JP2001313433A - Optical amplifier and method for optical amplification - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable use of a band, except a using band of an exiting optical amplifier for an optical transmission, to enlarge an amplifying band by using an exiting optical amplifying medium. SOLUTION: An excitation means for exciting to generate at least one gain peak and a gain equalizer for equalizing a peak of a gain of the optical amplifying medium are provided in the optical amplifying medium for amplifying a light, by inductively emitting the light through excitation. Thus, a flat gain is obtained in a wavelength band except the peak value of the gain. In order to increase the conversion efficiency of amplifying, a means for distributively or dispersively equalizing the gain is proved in the longitudinal direction of the medium, thereby improving the conversion efficiency of the optical amplification.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical amplifier for amplifying a wavelength division multiplexed signal light in which a plurality of signal lights having different wavelengths are multiplexed, and a multi-relay transmission of the wavelength division multiplexed signal light via a plurality of optical amplification repeaters. In particular, the present invention relates to an optical amplifier having a new band which has not been available.

[0002]

2. Description of the Related Art A band in which the loss of an optical fiber transmission line is small.
(About 0.3 dB / km or less) is from 1450 nm to 1650 nm, and various optical fiber amplifiers have been developed in this transmission band as shown in FIG.

[0003] At present, the demand for communication has exploded due to the spread of mobile phones and the rapid increase of Internet services. Research and development of technology for increasing the capacity that can be transmitted by one optical fiber has been conducted all over the world. It is being energetically advanced.

At present, an optical wavelength multiplexing technique utilizing the broadband characteristics of an optical fiber amplifier using a silica-based erbium-doped fiber (EDF) is an important technique.
nm band (1530-1560 nm) or C-band (Conventional-wa
velength band).

In addition, EDF has a 1580 nm band (1570 to 1600 nm),
An optical fiber amplifier of band (Longer-wavelength band) has been developed, and a total of 1.6 Tb / s is obtained by modulating each wavelength at 10 Gb / s and multiplexing about 160 waves in each band. The competition for commercialization and development of optical fiber communication systems that enable transmission is intensifying.

When the C-band and L-band bands are combined, about 8 THz
Therefore, if 10 Gb / s transmission signal channels are arranged at intervals of 25 GHz, the total transmission capacity can be further increased by 1.6 terabits, but the limit is about 3.2 Tb / s.

On the other hand, further increase in capacity is desired.
Current C? band, L? In addition to the band optical amplifier, an optical fiber amplifier having a new optical amplification band is required.

In FIG. 1, the amplification fiber in the S-band band from 1490 nm to 1530 nm has a GS-TDFA (gain shifted thul
Although an ium-doped fluoride-based fiber amplifier) is being developed, the band having a gain is in the 1475 to 1510 nm band, and it has been difficult to achieve amplification in the S-band band of 1510 to 1530 nm.

Further, the wavelength of 1610 to 1650 nm is limited to special fibers such as thulium and terbium-doped fluoride fibers.

In the optical amplifier described above, a portion serving as an optical amplification medium performs stimulated emission by energy level inversion distribution by excitation to perform optical amplification.

[0011] In addition, there is Raman fiber amplification utilizing the nonlinear effect of fiber.

Raman amplification has the advantage of being able to have gain in an arbitrary wavelength band by selecting the wavelength of the pumping light source because the nonlinear effect of the fiber is used, but the gain per unit length is small and several km. Therefore, there is a problem that a transmission line for optical amplification of several tens km is required, and miniaturization is difficult.

[0013]

SUMMARY OF THE INVENTION In the present invention, amplification is performed using stimulated emission caused by population inversion of energy formed in an optical amplification medium by excitation other than Raman amplifier (optical amplification utilizing nonlinear effects). Using an optical amplifier (rare-earth element doped fiber amplifier or semiconductor optical amplifier SOA), S +? band, 1490-1530nm band S? ba
nd or 1610? L + in the 1650nm band? An object of the present invention is to provide an optical fiber amplifier having an amplification band in band.

[0014]

As a first means, an optical amplifier comprises an optical amplifying medium for amplifying light, an exciting means for exciting the optical amplifying medium to generate at least one gain peak, and an optical amplifying medium. And a gain equalizer for equalizing the gain of the optical amplification medium. The gain equalizer equalizes the gain of the optical amplifying medium so as to generate a gain in an optical wavelength region where the gain is not maximized.

As a second means, an optical amplifying medium for amplifying light, an exciting means for exciting the optical amplifying medium to generate at least one gain peak, a gain of the optical amplifying medium over the entire optical amplifying medium, and the like. And a gain equalizer for equalizing the gain of the optical amplifying medium so as to generate a gain in an optical wavelength region where the gain of the optical amplifying medium is not maximized.

As a third means, a plurality of optical amplifying media for amplifying light, an exciting means for exciting at least one gain peak in the plurality of optical amplifying media, and an optical amplifier between the plurality of optical amplifying media are provided. A plurality of gain equalizers for equalizing the gain of the amplifying medium are provided, and the plurality of gain equalizers amplify light in a wavelength region where the gain generated by the plurality of optical amplifying media and the pumping unit is not maximized.

As a fourth means, an optical amplification medium doped with a rare earth element, an excitation light source for exciting the rare earth element of the optical amplification medium, and a wavelength characteristic of optical amplification gain generated when the amplification medium is excited by the excitation light source are described. A gain equalizer for equalizing is provided, and the optical pumping light source has a population inversion ratio in which gain occurs in the wavelength band of the signal amplified by the optical amplifying medium, and the wavelength band of the signal amplified by the optical amplifying medium is pumped. The gain equalizer attenuates the maximum value of the gain of the optical amplifying medium by using a band that does not include the maximum value of the gain generated in the optical amplifying medium by the light source.

As a fifth means, in the first to third means, the input and output of the optical amplifying medium are monitored, feedback is applied to the pumping means, and the gain of the optical amplifying medium is controlled to be constant.

As a sixth means, a resonator including an optical amplifying medium is provided in the first to third means.

As a seventh means, in the first to third means, the optical amplification medium is an optical amplifier which causes stimulated emission in the medium by excitation.

As an eighth means, the optical amplifier performs pumping so that the optical amplifying medium composed of erbium-doped fiber has a population inversion ratio of 0.7 to 1;
The wavelength band between 30 nm is transmitted, and the wavelength band is equalized in a distribution or dispersion manner to the amplification medium so as to have a substantially flat wavelength characteristic.

