JP2001156370A - Optical amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical amplifier miniaturized and economized by few components. SOLUTION: Signal light mixed by a mixer 2 is projected to an EDF4 together with excitation light from an excitation light source 3, excited and amplified by excitation light, emitted from the EDF4 and projected to the second circulator 5, emitted from the second port for the circulator 5, passed through an optical part 6, projected in the opposite direction to the incident direction to the EDF4 from the second circulator 5 and repassed through the EDF4, and emitted from the third port for the first circulator 1 through the mixer 2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical amplifier used in an optical fiber communication system or the like and using an active optical fiber as a gain medium.

[0002]

2. Description of the Related Art A conventional optical amplifier of this kind is, for example, shown in FIG.
1 (PFWysocki, et a)
l., IEEE Photon.Technol.Lett., vol.9, pp.1343-1345,19
97) and the second configuration shown in FIG. 22 (E.De.
survire, Erbium-Doped Fiber Amplifiers, Jhon Wiley &
Sons, Inc., Section 6.2, 1994). In the conventional optical amplifier having the first configuration shown in FIG. 21, the signal light incident on the first isolator 101 is transmitted to the first multiplexer 102.
Is multiplexed with the pumping light from the first pumping light source 103, enters a first erbium-doped fiber (hereinafter abbreviated as EDF) 104, is excited by the pumping light, and amplifies the signal light. The amplified signal light passes through the second isolator 105 and then enters the partial reflection type gain equalizer 106. The signal light emitted from the partial reflection type gain equalizer 106 passes through a third isolator 107, and is then multiplexed with a pump light from a second pump light source 109 by a second multiplexer 108. The light enters the second EDF 111, is excited by the pump light, is amplified, and is supplied to the fourth isolator 112.
And is emitted.

In the conventional second optical amplifier shown in FIG. 22, signal light enters a first port of a circulator 121, exits from a second port, and is output from a pump 123 by a multiplexer 123. The multiplexed signal light and the pump light are multiplexed with the pump light, and the multiplexed signal light and the pump light are used as erbium-doped fiber (EDF) 12
5, the signal light is excited by the pump light and amplified. The amplified signal light is used as a wavelength-independent reflector 126.
Incident on the EDF 125, is reflected in the direction opposite to the incident direction, passes through the EDF 125 again, and is incident on the second port of the circulator 121 via the multiplexer 123. From the gain equalizer 122
And exits from the gain equalizer 122.

In the optical amplifiers of the first and second conventional configurations shown in FIGS. 21 and 22, the erbium-doped fibers (EDFs) 104, 111, and 125, which are rare-earth-doped fibers, are used as gain media. But
The rare earth-doped fiber includes a praseodymium-doped fiber and a thulium-doped fiber in addition to the erbium-doped fiber depending on the type of the rare earth.

[0005] The partial reflection type gain equalizer 106 is installed at an intermediate position in order to equalize the gain while keeping the noise figure and optical output power of the amplifier at good values. The partial reflection type gain equalizer 106 is a black type fiber grating or the like, and gives a transmission loss by reflecting a part of the signal light backward with respect to the propagation direction of the signal light.
The gain spectrum is equalized by giving the transmission loss a wavelength dependency.

Before and after the partial reflection type gain equalizer 106,
Isolators 105 and 107 are provided. This is because the signal light or the amplified spontaneous emission light reflected by the partial reflection type gain equalizer 106 is the first and second EDF 1
It is installed for the purpose of removing noise caused by returning to 04,111. Also, the outer two isolators 10
Reference numerals 1 and 112 are provided to remove the instability of the present amplifier due to residual reflected light from the outside.

FIGS. 23A and 23B respectively show FIGS.
3 shows the gain spectrum of the amplifier and the loss spectrum of the gain equalizer 106. FIG. 23A shows spectra when a gain equalizer is not used and when it is used, and gain equalization is performed between wavelengths λ ₁ and λ ₂ .

In the second configuration of the optical amplifier shown in FIG. 22, the circulator 121 and the wavelength-independent reflector 126 are used to improve the pumping efficiency. Since the signal light reflected by the wavelength-independent reflector 126 passes through the erbium-doped fiber (EDF) 125 twice, the gain (in dB) is obtained.
Is twice as large as in the case of a single pass. Further, since the excitation light is also reflected by the wavelength-independent reflector 126, the excitation light intensity in the EDF is enhanced. FIGS. 24A and 24B show a reflectance spectrum and a loss spectrum of the wavelength-independent reflector 126, respectively. The reflectance is set high (close to 100%) and the loss is set small. E
The signal light that has reciprocated through the DF 125 is guided to a signal light output port by the circulator 121, and is output through the gain equalizer 122. The gain spectrum of the amplifier and the loss spectrum of gain equalizer 122 are the same as in FIG. The gain equalizer 122 used in the second configuration may be a partial reflection type or a non-reflection type. Non-reflection type gain equalizers include a Mach-Zehnder type filter and a long-period fiber grating.

FIGS. 25A and 25B show the configuration of the erbium-doped fiber 125 and the wavelength-independent reflector 126 used in the second configuration. FIG.
FIG. 25A shows the case of the silica-based erbium-doped fiber 125a, and FIG. 25B shows the case of the non-silica-based erbium-doped fiber 125b. In the case of the silica-based erbium-doped fiber (EDF) 125a, the EDF and the adjacent optical component pigtail fiber are generally fusion-spliced.

In general, the core diameter of the EDF is considerably smaller than the core diameter of the optical component pigtail fiber, and the fusion splicing of both requires labor and cost. On the other hand, in the case of the non-silica-based erbium-doped fiber (EDF) 125b, since the EDF and the pigtail fiber adjacent to the optical component cannot be fusion-spliced, the silica fiber having a high NA (high numerical aperture) and the EDF are obliquely polished. The end faces are butt-connected, and the high NA silica fiber and the optical component pigtail fiber are fusion-spliced.

[0011]

In the conventional optical amplifier described above, the optical amplifier having the first configuration shown in FIG.
The number of optical components such as the DFs 104 and 111, the pump light sources 103 and 109, and the multiplexers 102 and 108 is large, and there is a problem that the scale of the optical amplifier becomes large and the price becomes high.

Further, in the conventional optical amplifier having the second configuration shown in FIG. 21, the gain equalizer 122 is provided outside the EDF, which is a gain medium, on the output side of the signal light. However, there is a problem that the temperature is reduced by the loss of the gasifier 122.

Further, the EDFs 125a and 125 shown in FIG.
In the configuration of the reflector 5b and the wavelength-independent reflector 126, as described above, fusion splicing is required, or fusion joining is performed after butt-connecting using an obliquely polished end face. And the cost of parts and assembly is high.

The present invention has been made in view of the above,
It is an object of the present invention to provide an optical amplifier which is reduced in size and economical with a small number of components.

[0015]

According to a first aspect of the present invention, there is provided a first optical circulator for receiving a signal light from a first port and emitting the signal light from a second port. An excitation light source for generating excitation light;
A multiplexer for multiplexing the signal light emitted from the second port of the optical circulator and the pumping light from the pumping light source, and the signal light and the pumping light from the multiplexer are incident, and An active optical fiber that is a gain medium that is excited to amplify the signal light, and the signal light and the pump light that have passed through the active optical fiber are input from the first port and output from the second port. A second optical circulator configured to enter a third port, return to the first port, and exit from the first port; and a first port of the second optical circulator. The signal light and the excitation light emitted from the active optical fiber are re-passed in the direction opposite to the incident direction with respect to the active optical fiber, and the signal light and the excitation light re-passed through the active optical fiber in the opposite direction are passed through the multiplexer. A second port of the first optical circulator; Incident on, and subject matter to be emitted from the third port of the first optical circulator.

