KR100578165B1 - Gain flattening filter using the fused couplers - Google Patents

Abstract

The present invention has been proposed to provide a gain equalization filter using a fusion coupler that can be manufactured at low cost and compensates the entire band flatly to improve the gain flatness of the amplifier. It is configured to connect the fusion coupler in a predetermined form, and to adjust the gain spectrum of the amplifier by adjusting the center wavelength and period of the fusion coupler, wherein at least two fused couplers are connected in a ring form or in multiple stages It is connected in series and the gain spectrum is changed as the optical signal passes through the two or more fusion combiners.

Optical communication, wavelength division multiplex transmission system. Erbium-doped fiber amplifiers, gain equalization filters, fusion combiners

Description

Gain flattening filter using the fused couplers

1 is a gain spectrum according to a wavelength of a generally known erbium-doped fiber amplifier.

2 is a block diagram of an erbium-doped optical fiber amplifier having a gain equalization filter using a fusion coupler according to the present invention.

3 is a basic structural diagram of a gain equalization filter using a fusion coupler according to the present invention.

FIG. 4 is a modeling diagram of a gain equalization filter using the fusion coupler of the present invention shown in FIG. 3.

5 (a) and 5 (b) are detailed modeling structural diagrams of the first and second fusion couplers shown in FIG. 4.

(A) of FIG. 6 shows the permeation characteristics of the fusion coupler having a center wavelength of 1533 nm and a period of 66 nm, and (b) shows the permeation characteristics of the fusion coupler having a center wavelength of 1553 nm and a period of 70 nm.

7 is a graph showing the overall transmission characteristics of the gain equalization filter of the present invention shown in FIG.

8 is a gain spectrum of an erbium-doped optical fiber amplifier having a gain equalization filter according to the present invention.

9 is a block diagram of a gain equalization filter showing another embodiment of the present invention.

10A and 10B are diagrams showing a gain equalization filter showing still another embodiment of the present invention.

11 shows the structure of a fusion bond group.

<Explanation of symbols for the main parts of the drawings>

1: erbium-doped optical fiber 2: optocoupler

3a, 3b: optical isolator 4: gain equalization filter

5: pump laser diode 6: first fusion coupler

7: second fusion coupler 11: clad

12 core 13 fusion region

The present invention relates to a gain equalization filter for flattening a non-uniform gain spectrum of an erbium-doped fiber amplifier, and more particularly, to a gain equalization filter using a fusion coupler that can be made at low cost and compensates the entire band evenly. will be.

In general, in a wavelength division multiplexing system, the output strength, gain bandwidth, and gain flatness of an optical amplifier that amplify a signal to be transmitted are the factors that have the greatest influence on transmission capacity.

Among optical amplifiers, optical fiber amplifiers are mainly used in recent years because they can amplify optical signals without converting them into electrical signals by using optical fibers as amplifying media, and inductive inelastic scattering of glass which is a main component of optical fibers In some cases, and using the induced emission action of the rare earth element added to the optical fiber.

Among these, an erbium-doped fiber amplifier (also called EDFA) using an optical fiber containing erbium (Er) ions, a rare earth element, has an amplifiable wavelength band of 1550 nm, which corresponds to the low loss band of the optical fiber. In addition, since the lifetime of the erbium ion is 10 ms and high efficiency amplification (40 dB) and low distortion characteristics can be obtained, it is widely used in wavelength division multiplexing (WDM), which has formed the mainstream of optical communication in recent years.

However, as shown in FIG. 1, the erbium-doped optical fiber amplifier has an absorption and emission spectrum maximum at 1530 nm, but has an uneven characteristic in the entire transmission band. Due to this characteristic, the transmission bandwidth is limited in a system in which several amplifiers are connected. To overcome this, a gain equalization filter for flattening the gain spectrum must be inserted into the amplifier. Was done.

A gain equalization filter using a variable attenuator used in the related art is a filter for adjusting gain for each channel, and a variable attenuator equal to the number of channels is required. Therefore, in the wavelength division transmission system operating more than 40 channels, the volume becomes large and the manufacturing cost becomes high (A.R. Chraplyvy, IEEE Photon, Technol. Lett. 4, 428 (1993)).

As a method for solving the problem of the gain equalization filter using the variable attenuator, filters using a twin core optical fiber or filters using an etching grating have been developed, but this has a disadvantage in that the manufacturing process is very complicated and mass production is difficult (G. Grasso, Optical Fiber Communications Conference, vol. 4, 1991, M. Wilkinson, Electron. Lett. 28, 131 (1992).