As a ninth means, the optical amplification is carried out so that the optical amplifying medium comprising an erbium-doped fiber has an inversion distribution ratio of about 0.8 to 1 and about 1450 nm to about 1
The wavelength band between 490 nm is transmitted, and the wavelength band is distributed or dispersed in the amplification medium so as to have substantially flat wavelength characteristics.

As a tenth means, the optical amplifier pumps the optical amplifying medium composed of erbium-doped fiber so as to have a population inversion ratio of about 0.3 to 1, and transmits a light in a wavelength band of about 1610 nm to about 1650 nm. Then, the wavelength band is distributedly or dispersively equalized to the amplification medium so as to have a substantially flat wavelength characteristic.

As an eleventh means, there is provided an optical amplification method for an optical amplifier, comprising selecting a population inversion ratio of an optical amplification medium, expanding a band having a gain generated in the optical amplification medium, and determining a peak value of a gain generated in the optical amplification medium. Wavelength bands used for optical communication are set at different positions, gain equalization is performed so that the gain becomes flat in the wavelength band used for optical communication, and wavelength bands where gain occurs in optical amplification media other than the wavelength band used for optical communication Should be attenuated.

As a twelfth means, an inversion distribution of energy is generated by exciting the optical amplifier, and an optical amplifying medium having at least one peak of the amplification gain, and a substantially flat gain band at a wavelength lower than the peak value of the amplification gain. , And an equalizer for equalizing the gain characteristic of the amplification medium.

[0026]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 2 shows various inversion distribution ratios (Inversion r) of a silica-based erbium-doped fiber (EDF).
ate) shows the wavelength characteristic of the relative gain coefficient.

The population inversion ratio is defined as a ratio at which erbium ions are excited, and becomes 1.0 when all the electrons are excited (all the electrons are excited to the upper level). Level) and 0.0.

Relative gain coeff on the vertical axis
icient) is equal to the gain per unit length.

At present, a wavelength-division multiplexed optical fiber amplifier using a silica-based erbium-doped fiber (EDF), which is widely available, has an inverted distribution of the silica-based erbium-doped fiber EDF so as to have an amplification band in a 1550 nm band (1530 to 150 nm). The silica-based erbium-doped fiber (EDF) is operated by exciting the inversion rate to about 0.7.

This is realized by using a gain equalizer in combination to use a band having the maximum value of gain and realize a flat gain without wavelength dependence.

Similar to the C-band, a wavelength multiplexing optical amplifier that amplifies a long wavelength band L-band (1560 to 1610 nm) is almost at the commercial level.

This is because the inversion rate of the silica-based erbium-doped fiber (EDF) is purposefully reduced to about 0.4, so that the gain per unit length in the long wavelength band (L-band) is maximized. By producing a flat gain,
Constructs an L-band optical amplifier.

This silica-based erbium-doped fiber (E
As can be seen from Fig. 2, the L-band optical amplifier using DF) has a smaller unit gain of EDF than the C-band optical amplifier using EDF. By doing so, the required gain is realized.

The inversion rate in FIG.
For example, a close look at the wavelength characteristic of a gain of 0.9 reveals that there is a higher gain than the L-band optical amplifier in a band that has not been realized so far, for example, a 1450 to 1530 nm band or a 1610 to 1650 nm band.

However, when the inversion rate is 0.9, the gain per unit length is large in the C-band (1530 to 1570 nm), so that the C-band is dominant in the amplification effect.

Therefore, the present invention provides a gain equalizer (GEQ)
-Used for band suppression.

FIG. 3 shows the inversion distribution ratio (Inversion rat) in FIG.
e) shows the gain characteristic per unit length when extracting the gain wavelength characteristic of 0.9 and extracting and equalizing the S-band gain.

When the inversion rate in FIG. 3 is 0.9, the gain has a peak near 1530 nm.

By removing the hatched portion of the gain peak portion,
Gain equalization is performed in the S-band such that the gain characteristics are flat in the S-band and gain characteristics in which the other wavelength bands are deleted, as in the outlined portion.

In this method, the C-band having a large gain per unit length is removed by an equalizer, and the gain in the S-band is equalized flat to facilitate wavelength multiplexing of transmission signal light. , S-band gain per unit length is reduced.

However, the inversion distribution ratio (Inversion) in FIG.
The gain per unit length can be higher than the gain of the L-band optical amplifier that performs optical amplification using (rate) 0.4.

That is, considering that a practical optical fiber amplifier can be manufactured in the L-band, the population inversion ratio of the optical amplifying medium is increased, the band in which the optical amplifying medium generates a gain is widened, and the peak value in this band is increased. The gain equalization is performed so as to obtain a flat gain characteristic at a position different from the position (shoulder portion of the gain characteristic), and in order to obtain a desired gain, the length of the optical amplifying medium and the gain equalizer are set to By selecting based on the gain characteristics at the population inversion rate of energy, silica-based erbium-doped fiber (EDF) can be used to select S? band, S +? band, L +? band
An optical amplifier having a practical gain in the band can be realized.

In particular, in the case of an erbium-doped fiber, considering the characteristics of the gain and the population inversion ratio with respect to the wavelength, a substantially flat wavelength band is realized on the shorter wavelength side than the existing C-band amplifier and L-band amplifier. This is effective in realizing a possible optical amplifier.

FIG. 4 shows that S? gain equalizer (GE
Q, so-called optical filter).

The 950 nm to 1000 nm band has a transmission band so that an excitation light source of 980 nm can pass therethrough.
The band is configured such that the transmittance decreases as the wavelength goes to a longer wavelength so that the white gain characteristic shown in FIG. 3 can be realized in accordance with the amplification gain.

FIG. 4 shows the characteristics of the configured gain equalizer corresponding to FIG. 2, and shows a configuration that suppresses the gain peak centered on 1531 nm. band
When the population inversion is selected so as to have a higher gain, the peak wavelength shifts by about 1528 nm to 1535 nm depending on the dopant material (material such as Al and Ge) and the structure of the fiber effective cross section.

Therefore, since the peak center wavelength of the gain differs depending on the type of the amplification medium, the equalization amount with respect to the wavelength corresponds to the population inversion ratio of each amplification medium and the center wavelength of the gain. Need to be adjusted and configured.

