JP2004193587A - Optical amplifier - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical amplifier capable of amplifying with no deviation in gain in a plurality of wavebands in gain without increasing the number of parts by providing a gain equalizer which has a predetermined transmission characteristic and a reflection characteristic at the subsequent stage of an optical amplifying media and gain equalization while varying the path of light transmission for every waveband. <P>SOLUTION: The optical amplifier comprises optical amplifiers 13A and 13B, and the gain equalizers 5A and 5B having the transmission characteristic and the reflection characteristic connected to the subsequent stage of the optical amplifiers and related to the amplification characteristic of the optical amplifiers, and performing gain equalization while varying the path of the light transmission for every waveband. The optical amplifier further gets light which is not reflected by the gain equalizer 5A to pass through and gets light to act as the excitation light of the optical amplifier 13B to effectively amplify the light. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

The present invention relates to an optical amplifier that amplifies optical signals in a plurality of wavelength bands.

  2. Description of the Related Art In an optical communication system, a wavelength division multiplexing (WDM) communication system is used as a means for responding to a demand for increased transmission capacity and flexible network formation. WDM communication is a system that increases the transmission capacity by transmitting light of multiple wavelengths over a single optical fiber.WDM communication repeaters have the characteristic of amplifying light of multiple wavelengths collectively. Desired.

  In recent years, in order to further increase the transmission capacity, not only the 1.55 μm band (about 1530 nm to about 1560 nm) conventionally used for transmission but also the 1.58 μm band (about 1570 nm to about 1610 nm) has been used for WDM communication. . The 1.55 μm band and the 1.58 μm band are also called C-band and L-band, respectively. In a WDM communication system using both wavelength bands, a repeater is required to have characteristics of amplifying both wavelength bands.

  Generally, the gain of an optical amplifying medium used in a repeater is not constant with respect to wavelength, and when a wavelength-multiplexed signal is amplified collectively, the light intensity of each wavelength after amplification differs depending on the wavelength. When long-distance transmission is performed by a WDM communication system, multi-stage relaying is required. However, if the optical amplification characteristics of the repeater differ depending on the wavelength, the intensity difference between the wavelengths accumulates. When the intensity difference between the wavelengths of the WDM signal light increases, when the signal light is branched and received for each wavelength, a signal-to-noise ratio (SNR) degradation and a problem of setting a reception level of a receiver occur. Therefore, the optical amplifier used in the repeater needs to have a characteristic in which the gain wavelength dependence of the optical amplification medium is compensated and the gain is flat with respect to the wavelength.

  Here, the characteristics of optical amplification of an optical amplifier using an amplification optical fiber doped with Er, which is generally used as an optical amplification medium, will be described. FIG. 13 is a diagram showing wavelength characteristics with respect to gain per unit length of Er-doped optical fiber (EDF) for each population inversion ratio (0.0 to 1.0). When the population inversion ratio is about 0.7, the gain characteristic is that the wavelength region where the gain is flat is wide in the C band (about 1530 nm to about 1560 nm), and the gain is inclined with respect to the wavelength in the L band (about 1570 nm to about 1610 nm). As the wavelength increases, the gain decreases.

  On the other hand, when the population inversion ratio is about 0.4, the gain characteristic is negative at a wavelength of 1550 nm or less in the C band, and a part of the incident C band light is absorbed. On the other hand, although the gain is small in the L band, its characteristics are flat. Therefore, in order to amplify the C-band signal light and make the gain flat, it is sufficient to amplify the gain by setting the inversion distribution ratio of the EDF to about 0.7 and to perform gain equalization. In order to amplify the gain, the inversion distribution ratio of the EDF is set to about 0.4 and the gain is equalized. The gain in the L band of the EDF with the population inversion ratio of about 0.4 is smaller than the gain in the C band of the EDF with the population inversion rate of about 0.7, so that the EDF required to amplify the L band light flat with respect to the wavelength is obtained. The length is longer than the EDF length that amplifies the C-band light flat with respect to the wavelength.

(First conventional example)
As a conventional amplifier for amplifying WDM signal light in a plurality of wavelength bands, a configuration is known in which an input signal light is demultiplexed into a plurality of wavelength bands, amplified in each wavelength band, and then combined ( For example, Patent Document 1). In the configuration of the first prior art, since the input signal light is demultiplexed for each wavelength band, there is an advantage that each amplifier can amplify a plurality of wavelength bands at a time and does not need to be a wide band. .
FIG. 9 shows an optical amplifier according to a first related art, which includes a demultiplexer 8A for demultiplexing signal light into eight wavelength bands, and wavelength bands output from the demultiplexer 8A. Eight optical amplifiers 13A to 13H for amplifying the respective signal lights, pump light sources 3A and 3B for generating pump light, and branching pump light output from each pump light source to input to each optical amplifier. Devices 12A and 12B, optical variable attenuators 14A to 14H for adjusting the signal light intensity output from each optical amplifying unit, and a multiplexing unit 8B for multiplexing the signal light output from each optical attenuator. It is composed of optical isolators 6A and 6B arranged at the input terminal and the output terminal.

  The optical amplifying units 13A to 13H are each configured to excite light from the excitation light sources 3A and 3B by the excitation light branched by the splitters 12A and 12B, and amplify the input signal light split into each wavelength band by the optical splitter 8A. I do. The signal light output from each optical amplifier is adjusted in intensity by the optical variable attenuators 14A to 14H, multiplexed by the optical multiplexer 8B, and output through the isolator 6B.

(Second conventional example)
Next, as a conventional optical amplifier that demultiplexes input signal light for each wavelength band and amplifies WDM signal light in multiple wavelength bands, in order to compensate for imperfect isolation of the demultiplexer. A configuration is known in which a filter is connected downstream of a duplexer (for example, Patent Document 2). In the configuration of the second prior art, in addition to amplifying signal light for each wavelength band, an optical amplifier for amplifying light in a certain wavelength band is affected by an optical amplifier for amplifying light in another wavelength band. There is an advantage of not receiving.
FIG. 10 shows an example of an optical amplifier according to a second conventional technique, which includes an optical coupler 100, a C-band optical amplifier 200, an L-band optical amplifier 300, and an optical multiplexer 8B. . The optical coupler 100 includes an optical demultiplexing unit 8A and an optical filter 7, and the optical filter 7 transmits L band light and blocks C band light.

  The C-band optical amplifier 200 includes pump light sources 3A and 3B, WDM couplers 4C and 4D, amplification optical waveguides 2A and 2B, optical isolators 6A and 6B, and a gain equalizer 15A. The L-band optical amplifier 300 includes pump light sources 3C and 3D, WDM couplers 4E and 4F, amplification optical waveguides 2C and 2D, optical isolators 6C and 6D, and a gain equalizer 15B. The optical multiplexer 8B receives the C-band signal light optically amplified and output by the C-band optical amplifier 200 and the L-band signal light optically amplified and output by the L-band optical amplifier 300. Are combined and output.

  Here, in the case of an optical amplifier that amplifies a plurality of wavelength bands by demultiplexing for each wavelength band, the reverse ASE light generated at the time of optical amplification in the long wavelength side band optical amplifier is of a short wavelength side band. If the signal is input to the optical amplifier for the short wavelength side band, the noise factor is degraded.

  However, in the optical amplifier according to the second prior art, since the reverse ASE light generated from the L-band optical amplifier 300 is cut off by the optical filter 7, the inter-band isolation of the optical splitter 8A is incomplete. Also, the reverse ASE light is prevented from being input to the C-band optical amplifier 200, and the noise figure of the C-band optical amplifier 200 is prevented from deteriorating.

