JPH09265116A - Optical amplifier formed by using erbium added optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical amplifier formed by using an Er (erbium) added optical fiber with which the sufficient flat characteristics are obtd. over a wide range even at the time of a high gain. SOLUTION: An input side fiber 7a, lens 6a, isolator 5a, a WDM (wavelength division multiplex) circuit 3a and a lens 2a are connected in series between the input side of the Er added multicore fiber 1 and a signal light input end. The exciting light 10a from a semiconductor laser for excitation is supplied to the WDM circuit 3a. The input side fiber 7a is composed of a single mode fiber 11 and an Er added optical fiber 12 (length is <=1.5 meters). The gain near a wavelength region 1.530nm is attenuated before the signal is joined with the exciting light by the Er added optical fiber 12. The wavelength characteristics of the gain of the signal light after the amplification are thus flattened.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical amplifier using an Er-doped optical fiber which is required to have a flat gain wavelength characteristic.

[0002]

2. Description of the Related Art In recent years, optical fiber amplifiers in which rare earth elements such as Er (erbium), Pr (praseodymium) and Nd (neodymium) are added to the core of an optical fiber have come to a practical level. In particular, an Er-doped optical fiber amplifier has a high gain and a high saturation output in the 1.55 μm band, and is therefore expected to be applied to various systems. Among them, high-speed, large capacity, long-distance transmission and optical CATV (CaTV (Ca
ble Television) systems have attracted attention.
When the Er-doped optical fiber amplifier is applied to such a system, the gain of the Er-doped optical fiber amplifier in the above-mentioned wavelength band is flat in order to suppress the deterioration of the optical S / N characteristic and the crosstalk characteristic. This is very important.

In order to achieve such high gain and flattening, the present inventors have previously shown the Er-doped multicore fiber as shown in FIG. 5 and the Er-doped multicore fiber as shown in FIG. I am proposing an optical amplifier. First, the Er-doped multicore fiber 1 used is shown in FIG.
As shown in FIG. 3, a glass rod 103 covered with a primary cladding layer 102 and provided with a plurality of cores 101a to 101g co-doped with a rare earth element (for example, Er) and Al.
And the glass rods 103 are thickly covered with the secondary cladding 104. By using such an Er-doped multi-core fiber 1, it is possible to achieve a high gain and a flat gain wavelength characteristic.

There are two reasons for this achievement. First, the first reason is that the Er-doped multi-core fiber can have a sufficiently high Al-doped concentration relative to a conventional Er-doped fiber having one core. The second reason is that when the power of the pumping light in the core is lowered with the conventional optical fiber, the gain peak near the wavelength of 1.535 μm decreases, and the gain-wavelength characteristic gradually becomes flat. Wavelength 1.53 as power is further reduced
Since the gain in the short wavelength region on the μm side is reduced and the gain in the long wavelength region on the 1.56 μm side is increased, that is, gain-wavelength characteristics that rise to the right from the short wavelength to the long wavelength, the pumping light is lowered. It has been known that the gain becomes extremely low and it cannot be used as an optical amplifier as it goes, but this Er-doped multi-core fiber, on the contrary, actively uses this principle. That is, as shown in the drawing, the core diameter D and the core spacing d are set so that the pumping light and the signal light propagate substantially evenly in the Er-added cores 101a to 101g, respectively.
If is optimized, the amplification gain of the signal light propagating inside each of the cores 101a to 101g becomes low,
The wavelength characteristic becomes almost flat, and after propagating for a desired length, the amplified signal is superimposed inside each of the cores 101a to 101g, and the wavelength characteristic of the gain is almost equal. Utilizing flatness.