FIG. 5 shows a specific configuration example of the S-band optical amplifier.

In the figure, 1 is a silica-based erbium-doped fiber (EDF), 2 is a gain equalizer (GEQ), 31 and 32 are optical isolators, 4 is a pump light source, 5 is a wavelength multiplexing coupler, 8 is an input terminal, 9 Is an output end, 71 and 72 are optical branch couplers, 81 is an input monitor PD, 82
Denotes an output monitor PD, and 50 denotes a constant gain control circuit (AGC).

The wavelength-division multiplexed light input from the input terminal 8 is input through the optical isolator 31 and the wavelength-division multiplexing coupler 5 to stimulated emission by excitation into a silica-based erbium-doped fiber (EDF) 1 as an amplification medium.

The silica-based erbium-doped fiber (EDF) used here had a mode field diameter of 7 μm and an Er concentration of 50 μm.
0 ppm, fiber length 150 m.

This fiber configuration is an example, and a general EDF can be used. (The range of the mode field diameter of general EDF currently on the market is 5 μm
The concentration of Er may be 100 ppm to 1500 ppm.
The length of the fiber varies from 1 m to several tens of km because the length is adjusted in accordance with the gain and concentration amplified by the amplifier. Note that the fiber length is adjusted by adjusting the fiber length according to the gain of the used wavelength band per unit length based on the population inversion ratio and the gain to be obtained.

Silica-based erbium-doped fiber (EDF) 1
In, the wavelength-division multiplexed light input from the input terminal 8 is optically amplified by 0.98 μm pumping light from the pumping light source 4 input via the wavelength multiplexing coupler 5 and input to the gain equalizer 2.

At this time, the population inversion ratio of the silica-based erbium-doped fiber 1 is 0.9, and the AGC control of the pumping light power is performed so as to obtain the wavelength characteristic as shown in FIG.

The gain equalizer 2 basically has the gain equalization characteristic shown in FIG. 4 and performs gain equalization so that the gain has the outline characteristic shown in FIG. Since it is provided at the subsequent stage of the system erbium-doped fiber (EDF) 1, it may be a characteristic that does not transmit the excitation light.

The gain equalizer 2 can be realized by combining a plurality of Fabry-Perot etalon filters, a dielectric multilayer filter, and a fiber grating filter.

The output of the gain equalizer 2 outputs the amplified light from the output terminal 9 via the optical isolator 32.

The input end side optical splitting coupler 71 splits a part of the input light and inputs it to the input monitor PD81, and the output end side optical splitting coupler 72 splits and outputs a part of the light amplified by the EDF1. Input to monitor PD82.

The automatic gain control circuit (AGC) 50 is connected to the input monitor PD8.
The pump light source 4 is controlled so that the gain of the erbium-doped fiber 1 becomes a constant value based on the light detected by 1 and the output monitor PD82.
The optical power of the output of the semiconductor laser of 0.98 μm is controlled. By maintaining the gain of the EDF at a constant value, the population inversion ratio also becomes a constant value regardless of the value of the input power.

While using the automatic gain control circuit (AGC) 50 together,
If it is desired to control the output level to be constant, provide a variable attenuator at the position of the input terminal 8 or the output terminal 9 to control the optical signal level input to the optical amplifier or the output of the optical amplifier. Thus, even when the constant gain control is performed in the optical amplifier, the output of the optical amplifier can be ALC controlled to be constant.

In FIG. 5, a 0.98 μm excitation light source was used.
When combined with the current EDFA, the population inversion ratio can be reduced to approximately 1. If a gain occurs in the band in which communication is to be performed, a 1.48 μm band (1.45 to 1.49 μm ) May be used.

Further, in FIG. 5, a description is given of forward pumping in which excitation is performed from the input end of the EDF. However, a wavelength multiplexing coupler is provided between the gain equalizer 2 and the optical isolator 32, and pumping is performed from the output end of the EDF. Backward pumping or bidirectional pumping for pumping pump light to the EDF from both the input end side and the output end side can also be used.

In the case of bidirectional pumping, the pumping wavelength is set to 0.1.
It is also possible to use a combination of the 98 μm and 1.48 μm bands.

However, the wavelength characteristics of the equalizer are 1.48 band and 0.98 band.
It is necessary to be able to transmit the band excitation light.

When two pump light sources are used as described above, the pump light source of either wavelength may be the forward pump light.

The pumping light source here is not limited to a single semiconductor laser, but may be a light source that emits light of a plurality of semiconductor lasers by combining wavelengths and / or polarizations.

FIG. 5 shows an example in which the population inversion ratio is 0.9 by taking an S-band optical amplifier as an example. Since it is sufficient to perform gain equalization so as to lower the S-band optical amplifier in the case of the optical amplifying medium used in FIG. 5, the characteristic of FIG. 2 uses an inversion distribution ratio of 0.7 to 1. Can be.

Similarly, in the case of forming an S + -band optical amplifier of 1450 nm-1490 nm, in the case of the characteristic of FIG. 2, an inversion distribution ratio of 0.8 to 1 can be used.

In the case of forming an L + -band optical amplifier of 1610 nm-1650 nm and having the characteristics shown in FIG. 2, an inversion distribution ratio of 0.3 to 1 can be used.

Since the gain of the optical amplifier differs depending on the band, it is necessary to select the length of the optical amplifying medium EDF so as to meet the desired gain.

The S-band optical amplifier of the present invention and the already technically completed L-band optical amplifier have the following decisive differences.

Looking at the gain wavelength characteristic of the L-band optical amplifier using the inversion rate of around 0.4 in FIG.
Although the value itself is small, the gain of the L-band is the maximum gain.

This is because the constant gain control (AGC)
If the inversion rate is fixed to 0.4, the pump light power is naturally converted to L-band signal light, which means that the amplifier has high amplification efficiency in the band of light to be amplified.

On the other hand, an S-band is obtained by gain equalization using an amplifying fiber and its inversion rate of 0.9.
In the case where amplification is performed by using the gain equalizer, for example, the C-band, which is larger than the gain of the S-band, is suppressed.

When the GEQ2 is installed at the output end as shown in FIG. 5, the gain peaks out of the band through which the signal light passes (1.
(Around 53 μm), the ASE component generated on the input side of the optical fiber # 1 also acts to amplify the ASE generated on the input side in the middle stage and the output side. It is converted into Spontaneous Emission (ASE) light outside the band band, resulting in an optical amplifier with very poor conversion efficiency (conversion efficiency several% or less).

