be processed without distortion through the lens, and since there is no noise added by the coupling between modes, It can be usefully applied to the connection of the optical signal. In addition, since there is no problem of modal dispersion caused by the propagation time difference between modes, there is an advantage that it can be variously used in the field of optical communication.

Conventional methods for directing short wavelength light through a fiber optic to single mode, in particular base mode, are as follows.

The first is to use a new type of single-mode fiber with shorter wavelengths.

Fibers that are monomode at short wavelength can be fabricated by reducing the radius of the core or by reducing the refractive index difference between the core and the cladding.

However, this method has a problem in that it is necessary to prepare a special non-standard optical fiber which is difficult to obtain on the market. In addition, since the bend loss for long wavelength light of 1.5 μm or more is large, the use of long wavelength is bound to be abandoned even if a single mode transmission characteristic is obtained at short wavelength.

The second way to send short wavelength light through a single fiber is to remove the optical power from modes other than the base mode, that is, higher-order modes, through a special optical device.

In multimode transmission with as few as four modes, there is no inter-mode coupling in the optical fiber itself due to the large difference in effective index between modes. Therefore, if the higher-order mode except base mode can be eliminated, the transmission system is substantially single mode transmission. In this regard, it is possible to use a method of selectively removing the higher order mode from the optical fiber.

Although the fiber itself has multimode at short wavelengths, it can be called pseudo monomode transmission because the higher order mode rejection filter functions so that the actual optical signal is transmitted only through the base mode. This method is economical in that it does not require a new kind of optical fiber but only adds the proper higher-order mode elimination means to the existing standard single-mode fiber to obtain the characteristics of single-mode transmission in the short wavelength region.

In order to remove the higher-order mode, a conventionally proposed method is a removal method by bending first (see US Patent No. 5,013,118 "Filtering high order modes of short wavelength signals propagating in long wavelength single mode fibers"). The loss due to bending is higher in the mode with the lower effective refractive index, so that the higher order mode has the larger bending loss. This method is easy to implement because you only need to determine the appropriate amount of bending.

However, the long wavelength base mode also has a large bending loss compared to the short wavelength base mode. For example, bending to remove the higher order mode of 850 nm will inevitably cause loss even in light that is guided to the 1550 nm base mode.

Another method for removing the higher order mode is the removal method using optical coupling characteristics. Devices such as fiber-optic couplers and fiber grating devices all rely on these optical coupling characteristics, which allow you to combine only the lower or higher order modes into the desired mode. It can be designed to have the characteristics of higher-order mode rejection. For example, using the technique of optical coupling such that only the higher order modes are combined with the cladding mode, the higher order mode can be removed from the core.

This method has advantages in that it is easy to apply the technology, but it does not fully consider that the nature of the optical coupling is fundamentally wavelength dependent. For example, when the wavelength exhibits an ideal higher-order mode rejection at 850 nm, it will not be possible to completely remove the higher-order mode at 650 nm, which inevitably has the potential to eliminate long wavelengths, for example at 1550 nm base mode optical power. .

As described above, in the conventional method for transmitting light having a short wavelength in a single mode through an optical fiber, there are fundamental problems as described above, and therefore, more specific design and implementation techniques are required to minimize the problems.

 SUMMARY OF THE INVENTION The present invention has been made in view of the above-described requirements, and has a higher order mode rejection filtering function that effectively removes the higher order mode in a wider wavelength range and has a problem that the transmission capability of a single mode is longer in the longer wavelength region than the blocking wavelength. Its purpose is to provide a single mode optical fiber structure.

According to a preferred embodiment of the present invention for achieving this object, in an optical waveguide consisting of a core to which light is guided and a cladding portion having a lower refractive index than the core, a plurality of modes from an input single mode optical fiber for injecting light Light is incident through the transition region where the cross-sectional size of the core or the refractive index of the core or the cross-section and the refractive index are reduced together as the incident light propagates through the core, The single-mode region is reduced to the stage with only a single mode, and finally another transition region where the cross-sectional size of the core or the index of refraction of the core, or both, gradually increases and returns to its original value. Incident on the output single-mode optical fiber In the structure, the higher-order modes, except the incident base mode, are either cladding modes that are guided by the cladding or unguided radiation modes in the single-mode region, so that they do not re-enter the core again, so that higher-order modes It provides a single mode optical fiber structure having a high order mode rejection filtering function to remove light guided by.