Next, the structure of the optical amplifier of FIG. 6 based on the above-mentioned principle and the result of evaluation of the wavelength characteristic of the gain for this will be described. The Er-doped multi-core fiber 1 of the first example has a core spacing d of 1.3 μm, a core diameter of about 2 μm, a cladding diameter of 125 μm, a relative refractive index difference Δ between the core and the cladding of 1.45%, and a mode. Field diameter is about 8.8 μm (value at wavelength 1.55 μm), Er and Al addition amount in each core is 400 ppm and 8,500
A fiber having a ppm and a fiber length of about 45 m was used. Also,
In the Er-doped multi-core fiber 1 of the second example, the relative refractive index difference Δ between the core and the clad is 2.19%, the mode field diameter is about 5.2 μm (value at wavelength 1.55 μm),
Er and Al addition amount in each core is 400ppm and 1
A fiber having a fiber length of 7,000 ppm and a fiber length of about 20 m was used.

Isolators 105a and 105b for preventing light from propagating in the reverse direction are connected to both ends of the Er-doped multicore fiber 1, and WDM (Wavelength Division Mult) is provided inside each of them.
iplexing) couplers 106a and 106b are provided. The signal light S ₁ is input to the isolator 105a,
The amplified signal light S ₂ is output from the isolator 105b.

Optical fibers 108a and 108b connected to pumping semiconductor lasers 107a and 107b are coupled to the WDM couplers 106a and 106b, respectively, and pumping lights 109a and 109b generated by the pumping semiconductor lasers 107a and 107b. Are propagated to the Er-doped multi-core fiber 1. Isolator 105
When the signal light S ₁ is input via a, the pumping semiconductor laser 107a generates laser light 109a having a short wavelength, and the laser light 109a is generated by the WDM coupler 106a.
When it is made incident on the Er-doped multi-core fiber 1 via, the energy level of ions is excited, and an amplification effect by stimulated emission occurs. The amplified optical signal S ₂ is Er
The added multicore fiber 1 is output to the outside through the isolator 105b. Similarly, the laser beam 109b generated by the pumping semiconductor laser 107b is incident on the Er-doped multicore fiber 1 from the rear direction, and the light is amplified by the above-described principle. Here, the excitation light is applied from the front and back, but it may be applied either before or after.

Here, the wavelengths of the pumping lights 109a and 109b are set to 0.98 μm, the pumping light power of the pumping light 109a is set to 70 mW, and the pumping light power of the pumping light 109b is set to 80 mW. These values are values determined from the fact that the gain wavelength characteristic is suitable for flattening. Wavelength characteristics of gain (Al concentration dependence) in the optical amplifier having the configuration of FIG.
Is measured in FIG. 7 and FIG. FIG. 7 shows the measurement when the added amount of Al was 8,500 ppm.
Fig. 8 shows the measurement when the addition amount of 17,000ppm.
It is. Here, in the above two examples, the signal light power Sp is -37 dB, -27 dB, -17 dB, -9 dB.
The wavelength characteristic of each gain in each case was measured. As is clear from FIGS. 7 and 8, 1,540 nm-1,
It can be seen that the gain is flattened in the range of 560 nm.

[0009]

However, according to the conventional Er-doped multi-core fiber amplifier, although a flat wavelength characteristic can be obtained when the gain is below a certain level, it is 40 dB.
In the front and back high gain regions, satisfactory results were not obtained for the wavelength characteristics of gain. As is clear from FIG. 7 and FIG. 8, there remains a problem in the increase (lifting) of the gain particularly near 1530 nm.

The present invention has been made in view of the above-mentioned state of the art, and an object of the present invention is to provide an optical amplifier using an Er-doped optical fiber capable of obtaining a sufficient flat characteristic over a wide range even at a high gain. I am trying.

[0011]

In order to achieve the above object, the present invention provides an amplifying optical fiber which is doped with Er and which amplifies the signal light by inputting the signal light and the pump light. Multiplexing means for multiplexing the pumping light with signal light and supplying it to the amplification optical fiber, and the amplification optical fiber by attenuating the signal light supplied to the amplification optical fiber in a predetermined wavelength band An optical amplifier using an Er-doped optical fiber, comprising: a gain compensating means for compensating the wavelength dependence of the gain of the signal light amplified by.