Further, a GEQ having a large attenuation characteristic is required in order to attenuate unnecessary ASE generated.

In general, the conversion efficiency of the optical amplifier is about 60% in the C-band and about 40% in the L-band.

FIG. 6 shows an embodiment for improving the conversion efficiency with respect to the configuration of FIG.

In FIG. 6, the amplification medium 1 necessary for obtaining a predetermined gain in the configuration of FIG. 5 is divided into a plurality of parts, and gain equalizers are respectively arranged between them, and the entire optical amplifier in the optical amplifier is arranged. This figure shows a case where a gain equalizer is arranged in a distribution medium or in a longitudinal direction in an amplification medium.

In the drawing, 11, 12, and 13 are silica-based erbium-doped fibers (EDF), 21, 22, and 23 are gain equalizers (GEQ), and 31 and 32.
Is an optical isolator, 4 is an excitation light source, 5 is a wavelength multiplexing coupler, 8
Is an input terminal, 9 is an output terminal, 71 and 72 are optical branch couplers, 81 is an input monitor PD, 82 is an output monitor PD, and 50 is a constant gain control circuit (A
GC).

The wavelength-division multiplexed light input from the input terminal 8 passes through the optical branching coupler 71, the optical isolator, and the wavelength-division multiplexing coupler 5, and passes through a silica-based erbium-doped fiber as an amplification medium.
(EDF) 1 is input.

When a predetermined gain is obtained at a population inversion ratio of 0.9 and the length is set to 50 m, the EDF 1 is divided into silica-type erbium-doped fibers 11 (EDF1) to 13 (EDF50) every 1 m. .

Then, GEQ'1,21 to G
Connect EQ'50 and EQ'23 respectively.

For these GEQs, a long-period fiber grating filter is optimal.

At this time, the wavelength characteristic of the transmittance of GEQ ′ is
Will be included in 50 units, so the GEQ signal wavelength band (1490? 1
(530 nm) can be reduced to 1/50 the transmittance characteristic (in dB).

As described above, by dividing EDF1,
The ASE generated in each divided section is the G provided between EDF1.
Because it is removed by the EQ, the next EDF
Since the ASE generated at F is not input, the ASE component is not amplified by the pump light, so that the conversion efficiency of the amplifier to the signal light can be improved.

Silica-based erbium-doped fiber (EDF) 1
In, the wavelength-division multiplexed light input from the input terminal 8 is optically amplified by 0.98 μm pumping light from the pumping light source 4 input via the wavelength multiplexing coupler 5 and input to the gain equalizer 2.

At this time, the population inversion of the power of the pumping light in the silica-based erbium-doped fiber 1 is 0.9, and pumping is performed so as to obtain the wavelength characteristic as shown in FIG.

The gain equalizers 21 to 23 have the gain equalization characteristics shown in FIG. 4 as a whole and are made of a silica-based erbium-doped fiber.
The gain of the (EDF) 1 is made to have the outline characteristics shown in FIG. 3, the gain is equalized so as to allow the pumping light to pass, and the amplified light is output from the output terminal 9 via the optical isolator 32.

The individual gain equalizers may have different equalization characteristics, or even if they have the same equalization characteristics, if the resulting characteristics are as shown in FIG. is good.

The gain equalizers 21 to 23 can be realized by combining a plurality of Fabry-Perot etalon filters, dielectric multilayer filters, and fiber grating filters.

By using the gain equalizers 21 to 23 having a high reflection loss characteristic of approximately −60 dB, resonance between the gain equalizers is prevented.

The input end optical branching coupler 71 splits a part of the input light and inputs it to the input monitor PD81, and the output end side optical splitting coupler 72 splits and outputs a part of the light amplified by the EDF1. Input to monitor PD.

The automatic gain control circuit (AGC) 50 is an input monitor PD
And an excitation light source such that the gain of the optical amplifier (strictly speaking, the gain of the EDF) becomes constant based on the light detected by the output monitor PD.
The output of the 0.98 μm semiconductor laser, which is 4, is controlled for the optical power.

When it is desired to use an automatic gain control circuit (AGC) 50 to perform ALC control so that the output level becomes constant while maintaining the wavelength characteristic of the gain, the position of the input terminal 8 or the output terminal 9 can be changed. By providing an attenuator and controlling the level of an optical signal input to the optical amplifier or the output of the optical amplifier, the output of the optical amplifier can be kept constant even when the optical amplifier performs gain constant control.

In FIG. 6, a 0.98 μm excitation light source was used.
A μm excitation light source may be used.

Further, in FIG. 6, a description is given of forward pumping in which excitation is performed from the input end of the EDF. However, a wavelength multiplexing coupler is provided between the optical isolator 32 and the gain equalizer 23, and excitation is performed from the output end of the EDF. It is also possible to use backward pumping or bidirectional pumping in which pumping light is pumped into the EDF from both the input end side and the output end side.

In the case of bidirectional excitation, an excitation light source of 0.98 μm and an excitation light source of 1.48 μm can be used.

In this case, the excitation light source of either wavelength may be forward excitation.

The pumping light source here is not limited to a single semiconductor laser, but may be one that combines the wavelengths and / or polarizations of the light of a plurality of semiconductor lasers and outputs the combined light.

Further, when it is necessary to increase the population inversion ratio to 1 or the like, a large amount of optical power of the pumping light source is required. Therefore, a wavelength multiplexing coupler is provided between each EDF, and forward pumping, backward pumping, or You may comprise so that bidirectional excitation may be performed.

FIG. 6 shows an example in which the population inversion ratio is 0.9 by taking an S-band optical amplifier as an example. However, the value of the population inversion ratio is selected so as to have a gain in the band to be used, and the gain other than the band to be used is adjusted. Since it is sufficient to perform gain equalization so as to lower the value, in the case of the optical amplifying medium used in FIG. 6, an S-band optical amplifier can be configured by using the population inversion ratio of 0.7 to 1.

Similarly, when constructing an S + -band optical amplifier of 1450 nm-1490 nm, a population inversion ratio of 0.8 to 1 can be used.

In the case of forming an L + -band optical amplifier of 1610 nm-1650 nm, a population inversion ratio of 0.3 to 1 can be used.

