JPH11274623A - Optical amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical amplifier which can provide a stable gain, a small gain deviation between signal wavelengths and less leakage of internal oscillation light with a simple arrangement. SOLUTION: Signal light is demultiplexed by an optical wavelength demultiplexer/multiplexer 14 according to its wavelengths, demultiplexed light of the different wavelengths are gain equalized by optical attenuators 16, a demultiplexed output to which the signal light is not allocated, that is, the demultiplexed output of wavelengths outside of the wavelength band, is fed back to an amplification part which includes an optical demultiplexer 11, an exciting light source 12 and a rate-earth added optical fiber 13 for internal oscillation, thus stabling the gain. That is, a gain stabilization can be realized, a gain dependency on wavelength can be reduced, and it is prevented that the internal oscillation light externally leaks.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical amplifier using a rare earth-doped optical fiber.

[0002]

2. Description of the Related Art An optical fiber amplifier using a rare-earth-doped optical fiber has excellent characteristics such as high gain, high output, and low noise, so that the performance of an optical communication system has been greatly improved. The principle of the optical fiber amplifier is that when excitation light having a wavelength corresponding to the excitation level of the added rare earth ion is incident on the optical fiber, the stimulated emission phenomenon occurs due to the inverted distribution of the energy level of the rare earth ion, and the signal light is generated. It is amplified.

[0003] In particular, an optical fiber amplifier (EDFA) using an optical fiber doped with erbium (Er) has an amplification wavelength band of the lowest loss wavelength band (1.5 times) of a silica-based optical fiber.
(5 μm band), high efficiency, and high gain and low noise amplification characteristics can be easily obtained.

[0004] When an optical fiber amplifier such as an EDFA is used, a wavelength-multiplexed optical signal can be amplified collectively, so that a large-capacity and flexible transmission system using wavelength division multiplexing can be economically constructed. .

However, in such a wavelength division multiplexing transmission system, when the number of multiplexed channels fluctuates dynamically, the gain per wavelength also fluctuates.

In order to avoid such gain fluctuation,
A method has been proposed in which the signal light intensity input to the optical fiber amplifier and the amplified output signal light intensity are monitored, and the pump light intensity is controlled so that the ratio becomes constant.

However, in this method, in addition to the complicated structure, it is difficult to stabilize the gain with high accuracy over a wide input wavelength range, and there is a problem in the transient response characteristic to the time variation of the input signal light power. is there.

Therefore, as another method for solving the above problem, FIG. 4 shows a method for generating oscillation in an optical amplifier and stabilizing the gain by the oscillation.

FIG. 4 is a block diagram of a conventional optical amplifier.

This optical amplifier is, like an ordinary optical fiber amplifier, basically composed of a rare-earth-doped optical fiber 1, an excitation light source 2, an optical multiplexer 3 for multiplexing signal light and excitation light, and an input / output optical isolator. It consists of four and five. Optical fiber grating filters 6 and 7 are connected to both ends of the rare-earth-doped optical fiber 1 in order to cause oscillation.

Optical fiber grating filters 6, 7
Is a technique in which a diffraction grating is formed in an optical fiber containing a high concentration of germanium by irradiating a high-output short-wavelength laser beam by a holographic method. According to such optical fiber grating filters 6 and 7, it is possible to reflect only an optical signal having a desired wavelength and bandwidth by changing the period of the diffraction grating, the number of gratings, and the like. In addition, there is an advantage that the loss due to connection or the like is small as compared with the case where it is constituted by conventional optical components.

In the optical amplifier shown in FIG. 4, oscillation occurs at the wavelength selected by the optical fiber grating filters 6 and 7, and the gain of the amplifier is fixed. The magnitude of the gain is determined so that the gain of the feedback group including the amplifying unit and the reflecting unit becomes 1, and thus is determined by the reflectance of each of the optical fiber grating filters 6 and 7.

According to such a method using internal oscillation, a constant gain can be always maintained irrespective of the change in the input signal light power, and the transient response to the time change of the input signal light power is also stable. doing. Moreover, complicated electrical control is not required.

