JPH07202306A - Optical fiber amplifier for WDM transmission - Google Patents

Abstract

(57) [Abstract] [Purpose] In an optical fiber amplifier for wavelength division multiplexing, which amplifies signal light of two different wavelengths λ1 and λ2,
The gain or the optical output of the signal light between the respective wavelengths caused by the wavelength dependence of the gain of the amplification optical fiber is made substantially equal. [Configuration] An amplifier optical fiber and two amplifiers each composed of a pumping light source for inputting pumping light to the optical fiber are provided, and the amplifier optical fibers of the amplifiers are connected in series. Generally, in the amplification optical fiber, the rate of change in gain with respect to the change in pumping light differs depending on each wavelength. When this is assumed to be the wavelength dependence of the change in gain, two amplification optical fibers with different wavelength dependences should be connected in series and the optical output of the pumping light to each amplification optical fiber adjusted individually. Thus, the total gain or the optical output of the signal light of each wavelength can be set to be substantially equal.

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 the construction of an erbium-doped optical fiber amplifier used for wavelength division multiplexing transmission.

[0002]

2. Description of the Related Art Wavelength multiplex transmission is a method for expanding transmission capacity by using signal lights of a plurality of wavelengths in optical communication. In such a system, an optical multiplexer that multiplexes a plurality of lights of different wavelengths in the transmitter and an optical demultiplexer that demultiplexes the signal light of each wavelength in the receiver are required. Insertion loss occurs in the duplexer.
Therefore, an optical amplifier is used to compensate for the loss at the time of this optical multiplexing / demultiplexing.

Optical amplifiers include semiconductor amplifiers, fiber Raman amplifiers, fiber Brillouin amplifiers, and rare earth-doped fiber amplifiers. In particular, one of the rare earth-doped fiber amplifiers, the 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. In this, the wavelength-multiplexed amplified light of each wavelength is collectively amplified as it is by one amplification optical fiber.

[0005]

In the above-mentioned method of collectively amplifying the multiplexed signal lights, the gain of the optical fiber amplifier has wavelength dependence as shown in FIG. There will be a gain difference. 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.

In view of the above-mentioned drawbacks, the present invention is capable of adjusting the gain for each signal light of each wavelength and making the signal light output of each wavelength from the optical fiber amplifier almost equal. It is to provide an optical fiber amplifier for use.

[0007]

In order to eliminate the above-mentioned drawbacks, the optical fiber amplifier for wavelength division multiplex transmission of the present invention has two components.
A first amplification optical fiber that amplifies the input signal light in which the two signal lights having different wavelengths λ1 and λ2 are multiplexed and sends out the first amplification light; and a first amplification optical fiber. A first pumping light source for outputting a first pumping light for pumping;
A first optical multiplexer that multiplexes the first pumping light into the first amplification optical fiber and a transmission side of the first amplification optical fiber are connected in series to amplify the first amplification light. A second amplification optical fiber for transmitting the second amplification light, a second excitation light source for outputting the second excitation light for exciting the second amplification optical fiber, and a second excitation light for the second excitation light. A second optical multiplexer that combines the two amplification optical fibers, and a light that is arranged on the transmission side of the second amplification optical fiber and that branches a part of the second amplification light and transmits the branched light. A branching device, a light receiving module that receives the branched light and converts the branched light into an electric signal, and a control circuit that generates a control signal to the first pumping light source and the second pumping light source according to the electric signal. It has a feature.

According to the present invention, in the above construction, when the input light intensity of the pumping light to the amplification optical fiber is changed, λ2 with respect to the change of the gain with respect to the signal light of the wavelength λ1.
When the rate of change in the gain of the signal light is defined as α, the rate of change in gain α1 of the first amplification optical fiber and the rate of change in gain α2 of the second amplification optical fiber are different. It has a feature.