(Third conventional example)
In addition, as a conventional optical amplifier that amplifies WDM signal light in multiple wavelength bands and performs gain equalization, it reflects the signal light wavelength to the optical fiber amplifier so that the gain for each signal light wavelength is constant. A configuration in which means is provided is known (for example, Patent Document 3).
FIG. 11 shows an optical amplifier according to the third prior art, which includes an optical circulator 1, an optical fiber 16A for writing a grating, an optical fiber 2A for amplification, an excitation light source 3A, an optical multiplexer / demultiplexer. 4A, with a non-reflective termination 10. The grating writing optical fiber 16A is an optical fiber that has the same refractive index distribution as the optical fiber connected to the end thereof, and is easy to form a grating by doping Ge or the like. A grid 9A is formed. The amplification optical fiber 2A is an Er-doped silica-based optical fiber mainly doped with Er as an active material for light amplification in a core portion, and has diffraction gratings 9B to 9F. The diffraction gratings 9A to 9F are optical fiber gratings that irradiate interference fringes by ultraviolet rays from an Ar laser or the like based on a normal holographic method or a phase grating method to generate a pattern of a refractive index change in a clad portion or a core portion. is there. Each of these diffraction gratings 9A to 9F has a reflection wavelength range of the central wavelength λi with a sufficiently large suppression ratio. Here, i = 0, 1,..., 5.

  Excitation light generated from the excitation light source 3A excites the amplification optical fiber 2A via the optical multiplexer / demultiplexer 4A. The excitation light not absorbed by the amplification optical fiber 2A is incident on the diffraction grating writing optical fiber 16A, is reflected by the diffraction grating 9A formed on the diffraction grating writing optical fiber 16A, and is again amplified by the amplification light. The light enters the fiber 2A and excites the amplification optical fiber 2A.

The amplification optical fiber 2A is excited by the excitation light from the optical multiplexer / demultiplexer 4A and the excitation light reflected by the diffraction grating 9A formed on the diffraction grating writing optical fiber 16A. The signal light incident via the optical circulator 1 and the grating writing optical fiber 16A is amplified by the pumped amplification optical fiber 2A, but the signal light having the wavelength components λ _{1 to} λ ₅ of the signal light is not amplified. The light is reflected by the diffraction gratings 9B to 9F formed on the amplification optical fiber 2A, is amplified again, and is output to the emission end via the diffraction grating writing optical fiber 16A and the optical circulator 1. On the other hand, signal light containing other wavelength components passes through the amplification optical fiber 2A and is output to the non-reflection terminal 10 through the optical multiplexer / demultiplexer 4A.

Here, each fiber length L in the amplification optical fiber 2A wavelength components lambda ₁ to [lambda] ₅ of the signal light passing through each _{(λ 1) ~L (λ 5} ) is the active material for the wavelength lambda ₁ to [lambda] ₅ Based on the absorption and stimulated emission cross sections of Er, the intensity is set in consideration of the intensity of the excitation light, the concentration distribution of the active substance, and other fiber structures.

For example, the five by setting the optimum value of the optical amplification characteristics based on the input intensity of the excitation light to the wavelength lambda ₁ to [lambda] ₅ the fiber length, the gain of the wavelength components lambda ₁ to [lambda] ₅ of the signal light G (λ ₁ ) to G (λ ₅ ) are maximized. Further, by setting these five fiber lengths in an ascending order with respect to the wavelengths λ _{1 to} λ ₅ having gains arranged in a descending order in the optical amplification characteristics based on the intensity of the pumping light, the wavelength component of the signal light is set. lambda ₁ each gain G (λ ₁₎ of ~λ ₅ ~G (λ ₅₎ coincide.

(Fourth conventional example)
Further, as a configuration of a conventional optical amplifier that amplifies WDM signal light in a plurality of wavelength bands and performs gain equalization, a filter is provided between a first amplification optical fiber and a second amplification optical fiber, A configuration is known in which light in the first wavelength band is reflected by a filter, and light in the second wavelength band is transmitted through the filter, thereby amplifying the two wavelength bands without demultiplexing and multiplexing (for example, see, for example). Patent Document 4, Patent Document 5).
JP-A-2000-58953 FIG. 12 shows an optical amplifier according to a fourth embodiment, which includes an optical circulator 1, a C-band optical amplifier 18A, a reflection filter 7 that totally reflects the C-band light and transmits the L-band light, The L-band optical amplifier 18B includes a reflection mirror 11 that totally reflects L-band light. The C-band optical amplifier 18A includes an Er-doped fiber 2A, a pump light source 3A, a WDM coupler 4A, and a gain equalizer 15A. The L-band optical amplifier 18B includes an Er-doped fiber 2B, a pump light source 3B, a WDM coupler 4B, and a gain equalizer 15B. The gain equalizer 15A is set so as to equalize the gains of the signal lights of different wavelengths mainly belonging to the 1.55 μm band, that is, to keep the signal intensity level of the signal lights of the respective wavelengths constant. The equalizer 15B is set so as to equalize the gains of a plurality of signal lights belonging to the 1.58 μm band.

  The C-band light and the L-band light incident on the port A of the optical circulator 1 are emitted from the port B, enter the optical amplifier 18A, are optically amplified by the Er-doped fiber 2A, and gain-equalized by the gain equalizer 15A. Then, the light enters the reflection filter 7. Since the reflection filter 7 reflects the C band light and transmits the L band light, the signal light is separated into the C band light and the L band light. The C band light reflected by the reflection filter 7 re-enters the optical amplifier 18A, and the L band light transmitted through the reflection filter 7 enters the optical amplifier 18B.

  The L-band light transmitted through the reflection filter 7 is optically amplified by the Er-doped fiber 2B, reflected by the reflection mirror 11, is optically amplified again by the Er-doped fiber 2B, and is incident on the reflection filter 7. The reflection filter 7 transmits the amplified L-band light from the optical amplifier 18B again, and enters the optical amplifier 18A.

  Thus, the C-band light amplified by the optical amplifier 18A and reflected by the reflection filter 7 is amplified by the optical amplifier 18A, amplified by the optical amplifier 18B via the reflection filter 7, and reflected by the reflection mirror 11. Thereafter, the L-band light that has been amplified again by the optical amplifier 18B and passed through the reflection filter 7 is again amplified by the optical amplifier 18A, enters the port B of the optical circulator 1, and is output from the port C. As a result of these amplifications, the gains of the optical amplifiers 18A and 18B are adjusted so that the gains of the C-band light and the L-band light become equal.

  The optical amplifier according to the first conventional example has a configuration in which signal light is demultiplexed for each wavelength band and then amplified in each wavelength band, so that an optical amplifier in each wavelength band does not require a wide-band amplification unit. There are advantages. However, it is necessary to provide an excitation light source, an amplification unit, and a light intensity adjustment unit for each wavelength band, and the number of components increases. In order to reduce the number of pumping light sources, it is possible to use a configuration in which the pumping light source is split by a splitter and input to each optical amplifier, but the disadvantage that automatic gain control (AGC control) for each wavelength band cannot be used. Will have.

  In addition, since it is difficult to keep the isolation for each wavelength band of the demultiplexing unit high, additional means such as a filter as in the second conventional example is required.

  In the optical amplifier according to the third conventional example, the optical fiber gratings 9B to 9F are formed on the amplification optical fiber 2A to change the fiber length of each wavelength component of the signal light that passes through the amplification optical fiber 2A, thereby changing the length of the amplifier. Although the gain for each wavelength is equal, the optical fiber gratings 9B to 9F need to be provided for each signal light wavelength, and in order to increase the number of wavelengths of the used WDM signal light or to change the wavelength, ultraviolet light irradiation must be performed again. Therefore, it is necessary to provide an optical fiber grating. The optical fiber gratings 9B to 9F formed on the amplification optical fiber 2A have a characteristic of reflecting signal light of a target wavelength and transmitting light of other wavelengths. Although it is necessary to make the wavelength interval narrower than the wavelength interval, when the signal light wavelength fluctuates, there is in principle an operation instability that the intensity of the target signal light reflected by the optical fiber grating is greatly reduced. In Patent Document 3, as a means for reflecting a target wavelength component, a dielectric multilayer filter is cited in addition to the optical fiber grating. However, in the dielectric multilayer filter, a work of connecting filters for each signal light wavelength is performed. Is required.

  In the optical amplifier according to the fourth conventional example, a reflection filter is provided between the two amplifiers to separate the C band light and the L band light, and the gain equalizer in the preceding stage of the reflection filter has only the C band. There is an advantage that the gain is equalized, and the gain equalizer at the subsequent stage of the reflection filter may equalize only the gain of the L band. However, since the reflection filter does not transmit the light in the C band, the ASE light and the pump light of the optical fiber amplifier in the preceding stage which are not reflected by the reflection filter are not effectively used.

