JPH11112065A - Optical fiber amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical fiber amplifier wherein optical amplification is performed at stable output while polarization state of an input light is held. SOLUTION: An Er(erbium) doped optical fiber 2 of polarized wave hold type, an excitation light source 4a which excites the Er doped optical fiber 2, and a 1% branch coupler 7 wherein a part of the amplification light amplified through the Er doped optical fiber 2 is branched as a monitor light with a remaining part guided to an output end 10 side are provided to constitute an optical fiber amplifier. With a polarizer 6 provided on an incidence end 12 side of the 1% branch coupler 7, when for example, a linear polarized wave in the direction of slow propagation axis corresponding to a slow polarized wave component axis of slow optical propagation in a polarized wave holding optical fiber is inputted to the optical fiber amplifier, only a linear polarized wave in the slow axis direction is allowed to transmit with the polarizer 6 for output. Further, only the linear polarized wave is made incident on a photo- diode 8 as a monitor light. Based on the intensity of the monitor light, a drive current of the excitation light source 4a is controlled for an output of the optical fiber amplifier to be constant.

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 used for optical communication and the like.

[0002]

2. Description of the Related Art In recent years, Er-doped (erbium) -doped optical fibers for amplifying light as light have been developed, and optical fiber amplifiers using the Er-doped optical fibers have been proposed. This optical fiber amplifier includes an Er-doped optical fiber and an E-doped optical fiber doped with Er.
a pumping light source such as a laser diode for pumping the laser light, and a WDM (Wavelength Division Mult) for injecting light from the pumping light source into the Er-doped optical fiber.
iplex) It is configured by connecting optical passive components such as couplers and optical isolators for suppressing reflection.

In addition, an excitation light source, an optical isolator, a WDM
Each coupler is provided with an optical fiber for connection to another optical component, and the optical fibers for connection are fusion-spliced. Er used in such an optical fiber amplifier
The length of the doped optical fiber is set in consideration of amplification characteristics and noise characteristics, and is generally set to a length of several meters to several tens of meters in currently used optical fiber amplifiers. I have.

When an optical fiber amplifier is used together with an optical component having a large polarization dependence, such as an optical modulator, for example, the input light is input by the optical fiber amplifier while maintaining the polarization state of the input light to the optical fiber amplifier. Therefore, a polarization-maintaining optical fiber amplifier in which the Er-doped optical fiber and the connection optical fiber of the optical fiber amplifier are composed of polarization-maintaining optical fibers is used. The polarization-maintaining optical fiber is formed so that the refractive index distributions of the X-axis and the Y-axis at right angles to each other in the optical fiber cross section (transverse cross section) are different from each other. It has a slow axis, which is a component axis, and a fast axis, which is orthogonal to the slow axis and has a higher light propagation speed than the slow axis. As the input light to the polarization-maintaining optical fiber amplifier, linearly polarized light in one of the slow axis and the fast axis of the polarization-maintaining optical fiber is used.

[0005] One of the characteristics of the polarization maintaining optical fiber amplifier is an extinction ratio. The extinction ratio is an index that indicates the degree of leakage of light intensity from the slow axis to the fast axis or from the fast axis to the slow axis of the polarization-maintaining optical fiber. To perform amplification,
It is necessary to keep this extinction ratio large, and development of a polarization-maintaining optical fiber amplifier having a large extinction ratio is desired.

[0006]

However, in a polarization-maintaining optical fiber amplifier, in order to obtain a sufficient gain, a polarization-maintaining Er provided in the optical fiber amplifier is required.
The length of the doped optical fiber needs to be relatively long,
As the length of the Er-doped optical fiber becomes longer, the leakage of light intensity from the slow axis to the fast axis or from the fast axis to the slow axis of the Er-doped optical fiber increases by that much, and the extinction ratio tends to decrease. .