The improvement of the conversion efficiency from the pump light power to the S-band signal light will be described below by comparing the structure of FIG. 5 with the structure of FIG.

In the optical amplifier of FIG. 5, where the ASE in the form shown in FIG. 3 is very large, the hatched portion is deleted by the GEQ, so that the pump power is converted into unnecessary ASE and is wasted. .

On the other hand, in FIG. 6, the ASE of the form also shown in FIG.
However, the efficiency is improved by shaping with GEQ (ie, removing unnecessary parts before being amplified) before they are amplified and increased.

If this is expressed by a number, if the power (area) of the entire ASE is 100 mW in FIG. 5 and the hatched portion (90%) is discarded, about 90 mW is discarded (that is, about 90 mW). ,
Since this 90 mW is obtained by converting the excitation light, at least 90 mW
mW of pump light power is discarded).

The white part cannot be deleted because it is a signal band.

In FIG. 6, the power of the ASE is, for example, about 1/50.
When (area) is 2mW, 90% is discarded, so only 1.8mW is deleted at most.

Here, it is important to note that when the GEQ is not used for deletion, the silica-based erbium-doped fiber 11 (EDF
The 1.8mW ASE generated in 1) causes the next stimulated emission, wasting the pump light power.

In other words, placing one GEQ on the output side and performing equalization as shown in FIG. 5 means that unnecessary ASEs are grown as much as they grow, and then GEQs are deleted.

On the other hand, in FIG. 6, the ASE is deleted when it grows a little, and the excitation energy for further amplifying the ASE by the generated ASE can be directed to the amplified energy of the signal, thereby improving the conversion efficiency.

As described above, when the EDF is divided and a low-loss GEQ is inserted between the divided EDFs, the conversion efficiency is improved.

The location of the GEQ in the longitudinal direction of the amplification fiber is the key to improving the conversion efficiency.

As a modification of FIG. 5, a silica-based erbium-doped fiber may be divided into two parts, and a gain equalizer may be provided between the two parts so that the final output has the outline characteristics shown in FIG. .

With this configuration, the gain equalizer is connected to the output side.
The conversion efficiency from the pump light power to the S-band signal light can be improved as compared with the configuration in which one is provided.

FIGS. 5 and 6 show an example in which an existing silica-based EDF is used. However, as shown in FIG. 7, a fiber or an optical waveguide whose length is shortened by increasing the Er doping concentration is used. By providing a gray scale, distributed gain equalization can be performed in the optical amplifier as in FIG.

FIG. 7A shows a case where fluoride is used as a base material.
Increase the concentration of erbium element added per unit length,
A fiber or Er-doped optical waveguide with a reduced length is shown.

In the figure, reference numeral 14 denotes a core, 15 denotes a clad, and 16 denotes a gray tag.

In FIGS. 5 and 6, the EDF had a moat field diameter of 7 μm and the doping amount of the Er element was 500 ppm. At this time, if the population inversion ratio was 0.9, an intended gain of about 20 dB was obtained. Then the EDF needed a total length of 150m.

Therefore, the base materials of the fiber and the waveguide substrate are
By using a material that can dope Er at a high concentration and setting it to 15 × 10 ⁵ ppm, the same gain can be obtained with a total length of 5 cm.

As described above, when the light guide length is 5 cm, it functions as a GEQ for performing gain equalization so as to attenuate wavelengths other than the wavelength band to be amplified as shown in FIG. The grating 16 to be formed is formed on the core portion 14 of the optical fiber or the optical waveguide. In this case, it is important to use, for example, a long-period grating technique so that the light removed by the GEQ does not return to the core and cause resonance. That is, when the light deleted by the GEQ returns to the core, a resonator is formed in the Er-to-fiber which is the amplification medium, and unstable operation due to resonance or unnecessary laser oscillation occurs. It is necessary to build GEQ to prevent this.

At this time, the grating may be provided at a plurality of locations as in FIG. 6, or may be provided on the entire core 14.

In the case of this configuration, the same effect as in the case where the number of divisions of the grating in FIG. 6 is increased to infinity can be obtained.

With the configuration shown in FIG. 7A, an optical amplifier having a high conversion efficiency similar to the case where GEQs are distributed and arranged can be formed as in FIG.

FIG. 7B shows an example in which the configuration of FIG. 7A is applied as a system.

In the figure, (A) is an amplification medium such as a high-concentration erbium-doped fiber (EDF) or a high-concentration erbium optical waveguide substrate of FIG. 7 (A), 31 and 32 are optical isolators, 4 is an excitation light source, and 5 is a wavelength. Multiplex coupler, 8 is input, 9 is output, 7
Reference numerals 1 and 72 denote optical branch couplers, 81 denotes an input monitor PD, 82 denotes an output monitor PD, and 50 denotes a constant gain control circuit (AGC).

The output level of the pumping light source 4 is controlled by the optical branching couplers 71 and 72 based on the branched input and output monitor values so that the gain in the amplifying medium becomes constant in the automatic gain control circuit 50.

An optical fiber (for example, SMF) is connected between the WDM coupler 5 and the amplification medium (A) and between the amplification medium (A) and the optical isolator.

When it is desired to use an automatic gain control circuit (AGC) 50 to perform ALC control so that the output level becomes constant while maintaining the wavelength characteristic of the gain, the position of the input terminal 8 or the output terminal 9 can be changed. By providing an attenuator and controlling the optical signal level input to the optical amplifier or the output of the optical amplifier, the output of the optical amplifier can be kept constant even when the gain is controlled by the optical amplifier. .

FIG. 8 shows gain wavelength characteristics when the excitation current of the semiconductor optical amplifier is changed.

In the case of a semiconductor optical amplifier, a bias current is used instead of light as an excitation source.

As can be seen from FIG. 8, the semiconductor optical amplifier has an amplification peak, and by changing the pumping current to change the population inversion ratio, the wavelength position and the gain of the amplification peak change and the gain wavelength characteristic changes. You can see that.

Therefore, similarly to the case where the EDF is used, the current value is selected so as to have a population inversion ratio having a gain in the band where the optical amplification is performed (that is, to make the gain constant), and the light in the unnecessary band is selected. By performing gain equalization on the gain generated by the amplification using a plurality of gain equalizers, it is possible to realize an optical amplifier having high conversion efficiency in a band other than the maximum value of the gain.

FIG. 9 shows a specific example in which an optical amplifier is constituted by a semiconductor optical amplifier.