According to the first aspect of the present invention, the signal light that has passed through the active optical fiber via the first circulator and the multiplexer is returned via the second circulator, and the active optical fiber is inverted. Since the light is re-incident in the direction, the number of components can be reduced.

According to the second aspect of the present invention, the signal light enters from the first port and exits from the second port.
An optical circulator, and an excitation light source that generates excitation light,
A multiplexer for multiplexing the signal light emitted from the second port of the first optical circulator with the pumping light from the pumping light source; and the signal light and the pumping light from the multiplexer are incident on the multiplexer. An active optical fiber that is a gain medium that is amplified by the pumping light and amplifies the signal light, and the signal light and the pumping light that have passed through the active optical fiber are input from the first port and output from the second port. And a second optical circulator configured to enter the third port, return to the first port, and exit from the first port. The signal light and the pump light emitted from the first port are re-passed to the active optical fiber in the direction opposite to the incident direction, and the signal light re-passed through the active optical fiber in the reverse direction is passed through the multiplexer. A second port of the first optical circulator; Incident on, and subject matter to be emitted from the third port of the first optical circulator.

According to the second aspect of the present invention, the signal light that has passed through the active optical fiber via the first circulator and the multiplexer is returned via the second circulator, and the active optical fiber is inverted. Since the light is re-incident in the direction, the number of components can be reduced.

According to the third aspect of the present invention, the signal light enters from the first port and exits from the second port.
An optical circulator, and an excitation light source that generates excitation light,
A multiplexer for multiplexing the signal light emitted from the second port of the first optical circulator with the pumping light from the pumping light source; and the signal light and the pumping light from the multiplexer are incident on the multiplexer. An active optical fiber that is a gain medium that is amplified by the pump light and amplifies the signal light; and a signal light that has passed through the active optical fiber is input from the first port and output from the second port. A second optical circulator configured to enter a third port, return to the first port, and exit from the first port; and a first port of the second optical circulator. The signal light emitted from the first optical circulator is passed through the active optical fiber again in the direction opposite to the incident direction, and the signal light having passed through the active optical fiber is passed through the multiplexer to the second optical circulator. Incident on the first optical circuit And summarized in that emitted from the third port of the Regulator.

According to the third aspect of the present invention, the signal light passing through the active optical fiber via the first circulator and the multiplexer is returned via the second circulator, and the signal light is inverted through the active optical fiber. Since the light is re-incident in the direction, the number of components can be reduced.

According to a fourth aspect of the present invention, there is provided an optical circulator in which signal light enters from a first port and exits from a second port, an excitation light source for generating excitation light,
A multiplexer for multiplexing the signal light emitted from the second port of the optical circulator and the excitation light from the excitation light source;
An active optical fiber, which is a gain medium that is excited by the pump light and amplifies the signal light, is injected with the signal light and the pump light from the multiplexer, and the signal light and the pump light that pass through the active optical fiber are incident. A gain equalizer that is non-reflective to the signal light and the pump light, and is connected to an output of the gain equalizer, and the signal light and the pump light emitted from the gain equalizer are subjected to the gain equalization. A reflector for reflecting the signal light and the pump light reflected by the reflector and passing through the gain equalizer again in the direction opposite to the incident direction with respect to the active optical fiber. The signal light and the pump light, which have passed through the active optical fiber in the reverse direction, enter the second port of the optical circulator via the multiplexer, and exit from the third port of the optical circulator. That is the gist.

According to the present invention, the signal light passing through the active optical fiber via the circulator and the multiplexer is passed through the non-reflection type gain equalizer twice before and after being reflected by the reflector. It is configured to pass through the active optical fiber again in the opposite direction. Therefore, the number of components is relatively small, and the transmission loss is reduced by passing twice through the non-reflection type gain equalizer. Half is fine.

According to a fifth aspect of the present invention, there is provided an optical circulator in which signal light is incident from a first port and emitted from a second port, an excitation light source for generating excitation light,
A multiplexer for multiplexing the signal light emitted from the second port of the optical circulator and the excitation light from the excitation light source;
An active optical fiber, which is a gain medium that is excited by the pump light and amplifies the signal light, is injected with the signal light and the pump light from the multiplexer, and the signal light and the pump light that pass through the active optical fiber are incident. A gain equalizer that is non-reflective to the signal light, and is connected to the output of the gain equalizer, and outputs the signal light and the pump light emitted from the gain equalizer to the gain equalizer. Having a reflector that reflects, the signal light and the pump light that are reflected by the reflector and re-pass the gain equalizer are re-passed to the active optical fiber in a direction opposite to the incident direction, The gist is that the signal light that has passed through the active optical fiber again in the opposite direction enters the second port of the optical circulator via the multiplexer, and exits from the third port of the optical circulator.

According to the fifth aspect of the present invention, the signal light which has passed through the active optical fiber via the circulator and the multiplexer is passed through the non-reflection type gain equalizer twice before and after being reflected by the reflector. It is configured to pass through the active optical fiber again in the opposite direction. Therefore, the number of components is relatively small, and the transmission loss is reduced by passing twice through the non-reflection type gain equalizer. Half is fine.

According to a sixth aspect of the present invention, the signal light enters from the first port and exits from the second port.
An optical circulator, and an excitation light source that generates excitation light,
A multiplexer for multiplexing the signal light emitted from the second port of the first optical circulator with the pumping light from the pumping light source; and the signal light and the pumping light from the multiplexer are incident on the multiplexer. An active optical fiber that is a gain medium that is amplified by the pump light and amplifies the signal light; a gain equalizer that receives the signal light passing through the active optical fiber and is non-reflective to the signal light; A reflector connected to the output of the equalizer and reflecting the signal light emitted from the gain equalizer to the gain equalizer, and emitting the signal light by the reflector to obtain the gain equalization. The signal light re-passed through the optical fiber is re-passed to the active optical fiber in a direction opposite to the incident direction, and the signal light re-passed through the active optical fiber is passed through the multiplexer to the first optical circulator of the first optical circulator. Incident on two ports, a third port of the first optical circulator And summarized in that the al emitted.

According to the present invention, the signal light passing through the active optical fiber via the circulator and the multiplexer is passed through the non-reflection type gain equalizer twice before and after being reflected by the reflector. It is configured to pass through the active optical fiber again in the opposite direction. Therefore, the number of components is relatively small, and the transmission loss is reduced by passing twice through the non-reflection type gain equalizer. Half is fine.