FIG. 5 is a diagram showing the difference in the dependence of the gain on the input signal light power depending on the presence or absence of optical feedback. In the figure, the horizontal axis indicates the input signal light power, and the vertical axis indicates the gain.

In the case where there is no optical feedback, the gain greatly varies depending on the input signal light power. However, by performing the optical feedback control (oscillation), the input signal power range in which the gain is stabilized to a constant gain is obtained. It is enlarged. In addition, when there is no control, the wavelength dependence of the gain changes according to the input signal light power.However, by performing the control by the feedback loop, the wavelength dependence of the gain is reduced in a range where the gain is stabilized. Be stabilized.

[0016]

However, the conventional optical amplifier has a problem that the internal oscillation light is output to the outside together with the signal light. In the signal light power region where the gain is stabilized by the internal oscillation, the oscillating light has a higher optical power than the signal light, so that there is a problem that such light is emitted to the outside.

Further, in the wavelength multiplexing application, it is required that the amplification gain is constant irrespective of the signal wavelength. However, in the prior art, the gain deviation between the signal wavelengths depends on the wavelength dependence of the gain of the amplification medium. Occurs. In order to reduce such a gain deviation, a method of inserting a filter for gain equalization into the amplified output has been studied. However, in order to sufficiently reduce the gain deviation, complicated equalization filter characteristics are required. You.

An object of the present invention is to solve the above-mentioned problems and to provide an optical amplifier which has a stable configuration, a small gain deviation between signal wavelengths, a small leakage of internal oscillation light, and a simple configuration. .

[0019]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention provides a rare earth-doped optical fiber which is excited by pumping light and has a signal light having a different wavelength within an amplification wavelength band of the rare earth-doped optical fiber. An optical amplifier connected to an optical multiplexer / demultiplexer for multiplexing / demultiplexing the optical fiber, comprising coupling means for optically coupling one of the output ports of the optical multiplexer / demultiplexer to the input side of the rare-earth-doped optical fiber. .

According to the present invention, the terminal A is connected to the terminal B and the terminal B is connected to the output side of the rare-earth-doped optical fiber excited by the excitation light.
A terminal B of an optical circulator having a forward characteristic is connected to a terminal C and a signal C having a plurality of different wavelengths within the amplification wavelength band of the rare earth-doped optical fiber is coupled to the input side of the rare earth-doped optical fiber. An optical amplifier to which a demultiplexer is connected, wherein one of output ports of an optical multiplexer / demultiplexer is connected to a terminal A or a terminal C of an optical circulator.

According to the present invention, the terminal A is connected to the terminal B and the terminal B is connected to the output side of the rare-earth-doped optical fiber excited by the excitation light.
A terminal B of an optical circulator having a forward characteristic is connected to a terminal C and a signal C having a plurality of different wavelengths within the amplification wavelength band of the rare earth-doped optical fiber is coupled to the input side of the rare earth-doped optical fiber. An optical amplifier to which a demultiplexer is connected, wherein one of the output ports of the optical multiplexer / demultiplexer is connected to a terminal A of the optical circulator via an optical coupler, and at least one of the other output ports of the optical multiplexer / demultiplexer is connected. Are connected to a light reflector.

In addition to the above configuration, the optical amplifier of the present invention may include an optical attenuator having a variable attenuation between the optical reflector and the output port of the optical multiplexer / demultiplexer.

In addition to the above configuration, the optical multiplexer / demultiplexer of the optical amplifier of the present invention may be an integrated optical multiplexer / demultiplexer using an arrayed waveguide grating formed on a substrate.

According to the present invention, the signal light is demultiplexed for each wavelength by the optical multiplexer / demultiplexer, the gain is equalized by the optical attenuator for each demultiplexed wavelength, and the demultiplexer not allocated to the signal light is demultiplexed. The output, that is, the demultiplexed output of a wavelength outside the signal wavelength band, is fed back to the amplifying unit to internally oscillate, thereby stabilizing the gain. That is, it is intended to stabilize the gain and reduce the wavelength dependence of the gain and to prevent the internal oscillation light from leaking outside.