  The present invention has been made to solve the above problems, and includes a gain equalizer having predetermined transmission characteristics and reflection characteristics at a subsequent stage of an optical amplifying medium to change a light passage path for each wavelength band and to perform gain equalization. By performing the above, an optical amplifier capable of performing amplification without gain deviation in a plurality of wavelength bands without increasing the number of components is provided.

  Also, when changing the light passage path for each wavelength band, an optical amplifier that transmits light not reflected by the reflection filter and acts as excitation light for the subsequent optical amplification medium, thereby increasing the utilization efficiency of the excitation light. I will provide a.

  To achieve the above object, an optical amplifier according to a first aspect of the present invention is an optical amplifier that collectively amplifies light in which a plurality of signal lights having different wavelengths are multiplexed. An optical amplifier comprising one or more gain equalizers connected to a subsequent stage of an optical amplification unit and having transmission characteristics and reflection characteristics related to amplification characteristics of the optical amplification unit, wherein the gain equalization is performed. The device is characterized in that it changes the light passage path for each wavelength band and performs gain equalization.

  This allows the gain equalizer to perform reflection or transmission for each wavelength band of the signal light, change the passing optical amplifier for each wavelength band of the signal light, and perform gain equalization at the time of the reflection or transmission. ing. Because the gain equalizer changes the reflection and folding or transmission for each wavelength band, the optical amplifier is not a wide band that amplifies all the wavelength bands included in the input signal light. In addition, gain equalization is performed simultaneously with reflection and transmission, so that there is an advantage that an optical amplifier can be flexibly selected.

An optical amplifier according to a second aspect of the present invention is an optical amplifier that collectively amplifies light in which a plurality of signal lights having different wavelengths are multiplexed, the optical amplifier having first to third ports, and input to the first port. An optical component for outputting the multiplexed light to the second port, and outputting the light input to the second port to the third port;
A first optical amplifier having one end connected to a second port of the optical component and amplifying the multiplexed light; and a first optical amplifier having one end connected to the other end of the first optical amplifier. An optical amplifier comprising: a gain equalizer; and a second optical amplifier having one end connected to the other end of the first gain equalizer and amplifying an output of the first gain equalizer. The gain equalizer transmits light of the second wavelength band and transmits light of the first wavelength band based on continuous equalization characteristics related to the gain characteristics of the first optical amplifier. , And transmits light not reflected by the light of the first wavelength band.

  Here, the equalization characteristic is an output characteristic of an optical amplifier with respect to an input signal light, which is realized by combining characteristics such as transmission characteristics, reflection characteristics, amplification characteristics, and attenuation characteristics with respect to wavelengths of components constituting the optical amplifier. That is. Although the optical amplifying medium has wavelength dependence of gain, the above equalization characteristics are realized by combining elements such as gain equalizers whose characteristics can be changed with respect to wavelength. Determining a continuous equalization characteristic means not defining the equalization characteristic for a specific wavelength, but defining the equalization characteristic for a specific range of wavelengths.

  Thereby, the passage path of the light in the first wavelength band is amplified by the first optical amplifier, reflected by the first gain equalizer, and then amplified again by the first optical amplifier. At the same time, gain equalization is performed by the reflection characteristics of the first gain equalizer. By relating the reflection characteristic of the first gain equalizer to the gain characteristic of the first optical amplifier, the wavelength-intensity characteristic of light in the first wavelength band output from the third port of the optical component can be changed. Can be adjusted. Further, the passage path of the light in the second wavelength band is a passage path in which the light is amplified by the first optical amplifier and transmitted through the first gain equalizer, and is different from the light in the first wavelength band. As a path, by transmitting light not reflected by the first gain equalizer with light in the first wavelength band, it can be effectively used for assisting the excitation of the second optical amplifier.

  Since the light in the first wavelength band and the light in the second wavelength band are amplified through different passing paths, the first and second optical amplifiers can amplify both wavelength bands to a sufficient intensity. The optical amplifier does not need to have a wide band, and the optical amplifier can be flexibly selected.

  An optical amplifier according to a third aspect of the present invention is the optical amplifier according to the configuration of the second aspect of the present invention, wherein the equalization characteristic is related to gain characteristics of the first and second optical amplifiers, Is characterized by transmitting light in the second wavelength band based on the equalization characteristics.

  Thereby, the wavelength-intensity characteristics of light in the second wavelength band can be adjusted by relating the transmission characteristics of the first gain equalizer to the gain characteristics of the first and second optical amplifiers. .

  An optical amplifier according to a fourth invention is the optical amplifier according to the configuration of the second or third invention, wherein the equalization characteristic is related to a gain characteristic of the first and second optical amplifiers, One end is connected to the other end of the second optical amplifier, and a second gain equalizer that reflects the light of the second wavelength band based on the equalization characteristic is provided.

  Thereby, the passage path of the light in the first wavelength band is amplified by the first optical amplifier, reflected by the first gain equalizer, and then amplified again by the first optical amplifier. On the other hand, the light passing path of the second wavelength band is amplified by the first optical amplifier, transmitted by the first gain equalizer, amplified by the second optical amplifier, and gained by the second gain. The light is reflected by the equalizer, amplified again by the second optical amplifier, transmitted by the first gain equalizer, and amplified again by the first optical amplifier. In this way, by changing the passage of the light in the first and second wavelength bands, the light in the second wavelength band is amplified twice by the first and second optical amplification media, respectively. Optical amplification media and pumping light can be used efficiently, and when an amplification optical fiber is used as the optical amplification medium, the fiber length of the amplification optical fiber can be reduced to about 4/5. it can.

  An optical amplifier according to a fifth aspect is the optical amplifier according to the configuration of the second or third aspect, wherein the equalization characteristic is related to a gain characteristic of the first and second optical amplifiers, A second gain equalizer having one end connected to the other end of the second optical amplifying unit and transmitting the light of the second wavelength band based on the equalization characteristic; and a third port of the optical component. Combining means for combining the output of the second gain equalizer with the output of the second gain equalizer.

  Thereby, the passage path of the light in the first wavelength band is amplified by the first optical amplifier, reflected by the first gain equalizer, and then amplified again by the first optical amplifier. On the other hand, the light passing path of the second wavelength band is amplified by the first optical amplifier, transmitted by the first gain equalizer, amplified by the second optical amplifier, and gained by the second gain. This is a passage route through which light passes through the equalizer. As described above, by changing the passage paths of the light in the first and second wavelength bands, only the light in the first wavelength band is amplified twice by the first optical amplifying unit, and the second light is amplified. Since the light in the wavelength band is further amplified once by the second optical amplification unit, the characteristics of the first and second optical amplification media can be designed with a greater degree of freedom.

  An optical amplifier according to a sixth aspect is the optical amplifier according to the configuration of the fifth aspect, wherein the second amplifying unit includes an optical amplifying medium and an isolator. In the configuration of the fifth aspect, the second amplifying unit includes an optical amplifying medium, and the light in the second wavelength band passes through the second gain equalizer. At a later stage, the signal light travels in one direction, so that an isolator can be inserted. According to the sixth invention, for example, even when an amplification optical fiber having a large Rayleigh scattering is used as the optical amplification medium, it is possible to prevent a loop oscillation from occurring between the optical component and the amplification optical fiber. it can.

  An optical amplifier according to a seventh aspect is the optical amplifier according to the configuration of the fifth aspect, wherein the second gain equalizer converts the light of the third wavelength band in the light of the second wavelength band to the light of the third wavelength band. The light is reflected based on the equalization characteristic, and the light in the fifth wavelength band in the light in the second wavelength band is transmitted based on the equalization characteristic.

  Thereby, the passage path of the light in the first wavelength band is amplified by the first optical amplifier, reflected by the first gain equalizer, and then amplified again by the first optical amplifier. On the other hand, of the light in the second wavelength band, the light path in the third wavelength band is amplified by the first optical amplifier, transmitted through the first gain equalizer, and then transmitted through the second optical amplifier. Amplifying at the section, reflecting at the second gain equalizer, amplifying again at the second optical amplifying section, transmitting at the first gain equalizer, and amplifying again at the first optical amplifying section. The route. Further, of the light of the second wavelength band, the passage path of the light of the fourth wavelength band is amplified by the first optical amplifier, transmitted through the first gain equalizer, and transmitted to the second optical amplifier. , And passes through the second gain equalizer. As described above, by changing the passage paths of the light in the first, third, and fourth wavelength bands, light in a plurality of wavelength bands can be amplified and gain-equalized with a greater degree of freedom.

