JP2734969B2 - Optical fiber amplifier for WDM transmission - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber amplifier, and more particularly to an erbium-doped optical fiber amplifier used for wavelength division multiplex transmission.

[0002]

2. Description of the Related Art Wavelength division multiplexing is a method for expanding transmission capacity by using signal light of a plurality of wavelengths in optical communication. In such a system, an optical multiplexer for multiplexing a plurality of light beams having different wavelengths in the transmitting unit and an optical demultiplexer for demultiplexing the signal light of each wavelength in the receiving unit are required. Insertion loss occurs in the duplexer.
Therefore, an optical amplifier is used to compensate for the loss at the time of optical multiplexing and demultiplexing.

[0003] Optical amplifiers include semiconductor amplifiers, fiber Raman amplifiers, fiber Brillouin amplifiers, and rare earth doped fiber amplifiers. In particular, an erbium-doped fiber amplifier (E
DFA) is widely used because it has no polarization dependence and can be excited by a semiconductor laser.

As an optical fiber amplifier for amplifying and transmitting a plurality of wavelength-multiplexed signal lights having different wavelengths, for example, there is an optical fiber amplifier described in Japanese Patent Application Laid-Open No. 3-89644. This is to amplify the wavelength-multiplexed amplified light of each wavelength as it is with a single amplification optical fiber.

[0005]

In the method of amplifying the multiplexed signal light collectively, the gain of the optical fiber amplifier has wavelength dependence as shown in FIG. A gain difference occurs. As a result, an output difference occurs in the signal light output from the optical fiber amplifier, and the signal light having a relatively weak optical output cannot be sufficiently demultiplexed on the receiving side, and there is a problem that the crosstalk characteristic deteriorates.

SUMMARY OF THE INVENTION In view of the above-mentioned disadvantages, the present invention can adjust the gain for each signal light of each wavelength and can make the signal light output of each wavelength from the optical fiber amplifier substantially equal. The present invention provides an optical fiber amplifier.

[0007]

SUMMARY OF THE INVENTION In order to eliminate the above-mentioned drawbacks, an optical fiber amplifier for wavelength division multiplexing transmission according to the present invention has the following features.
A first amplifying optical fiber that amplifies an input signal light in which two different signal lights of wavelengths λ1 and λ2 are multiplexed and sends out a first amplified light; and a first amplifying optical fiber. A first excitation light source for outputting a first excitation light to be excited;
A first optical multiplexer for multiplexing the first pump light with the first amplifying optical fiber, and a first optical multiplexer connected in series to a transmitting side of the first amplifying optical fiber to amplify the first amplified light; A second amplification optical fiber for transmitting a second amplification light, a second excitation light source for outputting a second excitation light for exciting the second amplification optical fiber, and a second excitation light for the second amplification light. A second optical multiplexer for multiplexing with the second amplifying optical fiber, and a light which is disposed on the sending side of the second amplifying optical fiber and branches off a part of the second amplifying light and sends out the branched light. A splitter, a light receiving module for receiving the split light and converting the split light into an electric signal, and a control circuit for generating a control signal for the first excitation light source and the second excitation light source according to the electric signal. Features.

According to the present invention, in the above-mentioned configuration, when the input light intensity of the pumping light to the amplification optical fiber is changed, the λ2 with respect to the change in the gain with respect to the signal light of the wavelength λ1 is changed.
When the rate of change in the gain of the signal light is α, the difference α1 in the gain of the first amplification optical fiber is different from the rate α2 in the gain of the second amplification optical fiber. Features.

The rate of change α1 of the gain of the first amplification optical fiber and the rate of change α2 of the gain of the second amplification optical fiber are defined by the first amplification optical fiber and the second optical fiber. And a gain value in which the gain for the signal light of wavelength λ1 is equal to the gain for the signal light of wavelength λ2 within a range smaller than the gain of the signal light amplified by the above.

Further, in the above configuration, the optical output of the first pumping light and the optical output of the second pumping light are set such that the gain for the signal light of wavelength λ1 and the gain for the signal light of wavelength λ2 become equal. It is characterized by having.

[0011]

The wavelength division multiplexing transmission optical fiber amplifier according to the present invention comprises two sets of amplifiers each comprising an amplification optical fiber and an excitation light source for inputting excitation light into the optical fiber. Are connected in series. With such a configuration, the gain or the optical output for the signal light of each wavelength amplified by each amplification optical fiber is set independently by adjusting the optical output of the pump light, so that the signal light for each wavelength is just adjusted. The gain after passing through the two amplifiers can be set to be substantially equal.

The relationship of the gain of the amplifying optical fiber to two signal lights of different wavelengths λ1 and λ2 is represented by the following equation (for example, C. Randy Giles, E
mmmanuel Desurvie, "Modelli
ng Erbium-Doped Fiber Amp
LIFIERS ”, JORNAL OF LIGHTW
AVE TECHNOLOGY VOL9, NO. 2,
FEBRUARY 1991 (Reference 1)).