Further, according to the present invention, there is provided an optical circulator in which signal light is incident from a first port and emitted from a second port, an excitation light source for generating excitation light,
A multiplexer for multiplexing the signal light emitted from the second port of the optical circulator and the excitation light from the excitation light source;
An active optical fiber, which is a gain medium that is excited by the pump light and amplifies the signal light, is injected with the signal light and the pump light from the multiplexer, and the signal light and the pump light that pass through the active optical fiber are incident. A reflector that reflects the signal light and the pump light so as to re-pass the active optical fiber in a direction opposite to the incident direction, and has a reflectance depending on the wavelength of the signal light, and a gain and the like. And a reflector having a functionalization function,
The signal light and the pump light reflected by the reflector after being subjected to the gain equalization process are re-passed to the active optical fiber in a direction opposite to the incident direction, and the signal light re-passed through the active optical fiber in a reverse direction. And the pumping light enters the second port of the optical circulator via the multiplexer, and exits from the third port of the optical circulator.

According to the seventh aspect of the present invention, the signal light passing through the active optical fiber via the circulator and the multiplexer is reflected by a reflector having a gain equalizing function to cause the active optical fiber to travel in the reverse direction. , The gain equalizer as in the related art is not required, and the number of components can be reduced.

Further, according to the present invention, there is provided an optical circulator in which signal light is incident from a first port and emitted from a second port, an excitation light source for generating excitation light,
A multiplexer for multiplexing the signal light emitted from the second port of the optical circulator and the excitation light from the excitation light source;
An active optical fiber, which is a gain medium that is excited by the pump light and amplifies the signal light, is injected with the signal light and the pump light from the multiplexer, and the signal light and the pump light that pass through the active optical fiber are incident. A reflector that reflects the signal light and the pump light so as to re-pass the active optical fiber in a direction opposite to the incident direction, and has a reflectance depending on the wavelength of the signal light, and a gain and the like. And a reflector having a functionalization function,
The signal light and the pump light reflected by the reflector after being subjected to the gain equalization process are re-passed to the active optical fiber in a direction opposite to the incident direction, and the signal light re-passed through the active optical fiber in a reverse direction. Is input to the second port of the optical circulator via the multiplexer, and is output from the third port of the optical circulator.

According to the present invention, the signal light passing through the active optical fiber via the circulator and the multiplexer is reflected by a reflector having a gain equalizing function to cause the active optical fiber to travel in the reverse direction. , The gain equalizer as in the related art is not required, and the number of components can be reduced.

According to the ninth aspect of the present invention, the signal light enters the first port and exits from the second port.
An optical circulator, and an excitation light source that generates excitation light,
A multiplexer for multiplexing the signal light emitted from the second port of the first optical circulator with the pumping light from the pumping light source; and the signal light and the pumping light from the multiplexer are incident on the multiplexer. An active optical fiber that is a gain medium that is amplified by the pump light and amplifies the signal light, and the signal light that has passed through the active optical fiber is incident, and the signal light is incident on the active optical fiber in a direction opposite to the incident direction. A reflector that reflects so as to re-pass, and has a reflectance dependent on the wavelength of the signal light, and a reflector having a gain equalization function, and receives a gain equalization process by the reflector. The reflected signal light is re-passed through the active optical fiber in a direction opposite to the incident direction, and the signal light re-passed through the active optical fiber is passed through the multiplexer to a second port of the first optical circulator. Incident on
The point is that the light is emitted from the third port of the first optical circulator.

According to the ninth aspect of the present invention, the signal light that has passed through the active optical fiber via the circulator and the multiplexer is reflected by a reflector having a gain equalizing function to cause the active optical fiber to travel in the reverse direction. , The gain equalizer as in the related art is not required, and the number of components can be reduced.

The present invention according to claim 10 provides the invention according to claims 7 and 8.
In the present invention according to the ninth aspect, the reflector is provided in the active optical fiber and includes a fiber grating that reflects signal light, and the fiber grating has a reflectance independent of a signal light wavelength. Alternatively, the gist is to perform gain equalization with a reflectance depending on the signal light wavelength.

According to the tenth aspect of the present invention, since the reflector is provided in the active optical fiber and is constituted by the fiber grating for reflecting the signal light, the number of components can be reduced, and the economy can be reduced. Can be achieved.

Further, the present invention described in claim 11 is characterized in that, in the present invention according to any one of claims 1 to 10, the active optical fiber is a rare earth doped fiber or a Raman fiber.

According to the eleventh aspect of the present invention, the active optical fiber is a rare earth doped fiber or a Raman fiber.

Further, according to a twelfth aspect of the present invention, in the first, second or third aspect of the present invention, the second optical circulator is provided with an excitation light emitted from the excitation light source and passed through the active optical fiber. The gist of the present invention is that the light is transmitted with low loss and is incident on the active optical fiber again.

In the present invention according to claim 12, the second aspect
The optical circulator is emitted from the excitation light source, passes the excitation light having passed through the active optical fiber with low loss,
Since the light enters the active optical fiber again, the number of constituent parts may be small, and the economy can be achieved.

The present invention according to claim 13 provides the invention according to claims 4 and 5
Or the reflector according to claim 6, wherein the reflector is constituted by a reflecting means including a mirror that reflects the excitation light passing through the active optical fiber and passing through the gain equalizer and incident with high reflectance. That is the gist.

According to the thirteenth aspect of the present invention, the reflector is a reflecting means including a mirror which reflects the excitation light passing through the active optical fiber and passing through the gain equalizer with high reflectance. Since the signal light is transmitted through the gain equalizer, the signal light is amplified by the active optical fiber, and there is no reduction in the optical output power.

According to a fourteenth aspect of the present invention, in the seventh, eighth or ninth aspect of the present invention, the reflector having a reflectance depending on the wavelength of the signal light is provided with a pump having passed through the active optical fiber. The gist of the present invention is that it comprises a reflection unit including a fiber grating for excitation light and a fiber grating for signal light which are arranged in series so as to reflect light at a high reflectance.

According to the fourteenth aspect of the present invention, the reflector having the reflectance depending on the wavelength is constituted by the reflection means including the fiber grating for the excitation light and the fiber grating for the signal light. Excitation light that has passed through the fiber can be reflected with high reflectance.

According to a fifteenth aspect of the present invention, in the tenth aspect of the present invention, the fiber grating installed in the active optical fiber reflects the excitation light having passed through the active optical fiber at a high reflectance. The gist of the present invention includes a reflection unit including a pump light fiber grating and a signal light fiber grating arranged in series.

According to the fifteenth aspect of the present invention, the fiber grating installed in the active optical fiber is:
Since it is constituted by the reflection means including the fiber grating for the excitation light and the fiber grating for the signal light, the excitation light passing through the active optical fiber can be reflected at a high reflectance.

According to a sixteenth aspect of the present invention, there is provided the optical circulator according to any one of the first to fifteenth aspects, wherein the optical circulator is provided on the input side of the first port of the optical circulator to which the signal light is incident. Is shorter than the active optical fiber.
And a second optical amplifier having a gain sufficient to reduce the noise figure of the optical amplifier after the optical circulator.

According to the sixteenth aspect of the present invention, a short active optical fiber is provided before the optical amplifier, that is, on the input side of the optical circulator into which the signal light is incident, so that a sufficient gain can be obtained. Since the first optical amplifier is provided, the noise figure of the optical amplifier after the optical circulator can be appropriately reduced.

At this time, the second active optical fiber is
It is not always necessary to be shorter than the active optical fiber. That is, the simplification and cost reduction of the optical amplifier need not be prevented.

According to a seventeenth aspect of the present invention, in the invention according to any one of the first to sixteenth aspects, a reflector for excitation light is provided between the active optical fiber and the second optical circulator. Is the gist.