[0025]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

FIG. 1 is a block diagram showing one embodiment of the optical amplifier of the present invention.

In this optical amplifier, a three-terminal optical circulator 10 having forward characteristics from terminal A to terminal B and terminal B to terminal C, and one input port 11a is connected to terminal B of the optical circulator 10. An optical multiplexer 11; an excitation light source 12 connected to the other input port 11b of the optical multiplexer 11; a rare earth-doped optical fiber 13 whose input side is connected to an output port 11c of the optical multiplexer 11; An optical multiplexer / demultiplexer 14 connected to the output side of the additional optical fiber 13, and a coupling means inserted between one of the plurality of output ports of the optical multiplexer / demultiplexer 14 and the terminal C of the optical circulator 10. Optical attenuator 15 and optical multiplexer / demultiplexer 14
Optical attenuators 16 connected to the remaining output ports of
(16-0 to 16-N).

Next, the operation of the optical amplifier will be described. It is assumed that signal light having wavelengths λ _{0 to} λ _N is wavelength-multiplexed and input to this optical amplifier.

The signal light input goes from the terminal A to the terminal B of the optical circulator 10,
And multiplexed with the excitation light from the optical fiber 13 to enter the rare-earth-doped optical fiber 13. The signal light amplified by the rare-earth-doped optical fiber 13 is separated by an optical multiplexer / demultiplexer 14 for each of the channels having different wavelengths, and the gain of the amplification unit (including the optical multiplexer 11, the pumping light source 12, and the rare-earth-doped optical fiber 13) is improved. The light is output through an optical attenuator 16 that compensates for the wavelength dependency.

On the other hand, of the output ports of the optical multiplexer / demultiplexer 14, a demultiplexing output port outside the signal wavelength band (wavelength λ in the figure)
_{The (N + 1} output port) is connected to the terminal C of the optical circulator 10 via the optical attenuator 15. Therefore, of the spontaneous emission light generated in the amplifying unit, the light near the wavelength λ _{N + 1} reaches the terminal C from the terminal B of the optical circulator 10 and the light attenuator 1
5. Form a feedback loop for inputting the signal again to the amplifier through the optical multiplexer / demultiplexer 14. The amplifier oscillates near the wavelength by this feedback loop. Due to this oscillation, the gain of the amplifier is kept constant irrespective of the signal light input power as in the conventional example shown in FIG.

The gain of the amplifying section can be adjusted by changing the loss of the feedback loop by the optical attenuator 15.

With such a configuration, each channel can obtain a strictly constant gain irrespective of the input signal light power, and the internal oscillation light does not leak to the outside.

FIG. 2 is a block diagram showing another embodiment of the optical amplifier of the present invention.

While the optical amplifier shown in FIG. 1 has a demultiplexed signal light output, this optical amplifier is different in that both the input and output of the optical amplifier are wavelength-multiplexed signal light.

This optical amplifier comprises an optical coupler 17 into which signal light is input to an input port 17a, a three-terminal optical circulator 10 having a terminal A connected to an output port 17c of the optical coupler 17, and one input port 11a. Is an optical multiplexer 11 connected to a terminal B of the optical circulator 10, and an excitation light source 1 connected to the other input port 11b of the optical multiplexer 11.
2, an optical fiber 13 whose input side is connected to an output port 11c of the optical multiplexer 11, an optical multiplexer / demultiplexer 14 whose input port is connected to the output side of the optical fiber 13, and an optical multiplexer / demultiplexer. 14, an optical attenuator 15 inserted between one of the plurality of output ports and the input port 17b of the optical coupler 17, and an optical attenuator 16 connected to the remaining output ports of the optical multiplexer / demultiplexer 14. -0 to 16-N;
Light reflector 1 connected to each optical attenuator 16-0 to 16-N
8 (18-0 to 18-N). In addition,
The basic amplifying unit is the same as the optical amplifier shown in FIG.