The rate of change in gain α1 of the first amplifying optical fiber and the rate of change in gain α2 of the second amplifying optical fiber are the same as those of the first amplifying optical fiber and the second optical fiber. The gain of the signal light having the wavelength λ1 and the gain of the signal light having the wavelength λ2 are equal to each other in a range of a value smaller than the gain of the signal light amplified by.

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

[0011]

The optical fiber amplifier for wavelength division multiplex transmission of the present invention comprises two sets of amplifiers each including an amplification optical fiber and a pumping light source for making pumping light incident on the optical fiber. Are connected in series. With such a configuration, the gain or the optical output for the signal light of each wavelength that is amplified by each amplification optical fiber is set independently by adjusting the optical output of the pumping light, so that The gains after passing through the two amplifiers can be set to be almost equal.

The relationship between the gains of the amplifying optical fibers for two signal lights having different wavelengths λ1 and λ2 is expressed by the following equation (for example, C. Randy Giles, E.
mmanu Desurvire, "Modelli
ng Erbium-Doped Fiber Amp
lives ", JORNAL OF LIGHTW
AVE TECHNOLOGY VOL9, NO. Two
FEBRAUARY 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 the gain when the pump light is changed to the light of No. 2 and the ratio of the change in the gain when the pump light is changed to the light of the wavelength different from λ1 are always constant. It is shown that.

If the change of the gain with respect to the change of the pumping light is not dependent on the wavelength and is always the same, it is possible to amplify the wavelength-multiplexed signal light with a single amplification optical fiber at the same gain.

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

Therefore, according to the present invention, two amplification optical fibers are connected in series, the optical output of the pumping light is adjusted, and the amplification light wavelength-multiplexed signal lights of two different wavelengths are substantially equal to each other. The value is set so that any gain can be set.

First, from equation (1), ΔG2 = αΔG1 (α: constant) (2) The front and rear stages connected in series are subscripts f and b
When expressed by, the total gain of each wavelength due to both amplification optical fibers when both amplification optical fibers are connected in series is
It is shown in equations (3) and (4).

ΔG1 = ΔG1f + ΔG1b (3) ΔG2 = (αfΔG1f) + (αbΔG1b) (4) That is, when αf ≠ αb, the amplification optical fibers in the preceding and succeeding stages are connected to each other. The gain change can be adjusted independently by individually adjusting the optical output of the pumping light. Therefore, the total gain G1 of each wavelength by the two amplification optical fibers
And G2 can be set independently. This makes it possible to equalize the gains of the amplifiers of the respective wavelengths, and if there is a difference in the input level between the signal lights of the respective wavelengths, make the optical outputs of the signal lights of the respective wavelengths after amplification equal. It is also possible to set the gain of each wavelength to.

[0020]

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

FIG. 1 shows a block diagram of a first embodiment of an optical fiber amplifier for wavelength division multiplexing transmission of 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 pumping light from the first pumping light source 3 is multiplexed into signal light. Similarly, a second optical multiplexer is arranged between the first amplification optical fiber and the second amplification optical fiber 2, and the pumping light from the second pumping light source 4 is multiplexed. In order to avoid the influence of return light on the input side of the optical fiber amplifier, between the first amplification optical fiber and the second optical demultiplexer, and on the output side of the second amplification optical fiber, respectively, Each isolators are arranged.

On the output side of the second amplification optical fiber 2,
The optical branching device 10 is arranged and a part of the amplified light is branched.
The branched amplified light is transmitted by the optical demultiplexer 11 to each wavelength λ.
The light beams are demultiplexed into 1 and λ2 and are incident on the light receiving modules 12 and 13, respectively. The amplified light of each wavelength branched by each of the light receiving modules 12 and 13 is converted into an electric signal and sent to the control circuit 14.

On the other hand, an optical branching device 15 is arranged in front of the first optical multiplexer, and a part of the input light of the multiplexed signal light is branched. The branched input light is demultiplexed by the optical demultiplexer 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.