An optical amplifier according to an eighth aspect of the present invention is an optical amplifier that collectively amplifies light obtained by multiplexing a plurality of multiplexed signal lights of different wavelengths, the optical amplifier having first to third ports, and input to the first port. An optical component for outputting the multiplexed light to the second port, and outputting the light input to the second port to the third port;
One end is connected to a second port of the optical component, an optical amplifier for amplifying the multiplexed light, a gain equalizer having one end connected to the other end of the first optical amplifier, Multiplexing means for multiplexing the output of the third port of the optical component and the output of the other end of the gain equalizer, wherein the gain equalizer is related to a gain characteristic of the optical amplifier. The light of the first wavelength band is reflected based on the continuous equalization characteristic, and the light of the second wavelength band is transmitted based on the equalization characteristic.

  According to the eighth invention, a light path in the first wavelength band is amplified by the first optical amplifier, reflected by the first gain equalizer, and then amplified again by the first optical amplifier. In addition to the passing path, gain equalization is performed by the reflection characteristics of the first gain equalizer. At the same time, the passage path of the light of the second wavelength band is set as a passage path through which the light passes through the first gain equalizer, and the gain is equalized by the transmission characteristic of the first gain equalizer. As a result, the passing path is changed while amplifying light in both wavelength bands, and gain equalization is performed for the first wavelength band.

  According to the optical amplifier of the present invention, a gain equalizer having predetermined transmission characteristics and reflection characteristics is provided at the subsequent stage of the optical amplifying medium. By performing equalization, optical amplification in a plurality of wavelength bands becomes possible. Further, the operation of the gain equalizing means for a specific wavelength band reflects the first wavelength band by a predetermined reflection characteristic that makes the output light intensity uniform, and transmits the light not reflected in the wavelength band. By doing so, the unreflected signal light of the first wavelength band or the ASE light when the first optical amplification medium is an amplification optical fiber can be used for exciting the second optical amplification medium. it can.

  In addition, instead of reflecting light for each individual wavelength and adjusting the gain depending on the reflection position, the light is reflected and transmitted by predetermined transmission characteristics and reflection characteristics in the used wavelength band, so that the configuration is simple. It is possible to cope with an increase in the signal light wavelength or a change due to an external factor of the signal light wavelength.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(First embodiment)
FIG. 1 shows the configuration of the first embodiment of the present invention. The optical amplifier according to the first embodiment has first to third ports, outputs light input to the first port to the second port, and inputs light to the second port. It comprises an optical component 17 for outputting light to the third port, optical amplifiers 13A and 13B, and gain equalizers 5A and 5B. The optical component 17 includes the circulator 1. The circulator 1 outputs the light input to the port A from the port B, and outputs the light input to the port B to the port C. The optical amplification units 13A and 13B include amplification optical fibers 2A and 2B, excitation light sources 3A and 3B, and couplers 4A and 4B, respectively.

In the optical amplifier according to the first embodiment, the C band is transmitted as the first wavelength band that is reflected and transmitted by the first gain equalizer 5A, the second band is transmitted through the first gain equalizer 5A, and the second gain is transmitted. The L band is used as a second wavelength band that reflects the equalizer 5B. Also,
As the amplification optical fibers 2A and 2B, an optical fiber doped with a rare earth element or an optical fiber causing a Raman scattering effect, for example, a dispersion compensating fiber (DCF) or a dispersion shift fiber (DSF) is used. Can be used. The optical fiber doped with the rare earth element forms an inversion distribution by exciting the rare earth element added by the excitation light, and amplifies by causing a stimulated emission phenomenon by the signal light. On the other hand, the optical fiber that causes the Raman scattering effect induces nonlinear scattering in the optical fiber by the excitation light, and performs amplification by stimulated Raman scattering of the signal light.

  Hereinafter, in the optical amplifier according to the first embodiment, 15 waves having different wavelengths are respectively wavelength-multiplexed in the C band and the L band, the signal light intensity is WDM signal light of -25 dBm / ch, and the output signal light intensity is Explanation will be given based on a configuration equal to -9.2 dBm / ch. Here, the average transmittance in the C band of the gain equalizer 5A is set to 48.6%, the Er-doped optical fiber is 2.5 m as the first amplification optical fiber 2A, and the Er-doped optical fiber is 2D as the second amplification optical fiber 2B. The optical fiber is 21.45m. A semiconductor laser having a wavelength of 0.98 μm and an output of 15 mW is used as the excitation light source 3A, and a semiconductor laser having a wavelength of 0.98 μm and an output of 18.5 mW is used as the excitation light source 3B. When the amplification optical fibers 2A and 2B are excited by the excitation light from the excitation light sources 3A and 3B through the couplers 4A and 4B, respectively, the amplification optical fiber 2A and the second amplification light, which are the first amplification optical fibers. FIG. 2 shows the characteristics of the amplification optical fiber 2B which is a fiber.

  The C-band light is amplified by the amplification optical fiber 2A, reflected by the gain equalizer 5A, amplified again by the amplification optical fiber 2A, and output. Thus, the C-band light output from the port C of the circulator 1 is output. The light is amplified only by the amplification optical fiber 2A. Therefore, the amplification characteristic of the amplification optical fiber 2A is a characteristic in which the intensity-wavelength characteristic of the output light by amplifying the C-band light and equalizing the gain becomes flat. As shown in FIG. 2, the characteristic of the amplification optical fiber 2A, which is the first amplification optical fiber, is such that after the gain becomes maximum around 1530 nm, it is about 9 (dB) in the region of 1540 nm to 1560 nm. The gain decreases as the wavelength becomes longer. The gain decreases to 5 (dB) or less in the wavelength region of the L band (about 1570 nm to about 1610 nm), and the gain is flat in the C band (about 1530 nm to about 1560 nm). This is a characteristic that a wide area is widened.

  In the optical amplifier according to the first embodiment, a dielectric multilayer film is used as the gain equalizers 5A and 5B. As the continuous equalization characteristics of the optical amplifier according to the first embodiment of the present invention, the transmission characteristics and the reflection characteristics of the gain equalizers 5A and 5B indicate that the output light intensity from the port C of the circulator 1 is different from the wavelength. The flatness is determined in relation to the gain characteristics of the amplification optical fibers 2A and 2B. FIG. 3 shows the transmission and reflection characteristics of the first gain equalizer 5A and the reflection characteristics of the second gain equalizer.

  The C-band light output from the port C of the circulator 1 is amplified by the optical amplifier 13A, reflected by the gain equalizer 5A, amplified again by the optical amplifier 13A, and incident on the circulator 1. The reflection characteristic of the C band of the gain equalizer 5A is determined in relation to the amplification characteristic of the optical amplifier 13A, that is, the amplification characteristic of the amplification optical fiber 2A, and is amplified by reciprocating through the amplification optical fiber 2A. The characteristics of the C-band light are determined so that the output light intensity of the C-band light from the port C of the circulator 1 becomes flat with respect to the wavelength. On the other hand, the amplification characteristic of the amplification optical fiber 2A is not flat with respect to the wavelength as shown in FIG. 2, and after the gain becomes maximum around 1530 nm, it is about 9 (dB) in the region of 1540 nm to 1560 nm. Therefore, as shown in FIG. 3A, the reflectance of the gain equalizer 5A with respect to light in the C band has a reflectance of 0.2 or less near 1530 nm, and in the region of 1540 nm to 1560 nm, as shown in FIG. It is about 0.5 to 0.6, and has a characteristic approaching 1 near 1560 nm.