In other words, according to the present invention, a first optical fiber for optical amplification to which Er is added and W provided on at least one of the input side and the output side of the first optical fiber.
A DM circuit, a pumping light source for supplying pumping light to the WDM circuit, Er is added, has a length of 1.5 meters or less, and is a first optical fiber for inputting the input light. And a second optical fiber propagating to the side.

According to this structure, before the signal light is merged with the pump light, the wavelength region (1, 5) in which the gain in the signal light is projected is projected.
(Around 30 nm) is attenuated by the second optical fiber set to a specific condition. Therefore, the gain wavelength characteristic of the amplified optical signal becomes flat over a wide range. Also,
The optical amplifier using the Er-doped optical fiber of the present invention is Er
Er-doped optical fiber for optical amplification, to which is added, a pumping light source that generates pumping light, and Er-doped length 1.5.
The optical fiber coupler type merging circuit may be configured by using an optical fiber having a length of less than or equal to meters and including an input light and an excitation light from the excitation light source.

According to this structure, before the signal light is merged with the pump light, the wavelength region (1, 5) in which the gain in the signal light is prominent.
(Around 30 nm) is attenuated by an optical fiber coupler type merging circuit set to a specific condition, and preliminary optical amplification is also performed. Therefore, the wavelength characteristic of the gain of the optical signal amplified by the Er-doped optical fiber for main amplification becomes flat over a wide range.

In the second optical fiber or the optical fiber of the optical fiber coupler type merging circuit, the Er addition amount in the core is preferably 10 to 1,000 ppm. According to this configuration, the length of the optical fiber can be adjusted by changing the amount of Er added. That is, the length of the optical fiber can be shortened by increasing the Er addition amount. Therefore, by changing the Er addition amount or the length of the optical fiber, it is possible to arbitrarily adjust the lifting amount of the wavelength region in which the gain is projected.

The second optical fiber or the optical fiber of the optical fiber coupler type merging circuit preferably has a mode field diameter of 2 to 10 μm at a wavelength of 1.55 μm. According to this configuration, when a single-mode optical fiber or a dispersion-shifted optical fiber is used as the input side fiber, it is easy to make the mode field diameters together, and the connection with the input side fiber can be performed with low loss. it can. Further, the connection with the pigtail fiber of the isolator with pigtail fiber in the optical amplifier and the Er-doped optical fiber for amplification can be made low loss.

The second optical fiber or the optical fiber of the optical fiber coupler type merging circuit may be configured not to contain Al. According to this configuration, the wavelength is 1,530n
The power of the signal light within a narrow band near m can be attenuated. That is, the wavelength having the peak of loss of the Er-doped optical fiber in which Al is not added is about 1,53.
It is 5 nm, which can suppress the rise of the gain near the wavelength of 1,535 nm.

Further, in the optical amplifier provided with the optical fiber coupler type merging circuit, the second pumping light different from the pumping light is used, and the second pumping light is directed from the emission side of the signal light to the incident side. You may provide the means to make it convey. According to this configuration, since the pumping light can be supplied not only from the front direction but also from the rear direction, the gain can be further increased.

[0019]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a connection diagram showing a first embodiment of an optical amplifier using an Er-doped optical fiber according to the present invention. Lenses 2a and 2b are provided at both ends of the Er-doped optical fiber 1 for amplification, and WDM circuits 3a and 3b are provided so as to face the lenses 2a and 2b, respectively. Further, the lenses 4a and 4b are arranged at angular positions symmetrical with respect to the lenses 2a and 2b with respect to the vertical line with respect to the WDM circuits 3a and 3b.
Are arranged. Further, lenses 6a and 6b are provided outside the WDM circuits 3a and 3b via bulk-structured isolators 5a and 5b. The lenses 6a and 6b are provided.
Facing the input fiber 7a on the input side and fiber 7b on the output side
Are arranged. The input side fiber 7a has a configuration in which a single mode fiber (or dispersion shift fiber) 11 and an Er-doped optical fiber 12 are connected in series.