In the control circuit 14, the intensities of the signal lights of the respective wavelengths after amplification are compared with each other to calculate the total gain, and it is determined whether the gain is larger or smaller than the desired gain.
The injection current to the first and second pumping light sources is set by the optical output of each wavelength by the electric signal in the control circuit 14. In the figure, thick lines indicate optical signals, thin lines indicate electrical signals,
And the flow of the injection current to the semiconductor laser, respectively.

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

FIG. 2 shows a block diagram of a second embodiment of an optical fiber amplifier for wavelength division multiplex transmission according to the present invention. In the first embodiment, the optical branching device 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 gains are set to be equal. In contrast, the second
In this embodiment, the optical outputs of the optical signals of the respective wavelengths from the optical fiber amplifier are set to be equal. In this case, the optical branching device 15, the optical demultiplexer 16, the light receiving module 1
7 and 18 are not necessary, 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 demultiplexed into the signal light of each wavelength, and the light receiving module 1
At 2, it is converted into an electric signal and sent to the control circuit 14.
If 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 the ROM or the like built in the control circuit 14 and the signal light is substantially equal at each wavelength, the optical branching device 10 is used. Only the intensity of the branched light branched by is detected, and the gain to be amplified by each amplification optical fiber, that is, the injection current to each pumping light source is uniquely determined.

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

The optical fiber amplifier for wavelength division multiplex transmission of the present invention is configured to collectively amplify the signal light of wavelength 1552 nm and the signal light of wavelength 1562 nm. An optical semiconductor laser that outputs excitation light having a wavelength of 980 nm is used for both the first and second excitation light sources.

First amplification optical fiber 1 used in the previous stage
Has a narrow gain band, and 1562 nm is outside the gain flat region. As shown in FIG. 4, the gain at 1552 nm and the gain at 1562 nm match at a gain of 13 dB, but when the gain at 1552 nm changes by 3 dB, 1562 nm is obtained.
The gain in nm changes by 5 dB. On the other hand, the second amplification optical fiber 2 used in the latter stage has a wide gain band,
The gain at 552 nm and the gain at 1562 nm are almost always equal.

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

Wavelength of 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, a signal light of −21.5 dBm is incident on the first amplification optical fiber in the preceding stage due to the loss of the optical components on the input side, and each of them is amplified by 13 dB and amplified by −8 each. It becomes 0.5 dBm (operating point A in the figure). In addition, due to the interstage loss
Signal light of −10 dBm is incident on F, amplified by 11 dB to become +1 dBm (operating point B), and signal light of 0 dBm is emitted due to output side loss.

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

In the above-mentioned embodiment, the second used in the latter stage
The amplifying optical fiber 2 is used with G1 = G2, but the present invention is not limited to this. That is, in the amplification optical fibers of the front stage and the rear stage, the rate of change in the gain of the signal light of λ2 with respect to the change in the gain of the signal light of the wavelength λ1 when the input light intensity of the pump light changes may be different from each other. For example, any 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 amplification for wavelength division multiplex transmission of the present invention requires two pumping light sources, but each signal light is demultiplexed and combined as compared with the method of amplifying separately for each wavelength and multiplexing after amplification. It has features such as no need for an optical demultiplexer and an optical multiplexer to do so, and a small optical output of each pumping light source.

[0037]

As described above, the wavelength division multiplex transmission optical fiber amplifier of the present invention comprises two sets of amplifiers each including an amplification optical fiber and a pumping light source for making pumping light incident on the optical fiber. The amplification optical fibers forming the respective amplifiers have different wavelength dependences of the change in gain.
By connecting such amplification optical fibers in series and adjusting the optical output of the pumping light to each amplification optical fiber individually, the total gain or optical output of the signal light of each wavelength will be almost equal. Can be set to. Therefore, even with respect to the wavelength-division-multiplexed signal light, the signal light of each wavelength can be amplified with an arbitrary value so that the signal light of each wavelength has the same gain or the optical output of each wavelength is equalized. 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 drawings]

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

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

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

FIG. 4 is a diagram showing a relationship of a gain with respect to each wavelength of the 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]