  Further, as shown in FIG. 13, when the population inversion ratio is low in the EDF, the gain of the C band light is negative, the absorbed C band light acts as pump light, and amplifies the L band light. I do. In the first embodiment, the transmission characteristic of the gain equalizer 5A is set so that the transmission of light in the C band which is not reflected by the gain equalizer 5A and the incidence on the amplification optical fiber 2B for amplifying the L band are performed. With the characteristics, the excitation light can be reduced. For this reason, the transmission characteristics (○ in FIG. 3) of the gain equalizer are determined so that all the C-band light not reflected by the gain equalizer 5A is transmitted.

  The coupler 4A is a coupler for multiplexing the pump light from the pump light 3A and the input signal light from the port B of the circulator 1, and the coupler 4B is a coupler for pump light from the pump light 3B and light from the gain equalizer 5B. Are coupled. As the couplers 4A and 4B, a 3 dB coupler or a wavelength multiplexing coupler can be used. In the present embodiment where the wavelength band of the signal light is wide, a wavelength multiplex coupler is more preferable.

  The optical amplification unit 13A includes an amplification optical fiber 2A, an excitation light source 3A, and a coupler 4A, and has one end connected to the port B of the circulator 1 and the other end connected to the gain equalizer 5A, as shown in FIG. Have been. The optical amplifier 13B includes an amplification optical fiber 2B, an excitation light source 3B, and a coupler 4B. The other end of the gain equalizer 5A is connected to the other end of the gain equalizer 5B.

  Hereinafter, the process of changing the signal light intensity of the input WDM signal light in each configuration of the first embodiment will be described with reference to FIG. In the graph shown in FIG. 4, ● represents an input signal light input to port A of circulator 1, and ○ represents an output signal obtained by amplifying signal light output from port B of circulator 1 by first amplifying optical fiber 2A. Light, ★ is light transmitted and output through the first gain equalizer 5A, Δ is light reflected and output from the first gain equalizer 5A, and ☆ is light output from the gain equalizer 5A. Is amplified by the second amplification optical fiber 2B, □ is the light reflected and output from the second gain equalizer 5B, and Δ is the light output from the gain equalizer 5B. Represents the output light amplified by the amplification optical fiber 2B, and ◆ represents the output light amplified by the first amplification optical fiber 2A of the light output from the gain equalizer 5A.

  In FIG. 1, C-band and L-band input signal lights (● in FIG. 4) enter the optical amplifier 13A from the port A of the circulator 1 via the port B. The amplification optical fiber 2A is excited by the excitation light from the excitation light source 3A, and the C-band and L-band signal lights incident on the amplification optical fiber 2A through the coupler 4A are amplified according to the characteristics shown in FIG. You. Since the C-band light is amplified only by the amplification optical fiber 2A, its gain characteristic is determined so that the intensity of the C-band light becomes flat with respect to the wavelength. Is lower than the gain in the C band. Therefore, the signal light intensity (FIG. 4A) after passing through the amplification optical fiber 2A is such that the intensity in the L band is lower than the intensity in the C band.

  The light output from the amplification optical fiber 2A exits the optical amplification unit 13A, enters the gain equalizer 5A, and is reflected (FIG. 4A) and transmitted (FIG. 4A) in accordance with the characteristics shown in FIG. 4 ★). The reflection characteristics of the gain equalizer 5A in the C band (FIG. 3A) are related to the amplification characteristics of the optical amplifier 13A, that is, the amplification characteristics of the amplification optical fiber 2A (FIG. 2 ●). The light reflected by the optical modulator 5A (FIG. 4A) is again amplified by the amplification optical fiber 2A, so that the intensity is determined to be flat with respect to the wavelength (the C band region in FIG. 4A). I have. The C-band light not reflected by the reflection characteristics passes through the gain equalizer 5A (see FIG. 4). On the other hand, all light in the L band passes through the gain equalizer 5A.

  The C-band and L-band light (FIG. 4 *) transmitted through the gain equalizer 5A enters the optical amplifier 13B, and enters the amplification optical fiber 2B through the coupler 4B. The amplification optical fiber 2B is excited by the excitation light from the excitation light source 3B, and the L-band signal light is amplified according to the characteristics shown in FIG. The inversion distribution ratio of the amplifying optical fiber 2B is lower than that of the amplifying optical fiber 2A, and the amplifying characteristic thereof is different from the amplifying characteristic of the amplifying optical fiber 2A as shown in FIG. In the L band, the gain required for the output intensity of the L band light at the port C of the circulator 1 to be flat with respect to the wavelength is significantly different. As shown in FIG. 13, when the population inversion ratio is low in the EDF, the gain of the light in the C band is negative, and the absorbed light in the C band acts as the excitation light, and amplifies the light in the L band. In the first embodiment, the C-band light transmitted through the gain equalizer 5B acts as the pump light of the amplification optical fiber 2B together with the light from the pump light source 3B incident through the coupler 4B, and the L-band light is transmitted. Amplify light. Therefore, the output of the amplification optical fiber 2B (FIG. 4) hardly contains the C-band light acting as the pump light, while the L-band signal light is amplified and output.

  The light output from the amplification optical fiber 2B (Fig. 4 *) exits the optical amplification unit 13B, enters the gain equalizer 5B, and reflects according to the characteristics shown in Fig. 3A. The reflection characteristics of the gain equalizer 5B (FIG. 3A) are the amplification characteristics of the optical amplifiers 13A and 13B, that is, the amplification characteristics of the amplification optical fibers 2A and 2B (FIG. 2) and the gain characteristics of the gain equalizer 5A. The L-band light (FIG. 4 □) reflected by the gain equalizer 5B is again amplified by the amplification optical fiber 2B (FIG. 4 お り), which is related to the transmission characteristics (● in FIG. 3). 5A, and is again amplified by the amplification optical fiber 2A, so that the intensity becomes flat with respect to the wavelength (the L band region in FIG. 4A).

  The L-band light output to the port C of the circulator 1 is transmitted and reflected by the gain equalizers of both the gain equalizers 5A and 5B. Although gain equalization can be performed, in the optical amplifier according to the first embodiment, gain equalization of the L-band light is not performed by the gain equalizer 5A, and gain equalization is performed by the gain equalizer 5B. That is, the transmittance of the L-band light of the gain equalizer 5A is 1 as shown in FIG. 3 ●, and the transmittance of the L-band light of the gain equalizer 5B is 1 as shown in FIG. As shown by Δ, it is about 0.9 near 1570 nm, becomes 1 near 1580 nm, decreases to about 0.85 up to about 1590 nm, and becomes about 0.9 again at wavelengths longer than 1600 nm. When the amplifying optical fiber 2A is output and enters the gain equalizer 5A (FIG. 4A), there is a large intensity difference between the C-band light and the L-band light, but as shown in FIG. , L-band light are transmitted and reflected by the gain equalizers 5A and 5B with little loss, and the L-band light is input to the input of the amplification optical fiber 2A (C band region △, L band region ▽). Light is stronger.

  The light reflected by the gain equalizer 5B (FIG. 4 □) enters the optical amplifier 13B again, is amplified by the amplifying optical fiber 5B (FIG. 4 ▽), exits the optical amplifier 13B, and is output from the gain equalizer 13B. It is incident on 5A. Since the transmittance of the gain equalizer 5A with respect to the L band light (● in FIG. 3) is 1, all the light that has entered the gain equalizer 5A from the optical amplifier 13B is transmitted.

  The C-band light amplified by the optical amplifier 13A and reflected by the gain equalizer 5A (FIG. 4A) and the L-band light amplified by the optical amplifier 13B (FIG. 4A) and transmitted through the gain equalizer 5A are The light again enters the optical amplifier 13A, is amplified by the amplification optical fiber 2A (FIG. 4A), the output light intensity becomes flat with respect to the wavelength, and is output from the circulator 1 port B through the port C.

  As described above, the amplification characteristics of the optical amplifiers 13A and 13B, that is, the amplification characteristics of the amplification optical fibers 2A and 2B shown in FIG. 2, the transmission characteristics of the gain equalizers 5A and 5B shown in FIG. By defining the reflection characteristics so that the intensity of the output light from the circulator 1 becomes flat with respect to the wavelength (FIG. 4A), an optical amplifier having a flat gain in the C band and the L band is formed. The amplification characteristics of the amplification optical fibers 2A and 2B can be determined by changing the concentration of Er added, the fiber length, the intensity of the excitation light source, and the like.