Further, as described above, the optical fiber amplifier
It has a large number of optical components such as an excitation light source, an optical isolator, and a WDM coupler, and since there are many fusion spliced portions of connection optical fibers provided in these optical components,
The leakage of the light intensity occurs when light propagates through many optical components, and furthermore, the leakage of the light intensity also occurs due to the displacement of the main axis of the polarization maintaining optical fiber at the fusion spliced part, and so on. The extinction ratio tends to be small. Therefore, in the conventional optical fiber amplifier, when the input light propagates through the optical fiber amplifier, the polarization state of the light becomes unstable, and the optical amplification is performed while maintaining the input linear polarization state. It was difficult to output light from the output end of the optical fiber amplifier.

The present invention has been made to solve the above-mentioned conventional problems, and an object thereof is to stably perform optical amplification while maintaining the polarization state (linear polarization) of input light. It is to provide an optical fiber amplifier which can be used.

[0009]

In order to achieve the above-mentioned object, the present invention has the following structure to solve the problem. That is, the present invention relates to an optical fiber amplifier having a polarization-maintaining Er-doped optical fiber and a pumping light source for pumping the Er-doped optical fiber. An input end side of an optical branching coupler that connects a part of the amplified light amplified through the fiber as monitor light and guides the rest of the amplified light to an output end side of the optical fiber amplifier is connected to the optical branching coupler. An excitation light source drive circuit that controls the excitation light source so that the output of the optical fiber amplifier becomes constant by controlling the drive current of the excitation light source based on the intensity of the monitor light is provided on the branch end side. ,
Between the exit end of the Er-doped optical fiber and the entrance end of the optical branching coupler, there is a slow axis which is a polarization component axis in which the light propagation speed of the polarization maintaining optical fiber is slow, and the slow axis is orthogonal to the slow axis. Means for solving the problem is a configuration in which a linearly polarized wave transmitting component that transmits only linearly polarized light in one axial direction of a fast axis having a light propagation speed higher than that of the axis is provided.

Further, it is a characteristic feature of the present invention that the linearly polarized wave transmitting component is a polarizer and the linearly polarized wave transmitting component is a polarization dependent type isolator.

In the present invention having the above structure, a part of the amplified light amplified through the Er-doped optical fiber is branched by the optical branching coupler as monitor light, and the rest is guided to the output end of the optical fiber amplifier. However, between the input end of the optical branch coupler and the output end of the Er-doped optical fiber, only linearly polarized light in either the slow axis or the fast axis of the polarization maintaining optical fiber is transmitted. Since the linearly polarized light transmitting component to be provided is provided, even if the extinction ratio of the amplified light deteriorates (even if light leaks from the slow axis to the fast axis or from the fast axis to the slow axis). ,
Only the linearly polarized light in the slow axis direction or the fast axis direction corresponding to the input light to the optical fiber amplifier enters the optical branching coupler. Therefore, of the amplified light, light leaking from the slow axis to the fast axis or light leaking from the fast axis to the slow axis can be ignored, the polarization state of the input light is maintained and output, and the above problem is solved. Will be resolved.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a first embodiment of an optical fiber amplifier according to the present invention. Note that, in the figure, broken lines indicate the flow of electric signals. In FIG. 1, the input end 9 of the optical fiber amplifier is connected to the input end side of a polarization-maintaining Er-doped optical fiber 2 via a polarization-independent isolator 1a. The input end of the WDM coupler 3a is connected to the end 19, the polarization-independent isolator 1b is connected to one end of the output end of the WDM coupler 3a, and the excitation light source 4a is connected to the other end thereof. This embodiment is a backward pumping type optical fiber amplifier. In the figure, reference numeral 5 denotes a polarization maintaining type connection optical fiber, and reference numeral 13 denotes a single mode optical fiber.

At the output end side of the polarization independent isolator 1 b, a part of the amplified light that has been amplified through the Er-doped optical fiber 2 via the polarizer 6 is branched as monitor light, and the amplified light is separated. The input end 1 of the 1% branch coupler 7 as an optical branch coupler for guiding the remainder to the output end 10 side of the optical fiber amplifier.
Two sides are connected. The 1% branch coupler 7 is provided immediately before the output terminal 10 of the optical amplifier.
The splitting coupler 7 guides 99% of the amplified light incident on the 1% splitting coupler 7 to the output terminal 10 side, and splits 1% of the amplified light to the branching end 16 side as monitor light.