Optical fibers 8a and 8b (single mode fibers) are arranged facing the lenses 4a and 4b,
Pumping semiconductor lasers 9a and 9b (generating pumping lights 10a and 10b) are provided at the ends of the optical fibers 8a and 8b, respectively. Then, the signal light S ₁ is input to the optical fiber 7a, and the amplified signal light S ₂ is output from the optical fiber 7b. Each lens is used for converting non-parallel light into parallel light.

The isolators 5a and 5b have a function of transmitting only the signal light S _{1 in the} direction of the arrow in the figure and not propagating the light in the opposite direction. The WDM circuits 3a and 3b are optical circuits having wavelength characteristics that transmit the signal light S ₁ and reflect the pumping lights 10a and 10b. Next, the operation of the optical amplifier configured as shown in FIG. 1 will be described. Here, the signal light S ₁ has several waves in the wavelength range of 1,530 nm to 1,560 nm.
The signal light is multiplexed with 100 different wavelengths and propagates in the single mode fiber 11 in the right direction in the figure. The Er-doped optical fiber 12 has a length of 1.5 m.
The power of signal light having a wavelength near 1,530 nm is attenuated in the process of the signal light S ₁ propagating in the Er-doped optical fiber 12 as follows.

Here, the principle of attenuation will be described. When Er is added to the optical fiber, the light is attenuated with its own absorption (wavelength 1530 nm). So-called optical fiber absorption loss occurs. Er-doped optical fiber 12
Of the optical signal in the Er-doped optical fiber 1 for amplification
When an Er-doped multi-core fiber having the characteristics shown in FIG. 8 is used, it suffices that the signal light having a wavelength of 1,530 nm can be attenuated by about 5 dB. As a result, the wavelength characteristic of gain is 1,53
It can be planarized from 0 nm to 1,560 nm.

For example, as the Er-doped optical fiber 12,
When an Er-doped multi-core fiber having a length of 70 cm, an Er added amount of 400 ppm and an Al added amount of 8,500 to 17,000 ppm is used, as is known from FIG. 2, the loss is about 15 dB / 3 m at a wavelength of 1,530 nm. Yes,
Therefore, in order to suppress the gain protrusion at the wavelength of 1,530 nm, the fiber length may be set to about 50 cm to 100 cm.

The amount of Al added is 0 to 10,000 p.
It is selected from the range of pm. Er-doped optical fiber 12 A
When 1 is not added, the wavelength at which the peak value of the loss is reached shifts to around 1,535 nm, as shown in FIG. However, in the case where Al is not added, light absorption loss occurs in a very narrow wavelength band as compared with the case in which Al is added, and it is possible to intensively attenuate only a target portion. Is preferred.

The amount of Er added is 10 as described above.
It can be selected in the range of ppm to 1,000 ppm, and the length of the Er-doped optical fiber 12 can be adjusted according to the Er-doped amount. The Er-doped optical fiber 12
The Er-doped optical fiber 1 for amplification may be an Er-doped multi-core fiber or an Er-doped optical fiber having one ordinary core. As the single mode fiber 11, a normal low relative refractive index difference Δ or high normal refractive index difference Δ can be used. The Er-doped optical fiber 1
You may add P to 2 together and shift a loss peak to the short wavelength side.

The signal light S ₁ after being attenuated by the Er-doped optical fiber 12 is pumped by the WDM circuit 3a.
It joins the excitation light 10a (wavelength 0.98 μm or 1.48 μm band) output from the. The excitation light 10a is the lens 4
After being converted into parallel light by a, it joins the signal light S ₁ in the process of being incident on the lens 2a through the WDM circuit 3a. The combined light enters the lens 2a, is condensed by the lens 2a, then enters the amplification Er-doped optical fiber 1, and propagates in the right direction in the drawing.

Further, laser light 10b (wavelength 0.98 μm or 1.48 μm generated by the pumping semiconductor laser 9b is used.
The band) propagates in the optical fiber 8b, is converted into parallel light by the lens 4b, is incident on the WDM circuit 3b, is reflected here, is incident on the lens 2b, and is condensed and then Er-doped optical fiber for amplification. 1 propagates in the direction opposite to the propagation directions of the signal light S ₁ and the pump light 10a.

