JP2710199B2 - 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 dependency 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 provides an optical fiber amplifier for demultiplexing two multiplexed signal lights having different wavelengths .lambda.1 and .lambda.2. A demultiplexer, a first amplifying optical fiber for amplifying the demultiplexed signal light of wavelength λ1, and a demultiplexed wavelength λ
A second amplifying optical fiber for amplifying the second signal light;
A first pumping light source for inputting pumping light to the amplifying optical fiber, a second pumping light source for inputting pumping light to the second amplifying optical fiber, and a first pumping light from the first pumping light source. A first optical multiplexer for multiplexing with the amplification optical fiber, and a second optical multiplexer for multiplexing the excitation light from the second excitation light source with the second amplification optical fiber.
And a third optical multiplexer for multiplexing the amplified lights of the first and second amplifying optical fibers.

[0008] The optical fiber amplifier for wavelength division multiplexing transmission of the present invention also comprises two multiplexed wavelengths λ1 and λ2.
An optical splitter for splitting two signal lights, and a split wavelength λ1
A first amplifying optical fiber for amplifying the signal light of the above, a second amplifying optical fiber for amplifying the branched signal light of wavelength λ2, an excitation light source for outputting the excitation light, and an excitation light from the excitation light source An optical splitter that splits the light into a first split pump light and a second split pump light, a first optical multiplexer that couples the first split pump light to the first amplification optical fiber, A second optical multiplexer that couples the two branching pump lights to the second amplification optical fiber, multiplexes the amplified light from the first amplification optical fiber, and the amplified light from the second amplification optical fiber. A third optical multiplexer,
Means for adjusting the wavelength of the excitation light of the excitation light source.

In particular, in the above configuration, the insertion loss of the first optical multiplexer and the insertion of the second optical multiplexer are set in the difference between the gain of the first amplification optical fiber and the gain of the second amplification optical fiber. It is characterized in that the wavelength of the pumping light source is adjusted so that the difference in loss becomes substantially equal.

[0010]

In the optical fiber amplifier for wavelength-division multiplexing transmission of the present invention, the multiplexed signal light is once split into signal lights of each wavelength by an optical demultiplexer, and an individual amplification optical fiber is provided for each signal light. , The gain can be controlled independently.

Further, in the present invention, since the pump light emitted from one pump light source is branched and used for each amplifying optical fiber, the power consumption of the entire apparatus can be reduced. At this time,
The gain of each amplification optical fiber uses the wavelength dependence of the loss of the optical multiplexer for multiplexing the pump light to each amplification optical fiber, and the loss difference cancels the gain difference of each amplification optical fiber. To control the wavelength of the excitation light source.

[0012]

An embodiment of the present invention will be described in detail with reference to 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.
An example in which two wavelengths are multiplexed is shown. The two multiplexed signal lights are divided into wavelengths λ1 and λ by the optical demultiplexer 1.
The signal is split into two signal lights. The signal light having the wavelength λ1 enters the amplification optical fiber 4. The optical multiplexer 3 is connected to the amplification optical fiber 4, and the pumping light from the pumping semiconductor laser module 6 is multiplexed.

A part of the amplified signal light is split by the optical splitter 5, input to the light receiving module 8, converted into an electric signal, and sent to the control circuit 12. The intensity of the electric signal controls the injection current of the pumping semiconductor laser module 6 so as to obtain a desired gain, and determines the pumping light intensity.

The signal light of wavelength .lambda.2 is formed by an amplifying optical fiber 4b, a pumping semiconductor laser module 6b, and the like. The amplified lights of the wavelengths λ1 and λ2 are multiplexed again by the optical multiplexer 9 and transmitted to the transmission optical fiber.

As described above, the wavelength division multiplexing transmission optical fiber amplifier of the present invention amplifies signal light for each wavelength, so that gain control is easy and the output level of signal light transmitted from the transmission optical fiber is high. Can be made substantially equal.

In this embodiment, the specific signal light wavelengths are 1550 nm and 1560 nm. An erbium-doped optical fiber is used for the amplification optical fiber, and a wavelength of 980 nm is used for the excitation light source.
The semiconductor laser for excitation which outputs the excitation light of this type is used.