  In the first embodiment, the C-band average transmittance of the gain equalizer 5A is 48.6%, and the C-band light transmitted through the gain equalizer 5A is transmitted to the amplification optical fiber 2B together with the light from the excitation light source 3B. And amplifies L band light. Thus, the output of the excitation light source 3B can be reduced as compared with the case where the light of the C band is not transmitted.

Table 1 shows the output reduction ratio of the pump light source 3B when the C-band light transmittance of the gain equalizer 5A is changed in the configuration of the first embodiment of the present invention. FIG. 5 shows the reflection characteristic of the first gain equalizer 5A, and FIG. 6 shows the amplification characteristic of the first amplification optical fiber 2A. Note that, as shown in FIG. 3, since the reflectance of the first gain equalizer 5A in the L band is 1, the characteristics of only the C band are shown in FIGS.

  In Table 1, "Transmittance" indicates the average transmittance in the C band of the gain equalizer 5A, and "1st EDF length" and "2nd EDF length" indicate the amplification optical fiber 2A set to obtain a predetermined gain characteristic. The length of the excitation light from the excitation light source 3B to the amplifying optical fiber 2B at each transmittance is indicated by the length of the excitation light at the time of transmission in the C band. The “excitation light at the time of non-transmission in the C band” is the intensity of the excitation light from the excitation light source 3B when the transmission characteristic of the gain equalizer 5A at each transmittance is such that the C band is non-transmission. . The “reduction ratio” is calculated from the output of the excitation light source 3B when the C band is not transmitted and when the C band is transmitted.

  Here, comparing the reflection characteristic of the gain equalizer 5A in the first embodiment with respect to the C band light with the signal light intensity shown in FIG. 4, the gain of the amplification optical fiber 2A is equal to the input signal light (FIG. 4). ●) and the intensity difference between the forward amplified light (FIG. 4A) and the reflected light (FIG. 4 図) of the gain equalizer 5A and the reverse amplified light (FIG. 4 ◆). It appears as an intensity difference. The reflection characteristic of the gain equalizer 5A shown in FIG. 3 (３) is based on the intensity of the light after forward amplification (増 幅 in FIG. 4) and the intensity of the light after reverse amplification (図 in FIG. 4). The intensity of the reflected light (FIG. 4A) of the gain equalizer 5A determined from the amplification characteristic of the optical fiber 2A is determined. Therefore, when the gain characteristic of the amplification optical fiber 2A is changed without changing the intensity of the light (FIG. 4A) after the backward amplification is flat with respect to the wavelength, the reflection characteristic of the gain equalizer 5A is changed. Needs to be changed.

  FIG. 6 shows the amplification gain in the C band of the amplification optical fiber 2A when the average transmittance in the C band of the gain equalizer 5A is increased. As the average transmittance of the C band increases, the light intensity of the C band incident on the gain equalizer 5A needs to be increased, and the gain of the necessary optical fiber for amplification also increases. When the average transmittance of the C band is increased, the characteristics of the amplification optical fiber 2A increase as the average transmittance increases from 24.7% (Fig. 6 ●) to 38.0% (Fig. 6 △) and 48.6% (Fig. 6 ○). However, the increase amount is not uniform with respect to the wavelength, and the increase amount is large in a region of 1550 nm or less, particularly in a range of 1530 nm to 1540 nm. This is because, as shown in FIG. 13, the amount of increase in the amplification gain when the population inversion is increased and the amplification gain per unit length is changed is not uniform.

  Therefore, in order to change the C-band average transmittance while keeping the output intensity characteristic of the entire optical amplifier flat with respect to the wavelength, the reflection of the gain equalizer 5A is changed in accordance with the change in the characteristic of the amplification optical fiber 2A. It is necessary to change the characteristics.

  FIG. 5 shows the reflectance of the gain equalizer 5A, which is the first gain equalizer, when the C-band average transmittance is changed. As the average transmittance increases, as shown in FIG. 6, the gain of the amplifying optical fiber 2A is not uniform with respect to the wavelength and increases, so that the reflectance of the gain equalizer 5A decreases accordingly. It is not uniform. Since the increase in the gain of the amplification optical fiber 2A is large in the region of 1550 nm or less, especially in the range of 1530 nm to 1540 nm, the reflectivity of the gain equalizer 5A is greatly reduced in the region of 1550 nm or less, particularly in the range of 1530 nm to 1540 nm.

  As shown in FIG. 13, when the population inversion ratio of the amplification optical fiber 2A is increased to increase the average transmittance in the C band, the gain in the wavelength region of the L band (about 1570 nm to about 1610 nm) is reduced. Since the inclination with respect to the wavelength increases, the length of the amplification optical fiber 2B required to keep the output intensity characteristics of the entire optical amplifier flat with respect to the wavelength increases.

  Here, considering the case where the gain equalizer 5A does not transmit the light in the C band, as shown in Table 1, it is necessary to amplify the light in the L band so that the intensity becomes flat with respect to the wavelength. The length of the amplifying optical fiber 2B is as long as 20.55 m at an average transmittance of 24.7%, 20.97 m at 38.0%, and 21.45 m at 48.6%. As the length of the amplification optical fiber 2B becomes longer, the absorption of the excitation light also increases, so that the output of the excitation light required for the excitation also increases, and the excitation light source 3B required to excite the amplification optical fiber 2B becomes larger. The output is as large as 17.0 mW at an average transmittance of 24.7%, 17.9 mW at 38.0%, and 18.5 mW at 48.6%.

  On the other hand, considering the case where the gain equalizer 5A transmits C-band light, the C-band light transmitted through the gain equalizer 5A is used as excitation light of the amplification optical fiber 2B together with light from the excitation light source 3B. Therefore, the output of the excitation light source 3B can be reduced as compared with the case where the C band light is not transmitted. As shown in Table 1, the reduction rate is 11.3% at an average transmittance of 24.7%, 15.5% at 38.0%, and 17.1% at 48.6%. If the length of the amplification optical fiber 2B is increased when the C band is not transmitted, Although the necessary excitation intensity increases accordingly, the transmission of C-band light has the effect that there is almost no need to change the excitation intensity.

(Second embodiment)
FIG. 7 shows the configuration of the second embodiment of the present invention. The optical amplifier according to the second embodiment has first to third ports, outputs the light input to the first port to the second port, and inputs the light to the second port. It comprises an optical component 17 for outputting light to the third port, optical amplifiers 13A and 13B, and gain equalizers 5A and 5B. The optical component 17 includes the circulator 1. The circulator 1 outputs the light input to the port A from the port B, and outputs the light input to the port B to the port C. The optical amplification units 13A and 13B include amplification optical fibers 2A and 2B, excitation light sources 3A and 3B, and couplers 4A to 4C, respectively. The optical amplifier 13B includes an isolator 6A for preventing oscillation between the gain equalizers 5A and 5B.

  In the optical amplifier according to the second embodiment, the C band is used as a first wavelength band that is reflected and transmitted by the first gain equalizer 5A, and the first gain equalizer 5A and the second gain equalizer are used. The L band is used as the second wavelength band transmitting 5B. The circulator 1 outputs the WDM signal light input to the port A from the port B, and outputs the light input to the port B to the port C.

  As the amplification optical fibers 2A and 2B, an optical fiber doped with a rare earth element or an optical fiber causing a Raman scattering effect, for example, DCF or DSF can be used. The optical fiber doped with the rare earth element forms an inversion distribution by exciting the rare earth element added by the excitation light, and amplifies by causing a stimulated emission phenomenon by the signal light. On the other hand, the optical fiber that causes the Raman scattering effect induces nonlinear scattering in the optical fiber by the excitation light, and performs amplification by stimulated Raman scattering of the signal light.

  In the second embodiment, as in the first embodiment, the input light wavelength-multiplexed into the C band and the L band is amplified and gain-equalized and output, but the light path of the L band is the first embodiment. Is different from the L-band light path. That is, in the first embodiment, the light in the L band is reflected by the gain equalizer 5B, whereas in the second embodiment, the light in the L band is transmitted by the gain equalizer 5B.

