JP2728045B2 - Optical fiber amplifier - 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 for directly amplifying an optical signal as light using a rare-earth-doped fiber, and in particular, changes the intensity of pump light based on the optical intensity of the amplified optical signal. The present invention relates to an optical fiber amplifier that controls the amplification gain by performing the control.

[0002]

2. Description of the Related Art As an optical amplifier for amplifying an optical signal, an optical fiber amplifier for directly converting an optical signal directly into light has attracted attention in recent years. Optical fiber amplifiers amplify optical signals by using the stimulated emission phenomenon that occurs when an excitation light enters an optical fiber doped with a rare-earth element such as erbium to make it into an excited state, and an optical signal to be amplified is injected there. Is what you do. The amplification gain of such an optical fiber amplifier can be changed by the light intensity of the pump light supplied to the rare earth doped fiber.

Japanese Patent Laid-Open Publication No. Hei 3-127787 discloses an optical fiber amplifier in which the light intensity of output light from an optical fiber amplifier is monitored and the light intensity of pump light is feedback-controlled so that the light intensity becomes constant. ing.

FIG. 4 shows an outline of a configuration of a conventionally used optical fiber amplifier in which the intensity of pump light is feedback-controlled. In this optical fiber amplifier, an optical signal is amplified by an erbium-doped fiber. The optical signal to be amplified is input to the first optical coupler via the first optical isolator 101. Excitation light to be supplied to the erbium-doped fiber 103 is input from the excitation laser diode 104 to the first optical coupler 102. The first optical coupler 102 wavelength-multiplexes the input optical signal and the pump light and outputs the resultant signal. The output light of the first optical coupler 102 is an erbium-doped optical fiber 1
03 is input. The output light of the erbium-doped optical fiber 103 is input to the second optical coupler 106 via the second optical isolator 105. Second optical coupler 106
Has a function of splitting the amplified optical signal into two.

[0005] One optical signal 107 split by the second optical coupler 106 becomes output light of an optical fiber amplifier. The other optical signal 108 that has been split by the second optical coupler 106 is input to the photodiode 109. The photodiode 109 is a circuit element that outputs an electric signal having a magnitude corresponding to the light intensity of the received light signal. The electric signal from the photodiode 109 is output to the output stabilizing circuit 11.
1 has been entered. The output stabilizing circuit 111 compares the electric signal from the photodiode 109 with a predetermined reference signal, and supplies the drive current 11 to the excitation laser diode 104 so that the average value of the optical power of the amplified optical signal becomes constant.
2 is a circuit for supplying the same.

[0006] The light intensity of the amplified optical signal is detected by a photodiode 109. Output stabilization circuit 111
Compares the magnitude of the electric signal from the photodiode 109 with the magnitude of the reference signal, and adjusts the value of the excitation current supplied to the excitation laser diode so that the light intensity of the amplified optical signal becomes constant. The light intensity of the pumping light output from the pumping laser diode 104 changes in accordance with the supplied pumping current 112, and the amplification gain in the erbium-doped optical fiber 103 changes accordingly. As described above, by changing the intensity of the excitation light based on the output of the photodiode 109, feedback control is performed so that the light intensity of the amplified optical signal becomes constant.

FIG. 5 shows a wavelength characteristic of a gain when pumping light having a constant light intensity is supplied to an erbium-doped optical fiber. The erbium-doped optical fiber has a peak amplification characteristic 121 and a wavelength of 1.55.
In the vicinity of μm, a large amplification gain can be obtained. However, when the wavelength is 1.545 μm or 1.56 μm, almost no gain can be obtained. Therefore, the wavelength band of the optical signal that can be amplified by the erbium-doped optical fiber is usually limited to a very narrow range 122 of 1.548 μm to 1.555 μm.

Japanese Patent Application Laid-Open No. 3-196125 discloses an optical fiber amplifier improved to extend the wavelength band that can be amplified by an erbium-doped optical fiber. In this optical fiber amplifier, the optical signal after amplification or before amplification is passed through an optical filter having a transmission characteristic for smoothing the wavelength characteristic of the gain of the erbium-doped optical fiber. As a result, the peak distribution of the gain of the erbium-doped optical fiber is smoothed, and the wavelength band that can be equivalently amplified is expanded.

