KR100219711B1 - Optical fiber amplifier with flat gain property - Google Patents

Abstract

The present invention relates to an optical fiber amplifier using a reflector and a filter having a different reflectance according to a wavelength, comprising: a pump light source for applying pumping light to amplify incident signal light; A first WDM coupler coupling the incident signal light and the pump light; An optical fiber amplifying medium for amplifying the incident signal light and amplifying the reflected signal light by the reflected pump light in the reverse direction; A second WDM coupler for separating and outputting the amplified signal light and the remaining pump light and combining the remaining pump light and the reflected signal light reflected in the reverse direction; A filter which attenuates the signal light of the second WDM coupler in the forward direction and attenuates the reflected signal light in the reverse direction at an attenuation rate different according to the wavelength; A third WDM coupler for coupling the signal light of the filter with the remaining pump light and separating the remaining pump light and the reflected signal light reflected in the reverse direction, so that the reflected pump light is output to the 2WDM coupler and the reflected signal light is output to the filter; An optical fiber reflector reflecting the signal light at different reflectances according to the wavelength and totally reflecting the remaining pump light; And a circulator for transmitting the incident signal light to the first WDM coupler and outputting the reflected signal light of the first WDM coupler.

According to the present invention, by using a reflector and a filter coated with a different reflectance according to the wavelength, it is possible to flatten the gain in a wider wavelength range, eliminate the need for an input / output end isolator, and reduce the length of the EDF by half. .

Description

Fiber Optic Amplifier with Flat Gain

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber amplifier, and more particularly, to an optical fiber amplifier having a flat gain characteristic, having an optical fiber reflector having a different reflectance according to a wavelength and a filter for varying the intensity of an optical signal according to a wavelength.

In general, in the repeater for long-distance optical communication, the conventional optical communication relay method is a method of converting the weakened optical signal into an electrical signal to amplify and then converting it into an optical signal to communicate. In such a relay method, there are many problems such as excessively enlarged and increased noise of the relay amplifier system. An optical amplifier that amplifies the optical signal itself is required as a repeater to compensate for this problem and to efficiently perform the optical amplification.

As such optical amplifiers, Erbium Doped Fiber Amplifiers (hereinafter referred to as EDFAs) are receiving a lot of attention as next-generation optical communication optical repeaters. The EDFA is used to amplify the optical signal periodically to prevent attenuation of the optical signal due to the long distance transmission when a large amount of data is transmitted over a long fiber.

If the EDFA is classified according to the use, it can be divided into a pre-amplifier, an in-line amplifier, a post-amplifier, and a CATV power booster. In addition, the EDFA is 1.51 showing the lowest loss of the optical fiber when the rare earth element erbium (Er3 +) is added In addition to a wide amplification band in the m region, it is easy to splic with communication optical fibers, and has high gain characteristics, low noise, and low polarization dependence.

EDFA is used in a Wavelength Division Multiplexing (WDM) system in which information carried on different wavelengths of light is bundled and communicated on a single line. However, in order for EDFA to be used for wavelength division multiplexing (WDM), the gain of EDFA must be high, and a flat gain characteristic is required over a wide wavelength range.

FIG. 1 is a block diagram illustrating a general EDFA configuration, and includes two optical isolators 100 and 130 for preventing reverse flow of signals to prevent oscillation of light waves other than signals, and passive light for combining pump light and signal light into a single optical fiber. A wavelength division multiplexer (WDM) coupler (120), an erbium doped fiber (EDF, 120), which is an amplifying medium that amplifies the inductive radiation principle by doping erbium on a part of the optical fiber core; It is composed of a pumping laser diode 140, which is an active optical element that provides the energy required to excite the erbium ions in the ground state in the EDF. In the EDFA having such a configuration, a pumping laser diode 140 is connected to the EDF, and the pumping light of the pumping laser diode excites Er3 ⁺ , which is a rare earth element doped in the core of the optical fiber, to produce a stimulated emission. Get up. The center wavelength of the pumping laser diode 5 is 980 nm, and the pumping light output is incident on the erbium-doped optical fiber 120 through the wavelength division multiplexer 110 together with the weak optical signal through the isolator 100 on the input side. . The wavelength division multiplexer 110 combines an optical signal having a wavelength of 1530 nm to 1560 nm and a pumping light having a wavelength of 980 nm to enter the EDF 120. In the EDF 120, optical signals having a wavelength of 1530 nm to 1560 nm are amplified by pumping light having a wavelength of 980 nm. The isolators 100 and 130 prevent distortion of the input signal due to reflected light reflected by other photons and light traveling in the reverse direction among the output light by the ASE (amplified spontaneous emission) of the EDF.