[0013] Here, ΔGi is the amount of change in gain for each wavelength.

That is, according to Document 1, a certain wavelength λ1
The ratio of the change in gain when the pumping light is changed with respect to the light of the same wavelength and the ratio of the change in the gain when the pumping light is similarly changed with respect to light having a wavelength different from λ1 are always constant. It is shown that.

If the change in the gain with respect to the change in the pumping light is always the same without depending on the wavelength, the amplification of the wavelength-multiplexed signal light can always be amplified with the same gain using a single amplification optical fiber.

However, in general, the change in the gain with respect to the change in the optical output of the pump light differs depending on the wavelength. For example, for light of wavelength λ1 = 1552 nm, 13d
Assume that there is a gain of B. At this time, the wavelength λ2 = 15
It is assumed that a gain of 13 dB is obtained even for light of 62 nm. It is possible that the same gain is obtained only for a certain specific gain. However, when the optical output of the pump light is increased to increase the gain, the wavelength λ2 is increased despite the fact that the gain is 16 dB for the wavelength λ1.
, The gain is 18 dB, which is not the same.

Therefore, according to the present invention, two amplifying optical fibers are connected in series, the optical output of the pump light is adjusted, and the amplifying optical wavelength multiplexed signal light of two different wavelengths is substantially equal. Any gain can be set with a value.

First, from equation (1), ΔG2 = α △ G1 (α: constant) (2) The front and rear stages connected in series are denoted by subscripts f and b.
When the two optical fibers for amplification are connected in series, the total gain of each wavelength by both optical fibers for amplification is
Equations (3) and (4) show.

ΔG1 = ΔG1f + ΔG1b (3) ΔG2 = (αf △ G1f) + (αb △ G1b) (4) That is, in the case of αf ≠ αb, the signal is transmitted to the preceding and subsequent amplification optical fibers. By individually adjusting the optical outputs of the pump lights, the change in gain can be adjusted independently. Therefore, the total gain G1 of each wavelength by the two amplification optical fibers
And G2 can be set independently of each other. Thereby, the gains of the amplifiers of the respective wavelengths can be equalized. If there is a difference in the input level between the signal lights of the respective wavelengths, the optical outputs of the amplified signal lights of the respective wavelengths are equalized. It is also possible to set the gain for each wavelength.

[0020]

An embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a block diagram showing a first embodiment of an optical fiber amplifier for wavelength division multiplexing transmission according to the present invention. The first amplification optical fiber 1 and the second optical fiber 2 are connected in series. A first optical multiplexer 5 is arranged on the input side of the first amplification optical fiber 1, and the pump light from the first pump light source 3 is multiplexed with the signal light. Similarly, a second optical multiplexer is arranged between the first amplification optical fiber and the second amplification optical fiber 2, and the pump light from the second pump light source 4 is multiplexed. The input side of the optical fiber amplifier, between the first amplifying optical fiber and the second optical demultiplexer, and on the output side of the second amplifying optical fiber, to avoid the influence of the return light, respectively, Each isolators are arranged.

On the output side of the second amplification optical fiber 2,
An optical splitter 10 is arranged, and a part of the amplified light is split.
The split amplified light is converted by the optical demultiplexer 11 to each wavelength λ.
The light is divided into 1 and λ2, and is incident on the light receiving modules 12 and 13, respectively. The amplified light of each wavelength in each of the light receiving modules 12 and 13 is converted into an electric signal and transmitted to the control circuit 14.

On the other hand, an optical splitter 15 is arranged at a stage before the first optical multiplexer, and a part of the input light of the multiplexed signal light is split. The split input light is split by the optical splitter 16 and converted into electric signals by the light receiving modules 17 and 18, respectively. An electric signal indicating the intensity of the signal light of each wavelength is sent to the control circuit 14.

The control circuit 14 compares the intensities of the signal light of each wavelength with the input side and the amplified signal light to calculate the total gain, and determines whether the gain is larger or smaller than a desired gain.
In the control circuit 14, the injection current to the first and second excitation light sources is set by the optical output of each wavelength by the electric signal. In the figure, the thick lines indicate optical signals, the thin lines indicate electrical signals,
2 shows the flow of injection current to the semiconductor laser.

Next, a second embodiment of the present invention will be described.

FIG. 2 is a block diagram showing a second embodiment of the optical fiber amplifier for wavelength division multiplexing transmission according to the present invention. In the first embodiment, the optical splitter 15 is arranged on the input side, the intensity of the input light of the signal light of each wavelength is detected, the gain is calculated, and the gain is set to be equal. In contrast, the second
In the embodiment, the optical output of the optical signal of each wavelength from the optical fiber amplifier is set to be equal. In this case, the optical splitter 15, the optical splitter 16, the light receiving module 1
Steps 7 and 18 are unnecessary, and the injection current to each excitation light source may be set so that the electric signals from the light receiving modules 12 and 13 have the same intensity.