Here, for ease of explanation, the case of two wavelengths has been described. However, it is needless to say that the present invention is applicable even when the number of multiplexed wavelengths is three or more. In addition, although the pumping light is forward-pumped, the pumping light may be backward-pumping, or may be pumping from both front and rear.

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

The difference between the second embodiment and the first embodiment is that the first embodiment has a pump light source for each amplifying optical fiber, whereas the second embodiment has a pump light source. Is one. Hereinafter, only the differences between the configurations of the second embodiment will be described.

The light emitted from the pumping semiconductor laser 16 is split into two by an optical splitter 13 and connected to optical multiplexers 12a and 12b, respectively. In the optical multiplexers 12a and 12b, the branched pump lights are respectively amplified by optical fibers 14a and 14a.
Combines with 14b.

A part of the amplified signal light is supplied to the optical splitter 15.
a and 15b, respectively.
The signals are converted into electric signals at a and 18b and sent to the control circuit 20 and the temperature control circuit 21. The control circuit 20 monitors the intensity of the signal light of each wavelength, and the intensity of both light is below the desired optical output level, that is, the amplification optical fiber 1
It is determined whether the gains of 4a and 14b are small. When the gain is small, the current to the semiconductor laser module for excitation is increased, and the excitation light is increased. If the gain is too large, do the reverse.

On the other hand, when one of the gains of the signal light of each wavelength is larger than the desired value and the other is smaller, the temperature is adjusted by adjusting the current to the cooling element by the temperature control circuit 21. The wavelength of the excitation light of the semiconductor laser module for excitation 16 is controlled.

FIG. 4 shows the insertion loss with respect to the wavelength of the incident light from the pumping semiconductor laser module 16 of each of the optical multiplexers 12a and 12b connected to each amplifying optical fiber to the amplifying optical fiber 14a or 14b. FIG. Each of the optical multiplexers 12a and 12b is an interference film filter type, and reflects a light having a wavelength of about 1200 nm or less and transmits a light having a wavelength of 1200 nm or more, and a wavelength of about 980 nm which is the center wavelength of the semiconductor laser for excitation. It is composed of multilayer films having equal pass bands. Here, as shown in FIG. 4, the insertion loss of the optical multiplexer 12a becomes minimum at 970 nm, and gradually increases as the wavelength deviates from this wavelength. The pass bandwidth when the loss increases by about 3 dB is about 36 nm. On the contrary,
Similarly, the optical multiplexer 12b increases the loss as the wavelength shifts, with 990 nm as the center wavelength at which the minimum loss occurs.

Next, a description will be given of gain adjustment by a configuration having the optical multiplexers 12a and 12b having such pass band characteristics.

If the gains of the amplification optical fibers 14a and 14b are equal, the center wavelength of the pumping light of the pumping semiconductor laser module may be set to 980 nm. However, when the gains of the two are different, for example, when the gain of the amplifying optical fiber 14a is smaller than the gain of 14b and the difference L1 is 1.3 dB, the insertion loss of the optical multiplexer 12a is reduced. The wavelength should be selected so as to be 1.3 dB larger than the insertion loss. From FIG.
Since this wavelength is about 976 nm, the current of the control circuit 22 is adjusted so that the center wavelength of the semiconductor laser module 16 for excitation is shortened by 4 nm, and the cooling element 22 is adjusted.
May be temperature controlled. In this case, since the wavelength becomes shorter, the temperature may be lowered. Conversely, for example, when the gain of the amplification optical fiber 12a is smaller than that of the optical fiber 12b by about 2.5 dB, the wavelength at which the insertion loss of the optical multiplexer 12a is smaller than that of the optical multiplexer 12b by 2.5 dB, that is, What is necessary is just to adjust the center wavelength of the excitation light to be 988 nm. In this case, the semiconductor laser for excitation is heated. In the present embodiment, a Peltier element is used as the cooling element 22 so that the semiconductor laser for excitation can be quickly heated and cooled.

The insertion loss from the optical demultiplexer 11 to the amplification optical fibers 14a and 14b through which the signal light passes is designed to be extremely low at about 0.5 dB. Further, since the optical branching device 13 must always branch at a constant ratio, usually one half, a device having a small wavelength dependence of the branching ratio is used.