[0009]

The amplification gain differs depending on the wavelength of the optical signal, even if the optical intensity of the optical signal before being amplified by the erbium-doped optical fiber is constant. When feedback control is performed so that the light intensity of the amplified optical signal is constant, as the wavelength difference from the wavelength at which the maximum amplification gain is obtained increases, the light of the pump light increases in order to increase the amplification gain. Strength is increased. Therefore, when the wavelength shift becomes large, an excessive current flows through the pumping laser diode, and the life of the pumping laser diode is shortened. Further, the pump laser diode may be damaged by an excessive current, and there is a problem that the reliability of the optical fiber amplifier is reduced.

The wavelength at which the intensity of the pumping light can be changed to absorb fluctuations in the amplification gain within a range that does not cause damage to the pumping laser diode is generally limited to the range of 1.548 μm to 1.555 μm. Therefore, the light source used as the source of the optical signal amplified by the optical fiber amplifier must have a high accuracy for outputting light having a wavelength within this range. For this reason, the light source becomes expensive, and when a light source fails, it takes time to obtain a highly accurate light source,
The operation of the optical communication system may be greatly hindered.

In a case where a line that does not use an optical fiber amplifier and a line that uses an optical fiber amplifier are mixed in a transmission source station, a light source of a high-precision wavelength and a light source of a normal light source are used. Two types must be prepared, and the management becomes complicated. Further, a maintenance person may mix up the light source and attach a normal light source to a line-side device using the optical fiber amplifier. In such a case, since the wavelength of the light source is out of the appropriate band, there is a problem that an excessive drive current flows to the pumping laser diode in order to compensate for a decrease in amplification gain caused by the wavelength.

In JP-A-3-196125, the wavelength characteristic of the gain of an erbium-doped optical fiber is smoothed by an optical filter, but the optical signal output from the optical fiber amplifier to the next stage also passes through the optical filter. ing. For this reason,
There is a problem that the substantial amplification gain of the optical fiber amplifier is reduced.

An object of the present invention is to provide an optical fiber amplifier that does not impose an excessive load on a pumping light source even when the wavelength of an optical signal to be amplified deviates from an appropriate wavelength band.

[0014]

According to the first aspect of the present invention, a rare earth-doped fiber for amplifying an optical signal, excitation light supply means for supplying excitation light to the rare earth-doped fiber, and amplification by the rare earth-doped fiber are provided. Optical branching means for splitting the optical signal after the splitting, and one of the optical signals split by the optical splitting means is input and transmitted in a wavelength band in which a gain equal to or higher than a certain level is obtained by the rare-earth-doped fiber. An optical filter whose transmittance is smaller than the transmittance outside the wavelength band,
Photoelectric conversion means for outputting an electric signal corresponding to the intensity of the output light of the optical filter, comparison means for comparing the value of the electric signal output from the photoelectric conversion means with a predetermined reference value, The optical fiber amplifier is provided with a pumping light intensity changing means for changing the light intensity of the pumping light supplied from the pumping light supply means based on the comparison result of the means.

That is, according to the first aspect of the present invention, the optical signal after amplification is passed through the optical filter, and the light intensity of the optical signal incident on the photoelectric conversion means inside and outside the wavelength band in which a certain or more amplification gain is obtained. To reduce the difference. This allows
It is possible to prevent the intensity of the pump light from becoming excessive even when an optical signal outside the wavelength band in which an amplification gain equal to or more than a certain value is obtained is input.