In addition, U.S. Patent No. 6,542,666 (patent registered on April 1, 2003, titled 'Optical Component') proposes a gain equalization filter in which gratings are engraved in an optical fiber using a long period grating or a Bragg grating of an optical fiber. have. This structure is simpler and easier to manufacture than the conventional technologies described above, but has a disadvantage in that it is very sensitive to temperature. This is because the refractive index is sensitive not only to the grating but also to the temperature when the grating is engraved on the core in the optical fiber to change the refractive index of the core to induce interference between modes. In order to solve this temperature characteristic problem, a method of engraving a lattice by adding boron to the optical fiber core has been studied, but it is difficult to guarantee the probability of the characteristic in actual mass production. (ripple) is a serious problem for long distance transmission.

Accordingly, there is a continuing need for a wavelength division multiplexing system that operates multiple channels, and a gain equalization filter capable of flattening the entire transmission band while having a low fabrication cost and a simple structure for long distance transmission.

SUMMARY OF THE INVENTION The present invention has been proposed to solve the above-mentioned problems, and its object is to provide a gain equalization filter using a fusion combiner, which can be manufactured at low cost to improve the gain flatness of the amplifier by flatly compensating the entire band. It is.

As a construction means for achieving the above object of the present invention, the present invention comprises a gain equalization filter for compensating the gain spectrum of the amplifier by connecting two or more fusion couplers in a predetermined form, the center wavelength and period of the fusion coupler It characterized in that it is configured to adjust the gain spectrum of the amplifier by adjusting the.

In addition, the gain equalization filter of the present invention may combine two or more fusion bond groups in a ring form.

In addition, the gain equalization filter of the present invention may combine two or more fusion combiners in series.

In addition, the present invention provides an erbium-doped fiber amplifier provided with a gain equalization filter using the fusion coupler configured as described above before the output terminal of the optical signal.

Hereinafter, a gain equalization filter using a fusion coupler according to the present invention will be described with reference to the accompanying drawings.

2 is a structural diagram of an EDFA having a gain equalization filter according to the present invention.

FIG. 2 shows the most basic amplifier structure. In general, the first stage of the C-band EDFA is such a structure, and the gain equalization filter 4 is the fusion according to the present invention having the configuration and operation described below. It is a gain equalization filter using a coupler.

In addition, the structure of the EDFA shown in FIG. 2 may be changed as an embodiment, and accordingly, a configuration of the gain equalization filter 4 may be partially changed. Various embodiments of the gain equalization filter 4 will be described in more detail below.

Referring to FIG. 2, the erbium-doped optical fiber amplifier is provided with an optical isolator 3a for blocking reverse reflection of the optical signal at an input terminal of the optical signal, and a pump laser diode 5 having a wavelength of 980 nm transmits the optical signal. Direction and forward direction, and combine the exit optical fiber of the optical isolator (3a) and the pump laser diode (5) into the erbium-doped optical fiber (1) through the WDM optocoupler (2), the erbium-added optical fiber (1) The output side of is connected to the gain equalization filter 4 through the optical isolator 3b, and the output side of the gain equalization filter 4 is connected to the optical signal output terminal.

In the EDFA having such a structure, when the length of the erbium-doped optical fiber 1 as an amplification medium is 16 m and the output intensity of the pump laser diode 5 is 180 mW, the gain is 20 dB. That is, when the intensity of the input optical signal is -2 dBm, the intensity of the output optical signal is 18 dBm.

By the way, in the above structure, the gain spectrum when the gain equalization filter 4 according to the present invention is not provided is as shown in Fig. 1, and the gain variation with respect to the wavelength is about 3 dB.

The gain equalization filter 4 of the present invention reduces the gain variation for each wavelength as described above, and FIG. 3 shows an example of the structure.

As shown in FIG. 3, the gain equalization filter 4 of the present invention comprises first and second fusion couplers 6 and 7 having a coupling constant of K1 and K2, respectively, in a ring shape.

Here, the coupling constant is a coefficient representing the pulsation phenomenon of energy due to the interference between the two modes, the ring-shaped structure can occur in the loop form by preventing the time difference does not occur by traveling the same distance in two directions Eliminate distortion of the signal due to time differences.

That is, in FIG. 3, the amplified optical signal input through the optical isolator 3b is incident to the second fusion coupler 7, and then transferred to the first fusion coupler 6, and thus two fusions are performed. The signal passing through the coupler (7, 6) is again output through the second fusion coupler (7).