According to the seventeenth aspect of the present invention, there is an advantage that the reflection amount of the excitation light increases.

[0050]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram illustrating a configuration of an optical amplifier according to the first embodiment of the present invention. The first shown in FIG.
In the optical amplifier according to the embodiment, the signal light enters the first port of the first circulator 1 and the signal light emitted from the second port is combined with the pump light from the pump light source 3 in the multiplexer 2. Waved. The combined signal light and pump light enter an erbium-doped fiber (abbreviated as EDF) 4 which is a gain medium, and the signal light is pumped and amplified by the pump light, and the amplified signal light and pump light are amplified. Emerges from the EDF 4 and enters the first port of the second circulator 5. Then, the light exits from the second port of the second circulator 5, passes through the optical component 6, enters the third port of the second circulator 5, and enters from the first port into the EDF 4 in the incident direction. Incident in the opposite direction to
4 again, enters the second port of the first circulator 1 via the multiplexer 2, and exits from the third port.

In this embodiment, the gain medium uses an erbium-doped fiber (EDF) 4 which is an active optical fiber, but is not limited to this. The gain medium is a rare earth-doped fiber or a Raman fiber. Yes,
This embodiment shows a case where an erbium-doped fiber is used as the rare-earth-doped fiber.

The optical component 6 has a signal light loss such as a gain equalizer or an optical add / drop circuit. As described above, since the light is reflected through the circulator 5 having the isolator function, the optical component 6 has a directivity with respect to the signal light because of the use of the partial reflection type gain equalizer and the directional coupler. It may be an optical insertion / branching circuit or the like. Of course, the optical component 6 may be a non-reflection type gain equalizer.

Further, a configuration without the optical component 6 may be adopted. In this case, the signal light is simply reflected by the circulator 5, and the circulator 5 is a reflector (mirror) of the signal light.
Becomes However, since it is a reflector using the circulator 5, compared with a normal reflector in which a metal film is deposited on a glass plate,
There are advantages such as high resistance to optical power and low cost.

When the optical component 6 is the gain equalizer, it is easy to set the pumping light so as to transmit through the circulator 5 and the gain equalizer with low loss. In that case, as shown in FIG. 1, the excitation light that has passed through the EDF 4 can be introduced again into the EDF 4,
Excitation efficiency can be improved. The gain spectrum of the amplifier and the transmission loss spectrum of the gain equalizer in this configuration are the same as those in FIG.

FIG. 2 is a diagram showing a configuration of an optical amplifier according to a second embodiment of the present invention. The optical amplifier shown in FIG.
Second circulator 5 in the embodiment shown in FIG.
And a point that a non-reflective gain equalizer 7 and a wavelength-independent reflector 8 are used instead of the optical component 6,
Other configurations and operations are the same.

The optical amplifier according to the second embodiment shown in FIG. 2 is different from the conventional second configuration shown in FIG. 20 in that the non-reflection type gain equalizer 7 has the EDF 125 and the wavelength-independent type. The different point is that it is installed between the reflector 126. However, the gain equalizer 7 used in the second embodiment
Is limited to a non-reflective type.

In the optical amplifier of the present embodiment configured as described above, the signal light passes through the non-reflection type gain equalizer 7 twice before and after being reflected by the wavelength-independent type reflector 8. The signal light that has passed through the non-reflection type gain equalizer 7 is again amplified by the EDF 4 and then output through the circulator 1. The gain spectrum of the amplifier and the transmission loss spectrum of the non-reflection type gain equalizer in this configuration are the same as those in FIG. However, in this configuration, the signal light is reflected by the non-reflection type gain equalizer 7.
Is transmitted twice, the transmission loss may be about half that of FIG. 21 (b).

FIG. 3 is a diagram showing a configuration of an optical amplifier according to a third embodiment of the present invention. The optical amplifier according to the third embodiment shown in FIG. 13 differs from the optical amplifier according to the first embodiment shown in FIG. 1 only in that a wavelength-dependent reflector 9 is provided instead of the second circulator 5 and the optical component 6. And other configurations and operations are the same.

The optical amplifier according to the second embodiment shown in FIG. 3 is different from the conventional second configuration shown in FIG. 20 in that the signal light and the wavelength-independent reflector 126, which is a reflector, are different from each other. The wavelength-dependent reflector 9 differs greatly in that it is devised to have a gain equalizing function. Therefore, the gain equalizer 12 used in the second conventional configuration is used.
2 is unnecessary, and has an advantage that the number of components can be reduced.

The wavelength-dependent reflector 9 is, like the wavelength-independent reflector, a metal-deposited film or a dielectric multilayer film deposited on glass, or a broadband fiber grating. Is changed to give wavelength selectivity of reflectance. A specific example will be described later. FIG. 4A shows the reflectance spectrum of the wavelength-dependent reflector 9.
FIG. 4B shows the loss spectrum. The gain spectrum (before and after gain equalization) of the amplifier using the wavelength-dependent reflector 9 is the same as that shown in FIG.

As described above, in the first and third embodiments, the number of optical components such as the EDF and the pumping light source multiplexer, which has been a problem in the conventional first and second configurations, is large. The disadvantage that the size of the amplifier becomes large and the amplifier becomes expensive is solved. In the second embodiment, the gain equalizer, which has been a problem in the second conventional configuration, is installed outside the gain medium and on the output side of the signal light. This eliminates the drawback of lowering by the loss of the vessel.

FIG. 5 shows the configuration of the erbium-doped fiber (EDF) 4 and the reflector 9 shown in FIG. 3. A fiber grating 10 is used as the reflector 9, and FIG. ), EDF is silica-based EDF4
In the case of a, FIG. 5B shows that the EDF is a non-silica-based EDF.
(Such as fluoride EDF or tellurite EDF) 4b
Is the case. The difference from the prior art shown in FIG. 23 is that the fiber grating 10 as the signal light reflector is directly formed in the EDF. The fiber grating 10 may or may not reflect the excitation light in addition to the signal light. The rare earth-doped fiber is not limited to EDF, and the same holds for the praseodymium-doped fiber, thulium-doped fiber, and the like.
As compared with the case of the prior art shown in FIG. 23, there is an advantage that the number of constituent parts is reduced, and the cost of parts and assembly can be reduced.

FIG. 6 is a diagram showing a specific example of the first embodiment shown in FIG. In the specific example shown in the figure, a silica erbium-doped fiber (EDF) 4 is used as the gain medium.
And a black chirped fiber grating (CFG) is used as the optical component 6 including the partial reflection type gain equalizer and the like.

As already described with reference to FIG.
A considerable amount of the excitation light emitted from the F4 passes through the circulator 5 and the CFG 13 provided at a stage subsequent to the EDF 4 with low loss, and then enters the EDF 4 again to excite the EDF 4, thereby obtaining high excitation efficiency. The wavelength and power of the excitation light from the excitation light source 3 are 1.48 μm and 2 μm, respectively.
00 mW. The length of the EDF 4 is 50 m. FIG.
(A) and (b) show the amplifier gain spectrum and the transmission loss spectrum of the gain equalizer in this specific example, respectively. When the gain equalizer 6 (13) is not used, the gain spectrum is non-flat in the wavelength range of 1560 to 1575 nm, but is flattened in the wavelength range of 1560 to 1600 nm by using the gain equalizer. The flattened gain is 20 dB. The peak loss value of the gain equalizer is 12 dB.