However, the EDFA has a high gain only for a specific wavelength and has a large gain deviation depending on the wavelength. In particular, there is a problem that the gain spectrum greatly depends on the operating gain range of the EDFA. Thus, when the EDFA is mounted on a real system, it is not only possible to send signals of multiple wavelengths to a single optical line at the same time, but also to reduce the efficiency of optical transmission because the gain decreases depending on the wavelength when the intensity of the pump light decreases after a long time of use. . That is, the wavelength range of the signal light transmittable within the gain change of 0.5 dB is only about 3 nm.

In addition, optical communication aims to transmit a large amount of information quickly and accurately, and at the same time, many new concepts such as time division, code division, and wavelength division are emerging to send more information. In recent years, the optical transmission has been focused on a wavelength division transmission method, and research on this has been actively conducted. This requires a fiber amplifier with a constant gain regardless of wavelength. Since the EDFA has different amplification gains depending on the wavelength, even when the transmitter transmits the signal intensity having the same intensity regardless of the wavelength, the intensity of the signal received at the receiver varies depending on the wavelength. You cannot receive all of the original signals beyond. This phenomenon causes more serious use of more fiber amplifiers, and ultimately the fiber amplifiers cannot be used for communication except for signals of a certain wavelength.

In addition, the gain flattened optical fiber amplifier changes the degree of gain flattening when the signal light intensity changes, that is, when the average value of the gain changes. If a given EDFA's performance is degraded and the gain flattening function is slightly broken, the intensity of the input signal light entering the subsequent EDFA varies depending on the wavelength, and this effect accumulates over several EDFAs, resulting in a gain equalization effect for the entire system. Disappear. Therefore, when used in the actual WDM (Wavelength Division Multiplexer) communication, if the performance of a particular EDFA is lowered among several EDFAs and the gain is lowered, the gain leveling function of the entire system may be lost.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and improves the small signal gain in a wider wavelength band (1544 nm to 1559 nm), significantly reduces the gain difference according to the wavelength in the wavelength region, and independently of the intensity of the input signal light. In order to flatten the gain according to the wavelength in the band, the aim of the present invention is to provide an optical fiber amplifier with flat gain characteristics by using a Mach-Zehnder filter with periodic attenuation and an optical fiber reflector having a different reflectance depending on the wavelength. have.

Figure 1 shows a block diagram of the configuration of a typical EDFA.

2 is a block diagram showing the configuration of an optical fiber amplifier having a flat gain characteristic using an optical fiber reflector according to the present invention.

Figure 3 shows the reflectance characteristics of the optical fiber reflector according to the signal light wavelength of the optical fiber reflector used in the optical fiber amplifier according to the present invention.

Figure 4 shows the gain characteristics according to the wavelength of the EDFA and normal EDFA using the optical fiber reflector according to the present invention.

5 illustrates gain characteristics according to wavelengths of REDFAs measured while changing the intensity of the input signal light.

6 illustrates gain characteristics of the REDFA according to the intensity of the signal light measured while changing the input signal wavelength.

FIG. 7 is a block diagram illustrating a configuration of an optical fiber amplifier in which a second pump LD is added between the EDF and the second WDM coupler of FIG. 2.