In addition, the optical fiber structure according to the present invention is a single optical element connected to the input optical fiber and the output optical fiber at both ends thereof, and light is incident on the input optical fiber in multiple modes, whereas the light is no longer in the higher order mode on the output optical fiber side. It functions as a base mode pass filter that passes only the base mode without being loaded.

Hereinafter, with reference to the accompanying drawings will be described in detail a preferred embodiment of the present invention.

Prior to the description, the waveguide structure of the filter according to the present invention may be considered in various ways, but in order to avoid mode coupling during incidence, it is preferable to have a structure such as an input or output single mode optical fiber at the input and output portions of the transition region. Thus, the most ideal waveguide structure is the optical fiber itself to be used in conjunction with the higher order mode rejection filter proposed by the present invention. Also, realistically, the input single mode optical fiber and the output single mode optical fiber may be considered only if they are the same kind of optical fiber.

Such a filter can be fabricated by fabricating the same kind of single mode fiber as the input or output single mode fiber, and this method would be ideal. In particular, the progressive reduction of the cross-sectional size of the optical fiber core can be easily manufactured by secondary processing the already produced optical fiber, and in this way it is economical to create a structure for the higher-order mode removal proposed by the present invention.

In the present invention, a tapered waist-shaped structure is formed by pulling up and stretching in both directions while applying heat to a portion of a single mode optical fiber. In other words, the shape of the mortar is gradually reduced in the longitudinal axis and then recovered again.

This tapered waist structure can have a single mode region at the center and transition regions on both sides thereof. This tapered waist structure has a core and cladding structure, and each of the refractive indices is about the same as before. However, the radius of the core is reduced, which in turn reduces the waist V-number.

Consideration for creating a single-mode region is the minimum fiber diameter at the waist center. In order to bring the waist V-number into a single mode at 650nm wavelength, the core and cladding radius of the waist are about half as much as before.

Short wavelengths of light in this single mode region also have only a base mode in the core. The remaining modes are all combined in the cladding mode which is guided by the interface of the cladding. Higher modes, once removed by cladding, are less likely to recombine back into the core.

However, there is some possibility of recombination to the core through the transition region again, and since this recombination has wavelength dependency, the characteristic of higher-order mode elimination becomes wavelength dependent. Therefore, the process of reliably removing the higher order modes that become the cladding mode in the single mode region will have a more reliable higher order mode elimination characteristic.

Surround the cladding at the waist with a cladding coating, a material of the same or higher refractive index than the cladding, and remove the cladding modes towards the cladding sheath, where the attenuated action of scattering, absorption, etc. is removed. Can eliminate the possibility of recombination into the core.

That is, the higher order mode elimination method of the present invention maintains the difference between the original optical fiber structure and the refractive index at the intermediate point of the single mode optical fiber, but has a tapered waist shape in which the length is gradually reduced to its diameter axis. By creating a structure and covering the waist-shaped cladding outer wall with a material with a higher refractive index or the same material as the cladding, higher-order modes propagating within a single-mode fiber pass through this waist and are more likely due to the reduced core radius of that part. It is characterized in that the core mode guided by the abnormal core cannot be removed and the cladding and the cladding skin are removed.

At this point, the optical signal once removed by the cladding jacket cannot return back to the core, whereas the base mode of the core is almost unaffected as it passes through this narrow waist. Here, at the center of the narrow waist, the value of the lowest radius of the fiber is such that the light of short wavelength to be used can only excite the base mode to the core for this lowest radius value, i.e. the value of V-number less than 2.405. Should be smaller than the radius of the fiber.

Advantages of the method according to the present invention over the method of removing the higher order mode by bending or mode combining method are as follows.

First, the tapered waist is simple to manufacture because it can be produced by applying appropriate heat (about 1400-1700 ° C) to the optical fiber and stretching it from both sides.

Secondly, this method takes advantage of the fundamental characteristics of the base mode and the higher order mode of the core and has little or no dependence on wavelength.

Third, its operation is reliable without problems such as the temperature dependence seen by devices, often by mode coupling.

Fourth, the narrow waist has a short length of about 5 to 20 cm and can be manufactured as a small element, so its volume and weight are small.

Hereinafter, with reference to the accompanying drawings will be described a preferred embodiment of the present invention.

1 is a cross-sectional view of an optical fiber having a high order mode cancellation filtering function according to the present invention.