  The gain equalizers 5A and 5B are formed of a dielectric multilayer film. As continuous equalization characteristics of the optical amplifier according to the second embodiment of the present invention, the transmission characteristics and the reflection characteristics of the gain equalizers 5A and 5B are such that the output light intensity from the coupler 4C becomes flat with respect to the wavelength. As described above, it is determined in relation to the gain characteristics of the amplification optical fibers 2A and 2B. In the optical amplifier according to the second embodiment, the light in the L band is amplified only once each by the amplification optical fibers 2A and 2B, and therefore the amplification characteristics of the amplification optical fibers 2A and 2B and the gain equalizers 5A and 5B. Are different from those of the first embodiment in transmission and reflection characteristics.

  In the second embodiment, since the second amplifying optical fiber 2B amplifies only the light in the direction from the gain equalizer 5A via the coupler 4B, the optical amplifying unit 13B includes an isolator. This prevents oscillation due to reflection of the first and second gain equalizers, reflection of the end surface of the optical amplification medium, generation of reverse ASE light by the optical amplification medium, and the like.

  The coupler 4A is a coupler for multiplexing the pump light from the pump light 3A and the input signal light from the port B of the circulator 1, and the coupler 4B is a coupler for pump light from the pump light 3B and light from the gain equalizer 5B. Are coupled. The coupler 4C is a coupler for multiplexing the output of the port C of the circulator 1 and the transmission output of the gain equalizer 5B. As the couplers 4A to 4C, a 3 dB coupler or a wavelength multiplexing coupler can be used. In the present embodiment where the wavelength band of the signal light is wide, a wavelength multiplex coupler is more preferable.

  In FIG. 7, the input signal light of the C band and the L band enters the optical amplifier 13A from the port A of the circulator 1 via the port B. The amplification optical fiber 2A is excited by the excitation light from the excitation light source 3A, and the C-band and L-band signal lights incident on the amplification optical fiber 2A through the coupler 4A are amplified. Since the C-band light output from the port C of the circulator 1 is amplified only by the amplification optical fiber 2A, the gain characteristic is determined so that the intensity of the C-band light becomes flat with respect to the wavelength. The gain in the L band of the amplification optical fiber 2A is lower than the gain in the C band. Therefore, the signal light intensity (FIG. 4A) after passing through the amplification optical fiber 2A is such that the intensity in the L band is lower than the intensity in the C band.

  The light output from the amplification optical fiber 2A exits the optical amplification unit 13A, enters the gain equalizer 5A, and is reflected and transmitted. As in the first embodiment, the reflection characteristic of the gain equalizer 5A in the C band is related to the amplification characteristic of the optical amplifier 13A, that is, the amplification optical fiber 2A, and is reflected by the gain equalizer 5A. The coupler 4C is determined to output the coupler 4C with a flat intensity with respect to the wavelength by amplifying the reflected light again by the amplification optical fiber 2A. The C-band light not reflected by the above-mentioned reflection characteristics passes through the gain equalizer 5A. On the other hand, the transmission characteristics of the gain equalizer 5A in the L band are related to the amplification characteristics of the optical amplifiers 13A and 13B, that is, the amplification characteristics of the amplification optical fibers 2A and 2B and the transmission characteristics of the gain equalizer 5B. The light transmitted through the gain equalizer 5A is amplified by the amplification optical fiber 2B and transmitted by the gain equalizer 5B, so that the coupler 4C is output with a flat intensity with respect to the wavelength. Has been. Similarly to the first embodiment, the reflection characteristic of the gain equalizer 5A in the second embodiment is a characteristic in which the amplification characteristic of the amplification optical fiber 2A changes according to the wavelength.

  The C-band and L-band lights transmitted through the gain equalizer 5A enter the optical amplifier 13B, and enter the amplification optical fiber 2B through the coupler 4B. The amplification optical fiber 2B is excited by the excitation light from the excitation light source 3B, and the L-band signal light is amplified. As in the first embodiment, the inversion distribution ratio of the amplification optical fiber 2B is lower than that of the amplification optical fiber 2A, and its amplification characteristics are significantly different from those of the amplification optical fiber 2A. As shown in FIG. 13, when the population inversion ratio is low in the EDF, the gain of the light in the C band is negative, and the absorbed light in the C band acts as the excitation light, and amplifies the light in the L band. As in the first embodiment, also in the second embodiment, the C-band light transmitted through the gain equalizer 5B pumps the amplification optical fiber 2B together with the light from the pump light source 3B incident through the coupler 4B. Acts as light and amplifies L band light. Therefore, the output of the amplification optical fiber 2B hardly includes the light in the C band that has acted as the pump light, while the signal light in the L band is amplified and output.

  The light output from the amplification optical fiber 2B exits the optical amplification unit 13B, enters the gain equalizer 5B, and transmits according to the transmission characteristics of the gain equalizer 5B. The transmission characteristics of the gain equalizer 5B are related to the amplification characteristics of the optical amplifiers 13A and 13B, that is, the amplification characteristics of the amplification optical fibers 2A and 2B and the transmission characteristics of the gain equalizer 5A. The L-band light transmitted by the converter 5B is determined so as to output the coupler 4C with a flat intensity with respect to the wavelength. The light transmitted through the gain equalizer 5B enters the coupler 4C.

  On the other hand, the C-band light amplified by the optical amplifier 13A and reflected by the gain equalizer 5A again enters the optical amplifier 13A, is amplified by the amplification optical fiber 2A, and is coupled from the circulator 1 port B to the port C through the port C. 4C is incident.

  Therefore, the C-band light amplified by the optical amplifier 13A, reflected by the gain equalizer 5A, again amplified by the optical amplifier 13A, and gain-flattened, and amplified by the optical amplifiers 13A and 13B, The L-band light gain-equalized by the equalizers 5A and 5B and gain-flattened is input to the coupler 4C, multiplexed and output, so that output light gain-flattened in both wavelength bands. Have gained.

  As described above, the amplification characteristics of the amplification optical fibers 2A and 2B and the transmission characteristics and reflection characteristics of the gain equalizers 5A and 5B are determined so that the output light intensity from the coupler 4C becomes flat with respect to the wavelength. Thus, an optical amplifier having a flat gain in the C band and the L band is configured. The amplification characteristics of the amplification optical fibers 2A and 2B can be determined by changing the concentration of Er added, the fiber length, the intensity of the excitation light source, and the like.

(Third embodiment)
FIG. 8 shows the configuration of the third embodiment of the present invention. The optical amplifier according to the third embodiment has first to third ports, outputs light input to the first port to the second port, and inputs light to the second port. It comprises an optical component 17 for outputting light to the third port, an optical amplifier 13A, and a gain equalizer 5A. The optical component 17 includes the circulator 1. The circulator 1 outputs the light input to the port A from the port B, and outputs the light input to the port B to the port C. The optical amplifier 13A includes an amplification optical fiber 2A, an excitation light source 3A, and couplers 4A and 4B.

  As the amplification optical fiber 2A, an optical fiber doped with a rare earth element or an optical fiber causing a Raman scattering effect, for example, DCF or DSF can be used. The optical fiber doped with the rare earth element forms an inversion distribution by exciting the rare earth element added by the excitation light, and amplifies by causing a stimulated emission phenomenon by the signal light. On the other hand, the optical fiber that causes the Raman scattering effect induces nonlinear scattering in the optical fiber by the excitation light, and performs amplification by stimulated Raman scattering of the signal light.

  The coupler 4A is a coupler that combines the pumping light from the pumping light 3A and the input signal light from the port B of the circulator 1, and the coupler 4B is an output of the port C of the circulator 1 and a transmission output of the gain equalizer 5A. And a coupler for multiplexing. As the couplers 4A and 4B, a 3 dB coupler or a wavelength multiplexing coupler can be used. In the present embodiment where the wavelength band of the signal light is wide, a wavelength multiplex coupler is more preferable.

  In the third embodiment, as in the first and second embodiments, the C-band and L-band WDM signal lights are amplified and gain-equalized, but the amplification is performed by the optical amplifier 13A, that is, the amplification light. Only the fiber 2A is used, and the C band is used as a first wavelength band reflected by the gain equalizer 5A, and the L band is used as a second wavelength band transmitted through the gain equalizer 5A.