Explanation of symbols for the main parts of the drawings

200: circulator, 210: pump LD,

220: first WDM coupler, 230: EDF,

240: second WDM coupler, 250: mark-gender filter,

260: third WDM coupler, 270: optical fiber reflector

According to the present invention for achieving the above object, an optical fiber amplifier for amplifying an optical signal using a reflector having a different reflectance according to a wavelength includes a reflection reflected from the reflector in a direction in which an incident signal light travels to the reflector A pump light source for applying pumping light to amplify the incident signal light when the direction in which the signal light travels is reversed; A first WDM coupler which forwardly combines the incident signal light and the pump light of the pump light source and transmits the light to one optical fiber; An optical fiber amplifying medium for amplifying the incident signal light output from the first WDM coupler in a forward direction by the pumping light of the pump light source and amplifying the reflected signal light by the pump light reflected in the reverse direction; The signal light amplified in the optical fiber amplification medium and pumping the remaining pump light is output as it is in the forward direction, and the pumping and pumping the remaining pump light and the reflected signal light of the reflected pump light source in the reverse direction and outputs to the optical fiber amplification medium 2WDM coupler; A filter for varying the attenuation rate according to the wavelength with a predetermined period, attenuating the signal light of the second WDM coupler in a forward direction and attenuating the reflected signal light in a reverse direction according to the attenuation rate; In the forward direction, the signal light of the filter and the remaining pump light of the second WDM coupler are combined and output, and in the reverse direction, the remaining pump light and the reflected signal light are separated to output the reflected pump light to the second WDM coupler and the reflected signal light. A third WDM coupler outputting to the filter; The signal light output from the third WDM coupler and the remaining pump light are inputted, and the signal light outputs the reflected signal light reflected at different reflectances according to the wavelength to the third WDM coupler, and the remaining pump light is totally reflected to the third WDM coupler. An optical fiber reflector for outputting; And a circulator for transmitting the incident signal light to the first WDM coupler and outputting the reflected signal light of the first WDM coupler received through the first WDM coupler.

The optical fiber amplifier includes: a second pump light source for applying pumping light to amplify the incident signal light; And a fourth WDM coupler for forwarding the signal light output from the optical fiber amplification medium to the mark gender filter and outputting the reflected signal light to the optical fiber amplification medium in a reverse direction, wherein the optical fiber amplification medium is forward. Amplifies the incident signal light output from the first WDM coupler by the pumping light of the pump light source, and amplifies the reflected signal light by the pump light of the second pump light source input through the fourth WDM coupler in the reverse direction. It is done.

In the optical fiber amplifier, the optical fiber amplification medium has a large gain for wavelengths of 1530 nm, a small gain for wavelengths of 1550 nm, and the filter has a transmission loss for a wavelength of 1530 nm relative to a transmission loss for a wavelength of 1550 nm. The optical reflector is largely tuned, and the reflectance of the wavelength of 1530 nm is small, and the gain of the wavelength of 1550 nm is large.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. FIG. 2 is a block diagram showing the configuration of an optical fiber amplifier according to the present invention, and is used to excite Erbium ions in an EDF (Erbium Doped Fiber) 230, which is an optical fiber amplification medium, and a ground state in the EDF 230. A pump laser diode (LD) 220 which is a light source, a first wavelength division multiplexer (WDM) coupler 210, a second WDM coupler 240, a third WDM coupler 260, which combines or separates a signal light and a pump light, Mark-gender filter 250 having periodicity and varying attenuation rate according to wavelength, circulator 200 having three ports for input and output of signal light, and optical reflector having reflectance varying according to wavelength 270.

For convenience of description, the direction in which the incident signal light proceeds to the optical fiber reflector 270 is defined as a forward direction, and the direction in which the reflected signal light reflected from the optical fiber reflector 270 proceeds is defined as a reverse direction.

The pump LD 220 is a pump light source for applying pumping light to amplify the incident signal light. In addition, in order to compare the results of the present invention with a normal single forward EDFA, the pump LD 220 has a wavelength of 980 nm and forward pumping in the same direction as the signal light. Fixed with.

The first WDM coupler 210 is a fuse type that combines the incident signal light and the pump light of the pump LD 220 to be transmitted to the EDF 230.

The EDF 230 is an optical fiber amplifying medium for amplifying the incident light signal output from the WDM coupler 210 by the pumping light of the pump LD 220 and amplifying the reflected signal light. In addition, the EDF 230 is a high aluminum doped (High Al-doped) EDF, Erbium (Erbium) concentration of 165ppm used 24m.