The basic configuration is the same as that of FIG. 6, but a plurality of semiconductor optical amplifiers 33, 34 and 35 are used instead of EDF1.

The wavelength-division multiplexed light input from the input terminal 8 is input to the semiconductor optical amplifiers (SOAs) 33 to 35 as an amplification medium via the optical branch coupler 71 and the optical isolator.

Here, multiple stages of semiconductor optical amplifiers (SOAs) necessary to obtain a predetermined gain in a target wavelength band are provided.

Then, the gain equalizer 21 (GEQ'1) to the gain equalizer 23 (GEQ'50) are connected between the semiconductor optical amplifiers.

At this time, the wavelength characteristic of the transmittance of the GEQ ′ is
, The transmittance characteristics of the gain equalization amount when the gain equalization is performed collectively in the final stage as shown in FIG.
(dB units) is reduced to 1 / number.

Here, the individual gain equalizers may have different equalization characteristics, or even if they have the same equalization characteristics, the resulting characteristics may be in the target band. It is good if the characteristics are flat and a predetermined gain can be obtained.

The gain equalizers 21 to 23 can be realized by combining a plurality of Fabry-Perot etalon filters, dielectric multilayer filters, and fiber grating filters.

The input end optical branching coupler 71 splits a part of the input light and inputs it to the input monitor PD81, and the output end side optical splitting coupler 72 receives the light amplified by the semiconductor optical amplifiers SOA33-35. Branch and input to the output monitor PD.

The automatic gain control circuit (AGC) 50 is an input monitor PD
Further, based on the light detected by the output monitor PD, the excitation current bias level of the semiconductor optical amplifiers SOA33-35 is controlled so that the gain of the optical amplifier becomes a constant value.

When it is desired to control the output level to be constant using the automatic gain control circuit (AGC) 50, a variable attenuator is provided at the position of the input terminal 8 or the output terminal 9 to provide an optical amplifier. By controlling the input optical signal level or the output of the optical amplifier, the output of the optical amplifier can be kept constant even if the gain is controlled by the optical amplifier.

FIG. 10 shows a case where a silica-based erbium-doped fiber is used as an optical amplifying medium, and a Fabry-Perot resonator is constituted by two fiber grating reflectors (FG-Mirrors) including the optical amplifying medium. Automatic gain control by input / output monitor This is a constant gain system configuration that eliminates the need for AGC control.

FIG. 10 shows a 9: 1 coupler (CPL) 7 in the configuration of FIG.
Fiber grating mirrors 42 and 43 are provided at the ends where 10% is branched by 3, 74 and 9: 1 couplers (CPL) 73, 74.

The first 9: 1 coupler (CPL) 73 includes the wavelength multiplexing coupler 5 and the first silica-based erbium-doped fiber 21.
(EDF1).

A first fiber grating mirror (FG-Mirror) 4 is provided at a branch point of the first 9: 1 coupler (CPL) 73.
2 are provided.

The second 9: 1 coupler (CPL) 74 is provided between the gain equalizer 23 (GEQ50) and the optical isolator 32.

A second fiber grating mirror (FG-Mirror) 4 is provided at a branch point of the second 9: 1 coupler (CPL) 74.
Three are provided.

In the following, the above configuration is replaced by S? An operation example in the case of using the wavelength multiplexing optical amplifier for band will be described.

First, no signal light exists at a wavelength of 1530 nm.

Amplified by the silica-based erbium-doped fiber 1 and subjected to gain equalization by a plurality of gain equalizers, the S? The gain wavelength characteristic of the band is obtained and output.

The silica-based erbium-doped fiber 1
The light amplified by is output by the second 9: 1 coupler 74 to the optical isolator 32, and the remaining 10% is output to the fiber grating mirror (FG-Mirror) 43.

A fiber grating mirror (FG-Mirro
r) 43 reflects light of 1530 nm in a band of ± 0.1 nm at a wavelength of 1530 nm, and returns the silica-based erbium-doped fiber 1 via a second coupler 74.

The silica-based erbium-doped fiber 1 amplifies this return light, splits the split light by a first 9: 1 coupler 74, and inputs it to a first fiber grating mirror (FG-Mirror) 73.

Fiber grating mirror (FG-Mirro
r) 42 reflects light of 1530 nm in a band of ± 0.1 nm at a wavelength of 1530 nm, and returns to the silica-based erbium-doped fiber 1 again through the first coupler 73.

As a result, the 1530 nm Fabry-Perot resonator is composed of two fiber grating mirrors and a 9: 1 coupler (CPL) 2
And EDF, which is the amplification medium.

In this configuration, when the population inversion is formed by the excitation of the EDF, a laser beam of 1530 nm is output at a wavelength of 1530 nm while satisfying the laser oscillation conditions.

When the laser is oscillating, the population inversion ratio is fixed at a constant value (the gain is also constant), so that the wavelength characteristic of the gain and the gain are constant even if the input is changed.

As the signal input increases, the pump light power consumed for amplifying the signal light increases, and finally, the laser operation at 1530 nm stops.

When the laser operation stops, the gain does not become constant.

In this case, the fiber grating mirror is formed so as to oscillate at a wavelength of 1530 nm, and Fabry-Perot resonance is performed. However, a wavelength at which an energy inversion distribution occurs in the optical amplification medium at a wavelength capable of transmitting through the gain equalizer. Any location in the S-band where signal light does not exist may be used.

The structure of the resonator is not limited to Fabry-Perot resonance, but may be a ring type resonator.

The S-band has been described as an example.
However, the wavelength may be any wavelength in the amplification band as long as it is a wavelength at which energy inversion distribution occurs in the optical amplifying medium at a wavelength passing through the gain equalizer and where no signal light exists.

When the gain equalizer is configured as shown in FIG. 4, most of the portion where the gain is caused by the population inversion is discarded. Therefore, the gain equalizer that transmits a part of the wavelength of the discarded band is used. To resonate with the wavelength,
The reflection wavelength of the fiber grating mirror may be selected.

The reason why the gain of the signal light due to the laser oscillation becomes constant will be described with reference to FIGS.

FIG. 11 illustrates a case where a gain characteristic due to laser oscillation is made constant in a conventional C-band band optical amplifier.

A C-band optical signal is input to 1552 nm, and optical amplification is performed by an optical amplification medium.