 1 ... 1st amplification optical fiber 2 ... 2nd amplification optical fiber 3 ... 1st pumping light source 4 ... 2nd pumping light source 5 ... 1st optical multiplexer 6 ... 2nd optical multiplexer 7 ... 1st optical isolator 8 ... 1st optical isolator 9 ... 1st optical isolator 10 ... Optical branching device 11 ... Optical division Wave device 12 ・ ・ ・ Light receiving module 13 ・ ・ ・ Light receiving module 14 ・ ・ ・ Control circuit

Continuation of front page (51) Int.Cl. ⁶ Identification number Office reference number FI Technical display location H01S 3/094 H04J 14/00 14/02 H04B 10/17 10/16 // G02B 27/28 A 9372-5K H04B 9/00 J

Claims (8)

[Claims] 1. A first optical signal amplifying input signal light in which two signal lights of different wavelengths λ1 and λ2 are multiplexed.
A first amplification optical fiber for sending out the amplified light, a first pumping light source for outputting a first pumping light for pumping the first amplification optical fiber, and a first pumping light for the first pumping light A first optical multiplexer that multiplexes into one amplification optical fiber, and a first amplification optical fiber that is connected in series to the transmission side of the first amplification optical fiber and amplifies the first amplification light to obtain a second amplification light. And a second pumping light source for outputting a second pumping light for pumping the second amplifying optical fiber; and a second pumping light for amplifying the second pumping light. A second optical multiplexer that multiplexes into the optical fiber for use, and an optical branching unit that is arranged on the sending side of the second optical fiber for amplification and that splits a part of the second amplified light and sends out branched light. And a light-receiving module that receives the branched light and converts the branched light into an electric signal, and the first excitation unit according to the electric signal. Light source and the second and out granulation of the control signal to the excitation light source control circuit and the wavelength-multiplexed transmission optical fiber amplifier comprising the. 2. A first isolator is disposed on the input side of the first amplification optical fiber in the same direction as the transmission direction of the signal light, and on the transmission side of the first amplification optical fiber. A second isolator is disposed in the same direction as the signal light transmission direction, and a third isolator is disposed on the sending side of the second amplification optical fiber in the same direction as the signal light transmission direction. The optical fiber amplifier for wavelength division multiplex transmission according to claim 1, characterized in that 3. The first optical multiplexer is arranged on the input side of the first amplification optical fiber, and the second optical multiplexer is provided with the first amplification optical fiber and the second optical fiber. The optical fiber amplifier for wavelength division multiplexing according to claim 1, wherein the optical fiber amplifier is arranged between the amplification optical fibers. 4. The first optical multiplexer is arranged on the sending side of the first amplification optical fiber, and the second optical multiplexer is provided on the sending side of the second amplification optical fiber. "Claim 1" characterized by being arranged
An optical fiber amplifier for wavelength division multiplex transmission according to the above. 5. 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 wavelength λ1 when the input light intensity of the pumping light to the amplification optical fiber is changed is α. Sometimes, the rate of change α1 in gain of the first amplification optical fiber and the rate of change α2 in gain of the second amplification optical fiber are different from each other. Optical fiber amplifier for multiplex transmission. 6. A gain for the signal light of the wavelength λ1, and
The optical output of the first pumping light and the optical output of the second pumping light are set so that the gains with respect to the signal light of the wavelength λ2 are substantially equal to each other.
An optical fiber amplifier for wavelength division multiplex transmission according to the above. 7. The optical output of the signal light of the wavelength λ1 from the second amplification optical fiber and the optical output of the signal light of the wavelength λ2 from the second amplification optical fiber are substantially equal to each other. The optical fiber amplifier for wavelength division multiplexing transmission according to claim 5, wherein an optical output of the first pumping light and an optical output of the second pumping light are set in the optical fiber amplifier. 8. The optical fiber amplifier for wavelength division multiplexing transmission according to claim 6, wherein the first and second amplification optical fibers are erbium-doped optical fibers.