According to a second aspect of the present invention, a rare earth-doped fiber for amplifying an optical signal, pumping light supply means for supplying pumping light to the rare-earth-doped fiber, and an optical signal amplified by the rare earth-doped fiber And an optical signal that has been branched by the optical branching means is input, and the transmittance in a wavelength band in which a gain equal to or higher than a certain level is obtained by the rare-earth-doped fiber is out of the wavelength band. An optical filter having a transmittance smaller than the transmittance, an opto-electric conversion unit for outputting an electric signal corresponding to the intensity of the output light from the optical filter, and a setting change of the amplification gain for amplifying the electric signal output from the opto-electric conversion unit Possible amplifying means, comparing means for comparing the value of the electric signal amplified by the amplifying means with a predetermined reference value, and pumping light supply based on the comparison result of the comparing means. And the excitation light intensity changing means for changing the light intensity of the excitation light supplied by the means were provided in the optical fiber amplifier.

That is, according to the second aspect of the present invention, the optical signal after amplification is passed through an optical filter, and the light intensity of the optical signal incident on the photoelectric conversion means inside and outside the wavelength band where an amplification gain of a certain level or more can be obtained. To reduce the difference. This allows
It is possible to prevent the intensity of the pump light from becoming excessive even when an optical signal outside the wavelength band in which an amplification gain equal to or more than a certain value is obtained is input. Before the output signal of the photoelectric conversion means is compared with a reference value, the output signal is amplified by an amplifier whose gain can be changed. For example, in the case of amplifying an optical signal of a wavelength outside the wavelength band where a certain amplification gain can be obtained, if the gain of the amplifier is reduced, the difference from the reference value increases and the intensity of the pump light increases. It can be corrected so as to increase the amplification gain in the rare earth doped fiber.

According to the third aspect of the present invention, the optical filter comprises:
It has transmission characteristics that cancel out fluctuations in the amplification gain of the rare-earth-doped fiber due to changes in the wavelength of the optical signal to be amplified.

That is, according to the third aspect of the present invention, an optical filter having a transmission characteristic for canceling a change in amplification gain of a rare-earth-doped fiber due to a change in wavelength of an optical signal to be amplified is used. Thus, the light intensity of the optical signal incident on the photoelectric conversion means does not depend on the wavelength, and the intensity of the pump light can be kept constant even if the wavelength changes.

In the invention according to the fourth aspect, an erbium-doped fiber is used as the rare-earth-doped fiber.

That is, in the invention according to claim 4, an erbium-doped fiber is used as the amplification medium. The erbium-doped optical fiber is suitable for amplifying an optical signal having a wavelength near 1.55 μm with a small loss in the optical fiber.

According to the fifth aspect of the present invention, the amplification gain set in the amplifying means is such that when the wavelength of the amplified optical signal is outside the wavelength band in which the amplification gain is equal to or more than a certain value, the wavelength of the optical signal is within the wavelength band. Is set smaller than that in the case where the pump light supply means is not damaged.

In other words, according to the fifth aspect of the present invention, when the wavelength of the optical signal is in a wavelength band where the gain of the rare-earth-doped fiber is small, the gain of the amplifier arranged before the comparator is set to be small. As a result, the difference from the reference value increases, the intensity of the pump light increases, and correction can be made to increase the amplification gain in the rare-earth-doped fiber.

[0024]

FIG. 1 shows an outline of a configuration of an optical fiber amplifier according to an embodiment of the present invention. An optical signal 11 to be amplified is input to a first optical isolator 12. The output light 13 of the first isolator 12 and the excitation light 15 from the excitation laser diode 14 are both input to the first optical coupler 16. Optical signal 1
3 and the pumping light 15 are multiplexed by the first optical coupler 16 and output. Output light 17 of the first optical coupler 16 is input to an erbium-doped optical fiber 18. The erbium-doped optical fiber is excited by the energy of the excitation light. Here, when an optical signal is input, a stimulated emission phenomenon occurs, and the optical signal is amplified.

The optical signal 19 amplified by the erbium-doped optical fiber 18 is supplied to a second optical isolator 21
, And is input to the second optical coupler 22. The second optical coupler 22 has a function of splitting an input optical signal into two. One of the optical signals 23 after being split is output as output light of the optical fiber amplifier. The other optical signal 24 branched by the second optical coupler 22 is input to the optical filter 25.