On the branch end 16 side of the 1% branch coupler 7, a photodiode 8 for converting the light intensity of the monitor light into an electric signal is provided, and the voltage of the converted electric signal is converted to a reference voltage. By controlling the drive current of the pump light source 4a based on the intensity of the monitor light, the pump light source drive circuit 11 controls the drive of the pump light source 4a so that the output of the optical fiber amplifier becomes constant.
Is provided.

The polarizer 6 is provided between the output end 19 of the Er-doped optical fiber 2 and the input end 12 of the 1% branch coupler 7 as shown in FIG. As a linearly polarized light transmitting component that transmits only linearly polarized light in either the slow axis, which is a slow polarization component axis, and the fast axis, which is orthogonal to the slow axis and has a light propagation speed higher than the slow axis. It works. Note that this linearly polarized light transmission direction is set corresponding to the input light. When the polarization state of the input light is linearly polarized light in the slow axis direction, the light transmission direction of the polarizer 6 is slow. When the polarization state of the input light is linear in the fast axis direction, the light transmission direction of the polarizer 6 is set to transmit only the linear polarization in the fast axis direction. It is set to the direction to be made.

The present embodiment is configured as described above, and the input light input from the input end 9 of the optical fiber amplifier enters the Er-doped optical fiber 2 via the polarization independent isolator 1a. , The Er-doped optical fiber 2, and the amplified light is divided into a WDM coupler 3 a, a polarization-independent isolator 1 b, a polarizer 6, and a 1% branch coupler 7.
Are sequentially transmitted to the output terminal 10 side.

In this case, even if the optical component is of the polarization maintaining type, since the light intensity is transmitted and received between the slow axis and the fast axis (that is, the extinction ratio is not infinite),
The propagation of the Er-doped optical fiber 2 and the optical components such as the WDM coupler 3a causes the extinction ratio to deteriorate, and furthermore, the polarization maintaining optical fibers at the fusion splicing site of the polarization maintaining optical fiber 5 for connection. Although the extinction ratio deteriorates due to a slight shift in the main axis direction, in the present embodiment, the polarizer 6 is provided on the incident side of the 1% branch coupler 7, and only linearly polarized light in the same direction as the input light is emitted. In order to transmit the light, the leakage of the light intensity from the slow axis to the fast axis or the leakage of the light intensity from the fast axis to the slow axis, which is generated in the amplified light incident on the polarizer 6, can be neglected. Only the linearly polarized wave (in the direction of the slow axis or the fast axis) enters the 1% branch coupler 7 and is guided to the input terminal 10 side of the optical fiber amplifier.

Also, only one linearly polarized wave in the same direction as the input light is 1
The light is branched as monitor light by the% branch coupler 7 and is incident on the photodiode 8. Based on the light intensity of the monitor light, the drive control of the pump light source 4 a is accurately performed by the pump light source drive circuit 11. The driving of the optical fiber amplifier is accurately controlled so as to be constant.

According to the present embodiment, as described above, E
A polarizer 6 is provided on the incident side of a 1% branch coupler 7 for guiding the amplified light amplified by the r-doped optical fiber to the output end of the optical fiber amplifier, and only linearly polarized light in the same direction as the input light is transmitted by the polarizer 6. By causing the amplified light to enter the 1% branch coupler 7, it is possible to correct the deterioration of the extinction ratio that occurred before the amplified light entered the polarizer 6, and to perform optical amplification while maintaining the polarization state of the input light. It is possible to do
An optical fiber amplifier having a large extinction ratio can be obtained.

Further, according to the embodiment, as described above, only the linearly polarized wave in the same direction as the input light is made incident on the 1% branch coupler 7 by the polarizer 6, and a part thereof is branched as monitor light. However, since the drive control of the excitation light source 4a is performed based on the light intensity of the monitor light, the drive control of the excitation light source 4a can be performed very accurately, thereby suppressing the fluctuation of the output of the optical fiber amplifier. In addition, an excellent optical fiber amplifier capable of performing optical amplification with a very stable output can be provided.