By arranging an optical coupler at both ends of the optical amplifying medium as in FIG. 10 and providing a fiber grating mirror for reflecting 1530 nm at the branch destination to form a resonator, laser oscillation can be confirmed at 1530 nm.

FIG. 12 is a graph in which signal light having a wavelength of 1552 nm is used as in FIG. 11, and the gain is measured by changing the level at which the signal light is input to the optical amplification medium.

The characteristics indicated by Δ indicate the characteristics when there is no resonator and laser oscillation is not performed. It can be seen that the gain changes when the input level is changed.

The characteristics indicated by □ indicate characteristics when a resonator that resonates at 1530 nm is formed in the optical amplification medium and laser oscillation is performed.

From this characteristic, even if the input level is changed, -35 dBm
It can be seen that the gain is fixed over a wide input range of -10 dBm.

In FIG. 12, the laser operation stops at an input of about -7 dBm.

FIG. 13 shows an example in which a broadband optical amplifier is constructed using the present invention.

The wavelength-multiplexed optical signal is transmitted to the transmission line 57 composed of an optical fiber.

The transmission line 57 is a single mode fiber SMF (1.
3μm zero dispersion fiber), dispersion compensating fiber (fiber having a negative dispersion value with respect to SMF), dispersion shift fiber DSF (fiber whose zero dispersion value is within the transmission signal wavelength band), non-zero dispersion shift fiber NZ-DSF (Fiber having a zero dispersion value adjacent to the transmission signal wavelength band) can be used.

The transmission path 57 is excited by the excitation light source 56 via the wavelength division multiplexing coupler 66, thereby performing Raman amplification, and performing distributed amplification on the transmission path, thereby dividing the transmission path into each subsequent wave band. Noise figure (NF) in the case of amplification can be improved.

The wavelength-multiplexed optical signal Raman-amplified on the transmission line by the pump light source 56 is subjected to L-band and C-ba by a WDM filter.
It is divided into each wavelength band of nd and S-band. (Here L-ban
The wavelength band separation is not limited to d, C-band, and S-band, but may be performed according to the gain wavelength band of the optical amplifier. ) L-band optical amplifier 60, C-band optical amplifier 61, S-band optical amplifier 62
, And optical amplification is performed.

The C-band optical amplifier 61 is an L-band optical amplifier 61-1, 6
1-2, branch coupler 75, dispersion compensation fiber 53, variable attenuator 5
2. It is composed of an automatic gain control circuit 50 and an automatic level control circuit 51.

The automatic gain control circuit 50 comprises a C-band optical amplifier 61-
By detecting the input and output powers of the C-band optical amplifiers 61-1 and 61-2, the C-band optical amplifiers 61-1 and 61-2 are controlled so that the population inversion ratio is about 0.7 and the gain is constant.

As a result, amplification with a small gain deviation at 1530 nm to 1570 nm is realized.

The dispersion compensating fiber 53 is provided for compensating the dispersion of the transmission line.

The variable optical attenuator 52 is controlled by the automatic level control circuit 51 so that the output of the C-band optical amplifier 61 becomes a constant value.
The output of the band light amplifier 61-1 is attenuated.

The L-band optical amplifier 60 is an L-band optical amplifier 60-1, 6
0-2, branch coupler 75, dispersion compensation fiber 53, variable attenuator 5
2. It is composed of an automatic gain control circuit 50 and an automatic level control circuit 51.

The automatic gain control circuit 50 comprises a C-band optical amplifier 61-
The L-band optical amplifiers 61-1 and 61-2 are controlled so that the population inversion ratio is about 0.4 and the gain is constant by detecting the input and output powers of 1,61-2.

Thus, amplification with a small gain deviation at 1570 nm to 1610 nm is realized.

The L-band optical amplifying units 60-1 and 60-2 have the same gain as the C-band amplifying unit from 1570 nm to 1610 nm even if the inversion distribution ratio is low and the gain is kept constant by the automatic gain control circuit 50. The length of EDF, which is the amplification medium, is adjusted so that it can be obtained.

The dispersion compensating fiber 53 is provided for compensating the dispersion of the transmission line.

The variable optical attenuator 52 is controlled by the automatic level control circuit 51 so that the output of the L-band optical amplifier 60 becomes a constant value.
The output of the band light amplifier 60-1 is attenuated.

The S-band optical amplifier 62 includes S-band optical amplifiers 62-1, 6
2-2, branch coupler 75, dispersion compensation fiber 53, variable attenuator 5
2. It is composed of an automatic gain control circuit 50 and an automatic level control circuit 51.

The S-band optical amplifiers 62-1 and 62-2 make the automatic gain control circuit 50 equal to or higher than the inversion distribution ratio C-band based on the input / output power branched by the branch coupler, and The gain is controlled to be constant.

Each of the S-band optical amplifiers 62-1 and 62-2 has a gain equalizer therein as described in the previous embodiment, and has a gain of 1590 nm to 153 nm.
EDF-amplified light, which is the amplification medium, is equalized so that the same gain as that of the C-band amplifier is obtained at 0 nm.

The S-band optical amplifiers 62-1 and 62-2 can apply all the configurations and wavelength bands of the other embodiments of the present invention described above.

The dispersion compensating fiber 53 is provided for compensating the dispersion of the transmission line.

The optical variable attenuator 52 is controlled by the automatic level control circuit 51 so that the output of the S-band optical amplifier 62 becomes a constant value.
The output of the band light amplifier 60-1 is attenuated.

L-band optical amplifier 60, C-band optical amplifier 61, S-ba
The output of the nd optical amplifier 62 is optically multiplexed by the WDM coupler 55 and output to the transmission line.

Although FIG. 13 is described based on the combination of the S-band and the existing L-band and C-band, it may be combined with another wavelength band described in FIG.

[0202] The specific configuration to be implemented is not limited to only three wavelength bands, but may be a combination of two or more wavelength bands.

[0203]

According to the present invention, the population inversion ratio of the optical amplifying medium is increased, the band in which the gain is generated in the optical amplifying medium is widened, and the gain characteristic is flat at a position (shoulder portion of the gain characteristic) different from the peak value in this band. In order to obtain the desired gain, the length of the roughened medium is selected based on the gain characteristics of the gain equalizer and the optical amplifying medium. S? By fiber (EDF) band, S +? band
Obi, L +? An optical amplifier having a practical gain in the band can be realized.