The wavelength-multiplexed signal light input to one input port 17a of the optical coupler 17 reaches from the terminal A to the terminal B of the optical circulator 10 and is amplified by the optical multiplexer / demultiplexer 14 to each channel. The light is separated, totally reflected by the light reflector 18 via the optical attenuator 16 for each wavelength, and output again from the terminal B to the terminal C of the optical circulator 10 via the same optical path. Each optical attenuator 16 is adjusted so that the amplified optical output of each channel becomes equal.

On the other hand, of the output ports of the optical multiplexer / demultiplexer 14, the demultiplexed output port outside the signal wavelength band (the output port of the wavelength λ _{N + 1 in} the figure) passes through the optical attenuator 15 and is connected to the input signal by the optical coupler 17. Are multiplexed. Therefore, of the spontaneous emission light generated in the amplifying unit, light near the wavelength λ _{N + 1} is the optical multiplexer / demultiplexer 14,
A feedback loop is formed to be input again to the amplification unit via the optical attenuator 15, the optical coupler 17, and the optical circulator 10. Due to this feedback loop, the optical amplifier oscillates in the vicinity of the wavelength, and the oscillation keeps the gain of the amplifying unit constant irrespective of the signal light input power as in the optical amplifier shown in FIG. The gain of the amplifying section can be adjusted by changing the loss of the loop by the optical attenuator 15.

With such a configuration, similarly to the optical amplifier shown in FIG. 1, each channel can obtain a strictly constant gain regardless of the input signal light power, and the internal oscillation light leaks to the outside. Not even. In addition, since the signal light reciprocates in the amplification medium, the amplification efficiency is improved.

FIG. 3 is a block diagram showing another embodiment of the optical amplifier of the present invention.

The difference from the optical amplifier shown in FIG. 2 is that a signal light input / output port is provided instead of a part of the light reflector.

This optical amplifier comprises an optical coupler 17 into which signal light is input to an input port 17a, a three-terminal optical circulator 10 having a terminal A connected to an output port 17c of the optical coupler 17, and one input port 11a. Is an optical multiplexer 11 connected to a terminal B of the optical circulator 10, and an excitation light source 1 connected to the other input port 11b of the optical multiplexer 11.
2, an optical fiber 13 whose input side is connected to an output port 11c of the optical multiplexer 11, an optical multiplexer / demultiplexer 14 whose input port is connected to the output side of the optical fiber 13, and an optical multiplexer / demultiplexer. 14, an optical attenuator 15 inserted between one of the plurality of output ports and the input port 17b of the optical coupler 17, and an optical attenuator 16 connected to the remaining output ports of the optical multiplexer / demultiplexer 14. -1 to 16-N;
Optical reflector 1 connected to each optical attenuator 16-1 to 16-N
8-1 to 18-N.

The optical attenuator 1 connected to the demultiplexing port of the optical multiplexer / demultiplexer 14 for outputting the wavelength λ _K instead of the optical reflector.
The optical transmitter Tx and the optical receiver Rx are connected to 6-K via an optical circulator 19 having three terminals. With such a configuration, the wavelength λ of the signal light input to the optical amplifier is
Only the _K component is dropped and received, and another signal of the same wavelength can be newly inserted.

An optical attenuator 1 connected to the demultiplexing port of the optical multiplexer / demultiplexer 14 for outputting the wavelength λ _N instead of the optical reflector.
6-N, an optical transmitter Tx and an optical receiver Rx are connected via a 1 × 2 optical switch 20 and a three-terminal optical circulator 21 which can be switched with the optical reflector 18-N. With such a configuration, the operation of the corresponding channel can be selected from the relay / amplification of the input signal and the above-described drop / add by the switching operation of the optical switch.
Such a switchable wavelength channel dropping / adding function is necessary for realizing a highly functional and flexible wavelength multiplexing network in the future.

As described above, according to the present invention, (1) by providing a feedback loop in an optical fiber amplifier and oscillating, the gain of the amplifying section can be stabilized, and a constant gain can be obtained regardless of the fluctuation of the input signal light power. Is obtained.