As described above, according to this embodiment, E
The polarizer 6 that transmits only the linearly polarized light in the same direction as the input light among the amplified lights amplified by the r-doped optical fiber 2 is connected to one polarizer 6.
By providing the input side 12 of the% branching coupler 7, the optical amplification can be performed with a stable output while maintaining the polarization state of the light guided to the output side of the optical fiber amplifier in the same manner as the polarization state of the input light. An optical amplifier having such an excellent effect can be obtained.

FIG. 2 shows a second embodiment of the optical fiber amplifier according to the present invention. In the figure, the same reference numerals are given to the same parts as those in the first embodiment, and the duplicated description is omitted. This embodiment is different from the first embodiment in that a polarization-dependent isolator 14 is provided on the incident side of the 1% branch coupler 7 instead of the polarizer 6 as the linearly polarized wave transmitting component. That is. In the figure, 3b indicates a WDM coupler,
b denotes a pumping light source. In the present embodiment, the pumping light source 4b is also connected to the incident end 18 side of the Er-doped optical fiber 2 to perform bidirectional pumping.

The present embodiment is configured as described above. Also in this embodiment, for example, when linearly polarized light in the slow axis direction of a polarization maintaining optical fiber is input to an optical fiber amplifier as input light. The input light is amplified through the Er-doped optical fiber 2, and of the amplified light, only the linearly polarized light in the slow axis direction in the same direction as the input light passes through the polarization-dependent isolator 14 and is branched by 1%. 1 from the incident end 12 of the coupler 7
% Branch coupler 7 and the output terminal 1 of the optical fiber amplifier.
It is led to 0. Further, based on the monitor light branched by the 1% branch coupler 7, the drive control of the pump light sources 4a and 4b is performed by the pump light source driving circuit 11, and the output of the optical fiber amplifier is controlled to be constant. .

In this embodiment, the first operation is also performed by the above operation.
The same effects as in the embodiment can be obtained. Also,
According to the present embodiment, by using the polarization-dependent isolator 14 as the linear polarization transmission component, the polarization-independent isolator 1b can be omitted, and the number of components of the optical fiber amplifier can be reduced accordingly. The extinction ratio can be improved accordingly.

FIG. 3 shows a third embodiment of the optical fiber amplifier according to the present invention. Also in the figure, the same reference numerals are given to the same names as those in the first and second embodiments, and the duplicate description thereof will be omitted. This embodiment is different from the second embodiment in that the WDM in FIG.
This is to provide a composite module 15 in which the coupler 3a, the polarization dependent isolator 14 and the 1% branch coupler 7 are integrated.

The present embodiment is configured as described above, and this embodiment operates in the same manner as the second embodiment, and can provide the same effects. In the present embodiment, the composite module 15 is provided on the emission end 19 side of the Er-doped optical fiber 2, and the WDM coupler 3a, the polarization dependent isolator 14 and the 1% branch coupler 7 are integrated, so that the WDM coupler Since the polarization-maintaining connection optical fiber 5 for connecting the 3a, the polarization-dependent isolator 14 and the 1% branch coupler 7 can be omitted, the omitted light is used for the polarization-maintenance connection. It is possible to prevent the extinction ratio from deteriorating when the light propagates through the optical fiber 5,
Since the deterioration of the extinction ratio due to fusion splicing of the polarization-maintaining connection optical fibers 5 can be prevented, the extinction ratio can be further improved.

The present invention is not limited to the above-described embodiment, but can adopt various embodiments. For example,
In the first embodiment, the pump light source 4a is provided on the rear side of the Er-doped optical fiber 2 for backward pumping. In the second and third embodiments, the pump light sources 4a and 4b are The pump light source is provided only on the front side of the Er-doped optical fiber 2 for forward pumping.

In the first embodiment, the polarizer 6 is provided as a linearly polarized wave transmitting component on the incident end 12 side of the 1% branch coupler 7, and in the second and third embodiments, the linearly polarized light is transmitted. Although the polarization-dependent isolator 14 is provided as the transmission component, the linear polarization transmission component is not limited to the polarizer 6 and the polarization-dependent isolator 14, and the linear polarization transmission component has the same direction as the input light to the optical fiber amplifier. Any optical component that can transmit only linearly polarized light in one of the slow axis and the fast axis of the polarization maintaining optical fiber may be used.