The signal lights S ₁ are amplified by the pumping lights 10a and 10b propagating in the amplifying Er-doped optical fiber 1. In this optical amplification, an energy level of an ion is excited and the amplification effect by stimulated emission is used.
The amplified optical signal enters the output side fiber 7b from the amplification Er-doped optical fiber 1 through the lens 2b, the WDM circuit 3b, the isolator 5b and the lens 6b. And
The optical signal S ₂ is output from the output side fiber 7b to the outside.

Although the excitation light is applied from the front and the back in this case, it may be applied either before or after. FIG. 3 is a characteristic diagram showing measurement results of gain wavelength characteristics in the optical amplifier having the configuration of FIG. The characteristics shown here are those of the Er-doped optical fiber 1 for amplification shown in FIG. 1 with the high Al-doped Er (Al-doped amount = 17,000 ppm) Er shown in FIG.
A doped multi-core fiber was used. As the Er-doped optical fiber 12, an optical fiber (length 70 cm) having the characteristics shown in FIG. 2 was used. The powers of the pump lights 10a and 10b were 70 mW and 80 mW, and the pump wavelength was 0.98 μm.
m. As a result, as shown in FIG.
Gain flatness was obtained over a wavelength range from m to 1,560 nm.

FIG. 4 is a connection diagram showing a second embodiment of an optical amplifier using an Er-doped optical fiber according to the present invention. In FIG. 4, the same reference numerals are used for the same components as those shown in FIG. 1, and thus duplicated description will be omitted below. The difference from the configuration of FIG. 1 lies in the mixing means of the pumping light 10a by the pumping semiconductor laser 9a and the signal light S _{1. In} the configuration of FIG. 4, the isolator 5a and the amplifying E are used.
A characteristic is that an optical fiber coupler type merging circuit 13 is interposed between the r-doped optical fiber 1 and the optical fiber 8a.

The optical fiber coupler type merging circuit 13 is
An optical amplifier configured using an Er-doped optical fiber 14 having a length of 1.5 m or less, a function of attenuating the power of signal light having a wavelength near 1,530 nm, and a pumping light 10a.
It includes a function for combining the signal beam S _1, and the three functions of amplifying the signal light S ₁ preliminarily. The optical fiber coupler type merging circuit 13 includes an isolator 5a and an amplifying Er.
An Er-doped optical fiber 14 is used between the doped optical fibers 1. The Er-doped optical fiber 14 is divided into _three zones C ₁ , C ₂ and C ₃ from the signal light input side, and the zone C ₁ 1
The light in the 530 nm band is attenuated, and in the zone C ₂ , the signal light and the excitation light merge and a coupling occurs. At the same time, the signal light (wavelength 1
(530 to 1560 nm) is also partially amplified, but its amplification degree is low. After that, the signal light entering the zone C ₃ is amplified by the excitation by the excitation light. The amplification degree in this zone C ₃ depends on the product of the Er addition amount and the fiber length. When an Er-doped optical fiber is used as the optical fiber 8a, the excitation light is attenuated in this optical fiber,
Not preferred. The optical fiber coupler type merging circuit 13
Can be manufactured by almost the same method as a normal fiber coupler manufacturing method.

According to the configuration of FIG. 4, the overall configuration of the optical amplifier can be simplified, so that the cost can be reduced. Further, since the connection with the amplification Er-doped optical fiber 1 can also be performed by using the optical fiber having the same structure, it is possible to achieve extremely low loss. The isolator 5a
The output side fiber 15 may also be configured by using an optical fiber doped with Er.

[0033]

As described above, according to the present invention, the signal light is passed through the second optical fiber having a length of 1.5 m or less containing Er before the signal light and the pumping light are combined. Since the rising portion of the gain is attenuated, the wavelength characteristic of the gain of the amplified optical signal becomes flat over a wide range.