FIG. 3 shows a block diagram of the third embodiment. In this embodiment, the branched amplified light is not further split into signal lights of each wavelength, and the light receiving module 1
The signal is converted into an electric signal at 2 and sent to the control circuit 14.
The gain of each wavelength with respect to the optical output of the pumping light of each amplification optical fiber is stored in advance in a ROM or the like built in the control circuit 14, and if the signal light is substantially equal at each wavelength, the optical splitter 10 It is only necessary to detect the intensity of the split light branched by the above, and the gain to be amplified by each amplifying optical fiber, that is, the injection current to each pump light source is uniquely determined.

Next, specific experimental results will be described based on a third embodiment of the present invention.

The optical fiber amplifier for wavelength division multiplexing transmission of the present invention is configured to amplify the signal light having a wavelength of 1552 nm and the signal light having a wavelength of 1562 nm at a time. As the pumping light source, an optical semiconductor laser that outputs pumping light having a wavelength of 980 nm for both the first and second light sources is used.

First amplifying optical fiber 1 used in the preceding stage
Has a narrow gain band, and 1562 nm is outside the gain plateau. The gain at 1552 nm and the gain at 1562 nm match as shown in FIG. 4 at a gain of 13 dB, but when the gain at 1552 nm changes by 3 dB, the gain at 1552 nm is 1562 nm.
The gain in nm changes by 5 dB. On the other hand, the second amplification optical fiber 2 used in the subsequent stage has a wide gain band,
The gain at 552 nm and the gain at 1562 nm are always approximately equal.

Here, the loss of the input-side optical components (the first optical isolator 7 and the first optical multiplexer 5) is arranged between the first amplifying optical fiber and the second amplifying optical fiber. Loss of the optical component (the second optical isolator 8 and the second optical multiplexer 6) and loss of the output-side optical component (the third optical isolator 9) are 1.5 dB, 1.5 dB, 1. 0 dB.

Wavelength 15 incident on the present optical fiber amplifier
It is assumed that the optical output level of each signal light having a wavelength of 52 nm and a wavelength of 1562 nm is −20 dBm. When these signal lights are incident, the signal light of -21.5 dBm is incident on the first amplification optical fiber at the preceding stage due to the loss of the optical component on the input side, and the signal light is amplified by 13 dB at a time, and each of the signal lights is amplified by -8. .5 dBm (operating point A in the figure). Furthermore, the ED in the subsequent stage is
A signal light of -10 dBm is incident on F, is amplified by 11 dB and becomes +1 dBm (operating point B), and a signal light of 0 dBm is emitted due to an output-side loss.

In the above embodiment, the case where the optical output levels at the time of incidence of the signal lights of the respective wavelengths are equal has been described. However, when the output levels are different, for example, only the signal light of the wavelength of 1562 nm is incident at -22 dBm. . At this time, the first amplification optical fiber 1 in the preceding stage transmits the signal light having a wavelength of 1552 nm at 16 dB and the signal light having a wavelength of 1562 nm at 18 dB.
It is amplified to -5.5 dBm (operating point A '). Further, the signal light of -7 dBm is incident on the second amplification optical fiber 2 of the subsequent stage due to the interstage loss, and is amplified by 8 dB to be +1 dBm (operating point B '), and the signal of 0 dBm is caused by the loss on the output side. Light is emitted.

In the above-described embodiment, the second
Although the optical fiber 2 for amplifying is used in which G1 = G2, the present invention is not limited to this. That is, the ratio of the change in the gain of the signal light of λ2 to the change in the gain of the signal light of the wavelength λ1 when the input light intensity of the pump light changes in each of the amplification optical fibers at the front and rear stages is different from each other. For example, an arbitrary gain can be set independently for each wavelength.

In this embodiment, the pumping method for each amplification optical fiber is forward pumping, but backward pumping or bidirectional pumping may be used.

The optical fiber amplifier for wavelength division multiplexing transmission of the present invention requires two pumping light sources. However, as compared with a system in which each wavelength is amplified separately for each wavelength and multiplexed after amplification, each signal light is split and multiplexed. It does not require an optical demultiplexer or an optical multiplexer to perform the operation, and the optical output of each excitation light source can be small.