In the prior art configuration shown in FIG. 20 where the same gain spectrum and optical output power can be obtained, the length of the EDF is
50m each with 104 and EDF111, 10 in total
0 m, and the pump light power is about 100 mW for the EDF 104 and about 200 mW for the EDF 111, for a total of about 300 mW. Therefore, this example and the prior art 1
When the configurations are compared, this specific example has a clearly reduced number of components and is inexpensive.

FIG. 8 is a diagram showing a specific example of the second embodiment shown in FIG. In the specific example shown in the figure, a silica erbium-doped fiber (EDF) 4 is used as the gain medium.
And a long period fiber grating (LPFG: Long Period Fiber Grati
ng) 14, and a mirror (a metal film deposited on a glass plate) 15 is used as the wavelength-independent reflector 18. FIG. 9 shows a gain equalizer 7 (14) according to this example.
2 shows the transmission loss spectrum of the sample. The gain spectrum is the same as FIG. The peak loss value of the gain equalizer is 6 which is half that of the specific example (12 dB) shown in FIG.
dB. This is because the signal light passes through the gain equalizer twice, as described above.

In order to obtain the same gain spectrum as that of this example in the second prior art configuration of FIG. 21, the peak loss of the gain equalizer installed in the output stage of the amplifier in the second prior art configuration is 12 dB. The optical output power from the gain medium EDF is limited according to the pumping light power, and the limited optical output power suffers considerable loss in the gain equalizer. Reduce. In this specific example, the signal light is transmitted through the gain equalizer 14 (7) and then amplified and output by the EDF 4, so that the optical output power, which is a drawback of the prior art, is not reduced.

FIG. 10 is a diagram showing a specific example of the third embodiment shown in FIG. In the specific example shown in the figure, silica erbium-doped fiber (EDF) is used as the gain medium.
4 as the wavelength-dependent reflector 9,
Chirped fiber grating (CFG: Chirped F
iber Grating) 16 and 17 are used for signal light and pump light, respectively. FIGS. 11 (a) and 11 (b)
It shows the reflectance spectrum and the reflection loss of the CFG 17 for signal light in this specific example. The reflectance is kept high in the wavelength range of 1575 to 1600 nm and in the wavelength range of 1560 nm or less. Such a reflectance spectrum is obtained by making the distribution density of the chirp wavelength non-uniform. The peak loss value of the reflectivity is 12 dB. The gain spectrum is the same as FIG. on the other hand,
The excitation light CFG 16 reflects the excitation light of 1.48 μm with high reflectance (close to 100%).

This specific example has an advantage that the optical output power is not reduced as in the specific example of the second embodiment shown in FIG. 8, as compared with the second conventional configuration shown in FIG.

FIG. 12 is a diagram showing another specific example of the first embodiment shown in FIG. In the specific example shown in the figure, a Raman fiber 18 is used as the gain medium, and a black chirped fiber grating (CFG: Chirped Fiber Grating) 13 is used as the gain equalizer 6. The excitation light from the excitation light source 3 has two wavelengths, the wavelengths are 1.37 μm and 1.42 μm, and the total power is 300 mW. Raman fiber 18 is 3k long
m. FIGS. 13A and 13B show an amplifier gain spectrum and a gain equalizer 13 in this specific example, respectively.
6 shows a transmission loss spectrum of (6). When the gain equalizer is not used, the gain spectrum has a wavelength range of 1500 to 1
Although it is not flat at 520 nm, it is flattened in a wavelength range of 1470 to 1520 nm by using a gain equalizer. The flattened gain is 15 dB.
The peak loss value of the gain equalizer is 6 dB.

In the prior art configuration of FIG. 19 in which the same gain spectrum and optical output power can be obtained, the length of the Raman fiber is 3 km each for the first Raman fiber and the second Raman fiber, that is, 6 km in total, and the pump light power is , First
About 400 mW for the Raman fiber and about 400 mW for the second Raman fiber, about 800 mW in total.
Therefore, when the present example is compared with the first configuration of the related art, the present example clearly has a smaller number of components and is inexpensive.

FIG. 14 is a diagram showing another specific example of the third embodiment shown in FIGS. The specific example shown in the figure uses a tellurite erbium-doped fiber 19, which is a non-silica-based erbium-doped fiber, as the gain medium, and a black-chirp fiber grating as the fiber grating 10 constituting the wavelength-dependent reflector 9. (CFG: Chirped Fiber Grating) 20
And 21 are used one for signal light and one for pump light.

FIG. 15 shows an amplifier gain spectrum in this example. FIGS. 16A and 16B
Shows the reflectance spectrum and the return loss spectrum of the signal light CFG 21 in this specific example, respectively. When the gain equalizer is not used, the gain spectrum is non-flat in the wavelength range of 1550 to 1570 nm, but by using the gain equalizer, the gain range is 1550 to 1610 nm.
nm. The flattened gain is 2
0 dB. The peak loss value of the gain equalizer is 12 dB.

This embodiment has the advantage that the configuration is simpler and an inexpensive amplifier can be obtained as compared with the prior art shown in FIG.

FIG. 17 is a diagram showing a configuration of an optical amplifier according to the fourth embodiment of the present invention. In the optical amplifier of the present embodiment shown in the figure, a short erbium-doped fiber (EDF) 22 is provided at a stage preceding the optical amplifier 30 which is a specific example of the first embodiment shown in FIG. Provided with a multiplexer 23, an excitation light source 24, and an isolator 25,
This is a new pre-stage optical amplifier, which removes the degradation of the noise figure in the optical amplifier 30.

More specifically, for example, FIGS.
The optical amplifiers of the present invention shown in 3, 6, 8, 10, 12, and 14 and the conventional optical amplifier shown in FIG. 21 have a structure in which signal light is reciprocated in a rare earth-doped fiber or a Raman fiber. The signal light power increases on the input side of the signal light, that is, on the left side of the optical amplifier 30 in FIG. 17, and the noise figure of the optical amplifier deteriorates as compared with the optical amplifier shown in FIG. 19 which does not reciprocate the signal light. Therefore, the present embodiment provides an optical amplifier that can remove the noise figure degradation thereof while maintaining the effects of the optical amplifiers of the present invention described above. Therefore, the fourth signal shown in FIG.
The optical amplifier 30 used in the optical amplifier according to the embodiment is not limited to the specific example of the first embodiment shown in FIG.
The present invention can be applied to all the optical amplifiers shown in FIGS. 1, 2, 3, 6, 8, 10, 12, and 14.

That is, in the fourth embodiment, the EDF
In addition, a short EDF 22 is added to the front stage of the optical amplifier 30 using the optical amplifier 4 to form a multi-stage amplification configuration, thereby eliminating noise figure degradation. The short EDF 22 is excited by a newly provided excitation light source 24. In the optical amplifier of the specific example of the first embodiment shown in FIG.
In this embodiment, the length of the EDF 4 is 45 meters, and the length of the short EDF 22 is 1 meter.
0 meters. The pump light power for the EDFs 4 and 22 of 45 meters and 10 meters in the embodiment is 200 mW and 20 mW, respectively. In the optical amplifier of the specific example of the first embodiment in FIG. 6, the pump light power is 200 mW for the EDF of 50 meters. Therefore, the newly added EDF is the original EDF.
The excitation light source which is considerably shorter than DF and newly added is
Since the required pumping light power can be considerably smaller than that of the pumping light from the beginning, the increase in the price of the amplifier of the present embodiment is smaller than that of the amplifier of FIG. Therefore, when the present embodiment is compared with the first configuration of the related art, the present embodiment has a clearly reduced number of components and is inexpensive.