The optical filter 25 has a transmission characteristic that cancels out a wavelength-dependent gain variation in the erbium-doped optical fiber 18. The optical signal 26 that has passed through the optical filter 25 is received by the photodiode 27. The photodiode 27 is a circuit element that outputs a current signal 28 having a magnitude corresponding to the light intensity of the received light signal 26. Current signal 2 output from photodiode 27
8 is input to the output stabilizing circuit 29.

The output stabilizing circuit 29 includes a comparing circuit for comparing a reference value (not shown) with the value of the input current signal 28, and a driving circuit for generating an exciting current having a magnitude corresponding to the output value of the comparing circuit. , And a reference power supply that generates a current signal serving as a reference value. The drive current 31 output from the output stabilization circuit 29 is
Is input to The output stabilizing circuit 29 changes the magnitude of the drive current 31 so that the output current 28 of the photodiode 27 matches the reference value. Thus, if the wavelengths are the same, control is performed so that the intensity of the optical signal amplified by the erbium-doped optical fiber 18 becomes constant.

FIG. 2 shows the transmission characteristics of the optical filter shown in FIG. In a range 41 with a wavelength of 1.545 μm or less and a range 42 with a wavelength of 1.56 μm or more, the transmittance of the optical filter is almost close to “1”, and the input light passes therethrough with almost no loss. Also, 1.545 μm
From 1.55 μm to approximately 1.55 μm, the transmittance gradually decreases, and the wavelength range from 1.548 μm to 1.555 μm 43
Has a very low transmission rate. And 1.555
From μm to 1.56 μm, the transmittance gradually increases so as to approach “1”. The wavelength characteristic of the amplification gain of the erbium-doped optical fiber is, as shown in FIG.
Since the shape of the peak has a peak near 5 μm to 1.555 μm, the characteristic of the peak is canceled by passing through the optical filter 25. Therefore, the intensity of light input to the photodiode is 1.545 μm to
It hardly depends on the wavelength over the range of 1.56 μm, and becomes constant.

As described above, by passing through the optical filter 25, the current signal 2 from the photodiode 27 is output.
Since the value of 8 does not depend on the wavelength of the optical signal, the magnitude of the excitation current 31 output from the output stabilizing circuit 29 does not change due to the wavelength. For this reason, even when the wavelength of the optical signal is out of the band where the amplification gain is large, such as 1.545 μm, the excessive drive current 31 is not input to the pumping laser diode 14, so that deterioration or damage due to overload is prevented. Can be prevented. On the other hand, since the output light 23 of the optical fiber amplifier does not pass through the optical filter 25, it can be output as it is without lowering the gain obtained by the erbium-doped optical fiber 18.

If the output light of the optical fiber amplifier is passed through the optical filter, it becomes 1.55 μm to 1.5 μm.
Since the transmittance in the range of 55 μm is low, the amplification effect in the erbium-doped optical fiber is almost canceled.
However, since the optical signal is split into two by the second optical coupler 22 and only the optical signal input to the photodiode passes through the optical filter, the signal can be output without lowering the gain obtained by the erbium-doped optical fiber 18. .

Modification

In the optical fiber amplifier shown in FIG. 1, the intensity of the pump light is constant without depending on the wavelength of the optical signal to be amplified. Therefore, when the wavelength is outside the wavelength range 122 shown in FIG. 5, the light intensity of the output light from the erbium-doped optical fiber 18 becomes low. Since a general optical communication system is operated with a sufficient margin for the received optical power, there is no problem even if the optical power of the output light of the optical fiber amplifier is reduced to some extent. However, if a sufficient margin is not secured, or if the optical power of the output light of the optical fiber amplifier is reduced beyond the margin on the receiving side, normal communication cannot be performed, and Operation may be hindered.