(2) By using a free port (output port outside the signal band) of the signal light duplexer and forming a loop for cyclically feeding back the amplified signal light to the amplification unit via this port, The internal oscillation light can be automatically set outside the signal band, and the oscillation light can be prevented from leaking to the outside.

(3) The wavelength dependence of the gain of the amplifier can be strictly compensated by individually adjusting the demultiplexed outputs of the optical demultiplexer by the optical attenuator.

(4) The amplification efficiency can be improved by adopting an amplification configuration in which the amplified and demultiplexed signal light is reflected and passed again through the amplification medium.

(5) Functions required for realizing an advanced wavelength division multiplexing network, such as a function of selectively dropping / adding only signal light of a specific wavelength and a function of switching a channel to be dropped / added, are simplified. Can be realized.

(6) Since the number of parts is small and the configuration is simple,
Small size and low price are possible.

[0050]

In summary, according to the present invention, the following excellent effects are exhibited.

By returning the demultiplexed output of the wavelength outside the signal wavelength band to the amplifying unit and internally oscillating, the gain is kept constant regardless of the input signal light power and the gain deviation between the signal wavelengths is sufficiently reduced. It is possible to provide an optical amplifier which can be reduced in size, can suppress the leakage of the internal oscillation light, and has a simple configuration.

[Brief description of the drawings]

FIG. 1 is a block diagram showing one embodiment of an optical amplifier of the present invention.

FIG. 2 is a block diagram showing another embodiment of the optical amplifier of the present invention.

FIG. 3 is a block diagram showing another embodiment of the optical amplifier of the present invention.

FIG. 4 is a block diagram of a conventional optical amplifier.

FIG. 5 is a diagram illustrating a difference in characteristics of the dependence of gain on input signal light power depending on the presence or absence of optical feedback;

[Explanation of symbols]

 Reference Signs List 10 optical circulator 11 optical multiplexer 12 excitation light source 13 rare earth-doped optical fiber 14 optical multiplexer / demultiplexer 15, 16, 16-0 to 16-N optical attenuator

Claims (5)

[Claims] An optical amplifier having an output side of a rare-earth-doped optical fiber pumped by pumping light connected to an optical multiplexer / demultiplexer for multiplexing / demultiplexing signal lights of different wavelengths within an amplification wavelength band of the rare-earth-doped optical fiber. An optical amplifier, further comprising coupling means for optically coupling one of the output ports of the optical multiplexer / demultiplexer with the input side of the rare-earth-doped optical fiber. 2. A rare earth-doped optical fiber excited by pumping light is connected to a terminal B of an optical circulator having a forward characteristic from a terminal A to a terminal B and from a terminal B to a terminal C to the output side of the rare earth-doped optical fiber. An optical amplifier in which an optical multiplexer / demultiplexer that multiplexes / demultiplexes a plurality of different wavelengths of signal light within an amplification wavelength band of the rare-earth-doped optical fiber is connected to an input side of the optical fiber, and an output port of the optical multiplexer / demultiplexer. An optical amplifier characterized in that one of the above is connected to the terminal A or the terminal C of the optical circulator. 3. The rare earth-doped optical fiber pumped by the pumping light is connected to a terminal B of an optical circulator having a forward characteristic from a terminal A to a terminal B and from a terminal B to a terminal C on the output side of the rare earth-doped optical fiber. An optical amplifier in which an optical multiplexer / demultiplexer that multiplexes / demultiplexes a plurality of different wavelengths of signal light within an amplification wavelength band of the rare-earth-doped optical fiber is connected to an input side of the optical fiber, and an output port of the optical multiplexer / demultiplexer. An optical amplifier connected to a terminal A of the optical circulator via an optical coupler, and at least one of the other output ports of the optical multiplexer / demultiplexer is connected to an optical reflector. 4. An optical attenuator having a variable attenuation between said optical reflector and an output port of said optical multiplexer / demultiplexer.
An optical amplifier according to claim 1. 5. The optical amplifier according to claim 1, wherein said optical multiplexer / demultiplexer is an integrated optical multiplexer / demultiplexer using an arrayed waveguide grating formed on a substrate.