However, E provided before the optical amplifier 30
The DF 22 does not necessarily have to be shorter than the EDF 4 in the optical amplifier 30. For example, if EDF22 is EDF
4, it is sufficient that the simplification and cost reduction of the optical amplifier 30 according to the present invention do not prevent the simplification and cost reduction of the entire optical amplifier of FIG. Generally, the gain of the EDF 22 may be as small as about 10 dB, and the EDF 22 and the pump light source 24 are inexpensive. Further, the length of the EDF depends on the concentration of Er added, and the like.
Er concentration may be different between DF4 and EDF22.

The numerical example of the noise figure of the amplifier of the specific example of FIG. 6 is about 8 dB, and the numerical example of the noise figure of the amplifier of this embodiment is about 6 dB, and the noise figure is improved.

FIG. 18 is a diagram showing still another specific example of the optical amplifier according to the first embodiment of the present invention. The optical amplifier shown in the figure is a specific example of the optical component 6 provided in a loop formed between the second port and the third port of the second circulator 5 in the embodiment shown in FIG. The only difference is that a signal light insertion / separation circuit 26 is used, and the other configurations and operations are the same. In a conventional optical amplifier compared with the present embodiment, for example, FIG.
In the optical amplifier No. 9, the partial reflection type gain equalizer 106 is replaced with a signal light insertion / separation circuit 26.

In the signal light insertion / separation circuit 26, the AW
G26a and 26b are array waveguide gratings, and λ _out and λ _in are signal light wavelengths to be separated and inserted, respectively.
The first stage AWG 26a separates the multi-wavelength input signal light into a plurality of optical paths according to the wavelength, and the second stage AWG 26b
Multi-wavelength signal light incident on a plurality of optical paths is multiplexed. Since the separation / insertion of the signal light is generally performed using a directional coupler, the insertion / separation circuit 26 can obviously operate in a predetermined signal light input direction. Further, since the insertion / separation circuit 26 does not generally have a low loss with respect to the pumping light, the pumping efficiency is not improved by reusing the pumping light as in the first embodiment.

The wavelength and power of the pump light from the pump light source 3 are 1.48 μm and 200 mW, respectively.
The length of the EDF 4 is 50 m. In the prior art configuration of FIG. 19 in which the same gain and optical output power can be obtained, the length of the EDF is 50 m for the EDF 104 and the EDF 111, respectively.
In total, the pumping light power is EDF10
About 100 mW for 4 and about 20 for EDF111
0 mW, about 300 mW in total. Accordingly, when the present embodiment is compared with the first configuration of the related art, the present embodiment has
Obviously, the number of components is small and inexpensive.

FIG. 19 is a diagram showing another specific example of the optical amplifier according to the first embodiment of the present invention. The optical amplifier shown in FIG. 6 eliminates the optical component 6 provided in a loop formed between the second port and the third port of the second circulator 5 in the embodiment shown in FIG. And are directly connected. This configuration corresponds to the optical amplifier having the first configuration of the prior art shown in FIG. 20 in which the partial reflection type gain equalizer 106 is removed.

In the optical amplifier thus configured, the signal light and the pump light that have passed through the erbium-doped fiber 4
After entering the first port of the circulator 5 and exiting from the second port, the incident light further enters the third port, returns to the first port, and exits from the first port. Then, the signal light and the pump light emitted from the first port of the second optical circulator are converted into the erbium-doped fiber 4.
The signal light and the pump light, which have passed through the erbium-doped fiber 4 in the opposite direction, enter the second port of the first circulator 1 via the multiplexer 2. Then, the light is emitted from the third port of the first optical circulator.

However, it should be noted that the optical amplifier of the present invention shown in FIG. 19 has the same function as the conventional optical amplifier shown in FIG. That is, the optical amplifier of the present invention and the conventional optical amplifier are the same in that the signal light and the pump light emitted from the EDF are reflected back to the EDF, and the difference is that the optical amplifier of the conventional technology has no wavelength dependency. Although the light is reflected by using the type reflector, the light is reflected by using the circulator in the optical amplifier of the present technology. However, compared to the wavelength-independent reflector, the circulator has advantages such as higher resistance to high optical power.

The wavelength and power of the pump light from the pump light source 3 are 1.48 μm and 20 mW, respectively. The length of the EDF 4 is 50 m. In the first conventional configuration shown in FIG. 20 in which the same gain and optical output power can be obtained, the lengths of the EDFs 104 and 111 are 50 m and 100 m in total, and the pump light power is ED.
Since the total power is about 100 mW for the F104 and about 200 mW for the EDF 111, when this example is compared with the first configuration of the related art, this example clearly has fewer components and is inexpensive. It is.

FIG. 20 is a diagram showing the configuration of the fifth embodiment of the present invention. Although similar to the first embodiment of FIG. 1, the following points are different. That is, the active optical fiber E
A reflector for the excitation light is provided between the DF and the second optical circulator. An example of the reflector is a fiber grating (FG). The FG reflects only the excitation light with high reflectance, and transmits the signal light. Since the FG has a small insertion loss and a high reflectance, it has an advantage that the reflection amount of the excitation light is larger than that of the first embodiment. On the other hand, in the first embodiment, since the pumping light passes through the second optical circulator twice and passes through the optical component once, their insertion loss is smaller than that in the fifth embodiment using the FG. Pretty big.

[0088]

As described above, according to the present invention,
Since the signal light that has passed through the active optical fiber via the first circulator and the multiplexer is returned through the second circulator and re-enters the active optical fiber in the opposite direction, the component parts are The cost can be reduced, the cost of parts and the cost of assembly can be reduced, and economy can be achieved.

Also, according to the present invention, a circulator,
The signal light that has passed through the active optical fiber via the multiplexer passes through the non-reflection type gain equalizer twice before and after being reflected by the reflector, and re-enters the active optical fiber in the opposite direction. Therefore, the number of components can be reduced, the cost of parts and the cost of assembly can be reduced, the economy can be improved, and the transmission loss by passing twice through the non-reflection type gain equalizer can be reduced. Half of the conventional size is sufficient.

Further, according to the present invention, a circulator,
Since the signal light passing through the active optical fiber via the multiplexer is reflected by a reflector having a gain equalizing function and is re-entered into the active optical fiber in the opposite direction, the number of components is reduced. Therefore, the cost of parts and assembly can be reduced, and the cost can be reduced. In particular, the gain equalizer which has been conventionally required becomes unnecessary, and the number of components can be further reduced.

According to the present invention, since the reflector is provided in the active optical fiber and is constituted by the fiber grating for reflecting the signal light, the number of components can be reduced and the economy can be improved. .

According to the present invention, the second optical circulator emits the excitation light from the excitation light source, passes the excitation light having passed through the active optical fiber with low loss, and reenters the active optical fiber. The number of parts may be small, and economy can be achieved.