The wavelength of the optical signal amplified by the optical fiber amplifier may fluctuate greatly when the light source is replaced for maintenance. So, when you replace the light source,
If the light intensity of the excitation light is changed to a value suitable for the wavelength of the light output from the light source, appropriate amplification can be performed.
Therefore, in the optical fiber amplifier of the modified example, the output of the photodiode is once amplified by a variable gain amplifier and compared with a reference value, and the intensity of the pump light is changed by adjusting the gain of this amplifier. I can do it.

FIG. 3 shows an outline of the configuration of an output stabilizing circuit in an optical fiber amplifier according to a modification.
The configuration of the optical fiber amplifier according to the modified example is the same as that shown in FIG. 1 except for the output stabilizing circuit, and a description thereof will be omitted. The light output stabilizing circuit 51 includes an amplifier 53 that amplifies a signal 52 from the photodiode 27, a reference power supply 55 that outputs a reference signal 54, a comparison circuit 56 that compares the output of the amplifier 53 with a reference value, And a drive circuit 58 that generates a drive current 31 having a magnitude corresponding to the output value of the circuit 56.

The amplifier 53 is provided with a feedback variable resistor 59 whose resistance value can be changed. By changing the resistance value of the variable resistor 59, the amplification gain of the amplifier 53 can be changed. For example, when the wavelength of the optical signal is 1.545 μm, the amplification gain in the erbium-doped optical fiber is only several dB, as is clear from FIG. In such a case, the value of the variable resistor 59 is changed to reduce the gain of the amplifier 53. As a result, the difference between the output value 61 of the amplifier 53 and the reference value 54 increases, and the drive current 31 increases. As a result, the gain of the erbium-doped optical fiber 18 can be increased to some extent.

If the adjustment by the variable resistor 59 is performed when the light source is replaced for maintenance, even if the wavelength of the light source is out of an appropriate wavelength range to some extent,
The optical power of the output light can be set within a margin on the receiving side. Further, the amplification gain set by the variable resistor 59 needs to be adjusted to a range in which an excessive drive current 31 that accelerates deterioration of the pumping laser diode 14 or causes damage to the pumping laser diode 14 does not flow. . As described above, the amplifier 5 provided before the comparison circuit 56
By adjusting the gain of No. 3, a decrease in the output of the optical fiber amplifier can be compensated to some extent, and the line can be appropriately operated.

In the embodiments and the modifications described above, the optical filter having the transmission characteristic that cancels out the wavelength characteristic of the amplification gain of the erbium-doped optical fiber is used. Any filter having a high transmittance in other bands may be used. Therefore, the transmission characteristics do not necessarily have to cancel each other. Although the erbium-doped optical fiber is used as the amplification medium, an optical fiber to which a rare earth element other than erbium is added may be used. At this time, it goes without saying that the characteristics of the optical filter are those according to the characteristics of the amplification gain of the amplification medium.

[0038]

As described above, according to the first aspect of the present invention, the amplified optical signal passes through the optical filter and then enters the photoelectric conversion means. Even when amplifying an external optical signal, it is possible to prevent the intensity of the excitation light from becoming excessive. This makes it possible to use an inexpensive light source whose output wavelength is not so high in accuracy as an optical signal light source, and it is not necessary to strictly distinguish between a high-precision light source and a low-precision light source. Can be reduced. Furthermore, even if carelessly using light sources having greatly different wavelengths, the pump light source of the optical fiber amplifier is not damaged, so that the reliability of the optical communication system can be improved.

According to the second aspect of the present invention, before the output signal of the photoelectric conversion means is compared with the reference value, the output signal is amplified by the amplifier whose gain can be changed. For example, a constant amplification gain can be obtained. In the case of amplifying an optical signal having a wavelength outside the wavelength band, if the gain of the amplifier is reduced, the intensity of the pump light is increased and the decrease in the gain can be corrected to some extent.

Further, according to the third aspect of the present invention, since an optical filter having a transmission characteristic for canceling the fluctuation of the amplification gain of the rare earth-doped fiber due to the change of the wavelength of the optical signal is used, the light is incident on the photoelectric conversion means. The optical intensity of the optical signal becomes independent of the wavelength. Thereby, the intensity of the excitation light can be kept constant even if the wavelength fluctuates.