The second WDM coupler 240 forwards the signal light amplified by the EDF 230 and the remaining pump light when the remaining pump light is incident, and outputs the signal light to the mark gender filter and the pump light to the third WDM coupler in the reverse direction. The pumped remaining pump light of the pump LD 220 and the reflected signal light are combined and output to the EDF 230.

The third WDM coupler 260 combines the signal light of the mark-gender filter 250 and the remaining pump light of the second WDM coupler 240 in the forward direction, and outputs the remaining pump light and the reflected signal light reflected in the reverse direction. The reflection pump light is output to the second WDM coupler 240 and the reflection signal light is output to the mark-gender filter 250.

The mark-gender filter 250 varies the attenuation rate according to the wavelength with a predetermined period, and attenuates the signal light of the second WDM coupler 240 in the forward direction and attenuates the reflected signal light in the reverse direction according to the attenuation rate. Let's do it. In the present embodiment, as an Amp-Flat mark-gender filter, the Free Spectral Range (FSR) has 32 nm and the maximum extinction ratio can be varied to 36 dB. It is tuned for the largest loss at 1536 nm.

The optical fiber reflector 270 is one of the core elements of the present invention, and reflects the signal light amplified through the optical fiber amplification medium EDF 230 at different reflectances according to the wavelength of the signal light, and thus from the optical fiber reflector 270. The reflected reflection signal light is output to the EDF 230, and the remaining pump light of the pump LD 220 is reflected to be reincident to the third WDM coupler 260. The optical fiber reflector 270 is coated on the EDF with a different reflectance depending on the wavelength, and its characteristics are shown in FIG. 3. In addition, the optical fiber reflector 240 was coated to totally reflect the pump wavelength of 980 nm in order to efficiently use the intensity of the pump light remaining after pumping the EDF 230.

The circulator 200 causes the incident signal light to enter the EDF 230 through the first WDM coupler 210, and the final output light of the reflected signal light reflected from the optical fiber reflector 270, that is, the first WDM coupler 210. It outputs the reflected signal light of the EDF 230 received through), and has a three-port structure. Port 1 → port 2 of the circulator 200 passes the input signal light and port 2 → port 3 passes the output signal light. The insertion loss of the input / output port is 1.2 dB and 1.73 dB, respectively. Isolation of port 2 → port 1 and port 3 → port 2 is more than 60dB and includes the function of the isolator used in the input / output stage in the existing structure, so there is no need to use a separate isolator.

On the other hand, the operation of the present invention based on the above-described configuration will be described. The present invention is to planarize the gain of the optical fiber amplifier regardless of the wavelength and intensity of the signal light input in the wavelength region from 1544nm to 1559nm using the optical fiber reflector and Mach-Zehnder filter with different refractive index according to the wavelength.

The incident signal light enters the EDF 230 through the first WDM coupler 210 through the port 1-> port 2 of the circulator 200, and is amplified first. The amplified signal light passes through a Mark-Zehnder Filter whose wavelength is adjusted to have a peak absorption of 1.6 dB at 1530 nm, and is reflected by the optical fiber reflector 270 to return the EDF 230. After being amplified, the final output is performed from port 2 to port 3 of the circulator 200.

In general, at an optimized length, the EDF 230 has a large gain for wavelengths of 1530 nm and a small gain of 1550 nm. Therefore, the output of the signal light amplified first is 1530nm larger than 1550nm. However, the mark-gender filter 250 used has a large transmission loss at 1530 nm and a small loss at around 1550 nm. In addition, since the optical reflector 270 has a low reflectance at 1530 nm and a high reflectance at 1550 nm, the intensity of signal light reflected by the reflector is 1530 nm smaller than 1550 nm. Therefore, when the signal light reflected by the optical fiber reflector 270 passing through the mark-gender filter 250 is incident on the EDF 230 and amplified again, the gain can be flattened according to the wavelength.