As described above, the spectrum of the optical signal is changed by the entire transmission spectrum while passing through the first and second fusion couplers 7 and 6.

In general, as shown in FIG. 11, two optical fibers including the clad 11 and the core 12 are combined with a cladding layer at a predetermined portion, and the mode in which the core 12 is advanced is fused. In the region 13, the interference with the clad 11 mode causes energy to exchange with each other according to the phase difference.

In addition, since the fusion region 13 is made by attaching two strands of optical fibers gradually while applying heat, reflection does not occur with the optical fibers in the same direction. This means that the additional loss of fusion bond groups is very small.

In general, the process of making a fusion coupler, first, stripping the coating of the two strands of the optical fiber about 3cm, twisting the two optical fibers stripped off the coating and fixed, and then applying heat to the twisted portion Pull in. At this time, the fusion region 13, which is the distance between the two strands of optical fiber, depends on the pulling strength, and thus, adjusts the fusion distance according to the transmission spectrum.

4 shows the overall modeling structure for analyzing the transmission characteristics of the filter to know the output spectrum of the optical signal passing through the gain equalization filter 4.

In FIG. 4, an optical signal is incident to a point indicated by C and exits in three directions A, B, and D. First, the optical signal incident at the point C passes through the second fusion coupler 7 having the coupling constant K ₂ , and then proceeds in two directions, K23 and K21 (where reflection is not generated by the optical fibers in the same direction). Do not proceed in the direction of K24). The optical signal traveling in the K23 direction enters K14 of the first fusion coupler 6 having a coupling constant of K ₁ and then proceeds in the K11 direction to be outputted as point A, or K21 and the first fusion coupler 6. Passed through K24 of the two-fusion coupler (7) is output to the point D. In addition, the optical signal propagated to K21 of the second fusion coupler 7 is incident on the first fusion coupler 6 in the K12 direction, passes through the K13 direction to the point B, or exits to K14 to the second fusion. Pass K24 of combiner 7 to point D.

Therefore, at point D, the signals divided into two branches proceed as the above, and the signals are separated by the two paths. .

Next, FIGS. 5A and 5B are modeling structures for analyzing transmission spectra of the first and second fusion couplers 6 and 7, respectively.

In the first fusion coupler 6 having the coupling constant K ₁ , the electromagnetic field at each input / output point is represented as shown in Fig. 5A. In this case, the relationship between the electromagnetic fields is represented by Equation 1 below. Here, the subscript of the electromagnetic field (E) indicates the direction (position) of the electromagnetic field, and a denotes a signal exiting at each point of the fusion combiner, and t denotes a signal entering the fusion combiner.

Next, FIG. 5B shows an electromagnetic field at each input / output point in the second fusion coupler 7 having the coupling constant K ₂ , and the relationship between the electromagnetic fields is as shown in Equation 2 below.

In the gain equalization filter of the present invention, the optical fibers in the K12 and K14 directions of the first fusion combiner 6 having the coupling constant of K ₁ are connected to the optical fibers in the K21 and K23 directions of the second fusion combiner 7. Therefore, the following relation holds between them.

Therefore, by substituting the condition of Equation 3 into Equations 1 to 2, a transmission formula is obtained as in Equation 4 below.

Here, T _CD represents the degree of transmission between the points C and D. Here, the coupling constants K1 and K2 represent the degree of mode coupling due to the phase difference between the two modes, and the response to the wavelength of the single fusion coupler is expressed by Equation 5 below.

Here, Δλ represents a period and λ _q represents a center wavelength.

In addition, when the response equation for the wavelength of the fusion coupler shown in Equation 5 is substituted into the transmission spectrum of the first and second fusion couplers 6 and 7, the first and second fusions included in the gain equalization filter according to the present invention may be used. The transmission spectra of the couplers 6 and 7 are represented by Equation 6 below.

Therefore, using Equation 6, the variable of the gain equalization filter 4 can be adjusted so that the gain spectrum in the erbium-doped optical fiber amplifier of the present invention is the flatst. The design variables controlled here are the central wavelength and period of each transmission type.

For example, as a result of simulating the gain equalization filter 4 such that a conventional erbium-doped fiber amplifier having gain spectral characteristics as shown in FIG. 1 has a flat gain spectrum, a first fusion combiner having a coupling constant K ₁ ( The center wavelength of 6) was 1533 nm and the period became 66 nm. In addition, the center wavelength of the second fusion coupler 7 having a binding constant of K ₂ was 1553 nm, and the period was 70 nm.