Further, a signal light is passed through an optical fiber coupler type merging circuit constructed by using an optical fiber having a length of 1.5 m or less containing Er to attenuate the rising portion of the gain and perform preliminary amplification. If the configuration is such that the pump light is merged after this, the signal light to be merged with the pump light has a flat gain wavelength characteristic, and the gain of the optical signal amplified by the Er-doped optical fiber for main amplification is increased. Has a flat wavelength characteristic over a wide range. In addition, the amplification degree can be increased.

[Brief description of drawings]

FIG. 1 is a connection diagram showing a first embodiment of an optical amplifier using an Er-doped optical fiber according to the present invention.

2] Al addition amount when the Er addition amount is 400 ppm: 0 ppm, 8,500 ppm, 17,000 ppm
6 is a loss wavelength characteristic diagram of an Er-doped multicore fiber in FIG.

FIG. 3 is a characteristic diagram showing a result of measuring a wavelength characteristic of a gain of the optical amplifier having the configuration of FIG.

FIG. 4 is a connection diagram showing a second embodiment of an optical amplifier using an Er-doped optical fiber according to the present invention.

FIG. 5 is a cross-sectional view showing an example of a conventional Er-doped multicore fiber.

FIG. 6 is a connection diagram showing a configuration of an optical amplifier using the Er-doped multicore fiber of FIG.

7 is a wavelength characteristic diagram of a gain in the optical amplifier having the configuration of FIG. 6 when the added amount of Al is set to 8,500 ppm.

8 is a gain wavelength characteristic diagram in the case where the amount of Al added is 17,000 ppm in the optical amplifier having the configuration of FIG.

[Explanation of symbols]

 1 Er-doped optical fiber for amplification 3a, 3b WDM circuit 5a, 5b Isolator 7a Fiber 9a, 9b for input side Semiconductor laser for pumping 11 Single mode fiber 12 Er-doped optical fiber 13 Optical fiber coupler type merge circuit

Claims (10)

[Claims] 1. An amplification optical fiber to which Er is added and which amplifies the signal light by inputting the signal light and the pumping light, and the signal light is multiplexed with the pumping light and supplied to the amplification optical fiber. And a gain for compensating the wavelength dependence of the gain of the signal light amplified by the amplification optical fiber by attenuating the signal light supplied to the amplification optical fiber in a predetermined wavelength band. An optical amplifier using an Er-doped optical fiber, characterized by comprising a compensation means. 2. The multiplexing means is a WDM circuit, and the gain compensating means has an Er of 1.5 m or less.
An optical amplifier using an Er-doped optical fiber according to claim 1, which is a doped optical fiber. 3. The Er-doped optical fiber is 10 ppm to
An optical amplifier using an Er-doped optical fiber according to claim 2, which has a core doped with 1,000 ppm of Er. 4. The Er-doped optical fiber is 1.55 μm
3. The Er-doped optical fiber optical amplifier according to claim 2, which has a mode field diameter of 2 .mu.m to 10 .mu.m at the wavelength of. 5. An optical amplifier using an Er-doped optical fiber according to claim 2, wherein the Er-doped optical fiber does not contain Al. 6. The multiplexing means is an optical fiber coupler type merging circuit, and the gain compensating means constitutes the optical fiber coupler type merging circuit and has a length of 1.5 m or less. An optical amplifier using an Er-doped optical fiber according to claim 1, which is a fiber. 7. The Er-doped optical fiber comprises an attenuator for attenuating the signal light, a coupler for coupling the signal light and the pump light, and an amplifier for amplifying the signal light. An optical amplifier using the Er-doped optical fiber described. 8. The Er-doped optical fiber is from 10 ppm to
An optical amplifier using an Er-doped optical fiber according to claim 6, which has a core having Er doped with 1,000 ppm. 9. The Er-doped optical fiber is 1.55 μm
The optical amplifier using the Er-doped optical fiber according to claim 6, having a mode field diameter of 2 μm to 10 μm at the wavelength of. 10. The optical amplifier using an Er-doped optical fiber according to claim 6, wherein the Er-doped optical fiber does not contain Al.