Figure 4 shows the gain characteristics according to the wavelength of the EDFA and the normal EDFA using the optical fiber reflector according to the present invention, the intensity of the input signal light is -35dBm and the intensity of the pump light used is fixed to 65mW. In FIG. 4, Gr represents the gain of REDFA, and Gn represents the gain of normal EDFA. In order to compare the measured data with the normal Normal EDFA (Single forward EDFA), optical devices such as WDM used the same product, and the same length of EDF was used as the same product. The wavelength of the input signal light was measured for 1544 nm, 1547 nm, 1549 nm, 1552 nm, 1554 nm, 1557 nm, and 1559 nm, and the results are shown together in FIG. 4 to compare with the normal EDFA (Single forward EDFA). In FIG. 4, data indicated by ▲ is a gain characteristic of normal EDFA, and data represented by ◆ is a gain characteristic of Recycled EDFA (REDFA) using the optical fiber reflector 270 and the mark-gender filter 250. Comparing these two data between wavelength 1544nm-1559nm, the results of the present invention can be seen that the gain is more than 10dB higher in a given wavelength range than the conventional optical fiber amplifier. In addition, the small signal gain difference of the Normal EDFA (Single forward EDFA) in the 15nm wavelength range from 1544nm to 1559nm is about 3dB, but the result of the present invention is very small as +/- 0.2dB.

5 shows the result of measuring the relationship between the wavelength and the gain while changing the intensity of the input light to -35, -30, -25, -20, -15 dBm. It can be seen that the gain of the optical amplifier is flattened between 1544 nm and 1559 nm regardless of the wavelength of the same signal light intensity. 6 shows the relationship between the intensity of the input signal light and the gain of the optical amplifier. The intensity of the signal light into which the gain curve of the optical amplifier is input for seven wavelengths, namely, 1544 nm, 1547 nm, 1549 nm, 1552 nm, 1554 nm, 1557 nm, and 1559 nm. Matches regardless.

Meanwhile, FIG. 7 illustrates an optical fiber amplifier in which the second WDM and the third WDM coupler are removed and the second pump LD 700 and the fourth WDM coupler 710 are added between the EDF 230 and the mark gender filter 250. As shown in a block diagram, the second pump LD 700 for applying the pumping light to amplify the incident signal light and the signal light output from the EDF 730 in the forward direction to the Mark Gender filter, The fourth WDM coupler 710 may further include a fourth WDM coupler 710 configured to combine the reflected signal light and the remaining pump light and the pump light of the second pump LD 700 to the EDF 730 in the opposite direction.

The EDF 730 amplifies the incident signal light output from the first WDM coupler 220 in the forward direction by the pumping light of the pump LD 210 and in the reverse direction through the fourth WDM coupler 710. The reflected signal light is amplified by the pump light of the input second pump LD 700.

7 is an operation principle similar to that of FIG. 2, and thus the description of the operation will be omitted. Only the pump light used when amplifying the reflected signal light reflected by the optical fiber reflector 270 in the EDF 730 may be used as the second pump LD ( Its features are different in that it uses 700 pump lights.

In the present invention, the gain of the EDFA is flattened using a reflector coated with a different reflectance according to the wavelength and a mark-gender filter. Thus, the gain for the wavelength range of 1544 nm to 1559 nm can be flattened regardless of the intensity of the input signal light. Showed.

In the present invention, a high signal gain average of 36 dB or more in the 1544 nm to 1559 nm wavelength region (15 nm) is obtained, and at the same time, the gain difference is flattened to +/- 0.2 dB regardless of the wavelength and the intensity of the signal light. This means that the gain is flattened over a given wavelength range regardless of the magnitude of the average gain of the fiber amplifier.

Since the optical fiber reflector used in the present invention is designed to reflect the pump light at the same time as the signal light, it is more efficient because the EDF can be pumped and the remaining power can be recycled. The strength of the pump LD used in the present invention is 65mW, but the product of low power is used. However, since the power of the pump LD is gradually increasing, it is possible to obtain higher gains by using such LD.