The transmission spectrum of the fusion bond device designed as described above is as shown in (a) and (b) of FIG. The transmission spectrum of the gain equalization filter 4 formed by combining the first and second fusion couplers 6 and 7 is shown in FIG. 7.

When the transmission spectrum of the gain equalization filter 4 shown in FIG. 7 and the transmission spectrum of the conventional erbium-added optical amplifier shown in FIG. 1 are combined, the amplified signal finally output has the transmission spectrum as shown in FIG. .

Comparing the graphs of FIG. 8 and FIG. 1, it can be seen that the transmission spectrum of FIG. 8 is flatter over the entire band than in FIG. 1, and numerically, the gain flatness is improved by about 1.3 dB. .

As described above, in the gain equalization filter implemented using the fusion coupler, the two strands of optical fibers used to fabricate the fusion coupler are general single optical fibers, and since the same optical fiber is used, the refractive index changes with temperature. Therefore, even if the ambient temperature changes and the refractive index changes of the optical fiber, the phase difference between the two modes due to the temperature hardly occurs.

In addition, the gain equalization filter using the fusion coupler has a small ripple of the transmission spectrum, as shown in FIG. 7. As shown in FIG. 6, since the energy transfer due to the interference between the two modes is in the form of a sine curve, the entire transmission spectrum has a smooth curve.

In general, the overall deviation of the accumulated gain spectrum during long-distance transmission can be reduced to some extent with the Dynamic Gain Equalizer, but the gain equalization filter is difficult to compensate for the ripple of the transmission spectrum itself. The small bending of the transmission spectrum is a very important advantage.

The gain equalization filter described above can be variously modified depending on the structure of the erbium-doped fiber amplifier to be applied.

Figure 9 shows another embodiment of the gain equalization filter according to the present invention, connecting both ends of the first optical fiber of the second fusion coupler 7 having a coupling constant K ₂ at the point C and the point D, and the second Both ends of the second optical fiber of the fusion combiner 7 are connected to both ends of the first optical fiber of the first fusion combiner 6 having a bonding constant K ₁ . At this time, the signal is input to the point C, and output to the points A, B, D.

In the above, the optical signal inputted to the point C is output to the point D through the second fusion coupler 7 and / or the first fusion coupler 6 as shown in the direction of the two arrows shown in FIG. The spectral adjustment by the 1,2 fusion coupler (6,7) is similar to the operation of FIG.

10 (a) and 10 (b) show another embodiment of the gain equalization filter according to the present invention, and has a structure in which two fusion bond groups are connected in multiple stages.

First, the gain equalization filter shown in (a) of FIG. 10 connects one side of the first optical fiber of the first fusion coupler 6 having the coupling constant K ₁ to the point B, and the second of the first fusion coupler 6. One side of the optical fiber (located in a direction different from one end of the first optical fiber connected to the point B) to the point A, and the other side to one side of the first optical fiber of the second fusion coupler 7 having a coupling constant K ₂ The second optical fiber of the second fusion coupler 7 connected to the second side of the first optical fiber of the second fusion coupler 7 in the same direction as the other side of the first optical fiber connected to the point C. Configure one side of to connect to point D. At this time, the optical signal is incident to the point A, and the optical signal is output to the points B, C, and D.

The optical signal incident on the point A is transmitted to the second fusion coupler 7 through the second optical fiber of the first fusion coupler 6, and is also reflected by the second optical fiber of the first fusion coupler 7 to the point B. Is also output. The optical signal inputted to the second fusion coupler 7 is divided into points C and D through the first optical fiber and the second optical fiber positioned in opposite directions of the input direction and output.

The optical signal output from the points B, C, and D is flattened by the action of the fusion coupler as described above, compared to the optical signal incident to the point A.

Next, FIG. 10 (b) shows a direction in which one side of the second optical fiber of the first fusion coupler 6 having the coupling constant K ₁ is connected to the point A, and connected to the point A of the first fusion coupler 6. The other side of the first and second optical fibers located in the opposite direction to the first and second optical fibers of the second fusion coupler (7) of the coupling constant K ₂ , respectively, and the first, The other side of the two optical fibers connects to points B and C, respectively.

In the above, an optical signal is input to the point A, and an optical signal is output from the points B and C.

The optical signal inputted to the point A passes through the fusion region of the first fusion coupler 6 and is divided into two directions through the first and second optical fibers to be input to the second fusion coupler 7 and the second fusion coupler. Pass through coupler 7 and output to points B and C.