Further, in the above embodiment, the 1% branch coupler 7 is provided on the output end 10 side of the optical fiber amplifier, and Er is provided.
1% of the amplified light amplified through the doped optical fiber 2 is split as monitor light, and the remaining light is output to the output terminal 1.
Although the light is guided to the zero side, the ratio of the light to be branched as the monitor light is not particularly limited, and may be appropriately set, and may be set to the output terminal 10 side of the optical fiber amplifier depending on the ratio of the branched light. An appropriate optical branching coupler may be provided.

[0030]

According to the present invention, the input end of the optical splitting coupler which branches a part of the amplified light through the Er-doped optical fiber as monitor light and guides the rest to the output end of the optical fiber amplifier. Side, a linearly polarized light transmitting component that transmits only linearly polarized light in any one of the slow axis and the fast axis of the polarization maintaining optical fiber is provided. The linearly polarized wave in one of the slow axis and the fast axis is set as the linearly polarized wave, and only the linearly polarized light in the same direction as the direction of the linearly polarized light is transmitted through the linearly polarized wave transmitting component. Excellent light with a large extinction ratio that can output light while maintaining the polarization state of the input light without being affected by the traffic of light between the slow axis and the fast axis that occurs before the light enters. Fiber amplifier Door can be.

According to the present invention, the excitation light source is driven so that the output of the optical fiber amplifier becomes constant by controlling the excitation current of the excitation light source by the excitation light source drive circuit based on the monitor light. Therefore, the drive control of the excitation light source can be performed very accurately on the basis of the monitor light.Thus, while maintaining the polarization state of the input light, the optical amplification is always performed with a stable output. An excellent optical fiber amplifier that can be performed can be obtained.

Further, by using the linearly polarized wave transmitting component as a polarizer or a polarization dependent type isolator, an excellent optical fiber amplifier having the above effects can be easily formed. In particular, if the linear polarization transmission component is a polarization-dependent isolator, the polarization-independent isolator conventionally provided in the optical fiber amplifier is omitted,
Since an optical fiber amplifier can be configured by providing a polarization dependent isolator instead of a polarization independent isolator, the number of parts of the optical fiber amplifier can be reduced by that much, and extinction is further enhanced. The ratio can be improved.

[Brief description of the drawings]

FIG. 1 is a main part configuration diagram showing a first embodiment of an optical fiber amplifier according to the present invention.

FIG. 2 is a main part configuration diagram showing a second embodiment of the optical fiber amplifier according to the present invention.

FIG. 3 is a main part configuration diagram showing a third embodiment of the optical fiber amplifier according to the present invention.

[Explanation of symbols]

 2 Er-doped optical fiber 4a, 4b Excitation light source 5 Polarization-maintaining connection optical fiber 6 Polarizer 7 1% branch coupler 11 Excitation light source drive circuit 14 Polarization-dependent isolator 15 Composite module

Claims (3)

[Claims] 1. A polarization-maintaining Er-doped optical fiber,
An optical fiber amplifier having a pump light source for pumping the Er-doped optical fiber, wherein a part of the amplified light amplified through the Er-doped optical fiber is branched as monitor light at an emission end side of the Er-doped optical fiber. An input end of an optical branching coupler that guides the rest of the amplified light to the output end of the optical fiber amplifier is connected to the pumping light source based on the intensity of the monitor light. An excitation light source driving circuit for driving and controlling the excitation light source so that the output of the optical fiber amplifier becomes constant by controlling the driving current of the Er-doped optical fiber and the incident end of the optical branch coupler. Between the slow axis, which is the polarization component axis where the light propagation speed of the polarization-maintaining optical fiber is slow, and the fast axis, which is orthogonal to the slow axis and whose light propagation speed is faster than the slow axis. Optical fiber amplifier, characterized in that the linear polarization transmission components for only the transmit linearly polarized deviation or the other axial direction is provided. 2. The optical fiber amplifier according to claim 1, wherein said linearly polarized wave transmitting component is a polarizer. 3. The optical fiber amplifier according to claim 1, wherein said linearly polarized wave transmitting component is a polarization-dependent optical isolator.