Since the optical fiber reflector used in the present invention can coat several optical fibers at once, there is an advantage in mass production. According to the present invention, it is possible to fabricate an optical fiber amplifier having high gain and at the same time flattening the gain in a wide wavelength band (1544 nm to 1559 nm). When the results of the present invention are used in actual WDM communication, even if the performance of a particular EDFA is lowered and the gain is lowered, the average gain is lowered without affecting the gain flattening according to the wavelength. In the future, many applications are expected for Wavelength Division Multiplexer (WDM) communication.

Claims (8)

In an optical fiber amplifier that amplifies an optical signal by using a reflector having a different reflectance according to a wavelength, when the direction of incident signal light propagates to the reflector is a forward direction and the direction of the reflected signal light reflected from the reflector is a reverse direction, A pump light source for applying pumping light to amplify the incident signal light; A first WDM coupler which forwardly combines the incident signal light and the pump light of the pump light source and transmits the light to one optical fiber; An optical fiber amplifying medium for amplifying the incident signal light output from the first WDM coupler in a forward direction by the pumping light of the pump light source and amplifying the reflected signal light by the pump light reflected in the reverse direction; The signal light amplified in the optical fiber amplification medium and pumping the remaining pump light is output as it is in the forward direction, and the pumping and pumping the remaining pump light and the reflected signal light of the reflected pump light source in the reverse direction and outputs to the optical fiber amplification medium 2WDM coupler; A filter for varying the attenuation rate according to the wavelength with a predetermined period, attenuating the signal light of the second WDM coupler in a forward direction and attenuating the reflected signal light in a reverse direction according to the attenuation rate; In the forward direction, the signal light of the filter and the remaining pump light of the second WDM coupler are combined and output, and in the reverse direction, the remaining pump light and the reflected signal light are separated to output the reflected pump light to the second WDM coupler and the reflected signal light. A third WDM coupler outputting to the filter; The signal light output from the third WDM coupler and the remaining pump light are inputted, and the signal light outputs the reflected signal light reflected at different reflectances according to the wavelength to the third WDM coupler, and the remaining pump light is totally reflected to the third WDM coupler. An optical fiber reflector for outputting; And And a circulator for transmitting incident signal light to the first WDM coupler and outputting reflected signal light of the first WDM coupler received through the first WDM coupler. The method of claim 1, wherein the pump light source An optical fiber amplifier using an optical fiber reflector characterized in that the pump laser diode. The method of claim 1, wherein the optical fiber amplification medium is A fiber amplifier with flat gain characteristics, characterized in that it is an EDF. The method of claim 1, wherein the filter An optical fiber amplifier having a flat gain characteristic, characterized in that it is a mark-gender filter that attenuates the signal light according to the wavelength. The method of claim 1, wherein the optical fiber amplification medium is Gain for wavelengths of 1530 nm is large, gain for wavelengths of 1550 nm is small, The filter is The transmission loss for the wavelength of 1530nm is relatively larger than the transmission loss for the wavelength of 1550nm, The optical fiber reflector An optical fiber amplifier having a flat gain characteristic, characterized by a small reflectance for a wavelength in the 1530 nm band and a large gain for a wavelength in the 1550 nm band. The method of claim 1, A second pump light source for applying pumping light to amplify the incident signal light; And In the forward direction, the signal light output from the optical fiber amplification medium is transmitted to the mark-gender filter, and in the reverse direction, the reflected signal light output from the mark gender filter and the pump light of the second pump light source are combined and output to the optical fiber amplification medium. Further provided with a fourth WDM coupler, The optical fiber amplification medium is In the forward direction, the incident signal light output from the first WDM coupler is amplified by the pumping light of the pump light source, and in the reverse direction, the reflected signal light is amplified by the pump light of the second pump light source input through the fourth WDM coupler. An optical fiber amplifier having a flat gain characteristic. According to claim 1, wherein the optical fiber reflector In order to obtain a flat gain characteristic of the optical fiber amplification medium in a desired wavelength band according to the type and length of the optical fiber amplification medium, the reflectance is adjusted differently according to the coating design, the optical fiber amplifier having a flat gain characteristic. According to claim 1, wherein the optical fiber reflector And an optical fiber amplifier having a flat gain characteristic, characterized in that it is manufactured regardless of the wavelength of the pumping light source.

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