JPH07142798A - Optical fiber amplifier - Google Patents

Abstract

PURPOSE:To provide an optical fiber amplifier capable of reducing an amplification of natural release beams to realize an enhancement in a gain and a reduction in a noise index. CONSTITUTION:In an optical fiber amplifier 11 in which excited beams and signal beams are simultaneously entered into an amplifying fiber doped with a rare-earch element to convert energy of the excited beams into energy of the signal beams, and to amplify and emit the signal beams, a fiber 12, consisting of a polarization holding fiber doped with the rare-earth element, is used as an amplifying fiber, and also the signal beams are entered into a polarized light mode that allows no increase in optical loss by bending of the fiber 12.

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.

[0002]

2. Description of the Related Art FIG. 5 is a structural explanatory view of a conventional optical fiber amplifier, in which the optical fiber amplifier 1 is Er (erbium).
Fiber 2 for optical amplification doped with a rare earth element such as
A pump light source 3 is provided, and an optical isolator 4, a multiplexer (WDM) 5, and an optical isolator 6 are provided in the middle of the fiber 2.
And are connected in the arrangement shown. The pumping light from the pumping light source 3 enters the multiplexer 5.

In this optical fiber amplifier 1, the fiber 2
By simultaneously entering the pumping light and the signal light into the, the energy of the pumping light is converted into the energy of the signal light, and the signal light is amplified and emitted.

[0004]

However, in the above conventional structure, a part of the light emitted from the rare earth element as spontaneous emission light in the optical amplification fiber propagates through the fiber. The propagating spontaneous emission light is also amplified, and the energy used for amplifying the signal light is reduced accordingly, and there is a problem that the gain does not increase. Further, since the spontaneous emission light becomes large, the noise figure also becomes large.

It is an object of the present invention to provide an optical fiber amplifier which can reduce the amplification of spontaneous emission light to improve the gain and reduce the noise figure.

[0006]

In order to achieve the above object, in the present invention, the pumping light and the signal light are simultaneously incident on the amplification fiber doped with a rare earth element, so that the energy of the pumping light is changed to that of the signal light. In an optical fiber amplifier configured to convert to energy and amplify and emit signal light, a polarization-maintaining fiber doped with a rare earth element is used as the amplification fiber, and the fiber is bent by bending. A configuration (first configuration) is characterized in that the signal light is made to enter a polarization mode in which optical loss does not increase.

In addition, in the optical fiber amplifier having the optical fiber amplifier described in the first structure and an optical coupler for monitoring the output light output, a polarization-maintaining rare earth-doped fiber and the optical coupler are provided. The configuration (second configuration) is characterized by using a holding fiber.

Further, in the optical fiber amplifier according to the first configuration or the second configuration, an optical isolator is configured by a combination of a polarization maintaining fiber which is a fiber in the optical amplifier and a Faraday rotator. (3rd structure).

[0009]

In any of the above configurations, of the spontaneous emission light emitted by the rare earth element, the component orthogonal to the signal light has a large bending loss and is attenuated while propagating through the amplification fiber.
The spontaneous emission light is amplified only by the component of the same polarization as the signal light, which can be reduced to nearly half that of the conventional one.

Further, in the case of the second configuration, by using the polarization maintaining fiber, it is possible to suppress the fluctuation of the optical output, which has been a problem in the prior art, which is caused by the fluctuation of the monitor branching ratio due to the fluctuation of the polarization plane. .

Further, in the case of the third structure, the optical isolator forming the optical amplifier can be composed of only the Faraday rotator without the polarizer and the analyzer, and the number of partial points and the cost can be reduced. be able to.

[0012]

Embodiments of the present invention will be described below with reference to FIGS.

FIG. 1 is an explanatory view of the structure of an optical fiber amplifier.
The optical fiber amplifier 11 includes a fiber 12 for optical amplification.
And a pumping light source 13, and in the middle of the fiber 12,
The optical isolator 14, the multiplexer 15, and the optical isolator 16 are connected in the illustrated arrangement. The pumping light from the pumping light source 13 enters the multiplexer 15.
The fiber 12 is a polarization maintaining fiber doped with a rare earth element.

Signal light of a predetermined polarization mode from the signal light source 17 enters the fiber 12 after passing through the optical isolator 14. This incident light is amplified in the fiber 12 and emitted. In this case, due to the characteristics of the polarization-maintaining fiber, the component of the spontaneous emission light, which has been a problem in the past, orthogonal to the signal light is attenuated, and the amplification of the spontaneous emission light can be suppressed to a low level. Therefore, the gain of signal light is improved and the noise figure is reduced.

Next, this will be described in detail with reference to FIG. When the polarization-maintaining fiber is bent with an appropriate diameter, there is a difference in bending loss between two orthogonal polarization modes. FIG. 2 is a characteristic diagram of a polarization maintaining fiber, with a winding diameter D = 5.
The loss wavelength characteristics of a fiber having a length of 0 mm and a length of 5 m are shown. L ₁ and L ₂ represent bending loss that occurs between two orthogonal polarization modes, and L ₂ has a smaller increase in bending loss.