[0037]

As described above, the optical fiber amplifier for wavelength-division multiplexing transmission of the present invention has a configuration provided with two sets of amplifiers each including an amplification optical fiber and a pumping light source for inputting pumping light into the optical fiber. The amplification optical fibers constituting each amplifier differ from each other in the wavelength dependence of the change in gain.
By connecting such amplifying optical fibers in series and individually adjusting the optical output of the pump light to each amplifying optical fiber, the total gain or the optical output of the signal light of each wavelength becomes almost equal. Can be set to Therefore, even with respect to the wavelength-multiplexed signal light, it is possible to amplify the signal light of each wavelength at an arbitrary value so as to have the same gain, or to equalize the optical output of each wavelength. As a result, even if optical amplification is applied to wavelength division multiplexing transmission, the difference in reception level is small, and high quality transmission becomes possible.

[Brief description of the drawings]

FIG. 1 is a block diagram of a first embodiment of an optical fiber amplifier for wavelength division multiplexing transmission according to the present invention.

FIG. 2 is a block diagram of a second embodiment of the optical fiber amplifier for wavelength division multiplexing transmission of the present invention.

FIG. 3 is a block diagram of a third embodiment.

FIG. 4 is a diagram showing a relationship between gains and respective wavelengths of an amplification optical fiber.

FIG. 5 is a diagram showing the relationship between the wavelength of signal light and the gain of an erbium-doped optical fiber.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... 1st amplification optical fiber 2 ... 2nd amplification optical fiber 3 ... 1st excitation light source 4 ... 2nd excitation light source 5 ... 1st optical multiplexer 6 second optical multiplexer 7 first optical isolator 8 first optical isolator 9 first optical isolator 10 optical splitter 11 optical component Wave receiver 12 ... light receiving module 13 ... light receiving module 14 ... control circuit

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI H04J 14/00 14/02

Claims (8)

(57) [Claims] 1. Amplifying an input signal light obtained by multiplexing two signal lights having different wavelengths λ1 and λ2 with each other to obtain a first signal light.
A first amplifying optical fiber that sends out the amplified light, a first pumping light source that outputs a first pumping light that pumps the first amplifying optical fiber, A first optical multiplexer for multiplexing with the first amplifying optical fiber; a first optical multiplexer connected in series with a transmitting side of the first amplifying optical fiber, for amplifying the first amplified light and a second amplified light A second amplifying optical fiber that sends out the second amplifying light; a second pumping light source that outputs a second pumping light that pumps the second amplifying optical fiber; a second optical multiplexer and the wavelength-multiplexed transmission optical fiber amplifiers with for combining the use optical fiber, changing the input light intensity of the excitation light to the amplification optical fiber
The change in gain for the signal light of wavelength λ1
When the rate of change of the gain of the signal light of λ2 is α,
The rate of change of the gain of the first amplification optical fiber.
α1 and the change in gain of the second amplification optical fiber.
2. The optical fiber amplifier for wavelength division multiplexing transmission according to claim 1, wherein α2 is different . 2. An optical fiber for wavelength multiplex transmission according to claim 1,
A amplifier, further disposed on a transmission side of the second amplification optical fiber,
Optical branching for branching a part of the second amplified light and transmitting the branched light
And a light receiving module for receiving the split light and converting it into an electric signal
And the first excitation light source and the second excitation light source in response to the electrical signal.
A control circuit for generating a control signal in the excitation light source;
An optical fiber amplifier for wavelength division multiplexing transmission. 3. A gain for the signal light having the wavelength λ1;
3. The wavelength according to claim 2, wherein an optical output of the first pump light and an optical output of the second pump light are set such that gains for the signal light of the wavelength λ2 are substantially equal. Optical fiber amplifier for multiplex transmission. 4. The optical output of the signal light of the wavelength λ1 from the second amplifying optical fiber and the optical output of the signal light of the wavelength λ2 from the second amplifying optical fiber are substantially equal. 3. The optical fiber amplifier for wavelength division multiplex transmission according to claim 2, wherein an optical output of said first pump light and an optical output of said second pump light are set. 5. The first amplifying optical fiber and a second amplifying optical fiber .
Claim 1 or claim 2 WDM optical fiber amplifier according amplifying optical fiber is characterized in that the erbium-doped optical fiber. 6. A first isolator is arranged on an input side of the first amplification optical fiber in the same direction as a transmission direction of the signal light, and is provided on a transmission side of the first amplification optical fiber. A second isolator is disposed in the same direction as the signal light transmission direction; The optical fiber amplifier for wavelength division multiplex transmission according to claim 2, wherein 7. The first optical multiplexer is arranged on an input side of the first amplifying optical fiber, and the second optical multiplexer is configured to include the first amplifying optical fiber and the second amplifying fiber. 3. An optical fiber amplifier for wavelength division multiplexing transmission according to claim 2 , wherein said optical fiber amplifier is disposed between said amplifying optical fibers. 8. The first optical multiplexer is disposed on a transmission side of the first amplification optical fiber, and the second optical multiplexer is disposed on a transmission side of the second amplification optical fiber. 2. The optical fiber amplifier for wavelength division multiplex transmission according to claim 1 , wherein the optical fiber amplifier is arranged.