According to the fourth aspect of the present invention, an erbium-doped fiber is used as the amplification medium. The erbium-doped optical fiber is suitable for amplifying an optical signal having a wavelength near 1.55 μm with a small loss in the optical fiber.

Further, according to the fifth aspect of the invention, when the wavelength of the optical signal is in a band where the gain of the rare-earth-doped fiber is small, the gain of the amplifier is set small. As a result, the difference from the reference value increases, the intensity of the pump light increases, and correction can be made to increase the amplification gain in the rare-earth-doped fiber.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating an outline of a configuration of an optical fiber amplifier according to an embodiment of the present invention.

FIG. 2 is a characteristic diagram showing transmission characteristics of an optical filter.

FIG. 3 is a block diagram illustrating an outline of a configuration of an output stabilizing circuit in an optical fiber amplifier according to a modified example.

FIG. 4 is a block diagram showing an outline of a configuration of a conventionally used optical fiber amplifier.

FIG. 5 is a characteristic diagram showing wavelength characteristics of gain of an erbium-doped optical fiber.

[Description of Signs] 14 Laser diode for excitation 16, 22 Optical coupler 18 Erbium-doped optical fiber 25 Optical filter 27 Photodiode 29 Output stabilization circuit 31 Driving current 53 Amplifier 55 Reference power supply 56 Comparison circuit 58 Driving circuit 59 Variable resistor

Claims (5)

(57) [Claims] 1. A rare-earth-doped fiber for amplifying an optical signal, pumping light supply means for supplying pumping light to the rare-earth-doped fiber, and an optical branch for splitting an optical signal amplified by the rare-earth-doped fiber Means, and one of the optical signals after being branched by the optical branching means is inputted, and the transmittance in a wavelength band in which an amplification gain equal to or higher than a predetermined value is obtained by the rare-earth-doped fiber is higher than the transmittance outside the wavelength band. A small optical filter, a photoelectric conversion means for outputting an electrical signal corresponding to the intensity of the output light of the optical filter, and a value of the electrical signal output from the photoelectric conversion means is compared with a predetermined reference value. An optical fiber, comprising: a comparison unit; and an excitation light intensity changing unit that changes a light intensity of the excitation light supplied by the excitation light supply unit based on a comparison result of the comparison unit. amplifier. 2. A rare earth-doped fiber for amplifying an optical signal, excitation light supply means for supplying excitation light to the rare-earth-doped fiber, and an optical branch for splitting the optical signal amplified by the rare-earth-doped fiber Means, and one of the optical signals after being branched by the optical branching means is inputted, and the transmittance in a wavelength band in which an amplification gain equal to or higher than a predetermined value is obtained by the rare-earth-doped fiber is higher than the transmittance outside the wavelength band. A small optical filter, a photoelectric conversion means for outputting an electric signal corresponding to the intensity of the output light of the optical filter, and an amplification capable of changing the setting of the amplification gain for amplifying the electric signal output from the photoelectric conversion means. Means for comparing the value of the electric signal amplified by the amplifying means with a predetermined reference value; and supplying the pumping light based on a comparison result of the comparing means. Optical fiber amplifier characterized by comprising an excitation light intensity changing means for changing the light intensity of the supplied excitation light stage. 3. The optical filter according to claim 1, wherein the optical filter has a transmission characteristic for canceling a change in amplification gain of the rare-earth-doped fiber due to a change in wavelength of an optical signal to be amplified. Optical fiber amplifier. 4. The optical fiber amplifier according to claim 1, wherein said rare earth-doped fiber is an erbium-doped fiber. 5. An amplification gain set in the amplification means,
When the wavelength of the optical signal to be amplified is outside the wavelength band where the amplification gain of the predetermined value or more is obtained, the wavelength of the optical signal is smaller than the wavelength within the wavelength band as long as the pumping light supply unit is not damaged. 3. The optical fiber amplifier according to claim 2, wherein the optical fiber amplifier is set.