Further, according to the present invention, the reflector is constituted by reflecting means including a mirror which reflects the excitation light passing through the active optical fiber and passing through the gain equalizer with high reflectance. In addition to the fact that the number of components is small and the cost can be reduced, the signal light is amplified by the active optical fiber after passing through the gain equalizer, and the optical output power is not reduced.

According to the present invention, the reflector having the reflectance depending on the wavelength is constituted by the reflection means including the fiber grating for the excitation light and the fiber grating for the signal light. In addition to that, the pump light that has passed through the active optical fiber can be reflected at a high reflectance.

Further, according to the present invention, the fiber grating provided in the active optical fiber is constituted by reflecting means including the fiber grating for the excitation light and the fiber grating for the signal light. In addition to being economical, the pumping light passing through the active optical fiber can be reflected at a high reflectance.

Further, according to the present invention, a stage before the optical amplifier,
That is, since the input side of the optical circulator into which the signal light is incident includes the short second active optical fiber and the first optical amplifier having a sufficient gain is provided, the noise of the optical amplifier subsequent to the optical circulator is reduced. The index can be reduced appropriately.

[Brief description of the drawings]

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

FIG. 2 is a diagram illustrating a configuration of an optical amplifier according to a second embodiment of the present invention.

FIG. 3 is a diagram illustrating a configuration of an optical amplifier according to a third embodiment of the present invention.

4 is a diagram showing a reflectance spectrum and a return loss spectrum of a wavelength-dependent reflector used in the optical amplifier shown in FIG.

FIG. 5 is a diagram showing a specific configuration of an erbium-doped fiber and a reflector used in the optical amplifier shown in FIG.

FIG. 6 is a diagram illustrating a specific example of the optical amplifier according to the first embodiment illustrated in FIG. 1;

7 is a diagram showing a gain spectrum of an optical amplifier and a transmission loss spectrum of a gain equalizer in the specific example shown in FIG. 6;

FIG. 8 is a diagram showing a specific example of the optical amplifier according to the second embodiment shown in FIG.

9 is a diagram showing a transmission loss spectrum of the gain equalizer in the specific example shown in FIG.

FIG. 10 is a diagram showing a specific example of the optical amplifier according to the third embodiment shown in FIG.

11 is a signal light CF in the specific example shown in FIG.
It is a figure which shows the reflectance spectrum of G, and a reflection loss spectrum.

FIG. 12 is a diagram illustrating another specific example of the optical amplifier according to the first embodiment illustrated in FIG. 1;

13 is a diagram showing a gain spectrum of an optical amplifier and a transmission loss spectrum of a gain equalizer in the specific example shown in FIG.

FIG. 14 is a diagram illustrating another specific example of the optical amplifier according to the third embodiment illustrated in FIGS. 3 and 5;

15 is a diagram showing a gain spectrum of the optical amplifier in the specific example shown in FIG.

16 is a signal light CF in the specific example shown in FIG.
It is a figure which shows the reflectance spectrum of G, and a reflection loss spectrum.

FIG. 17 is a diagram illustrating a configuration of an optical amplifier according to a fourth embodiment of the present invention.

FIG. 18 is a diagram showing still another specific example of the optical amplifier according to the first embodiment of the present invention.

FIG. 19 is a diagram illustrating another specific example of the optical amplifier according to the first embodiment of the present invention.

FIG. 20 is a diagram illustrating a configuration of an optical amplifier according to a fifth embodiment of the present invention.

FIG. 21 is a diagram illustrating a first configuration of a conventional optical amplifier.

FIG. 22 is a diagram illustrating a second configuration of a conventional optical amplifier.

23 is a diagram showing a gain spectrum of the conventional optical amplifier and a loss spectrum of the gain equalizer shown in FIG. 21.

FIG. 24 is a diagram showing a reflection spectrum and a loss spectrum of a wavelength-independent reflector used in the conventional optical amplifier shown in FIG.

FIG. 25 is a diagram showing a specific configuration of an erbium-doped fiber and a wavelength-independent reflector used in the second configuration of the conventional optical amplifier shown in FIG.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 first circulator 2 multiplexer 3 pumping light source 4 erbium-doped fiber (EDF) 5 second circulator 6 optical component 7 non-reflective gain equalizer 8 wavelength-independent reflector 9 wavelength-dependent reflector 10, 13, 14, 16, 17, 20, 21 Fiber grating 18 Raman fiber 22 Short erbium-doped fiber (EDF) 26 Insertion / separation circuit for signal light

Claims (17)