Furthermore, the efficiency of conversion from pump light to signal light is improved by dividing the optical amplifying medium into a plurality of parts and distributing or distributing gain equalizers between the optical amplifying mediums. be able to.

If the optical amplifying medium can be made compact, a grating is provided in the optical waveguide portion of the optical amplifying medium and the gain is equalized, so that the conversion efficiency is the same as that of the configuration in which the gain equalizers are arranged in a distributed manner. Can be improved.

Therefore, an optical fiber amplifier having a new band other than the C-band and L-band can be realized, which contributes to a further increase in capacity.

[Brief description of the drawings]

FIG. 1 shows an amplifying band of an optical fiber amplifier.

[Figure 2] Silica-based erbium-doped fiber (EDF)
5 shows the wavelength characteristic of gain at the inversion rate of Inversion.

FIG. 3 shows a gain wavelength characteristic with an inversion rate of 0.9 in FIG. 2 and a gain characteristic for extracting and equalizing an S-band gain.

FIG. 4 is a diagram illustrating wavelength characteristics of a gain equalizer.

FIG. 5 is a diagram showing a first embodiment.

FIG. 6 is a diagram showing a second embodiment.

FIG. 7 is a diagram showing a third embodiment.

FIG. 8 is a diagram illustrating gain characteristics of the semiconductor optical amplifier.

FIG. 9 is a diagram showing a fourth embodiment.

FIG. 10 is a diagram showing a fifth embodiment.

FIG. 11 is a diagram showing spectrum characteristics when laser oscillation is performed.

FIG. 12 is a view for explaining the principle that the gain becomes constant by laser oscillation.

FIG. 13 is a diagram showing a broadband optical amplifier using the present invention.

[Explanation of symbols]

 1,11,12,13 are silica-based erbium-doped fibers (EDF) 2,21,22,21 are gain equalizers (GEQ) 31,32 are optical isolators 33,34,35 are optical semiconductor amplifiers 4, pumped Light source 5 is wavelength multiplexing coupler 8 is input end 9 is output end 71,72,73,74 is optical branching coupler 81 is input monitor PD 82 is output monitor PD 50 is constant gain control circuit (AGC) 42,43 is fiber grating mirror

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04J 14/00 14/02 H04B 10/17 10/16 F term (Reference) 5F072 AB09 AK06 HH02 HH06 JJ05 KK07 KK08 KK15 KK30 MM03 MM07 PP07 QQ07 RR01 YY17 5K002 AA06 BA05 BA13 CA10 CA13 DA02 FA01

Claims (12)

[Claims] 1. An optical amplifying medium for amplifying light, excitation means for exciting at least one gain peak in the optical amplifying medium, and a gain equalizer for equalizing the gain of the optical amplifying medium. Wherein the gain equalizer performs equalization so as to generate a gain in an optical wavelength region where the gain of the optical amplifying medium does not become maximum. 2. An optical amplifying medium for amplifying light, excitation means for exciting the optical amplifying medium to generate at least one gain peak, and equalizing the gain of the optical amplifying medium to the entire optical amplifying medium. An optical amplifier, comprising: a gain equalizer that performs gain in an optical wavelength region where the gain of the optical amplifying medium is not maximized. 3. A plurality of optical amplifying media for amplifying light, pumping means for exciting the plurality of optical amplifying media to generate at least one gain peak, and the optical amplifying device between the plurality of optical amplifying media. A plurality of gain equalizers for equalizing the gain of the medium are provided, and the plurality of gain equalizers amplify light in a wavelength region where the gain generated by the plurality of optical amplifying media and the pumping unit is not maximized. Characteristic optical amplifier. 4. An optical amplification medium doped with a rare earth element, an excitation light source for exciting the rare earth element of the optical amplification medium, and a wavelength characteristic of an optical amplification gain generated when the amplification medium is excited by the excitation light source. The optical pumping light source has a population inversion ratio in which a gain occurs in a wavelength band of the signal amplified by the optical amplification medium, and a wavelength band of the signal amplified by the optical amplification medium. An optical amplification method using a band that does not include a maximum value of a gain generated in the optical amplifying medium by the pumping light source, and wherein the gain equalizer attenuates a maximum value of a gain of the optical amplifying medium. 5. The optical amplifier according to claim 1, wherein an input and an output of said optical amplification medium are monitored, feedback is applied to an excitation means, and the gain of said optical amplification medium is controlled to be constant. 6. The optical amplifier according to claim 1, further comprising a resonator including said optical amplifying medium. 7. The optical amplifying medium according to claim 1, wherein the optical amplifying medium is an optical amplifier that causes stimulated emission in the medium by excitation.
An optical amplifier according to any one of the preceding claims. 8. An optical amplifying medium comprising an erbium-doped fiber is excited so as to have a population inversion ratio of 0.7 to 1, transmits a wavelength in a wavelength band of about 1490 nm to about 1530 nm, and converts the wavelength band to a substantially flat wavelength. An optical amplifier characterized in that the amplification medium is equalized so as to have characteristics. 9. An optical amplifying medium comprising an erbium-doped fiber is excited so as to have a population inversion ratio of about 0.8 to 1, transmits a wavelength in a wavelength band of about 1450 nm to about 1490 nm, and substantially transmits the wavelength band. An optical amplifier characterized in that the amplification medium is equalized so as to have a flat wavelength characteristic. 10. An optical amplifying medium comprising an erbium-doped fiber is pumped so as to have a population inversion ratio of about 0.3 to 1, transmits a wavelength in a wavelength band of about 1610 nm to about 1650 nm, and substantially transmits the wavelength band. An optical amplifier characterized in that the amplification medium is equalized so as to have a flat wavelength characteristic. 11. A wavelength band used for optical communication is selected by selecting a population inversion ratio of the optical amplifying medium, expanding a band having a gain generated in the optical amplifying medium, and at a position different from a peak value of the gain generated in the optical amplifying medium. The gain is equalized so that the gain becomes flat in the wavelength band used for the optical communication, and the wavelength band where the gain occurs in the optical amplification medium other than the wavelength band used for the optical communication is attenuated. Characteristic optical amplification method. 12. An optical amplifying medium having at least one peak of an amplification gain due to the generation of a population inversion of energy by excitation, and an amplifying medium having a substantially flat gain band at a wavelength lower than the peak value of the amplification gain. An optical amplifier comprising an equalizer for equalizing a gain characteristic.