As described above, by controlling the center wavelength of the pumping light of the pumping semiconductor laser, the insertion loss of each optical multiplexer can be changed, thereby making the gain of each amplifying optical fiber equal. In the above-described second embodiment, the gains of the amplifying optical fibers are adjusted to be equal. However, if there is already a level difference between the multiplexed signal lights, the light receiving modules 18a and 18b It is also possible to control the gain so that the light output levels received at the same time become equal.

[0029]

As described above, in the optical fiber amplifier for wavelength division multiplexing transmission of the present invention, in wavelength division multiplexing transmission, signal light of each wavelength is once demultiplexed, and an optical fiber amplifier is provided for each signal light. Since the amplification is performed, the pumping state of each optical fiber amplifier can be controlled independently by individually adjusting the output of the pumping light, and the gain of each wavelength can be controlled.

Further, in the present invention, since one pumping light source is branched and shared by each optical fiber amplifier, it is possible to reduce power consumption and cost.

The optical fiber amplifier for wavelength division multiplex transmission of the present invention is particularly useful in an optical repeater of a transmission system using a wavelength division multiplex transmission system.

[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 diagram showing the insertion loss characteristics of each optical multiplexer in the second embodiment shown in FIG. 2;

FIG. 4 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 ... Optical demultiplexer 2a, 2b ... Optical multiplexer 4a, 4b ... Amplification optical fiber 5 ... Amplified light optical coupler 6 ... Monitoring optical splitter 7a, 7b .. Excitation semiconductor laser module 9 ... light receiving module 10a, 10b ... control circuit 11 ... optical demultiplexer 12a, 12b ... optical multiplexer 13 ... optical splitter 14a, 14b ... Amplifying optical fibers 15a, 15b Optical splitter 16 Pumping semiconductor laser module 22 Peltier element 18a, 18b Light receiving module 20 Control circuit 21 Temperature control circuit 22 ... cooling element

Claims (6)

(57) [Claims] 1. The multiplexed wavelengths λ1 and λ different from each other.
2, a first optical fiber for amplifying the demultiplexed signal light having the wavelength λ1, and an amplifying the demultiplexed signal light having the wavelength λ2. A second optical fiber for amplification, an excitation light source for outputting excitation light, an optical splitter for splitting the excitation light from the excitation light source into a first split excitation light and a second split excitation light, A first pump light multiplexer for coupling the first branch pump light to the first amplification optical fiber; and a second pump for coupling the second branch pump light to the second amplification optical fiber. an optical multiplexer, and amplified light from said first amplifying optical fiber, an optical multiplexer for multiplexing the amplified light of the second amplifying optical fiber
And vessels, and a means for adjusting the wavelength of the excitation light of the excitation light source
An optical fiber amplifier for wavelength-division multiplexing, wherein the first pumping light multiplexer includes a center wavelength λ0 of the pumping light.
From the excitation light source at a wavelength λa slightly shorter than
Insertion loss into the optical fiber for width is minimized, and the wavelength λ
The distance increases from the distance a, and the second pumping light multiplexer has a smaller wavelength than the wavelength λ0.
From the pumping light source at a longer wavelength λb.
Insertion loss into the fiber
Characterized by increased losses due to
Optical fiber amplifier for WDM transmission. Wherein said excitation light source is a semiconductor laser, means for adjusting the wavelength of the excitation light, according to claim 1 wavelength division multiplexing transmission optical fiber amplifier wherein the heating or cooling the semiconductor laser . 3. A semiconductor device according to claim 2, wherein said means for heating or cooling said semiconductor laser comprises a Peltier device.
An optical fiber amplifier for wavelength division multiplexing transmission as described in the above. 4. An insertion loss of the first optical multiplexer and an insertion of the second optical multiplexer in a difference between a gain of the first amplification optical fiber and a gain of the second amplification optical fiber. 4. The wavelength division multiplexed optical fiber amplifier according to claim 3 , wherein the wavelength of said pumping light source is adjusted so that the difference in loss becomes substantially equal. 5. The method according to claim 1, wherein a wavelength of the pump light source is adjusted so that an optical output level of the first amplification optical fiber is substantially equal to an optical output level of the second amplification optical fiber. The wavelength division multiplexing optical fiber amplifier according to claim 4, wherein: 6. The method of claim 1 wavelength division multiplexing transmission optical fiber amplifier, wherein the amplification optical fiber is an erbium doped optical fiber.