The optical fiber having the higher order mode elimination filtering function in FIG. 1 is basically an optical fiber device made using a single mode optical fiber, and includes a cladding 10 and a core 12 having standard single mode optical fiber characteristics. , A cladding mode removal envelope 14 having a refractive index similar to or slightly higher than cladding 10.

As shown, the optical fiber according to the present embodiment is characterized in that the intermediate point of the standard single-mode optical fiber having the core 12 and the cladding 10 is formed in a tapered waist shape.

If the optical fiber structure in the figure is classified by function, part I is the input fiber to which light is incident, A is the first transition region, B is the single mode region, C is the second transition region, and O is the higher order. It is an output fiber that emits light output without the mode.

The dotted line arrow 16 indicates the flow of the hypothetical base mode by the geometric optical diagram, and passes from the input optical fiber to the output optical fiber without changing the characteristics.

The dashed line arrow 18 denotes the flow of the higher order mode, which is removed by the cladding 10 in the single mode region and then removed by the cladding mode elimination envelope 14 and then dissipated through scattering and absorption in its interior and surface. do.

When the diameters of the core and the cladding before the tapered waist shape are d and D, respectively, the core and the cladding diameters in the waist portion, that is, the single mode region B, are reduced to d _w and D _w . The ratio of core diameter to cladding diameter in the waist is the same as in the absence of waist. That is, d _w / D _w = d / D. This is also referred to as 'reduction rate'.

This waist structure can be realized by giving a high temperature of more than 1400 degrees to the midpoint of a standard single-mode fiber and pulling it slightly on both sides of the fiber. At this time, the width of the waist is the length of both ends of the point where the diameter of the optical fiber begins to decrease may be denoted as W as in FIG. The width of the waist and the core diameter of the waist can be determined by how wide the high temperature is given to the optical fiber during manufacture and how long it is pulled on both sides.

In the manufactured waist as shown in FIG. 1, the V-number is reduced at the same rate as the core diameter is reduced. In other words, the normalized frequency, V, decreases in proportion to the reduction ratio.

This allows the waist to have a single-mode condition for short wavelengths around 650 nm. For a standard single-mode fiber with a cutoff wavelength at 1.2 μm, d _w becomes half of d and becomes single mode at a wavelength above 600 nm. Therefore, considering the use of shorter wavelengths from 650nm to 890nm, the core and cladding diameters should be reduced to half at the waist.

Since the main short wavelength to be used is 850 nm, the minimum fiber diameter at the waist center should be less than 3/4 of the fiber diameter before reduction.

In this waist portion, the higher order modes become the cladding mode. If the cladding modes are not removed, these cladding modes can recombine with the core's mode as they pass back around the waist. To prevent this, the cladding mode must be removed from the waist.

Since the skin 13 has the same or higher refractive index than the cladding 10, the modes of the cladding 10 are stripped out toward the skin 13. Care must be taken in the shape and material of the shell 14 to prevent such light from re-entering the core in combination with the mode of the core.

First, the diameter of the cladding sheath 14 is sufficiently larger than twice the cladding to lower the bond with the core mode. In addition, it is preferable that the cladding is arranged in an eccentric form at the center of the cladding sheath 14 so that the arrangement of the cladding sheath 14 and the cladding does not have rotational symmetry. The same effect occurs when the shape of the outer surface of the cladding skin 14 does not have rotational symmetry.

As such, the shape of the cladding sheath 14 or the placement of the fiber within the cladding sheath 14 may be designed to minimize the probability of light re-entering the fiber core using mode coupling theory.

Second, if the material of the cladding jacket 14 is a material that absorbs or scatters light well, light from the cladding jacket 14 may be removed before being combined with the core mode. The same effect can be obtained by roughly treating the surface of the cladding shell 14 so that scattering occurs well.

In the transition regions A and C, the diameter of the core gradually decreases and then increases in the longitudinal direction. At this time, the rate of change of the core diameter should be quite small so that the base mode does not lose. The length of the transition region should be at least 100 times longer than the wavelength and should be at least 100 times the length of the transition region.

On the other hand, the longer the length of the single-mode region, the more surely the higher-order mode will be eliminated, but it is better not to be too long because the base mode can also lose the single-mode region. Therefore, the width of the waist, W should be at least 200μm and substantially 1 to 10mm is preferred.

The loss of the ground mode is very small if the length of the waist is long enough and thus the angle of inclination, which reduces the diameter to make the waist, is small enough. Therefore, chromatic dispersion or non-linear optical characteristics in this section can be neglected, so this waist structure does not degrade the short wavelength and the multi-wavelength transmission capability of the long wavelength. However, it may cause some optical power loss.