Further, the optical signal outputted to the points B and C is flattened in the gain spectrum compared to the optical signal inputted to the point A due to the interference between the modes due to the phase difference in the first and second fusion combiners 6 and 7.

In the above description, an example using two fusion couplers has been described, but the number of fusion couplers is not limited thereto, and two or more may be used depending on the structure of the erbium-doped optical fiber amplifier.

 As described above, the gain equalization filter of the present invention combines two fusion combiners to flatten the non-uniform gain spectrum through the mode-to-mode interference through the phase difference. By using the coupler, the structure is simple and the manufacturing cost is reduced.

In addition, the gain equalization filter according to the present invention is advantageous in that it is advantageous for long-distance transmission because it is less influenced by temperature and less bend of the spectrum.

Claims (7)

A gain equalization filter that compensates for the gain spectrum of an optical fiber amplifier, The cladding layer of the two general optical fibers has a fusion region coupled to each other and is composed by connecting the second and second fusion couplers having different coupling constants in a ring form, It is configured to have a gain spectrum opposite to the gain spectrum of the optical fiber amplifier by adjusting the center wavelength and the period of the fusion coupler by adjusting the tensile strength when forming the fusion region of the first and second fusion couplers, respectively. Gain equalization filter using a fusion bonder. The method of claim 1, wherein the gain equalization filter Each having a _first and second fused bond group having a bonding constant K ₁ and K ₂ , Both sides of the first optical fiber of the first fusion coupler of the coupling constant K ₁ are connected to points A and B, respectively, and both sides of the remaining second optical fiber are the same direction of the first and second optical fibers of the second fusion coupler of the coupling constant K ₂ . Respectively connected to one side of, and the other side of the first and second optical fibers of the second fusion coupler are configured to connect to points C and D, respectively, A gain equalization filter using a fusion coupler in which an optical signal is input to the point C, and an optical signal is output from at least one of the points A, B, and D. The method of claim 1, wherein the gain equalization filter Both sides of the first optical fiber of the first fusion combiner of the coupling constant K ₁ are connected to points A and B, respectively, and both sides of the remaining second optical fiber are respectively connected to both sides of the first optical fiber of the second fusion combiner of the coupling constant K ₂ . And connecting both sides of the second optical fiber of the second fusion coupler to points C and D, respectively, A gain equalization filter using a fusion coupler in which an optical signal is input to the point C, and an optical signal is output from at least one of the points A, B, and D. A gain equalization filter that compensates for the gain spectrum of an optical fiber amplifier, The cladding layers of two general optical fibers have a fusion region coupled thereto, and are formed by cascading first and second fusion couplers having different coupling constants. It is characterized by having a gain spectrum opposite to the gain spectrum of the optical fiber amplifier by adjusting the center wavelength and the period of the fusion coupler by adjusting the tensile strength when forming the fusion region of the first and second fusion coupler, respectively. Gain equalization filter using fused coupler. The method of claim 4, wherein the gain equalization filter Each having a _first and second fused bond group having a bonding constant K ₁ and K ₂ , _One side of the first optical fiber of the first fusion coupler of the coupling constant K ₁ is connected to the point B, and one side of the second optical fiber located in the same direction as one side of the first optical fiber connected to the point B in the first fusion coupler. Connected to A, and the other side of the second optical fiber of the first fusion coupler is connected to one side of the first optical fiber of the second fusion coupler having a coupling constant K _2, and the other side of the first optical fiber of the second fusion coupler is a point Connected to C and configured to connect one side of the second optical fiber of the second fusion coupler located in the same direction as the other side of the first optical fiber connected to the point C to the point D, A gain equalization filter using a fusion coupler in which an optical signal is incident to the point A and an optical signal is output to the points B, C, and D. 5. The method of claim 4 wherein the gain equalization filter is Each having a _first and second fused bond group having a bonding constant K ₁ and K ₂ , _One side of the second optical fiber of the first fusion coupler having a bonding constant K ₁ to point A, and the first and second optical fibers of the first and second optical fibers positioned opposite to one side of the second optical fiber connected to the point A of the first fusion coupler. The other side is connected to one side of the same direction of the first and second optical fibers of the second fusion coupler of the coupling constant K ₂ , and the other side of the first and second optical fibers of the second fusion coupler are connected to points B and C, respectively. and, A gain equalization filter using a fusion coupler in which an optical signal is input to the point A and an optical signal is output from points B and C. An erbium-doped optical fiber amplifier comprising the gain equalization filter according to any one of claims 1 to 6, wherein the gain spectrum of the amplified optical signal is adjusted.

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