Therefore, in the present invention, the signal light is incident on the linear polarization mode in which the bending loss of the polarization-maintaining amplification fiber does not increase and the light is amplified. By doing so, the component of the spontaneous emission light, which is orthogonal to the signal light, has a large bending loss and is attenuated while propagating through the amplification fiber. Therefore, the amplification of the spontaneous emission light has the same polarization as the signal light. It is only the component, and it can be reduced to nearly half that in the case of using a conventional non-polarized fiber.

When an optical monitor is attached to the output side to monitor the optical output, if a fiber fusion type coupler having a large branching ratio such as 10: 1 or 20: 1 is used, the polarization dependence is large and the polarization plane is large. The branching ratio fluctuates due to fluctuations in the.

In such a case, as shown in FIG.
A polarization maintaining rare earth fiber 12 similar to that shown in FIG. 1 is used as the amplification fiber, and the amplification optical fiber 2 is connected to the monitoring optical coupler 2.
By using the polarization-maintaining fiber 22 up to 1, the fluctuation of the branching ratio of the monitor coupler can be reduced. This reduces fluctuations in the actual light output due to output control using the monitor light output. Reference numeral 23 is a controller.

Further, since the optical isolator used for the optical amplifier is inserted between the fibers, conventionally,
As shown in FIG. 4A, a combination of a wedge-shaped polarizer 7, an analyzer 8 and a Faraday rotator 9 which have no polarization dependency is used. Reference numerals 10 and 10 are lenses.

If the polarization maintaining fiber is applied to the propagation path as in the present invention, the function of the optical isolator can be achieved only by the Faraday rotator 9 as shown in FIG. Can be reduced.

[0021]

As described above, according to the present invention, by using the polarization-maintaining light amplification fiber, the amplification of spontaneous emission light can be reduced, and the gain and the noise figure can be reduced. Can be realized.

Further, in the conventional optical amplifier, the fluctuation of the monitor branching ratio due to the fluctuation of the polarization plane causes the fluctuation of the optical output, but the fluctuation of the optical output is suppressed by using the polarization maintaining optical amplification fiber. be able to.

Furthermore, by using the polarization-maintaining fiber, the optical isolator that constitutes the optical amplifier can be configured only by the Faraday rotator without the polarizer and the analyzer, and the number of parts can be reduced and the cost can be reduced. Can be realized.

[Brief description of drawings]

FIG. 1 is a structural explanatory view of an optical fiber amplifier according to an embodiment of the present invention.

FIG. 2 is a characteristic diagram of a polarization maintaining fiber according to an embodiment of the present invention.

FIG. 3 is a structural explanatory view of another optical fiber amplifier according to the embodiment of the present invention.

4A and 4B are structural explanatory views of an optical isolator according to an embodiment of the present invention, FIG. 4A showing a conventional one and FIG. 4B showing the present invention.

FIG. 5 is a structural explanatory view of a conventional optical fiber amplifier.

[Explanation of symbols]

 9 Faraday rotator 11 Optical fiber amplifier 12 Polarization-maintaining rare earth-doped fiber 13 Excitation light source 14, 16 Optical isolator 15 Combiner 21 Optical coupler for monitor 22 Polarization-maintaining fiber

Claims (3)

[Claims] 1. The energy of pumping light is converted into the energy of signal light by simultaneously entering pumping light and signal light into an amplifying fiber doped with a rare earth element, so that the signal light is amplified and emitted. In the optical fiber amplifier configured as described above, a fiber (12) obtained by doping a polarization maintaining fiber with a rare earth element is used as the amplification fiber, and the polarization mode is set so that optical loss does not increase due to bending of the fiber (12). An optical fiber amplifier characterized in that signal light is made incident. 2. The energy of the pumping light is converted into the energy of the signal light by simultaneously entering the pumping light and the signal light into the amplification fiber doped with a rare earth element, and the signal light is amplified and emitted. An optical fiber amplifier configured to monitor the output light output using an optical coupler (21), wherein a fiber (12) obtained by doping a polarization maintaining fiber with a rare earth element is used as the amplification fiber, and The signal light is made to enter a polarization mode in which optical loss does not increase due to bending of the fiber (12), and a fiber between the polarization maintaining rare earth-doped fiber (12) and the optical coupler (21) is provided. Polarization maintaining fiber (2
An optical fiber amplifier characterized by using 2). 3. The optical fiber amplifier according to claim 1 or 2, wherein an optical isolator is constituted by a combination of a polarization maintaining rare earth doped fiber (12) which is a fiber in the optical amplifier and a Faraday rotator (9). An optical fiber amplifier characterized by the above.