  In FIG. 8, the input signal light of the C band and the L band enters the amplifying optical fiber 2A from the port A of the circulator 1 via the port B and the coupler 4A. The amplification optical fiber 2A is excited by the excitation light from the excitation light source 3A, and the C-band and L-band signal lights are amplified.

  The light emitted from the amplification optical fiber 2A is incident on the gain equalizer 5A, and is reflected and transmitted. The reflection characteristic of the gain equalizer 5A in the C band is related to the amplification characteristic of the amplification optical fiber 2A, and the light reflected by the gain equalizer 5A is amplified again by the amplification optical fiber 2A. It is determined that the coupler 4B is output with a flat intensity with respect to the wavelength. The transmission characteristic of the gain equalizer 5A in the L band is related to the amplification characteristic of the amplification optical fiber 2A, and is determined so that the light transmitted through the gain equalizer 5A becomes flat with respect to the wavelength. Has been.

  The L-band light transmitted through the gain equalizer 5A enters the coupler 4B. On the other hand, the C-band light reflected by the gain equalizer 5A is again amplified by the amplification optical fiber 2A, and enters the coupler 4B through the ports B and C of the circulator 1. The C-band light and the L-band light are multiplexed by the coupler 4B, and light having a flat intensity characteristic with respect to wavelength is output in both wavelength bands.

  In the third embodiment, the C band is used as the first wavelength band reflected by the gain equalizer 5A, and the L band is used as the second wavelength band transmitted through the gain equalizer 5A. By changing the characteristics of the unit 13A and the gain equalizer 5A, the L band is used as the first wavelength band reflected by the gain equalizer 5A, and the C band is used as the second wavelength band transmitted through the gain equalizer 5A. A band may be used.

In the first to third embodiments, the amplification optical fiber is forward-pumped. However, the optical amplifier according to the present invention can be implemented by a configuration in which the amplification optical fiber is backward-pumped or bidirectionally pumped. It is possible. By changing the pumping method of the amplification optical fiber from forward pumping, it is possible to change noise characteristics, gain characteristics, band and wavelength characteristics.

In addition, since there is a gain equalizer that makes the intensity of the signal light linear with respect to the wavelength of the input light whose intensity is linear, there is a gain equalizer, so that the output light intensity is in both the C band and the L band. In addition to the flattened configuration, the configuration may be such that the signal light intensity of the output light becomes linear with respect to the wavelength.

  Further, the output characteristics of the optical amplifier according to the present invention can be changed by the amplification characteristics of the optical amplifier and the reflection characteristics and transmission characteristics of the gain equalizer, so that the intensity of the output light is flat with respect to the wavelength. In addition to being linear, the characteristics of the gain equalizer are related to the wavelength characteristics of the dispersion compensator and optical amplifier connected to the subsequent stage, and the characteristics of the dispersion compensator and optical amplifier The characteristics may be such that the output is flat or linear.

  Further, in the first to third embodiments, the gain equalizers 5A and 5B are filters using a dielectric multilayer film. However, if the filters can determine the transmission characteristics and the reflection characteristics, the FBGs may be used. And the like.

FIG. 1 is a configuration diagram showing a first embodiment of the present invention. FIG. 4 is a characteristic diagram showing amplification characteristics of first and second amplification optical fibers according to the first embodiment of the present invention. FIG. 3 is a characteristic diagram showing transmission characteristics and reflection characteristics of a first gain equalizer and reflection characteristics of a second gain equalizer according to the first embodiment of the present invention. FIG. 4 is a diagram illustrating signal strength at each stage of the first embodiment of the present invention. FIG. 4 is a diagram showing reflection characteristics of the first gain equalizer when the average transmittance in the C band is changed in the configuration of the first embodiment of the present invention. FIG. 4 is a diagram illustrating amplification characteristics of a first amplification optical fiber when the average transmittance in the C band is changed in the configuration of the first embodiment of the present invention. FIG. 5 is a configuration diagram showing a second embodiment of the present invention. FIG. 9 is a configuration diagram showing a third embodiment of the present invention. FIG. 1 is a configuration diagram showing an optical fiber amplifier according to a conventional technique. FIG. 1 is a configuration diagram illustrating an optical fiber amplifier according to a conventional technique. FIG. 1 is a configuration diagram illustrating an optical fiber amplifier according to a conventional technique. FIG. 1 is a configuration diagram illustrating an optical fiber amplifier according to a conventional technique. The figure which shows the inversion distribution rate of EDF, and the gain characteristic per unit length.

Explanation of reference numerals

1: circulators 2A to 2D: amplification optical fibers 3A to 3D: pumping light sources 4A to 4F: couplers 5A and 5B: gain equalizers 6A to 6D: isolators 7: filters 8A and 8B: duplexers / combiners 9A 9F: Optical fiber grating 10: Non-reflection terminal 11: Reflector mirrors 12A, 12B: Dividers 13A to 13H: Optical amplifiers 14A to 14H: Variable optical attenuators 15A, 15B: Conventional gain equalizer 16A: FBG Writable amplification optical fiber 17: Optical components 18A, 18B: Conventional optical amplifier

Claims (8)

In an optical amplifier that collectively amplifies light multiplexed signal light of a plurality of different wavelengths,
One or more optical amplifying units, and one or more gain equalizers connected downstream of the optical amplifying units and having transmission characteristics and reflection characteristics related to amplification characteristics of the optical amplifying units. An optical amplifier characterized in that the equalizer changes a light passage path for each wavelength band and performs gain equalization.
You In an optical amplifier that collectively amplifies light multiplexed signal light of a plurality of different wavelengths,
Having a first to a third port, outputting the multiplexed light input to the first port to the second port, and outputting the light input to the second port to the third port. An optical component that outputs to the port of
A first optical amplification unit having one end connected to a second port of the optical component and amplifying the multiplexed light;
A first gain equalizer having one end connected to the other end of the first optical amplifier,
A second optical amplification unit having one end connected to the other end of the first gain equalizer and amplifying an output of the first gain equalizer;
The gain equalizer transmits light in a second wavelength band and reflects light in the first wavelength band based on continuous equalization characteristics related to the gain characteristics of the first optical amplifier. And an optical amplifier that transmits light that is not reflected by the light of the first wavelength band.
3. The optical amplifier according to claim 2, wherein the equalization characteristic is related to gain characteristics of the first and second optical amplifiers, and wherein the first gain equalizer includes the second wavelength. An optical amplifier for transmitting light in a band based on the equalization characteristics.
4. The optical amplifier according to claim 2, wherein the equalization characteristic is related to a gain characteristic of the first and second optical amplifiers, and one end is provided at the other end of the second optical amplifier. And a second gain equalizer for reflecting light in the second wavelength band based on the equalization characteristic.
4. The optical amplifier according to claim 2, wherein the equalization characteristic is related to a gain characteristic of the first and second optical amplifiers, and one end is provided at the other end of the second optical amplifier. Are connected, a second gain equalizer that transmits the light of the second wavelength band based on the equalization characteristic, an output of a third port of the optical component, and a second gain equalizer. An optical amplifier, comprising: multiplexing means for multiplexing the output of the other end.
6. The optical amplifier according to claim 5, wherein said second amplifying unit includes an optical amplifying medium and an isolator.
6. The optical amplifier according to claim 5, wherein the second gain equalizer reflects light of a third wavelength band in the light of the second wavelength band based on the equalization characteristic, and An optical amplifier for transmitting light of a fourth wavelength band in light of the wavelength band based on the equalization characteristic.
In an optical amplifier that collectively amplifies light multiplexed signal light of a plurality of different wavelengths,
Having a first to a third port, outputting the multiplexed light input to the first port to the second port, and outputting the light input to the second port to the third port. An optical component that outputs to the port of
An optical amplification unit having one end connected to a second port of the optical component and amplifying the multiplexed light;
A gain equalizer having one end connected to the other end of the first optical amplifier;
Multiplexing means for multiplexing the output of the third port of the optical component and the output of the other end of the gain equalizer,
The gain equalizer reflects light in a first wavelength band based on a continuous equalization characteristic related to a gain characteristic of the optical amplifier, and converts light in a second wavelength band into the equalization characteristic. An optical amplifier, which transmits light based on the following.