Such a higher order mode elimination filter may be attached to a point of a single mode optical fiber to be used in the same manner as fusion splicing. Melt splices are commonly used in fiber optic connections, with alignment errors of less than 0.5 μm. Due to this small alignment error of the melt joint, it is very unlikely that the removed higher order mode is supplied with optical power again by mode coupling with the base mode. Therefore, the higher-order mode elimination filter can be easily attached to a single mode fiber in this manner.

FIG. 2 shows an impulse response curve graph (a) with the higher-mode cancellation filter presented by the present invention in a single mode optical fiber link of 800m length for 790nm wavelength, and an experiment showing the impulse response when the present filter is not installed. Result graph (b).

An impulse response is a time signal input from an optical signal pulse having a very short time width, passed through a 800m long single mode optical fiber, and received by a high speed optical receiver.

In graph (b) without using a higher order mode rejection filter, the optical signal propagates through multiple modes (two here) in a single mode optical fiber. Since the propagation speed is different between modes, two pulses come out as an impulse response as shown in the figure, and each corresponds to one mode.

On the other hand, in the graph (a) with the high-mode rejection filter proposed by the present invention at one point (in this case, the input side) of the single-mode fiber, only one pulse is observed, which means that the lower-order mode is removed.

In the present invention, a transmission link is constructed by installing a high-order mode cancellation filter having a waist length of 1 mm and a waist width of 50 μm on a transmitting side of a standard single mode optical fiber having a blocking wavelength at 1100 nm of 800 m length. The above graph was obtained by applying an impulse optical signal having a wavelength of 790 nm and observing the response at the receiving side.

The experimental results show that the fabricated higher-order mode rejection filter completely removes the higher-order mode optical power and the base mode optical power is almost not lost. In principle, this characteristic will appear from 790nm to 1100nm, the blocking wavelength.

Experimental results also show that these filters produce less additional losses of less than 1dB in the region from 1300nm to 1600nm. That is, the filter action proposed by the present invention has no loss for the base mode of the entire wavelength region which is often considered in optical communication. In theory, the loss can be reduced by increasing the length of the transition region so that the change in diameter becomes more gradual, and can theoretically be eliminated.

According to the present invention, since the high-order mode is eliminated, the single mode optical fiber link with the filter transmits the optical signal only by the single mode transmission, so the gain by the single mode transmission is large even if some loss is obtained.

In addition, the higher-mode elimination means proposed by the present invention will lead to a dramatic increase in its information transmission capability in optical communication using a widely used standard single-mode optical fiber and short wavelength optical signal, for example, an optical signal of 850 nm. . This is possible because the transmission in single mode eliminates mode dispersion.

In optical experimental devices, especially optical measuring devices, single-mode optical fibers are of great use. This allows light to be sent over long distances without compromising the quality of the beam. Fiber optics are flexible and do not lose their ability to transmit light despite mechanical movement. And the base mode is similar to the Gaussian mode, which is good for handling through the lens. For this reason, non-standard short-wavelength single-mode fibers are being used. The high-order mode elimination filter proposed by the present invention can transmit short wavelength light in the base mode by using a standard single-mode optical fiber, which is easy to obtain, thereby significantly reducing the manufacturing cost.

As mentioned above, although this invention was demonstrated concretely based on the Example, this invention is not limited to such an Example, Of course, various deformation | transformation are possible for it within the following Claim.

Claims (10)

delete delete delete delete delete delete delete An optical waveguide comprising a core to which light is guided and a cladding having a lower refractive index than the core, An input fiber region in which light is incident through a plurality of modes, A first transition region in which light incident from the input optical fiber region propagates through the core and gradually decreases in cross-sectional size of the core; A single mode region that is reduced to the step of having only a base mode in the core, A second transition region where the cross-sectional size of the core is gradually increased to return to its original value; An output optical fiber region for outputting only light in a base mode from light output through the second transition region; A cladding jacket coated with a material having a refractive index equal to or higher than that of the cladding and the core including the input optical fiber region, the first transition region, the single mode region, the second transition region, and the output optical fiber region. Single-mode optical fiber structure having a higher-order mode cancellation filtering function comprising a. delete The method of claim 8, Wherein said cladding envelope has a property of absorbing or scattering said incident light therein or at its surface.

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