[Claims] A first optical circulator for receiving signal light from a first port and emitting from a second port; an excitation light source for generating excitation light; and a second optical circulator for the first optical circulator. A multiplexer for multiplexing the signal light emitted from the port and the pumping light from the pumping light source; the signal light and the pumping light from the multiplexer are incident, and are pumped by the pumping light to amplify the signal light An active optical fiber that is a gain medium to be transmitted, and signal light and pump light that have passed through the active optical fiber are input from a first port and output from a second port.
A second optical circulator configured to enter the third port, return to the first port, and exit from the first port; and a first optical circulator of the second optical circulator. The signal light and the pump light emitted from the port are re-passed to the active optical fiber in a direction opposite to the incident direction, and the signal light and the pump light re-passed through the active optical fiber in the reverse direction are passed through the multiplexer. An optical amplifier that enters a second port of the first optical circulator and exits from a third port of the first optical circulator. 2. A first optical circulator in which a signal light is incident from a first port and emitted from a second port; an excitation light source for generating excitation light; and a second optical circulator of the first optical circulator. A multiplexer for multiplexing the signal light emitted from the port and the pumping light from the pumping light source; the signal light and the pumping light from the multiplexer are incident, and are pumped by the pumping light to amplify the signal light An active optical fiber that is a gain medium to be transmitted, and signal light and pump light that have passed through the active optical fiber are input from a first port and output from a second port.
A second optical circulator configured to enter the third port, return to the first port, and exit from the first port; and a first optical circulator of the second optical circulator. The signal light and the pump light emitted from the port are re-passed to the active optical fiber in the direction opposite to the incident direction, and the signal light re-passed through the active optical fiber in the reverse direction is passed through the multiplexer through the multiplexer. An optical amplifier, which enters the second port of one optical circulator and exits from the third port of the first optical circulator. 3. A first optical circulator in which a signal light is incident from a first port and emitted from a second port; an excitation light source for generating excitation light; and a second optical circulator of the first optical circulator. A multiplexer for multiplexing the signal light emitted from the port and the pumping light from the pumping light source; the signal light and the pumping light from the multiplexer are incident, and are pumped by the pumping light to amplify the signal light An active optical fiber, which is a gain medium to be changed, and a signal light having passed through the active optical fiber is input from a first port, is output from a second port, and is then output to a third port.
And a second optical circulator configured to enter the first port, return to the first port, and exit from the first port, and exit from the first port of the second optical circulator. The signal light is re-passed to the active optical fiber in a direction opposite to the incident direction, and the signal light re-passed through the active optical fiber is passed through the multiplexer to the second port of the first optical circulator. Incident and the third of the first optical circulator
An optical amplifier, which emits light from a port. 4. An optical circulator that receives a signal light from a first port and emits from a second port, an excitation light source that generates excitation light, and a signal that is emitted from a second port of the optical circulator. A multiplexer for multiplexing light and pumping light from the pumping light source; and an active medium serving as a gain medium for receiving signal light and pumping light from the multiplexer and for amplifying the signal light by being pumped by the pumping light. An optical fiber, a signal light and a pump light that have passed through the active optical fiber are incident, a gain equalizer that is non-reflective to the signal light and the pump light, and connected to an output of the gain equalizer; A signal light emitted from the gain equalizer and a reflector for reflecting the pump light to the gain equalizer, and the signal light reflected by the reflector and passed through the gain equalizer again And the pump light is re-directed to the active optical fiber in a direction opposite to the incident direction. The signal light and the pump light, which have passed through the active optical fiber in the reverse direction, enter the second port of the optical circulator via the multiplexer, and exit from the third port of the optical circulator. An optical amplifier, characterized in that: 5. An optical circulator that receives a signal light from a first port and emits from a second port, an excitation light source that generates excitation light, and a signal that is emitted from a second port of the optical circulator. A multiplexer for multiplexing light and pumping light from the pumping light source; and an active medium serving as a gain medium for receiving signal light and pumping light from the multiplexer and for amplifying the signal light by being pumped by the pumping light. An optical fiber, a signal light and a pump light that have passed through the active optical fiber are incident, and a gain equalizer that is non-reflective to the signal light; and a gain equalizer that is connected to an output of the gain equalizer. A reflector that reflects the signal light and the pump light emitted from the device to the gain equalizer, and the signal light and the pump light that are reflected by the reflector and pass through the gain equalizer again. Through the active optical fiber in a direction opposite to the incident direction. The signal light re-passed through the active optical fiber in the reverse direction enters the second port of the optical circulator via the multiplexer, and exits from the third port of the optical circulator. Optical amplifier. 6. A first optical circulator in which signal light is incident from a first port and emitted from a second port; an excitation light source for generating excitation light; and a second optical circulator of the first optical circulator. A multiplexer for multiplexing the signal light emitted from the port and the pumping light from the pumping light source; the signal light and the pumping light from the multiplexer are incident, and are pumped by the pumping light to amplify the signal light An active optical fiber that is a gain medium to be transmitted, a signal light that has passed through the active optical fiber is incident, and is connected to an output of the gain equalizer that is non-reflective to the signal light; A reflector for reflecting the signal light emitted from the gain equalizer to the gain equalizer, wherein the signal light emitted by the reflector and passed again through the gain equalizer is activated. Re-pass the optical fiber in the direction opposite to the incident direction, and An optical amplifier characterized in that the signal light having passed through the fiber again enters the second port of the first optical circulator via the multiplexer, and exits from the third port of the first optical circulator. . 7. An optical circulator that receives a signal light from a first port and emits from a second port, an excitation light source that generates excitation light, and a signal that is emitted from a second port of the optical circulator. A multiplexer for multiplexing light and pumping light from the pumping light source; and an active medium serving as a gain medium for receiving signal light and pumping light from the multiplexer and for amplifying the signal light by being pumped by the pumping light. An optical fiber, and a reflector that receives the signal light and the excitation light that have passed through the active optical fiber and reflects the signal light and the excitation light so as to re-pass the active optical fiber in the direction opposite to the incident direction. A reflector having a reflectance depending on the wavelength of the signal light and having a gain equalization function, wherein the signal light and the excitation light reflected by the reflector after being subjected to the gain equalization process are reflected by the reflector. How to enter the active optical fiber The signal light and the pump light, which have passed through the active optical fiber in the reverse direction, are incident on the second port of the optical circulator through the multiplexer, and the third light of the optical circulator is An optical amplifier, which emits light from a port. 8. An optical circulator that receives a signal light from a first port and emits from a second port, an excitation light source that generates excitation light, and a signal that is emitted from a second port of the optical circulator. A multiplexer for multiplexing light and pumping light from the pumping light source; and an active medium serving as a gain medium for receiving signal light and pumping light from the multiplexer and for amplifying the signal light by being pumped by the pumping light. An optical fiber, and a reflector that receives the signal light and the excitation light that have passed through the active optical fiber and reflects the signal light and the excitation light so as to re-pass the active optical fiber in the direction opposite to the incident direction. A reflector having a reflectance depending on the wavelength of the signal light and having a gain equalization function, wherein the signal light and the excitation light reflected by the reflector after being subjected to the gain equalization process are reflected by the reflector. How to enter the active optical fiber The signal light re-passed in the reverse direction through the active optical fiber is incident on the second port of the optical circulator through the multiplexer, and is transmitted from the third port of the optical circulator through the multiplexer. An optical amplifier, which emits light. 9. A first optical circulator in which signal light is incident from a first port and emitted from a second port; an excitation light source for generating excitation light; and a second optical circulator of the first optical circulator. A multiplexer for multiplexing the signal light emitted from the port and the pumping light from the pumping light source; the signal light and the pumping light from the multiplexer are incident, and are pumped by the pumping light to amplify the signal light An active optical fiber which is a gain medium to be transmitted, and a reflector which receives the signal light passing through the active optical fiber and reflects the signal light so as to re-pass the active optical fiber in a direction opposite to the incident direction. A reflector having a reflectance depending on the wavelength of the signal light and having a gain equalization function, and the signal light reflected by the reflector after being subjected to gain equalization processing by the active optical fiber. In the direction opposite to the incident direction. And the signal light that has passed through the active optical fiber again enters the second port of the first optical circulator via the multiplexer, and exits from the third port of the first optical circulator. An optical amplifier, characterized in that: 10. The reflector is provided in the active optical fiber and comprises a fiber grating for reflecting signal light, wherein the fiber grating has a reflectance independent of a signal light wavelength or a signal light. 10. The optical amplifier according to claim 7, wherein gain equalization is performed with a reflectance depending on a wavelength. 11. The optical amplifier according to claim 1, wherein the active optical fiber is a rare earth doped fiber or a Raman fiber. 12. The second optical circulator is configured to pass the excitation light emitted from the excitation light source and having passed through the active optical fiber with low loss, and to be incident on the active optical fiber again. The optical amplifier according to claim 1, 2 or 3, wherein: 13. The reflector according to claim 1, wherein the reflector includes a mirror including a mirror that reflects the excitation light passing through the active optical fiber and passing through the gain equalizer with high reflectance. The optical amplifier according to claim 4, 5 or 6, wherein 14. A fiber grating for excitation light, wherein the reflector having a reflectance depending on the wavelength of the signal light is arranged in series so as to reflect the excitation light having passed through the active optical fiber at a high reflectance. 10. The optical amplifier according to claim 7, wherein the optical amplifier comprises a reflection unit including a signal light fiber grating. 15. A fiber grating for excitation light and a signal light arranged in series so as to reflect at a high reflectance the excitation light passing through the active optical fiber, wherein the fiber grating provided in the active optical fiber is provided. 11. The optical amplifier according to claim 10, wherein said optical amplifier comprises a reflection unit including a fiber grating for use. 16. An optical circulator to which the signal light is incident is provided on an input side of a first port of the optical circulator, and includes a second active optical fiber having a shorter length than the active optical fiber. 16. The optical amplifier according to claim 1, further comprising a second optical amplifier having a gain sufficient to reduce a noise figure of the optical amplifier after the optical circulator. 17. The optical amplifier according to claim 1, further comprising a pump light reflector disposed between said active optical fiber and said second optical circulator.